Continuous digester
A continuous pulp digester having an elongate horizontally aligned pressure vessel through which the wood chips and digesting fluid flow in a forward direction. In a first and second embodiments within the pressure vessel there is an inner container defining an elongate chamber or passageway having a square cross sectional configuration. In a third embodiment the digesting chamber is cylindrical. At locations along the digester, there are several pair of liquid flow inlets and liquid flow outlets which enable filtrate from a pulp washer and a digesting agent to be moved into and across the digesting chamber to flow out the flow outlets. These are recirculated in a net upstream flow pattern toward the outlet end of the digester, and discharged as black liquor at different locations in the digester.
This application claims priority benefit of U.S. Provisional Patent Application No. 60/000,830 filed Jun. 29, 1995, Provisional Patent Application No. 60/004,474 filed Sep. 28, 1995, U.S. patent application Ser. No. 08/672,458, filed Jun. 28, 1996, and U.S. patent application Ser. No. 10/084,289 filed Feb. 25, 2002.
a. BACKGROUND OF THE INVENTIONThe present invention relates generally to the art of wood pulp digesters, and more particularly to a continuous digester.
b. BACKGROUND ARTIn a typical continuous pulp digester, the wood chips and the white liquor are fed into the upper end of a vertically aligned digester, with the interior of the digester defining a cylindrical digesting chamber maintained at a relatively high pressure (e.g. 200 PSI) and high temperature (e.g. approximately 380° F.). The ixture of chips and white liquor moves slowly and downwardly through the digester so that the total dwell time within the digester is generally between about two to four hours. During the period that the wood ships are in the digester, the white liquor reacts with the material in the wood chips to break down certain organic compounds in the wood chips so as to “delignify” the pulp.
At several locations along the length of the digester, portions of the liquid are extracted, either to be recirculated back into the digester, sent to an evaporator, or possibly to be processed elsewhere in the system. To retain the wood chips that are being processed in the digester, the liquid is extracted through sets of screens which are generally placed in sets at vertical locations circumferentially around the digester.
SUMMARY OF THE INVENTIONThe continuous digester system of the present invention comprises a pressure vessel having a lengthwise axis, a rear upstream inlet end having a wood chip intake means, and a front outlet means having a pulp outlet means. The vessel has an elongate processing chamber through which wood chips travel forwardly in the presence of a digesting agent while being transformed into pulp, with the pulp being discharged from the pulp outlet means at the front outlet end of the vessel. There is a liquid flow means to circulate the processing liquid through said digester system to carry dissolved solids with said processing liquid, said flow means comprising the following:
-
- i. an initial inlet means to initially introduce processing liquid into the pressure vessel at an an initial inlet downstream location;
- ii. a plurality of processing liquid inlet means at inlet locations along the lengthwise axis of the pressure vessel to introduce processing liquid into the processing chamber;
- iii. a plurality of processing liquid outlet means at outlet locations along the lengthwise axis of the pressure vessel to extract processing liquid from said processing chamber, said outlet locations being spaced laterally from said inlet location so that flow of said processing liquid from each of said inlet means to related outlet means has a lateral flow component through said processing chamber;
- iv. recirculating means comprising a plurality of interconnecting line means, at least some of said interconnecting line means connecting at least some of the outlet means with related inlet means at further upstream locations to direct processing liquid from said at least some of said liquid outlet means through related interconnecting line means to further upstream locations to flow through the related inlet means into the processing chamber and laterally in the processing chamber to other outlet means to again be recirculated through related interconnecting line means to other inlet means;
- v. liquor outlet means to discharge liquor for further processing, said liquor outlet means being upstream of the initial downstream location and upstream of at least some of said liquid inlet means and said liquid outlet means.
The digesting system is characterized in that the processing liquid moving in a recirculating pattern through the processing chamber and through the recirculating means carries dry solid content extracted from the wood chips during processing in the processing chamber in a net upstream flow pattern to be discharged from the processing chamber at said liquor outlet means.
In the preferred system, there is a washer to receive pulp from the digester and to dewater and wash the pulp. A substantial portion of filtrate from the washer is directed into the initial inlet means to move through the recirculating means into said net upstream direction.
Also, in the preferred form, the digesting agent is introduced into the liquid flow means to flow through the recirculating means in a net upstream direction to extract dry solids content from the wood chips being processed and carry these in the net upstream direction.
In the preferred form, the system further comprises an evaporation and recovery means to receive liquid discharged from the pressure vessel at a plurality of discharge locations at different operating locations in said pressure vessel so as to extract liquor having different characteristics from different extraction locations.
In the preferred embodiments of the present invention, the digesting agent is alcohol. The evaporation and recovery means extracts the alcohol from the liquor and recirculates the recovered alcohol back to liquid flow means to be recirculated into the liquid flow means.
Also, in the preferred form the system comprises an impregnation zone located in the pressure vessel at a more upstream location. There is at least one cooking zone located downstream of the impregnation zone, and at least one wash displacement zone located downstream of the cooking zone. At least some of the liquid inlet means and liquid outlet means are located at said displacement wash zone to receive the processing liquid and recirculate the processing liquid sequentially through related pairs of the liquid inlet means and the liquid outlet means. The flow means further comprises means to move the processing liquid from the wash displacement zone to an upstream location to be directed into the cooking zone to flow in a downstream direction in the processing chamber toward the displacement wash zone.
Desirably, processing liquid from the dislacement wash zone is recirculated to the impregnation zone to flow downstream in said vessel through said impregnation zone and into said cooking zone. Also, liquor is extracted from the impregnation zone and directed to the evaporation and recovery means for processing. In this arrangement, liquor also is extracted from the cooking zone and directed to the evaporation and recovery means for processing.
In the system of the present invention, the net upstream flow created by the liquid flow means comprises at least one displacement zone having a downstream end and an upstream end, with a plurality of a liquid outlet means being positioned at longitudinally spaced locations along a length of the displacement wash zone and a plurality of the outlet means positioned at spaced locations along the length of the displacement zone. The pluralities of liquid outlet and liquid inlet means are arranged so that there are related first downstream and second upstream liquid inlet means being arranged relative to related first downstream and second upstream outlet means in a manner that at least a portion of flow from the first downstream inlet means flows through the processing chamber to pass into the first downstream outlet means. Then at least a portion of the flow into the first downstream inlet means is recirculated to the second upstream inlet means, with at least a portion of the flow from the second liquid inlet means flowing across the processing chamber to the second upstream inlet means. At least a portion of the flow from the second upstream inlet means is recirculated by the recirculating means in an upstream direction. In this manner, the net upstream flow of processing liquid is accomplished.
The pressure vessel has a generally cylindrical cross sectional configuration transverse to its lengthwise axis. In two embodiments, there is inside the pressure vessel an inner-containing means, defining the elongate processing chamber, and comprising at least in part planar wall surfaces. Screen means are located at longitudinally spaced locations at the planar wall surfaces, so that at least some of the liquid inlet means passes liquid into the processing chamber through the screen means, and at least some of the liquid outlet means discharges processing liquid through related screen means. Desirably, there is propeller blade means which move across related screen means to prevent obstruction of flow through the screen means.
In another configuration, the cylindrical sidewall itself defines the processing chamber. At least one of the liquid inlet means and outlet means comprises liquid passageway means formed in the cylindrical sidewall, with the passageway means having flow axes which are slanted in a radially inward and forward direction. This flow means desirably comprises a plurality of circumferential ring assemblies positioned at longitudinally spaced locations along the sidewall. Each ring assembly defines a flow chamber to communicate with related passageway means extending through the wall member.
Also, in a preferred form, adjacent pairs of aligned liquid inlet means and liquid outlet means are arranged in an angularly alternating relationship, so that a cross flow of processing liquid between such adjacent alternating pairs have different flow directions through the processing chamber.
In a preferred form, the evaporation and recovery means comprises at least first and second heat exchange means and first and second separator means. The first evaporator means initially receives the liquor from the pressure vessel and after evaporation discharges liquor which is then directed to the first separator means, where a portion of the liquor is separated. The remaining liquor is directed to the second heat exchange. Then the liquor from the second heat exchange means is directed to the second separator means to extract another portion of the liquid from the second heat exchange means.
In preferred form, the pressure vessel is aligned so its major alignment component is horizontal.
In the method of the present invention, a pressure vessel and flow system is provided as described above. The flow of the wood chips is in a downstream direction, while there is a recirculation of the processing liquid in an upstream direction, as described above.
Other details of the present invention will become apparent from the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
1. Typical Prior Art Digesting Process
It is believed that a clearer understanding of the present invention will be obtained by first reviewing the digesting portion of a typical pulp mill for which the present invention is particularly adapted. With reference to
The feeder 4 directs the chips into a steaming vessel 5 that is kept at between 15 to 20 PSI where the chips are pre-steamed. The chips are then directed from the steaming vessel 5 into the chip chute 6, from which the chips move to a high pressure feeder 7. The chips are flushed into the feeder by means of a chip chute circulating pump 8. As seen in
In general, the digester pressure is controlled so as to be at about 200 PSI. The chips and the cooking liquor gradually move downwardly in the digester, first passing into an upper impregnation Zone I and then to the heating Zone II.
The temperature is raised in two steps by two cooking circulating systems, which comprise extraction strainers, pumps and central circulating chambers. Three heaters 13 are shown. After heating, the chips and liquor pass downwardly through the cooking zone III of the digester. As the chips then pass into the lower washing zone IV of the digester, extracted wash liquor is circulated through the chips to provide a quench of the cooking reaction. The chips continue to pass downwardly in the washing zone IV, then to be discharged. The entire sequence is arranged so that the duration of the digesting process is about one and one half to four hours.
Wash liquor from a subsequent filtrate tank or fresh hot water is pumped into the bottom of the digester and flows inwardly countercurrently to the chip flow. Elevated temperatures of 125° C. (to 135°) are controlled in the diffusion zone by an auxiliary wash liquor circulation and heater system.
At various locations in the digester, the liquor is recirculated to an upper location. A portion of the liquor that is extracted between zone III and zone IV is directed to a flash tank 17, and thence to flash heat evaporators. The pulp that is extracted from the bottom of the digester is directed to a blow unit 16 which has a pressure reducing function, and then further directed to a brown stock washer 19 and/or to some other location for further processing as indicated schematically at 20.
A. First Embodiment
2. The Overall Structure and Operation of a First Embodiment of the Digester
In this section there will be a brief description of the overall construction and operation of the digester and the digesting system of the first embodiment of the present invention. Then in the following sections there will be descriptions of the main structural components which are particularly adapted for use in this first embodiment, and also a more detailed description of the overall operation of the digesting system and other aspects and variations of the same.
To describe the first embodiment of the present invention, reference is first made to
The wood chips are introduced into the inlet end 104 through an inlet of the digester 102 by conventional means and are mixed with the liquor in the digester. Over a period of several hours (e.g. usually two to four hours), the wood chips move continuously down the length of the digester 102 and proceed through various processing zones. When the wood chips reach the exit end 106, these have been substantially delignified, and the pulp is diluted with filtrate from the washer 108 and then passed into the washer 108.
In the following description, the term “wash water” means the fresh water which is introduced into the washer 108. The term “filtrate” shall refer to the liquid which is removed from the pulp in the washer 108 during the dewatering operation (which will be called the “dewatering filtrate”), and the effluent which is discharged from the washer to be utilized at another location of the digester (this being called the “washer discharge filtrate”. The term “liquor” or “liquors” shall refer to all of the liquid which is in the digester and has as one of its ingredients the digesting ingredient (which in this preferred embodiment is ethyl alcohol). Finally, the term “black liquor” shall refer to the liquor which is discharged from the digester for further processing. This discharge of black liquor takes place adjacent to the wood chip inlet end 104 of the digester 102.
The term “forward” shall denote a direction extending from the wood chip intake end 104 to the pulp discharge end 106, so that the rear end will be at 104 and the forward end will be the end at 106. The term “downstream” shall denote the direction of flow of the wood chips which are being processed in the digester, this direction being from the end 104 to the end 106, and the term “upstream” shall denote the opposite direction. The term “inner” shall denote proximity to a longitudinal center axis of the digester 102, and the term “outer” or “outward” shall denote a direction away from the longitudinal center axis of the digester 102 and/or a location more distant from the longitudinal center axis or line of the digester 102.
The filtrate from the washer in addition to being recirculated to dilute the pulp that exits from the digester end at 106, provides part of the liquid to form the liquor which is used in the digesting process within the digester 102. This is accomplished in a manner that the filtrate from the washer 108 enters the digester vessel 102 adjacent to the downstream end, and then is recirculated through the digester in a manner that the “net flow path” is in an upstream direction. This will be described in more detail later herein, but the following will give a brief summary of how this is accomplished.
In the preferred embodiment, the digesting ingredient is ethyl alcohol. As will be discussed more fully hereinafter, the present invention particularly adapts itself for the effective use of ethyl alcohol and solves problems which have been experienced in the prior art where ethyl alcohol is used as the digesting ingredient. However, within the broader scope of the present invention, it is to be understood that other digesting ingredients could be used and derive a good portion of the benefits of the present invention.
There are five main components in the digester 102 which are combined with other components of the system. As indicated above, there will now be descriptions of each of these five components in more detail.
3. Typical Cross Sectional Configuration of the Digester 102
The term digester 102 shall refer not only to the high pressure vessel which is the containing structure, but also to those components within the containing structure. Thus, with reference to
The rear end of the digester vessel 110 is closed by a substantially hemispherical rear wall 112, and the front end is closed by a substantially hemispherical forward wall 114.
Positioned within the pressure vessel 110 is an inner container 116 which has a substantially square cross sectional configuration and which extends substantially the entire length of the digester 102 (See
This inner container 116 defines an elongate chamber or passageway 118 (also having a square cross sectional configuration) which is the digesting area. This area 118 contains what are initially the wood chips and the digesting liquid (i.e. the liquor). The inner container 116 has an upper wall 120, a lower wall 122 and right and left sidewalls 124 and 126, respectively. These walls are joined to one another at corner locations which are designated (beginning at the upper right hand corner of
Positioned between the inner surface 136 of the vessel 110 and the inner container 116, there is a plurality of reinforcing plates 138 welded or otherwise joined both to the outer pressure vessel 110 and the inner container 116. These plates 138 are positioned around all four walls of the inner container 116 at longitudinally spaced intervals along the length of the digester 102. These plates are provided with openings 140 so that a pressure equalizing fluid (either gaseous or liquid) can communicate between the areas 142 outside of the square container 116 between adjacent pairs of plates 138.
As indicated previously, the digesting process takes place at pressures as high as 200 to 500 PSI and temperatures as high as 150 to 200° C. The pressure vessel 110 is designed to withstand these high pressures and also to provide thermal insulation. Accordingly, the temperature and pressure levels within the inner chamber 118 should be substantially the same as the pressure and temperature of the areas 142 between the inner container 116 and the pressure vessel 110. This is accomplished by filling the areas 142 (which areas 142 have in cross section the shape of a segment of a circle) with a liquid or gaseous medium which would be kept at the same pressure as exist with the chamber 118 of the inner container 116. For this purpose, there are shown nozzles 144 which communicate with a fluid such as steam, or possibly a suitable liquid (e.g. condensate) to fill these areas 142 surrounding the inner container 116. These nozzles 144 will be provided for all space or areas of the vessel 110 which surround or are adjacent to the inner container 116.
As indicated above, what is shown in
4. Fluid Inlet Unit of the Digester 102
Reference is made to
In place of the top wall 120, there is provided an inlet screen 152 which is positioned in a plane extending between the two top corner lines 128 and 130. This screen 152 has a circular configuration and is mounted to a plurality of radially extending bracing arms 154 which in turn connect to a central hub 156. The screen assembly 158 (made up of the screen 152, the bracing arms 154 and the hub 156, is rotatably mounted within a surrounding plate 160 which has an outer square perimeter and a circular cutout to receive the screen 152 and the bracing arms 154.
The hub 156 is connected (e.g. by the nut and drive shaft connection 162) to a hydraulic actuator 164. This hydraulic actuator 164 has an output shaft 166 which is caused to rotate in a reciprocating manner through 180° of rotation. Thus, the actuator 164 will rotate the screen assembly 158 180° in one direction, then 180° in the opposite direction.
There is provided a longitudinally aligned wiper blade 168 (
The screen assembly 158 and the rotary actuator 164 are mounted to a flat circular mounting plate 172 that is in turn mounted to a cylindrical plate 174 that is in turn welded or otherwise joined at 176 to an opening in the vessel wall 110. This mounting plate 172 and the cylindrical plate 174 are of steel construction and of sufficient strength to withstand the pressures within the vessel 110.
There are also partial bracing plates 178, but the inner edge 180 of these plates 178 is spaced a short distance outwardly of the arms 154. There is a fluid inlet nozzle 182 which leads into the area or space 184 which is between the screen assembly 158 and the adjacent wall portion 186 of the vessel 110.
The plate 160 which surrounds the screen assembly 158 has a square configuration around its perimeter. At the forward and rear edges of the plate 160, there are forward and rear isolating plates 188 which form an isolated chamber which is defined at the inner location by the screen 152 and the surrounding plate 160, on the outside by the adjacent portion 186 of the vessel 110, and at the forward and rear ends by the forward and rear plates 188.
In operation, the effluent which is to be directed into the vessel 110 is directed through the inlet nozzle 182 at a pressure slightly higher than the fluid within the chamber 118. This fluid entering through the nozzle 182 distributes itself throughout the area or volume 184 behind the screen assembly 158 and thus passes substantially uniformly through the screen 152 into the chamber 118. As this happens, the screen assembly 158 is rotated by the actuator 164 through the 180° arcuate path of travel to wipe the screen 152 free of any material which might clog the screen 152. This reciprocating rotation of the screen assembly 158 may not need to be done continuously, but could be done intermittently to keep the screen 152 open. (e.g. every two to fifteen minutes or longer). The screen is rotated slowly (e.g. one to five revolutions per minute depending on the position in the digester, faster at the forward end and slower at the rear end of the digester)
As indicated above, this modular fluid inlet unit 148 is used at different locations along the length of the digester 102 to direct fluid into the digester 102. The various functions performed by this unit 148 will be discussed later herein.
5. Fluid Outlet Unit of the Digester 102
Reference is made to
Thus, there is a screen assembly 158a comprising the screen 152a, the support arms 154a, and the hub 156a. There is a rotary actuator 164a along with its output shaft 166a, and also the wiper arm 168a. Other components will simply be given numerical designations with the “a” suffix without further verbal description.
The main difference in this unit 190 is that the nozzle 182a, instead of being an inlet nozzle is an outlet nozzle. Thus, the nozzle 182a is operated at a pressure slightly below that which exists within the digesting chamber 118.
Also, since the flow from the digesting chamber 118 is outwardly through the screen 152a, there may be some tendency for the wood chips which are being processed into pulp to tend to accumulate on the screen 152a. The rotation of the screen assembly 158a relative to the blade 168a alleviates this problem.
As will be discussed later herein, most all of the inlet units 148 are positioned so that each is adjacent to and immediately upstream of, a related outlet unit 190. The pressure differential from the chamber 184 of the inlet unit 148 to the chamber 118 and thence to the outlet chamber 184a is such that it causes a fluid flow into the chamber 118 (through the screen 152) and outwardly from the chamber 118 (through the screen 152a). As the wood chips/pulp and liquor move forwardly in the chamber 118, the fluid flows outwardly through the screen 152 displaces the liquor in the chamber 118 downwardly so that as the displaced fluid moves downwardly through the chamber 118 it is also moving forwardly. As this liquor reaches the lower wall of the inner container 116, it is then adjacent to the outlet screen 152a so that this displaced liquid then flows out the lower conduit 182a. This operation is accomplished at various locations along the length of the digester 102 and will be described more completely in the next section.
To describe the rotary actuators 164 and 164a, reference is made to
In the area of the rack and pinion gear teeth 194 and 196, there is maintained an oil pressure moderately higher than the pressure in the digesting chamber 118. Seals are provided around the shaft 166 to prevent the oil in this area from flowing into the interior of the digester 102. Also, a seal is provided between the housing of the actuator 164 and the plate 172.
6. Flow Patterns in Displacement Zones
Reference is now made to
As the wood chips are introduced into the rear end 104 of the digester 102 and progress through the digester 102, they lose their character as wood chips and become delignified pulp fibers. The consistency of the pulp in the digester is typically about 12.5%, which means that there are two parts liquid within the fibers and five parts liquid surrounding the fibers.
In the right hand part of
The diagonal cross flow between the units 148 and 190 does not have any significant tendency to compress this plug of pulp fibers, but the flow passes through the spaces surrounding the pulp fibers.
With further reference to
It is to be understood that the interface plane 208 is not a clearly defined plane and the adjacent liquors tend to combine to some extent in a mixing zone along the plane 208.
A portion 212 of the flow of liquor from the screen 152 follows a flow path to the screen 152a at 214. Another portion of the liquor from the screen 152 follows more of a diverging downstream flow, indicated by the lines 216.
From the above description, it is apparent that a high percentage of the liquid flow at 202 is displaced into the screen 152a for recirculation in an upstream direction. Also, a significant percentage of the flow from the screen 152 is directed in a downstream direction. However, this flow at most locations is subjected to a further downstream displacing action to be recirculated back up to an upstream location. The advantage of this will become more apparent in reviewing the later description of the overall operation of the system 100 of the present invention.
7. Pulp Diluting Unit of the Digester
Reference is made to
The function of this diluting unit 220 is to deliver a large portion of the filtrate from the washer 108 into the front end of the chamber 118 to bring the consistency of the wood pulp/liquor mix (which is at about 12½% consistency in the digester 102) to about 2% to 4% consistency. It can be seen that the four walls of the inner container 116 are removed and replaced by four inlet flow assemblies 150b, each with its screen assembly 158b. Each screen assembly 158b has, as in the prior modular units 148 and 190 the hydraulic actuator 164b (or hydraulic motor) and the associated mounting plates 172b and 174b. All of the nozzles 182b are inlet nozzles so that there is a net inflow of filtrate from the washer 108 into the chamber 116 from all four sides.
There is connected to each rotary actuator shaft 166b a related mixing arm 222 which has a radially outward right angle elbow section 224 positioned laterally of the axis of the shaft 166. This elbow section 224 connects to a forearm section 226 that terminates at a middle location 228. The hydraulic actuators 164b could be arranged to rotate through 360° paths of travel, in an alternating pattern. Or each actuator 164b could be a continuously rotating motor, rotating only in one direction. These rotating arms 222 mix the incoming filtrate with the pulp in the dilution zone.
8. Liquid Outlet Unit for the Wood Chip Inflow
Reference is made to
When the wood chips are introduced into the rear end 104 of the digester 102, they are first mixed with liquid in a prior art manner so that they flow readily into the digester 102. Black liquor is used for this purpose. However, after the wood chips are introduced, it is desirable to displace this liquor to maintain the derived liquor to wood ratio in the digester 102.
Positioned above the module 230 is an accumulator tank 232 which is filled to about its mid-height with this recirculation liquor, as at 234. The upper half of the chamber defined by the container 230 is designated 236 and contains a pressurized gas, typically nitrogen. This communicates through two tubes 238 to the uppermost fluid assembly 150c which functions either as an inlet assembly or an outlet assembly, depending upon conditions in the digesting chamber 116. If the inflow of fluid into the inlet 104 of the digester 102 is for a period of time greater than the outflow at the opposite end 106, then this extra fluid is able to pass upwardly through the conduits 238 into the accumulator tank 232. If the opposite situation occurs, then fluid will flow from the tank 232 into the chamber 118.
In addition of the function as an accumulator, the module 230 acts also as a separator of air and gases that are coming in with the wood chips. These gases are vented from the area 236 periodically.
The other three assemblies 150c are all fluid outlet assemblies. These function to carry away the excess liquor which accompanies the wood chips that are being introduced into the digester 102.
9. Overall Operation of the Digesting System 100
Reference is now made back to
As indicated earlier, the wood chips are introduced at 104 and these move continuously downstream along the length of the digester 102. As the wood chips pass through various zones, they are subjected to several processing steps to delignify the wood chips, and to cause these to become pulp fibers. The pulp with the liquor carrying the pulp to the front discharge end 106 is first diluted and cooled with the filtrate from the washer 108 and then discharged into the washer for dewatering and washing.
As described previously, the filtrate from the washer 108 serves to dilute the pulp near the discharge end 106 so that it can be discharged at a consistency of 2 to 4%. Further, the filtrate from the washer 108 is delivered into the digester 102 near the outlet 106 end first to accomplish displacement washing of the pulp near the outlet end 106, and then to be moved further upstream to be combined with digesting ingredients (in this preferred embodiment ethyl alcohol) to provide the proper concentration of the digesting ingredient(s).
As indicated previously, the digesting liquid, hereinafter called the “liquor”, is recirculated through the digester in a fashion so that there is a net upstream movement of the liquor from the front discharge end 106 toward the rear inlet end 104, this being accomplished by extracting the liquor from the digester 102 at downstream locations and moving it upstream to be reinjected into the digester 102. As the liquor is moved further upstream, the liquor acquires a higher concentration of the lignin and other organic matter extracted form the wood chips, and thus, in the terminology of the pulp industry, becomes higher in dry solids (D.S.) content as it is recirculated in a continuous upstream fashion. The liquor is eventually discharged as black liquor at an upstream location indicated at 240.
With the foregoing being given as introductory comments, there will now be a more detailed description of this process. This will be done with reference to
10. Operation at the Dilution Zone and the Hot Wash Zone
Reference is first made to
Typically, the consistency of the pulp being processed in the digester is at about 12-13%. Thus, there are seven parts of liquor to one part wood fibre. To cause the proper discharge of the pulp into the washer 108, it is generally desirable to dilute the pulp to about 2% consistency. This is accomplished in the present invention by directing the major portion of the filtrate from the washer 108 by pumps 241 through a pair of heat exchangers 242 (to extract heat from the filtrate) into the filtrate inlet module 220. As the filtrate enters through the inlet nozzles 182b, it flows through the screens 152b into the chamber 118. The mixing arms 222 rotate slowly so that these mix the wood pulp with the filtrate. Then the filtrate is continuously discharged through a digester blow nozzle which is indicated schematically at 244.
In the schematic drawing of
From the above description, it is apparent that the major part of the filtrate is simply recirculated from the washer 108 through the heat exchangers 242 into the chamber 116 by means of the module 220 and then flows with the pulp fiber through the outlet nozzle 244 (blow nozzle) into the washer 108. In the washer 108, the diluted wood pulp is first dewatered, then washed through several cycles, and discharged. Fresh wash water is directed into the washer 108 through the inlet 246.
A substantially smaller portion of the filtrate is directed by a pump 247 first through a heat exchanger 248 which adds heat to the filtrate, and then is directed into the furthest downstream fluid inlet module 148-1 so that the filtrate flows downwardly through the flow inlet assembly 150 at the top of the module 148-1. The filtrate flowing from the assembly 150 moves downwardly, and at the same time, due to the forward flow of the liquor carrying the pulp in a downstream direction, the net flow of the filtrate is in a slanted downward and forward direction. The effect of this is, as described above, that the liquor presently in the digester 102 directly below the inlet assembly 150 is displaced downwardly and outwardly through the outlet assembly 150a of the unit 190-1. This initial displacement of the liquor accomplished by the combined action of the modules 148-1 and 190-1 is one stage of a final hot displacement wash accomplished by the filtrate derived directly from the washer 108.
Then the liquor which flows into the module 190-1 is moved by the pump 250 into a further upstream module 148-2 which moves the liquor displaced by the module 148-1 into the liquor stream to displace the liquor immediately below the inlet assembly 150 of the module 148-2 downwardly to flow outwardly through the screen 150a of the module 190-2.
The action of these modules 148-1 and 2 and 190-1 and 2 have thus accomplished a two stage hot wash, and this is accordingly designated in
At this point, it should be noted that the wash has been accomplished by the filtrate which enters at 148-1. The wash liquid that remains in the pulp moves downstream to the area of the dilution module 220. The liquor collected in the module 190-2 is moved by means of a pump 252 upstream toward a third inflow module 148-3.
11. Operation of the Diffusion Wash Zone, High Heat Alcohol Wash Zone Cooking Zone 3 and Displacement—Zone A
At a location between the pump 252 and the module 148-3, there is shown schematically at 254 an inlet nozzle where ethyl alcohol is added in a sufficient quantity to the liquor from the module 190-2 to cause the liquor flowing into the digester 102 to be sixty percent ethyl alcohol by weight and forty percent water by weight. The liquor then flows through a heat exchanger 256 to raise the temperature, and then is delivered to what is designated as the “high heat alcohol wash” zone.
There are at that high heat alcohol wash zone two flow inlet modules 148-3 and 148-4 and two flow outlet modules 190-3 and 190-4. The liquid displacement operation proceeds in this high heat alcohol wash zone in the same manner as described previously relative to the further downstream hot wash zone. The net effect is that the alcohol liquor mixture displaces most of the liquor which is flowing into the high heat alcohol wash zone and delivers it through a pump 256 in an upstream direction. Also the rate of flow of the liquor into and through the inlet assembly 150 of the module 148-3 is sufficiently high so that a substantial amount of this liquor ends up flowing out of the module 190-4 and thence is pumped at 257 in an upstream direction. At a location upstream of the pump 257 there is an injection nozzle 258 by which an alcohol water combined liquor can be delivered into the line as needed to adjust the pH if this is required. The liquid then flows through the heat exchanger 260 and is delivered to a further upstream location which is designated “displacement zone A”.
These modules 148-3 and 4 and 190-3 and 4 which comprise the high heat alcohol wash zone are spaced a distance upstream from the hot wash zone so that there is between these two zones an interconnecting section of the digester which has the typical cross section, as shown in
As indicated above, the liquor that flows through the heat exchanger 260 is then delivered to that section of the digester which is designated as displacement zone “A”. At this zone, there are two liquor inlet modules 148-5 and 6, and two liquid outlet modules 190-5 and 6. The liquor entering the module 148-5 discharges the liquor through the fluid flow assembly 150 to displace the liquor in the chamber 118 outwardly through the module 190-5 from which the liquor is delivered by the pump 261 to the module 148-6 which in turn delivers the liquor through the chamber 118 to displace the liquor in the chamber 118 to flow into the unit 190-6, where the pump 262 moves the liquor upstream.
Between the displacement zone A and the high heat alcohol wash zone, there is a section of the conduit which is designated “cooking zone 3”. At this cooking zone 3, the digester section is of sufficient length so that the dwell time of the liquor pulp mixture in traveling from the displacement zone “A” to the high heat wash zone is between about thirty to forty minutes.
12. Operation at Cooking Zone 2, Displacement Zone B, Cooking Zone 1 and Displacement Zone C
Reference is made to
Further upstream from the displacement zone “B”, there is what is called a displacement zone “C”. This comprises the modular units 148-9 and 10 and 190-9 and 10. The mode of operation of these modular units is substantially the same as those in the displacement zone “A” and displacement zone “B”, so that operation will not be described further herein. The liquor from the unit 190-8 is delivered by a pump 266 by an ethanol inlet nozzle which optionally can be used to add ethanol, through a heat exchanger 268 to the unit 148-9, and after passing through the units 160-9, 148-10, and 190-10, the liquor from 190-10 is delivered further upstream by a pump 269. The section of the digester between the displacement zone “B” and the displacement zone “C” comprises the cooking zone 1, and this section of the digester is sufficiently long so that the dwell time of the liquor and pulp mixture from the displacement zone “C” to displacement zone “B” is approximately thirty to forty minutes.
13. Operation of the Impregnation Zone, Initial Heating Zone, Wood Chip and Liquid Recirculation and Black Liquor Removal
Immediately upstream of the displacement zone C there is the impregnation zone. The section of the digester defining this impregnation zone is sufficiently long so that the dwell time of the wood chip liquor mixture is about ten to twenty minutes.
The liquor from the module 190-10 is pumped upstream through a heat exchanger 270 to add heat and this is delivered into the initial heating zone. Also there is a nozzle 271 for optional addition of ethanol as needed. At this initial heating zone, there are the modules 148-11 and 12 and 190-11 and 12. The displacement operation takes place as described with regard to the previous displacement zones. Most of the liquor passing through the initial heating zone moves through the modules 148-11, 190-11, 148-12, and 190-12 and is discharged at 240 as black liquor, for further processing. A smaller portion of the black liquor flows upstream to the location of the liquid outlet unit 230 (shown in
The wood chips are, as indicated earlier, mixed with a carrying liquid (i.e. black liquor) and introduced into the wood chip inlet 105 in a conventional manner. Most of this black liquor that carries the wood chips is discharged through the module 230 to be recycled to the wood chip feeder unit again carry additional wood chips into the inlet 105. Since the manner in which this is a done is already known in the prior art, this will not be described further herein.
The black liquor discharged at 240 is further processed for alcohol recovery and evaporation to recover the ethyl alcohol for reuse and also to provide the by-product or by-products from the black liquor More particularly the black liquor can have the liquid content reduced (e.g. by evaporation) and then be spray dried or otherwise dried to produce a by-product in a powder form which has desirable properties as an animal feed supplement or other uses.
14. Possible Modifications
It is to be recognized, of course, that within the scope of the present invention, various modifications could be made in the present invention without departing from the basis teachings thereof. One such modification is illustrated in
In
The other modification as shown in
It is believed that the mode of operation of these modules
B. Second Embodiment
15. Introduction to the Second Embodiment
This second embodiment of the present invention is shown in
In the second embodiment there is a further modified version of an inlet module as shown in
Components of this modified version of the inlet module will be given numerical designations similar to the prior embodiments, with an “f” suffix distinguishing those components of the modified version of
In
The basic operation of this second embodiment is in many respects substantially similar to the operation of the first embodiment as shown in
In
16. Description of the Second Embodiment
In
In the right half of
It was indicated earlier herein that the modified arrangement of the flow inlet and flow outlet modules as shown in
Also, it is possible that the vertical flow can be accomplished at different locations and in somewhat different manner in that at one location the flow could be at a downward slant, and in another location the flow from the inlet flow assembly to the outlet flow assembly could be at an upward slant. Further, with regard to the laterally disposed pairs of flow inlet and flow outlet assemblies, these could be arranged so that the flow would first be laterally across the digester in one direction, an din a subsequent pair flow inlet and flow outlet assembly, the lateral cross flow would be in the opposite direction.
In the right hand portion of
In
In the left part of
In the right hand part of
In the left hand side of
It will be noted that at the upstream end of the displacement wash zone of cooking zone 1, there is an inflow of the filtrate that was discharged further downstream at location “B”, and there is also an inflow of additional liquid at 56% alcohol content. There is a corresponding outflow at location “D”.
In the right hand part of
In
In the right part of
In the description of the first embodiment, it was indicated that a pressure equalizing fluid (either gaseous or liquid) surrounding the square container 116 is utilized. It is believed to be desirable to utilize a pressurized liquid which has substantially the same composition as the filtrate which is inside the square container. This would better insure pressure equalization at different temperature levels.
C. Third Embodiment of the Invention
17. Introduction to the Third Embodiment
The third embodiment is shown in
-
- a. the cross flow rings and their mode of operation;
- b. the wood chip feed assembly, and filtrate recirculation zone;
- c. the impregnation zone and its mode of operation.
After this, in Section 21 there will be a general description of the overall operation of the third embodiment, and in Section 22 there will be a description of the evaporating and recovery system, followed by a brief summary of Section 23.
To give an overview of this third embodiment, reference is made first to
The filtrate from the washer 408 is in turn directed into the digester 402 at its outlet end 406, and the manner in which this is accomplished will be described later herein. While various pulp washers that already exist in the prior art could be used in the present invention, particular advantages can be obtained by the washer 408 being the same as, or similar to, the washer described in U.S. Pat. No. 5,482,594 entitled “LIQUID REMOVAL APPARATUS AND METHOD FOR WOOD PULP”, issued on Jan. 9, 1996, the inventor being the same as the inventor in the present patent application. One of the reasons for this is that this particular washer enables the pulp that is received from the digester 402 to be washed at several atmosphere pressure and at a high pulp consistency (7-10 bars and 20% to 30%), respectively). This is accomplished in a quite effective manner so that the amount of alcohol which is lost (ethyl alcohol being the preferred digesting agent) and carried out with the washed pulp is relatively small and the loss due to evaporation is practically nil, thus enhancing the economy of operation of the present invention.
The digester 402 comprises an elongate pressure vessel 410 having a cylindrical sidewall 411, a rear wall 412 into which the wood chips carrying filtrate is directed, and a front wall 414 through which the diluted pulp is discharged through an appropriate blow valve 416. One significant difference in this third embodiment is that it does not have the square cross section inner container that is present in the first two embodiments. Nor does this third embodiment have the inflow and outflow modules as described in the first two embodiments. Instead, their is provided a cross flow ring system (mentioned very briefly above) which will be described in more detail later herein in Section 18.
In
At the inlet end of the digester 402, there is the wood chip feed assembly 422 which comprises a wood chip and filtrate supply section 424 and a pump section 426 to receive the diluted wood chips from the supply section 424 and direct these into the inlet end 404 of the digester 402.
The digester 402 comprises, in terms of function, seven sections, which will be identified below in the order in which they are placed, beginning at the forward end 406 of the digester 402, and proceeding on to the rear end 404, these being:
-
- a. dilution zone;
- b. displacement wash zone A
- c. cooking zone 2
- d. displacement wash zone B
- e. cooking zone 1
- f. impregnation zone
- g. wood chip filtrate recirculation zone
It is believed that the operation of these seven zones would be in large part understood from a review of the description of the first embodiment. In the later sections, these will be described in more detail. In the following three sections, there will be, as indicated above, a description of three sections of the third embodiment.
18. Cross Flow Ring System
This cross flow ring system will be described with reference to
The cross flow function in the present invention is accomplished by sets or sections of cross flow rings 430 (
Each ring 430 extends entirely around the circumference of the digester sidewall 411. The chamber 440 is actually divided into two arcuate sections. As can be seen in both
There is an inlet fitting 446a leading through the outer wall 432 at the location of the inflow chamber 440a, this defining an inlet passageway into the chamber 440a. In like manner, there is an outflow fitting 446b leading from the outflow chamber 440b.
As can be seen in
At this point, a clarification should be made with reference to the flow lines shown in
To follow this line of thought further, reference is now made to
The outflow from the ring 430-1 is pumped through a recirculating conduit positioned outside the digester vessel 410 to the inlet port at 430-2 (in). It can be seen that there is again cross flow where one of the flow lines 456 travels from the port 430-2 (in) across to the outlet port 430-2 (out), while other flow lines 458 travel across the chamber 448 and somewhat downstream toward the outlet port at 430-1 (out). This pattern repeats itself relative to the next rings 430-3 and 430-4. The flow from 430-4 (out) is directed upstream to re-enter into the digester at the inlet port of the most forward ring 430 in the second filtrate displacement wash zone (which is referred to in a later portion of this text as wash station 552) in displacement wash zone “A”. (This can be seen by examining
To explain the effect of this flow pattern, let us examine the flow which goes out the port 430-1 (out) and is directed into the port 430-2 (in). It can be seen that part of the flow (indicated by the broken lines 458) passes through the chamber 448 to go to the outlet port at 430-1 (out). However, a portion of this flow (indicated by the broken line 456) travels straight across to pass outwardly through the outlet port 430-2 (out). Further, the flow from the port 430-2 (out) moves through a recirculating conduit upstream to re-enter at the inlet port 430-3 (in). Thus, it can be seen that there is an overall circulating pattern where a greater portion of the flow from one inlet port migrates downstream to the next adjacent outlet port, and another portion of the flow coming through the inlet port passing straight across the chamber 448 to exit from the outlet port straight across and then to travel upstream through a recirculating conduit to next upstream inlet port. The overall effect of this pattern is that there is a net movement of the filtrate in an upstream direction in that there is an increment of downstream travel inside the vessel 410, and then a slightly greater increment of upstream travel through the recirculating pattern of the filtrate. Thus, in the digester vessel 410 there is a constant flow of filtrate downstream toward the outlet end 406. Outside the digester vessel 410 there is a counter flow upstream in the recirculating conduits, then across and through the chamber 448, then further upstream through the outside recirculating conduits, etc. The overall effect of this will be described more fully later herein.
It should be pointed out that
19. Wood Chip Feed Assembly and Filtrate Recirculating Zone
The wood chip feed assembly 422 can best be explained with reference to
The out flow from the feed tube 462 is directed through an outlet 482 to the aforementioned pump section 426 of the wood chip feed assembly 422. There are four centrifugal pumps 484 which operate in series in order to raise the wood chip and filtrate slurry to a sufficiently high pressure to enter the digester at the inlet valve 486.
In operation, when the pumps 484 start operating, the wood chip and filtrate slurry is drawn from the flow passageway 474, so that the liquid level in the passageway 474 drops. The arrangement of the feed tube 462 with its vertically spaced openings automatically adjusts the level of the filtrate within the feed tube passageway 474, since when the filtrate level in the passageway 474 drops to a lower level, the higher level of the filtrate 472 in the tank 460 causes an increased flow through the sidewall openings of the feed tube 462 into the passageway 474.
To enable the centrifugal pumps 484 to pump the wood chip/filtrate slurry, it is necessary that there be approximately twenty four parts filtrate to one part wood chips by weight. This ratio is maintained by operating the measuring auger 464 so as to obtain a desired rate of feeding for the wood chips, and also sizing and operating the pumps 484 so that the volumetric flow through the pumps 484 properly matches the feed rate of the wood chips to obtain this ratio.
Since a wood chip to liquid ratio of twenty four to one is substantially greater than the desired wood chip to liquid ratio which is directed into the impregnation zone of the digester, it becomes necessary to withdraw most of the filtrate from the wood chip filtrate slurry entering the digester 402 and return this filtrate to the tank 460. This is accomplished at the wood chip filtrate recirculating zone which is immediately downstream of the inlet valve 486. This is accomplished by two recirculating rings 488. Each of the two recirculating rings 488 has substantially the same configuration as the cross flow rings 430, except that the plenum chamber extends entirely around the circumference of the digester sidewall 411. Also, each ring 488 has four extraction fittings 490 which withdraw the filtrate from the interior of the digester vessel 410. This filtrate flows through the lines 492 into the line 494 and back to the tank 460 through an inlet at 496.
It is necessary to supply makeup filtrate to the tank 460. The reason for this is as follows. It takes approximately two tons of oven dry wood chips to produce (at fifty percent yield) one ton of pulp. The moisture content of two tons of oven dry wood chips is usually between about one ton to two tons of water, depending upon how dry or wet the wood chips are. If the wood chips are rather dry, they absorb liquid as they pass downwardly through the feed tube 462 and through the pumps 484. Thus, the extra liquid which is absorbed into the wood chips needs to be made up. This makeup liquid is supplied from the upstream end of the impregnation zone. Thus, there is shown in
20. The Impregnation Zone
In the impregnation zone (see
The filtrate from the alcohol displacement wash station of displacement wash zone “B” (See
The outflow of the filtrate from the third displacement wash section in the displacement wash zone “B” flows through a line 524 to pass through the heat exchanger 514 and thence pass through the two cross flow rings of the middle displacement wash section 508. The filtrate discharge from displacement wash section 508 travels through line 526 to the evaporation and recovery plant 418.
The heat exchanger 512 brings the temperature of the filtrate up to 205° C. so that the temperature of the filtrate flowing downstream through the cooking zone 1 is at a sufficiently high temperature, for example 195° C. The remaining two heat exchangers 514 and 516 are used to adjust the filtrate temperature for the two impregnation sections 502 and 504.
The withdrawal of the filtrate through the lines 497 and 526, which has a dry solids content of about seven percent, has certain advantages. In the impregnation zone there is being extracted from the wood chips a relatively large percentage of extraneous materials present in the wood chips. Examples of extraneous materials are volatile acids, volatile oils, resin and fatty acid fractions, etc. Volatile oils have considerable economic value because of these being a source of turpentine, pine oil, and other organic matter. Softwood species, such as pines and cedars, are particularly rich in this fraction. These are often termed “essential oils”. The major components of resin and fatty acid fractions include resin acids, fatty acids, turpene, and other esters and unsaponifables (waxes, sterols, etc.). Sterols include B-sitrosterol which has been researched for medical purposes and has proven to be extremely valuable. These are discussed in more detail in the book, “Second Edition, Volume 1, The Pulping of Wood” by Ronald G. MacDonald and John N. Franklin, published by McGraw-Hill Book Company.
Removal of extraneous materials opens up the pores and cavities in the wood chips to allow the alcohol digesting liquid to enter into the wood chips and accomplish the delignification. It is very valuable to extract these extraneous materials early so that they are not subjected to possible deterioration in subsequent cooking zones in the digester and that they are not diluted by other organic matter dissolved later in the cooking zones 1 and 2. Also, some of the extraneous materials can also have negative effects in the delignification process if these remain and are present in the cooking zones.
Thus present analysis indicates that by extracting the filtrate flow from the impregnation zone and treating this separately certain advantages would be produced in that this portion of the organic material has a quite different composition and value than other portions of organic matter which are later extracted from the wood chips. In the present invention, by diverting this organic material that is extracted in the impregnation zone at an early stage and recovering it separately, with the alcohol being evaporated, the further processing of this organic material into certain desirable by-products is substantially enhanced.
Another benefit derived in this impregnation zone is that the filtrate that flows through the impregnation section 502 is recycled back to the upstream end of the impregnation section 504. By maintaining some of this extracted organic material present in the impregnation zone, in accordance with present analysis, an enhanced extraction process in the impregnation zone is accomplished. Thus, the term “impregnation zone” is somewhat incomplete, since this zone serves a dual function of both impregnation and extraction. To comment on this further, reference is made to
The liquor at the end of the first cooking zone is high in lignin content and low in extraneous materials, and is therefore heavier in composition. This is the liquor that is directed through the line 586 to be evaporated through an evaporating system separate from the impregnation zone.
Most of the liquor that is removed immediately after cooking zone 2 remains in the pulp/liquor flow in the middle of the displacement wash zone B and is pumped to the end of the impregnation zone. This liquor is rich in hemicellulose, and some of this migrates into the impregnation zone discharge liquor, thus adding hemicellulose to the extraneous materials stream going to the evaporation and recovery system 418.
Delignification is accomplished mainly in the cooking zones 1 and 2. Thus, the liquor that is directed from the end of cooking zone 1 and is directed through the line 480 to the evaporation and recovery system 418 provides a high percentage of the lignin that is extracted from the wood chips.
21. Overall Operation of the Third Embodiment
This section is not only to describe the overall operation of the present invention, but also to provide more specific information about the other sections (i.e., zone) of the digester system which have not been described in detail in the prior sections related to this third embodiment. In sections 19 and 20, there was presented a more detailed description of the wood chip feed assembly, and also the seventh and sixth zones of the digester 402, namely the wood chip feed assembly and filtrate recirculation zone, and the impregnation zone. In the following description, each of the five other zones will be described in sequence, beginning at the front end of the digester where there is the dilution zone and then proceeding through the following zones in sequence to and including cooking zone 1. As this is done, reference will be made to the other components related to the operation in those zones.
Reference is first made to
As indicated previously, the washer 408 is desirably the same as (or quite similar to) the washer described in U.S. Pat. No. 5,482,594, the subject matter of which is hereby being incorporated by reference into the present patent application. In the normal mode of the operation of the washer 408, the pulp slurry enters the washer at about a 2% consistency, which means that there is one part pulp to forty-nine parts liquid. In this instance, for purpose of description, it will be assumed that the washer 408 is being operated so that the pulp entering the washer 408 is at 2% consistency. Assuming a dilution factor of 1, and the pulp discharge from the washer 408 at 20% consistency (which means 4 parts water to one part pulp), there is a total wash water quantity of five parts wash water to one part pulp. When this is combined with the forty nine parts water for one part pulp (49 plus 5), the total liquid entering the washer 408 equals 54 parts liquid to 1 part pulp.
The pressure in the digester is at about thirty bars, and there is a pressure reduction of about fifteen to twenty bars, when the pulp passes through the blow valve 416. The blow tank 407 stores the pulp and the liquid so that liquid can be properly fed into the washer 408. (The washer 408 may not be operating at the very same time that there is a discharge from the blow valve 416 of the digester 402, so the blow tank 407 also acts as a buffer.) There is a line 529 that directs the flushed alcohol/water vapors from the blow tank 407 to evaporation and recovery system 418.
With reference to
The components in the five zones beginning from the dilution zone through to cooking zone 1 will now be described under appropriate headings.
a. The Dilution Zone
With reference to
The two inlet rings 534 each have an outer ring structure which is the same as (or similar to) the ring structure of the cross flow ring as shown in
b. Displacement Wash Zone A
Reference is made to
With reference to
Flow from the washer 408 flows through the line 536 into the heat exchanger 538. The heat exchanger 538 operates in a manner that the flow from the heat exchanger 538 going into the digester is at about 68° C. This is to maintain the blowout temperature of the pulp/filtrate mixture that passes out the blow valve at about 74° C.
Also, as indicated previously, the outflow from the fourth cross flow ring (designated 430-4 in
Also, it can be seen in
The middle alcohol displacement wash zone 550 receives a flow of alcohol from the accumulator tank 420 through a pump 563 and a line 564, which directs the alcohol through a heat exchanger 566 and thence through a pump 554 to flow into the most forward ring 430 of the alcohol displacement wash station 550. The heat exchanger 566 raises the temperature of the alcohol to about 135° C. The tank 420 derives its alcohol from the evaporation and recovery plant, as does the aforementioned tank 421.
The alcohol passing into the most forward ring 430 of the alcohol wash station 550 recirculates in a cross flow pattern through the digester in substantially the same manner as the flow pattern that was described previously herein relative to the cross flow pattern in the most forward filtrate displacement wash section 538, as illustrate in
It is believed that in order to present a better understanding of the present invention, it would be helpful to review at this time the first part of the table presented in
However, it is within the capability of the washer 408, as described in the U.S. Pat. No. 5,482,594, to obtain even higher consistencies of the pulp being discharged. Also, it should be noted that the only net outflow of liquid from the washer 408 is the liquid which remains in the pulp which is being discharged from the washer 408. The rest of the filtrate from the washer 408 is directed back into the digester 402. As indicated previously, in the overall digester apparatus, there is a net upstream flow of filtrate. This occurs as follows there is a substantially constant volumetric flow downstream from the rear end 404 to the front end 406 in the chamber 448 of the digester 402. However, at the same time there is a grater flow through the recirculating conduits 558 and in the other lines (i.e., 556, 568 and others) that carry the filtrate to further upstream locations).
Reference is now made back to the table in
With further reference to lines 3 through 8 of
c. Cooking Zone 2
The pulp and filtrate mixture which leaves the displacement wash zone B travels downstream in the chamber 448 through the cooking zone 2 is maintained at a temperature of approximately 195° C. (As can be seen in
The filtrate in the chamber 448 leaving the displacement wash zone B has just traveled through the second alcohol displacement wash section (to be identified and discussed in the next section of this text) and thus has a relatively high alcohol content (59%), and also a rather low level of dry solids (0.3%).
As the pulp/liquor flow proceeds in a continuous manner downstream in the cooking zone 2, the dry solids content in the liquor increases, so that the pulp/liquor passing from cooking zone 2 and into displacement wash zone A has the dry solids content increase from 0.3% (at the start of cooking zone 2) to 5.9% (at the end of cooking zone 2). The time which it would take for a portion of the pulp/liquor mixture to pass through the cooking zone 2 would generally be between about 30 to 60 minutes.
d. Displacement Wash Zone B
Reference is made to
The inflow into the most downstream cross flow ring of the third filtrate displacement wash zone 574 (which is in the middle of displacement wash zone B) is from the line 568 which receives the outflow from the furthest upstream ring 430 of the first alcohol displacement wash section 550. The flow through the line 568 passes through a heat exchanger 582 which raises the temperature of this filtrate to about 195° C. The outflow from the third filtrate displacement wash section 574 is from its furthest upstream ring of that section 574 through the aforementioned line 524 to pass through the heat exchanger 514 into the downstream cross flow ring of the middle displacement wash zone 508 of the impregnation zone (see
The outflow from the most upstream cross flow ring of the second filtrate displacement wash zone 552 (see
e. Cooking Zone 1
Reference is made to
To appreciate some of the benefits derived from the present invention, it would be helpful to pause at this point and review some of the values presented in the chart of
f. Other Modifications and Further Comments
It is apparent that various modifications could be made in the present invention without departing from the basic teachings thereof. For example,
The ring 430″ has the chamber 440″??? cut out of the wall 411″, as in
Another possibility is that during the operation of the digester 402, it could be subjected to vibrations for various purposes (e.g., to enhance the diffusion of dry solids from the inside of pulp fibers and/or chips and to dislodge chips that may have been stuck in inlets and outlets). Also, the digester could be rotated about its lengthwise axis back and forth for this same purpose or other purposes.
22. The Evaporation and Recovery System
As a further modification, as shown in
The system 418 comprises an alcohol recovery system 702, which may be conventional and in this instance comprises a condensate stripper 704, an alcohol distillation column 706, and an alcohol condenser 708. The system 418 also comprises an evaporating system which comprises three sets of evaporating units. These evaporating units (which in the industry are called “vapor bodies”) can be conventional, and each comprise a containing tank, a heat exchanger, a liquor circulation means, a vapor supply line and a condensate removal system. There is a first set, comprising first and second stage evaporator units designated E-1i and E-2i. These two evaporators E-1i and E-2i receive liquor from the impregnation zone. There is a second set of evaporator units comprising two evaporator units E-1C and E-2C which receive liquor from the downstream end of cooking zone 1. Then there is a third evaporating section having three stages or units, these being designated, respectively, E-3, E-4 and E-5.
In
The flow of liquid from the impregnation zone flows through the inlet lines from 526 and 497 into a blow tank 710, and thence into the tank of the first evaporator stage E-1i. The liquid is recirculated by the pump P-1 upwardly through a flow line and back into the tank of E-1i, and into the upper end of a heat exchanger 712. Also, a portion from the flow from the pump P-1 is conveyed by the pump P-2 into a first separator S-1. This separator S-1 can be one of a number of different types of separators. The portion of the liquor which is extracted in the separator S-1 is indicated by the arrow 714. The remaining portion of the liquid from the separator S-1 is directed through the line 716 into the second stage evaporator E-2i.
The second stage evaporator unit E-2i has pumps P-3 and P-4 which operate in substantially the same manner as the pumps P-1 and P-2, with a portion of the liquor being directed to the second stage separator S-2. The portion extracted from the separator is discharged through the discharge line 720. The other portion of the liquor is recirculated upwardly and into the heat exchanger 722 of the second stage E-2i. The unextracted liquor from the separator section S-2 is directed through the line 724 and thence into a line 726 leading into the middle evaporator section comprising the three evaporator stages or units, E-3, E-4 and E-5.
Attention is now directed to the evaporator units in the second section, namely units E-1c and E-2c. These have pumps P-5, P-6, P-7 and P-8. Also, there are two by-product separators S-3 and S-4. The flow from the line 586 at the downstream end of cooking zone 1 enters the blow tank 728, with the liquid passing through the evaporator units E-1c and E-2c in substantially the same manner as described previously with respect to the evaporators of the first section, namely E-1i and E-2i. The liquor stream that is extracted in the separation process from the two separators S-3 and S-4 are designated 730 and 732, respectively. The flow from the separator S-3 which is not extracted in the separation process goes through line 734 into the line 726 to flow into the center evaporator section.
Various separating techniques could be used in one or more of the separators S-1, S-4. For example, a conventional centrifuge could be utilized, where oils are being separated since the oils are less dense than the lignin. Conventional filters also could be used, or systems where an added substance reacts with the desired by-products, making these heavier or lighter so that they either sink to bottom or flow to the top. Or the added substance could make the desired by-product stickier, or possibly heavier so that it could be more easily separated by a centrifuge. Further, in the evaporation process, the alcohol will evaporate more rapidly than the water because of its lower boiling point and other characteristics. Thus, since alcohol is the dissolving agent, when it evaporates it frees the organic solids from suspension. This better enables the lignin to be spun off by centrifugal force to free most of the extraneous materials (oils, etc.).
The by-product(s) removed by separator S-1 have a rather different composition than those separated by the separator S-2, since the liquor which goes into the separator S-2 has practically all of the alcohol removed therefrom. This is also true with regard to the separation that takes place at the Separators S-3 and S-4, with most all of the alcohol being removed from the liquor that goes to S-4.
While the evaporating system shown in
In the center evaporator section, there are three evaporating units/stages E-3, E-4 and E-5, and each stage comprises its own heat exchangers and recirculating components as previously described. The liquor flow in the stages are each handled separately, but the vapors are mixed to comprise one vapor stream. The three heat exchangers are each designated 736. The flow of liquor from the line 326 flows in a recirculating pattern through all three of the heat exchangers 736 in series, which recirculating pattern is or may be conventional in the prior art. Accordingly, this will not be described in detail herein. Specifically the liquor flow progresses from stage 3 to stage 4 and then to stage 5. The discharge of liquor from evaporation unit E-5 is through a line 738, into a blow tank 740, with the liquor being discharged through the line 741 to be delivered to the spray dryer.
Attention is now directed toward the flow of steam and vapor in the evaporation and recovery system 418. The steam is directed into the system 418 through a steam line 742 to the center section of evaporator units and is directed through three steam lines into the three heat exchangers 736 in the third, fourth and fifth stages E-3, E-4 and E-5. The vapors resulting from evaporators in E3, E4 and E5 is then directed through the lines 746, 748 to, respectively, heat exchangers in the two evaporating sections E-2i and E-2c. Then vapor from the evaporator units E-2i and E-2c of the second evaporating section. The vapor is then directed through the two heat exchangers of the evaporating sections E-1i and E-1c,
The vapor collected in the evaporators E-1i and E-1c into the line 750, which leads directly into the heat exchanger of condensate stripper 704. For convenience of illustration, since the line 750 begins at the right hand part of
The vapor discharged from the blow tank 740 travels through two lines 752 and 754 to be delivered to, respectively, the heat exchanger 712 of the evaporator E-1i and to the heat exchanger in the evaporator E-1c.
To review the overall operation of the recovery system of
The condensate from the heat exchangers in the seven evaporating units can be treated in a conventional manner. Any condensate which has such a low percentage of alcohol content so that further alcohol recovery would be uneconomical would be discharged from the recovery system. The condensate that has a sufficiently high percentage of alcohol therein for economical recovery is directed to the alcohol recovery section.
The non-condensable gases which enter the recovery system through the line 562 can be treated in a conventional manner in the recovery system. Accordingly, these will not be discussed further herein.
It is to be understood that within the scope of the present invention, there could be additional liquor streams from other portions of the digester entering into the recovery system, and these could be treated in separate evaporator sections, so that there would be additional sets of evaporators, such as indicated in the first section at E-1i and E-2i, and also in the second evaporator section (evaporators E-1c and E-2c). Likewise, there could be additional by-product separator sections such as those shown at S-1 through S-4.
23. Final Comments
To review briefly some of the desirable features of the present invention, it will be noted that except for the flow area at 498 (see
While alcohol is a preferred digesting agent in the present invention, other digesting agents could be used. For example, the present invention could be adapted for the Kraft process, sulfite process, or other digesting processes. Further within the scope of the present invention, while the present invention is particularly adapted for the digesting of wood products, it could be utilized for other materials such as hemp, linen and other plant material. Also, while in the preferred form, the digester has its longitudinal axis horizontally aligned, within the broader scope, the digester could be positioned vertically, or on a slant to both the horizontal and vertical.
As indicated above, there are various modifications which can be made to the present invention without departing from the basic teachings thereof.
Claims
1. A continuous digester system comprising:
- a. a pressure vessel having a lengthwise axis, a rear upstream inlet end having a wood chip intake means, and a front outlet end having a pulp outlet means, said vessel having an elongate processing chamber through which wood chips travel forwardly in the presence of a digesting agent while being transformed into pulp, with the pulp being discharged from the pulp outlet means at the front outlet end of the vessel;
- b. liquid flow means to circulate processing liquid through said digester to carry dissolved solids with said processing liquid, said flow means comprising; i. initial inlet means to initially introduce processing liquid into the pressure vessel at an initial inlet downstream location; ii. a plurality of processing liquid inlet means at inlet locations along the lengthwise axis of the pressure vessel to introduce processing liquid into the processing chamber; iii. a plurality of processing liquid outlet means at outlet locations along the lengthwise axis of the pressure vessel to extract processing liquid from said processing chamber, said outlet locations being spaced laterally from said inlet locations, so that flow of said processing liquid from each of said inlet means to related outlet means has a lateral flow component through said processing chamber; iv. recirculating means comprising a plurality of interconnecting line means, at least some of said interconnecting line means connecting at least some of the outlet means with related inlet means at further upstream locations to direct processing liquid from said at least some of said liquid outlet means through related interconnecting line means to further upstream locations to flow through the related liquid inlet means into the processing chamber and laterally in the processing chamber to other outlet means to again be recirculated through related interconnecting line means to other inlet means; v. liquor outlet means to discharge liquor, said liquor outlet means being upstream of the initial downstream location and upstream of at least some of said liquid inlet means and said liquid outlet means;
- c. said digester system being characterized in that the processing liquid moving in a recirculating pattern through the processing chamber and through said recirculating means carries dry solid content extracted from the wood chips during processing in the processing chamber in a net upstream flow pattern to be discharged from the processing chamber at said liquor outlet means.
2. The system as recited in claim 1, wherein there is a washer to receive pulp from the digester and to dewater and wash the pulp, a substantial portion of filtrate from the washer being directed into the initial inlet means as said processing liquid to move through said recirculating means in said net upstream direction.
3. The system as recited in claim 2, wherein a digesting agent is introduced into said liquid flow means to flow through said recirculating means and through said processing-in a net upstream direction to extract dry solids content from said wood chips being processed and carry said dry solids content in a net upstream direction.
4. The system as recited in claim 3, said system further comprising an evaporation and recovery means to receive liquor discharged from said pressure vessel at a plurality of discharge locations at different operating locations in said pressure vessel so as to extract liquor having different characteristics from different extraction locations.
5. The system as recited in claim 4, wherein said digesting agent is alcohol, with said evaporation and recovery means extracting said alcohol from said black liquor and recirculating said recovery alcohol back to said liquid flow means to be recirculated into said liquid flow means.
6. The system as recited in claim 1, wherein said system comprises an impregnation zone located in said pressure vessel at an upstream location, at least one cooking zone located downstream of said impregnation zone, and at least one wash displacement zone located downstream of said cooking zone, at least some of said liquid inlet means and said liquid outlet means being located at said displacement wash zone to receive said processing liquid and recirculate said processing liquid sequentially through related pairs of said liquid inlet means and said liquid outlet means, said flow means further comprising means to move the processing liquid from the wash displacement zone to an upstream location to be directed into said cooking zone, to flow in a downstream direction in the processing chamber toward said displacement wash zone.
7. The system as recited in claim 6, wherein at least some processing liquid from said displacement wash zone is recirculated ultimately to said impregnation zone to flow downstream in said vessel through said impregnation zone and into said cooking zone.
8. The system as recited in claim 7, wherein liquor is extracted from the impregnation zone and directed to said evaporation and recovery means for processing.
9. The system as recited in claim 8, wherein liquor is extracted from said cooking zone and directed to said evaporation and recovery means for processing.
10. The system as recited in claim 1, wherein said liquid flow means comprises at least one displacement wash zone, having a downstream end and an upstream end, with a plurality of said liquid inlet means being positioned at longitudinally spaced inlet locations along a length of said displacement wash zone and a plurality of said liquid outlet means positioned at spaced locations along a length of said displacement wash zone, said pluralities of liquid outlet and liquid inlet means being arranged so that there are a related first downstream and a second upstream liquid inlet means being arranged to a related first downstream and second upstream outlet means in a manner that at least a portion of processing liquid from said first downstream inlet means flows through said processing chamber to pass into said first downstream outlet means, with at least a portion of flow into said first downstream inlet means being recirculating through said recirculating means to said second upstream inlet means, with at least a portion of the flow from said second liquid inlet means flowing across said processing chamber to said second upstream inlet means, with at least a portion of the flow from said second upstream inlet means being recirculated by said recirculating means in an upstream direction, thus accomplishing said net upstream flow of processing liquid.
11. The system as recited in claim 1, wherein said pressure vessel has a generally cylindrical cross sectional configuration transverse to its lengthwise axis, and said digester system comprises an inner-containing means positioned within said pressure vessel, with said inner-containing means defining the elongate processing chamber, said inner-container means comprises at least in part planar wall surfaces.
12. The system as recited in claim 11, wherein there are inlet screen means and outlet screen means located at longitudinally spaced locations at said planar wall surfaces, at least some of said liquid inlet means passing liquid into said processing chambers through related screen means, and at least some of said liquid outlet means discharging processing liquid through related screen means at least some of said screen means having propeller blade means which move across related screen means to prevent obstruction of flow through said screen means.
13. The system as recited in claim 1, wherein said pressure vessel comprises a generally cylindrical sidewall, which defines the processing chamber as a generally cylindrical processing chamber, at least one of said liquid inlet means and liquid outlet means comprises liquid passageway means formed in said cylindrical sidewall, said liquid passageway means having flow axes, said flow axes being slanted in a radially inward and forward direction.
14. The system as recited in claim 13, wherein said liquid flow means comprises a plurality of circumferential ring assemblies positioned at longitudinally spaced locations along said sidewall, each of said ring assemblies defining a flow chamber to communicate with related passageway means extending through said wall member.
15. The system as recited in claim 1, wherein said plurality of liquid inlet means and said plurality of liquid outlet means are arranged in alignment pairs, having an alignment flow path between the liquid inlet means and the liquid outlet means of a related pair, at least some of said pairs of liquid inlet and liquid outlet means being arranged in an alternating pattern, whereby cross flow of processing liquid between adjacent alternating pairs have different flow directions through said processing chamber.
16. The system as recited in claim 1, wherein said elongate processing chamber is defined by a longitudinally extending chamber wall means, said liquid inlet means and said liquid outlet means being positioned at said chamber wall means in a manner that said liquid inlet means causes processing liquid to flow through said chamber wall means into said processing chamber, and said liquid outlet means extracts processing liquid from said processing chamber through said chamber wall means, said liquid inlet means and said liquid outlet means being arranged in related alignment pairs where at least some of the liquid from the liquid inlet means flows in a flow path substantially across said processing chamber to its related liquid outlet means.
17. The method as recited in claim 1, wherein there is an evaporation and recovery means to receive liquor discharged from said pressure vessel, said evaporation and recovery means comprising at least first and second heat exchange means to cause evaporation of liquid from said liquor, and first and second separator means, said first evaporator means being arranged to initially receive liquor from said pressure vessel and to discharge liquor from said first heat exchange means, means to direct liquor from said first heat exchange means to said first separator means said first separator means to separate a portion of the liquor from to the first evaporator means, means to direct remaining liquor from said first separator means to said second heat exchange means where said remaining liquor is subjected to a further heat exchange process, means to direct liquor from said second evaporator means to said second separator means to extract a portion of the liquor from said second heat exchanger means.
18. The system as recited in claim 1, wherein there are:
- a. at least one impregnation zone at an upstream location in said pressure vessel;
- b. first and second cooking zones, with said first cooking zone being positioned downstream of said impregnation zone, and said second cooking zone being located downstream of said first cooking zone;
- c. first and second displacement wash zones, with said first displacement wash zone being positioned downstream of said second cooking zone, and said second displacement wash zone being positioned between said first and second cooking zones;
- d. each of said displacement wash zones having a downstream end and an upstream end, with a plurality of said liquid inlet means being positioned at longitudinally spaced inlet locations along a length of said displacement wash zone and a plurality of said liquid outlet means positioned at spaced locations along a length of said displacement wash zone, said pluralities of liquid outlet and liquid inlet means being arranged so that there are a related first downstream and a second upstream liquid inlet means being arranged to a related first downstream and second upstream outlet means in a manner that at least a portion of processing liquid from said first downstream inlet means flows through said processing chamber to pass into said first downstream outlet means, with at least a portion of flow into said first downstream inlet means being recirculated through said recirculating means to said second upstream inlet means, with at least a portion of the flow from said second liquid inlet means flowing across said processing chamber to said second upstream inlet means, with at least a portion of the flow from said second upstream inlet means being recirculated by said recirculating means in an upstream direction, thus accomplishing net upstream flow of processing liquid in said displacement wash zone;
- e. said recirculating means interconnecting said first and second wash zones with said first and second cooking zones and said impregnation zone in a manner that within said processing chamber, there is a substantially continuous flow of pulp and processing liquid in a downstream direction from the inlet end to the outlet end, and a substantially continuous flow of processing liquid from said displacement wash zones through said recirculating means to upstream locations into said first and second cooking zones and into said impregnation zone,
- whereby dissolved solids are carried through said recirculating means in a net upstream direction, while wood chips being processed into pulp and the processing liquid in the digester travel in downstream direction.
19. The system as recited in claim 1, wherein said pressure vessel is aligned so that its major alignment component is horizontal.
20. A method of digesting wood chips, said method comprising:
- a. providing a pressure vessel having a lengthwise axis, a rear upstream inlet end having a wood chip intake means, and a front outlet end having a pulp outlet means, said vessel having an elongate processing chamber;
- b. feeding wood chips through said wood chip intake means into said processing chamber and causing said wood chips to travel forwardly in said processing chamber in the presence of a digesting agent while being transformed into pulp, and discharging the pulp from the pulp outlet means at the front outlet end of the vessel;
- c. circulating processing liquid through said digester to carry dissolved solids with said processing liquid by: i. initially introducing processing liquid into the pressure vessel at an initial inlet downstream location; ii. directing processing liquid through a plurality of processing liquid inlet means at inlet locations along the lengthwise axis of the pressure vessel into the processing chamber; iii. directing processing liquid from said processing chamber through a plurality of processing liquid outlet means at outlet locations along the lengthwise axis of the pressure vessel to extract processing liquid from said processing chamber, with said outlet locations being spaced laterally from said inlet locations, so that flow of said processing liquid from each of said inlet means to related outlet means has a lateral flow component through said processing chamber; iv. recirculating said processing liquid through a plurality of interconnecting line means, with at least some of said interconnecting line means connecting at least some of the outlet means with related inlet means at further upstream locations, by directing processing liquid from said at least some of said liquid outlet means through related interconnecting line means to further upstream locations to flow through the related liquid inlet means into the processing chamber and laterally in the processing chamber to other outlet means to again be recirculated through related interconnecting line means to other inlet means; v. discharging liquor through liquor outlet means at at least one location upstream of the initial downstream location and upstream of at least some of said liquid inlet means and said liquid outlet means;
- d. said method being characterized in that the processing liquid moving in a recirculating pattern through the processing chamber and through said recirculating means carries dry solid content extracted from the wood chips during processing in the processing chamber in a net upstream flow pattern to be discharged from the processing chamber at said liquor outlet means.
Type: Application
Filed: Nov 8, 2004
Publication Date: Mar 24, 2005
Inventor: Reijo Salminen (Bellingham, WA)
Application Number: 10/984,622