Note: Descriptions are shown in the official language in which they were submitted.
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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