# Patent

## Title
Lignin product and process for making same

## Abstract

There is provided a process for recovery of lignin from black liquor that contains either soluble or dispersed lignin, generating a “liquid-lignin” at high yield. Lignin at elevated pH is precipitated by reducing the pH of the black liquor stream with carbon dioxide, at elevated temperature and pressure, creating two bulk fluid phases: a dense liquid-lignin phase and a light lignin-depleted phase. The dense liquid-lignin phase is separated and washed with a strong acid to displace metal cations from the lignin, creating a low-salt, randomly shaped lignin. If needed, especially for lignin recovered from kraft papermaking black-liquor streams, an oxidation step is included to eliminate negative odor for high value green chemistry applications.

## Background
CROSS-REFERENCE TO RELATED APPLICATION(S)
This application is a continuation-in-part of application Ser. No. 13/147,785, filed Aug. 3, 2011 which claims the benefit of PCT Application No. US10/49773, filed Sep. 22, 2010, which claims the benefit of priority from U.S. provisional application Ser. No. 61/245,853, filed Sep. 25, 2009 on which the present application is based and benefits claimed under 35 U.S.C. §119(e).


BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to recovering lignin from a crude lignin waste stream such as the black liquor from a papermaking operation or from a biomass enzymatic conversion process. More particularly, the present invention relates to processes for recovering and purifying lignin to produce a low-salt, granular lignin.
(2) The Prior Art
Lignin, a component of wood, is the second most abundant polymer in the world behind cellulose. Lignin is primarily recovered from the black liquor stream within pulp and paper mills, such as from the kraft pulping process. Black liquor is removed from the host paper mill's recovery system downstream of an efficiently performing soap separator, since tall oil impurities are deleterious to the operation of the unit operations of the process and the downstream applications, especially the high-value applications other than fuel pellets. Additionally, crude lignin is a byproduct stream from the plethora of technologies using enzymes being developed which convert the cellulose in biomass to ethanol or other products. Those enzymes do not affect lignin which exits those processes in various forms, generally low in solids and with various pH depending on upstream treatments.
With its high energy density and variety of functional groups and structure, lignin holds promise to be an efficient biofuel source or green-chemical precursor. Thus, one use for lignin is to recover lignin as a solid and burn the solid lignin as a fuel, to or use the lignin as a binder for energy pellets. Another use is to provide a process to recover a high-purity low-salt lignin that is used to replace phenol used in resins for composites, to be a natural polymer for making polyurethanes, or to be used in a wide variety of alternative downstream chemical applications.
Currently wood pellets are burned, but the ash content and lower energy density limit their use as a fuel. Lignin pellets have approximately the same energy content as coal, about 12,000 Btu/lb, which is about 50% higher energy per mass of low-moisture wood pellets having about 8,000 Btu/lb. Lignin pellets may be used alone or blended directly with the coal feed with the only additional capital being the separate storage and feeding equipment for the pellets. Also lignin has demonstrated potential as an improved binder for wood or grass pellets, decreasing the dust levels generated in processing of the pellets, improving the water resistance of pellets which is important for outside storage of pellets, and increasing the energy density of the pellets and increasing the life time of dies through the lubricity properties in the lignin added to the biomass feed to the pelletizers.
One lignin recovery method from papermaking black liquor was implemented in the 1940s adjacent to a host kraft mill in Charleston S.C., makes powdered lignin containing a high-salt content, which is difficult for power companies to handle since the salt creates issues with high ash within power furnaces. Additionally, the high ask contents can negatively affect the properties of green chemical applications that incorporate the lignin.
Removing a fraction (up to 30%) of the lignin from black liquor allows pulp and paper mills that have reached the maximum throughput of their recovery boilers to increase production by the same fraction of lignin removed. For example, a large paper mill recovering 30% of their lignin from black liquor could produce >50,000 tons of lignin pellets per year. If a papermaking facility makes 50,000 ton/yr of lignin, and that lignin energy value is replaced by burning residual wood, then that lignin is used to displace coal, then the overall green-house gases are reduced by 125,000 ton/yr.
Most pulp and paper mills have the infrastructure to gather residual wood within an economically-effective radius ( ̃70 miles) of the mill. Many of these mills have reached the limit of their recovery furnaces because of heat-transfer limitations within the furnace. The multiple tubes within the furnace that generate steam on the inside with heat transferred from the burning concentrated black liquor on the outside reach their upper limit of heat flux. Increasing that heat flux risks catastrophic consequences (recovery furnace explosions); thus mills don't exceed that limit. Removing a fraction (≦30%) of the lignin allows the mills to increase their overall production rate of paper by that same fraction.
Many states are implementing renewable energy thresholds on electricity-generating power furnaces, many of which burn coal. However, burning significant fractions of residual wood, as the paper industry does, requires a different design of the furnace, which would have a larger footprint and would require more capital than a coal-burning furnace. A major factor is the lower energy content of residual wood containing significant levels of water (≧40%); wet residual wood has as low as 50% the energy density (Btu/lb) as coal or lignin pellets. To produce energy pellets, the wood has to be dried to moisture contents of 10-20%, but still the energy density of cellulose is still 2⁄3 that of coal. And residual wood contains significant levels of inorganics, which result in much higher levels of ash within the fuel, which requires either specialized equipment to continuously remove the ash or periodic shut-down to remove the ash. The paper industry historically has built power furnaces capable of burning large fractions of residual wood; the power industry has not. The power industry can add small fractions of residual wood to their furnaces, but a practical upper limit is soon reached. In Europe, power furnaces burning wood pellets have been designed, and millions of tons per year of pellets are being shipped from North America to meet the biomass burning requirements of those furnaces. The power industry in Europe and paper industry are frequently at odds, competing for the same supply of residual wood.
SUMMARY OF THE INVENTION
In accordance with the present invention there are provided processes for recovering lignin from black liquor to form a liquid-lignin phase, purifying the lignin to requisite low-ash levels, and producing a lignin particle. Further, the processes provide for producing a lignin pellet to replace coal in existing power furnaces. Alternatively, lignin in the form of randomly-shaped particles exits one of the embodiments of the process, saving the cost of extruder operation. The randomly-shaped particles or pellets of lignin may be used as an improved binder for the biomass-based energy pellet market.
Broadly speaking, the present invention provides processes for recovering a liquid-lignin from a lignin-containing stream, such as a black liquor stream from a paper making process or the crude lignin stream within an enzymatic biomass conversion process by carbonating, acidifying and recovering the lignin. More specifically, the processes comprise as a first step, pressurizing black liquor to between 50 and 200 psig. As an optional step, sufficient oxygen may be reacted with the black liquor to reduce and/or eliminate odors. The soluble lignin at a pH between 12 and 14 is precipitated by introducing the pressurized black liquor into an absorption column and treating the black liquor, which is at an elevated temperature and pressure, countercurrently with CO2, to reduce the pH to between about 9 and 10 to neutralize the NaOH and other basic components within the black liquor. The carbon dioxide also converts much of the sodium (and other metals) phenolic groups on the lignin molecules to the hydrogen form. 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 light lignin-depleted phase, being mostly black liquor, is returned to the recovery process of the host paper mill at a temperature higher than the temperature of the black liquor received, thus, removing a major impediment for commercial implementation by paper mills.
The dense lignin-rich phase is washed countercurrently with a strong acid, such as sulfuric acid to displace remaining sodium ions from the lignin and further acidify the residual NaOH, other basic components, and the residual NaHCO3 salt formed in the carbonation column, creating a low-salt lignin at a pH less than 4. The low-salt lignin is extracted or washed with water to remove the residual acid and inorganic salts and then used as is or is pelletized to form a low-dust, high bulk-density lignin fuel.
An alternative is to take the dense liquid-lignin phase directly into another pressurized reactor where the stream is mixed with sulfuric 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. 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 randomly-shaped lignin having an ash content less than 4.0%.
In either alternative, the off-gases from the acidification reaction will be rich in CO2 from the reversal of the sodium bicarbonate contained within the dense liquid-lignin phase formed in the carbonation system. Since this is a continuous process, this CO2-rich vent stream can be recycled to the carbonation system, reducing the overall process requirement of CO2.
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.
It is therefore the general object of the present invention to provide a novel processes for recovering and purifying lignin to produce a low-salt, high-energy-content lignin pellet, especially useful as a fuel.
Another object of the present invention is to provide a process that is suitable for high-value green-chemistry applications such as replacing phenol in resins, providing a base polymer for polyurethanes, and other end-use applications where the chemical functionalities of lignin are employed.
Yet another object of the present invention is to produce a randomly-shaped, low ash lignin.
Other objects features and advantages of the invention will be apparent to those skilled in the art from the following detailed description taken in conjunction with the drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
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:
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;
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
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)
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 through and complete and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to the elements throughout.
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 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, is preferably at a solids content of 30% to 45% and 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.
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 of black liquor 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 is shown in FIG. 2, is to oxidize the liquid-lignin exiting the carbonation column C2 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.
Lignin begins to precipitate immediately at the black liquor entrance near 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 pH 9-10, more and more lignin coalesces within the column. Countercurrently contacting the incoming black liquor with CO2, 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.
Pressurized black liquor is introduced via line 2 into the top of a two part CO2 absorption column C. 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 C1 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 CO2 to form NaHCO3. The column operates 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 9 to 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 C1 via line 4 and are vented to the atmosphere. When the option of oxygenating is used, the combined temperature rise of oxygenated and carbonated black liquor is typically about 20° C. or more.
The carbonated black liquor and soluble lignin pass into the bottom portion of the carbonation column C2, where the lignin undergoes phase separation, forming a dense liquid-lignin phase. The high temperature and pressure separation preserves heat from the heat of reaction of the sequential reaction of O2, when the oxygenating step is used, and CO2 that enables sending that heat back to the recovery operation in the black liquor. The lower portion C2 of the CO2 column is larger than the upper portion. For example, the lower portion may be approximately 8′ in diameter and 10′ 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 soluble 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 C2 via line 6.
A safety re-circulating loop is provided within column C1 to remove excess heat if needed. The loop includes pump D1 and heat exchanger E1. 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.
The dense liquid-lignin leaving the bottom of C2 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 dense liquid-lignin from flash tank F passes through line 7 into an attrition unit G, such as a screw conveyor. The lignin is passed via line 9 to belt filter H. The dense liquid-lignin does not slow the filtration. The belt filter H separates out any residual black liquor occluded inside the dense liquid-lignin 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.
The dense liquid-lignin is then transferred via line 10, preferably by a screw conveyor from the belt filter outfall to a mix tank K where the dense liquid-lignin is washed with a strong acid, such as sulfuric acid, to neutralize the residual NaOH. 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. 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.
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 sulfuric acid. A centrifugal pump P is used to pump the wet lignin to another filter Q, where it may be recovered as dry, irregularly shaped lignin particles.
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 4.0%, preferably less than 2.0%, and most preferably below 1.0%.
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 CO2 to lower the pH. In the embodiment shown in FIG. 2 the liquid lignin leaves the bottom portion C2 of the CO2 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 sulfuric 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.
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 CO2 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 CO2-rich vent stream 18 can be recycled to the carbonation column C, reducing the overall process requirement of CO2. Additional CO2 is added through line 3.
Example 1
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.














Black Liquor A
Black Liquor B



at 38% solids
at 48% solids












1st Pass on
2nd Pass on
1st Pass on
2nd 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 Run at 150° C. and 210 psig
A one-liter Parr reactor was charged with 1074 grams of black liquor A having 37.3% solids. The black liquor was heated to 150° C., introducing CO2 to raise the pressure to 210 psig. CO2 was allowed to enter the reactor through the sparger to maintain the pressure until the reaction was complete. The agitator speed was set at 60 rpm. A liquid-lignin phase was collected from the bottom of the reactor; a clean, smooth interface was observed for the interface. Analyses indicated that 37% of the lignin in the black liquor had been precipitated. The solids content of the liquid-lignin was 61%.
Example 3
Carbonation Run at 115° C. and 140 psig
The one-liter Parr reactor was charged with 979 grams of Black Liquor A. The agitator was set at 60 rpm. The black liquor was heated to 115° C. and the pressure was maintained at 140 psig by the CO2. The reaction was allowed to continue to completion, which was essentially complete after 75 minutes. Liquid-lignin was collected from the bottom of the reactor; again a clean interface was observed. Analyses indicate that 33% of the lignin had been recovered in the liquid-lignin phase. The solids content of the liquid-lignin was 61%.
Example 4
Carbonation at 150° C. with High-Solids Black Liquor
A special two-liter reactor was fabricated with a 30-degree conical bottom to allow better removal of dense phases. Into this reactor, 2234 gram of Black Liquor B were charged having a solids of 47.5%. The temperature was increased to 150° C., and CO2 pressure of 210 psig. The CO2 reaction was allowed to continue to completion. Liquid-lignin was collected from the bottom of the reactor; analyses indicate that 58% of the lignin had been recovered. Samples of the liquid-lignin ranged from 60-70% solids. These higher yields indicate the strong affect that black liquor solids has on lignin recovery; the higher the black liquor solids, the higher the recovery.
Example 5
Carbonation at 115° C.; Acidification at Ambient Temperature
A portion of the liquid-lignin from the carbonation at 115° C. using Black Liquor A was allowed to cool at ambient temperature and then ground into particles. 100 grams of these particles were then mixed with 185 ml of 1N sulfuric acid, sufficient to reduce the pH of the mixture to 3.4. The wet particles were then separated by centrifugation, mixed with water and centrifuged again. These wet particles were then mixed with fresh water and centrifuged again. These wet particles were dried, and the lignin weighed and analyzed. The ash level of the resultant lignin was 0.7% of the sample, demonstrating the ash could be washed effectively from the carbonated liquid lignin even at ambient temperature.
Example 6
Carbonation and Acidification at 115° C.
The two-liter reactor was charged with 1550 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 210 psig for 42 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 1N sulfuric acid to a pH of 2.6. The acidified supernatant phase was collected, and the acidified dense phase was removed and allowed to reach ambient temperature. A portion of the dense phase was washed with water at ambient temperature. The ash content of the final lignin product was 3.5%.
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.

## Claims

1. A process for recovering lignin from a crude lignin-containing waste stream comprising:
(a) carbonating said crude lignin-containing waste stream liquor at a temperature sufficient and pressure to neutralize NaOH and other basic components with an amount of carbon dioxide sufficient to reduce the pH to between 9 and 10.5;
(b) subjecting said carbonated lignin-containing waste stream to a phase separation wherein a dense liquid-lignin phase is formed, a lignin-depleted liquid phase is formed, and NaOH and other basic components are neutralized;
(c) recovering said lignin-depleted liquid phase;
(d) removing said dense liquid-lignin phase and neutralized NaOH and other basic components from the bottom of said column and acidifying said dense liquid-lignin phase with sulfuric acid at a temperature between about 100° C. and 130° C. to a pH between 1.5 and 3.5, thereby generating a granular lignin; and
(e) recovering the granular lignin.

2. The process according to claim 1 wherein said crude lignin-containing waste stream at a temperature and pressure sufficient to neutralize NaOH and other basic components contained therein at a temperature between 100° C. and 150° C. and a pressure between about 50 psig and about 200 psig.

3. The process according to claim 1 wherein said carbonation of said crude lignin-containing waste stream is carried out by contacting said crude lignin-containing waste stream with carbon dioxide countercurrently.

4. The process according to claim 1 wherein said crude lignin-containing waste stream is black liquor from a paper making operation and 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.

5. 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.

6. The process according to claim 4 wherein said papermaking black liquor is at a solids content between about 10% to about 70%.

7. The process according to claim 4 wherein said black liquor has a concentration between 30% and 60%.