Note: Descriptions are shown in the official language in which they were submitted.
MD-Papierfabriken
Heinrich Nicolaus GmbH
Ostenstr. 5
8060 Dachau
A Process and Apparatus for Continuously
Digesting Plant ~iber Ma-terial
~ACKGROUND OF lHE I~VE~ION
~his invention relates to a process and an apparatus for
~25 continuously digesting plant fiber material with an or-
ganic solven-t.
~he economically important processes for digesting cellu-
lose-containing raw fiber substances currently operate
according to the sulfate or sulfite process.
~ssential disadvantages of these process, however~ are the
required quantities of approximately 800 to 1000 tons per
day for economical and rational operation as well as the
enormous amount of environmental pollution in the form o~
waste water and exhaust gases, particularly because these
` contain sulfur compounds.
DESCRIPTION 0~ THE PRIOR ART
The possibility of digesting cellulose-containing plant
fiber materials with organic solvents was already recited
in US patent specification 1,856,567.
German laying-open print D~-OS 2,637,~49 describes a sol-
vent extraction process for producing cellulose pulp, in
which lignin is extracted from finely divided fibrous plant
material at elevated temperature and pressure using an aque-
ous solution of a lower aliphatic alcohol. This process op-
erates with a plurality of batch extraction vessels, a ser~
ies of working steps succeeding one another in each extrac-
tion vessel. ~his known process, however, is not suitable
for the ccntinuous, economical production of pulp from
plant fiber material. Ihe prime costs are disproportionate-
ly high owing to the large number of extraction vessels re-
quired. In particular, unless disproportionately large
amounts of solvent are used, the heating interval per charge
is very long so -that hydrolysis of the cellulose and hemi-
cellulose commences even before the hlgh temperature is at-
tained which is required to dissolve the lignin. ~or this
reason, it is not possible to obtain pulp with sufficient
strength for paper production using this known process.
US patent specification 3,5859104 and German opposition
print DE-AS 2,64~,155 teach continuous digesting and re-
covery processes for raw plant fiber materials for pulp
production using organic solvents. No pre-impregnation
is employed in these processes. ~or this reason, the flow
o~ the material is easily blocked, particularly since pos-
iti~e or forced transport through the reactor is not en-
visioned. In the known processes, the digested material
is cooked and then washed with cold organic solvent. Wash-
ing with cold solvent, however, does not remove residual
lignin. The organic solvent is subsequently pressed out
of the digested material in a worm extruder. The digested
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material is thereafter washed with an aqueous sodium
hydroxide solution in a separate means~ ~his solution
is finally pressed out of the material in a second worm
extruder. ~his approach is hardly effective~ but very
expensive. In particular, the necessity of driving the
organic solvent out of the cold~ digested material re-
sults in a higher expenditure of energy.
SUMMARY 0~ ~HE IMVEN~ION
~he object of the invention is to provide a process and
an apparatus for continuously digesting plant fiber mater-
ial with orgarlic solvent, the essential components of -the
plant fiber material9 i.e. lignin7 hemicellulose and cel-
1~ lulose being separated and extracted in pure form. ~his
process and apparatus operates economically on a large
industrial scale 9 iS characterized by a simple procedur-
al approach, in particularly avoiding the afore-mentioned
drawbacks of the known processes, and yields a high-grade
pulp adapted for easy bleachingO
~he subject matter of the invention is a process for con-
tinuously digesting plant fiber material wherein the plant
fiber material is treated with an organic solvent at temp-
~5 eratures of 130 to 210C in a counter-current manner9 which
` is characterized in that the plant fiber material is impreg-
nated with an organic solvent in an impregnating meansV tha-t
the impregnated plant fiber material is introduced in the
` top reactor zone by a feeding means and is conducted posi-
tively thereby in a downward direction, that cellulose is
produced in the bottom reactor zone, that organic solvent
at a temperature between 130 and 210~ is introduced into
the middle reactor zone as an extr~tion liquor, is conduc-
ted upwardly to the plant fiber material in a counter-cur-
rent and is withdrawn at the top reactor zone~ and that
water is supplied in the bottom reactor zone as a wash
liquor, is conducted upwardly to the plant fiber material
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in a counter-current and is withdrawn in the middle re-
actor zone and that the laden extraction llquor i5 gep-
arated into the essential components lignin, hemicellu-
lose and solvent.
~he subject matter of the invention is furthermore an
apparatus for continuously digesting plant fiber material
with an organic solvent, said apparatus comprising an
impregnating means, a reactor for digesting the plant
fiber material, means for washing the digested plant fi-
ber material and separa-ting means, which is characterized
in that the impregnating means, in which the plant fiber
material is impregnated with an organic solvent, is de-
signed as a screw conveyor and communicates directly with
the top reactor zone through a pressurized lock, that a
digesting zone and a washing zone are provided integrally
in said reactor3 the reactor having in its base one or
more inlet ports for the wash liquor and a discharge port
for the digested plant fiber material, an outlet in the
middle reactor zone for the wash liquor and one or more
inlet ports for the extraction liquor, a feeding means in
the top reactor zone for supplying the plant fiber mater-
ial as well as an outlet for the laden extraction liquor,
and that separating means are provided for extracting lig-
nin, hemicellulose and solvent.
The advantages of the inventive process are to be seen in
particular in the fact that economical quantities are at-
tained with as little as 50 to 100 tons per day, that the
rawsubstanceæ contained in addition to cellulose in the
raw material, i.e. hemicellulose and lignin, are obtained
primarily in their native form, and that the organic sol-
vents used for extraction can be recovered and recycled
`- by simple distillation and subsequent condensation, for
example.
The continuous recycling of the solvent used is prac-tically
free of losses.
The process is suitable for all cellulose-con-taining raw
materials which can be pumped and poured. In particular~
possible raw materials are all na-tive and tropical types
of wood9 as well as annular plants such as straw, hay and
bagasse.
~he cellulose obtained with the aid of the inventive pro-
cess is excellent for paper producing owing to its high
quality.
~he inventive process, however, also successfully produces
in addition to cellulose the other essential components of
the plant fiber material in their pure, native form so that
these products are available for further processing in many
different sectors.
In the case of the digestion processes which up until now
ha~e been the only ones employed on a large scale, i e.
the sulfite and sulfate processes, the hemicelluloses and
the lignin accumulated in the form of derivatives which,
however, were unsuitable as raw materials for further pro-
cessing. ~hese substances, valueable in themselves, have
hitherto been trea-ted or burned as being nothing better
than waste products.
~y contrast, the inventive process for the first time per-
mits the other essential components, i.e. lignin and hemi-
celluloses, to be extracted in the form o~ pure, valueable
raw materials.
~he hemicelluloses are suitable for a multitude of applica-
tions, their use being emphasized in particular in -the fol-
lowing areas: sugar production, especially sorbitol and man-
nitol; the production of -thickening agents, adhesives) smooth-
ing and thickening agents for the paper~ textile and varnish
industry; the further processing to form alcohol by fermen-
tation. It should be emphasi~ed that it can be advantageous
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to recycle the alcohol obtained from the hemicelluloses
as a solvent in the inventive proces~.
Ihe inventive process succeeds in extracting lignin in
its native and substantially pure form. ~ignin is an ex-
tremely valueable raw material. It accumulates during
the process as polyol and is especially suitable for the
production of synthetic resins. In particular~ a reaction
with aldehydes is feasible. Special emphasis ig placed on
polyurethanes, acetal resins, epoxy resins and phenol resins
as synthetic resins which can be obtained star-ting with
lignin~
:
Organic solvents are employed as the extraction liquor, ~he
- organic solvents should be miscible preferably with water
in any arbitrary ratio Of special interest are aliphatic
alcohols with 1 - 6 carbon atoms, including polyvalent al-
cohols such as glycol and glycerin, aromatic and aliphatic
amines, ketones, such as acetone and methylethyl ketone,
ethers, such as tetrahydrofurane and dioxane, and solvents
such as dimethyl formamide and dimethyl sulfoxide. Preferred
as solvents are in particular aliphatic alcohols 9 especially
ethyl alcohol and isopropyl alcohol~ ~he solvents can be
added either in their pure form or as mixtures with water.
Possible aqueous mixtures can have solvent concentrations
in water amounting to 10 - 100 %, preferably between 40 and
~` 60 %.
~he operation is preferably carried out with a two- to five-
fold volume of extrac-tion agent, based on the plant fiber
material to be digestedl
Impregnation can be conducted at atmospheric pressure - this
is preferred - although it can also occur under excess pres-
~5 sure. It can be performed at room temperature or at an ele-
vated temperature, a temperature between 40 and 80C, in
particular about 60C, being preferred.
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~xtraction is performed in the digestion or ex-tract-ion
zone preferably at temperatures between 13~ and 21~Cg
in particular between 180 and 200C. ~he pressure in the
reactor ranges betweèn 1 and 40 bar, preferably between
5 15 and 25 bar.
~he digested and washed plant fiber material, i,e. the
cellulose, which is discharged from the reactor is pro-
cessed further in the customary fashion so that it is
10 superfluous to go into further detail in this context.
lhe extraction liquor which is discharged from the re-
actor and which is laden with the substances which were
extracted from the plant fiber material is separated into
15 its components, i.e. essentially lignin, hemicellulose
and solvent.
~he solvent, preferably the alcohol, can be separated
easily by distillation, stripping or flash evaporation~
20 ~he remaining aqueous phase which contains the lignin
and hemicellulose can be separated into its components,
for instance, by causing the lignin -to be precipitated
out of solution due to the concentration shift caused
by lowering the temperature and by removal of the alcohol.
25 ~he lignin is then suitably isolated by means of a worm
extruder~ a centrifuge or a heavy-phase separator? for
example. ~he residual hemicelluloses can then be supplied
in turn to purified production and further processing.
30 ~he impregnation of plant fiber material, which is in most
cases present in the form of wood chips, is of importance
in the inventive process. Unlike conventional, continuous
pulping processes according to the sulfate or sulfite pro-
cedures, in which an expensive impregnation is indispen-
35 sible to ensure adequate pulp qualit~7 the impregnation
can be carried out more simpl~ in the inventive processO
In the ~onventional sul~ate or ~ul~ite procedures, the
impregnation operation basically fulfills the following
functions. Uncondensable gases and terpenes are driven
out of the wood chips so that the thermal transition
and the diffusion of the digesting chemicals into the
wood chips is not affected. ~he wood chip~ adsorb the
condensate vapor and the density increases at the same
time. ~he moisture displaces the air away from the sur-
face of the wood chip. ~hese two effects ensure that the
wood chips will sink into the digesting liquor relatively
quickly. ~he tempera-ture and the moisture content of the
wood chips are raised and evened out so that uniform di-
gestion occurs in the reactor. The temperature and mois-
; ture cause the wood chips to swell so that the impregna-
~5 tion of the chips with the inorganic components of the
digesting liquor is facilitated.
Inadequate impregnation of the wood chips, viz. when there
is no digesting liquor present in the core of the chip
while the wood chips are at the digesting temperature, can
result in an excessive condensation of the lignin and thus
in the notorious "~lack sludge".
The pre-impregnation of the wood chips is usually conduc-
ted according to the following procedure. The wood chips
are introduced into what is termed a steaming vessel via
an appor-tioning or dosing means. ~he wood chips are treat--
ed here with saturated steam at pressures ranging ~rom 1
to ~ bar. The steam pretreatment is carried out ~or the
reasons already mentioned above. The wood chips heated
by the steam are then transported into an impregnation
tank where the actual impregnation with the digesting
liquor commences. The wood chips saturated with digest-
ing liquor are then hydraulically pumped out o~ the im-
pregnation tank by means of a high~pressure conveyor into
an inclined separation pipe in the -top reactor zone. The
wood chips are conveyed into -the upper reactor ~one by
means of a screw conveyor. Any excess impregna-tion liquor
~: '
will be returned to the impregnation tank by way of a
cylindrical sieve located in the separation pipe.
Essential simplifications in the impregnating means re-
sult in the case of the inventive process, where solvents
or solvent/water mixtures are utilized instead of inor-
ganic digestive liquors. ~he essential advantage of the
solvent/water mixtures is to be found in the considerably
higher wetting rate of and diffusion into the wood chips
compared to conventional digestive liquors based on solu-
tions of inorganic salts.
.
~erpenes and resins which disrupt and impair the penetra-
tion of the digestive liquor are removed by the organic
solvent. ~he use of a steaming means operated at excessive
pressure can therefore be eliminated en-tirely~ It goes with-
out saying that the wood chips can nevertheless be steamed
in the inventive process if so desired.
~o achieve satisfactory impregnation with the organic sol-
ven~, the entire impregnation device can be reduced to an
inclined impregnating pipe located in the upper reactor
zone. ~he wood chips are pushed upwardly by a screw in the
ascending pipe which is filled with the extraction liquor.
Another improvement in the impregnation is achieved by pre-
heating the extraction liquor, for example~ using saturated
steam7 to temperatures of about 60C. '~he residence time of
the wood chips in the impregnation zone amounts -to between
1 and 15 minutes per batch, for example, preferably 2 to 5
minutes. ~he wood chips are then adequately impregnated with
extraction liquor to ensure immediate submersi,on in -the di-
gesting zone. A uniform distribution of the concentration
of the solvent in the raw material is achieved and thus a
clearly defined starting basis for the extraction stage is
obtained due to the pressureless impreg~lation with the or-
ganic solvent.
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~he wood chips are introduced against the high pressure pre-
vailing in the extraction vessel preferably by means of a
clock-controlled pressurized lock. It is advantageous to
actuate the pressurized loc~ with a pressurized medium,
preferably steam, in order to minimize the pressure dif-
~erence between the pressurized lock and the reac-tor, there-
by avoiding pressure fluctuations in the reactor. If the pre-
impre~nation is conducted under zero pressure and if the re-
actor operates at 18 bar, it is advantageous, for ins-tance,
to introduce steam with 16 bar pressure into the pressurized
lock.
~he impregnated wood chips can be submerged beneath the
li~uor level in the extraction reactor without any prob-
lems by means of a feeding means, advantageously a screw.
~his feeding means also causes the wood chips to be con-
veyed posi-tively through the reactor. lhe pre-impregnation
and the positive conveyance of the chips reliably ensures
that no wood chip jams will be formed at the inlet port
and thus no obstruc-tion to the ~low of the material will
occur here.
In the continuous pulp digesting processes on a sulfite or
sul~ate basis, the digester can be subdivided into four re-
action zones. lhe first zone is an impregnation zone. An im-
pregnation zone is necessary in the conventional sulfite
or sulfate digesting process in spite o~ pre-impregnation
in order -to achieve proper saturation of the wood chips with
the digesting chemicals. Without absolutely uni~orm pre-im~
pregnation, the result will be "black sludge" owing to the
lignin condensation which runs parallel to the lignin decom-
position. ~he impregnation zone is followed by a heating
zone in which the wood chips are heated to the desired di-
gesting temperature. ~his is followed by the digestion zone
in which the actual decomposition of lignin occurs~ ~his is
followed in turn by the washing and cooling zones in which
the dissolved decompositiorl products are removed by counter-
current extraction with hot water. The wash water is also
utilized as a quenching liquid to terminate the chemical
decomposition reaction.
The procedure of the digestion reaction can be simplified
substantially in the inventive process which uses an or-
ganic solvent as the extraction liquor. ~he reactor in-
cludes only two zones, i.e. a digesting ~one and a washing
zone.
; ~o impregnation ~one is required owing to the easy impreg-
nability of the wood chips with the organic solvent.
The wood chips are introduced into the digesting liquor in
the upper reactor zone. ~he operation is carried out at ex-
traction liquor temperatures between 130 and 210C. Diges-
tion occurs in a counter-current manner, the delignification
of the wood chips commencing at almost the very moment they
come into contact with the hot extraction liquor. ~o achieve
sufficient delignifica-tion rates9 it is necessary to reach
an adequately high temperature of -the wood chips as quickly
as possible. The lignin decomposition by means of organic
solvents or solvent/water mixtures is essentially a radical,
thermal decomposition reactor, although rates sufficient for
practical use are only reached at elevated temperatures.
Since lignin condensation, thus rendering the wood chips in-
soluble, occurs even at lower temperatures, an excessively
` long heating interval means tha-t the lignin can be extracted
only imcompletely. ~he requisite, rapid heating of the wood
chips is achieved in the inventive process by conveying the
impregnated wood chips in a counter-current with respect to
the ho-t extraction liquor.
The fresh ex-traction liquor is introduced in the inventive
process preferably by means of an annular line from the
periphery of the reactor. Instead of or, preferably9 in ad-
dition to introducing the liquor from the reactor periphery~
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_ 12 -
the extraction liquor can also be supplied into the middle
of the reactor through a hollow shaft, for instance. ~he
extraction liquor which ascends in a counter-current with
respect to the wood chips is withdrawn in the top reactor
zone. ~he organic solvent is recovered by distillation and
condensation and is then recycled.
Water, which can be warm or cold, is supplied in the bottom
reactor zone as the wash liquor which is heated when it comes
into contact with the wood chips which ~igrate downwardly.
~he water is supplied preferably through a peripheral annu-
lar line and/or a hollDw shaft located in the axis of the
reactor. Owing to the high diffusion rate of the organic sol-
vents, the solvent is almost completely removed from the ex-
tensively delignified material in the wood chip counter-cur-
rent~ lhe wash liquor which is laden with solvent and decom-
position products is generally withdrawn together with part
of the hot digesting liquor in the middle reactor zoneO
~he withdrawn wash liquor is admixed with a solvent by means
of a dosage pump, for example, the amount of said solvent
being such that the desired composition o~ -the extraction
liquor is restored. ~he extraction liquor is then reheated
to the requisite temperature, e.g. by a heat exchanger~
and is resupplied to the middle reactor zone through a per-
ipheral, annular line and/or through a hollow shaf-t~
lhe gist of the inventive process is that the washing liquor
does not serve to quench the reaction, but rather constitutes
one component of the extraction liquor supplied to the reactor.
Any fluctuations in the process and in the raw material
caused by alterations in the composition of the extraction
liquor can be easily compensated for in the inventive pro-
cess.
. .
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It is also advantageous that the wood chips are washed in
a counter-current with pure water at a relatively high temp-
erature. Owing to the low concentration of organic solvent
in the wash water, such a washing is much more effective
than a washing in which pure extraction agent is used as
the washing liquor.
'!
~he digested material is discharged as a rule by mean~ of
a blast tank, can be rewashed if desired, then sorted in
a conventional manner and processed further.
~RIE~ DESCRIP~ION 0~ IHE DRhWING
~he invention will now be explained in detail in the follow-
ing with reference to the drawing, in which the sole figure
shows a flow diagram of the impregnation9 extraction, washing
and separation stages of the inventive process.
DESCRIP~ION 0~ ~HE PREFERRED EMBODIMENT
~he plant fiber material, e.g. wood chips, is introduced
by a feeding means 16 into the impregnator 1 which oper~
ates at atmospheric pressure and a temperature of 60C.
An ethanol/water mixture is supplied to the impregnator
through line l7. ~he perfused wood chips are conveyed in
an upward direction through the inclined impregnator by
means of the screw conveyor 18 and are supplied to a
pressurized lock 2 by means of a discharge means 19. ~he
pressurized wood chips are then introduced into the reac-
; 30 tor 3 at the top thereof.
In the reactor, the wood chips are submerged beneath the
level of the liquor by the vertical worm 20 and are then
conducted downwar~ly through the reactor. During their
movement through said reactor, the chips are first tra-
versed in a counter-current by the hot extraction liquor.
- In so doing, the dissolving lignin and other cellulose
; associates are removed from the chips by diffusion and
.
. . .
., .
`~
- 14 ~
are transported further by the ascending extraction liquor.
~he extraction liquor enriched with extract leaves the re-
actor through line 10. The ex-traction liquor is supplied
through a cooler 23 to a spent liquor ~epres~uri~ing ves-
sel 24. ~he exhaust solvent vapors released are condensed
in an exhaust vapor condenser 25 and the solvent recycled
to the process through line 33.
~he residual aqueous solution consistin~ of hemicelluloses
and lignin is supplied by a pump 26 through another cool-
er 27 to a worm extruder 28. A centrifuge or a heavy-phase
separator can also be used in place of the worm extruder.
~he lignin precipitates in the worm extruder at a lower
temperature and is separated from the h~icelluloses. ~he
hemicelluloses and the lignin, which are withdrawn via
lines 29 and 30, can then be processed separately.
~he dissolution of the associated cellulose compounds is
concluded at the end of the digesting zone 4. ~he digest-
ing liquor is washed out of the digested material by as-
cending water in the ensuing washing zone 5 and is with-
drawn from the middle reactor ~one via the outlet 8, the
line 11 and pump 21. ~rish solvent is added to the washing
liquor via line 15 and, after being heated in the heat ex-
changer 14, it is recycled as extraction liquor through
lines 13, 13' and the hollow shaft 22 extending into the
reactor at the top, through the inlet port 9 into the mid-
dle of the reactor as well as through the annular pipe 31.
~resh solvent is supplied to the process via line 34O
~he washed pulp is discharged from the reactor at the end
o~ the washing æone 5 via discharge port 7. ~he washing
~5 liquor is supplied to the reactor in the form of water via
the peripheral, annular line 32 and/nr the hollow sha~'t 6
located in the bottom reactor zone.
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It is self-evident that the individual material flows
are controlled by means of suitable measuring and regu-
lating means,
~he inventive process yields pulps of excellent quality
and bleachability which are most suitable as the raw
material for paper production. ~he hemicelluloses and
lignin are obtained in pure form and can be utilized as
valueable raw materials in a multitude of applica-tionsO
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