U.S. patent application number 10/728976 was filed with the patent office on 2004-12-09 for method for producing pulp and lignin.
Invention is credited to Tarasenko, David.
Application Number | 20040244925 10/728976 |
Document ID | / |
Family ID | 33493399 |
Filed Date | 2004-12-09 |
United States Patent
Application |
20040244925 |
Kind Code |
A1 |
Tarasenko, David |
December 9, 2004 |
Method for producing pulp and lignin
Abstract
The invention provides for methods for producing pulp
(comprising cellulose) and lignin from lignocellulosic material,
such as wood chips. The methods involve acid catalyzed hydrolysis.
Lignocellulosic material having a relatively high moisture
concentration can be used as the starting material. The
lignocellulosic material is impregnated with an acid (preferably
nitric acid) and heated. During the heating lignin is depolymerized
at relatively low temperatures, and the acid catalyst is distilled
off. The acid catalyst can be collected and recycled after
impregnation and heating. The lignocellulosic material is then
digested in an alkaline solution under heat, dissolving the lignin
and allowing the pulp to be removed. Acid is added to the black
liquor to precipitate the lignin which is then removed. The
resultant amber liquor can be further processed into other
ancillary products such as alcohols and/or unicellular
proteins.
Inventors: |
Tarasenko, David; (Kelowna,
CA) |
Correspondence
Address: |
OYEN, WIGGS, GREEN & MUTALA
480 - THE STATION
601 WEST CORDOVA STREET
VANCOUVER
BC
V6B 1G1
CA
|
Family ID: |
33493399 |
Appl. No.: |
10/728976 |
Filed: |
December 8, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60474961 |
Jun 3, 2003 |
|
|
|
60508308 |
Oct 6, 2003 |
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Current U.S.
Class: |
162/16 ; 162/29;
162/37; 162/81; 162/90; 162/98 |
Current CPC
Class: |
D21C 1/04 20130101; D21C
3/02 20130101; D21C 11/04 20130101 |
Class at
Publication: |
162/016 ;
162/081; 162/090; 162/029; 162/037; 162/098 |
International
Class: |
D21C 003/02; D21C
011/00; D21C 003/16 |
Claims
What is claimed is:
1. A method for producing pulp and lignin from lignocellulosic
material, the pulp comprising cellulose, the method comprising: (a)
contacting the lignocellulosic material with an aqueous acid
solution to impregnate the lignocellulosic material, the aqueous
acid solution comprising from about 10% to about 40% by weight of
the acid; (b) heating the lignocellulosic material in two stages,
the first heating stage being carried out for a period of time
which is sufficient to depolymerize lignin within the
lignocellolosic material without substantially degrading the
cellulose or lignin in the ligncellulosic material, the second
heating stage being carried out at or above the boiling point of
the acid to distill off the acid; (c) contacting the
lignocellulosic material with an aqueous alkaline solution under
heat to solubilize lignin in the alkaline solution, leaving a black
liquor; (d) removing the pulp from the black liquor; (e) adding
sufficient acid to the black liquor to precipitate the lignin; and
(f) removing the lignin from the liquor.
2. A method according to claim 1 wherein the acid is nitric
acid.
3. A method according to claim 1 wherein the lignocellulosic
material is contacted with the aqueous acid solution in step (a)
for at least 30 minutes.
4. A method according to claim 3 wherein the lignocellulosic
material is contacted with the aqueous acid solution in step (a)
from about 12 hours to about 24 hours.
5. A method according to claim 1 wherein the lignocellulosic
material is contacted with the aqueous acid solution in step (a)
under heat.
6. A method according to claim 2 wherein the lignocellulosic
material is contacted with the aqueous nitric acid solution in step
(a) at a temperature from about 50.degree. C. to below the boiling
point of the nitric acid.
7. A method according to claim 2 wherein the aqueous nitric acid
solution in step (a) comprises from about 10% to about 30% by
weight of the nitric acid.
8. A method according to claim 7 wherein the aqueous nitric acid
solution comprises from about 15% to about 30% by weight of the
nitric acid.
9. A method according to 8 wherein the aqueous nitric acid solution
comprises from about 20% to about 25% by weight of the nitric
acid.
10. A method according to claim 2 wherein the aqueous nitric acid
solution comprises from about 10% to about 15% by weight of the
nitric acid.
11. A method according to claim 2 wherein the temperature during
the second heating stage of step (b) is from about 73.degree. C. to
below 100.degree. C.
12. A method according to claim 11 wherein the temperature during
the second heating stage of step (b) is from about 90.degree. C. to
about 95.degree. C.
13. A method according to claim 2 wherein the temperature during
the first heating stage of step (b) is up to about 75.degree.
C.
14. A method according to claim 13 wherein the temperature during
the first heating stage of step (b) is from about 50.degree. C. to
about 75.degree. C.
15. A method according to claim 14 wherein the temperature during
the first heating stage of step (b) is from about 60.degree. C. to
about 70.degree. C.
16. A method according to claim 14 wherein the temperature during
the first heating stage of step (b) is from about 50.degree. C. to
about 60.degree. C.
17. A method according to claim 14 wherein the temperature during
the first heating stage of step (b) is from about 70.degree. C. to
about 75.degree. C.
18. A method according to claim 2 wherein the first heating stage
in step (b) is carried out for at least 15 minutes.
19. A method according to claim 2 wherein the aqueous alkaline
solution comprises sodium hydroxide or potassium hydroxide or a
combination of sodium hydroxide and potassium hydroxide.
20. A method according to claim 2 wherein the aqueous alkaline
solution comprises an amount of alkali solute which is at least the
normal equivalent of the nitric acid in the aqueous acid solution
in step (a).
21. A method according to claim 2 wherein the aqueous alkaline
solution comprises an amount of alkali solute which is at least the
molar equivalent of the nitric acid in the aqueous acid solution in
step (a).
22. A method according to claim 2 wherein the acid added in step
(e) is sulfuric acid.
23. A method according to claim 2 wherein the amount of acid added
in step (e) is at least the molar amount of the alkali in the
aqueous alkaline solution in step (c).
24. A method according to claim 1 wherein the black liquor is
cooled from the temperature in step (c) before the acid is added in
step (e).
25. A method according to claim 2 wherein the temperature of the
black liquor when the acid is added in step (e) is up to about
75.degree. C.
26. A method according to claim 25 wherein the temperature of the
black liquor when the acid is added in step (e) is from about
5.degree. C. to about 75.degree. C.
27. A method according to claim 26 wherein the temperature of the
black liquor when the acid is added in step (e) is from about
5.degree. C. to about 50.degree. C.
28. A method according to claim 27 wherein the temperature of the
black liquor when the acid is added in step (e) is from about
25.degree. C. to about 50.degree. C.
29. A method according to claim 25 wherein the temperature of the
black liquor when the acid is added in step (e) is from about
50.degree. C. to about 75.degree. C.
30. A method according to claim 1 wherein any aqueous acid solution
not absorbed by the lignocellulosic material in step (a) is removed
prior to heating the lignocellulosic material in step (b).
31. A method according to claim 30 comprising collecting any
aqueous acid solution which is removed prior to step (b) and
recycling the collected aqueous acid solution for use in step (a),
and comprising collecting any acid which is distilled off in step
(b) and recycling the collected acid for use in step (a).
32. A method according to claim 1 comprising contacting the
lignocellulosic material with water before step (a) to increase the
moisture content in the lignocellulosic material.
33. A method according to claim 1 wherein the starting moisture
content of the lignocellulosic material is from about 30% to about
55% by weight of water.
34. A method according to claim 1 wherein the starting
lignocellulosic material comprises undigested lignocellulosic
material previously subjected to the method.
35. A method according to claim 1 wherein the starting
lignocellulosic material comprises wood chips, wood shavings,
sawdust or a combination of two or more wood chips, wood shavings
and sawdust.
36. A method according to claim 1 wherein the starting
lignocellulosic material comprises pieces of rye, wheat, hemp or a
combination of two or more of rye, wheat and hemp.
37. A method according to claim 1 wherein an amber liquor is left
following the removal of the lignin, and comprising processing the
amber liquor after the lignin has been removed.
38. A method according to claim 37 wherein the amber liquor is
processed to produce unicellular proteins or alcohols or both.
39. A method according to claim 1 wherein any water which is
produced as a reaction byproduct in one or more of the steps is
collected and recycled for use in the method.
40. A method according to claim 2 wherein steps (a) and (c) and the
first heating stage in step (b) are each carried out at a
temperature from about 50.degree. C. to about 75.degree. C.
41. A method according to claim 1 wherein at least one of the
heating of the impregnated lignocellulosic material in step (b),
the contacting the lignocellulosic material with the aqueous
alkaline solution in step (c) and the adding the acid in step (e)
is carried out with agitation.
42. A method according to claim 1 wherein the heating of the
impregnated lignocellulosic material in step (b), the contacting
the lignocellulosic material with the aqueous alkaline solution in
step (c) and the addition of the acid in step (e) are each carried
out with agitation.
43. A method according to claim 1 wherein the method is carried out
at atmospheric pressure.
44. A method according to claim 1 comprising washing, pressing,
bleaching and drying the pulp removed in step (d).
45. A method according to claim 1 comprising drying the lignin
removed in step (f).
46. A method according to claim 2 wherein step (c) is carried out
at a temperature up to about 75.degree. C.
47. A method according to claim 46 wherein step (c) is carried out
at a temperature from about 5.degree. C. to about 75.degree. C.
48. A method according to claim 47 wherein step (c) is carried out
at a temperature from about 50.degree. C. to about 75.degree.
C.
49. A method according to claim 46 wherein step (c) is carried out
at a temperature from about 20.degree. C. to about 50.degree.
C.
50. A method according to claim 47 wherein step (c) is carried out
at a temperature from about 30.degree. C. to about 40.degree.
C.
51. A method according to claim 47 wherein step (c) is carried out
at a temperature from about 40.degree. C. to about 50.degree.
C.
52. A method according to claim 1 wherein the lignocellulosic
material is immersed in the aqueous acid solution in step (a).
53. A method according to claim 1 wherein the lignocellulosic
material is sprayed with the aqueous acid solution in step (a).
54. A method for producing pulp and lignin from lignocellulosic
material, the pulp comprising cellulose, the method comprising: (a)
contacting the lignocellulosic material with an aqueous nitric acid
solution to impregnate the lignocellulosic material, the aqueous
nitric acid solution comprising from about 10% to about 40% by
weight of the nitric acid, (b) heating the impregnated
lignocellulosic material in two stages, the first heating stage
being carried out a temperature from about 50.degree. C. to about
75.degree. C. for a period of time which is sufficient to
depolymerize lignin within the lignocellolosic material without
substantially degrading the cellulose or lignin in the
ligncellulosic material, the second heating stage being carried out
at or above the boiling point of the nitric acid to distill off the
nitric acid; (c) contacting the lignocellulosic material with an
aqueous alkaline solution at a temperature from about 50.degree. C.
to about 75.degree. C. to solubilize lignin in the alkaline
solution, leaving a black liquor, the aqueous alkaline solution
comprising an amount of alkali which is at least the normal amount
of the nitric acid in the aqueous acid solution in step (a); (d)
removing the pulp from the black liquor; (e) cooling the black
liquor and then adding an acid to the black liquor to acidify the
solution to precipitate the lignin; (f) removing the lignin,
leaving an amber liquor; and (g) processing the amber liquor to
produce unicellular proteins or alcohols or both, wherein any
aqueous nitric acid not absorbed by the lignocellulosic material in
step (a) is removed and collected following step (a) prior to
heating the lignocellulosic material in step (b) and then recycled
for use in step (a), and wherein any nitric acid which is distilled
off is collected prior to contacting the lignocellulosic material
with the alkaline solution in step (c) and then recylcled for use
in step (a), and wherein the heating of the lignocellulosic
material in step (b), the contacting the lignocellulosic material
with the aqueous alkaline solution in step (c) and the addition of
the acid in step (e) are each carried out with agitation.
55. A method for producing pulp and lignin comprising the steps of
contacting lignocellulosic material with an acid and, after
removing any acid not absorbed by the lignocellulosic material,
heating the lignocellulosic material at a temperature up to about
75.degree. C. to effect the acid-catalyzed hydrolytic
depolymerization of the lignin in the lignocellulosic material
without substantially degrading the cellulose or lignin in the
lignocellulosic material, the acid-contacting and heating steps
being carried out before the lignocellulosic material is digested
in an alkaline liquor, the pulp being removed following the
digestion of the lignocellulosic material in the alkaline liquor,
the lignin being removed after being precipitated out with the
addition of an acid to the black liquor produced following the
digestion of the lignocellulosic material in the alkaline liquor,
wherein the pulp comprises cellulose.
56. A method according to claim 55 wherein the acid-contacting step
comprises immersing the lignocellulosic material in an aqueous
solution of the acid, and wherein the acid is nitric acid and the
aqueous solution comprises from about 10% to about 40% by weight of
nitric acid.
57. A method according to claim 56 comprising heating the
lignocellulosic material after depolymerizing the lignin but before
digesting the lignocellulosic material, at a temperature above the
boiling point of the acid in order to distill off the acid.
58. A method according to claim 44 wherein any liquor that is
removed from the pulp by pressing is collected and added to the
black liquor prior to adding the acid in step (e).
59. A method according to claim 45 wherein any liquor which is
removed from the lignin during drying is collected and added to the
liquor after step (f), and wherein the liquor is processed after
the lignin has been precipitated and removed.
60. The method of claim 1 wherein enough acid is added to the
solution in step (e) to lower the pH of the solution to an acidic
pH.
61. A method for treating lignocellulosic material comprising: (a)
contacting the lignocellulosic material with an aqueous acid
solution to impregnate the lignocellulosic material, the aqueous
acid solution comprising from about 10% to about 40% by weight of
the acid; (b) heating the lignocellulosic material in two stages,
the first heating stage being carried out for a period of time
which is sufficient to depolymerize lignin within the
lignocellolosic material without substantially degrading the
cellulose or lignin in the ligncellulosic material, the second
heating stage being carried out at or above the boiling point of
the acid to distill off the acid, wherein any aqueous acid solution
not absorbed by the lignocellulosic material in step (a) is removed
prior to heating the lignocellulosic material in step (b); (c)
contacting the lignocellulosic material with an aqueous alkaline
solution under heat to solubilize lignin in the alkaline solution,
leaving a black liquor; and (d) removing the pulp from the black
liquor, the pulp comprising cellulose.
62. A method according to claim 2 wherein the amount of acid added
in step (e) is at least the normal amount of the alkali in the
aqueous alkaline solution in step (c).
63. A method for producing pulp and lignin from lignocellulosic
material, the pulp comprising cellulose, the method comprising: (a)
contacting the lignocellulosic material with an aqueous nitric acid
solution to impregnate the lignocellulosic material, the aqueous
acid solution comprising from about 10% to about 40% by weight of
the nitric acid; (b) heating the lignocellulosic material in two
stages, the first heating stage being carried out for a period of
time which is sufficient to depolymerize lignin within the
lignocellolosic material without substantially degrading the
cellulose or lignin in the ligncellulosic material, the second
heating stage being carried out at or above the boiling point of
the acid to distill off the acid, wherein any aqueous acid solution
not absorbed by the lignocellulosic material in step (a) is removed
prior to heating the lignocellulosic material in step (b); (c)
contacting the lignocellulosic material with an aqueous alkaline
solution under heat to solubilize lignin in the alkaline solution,
leaving a black liquor; (d) removing the pulp from the black
liquor; (e) adding sufficient acid to the black liquor to
precipitate the lignin; and (f) removing the lignin from the
liquor.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Application No. 60/474,961, filed Jun. 3, 2003, and U.S.
Provisional Application No. ______ unassigned, filed Oct. 6,
2003.
TECHNICAL FIELD
[0002] This invention pertains to a method for producing pulp
(comprising cellulose) and lignin from lignocellulosic material
such as wood chips.
BACKGROUND
[0003] Wood is a composite material comprising cellulose, lignin
and hemicellulose.
[0004] Cellulose is the strong, fibrous component which consists of
long chains of glucose molecules and which is used to make paper.
Lignin is a phenolic polymeric matrix which holds the cellulose
fibres together. Hemicellulose is the component comprised of short,
branched chains of glucose and other like molecules.
[0005] In general terms, pulping processes involve converting wood
chips (or other suitable vegetative material) into a fibrous form
in order to produce pulp. Pulp refers to cellulose fibres or
material containing cellulose fibres which may be used in the
production of paper or paper containing products.
[0006] The two main types of pulping processes in use today are
mechanical pulping and chemical pulping though there are other
pulping processes, such as chemi-thermal mechanical pulping, which
are a combination thereof.
[0007] Mechanical pulping involves the physical separation of
individual fibres of the wood by forcing debarked logs and hot
water between large rotating steel discs with teeth that tear the
wood apart, or alternatively, by pressing logs against grindstones.
Generally, pulps produced by mechanical pulping are of lower
quality than pulps produced according to chemical pulping, and thus
are used to produce newsprint and cardboard type products.
[0008] On the other hand, chemical pulping involves subjecting wood
to heat and chemicals in order to dissolve the lignin and
hemicellulose binding materials, thereby separating the cellulosic
fibres. Kraft pulping is an example of a chemical pulping process,
and involves involves cooking wood chips in a pressurized vessel
known as a digester in the presence of hot caustic soda (NaOH) and
sodium sulfide (Na.sub.2S).
[0009] Kraft pulping is a variation of the soda process where only
NaOH is used to break down and remove the lignin. Relative to the
soda process, the addition of the sodium sulfide in the Kraft
process assisted in dissolving the lignin with less damage to
cellulosic fibres.
[0010] In Kraft pulping, the digestion process dissolves the lignin
that "glues" the cellulosic fibers to each other in the wood. The
resultant solution containing the dissolved ligneous material is
referred to as a "black liquor". After digestion, the
cellulose-containing pulp is separated from the black liquor and
washed. At this stage, the resultant colored slurry of cellulose
fibres is referred to as "brownstock". The color of the slurry is
the result of some residual lignin remaining within the slurry with
the cellulosic fibres. If desired, a further step of bleaching the
slurry can be carried out to remove some additional components of
the lignin, thereby brightening the pulp. Typically, the pulp is
bleached in multiple stages with various combinations of oxygen,
hydrogen peroxide and sodium hydroxide. Chelation stages can be
utilized in the bleaching process to remove transition metals in
the pulp which may otherwise interfere with peroxide bleaching.
[0011] One disadvantage with Kraft pulping is that it is carried
out under extreme conditions which adversely affect yields. Kraft
pulping typically involves pressures of approximately 120 p.s.i.,
temperatures of approximately 160-180.degree. C. and initial pH
values over 12 in the digestion stage. In part, these reaction
conditions are required by the fact that reactions carried out in
Kraft pulping are oxidation reactions. These harsh conditions
result in a loss of some cellulose and the destruction of much of
natural form lignin from the wood chips.
[0012] Lignin typically is not recovered as an end-product in
typical Kraft pulping processes. Instead, in Kraft pulping
processes the black liquor is typically condensed in recovery
boilers in the process of recovering chemicals such as sodium
hydroxide for reuse. Solids such as lignin are burned as fuel to
run the pulp mill utilizing the Kraft process. It would be
advantageous to recover as much lignin as possible in a pulping
process since lignin is a valuable component in its own right,
having a wide range of industrial applications such in the
manufacture of fertilizers, asphalt emulsifiers, soil stabilizers,
wood products (such as plywood and particle board), oil well
drilling fluids and dispersing agents for preparing concrete.
SUMMARY OF INVENTION
[0013] In one aspect the invention provides for methods for
producing pulp and lignin from lignocellulosic material, the pulp
comprising cellulose. The methods comprise: (a) contacting the
lignocellulosic material with an aqueous acid solution to
impregnate the lignocellulosic material, the aqueous acid solution
comprising from about 10% to about 40% by weight of the acid; (b)
heating the lignocellulosic material in two stages, the first
heating stage being carried out for a period of time which is
sufficient to depolymerize lignin within the lignocellolosic
material without substantially degrading the cellulose or lignin in
the ligncellulosic material, the second heating stage being carried
out at or above the boiling point of the acid to distill off the
acid; (c) contacting the lignocellulosic material with an aqueous
alkaline solution under heat to solubilize lignin in the alkaline
solution, leaving a black liquor; (d) removing the pulp from the
black liquor; (e) adding sufficient acid to the black liquor to
precipitate the lignin; and (f) removing the lignin from the
liquor.
[0014] The acid in step (a) may be nitric acid. The aqueous acid
solution in step (a) may comprise, without limitation, from about
10% to about 30%, from about 15% to about 30%, from about 20% to
about 25%, or from about 10% to about 15%, by weight of the
acid.
[0015] The lignocellulosic material may be contacted with the
aqueous acid solution in step (a) for at least 30 minutes,
including without limitation, from about 12 hours to about 24
hours. The lignocellulosic material may be contacted with the
aqueous acid solution in step (a) under heat. The lignocellulosic
material may be contacted with the aqueous nitric acid solution in
step (a) at a temperature from about 50.degree. C. to below the
boiling point of the nitric acid.
[0016] The temperature during the second heating stage of step (b)
may be, without limitation, from about 73.degree. C. to below
100.degree. C. or from about 90.degree. C. to about 95.degree. C.
The temperature during the first heating stage of step (b) may be,
without limitation, up to about 75.degree. C., from about
50.degree. C. to about 75.degree. C., from about 60.degree. C. to
about 70.degree. C., from about 50.degree. C. to about 60.degree.
C., or from about 70.degree. C. to about 75.degree. C. The first
heating stage in step (b) may be carried out for at least 15
minutes.
[0017] The aqueous alkaline solution may comprise sodium hydroxide
or potassium hydroxide or a combination of sodium hydroxide and
potassium hydroxide. The aqueous alkaline solution may comprise an
amount of alkali solute which is at least the normal equivalent of
the nitric acid in the aqueous acid solution in step (a). The
aqueous alkaline solution may comprise an amount of alkali solute
which is at least the molar equivalent of the nitric acid in the
aqueous acid solution in step (a).
[0018] The acid used in step (e) may be sulfuric acid. The amount
of acid added in step (e) may be at least the molar amount of the
alkali in the aqueous alkaline solution in step (c). The amount of
acid added in step (e) may be at least the normal amount of the
alkali in the aqueous alkaline solution in step (c).
[0019] The black liquor may cooled from the temperature in step (c)
before the acid is added in step (e). The temperature of the black
liquor when the acid is added in step (e) may be, without
limitation, up to about 75.degree. C., from about 5.degree. C. to
about 75.degree. C., from about 5.degree. C. to about 50.degree.
C., from about 25.degree. C. to about 50.degree. C., or from about
50.degree. C. to about 75.degree. C.
[0020] Any aqueous acid solution not absorbed by the
lignocellulosic material in step (a) may be removed prior to
heating the lignocellulosic material in step (b). The methods may
comprise collecting any aqueous acid solution which is removed
prior to step (b) and recycling the collected aqueous acid solution
for use in step (a), and comprising collecting any acid which is
distilled off in step (b) and recycling the collected acid for use
in step (a).
[0021] The starting moisture content of the lignocellulosic
material may be from about 30% to about 55% by weight of water. The
method may comprise contacting the lignocellulosic material with
water before step (a) to increase the moisture content in the
lignocellulosic material. The starting lignocellulosic material may
comprise, without limitation, wood chips, wood shavings, sawdust,
pieces of rye, wheat, hemp and combinations of two or more of the
foregoing. The starting lignocellulosic material may comprise
undigested lignocellulosic material previously subjected to the
method.
[0022] An amber liquor may be left following the removal of the
lignin, and the method may comprise processing the amber liquor
after the lignin has been removed, for example, to produce
unicellular proteins or alcohols or both. Any water which is
produced as a reaction byproduct in one or more of the steps in the
method may be collected and recycled for use in the method.
[0023] Steps (a) and (c) and the first heating stage in step (b)
may each carried out at a temperature from about 50.degree. C. to
about 75.degree. C. One or more of the heating of the impregnated
lignocellulosic material in step (b), the contacting the
lignocellulosic material with the aqueous alkaline solution in step
(c) and the adding the acid in step (e) may be carried out with
agitation. The methods may be carried out at atmospheric
pressure.
[0024] The pulp removed in step (d) may be washed, pressed,
bleached and dried. Similarly, the lignin removed in step (f) may
be dried. Any residual liquor that is removed from the pulp by
pressing may be collected and added to the black liquor prior to
adding the acid in step (e). Similarly, any residual liquor which
is removed from the lignin during drying may be collected and added
to the liquor after step (f), and wherein the liquor is processed
after the lignin has been precipitated and removed.
[0025] Enough acid may be added to the solution in step (e) to
lower the pH of the solution to an acidic pH.
[0026] Without limitation, step (c) may be carried out at a
temperature up to about 75.degree. C., from about 5.degree. C. to
about 75.degree. C., from about 50.degree. C. to about 75.degree.
C., from about 20.degree. C. to about 50.degree. C., from about
30.degree. C. to about 40.degree. C., or from about 40.degree. C.
to about 50.degree. C.
[0027] Step (a) may comprise immersing the lignocellulosic material
in the aqueous acid solution in step (a), or spraying the
lignocellulosic material with aqueous nitric acid solution.
[0028] In another aspect the invention provides for methods for
producing pulp and lignin from lignocellulosic material, the pulp
comprising cellulose, the methods comprise: (a) contacting the
lignocellulosic material with an aqueous nitric acid solution to
impregnate the lignocellulosic material, the aqueous nitric acid
solution comprising from about 10% to about 40% by weight of the
nitric acid; (b) heating the impregnated lignocellulosic material
in two stages, the first heating stage being carried out a
temperature from about 50.degree. C. to about 75.degree. C. for a
period of time which is sufficient to depolymerize lignin within
the lignocellolosic material without substantially degrading the
cellulose or lignin in the ligncellulosic material, the second
heating stage being carried out at or above the boiling point of
the nitric acid to distill off the nitric acid; (c) contacting the
lignocellulosic material with an aqueous alkaline solution at a
temperature from about 50.degree. C. to about 75.degree. C. to
solubilize lignin in the alkaline solution, leaving a black liquor,
the aqueous alkaline solution comprising an amount of alkali which
is at least the normal amount of the nitric acid in the aqueous
acid solution in step (a); (d) removing the pulp from the black
liquor; (e) cooling the black liquor and then adding an acid to the
black liquor to acidify the solution to precipitate the lignin; (f)
removing the lignin, leaving an amber liquor; and (g) processing
the amber liquor to produce unicellular proteins or alcohols or
both, wherein any aqueous nitric acid not absorbed by the
lignocellulosic material in step (a) is removed and collected
following step (a) prior to heating the lignocellulosic material in
step (b) and then recycled for use in step (a), and wherein any
nitric acid which is distilled off is collected prior to contacting
the lignocellulosic material with the alkaline solution in step (c)
and then recylcled for use in step (a), and wherein the heating of
the lignocellulosic material in step (b), the contacting the
lignocellulosic material with the aqueous alkaline solution in step
(c) and the addition of the acid in step (e) are each carried out
with agitation.
[0029] In yet another aspect the invention provides for methods for
producing pulp and lignin comprising the steps of contacting
lignocellulosic material with an acid and, after removing any acid
not absorbed by the lignocellulosic material, heating the
lignocellulosic material at a temperature up to about 75.degree. C.
to effect the acid-catalyzed hydrolytic depolymerization of the
lignin in the lignocellulosic material without substantially
degrading the cellulose or lignin in the lignocellulosic material,
the acid-contacting and heating steps being carried out before the
lignocellulosic material is digested in an alkaline liquor, the
pulp being removed following the digestion of the lignocellulosic
material in the alkaline liquor, the lignin being removed after
being precipitated out with the addition of an acid to the black
liquor produced following the digestion of the lignocellulosic
material in the alkaline liquor, wherein the pulp comprises
cellulose. The acid-contacting step may comprise immersing the
lignocellulosic material in an aqueous solution of the acid, and
wherein the acid is nitric acid and the aqueous solution comprises
from about 10% to about 40% by weight of nitric acid. The
lignocellulosic material may be heated, after depolymerizing the
lignin but before digesting the lignocellulosic material, at a
temperature above the boiling point of the acid in order to distill
off the acid.
[0030] In yet another aspect the invention provides methods for
treating lignocellulosic material comprising: (a) contacting the
lignocellulosic material with an aqueous acid solution to
impregnate the lignocellulosic material, the aqueous acid solution
comprising from about 10% to about 40% by weight of the acid; (b)
heating the lignocellulosic material in two stages, the first
heating stage being carried out for a period of time which is
sufficient to depolymerize lignin within the lignocellolosic
material without substantially degrading the cellulose or lignin in
the ligncellulosic material, the second heating stage being carried
out at or above the boiling point of the acid to distill off the
acid, wherein any aqueous acid solution not absorbed by the
lignocellulosic material in step (a) is removed prior to heating
the lignocellulosic material in step (b); (c) contacting the
lignocellulosic material with an aqueous alkaline solution under
heat to solubilize lignin in the alkaline solution, leaving a black
liquor; and (d) removing the pulp from the black liquor, the pulp
comprising cellulose.
[0031] In yet another aspect the invention provides methods for
producing pulp and lignin from lignocellulosic material, the pulp
comprising cellulose, the methods comprising: (a) contacting the
lignocellulosic material with an aqueous nitric acid solution to
impregnate the lignocellulosic material, the aqueous acid solution
comprising from about 10% to about 40% by weight of the nitric
acid; (b) heating the lignocellulosic material in two stages, the
first heating stage being carried out for a period of time which is
sufficient to depolymerize lignin within the lignocellolosic
material without substantially degrading the cellulose or lignin in
the ligncellulosic material, the second heating stage being carried
out at or above the boiling point of the acid to distill off the
acid, wherein any aqueous acid solution not absorbed by the
lignocellulosic material in step (a) is removed prior to heating
the lignocellulosic material in step (b); (c) contacting the
lignocellulosic material with an aqueous alkaline solution under
heat to solubilize lignin in the alkaline solution, leaving a black
liquor; (d) removing the pulp from the black liquor; (e) adding
sufficient acid to the black liquor to precipitate the lignin; and
(f) removing the lignin from the liquor.
[0032] Further aspects of the invention and features of specific
embodiments of the invention are described below.
BRIEF DESCRIPTION OF DRAWINGS
[0033] FIG. 1 is an illustration of the mechanism of acid-catalyzed
hydrolysis of an ester; and
[0034] FIG. 2 is a schematic illustration of a method according to
the present invention.
DESCRIPTION
[0035] Throughout the following description specific details are
set forth in order to provide a more thorough understanding of the
invention. However, the invention may be practiced without these
particulars. In other instances, well known elements have not been
shown or described in detail to avoid unnecessarily obscuring the
present invention. Accordingly, the specification and drawings are
to be regarded in an illustrative, rather than a restrictive,
sense.
[0036] Most existing pulping processes involve high pressure and/or
high temperature conditions or concentrated chemicals. Not only do
these factors increase costs, but they also reduce the yield and
purity of cellulose-containing pulp and lignin end-products since
the cellulose and lignin can be degraded or destroyed during the
pulping processes. The present invention addresses these
deficiencies by utilizing the acid catalyzed hydrolysis of
lignocellulosic material in methods for producing pulp and lignin
from lignocellulosic material. The hydrolysis reaction allows the
controlled de-polymerization of lignin within the starting
lignocellulosic material, utilizes much lower concentrations of
pulping chemicals and lower temperatures than existing Kraft
pulping processes and may be carried out at atmospheric pressure.
These modest reaction conditions not only result in reduced costs
relative to existing Kraft pulping processes, but also increase the
yield and purity of the cellulose-containing pulp and lignin since
there is less degradation and and destruction of these components
during the pulping process relative to existing Kraft pulping
processes.
[0037] The invention includes methods involving contacting
lignocellulosic material with an acid, preferably nitric acid, in
order to impregnate the lignocellulosic material and then heating
the lignocellulosic material to de-polymerize the lignin within the
lignocellulosic material prior to digesting the lignocellulosic
material in an alkaline solution to solubilize lignin in the
alkaline solution, leaving a black liquor (comprising the
solubilized lignin) and pulp. The pulp is then removed and
processed (e.g. pressing, washing, bleaching, drying). The
solubilized lignin is precipitated out of the black liquor with the
addition of an acid and then removed for processing. The remaining
solution can be further processed to produce other useful
by-products such as unicellular proteins, alcohols or both.
[0038] As used herein, "lignocellulosic material" refers to any
material which contains cellulose and lignin and, without
limitation, includes pieces or particles (for example wood chips,
wood shavings, sawdust) from any type of tree (for example pine,
oak, maple, fir, spruce) and other vegetative material (for example
rye, wheat hemp) and combinations thereof.
[0039] As used herein, "impregnation" refers to the absorption of
an impregnating material by lignocellulosic material. For example,
impregnating wood chips can be accomplished by immersing or soaking
the wood chips in a sufficient volume of the impregnating material
(e.g. nitric acid solution) to saturate all or part of the wood
chips. Other methods such as spraying could also be used to
accomplish the impregnation.
[0040] The separation of lignin and cellulose in their natural
forms has proven difficult in practice. Many existing pulping
processes are inefficient, with valuable cellulose and lignin being
degraded, modified or destroyed during pulping, or less than
optimal separation between cellulose and lignin being achieved in
the pulping process. The difficulty in separating lignin and
cellulose arises from the inherent instability and fragile nature
of pure lignin. Lignins are formed by removal of water from sugars
to create aromatic structures and these reactions are irreversible.
There are many possible monomers of lignin, and the types and
proportions depend on the source in nature. Some typical monomers
of lignin are shown below: 1
[0041] The hydroxyl (--OH) groups, either the hydroxyl groups on
the chains or the hydroxyl groups on the aromatic rings, can react
with each other or with the aldehyde or ketone groups. A ether
linkage is formed when a hydroxyl group reacts with another
hydroxyl group, a hemiacetal is formed when a hydroxyl group reacts
with an aldehyde and a ketal is formed when a hydroxyl group reacts
with a ketone. An early stage in the condensation of various
monomers to form lignin is shown below, wherein there are several
groups shown that can react further: 2
[0042] Some monomers will simply extend the polymer while others
will establish cross-linking. The shaded monomer has three of its
functional groups linked to other monomers, so it is starting a
branch or cross-link. Lignin molecules are three-dimensional and
heavily cross-linked. A typical lignin molecule can have a
molecular weight of about 15,000 amu. Lignin molecules are
susceptible to harsh chemical concentrations and temperature
exposure. A lignin polymeric matrix may be broken down at
temperatures as low as 100.degree. C. Lignin is unstable, light
sensitive, and breaks down into acid compound as it ages, and its
presence in paper items contributes to their degradation over
time.
[0043] As mentioned above, the present invention utilizes the
acid-catalyzed hyrolysis of lignocellulosic material. The lignin
within lignocellulosic material includes ester functional groups.
FIG. 1 illustrates the mechanism of the acid-catalyzed hydrolysis
of an ester. The rate of ester hydrolysis is very slow because
their leaving groups are basic. The rate can be increased by the
presence of a catalyst, such as an acid. In such an acid catalyzed
reaction all organic reactants, intermediates, and products are
positively charged. The following steps take place within the
hydrolysis of the ester:
[0044] (A) An acid/base reaction is illustrated. Since there is
only a weak nucleophile and a poor electrophile the ester needs to
be activated. The acid protonates the ester carbonyl, making it
more electrophilic.
[0045] (B) The oxygen atom in the water molecule functions as the
nucleophile, attacking the electrophilic carbon atom in the C=0.
Electrons move towards the oxonium ion, thereby creating a
tetrahedral intermediate.
[0046] (C) Another acid/base reaction is illustrated. The oxygen
atom in the water molecule is deprotonated to neutralize the
charge.
[0047] (D) Another acid/base reaction is illustrated. The
--OCH.sub.3 must leave, but first must be converted into a more
stable group by being protonated.
[0048] (E) The electrons from the adjacent oxygen atom assist the
formation of the leaving group, a neutral methanol molecule
CH.sub.3OH.
[0049] (F) Another acid/base reaction. The oxonium ion is
deprotonated, which exposes the carbonyl C.dbd.O carboxylic acid
product and regenerates the acid catalyst.
[0050] The carboxylic acid product obtained in (F) is a
de-polymerized form of lignin.
[0051] Because water (H.sub.2O) and methanol (CH.sub.3OH) have
approximately the same basicity, it will be equally easy for the
tetrahedral intermediates to collapse to reform the ester as it
would be for the formation of the carboxylic acid. Hence, at
equilibrium both the ester and the carboxylic acid will be
obtained. However, an excess of water will shift the equilibrium
towards the formation of the carboxylic acid product,
de-polymerized lignin. Thus, methods of the present invention will
render higher yields with the addition of water and, for this
reason, it is possible to use lignocellulosic material which has a
relatively high moisture content (e.g. "green" wood chips) or which
has been previously contacted with water in order to increase the
moisture content prior to impregnating the lignocellulosic material
with the acid.
[0052] The ability to use lignocellulosic materials with a
relatively high moisture content in the present invention is an
advantage compared to existing Kraft type pulping processes. In
many existing processes it is necessary to dry wood chips prior to
pulping in order to remove or reduce moisture to meet stringent
requirements. In fact, chips are preferred in many existing
processes on a BDU (bone dry unit) basis. Requiring a drying step
adds expense to such processes. The present invention overcomes
this disadvantage since "wet" starting materials can be used, thus
removing the need to dry the wood chips or other starting
materials. Moreover, as explained above in relation to FIG. 1,
since excess water drives the depolymerization reaction to favor
the formation of a carboxylic acid instead of collapsing to reform
an ester, additional moisture in the chips improves yields in
methods according to this invention.
[0053] Following depolymerization of lignin, the lignocellulosic
material is digested in the presence of an alkali such as sodium
hydroxide (NaOH) and/or potassium hydroxide (KOH). The positive
cations (K.sup.+ or Na.sup.+ or another suitable ion) then attacks
the exposed --OH on the lignin product (carboxylic acid) bumping
out the hydrogen cation (H.sup.+). The hydrogen cation then bonds
with the negatively charged hydroxyl ion to form water. The lignin
is then water-soluble and drops into solution, thereby separating
the dissolved lignin from the cellulose-containing pulp. Due to
more modest temperature, pressure and reactant concentrations
involved, the lignin and cellulose produced are subject to less
degradation than products obtained through a Kraft type process,
thereby providing increased yield and purity.
[0054] FIG. 2 is a schematic illustration of a method 10 according
to the present invention. Lignocellulosic material, such as wood
chips (not shown), is used as the starting raw material in an
impregnating tank 12. The wood chips are prepared by chipping wood
into a convenient size, for example, chips of roughly 0.5-2.0
inches in diameter. This can be done with a standard mill. Other
sizes and shapes of lignocellulosic material can be used. Given the
hydrolysis involved in the method, preferably "green" wood chips
are used, or in other words, chips having a starting moisture
content of approximately 30-55%, or even higher, by weight of
water. A pre-treatment stage could be utilized wherein the wood
chips were immersed or otherwise contacted with water prior to
being placed in the impregnating tank in order to increase the
moisture content within the chips up to or beyond 30-55% by weight
of water, though this step is not necessary to produce lignin, pulp
and other ancillary products according to this invention.
Impregnating bone dry chips with an acid solution could be done,
but this will result in higher consumption rates of the acid
impregnant, thereby increasing costs.
[0055] Wood chips are loaded into impregnating tank 12 where they
are contacted with an acid in order to impregnate the wood chips
with the acid. The acid is preferably provided within an aqueous
solution, and in such a case, the acid impregnates the wood chips
as part of the solution. That is, the wood chips absorb both acid
and water. According to one aspect of the invention, the aqueous
solution comprises from about 10% to about 40% by weight of the
acid. The invention also includes utilizing other, narrower acid
concentration ranges within this range. For example, the aqueous
acid solution may comprise from about 15% to about 30%, from about
10% to about 25%, from about 10% to about 15%, or from about 20% to
about 25%, by weight of the acid.
[0056] The aqueous acid solution is provided to impregnating tank
12 from acid solution tank 14 in a sufficient volume to immerse the
wood chips. The acid used is preferably nitric acid, but those
skilled in the art will appreciate that other acids could instead
be used within the scope of the present invention. Nitric acid is
used herein for illustration purposes.
[0057] The wood chips are impregnated by allowing them to soak in
the aqueous nitric acid solution in the impregnating tank 12. The
impregnation time may last for as little as 30 minutes or less and
may extend for any length of time before the degradation of the
wood chips begins to occur. For example, impregnation may last from
about 2 hours and 45 minutes to about 48 hours. It is contemplated
that the impregnation time may fall within a narrower time ranges
such as, for example, from about 12 to about 24 hours and ranges
within this range.
[0058] Heating the chips during the impregnation step may decrease
the impregnation time. For example, impregnation of the wood chips
could occur from about 30 minutes or less to about 13 hours with
the addition of heat, or up to 48 hours or longer, without the
addition of heat. Heating during impregnation can be achieved by
any number of well known techniques that could be used to heat the
wood chips within impregnating tank 12. For example, the exterior
of impregnating tank could be heated with steam to heat the tank
12. After sufficient time has passed to complete impregnation, any
excess nitric acid solution is removed from the wood chips, by
drainage for example, and passed to spent solution tank 16. Any
collected excess nitric acid solution may then be recycled for use
in the impregnating tank 12. Preferably any excess nitric acid
solution is filtered and purified using conventional techniques
before being reconstituted back up to strength. Filtration and/or
purification could be accomplished by passing any spent nitric acid
solution from spent acid solution tank 16 to a conventional
filtration and/or purification apparatus (not shown) before the
solution is passed to acid solution tank 14. However, it may not be
necessary to filter and purify the excess nitric acid solution if
there are not significant amounts of contaminants present.
[0059] The impregnated wood chips are then passed to a heating tank
18. This transfer can be accomplished by gravity feed, if
impregnating tank 12 is located above the heating tank 18, or by a
suitable transfer mechanism if not. The impregnated chips are
heated within heating tank 18. The exterior of heating tank 18 is
heated with steam produced by steam boiler 20, thereby heating the
chips within heating tank 18. Many other known apparatus or
techniques could alternatively be used to heat heating tank 18.
Heating of the chips in heating tank 18 may be carried out in two
heating stages; the first heating stage to depolymerize the lignin
within the wood chips without substantially degrading the cellulose
or lignin, and the second heating stage to distill off the acid
following depolymerization of the lignin. The distilled off nitric
acid is recycled for use in the impregnation stage. In particular,
the distilled off nitric acid is collected and passed to a
condenser 22 and then to spent solution tank 16 to be reconstituted
to a desired strength and then passed back to nitric acid tank 14
for eventual use in impregnating tank 12.
[0060] The nitric acid attacks the cross-links or ester groups of
the lignin polymeric matrix during the first heating stage. The
first heating stage is carried out for a period of time which is
sufficient to depolymerize the lignin within the wood chips without
substantially degrading the cellulose or lignin. When nitric acid
is used as the impregnating acid, preferably the first heating
stage is carried out at a temperature up to about 75.degree. C.,
but above the freezing point of the aqueous acid solution It is
within the scope of the invention for the first heating stage to be
carried out within a narrower temperature range, for example from
about 50.degree. C. to about 75.degree. C., from about 50.degree.
C. to about 60.degree. C., from about 70.degree. C. to about
75.degree. C., or from about 60.degree. C. to about 70.degree. C.,
when nitric acid is used as the impregnating acid. It is desirable
to maintain these more modest temperatures (relative to Kraft
pulping temperatures) in order to prevent the unwanted degradation
and destruction of the lignin and cellulose within the wood chips,
thus providing improved product yield and purity.
[0061] Preferably heating tank 18 is fitted with an agitator 24 to
provide agitation during the heating of chips within heating tank
18. Aggressive agitation by agitator 24 helps to both bring about a
homogeneous temperature within heating tank 18 and to begin
breaking apart the fibers in the wood chips.
[0062] During the second heating stage, the temperature is then
brought above the boiling point of the acid for a sufficient time
(for example, 30 minutes) to distill off the nitric acid. It is
important to note that the temperature of the boiling point of the
acid in this stage may depart slightly from the temperature that
one would expect the acid to boil. This discrepancy results from
various factors including the interaction of the acid with the
wood, the moisture content within the wood, and, for nitric acid,
the formation of azeotropes with water. For example, whereas some
scientific literature establishes the boiling point of nitric acid
to be 83.degree. C., the inventor has observed boiling for nitric
acid with wood chips as low as 73.degree. C. Thus, it is preferred
for the temperature of the second heating stage within heating tank
18 be at least about 73.degree. C. where nitric acid is utilized.
Similarly, it is also preferred to maintain the temperature during
the second heating stage below the boiling point of water,
100.degree. C., to avoid the accidental distilling off of wood chip
moisture, which would initiate the burning of the wood chips. Thus,
according to one aspect of the invention, the second heating stage
could be carried out from about 73.degree. C. to below 100.degree.
C. Again, it is within the scope of the invention to utilize
narrower ranges, for example from about 90.degree. C. to about
95.degree. C., within this larger range.
[0063] The amount of time for the first and second heating stages
will vary depending upon a number of factors, including the
particular temperatures involved. For example, the first heating
stage may be carried out for as little as 15 minutes, and could be
carried out for a longer period of time.
[0064] Following the heating in the heating tank 18, the wood chips
are then transferred to a digester 26. There the wood chips are
digested by being contacted with a caustic solution, or in other
words, an aqueous alkaline solution wherein the solute is an alkali
such as sodium hydroxide, potassium hydroxide, or a combination of
these.
[0065] Those skilled in the art will appreciate that other bases
having similar chemical properties as these bases could be used.
The wood chips are contacted with the aqueous alkaline solution
under heat to solubilize the lignin in the solution. The resultant
solution is darker in color due to the solubilized lignin and is
thus referred to as black liquor. This digestion step thereby
separates the lignin and the black liquor from the pulp. The
alkaline solution is provided to digester 26 from caustic solution
tank 28. The concentration of the alkaline solution is such to just
make the lignin water soluble. This is accomplished by the positive
cations (K.sup.+ or Na.sup.+ or another suitable ion) attaching to
the exposed cleavage sites on the lignin monomers and the lignin
then dissolving into the water.
[0066] The temperature during the digestion stage is preferably up
to about 75.degree. C., but above the freezing point of the aqueous
alkaline solution. It is however within the scope of the invention
to increase the temperature above 75.degree. C. up to the boiling
point of the alkaline solution. For example, where the alkaline
solution comprises an aqueous sodium hydroxide solution, it is
possible for the temperature during the digestion step to be from
about 75.degree. C. to about 95.degree. C. It is also within the
scope of the invention for the digestion stage to be carried out
within a narrower temperature range, for example, from about
5.degree. C. to about 75.degree. C., from about 50.degree. C. to
about 75.degree. C., from about 20.degree. C. to about 50.degree.
C., from about 30.degree. C. to about 40.degree. C., or from about
40.degree. C. to about 50.degree. C. Heat is provided to digester
26 by heater 30, which may be any conventional apparatus or
arrangement for providing heat to the digester 26.
[0067] Digester 26 may be fitted with an agitator 32 to provide
agitation during the digestion of the chips. Aagitation helps to
bring about a homogeneous temperature within the digester 26 and to
break up the wood chips. Agitation also increases the rate that the
cation (e.g. K.sup.+, Na.sup.+, etc.) binds to the exposed lignin
cleavage sites thus minimizing any possible caustic action on the
cellulose.
[0068] For cost and yield reasons, it is desirable to use as little
alkali as possible within the caustic solution in the digestion of
the wood chips. In one aspect of the invention, the alkaline
solution may include an amount of alkali which is at least the
normal equivalent of the nitric acid in the solution used in the
impregnation stage. Adding excess alkali will increase costs by
requiring more acid to be added at a later step. Further, the
alkali concentration in the aqueous alkaline solution should be
maintained below 17.5% by weight of the solution since beta and
gamma cellulose dissolve at this level.
[0069] Following digestion, the resultant pulp (containing
cellulose) and black liquor (containing solubilized lignin) is
transferred to pulp tank 34. The black liquor is removed, for
example by drainage, and transferred to black liquor tank 36. The
cellulose-containing pulp is then processed according to the user's
needs.
[0070] Typically, processing would involve washing, pressing,
bleaching and drying the pulp following the separation from the
black liquor. For example, in FIG. 2, the pulp would be pressed in
a pulp press 38 to remove any residual black liquor (which would
also be transferred to black liquor tank 36), and then bleached and
dried with bleaching apparatus 40 and dryer 42. Bleaching apparatus
40 and dryer 42 could be any conventional known apparatuses for
these purposes.
[0071] The lignin is precipitated out of the black liquor in black
liquor tank 36 after the addition of an acid from acid solution
tank 44. Preferably the acid added is a mineral acid, such as
sulfuric acid. The acid added to the black liquor strips off the
cation from the caustic solute, thereby precipitating out the
lignin and a caustic salt (e.g. where sulfuric acid is used, the
salts Na.sub.2SO.sub.4, K.sub.2SO.sub.4, or other similar salts
depending upon the alkali solute used in digestion, would result).
Addition of the acid to the black liquor lowers the pH of the
liquor until it is just slightly acidic. The acid may be added to
the black liquor within an aqueous solution. For cost and yield
reasons, it is advantageous if the acid solution is as dilute a
solution as possible. Black liquor tank 36 may be fitted with an
agitator 46 to provide agitation during the addition of the acid
from tank 44. The contents of black liquor tank 36 may be cooled,
for example by cooler 48, in order to faciliate the precipitation
of lignin. Cooler 48 can be any known apparatus for cooling
purposes. The black liquor may be cooled from the digestion
temperature before the addition of the acid. Preferably the
temperature is up to about 75.degree. C., but above the freezing
point of the black liquor. Other narrower temperature ranges could
be used at this step such as from about 5.degree. C. to about
75.degree. C., 5.degree. C. to about 75.degree. C., from about
50.degree. C. to about 75.degree. C., or from about 25.degree. C.
to about 50.degree. C.
[0072] The precipitated lignin can then be removed from the
resultant amber liquor (which is lighter in color than the black
liquor due to the removal of the lignin) and processed using any
conventional technique. For example, the resultant amber liquor and
lignin slurry may be passed from black liquor tank 36 through a
lignin filter 50, with the lignin then being transferred to a
lignin dryer 52 and dried at temperatures that will not degrade the
lignin. The resultant lignin contains natural form lignin.
[0073] The filtrate passing through lignin filter 50 is the amber
liquor, which is then transferred to an amber liquor tank 54. Any
residual amber liquor removed during the drying of lignin in lignin
dryer 52 is also transferred to amber liquor tank 54, as
illustrated in FIG. 2. Amber liquor is an aqueous solution
containing starches, sugars, and other minerals and compounds found
in plants that are not cellulose or lignin. In amber liquor tank
54, the amber liquor can be processed to yield various other
products of value including alcohols, unicellular proteins or both.
For example, the amber liquor could be used as a starting culture
for various bacteria to produce various products, such as animal
feeds and alcohols. Amber liquor tank 54 may be fitted with an
agitator 56 to provide agitation if desired. Any proteins produced
during the processing of the amber liquor may be removed by
filtration (via protein filter 58) and dried (via protein dryer
60). Suitable apparatus modifications and/or additions could be
made to accommodate alcohol processing and/or the production of
other desired end products. The final filtrate can be recovered and
treated in water treater 62 to remove any contaminants, thereby
allowing the recycling of water within various steps in the method,
for example in reuse in washing in pulp tank 34, preparation of the
alkaline solution in caustic solution tank 28 and in steam boiler
20. Any recovered water could also be used in steam boiler 20 or in
the preparation of the aqueous acid solutions in acid solution tank
14 and/or acid solution tank 44.
[0074] Without limitation, the present invention provides the
following benefits:
[0075] 1) Wet starting materials can be used. Since water is
essential to the hydrolysis of the lignocellulosic material, this
removes the need to dry the starting materials prior to the pulping
process, thus resulting in lower costs relative to certain existing
pulping methods.
[0076] 2) The method of the present invention does not require
added pressure, but may be carried out at atmospheric pressure.
This reduces costs relative to certain existing pulping methods, as
does the fact that the method of the present invention does not
require the addition of heat at temperatures approaching those used
in typical Kraft type pulping processes.
[0077] 3) Weak concentrations of strong acids and strong bases may
be used, thereby minimizing raw material costs and degradation of
final products.
[0078] 4) The acid catalyst can be recovered and recycled for
reuse, allowing improved cost efficiency. These factors also allow
a closed system, minimize pollution, and therefore environmental
impact. Very little pollution is caused by methods according to
this invention.
[0079] 5) Only small amounts of chemicals are needed to bring back
to strength each recovered chemical before being reused in the
method. Moreover, unlike recovery stages in certain Kraft type
processes, external energy (and the resultant expense) is not
required during the recovery of chemicals in methods according to
the present invention.
[0080] 6) Water used in the method may be recovered in saleable
byproducts (such as alcohols and animal feeds) treated and reused
or vented as steam, with suitable apparatus and process
modifications. The vented steam could be used in providing energy
for the method, thereby eliminating even this small loss of
water.
[0081] 7) The method is flexible in terms of starting materials.
The ability to process a wide variety of lignocellulosic material
without retooling any apparatus or changing the methods involved
gives flexibility in pulp production. Currently, mills are
typically designed to produce specific pulp types and utilize
specific wood species as raw materials. Furthermore, most Kraft
mills require chips meeting stringent quality specifications to
remain economically viable. The present method may utilize not only
any number of differently sized chips, but also sawdust, and also
chips that would be considered "green" and unusable by current pulp
mill standards. The flexibility provided by methods of this
invention eliminates or simplifies the need for chip mills designed
to produce chips of stringent standards, thereby offering the
potential for lower operating and capital costs.
[0082] 8) The yield of alpha cellulose is high using the method,
while the method also allows for a high yield of lignin, which is a
valuable component itself.
[0083] 9) Aside from cellulose and lignin, other useful and
potentially valuable by-products may be recovered using methods of
this invention. For example, the amber liquor is suitable for
fermentation of unicellular protein following precipitation and
removal of lignin. The protein can be used in animal feeds or for
research purposes.
[0084] 10) Methods of the invention may be applied to different
sized and configured apparatuses, thereby improving the flexibility
of use.
[0085] 11) Mills utilizing a method of the present invention will
be highly efficient, with lower operating costs than mills using
typical Kraft pulping processes.
[0086] The following examples are presented by way of illustration
and not by way of limitation.
EXAMPLE 1
[0087] This example comprises 19 trials which were run to
illustrate the yields of pulp and lignin obtained using methods
according to this invention, and also to illustrate the recovery of
the acid used in the impregnation stage.
[0088] The starting lignocellulosic material in Trials 1-18
comprised 200 grams of a mixture of hardwood and softwood wood
chips obtained from a sawmill and chip mill operation in Kelowna,
British Columbia, Canada. 600 grams of wood chips were used as the
starting material in Trial 19. All of the different species of
trees that the chips originated from were not ascertained, but at
least some of the chips came from Ponderosa Pine, Douglas Fir,
Maple, Oak and Spruce.
[0089] The moisture content of the wood chips was between 35-50%
water by weight.
[0090] The moisture content was calculated by drying separate
samples of the wood chips (which were not subjected to the steps of
the method) and measuring the weight difference in the wood chips
following the drying step.
[0091] The acid used in the impregnating step was nitric acid
(HNO.sub.3). Nitric acid solutions were prepared by diluting an
amount (chosen depending upon the strength of solution desired) of
70% (w/w) nitric acid with distilled water. In each trial, 1 L of
an aqueous nitric acid solution was used in the impregnating step,
with the exception that Trials 17 and 18 each used 750 mL, and
Trial 19 used 2250 mL. A 2500 mL beaker sealed with laboratory film
was used as the impregnating tank.
[0092] Following the impregnation step, excess nitric acid solution
was removed from the wood chips before they were heated in the
heating step. The heating step was carried out in a 2000 ml round
bottom triple neck boiling flask, which was fitted with a
distillation setup so that nitric acid distilled out in the heating
step would be collected.
[0093] In order to calculate the efficiency of the method in
recovering the nitric acid, samples from the excess acid solution
collected following the impregnating step and from the distilled
nitric acid which was collected during the heating step were each
titrated using a 10% (w/w) % solution that was prepared by
dissolving 100 g NaOH in distilled water and topped up in a 1 L
volumetric flask. Titration of the excess nitric acid solution
collected from after the impregnation step permitted the
calculation of the number of moles of nitric acid recovered after
the impregnating step, which in turn permitted the calculation of
the number of moles of nitric acid which were absorbed by the wood
chips, since the moles of nitric acid in the initial 1 L solution
was known. After titrating the nitric acid that was distilled out
and collected from the heating step, it was possible to calculate
the total moles of nitric acid recovered in the method. By
comparing this number to the amount of moles in a particular
starting nitric acid solution allowed the calculation of the
percentage of starting nitric acid which was recovered by the
method. Also, it was found that red fuming nitric acid (RFNA) and
nitrogen dioxide (NO.sub.2) formed during the heating step and
these vapors were lost to the atmosphere. These vapors could have
been retained with better equipment, and if that were the case, the
amount of nitric acid actually recovered would have greatly
increased. To account for this, the amount of mass lost from the
heating tank was determined by weighing the tank before and after
the heating stage, with the difference which was lost as RFNA and
NO.sub.2. On the assumption that this amount could be recoverable
as HNO.sub.3 with appropriate equipment modifications, it is then
possible to calculate what the total moles of HNO.sub.3 which would
be recoverable using the method by adding the amount actually
recovered with the amount lost as RFNA and NO.sub.2, and expressing
this as a percentage of the starting moles of HNO.sub.3. This is
shown in Tables 1-5 for Trials 1-19. All titrations used 2 drops
phenolphthalein as an indicator.
[0094] Following the heating step, the chips were digested in a
round stainless steel tank with a 9.283 liter capacity (22.86 cm
diameter, 11.43 cm height). The alkaline solution used in digestion
was prepared by diluting an amount of 2.5 molar sodium hydroxide
(NaOH) solution, 2.5 molar potassium hydroxide (KOH) solution, or a
combination thereof, with distilled water. Various volumes of the
initial 2.5 molar solutions were added to make the final alkaline
solution used in digestion and various volumes of the final
alkaline solution prepared were used for the various trials. The
moles of each alkali solute(s) and the total volume of the aqueous
alkaline solution prepared for Trials 1-19 are found in Tables
1-5.
[0095] In each of Trials 1-19 it took about 10 minutes or less to
digest approximately 95% or more of the wood chips. In certain
trials, a portion of the chips remained undigested, and the amounts
are indicated in Tables 1-5 for such trials, as are the total
digestion time for each trial.
[0096] Following the digestion step, the black liquor (containing
solubilized lignin) was vaccum filtered in a buchner funnel without
filter paper to remove the pulp, which was washed with distilled
water and then air-dried at room temperature and weighed. The black
liquor was transferred to a 2500 mL beaker, where the lignin was
precipitated with the addition of a sulfuric acid (H.sub.2SO.sub.4)
solution. The sulfuric acid solution was prepared by diluting an
volume (depending upon the concentration sought) of pure sulfuric
acid in a 1 L volumetric flask with distilled water. In most cases,
the temperature of the black liquor was allowed to cool from the
digestion step for the lignin precipitation step. The various
temperatures are shown in Tables 1-5.
[0097] The lignin was vacuum filtered in a buchner funnel without
filter paper and thus removed from the amber liquor. The lignin was
air-dried at room temperature and weighed. The lignin was not
however dried and weighed for Trials 17 and 18. In these trials,
the lignin did precipitate and was, by visual inspection, found to
have a similar relative amount and appearance as the lignin
produced in Trials 1-16.
[0098] As set out below, heat was applied during the heating and
digestion steps and in five trials, during the impregnation step.
Where so indicated, the heat was supplied in various steps by using
a heating mantle made by Glas-Col Apparatus Co., Cat 10 0410,
having 465 total watts and 110 volts. The mantle controller was a
Powerstat variable autotransformer, Type 3PN116B, made by The
Superior Electric Co. of Bristol, Conn. Agitation was provided
during the digestion step by a cold steel impeller which was 8
inches in diameter with {fraction (1/2)} inch separation between
45.degree. offset blades. A 18 volt DEWALT.TM. XRP cordless drill
was used to power the agitator. Agitation was provided during the
lignin precipitation step by manual stirring using a glass stir rod
for approximately 30-45 seconds in each trial.
[0099] The conditions and results of Trials 1-19 are found in
Tables 1-5.
1TABLE 1 Trial 1 Trial 2 Trial 3 Trial 4 IMPREGNATION STEP %
HNO.sub.3 (w/w) 15 24.15 25 30 Impregnating Time (hours) 24 48 48
13 HEATING STEP Time to reach 50.degree. C. (min.) 55 41 40 30 Time
between 50-75.degree. C. (min.) 76 91 30 75 Time above 75.degree.
C. (min.) 59 19 35 32 Minimum HNO.sub.3 Distillation Temp.
(.degree. C.) 85 81 81 73 Distillation time (min.) 59 19 35 32
DIGESTION STEP Alkaline Solution moles NaOH 0.425 0.625 0 0.435
moles KOH 0 0.125 0.625 0.19 Vol. of Alkaline Solution (mL) 2670
3300 3000 3000 Digestion Temp. (.degree. C.) 60 63 50 70 Digestion
Time (min.) 30 50 40 25 Amount of Undigested Chips (g) 0 0 5.4 0
LIGNIN PRECIPITATION STEP % H.sub.2SO.sub.4 Solution added to black
liquor 20 10 10 20 (v/v) Vol. of Acid Sol'n added to black liquor
35 140 140 75 (mL) Temp. during Addition of Acid (.degree. C.) 25
45 45 25 RESULTS Pulp recovered (g) 92.1 104.6 99.6 94.5 Pulp
recovered (%) 46.05 52.3 49.8 47.25 Lignin recovered (g) 41.4 42.1
39.8 43.1 Lignin recovered (%) 20.7 21.05 19.9 21.55 HNO.sub.3
recoverable (%) 98.44 93.2 91.7 77.7
[0100]
2TABLE 2 Trial 5 Trial 6 Trial 7 Trial 8 IMPREGNATION STEP %
HNO.sub.3 (w/w) 15 15 11.5 15 Impregnating Time (hours) 24 21 48
13, at 75-80.degree. C. HEATING STEP Time to reach 50.degree. C.
(min.) 15 19 31 35 Time between 50-75.degree. C. (min.) 20 90 65 25
Time above 75.degree. C. (min.) 45 12 15 20 Minimum HNO.sub.3
Distillation Temp. 83 81 83 83 (.degree. C.) Distillation time
(min.) 45 12 15 20 DIGESTION STEP Alkaline Solution moles NaOH 0.55
0 0.25 0.3125 moles KOH 0 0.625 0.3 0.3 Vol. of Alkaline Solution
(mL) 3220 3250 3220 3245 Digestion Temp. (.degree. C.) 50 50 60 70
Digestion Time (min.) 30 60 15 30 Amount of Undigested Chips (g) 7
8.6 14.2 7 LIGNIN PRECIPITATION STEP % H.sub.2SO.sub.4 Solution
added to black 10 10 10 10 liquor (v/v) Vol. of Acid Sol'n added to
black 90 132 150 125 liquor (mL) Temp. during Addition of Acid
(.degree. C.) 40 32 34 61 RESULTS Pulp recovered (g) 92.2 90.1 88.6
101.4 Pulp recovered (%) 46.1 45.1 44.3 50.7 Lignin recovered (g)
37.9 39.1 38.3 42.2 Lignin recovered (%) 19 19.6 19.2 21.1
HNO.sub.3 recoverable (%) 93.4 92.1 93.4 88.1
[0101]
3TABLE 3 Trial 9 Trial 10 Trial 11 Trial 12 IMPREGNATION STEP %
HNO.sub.3 (w/w) 15 20 15 24.15 Impregnating Time (hours) 17 at 2:45
at 31 30 75-80.degree. C. 75.degree. C. HEATING STEP Time to reach
50.degree. C. (min.) 35 19 37 41 Time between 50-75.degree. C.
(min.) 24 85 32 105 Time above 75.degree. C. (min.) 22 12 114 30
Minimum HNO.sub.3 Distillation Temp. (.degree. C.) 83 75 80 83
Distillation time (min.) 22 12 20 30 DIGESTION STEP Alkaline
Solution moles NaOH 0.625 0.625 0 0.3125 moles KOH 0 0 0.625 0.3125
Vol. of Alkaline Solution (mL) 3000 3000 2500 2250 Digestion Temp.
(.degree. C.) 50-63 70 55 60 Digestion Time (min.) 30 15 15 20
Amount of Undigested Chips (g) 0 20.3 24.4 0 LIGNIN PRECIPITATION
STEP % H.sub.2SO.sub.4 Solution added to black 10 10 20 10 liquor
(v/v) Vol. of Acid Sol'n added (mL) 150 150 65 125 Temp. during
Addition of Acid (.degree. C.) 29 38 27 31 RESULTS Pulp recovered
(g) 85.6 87.1 77.1 103.5 Pulp recovered (%) 42.8 43.6 38.6 51.75
Lignin recovered (g) 38.9 43.9 33.9 44.1 Lignin recovered (%) 19.5
22 17 22.05 HNO.sub.3 recoverable (%) 93.4 92.61 93.8 65.05
[0102]
4TABLE 4 Trial 13 Trial 14 Trial 15 Trial 16 IMPREGNATION STEP %
HNO.sub.3 (w/w) 20 30 15 15 Impregnating Time (hours) 13 15 12 14
HEATING STEP Time to reach 50.degree. C. (min.) 19 23 14 6 Time
between 50-75.degree. C. (min.) 95 55 48 57 Time above 75.degree.
C. (min.) 15 20 35 18 Minimum HNO.sub.3 Distillation Temp.
(.degree. C.) 81 80 81 79 Distillation time (min.) 15 20 45 25
DIGESTION STEP Alkaline Solution moles NaOH 0.625 0 0.5 0.5 moles
KOH 0 0.625 0 0 Vol. of Alkaline Solution (mL) 2500 2750 2700 2700
Digestion Temp. (.degree. C.) 55 61 73 41 Digestion Time (min.) 20
25 10 13 Amount of Undigested Chips (g) 4.9 11.6 28.7 102.3 LIGNIN
PRECIPITATION STEP % H.sub.2SO.sub.4 Solution added to black liquor
(v/v) 10 10 20 20 Vol. of Acid Sol'n added to black liquor 150 150
110 130 (mL) Temp. during Addition of Acid (.degree. C.) 26 36 18 5
RESULTS Pulp recovered (g) 96.4 87 71.2 35.2 Pulp recovered (%)
48.2 43.5 35.6 17.6 Lignin recovered (g) 47 38.6 27.2 17.8 Lignin
recovered (%) 23.5 19.3 13.6 8.9 HNO.sub.3 recoverable (%) 91.1
90.6 83.7 84.1
[0103]
5TABLE 5 Trial 17 Trial 18 Trial 19 IMPREGNATION STEP % HNO.sub.3
(w/w) 15 15 15 Impregnating Time (hours) 1:25 at 50-55.degree. C.
2:10 at 50-55.degree. C. 12 HEATING STEP Time to reach 50.degree.
C. (min.) 12 10 20 Time between 50-75.degree. C. (min.) 60 (held
between 55 (held between 35 70-75.degree. C.) 60-70.degree. C.)
Time above 75.degree. C. (min.) 11 22 27 Minimum HNO.sub.3
Distillation 83 82 83 Temp. (.degree. C.) Distillation time (min.)
11 15 32 DIGESTION STEP Alkaline Solution moles NaOH 0.5 0.475
1.2625 moles KOH 0 0 0 Vol. of Alkaline Solution 2500 2690 3050
(mL) Digestion Temp. (.degree. C.) 70 63 83 Digestion Time (min.)
10 3 10 Amount of Undigested Chips 31.6 37.8 166 (g) LIGNIN
PRECIPITATION STEP % H.sub.2SO.sub.4 Solution added to black 20 20
20 liquor (v/v) Vol. of Acid Sol'n added to 130 115 300 black
liquor (mL) Temp. during Addition of Acid (.degree. C.) 7 10 32
RESULTS Pulp recovered (g) 71 67.6 169.3 Pulp recovered (%) 35.5
33.8 28.2 Lignin recovered (g) Not Weighed Not Weighed 72.5 Lignin
recovered (%) Not Weighed Not Weighed 12.1 HNO.sub.3 recoverable
(%) 80.2 83.2 78.8
EXAMPLE 2
[0104] This example comprised a trial that was similar to Trials
1-19 in example 1 except that the starting lignocellulosic material
was 150 grams of shavings and sawdust from Hemlock, Oak and Pine
tree species. Impregnation was done for 30 minutes in 15% (w/w)
HNO.sub.3 solution under heat of 50.degree. C. The impregnated
shavings and sawdust were heated for 60 minutes between
50-75.degree. C., and then for 10 minutes over 80.degree. C.
Following the heating step, the shavings and sawdust were contacted
with an alkaline solution having 1.25 moles of NaOH and 2600 mL
total volume at a temperature of 52.degree. C. for 7 minutes.
Following pulp removal, lignin was precipitated with the addition
of 20% (v/v) H.sub.2SO.sub.4 The pulp and lignin were not weighed,
but a visual inspection showed satisfactory appearance and yield
for each, proving that the method of this invention can be carried
out with relatively small pieces of lignocellulosic material such
as shavings and sawdust.
EXAMPLE 3
[0105] The pulp samples produced in Trials 5, 7 and 2 from example
1 were subsequently analyzed to determine the relative amounts of
alpha cellulose, beta cellulose and gamma cellulose therein. The
results are found in Table 6 below.
6 TABLE 6 Trial 5 Trial 7 Trial 2 Alpha Cellulose (%) 81.3 79.6
79.2 Beta Cellulose (%) 4.4 3.7 7.5 Gamma Cellulose (%) 14.3 16.7
13.3
[0106] In addition to the methods herein, the present invention
also includes pulp produced according to the methods herein, as
well as paper products comprising cellulose from pulp produced
according to the methods herein. Similarly, the present invention
also includes lignin produced according to the methods herein, as
well as numerous other products and compositions comprising lignin
produced according to the methods herein, including for example,
fertilizers, asphalt emulsifiers, soil stabilizers, wood products
(such as plywood and particle board), oil well drilling fluids and
dispersing agents for preparing concrete.
[0107] As will be apparent to those skilled in the art in the light
of the foregoing disclosure, many alterations and modifications are
possible in the practice of this invention without departing from
the scope thereof. Accordingly, the scope of the invention is to be
construed in accordance with the substance defined by the following
claims.
* * * * *