METHODS FOR THE OXYGEN-BASED DELIGNIFICATION OF PULP
Pulp is delignified by forming a mixture of pulp and caustic, adding oxygen and feeding this oxygen-containing mixture to a first step reactor. The lignin in the oxygen-containing mixture will be partially delignified and will be fed to a second step reactor where the remainder of the lignin in the pulp will be delignified. In this manner, fast reacting lignin can be treated in the first step reactor and slow reacting lignin can be treated in the second reactor. The delignified pulp is recovered from the apparatus and after washing can be forwarded to a bleaching unit.
The present application claims priority from U.S. provisional application 62/020,446 filed Jul. 3, 2014.
BACKGROUND OF THE INVENTIONOxygen delignification is the step between digesting wood chips in pulp making and pulp bleaching operations. Oxygen delignification is designed to dislodge and disintegrate residual lignin left in the pulp after the digestion step using oxygen and alkali. It is the direct extension to delignification that occurs during digestion. Contacting pulp in an aqueous alkaline medium with oxygen causes oxidation of lignin which not only breaks molecules but also makes it water soluble. Oxidation of color imparting groups reduces the Kappa Number lowering the consumption of bleach chemicals in the bleach plant. Delignification with oxygen is a more gentle way of reducing the Kappa Number than by extended digesting and bleaching. In recent decades, new pulp mills have been increasingly adopting oxygen delignification systems as an advantageous step in reducing environmental impact and achieve a better return on economic investment.
The most commonly practiced oxygen delignification consists of the following steps. The first step involves adding washed pulp into a mixer, adding caustic, adding oxygen and steam to bring the temperature to a range of 70° to 95° C. and introducing this pulp mixture into the bottom of a vertical tall reactor in a continuous fashion. The pulp will flow upwards while lignin in the reactor gets oxidized in the alkaline medium thereby dissolving and disintegrating the lignin and dislodging it from the pulp fibers. The reactor is maintained at 5 to 10 bar pressure to improve on oxygen update. The residence time for pulp flowing through a commercially practiced reactor is in the range of between 20 and 100 minutes.
Oxygen delignification can be performed with both medium as well as high consistency pulp. Due to limited effectiveness, difficulty in mixing of oxygen and other operating problems with high consistency pulp, oxygen delignification has not achieved widespread success when compared with medium consistency pulp.
Oxygen delignification works with pulps from both types of woods, hardwood and softwood, reducing the Kappa Number up to 35% and 50% respectively. In the case of hardwood, a two stage approach is needed where two reactors are placed in series. The first stage is maintained at a higher pressure and lower temperature with less residence time while the second stage is usually maintained at lower pressure but at higher temperatures and greater residence times.
There has been limited success using a single stage high efficiency reactor towards achieving short term delignification at smaller and experimental scales. Consequently, there has not been any commercial apparati embodying this concept.
SUMMARY OF THE INVENTIONIn a first embodiment of the invention, there is disclosed a method for delignifying pulp comprising the steps of:
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- (a) Forming a mixture of the pulp and caustic;
- (b) Feeding oxygen to the mixture and feeding the oxygen-containing mixture to a first step reactor wherein lignin will delignify;
- (c) Feeding the oxygen-containing mixture to a second step reactor wherein lignin will delignify; and
- (d) Recovering the delignified pulp.
In step b) the fast reacting lignin or that lignin which contains the more fast reacting components will delignify first when mixed with the oxygen. This delignified lignin as well as that lignin that was not dellgnified will be passed through with the remainder of the pulp to the second step reactor in step c) where the lignin containing the slow reacting components will delignify. Some fast reacting lignin may of course carry over from the first step reactor to the second step reactor where it will delignify along with the slow reacting lignin.
The pulp that is treated is selected from the group consisting of medium and high consistency pulp.
The caustic employed is typical of that used in pulp and paper operations and typically is sodium hydroxide.
The oxygen is generally pure oxygen but oxygen concentrations greater than 80% purity will be effective for creating the oxygen-containing mixture. Supplemental oxygen may be added to the mixture of pulp and caustic after step (b) and before the oxygen-containing mixture is fed to the second step reactor.
Steam may be added to the mixture of pulp and caustic before being fed into the first step reactor in order to increase its temperature.
The first step reactor is a reactor where the gas phase is the continuous phase and the liquid phase is the dispersed phase.
The oxygen-containing mixture is present in the first step reactor for about 0.25 to 5 minutes.
The second step reactor is preferably a reactor column.
The oxygen-containing mixture is present in the second step reactor for about 2 to 40 minutes.
After the delignified pulp is recovered in step (d), it may be fed to a washing unit where water may be added to the washing unit.
The delignified pulp may also be fed to a pulp bleaching operation.
In another embodiment of the invention, there is disclosed an apparatus for delignifying pulp comprising a mixer, a first step reactor and a second step reactor.
In the apparatus the first step reactor is a reactor where the gas phase is the continuous phase and the liquid phase is the dispersed phase.
The second step reactor is a reactor column.
The mixer is in fluid communication with the first step reactor while the first step reactor is in fluid communication with the second step reactor.
This apparatus can further comprise a washing unit
Studies have demonstrated that the addition of oxygen utilized in delignification will also cause oxygen to react with cellulose and hemicelluloses and no-lignin components in the pulp system.
The residual in the pulp after digestion is often characterized as containing “fast reacting” and “slow reacting” lignin. Many non-lignin based oxidizable structures present in the pulp that consumes oxygen have been described by pulp and paper scientists.
The present invention divides the oxygen consuming reaction in oxygen delignification as “fast” and “slow” oxidation without specifying them to be either specific lignin or non-lignin reactions. Thus fast reacting lignin will be that part of the lignin in the pulp that contains more of the faster reacting components when the lignin is delignified. The slow reacting lignin will be that part of the lignin in the pulp that contains more of the slower reacting components when the lignin is delignified.
The methods of the present invention achieve high levels of oxygen delignification by using two steps. In the first step, a reactor is employed that offers high efficiency with respect to mass transfer. The reactor is capable of transporting oxygen from the gas phase to the liquid phase at extremely high rates. The pulp that is introduced into the first reactor has the highest concentration of both the fast and the slow reacting components. Oxygen is a sparingly soluble gas and by utilizing a high efficiency mass transfer reactor and elevated pressure, the aqueous medium surrounding the pulp can be maintained to saturation levels which maximize and maintain rapid reaction rates between the fast reacting components and oxygen.
The residence time of the pulp in the first step reactor or the feed rate of the pulp to the high efficiency reactor is adjusted accordingly to ensure depletion of almost all the fast reacting components or fast reacting lignins of the pulp before it exits the reactor.
The typical high efficiency reactor may offer 10 to 1000 times greater mass transport per unit volume compared to conventional gas-liquid reactors such as continuously stirred tank reactors or mechanically agitated contactors, bubble columns, etc.
The reactor employed for the first step of the invention is one where the gas phase is the continuous phase and the liquid phase is the dispersed phase. The power required for the gas liquid contacting is provided to the gas phase. In such a reactor, the gas phase has extremely high turbulence for transporting gas to the aqueous phase and only a partial amount of the energy from this turbulence is conveyed to the liquid phase thereby avoiding undesirably loss of fiber strength of the pulp.
Some of the manufacturers of such high efficiency reactor's attribute unusually high mass transport to ultrasonic phenomena. One such reactor available is made by Quantum Technologies of Akron, Ohio. However, for purposes of the invention, the method described herein is not limited solely to this one reactor.
Once the fast reacting components are oxidized, the fast mass transfer of oxygen of gas to the aqueous phase is not critical. The slow reacting components can be oxidized by holding the pulp exiting the first step in a chamber or a column type reactor with a predefined residence time as the second step of the invention. As discussed previously, the pulp leaving the first step will always be saturated with oxygen if operated as described for the first step. Since in the second step, the oxidation reaction is very slow, supplemental oxygen is limited and also very slowly consumed. Providing supplemental oxygen in the second step can be accomplished by allowing the complete output from the first step containing both phases. The gas phase consisting of excess oxygen and the aqueous phase consisting of pulp can be introduced at the bottom of the chamber or column utilized in the second step. Both the aqueous and gas phase in the chamber or column of the second step will flow co-currently upward. As the gas phase is lighter, it flows upward in the form of bubbles faster and rises through the aqueous pulp in the chamber/column supplementing the depleted oxygen and agitating the pulp.
The pulp exiting the second step reactor is then washed and sent to further processing steps such as bleaching.
The combination of the first step and the second step provide high efficiency oxygen delignification at lower costs than conventional techniques.
The figure is a schematic of a pulp delignification process according to the methods of the invention.
Turning to the figure, a schematic of a pulp delignification system according to the methods of the invention is shown. A mixer A receives pulp through line 1 and caustic through line 2. The combination of pulp and caustic is mixed in the mixer A and fed through line 3 and a pump (not shown) to the first step reactor B. The pump will raise the pressure of the pulp and caustic mixture.
Oxygen is fed through line 4 to join with the pulp caustic mixture in line 3 before being fed into the first step reactor B. Steam may also be introduced upstream of the first step reactor B to raise the pulp and caustic mixture temperature to between 50° C. and 100° C.
As discussed, there is an improved rapid reaction between the fast reacting lignin components or fast reacting lignin and the oxygen thereby resulting in the depletion of all the fast reacting components of the pulp before it exits the first step reactor B.
The pulp which now contains mostly the slow reacting lignin components will be fed from the first step reactor B through line 5 into the bottom section of the second step reactor chamber or column C. The second step reactor chamber or column C is designed to allow for additional retention time for the pulp to contact the oxygen thereby delignifying the slow reacting components.
The excess oxygen from the first step reactor B will be adjusted so as to provide the required oxygen feed rate in the second step reactor chamber or column C.
The gas and aqueous phase streams will rise through the reactor chamber or column C and the gas will be exhausted from the top through line 10. The mix of water and delignified pulp will be fed from the second step reactor chamber or column C through line 6 and will be washed in washer D which is assisted in this by the addition of fresh water through line 7. The pulp will be separated from the water and fed through line 9 to a pulp bleaching operation (not shown). The remaining water from the washing step can be discharged from the washing unit D through line 8 where it can be reused or cleaned up prior to discharge into the environment.
The advantages of the present invention are manifest in a number of areas. A smaller reactor column can be employed thereby reducing the capital required for equipment. Due to residency time of between 0.25 and 5 minutes for the first step reactor and 2 to 40 minutes for the second step reactor, the amount of feedstock in progress in the vessel is lessened.
The equipment is one twelfth to one half the size of conventional oxygen delignification equipment making for an easier retrofit into existing mills that do not have oxygen delignification.
The methods of the present invention will provide for a better Kappa Number reduction that that achieved by traditional oxygen delignification systems.
The pulp produced by this invention has better pulp strength as it exposes the cellulose fiber in the pulp to the caustic and oxygen for shorter durations of time.
In depth analysis of feed stock and its complex chemistry is not necessary to implement the two step method of the invention.
While this invention has been described with respect to particular embodiments thereof, it is apparent that numerous other forms and modifications of the invention will be obvious to those skilled in the art. The appended claims in this invention generally should be construed to cover all such obvious forms and modifications which are within the true spirit and scope of the invention.
Claims
1. A method for delignifying pulp comprising the steps of:
- (a) Forming a mixture of the pulp and caustic;
- (b) Feeding oxygen to the mixture and feeding the oxygen-containing mixture to a first step reactor wherein lignin will delignify;
- (c) Feeding the oxygen-containing mixture to a second step reactor wherein lignin will delignify; and
- (d) Recovering the delignified pulp.
2. The method as claimed in claim 1 wherein the lignin in step b) is fast reacting lignin.
3. The method as claimed in claim 1 wherein the lignin in step c) is slow reacting lignin.
4. The method as claimed in claim 1 wherein the pulp is selected from the group consisting of medium and high consistency pulp.
5. The method as claimed in claim wherein the caustic is sodium hydroxide
6. The method as claimed in claim 1 wherein the oxygen is greater than 80% purity.
7. The method as claimed in claim 1 wherein steam is added to the mixture of pulp and caustic before being fed into the first step reactor.
8. The method as claimed in claim 1 wherein the oxygen-containing mixture comprises a gas phase and a liquid phase.
9. The method as claimed in claim 8 wherein the first step reactor is a reactor where the gas phase is the continuous phase and the liquid phase is the dispersed phase.
10. The method as claimed in claim 1 wherein supplemental oxygen may be added to the mixture of pulp and caustic after step (b).
11. The method as claimed in claim 1 wherein the oxygen-containing mixture is present in the first step reactor for about 0.25 to 5 minutes.
12. The method as claimed in claim 1 wherein the second step reactor is a reactor column.
13. The method as claimed in claim 1 wherein the oxygen-containing mixture is present in the second step reactor for about 2 to 40 minutes.
14. The method as claimed in claim 1 wherein the delignified pulp is fed to a washing unit.
15. The method as claimed in claim 13 wherein water is added to the washing unit.
16. The method as claimed in 13 wherein the delignified pulp is fed to a pulp bleaching operation.
17. An apparatus for delignifying pulp comprising a mixer, a first step reactor and a second step reactor.
18. The apparatus as claimed in claim 17 wherein the first step reactor is a reactor where the gas phase is the continuous phase and the liquid phase is the dispersed phase.
19. The apparatus as claimed in claim 17 wherein the second step reactor is a reactor column.
20. The apparatus as claimed in claim 17 wherein the mixer is in fluid communication with the first step reactor.
21. The apparatus as claimed in claim 17 wherein the first step reactor is in fluid communication with the second step reactor.
22. The apparatus as claimed in claim 17 further comprising a washing unit.
Type: Application
Filed: Jul 1, 2015
Publication Date: Jan 7, 2016
Inventor: Naresh J. Suchak
Application Number: 14/789,359
