Process and apparatus for treating green liquor

Abstract

A method of and apparatus for treating a green liquor stream produced in a kraft pulp process to separate green liquor from dregs present in the stream by utilizing a filtration vessel that has a pressure differential between the green liquor inlet and outlet to drive the dregs from the green liquor. In addition, negative pressure is utilized in the filtration vessel to thereby drive water vapor from and consequently cool the filtered green liquor.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method of and an apparatus for treating green liquor formed in a kraft sulphate or soda pulp mill.

The kraft or sulfate process produces a high percentage of the chemical pulp produced annually in the world. In the kraft process, wood chips are cooked (digested) at an elevated temperature and pressure in “white liquor”, which is a water solution made up of primarily sodium sulfide (Na₂S) and sodium hydroxide (NaOH). The white liquor chemically dissolves lignin from the wood. Spent cooking liquor and the pulp wash water are combined to form a weak black liquor which is then concentrated in an evaporator. The concentrated black liquor is fired in a recovery furnace or recovery boiler. Inorganic chemicals present in the black liquor collect as a molten smelt at the bottom of the furnace. The smelt, consisting primarily of sodium carbonate (Na₂CO₃) and sodium sulfide (Na₂S) is then removed from the recovery furnace/boiler and dissolved in water in the smelt tank to create green liquor.

An essential part of the process of producing kraft pulp is chemical recovery. The chemical recovery process includes the production of white liquor used in the pulping process. White liquor is produced by a causticizing process, whereby green liquor is reacted with calcium hydroxide, i.e. slaked lime, to regenerate the white liquor through the following equilibrium reaction:

Na₂CO₃+Ca(OH)₂2NaOH+CaCO₃

Green liquor also contains insoluble compounds, know as dregs, including some inorganics of all types, flecks of carbon, and unburned organics. Dregs must be removed in the green liquor filter or they continue through the recovery cycle and become a dead load in the system. Clarifying the green liquor to remove dregs improves white liquor clarification, mud settling, mud washing, and soda removal on the lime kiln precoat filter.

It is known in the art to use a green liquor filter that uses pressure differential across the filter element to drive the separation of dregs solids from green liquor. Ordinarily both the high and low pressures in the filter are above atmospheric pressure, in accordance with the process requirements in the downstream white liquor filter, where pressurized operation always above atmospheric prevents heat loss by flashing of water to vapor and the subsequent loss of heat from the white liquor.

As indicated, green liquor is reacted with lime in a slaker and is converted to white liquor which is subsequently carried to a digester. The slaker is operated as close to boiling as possible to produce a high grade lime mud slurry. The lime mud slurry can be washed in water in one or several steps before it is dewatered and fed into a lime mud kiln. Calcination of the lime mud takes place in the lime mud kiln, which converts the content of calcium carbonate to lime, which principally consists of calcium oxide (CaO). This lime can subsequently be used in a lime slaker as described above.

Green liquor exiting the filter is typically hot, normally about 95° C., and at times higher, and if not cooled sufficiently it can cause the slaker to boil over. In prior art processes the hot green liquor solution is typically directed from the filter to a separate cooling device to reduce its temperature. The cooling device can be either a heat exchanger or vacuum flash cooler system. However, because of the chemical content of the green liquor solution, scaling, i.e., the deposition of chemicals in solution as a solid on pipe and vessel walls, can be a significant problem with the shell and tube exchangers. Vacuum flash coolers do not scale as quickly but are an expensive alternate to heat exchangers In addition, while dregs cake washing is increasingly required to meet stricter environmental requirements for dregs discharged from the filter, an undesirable result is that the strength of the green liquor within the filter is sometimes weakened.

Therefore, one objective of the invention is to have a method of efficiently cooling green liquor before it is fed to the slaker. Another objective is to have a cooling step for green liquor prior to the slaker in which scaling does not occur in the cooling apparatus. A further objective is to concentrate the green liquor back to the optimal strength that existed when it was fed to the green liquor filter prior to the dregs cake washing.

These objects and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawing.

DESCRIPTION OF THE DRAWING

The FIGURE is a diagrammatic view of a green liquor treatment system and method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides for a novel approach in which the green liquor filtering step is combined with the green liquor cooling step in a single apparatus.

Green liquor as produced from the smelt derived from black liquor in chemical recovery of a conventional Kraft liquor cycle is directed to an agitated stabilization tank 11. The agitation provided turns the contents of tank 11 over fairly quickly but not enough to shear any dregs that are present. The agitated green liquor is thereafter directed to green liquor filter 12 adapted for removing dregs from the green liquor solution. A green liquor filter that can be advantageously utilized in the present invention is FLSmidth Dorr-Oliver Eimco Inc.'s Green Liquor ClariDisc® Filter. Green liquor filter 12 has solution inlet 13, solution outlet 14 and dregs outlet 15 and one or more internal filtration element (not shown) to filter dregs from the green liquor solution wherein there is a pressure differential across the filtration element, and the filter unit as a whole, from a relatively higher pressure at or adjacent to the filter inlet (i.e. the “upstream” side, relative to the movement of the green liquor solution therethrough) to a relatively lower pressure at or adjacent to the filter outlet (i.e. the “downstream” side). Dregs removed from the green liquor solution then exit through outlet 15 and are typically disposed.

Because of the low pressure conditions, the flashing off of vapor from the green liquor and the resultant cooling of the green liquor is primarily achieved within green liquor filter unit 12 and starting on the immediate downstream side of the unit's filtration element. As the heat of vaporization for water is approximately 1000 BTU per pound, for every pound of water removed as vapor, approximately 1000 BTUs are removed from the solution and the loss of one BTU will drop the temperature of one pound of water one degree F. Since green liquor is predominantly made up of water, it will cool with the removal of BTUs from the solution.

The green liquor and vapor are thereafter delivered to a green liquor separator vessel 16. In separator vessel 16 the vapor is separated from the liquid. Vessel 16 optionally may also be exposed to negative pressure. Since there is no heat exchange surface per se in the separator vessel, scaling is not a problem.

The removal of water vapor from separator 16 via outlet duct 19 increases the chemical concentration in the Green Liquor. The negative pressure condition that causes the flashing is achieved with a source 17 of differential pressure such as a liquid ring vacuum pump or a compressor. The vapor load created can be huge while the noncondensible load (air and other gases that do not condense) is very small. Therefore, the liberated vapor stream, i.e., the flash vapor and non-condensables that are present in the vapor phase off the separator vessel are sent to condenser 18 where most of the flash vapor is condensed either by direct contact with a cooling water stream or indirectly by a heat exchanger. But in this case, the heat exchanger is fed only vapor which is free of dissolved solids so scaling is not an issue. The method of condensation employed can determine where to direct the discharge from the condenser. For example, if a barometric condenser is employed the discharge is typically directed to weak wash or another process step demanding water. If an indirect condenser is used, then all, none or a portion of the condensate produced can be directed back to mix with the clarified green liquor.

The concentrated and cooled green liquor is removed from separator 16 via outlet duct 20. Within condenser 18 the vapor is condensed back into water. As indicated, in one embodiment some or all of this water can be returned to the concentrated green liquor via duct 21 depending upon the desired strength of the green liquor solution to be delivered to the slaker, that is, if the cooled green liquor is stronger than optimum. If, because of dregs cake washing the green liquor has been diluted below its optimum strength than it may be determined that none of the water is to be combined with the concentrated green liquor. As indicated, some or all of the condensed water can also be utilized elsewhere in the system, such as being directed back to the filter unit to effect the dregs cake wash and is thereafter discharged from the filter unit.

The noncondensibles (that is essentially dry gas), are preferably recirculated back to filter 12 to prevent oxidation of the liquor. Unlike in prior art green liquor filtration processes that use a separate downstream flash cooling device, the non-condensibles need not be vented to atmosphere on a continuous basis.

The pressure differential across the filtration element (and between the green liquor inlet and outlet of the filtration unit) in the filter can range from about 0.2 to about 1.8 bar, and will preferably range from about 0.5 to about 1.5 bar. Unlike what is done in the prior art, it is important to maintain the low pressure end of the filter at a negative pressure to facilitate subsequent flash cooling of the green liquor solution. By controlling how far below atmospheric the low side of the filter unit is allowed to go, the practitioner can control how much flashing (and subsequent cooling) occurs in the downstream flash cooler. Preferably, the negative pressure utilized on the low pressure side of the filter will range from just below atmospheric down to about −0.7 bar. Normally, the green liquor will be cooled approximately 10° C. by the process of the present invention. Additionally, the cleaned green liquor will be concentrated in this process. This can be a benefit if the liquor is undesirably diluted within the filter due to the requirement of dregs cake washing. By controlling how much of the condensed water is returned to the cleaned green liquor, the practitioner can ensure the final strength of the green liquor leaving the system is optimum.

It is to be understood that the form of this invention as shown is merely a preferred embodiment. Various changes may be made in the function and arrangement of parts; equivalent means may be substituted for those illustrated and described; and certain features may be used independently from others without departing from the spirit and scope of the invention as defined in the following claims.

Claims

1. A method of treating a stream containing green liquor and solid dregs that is produced in a kraft pulp process, comprising

(a) directing the stream to a filter unit for filtering dregs from the stream having an inlet for the stream, an outlet for filtered green liquor and an outlet for dregs, wherein the unit has at least one filtration element and a pressure differential between the filtered green liquor outlet and the inlet, with the filtered green liquor outlet being maintained at a lower pressure than the inlet, to thereby facilitate the removal of dregs from the green liquor by filtration; and

(b) maintaining that portion of the filter unit downstream from the filtration element at a negative pressure to thereby drive water vapor from and cool and concentrate the filtered green liquor.

2. The method of claim 1 comprising the further step of directing the filtered green liquor and water vapor to a separator vessel in which the vapor is separated from the green liquor.

3. The method of claim 2 wherein the separator vessel is maintained at a negative pressure.

4. The method of claim 2 further comprising utilizing the cooled green liquor in a recausticizing step to generate white liquor.

5. The method of claim 1 wherein the pressure differential between the green liquor inlet and green liquor outlet ranges from about 0.2 Bar to about 1.8 Bar.

6. The method of claim 5 wherein the pressure differential ranges from about 0.5 to about 1.5 bar.

7. The method of claim 1 wherein the negative pressure at the green liquor outlet will range from just below atmospheric to about negative 0.7 bar.

8. The method of claim 2 wherein the water vapor removed from the mixture is condensed back into water.

9. The method of claim 8 wherein at least a portion of said condensed water is combined with the separated green liquor downstream from the separator vessel and prior to the recausticizing step.

10. The method of claim 8 wherein none of said condensed water is combined with the separated green liquor.

11. The method of claim 8 wherein at least a portion of the condensed water is directed to the filter unit and is utilized as dregs cake wash to wash dregs separated from the green liquor and is thereafter discharged from the filter unit.

12. A method of treating a green liquor stream produced in a kraft pulp process to separate green liquor from the stream for use in a recausticizing step to generate white liquor, comprising

(a) directing the stream to a filter unit having a green liquor inlet and a green liquor outlet, wherein the unit has a pressure differential between the green liquor inlet and green liquor outlet to facilitate the removal of dregs from the green liquor by filtration, with the green liquor outlet being maintained at a negative pressure to thereby drive water vapor from and cool the filtered green liquor;

(b) directing the filtered green liquor and water vapor to a separator vessel in which the vapor is separated from the green liquor; and

(c) condensing the water vapor removed from the green liquor back into water.

13. The method of claim 12 wherein at least a portion of the condensed water is combined with the green liquor downstream from the separation step and prior to the recausticizing step.

14. The method of claim 12 wherein none of the condensed water is combined with the green liquor.

15. The method of claim 12 wherein at least a portion of the condensed water is directed to the filter unit and is utilized to wash dregs separated from the green liquor and is thereafter discharged from the filter unit.

16. The method of claim 12 wherein the pressure differential between the green liquor inlet and green liquor outlet is about one bar.

17. The method of claim 12 wherein the negative pressure at the green liquor outlet will range from just below atmospheric to about negative 0.7 bar

18. The method of claim 12 wherein the separator vessel is maintained at a negative pressure.

19. Apparatus for use in a recausticizing process for separating green liquor from dregs comprising

a filtration vessel containing a green liquor inlet and a green liquor outlet;

means to maintain a pressure differential between the green liquor inlet and the green liquor outlet to facilitate the removal of dregs from the green liquor by filtration;

means to maintain the green liquor outlet at a negative pressure to thereby drive water vapor from and cool the filtered green liquor;

means to remove the filtered dregs from the filtration vessel; and

means to remove the water vapor and the filtered green liquor from the filtration vessel.