CA1151644A - Process for separating ketose-aldose mixtures by selective adsorption - Google Patents

Abstract

abstract A process for separating components of a feed mix-ture comprising a ketose and an aldose which process comprises contacting the mixture at adsorption conditions with an adsor-bent comprising a crystalline aluminosilicate selected from the X and Y zeolites and containing one or more selected cations at the exchangeable cationic sites, thereby selectively adsorbing one of said components and thereafter recovering the same. Preferably the adsorbed component will be recovered by contacting the adsorbent with a desorbent material.

Description

llS164~

The field of art to which this invention per-tains is the solid-bed adsorptive separation of monosac-charides. More specifically the invention relates to a process for separating one component from a mixture com-prising a ketose and an aldose which process employs an adsorbent comprising a crystalline aluminosilicate which selectively adsorbs either the ketose or the aldose from the feed mixture.

It is well known in the separation art that cry-stalline aluminosilicates can be used to separate cer-tain hydrocarbon types from mixtures thereof, such as the separation of normal paraffins from branched-chain paraf-fins and the separation of olefinic hydrocarbons from paraffinic hydrocarbons. The X and Y zeolites have been employed in processes to separate individual hydrocar-bon isomers. Thus, adsorbents comprising X and Y zeolites are used to separate alkyl-trisubstituted benzene`isomers (U.S. Patent 3,114,782); to separate alkyl-tetrasubsti-tuted monocyclic aromatic isomers (U.S. Patent 3,864,416);
and to separate specific alkyl-substituted naphthalenes (in U.S. Patent 3,668,267). Perhaps the most extensively used processes are those for separating paraxylene from a mixture of C8 aromatics. (U.S. Patents Nos. 3,558,730;
3,558,732; 3,626,020; 3,663,638; and 3,734,974).
In contrast, our invention relates to the sep-aration of non-hydrocarbons and more specifically to the separation of monosaccharides. We have discovered that adsorbents comprising certain zeolites containing one or more selected cations at the exchangeable cationic sites exhibit adsorptive selectivity for a ketose with respect to an aldose, while certain other cationic-exchanged zeo-lites e~hibit selectivity for an aldose with respect to ,. ~

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~5:16~4 a ketose, thereby making separation of a ketose (or al-dose) from a mixture comprising a ketose and an aldose by solid~bed selective adsorption possible. In a speci-fic embodiment our process is directed to separating fructose from a mixture comprising fructose and ~lucose.
Fructose is considered to be the most soluble and the sweetest of the sugars. Relative to sucrose having a sweetness of 1.0, fructose has a relative sweet-ness of about 1.4 while that of glucose is 0.7. The literature indicates that one of the uses of fructose in pure form is as a source of calories for patients who must be fed intervenously, whereas glucose is not suitable for intervenous feeding. While fructose exists widely in nature, the methods for isolating high-purity fructose are, however, more difficult than the primary method for obtaining high-purity glucose. High-purity glucose is readily manuEactured from starch by hydrolysis with min-eral acids at elevated temperature followed by refining and crystallization, while one method of obtaining high-purity fructose involves hydrolysis of sucrose, separa-tion of an insoluble lime-fructose complex, acidification of the complex with acids that form insoluble calcium salts, removal of cation and anion contaminants, concen-tration of the resulting solution, and finally crystal-lization of fructose. Extensive studies have been made on the production of fructose by hydrolysis of fructose-bearing polysaccharides extracted from the Jerusalem artichoke. Several methods of separating glucose from invert sugar, leaving fructose, have also been attempted, such as formation of insoluble benzidine dèrivatives of glucose and sodium chloride addition compounds of glucose, but these have not been practicable.

It is accordingly a broad object of the pre-sent invention to provide a process for separating the ketose and aldose from a feed mixture containing both components to produce ketose and aldose product streams containing higher concentrations of the ketose and al-dose, respectively, than were contained in the feed mix-ture. More specifically it is an objective of the in-vention to provide a process for producing concentrates - of the fructose and the glucose from a feed mixture, such as an invert sugar solution or a high fructose corn syrup, containing the two components.
Accordingly, the present invention is directed to a process for separating components of a feed mixture comprising a ketose ànd an aldose which process comprises contacting said mixture at adsorption conditions with an adsorbent comprising a crystalline aluminosilicate selec-ted from (1) an X zeolite containing at exchangeable cationic sites a cation selected from the group consist-ing of sodium, potassium, barium and strontium, and

(2) a Y zeolite containing at exchangeable cationic sites at least one cation selected from the group consisting of ammonium, sodium, potassium , calcium, strontium, barium and combinations thereof, thereby selectively adsorbing one of said com-ponents, and thereafter contacting the adsorbent con-taining the adsorbed component with a desorbent and re-covering the resultant desorbed component.
In another embodiment of the invention the feed mixture is contacted with an adsorbent comprising a X
zeolite containing at exchangeable cationic sites a ab~ C

115~6~

cation selected from the group consisting of sodium, barium and strontium thereby selectively adsorbing the ketose and thereafter recovering the ketose by desorp-tion.
In a further embodiment the feed mixture is contacted with an adsorbent comprising a X zeolite con-taining at exchangeable cationic sites a cation pair selected from the group consisting of barium and potas-sium and barium and strontium thereby selectively adsor-bing the ketose and thereafter recovering the ketose by desorption.
In a still further embodiment, the feed mlx-ture is contacted at adsorptlon conditions with an ad-sorbent comprising a ~ zeolite containing potassium cations at exchangeable cationic sites, thereby selec-tively adsorbing the aldose and thereafter recovering the aldose by desorption.
In still another embodiment the feed mixture is contacted with an adsorbent comprising a Y zeolite containing at exchangeable cationic sites at least one cation selected from the group consisting of ammonium, sodium, potassium, calcium, strontium, barium and com-binations thereof; thereby selectively adsorbing the ketose and thereafter recovering the ketose by desorp-tion.
In a step-wise embodiment, the invention in-volves: (a) contacting the feed mixture with the adsor-bent to thereby selectively adsorb one component (ketose or aldose); (b) removing from the adsorbent a raffinate stream comprising the other component; ~c) contacting said adsorbent with a desorbent material to effect the desorption of the adsorbed component from mabj ~lSl~

said adsorbent; and, (d) removing from said desorbent an l'extract"
stream comprising said adsorbed component.
Preferably the step-wise process comprises the steps of: (a) maintaining net fluid flow through a column of said adsorbent in a single direction, which column contains at least three zones having separate operational functions occurring therein and being serially interconnected with the terminal zones of said column connected to provide a conti.nuous connection of said zones; (b) maintaining an adsorption zone in said column, said zone defined by the adsorbent located between a feed input stream at an upstream boundary of said zone and a raffinate out--put stream at a downstream boundary of said zone; (c) maintaining a purification zone immediately upstream from said adsorption zone, said purification zone defined by the adsorbent located between an extract output stream at an upstream boundary of said purification zone and said feed input stream at a downstream boundary of said purification zone; (d) maintaining a desorption zone immediately upstream from said purification zone, said desorption zone defined by the adsorbent located between a desorbentinput stream at an upstream boundary of said zone and said extract output stream at a downstream boundary of said zone;
(e) passing said feed mixture into said adsorption zone at adsorption conditions to effect the selective adsorption of one component (ketose or aldose) by said adsorbent in said adsorption zone and withdrawing a raffinate output stream comprising the non-adsorbed component from said adsorption zone; (f) passi.ng a desorbent material into said desorption zone at desorption con-ditions to effect the displacement of the adsorbed compound ~rom 115~6~4 the adsorbent in said desorption zone; (g) withdrawing an extract output stream comprising adsorbed component and desorbent material from said desorption zonei (h) passing at least a portion of said extract output stream to a separation means and therein separating at least a portion of said desorbent material to produce an extract product stream having a reduced concen-tration of desorbent material; and, (i) periodically advancing through said column of adsorbent, in a downstream direction with respect to fluid flow in said adsorption zone, the feed input stream, raffinate output stream, desorbent input stream, and extract output stream to effect the shifting of zones through said adsorbent and the production of extract output and raffinate output streams.
Other objectives and embodiments of our invention encompass details about feed mixtures, adsorbents, desorbent materials and operating conditions all of which are herein-after disclosed in the following discussion of each of the facets of the present invention.
The definitions of various terms used throughout the specification will be useful in making clear the operation, objects and advantages of our process.
A feed mixture is a mixture containing one or more extract components and one or more raffinate components to be separated by our process. The term "feed stream" indicates a stream of a feed mixture which passes to the adsorbent used in the process.
An "extract component" is a compound or type of com-pound that is more selectively adsorbed by the adsorbent while a 11516~

"raffinate component'1 is a compound or type of compowld that is less selectively adsorbed. In this process, when a ketose is an extract component, an aldose is a raffinate component, and vice versa. The term "desorbent material" shall mean generally a material capable of desorbing an extract component. The term "desorbent stream" or "desorbent input stream" indicates the stream through which desorbent material passes to the adsorbent. The term "raffinate stream" or "raffinate output stream" means a stream through which a raffinate component is removed from the adsorbent. The composition of the raffinate stream can vary from essentially 100% desorbent material to essentially 100~ raffinate components. The term "extract stream" or "extract output stream" shall mean a stream through which an extract material which has been desorbed by a desorbent material is removed from the adsorbent. The composition of the extract stream,likewise, can vary from essentially 100%
desorbent material to essentially 100% extract components. At least a portion of the extract stream and preferably at least a portion Of the raffinate stream frorn the separation process are passed to separation means, typically fractionators, where desorbent material is separated to produce an extract product and a raffinate product. The terms "extract product" and "raffinate product" mean products produced by the process con-taining, respectively, an extract component and a raffinate component in higher concentrations than those found in the extract stream and the raffinate stream. Although it is possible to produce a high purity ketose product or aldose product (or both) at high recoveries, it will be appreciated that an extract ~lS~6~

component is never completely adsorbed by the adsorbent, nor is a raffinate component completely non-adsorbed by the adsorbent.
Therefore, varyin~ amounts of a raffinate component can appear in the extract stream and,likewise, vaxying amounts of an extract component can appear in the raffinate stream depending upon the process operating conditions employed. The extract and raffinate streams then are further distinguished from each other and from the feed mixture by the ratio of the concen-trations of an extract component and a raffinate component appearing in the particular stream. ~lore specifically, the ratio of the concentration of a ketose for example, to that of a less selectively adsorbed aldose will be lowest in the raffinate stream, next highest in the feed mixture, and the highest in the extract stream. Likewise, the ratio of the concentration of a less selectively adsorbed aldose to that of the more selectively adsorbed ketose will be highest in the raffinate stream, next highest in the feed mixture, and the lowest in the extract stream.
The term "selective pore volume' of the adsorbent is defined as the volume of the adsorbent which selectively adsorbs an extract component from the feed mixture. The term "non-selective void volume" of the adsorbent is the volume of the adsorbent which does not selectively retain an extract component from the feed mixture. This volume includes the cavities of the adsorbent which contain no adsorptive sites and the inter-stitial void spaces betweenadsorbent particles. The selective pore volume and the non-selective void volume are generally expressed in volumetric quantities and are of importance in ~1516~

determininy the proper flow rates of fluid required to be passed into an operational zone for efficient operations to take place for a given quantity of adsorbent. When adsorbent "passes"
into an operational zone (hereinaf-ter defined and described) employed in one embodiment of this process,its non-selective void volume together with its selective pore volume carries fluid into that zone. The non-selective void volume is utilized in determining the amount of fluid which should pass into the same zone in a counter-current direction to the adsorbent to displace the fluid present in the non-se]ective void volume.
If the fluid flow rate passing into a zone is smaller than the non-selective void volume rate of adsorbent material passing into that zone, there is a net entrainment of liquid into the zone by the adsorbent. Since this net entrainment is a fluid present in non-selective void volume of the adsorbent, it in most instances comprises less selectively retained feed compo-nents. The selective pore volume of an adsorbent can in certain instances adsorb portions of raffinlte material from the fluid surrounding the adsorbent since in certain instances there is competition between extract material and raffinate material for adsorptive sites within the selective pore volume. If a large quantity of raffinate material with respect to extract material surrounds the adsorbent, raffinate material can be competitive enough to be adsorbed by the adsorbent.
Feed mixtures which can be charged to the process of the invention will be those comprising a ketose and an aldose and more specifically and preferably will be aqueous solutions of a ketose and an aldose. While the feed mixture may contain more than one ketose and more than one aldose, typically the ~lS~

feed mixture will contain one ketose and one aldose each in concentrations of from about 0.5 wt.% to about 30 wt.~ and more preferably from about 1 to about 15 wt.%. The process may be used to separate a ketopentose from an aldopentose but more typically will be used to separate a ketohexose from an aldohexose.
Well known ketohexoses are fructose (levulose) and sorbose;
well known aldohexoses are glucose (dextrose), mannose and galactose while lesser-known aldohexoses are gulose, talose, allose, altrose, and idose. Preferred feed mixtures containing hexoses will be aqueous solutions of invert sugar formed when sucrose is hydrolyzed by acidic materials into equi-molar amounts of fructose and glucose. Other preferred feed mixtures will be aqueous solutions of high fructose (typically about 40-45%) corn syrup produced by the enzymatic isomerization of glucose solutions.
The desorbent used in the process of the invention should satisfy several criteria. First, it should displace an extract component from the adsorbent with reasonable mass flow rates without itself being so strongly adsorbed as to unduly prevent an extract component from displacing the desorbent material in a following adsorption cycle. It should also be more selective for all of the extract components with respect to a raffinate component than it is for the desorbent material with respect to a raffinate component. It must be compatible with the particular adsorbent and the particular feed mixture.
It must not reduce or destroy the critical selectivit~ of the adsorbent for an extract component with respect to a raffinate component. It should be easily separable from the feed mixture 1~S~64~

that is passed into the process, preferably by distillation.
Since the raffinate and extract products are oodstuffs intended for human consumption, desorbent materials should also be non-toxic. Finally, desorbent materials should be rea~ily availahle and therefore reasonable in cost. Water satisfies these cri-teria and is a suitable desorbent material for the present process.
It is known that certain characteristics of adsorbents are necessary to the successful operation of a selective ad-sorption process. Among such characteristics are: adsorptive capacity for some volume of an extract component per volume of adsorbent; also sufficiently fast rates of adsorption and desorption. Another necessary characteristic is the ability of the adsorbent to separate components of the feed; in other words, that the adsorbent possess adsorptive selectivity, (B), for one component as compared to another component. The selectivity, (B), as used throughout this specification is defined as the ratio of the two components of the adsorbed phase over the ratio of the same two components in the unadsorbed phase at equilibiium conditions. Relative selectivity is shown as Equation 1 below.

~quation 1 Selectivity = (B) = [vol. percent C/vol. percent D]A
[vol. percent C/vol. percent D]u where C and D are two components of the feed represented in volume percent and the subscripts A and U represent the adsorbed and unadsorbed phases respectively. T~here selectivity as be-tween two components approaches 1.0 there isno preferential adsorption of one component by the adsorbent with respect to the ~5~4~

other, When comparing the selectivity by the adsorbent of one component C over component D, a value of (B) larger than ].0 indicates preferential adsorption of component C within the adsorbent. A value of (B) less than 1.0 would indicate that component D is preferentially adsorbed leaving an unadsorbed phase richer in component C and an adsorhed phase richer in component D. Ideally,desorbent materials should have a selec-tivity equal to about 1 or less than l with respect to all extract components so that all of the extract components can be extracted as a class and all raffinate components cleanly rejected into the raffinate stream. While separation oE an extract component from a raffinate component is theoretically possible when the selectivity of the adsorbent for the extract component with respect to the raffinate component is greater than 1, it is preferred that such selectivity approach a value of 2.
Like relative volatility, the higher the selectivity the easier the separation is to perform.
A dynamic testing apparatus is employed to test various adsorbents with a particular feed mixture and desorbent material to measure the adsorbent characteristics of adsorptive capacity, selectivity and exchange rate. Such an apparatus may consist of an adsorbent chamber of approximately 70 cc volurne having inlet and outlet at opposite ends, temperature and pressure control equipment to assure a constant predetermined pressure, analytical equipment such as refractometers, polarimeters and chromatographs to determine quantitatively or qualitatively one or more components in the effluent stream leaving the chamber.
A pulse test using this apparatus will determine selectivities ~15~644 and other data for various adsorbent systems. The adsorbent is filled to equilibrium with a particular desorbent material. At a convenient time, a pulse of feed containing known concentrations of a tracer and of a particular ketose or aldose, or both, all diluted in desorbent, is injected for a duration of several minutes. Desorbent flow is resumed, and the tracer and the ke-tose and aldose are eluted as in a liquid-solid chromatographic operation. The effluent is analyzed to determine traces of the envelopes of corresponding component peaks developed.
From information derived from the test, adsorbent performance can be rated in terms of void volume, capacity index for an extract component, selectivity for one component with respect to the other, and the rate of desorption of an extract component by the desorbent. The capacity index of an extract component may be characterized by the distance between the center of the peak envelope of the extract component and the peak envelope of the tracer component or some other known reference point. It is expressed in terrns of the volume in cubic centimeters of desorbent pumped during this time interval represented by the distance between the peak envelopes. Selecti-vity, (B), for an extract component with respect to a raffinate component may be characterized by the ratio of the distance be-tween the center of the extract component peak envelope and the tracer peak envelope (or other reference point) to the corresponding distance between the center of the raffinate component peak envelope and the tracer peak envelope. The rate of exchange of an extract component with the desorbent can generally be charac-teri~ed by the width of the peak envelopes at half intensity.

~l~5~4 The narrower the peak width the faster the desorption rate.
The desorption rate can also be characterized by the distance between the center of the tracer peak envelope and the disap-pearance of an extract component which has just been desorbed.
This distance is again the volume of desorbent pumped during this time interval.
Adsorbent systems can also be evaluated by actual testing in a continuous countercurrent liquid-solid contacting device, such as described in U.S. Patents 2,985,589 and 3,706,812.
Additional details on adsorbent evaluation techniques may be found in the paper "Separation of C8 Aromatics by Adsorption"
by ~. J. deRosset, R. W. Neuzil, D. J. Korous, and D. l~. Roshack presented at the American Chemical Society, Los Angeles, Califor-nia, March 28 through April 2, 1971.
Adsorbents to be used in the process of this invention include crystalline aluminosilicate cage structures in which the alumina and silica tetrahedra are intimately connected in an open three dimensional network. The crystalline aluminosilicates are often referred to as "molecular sieves" particularly when the separation which they effect is dependent essentially upon differences between the sizes of the feed molecules. In the process of this invention, however, the term "molecular sieves"
is not strictly suitable since the separation of a ketose from an aldose is apparently dependent on differences in electro-chemical attraction between a ketose and the adsorbent on the one handand between an aldose and the adsorbent on the other, rather than on physical size differences in the molecules.
In hydrated form, the crystalline aluminosilicates llS~6~4 generally encompass those zeolites represented by the Formula 1 below:
Formula 1 M2/nO :A1203 :wsio2 :yH20 where "M" is a cation which balances the electrovalence o~ the aluminum-centered ~etrahedra and which i.s generally referred to as an exchangeable cationic site, "n" represents the valence of the cation, "w" represents the moles of SiO2, and "y"
represents the moles of water. The generalized cation "M"
may be monovalent, divalent or trivalent or mixtures thereof.
The X zeolite in the hydrated or partially hydrated ~orm can be represented in terms of mole oxides as shown in Formula 2 below:
Formula 2 (0.9+0.2)M2/nO:A1203:(2.5+0.5)SiO2;yH20 where "M" represents at least one cation having a valence of not more than 3, "n" represents the valence of "M", and '~y" is a value up to about 9 depending upon the identity of "M" and the degree of hydration of the crystal. As noted from Formula 2 the SiO2/A1203 mole ratio of X zeolite is 2.5+0.5. The cation "M" may be one or more of a number of cations such as a hydrogen cation, an alkali metal cation, or an alkaline earth cation, or other selected cations, and is generally referred to as an exchangeable cationic site. As the X zeolite is initially prepared, the cation "M" is usually predominately sodium and the zeolite is therefore referred to as a sodium-X zeolite. De-pending upon the purity of the reactants used to make the zeolite, other cations mentioned above may be present, however, as impurities.

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The Y zeolite in the hydrated or partially hydrated form can be similarly represented in terms of mole oxides as in Formula 3 below:
Formula 3 (o.g+o~2)M2/no:Al2o3:wsio2 y~2o where "M" is at least one cation having a valence not more than

3, "n" represents the valence of "M", "w" is a value greater than about 3 up to 6, and "y" is a value up to about ~ depending upon the identity of "M" and the degree of hydration of the crystal. The SiO2/A1203 mole ratio for Y zeolites can thus be from about 3 to about 6. The cation "M" may be one or more of several cations as in the case of the X-zeolite but, as the Y zeolite is initially prepared, the cation "M" is also usually predominately sodium. A Y zeolite containing predominately sodium cations at the exchangeable cationic sites is therefor referred to as a sodium-Y zeolite.
Cations occupying exchangeable cationic sites in the zeolite may be replaced with other cations by well known ion exchange methods, for example by contacting the zeolite, or a base material containing the zeolite, with an aqueous solution of a soluble salt of the cation or cations desired to be placed upon the zeolite. After the desired degree of exchange takes place the sieves are removed from the aqueous solution, washed, and dried to a desired water content. By such methods the sodlum cations and any non-sodium cations which might be occupying exchangeable sites as impurities in a sodium~X or sodium-Y
zeolite can be partially or essentially completely replaced with other cations.

The term "base material" as used herein shall refer to a material containing X or Y zeolite which can be used to make the special adsorbents described below. The zeolite will typically be present in the base material in amounts ranging from about 75 wt. % to about 98 wt. % of the base material based on volatile free composition. Volatile free compositions are generally determined after the base material has been calcined at 900 C. in order to drive off all volatile matter. The remainder of the base material will generally be amorphous material such as silica, alumina or silica-alumina mixtures or compounds, such as clays, which material is present in intimate mixture with the small particles of the zeolite material. This amorphous material may be an adjunct of the manufacturing process for X or Y zeolite (for example, intentionally incom-plete purification of either zeolite during its manufacture) or it may be added to relatively pure X or Y zeolite, but in either case its usual purpose is as a binder to aid in formlng or agglomerating the hard crystalline particles of the zeolite.
Normally the base material will be in the form of particles Z0 such as extrudates, aggregates, tablets, pills, macrospheres, or granules produced by grinding any of the above to a desired particle size range. The adsorbent to be used in our process will preferably have a particle size range of about 16-60 mesh, and more preferably about 30 to about 50 mesh (Standard U.S. Mesh), Examples of suitable base materials which can be used to make the adsorbents employed in our process are "Molecular Sieves 13X" and "SK-40" both of which are available from the Linde Company, Tonawanda, New York. The first material contains X zeolite, while the latter material contains Y
zeolite.

1~5~6~4 It has heen diseovered that X and Y zeolites con-taining speeified cations at the exchangeable cationic sites possess the requirements previously discussed and are therefore suitable for use in the process. Some of the suitable zeolites contain essentially a single cation species at the exchangeable cationie sites while others are essentially completely exchanged with selected eation pairs. A zeolite is deemed to be essentially eompletely exehanged when the residual sodium eontent of the zeolite after ion exchange is less than about 2 wt.% Na20. Spe-eifieally we have found that the adsorbents comprising an X
zeolite eontaining at exchangeable cationic sites sodium cations or barium eations or strontium eations all possess seleetivity for a ketose with respec~ to an aldose. Other adsorbents having similar selectivity are X zeolites containing the eation pair ba~ium and potassium or the cation pair barium and strontium at exehangeable cationie sites. A zeolite X
adsorbent eontaining barium and potassium at the exchangeable eationie sites will preferably have a weight ratio of barium to potassium within the range from about 1:1 to about 100:1 and more preferably within the range of from about 1:1 to ahout 10:1.
A zeolite X adsorbent eontaining barium and strontium at the exehangeable cationic sites will preferahly have a weight ratio of barium to strontium within the range of from about 1:1 to about 15:1 and more preferably from about 5:1 to ahout 15:1.
On the other hand it has been found that an adsorbent eomprising anX zeolite eontaining potassium at the exchangeable eationie sites is suitable for use in our proeess by virtue of its seleeti~lity for an aldose with respeet to a ketose. Such ~S.16~4 adsorbent may be manufactured by essentially completely ion exehanging "Moleeular Sieves 13X" (Na-X zeolite) with potassium cations (typically with a KCl solution), washing the exchanged material with water to remove excess ion exehange solution and drying the adsorbent to less than about 10 pereent weight loss on ignition (LOI) at 900C.
Further, it has been discovered that adsorbents com-prising a Y zeolite eontaining at exehangeable eationie sites at least one eation seleeted from the group eonsisting of ammonium, sodium, potassium, ealcium, strontium, barium and combinations thereof are suitable for use in our process beeause of their selectivity for a ketose with respect to an aldose. Preferably, the zeolites will be essentially eompletely exchanged with the seleeted cation or cations. A partieularly preferred adsorbent is a base material eomprising Y zeolite and amorphous material eontaining ealeium eations at the exehangeable eationie sites.
There is a surprising lack of predietability regarding the suitability of adsorbents for use in our process. Many adsorbents comprising X or Y zeolites and amorphous material in faet exhibit no selectivity for either a ketose or an aldose and are therefore not suitable for use in the process. For instanee, a Y zeolite eontaining ammonium cations at exehangeable eationie sites exhibits seleetivity for a ketose with respeet to an aldose, but a Y zeolite containing hydrogen eations at the exchangeable cationic sites exhibits no selectivity for either a ketose or an aldose.
An X zeolite containing potassium at the exchangeable cationie sites appears unique among the X zeolites in its ~L5 ~6~

ability to selectively adsorb an aldose with respect to a ketose. An adsorbent comprising either a cesium-exchanged X
or Y ~eolite exhibits selectivity for neither an aldose nor a ketose. A potassium-exchanged Y zeolite~ unlike the potassium-exchanged X zeolite, also exhibits selectivity for a ketose with respect to an aldose. Adsorbents comprising Y zeolites con-taining at exchangeable cation sites either barium or strontium or barium and strontium or barium and potassium cations exhibit selectivity for a ketose with respect to an aldose, while adsorbents comprising X zeolites containing at exchangeable cationic sites either calcium or magnesium exhibit selectivity for neither an aldose nor a ketose.
Considering adsorbents comprising Y zeolites containing at exchangeable cation sites cations of metals of Group IIA of the Periodic Table of Elements, those containing calcium, strontium or barium all exhibit selectivity for a ketose with respect to an aldose but a Y ~eolite containing magnesium exhibits selectivity for neither a ketose nor an aldose. Of those suitable adsorbents comprising Y zeolites containing Ca, Sr, or Ba cations at exchangeable cationic sites, we have dis-covered that an adsorbent comprising a Y zeolite containing Ca cations at such sites is much superior to adsorbents containing Sr or Ba cations at the same sites. l`he reasons why some adsorbents are acceptable for use in our process while others are not is not fully understood at the present time.
The adsorbent may be employed in the form of a dense compact fixed bed which is alternatively contacted with the feed mixture and desorbent materials. In the simplest embodiment 6~4 of the invention the adsorbent is employed in the form of a single static bed in which case the process is only semi-continuous~ In another embodiment a set of two or more static beds may be employed in fixed-bed contacting with appropriate valving so that the feed mixture is passed through one or more adsorbent beds while the desorbent materials can be passed through one or more of the other beds in the set. The flow of feed mixture and desorbent materials may be either up or down through the desorbent. Any of the conventional apparatus employed in static bed fluid-solid contacting may be used.
Countercurrent moving-bed or simulated moving-bed countercurrent flow systems, however, have a much greater separation efficiency than fixed adsorbent bed systems and are therefore preferred. In the moving-bed or simulated moving-bed processes the adsorption and desorption operations are continuously taking place which allows both continuous pro-duction of an extract and a raffinate stream and the continual use of feed and desorbent streams. The operating principles and sequence of the simulated moving-bed countercurrent flow system are described in U.S. Patent 2,985,589. In such a system it is the pro~ressive movement of multiple liquid access points down an adsorbent chamber that simulates the upward movement of adsorbent contained in the chamber. Only four of the access lines are active at any one time; the feed input stream, desor-bent inlet stream, raffinate outlet stream, and extract outlet stream access lines. Coincident with this simulated upward movement of the solid adsorbent is the movement of the liquid occupying the void volume of the packed bed of adsorbent. So ~SJ.644 that countercurrent contact is maintained, a li~uid flow down the adsorbent chamber may be provided by a pump. As an active liquid access point moves through a cycle, that is, from the top of the chamber to the bottom, the chamber circulation pump moves through different zones which require different flow rates.
A programmed flow controller may be provided to set and regulate these flow rates.
The active liquid access points effectively divide the adsorbent chamber into separate zones, each of which has a different function. In this embodiment of our process it is generally necessary that three separate operational zones be present in order for the process to take place, although in some instances an optional fourth zone may be used The adsorption zone, zone 1, is defined as the ad-sorbent located between the feed inlet stream and the raffinate outlet stream. In this zone, the feed stock contacts the adsorbent, an extract component is adsorbed, and a raffinate stream is withdrawn. Since the general flow through zone ] is from the feed stream which passes into the zone to the raffinate stream which passes out of the zone, the flow in this zone is considered to be a downstream direction when proceeding from the feed inlet to the raffinate outlet streams.
Immediately upstream with respect to fluid flow in zone 1 is the purification zone, zone 2. The purification zone is defined as the adsorbent between the extract outlet stream and the feed inlet stream. The basic operations taking place in zone 2 are the displacement, from the non-selective void volume of the adsorbent, of any raffinate material carried into zone 2 by the shifting of adsorbent into this zone and the il~l64~

desorption of any raffinate material adsorbed within the selective pore volume of the adsorbent or adsorbed on the sur-faces of the adsorbent particles. Purification is achieved by passing a portion of extract stream material leaving zone 3 into zone 2 at zone 2's upstream boundary, the extract outlet stream, to effect the displacement of raffinate material. The flow of material in zone 2 is in a downstream direction from the extract outlet stream to the feed inlet stream.
Immediately upstream of æone 2 with respect to the fluid flowing in zone 2 is the desorption zone or zone 3.
The desorption zone is defined as the adsorbent between the desorbent inlet and the extract outlet stream. The function of the desorption zone is to allow a desorbent material which passes into this zone to displace the extract component which was adsorbed upon the adsorbent during a previous contact with feed in zone 1 in a prior cycle of operation. The flow of fluid in zone 3 is essentially in the same direction as that of zones 1 and 2, In some instances an optional buffer zone, zone 4, may be utilized. This zone, defined as the adsorbent between the raffinate outlet stream and the desorbent inlet stream, if used, is located immediately upstream with respect to the fluid flow to zone 3. Zone 4 would be utilized to conserve the amount of desorbent utilized in the desorption step since a portion of the raffinate stream which is removed from zone 1 can be passed into zone 4 to displace desorbent material present in that zone out of that zone into the desorption zone. Zone 4 will contain enough adsorbent so that raffinate material present in the ~1516~

raffinate stream passing out of zone 1 and into zone 4 can be prevented from passing into zone 3 thereby contaminating extract stream removed from zone 3. In the instances in which the fourth operational zone is not utilized the raffinate stream passed from zone 1 to zone 4 must be care~ully monitored in order that the flow directly from zone 1 to zone 3 can be stopped when there is an appreciable quantity of raffinate material present in the raffinate stream passing from zone 1 into zone 3 so that the extract outlet stream is not contaminat-ed.
A cyclic advancement of the input and output streams through the fixed bed of adsorbent can be accomplished by utilizing a manifold system in which the valves in the manifold are operated in a sequential manner to effect the shifting of the input and output streams thereby allowing a flow of fluid with respect to solid adsorbent in a counterCurrent manner.
Another modeof operation which can effect the countercurrent flow of solid adsorbent with respect to fluid involves the use of a rotating disc valve in which the input and output streams are connected to the valve and the lines through which feed input, extract output, desorbent input and raffinate output streams pass are advanced in the same direction through the adsorbent bed. Both the manifold arrangement and disc valve are known in the art. Specifically rotary disc valves which can be utilized in this operation can be found in U.S. Patents 3,040,777 and 3,422,848.
It is contemplated that at least a portion of the extract output stream will pass into a separation means wherein ~5~6~9, at least a portion of the desorhent is removed to produce an extract product containing a reduced concentration of desorbent.
Preferably at least a portion of the raffinate output stream will also be passed to a separation means wherein at least a portion of thedesorbent is removed to produce a desorbent stream which can be reused in the process and a raffinate product con-taining a reduced concentration of desorbent. The separation means will typically be a fractionation column.
Although both liquid and vapor phase operations can be used in many adsorptive separation processes, liquid-phase operation is preferred for the present process because of the lower temperature requirements and higher yields of extract product that can be obtained. Adsorption conditions will include a temperature range of from about 20C. to about 200C.
with about 20C. to about 100C. being preferred and a pressure range of from about atmospheric to about 35 atmospheres absolute, with from about atmospheric to about 17.5 atmospheres being preferred to insure liquid phase. ~esorption conditions will include the same range of temperatures and pressures as used for adsorption conditions.
The following examples are presented to illustrate the unique selectivity relationships that makes the process of the invention possible, and are not intended to unduly restrict the scope and spirlt of the claims attached hereto.
EXAMPLE I
..
This example presents retention volume and selectivity results obtained by pulse tests with eleven adsorbents, one comprising an A zeolite, one comprising a Y zeolite, and nine ~S~6~

comprising X zeolites. More specifically the adsorbent com-prising the A zeolite was Linde 5A Molecular Sieves (a calcium-exchange A zeolite), the adsorbent comprising a Y zeolite was pr0pared by essentially Gompletely ion-exchanging Linde SK-40 with potassium and the adsorbents comprising X zeolite were portions of Linde 13X Molecular Sieves essentially completely exchanged with the cations of metals K, Cs, Mg, Ca, Sr, sa, Ba+K and Ba+Sr. All adsorbents had a particle size range of approximately 20-40 U.S. Mesh. (Na-X = unexchanged 13X sieves.) The general pulse-test apparatus and procedure have been previously described. The adsorbents were tested in a 70 cc.coiled column maintained at 55C. and 4.4 atmospheres absolute pressure, and using pure water as the desorhent material.
The sequence of operations for each test was as follows: De-sorbent material (water) was continuously run through the columm containing the adsorbent at a nominal liquid hourly space ve-locity (LHSV) of about 1Ø At a convenient time desorbent flow was stopped, a 4.7 cc sample of 10 wt.% fructose in water was injected into the column via a sample loop, and the desorbent flow was resumed. The emergent sugar was detected by means of a continuous refractometer detector and a peak envelope trace was developed. Another pulse containing 10 wt.% glucose was similarly run. A saturated water solution of benzene was also injected to serve as a tracer from which the void volume of the adsorbent bed could be determined. Thus for each adsorbent tested three peak traces were developed, one for glucose, one for fructose and one for benzene. The retention volume for glucose is calculated by measuring the distance from time zero ~L~Sl~i44 or the reference point to the midpoint of the glucose peak and subtractin~ the distance representing the void volume of the adsorbent obtained by measuring the distance from the same reference point to the mid-point of the benzene peak. For some adsorbents both the fructose and glucose peaks were essentially on top of the benzene peak envelope indicating that both monosaccharides were relatively unadsorbed by the particular adsorbents in the presence of water. The selectivity of an adsorbent for fructose with respect to glucose is the quotient obtained by dividing the fructose retention volume by the glucose retention volume. The results for these tests are shown in Table No. 1 below.
Table No. 1 Selectivities of Various Adsorbents for Fructose with Respect to Glucose RETENTION VOL. OF RETENTION VOL. OF SELECTIVITY
TEST ADSORBENT FRUCTOSE, CC. GLUCOSE, CC. (B) 1 Na-X 7.1 5.0 1.42 2 K-X 11.9 21.6 0.55 3 K-Y 10.8 4.9 2.21

4 Cs-X Both were relatively unadsorbed

5 Mg-X Both were relatively unadsorbed

6 Ca-X Both were relatively unadsorbed

7 Ca-A Both were relatively unadsorbed

8 Sr-X 8.0 1.3 6.15

9 Ba-X 27.1 9.6 2.82 10Ba-K-X 16.4 7.5 2.19 11Ba-Sr-X 21.3 4.2 5.0 The adsorbents used for tests 1 through 4 were three X zeolites and one Y zeolite each containing at the exchangeable cationic sites cations of metals from Group IA
of the Periodic Table of Elements. The K-X adsorbent used for test 2 had a "selectivity" of 0.55 (for fructose with respect to glucose), and therefore actually exhibited selectivity for glucose with respect to fructose. The Na-X adsorbent used for test 1 (selectivity of ' 42) and the K-Y adsorbent used for test 3 (selectivity of 2.21) both exhibited selectivity for fructose with respect to glucose, while the Cs-X adsorbent used in Test 4 exhibited relative selectivity for neither.
The adsorbents used for Tests 5, 6, 8 and 9 were X
zeolites containing at exchangeable cationic sites cations of metals from Group IIA of the Periodic Table of Elements, while the adsorbent used for TeSt 7 was a calcium-exchanged A zeolite.
Both fructose and g~ucose were relatively unadsorbed with the Mg-X, Ca-X and Ca-A adsorbents used in the presence of water for Tests 5, 6 and 7 respectively, but both the Sr-X and Ba-X
adsorbents used in Tests 8 and 9, respectively, exhibited selectivity for fructose with respect to glucose. While not definitely established, it is believed that adsorbents com-prising X zeolites containing at the exchangeable cationic sites a Group IIA cation generally become less acidic as one moves downward from Period 3 to Period 6 of the Periodic Table of Elements in selecting the Group IIA cation. Thus adsorbents comprising Ca- or Mg-exchanged X zeolites are unsuitable for use in the present process because they are more acidic, while adsorbents comprising a Ba- or Sr-exchanged X zeolite are suitable for use in our process because less acidic.

The adsorbents used for Tests 10 and ll were X zeo-lites containing at exchan~eable cationic sites the cation pairs Ba and K, and Ba and Sr, respectively. The Ba-K-X
adsorbent used in Test lO exhibited selectivity for fructose with respect to glucose, while the K-X adsorbent used in Test 2 did not, but the selectivity of the sa-K-x adsorbent was not as high as that of the Ba-X adsorbent used in Test 9. The Ba-Sr-X adsorbent used in Test ll exhibited fructose-to-glucose selectivity less than the Sr-X adsorbent used in Test 8 but higher than the Ba-X adsorbent used in Test 9.
EX~MPLE II
To assure that fructose could be separated from an actual mixture containing fructose and glucose a solution con-taining 20 wt.~ each of fructose and glucose in water wa~
pulse-tested over a 440 cc. bed of adsorbent comprisin~ barium-~
exchanged X zeolite contained in a column having a 1.27 cm-inside diameter and 2.28 m. in heiyht. The adsorbent was the same as that used in Test 9 of Example I above and the same operatin~ temperature and pressure as those of Example I were employed. Water as the desorbent material was first passed over the adsorbent, then the pulse of feed was injected, and then desorbent material flow was resumed. The effluent was analyzed by both refractive index and polarimetry, and with this combina-tion quantitative rather than qualitative determinations of the fructose and glucose in the effluent were determined. The larger sample sizes required for these analyses was the reason for using a column larger than that used in Example I. The results obtained from this example, along with those of Test 9 of Example I twhich used the same adsorbent), are shown in Table 2.

~15~44 Selectivity Comparison with Ba-X Adsorbent RETENTION VOL. OF RETENTION VOL. OF SELECTIVITY, TEST FRUCTOSE, CC. GLUCOSE, CC. (~) Example II 105. 35 3.0 Test 9 of Example I 27.1 9.6 2.82 The selectivity obtained when the fructose and glucose were processed together is considered to be substantially the same as that obtained when they were processed separately.
EXAMPLE III
This example presents glucose and fructose peak widths and retention volumes and selectivities for fructose with respect to glucose and with respect to water which were lS obtained by conducting pulse tests with ten different adsor-bents. Of the ten adsorbents, one comprised an X zeolite, and nine comprised Y zeolites. More specifically the adsorbent comprising X zeolite was a portion of Linde 13X Molecular Sieves which had been essentially completely exchanged with Ca cations and the nine adsorbents comprising Y zeolite were nine portions of Linde SK-40 which had been essentially completely ion exchanged with hydrogen, ammonium, Na, K, Cs, Mg, Ca, Sr, and ~a cations. These ten adsorbents are hereinafter referred to as NI14-Y, ~I-Y, Na-Y, K-Y, Cs-Y, Mg-Y, Ca-Y, Ca-X, Sr-Y and Ba-Y zeolite adsorbents. All adsorbents had a particle size range of approximately 20-40 U.S. Mesh.
The adsorbents were tested in a 70 cc.coiled column maintained under the same conditions as in Example I, and using the same procedure as in Example I, with the exception that ~5:16/~4 after the pulse containing 10 wt.% glucose was run, a pulse of deuter.ium oxide was injected. Deuterium oxide has a different index of refraction than does water; thus deuterium oxide can be detected with the refractometer in the same way as is done for the sugars. For each adsorbent tested four peak traces were developed, one for glucose, one for fructose, one for deuterium oxide and one for benzene. ~etention volumes for glucose, fructose and water, and also for deuterium oxide were obtained by the method described in Example I. The results for these pulse tests are shown in Table No. 3.
The NH4-Y zeolite adsorbent used for Test 1 exhibited a good selectivity of 6.5 for fructose with respect to glucose and an acceptable -- although somewhat low -- selectivity of 0.71 for fructose with respect to water. Preferred selectivities for an extract component with respect to a desorbent material are from about 1.0 to about 1.5 so that an extract component can readily displace desorbent material from the adsorbent in the adsorption zone while still permitting an extract component to be removedwith reasonable amounts of desorbent material from adsorbent in the desorption zone. The H-Y zeolite used for Test 2 exhibited selectivity for neither fructose nor glucose in the presence of water; both eluted simultaneously. Both the Na-Y zeolite adsorbent used for Test 3 and the K-Y zeolite adsorbent used for Test 4 exhibited fructose selectivity with respect to glucose, although less than that obtained with the NH4-Y adsorbent, but the Cs-Y zeolite adsorbent used for Test 5 exhibited selectivity for neither. Fructose selectivities with respect to water for the Na-Y and the K-Y zeolite adsorbents were again less than 1Ø

~15:~64~

Both the Mg-Y zeolite used for Test 6 and the Ca-x adsorbent used for Test 8 exhibited no selectivity for ylucose or fructose since both eluted simultaneously. The Sr-Y æeolite and the Ba-Y zeolite used for Tests 9 and 10 respectively both exhibited acceptable selectivity for fructose, but they also exhibited the highest selectivity for fructose with res-pect to water of all the adsorbents tested, indicating that larger amounts of desorbent material (watex) would be required to desorb the extract component fructose. The best overall performance as measured by the pulse test was obtained with the Ca-Y zeolite adsorbent used for Test 7. This adsorbent has the best selectivity for fructose with respect to glucose, an ideal selectivity for fructose with respect to water, and peak widths which indicate reasonably fast transfer rates. For these reasons the Ca-Y zeolite adsorbent is the preferred adsorbent for the process of this invention.

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~ O n 6~4 EXAMPLE IV
_ ~his example illustrates the ability of the process of the invention to separate a ketose from an aldose when operat-ed in a preferred embodiment which utilizes a continuous, simulated-moving bed, countercurrent-flow system. Specifically the example presents test results obtained when a synthetic blend of 16.5 wt.% each of fructose and glucose in water was processed using a barium-exchanged X zeolite adsorbent of approximately 20-40 U.S. Mesh particle size range and water as a desorbent material in a pilot-plant-scale testing apparatus (described in U.S. Patent 3,706,816). ~riefly the apparatus consists essentially of 24 adsorber,t chambers each havinq ahout-18.8 cc. volume. The individual adsorbent chambers are serially connected to each other with small-diameter connecting piping, and to a rotary-type valve with separate piping. ~he valve has inlet and outlet ports which direct the flow of feed and desor-bent material t~ the chambers, and extract and raffinate streams from the chambers. By manipulating the rotary valve and main-taining given pressure differentials and flow rates through the various lines passing into and out of the series of chamber.s, a simulated countercurrent flow is produced. The adsorbent remains stationary while fluid flows throughout the serially connected chambers in a manner which when viewed from any position within the adsorbent chambers is steady countercurrent flow. The movir,g of the rotary valve is done in a periodic shifting manner to allow a new operation to take place in the adsorbent beds located between the active inlet and outlet ports of the rotary valve. Attached to the rotary valve are ~15~6~4 input lines and output lines through which fluids flow to and from the process. The rotary valve contains a feed input line through which passes a feed mixture containing an extract and a raffinate component, an extract stream outlet line through which passes desorbent material in admixture with an extract component, a desorbent material inlet line through which passes desorbent ma~erial and a raffinate stream outlet line through which passes a raffinate component in admixture with desorbent material. Additionally, a flush material inlet line is used for the purpose of flushing feed components from lines which had previously contained feed material and which will subse-quently contain a raffinate or extract output stream. The flush material employed is desorbent material which then leaves the apparatus as part of the extract and raffinate output streams. The raffinate and extract output streams were collected and analyzed for fructose and glucose concentrations by chroma-tographic analysis, but no attempt was made to remove desorbent material from them. Fructose yield was determined by calculating the amount of fructose "lost" to the raffinate stream, deter-mining this quantity as a percentage of the fructose fed to the unit over a known period of time and subtractin~ this percentage from 100 percent. The operating pressure for the tes~s was 10.2 atms., gauge, and the operating temperatures were 50C. and 75C. r respectively, for Tests 1 and 2. The fructose purity (as a percent of total sugars present) of the extract output stream, and the fructose yield, are shown below in Table 4.

llS16~4 EXTRACT STREAM
TEST FRUCTOSE PURITY, % FRUCTOSE YEILD, The results of Tests 1 and 2 above do not necessarily represent the optimums that might be achieved.
EXAMPLE V
-In this example the procedure of Example IV was essentially repeated but using a Ca-Y zeolite adsorbent to separate a ketose from an aldose. Specifically the example presents test results obtained when a water solution of corn syrup was processed using the Ca-Y zeolite adsorbent described in Example IIIand using deionized water as a desorbent material.
The feed was processed as a 50% sugar solution in water. The solids content of the feed was 52~ glucose, 42~
fructose and 6% higher saccharides. The operating temperature was 60C. The fructose purities (as a percent of total sugars present) of the extract output stream, and the fructose yields, are shown below in Table 5. (Pressure used was 10.5 atms. gauge.) TA~LE 5 EXTRACT STREAM
TEST FRUCTOSE PURITY, % FRUCTOSE YIELD, '~

6~

By way of illustration, analysis of the extract and the raffinate streams at one point on the fructose purity-yield curve, 85% fructose yield point, were as shown in Table 6 below.

5Extract and Raffinate Stream Analysis at the 85% Yield Point EXT~CT STREAM RAFFINATE STREAM
~ Fructose 88.3 10.7 % Glucose 11.7 79.5 % Higher Saccharides Trace 9.8 % Sugars 14.9 13.3 Again, the results of the tests above do not necessarily repreeent the optimums that might be achieved.

Claims (5)

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

1. A process for separating components of a feed mixture comprising a ketose and an aldose which pro-cess comprises contacting said mixture at adsorption con-ditions with an adsorbent comprising a crystalline alumino-silicate selected from (1) an X zeolite containing at exchangeable cationic sites a cation selected from the group consisting of sodium, potassium, barium and strontium, and (2) a Y zeolite containing at exchangeable cationic sites at least one cation selected from the group consisting of ammonium, sodium, potassium, calcium, stron-tium, barium and combinations thereof, thereby selectively adsorbing one of said components, and thereafter contacting the adsorbent containing the adsorbed component with a desorbent and recovering the resultant desorbed component characterized by the steps of:
(a) maintaining a net fluid flow in a single direction through a column of the adsorbent, which column contains at least three serially-interconnected zones, each having a separate operational function occurring there-in, with the terminal zones of said column being connected to provide a continuous cyclic flow through said zones;
(b) maintaining in said column an adsorption zone comprising the adsorbent located between a feed input stream as upstream boundary of said zone and a raffinate output stream as downstream boundary of said zone;
(c) maintaining a purification zone immediately upstream of said adsorption zone and located between an extract output stream as upstream boundary thereof and said feed input stream as downstream boundary thereof;
(d) maintaining a desorption zone immediately mab/

upstream of said purification zone and located between a desorbent input stream as upstream boundary thereof and said extract output stream as downstream boundary thereof;
(e) passing the feed mixture into said adsorp-tion zone and therein selective adsorbing the one component, and withdrawing from said adsorption zone a raffinate out-put stream comprising the other component;
(f) passing the desorbent into said desorption zone and displacing from the adsorbent therein the one component which had been selectively adsorbed thereon in a previous cycle;
(g) withdrawing from said desorption zone an extract output stream comprising desorbent and the selec-tively adsorbed one component;
(h) passing at least a portion of said extract output stream to a separation means and therein separating at least a portion of said desorbent material and producing a product stream comprising the selectively adsorbed com-ponent;
(i) periodically advancing through said column of adsorbent in a downstream direction with respect to fluid flow in said adsorption zone the feed input stream, raffinate output stream, desorbent input stream, and ex-tract output stream to effect the shifting of zones through said adsorbent and the production of extract output and raffinate output streams.

2. The process of Claim 1 further characterized in that it includes the step of passing at least a portion of the raffinate output stream to a separation means and therein separating at least a portion of the desorbent to produce a raffinate product having a reduced concentration of desorbent.

3. The process of Claim 1 further characterized in that it includes the step of maintaining a buffer zone mab/

immediately upstream from said desorption zone and located between the desorbent input stream as downstream boundary thereof and the raffinate output stream as upstream boundary thereof.

4. A process for continuously separating, in liquid phase, fructose from a liquid feed mixture of sugars containing essentially fructose and glucose, fructose being selectively sorbed by contact with solid sorbent particles of crystalline alumino-silicate or zeolite, utilizing a simulated counter-current flow system, wherein liquid streams are allowed to flow through three serially and circularly interconnected zones which are a desorption zone, a rectification zone and a sorption zone, each zone being divided into a plurality of serially interconnected sections, each section being packed with a mass of said solid sorbent particles, by introducing said liquid feed mixture into the first section of said sorption zone, introducing water as a desorbent into the first section of said desorption zone, withdrawing a portion of a desorption effluent comprising the sorbate and the desor-bent from the last section of said desorption zone for obtaining a product of fructose, and withdrawing a portion of a raffinate effluent comprising less sorbed surgars and the desorbent from a point such that at least one section of said sorption zone remains downstream therefrom, and all of the points of introducing and withdrawing said liquid streams into and from said sections are simultaneously shifted one section at a time at predetermined intervals of time, in a downstream direction while maintaining the same order of continuity and the same spatial relationship between said points.

5. A process for continuous separation of fruc-tose in accordance with Claim 4, wherein said liquid mab/

streams flowing in said three zones are interrupted at a point between said desorption zone and said rectifi-cation zone, while the first portion of said desorption effluent flowing out from said last section of said de-sorption zone is directly, or after being subjected to an evaporation, circulated as reflux, and the second por-tion thereof is withdrawn from the system.

mab/