CA1076109A

CA1076109A - Hydrolysis of material containing pentose and procedure in producing furfural

Info

Publication number: CA1076109A
Application number: CA268,966A
Authority: CA
Inventors: Gote E. Savo; Gustaf A. Nyman
Original assignee: ANSTALT GEMASS
Current assignee: ANSTALT GEMASS
Priority date: 1976-12-30
Filing date: 1976-12-30
Publication date: 1980-04-22

Abstract

ABSTRACT OF THE DISCLOSURE :

This invention is concerned with a method for continuous hydrolysis of a vegetable solid pentosan containing material in a temperature range between 80° and 120°C, and at a pH lower; than 5, with extraction of a usable hydrolysis residue by feed-in of raw material at one end and feed-out at the other end of a hydrolysis vessel. According to the invention, parts of the hydrolysis fluid are recirculated into the hydrolysis vessel at two different concentrations at different or identical level, and the supplementation necessary for hydrolysis of the fluid quantity takes place together with the recirculated hydrolysis fluid having the lower concentration. This method advantageously allows both the hydrolysis products and the hydrolysis residue to be recovered and used in a manner which is economically favorable. The apparatus for carrying out this method is also disclosed.

Description

1~176~9 `'.; ' ,, .
This invention relates to the continuous hydrolysis of vegetable solid materials containing pentosan and to the procedure in producing furfural. ~ ' Much effort has been expended in treatment of'waste material containing cellulose in order to gain therefrom useful products, such as e.g. furfuralj with simultaneous useful con-version of the residual product emerging from the hydrolysis. ~' Large quantities of such waste material are burned or are squandered by using them towards less qualified aims than those which in fact are attainable. An example of such waste materialis bagasse, that is the fibrous residue of the cane sugar in-sustr'y, which contains a uery high proportion of non-fibrous pith, as a rule approximately 30%. The pith is removed and disposed of by wastefu.l mechanical process e.g. by burning, while the fibrous component is processed to give cellulose :, .
products. However, bagasse may also be subjected to acid hydro-Iysis to gain furfural, but then the remaining fibre will be s~tiff -~ and brittle owing to the radical reaction conditions required, and therefore during refining it is largely lost in the form of .
short fibres, which are strained off together with the pentosans and pith constituents, etc.
~' Another product which is similarly very inefficiently uti'lized is corncobs, which are obtained in immerse quantities after the corn has been removed. Such corncob residues are conventionally hydrolyzed, and have been hydrolyzed for the last 50 years with 3 to 5% sulphuric acid under pressure for the producing of furfural, whereupon the resulting residue is usable only as fuel.
Othe'r materials which are similarly wasted or poorly utilized include straw, chaff, reeds, husks of various ce'reals, wood residues such as sh~vings, ships, etc.

One object of the present invention is to allowsuch vegetable solid .
~ ' , ,'. .. . .

- 1~76~9 materials containing pentosan ~o be hydrolysed in such manner that both the hydrolysis products and the hydrolysis residue can be recovered and used in a manner which is economically favourable.
A discontinuous process for producing furfural and high polymerized cellulose is disclosed in~the Swedish Patent ~
Specification No. 113 076, wherein the raw material used is ~ -vegetable matter from annual plants, such as rye straw choppings and oat awns, and this is hydrolyzed in a sparing way.
- The hydrolysis of chaff, straw, reeds, wood chips etc.

is~also described in the Finnish-Patent Specification No 23 ~71.
` The process is continuously carried out with weak acids at a - temperature over 100C in a vertical digester, with countercurrent.
similar procedure using a compression ~luid is disclosed in the Finnish Patent Specification No. 22 455.
From the Swedish Patent Specification No. 143 132 is .: - . : .
known a continuous hydrolysis method in connection with the manufacturing of furfural, and in which the hydrolysis temperature and acidity range are similar. A plurality of autoclaves are mounted side by side and the hydrolysis ~luid may be conducted from one autoclave to another as the material in introduced and taken out. However, this method~is not equivalent to a counter-: . . ,: :
s current principle in the true sense of the word nor with a continuously progressing feed of material through the hydrolysis - vessel. The procedure cannot be characterized as a true con-~ - current process either. A great amount of manual work is ~. :
' necessary in empyting and filling the autoclaves, and the effec-tive hydrolysis time will also be substantially shorter than in ~ -the procedure of the present invention, which furthermore requires a smaIler equivalent hydrolysis ~lume.
- 30 According to the present invention is employed a method for continuous hydrolysis of ~egetable solid pentosan-containing material in a temperature range between 80 and 120C, and at a pH lower that 5, _ _ _ _ _ -.' . .

-2-, .. ;
.

1076~L~9 with extraction of a usable hydrolysis residue by feed~in of raw ~aterial at one end and feed-out at the other end of a hydrolysis vessel, characterized in that parts o~ the hydrolysis fluid are recirculated into the hydrolysis vessel at two different concen-trations at different or identical levels, and that the supplement-ation necessary for hydrolysis of the fluid quantity takes place together with the recirculated hydrolysis fluid having the lower ~ concentration.
; For effecting the last-described method the invention -; 10 provides an apparatus comprising a hydrolysis vessel with a `~ feed-in means at one end and a feed-out means at the other end and ; with a drain for hydrolysate, characterized by a means for recir-culation of concentrated hydrolysate at a point between the feed-in ~ ~ .
and feed-out of the raw material, and closer to the feed-in means for the return of weaker hydrolysate or washing fluid, at a point .. ~., ~, .
immediately adjacent to the feed-in of the raw material in concurrent flow operation and immediately adjacent to the feed-out in countercurrent flow operation, the feed-out means being cons-tructed so that the hydrolysis fluid is prevented from flowing out by a continuous plug formed from the hydrolyzed solid matter.
Thus a continuous method can be effected in a hydrolysis vessel into which the hydrolysis fluid is returned at di~`ferent solution concentrations at different or equal levels. It is hereby . , possible to considerably increase the concentration in the hydrolysis fluid, which impies an improved heat economy and lower ohemicals costs, in addition to which the possibilities of further treatment of the organic substance extracted improve. It is then ; also possible to perform a selective hydrolysis in a second step at temperatures between ~40 and 180C, preferably in the range 155 to 165C, which in many instance~ have proved appropriate for the extraction of pentosans difficult to hydrolyze. If temperature exceeds 180C there occurs simultaneous cleaving off ..
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, 7~ 9 ~ ~ water, with formation of furfur~
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Initial materials rich in pith, such as bagasse, are - :
depithed by hydrolysis and go into solution, whereat the monosaccharide can be converted into a useful and usable substance.
~he depithed residue, which in such instances consists of fibres, - can be further converted into cellulose, animal fodder or `~
monosaccharides.
At present bagasse is used for paper manufacturing and -~
.. . .
the pith is used only fuel. Since the pith content amounts to 30~ of the dry weight of bagasse it may be understood that the invention offers an appreciable alternative.
How the invention may be put in practice is more closely described with reference to the accompanyins drawings, in which Fig. 1 is a diagram of an apparatus for putting the invention into effect as a concurrent process, and :-" ' : ' ~ig. 2 is a diagram of an apparatus for putting the - invention into effect with a countercurrent -arrangement `~ Fig. 3 is a diagram of an apparatus for producing fur~ural in accordance with the method described in ~ig. 1 and 2.
~he apparatus schematically represented in ~ig. 1 for ; hydrolysis with concurrent flow has a funnel-shaped hopper 1 for introduction of the raw material into the hydrolysis vessel 3 by ~ means of a feed device consisting of a variable speed conveyor -~
- screw 2 within a tubular housing. In the upper part of the hydrolysis ves~el 3 terminates a tube 20 carrying washing fluid, which is conducted from a container 16 through a pipe 17 and an adjustable valve 18 and a pump 19 and is preheated in a heat exchanger 25 prior to entering the hydrolysis vessel 3.

The concentrated hydrolysate i~ returned from the container 8 to the lower part of the hydrolysis vessel 3 by a pipe .; . .
' " '' ~' :' ;.
, 76~ [)9 .', . `:
, 22 with the aid of a pump 23 and a regulating valve 21.
A feed-out means 4, consisting e.g. of a variable speed conveyor screw encircled by a tube on the bottom of the hydrolysis vessel 3, dams up the hydrolysis fluid by a continuous formation of a plug of the material already treated,'the conveyor ~ ' ;, ' screw being shorter than 'the surrounding tube.
.
' ' From the feed-out means 4 the process material drops .
into a separator 6, where fluid and solid material are separated , as completely as this can be done. The hydrolysate is conducted ~ 10 into the container 8 by a conduit 7, and the treated, solid '' ` ' mass is transported through a conduit 9 to a diffusor 10, where ~ washing water is- added from a pipe 12. The diffusor 10 has a ,~, feed-out means 11 for solid matter on the bottom, such as for '~ `

~ instance a conveyor which carries the treated mass further to . . . .
a dewaterlng means 14. From the dewatering means 14 the fluid is conducted to a container 15 through a conduit 15, and the solid mass is carried off for further conversion. lb the con-. :
~ tainer 16 is connected a conduit 28 supplying acid to the system~ -, .
Hydrolysate is pumped from the container 8 through aconduit ' 20 27, containing a:regulating valve 26, for further conversion,.

The mode of operation of the concurrent flow arrangement ~ , .
is that the raw material is introduced into the hydrolysis vessel

3 through the hopper I and conveyor screw 2, and admixed with he ., , :
'' ~ acid washing fluid at between 80 and 120C, preferably approxi-mately 100C, recirculated by a,conduit 20 from thecontainer 16.

' , ' The conduit 20 discharges into the hopper 1 or as shown , into the upper part of the hydrolysis vessel 3. The raw material ... .
' ` descends under its own weight down through the hydrolysis vessel ,. . .
', 3 during a period of a length determined by the quantities fed ,' 30 in and taken ou`t. A hydrolysis period between three and six ' `

'~ hours is preferred.

` ` The w-a,shing-fluid from the conduit 20 contains the ~ ~ .
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76~g . '.
greater part of the organic substances washed out in the diffusor 10. Fresh washing water at 50 to 100C is conducted into the diffusor 10 at a quantity equal to that of the fluid removed from the system.
In the dewatering means 14, which may be a screw press or a suction filter of previously known -type, the mass is dewatered to approximately 50% dry matter content by weight and it is subsequently conducted away for further conversion e.g. into cellulose, animal fodder, monisaccharldes, etc.
In the container 16 the acid concentration is regulated by adding new acid to replace the small amount of acid consumed -~; in the process. -The temperature of the washing fluid is regulated, before ;
its introduction into the hydrolysis vessel 3, with the aid of a heat exchanger 25, and the temperature in the hydrolysis vessel : , -: . , , 3 is kept constant by means of heat introduced by a conduit 29.
, . ,: .
The separator 6 may be, like the dewatering means 14, a screw press or a suction filter. Part of the hydrolysis fluid ~;
separated in the separator 6 and collected in the container 8 is 20 returned by the conduit 22 to the hydrolysis vessel 3, approximatel~ ~-at the middle of the vessel 3.
, ~. , lhe quantity of hydrolysis fluid recirculated, which is ~, distributed in the hydrolysis vessel 3 in a suitable manner, may ~;be regulated by means of a pump 23 to increase the concentration ; ~of organic substance in the hydrolysate as far as is technioally possible. It is obvious that the foaming of the hydrolysate imposes a limit on the concentration.
From the container 8 the hydroly3ate i9 drawn or pulnped for further conversion into desired products, for instance to obtain crystallized pentose, furfural, etcl. or for producing e.g. yeast, etc.
Fig 2 shows a schematic diagram of a countercurrent flow :.
.
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76~
arrangement for effecting the mether~ of the invention. From -the hopper 1, the raw material is fed into the process compartrnent of the hydrolysis vessel 3 through the feed-in device 2. In the upper part of the hydrolysis vessel 3 the hydrolysate is conducted to the container 8 through an aperture 31 provided with a wire screen to prevent solid matter from following along with the hydrolysate. A pipe 22 connected to the lower part of the hydrolysis vessel 3 serves for returning concentrated hydrolysate from the container 8 with the aid of the pump 23 and the regulating valve 21.
A conduit 29 connected to the lower part of the hydrolysis vessel 3 maintains a constant temperature during the hydrolysis. In the bottom portion of the hydrolysis vessel 3 is a transport screw 4 with variable speed which transports the material from ~he process ~- compartment to the washing compartment in the hydrolysis vessel 3.
; In the washing compartment the material is lifted by ~ ~ means of a vertical conveyor screw 32, or by another suitable ;; means, up to the top, where feed-out means 11, for instance a variable speed horizontal conveyor screw enclosed within a tube ~ -located outside the vessel, transports the process material on to the dewatering means 14. In the feed-out means 11 a continuous plug formation of the process material i9 obtained because the conveyor screw is shorter than the tube.
From the dewatering means 14 the washing fluid is conducted to the container 16 through the pipe 15, and the , solid matter is carried by conveyor means 30 to further conversion steps.
From the container 16, washing fluid i3 recirculated by the pipe 17 and re~ulating valve 18 and pump 1~ and throu~h -the -~
heat exchanger 25 into the hydr~lysis vessel 3. Acid can be introduced into the container 16 through a pipe 28. Fresh washing water is introduced into the pipe 20 by a pipe 12.
~he apparatus of Figure 2 operates with countercurrent . ;~
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76~Lal9 , -` ~ . .
: flow. The raw material supplied through the hopper 1 and ... .
conveyor screw 2 encounters the hydrolysate which is on its way out and which flows through strainer means ~1 to the container.
8. The material descends by its own weight through the process compartment of the hydrolysis vessel 3, while the hydrolysate has an opposite direction of flow because the washing compartment of the hydrolysis vessel is higher than the process compartmen-t.
Part of the hydrolysate is recirculated from the container 8 `
through the regulating valve 2~ and conduit 22 and pump 23 to the ~- `
bottom of the process compartment in the hydrolysis vessel ~.- The .. . ...
process mass carried over to the washing compartment of the .~ . .
hydrolysis vessel 3 by the transport means 4 has an upward direction of flow in the washing compartment, while the washing fluid ~lows downwardly. The washing fluid is introduced in the topmost part of the hydrolysis vessel through the conduit 20 and it co~es ~-~
from the dewatering of the process mass in the dewatering means 14, through the conduit 15j container 16, suction 17, regulating ~`
valve 18 and pump 19. Furthermore, fresh washing water is added -~
through the conduit 12 in such quantity that the system is kept filled at all times. The temperature of the washing fluid is .~ . .
regulated by means of the heat exchanger 25 to `be as desired.
The temperature in the hydrolysis vessel is kept constant by supplying heat through the conduit 29 on the bottom of the process compartment. The acid content is maintained at a suitable level by subblying acid through the conduit 29. The process mass is conveyed away from the washing compartment of the hydrolysis vessel by the feed-out means 11 and at a dry matter content of about 50~0 by weigh~ for fur-ther conversiorl, to become cellulose or animal fodder, for continued hydroly~is to become monosaccharides, etc.
- From the container 8 the hydrolysate is pumped through the regulating valve 26 and the conduit 27 to be subjected to . '., -8 ~ ;

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~L0761(~9 conversion, for instance to become crystallized pantose, preferably zylose, furfural, or toyyeast preparation, etc. The hydrolysis period is preferably from 3 to 6 hours.
In experiments the invention gave results as in the following Examples, all percentages being by weight.

Example 1 50 kg of residue after extraction of sugar from sugar cane, that is socalled bagasse, having a water content of 11 . 2~o~
` ~ were hydrolyzed with 435 kg of 0.5% sulphuric acid during 5 hours ;~
at 100C with fluid G~irculation in concurrent flow mode. ~he quantity of organic substance extracted, referred to the estimated .
dry bagasse quantity, was 26.9~. The concentration in the hydrolysate amounted to 3.1%, mainly pentoses.

Example 2 50 kg bagasse were hydrolyzed in countercurrent flow, but 300 kg of the hydrolysate from Experiment 1 were included and supplemented with new 0.5% sulphuric acid to make 435 kg. ~he quantity of extracted organic subtance was found to be 25~0% and ZO the concentration in the hydrolysate, 4.68%, mainly pentose.

Example 3 , .. . .
33 kg bagasse having 9.7~ water votent were hydrolyzed with 43~7 kg of 0~5% sulphuric acid at 98C, with fluid circulation.
e pH value of the hydrolysis fluid was 1.41. After 4 hours the quantity of substance extracted was 22~ after 4 hours it was 25.2%

and after 6 hour~ 26~8~o~ ~he organic substance extracted consisted in its main part of monosaccharides from the pentose group. On l ~ extension of the hydrolysis time up to ten hour~, with ba~asse and .~ with the reaction conditions stated, only an insigni~icant increase .
; 30 of the pentosan extracted was noted. On the other hand an -; increase of the temperature to 150C resulted in continued i hydrolysis of pentosan to soluble pentose, but this imposes more ,' -: --9-- . . :
.... . .

.. . . . .. . . . .. . . . . .

~ ~ ~, 1076~09 onerous demand 9 on the apparatus employed.

Example 4 ~;
100 g of residue from corncobs, with 9.1~o water content, `` were disme~bered into pieces o~ walnut size and hydrolyzed with 470 g of 0.5% sulphuric acid by cooking at normal pressure with reflux. After 4 hours the quantity of substance extracted amount to 27~ and after 5 hours to 29%, whereafter only minute quantities were extracted by continued hydrolysis. ~he hydrolysate ;~
thus obtained mainly consisted of a pentose solution.
~he present invention concerns also a way of producing ` furfural in accordance with the method described in Fig. 1 and 2.
According to the invention, hydrolysate containing pentoses is conducted from a process as described in Fig. 1 and 2 - to a steam-heated dehydration container, whence the evaporated furfural mixture is further conducted through heat exchanging means and concentration steps to give a final product containing , , 95 per cent furfural.
According to the invention the solid residue from hydrolysis is also processed into cellulose, animal feed or monosaccharides.
~- According to the invention the hydrolysate from the first step is dehydrated at the same acid concentration, 0.5%
H2S04, as the hydrolysis, that is the hydrolysate is directly pumped from the first to the second step. Subsequent to - , dehydration, the substantial part of the pentose-free aaid solution ; is recirculated to the first hydrolysis step ~or reuse o~ the acid . !
and in order to increase the content of organic substances to the highest possible concentration, which is limited among other `~ things by the foaming of the liquid. ~he hydrolysis may in certain cases be performed with acid concentration 0.25% EI2S04, in which instance the dehydration then also takes place at this same con-centration.
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~ ~ ' ''' ' ,'' ' .', , ''. ' '' ', ' '.," , ,.. . '.~.'"' ' , , , 1~76~9 ~ he optimum temperature ~or dehydration is determined by a number of reactions which are competitive among themselves.
~he rate at which the pentose is transformed into fur~ural under cleavage of water increases with increasing temperature, but at the same time also the rate increases at which the furfural is converted into other substances. ~owering of pH exerts the same i~fluence as increasing temperature on the reactions.
It has been found that in application of the invention the optimum conditions lie at 155 to 170C, and preferably about 165C.
~ At 165~ the dehydration rate is acceptable in view -~ of apparatus capacity AS well as the polymerizing tendency of the furfural and its disposition to other reactions. By soonest possible removal of the furfural that has been formed from the aoid reaction environment, the yield is improved, and therefore an ;
apparatus design has been developed according to the invention which provides a very high evaporation rate of the furfural produced.
Since the departing furfural solution ~s dilute, the major part of the evaporated water has to be returned b~ condensation in order to obtain correct azeotrope composition at the temperature in question. Herefor a great amount of heat is expended, which is utilized in the condensor the generate secondary steam, this .~, .
steam being further used to purposes of distillation and refining of the furfural.
., I . . :
According to the invention the condensing of furfural , , .:
is combined with producing of process steam. Furthermore, the combining of dehydration and azeotrope formation results in very remarkable savings of steam.
Since the detaching of water in the dehydration container requires a certain reaction time, 0.5 to 3 hours, preferably 1 5 hours, the capacity of the apparatus is determined by the reaction volume. Because the apparatus volume is comparatively ;

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expensive, the total volume available for dehydration may be :
increased in that the initial step of the reactions takes place in ~ ~
a preheater, where the hydrolysate is heated to operating tempe- -. rature.
. The procedure in producing furfural shall be described .
in greater detail by means of an embodi~ent example, and in :
connection with Fig. 3.
~he hydrolysate from the preceding (not depicted) first : ~ .
stage of furfural preparation, which contains pentosans, is .~
10 conducted into a cylindrical dehydration container 31, which has ~ : .
been divided by transversal partitions 32, 33 into smaller 34.
~. Every second partition 3Z is sealed against the bottorn of.the .
container 31, while every second partition 3.3 ends at a distance.; above the bottom, leaving an intermediate space In the bottom , :;
:~ of~each chamber 34 a steam inlet 35 has been provided.
.. . .
~ ~he hydrolysate is conducted into the cylindrical . .
container 31 through a stud 36 at one end, and it passes in zigzag flow through the container 31, over every second partition , `l ~ 32 and under every second partition 33, to be finally canducted .~ ~20~ out through a pipe stud 37 on the other end, and back to the. j - .
~ first step, or following cooling to 100C into the container 20 if ;-~ the furfural content has not yet attained the desired minimum.~:
. : .
. ~ ~he hydrolysate is heated by introduction of steam through the ` stea~ entry ports 35, to about 165C at a pressure of about 6 kg/cm2 ; ~ gauge in container 31. In connection herewith steam is distilled : ;
: off from the hydrolysate, containing 4 to 5% furfural, and . ~ :
` drained through the conduit 38~ carrying the valve 39, to a .. condensor 40. Owing to the fact that the iurfural th.at has been : :
for~ed can be expelled virtually instantaneously without its :
. 30 having topass any longer distances through the acid hydrolysate . solution, owing to the design wherein the hydrolysate runs in .
. a thin layer over the upper edges of the partitions 32, the : 12 . .,: . , . . , . -, . . .. .
., ". :
. ,,; ,. . ., ~ . . . , ~ .

1076~9 further reactions of the furfural are minilni.zed.
The pressure of the furfural containing steam is ~ reduced by the aid of the valve S9 to 3 kg/cm2 gauge pressure in the condensor 40, and from the top of the condensor pure steam is withdrawn through the conduit 41 to a steam container 42, and from the bottom of the condensor dilute furfural is withdrawn in liquid form through the conduit 43, containing the valve 44.
From the top of the steam container, process steam is withdrawn by the conduit 45, with valve 46, and from the bottom of the steam containe,r the steam goes through the conduit 47 back !, .
~ b the bottom of condensor 40. ~
. .. ~ .
The furfural solution from condensor 40, having a temperature about 140C, passes through the conduit 4~ to a cooler 48, where it ls cooled to about 100C, and further through conduit 49 ~to a container 50. ~'rorn this container 50 the dilute furfural solution is then introduced through the conduit 51 at the centre ~ -of a stripper 52, from the top of which low-boiling constituents are taken off by the conduit 53. ~he drain 60 of the stripper ~
may be selectively conducted to the first step, or out frorn the :: :
system. Steam for the heating of the stripper is introduced from ~:
the steam container 42 by conduit 45 in the bottom space of the .~, ~, .
: ~ stripper through the loop 54. At a point somewhat higher than . : ~
~, . .
the centre of the stripper, an azeotropic furfural solution is withdrawn through conduit 59, and it is cooled in the cooler --55. In a phase separator 56, the solution is split into two : -~
phases, the heavier phase containing 95~0 furfural, which is :~
. . .: .
withdrawn at the bottom of the separator 56 through the conduit ~ :

57, and the remainder containin~ about 8.5~o furfural, which is :~
.. ..
withdrawn at the top of the separator 56 and is returne~d by the ~ . :

conduit 58 to the container 50.
. : :
By the combination of the distilling of the furfural produced in several steps in connection with steam production ~:
.
~ -1 3~
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1076~1~)9 . , :
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and by using the ~econdary steam produced as process steam, ~ ;
a very good heat economy of the installation is achieved. ~he heat economy may be further improved by also incorporating a ;~
distilling step after the dehydration container and before the condensor, comprising a few bottoms.
The discharge from dehydration is reconducted to hydrolysis in step 1, and the lnherent heat is recovered in the condensor at the same time. There occurs hereby an elevation of the concentration of organic substance, from which other organic products may be made.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method for continuous hydrolysis of a vegetable solid pentosan-containing material in a temperature range between 80° and 120°C, and at a pH lower than 5, with extraction of a usable hydrolysis residue by feed-in of raw material at one end and feed-out at the other end of a hydrolysis vessel, character-ized in that parts of the hydrolysis fluid are recirculated into the hydrolysis vessel at two different concentrations at differ-ent or identical levels and the supplementation necessary for hydrolysis of the fluid quantity takes place together with the recircualted hydrolysis fluid having the lower concentration.

2. A method according to claim 1, wherein the hydrolysis is effected in a temperature range between 98° and 102°C.

3. A method according to claim 1, wherein the solid hydrolysis residue is utilized by means of further hydrolysis in a second step at temperatures between 140° and 180°C where-by pentosans difficult to hydrolyze go into solution and pentose is dehydrated to become furfural.

4. A method according to claim 3, wherein the further hydrolysis is effected in a temperature range between 155°
and 165°C.

5. A method according to claim 1, 2 or 3, wherein the raw material is bagasse, which is depithed by the hydrolysis, whereby the pith is detached from the fibres.

6. A method according to claim 1, 2 or 3, wherein the solid, insoluble residue material is converted into cellulose, animal fodder or monosaccharidos.

7. A method according to claim 1, wherein part of the departing hydrolysate is returned to the hydrolysis vessel at a point closer to the feed-in than to the feed-out the raw metarial.

8. A method according to preceding claims 1 or 7, wherein washing fluid is recirculated into the hydrolysis vessel together with the feed-in of raw material in a concurrent flow method or in the immediate vicinity of the feed-out in a countercurrent flow method.

9. A method according to claims 1 or 7, wherein the replacement of losses of fluid and acid takes place by additions to the washing fluid prior to its entry into the hydrolysis vessel.

10. An apparatus for the continuous hydrolysis of a vegetable solid pentosan-containing material in a temperature range between 80° and 120°C, and at a pH lower than 5, with extraction of a usable hydrolysis residue by feed-in of raw material at one end and feed-out at the other end of a hydro-lysis vessel, said apparatus comprising a hydrolysis vessel with a feed-in means at one end and a feed-out means at the other end and with a drain for hydrolysate, means for recirculation of concentrated hydrolysate at a point between the feed-in and feed-out of the raw material, and closer to the feed-in, means for the return of weaker hydrolysate or washing fluid, at a point immediately adjacent to the feed-in of the raw material in concurrent flow operation and immediately adjacent to the feed-out in countercurrent flow operation, the feed-out means being constructed so that the hydrolysis fluid is prevented from flowing out by a continuous plug formed from the hydrolyzed solid matter.

11. A method according to claim 3, wherein the second step comprises a first removal by distillation of furfural from the hydrolysate at a pressure about 6 kg/cm2 gauge and a temperature about 155 to 170°C, that the distillate having a concentration of 4 to 5% furfural is condensed with withdrawal of secondary steam, that the condensate is cooled to below 100°C, that a furfural azeotrope is stripped at about 98°C in a stripper, and that a 95% furfural is separated in a separator with utiliza-tion of the mixing gap between furfural and water, and that dilute furfural solution is reconducted to the input conduit to the stripper.

12. A method according to claim 11, characterized in that the temperature of the first distillation is 165°C.

13. An apparatus according to claim 10 characterized in that it further comprises a dehydration container with transversally positioned partitions and steam entry ports, a condensor for withdrawal of secondary steam and of furfural condensate, a cooler for the furfural condensate to be below 100°C, a stripper heated by means of introducing steam from the condensor through a loop in the bottom of the stripper, a cooler for the stripped, concentrated furfural and a separator for phase separation and withdrawal of a 95% furfural and provid-ed with return conduit for reconduction of dilute furfural to the cooler.

14. An apparatus according to claim 13, characterized in that the transversal walls in the dehydration container alternatingly leave a free space for passage on top and at the bottom, whereby the hydrolysate residue passing through is partly assigned a long perfusion path in the dehydration container and partly the furfural that is formed finds a very short path for its passage through the acid reaction solution before it escapes in vapour form.

15. A method according to claim 1, for further process-ing of the hydrolysate residue, characterized in that the solid residual material is converted into cellulose, animal feed or polysaccharides by digesting the hydrolysate residues with 5 to 25% alkali at 50 to 100°C during 0.5 to 3 hours, whereupon the solid residues are filtered off and washed.