Process for treating lignin

Abstract

A process for recovery of lignin from black liquor that contains either soluble or dispersed lignin by generating a “liquid lignin” at high yield is disclosed. Soluble lignin at a high pH is precipitated by reducing the pH of the black liquor stream by countercurrent reaction with carbon dioxide, at elevated temperature and pressure, creating a dense liquid-lignin phase and a light lignin-depleted phase. The dense lignin-rich phase is separated and washed countercurrently with a non-sulfur containing acid, such as acetic acid or formic acid, to displace metal cations from the lignin, creating a low-salt lignin, which is then formed into a low-dust, high-bulk density lignin fuel pellet. If desired, an oxidation step may be used to eliminate odor for lignins having high value green chemistry applications.

Claims

1. A process for recovering lignin from paper making black liquor comprising: (a) carbonizing said black liquor at a temperature and pressure sufficient to neutralize NaOH and other basic components contained therein with carbon dioxide sufficient to reduce the pH to between pH 9.0 and 10.5; (b) recovering a dense liquid-lignin phase; (c) acidifying said carbonated liquid-lignin phase with a non-sulfur containing acid at a temperature up to 200° C. to neutralize residual NaOH and other basic components, thereby generating an acidified dense liquid-lignin phase; (d) recovering lignin from said acidified dense-lignin phase to remove residual acid and ash content, thereby generating purified lignin; and (f) recovering said purified lignin.

2. The process according to claim 1 wherein said black liquor is pressurized to between 50 psig and 200 psig.

3. The process according to claim 1 wherein said carbonation of said black liquor is carried out by contacting said black liquor with carbon dioxide countercurrently.

4. The process according to claim 1 wherein said carbonating step is carried out at a temperature between about 80° C. and 200° C.

5. The process according to claim 1 wherein said carbonating step is carried out at a temperature between about 100° C. and 150° C.

6. The process according to claim 1 wherein carbon dioxide from the acidification step is recycled to the carbonation step.

7. The process according to claim 1 wherein an oxidizing agent is reacted with said black liquor prior to carbonation in an amount sufficient to eliminate or substantially reduce the odor of the resulting lignin product.

8. The process according to claim 1 wherein an oxidizing agent is reacted with said liquid-lignin phase in an amount sufficient to eliminate or substantially reduce the odor of the resulting lignin product.

9. The process according to claim 1 wherein said non-sulfur containing acid is present in an amount sufficient to reduce the pH to less than pH 4.

10. The process according to claim 1 wherein said non-sulfur containing acid is present in an amount sufficient to reduce the pH to between pH 1.5 and pH 4.

11. The process according to claim 1 wherein said acidifying step is carried out at a temperature from about 90° C. to about 110° C. to form a dense liquid-lignin phase.

12. The process according to claim 1 wherein said papermaking black liquor is at a solids content between about 10% and about 70%.

13. The process according to claim 1 wherein said papermaking black liquor is at a solids content between about 30% and about 60%.

14. The process according to claim 1 wherein said black liquor feed from a papermaking operation is removed downstream from a tall oil soap separator.

15. The process according to claim 1 wherein said lignin product from step (f) is pelletized.

16. The process according to claim 1 further comprising washing the extraction of said acidified dense lignin phase to remove residual acid and ash content, thereby generating purified lignin.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Having described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

(2) FIG. 1 is a schematic flow diagram which illustrates an embodiment of the process of the present invention showing the optional oxygenating step, the carbonating step, the acidifying step and the extracting step;

(3) FIG. 2 is a schematic diagram of an alternative embodiment of the process of the present invention showing the application of oxygenating after the carbonating step; and

(4) FIG. 3 is a schematic diagram of an alternative embodiment of the process of the present invention showing recycle of carbon dioxide from the acidification settling tank to the carbonation column.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

(5) The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to the elements throughout.

(6) Referring to FIG. 1, there is shown a schematic diagram of an embodiment of a process of the present invention showing the steps, from a lignin containing stream, of carbonating to form a liquid-lignin phase, acidifying and recovering the lignin. Black liquor, leaving the soap separator in the pulp and paper plant, is introduced through line 1 to pump A where the black liquor is preferably pressurized to between about 50 psig to about 200 psig, preferably about 150 psig. Typically the black liquor is removed midway in the evaporator train, preferably at a solids content of 30% to 45% and has a temperature of about 80° C. to about 120° C. Keeping the heat of reaction in the pressurized system raises the temperature significantly. It should be understood that the solids content of the black liquor ranges from about 10% to about 70%, but more normally is from 25% to 60%. The melt point of lignin depends strongly on the level of sodium ions, the source of the lignin, and the level of occluded black liquor in the lignin phase, hence its viscosity is difficult to predict. Alternatively, the black liquor may be taken downstream from the tall soap separator.

(7) Also, as an option, the pressurized black liquor may be reacted with an oxidizing agent, such as oxygen, peroxide or the like, in an amount sufficient to reduce or eliminate the odor level in the black liquor so that there will be little or no odor in the final lignin product. Only the odorous materials are intended to be oxygenated, not the lignin material. This step removes the odor, by reacting with the mercaptans (methyl, ethyl, dimethyl, and diethyl) and other malodorous components. Preferred equipment for this reaction is a Hydrodynamics Shockwave Power Reactor®, shown at B in FIG. 1. The oxygenation also has a substantial heat of reaction, raising the temperature of the stream about 50° C. depending on the reactants within the aqueous stream and its solids content. An alternative location in the process, that shown in FIG. 2, is to oxidize the liquid lignin exiting the carbonation column C.sub.2 in line 6, and thereby conserving oxygen by not oxidizing the entire black liquor flow. Another alternative is to not oxidize the black liquor when applications are insensitive to the odor of the final product, as typically would be the case when the lignin is to be used as a fuel or as a binder for energy pellets.

(8) Lignin begins to precipitate near the black liquor entrance at the top of the column as the pH begins to be reduced by carbon dioxide. As the pH decreases from its high (12-14) near the top to the exit at the bottom at a pH below 11, preferably between a pH of from pH 9 to pH 10, more and more lignin becomes insoluble and coalesces within column. Countercurrently contacting the incoming black liquor with CO.sub.2, creates a pH gradient in a column so that liquid-lignin droplets are created near the top that sweep and collect other liquid-lignin droplets that are forming at the lower pH in the lower zone of the column. The liquid-lignin particles have a natural affinity for other liquid-lignin particles, facilitating coalescence as they fall within the column. As the liquid-lignin particles fall through the column, they collect other particles that are forming at the lower pH within the lower zones of the column. The dense particles then coalesce into a bulk liquid-lignin phase which accumulates at the bottom of the column.

(9) Pressurized black liquor is introduced via line 2 into the top of a two part carbonation absorption column C and CO.sub.2 is introduced via line 3. The size of the column will depend upon the volume of black liquor being treated. For example, in a column designed to process 50,000 tons of lignin per year, the upper portion of the column C.sub.1 may be approximately 4′ diameter and 40′ tall. The black liquor, with a high NaOH content and a pH of near 14, reacts with the CO.sub.2 to form NaHCO.sub.3. The column may operate at a nominal pressure of 150 psig and a temperature between about 80° C. and 200° C., preferably about 100° C. to 150° C. In the column, the NaOH is neutralized, lowering the pH to less than pH 11, preferably pH 8 to 11, more preferably from pH 9 to pH 10. This reaction causes the release of a substantial exotherm, increasing the temperature of the stream depending on the NaOH content and the solids level of the stream. Malodorous gases leave the top of column C.sub.1 via line 4 and are vented to a vapor control system. When the option of oxygenating is used, the combined temperature rise of oxygenated and carbonated black liquor is typically about 20° C. or more.

(10) The black liquor and lignin solution pass into the bottom portion of the carbonation absorption column C.sub.2, where the lignin undergoes phase separation, forming a heavy liquid-lignin phase. The high temperature and pressure separation preserve heat from the heats of reaction of the sequential reaction of O.sub.2, and lignin, when the oxygenating step is used, and CO.sub.2 and lignin that enables the process to send that heat back to the recovery operation in the black liquor via line 5. The lower portion C.sub.2 of the CO.sub.2 column is larger than the upper portion. For example, the lower portion may be approximately 8 feet in diameter and 10 feet tall for a 50,000 ton per year column. The carbon dioxide also converts much of the sodium (and other metals) and phenolic groups on the lignin molecules to the hydrogen form, causing the lignin to become insoluble. The carbonated black liquor and lignin undergo a phase separation creating a dense lignin-rich “liquid-lignin” phase and a light lignin-depleted phase. The black liquor separates into the light (top) phase and is returned to the recovery operation of the host paper mill via line 5. The dense liquid-lignin phase leaves the bottom of the column C.sub.2 via line 6.

(11) A safety re-circulating loop can be provided within column C.sub.1 to remove excess heat if needed. The loop includes pump D.sub.1 and heat exchanger E.sub.1. Alternatively, the temperature within the column can be controlled with a heat exchanger on the inlet black liquor line, controlling the temperature within the column to provide optimum separation.

(12) The lignin solution leaving the bottom of C.sub.2 via line 6 contains approximately 30% aqueous phase and goes to a tangential entry cyclonic flash tank F. In the flash tank F, the liquid-lignin solution is flashed down to atmospheric pressure with the evolution of steam which is vented to the atmosphere through line 8. Typically, about 85% of the aqueous phase is removed in this step. The relatively dry lignin solution from flash tank F passes through line 7 into an attrition unit G, such as a screw conveyor, which pulverizes the lignin into a smaller size range. The lignin particles are passed via line 9 to belt filter H. The lignin particles remain large enough not to slow the filtration. The belt filter H separates out any residual black liquor occluded inside the lignin particles that was not previously removed. The residual black liquor is returned to the pulp mill via a pump tank I followed by intermittent service transfer pump J.

(13) The lignin is then transferred via line 10, preferably by a screw conveyor from the belt filter outfall to a mix tank K where the lignin is washed with a sulfur-free acid, such as acetic acid of formic acid, to neutralize the residual NaOH. The acid is may be used at high concentration, say 3.5 molar (M), to a low concentration, say about 1 molar. During this step the pH is reduced to a pH less than 4, preferably from about 1.5 to about 3.5. An agitator L provides a high level of mixing within a short residence time. The acidified lignin slurry is then pumped M to drum filter N, where the lignin is separated from the acid water, which is removed through line 11. The acidifying step is carried out at a temperature up to 200° C. to form a dense liquid-lignin phase. When the acidifying temperature is between about 90° C. and about 130° C. lignin granules are formed. When the acidifying step is carried out at a temperature above about 130° C. a dense taffy-like lignin is formed. These temperatures are dependent upon the specific nature of the lignin.

(14) Either of these lignin forms can be pumped or discharged through a pressure-reducing valve into a countercurrent water extraction system, where residual acid and salt are removed, creating a low-ash lignin. For example, from the filter N, the lignin filter cake is passed through line 12, preferably via a screw conveyor to a second agitated mix tank O. Water is fed to the mix tank via line 13 for thorough removal of acid. A centrifugal pump P is used to pump the wet lignin to another filter Q, where it may be recovered and used as is.

(15) Alternatively, the dried lignin is then conveyed through line 14, preferably via a screw conveyor, to a pelletizer R, where the lignin is pelletized. The pellets are then transferred to pellet storage bin S using line 15. The dried lignin has an ash content less than 2.0%, preferably less than 1.0%.

(16) In an alternative of the processes of this invention, black liquor is passed through line 2 to the two part absorption column C where it is treated countercurrently with CO.sub.2 to lower the pH. In the embodiment shown in FIG. 2 the liquid lignin leaves the bottom portion C.sub.2 of the CO.sub.2 column through line 6 where it is oxygenated. The oxygenated liquid-lignin phase is pumped through line 10 into another pressurized mixer K where the stream is mixed with a sulfur-free acid. Depending on the nature of the lignin and the temperature of the reactor, the lignin forms either another dense liquid-lignin phase or heavy solid granules that separate by settling, such as in settling tank W. A stream of acid brine is removed through line 16 and a stream of off-gases including malodorous gases and carbon dioxide is removed through vent line 18. The dense liquid-lignin is passed through line 12 to an extraction column T where water through line 13 is fed countercurrently through the column. Being a countercurrent continuous washing or extraction system, the minimum levels of water will be required to achieve the target ash level in the final product. Also a portion of the extraction or wash water can be recycled to the acidification reactor to reduce the process water requirements of the process. A low ash lignin is removed from the bottom of the column and brine is removed from the top.

(17) In FIG. 3 there is shown a variation of the processes shown in FIG. 1 and FIG. 2. In either process, the off-gases from the acidification reaction K will be rich in CO.sub.2 from the reversal of the sodium bicarbonate contained within the heavy liquid-lignin phase formed in the carbonation system. Since this is a continuous process, this CO.sub.2-rich vent stream 18 can be recycled to the carbonation column C, reducing the overall process requirement of CO.sub.2. Additional CO.sub.2 is added through line 3.

EXAMPLE 1

(18) Black liquor was oxidized using the Shockwave Power Reactor (SPR Hydrodynamics, Rome, Ga.). A single-pass and a two-pass operation were run on each of the two kraft papermaking black liquors. Data from the runs are shown in Table 1. The two-pass oxidized black liquor samples were used for the following examples.

(19) TABLE-US-00001 Black Liquor A at Black Liquor B at 38% solids 48% solids 1.sup.st Pass on 2.sup.nd Pass on 1.sup.st Pass on 2.sup.nd Pass on SPR SPR SPR SPR Black Liquor Flow 1.8 1.8 2.2 2.2 (gpm) Oxygen Flow (scfm) 3.0 2.7 4.0 3.8 T inlet ° C. 24 54 24 55 T outlet ° C. 93 75 98 99

EXAMPLE 2

Carbonation and Acidification at 115° C.

(20) The two-liter reactor was charged with 1450 grams of Black Liquor A. Agitation was set at 60 rpm, temperature was increased to 115° C., and carbon dioxide was added to maintain pressure of 150 psig for 180 minutes. Agitation was ceased and the reaction mix was allowed to settle for one hour. The supernatant phase was removed. The agitator was restarted at a rate of 180 rpm. The carbonated liquid-lignin phase was acidified with 8.7M acetic acid to a pH of 3.6. The acidified supernatant phase was collected, and the acidified dense phase was removed and allowed to reach ambient temperature. The ash content of the acidified lignin product was 7.5%.

EXAMPLE 3

Carbonation and Acidification at 115° C.

(21) The two-liter reactor was charged with 1450 grams of Black Liquor A. Agitation was set at 60 rpm, temperature was increased to 115° C., and carbon dioxide was added to maintain pressure of 150 psig for 180 minutes. Agitation was ceased and the reaction mix was allowed to settle for one hour. The supernatant phase was removed. The agitator was restarted at a rate of 180 rpm. The carbonated liquid-lignin phase was acidified with 1.3 liters of 3.5 M acetic acid. The agitation was stopped and the sample allowed to stand for 30 minutes. The supernatant phase was removed. The liquid-lignin phase was acidified again with 1.3 liters of 3.5 M acetic acid, with agitation and then allowed to settle for 30 minutes. The acidified supernatant phase was collected, and the acidified dense phase was removed and allowed to reach ambient temperature. The ash content of the acidified lignin product was 4.2%.

EXAMPLE 4

Carbonation, Acidification, and Water Wash

(22) The two-liter reactor was charged with 1450 grams of Black Liquor A. Agitation was set at 60 rpm, temperature was increased to 115° C., and carbon dioxide was added to maintain pressure of 150 psig for 180 minutes. Agitation was ceased and the reaction mix was allowed to settle for one hour. The supernatant phase was removed. The agitator was restarted at a rate of 180 rpm and the carbonated liquid-lignin phase was acidified with 1.3 liters of 3.5 M acetic acid. The agitation was stopped and allowed to settle for 30 minutes. The acidified supernatant phase was collected. The agitation was re-started and 1 liter of water was added, and the system was mixed for 30 minutes. The agitation was stopped and the system allowed to settle for 30 minutes. The supernatant was collected, and the washed dense phase was removed and allowed to reach ambient temperature. The ash content of the acidified lignin product was 5.2%.

EXAMPLE 5

Carbonation, Acidification, and Water Wash

(23) The two-liter reactor was charged with 1450 grams of Black Liquor A. Agitation was set at 60 rpm, temperature was increased to 115° C., and carbon dioxide was added to maintain pressure of 150 psig for 180 minutes. Agitation was ceased and the reaction mix was allowed to settle for one hour. The supernatant phase was removed. The agitator was restarted at a rate of 180 rpm and the carbonated liquid-lignin phase was acidified with 1.3 liters of 3.5 M acetic acid. The agitation was stopped and allowed to settle for 30 minutes. The acidified supernatant phase was collected. The agitation was re-started and 1 liter of water was added, and the system was mixed for 30 minutes. The agitation was stopped and the system allowed to settle for 30 minutes. The supernatant was collected. The agitator was restarted at a rate of 180 rpm and the carbonated liquid-lignin phase was acidified with 1.3 liters of 3.5 M acetic acid. The agitation was stopped and allowed to settle for 30 minutes. The acidified supernatant phase was collected. The agitation was re-started and 1 liter of water was added, and the system was mixed for 30 minutes. The agitation was stopped and the system allowed to settle for 30 minutes. The supernatant was collected, and the washed dense phase was removed and allowed to reach ambient temperature. The ash content of the acidified lignin product was 1.1%.

EXAMPLE 6

Carbonation, Acidification, and Water Wash of Soda Black Liquor

(24) The two-liter reactor was charged with 2150 grams of Soda Black Liquor. Agitation was set at 60 rpm, temperature was increased to 115° C., and carbon dioxide was added to maintain pressure of 150 psig for 180 minutes. Agitation was ceased and the reaction mix was allowed to settle for one hour. The supernatant phase was removed. The agitator was restarted at a rate of 180 rpm and the carbonated liquid-lignin phase was acidified with 1.3 liters of 3.5 M acetic acid. The agitation was stopped and allowed to settle for 30 minutes. The acidified supernatant phase was collected. The agitation was re-started and 1 liter of water was added, and the system was mixed for 30 minutes. The agitation was stopped and the system allowed to settle for 30 minutes. The supernatant was collected. The agitator was restarted at a rate of 180 rpm and the carbonated liquid-lignin phase was acidified with 1.3 liters of 3.5 M acetic acid. The agitation was stopped and allowed to settle for 30 minutes. The acidified supernatant phase was collected. The agitation was re-started and 1 liter of water was added, and the system was mixed for 30 minutes. The agitation was stopped and the system allowed to settle for 30 minutes. The supernatant was collected, and the washed dense phase was removed and allowed to reach ambient temperature. The ash content of the acidified lignin product was 0.14%.

EXAMPLE 7

Carbonation, Acidification, and Water Wash

(25) A 100 mL Parr reactor was charged with 100 g of Kraft Black Liquor. Agitation was set at 60 rpm, the temperature was increased to 125° C., and carbon dioxide was added to maintain the pressure at 140 psig for 30 minutes. Agitation was ceased and the reactor allowed to settle for 1 hour at temperature. The reactor was allowed to cool to 40° C. before the supernatant was decanted off 5 mL of DI water was charged into the reactor and the temperature increased to 125° C. The agitator was restarted at 120 rpm. The carbonated liquid-lignin phase was acidified with 20 mL of 10 wt % acetic acid. The agitation was stopped and the temperature allowed to cool to 40° C. The lignin slurry was then filtered using a medium porosity filter paper. The lignin was then collected and suspended in DI water and allowed to sit for 1 hour. The slurry was then centrifuged to collect the lignin. The ash content of the lignin was 9.3%. The above procedure was repeated with sulfuric acid in place of the acetic acid. The ash content of the lignin acidified with sulfuric acid was found to be 7.9%.

EXAMPLE 8

Carbonation, Acidification, and Water Wash

(26) A 100 mL Parr reactor was charged with 100 g of Kraft Black Liquor. Agitation was set at 60 rpm, the temperature was increased to 125° C., and carbon dioxide was added to maintain the pressure at 140 psig for 30 minutes. Agitation was ceased and the reactor allowed to settle for 1 hour at temperature. The reactor was allowed to cool to 40° C. before the supernatant was decanted off. 5 mL of DI water was charged into the reactor and the temperature increased to 85° C. The agitator was restarted at 120 rpm. The carbonated liquid-lignin phase was acidified with 20 mL of 10 wt % acetic acid. The agitation was stopped and the temperature allowed to cool to 40° C. The lignin slurry was then filtered using a medium porosity filter paper. The lignin was then collected and suspended in DI water and allowed to sit for 1 hour. The slurry was then centrifuged to collect the lignin. The ash content of the lignin was 8.4%. The above procedure was repeated with sulfuric acid in place of the acetic acid. The ash content of the lignin acidified with sulfuric acid was found to be 7.9%.

(27) Many modifications and other embodiments of the inventions set forth herein will come to mind to one skilled in the art to which these inventions pertain having the benefit of the teachings presented in the foregoing descriptions. Therefore, it is to be understood that the inventions are not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.