U.S. patent application number 14/748653 was filed with the patent office on 2016-01-21 for process for lowering molecular weight of liquid lignin.
The applicant listed for this patent is Michael A. Lake. Invention is credited to Michael A. Lake.
Application Number | 20160017541 14/748653 |
Document ID | / |
Family ID | 54938767 |
Filed Date | 2016-01-21 |
United States Patent
Application |
20160017541 |
Kind Code |
A1 |
Lake; Michael A. |
January 21, 2016 |
PROCESS FOR LOWERING MOLECULAR WEIGHT OF LIQUID LIGNIN
Abstract
Processes and systems for lowering molecular weight of lignin
generally includes first isolating a dense liquid lignin phase from
black liquor and subjecting the dense liquid lignin phase to a
temperature and pressure for a period of time to effect an average
molecular weight distribution of the lignin. Solid lignin produced
with the lowered molecular weight is then isolated. The systems and
processes may further include an oxidation unit for oxidizing the
black liquor and intermediate streams to remove or mitigate
malodorous or toxic emissions.
Inventors: |
Lake; Michael A.; (Mount
Pleasant, SC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lake; Michael A. |
Mount Pleasant |
SC |
US |
|
|
Family ID: |
54938767 |
Appl. No.: |
14/748653 |
Filed: |
June 24, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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62016833 |
Jun 25, 2014 |
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Current U.S.
Class: |
162/16 |
Current CPC
Class: |
D21C 11/0007 20130101;
D21C 11/0057 20130101 |
International
Class: |
D21C 11/00 20060101
D21C011/00 |
Claims
1. A process for reducing molecular weight of lignin comprising:
carbonating a black liquor stream at a temperature between about
80.degree. C. and 200.degree. C. and adjusting the pH above 10;
recovering a dense liquid-lignin phase, wherein lignin within the
dense liquid-lignin phase has a first average molecular weight; and
exposing said dense liquid-lignin to heat and pressure for a
predetermined period of time to reduce an average molecular weight
of said dense liquid-lignin to a second average molecular weight,
wherein the second average molecular weight is less than the first
average molecular weight.
2. The process according to claim 1 further comprising, acidifying
the dense liquid lignin with an acid to a pH between 1.5 and
3.5.
3. The process according to claim 1, wherein said carbonating of
the black liquor is effective to adjust the pH to between 11 and
13.
4. The process according to claim 1, wherein exposing said dense
liquid-lignin to heat and pressure is at a temperature of about
150.degree. C. to about 300.degree. C.
5. The process according to claim 1, wherein exposing the dense
liquid lignin to heat and pressure is at a temperature of about
150.degree. C. to about 190.degree. C.
6. The process according to claim 1 where a strong base is added to
the dense liquid-lignin to raise its pH and catalyze lowering of
the average molecular weight.
7. The process according to claim 1, further comprising adding an
oxidizing agent to said black liquor prior to carbonating in an
amount effective to eliminate or substantially reduce the odor of
the resulting lignin product.
8. The process according to claim 7, wherein said oxidizing agent
is a member of the group consisting of oxygen, air, a peroxide, or
mixtures thereof.
9. The process according to claim 1, wherein exposing said dense
liquid-lignin to heat and pressure for the period of time to reduce
the average molecular weight of the lignin to a second average
molecular weight comprises introducing the dense liquid-lignin into
a plug-flow reactor.
10. The process according to claim 1, wherein said black liquor has
a solids content between 10 to 70 weight percent.
11. A process for reducing molecular weight of lignin, comprising:
acidifying a black liquor stream to a pH above 10 to form a dense
liquid-lignin phase; isolating the dense liquid-lignin phase from
the black liquor stream, wherein lignin within the dense
liquid-lignin phase has a first average molecular weight; and
exposing said lignin to heat and pressure at a temperature of about
150.degree. C. to about 250.degree. C. for a period of time to
reduce an average molecular weight of the lignin to a second
average molecular weight, wherein the second average molecular
weight is less than the first average molecular weight.
12. The process according to claim 11, wherein subsequent to
exposing the dense liquid-lignin to heat and pressure for the
period of time to reduce the average molecular weight of the lignin
to the second average molecular weight acidifying the dense
liquid-lignin with an acid to a pH less than 4.
13. The process according to claim 11 wherein a strong base is
added to the dense liquid-lignin to raise its pH and catalyze
lowering the average molecular weight.
14. The process according to claim 11, further comprising adding an
oxidizing agent to the black liquor prior to acidifying the black
liquor in an amount effective to eliminate or substantially reduce
the odor of the resulting lignin product.
15. The process according to claim 11, wherein said oxidizing agent
is a member of the group consisting of oxygen, a peroxide, or
mixtures thereof.
16. The process according to claim 11, wherein said black liquor
has a solids content between 10 to 70 weight percent.
17. A process for reducing molecular weight of lignin comprising:
carbonating a black liquor stream at a temperature between about
80.degree. C. and 200.degree. C. and adjusting the pH above 10;
recovering a first dense liquid-lignin phase, wherein lignin within
the first dense liquid-lignin phase has a first average molecular
weight; exposing the first dense liquid-lignin to heat and pressure
for a pre-determined period of time to reduce a first average
molecular weight of the lignin to a second average molecular
weight, wherein the second average molecular weight is less than
the first average molecular weight, further carbonating the less
dense liquid stream isolated from the first carbonation to lower a
pH to a range of about 9 to about 10 recovering a second dense
liquid-lignin phase.
18. The process according to claim 17 further comprising,
acidifying the dense liquid-lignins, either combined or separately,
with an acid to a pH less than 6.
19. The process according to claim 17, further comprising,
acidifying the dense liquid-lignins, either combined or separately,
with an acid to a pH between 1.5 and 3.5.
20. The process according to claim 17, wherein said carbonating of
said black liquor is carried out by contacting said black liquor
with carbon dioxide countercurrently.
21. The process according to claim 17, wherein said carbonating of
the black liquor is effective to reduce the pH to between 9.0 and
10.5.
22. The process according to claim 17, wherein exposing the dense
liquid lignin to heat and pressure is at a temperature of about
150.degree. C. to about 300.degree. C.
23. The process according to claim 17, wherein exposing the dense
liquid-lignin to heat and pressure is at a temperature of about
150.degree. C. to about 190.degree. C.
24. The process according to claim 17 where a strong base is added
to the dense liquid lignin to raise its pH and catalyze lowering of
the average molecular weight.
25. The process according to claim 17 further comprising, adding an
oxidizing agent to the black liquor prior to carbonating the black
liquor in an amount effective to eliminate or substantially reduce
the odor of the resulting lignin product.
26. The process according to claim 17, wherein the oxidizing agent
is a member of the group consisting of oxygen, air, a peroxide, and
mixtures thereof.
27. The process according to claim 17, wherein exposing the dense
liquid- lignin to heat and pressure for the period of time to
reduce the average molecular weight of the lignin to a second
average molecular weight comprises introducing the dense liquid
lignin into a plug-flow reactor.
28. The process according to claim 17, wherein said black liquor
has a solids content between 10 to 70 weight percent.
29. A process comprising the steps of pH fractionation, using
controlled acidification of black liquor to separate a first
liquid-lignin fraction of relatively high molecular weight, further
acidification of produce a second liquid-lignin fraction of lower
molecular weight, heating the first liquid-lignin fraction to lower
its molecular weight, and blending the resulting two fractions to
produce a blend of desirably lower molecular weight.
30. A process according to claim 29, where said black liquor
feedstock is oxidized.
31. The process according to claim 30, wherein said oxidizing agent
is a member of the group consisting of comprises oxygen, air, a
peroxide, and mixtures thereof.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The contents of Provisional patent Application U.S. Ser. No.
62/016,833 filed Jun. 25, 2015, on which the present application is
based and benefits claimed under 35 U.S.C. .sctn.119(e), is herein
incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention generally relates to a process for
lowering the molecular weight of liquid lignin.
[0003] Lignin, a component of wood, is the second most abundant
polymer in the world behind cellulose. With its high energy density
and variety of functional groups and structure, lignin is an
efficient biofuel source or green-chemical precursor. One use for
lignin is to burn the solid lignin as a fuel, to or use the lignin
as a binder for energy pellets. 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.
[0004] Lignin also has great potential as a chemical feedstock for
adhesives, plastics, coatings, fibers, and carbon fibers. Lignin is
the most abundant source of aromatic chemistry found in nature, and
as such, is a valuable potential source of benzene-, toluene-, and
xylene-type chemistry. In the non-fuel applications, lignin
molecular weight (MW) can impact end use properties. Lower
molecular weight can, for example, lower lignin viscosity thereby
promoting flow, spreading or wetting of a binder, coating or
composite component, while higher molecular weight enhances other
properties, like glass transition temperature and rigidity. An
important application attribute in composites is the ability of the
resin to flow and penetrate the fibrous matrix, completely
saturating the interstitial volume with resin to minimize "dry
spots" which severely limit strength properties of the final
composite. Generally lignin is incorporated into a resin to
displace a fraction of the petrochemical polymer used normally in
the resin, and high molecular weight lignin can increase viscosity
and limit flow of the resin. Decreasing molecular weight of lignin
can significantly reduce its viscosity when included as a component
in the resin. Thus, there is interest in developing methods to make
lignins of controlled molecular weight. To understand the methods
used to control molecular weight, it is useful to begin by
explaining why and how lignin is prepared in the papermaking
industry.
[0005] In the kraft papermaking process, wood chips are cooked in a
strongly basic aqueous solution, typically an aqueous sodium
hydroxide (NaOH) solution, and which may also contain sodium
hydrosulfide (NaSH) to cleave and dissolve lignin and hemicellulose
and leave the cellulose fibers that are subsequently filtered,
washed, and formed into paper. The severity of pulping is the first
determinant of molecular weight in the lignin produced. In general,
the more severe the pulping, the lower is the molecular weight of
the resulting lignin. The liquid containing soluble lignin,
hemicellulose and spent pulping chemicals is called black
liquor.
[0006] Many pulp mills today could increase their production of
pulp and paper and hence their profitability were they not limited
by capacity of the recovery boiler used to burn black liquor to
recover heat and the valuable pulping chemicals (NaOH and NaSH).
Removing a fraction (up to 30%) of the lignin from black liquor
allows mills that have reached the maximum throughput of their
recovery boilers to increase production by approximately the same
fraction of lignin removed. A general rule-of-thumb is that 60
units of lignin is dissolved in black liquor for each 100 units of
pulp fiber generated. For example, in a large mill making a million
tons/yr. of pulp, the lignin dissolved in the black liquor would be
about 600,000 tons/yr. Recovering 30% of the lignin from black
liquor could produce 180,000 tons of lignin per year. Papermaking
facilities generally have power boilers that are designed to burn
residual wood (bark, limbs) from forest logging operations. Lignin
is very similar to coal and can be burned in a utility company's
power boilers generating electricity that are designed to burn coal
but not residual wood. If a papermaking facility makes one unit of
lignin, then replaces that lignin energy value is replaced in their
operations by burning residual wood, then uses that lignin to
displace coal at a utility company, green-house gases are reduced
by an overall 2.5 units. In this manner, a large mill recovering
180,000 ton/yr. lignin could reduce green-house gases by 450,000
ton/yr. if that mill replaced the energy lost by burning residual
wood and that lignin was used to displace coal. Typically, the
black liquor that is fed to a separate lignin recovery process is
removed midway in the evaporator train, is preferably at a solids
content of 30% to 45% and has a temperature of about 80.degree. C.
to about 120.degree. C. It should be understood that the solids
content of the black liquor serving as feed for a lignin-recovery
process ranges from about 20% to about 60%, but more normally is
from 30% to 50%.
[0007] Lignin may be recovered from papermaking black liquor by
several processes. One such process makes powdered lignin
containing high-salt content (about 4% ash), which creates issues
with high ash within utility power boilers. Also this method cools
the black liquor and dilutes the black liquor that is returned to
the host paper mills, which creates a high energy penalty.
[0008] A second process is described by U.S. Pat. Nos. 8,172,981
and 8,486,224. In this process, the black liquor is first cooled
and contacted with carbon dioxide to reduce the pH from 13-14 to
9-10. As a result, lignin becomes insoluble in the black liquor and
precipitates as solid particles that are filtered to remove them
from the residual black liquor which is returned to the host mill.
The lignin filter cake is then re-slurried, mixed with an acid to
reduce the dispersion's pH from 9-10 to 2-3. At this lower pH, the
cations (primarily sodium) are displaced from the phenoxy- and
carboxylic-groups on the lignin polymer. The lignin dispersion is
subjected to a second filtration, and the cake is washed in-situ
within the filter with water to reduce the ash content of the final
product lignin. This low-salt lignin (about 1%) can be used as a
fuel.
[0009] The third process is similar to the second process since the
black liquor is first cooled then reacted with carbon dioxide to
lower the pH and precipitate the lignin as solid particles, which
are then filtered to separate the lignin from the residual black
liquor. The major difference is that this third process does not
re-slurry the lignin after the carbonation. Instead, sulfuric acid
is pumped through the filter cake to displace the sodium cations
from the lignin phenoxy- and carboxylic-groups, and the cake is
subsequently washed with water to remove the salt. Also, this
process uses oxidation of the black liquor to improve the
filtration properties. This process has similar disadvantages with
respect to energy penalty to the host mill as the other two
processes, in that the black liquor is cooled to recover the lignin
and that a significant amount of water is used.
[0010] US Pat. Pub. No. 2011/0294991, incorporated herein by
reference in its entirety, describes a fourth process for removing
lignin from black liquor that includes, similar to the other
processes removing lignin from papermaking black liquor, lowering
system pH with carbon dioxide from the initial pH 13-14 down to pH
9-10. This process maintains the high process temperature from the
host papermaking facility, separating lignin from the black liquor
as a true liquid phase which is dense and separates from the
residual black liquor by gravity. The heat of reaction of carbon
dioxide is preserved so that residual black liquor, from which the
lignin has been removed, returns to the host mill at a higher
temperature than the black liquor fed. The lignin is concentrated
in the liquid-lignin phase. The liquid-lignin pH is further lowered
to pH 2-3 so that sodium cations are displaced from the carboxylic-
and phenoxy-group functionality of the lignin polymer. This process
is continuous, thus smaller equipment can be used which requires
lower capital cost relative to the two competitive processes.
[0011] At least two methods are being developed to remove lignin
from wood chips, which allows stand-alone operation separate from a
host Kraft mill. The first process uses ethanol, co-solvents and
catalysts at elevated temperature and pressure to separate the
lignin and hemicellulose as a liquid phase from the solid
cellulose. The lignin produced has extremely low-salt levels, less
than 0.1%. This process produces two separate lignin product
streams, one having a relatively low molecular weight compared to
the other. Also, unlike the previously-discussed processes in which
the lignin molecular weight is set by the upstream pulping process,
the molecular weight of the lignin can be adjusted via adjusting
the time, temperature, solvent concentrations and catalyst
concentrations within the process. However the capital and
operating costs are extremely high, such that the lignin cannot be
considered as a fuel to replace coal.
[0012] A second process is described in US Pat. Pub. No.
2013/0239954. This process uses near-critical water and carbon
dioxide to convert cellulose in biomass to sugars, leaving lignin
as a residual stream. This process is reported to produce sugars at
competitive value to corn, so the resultant lignin likely could be
used as a bio-fuel.
[0013] Biodegradation of lignin, sometimes termed enzymatic
degradation of lignin, is a key aspect of wood rotting. Research in
this area continues, with added impetus provided by the need to
develop better cellulosic ethanol technology. Because enzymes are
thermally fragile, temperatures must be kept low, so rates are
generally slow, the enzymes are costly, carbon dioxide byproduct
formation costs yield, and the lignin-derived products are
contaminated with residual enzyme and its degradation products.
Thus, there is a continuing search for more effective and less
costly methods for controlling lignin molecular weight.
[0014] Methods for lowering the molecular weight of isolated lignin
have been disclosed U.S. Pat, Pub. 2008/0050792 and U.S. Pat. No.
7,964,761B2. This concept requires isolation of solid lignin as
feedstock for the molecular weight lowering process. As a result,
its use suffers all the drawbacks noted above for processes that
isolate lignin. Thus, there is a continuing search for ways to
simplify the process and make it more economical.
[0015] U.S. Pat. Pub. No. 2013/0131326 discloses a process for
reducing the molecular weight of lignin by increasing the
temperature of black liquor to 170.degree. C. to 190.degree. C. for
a period of time of about 1 to about 60 minutes followed by lignin
precipitation. This temperature is higher than the normal pulping
temperature of about 150.degree. C. that is generally considered to
determine the molecular weight of the lignin in the untreated black
liquor. One of the problems with reducing molecular weight in this
manner is that the process is energy intensive since all of the
black liquor is being treated to effect molecular weight reduction.
As a result, the equipment is relatively large and requires
pressurization to prevent boiling, and thus relatively expensive.
Moreover, because all of the black liquor is exposed to the
elevated temperatures to effect molecular weight reduction, the
non-lignin components within the black liquor are also exposed to
the higher temperatures, which can degrade and significantly
decrease the value for some of these components.
SUMMARY OF THE INVENTION
[0016] Disclosed herein are processes and systems for lowering
molecular weight of lignin. In one embodiment, a process for
reducing molecular weight of lignin comprises: carbonating a black
liquor stream and adjusting the pH above 10; recovering a dense
liquid-lignin phase, wherein lignin within the dense liquid-lignin
phase has a first average molecular weight; and exposing the dense
liquid lignin to heat and pressure for a period of time to reduce
an average molecular weight of the lignin to a second average
molecular weight, wherein the second average molecular weight is
less than the first average molecular weight.
[0017] In another embodiment, the process for reducing the
molecular weight of lignin comprises: acidifying a black liquor
stream to a pH above 10 to form a dense liquid- lignin phase;
isolating the dense liquid-lignin phase from the black liquor
stream, wherein lignin within the dense liquid-lignin phase has a
first average molecular weight; and exposing the lignin to heat and
pressure for a period of time to reduce an average molecular weight
of the lignin to a second average molecular weight, wherein the
second average molecular weight is less than the first average
molecular weight.
[0018] In another embodiment, a system for reducing molecular
weight of lignin comprises: a source of black liquor; a pump in
fluid communication with the source of black liquor; a carbonation
column configured to countercurrently feed carbon dioxide into the
carbonation column and adjusting the pH of the black liquor above
about 9 to about 10 and isolate dense liquid-lignin phase from the
black liquor, wherein the dense liquid-lignin phase comprises
lignin having a first average molecular weight; a first reactor
configured to heat the dense liquid-lignin phase at a pressure and
for a residence time effective to reduce the first average
molecular weight of lignin to a second average molecular weight,
wherein the second average molecular weight is less than the first
average molecular weight; and a second reactor in fluid
communication with an acid source that is configured to reduce a pH
of the dense liquid lignin to less than 4.
[0019] In yet another embodiment, a system for reducing molecular
weight of lignin comprises: a source of black liquor; a pump in
fluid communication with the source of black liquor; a carbonation
column configured to countercurrently feed carbon dioxide into the
carbonation column and reduce a pH of the black liquor to a range
of about 11 to about 12 and isolate a first dense liquid-lignin
phase from the black liquor, wherein the dense liquid-lignin phase
comprises lignin having a first average molecular weight; a first
reactor configured to heat the first dense liquid lignin phase at a
pressure and for a residence time effective to reduce the first
average molecular weight of lignin to a second average molecular
weight, wherein the second average molecular weight is less than
the first average molecular weight; a second carbonation column
wherein carbon dioxide is fed to countercurrently contact the
partially carbonated black liquor stream from the first carbonation
column to adjust the pH of the stream to above about 9 to about 10
to isolate a third dense liquid-lignin phase where in the third
dense liquid-lignin phase comprises lignin having a third average
molecular weight; and a second reactor in fluid communication with
an acid source that is configured to reduce a pH of the combined
second and third dense liquid lignin phases to less than 4 or
optionally a second reactor in fluid communication with an acid
source that is configured to reduce a pH of the second dense liquid
lignin phase to less than 4 and a third reactor in fluid
communication with an acid source that is configured to reduce a pH
of the third dense liquid lignin phase to less than 4.
[0020] Optionally, the system may further include an oxidation unit
configured to introduce an oxidizing agent into the system and
react the oxidizing agent with impurities within the black liquor
or dense liquid-lignin phase in an amount effective to eliminate or
substantially reduce the odor of the resulting lignin product
having the second average molecular weight.
[0021] The disclosure may be understood more readily by reference
to the following detailed description of the various features of
the disclosure and the examples included therein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] Referring now to the Figures wherein the like elements are
numbered alike:
[0023] FIG. 1 illustrates an example of the process for producing
lignin disclosed in U.S. Pat. Pub. No. 2011/0294991;
[0024] FIG. 2 illustrates an exemplary process flow and system for
reducing the molecular weight of liquid-lignin in accordance with
the present invention;
[0025] FIG. 2b illustrates an exemplary process flow and system for
reducing the molecular weight of liquid-lignin showing the option
of adding a strong base to adjust the pH;
[0026] FIG. 3 illustrates a process flow and system including an
optional oxidation step added prior to carbonation in accordance
with another embodiment of the present disclosure;
[0027] FIG. 4 illustrates a process flow and system including an
optional oxidation step added after carbonation in accordance with
another embodiment of the present disclosure;
[0028] FIG. 5 illustrates a process flow and system including an
optional oxidation step added following the heating step; and
[0029] FIG. 6 illustrates a process flow and system including an
optional oxidation step of the lignin product following
solid-liquid separation in accordance with another embodiment of
the present disclosure.
DETAILED DESCRIPTION OF THE INVENTION
[0030] Disclosed herein are processes and systems for selectively
reducing the molecular weight and/or molecular weight distribution
of liquid lignin. The processes and systems are generally
configured to first separate liquid-lignin such as by a carbonation
process, or by an acid addition process, and heating the separated
liquid lignin at an elevated temperature for a predetermined period
of time and pressure effective to reduce an average molecular
weight of the lignin as well as affect the molecular weight
distribution of the lignin contained within the liquid-lignin. If
necessary, sodium hydroxide (NaOH) or another strong base can be
added to the liquid-lignin to raise its pH and further catalyze the
reaction. This additional sodium can be recovered as sodium sulfate
and returned to the host papermaking mill.
[0031] Advantageously, reducing the average molecular weight and/or
molecular weight distribution by first separating the liquid-lignin
phase from black liquor followed by heating overcomes many of the
problems associated with the prior art. For example, by separating
the liquid-lignin beforehand, the non-lignin components typically
contained within the black liquor are not subjected to the
time-temperature history employed to reduce the molecular weight of
the lignin to the desired amount. Thus, avoiding thermal
degradation of non-lignin components such as sugars and other
labile components is a clear advantage of this invention. Moreover,
the liquid-lignin phase comprises only about 20% of the total mass
of the original black liquor, which offers the clear advantage of
utilizing a smaller reactor system. Additionally, the liquid-lignin
already has an elevated temperature since the heat of reaction of
the carbon dioxide is retained within the liquid-lignin, elevating
its temperature above the incoming black liquor by typically more
than 10.degree. C. Thus a process treating the liquid-lignin has
much better energy efficiency than the prior art which requires
elevating the temperature of the entire black liquor phase.
[0032] For ease in understanding, reference will now be made to
carbonation processes. However, it should be apparent that an acid
addition process can be employed to form and separate the dense
liquid-lignin phase. Acid addition processes generally include
adjusting the pH to effect phase separation of the dense
liquid-lignin, which can then be processed as described in greater
detail below with reference to the carbonation process to effect
molecular weight reduction of the lignin.
[0033] In a carbonation process, lignin begins to precipitate
immediately near the black liquor entrance and near the top of the
column as the pH begins to be reduced by carbon dioxide (CO.sub.2).
As the pH decreases more and more lignin becomes insoluble and
coalesces within column as liquid-lignin droplets, which settle
rapidly from the residual carbonated black liquor under simple
gravitational force. Countercurrently contacting the incoming black
liquor with CO.sub.2 creates a pH gradient in the 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, the liquid-lignin
particles 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. The melt point of the liquid lignin phase
depends strongly on the concentration of cations (mainly sodium),
the source of the lignin (the species of trees being pulped), and
the level of water in the phase, hence its viscosity is difficult
to predict.
[0034] The lower-density lignin-depleted phase, containing most of
the sugars and valuable pulping chemicals, is returned to the
recovery process of the host paper mill at a temperature higher
than the temperature of the black liquor received. This eliminates
the loss of energy as a major impediment for commercial
implementation of lignin recovery by pulp mills, another clear
advantage of the present invention. A carbonation process, such as
the one disclosed in FIG. 1, employs a temperature profile within a
range of about 90.degree. C. to about 150.degree. C. Practice of
this invention to lower molecular weight of liquid-lignin requires
heating to a higher temperature. In one embodiment, the temperature
profile in the present process is within a range of about
150.degree. C. to 300.degree. C. and in another embodiment, the
temperature profile is within a range of 150.degree. C. to
200.degree. C. In still other embodiments, the temperature profile
in the present process is within a range of about 150.degree. C. to
about 190.degree. C.; and in yet other embodiments, the temperature
profile in the present process is within a range of about
160.degree. C. to about 170.degree. C.
[0035] As the temperature and/or pressure increases, the length of
time to achieve the desired lower molecular weight decreases.
Depending on the selected temperature, pressure, and time profile
and the desired molecular weight and molecular weight distribution,
the isolated liquid-lignin is heated for a period of time of about
1 minute to about 360 minutes, in other embodiments, from about 1
minute to 80 minutes, and in still other embodiments, from 1 minute
to about 30 minutes. Molecular weight reduction may be accelerated
by further increasing pH of the liquid lignin. In view of the
foregoing, the carbonation processes generally include as a first
step, pressurizing black liquor to between 50 and 3200 psig.
[0036] Methods and equipment for performing the heating step are
well known to those skilled in the art. By way of example, a plug
flow reactor may be employed to provide the desired time,
temperature, and pressure profile to effect the molecular weight
reduction. Other reactor configurations, including but not limited
to batch stirred tank reactor, continuous stirred tank reactor
(CSTR), ebullated bed reactor, and trickle bed reactor, will be
obvious to one skilled in the art. Because of the high reactivity
of lignin toward solids formation it is important to achieve good
temperature control in the heating unit, especially avoiding hot
spots, areas where the temperature is more than about 10.degree. C.
above the set temperature to minimize reactor fouling and plugging.
Preferred methods for heating include heat transfer fluid (hot
oil), electric resistance heating, and the like. Direct steam
injection is another method which eliminates heat-transfer surfaces
and the associated fouling issues and is compatible with high
viscosity of the liquid lignin. An in-line mixer following the
heating unit or the point of steam injection will facilitate more
even temperature distribution within the viscous liquid-lignin
phase. After heating, the dense lignin-rich phase can then be
cooled and/or acidified to precipitate lignin with the desired
molecular weight and desired molecular weight distribution.
[0037] A strong base could be added to the liquid-lignin,
increasing its pH and catalyzing the molecular weight reduction.
Sodium hydroxide is preferred as a strong base, since the sodium
would be captured in the acid brine from the downstream lignin
recovery process and returned to the host mill's recovery cycle
where the sodium is recovered and used in NaOH and NaSH. Sufficient
sodium hydroxide must be added to overcome the buffering effect of
the sodium bicarbonate contained in the liquid-lignin. This may
approach or exceed pH 12, starting with the carbonated
liquid-lignin stream at pH 9-10.
[0038] As an optional step, sufficient oxidant may be reacted with
the black liquor to mitigate and/or eliminate odors and species
like sodium hydrosulfide (NaSH) that can evolve toxic gas (e.g.,
H.sub.2S) if contacted with an acid much stronger than CO.sub.2.
Alternatively, oxidation of liquid lignin after separation offers
the clear advantage of treating a smaller volume, about 20% of the
total black liquor volume, so requires smaller, less costly
equipment, and requires less oxygen since it avoids unwanted
oxidation of sugars and other components of the carbonated black
liquor.
[0039] Once the liquid lignin is separated, molecular weight and
molecular weight distribution of the liquid lignin can be
determined using size exclusion chromatography or other methods
known to those skilled in the art. Similarly, after the heat
treatment described above, molecular weight and molecular weight
distribution of the treated liquid lignin can be determined.
Likewise, after the lignin or lignin fractions have been recovered
by acidification and separation, the molecular weight and molecular
weight distribution of each material can be determined.
[0040] After the desired degree of molecular weight reduction is
effected, the dense liquid-lignin is further acidified to a pH less
than 4 in some embodiments, and to a pH of about 1.5 to about 3.5
in other embodiments to form solid liquid lignin. The acidification
step can be performed by adding an acid (e.g., sulfuric acid). The
particular acid is not intended to be limited. For instance,
organic acids such as formic or acetic acid could be used. Protic
acids, such as sulfuric acid, are favorable since their cost is low
and because the sulfur can often be used in the host pulp mill to
offset the normal sulfur make-up used by the mill to replace sulfur
losses in the mill system, which produces internally the sodium
hydrosulfide used as a pulping catalyst. For example, the dense
liquid-lignin phase may be fed 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 low-ash lignin.
[0041] Turning now to FIG. 1, black liquor 12 is pressurized and
fed through line 14 to pump 16, which feeds the pressurized stream
through line 18 into the upper region of carbonation column 30.
Carbon dioxide (CO.sub.2, 31) is fed into column 30 and flows
upward countercurrent to the downward flowing black liquor, while
the pH is lowered to a pH of about 9-10 and entrained acid gases
flow out of column 30 with any excess CO.sub.2 through vent 32 to
an effective scrubbing system, typically using strongly basic white
liquor to remove toxic and malodorous gases. The lower end of
column 30 is enlarged to form a settler 34 where the less dense
lignin-depleted liquid rises to the top and exits through line 36,
while the more dense liquid lignin exits through line 38 into the
acidification vessel 70. Where acid 72 is added and the resulting
slurry moves through line 74 into the solid-liquid separation unit
80, lignin. Lignin product 100 exits the solid liquid separation
unit either as solid or as slurry through conduit 82, the precise
nature of which will depend on the nature of the product stream,
typically a pipe for slurry or a bin or conveyor for solid
lignin.
[0042] Turning now to FIG. 2, there is shown a schematic diagram of
an embodiment of an exemplary system of the present invention
showing the steps, from a lignin containing stream, of carbonating
to form a liquid-lignin, reducing the molecular weight of the
liquid lignin, acidification and solid-liquid separation to recover
lignin product. In this case, a heating unit 50 is inserted between
settler 34 and acidification unit 70. The final lignin product will
be suitable for applications that are generally 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.
[0043] Also shown in FIG. 2b is the optional addition of a strong
base 46, typically NaOH, to adjust pH of the stream. The strong
base 54 is transported through line 56 into the liquid lignin in
line 38.
[0044] Turning now to FIG. 3, there is shown a schematic diagram of
an embodiment of an exemplary system of the present invention
showing the optional step of reacting the black liquor stream with
an oxidant, e.g., air, oxygen, peroxide or the like, prior to
carbonation. In this case, oxidation unit 20 is inserted between
pump 16 and carbonation column 30. Pressurized black liquor is fed
into oxidation unit 20 through line 16, oxidant is fed through line
22 and the oxidized liquid is fed into carbonation column 30
through line 24. Exemplary equipment for this reaction is a
Hydrodynamics Shockwave Power Reactor.RTM., shown at 20 in FIG. 3.
Oxidation also has a substantial heat of reaction, raising the
temperature of the stream, typically about 50.degree. C. depending
on the reactants within the aqueous stream and its solids
content.
[0045] Turning now to FIG. 4, there is shown a schematic diagram of
another embodiment of an exemplary system of the present invention
showing the optional step of oxidizing the liquid lignin stream
after carbonation. This option offers the advantage of not
oxidizing the entire black liquor stream, so equipment can be
smaller and less oxidant is needed. In this case, oxidation unit 40
is inserted between settler 36 and heating unit 50. Liquid lignin
is fed into oxidation unit 40 through line 38. Oxidant is fed
through line 42 and the oxidized liquid is fed into heating unit 50
through line 44. Depending on the heat of reaction with the oxidant
with the liquid lignin, heating unit 50 may not be required due to
the inherent temperature increase.
[0046] Turning now to FIG. 5, there is shown a schematic diagram of
an embodiment of an exemplary system of the present invention
showing the optional step of oxidizing the liquid lignin stream
after heating to lower molecular weight. In this case, oxidation
unit 60 is inserted between heating unit 50 and acidification unit
70. This option may be advantageous when heating produced
especially reactive species that complicate downstream operations.
Heated, and optionally cooled, liquid-lignin is fed into oxidation
unit 60 through line 52. Oxidant is fed through line 62 and the
oxidized liquid is fed into acidification unit 70 through line
64.
[0047] Turning now to FIG. 6, there is shown a schematic diagram of
an embodiment of an exemplary system of the present invention
showing the optional step of oxidizing the liquid lignin stream
after heating to lower molecular weight. In this case, oxidation
unit 90 is located after solid-liquid separation unit 80. This
option may be advantageous when earlier oxidation produces species,
surfactants for example, that adversely affect downstream
operations. Solid lignin, optionally as slurry, is fed into
oxidation unit 90 through line 82, oxidant is fed through line 92
and the oxidized lignin product exits through line 100.
[0048] This written description uses examples to disclose the
invention, including the best mode, and also to enable any person
skilled in the art to make and use the invention. The patentable
scope of the invention is defined by the claims, and may include
other examples that occur to those skilled in the art. Such other
examples are intended to be within the scope of the claims if they
have structural elements that do not differ from the literal
language of the claims, or if they include equivalent structural
elements with insubstantial differences from the literal languages
of the claims.
* * * * *