Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a method of renderin~
lignin separable from cellulose and hertlicellulose and the
product so procluced.
It has already been proposed in Canadian Patent
No. 1,096,37~ dated February 24, 1981 "Method of Rendering
lignin separable from cellulose and hemicellulose in
lignocellulosic material and the p.roduct so produced", E.A.
DeLong, to pack a pressure vessel, having a closed valved
outlet, with lignocellulosic material in a divided, exposed,
moist form, then to rapidly fill the pressure vessel with steam
at a pressure of at least 500 psi (3450 kilo Pascals) to raise
the temperature of the lignocellulosic material in less than 60
seconds to a temperature in the range 185C to 240C and
thermally soften the lignocellulosic material into a plastic
condition, and then, as soon as the plastic condition has been
attained, opening the valved outlet and instantly expelling the
lignocellulosic material to atmosphere. The lignocellulosic
:material issues from the outlet in a particulate form with the
lignin therein rendered into particles substantially in the
range 1 to 10 microns and substantially without thermal
degradation occuring to produce cross-links between the lignin,
cellulose and hemicellulose, so that a substantial portion of
the lignin is separable from the cellulose and hemicellulose by
solvent extraction with, for example, ethanol or methanol at
room temperature.
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The Derlon~ process not only has the unique feature c~E
renderin~ a surprisin~ly high proportion of thermally
undegraded lignin separable from the cellulose and
hemicellulose, but has the additional unique Eeature that this
-thermally undegraded lignin can be solven-t extracted without
thermal degradation occuring of the lignin, cellulose and
hemicallulose during khe solvent extraction step.
While the DeLong process has proved to be useful, it
has been observed that when the starting material is nearly air
dry as is often the case with straw and bagasse, and may be the
case with a percentage of wood chips depending on how they have
been stored, depolymerization of both lignin and xylan occurs
producing toxic monomers of lignin and degrada~.ion components
of xylan which, in some instances render the particulate
material as it has issued from the outlet unacceptable for use
in any form of fermentation process. These toxic components
can be extracted by water or by the normal lignin solvents such
as ethanol, methanol and sodium hydroxide. However, it may be
desirable to use the particulate material as a ~ermentation
feed stock, or as an animal feed, directly from the outlet, in
which case it i5 desirable to minimize the depolymerization of
both lignin and xylan so that the production of toxic
components of lignin in the particulate material that issued
rom the outlet is minimal and the depolymerization of the
xylan is also minimized.
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According to the present inven-tion there is provided
a method o~ renclering :lignirl separab:Le from cellulose and
hemicellulose in lignocellulosic material, comprising:
a) packing the lignocellulosic material in a divided, exposed,
moist form in a pressure vessel having a valved outlet,
b) with the outlet closed, rapidly filling the pressure vessel
with steam to a pressure of at least 500 psi to bring, by
means of the pressurized steam, substantially all of the
lignocellulosic material to a temperature in the range of
185 to 240C in less than 60 seconds and thermally soften
the lignocellulosic material into an plastic condition,
c) as soon as the said plastic condition has been attained,
gently venting the pressure vessel above the level of
the lignocellulosic material to atmosphere to reduce the
steam pressure in the pressure vessel to between 200 psi
and 450 psi, and then immediately opening the valved
outlet and e~plosively expelling the lignocellulosic
material, in the said plastic condition, from the
pressure vessel through the outlet to atmosphere so that
the said material issues from the outlet in particulate
form with lignin therein rendered into particles
substantially in the range of 1 to 10 microns and
separable from the cellulose and hemicellulose, the
particulate lignin and cellulose being together in
dissociated form havinq the appearance of potting soil
with negligible cross linking oE the lignin ancl xylan
having reoccurred so that the particulate material is
directly useful in fermentation processes, a major portion
oE the lignin being soluble in methanol or ethanol at room
temperature and being thermoplastic, the cellulose being in
the form o~ crystalline alpha cellulose microfibrils and
suitable for diges-tion by microorganisms and enzymes.
The present invention also includes the product in
particulate form when produced by the above method.
The pressure vessel is preferably vented to
atmosphere over a period in the range of the order of 5 seconds
to of the order of 30 seconds. The pressure vessel is
preferahly rapidly filled with the said steam to bring the
lS lignocellulosic material to a temperature in the range of the
order of 200C to of the order of 238C, more specifically to a
temperature of the order of 234Co The lignocellulosic
màterial is preferably brought to the said temperature in less
than of the order o-f 45 seconds. Preferably the expulsion o~
the lignocellulosic material to atmosphere is accomplis~ed in
less than of the order of 500 milli-seconds.
In some embodiments of the present invention, the
lignin in native form, having a purity of the order of 93
weight %, is solvent extracted from the particulate material,
at a temperature of less than of the order of 45C, preferably
room temperature, by a solvent selected from the group
consisting of ethanol and methanol, the lignin so extracted
~eing readily reactive chemically, thermoplastic, having an
in~rared spectrum approaching that of so called native lignin
and a number average molecular weight of the order of 600 to o~
the order of 1000.
When the lignocellulosic material is freshly
harvested moist wood, then the pressure vessel is preferably
rapidly filled with the said steam to a pressure in the range
of the order of 500 to of the order of 700 psi.
In other embodiments o-f the present invention the
lignocellulosic material is a substance selected from the group
consisting of wood and bagasse dried below the fibre saturation
to a moisture content of less than of the order of 20 weight %
and the pressure vessel is rapidly filled with the said steam
at a pressure in the range of the order of 400 to of the order
of 600 psi.
In some embodiments of the present invention the
lignocellulosic material is hardwood.
In other embodiments of the present invention the
lignocellulosic material is annual plant material and the
pressure vessel is rapidly filled with the said steam to a
pressure in the range of the order oE 500 to of the order of
600 psi.
When the lignocellulosic material is annual plant
materialt the pressure vessel is preferably rapidly filled with
the said steam to a pressure of the order of 550 psi.
Preferably, condensate is removed from a bottom
portion of the pressure vessel as it is formed.
Prefe~ably, the pressure vessel is gent:Ly ~ented to
atmosphere -to a pressure of the order of 300 psi be~Eore the
valved outlet is opened.
In the accompanying drawings which are provided to
give an understa~ding of the present invention and, by way of
example, to describe embodiments o-f the present invention;
Figure 1 is a sectional side view oE a pressure
vessel having outlet,
Figure 2 is a graph of the time t in seconds to heat
lignocellulosic material plotted against the temperature T in
C, with viscosity/temperature curves added for various
constituents of the lignocellulosic material, and
Figure 3 is a schematic representation of the cell
wall structure o~ a ~ibre of lignocellulosic material.
In figure 1 there is shown a pressure vessel 2,
having a valved outlet which in the embodiment illustrated is
an extrusion die outlet 6, an extrusion die closure plug 30, a
loading end closure flap ~ and steam inlet orifices 10 to 12.
The pressure vessel 2 has a bottle neck portion 14
leading to the die 4 and entry ports 16 and 18 ~or temperature
probes (not shown).
The front end of the pressure vessel 2 containing the
die outlet 4 has a flange 20 to which is sealed a curved
impinging tube 22 which gradually reduces in cross-section in a
downsteam direction. The curved impinging tube 22 has a
spindle inlet sleeve 24 provided with a flange 26. A pneumatic
xam 28 is attached to the flange 26 and has a die closure plug
.
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30 mountecl on the spindle 32 oE the ram 28. A condensate
drainage tank 31 is provided having an outlet valve 33.
The rear end 34 of the pressure vessel 2 is sealed to
the remainder by flanges 36 and 38 and has the loading end
closure flap 8 hinged thereto by a hinge 30 and sealable
therewith by a clamp 42. The rear end 34 has a venting valve
44.
In operation the loading end closure flap 8 is opened
and the pressure vessel 2 is loaded with lignocellulosic
material in a divided form with the die closure plug 30 closing
the outlet 4 and the valve 33 closed. A rod (not shown) is
used to pack the lignocellulosic material in the pressure
vessel 2.
With the pressure vessel 2 completely filled with
lignocellulosic material the die closure plug 30 is sealed by
the pneumatic ram 28 and the closure plug 8 is sealed to the
rear end 34 by the clamp 42 and then the pressure vessel is
filled with steam at a pressure of at least 500 psi, preferably
in the range 500 to 700 psi, and at a sufficient temperature to
raise the temperature of the lignocellulosic material to a
temperature in the range 185 to 240C, preferably 200 to 238C
and in particular 234C, in less than 60 seconds (preferably
less than 45 seconds) to plasticize the hemicellulose and the
lignin in the lignocellulosic material so that the
lignocellulosic material is thermally softened into a
plastic condition, by injecting steam into the steam
inlet oriEices lO to 12 to form a source (not shown).
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The temperat-lre pro~es (rlot shown) in the ports 1~ and L8 are
used to monitor the temperature oE the lignocellulo.sic material
in the pressure vessel 2 to determine when the lignocellulosic
material has reached the chosen temperature.
As soon as the lignocellulosic material in the
pressure vessel 2 has reached the desired temperature the top
of the pressure vessel 2 is gently vented to a-tmosphere by
opening the valve 44 to reduce the steam pressure to between of
the order of 200 psi to of the ordex of 450 psi, preEerably 300
psi and then the pneumatic ram 28 is immediately actuated to
withdraw the closure plug 30 and immediately open the die
outlet 4 to atmosphere so that the lignocellulosic material is
explosively expelled through the die outlet 4 in the
plastic condition along the curved impinging tube 22. The
sudden release to atmosphere explosively expels the
lignoceIlulosic material in a plasticized condition and
produces a particulate material having .he appearance of
potting soil which stains the fingers brown and has a high
enough specific gravity to sink like a stone in water.
While the curved impinging tube is not essential it
has the advantage of utilizing some of the ex,pulsion force to
further comminute the lignocellulosic material in addition to
the comminution obtained by extrusion.
During the heating cycle, condensate from the steam
wiIl drain into tank 31 which is at the same temperature as the
pressure vessel 2, and the condensate can be released Erom the
tank 31. by the valve 33 after the lignocellulosic material has
been expelled through the die outlet ~.
In Figure 2,
a) Is a curve showing the relationship between time (t)
seconds taken in the DeLong process, using an input steam
temperature of 255C and pressure of 630 psi to reach a
temperature (TC) of the lignocellulosic material.
b) Is a curve showing the relationship between lignin
viscosity (N), which is higher in the direction of arrow H and
lower in the direction of arrow L, plotted against the
temperature (TC) of the lignin,
c) Is a similar curve to (b) but of the hemicellulose
(xylan) viscosity (N) plotted against the temperature (TC) of
the hemicellulose, and
l~ d) Is a similar curve to (b) but of the cellulose
viscosity (N) plotted against the temperature (TC) of the
cellulose .
For all of the curves (b) to (d), the point D'
represents the degradation temperature for each component of
the lignocellulosic material while point P on curve (a)
represents the optimum time (t) in seconds and temperature (T)
in C.
Referring to Figure 2, for a better understanding of
the present invention, it is useful to review the chemical and
physical effects which occur during the DeLong process:
At ~J125C, depending Oll moisture content, curve (b~
shows that the llgnin passes through its softening temperature.
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It wants to become a 1iquid oE about the consisterlcy o~ table
syrup.
At ~V165C, depending on moisture content, curve (c)
shows that the xylan component of the hemicellulose passes
through its softening temperature. It wants to become a liquid
of about the consistency of tooth paste.
At r~234C, and affected very little by moisture, the
crystalline alpha cellulose passes through its softening
temperature.
It wants to yield like a soft willow. This is point
P on curve (a).
Tests have shown that if too high a steam temperature
is used, the material, depending on moisture content, will
begin to pyrolize. This effect becomes progressively more
pronounced above input steam pressures of 700 psi (262C). The
material being treated is difficult for the steam to access,
and it is an insulator. Thus, it requires about 45 seconds to
raise the materi~l homogeneously to 234C. If the pressure is
too high the outside material will burn before the inside of a
wood chip achieves the required 234C~ If the steam
temperature is too low then the time taken to raise the
material to a homogenous 234C will be longer than 45 seconds.
After 30 seconds above 180C the lignin begins to
condense, with the hemicellulose probably forming crosslinks
which reduces the yield of pure lignin. Also, the xylan
component of the hemicellulose is degraded above 220C. The
degradation reactions become serious when these materials are
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above their clegradation temperature for times beyoncl 30
seconds. Figure 2 shows that both softeniny temperatures
(measured by viscosity) and degradation temperatures of lignin
and hemicellulose (xylan) are achieved with tha DeLong process
in 30 seconds or less.
At 180C the lignin begins to repolymerize. At 220C
the hemicellulose begins to degrade. Thus it is e~sential to
spend as little time as possible above 180C, and no time above
240C (preferably no time above 234C), plus a small allowance
for measurement error. It is the above physical factors which
determine bringing the lignocellulosic material to a
~ temperature in the range 185C to 240C in less than 60 seconds
; and the preferred temperature range 200C - 238C tbetter still
234C) in less than 45 seconds.
In Flgure 3 the cell wall structure of a fibre of
lignocellulosic material is depicted comprising a middle
lamella 46, a primary wall 48 a secondary wall composed of
layers 50 to 52, and a central lumen 54.
The effect of the ejection through the die outlet 3
and the instantaneous decompression of the lignocellulosic
material is to produce a profound morphological and
topochemical change, to quench the wood components in their
transformed state and stop all further reaction. Some lignin,
coalesced into droplets, i5 propelled along the fibre until it
exits from the cell wall along with some hemicellulose.
Movement of coalesced lignin and hemicellulose along and out of
the S2 layer 51 of the Eiber opens up the internal structure by
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spreaclin~ the structuralIy weakened (softened) microEibrils in
the fibre bundles~ Wherl the coalesced liynin reaches a weak
spot in the ceLl wall it is ejected leaving a number of voids
in a generally loosened cellulose structure. These voids, as
well as the lack of the lignin protective coating accounts for
the radically increased accessihility of the carbohydrate
fraction to enzymes, microflora and acids. The exiting steam
drops the temperature instantly to 100C which prevents further
chemical change or re-combinations and stabilizes the cell
structure in an open and easily accessible state.
The role of prehydrolysis is difficult to judge. It
is clear that it is a Eactor. When the steam enters the deep
cell structure the material is heated, and in the case of
poplar, both acetic and uronic acid are produced (9.9~ of the
dry weight of the starting material). Hydrolysis occurs at an
acceleratir.g rate and the degree of polymerization of both the
lignin and the xylan is reduced. The totally unexpected
phenomenon which occurs during the process and subsequent
; decompression is that the hemicellulose and the lignin are
20 cleanly separated. This clean separation renders the lignin
soluble in a mild organic solvent. The molecular weight of the
lignin is reduced to the order of 800 (number average) and the
DP of the xylan is reduced from ~v220 to rJ7.
In summary, the explosion process involves control of
competing chemical and physical reactions:
a) hydrolysis and m~chanical severing of lignin/hemicellulose
links;
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b) repolymerization of lignln,
c) degradation of organic matter;
d) uni~orm di~fusional penetration of heat and steam.
Because of the specific (temperature and pressure)
and short reaction time, reaction (a) and process (d~ are
favoured over the other two so that the desirable product is
obtained.
It has been observed that two effects of the
explosion are more pronounced when the starting material is
below fibre saturation (20~ moisture). The first is the
desirable effect of breaking of the cross links between the
three major components of the lignocellulosic material. The
second i~ the less desirable but often times useful effect o
depolymerization of the three main polymers contained in
lignocellulosic material (lignin, xylan and cellulose). Taken
to e~treme and most pronounced with dry material, water soluble
low molecular weight components of the lignin, such as phenol
and benzene are produced. Phenol and benzene are toxic to
microflora. In the case of the xylan some furfural may be
produced.
Thus, for some applications it is desirable to
achieve the dissociation of the lignin, xylan and cellulose by
severing their cross links but reduce the amount of
depolymerization of the lignin, xylan, and cellulose. This
effect is achieved by processing the material in the reactor in
the normal way until the optimum temperature (234) is achieved
(usualLy in about 40 seconds) then gently reducing the pressure
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in the reactor by venting the steam through the valve ~ to a
pressure in the range of the order of 200 psi to of the order
of ~50 psi (preferably o~ the order of 300 psi) preferably over
a period of the order of 5 to of the order of 30 seconds
(better still of the order of 15 seconds). Then release the
material instantly to atmosphere in the normal way.
This procedure produces the following three effects:
a) the reduction in pressure while retaining the material in
the vessel causes the steam in the microstructure of the
cell to escape gradually. The temperature reduction in the
material is also gradual because the steam temperature does
not fall below the optimum temperature of the material
until a pressure of 450 psi is reached. Thus, if the
venting takes place at a substantially uniEorm rate, the
microstructure of the material is subjected to the
mechanical action of the escaping steam for a period of at
least 10 seconds before the cellulose passes below its
softening temperature. Thus while the mechanical effect on
the layer 51 is milder, its duration is much longer and the
net effect is the same.
b) At approximately 300 psi the steam temperature is 214C and
the temperature of the material is below 220C. At these
material temperatures, the depolymerizing effect oE the
explosion is reduced and the explosive force because of the
reduced pressure is also less. The primary e~fect,
however, is undoubtely the reduced temperature of the
material. The reduced temperature is sufficiently high ~o
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break the cross links but will reduce the depolymerization
effect Oe the explosion.
c) The gradual venting of the reactor will vent off a major
percentage of any toxic vapors such as furfural, which may
have been produced. The boiling point of furfural is
161.7C. Thus it will vent quite readily and can be
recovered as a valuable byproduct.
As a general rule, the total time taken for heating
the lignocellulosic material to the desired temperature plus
the valve down time should not be allowed to exceed about one
minute. Beyond that time, the lignin will begin to degrade and
the process will be less effective. By controlling the valve
c~own time and the valve down pressure, some control over the
degree of polymerization of the end products can be achieved.
If it is suspected that a significant percentage of
the material may be dried below fibre saturation in the case of
wood or bagasse, the input steam pressure should preferably be
reduced from 650 psi to 550 psi as is preferably done for
straw.
During the first few seconds after the steam is
introduced into the reactor, contact with the relatively cool
lignocellulosic material produces a condensate. This
condensate covers between 10 and 20 percent of the
; lignocellulosic material depending on the moisture content and
temperature of the starting lignocellulosic material,
preventing proper processing of the submerged lignocellulosic
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material. Thu~, the tank 31 for removing khat condensate as it
occurs substantially improves the performance of the process.
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