Method of producing xylose

Details Method of producing xylose Method of producing xylose

1999-08-06

2001-05-29

Lee, Howard C. (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carbohydrates or derivatives

C536S127000

Reexamination Certificate

active

06239274

ABSTRACT:

The present invention relates to a method of producing xylose by chromatographic separation of the xylose from a sulphite cooking liquor containing for example xylose and xylonic acid and having lignosulphates as its main components. It is known to use xylose as a raw material in e.g. xylitol, aromatic products and animal feed.
Xylitol is a sugar alcohol occurring in nature and having a sweetness corresponding to that of “ordinary sugar”, but a calorie content (2.4 kcal/g) lower than that of ordinary sugar. Small amounts of xylitol occur in many fruits and vegetables, and it is even produced by the human body as a normal metabolic product. Certain metabolic, odontological and technical properties of xylitol make it an extremely good special sweetener for various uses, such as chewing gum and candy. An example is the independence of xylitol metabolism of insulin metabolism, allowing xylitol to be used by diabetics, too. Xylitol also has a slowing intestinal effect, making it usable in slimming diets. Furthermore, xylitol has been proved to be noncariogenic, even anticariogenic.
The recognized advantageous properties of xylitol have increased its demand recently, resulting in an increased need for an inexpensive and suitable raw material for the production of xylitol.
Previously xylitol was produced mainly by hydrolyzing xylan-containing materials. This way a monosaccharide mixture, containing for example xylose, is obtained. After different purification stages the xylose is then reduced into xylitol by catalytic reduction (hydrogenation), usually in the presence of a nickel catalyst, such as Raney nickel. The literature of the field describes numerous methods of producing xylose and/or xylitol from xylan-containing materials. As examples can be mentioned U.S. Pat. No. 3,784,408 (Jaffe et al.), U.S. Pat. No. 4,066,711 (Melaja et al.), U.S. Pat. No. 4,075,406 (Melaja et al.), U.S. Pat. No. 4,008,285 (Melaja et al.), and U.S. Pat. No. 3,586,537 (Steiner et al.).
In several plants the majority of hemicellulose is xylan which can be hydrolyzed into xylose. Particularly the hemicellulose of hardwood is rich in xylan. Consequently it is possible to obtain xylan and xylose as by-products from cellulose industry using hardwood, the utilization of which has been previously suggested in e.g. Finnish Pat. 55,516 and U.S. Pat. application 60/049,065.
Xylan, the hemicellulose used as raw material in xylose production, occurs particularly in acid hardwood sulphite cookings mainly as monomers, in which cookings Mg
2+
, Ca
2+
, NH
4
+
and Na
+
are typically used as the base. The concept “cooking liqucr” refers herein to the solution used in the cooking or obtained after the cooking, or to a part thereof (e.g. what is known as side removal or a cooking liquor from multi-stage cooking). Besides hardwood, other xylan-containing biomass, such as straw or bagasse, can also be used as raw material in sulphite cookings.
When using sulphite cooking liquor as a raw material of xylose, the problem is the varying amount of xylose in the cooking liquor, due to e.g. different wood hemicellulose compositions and hemicellulose reactions during cooking. Consequently, in addition to xylose and components dissolved from wood, sulphite cooking liquors contain for example xylonic acid, occurring in acid conditions mainly as lactone. The xylonic acid in the liquor complicates in itself xylose separation by reducing the xylose content in the cooking liquor and the xylose yield from the cooking liquor. This again reduces for example the xylose crystallization yield from a xylonic acid-containing xylose concentrate, a problem dealt with in pending PCT Patent Applications PCT/FI97/00402 and PCT/FI97/00403.
It has now been observed that as the amount of xylonic acid in the cooking liquor increases, the amount of xylose in the liquor diminishes. In other words, the xylose yield is the better the less xylonic acid is present in the liquor. In addition to said xylonic acid and xylose, the cooking liquors of acid sulphite cookings also contain degradation products of xylose and other saccharides, the amounts of which are affected by cooking conditions, such as cooking time and cooking temperature. Furthermore, part of the potentially obtainable xylan/xylose remains in the wood.
As presented above, cations Mg
2+
, Ca
2+
, NH
4
+
and Na
+
are used as base in sulphite cookings and all of these can be used in acid cookings. NH
4
+
and Na
+
cations can also be used in neutral and even basic conditions. The cations most commonly used in acid sulphite cookings are Ca
2+
and Mg
2+
. With stricter environmental norms, Mg
2+
cooking is becoming more common, since Mg
2+
can be recycled by evaporation and burning the cooking liquor after cooking and by recovering the Mg as MgO. Sulphur is recovered simultaneously. there is no corresponding economic recycling method for Ca
2+
. On the other hand, the use of Mg
2+
base enables a somewhat higher pH in cooking, which is advantageous as regards the production of chemical pulp. However, it has now been noted that as a result of a higher pH, less xylose is formed in the cooking liquor.
The xylose content in Ca
2+
and Mg
2+
cooking liquors has been analyzed to be even less than 10% of the dry solids in the cooking liquor, the theoretical amount being as much as over 20% of dry solids, Factors directly affecting the xylose content of the cooking liquor include the tree species and the quality of wood used in the cook and its hemicellulose composition. Usable tree species include birch, beech, aspen, maple, elm, rowan, eucalyptus and acacia. Birch and beech are known to be particularly good xylose raw materials, birch being especially advantageous in this respect. If the wood raw material is pure birch, even over 20% xylose contents have been measured from the dry solids of the cooking liquor. However, in practice the raw material used in hardwood pulp is often mixed hardwood formed of various hardwood species, resulting in a varying amount of xylose in the cooking liquor with a content of even less than 10%. However, pulp producers do not usually select the raw wood material mainly based on the xylose content of the cooking liquor, but instead the decision is naturally affected by other factors related to the production of pulp, such as availability of raw wood material, its price, and the quality and price of the pulp to be produced. Other factors affecting the xylose content include cooking time and cooking temperature and the total amount of SO
2
used. All these factors also affect the quality of the produced pulp. Acid sulphite cookings are used to produce paper pulp and special pulp, such as dissolving pulp.
Chromatographic separation of xylose is described in e.g. the above PCT Patent applications PCT/FI97/00402 and PCT/FI97/00403 and U.S. Pat. No. 4,631,129. Chrorriatographic separation of pure xylose from a cooking liquor rich in xylonic acid is difficult, for example because xylonic acid is not easily separated from xylose. In chromatographic separation, organic carboxylic acids, such as xylonic acid in the cooking liquor, are eluted below their dissociation pH as uncharged substances. Elution order is affected by e.g. molecular size, i.e. acids with larger molecular size are eluted first. The molecular sizes of xylonic acid and xylose are substantially identical. Consequently, xylonic acid is eluted from the column at roughly the same time as xylose, or somewhat later, (as lactone), whereby the purity of the xylose fraction is not high. When pH is raised over the dissociation pH, the acids are dissociated, while xylonic acid lactone is also partially split. If the separation resin in the column is in a monovalent (Na
+
, K
+
) resin form, the dissociated acids or their salts would be eluted at the beginning of the separation profile, before xylose. When resin is used in a divalent form, acids and their salts are, however, more strongly retained, particularly at an elevated

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