U.S. patent application number 13/088842 was filed with the patent office on 2011-10-20 for separation of lignin from lignocellulosic materials.
This patent application is currently assigned to Savannah River Nuclear Solutions, LLC. Invention is credited to Maximilian Boris Gorensek, Steven Randall Sherman.
Application Number | 20110253326 13/088842 |
Document ID | / |
Family ID | 44787289 |
Filed Date | 2011-10-20 |
United States Patent
Application |
20110253326 |
Kind Code |
A1 |
Sherman; Steven Randall ; et
al. |
October 20, 2011 |
Separation of Lignin From Lignocellulosic Materials
Abstract
A method is described for separating lignin from liquid
solutions resulting from the pretreatment of lignocellulosic
materials such as switchgrass with ammonium hydroxide. The method
involves a sequence of steps including acidification, evaporation,
and precipitation or centrifugation that are performed under
defined conditions, and results in a relatively pure, solid lignin
product. The method is tested on ammonium hydroxide solutions
containing lignin extracted from switchgrass. Experimental results
show that the method is capable of recovering between 66-95% of
dissolved lignin as a precipitated solid.
Inventors: |
Sherman; Steven Randall; (N.
Augusta, SC) ; Gorensek; Maximilian Boris; (Aiken,
SC) |
Assignee: |
Savannah River Nuclear Solutions,
LLC
Aiken
SC
|
Family ID: |
44787289 |
Appl. No.: |
13/088842 |
Filed: |
April 18, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61342751 |
Apr 19, 2010 |
|
|
|
Current U.S.
Class: |
162/55 ;
162/90 |
Current CPC
Class: |
D21C 3/024 20130101;
D21C 11/0007 20130101; C08H 6/00 20130101; C08H 8/00 20130101 |
Class at
Publication: |
162/55 ;
162/90 |
International
Class: |
D21C 3/20 20060101
D21C003/20; D21D 5/02 20060101 D21D005/02; D21D 5/18 20060101
D21D005/18 |
Goverment Interests
STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSORED
RESEARCH AND DEVELOPMENT
[0002] This invention was made with Government support under
Contract No. DE-AC09-08SR22470 awarded by the United States
Department of Energy. The Government has certain rights in the
invention.
Claims
1. The process of recovering lignin from a biomaterial comprising
the steps of: providing a lignin-containing cellulosic biomaterial;
treating said cellulosic biomaterial with a solution of ammonium
hydroxide, thereby extracting lignin from cellulosic biomaterial
and providing an ammonium hydroxide lignin solution; separating the
treated cellulosic biomass from the solution of ammonium hydroxide
lignin solution; evaporating ammonia from the separated ammonium
hydroxide lignin solution while maintaining a pH greater than 7.0;
acidifying the separated ammonium hydroxide lignin solution to a pH
level below 4.0, thereby forming a colloidal suspension of lignin;
reducing a volume of said colloidal suspension of lignin by
evaporation of water, thereby forming a coagulation of particles of
lignin within said reduced volume solution; separating the lignin
from the reduced volume solution; optionally washing the separated
lignin with dilute sulfuric acid; and, drying said lignin.
2. The process of claim 1 wherein said treating step with a
solution of ammonium hydroxide further includes supplying the
ammonium hydroxide at a volume to mass ratio of between about 1:1
to about 16:1.
3. The process according to claim 1 wherein said step of
evaporating ammonia further includes the step of heating the
separated ammonium hydroxide within the solution until a boiling
point of the solution reaches about 96 to 98.degree. C. at ambient
pressure.
4. The process according to claim 3 wherein said acidifying the
step is carried out prior to the cooling of the separated ammonium
hydroxide lignin solution following evaporating of ammonia.
5. The process according to claim 1 wherein said step of reducing a
volume of said colloidal suspension of lignin further includes
reducing the volume by about 50%.
6. The process according to claim 1 wherein said step of separating
the lignin from the reduced volume solution further includes
separating the lignin by at least one of a centrifugation step or a
filtration step.
7. The process according to claim 1 wherein said drying step
further includes drying the lignin in a vacuum oven at a
temperature of less than about 65.degree. C.
8. The process according to claim 1 wherein said step of separating
the treated cellulosic biomass from the ammonium hydroxide lignin
solution is via a filtration step.
9. The process according to claim 1 wherein said treating step with
a solution of ammonium hydroxide further includes supplying the
ammonium hydroxide at a volume to mass ratio of at least about
6:1.
10. The process according to claim 1 wherein said treating step
with a solution of ammonium hydroxide further includes supplying
the ammonium hydroxide with a volume to mass ratio of between about
8:1 to about 16:1.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Application Ser.
No. 61/342,751, entitled "Separation of Lignin from Aqueous
Ammonia", filed on Apr. 19, 2010, and which is incorporated herein
by reference.
FIELD OF THE INVENTION
[0003] This invention is directed towards a method for separating
lignin from liquid solutions that result from the pretreatment of
lignocellulosic materials. The method involves a sequence of steps
including acidification, evaporation, and precipitation or
centrifugation that are performed under defined conditions, and
results in a relatively pure, solid lignin product.
BACKGROUND OF THE INVENTION
[0004] This invention relates to Biofuels production processes that
use fermentation to convert lignocellulosic materials (e.g., grass,
leaves, wood) into ethanol and other chemicals. Lignocellulosic
materials usually need a pretreatment step to facilitate the
chemical or enzymatic hydrolysis of cellulose into fermentable
sugars. Lignocellulosic materials contain cellulose and
hemicellulose bound into a rigid composite structure that is
surrounded by lignin. The lignin layers must be disrupted during
the pretreatment step to expose the underlying cellulose structure
to chemical attack. Lignin is a highly cross-linked polymeric
material found in all land-based plants (except bryophytes) that
aids in water transport, adds stiffness to plant structures, and
helps protect the plant material against disease and attack by
pests. The amount of lignin varies according to plant structure,
species, and season, but harder lignocellulosic material typically
correlates with higher lignin content, and the hardest of the
hardwoods, piratinera guianensis or snakewood, may contain up to
39% lignin by weight in the heartwood. Disruption of the
lignocellulosic structure may be achieved by physical or chemical
means.
[0005] If hydrolysis of cellulose into sugars is the only concern
in a biofuels production process, then multiple pretreatment
methods may be effective for any given lignocellulosic feedstock.
In that case, a pretreatment method should be chosen to provide the
highest conversion of cellulose into sugars at the least cost for a
given feedstock. However, lignin is also a useful material, and the
economics of a biofuels plant may be improved if lignin is
recovered as a separate product.
[0006] Lignin has many current and potential commercial uses. It
has a moderate net heat of combustion (lower heating value, or LHV)
of approximately 20 MJ/kg when purified, and it may be burned to
generate steam for process heating instead of lignite, coal or
natural gas, which have heats of combustion of about 11, 29, and 37
MJ/kg, respectively. Lignin may be used as a starting material for
the manufacture of useful higher-value chemicals (e.g.,
lignosulfonates, dispersants, vanillin, dimethylsulfoxide). It may
be pyrolyzed to make an oil-like product or a pyrolytic solid which
may be further processed using standard petroleum refining
techniques to make gasoline, diesel, and jet fuel. Lignin is also
being studied and used as a polymer additive for the manufacture of
bio-based or "green" polymers, and as a low-cost source material
for the manufacture of carbon fibers.
[0007] Of the available pretreatment methods, only a sub-set of
chemical pretreatment methods--those that operate in part by
separating lignin from the lignocellulosic structure rather than
destroying it--can be used to generate a relatively pure lignin
side stream during the pretreatment stage. Though lignin may also
be recovered downstream after cellulose and hemicellulose have been
hydrolyzed into sugars, the hydrolysis of cellulose rarely exceeds
90% conversion, and any lignin remaining after hydrolysis is still
intermingled with cellulose and other materials.
[0008] Lignin removal and recovery by chemical means is not a new
concept, and it is performed on an industrial scale in wood pulping
mills that use the Sulfite Process (acid-based), from which
lignosulfonates can be produced, and the Kraft Process
(alkaline-based), from which a purer lignin stream may be isolated,
albeit with a relatively high ash content. Because of the use of
harsh chemicals and high capital equipment costs, these wood
pulping processes have not yet been deployed in biofuels
production.
[0009] Accordingly, there remains room for improvement and
variation within the art.
SUMMARY OF THE INVENTION
[0010] It is one aspect of at least one of the present embodiments
to provide a process of recovering lignin from a biomaterial
comprising the steps of providing a lignin-containing cellulosic
biomaterial; treating said cellulosic biomaterial with a solution
of ammonium hydroxide, thereby extracting lignin from cellulosic
biomaterial and providing an ammonium hydroxide lignin solution;
separating the treated cellulosic biomass from the solution of
ammonium hydroxide lignin solution; evaporating ammonia from the
separated ammonium hydroxide lignin solution while maintaining a pH
greater than 7.0; acidifying the separated ammonium hydroxide
lignin solution to a pH level below 4.0, thereby forming a
colloidal suspension of lignin; reducing a volume of said colloidal
suspension of lignin by evaporation of water, thereby forming a
coagulation of particles of lignin within said reduced volume
solution; separating the lignin from the reduced volume solution;
optionally washing the separated lignin with dilute sulfuric acid;
and, drying said lignin.
[0011] It is a further aspect of at least one of the present
embodiments of the invention to provide for a process of reducing
and recovering lignin content from a bio-feedstock, thereby
increasing the conversion efficiency of subsequent hydrolysis steps
of the bio-feedstock into a sugar or fuel.
[0012] It is yet a further object of at least one embodiment of the
present invention to provide for a process of recovering lignin
from a bio-feedstock which allows for a greater than 80% hydrolysis
of the treated bio-feedstock into a sugar or fuel.
[0013] It is a further aspect of at least one of the present
embodiments of the present invention to provide for a treatment
step of treating a lignin-containing cellulosic material which is
treated with a solution of ammonium hydroxide in a volume to mass
ratio of between about 1:1 to about 16:1 and at a temperature of
about 80.degree. C.
[0014] It is a further aspect of at least one embodiment of the
present invention to provide for a process of recovering lignin
wherein an ammonium hydroxide solution containing an extracted
lignin component is heated to a temperature of about 96 to about
98.degree. C. at ambient pressure so as to evaporate ammonia from
the solution.
[0015] It is yet a further aspect of at least one embodiment of the
present invention to provide for a process of separating lignin
from a bio-material source following a prior step of evaporating
ammonia, the heated lignin containing solution is acidified to form
a colloidal suspension of lignin.
[0016] These and other features, aspects, and advantages of the
present invention will become better understood with reference to
the following description and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] A fully enabling disclosure of the present invention,
including the best mode thereof to one of ordinary skill in the
art, is set forth more particularly in the remainder of the
specification, including reference to the accompanying
drawings.
[0018] FIG. 1 is graph showing lignin recovery from various
biomaterial feedstocks using a 8:1 pretreatment ratio of extraction
volume to mass.
[0019] FIG. 2 is a graph setting forth lignin recovery of various
feedstocks using a 16:1 pretreatment ratio.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0020] Reference will now be made in detail to the embodiments of
the invention, one or more examples of which are set forth below.
Each example is provided by way of explanation of the invention,
not limitation of the invention. In fact, it will be apparent to
those skilled in the art that various modifications and variations
can be made in the present invention without departing from the
scope or spirit of the invention. For instance, features
illustrated or described as part of one embodiment can be used on
another embodiment to yield a still further embodiment. Thus, it is
intended that the present invention cover such modifications and
variations as come within the scope of the appended claims and
their equivalents. Other objects, features, and aspects of the
present invention are disclosed in the following detailed
description. It is to be understood by one of ordinary skill in the
art that the present discussion is a description of exemplary
embodiments only and is not intended as limiting the broader
aspects of the present invention, which broader aspects are
embodied in the exemplary constructions.
[0021] In describing the various figures herein, the same reference
numbers are used throughout to describe the same material,
apparatus, or process pathway. To avoid redundancy, detailed
descriptions of much of the apparatus once described in relation to
a figure is not repeated in the descriptions of subsequent figures,
although such apparatus or process is labeled with the same
reference numbers.
[0022] Biomass Source Material
[0023] Samples of switchgrass (Alamo variety) were obtained from
Clemson University's Pee Dee Research and Education Center, located
near Florence, S.C., USA. The switchgrass was in its second season
of growth, and had been harvested in December after the switchgrass
tract had gone dormant for the winter. Switchgrass bales consisting
of stems and leaves were collected from this tract and chopped
using a diesel-powered rotary "bale buster" to reduce the lengths
of grass to between approximately 3 and 10 cm. Samples of the
chopped switchgrass were milled further to a particle size of
between 0.5 and 1.0 mm in length by passing the material multiple
times through a Thomas Model 4 Wiley Mill.
[0024] Two samples of milled switchgrass taken from random lots of
switchgrass were submitted to the Georgia Institute of Technology's
Institute of Paper Science and Technology (IPST) for compositional
analysis. Analytical tests were performed to identify structural
carbohydrate, lignin, and ash content. The IPST used analytical
methods that are prescribed by NREL and IPST protocols. The results
of these analytical tests are shown in Table 1.
TABLE-US-00001 TABLE 1 Switchgrass composition (dry basis).
Switchgrass Switchgrass Analyte sample #1, wt % sample #2, wt %
Arabinan 3.5 3.4 Galactan 1.3 1.3 Glucan 32.3 31.4 Xylan 21.4 20.9
Mannan <0.1 <0.1 Lignin (acid soluble) 3.4 3.4 Lignin (acid
insoluble) 23.2 23.3 Ash (525.degree. C.) 3.7 3.6 Total 88.9
87.4
[0025] A weighed sample of milled switchgrass was dried in a drying
oven at atmospheric pressure and 65.degree. C. overnight and
weighed again the following day in order to determine a dry weight.
The dried sample weighed 10% less, which indicated that the
moisture content of the sample was 10%.
[0026] Reagent Preparation
[0027] Totally, 15 vol % ammonium hydroxide solution, which is
needed to perform pretreatment, was prepared by mixing 50% v/v
ammonium hydroxide obtained from the Ricca Chemical Company with
the appropriate amount of deionized water. A 10 wt sulfuric acid
solution was prepared by mixing 96-98% sulfuric acid from Acros
Organics with the appropriate amount of deionized water.
[0028] Equipment
[0029] Pretreatment operations were performed using laboratory
glassware that was heated in a water bath. Boiling of fluids was
performed by heating on a hot plate equipped with a magnetic
stirrer. Titrations were performed using a graduated glass burette.
Measurements of pH were performed using Colorphast pH-indicator
strips, range 2.0-14.0, that were manufactured by EMD Chemicals,
Inc. Filtrations were performed using a ceramic perforated Buchner
funnel either by itself, or with filter paper circles. Two types of
filter paper were used, when appropriate: Coarse Fisherbrand Filter
Paper Grade P8 (particle retention >20-25 .mu.m), and Fine
Fisherbrand Filter Paper Grade G6 (particle retention >1.5
.mu.m). Whenever high-speed centrifugation was performed for the
purpose of recovering precipitated lignin, 50-mL conical bottom
disposable centrifugation tubes obtained from Cole-Parmer were
used. All work was performed in or exhausted to a chemical fume
hood to avoid exposure to ammonia vapors.
[0030] Experiments
[0031] Fifteen trials were performed to study how effectively
lignin could be recovered using the described method (see Table 2).
As a necessary first step, ammonium hydroxide solutions containing
lignin were generated, and this was performed by mixing 50-g
batches of milled switchgrass with 15 vol % ammonium hydroxide in
500-mL glass filter flasks at a 6.6:1 or an 8:1 vol (ml):mass (g)
ratio. Twelve trials (Trials 1-12) were performed using a 6.6:1
volume:mass pretreatment ratio, and three trials (Trials 13-15)
were performed using an 8:1 volume:mass pretreatment ratio. Trials
8-12 were performed to study process scale-up, and used larger
amounts of switchgrass than the other trials. For Trials 8-11,
three 50-g batches of switchgrass were pretreated for each trial,
and the pretreatment solution obtained from each batch was combined
for subsequent processing steps. For Trial 12, twenty-two 50-g
batches of switchgrass were pretreated, and the pretreatment
solution obtained from each batch was combined for subsequent
processing steps.
TABLE-US-00002 TABLE 2 Experimental results. Calc. % lignin
recovered Method Calc. % from Initial Pretreat sequence Lignin
original pretreat- mass, Vol:Mass variation product lignin ment
Trial # g ratio # mass, g recovered solution 1 50 6.6:1 2 6.15 43
94 2 50 6.6:1 1 6.05 43 92 3 50 6.6:1 1 6.1 43 93 4 50 6.6:1 2 8.5
60 130 5 50 6.6:1 2 6.25 44 95 6 50 6.6:1 1 6.2 43 95 7 50 6.6:1 1
4.3 30 66 8 150 6.6:1 1 13.35 31 68 9 150 6.6:1 1 15.45 36 79 10
150 6.6:1 1 14.85 35 76 11 150 6.6:1 1 13.10 31 67 12 1200 6.6:1 1
145 46 101 13 50 8:1 1 8.58 60 131 14 50 8:1 1 8.49 59 130 15 50
8:1 1 10.11 71 154
[0032] During pretreatment, the flasks were partially submerged in
a water bath for 24 h at a bath temperature set point of 80.degree.
C. The choice to use a 50-g batch size during the pretreatment step
was somewhat arbitrary, but could not be much exceeded due to the
height limit of the water bath. The lid of the water bath would not
close when 1-L filter flasks were used, and so 500-mL filter flasks
were used instead, thus limited the pretreatment fluid volumes to
500 ml or less. As a result, multiple 50-g batches were needed to
perform the scale-up trials. Although the flasks were stoppered,
they were also vented through the side port of the filter flasks to
the fume hood to relieve pressure in the filter flask and to
capture escaping ammonia vapors. Occasionally the flasks were
shaken by hand to increase mixing. At the end of 24 h, the flasks
were removed from the water bath and allowed to cool, and then the
material in each flask was filtered using a ceramic Buchner funnel
without filter paper to separate the pretreated solids from the
pretreatment fluid. In a few cases, the pretreatment solution had
to be filtered again using coarse filter paper to capture small
particles, and to ensure a solids-free pretreatment fluid.
[0033] The lignin recovery method set forth below was then applied
to each pretreatment solution. Two variations of the method were
tried. In Variation 1, the steps were followed in the order
described. In Variation 2, the first step (Ammonia evaporation) and
the second step (Acidification) were interchanged in order to
determine whether the order of these steps made any difference in
the amount of lignin recovered. In the performance of Step 4,
either filtration with fine filter paper, or centrifugation
followed by decantation was used to separate the coagulated lignin
particles from the liquid.
[0034] Step One--Ammonia Evaporation
[0035] The ammonium hydroxide solution containing lignin is heated
in an open vessel (or in a distillation column) to boiling for the
purposes of evaporating ammonia. The solution is heated until the
boiling point at atmospheric pressure reaches 96-98.degree. C. At
this point, nearly all of the ammonia in solution has been removed,
while the solution still retains a sufficient amount of residual
ammonia to keep the pH above 7, which prevents premature
condensation of lignin from solution.
[0036] Step Two--Acidification
[0037] While the solution is still hot, it is titrated with
sulfuric acid until the solution pH falls to below pH 4. The color
of the solution changes from black to brown upon crossing the pH
neutral point, and a colloidal suspension of lignin forms.
[0038] Step Three--Water Evaporation
[0039] The solution is allowed to boil and water is evaporated
until the volume of the solution is reduced to approximately half
of the starting amount, and a "skin" of lignin begins to form on
the surface. The formation of a skin indicates that lignin
particles have coagulated to form larger, more easily separable
particles. In cooking terminology, this step is known as a
reduction.
[0040] Step Four--Cooling, and Solid/Liquid Separation
[0041] Once the solution has cooled, the lignin particles may
settle out of solution on their own, in which case the liquid may
be directly decanted, or the separation process may be aided by
using a vacuum filter or high-speed centrifuge.
[0042] Step Five (Optional)--Washing with Dilute Sulfuric Acid
[0043] If a very low-ash lignin product is desired (less than 1000
ppm), then the lignin may be washed several times with dilute
sulfuric acid, followed by a water rinse. The washing step helps
remove salts adhering to the precipitated material, and may help
remove entrained ash that was carried along from the pretreatment
step.
[0044] Step Six--Drying
[0045] The lignin may be dried in the open air, or placed in a
drying oven. The temperature of the drying oven should not exceed
65.degree. C. to avoid glazing or sintering of the recovered lignin
particles.
[0046] A sample of milled switchgrass that had been pretreated
using a 6.6:1 ratio of 15 vol % ammonium hydroxide was submitted to
the IPST for analysis of structural carbohydrate, lignin, and ash
content, as described in the Biomass Source Material section. A
lignin sample was also drawn from a random trial and was submitted
to the IPST for compositional analyses. The analytical results for
these samples are shown in Table 3.
TABLE-US-00003 TABLE 3 Compositional analyses of pretreated biomass
and recovered solid (dry basis). Pretreated Pretreated Recovered
Recovered Switchgrass Switchgrass solid solid replicate replicate
replicate replicate Analyte #1 wt % #2 wt % #1 wt % #2 wt %
Arabinan 3.7 3.8 0.4 0.3 Galactan 1.2 1.2 0.3 0.3 Glucan 39.9 40.1
2.1 2.1 Xylan 21.2 21.4 1.5 1.5 Mannan <0.1 <0.1 <0.1
<0.1 Lignin (acid 2.9 2.9 7.4 7.1 soluble) Lignin (acid 14.9
15.5 76.5 77.3 insoluble) Ash (525.degree. C.) 2.8 3.2 0.4 0.4
Total 86.7 88.2 88.7 89.1
[0047] The results of the experimental work are shown in Table 2.
During the performance of Step Four of the method, it was observed
that the cleanest lignin separation could be achieved by using fine
filter paper, rather than centrifugation. The filtrate produced was
clear and amber-colored, whereas the supernatant liquid produced by
centrifugation tended to be a little cloudier, though still
amber-colored.
[0048] The sample analyses provided in Tables 2 and 3 may be used
to calculate two numbers, the percentage of the original lignin
content recovered, and the percentage of the extracted lignin
recovered from the pretreatment fluid. The percentage of the
original lignin content recovered is calculated in the following
manner. Since the milled switchgrass was observed to have an
approximate moisture content of 10 wt %, each 50-g batch of milled
switchgrass has 45 g of dry biomass. Table 1, which was also
measured on a dry basis, shows that the milled switchgrass contains
approximately 26.6 wt % lignin. Therefore, each 50-g batch contains
approximately 12 g of lignin. In Table 3, chemical analysis of a
sample of recovered. solid shows that the recovered solid contains
.about.84 wt % lignin. Therefore, the calculated percentage of the
original lignin content recovered is determined by multiplying the
mass of recovered solid by 0.84, dividing by the mass of lignin
originally present in the biomass, and then multiplying by 100%.
For example, if the amount of recovered solid is 5 g, the amount of
lignin in the recovered solid is 5 g.times.0.84=4.2 g. Then, if the
amount of lignin in a 50 g sample of biomass is 12 g, the amount of
original lignin recovered is 4.2 g/12 g.times.100%=35%.
[0049] The second number, the calculated percentage of the
extracted lignin recovered from the pretreatment solution, is a
more precise number, and shows how well the recovery method works
in removing dissolved lignin from the pretreatment solution.
According to the literature, ammonium hydroxide dissolves lignin
and hemicellulose but does not significantly dissolve cellulose
(glucan). So the mass of cellulose in the pretreated biomass should
be the same before and after pretreatment. According to Table 1,
each 50-g batch of switchgrass contains approximately 45 g (dry
basis).times.0.32=14.3 g glucan. After pretreatment, the glucan
content should be the same (.about.14 g), but the total mass of
pretreated biomass should be less due to removal of lignin and
hemicellulose during pretreatment. Table 3 shows that the glucan
content has been increased to 40 wt % as a result of pretreatment.
If it is assumed that the mass of glucan in the pretreated biomass
is the same before and after pretreatment, then the total mass of
the biomass after pretreatment is approximately 14.3 g/0.40=35.7 g.
From Table 2, it is also known that the pretreated material
contains 18.1 wt % lignin (soluble and insoluble). Therefore the
mass of lignin remaining in the pretreated material is 35.7
g.times.0.181=6.5 g. Therefore, pretreatment dissolves .about.12 g
(initial)-6.5 g (final)=5.5 g of lignin per 50-g batch. From Table
2 it is also known that the solids collected using this method
contain 84% lignin, and so the largest amount of solid that should
be recoverable is 5.5 g/0.84=6.5 g. So, if 6.5 g of solids are
recovered by this method from 50 g of switchgrass, then 100% lignin
of all of the lignin dissolved during pretreatment has been
recovered. If only 5 g of solids, for example, are recovered per 50
g batch instead, then the calculated percentage of lignin recovered
from the pretreatment solution is (5 g.times.0.84)/5.5
g.times.100%=76%.
[0050] Statistically, there is no difference between the amount of
lignin recovered when Steps 1 and 2 of the lignin separation method
are interchanged. Both variations work equally well in recovering
lignin from solution. However, it was readily apparent that
acidifying the pretreatment solution before the ammonia is
evaporated is less efficient because a greater amount of acid is
needed to neutralize the ammonia in solution. Also, evaporation of
ammonia takes more energy because ammonia is less volatile when it
is protonated. Furthermore, the presence of sulfate ion leads to
the formation of ammonium sulfate salt, which is not volatile and
is retained in the waste liquid.
[0051] With regard to the calculated percentage of lignin recovered
from the pretreatment solution, there is significant variability,
though the method is proficient in all cases at recovering at least
66% all lignin dissolved into solution. The drop in the calculated
percentage of lignin recovered for Trials 8-11 may be due to a
switch from filtration to centrifugation as a means to separate
precipitated lignin from solution. While filtration proved to be
effective at achieving a very clean separation of lignin from
solution, centrifugation was faster to perform and was more
convenient, since no filter media was needed. For Trial 12;
filtration was again used as a means of separating lignin particles
from solution, and the percentage of lignin recovered increased
accordingly. Though a clean separation may be achieved using
centrifugation, the technique requires careful decantation of the
supernatant liquid after centrifugation to avoid loss of lignin, an
the decreased lignin recovery for Trials 8-11 is likely due to
variation in the performance of the decantation step.
[0052] In Table 2, the percentage of lignin recovered from the
pretreatment solution is lower for Trials 8-12, and the cause for
this drop is not certain. One possibility is that centrifugation is
less effective than filtration in recovering precipitated lignin
from solution. Centrifugation was used in these trials instead of
filtration in an effort to decrease separation time, and some
precipitated solid particles may have been lost when the
supernatant liquid was decanted from the centrifugation tubes.
Increased solids loss during the solids recovery step would also
explain the drop in the percentage of original lignin recovered for
these trials.
[0053] For Trial 4 and Trials 12-15, the percentage of lignin
recovered from the pretreatment solution is significantly greater
than 100%, which is not possible unless ammonium hydroxide
pretreatment for these particular trials was more successful at
extracting lignin from switchgrass than was determined using the
data in Table 3. While there isn't a clear explanation as to why
this might have occurred for Trial 4, it is suspected that
increasing the pretreatment volume:mass ratio in Trials 13-15 from
6.6:1 to 8:1 improved the effectiveness of the lignin extraction,
thus making more lignin available in solution for recovery.
[0054] As set forth in FIG. 1, the amount of lignin recovered
varies according to the properties and type of biomass. Using a
pre-treatment ratio (volume; mass) of 8:1, the lignin extraction
method works best on switchgrass and was least effective on
loblolly pine. When the pre-treatment ratio was increased from 8:1
to 16:1, the performance of the lignin extraction method was
improved, and, for loblolly pine, became statistically
indistinguishable in the results obtained for switchgrass and as
further set forth in FIG. 2.
[0055] While the pre-treatment ratios and trials show efficacy at
6:1 to 16:1 it is believed that much lower ratios, such as 1:1, are
also useful depending upon the processing conditions, the type of
bio-feedstock utilized, and the degree of mechanical mixing or
agitation that might be employed. For instance, one pre-treatment
option may involve percolating ammonium hydroxide through a solid
bed of bio-mass. In such conditions, the mixing ratio of liquid to
solid may be about 1:1, yet the fluid may be saturated with
dissolved lignin which can be recovered as described herein.
Accordingly, the actual mixing ratio of liquid to solid can be
optimized around variables of lignin content of the bio-mass, the
physical size dimensions of the bio-mass, extraction processing
temperatures and the amount of mixing or agitation which may be
employed.
[0056] Without being limited by theory, it is believed that the
increase in lignin removal when the pre-treatment ratio was
increased suggests that the ammonium hydroxide pretreatment step is
responsible for the poor lignin recovery from the biomaterials such
as sweet sorghum, sweet gum, and loblolly pine. It is believed that
the ammonium hydroxide pretreatment process may be affected by a
lignin stability limit, the composition of the biomass, or perhaps
mass transfer of limitations that are alleviated using a higher
liquid to solid ratio for the pretreatment step.
[0057] The process described herein has been found useful for
resolving between 40-60% of lignin in the feedstock materials. It
has also been observed that the treatment methodology will extract
less than 20% of any cellulose and less than 5% of cellulose from
the pretreated biomass. While other methodologies may remove larger
amounts of lignin, it has been found that the current approach
provides for a cost-effective removal of a sufficient amount of
lignin such that greater than 80% hydrolysis of the pretreated
biomass may be converted into useful figures. As a result, the
lignin removal process allows for a high percentage of hydrolysis
of biomass into useful sugars while generating a easily purified
lignin stream that has additional commercial value.
[0058] A break-even point was calculated for a lignin separation
train for a plant of two different sizes: (1) a pilot plant
processing 1 metric ton/day of switchgrass (dry basis), and (2) a
production-scale plant processing 350,000 metric tons/year. This
amount of switchgrass could be produced in one year by cultivating
about 18,000 hectares and collecting it for processing in a
regional ethanol production facility. To calculate a break-even
point, an existing Aspen Plus flowsheet of a lignocellulosic
ethanol production plant was modified to include a representation
of the lignin separation process (see FIG. 1). The Aspen Plus
simulation was performed at the scale of 1-metric ton
switchgrass/day, and the process mass and energy balances for the
lignin separation train were calculated. The normalized marginal
material and utility costs were then calculated using a mass and
energy balance information obtained from the Aspen Plus simulation
and cost information and assumptions provided in Table 4. Predesign
costs for the pilot-scale plant were calculated using the
capacity-ratio exponent method and order-of-magnitude estimates and
multiplying factors were used to determine the costs of
installation, instrumentation, piping, and electrical wiring. The
costs were then escalated to 2010 using the Marshall and Swift
Equipment Cost Index.
TABLE-US-00004 TABLE 4 Assumptions table for pilot-scale process
cost calculations. 1 Metric ton/day switchgrass feed (dry basis)
274 kg/day lignin content in switchgrass feed 240 kg/day lignin
recovery rate (dry basis) 5.69 $/GJ, cost of natural gas
($6/Million Btu) 1400 Scaling factor for increase in lignin
production rate from the 1-ton/day pilot plant to a 100 .times. 106
L/year biofuels facility 0.06 $/kWh, cost of electricity 90%
Percentage of time lignin separation train is operating 150 $/ton,
cost of 98% sulfuric acid 633 MJ/hour, capacity of chiller unit
(50-ton chiller) 83% Energy efficiency of lignin chiller unit 0%
Percentage of heat recovery from evaporator vapor 10 kW,
non-specific electrical load of lignin recovery equipment
(instrumentation, pumps, tank stir, etc.) 1000 Marshall and Swift
Equipment Cost Index for baseline costs 1481 Marshall and Swift
Equipment Cost Index, 1st Quarter 2010
[0059] For the larger-scale plant, the marginal utility costs
remain the same per unit lignin produced, but determining the cost
of equipment becomes more complicated. Once a particular piece of
equipment reaches its maximum manufactured size, then multiple
units must be deployed to achieve higher throughput, and equipment
costs scale linearly with the number of units beyond that point.
The point at which a particular piece of equipment reaches its
maximum size is not known without consulting specific equipment
manufacturers. As a result, only order-of-magnitude marginal costs
could be calculated for the larger plant size.
[0060] For the 1-ton/day switchgrass plant, which is the size
typically discussed for the performance of integrated demonstration
facilities, the normalized marginal material and utility cost is
$0.56/kg lignin. The individual contributions to this cost are
shown in Table 5. Totally, 75% of the marginal this cost is due to
operation of the evaporator, which requires the burning of natural
gas to heat the evaporator, and the use of an electrically powered
chiller to condense the evaporated water vapor. The cost could be
reduced if the heat of evaporation were recuperated and used
elsewhere in the plant, but no credit was taken for such a step in
this analysis. The total estimated marginal cost of equipment for
the 1-ton/day switchgrass plant is shown in Table 6. For this
plant, the equipment costs are calculated to be $1.80/kg
lignin.
TABLE-US-00005 TABLE 5 Normalized marginal material and utility
costs. Process unit Consumption Marginal cost, $/kg Lignin 98%
Sulfuric acid 84 kg/day 0.06 Gas-fired evaporator 11,500 MJ/day
0.27 Electric vacuum drier 65 kWh/day 0.02 Electric chiller unit
29.9 kWh/day 0.16 General electrical load 217 kWh/day 0.05 Total
0.56
[0061] For the larger, industrial-scale plant, the marginal utility
costs remain the same, since they scale linearly with the
throughput of the plant, but the marginal equipment costs are
reduced. For this larger-scale plant, the total capital costs of
the lignin separation train are estimated to be in the range of
$25M, which corresponds to a normalized equipment cost of
approximately $0.22/kg lignin.
[0062] At these normalized marginal costs, it is not economical to
burn the lignin for process heat. Ignoring capital costs
altogether, burning the lignin to generate process heat instead of
burning natural gas is the equivalent of buying natural gas at a
price of $28/GJ, which is more than five times its current market
cost. Therefore it is not economical to burn the separated lignin
at any scale. The better option would be to find a market for the
purified lignin where it could be sold at least for its break-even
price as a feed material for the manufacture of other higher-value
chemicals or materials. At the 1-ton/day scale, it is not likely
that creating a separate lignin stream by this process would be
economical. The breakeven point is greater than $2.36/kg, which is
a tolerable feedstock price only if the products made from it are
not commodity chemicals. At the industrial scale, however, the
break-even point is greater than $0.78/kg, and it is possible that
lignin produced by this method could be used to make commodity
chemicals such as phenol, for example, which currently sells for
approximately $1.56-1.74/kg on the commodity markets.
[0063] According to the Aspen Plus simulation, the waste stream
from the process contains 4% dissolved xylan, 3.5% ammonium
sulfate, and 1.7% protein, so there is the potential to further
exploit this stream to provide additional revenue for the plant
from the generation of value-added coproducts.
[0064] Although preferred embodiments of the invention have been
described using specific terms, devices, and methods, such
description is for illustrative purposes only. The words used are
words of description rather than of limitation. It is to be
understood that changes and variations may be made by those of
ordinary skill in the art without departing from the spirit or the
scope of the present invention which is set forth in the following
claims. In addition, it should be understood that aspects of the
various embodiments may be interchanged, both in whole, or in part.
Therefore, the spirit and scope of the appended claims should not
be limited to the description of the preferred versions contained
therein.
TABLE-US-00006 TABLE 6 Pilot-scale lignin recovery train estimated
predesign equipment costs. $65,000 15 m3 jacketed lignin
precipitation vessel with mixer $50,000 Evaporator
(order-of-magnitude estimate) $75,000 46.5 m2 filter press $20,000
Vacuum drier $20,000 (2) Slurry pumps $10,000 Wet lignin conveyer
$10,000 Lignin slurry coolers, pipe-in-pipe (prder-of-magnitude
estimate) $75,000 633 MJ/hour capacity refrigerated chiller unit
$127,000 Installation $85,000 Instrumentation $146,000 Piping
$33,000 Electrical $716,000 Total Equipment Cost, Installed
* * * * *