Note: Descriptions are shown in the official language in which they were submitted.
7:~LVS
This invention relates to lignin treatment, and moxe
particularly to a process whereby lignin, a naturally
occurring polymeric material, may be modified and reduced in
molecular weight and thereby rendered into a form more suited
to further utilization~ The invention also relates to
separation methods for separating such modified lignin from
other components of the initial woody material in which the
lignin naturally occurs.
The production of chemicals from wood feed stock is
currently substantially restricted either to the production
and isolation of special fractions such as extractives,
furfural etc., or to by-products of other operations conducted
on the wood, such as pulping by chemical means, which are
directed to wood fiber utilization. In the first type nf
processes, very inefficient use is made of the wood resource.
In the second type of processes, the process conditions and
yields are optimizPd with respect to fiber quality. The
lignin fraction, whi-ch comprises fully 20-25% of the raw
material, is degraded in such processes in a manner not
conducive to further use thereof. The major chemical pulping
processes employed today produce vast quantities of "lignins"
of which only a miniscule fraction finds uses other than fuel.
When one considers the enormous scale upon which wood pulping
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is conducted commercially, in the manufacture of wood pulp for
paper, textiles and the like, the amount of lignin which is
wasted, as an essentially useless by-product of such processes,
is enormous and represents a tremendous waste of a potentially
valuable natural resource.
There is an increasing interest in wood/woody materials
as a chemical feed st~ck, since they are a renewable resource.
When the primary object of the wood treatment is to obtain and
separate chemical raw materials therefrom, many of the
previously accepted and adopted considerations and constraints
of conventional pulping, directed towards fiber quality and
production, do not apply. The present invention is of interest
in connection with the use of wood/woody materials as a
chemical feed stock, and esp~cially in the obtaining
of useful lignin products therefro~.
Lignin is a potentially valuable source of chemical
raw materials. As a natural constituent of wood~ it is a
renewable natural resource. Whilst their full constitutio~
has not been clarified with certainty in all cases, lignins
are known to be complex non-polysaccharides, containing in
their structure various substituted phenol structural units. On
degradation, lignins of dlfferent types, from different sources,
yield different substituted phenols, of significant interest as
chemical raw materials. The difficulty, however, is to deuise
an economically satisfactory process for separating the lignins
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from other constituents of wood, without at the same time
degrading the lignins or otherwise chemically treating them to
convert them to non-useful chemical products.
The present invention is based upon our discovery
that the lignin of hardwood species ~such as aspe", birch, etc.
undergoes a depolymerization reaction at high temperature, but
that this reaction is closely followed by a second, condensation
reaction. Unless time and temperature conditions are closely
controlled, the second reaction masks and cannot effectively
be separated from the first reaction, with the result that a
condensed, cross-linked and possibly repolymerized lignin
residue of no practical value is obtained. According to the
process of our invention, however, we are able effectively to
depolymerize the lignin in situ in the hardwood starting
material, without permitting the condensation reaction to occur
to any significant extent. This is accomplished by a careful
selecti~n of cooking conditions. The result is a depolymeri.7ed
form of lignin which is very readily separated from the other
components of the wood, e.g. by simple so]vent extraction, and
which is highly useful as a chemical raw material.
Thus according to one aspect o~ the present invention,
there is provided a process for rendering ~he lignin content
of natural hardwood materials substantially soluble in solvents
such as dioxane-water mixtures so as to permit ready solvent
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extractions thereo to separate at least a substantial
proportion of said lignin content from the remainder of said
hardwood material, which comprises cooking said hardwo~d
material in divided form in the presence of steam at a pre~
selected temperature and for a preselected time chosen in
combination with one another within the temperature range 170-
240C, and within the time range of 2-80 minutes, 50 as to
produce a steam digested hardwood residue which, after
subsequent treatment with a 90/10 v/v mixture of dioxane/water
under reflux, contains not more than 10% lignin residue.
The process of the present invention depolymerizes
the lignin portion of the hardwood starting material in situ,
thereby rendering it readily soluble in common solvents such as
dioxane, alcohols, ketones, glycols, ethers, mi.xtures thereof
and with water, and dilute (0.1-10%) aqueous alkalis. As such,
it is readily extractable with such solvents, to separate it
from the woody residues, and recoverable as a useful chemical
product. The solubiliky of the lignin product in such .
. substantial amounts aemonstrates that~ by the process of the
invention, it has been depolymerized but that the subsequent
condensation reaction has not been allowed to proceed to any
significant extent.
The process of the present invPntion may be carried
out simply and effectively by heating hardwood chips, or other
7~i~35
similarly divided forms of hardwood such as shavings, sawdust
etc., in ~ pressure ~essel by mea~s of direct exposure to
saturated steam. As is well known, in such processes the steam
pressures determine the cooking temperatures, and can be used
for monitoring cooking conditions. After an appropriate time,
preselected and chosen in combination with the selected il
temperature of cooking, the woody material is removed from the 'i
vessel~ By means of the process of the present invention, the ¦
lignin fraction thereof has been renaered soluble, and m~y in a
second stage be extracted under mild conditions as described
below. The process of the invention may be carried out batch- ,
wise, but it is most advantageous to carry out the process ~1
continuouc:ly in a suitably equipped vec;sel which permits li
continuous feeding of the material thereto and discharge of ¦~
material therefrom, without loss of steam pressure, for l;
economic reasons.
In practicing the present invention, the woody material
should initially contain its original moisture content, or
alternatively, it should be rewet prior to entering the process.
The precise amount of water is not critical, but it is
preferred that the material be fiber saturated with water at
the commencement o~ the process, 50 as substantially to avoid
overcooking of portions of the lignin therein.
The process of the present invention i~ preferably
carried out in the absence of other added chemical digestion
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reagents than water. This reduces the risk that side chemical
reactions of the liqnin content may occur which will reduce
the solubility of the final treated lignin. In addition, of
course, it is of economic advantage to avoid the use of added
chemical reagents, not only in terms of their cost of
consumption, but also in terms of the effluent by-products
which tend to be produced from chemical reagents, which cause
expenses of disposal.
Features of the process of the present invention are
graphically illustrated in the accompanying drawings in which:
FIGU~E 1 is a graph o~ temperature against time,
depicting the preferred cooking conditions for use in the
process of the present inVentiQn;
FIGURE 2 is a series of graphs of percenta~e lignin
remaining in a cooked wood sample after treatment by the
process of the invention, including solvent extraction thereof,
agai.nst time of cooking, at a selection of fixed temperatures.
The process of the present .invention is eminently
useful for a production both of lignin for use in subsequent
2~ chemical synthesis as a source of chemical raw materials, such
as vanilli.n, and also in the production of pulps from hardwoo~,
in a simple, economical manner. Thus, both the lignin residues
which are formed, and the cooked woody material, are usefult
commercial products. The woody product after cooking is effect-
ively a hardwood pulp, which is useful ~or adding in small
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amount~ to other cellulose pulps for the making of various
types of paper. The original hardwoods which can be used in
the process of the invention, including aspen, poplar,
eucalyptus, birch and the like, are very fast growing materials,
thereby enhancing the general economics o~ the process.
Fig. 1 of the accompanying drawings shows the extent
of combinations of temperatures and times for cooking, for use
in the process of the present invention, to obtain a readily
soluble lignin fraction. Within the overall area designated A,
substantially any combination of temperature and time can be
chosen for cooking, so as to yield a cooked woody product, the
lignin content of which is substantially extractable with 90/10
dioxane/water mixtures under reflux or a dilute aqueous alkali
at about 60C, to leave a woody material of less than about
- 10~ of lignin residue. Within the smaller area B designated
on Fig. 1, i.e. the shorter time range of area A, temperature
and time condition combinations are found which are preferred,
because of the lower energy requ,irements thereby leaaing to
enhanced economics of the process, and also reducing the risk
of over cooking the material so as to increase the non-soluble
portion of the lignin. Within this area, temperature and time
conditions are found which, when employed, lead to the
production of cooked hardwood residues which after the
aforesaid solvent extraction have less than about 7% lignin
residues.
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Fig. 2 depicts the extractability of the liqnin
depol~meriæation products resulting from treatment according
to the process of the present invention. As can be seen, the
exact time and temperature conditions are critical, and this `
relationship is a key element of the process according to the
invention. It is evident that, at each temperature, there is
a cooking time at which depolymerization (extractabi-ity) of l
the lignin is maximized. It is further evident that the ii
extent of desired reaction increases with temperature, but
there is a decrease in the difference between results of
successive increases of given temperature towards the upper end
of the range. This is in accordance with the believed
mechanism of the successive first order reactions.
The followinq table illustrates oPtimum time/
temperature values for aspen wood. In the table, L values
indicatP percent residual liqnin after extraction with a
solvent comprising 90~ dioxanP, and 10~ water by weight.
Temp C Time (opti~al) L
175 80 7.1
185 60 4.3
195 25 2.4
205 ~ 16 1.8
215 4 1.7
The invention and conditions for carrying the
process into practice are further described in the followinq
specific examples:
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EXAMPLE 1
30G (dry weight) of chips of aspen (Populus
tremuloides), apProximately 1/8 inch thick, 5/8 inch long and
1/4 to 3/4 inch wide, containing 50~ moisture were placed i~
an autoclave and heated to 205C. After 1~ minutes at this
temperature, the vessel was rapidly cooled and opened. The
chips were found to be completely softened and readily
disintegrated, e.g. between the fingers or by the action of a
small laboratory stirrer. The resulting wood pulp was
essentially free from lumps and undisintegrated wood.
Extraction with 0.5~ sodium hydroxide solution
solubilized approximately half of the sample. The residual
lignin in the solid, woody material was 4.7%, as compared to
approxlmately 20% in the starting material. Accordingly, ~8%
of the initial lignin had been rendered solublel i.e. de-
polymerized, without subsequent condensation by the process of
the example.
Example 2. The aspen woodpulp of example 1 was extracted in a
Soxhlet extractor with boiling 90% dioxane -10~ water, which
dissolved most of the lignin present. The woodpulp, after washing
and drying, was found to weigh 14.6g, for a yield o 49 percent
on the original wood. The fibers were light brown in colour, and
contained 3.7 percent lignin and 1.4 percent xylan. The starting
wood had contained 17.1 percent lignin and 14.9 percent xylan.
When the woodpulp of Example 1 was extracted with
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pure ioxane, or with dioxane diluted with water to 50% dioxane,
higher yields of woodpulp were obtained, 51.5 and 52~5 percent
respectively. These woodpulps contained higher lignin
concentrations, 5.8 and 5.0 percent respectively, but the same
xylan content, 1.4 percent.
Example 3. 30 g tOD) of chipped eucalyptus (Eucalypt~ls saligna~
containing 50~ moisture were placed in an autoclave and heated
to 200C. After 30 minutes the autoclave was rapidly cooled
and opened. The chips were found to be softened and readily
10 aisintegrated.
Extraction of the treated chips with boiling 90%
dioxane - 10% water solubilized 43% of the raw material and
the residue contained 10% lignin. Accordingly as the raw
material had a lignin content of 24~, 76% of the lignin was
solublized by our process.
Example 4. Aspen chips at 50~ moisture were fed at the rate
of 280 pounds per hour into a horizontal continuous digestor
equipped with an internal screw conveyor. Steam at 265 psig
~as admitted near the entrance to the digestor to raise the
temperature of the chips to 208C. The screw conveyor rate
was adjusted to give a residence time in the digestor of 10
minutes. The cooled chips were disintegrated after release
from the digestor~ The resulting woodpulp was filtered and
washed on conventional equipment. The yield of fiber was
calculated to be 70 percent based on the starting wood. It
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was brown in color~ The woodpulp was then extracted with 0.2~
hot sodium hydroxide solution in an open vessel. The resulting
woodpulp was 1ight brown in color. The yield based on wood
was 45% and the lignin content thereof was about 4.5%, as
compared to a starting lignin content in the wood of about 20%.
Example 5. Chips of fast-growing poplar Clone No. 1 45/51
were analyzed and found to have a lignin content of 21.5%.
This poplar is notable in that in the cold climate of Canada
it grows rapidly, the measured rate of wood production being
11 metric tons per hectare each year. Upon heating rapidly to
200C, and maintaining at that temperature for 30 minutes,
brown woodpulp was obtained in 73~ yield. After extraction
with 0.5% hot sodium hydroxide solution for 90 minutes in an
open vessel, the yield was reduced to 47%. The light brown
woodpulp had a lignin content of 13.9%.
It should be noted that in the example cited above,
a substantially greater percentage of the wood than that
accounted for by the lignin is in fact solublized. This is a
consequence of the degradation of the pentasans and other
components comprising the hemicelluloses. These reactions are
well known and form the basis for certain established
silvichemical production processes
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