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WO 99/60027 PCT/CA98/00479
1
PROCESS FOR PREPARING A HIGH PURITY CHEMICAL-FREE
MICROCRYSTALLINE CELLU7GOSE FROM A CHEMICALLY PRODUCED CELLULOSE
FIELD OF THE INVENTION
The present invention relates to a process for preparing high
purity chemical--free microcrystalline cellulose having a low
polymerization degree (DP) from a chemically produced
cellulose.
BACKGROUND OF THE INVENTION
Microcrystalline: cellulose is obtained by depolymerization of
cellulosic fibres down to their constitutive microcrystals.
Such a depolyme:rized cellulose is also known as level-off
degree of polymerizai~ion cellulose (LODP) .
The LODP accounts for the fact that cellulosic fibres are
constituted of fibrils held together by significant amounts of
disordered amorphous cellulose. The fibrils are mainly formed
of highly ordered cellulose crystals. The latter are held
together by adhesion and by small amounts only of amorphous
cellulose in between 'the crystals .
The preparation of microcrystalline cellulose involves two very
different steps:
(a) production o:E cellulose fibres; and
(b)transformation of the fibres into microcrystalline
cellulose.
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1 S, Juli 1999
Existing processes far the production of microcrystalline
cellulose start with the purchase of cellulose fibres and aa_e
followed by carrying out a conventional process such <as
hydrolysis or its variants to obtain the des~.red end product.
Such processes are not environmentally friendly since the
cellulose fibres is derived from conventxol~al processes using
sulfur chemicals, such as SOz (sulfite process) , Na~S (Kraf:t
process), cad sulfuric acid (ethanol pulping process, or
~.0 Allaell) . Fztrtherz~toxe, the depolymerization of cellulose f~.bxes
to yiel.d~ microcrystalline Cellulose via an acid hydrolys~_s
process uses mineral acids (HCl and HZSO~ being common acids).
Canadian patent No.2,~.37,890 teaches the conversion c>f
cellulose fibres (derived from conventsonal pulp~.ng processe:~)
into microcrystalline cellulose by using benign reagent~~,
namely Oz and C02. More particularly, this patent teaches that
low degrees of polymerization, DP, may be obtained by a high
pressuxe treatment of aqueous suspensions of cellulose wa,th c~2
20 and C02 within autoclaves operated in batch mode. The treat~:d
product is subjected to centrifugal washing and spray dryirsg
to yield rnicrocxystalline cellulose with a DP of about 300.
Canadian patent No. 2,139,00 discloses a process wherein a
fibrous cellulose 2-In wt~ suspension is subjected to repeated
passages through a high shear zone (two confronting surface:>,
one fixed and the other having a rotational movement such as
a double disk ref~.nerj at temperatures not greatez than 200"F
(93.3°C). The obtained product is a cellulose having a high
30 reterit~.on of water (about 3300. This cellulose is called
"microdenorninated cellulose"_
A~f,END~D ~,~ .
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~:.anadian patents No. 1,198,703 discloses a process for the
;simealtarzeous production of LoDP rnicrocrystalli.ne cellulose and
glucose by carrying nut in a sequential. manner using hardwoods
s~rld annual plants (such as stx'aw and bagasse) as starting
Z.naterial , The separation of the hem~,cell~ulose, lignin and
~~ellulose is accomplished via steam treatment (which libe=ate~s
a significant fraction of the hemicelluloses), delignifica.tion
(with benign organic solvents or alkaline solutions), and
:removal of residual heraicellulose from cellulose by an alka~_i
treatment. The cellulose thus produced i.s soaked/impregnated
,with either HCl, H~504 or Sox, and introduced into a reactor
~~he.re saturatEd steam is added to .Leach 185°-240°C in less
than
' 60 seconds. The treated material is withdrawn (i.e. expelled)
from the reactor through a die by opening a valve. The expelled
~cellulosic solids are adiabat~.cally quenched to 100°C :in a
fract~.on o~ a second. The product that is sa obtained consist
of disegregated cellulose fibrils (-20-50um 3..n diameter and, 1--2
m in length) and glucose presumably produced from the
hydrolysis of amorphous cellulose.
'WO-A-92 14 760 and SU-A-2792942 alav disclose a process fc>r
producing xuicrocrystalline cellulose wuhich uses steam explosion
treatment. F~owever, such processes ~,rlvol~rc: a step where the
cElluiose is treated with a chemical agent such as an acid.
In view of the above, there would be an interest to develop an
environmentally friendly process for producing a "greeri"
(env~.rotunentally friendly) lnicrocrystallille celJ.ulose, w;hic:h
would use chemically produced eellul.ose such as a
premanufactured apec~.alty pulp as the starting material bun
taould avoid the use of C1- or S- containing acid reagents.
The Applicant has already proposed a process for producing
chemical free cellulose which in turn may be used for produ:ir.;g
microcxystalline cellulose- In this process, any suitable ra.w
~caaterial such as softwoods, hardwoods, kenaf, bagasse anal
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straws may be us;ed as starting material. The selected starting
material is in the form of shavings, residues or sawdust are
first subjected to heating under water vapour pressure for a
certain period of time. Then, a violent depressurization is
created thereby allowing an instant vaporization of the water
from the wood which in turn causes the defibration of the wood
matrix. The defibrate:d wood is then forwarded into a refiner
and is subsequently soaked in water to wash away excess of
hemicellulose. The li.gnocellulosic material obtained is then
subjected to all~;aline or alcohol extraction of the lignin. The
latter can be further converted to phenolic products which can
be used in the manufacture of adhesives for wood panels, or as
raw material fo:r pharmaceutical and nutritional products.
A purification step is then carried out for the purpose of
concentrating and recovering the "good" fibres. The cellulose
fibres so obtained are subjected to a complexation step. This
step is necessa~:y to eliminate the presence of ions that could
interfere with the next step which is the bleaching of the
cellulose.
After bleaching the cellulose, a fibrous cellulose is obtained.
This fibrous cellulose undergoes a second steam explosion
treatment, like the one previously described. This cellulose
is finally ble~~ched and microcrystalline cellulose is then
recovered.
Thus, the prior process comprises the steps of:
a) mechanical and/or pneumatic purification of the shavings,
residues or sawdust;
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b)primary steam explosion treatment (pressurization of the wood
by steam);
c)water extract_Lon (for separating hemicellulose from the
wood) ;
5 d) refining;
e) alkaline or alcohol. treatment (for extracting lignin);
f) purification (for concentrating and recovering the "good"
fibres) ;
g) complexation (ion elimination);
h) bleaching of the fibrous cellulose that is so obtained;
i) secondary steam explosion treatment, and
j) bleaching of the fibrous cellulose.
The Applicant has now~found a way to produce a high purity
chemical free micrc>crystalline cellulose having a low
polymerization degree :by using a chemically produced cellulose.
SUMMARY OF THE I:NVENT:ION
Thus, it is an object of the present invention to provide a new
process for producing microcrystalline cellulose having a low
degree of polym~srization (DP) by using a chemically produced
cellulose (kraft, sulfite) as starting material.
More particularly, the object of the present invention is to
provide a proce:3s for preparing microcrystalline cellulose by
using as a starling material, a chemically produced cellulose
which may be in the form of a premanufactured specialty pulp.
This process comprises the steps of:
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a) preparing a ~~ulp by repulping, filtration and trituration
of the chemicall.ly produced cellulose;
b) subjecting the pulp obtained in step a) to a steam explosion
treatment in order to obtain a microcrystalline cellulose;
c) filtering and washing the microcrystalline cellulose;
d) bleaching the: micrc:rystalline cellulose obtained in step c)
to obtain a bleached microcrystalline cellulose;and~
e) filtering and washing the bleached microcrystalline
cellulose of step d) to obtain the high purity chemical free
microcrystallin~~ cellulose.
In accordance w:~th the invention, the process is characterized
in that at the e:nd of the steam explosion treatment of step b),
a shearing force is applied onto the pulp.
Thus, it has been found a way to depolymerize cellulose to low
DPs by treating a ce7.lulose containing aqueous suspension via
an appropriately designed floating fixed head high shear
devices such as a valve, near the glass transition temperature
of cellulose, typically in the 230°-240°C range. The shearing
force applied on the pulp is the result of the combined effect
of instant depressurization and sudden flow of the pulp through
a discharge va7.ve .
Hence, through this process, it is possible to transform
chemically produced celluloses (Kraft, sulfite), via water
soaking/impregnation, and aqueous/steam treatments using only
water (liquid and steam), into low DP microcrystalline
cellulose.
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As can be appreciated, the process of the invention comprises
a bleaching ste~> (d) where the microcrystalline cellulose is
bleached by using benic3n chemicals such as caustic and hydrogen
peroxide. Hence, the process of the present invention does not
require the use: of acid. Nonetheless, classical bleaching
reagents may be used for bleaching the microcrystalline
cellulose if de~;ired.
The process of the present invention allows the production of
microcrystalline~ cellulose having a DP comprised between 150
and 260, a whiteness higher than 80 and a cristallinity index
of about 83% to 87%.
DETAILED DESCRII?TION OF THE INVENTION
In the process of the present invention, a premanufactured
pulp, such as a specialty pulp, which is a bleached cellulose
from angyosperm or gymnosperm obtained from kraft, bisulfite
chemical processes ox- any conventional pulping processes, is
used as the starting material.
Softwoods provide mic:rocrystalline cellulose with a level of
DP (LODP) >200, whereas hardwoods and straws yield
microcrystalline cellulose having level of DP(LODP) <200. The
degree of polymerisation can be tailored and optimized that is
reduced by subsequent aqueous/steam treatments. However, this
will cause a~ reduction in the final yield of the
microcrystalline cellulose.
When carrying ~~ut th.e process of the invention, the starting
material must not be in a condensed state so that it is easily
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transported. It must contain a residual humidity varying from
35 to 65%. Hence,, if the starting material is a premanufactured
pulp, it must be repulped in order to increase its humidity and
to render it ir.~ a less condensed state. This in turn will
increase its rea.ctivit:y.
The cellulose having .a proper level of humidity is subjected
in a reactor to a steam explosion treatment in a continuous or
discontinuous batch mode.
The steam explosion treatment is carried by pressurization of
the reactor. This is done by injecting steam in the reactor.
The various conditions such as temperature, pressure and
residence time are perfectly determined and controlled so that
it is possible to develop a mathematical model, referred to as
the "severity" factor,. Ro, for determining the final quality
of the treated cellulose.
"Severity" stands for a hydrolysis control parameter
represented by the symbol Ro and expressed by the mathematical
formula which combines two variables, namely the time of
heating and the temperature or vapour pressure.
R = t x a T-100f14.75
0
t is the time o:f heating in minutes and T is the temperature
in Celsius.
A shearing force: is applied at the end of the steam explosion
treatment and the treated material is subjected to filtration.
The shearing force :is the result of the combined effect of a
violent vaporization induced by an instant depressurization
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and a sudden flow of the pulp out of the reactor through a
discharge valve. The cellulose is simultaneously washed :i.n
order to prepare the pulp for bleaching.
During the steam treatment, it is recommended to work at a
pressure varying from 215 to 440 psig (about 15 to 31 atm), at
a temperature varying between 200° and 240°C, and at a contact
time varying from ~1 to 24 minutes .
As mentioned above, an important ,aspect of the present
invention lies in that at the end of the steam explosion
treatment, a shearing force is applied on the treated pulp.
The final bleaching using sod~.um hypochlorite at 40°C, p~I > 9
during 1 or 2 hours or hydrogen peroxide i.s carried out at a
pH comprised between 10 and 11, at a temperature of 60 ° ~ or
110° to 120°C under riitxogen or oxygen pressure varying from
60 to 120 psi (4:2 to 8.4 atm), and in the presence of
magnesium sulfate and sodium silicate.
EXAI~PI~E 1
(I)PREPARATTON OF THE PUPULUS TREMUZ,OIDES SOhUBLE PULP
The method to prepare this type of pulp comprises a step of
steam explosion which could be carried out in a continous or
discontinuous batch mode. The pressure, temperature and contacts
time are perfectly determined and controlled, since they ;have
been modeled for each type and form of biomass and each~~type
arid size of reactor.
Lj
-,_,
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The poplar chips (6 mm of thickness and 1 to 2 inches in width)
must contain a humidity level of between 35% and 65% calculated
as base humidity.
5 Their dry composition are the following:
Total extract . 5, 03%
Alpha cellulose . 44, 92%
Holocellulose . 77, 30%
Pentosans . 17, 82%
10 Lignin . 17, 67%
Ash . 0, 19%
Dp _ 1352
A 6 litre reactor is charged with the wood and is supplied by
steam. The reactor is maintained at a pressure of 380 psi for
3 minutes before carrying out instant depressurization. The
resulting pulp from the cyclone is refined at a pulp
consistency of 2% for 90 seconds at 18000 rpm in a blaring
Blender Commercial Model 38 BL.19* with water at 60°C or up, so
as to eliminate the hemicellulose liberated during cooking.
The Iignocellulose obtained is mixed in a solution of sodium
hydroxide at 22 grams per litre so as to acheive a pulp
consistency of 10%. The treatment is carried out at 160 °C for
90 minutes.
After washing, the partially delignified cellulose has a kappa
index between 6 and 8. A purification step of the pulp allows
to separate fines and other impurities so to obtain a very low
kappa index which is between 3 and 5.
* Trademark
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The cellulose is then bleached either by the classical
sequences of bleaching CepHD:
-chlorination at pH<2 with 0,08 g/1 of active chlorine at a
temperature of 25°C
-extraction of c:hloro-lignin by washing the fibres at 40°C/by
means of a hydroxide sodium solution at 3 g/1 and containing
hydrogen peroxide at 0, 2 g/1
-hypochlorination at ~40°C, 0,3 g/1 of active chlorine at a
temperature of 70°C anal a pH varying from 3,5 to 7,0.
The bleached cellulose is obtained with an output of 35% on the
original chips (outpu.t calculated from the dry bases) and
embodies the following characteristics:
S 10 . 4, 89%
S 18 . 2, 26%
Pentosans . 1, 27%
Alpha cellulose: 96, 40%
Ash . 0, 10%
DP 787
This high purity cellulose may be used as a soluble pulp in the
production of viscose, cellulose acetate and microcrystalline
cellulose.
~T_I) PROCESS FOR P~tEPA_R-~TION OF MICROCRYSTALLINE CEL ULO~F
The bleached cellulose pulp obtained herein above is
transformed into microcrystalline cellulose by a process which
is an object of the present invention.
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The soluble pulp is brought to a residual humidity comprised
between 35~ and 65%, preferablx 50% before being treated with
steam at 350 psi (29.6 atm) for B minutes, washed and b~.eached
with hydrogen peroxide at pH 10,5 during 50 mZnutes at 60°C and
preferably at '120°C undex nitrogen or oxygen pressure.
The so obtained microcrystaJ.line cellulose has the following
characteristics:
PP: 170
Crystallinity index (RX) . 87~
F~XAMPS~E 2
(I)PRS$ARATION GF CEhhDhOSE PULP FROM SUGARMAPhE SAWDUST
The cellulose of this example was prepared from. sElected
sugarmaple sawdust. The pulping process is sixnj.lar to the one
described in example l, wherein the heating step is carried out
at a pressure of 350 psi (24_6 atm) fox 4 minutes. The
purification step is maintained so as to have a pulp
consistency of 2%. The lignocellulose is treated at a pulp
consistency of 12% in a sodium hydroxide solution (28g/1) at
160°C for 90 minutes. The lignin is solubilized in the form of
sodium lignate.
The cellulose so obtained has a kappa index of 10. A chelating
agent is added to the treatment so as to prepare the b7.eaching
with hydxogen peroxide (2~ from dry base) at pH 10,5 zn a pulp
consistency of 12~. After washing and filtxati.on, the cellulose
comprises the following characteristics:
DE 700
Pentosans 1,20
n fir... ~
~va..:...',:c 3. .,: ~.,Q:r,' 9
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Holocelluloses: 98,05%
Ash: 0,70%
kappa index: 3
PCT/CA98/00479
, This cellulose nay be used as a food fibre due to its high
purity and its method of production which do not require any
use of chlorinated and/or sulphurated products. This cellulose
may also be used as a starting material for the application of
specialty pulp p~:oduction, viscose, cellulose acetate, etc.
( I I ) PROCE88 FnR z?REPA_RING THE MICRO _RVSTALLINE CELLULOSE
The cellulose obtained herein above is subjected to a reactor
for the steam explosion treatment. This treatment is carried
out at 320 psi for 8 minutes. After this treatment, an instant
depressurization is carried out in the reactor. A bleaching
step is carried out and the resulting microcrystalline
cellulose embodiea the following characteristics:
DP . 182
Crystallinity inolex (RX) . 85
Density: 0, 28g/cm3
Ash: 0, 10%
The output of this process on the initial cellulose is 80%.
EXAMPLE 3
Cellulose HV 20* from ~culphite pulp production from softwoods
having the following characteristics was used as the starting
material:
* Trade-marks
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S 10 at 25°C . 5,8%
S 18 at 25°C . 5,2%
Intrinsic viscoF;ity . 14,46 dl/g
Pentosans . 4,4~c
Ash: 0,17%
Whiteness: 87,5$
DP 1966
PCT/CA98/00479
The treatment that was applied for a pulp having a high DP was
16 minutes under 320 psi. The obtained cellulose was sieved on
70 mesh. This cE~llulose can be bleached at 60°C using 2% of
hydrogen peroxide in the presence of magnesium sulfate and
sodium silicate .at pH 10,5. In the case where it is bleached
with sodium hypo~~hlorite 1% on dry base, it is carried out at
40°C for 2 hours with a pulp consistency of 10% and at a pH >9.
The output of these operations is 80% on dry cellulose as the
starting material.
The so obtained microc:rystalline cellulose has the following
characteristics:
DP 253
Cristallinity index (RX) : 85%
Ash . <500ppm
EXAMPLE 4
Cellulose Q90*, a resinous kraft pulp (20% Jack pine, 80% Hlack
Spruce) having the following characteristics was used as a
starting material. for ithe present example.
*Trade-marks
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WO 99/60027
Viscosity . 21,4 cps
Whiteness Iso . .B9%
DP . 1156
PCT/CA98/00479
5 Two consecutive treatments of 8 minutes at 320 psi were carried
out. After siev:Lng on 70 mesh and bleaching with hydrogen
peroxide 2%, the mic:rocrystalline cellulose embodied the
following characteristics:
10 DP . 246
Cristallinity index (R~): 83%
Ash: <500 ppm
Although the present invention has been explained hereinabove
15 by way of a preferred embodiment thereof, it should be
understood that t:he invention is not limited to this precise
embodiment and treat various changes and modifications may be
effected therein ~Nithout departing from the scope or spirit of
the invention.
