CA1234803A - Procedure for recovering soluble carbo-hydrates contained in wood - Google Patents

Abstract

Abstract of Disclosure The invention concerns a procedure for recovery of soluble carbo-hydrates present in wood, in connection with the alkaline cellulose digesting process. According to the invention, recovery of carbo-hydrates takes place at the initial phase of the cellulose digest-ing process, where the digesting solution contains dissolved poly-saccharides in abundance, while the content of polymeric lignin in the solution is low. A situation like this exists at the stage when the digesting solution has not yet reached the temperature at which the digestion process mainly takes place. As taught by the inven-tion, the carbohydrates are recovered by conducting digesting solu-tion to an ultrafilter, which separates the carbohydrates there-from, and by returning the filtered solution to the cellulose digestion. The carbohydrates that are obtained may be used e.g. in the manufacturing of sugar alcohols, or as raw materials for biotechnical processes.

Description

12~8~3 PROCEDURE FOR RECOVERIN~ SOLU~LE CARBOHYDRATES CONTAINED IN WOOD

The present invention concerns a procedure for recovering ~oluble carbohydrates contained in wood, wherein the carbohydrate~ contain-ed in wood that i~ to be diyested to cellulo~e and which hall b~
recovered are brought into a solution from which they are separat-ed, and wherein the proces~ng of the wood continues therea~ter as an alkaline oellulo~e digesting process.

Th~ ob~ect of the invention i5 to teach a procedure by which it is possible in connection with manufacturin~ chemical or chemical-mechanical cel1ulose pu1p to separate a remarkable part o~ those polysaccharides which are dissolved in the dige~tin~ liquor and which in conventional çulphate digesting 1argely undergo cleavln~
into saccharic acids and other decomposition product~ during the digesting process, to end up with the waste 11quor in combu3tion in the digesting ch~micals recovery process. Th~ carbohydrates which are solved ~rom the wood or other raw material containing li~no-cellulose are th~n recovered in such form that they can be used e.g. ln manufacturing mono~accharidæ~ or sugar d~rivative~l as cattle ~odderJ or for manufacturing alcohol and protein~.

~t i~ previously known that so1uble carbohydrate~ can b- manufac-tured from wood or other lignocellulo~ic raw materials by hydroly~-lng thQm with a mtneral acid~ such as dllut~ or concentrat~d sul-phuric acid or hydrochlDric acid. Howev~r, the~ processes have provQd un~conomical compared with th~ productng of c~llulose frD~
th~ r~w material~ mentioned.

It i~ also alternatively possible in connection with the manufac-turing of cellulo~ pulp by hydrolyztng wood chlp~ with th~ aid of steam and minQral acid3~ tn a particular pr~hydroly~1~ Btep priar to th~ c~l lu105~ dl~e~ting proper, to produc~ solubl~ carbohydrate~

~2~ )3 originating in the hemicellula~es. Dependlng on hydrQlysis condl-tions, these carbohydrates are more or less completely hydrolyzed, that is, they are in monosaccharide or polysaccharide form. If softwood ha~ been used fcr wood material, the carbohydrates wlll containl among others~ glucose, mannose, galactos~l xylos~ an4 arabinose. If the wood material that has been used is hardwood, the carbohydrate~ cantaln mainly xylose.

Hydrolysates of this kind may be used for instance ln manufacturing sugar alcohols, such as xylitol or sorbltol, as a raw Material in biotechnical processes~ for cattle fodderl etc. However, the pre-hydrolysis procedure is encumbered by the drawback that the yleld and quality of cellulose pulp su~er from the hydrolysis and the range of application of this type of pulps i~ narrow, being prl-marily restrlcted to viscose manufacturing.

It 15 possible to isolate polysacchariies in connectlon with the mercerizing process included in tha manu~acturing of viscos2 fibres. In the mercerizing process~ dis~olving pulp i5 treated with NaOH solution, thQ aim being to dissolv~ the hemicellulose cDmpo-nents from the cellulose. From the solution thus obtained, the solved polysaccharides can be recDvered with the aid Df ultra-filtration, as these compounds cannot pass through the ultrafiltra-tion membranes and are therefore enriched in the ultrafi1tration cDncentrate. We refer ln thls context to the U.S. Patent No.
4,~70,914 and to Hurlen, S. - Olsenl A.: Removal o~ Hemlcel 1U1DS~
from Steeping Lye by UltrafiltratlDn. TAPPI~OEZEPA 5th Int. Di~-solving Pulps Conf. ~Vienna) Papers: ~-S8 ~Oct. 8-10 1~80). The greater part o~ the polysaccharide~ orlginating in th~ wood hav~
however been dissolv~d in connec~ion with the cellulose manufactur-ing proces~5 in th~ waste liquor, and th~ poly~accharide quantitle~
obtained in connec~ion with the mercerizing proces~ are th~r~fore minor. It should moreover be kept in mind that the demand of di~-solving pulp is limited.

It is known that wh~n c~llulo~e pulp i~ being manufactur~d by dlgestlng wood chips or other lignoc~llulo~e-containing raw materl-L23~803 als by alkaltne digestlng method~, e.g. by the sulphate process, bysoda dige3ting or by the alkaline sulphlt~ prQCesS~ pa~t of thR
pDIysaccharide~ present in the raw material Ith~ cellulose and hemicellulo~e) 9Q into solution in the dig~sting llquor. At th~
Initlal stages of digesting, thQ hemlcel lulose5 in particular ar~
solved in large quantlties in the orlglnal polymerized form. It has been shown for instance in 5. AxQlsson, I. Croon and B.E. Eng~t~m, Dlssolution of Hemicellulose~ During Sulphato Pulping, Svensk Papperstidning 6~ ~19~2): 18~ 693-697, that when blrch wood was digested by the sulphate method the digesting flui~ contalned at the beginning of dlgesti~n 11 9~l Df precipitating polysaccharide~.
This fraction increa~es with progresslng digestion, and at l~OQC
about 8% of the wood could be isolated ln the form of polysacchar-ides. The polysaccharide concentration of the dige~ting fluid was then about 20 9~1. On continued digestion and with lncreasing temperature, th~ polysaccharide fraction wa~ reduced, and the final black liquor contained only about 2 9~1 of precipitable polysacch-arides. We have now in our own studies obser~ed, additionally~ that at the initial stage of digestion less than half of the lignln ln the wood has been dissolved malnly in the form of low-molæcular derivatives, and therefore~ when the polysaccharide content o~ the digesting liquor i 5 at its highest, thQ liquor contains hardly any other polymeric compound~.

The above-mentioned observation is the basis of the pre~ent inven-tion) which is characterized in that recQvery of the soluble carbo-hydrates in wood is carried out at the initial phase ~f the cellu-1D3~ digesting proc~ss, at which th~ polysaccharid~ cantent of th~
dlgesting ~olution i~ high and the content oi polymeric lignin i~
low, by conducting digesting ~olution into an ultrafilter~ which aeparate~ therefrom the caroohydratæ~, and by returnlng th~ filter-ed solution to the cellulos- digæ~tion.

Recovery of the carbohydrates take~ place, a~ taught by the inven-tion~ most adv~n~ageously at th~ pha~e when the digesting Yolut~on ha~ not yet reach~d th~ temporatur~ ~t which the dige~ting proc4~
mainly takes placeO This temperatur- is usually ln the range o~ 170 .

~L23~303 to 180~C, and the temperature at which separation of carbohydrates takes place may then be 100-l~O~C, preferably 120-160C, and most preferably 140-lS0C. In order that reasonable yield ~ight be attained in carbohydrate recov~ry, at the separatlon stagR at ~ea~t about 8% of the original dry matter o~ the wood ~hould be present in the digesting solution ln the form of dissolved carbohydrat~3.
In practice, the amount of carbohydrates may vary wlthin the rang~
of 8-30~. Similarly~ the quantity o~ lignin in the dige~ting ~olu-tiDn should b~ at mo~t lS%, calculated on the original dry matt~r of the wood, the lign~n then occurring in the solution mainly in the form o~ low-molecular compounds which do not int~rfere with the polysaccharide ~paration.

The present invention ha3 the advantage that ~oluble polysacchar-ides can be manufactured without any particular prehydrolysl~
process ln connection with manufacturing cellulose pulp. Thc proce-dure o~ the lnvention is advantageous both technically and economi-cally because no pressure and acid resi~tant hydroly~is apparatus i5 required, nor any extra cheMlcals such as mineral acid~ for instance. In view of the environm~nt~ lt i5 to be not~d that in contra~t with the hydrolysls proce3s~ in th~ present proce5~ no harmful waste waters nr other waste~ are produced. Separation o~
the polysaccharldes at the initial stage o~ dige~tion 13 a1so favourable from th~ viewpoint o~ cellulose manu~acturing~ sinc~
their decomposition in connRction with cellulos~ dige~tion to saecharic acid~ do~s not take place wlthout consumptiGn of energy and digesting chemicals.

Removal of the carbohydrat~ from the dige~ting sDlution i~ also advantageou~ in vi~w of chQmicals regen~ratlon, because part of th~
soda boiler'~ capacity i3 set fre~, in other word~, the cellulose manufacturing capacity may be incr~as2d.

In th~ following, th~ invention is described by an embodiment example~ referrlng to the attached dnawlng9 which present~ an apparatus usable in applyin~ th- lnv~ntion.

- 5 - 1~34803 Exam~le The object of the experi~ent that was carrled out was by ul~rafil-tration to separate in industrial con~ition~ xylan from the reflux liquor in birch digestion. The carbohydrate concentration wa~ high at this ~tage in the dlgestion, 13.1 9/1, and th~ llgnln content was low according to absorptlon mRagUrement~ A~o ~S410. The equivalent absorbance value of the dlge~ting llquor at the final stage of digestion would be about L800t i.e.~ not more than about 23~ of the lignin had been dis~olved at this stage.

The ultra4iltration equipm~nt was o~ type DDS-2S-2~ 2S, with dia-phragm area 2.2S m~ and dlaphragm~ of type GR8P being us~d~ and thls apparatus being surrounded by peripheral apparatus and con-tainers as shown in the drawing.

The reflux liquor was drawn during blrch wood dlgestion ~rom the reflux between the lower end of the absorption tower and the top end of the Kamyr dlg~ter, into the contain~r Sl.

The container Sl was fitted wlth mixer and served a~ chemlcals addition Ve~5el. The liquor was pumped frum thc container Sl by the pump Pl thrDugh thQ pressure filter Rl into the container S2. The pr~ssur~ filt~r was type Seitz-Zenlth, and filter plate~ K 150 wer~
b~ing used. Thæ contain~r 52 was likewi e 4itted with mi~ær~ and from this contain~r the llquor wa~ drawn~ through l~el control~ to th~ supply container S~ of the UF module.

The UF modul~ proper comprl~ed, in addition to thR container S~, a clrculation pump P2, a h~at ~xchanger and a diaphragm unit R29 from which th~ concentr~te W~5 re~luxed to th~ container S3. The 4il-trat~ depart~d from th~ ~y~t~m through th~ rotameter Fl-7. Th~
supply pr~ssur~ of th~ module during ~h~ experimsnts wa~ 0.8-0.9 MPa a~d the temperature in the circulation9 8~-8~C. Th~ r~flux through the modulo amount~g to S-8 m3/h. Th~ re~ult~ are seen in Table 1.

~ ~, - 6 - ~Z3~8~3 Table 1. Separation o~ carbohydrates from birch sulphate digesting 50 lution.

Volume, Dry matter conc., Dry matter, Carbohyd~ate~g~;
I 9/l kg 9/l kg ~ of dry matter __________ _______________________ --Dlgesting solution190 1~8 31.9 1~.1 2.5 7.8 UltrafiItration concentrate 37 224 8.3 ~.9 2.4` 2~.9 Ultraflltration filtratelS0 140 21.4 1.1 0.2 0.9 a) The carbohydrate concentration wa~ determined by liquld chromatography after total hydrolysis ~with sulphuric acid~.

The table reve~ls that the volume of th~ ultra~iltr~tiQn concen-trate contain~ng 96~ of thQ carbohydrate~ of the origin~l volumoe, and it i5 evident that ~6~ o~ the dry matter have been tr~nsf~rred to the ultrafiltration filtratæ~ containing mainly digesting chemicals which can be returned to the cellulo~ prQc~s3.

The composition of the carbohydrates in the ultrafiltratlon concen- ;
trat~ i~ given in Table 2.

~ . : :

123~8(:~3 , Table 2. CarbDhydrate CDmpO~itiDn of the ultrafiltratiun concentrate.

Mannose O.Z~
Glucose 0.3%
Rhamnosæ 0.8%
Arabino~e 2.4%
Galactose 3.0 Xylose 93.3 _ _ _ _ _ _ 1 00 . 0%

Since th~ ultra*iltratlon do~s not substantially alter th~ carbo-hydrate composition ~ascertalned by Measurements) J the observation can be made by ~uxtapositlon with the data in Table 1 that by ultrafiltration of the reflux broth in blrch digestion the xylose content o~ the dry matter in the broth could be rai3ed from 7X to 27%~ while at the samP time the volume of the solution that had to be handled decreased 4rom 190 to 37 litres.

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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a procedure for recovery of soluble carbohy-drates present in wood wherein the carbohydrates to be recovered which are contained in wood to be digested to cellulose are brought into a solution from which they are separated, and wherein the treatment of the wood thereafter continues as an alkaline cellulose digestion process, the improvement which com-prises performing the recovery of the carbohydrates at the ini-tial phase of the cellulose digesting process, where the polysac-charide content of the digesting solution is high and the content of polymeric lignin is low, by conducting the digesting solution into an ultrafilter which separates the carbohydrates therefrom, and returning the filtered solution to the cellulose digestion and wherein the separation of carbohydrates is carried out before the digesting solution has reached that temperature at which the digesting process mainly takes place.

2. The procedure according to claim 1, wherein the separation of carbohydrates is carried out at that stage when the temperature of the digesting solution is in the interval from 100-170°C.

3. The procedure of claim 2, wherein the temperature is from 120 to 160°C.

4. The procedure of claim 2, wherein the temperature is from 140 to 150°C.

5. In a procedure for recovery of soluble carbohy-drates present in wood wherein the carbohydrates to be recovered which are contained in wood to be digested to cellulose are brought into a solution from which they are separated, and wherein the treatment of the wood thereafter continues as an alkaline cellulose digestion process, the improvement which com-prises performing the recovery of the carbohydrates at the ini-tial phase of the cellulose digesting process, where the polysac-charide content of the digesting solution is high and the content of polymeric lignin is low, by conducting the digesting solution into an ultrafilter which separates the carbohydrates therefrom, and returning the filtered solution to the cellulose digestion and wherein the separation of carbohydrates is carried out at a stage at which at least 8% of the original dry matter of the wood are in the solution in the form of dissolved carbohydrates.

6. In a procedure for recovery of soluble carbohy-drates present in wood wherein the carbohydrates to be recovered which are contained in wood to be digested to cellulose are brought into a solution from which they are separated, and wherein the treatment of the wood thereafter continues as an alkaline cellulose digestion process, the improvement which com-prises performing the recovery of the carbohydrates at the ini-tial phase of the cellulose digesting process, where the polysac-charide content of the digesting solution is high and the content of polymeric lignin is low, by conducting the digesting solution into an ultrafilter which separates the carbohydrates therefrom, and returning the filtered solution to the cellulose digestion, wherein the separation of carbohydrates is carried out before the digesting solution has reached that temperature at which the digesting process mainly takes place and at a stage at which at least 8% of the original dry matter of the wood are in the solu-tion in the form of dissolved carbohydrates.

7. The procedure of claim 1, 5 or 6, wherein the sepa-ration of carbohydrates is carried out at a stage at which the solution contains lignin at most 15%, calculated on the original dry matter quantity of the wood.

8. The procedure of claim 2, 3 or 4, wherein the sepa-ration of carbohydrates is carried out at a stage at which the solution contains lignin at most 15%, calculated on the original dry matter quantity of the wood.