Note: Descriptions are shown in the official language in which they were submitted.
1284637
CROSS REFERENCE TO REI,ATED APPLICATION
This application is related to Canadian Serial
No. 44~,504 filed December 30, 1'383.
BACKGROUND OF THE INVENTION
This invention relates to an organosolv progress
for the delignification of lignocellulosic material to
produce a nigh yield of fibrillateable cellulose pulp haJing
a low residual content of lignin while also obtaining
reactive lignin as a by-product. In contrast to Qrior art
processes, the method of this invention is directed to a high
temperature process controlled so as to cause solvation of
reactive lignin to predominate while suppressing degradation
of cellulosic material in order to enable recovery of high
quality cellulose pulp and of reactive lignin.
Numerous commercial processes exist for producing
cellulose pulp from wood. Generally stated, those processes
which rely on inorganic chemicals as the extracting agent do
not yield lignin as a valuable by-product. The liquor used
may take many different forms but the extràction process
normally proceeds in an acidic or in a basic environment. If
a high quality lignin by-product is to be recovered, the
process used is usually of the so-called organosolv type, in
which the liquor is an aqueous solution of an organic
solvent. Exemplary of such a process is U. S. Patent
3,585,104 which teaches the use of a digesting liquor
containing an aqueous mixture of lower aliphatic alcohols
such as methanol, ethanol, propanol or aqueous mixtures of
the lower aliphatic ketones such as acetone, as appropriate
digesting or pulping agents. As is conventional in the prior
art with respect to such liquors, Patent 3,585,104 teaches a
pulping temperature of between 150C to 200C and common
residence times of an hour or more at the cooking
lZ84637
temperature. No consideration is given with respect to the
overall cooking time during which the liquor is subjected to
a temperature which will promote cellulose/hemicellulose
degradation, i.e., the time during which the liquor and
consequently the wood material therein is subjected to
temperatures exceeding about 150~C.
Also exemplary is U. S. Patent 2,037,001 to
Aronovsky wherein it is taught to employ an aqueous liquor
containing a mono-hydroxy alcohol which has at least four
carbon atoms of which at least three are in a straight chain.
The invention disclosed in that patent is the use of an
alcohol, as aforesaid, which is not miscible with water in
all proportions at higher temperatures. Aronovsky teaches
that when the aqueous liquor is at an elevated digesting
temperature, the water and alcohol form a 'nomogeneous
solution in which the pulping takes place, but upon cooling
to room temperature, the liquor forms two immiscible phases.
One phase is the water phase containing inorganic chemicals
in solution, and the other phase is the alcohol phase
containing extracted lignin and other by-products. The pulp
is in a solid phase and is separated from the liquid phase
for recovery. It is taught that at the lower temperature at
which the pulp is recovered, there is less opportunity for
lignin to precipitate back onto the pulp because it remains
solubilized in the alcohol layer which is stronger than when
miscible with and hence diluted by water at the higher
temperatures. It is taught that a purer lignin extract can
be recovered from the aLcohol layer and that more economical
recovery of the alcohol is possible.
Aronovsky also teaches that extractive digestion
of ligneous materials in an aqueous alcohol solution proceeds
best in acidity. This patent recognizes that too much
acidity exerts a hydrolytic action on the cellulose but that
if alkalis such as caustic soda, sodium sulphide and sodium
carbonate are added to avoid too low a pH, they exert an
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inhibiting action on the process of solvating lignin. In
contrast, and in order to avoid this inhibitive action, urea
in relatively small quantities is taught to be added as an
agent to prevent high acidity in the digester. The result is
that the digestion takes place at a higher pH than where urea
is not used. The patent teaches the use of less than 5~ o~
urea (yielding an acidic final pH) and that this amount of
urea is fully as effective as 40% urea (which would yield a
basic final pH).
Example 1 of the Aronovsky patent teaches a
digestion temperature of 185C and a coo~ing time of 2 hours
at that temperature with no urea addition to yield 39.2% pulp
with 2% lignin content, which pulp is noted to be comparable
to that obtained by "the known 'soda process"'. In Example
2, the process of Example 1 was varied by displacing the
cooking liquor and dissolved content with hot fresh liquor
after one hour cooking time and continuing the cooking for an
additional hour, yielding a higher percentage (51%) of pulp,
residual lignin content being unspecified. Examples 3 and 4
of the patent employ the process but with different woods,
teaching digestion temperature of 185C and coo~ing time of 5
hours at that temperature, without urea addition, and
yielding 45.7% pulp with 10% residual lignin and 50.5% pulp
with 11.9% residual lignin respectively. Examples 5 and 6
use different alcohols, teaching digestion temperature of
175C and a cookiing time of 4 hours at that temperature,
without urea addition, to yield 53% and 54% pulp
respectively, no residual lignin content being specified.
Example 7 of the patent teaches the use of 4.5% urea based on
dry wood at a digestion temperature of 175C and cooking time
of 4 hours at that temperature in order to ac`nieve a high
yield of cellulose pulp. A yield of 67.2% pulp is specified
but residual lignin content thereof is not mentioned. In
discussing Example 7 in comparison with the same process
without urea, it is noted that "urea functions by
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decomposition to provide ammonia which is ava-ilable to
neutralize acid formed in the digestion, thus preventing a
low pH (high acidity) in the digester. The result is that
the digestion takes place at a higher pH than where urea is
not used. This minimizes loss of and degradation and
alteration of celluloses and pentosans of the original wood."
These examples are in accord with the conventional pH,
temperatures and times at cooking temperatures, i.e. an
acidic environment, temperatures less than 200C and long
cooking times at the cooking tem~erature.
3RIEF SUMM~RY OF THE INVENTION
It is considered that in the prior art, there is
no teaching of the critical importance of controlling an
organosolv process with respect to the manner in which the
temperature, total time of the cooking process and the pH of
the liquor are interrelated so as to obtain the objectives of
this invention.
The cellulosic content of wood, particularly
hemicellulose, is very susceptible to degradation under any
conditions in which organosolv pulping is normally carried
out. Commercial pulping processes are usually either acidic
or basic. These acids or bases promote lignin fragmentation
and solvation but, unfortunately, acids and bases also
promote undesirable side reactions such as lignin
condensation and depolymerization of hemicellulose/cellulose
(herein "cellulose degradation"). Cellulose degradation not
only reduces the yield and quality of recovered pulp but it
also creates the condition in which condensation of lignin
with the products of cellulose degradation occurs, thereby
degrading the quality of recovered lignin as well.
Therefore, as an ideal, it would be desirable to conduct the
pulping under the conditions in which cellulosic degradation
is minimized while solvation of reactive lignin is maximized
but with a minimum of lignin condensation.
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We have discovered that delignification of
lignocellulosic materials in an organosolv liquor can be
controlled as to pH, temperature range, heating and cooling
rates and consequent cooking time so that the process
predominantly becomes one of reactive lignin solvation with
minimized carbohydrate degradation. As a result, the
residual lignin content of the cellulose pulp decreases at a
rapid rate while the yield of high quality cellulose pulp
decreases at a slower rate, whereupon the process is rapidly
terminated to yield the requisite combination of pulp yield
and residual lignin content. The process is significantly
improved if the liquor contains an additive such as methylan-
thraquinone (.~AQ) which not only increases the rate of
reactive lignin solvation but also blocks lignin condensation
reactions. These additives may be aromatic additives
containing electron donating groups and fused aromatic ring
compounds capable of undergoing a single electrophylic
substitution, of which anthraquinone and 2-naphthol are
additional examples.
This invention concerns the delignification of
lignocellulosic material such as wood by a process which is
predominantly one of reactive lignin solvation to recover a
high yield of carbohydrate fraction in the form of chemical
grade, fibrillateable, high molecular weight
cellulose/hemicellulose pulp with low residual lignin content
and a lignin fraction in the form of reactive lignin. By
"high yield" is meant a yield of at least about 50 wt % pulp
based upon dry wood (lignocellulosic material) and by "low
residual lignin content" is meant not more than about lO wt %
residual lignin content based upon the amount of recovered
pul p .
The invention is based on several interrelated
discoveries concerning the relationships among process steps
essential to achieve the objectives sought. The solvation
process employs a digesting liquor which is an aqueous
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organic solvent containing an acid neutralizi-ng agent and,
preferably, an additive such as methylanthraquinone,
2-naphthol or anthraquinone. Lignocellulosic material is
immersed to impregnate it with the liquor. The liquor with
the immersed material is next controllably heated to a high
digesting temperature followed by rapid cooling to promote
lignin solvation and then the ultimate separation and
recovery of the fractions of interest.
One discovery is that the liquor must be provided
with an acid neutralizing agent which causes the liquor,
during an early or first stage of the cooking process, to
attain a substantially neutral pH which is thereafter
retained during the cooking process. By "substantially
neutral" is meant a pH of from about 6.0 to about 8.0 and
preferably 6.8 to 7.5. By "early or first stage" is meant
that stage of the cooking process during which the liquor is
being heated through a temperature range of about 150C to
about 175C. By "cooking process" is meant that portion of
the overall process which takes place while the temperature
of the liquor is above a temperature of approximately 150C,
i.e., above that temperature at which significant
cellulose/hemicellulose degradation begins to occur.
By "total cooking time" is meant the time period
during which the cooking process is being carried out and
represents the time necessary to yield both the pulp of high
molecular weight with low residual lignin content and the
rezctive lignin. In accord with this invention, the total
cooking time is separated into stages, namely, a first stage
in which the liquor is heated through the temperature range
of about 150C to about 175C, a second stage in which the
liquor is heated through a temperature range of about 175C
to the maximum cooking temperature which is within the range
of about 200C to about 280C, a third stage (if present) in
which the liquor is maintained at the maximum cooking
temperature for a finite residence time, and a last or final
1284637
stage in which the liquor is cooled to a temperature of less
than about 150C.
Further discoveries are that the maximum tempera-
ture attained during the second stage (above about 175C) of
the cooking process must be high, i.e., in the order of about
200C to about 230C, that the heating rate during the second
stage should be as high as possible and in any case prefer-
ably higher than the heating rate during the first stage, and
that the cooking process must be terminated by rapid cooling
to a temperature of less than about 150C so that the total
cooking time during which the liquor remains at a temperatlre
of more than a~out 150C is controlled to yield chemical
grade cellulose pulp of a desired low content of residual
lignin with a minimum of cellulose/hemicellulose degradation
and to yield by-product lignin which is reactive.
Preferably, the first stage of cooking should
proceed at a heating rate of about 3.0C to about
6.0C/minute over the temperature range of about 150C to
about 175C in order to allow sufficient time for acids being
formed in the lignocellulosic material (i.e., acetic and
formic acids) to escape from the lignocellulosic material and
become dissolved in and neutralized by the solvent system.
Thereafter, the second stage of the cooking
process may proceed at the same heating rate as the first
stage. However, it is preferable that it proceed at a higher
he ting rate of at least about 6C/minute or more until the
maximum cooking temperature is reached and the final stage
should proceed at a cooling rate which is as rapid as
possible.
~ further discovery is that the cooking process is
most effective when the heating rate during the second stage
of cooking is high enough to permit the aforesaid third stage
of the cooking process to exist before the cooking process
must be terminated due to the incipient attainment of the
desired percentage yield of high quality pulp with the
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desired low residual lignin content. That is~, the process is
most effective with respect to the high pulp yield with low
residual lignin content when the liquor may be held at the
maximum cooking temperature for 2 finite period of time (the
residence time) before the cooking process must be terminated
by cooling. This is related to the discovery that, at any
given temperature within the maximum cooking temperature
range herein, the ratio of the rate of reactive lignin
solvation to the rate of cellulose/hemicellulose degradation
is most favorable and thus optimum during some finite
residence time during the third stage of the cooking process
We have discovered that whereas the rate at which
cellulose/hemicellulose degradaticn proceeds at any given
temperature is substantially uniform with time, i.e.
temporally independent, the rate of reactive lignin solvation
at the same given temperature is temporally dependent.
Specifically, we have found that the rate of reactive lignin
solvation is initially very high at any such given
temperature followed a short time thereafter by a sharply
decreased rate of reactive lignin solvation. Thus, if the
time taken during the second stage of cooking is shortened by
maximizing the heating rate during that stage, less
cellulose/hemicellulose degradation takes place during that
time and a residence time or third stage of cooking is
possible. In regard to this, it is to be understood that the
higher the maximum cooking temperature, the less residence
time is permissible before cooling is necessary. Thus, at
the maximum cooking temperature of about 280C, it is
entirely possible that no residence time is available before
cooling must be effected, depending upon the rate of heating
during the second stage of the cooking process.
This invention is directed to high temperature
and, if possible, residence time at the maximum cooking
temperature, delignification of cellulosic material such as
wood in a liquor which is an aqueous organic solvent, with or
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without the presence of additive which aids lignin extraction
and stabilization of cellulose, but in the presence of an
acid neutralizing agent which causes the reaction to proceed
under substantially neutral conditions.
We have discovered that in a liquor containing
neutralizing agent as aforesaid, whereas the rate of cellu-
lose degradation is temperature dependent (increasing with
temperature) it is substantially temporally independent,
whereas the rate of reactive lignin solvation, although also
increasing with temperature, proceeds at each temperature at
an initially high rate for a short period of time, followed
by a sharply defined and decreased rate of reactive lignin
solvation. The transition at each temperature from the high
rate of reactive lignin solvation to the decreased rate of
reactive lignin solvation has been found to occur after only
a few minutes for the temperature range of 200C to about
280C, the time required to attain the transition varying
inversely with temperature.
We have also discovered that whereas recovery o~ a
high yield of cellulose pulp with low residual lignin content
and the recovery of reactive lignin is attained in accord
with this invention when the heating of the liquor is carried
out with relatively rapid and even with a uniform rate of
heating from about 150C to the maximum cooking temperature
followed immediately by rapid cooling to a temperature of
less than 150C, both the yield of pulp and the low content
of residual lignin therein is improved if it is possible to
hold the liquor at the maximum cooking temperature for some
period of time before the cooling is carried out. That is to
say, if the liquor is heated to the maximum cooking tempera-
ture within the maximum cooking temperature range of 200C to
280C with sufficient rapidity from about 175C to the maxi-
mum cooking temperature, a residence time at the maximum
cooking temperature, ranging inversely with maximum cooking
~2;8~63~
temperature between instantaneous or zero time to about 30
minutes is possible and extremely beneficial.
Since the problems of ce]]ulose degradation and rate
of solvation are of primary concern, we may use conventional
additives such as rnethy]anthraquinone (MAQ) which are known to
reduce lignin condensation reactions and to stabilize
carbohydrate pu]p. Thus, a high yield of re:Latively undergraded
cellulose pulp with low residual lignin content and the recovery
of high quality lignin can be idealized by selecting the
combination of temperature and residence time which completes the
delignification. In this regard, it is important to heat the
liquor as rapidly as possible and, ~hen the reaction is
substantially complete, to cool the liquor as rapidly as
possible, thus to minimize unwanted cellulose degradation and
lignin condensation.
Although various aspects of the invention have been
described previously, the invention in one of its broader aspects
provides a process for extracting lignin from lignocellulosic
material to recover reactive lignin and to yield high quality
cellulosic pulp, which comprises the steps of (a) impregnating
lignocellulosic material with a liquor which comprises an aqueous
organic solvent and an acid neutralizing agent, (b) heating the
liquor with the impregnated lignocellulosic material submerged
therein from a temperature of about 150C to a maximum
temperature in the maximum temperature range of 200'C to 280C
and then cooling the liquor with material submerged therein to a
temperature of less than about 150C., (c) maximizing reactive
lignin solvation while suppressing cellulose degradation during
step (b) by (i) controlling the amount of neutralizing agent in
step (a) to achieve a final pH for step (b) of from 6.0 to 8.0 in
the liquor during heating in step (b) from about 150C to about
175C, and (ii) maximizing the time at which the liquor is within
the maximum temperature range during step (b) by rapidly cooling
the liquor to terminate step (b), and (d) recovering reactive
lignin and high quality cellulose pulp from the cooled liquor.
In a further aspect the invention pertains to a
process for extracting lignin from lignocellulosic material to
recover reactive lignin and to yield quality cellulosic pulp,
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lZ84637
which cornprises the steps of (a) impreynating lignocellulosic
material with a ]iquor comprising an aqueous organic solvent, an
aromatic additive containing electron donating groups and fused
aromatic r:ings capable of undergoing a single electrophylic
substitution, and an acid neutralizing agent, (b) heating the
liquor with the impregnated lignocellulosic material submerged
therein from a temperature of about: 150C to a maximum
temperature in the temperature range of 200C to 280C, holding
the liquor at the maximum temperature for a period of time of up
to 30 minutes, andthen cooling the liquor with the material
submerged therein to a temperature of less than about 150C.,
(c) maximizing reactive lignin solvation while suppressing
cellulose degration during step (b) by (i) controlling the amount
of neutralizing agent in step (a) to achieve a final pH for step
(b) of from 6.0 to 8.0 in the liquor during heating in step (b)
from about 150C to about 175C, and (ii) maximizing the time at
which the liquor is within the maximum temperature range during
step (b) by rapidly heating the liquor from about 175C to the
maximum temperature, and rapidly cooling the liquor to terminate
step (b) so that at least about 50% of pulp based upon dry weight
of the lignocellulosic material and containing less than about 10
weight % residual lignin based upon the amount of recovered pulp
is present in the final liquor, and (d) recovering reactive
lignin and high quality cellulose pulp containing less than 10
weight % residual lignin based upon the amount of pulp recovered
from the cooled liquor.
Further still the invention comprehends a process for
the delignification of lignocellulosic materials to extract
lignin therefrom in reactive form to yield a high percentage of
cellulosic pulp containing high molecular weight cellulose with a
low content of residual lignin, comprising the steps of
(a) cooking lignocellulosic material while submerged in a liquor
including an aqueous organic solvent and a minor amount of acid
neutralizing agent by heating the liquor and lignocellulosic
material submerged therein from a temperature of about 150C to a
maximum temperature in the cooking temperature range of about
200C to about 280C and thereafter rapidly cooling the liquor to
a temperature below about 150C to terminate the cooking,
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~28~637
(b) maintaining an optimum ratio of the rate of reactive lignin
solvation relative to the rate of cellulose degradation during
s-tep (a) by heating the liquor through a first temperature range
of about 150C to about ~75C at a heating rate in the order of
about 3~C/minute to about 6C/minute, the arnount of acid
neutralizing material in the liquor being sufficient to attain a
final pH for step (a) of from 6.0 to 8.0 during the heating
through the first temperature range, heating the liquor through
the temperature range of about 175C to the maximum temperature
at a rate of at least about 6C/minute, and terminating step (a)
by rapidly cooling the liquor when the cellulose has attained
less than about 10 weight % of residual lignin content based upon
the amount of recovered pulp, and (c) separately recovering the
cellulosic pulp having high molecular weight cellulose with less
than about 10% residual lignin content and the reactive lignin
from the cooled liquor.
Another aspect of the invention pertains to a process
for the delignification of lignocellulosic materials to extract
lignin therefrom in reactive form to yield a high percentage of
cellulosic pulp containing high molecular weight cellulose with a
low content of residual lignin. The process includes the steps
of (a) cooking lignocellulosic material while submerged in a
liquor including an aqueous organic solvent and an acid
neutralizing agent by heating the liquor and lignocellulosic
material submerged therein from a temperature of about 150C to a
maximum temperature in the cooking temperature range of about
200C to about 280C and thereafter rapidly cooling the liquor to
a temperature below about 150C to terminate the cooking, and
(b) maintaining an optimum ratio of the rate of reactive lignin
solvation relative to the rate of cellulose degradation during
step (a) by (i) heating the liquor to a first temperature in the
range of about 150C to about 175C and holding the liquor at the
first temperature for from about 4 minutes to about 10 minutes,
the amount of acid neutralizing agent in the liquor being
sufficient to attain a final pH for step (a) of from 6.0 to 8.0,
(ii) heating the liquor from the first temperature to a second
temperature in the range of about 200C to about 280C and
holding the liquor at the second temperature for up to 30
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~284637
minutes, and (iii) terminating step (a) by rapidly cooling the
liquor when the cellulose has attained ]ess than abou-t 10 weight
% of residual lignin content based upon the amount of recovered
pulp .
The above and further advantages of this invention
will be more apparent as this description proceeds.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
Figure 1 is a graph of temperature vs yield and
residua] lignin content of the pulp for a typical process of this
invention in which no third cooking stage or residence time is
employed;
Figure 2 is a graph of time vs the logarithm of
percent lignin content comparing residual lignin content of this
invention with and without residence time;
Figure 3 is a plot of the data of Figure 2
illustrating the transition from higher to lower rate of reactive
lignin solvation at vari.ous temperatures;
Figures 4 - 6 are graphs of residence time vs lignin-
free pulp yield and carbohydrate content of the pulp at different
maximum cooking temperatures;
Figure 7 is a graph of temperature vs % residual
lignin comparing heating rates at zero residence time and various
residence times for a given heating rate.
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1~84637
DETAILED DESCRIPTION OF THE PREFERRED ~BODIMENTS
With reference to Figure 1, the results shown
therein were in relation to a series of cooks in which the
mixture of wood material and liquor was heated rapidly to a
maximum temperature in the range of from 100C to 2~0C and
then immediately cooled very rapidly. As will be recognized,
such cooking omits the third stage of cooking. The
lignocellulosic material was tulip popular chips which were
reduced to a nominal 1 mm particulate size by Wiley milling.
Pulping cooks were performed using a 650 ml stirred batch
reactor manufactured by Pharr Instruments, Inc. The material
was first degassed followed by soaking in a pul?ing liquor
composed of ethanol, water, sodium bicarbonate and methylan-
thraquinone in proportions as follows:
Ethanol 180 ml
Water 120 ml
Sodium bicarbonate 7.7 g
Wood (dry) 33 g
MAQ 1.2 g
The mixture of liquor and material submerged
therein was charged into the reactor, rapidly heated to the
desired temperature and immediately cooled rapidly. The
results of such cooks are shown in Figure l wherein it will
be seen that delignification essentially does not occur until
the temperature exceeds about 160C, but thereafter proceeds
quite rapidly. The residual lignin content of the pulp does
not follow the delignification curve except as to general
trend. In these cooks, which are identified as nonisothermal
cooks for convenience, the maximum time taken to heat to
temperature was about 32 minutes and the maximum time taken
for cooling was about 12 minutes.
~ further series of cooks was undertaken, these
being named for convenience as isothermal cooks. In thes
isothermal cooks, the reactor was heated to a specific
temperature of from 200C to 260C, held at that temperature
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for up to 45 minutes and then rapidly cooled.~ As will beapparent, the so called isothermal cooks are of the type
which include the third stage of cooking in accord with this
invention. A comparison between the isothermal and the
nonisothermal cooks is shown in Figure 2. Consider, for
example, the upper curve of this graph. The time period
through the first 5 or so minutes represents the first stage
of the cook, i.e., the time taken to heat from about 150C to
about 175C. The second cooking stage is the time taken to
heat from about 175C to the ma~imum cooking temperature,
200C in this case, and the second stage time is about 5
minutes (10 minutes total for the first and second cooking
stages combined). Thereafter, the temperature of 200C is
held for a third cooking stage which lasts about 45 minutes.
The final or cooling stage takes about five minutes. The
total cooking time, in this case, is about one hour. In all
of the isothermal cooks, it will be seen that the delignifi-
cation proceeds more slowly during the third cooking stage in
comparison to continued heating during the second cooking
stage. However, since cellulose degradation proceeds more
rapidly the higher the temperature reached at the maximum
cooking temperature, it will be apparent that the third cook-
ing stage offers the possibility for a slowing of cellulose
degradation due to the temperature being held steady, even
though the factor of time-at-temperature is introduced.
Figure 3 is a plot of the data points of Figure 2
with straight lines drawn through the isothermal points to
the extent possible. As reference to Figure 3 will show, at
any given temperature within the maximum cooking temperature
range, the rate of delignification has been found to proceed
initially at a very high rate as illustrated by the straight
line to the left having a relatively steep slope, followed
within a few minutes by a transition, marked by an arrow in
Figure 3, to a sharply decreased rate as illustrated by the
line to the right having a lesser slope. Since the rate of
~Z8~6.3~
cellulose degradation for the ternperature in question has
been found to be substantially time-independent, the ratio of
reactive lignin solvation to celLulose degradation is highly
favorable all during the rapid rate of delignification shown
in Figure 3 and transposes to a less favorable ratio at the
transition point between the two lines for any given
temperature. Since the end point of any cook is the total
cooking time at which the desired combination of yield of
high quality pulp and low content of residual lignin is
reached, the faster the ma~i~um coo'~ing temperature can be
reached, the longer the time period for the thir~ s~age
cooking is possible.
Generally, it is preferable to raise the
temperature of the pulping mixture as quickly as possible.
The pulping mixture may be heated rapidly to 150C, but the
process is relatively indifferent to the manner in which the
mixture is heated to this temperature. However, it is
especially important to conduct the heating between the
temperatures of about 150C to about 175C at a heating rate
which allows about 4 to lO minutes for this first cooking
stage ln order to allow acids being formed in the wood to
emerge therefrom and be immediately neutralized by the liquor
containing the acid neutralizing agent. Consequently, the
heating rate during the first stage of cooking should be in
the order of about 3C/minute to about 6C/minute. For the
second cooking stage one should heat rapidly above 175C to
the maximum cooking temperature. Although heating at a rate
of only about 3C/minute during the second cooking stage will
work, much higher heating rates are desired in order to
minimize the second stage time and allow more time for the
third stage of cooking. Similarly, it is preferable to
reduce the pulping temperature of the mixture to terminate
the cooking process as quickly as possible in order to reduce
the amount of time during which the wood chips are above
about 150C.
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While the wood chips may be rapidly heated to a
temperature of 200C or greater, it is preferred that the
wood chips be rapidly heated above 175C at greater than
3C/min to within the range of 200C to 280C. Temperatures
in excess of about 280C may result in degradaton of the
liberated cellulose. However, it is especially preferred to
heat the submerged wood chips from about 175C rapidly to the
maximum cooking temperature in tne ma~imum cooking
temperature range of 200C to 280C at a heating rate of at
least 6C/min. and most preferably at a heating rate of a
least about 10C/minute.
Since the cooling rate effects the termination of
the cooking process, it is important to effect the cooling at
as rapid a rate as is possible, i.e., at a cooling rate of
more than 10C/minute and preferably at a rate of at least
about 20C/minute.
The digestion of the wood chips in the liquor
composition during the third stage of cooking is performed
for a residence time ranging from substantially instantaneous
or zero time up to a residence time of 30 minutes. The
higher the maximum cooking temperature, the lesser the
residence time needed.
It has been found that by utilizing the conditions
noted above, it is possible to effect the separation of the
cellulose from the lignin with the least damage to the
liberated cellulose. The use of high temperatures ensures
rapid lignin extraction whereas the rapid heating during the
second stage of cooking allows the use of third stage cooking
which ensures minimal product degradation. These conditions
allow high production rates per unit volume thus allowing low
processing costs.
The effect of heating rate on the residual lignin
content is illustrated in Figure 7. The curve indicating a
heating rate of 3.6C/minute with zero residence time (no
third stage cooking) shows an apparent improvement over the
_ -14-
34637
higher heating rate of 5.6C/minute with zero residence time,
but the two curves relating to this higher heating rate with
the short residence times of S and 10 minutes clearly
demonstrate that the introduction of the third stage of
cooking quickly establishes superiority over the lower
heating rate with no third stage cooking.
Figures 4-6 also demonstrate that the third stage
residence time does not significantly affect cellulose
degradation.
Systems which may be utilized to heat and cool the
pulping liquor mixture of the present invention are ~nown to
those skilled in the pulping art. For example, rapid heating
can be achieved by vapor injection and rapid cooling by
vaporization at reduced pressure.
Wood chips or other lignocellulosic material are
degassed to remove gases which cause oxidation of the wood
chips and to promote uniform penetration of the pulping
liquor. The wood chips are rapidly heated in a liquor
containing water, an organic solvent and an acid neutralizing
agent such that the wood chips are submerged in the liquor,
from a temperature of about 150C to a maximum cooking
temperature within the range of 200C to 280C, held at such
maximum cooking temperature for a period of time which ranges
from zero or instantaneous up to 30 minutes, and then the
mixture is rapidly cooled.
It is important to degas the wood chips prior to
rapidly heating the submerged wood chips in the digesting
liquor. By degassing the wood chips, the rate of oxidative
degradation of the liberated cellulose is reduced, so as to
ensure the attainment of a cellulose of high molecular weight
in the cellulose pulp. Also degassing facilitates liquor
penetration into the chips, providing more uniform
delignification. In degassing the wood chips, an ordinary
aspirator apparatus may be used to produce a vacuum. These
devices are well known, and their operation is understood by
~2~
those skilled in the laboratory sciences. Alternatively, hot
soaking by means such as hot liquor reflux at atmospheric
pressure or steaming may be emp]oyed to accomplish degassing
and uniform pulping liquor penet:ration.
To terrninate the digestion or cooking process, the
mixture is rapidly cooled to a temperature below 150C at a
cooling rate of at least 10C/min. However, it is especially
preferred to cool the mixture to a temperature in the range
of 20C to 90~C at a cooling rate of greater than 20C/min.
Digestion of the wood chips in a liquor of water
and an organic solvent rnaintained in a neutral state allows
the lignin to solubilize, reduces the degradation of
carbohydrate materials and reduces lignin condensation.
A wide variety of organic solvents may be used.
However, preferred as the organic solvents are the
straight-chain or branched-chain alcohols having up to about
8 carbons. For example, alcohols such as isopropyl alcohol
or ethyl alcohol can be used. However, ethanol is the
preferred alcohol. A solvent to wood ratio in the range of 3
to 30 ml of solvent to 1 gram of wood may be used. However,
it is preferred to use a solvent to wood ratio in the range
of 4 to 10 ml per 1 gram of wood.
In addition to using the preferred lower alcohols
having up to 8 carbons, any of the other conventional organic
solvents used in the delignification of cellulosic materials
may be used in the present process. For example, aqueous
solutions of lower aliphatic ketones may also be used in the
present invention. Aqueous solutions of other organic
solvents such as ethylene glycol, or dioxane may be used as
well.
The digesting process of the present invention is
principally carried out as a neutral process. More
particularly, the neutral solvent extraction of lignin from
cellulosic materials reduces the formation of undesired
by-products such as furfural and condensed lignins.
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lX8~6'~
A neutral solvent extraction is carried out by the
addition of a acid neutralizing agent to adjust pH. Any acid
neutralizing agent may be used which functions to maintain a
substantially neutral solvent extraction. Sodium bicarbonate
or sodium carbonate are preferred acid neutralizing agents.
Other possible acid neutralizing agents are sodium phosphate,
sodium citrate and calcium phosphate. The controlled
addition o~ a base, such as sodium hydroxide may also be
employed to obtain a neutral state. The selection of an
appropriate acid neutralizing agent may easily be made bv
anyone skilled in the chemical arts. The amount o~ acid
neutralizing agent used may be substantially reduced by
recycling the salts, such as sodium acetate and sodium
formate, from the pulping liquor, due to the requirements of
chemical equilibria.
While the initial pH of the reaction mixture may
be as high as about 10.0, it is important that the pH of the
product mixture be maintained at a pH range of about 6.0 to
8.0 during operation above 175C, as measured by a conven-
tional pH meter at ambient temperature. It is preferred that
this pH be in the range of about 6.8 to 7.5.
Additionally, an additive in the form of an
organic stabilizer and/or inhibito_ may be added to the
liquor to help increase pulp yield while reducing the yield
of undesired by-products. For example, the use of methylan-
thraquinone (~AQ) as an additive at elevated temperatures
protects carbohydrate peeling and reduces lignin condensa-
tion. Another additive found to work well is 2-napthol. As
a general rule, quinones containing electron donating groups
and fused aromatic ring compounds capable of undergoing a
single electrophylic substitution would serve as additives in
the present invention.
Different additives appear to inhibit different
side reactions and, therefore, it may be preferrable to use a
~2~34637
combination of additives in an amount up to 15~ of the
lignocellulosic material on a dry weight basis.
The obtained lignin has a high market potential as
it is more reactive than that obtained by present commercial
pulping processes. For example, it can be readily converted
to monoaromatic compounds in hign yield. One approach for
such conversion is catalytic hydrogenolysis as proposed by
Hydrocarbon Research, Inc. Gendler, Huibers and Parkhurst,
Wood and Aqricultural Residues: Research on Use for Feed,
Fuels, and Chemicals, E. Soltes, ed., Academic Press, pp
391-400 (1983).
Moreover, the reactive lignin of the present
invention can be directly substituted for phenol in numerous
polymer applications. The lignin obtained by the present
process can be reacted with condensation polymers due to the
reduced degree of lignin condensation occuring in the present
extraction liquor which maintains a substantially neutral pH
during delignification. Thus, both the cellulose pulp and
lignin obtained from this invention have hig`n market value,
whereas in commercial pulping processes only the cellulose
pulp is of value.
The cellulose pulp is produced in high yield by
the present process. Moreover, the cellulose in the
liberated pulp has a high molecular weight. Also, by using
the process of the present invention, it is possible to
obtain a cellulose pulp having less degradation of the
hemicelluloses fraction. ~s the hemicelluloses are the most
abundant of the polymeric cementing materials which hold the
cellulose fibers together, their preservation results in the
isolation of a superior fiber.
By maintaining the fibrous structure of the
cellulose, the process of the present invention is useful for
producing cellulose derivatives as well as nonwoven cellu-
losic structures. When the present process is performed
under its preferred condition, it will typically yield an
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unbleached cellulose pulp leaving less than 10~ lignin based
upon weight 3 of the cellulose pulp and more than 20
hemicellulose by weight.
Batch digesters mav be used as well as continuous
digesters. A varying temperature pro~ile may be used as well
as a sequential addition to solvent, acid neutralizing age~t,
or base. Compared to conventional organosolv pulping, the
present invention provides a higher yield of cellulose pulp
and higher production raLes ?er unit of reactor volume.
In the alternative, instead of placing the liquor
and lignocellulosic material in a single digester, the liquor
and material may be placed in a first digester heated to a
temperature in the range of about 150C to about 175C and
held in that digester for from about 4 minutes to about 10
minutes. Then the liquor and material may be transferred
from the first digester to a second digester and held there
for up to 30 minutes, the second digester being heated to a
temperature in the range of about 200C to about 280C.
Also, practically any hard or soft wood or other
lignocellulosic material may be treated according to the
present invention. Of course, by treating wood chips it is
possible to expose a large surface area of the wood to the
pulping liquor, thereby producing better results.
The present invention will be further illustrated
by certain examples and references, tabulated below, which
are provided for purposes of illustration only and are not
intended to limit the present invention.
--19--
lX8D~637
TABLE 1
Example 1 (control) 2(control) 3(control) 4
Neutralizing
Agent, Sodium
bicarbonate
(wt ~ wood) 0 0 20 27
Alcohol (ethanol)
(vol ~ liquor) 0 55 0 5
Liquor/wood ratio
(ml/g) 10 10 10 lO
Max. coo,cing
temp., C 230 230 230 230
Residence
time, Min. 15 15 15 15
Initial pH 6.0 6.1 8.5 9.5
Final pH 3.3 4.1 5.6 7.3
Pulp Yield
(wt % wood)45.5 48.6 54.8 57.1
Residual Lignin
(wt % pulp)21.8 7.0 17.1 6.5
In the above Table 1, the starting wood material
was Tulip Poplar (lirodendron tulipfera) chips comminuted to
a nominal lmm particle size and which were degassed, immersed
and soaked in the relevant liquor to impregnate the material
prior to the cooking. In Example 1, the liquor consisted of
water only. In Examples 2 and 4, 55 vol % ethanol was also
present in the liquor plus, in Example 4, acid neutralizing
agent in amount of 27 wt % of dry wood. In Example 3, the
liquor consisted of water and acid neutralizing agent in
amount of 20 wt. % of dry wood. The reactants were charged
into the reactor and heated at a rate of about 6C/minute to
the maximum temperature, held at that temperature for 15
minutes and rapidly cooled at a rate of about 10C/minute to
a temperature of less than 150C. The products were
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~Z84637
filtered, washed and dried to obtain pulp yield. The lignin
content of the pulps was either determined directly using the
Klason method (TAPPI Standard T222) or indirectly by
determining the Xappa number of the pulp using TAPPI Standard
T235 OS-76.
Table 1 illustrates that digestion in water only,
Example 1, provides an acid final pH of 3.3 which is
responsible for the low yield o, pulp (45.5~ based on dry
wood) and the high residual lignin content (21.8 wt % of
.10 pulp). The results of Example 2 illustrate that the addition
of alcohol reduces the residual lignin content signi~ican.ly
(to 7.0 wt ~ of pulp) but increases the pulp yield only
slightly (to 48.6 wt % of wood) due primarily to the acid
final pH of 4.1. The results of Example 3 illustrate that
the addition of acid neutralizing agent to the water,
although increasing the pulp yield significantly (to 54.8 wt
% of wood) has only a slight effect on the residual lignin
content (17.1 wt % of pulp). However, Example 4 demonstrates
that the addition of both the alcohol and the acid
neutralizing agent provides the best pulp yield (57.1 wt % of
wood) and the lowest residual lignin content (6.5 wt % of
pulp), the final pH being 7.3
lZ8463~
TABLE 2
Example 5 6 7
Neutralizing
Agent, Sodium
bicarbonate
(wt % wood) 13.333.3 7.9*
Alcohol (ethanol)
(vol % liquor) 70 30 50
Liquor/wood ratio
(ml/g) 10 10 10
Max. cooking
temp., C 230230 230
Residence
time, Min. 15 15 15
Final pH 6.67.3 6.8
Pulp Yield
(wt % wood) 59.252.5 60.2
Residual Lignin
(wt % pulp) 9.36.0 9.8
* Acid neutralizing agent is sodium carbonate.
The conditions for Example 5-7 were the same as
those for Examples 1-4, except that, as noted in Table 2, the
amounts of alcohol tethanol) were different from those of
Examples 2 and 4 of Table 1. Examples 5 and 6 illustrate
that varying the amounts of a different acid neutralizing
agent alters the final pH and in combination with the varying
amounts of alcohol and the higher maximum cooking temperature
provides different pulp yields and residual lignin contents.
Example 7, using about the same amount of alcohol as in
Example 4 but considerably lesser amount of acid neutralizing
agent (7.9 wt. % of wood as opposed to 27 wt % of wood) again
alters the final pH and in combination with the higher
maximum cooking temperature provides both a slightly higher
pulp yield and a substantially greater
~34S37
residual lignin content as compared with Example 4. Examples
5~7 also show that varying the amount of alcohol from 30-70
vol. ~ of liquor had little effect on the process.
TABLE 3
Example 8 9 10 11
Neutralizing
Agent, Sodium
bicarbonate
(wt ~ wood) 17.523.0 23.0 23
~lcohol (ethanol)
(vol ~ liquor) 60 60 60* 63
Liquor/wood ratio
(ml/g) 20 5 10 30
Max. cooking
temp., C 215 230 230 230
Residence
time, Min. 15 15 15 15
Final pH 7.6 7.8 7.1 7.2
Pulp Yield
(wt ~ wood) 64.563.3 61.5 57.0
Residual Lignin
(wt ~ pulp) 9.9 7.9 5.9 5.8
* n-propanol
Examples 9, 10 and 11 show the effect of changes
in the liquor to wood ratio, all other conditions remaining
the same with the lone exception of using n-proponal as the
alcohol in Example 10 in place of ethanol. A two-fold
increase in the liquor/wood ratio, Examples 9 and 10,
resulted in a slight decrease in pulp yield while lowering
the residual liqnin content. Example 11 shows that there is
little to be gained from increasing the liquor/wood ratio
above 10 ml/g. Example 8 shows that a lower maximum cooking
temperature of 215C slightly increased the pulp yield but
~Z8~637
resulted in a greatly higher residual lignin content over
what would be expected for a maximum cooking te~perature of
230C based on Examples 9-11.
TA~LE 4
Example 12 13 14 15*
Neutralizing
Agent, Sodium
bicar~onate
(wt ~ wood)26.7 23.0 23.0 23.0
Alcohol (ethanol)
(vol ~ liquor) 55 60 60 60
Liquor/wood ratio
(ml/g) 10 10 10 10
Max. cooking
temp., C 240 255 270 270
Residence
time, Min. 0 0 0 3
Final pH 7.? 7.6 7.0 7.1
Pulp Yield
(wt % wood)60.2 59.3 53.4 53.2
Residual ~ignin
(wt ~ pulp)8.3 5.6 2.5 1.3
* This Example also contains MAQ in amount of 4 wt % of wood.
Table 4 illustrates various Examples in which the
third cooking stage is omitted (no residence time) at various
high temperatures. This Table clearly shows that yields of
greater than 50~ with very low residual lignin content are
possible for the higher temperatures in the maximum cooking
temperature range. Some sacrifice in ~ yield by going to the
higher temperatures will allow greater reduction in the
residual lignin content.
lZ8~
TABLE 5
Example 4 16 17 18
Type of
Additive* none 2-~ap AQ MAQ
(wt % wood) 0 4 4 4
Neutralizing
Agent, Sodium
bicarbonate
(wt. % liquor) 27 21 20 23.3
Alcohol (ethanol)
(vol ~ liquor) 5, 60 60 60
Liquor/wood ratio
(ml/g) 10 10 10 10
Max. cooking
temp., C 230 230 230 230
Residence
time, Min. 15 15 15 15
Final pH 7.3 6.8 7.3 7.3
Pulp Yield
(wt % wood) 57.1 58.9 60.6 61.0
Lignin-free
pulp
(wt % wood) 53.4 55.8 57.8 59.4
Residual Lignin
(wt % pulp) 6.5 5.3 4.7 2.7
* 2-Nap = 2-Napthol
AQ = Anthraquinone
MAQ = Methylanthraquinone
Investigation of the process was performed to
determine the effects of various additives to the solvent
system. A summary of these results is included in Table 5.
The additives tested, methylanthraquinone, anthraquinone, and
2-naphthol, increased the yield primarily by increasing the
quantity of the lignin-free pulp, reflecting improved
carbohydrate stabilization. Quinonoid type compounds, in
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lZ8~i37
particular, are known to block carbohydrate peeling reactions
by oxidizing the reducing end of carbohydrate polymers which
would otherwise degrade. As a result, significant increases
in the pulp yield were observed. Also noteworthy is the
reduction of the residual lignin content despite the higher
retention of carbohydrate. .~ethylanthraquinone (MAQ),
Example 18, is the most beneficial additive of the three,
providing the highest pulp and the lowest lignin content.
All three of these additives increase the extent of
delignification by blocking lignin condensation reactions.
Table 5 compares the results of Example 4 (no
additive) with several Examples which do employ additives and
the comparison clearly illustrates the superior results in
both % yield and residual lignin content which is possible
with the use of an additive. Thus, the use of an additive
contributes both to preservation of carbohydrates and
delignification of the material.
TABLE 6
Example Tulip poplar 4 18
CARBOHYDRATES:*
Glucan 49.5 39.4 44.4
Xylan 17.2 9.5 12.4
Mannan 3.7 1.3 1.2
Galactan 0.98 0.26 0.64
Arabinan 0.69 0.30 0.64
Total
Carbohydrates 72.1 50.8 58.9
* Carbohydrate analysis done by alditol acetate method.
Table 6 compares the original wood carbohydrate
content with the carbohydrate content of Examples 4 and 18,
the former employing no additive and the latter employing the
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additive MAQ. As is the case with Table 5, Table 6
demonstrates the superiority of additive usage.
The stablization of carbohydrates through acid
neutralization and the addition of MAQ is illustrated in
Table 6. Comparing Example 18 with the original wood, 90~ of
the original glucan and over 70~ of the original xylan has
been preserved after pulping wi-th MAQ. Even without MAQ,
Example 4 retains over 50~ of the original xylan. The losses
of glucan an x~lan incurred may be due to the initial pH
greater than 9 (see Example 4 Table 1) and dissolution of
hemicellulose bonded to soluble lignin. Improved control of
the pH beyond the use of acid neutralizing agent is limited
by the reactor used in these studies which does not permit
controlled additions of acid neutralizing agent to the
reacting mixture once the heating cycle has started.
84~
TABLE 7
Example 19 20 21
MAO/
Type of Additive MAQ 2-naphthol MAQ
(wt % wood) 1 4/4 15
Neutrali~ing
Agent, Sodium
bicarbonate
(wt ~ wood) 26.7 23 23
Alcohol (ethanol)
(vol % liquor) 50 60 60
Liquor/wood ratio
(ml/g) 13 10 10
Max. cooking
temp., C 200 230 230
Residence
time, Min. 25 10 10
Final pH 7.2 7.6 7.5
Pulp Yield
(wt % wood) 61.3 59.7 60.2
Residual Lignin
(wt % pulp) 9.8 2.8 3.3
Table 7 shows that the addition of MAQ of up to 15
weight % based upon dry wood serves the dual function of
increasing pulp yield while decreasing residual lignin
content. As illustrated by Example 20 it is also possible to
mix additives. However, in view of Example 18, it does not
appear advantageous to use more than 4 wt % I~AQ.
Having now fully described our invention, it is to
be understood that the following claims are to be construed
not by the specific Examples noted above, but within the
wider latitude afforded by the description as a whole.
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