CA1195171A - Sweetner solution purification process - Google Patents

Abstract

TITLE OF THE INVENTION
SWEETENER SOLUTION PURIFICATION PROCESS

ABSTRACT OF THE DISCLOSURE
An improved process for purifying sweetener solutions is described. This process features passing crude sweetener solution through a series of adsorption materials, preferably arranged in the following order; cation exchange resin, microporous adsorbent, anion exchange resin.

Description

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SWEETENER SOLUTION PURIFICATION PROCESS

BACKGROUND OF THE INVENTION
Ion-exc~ange resins may be poisoned by t~e adsorption of substances tbat are either very difficult to remove or are very easily removed from th~ polymeric structure of the resin. A guard chamber containing activated carbon has been employed for the protection of an anion exchange resin column used in the purification of uranium values. This system operates in a pH range of 5-lû. See, U.S.
Patent No. 4,296,075.

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Cation and anion exc~ange resin pairs (or systerns) are used extensively in the purification of sweetener solutions, for exarnple, corn syrups, cane syrups, beet syrups and in other s~/eetener applications. These ion~exchanye resin systems are especially useful in purif`ying dextrose syrups for the production of high fructose corn syrups (HFCS).
These resins are employed for the removal of ash, specifically conductometric ash as measured by the beverage industry. A key cause of unwanted syrup conductance is the presence of salts resulting from neutralization reactions during the processing of the syrup. While the cation and anion exchange resins are effective in removing these salts, other components present in crude sweetener solution also contribute to this undesirable syrup conductivity. These other components are generally weak organic acids, generated by the breakdown of starches or proteins. These acids include citric, glutamic, lactic, tartaric and others.
Generally, the specific acids are not identified.
Rather, the mixture of acids in the syrup are termed "titratable acidity". These weak organic acids are removed by the anion exchange resin as negatively charged organic anions. However, due to the low charge density of some of these acids, they are easily displaced from t~ne anion exchange resin. T~is results in an early rollover of the organic acids from the anion exchange resin causing an unacceptable rise in syrup conductance. This rollover or bleed of organic acids into the syrup triggers a need for regeneration of the anion exchange resin before it has been fully utilized. Others of these acids will not be readily displaced and will occupy exchange

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sites which are then unavailable to the inorganic ash constituents -thereby also shorteniny the useflJl resin cycle. Such exchanye resin system downtirne is wasteful bot~ from a processing (tirne) standpoint and an economic standpoint.
It ~as been discovered, that the ion-exchanye system downtime can be reduced, and ~ence, overall process efficiency increased, if the weak undissociated acids in the sweetener syrup are removed before -they enter the anion exchange resin.
These acids are adsorbed on a properly posi-tioned microporus adsorbent within the ion exchanye resin system.

SUMMARY OF T~IE INVENTION
Under low pH conditions present in a sweetener purification process, the weak organic acids contributiny to t~e titratable acidity of the syrup can be removed from the system by a microporous adsorbent. From this discovery, it has been theorized that numerous other applications are possible for t~e use of microporous adsorbents such as activated carbon for the protection of ion exchanye resins. These other applications are described in greater detail herein below.
Thus t~ere is provided an improved sweetener solution purification process which comprises: (a) passiny crude sweetener solution t~rough a bed containing a cation exchange resin, thereby loweriny the syrup pH; (b) passing the effluent of said cation exc~anye resin t~roug~ a bed containing a microporous adsorbent, thereby adsorbing weak oryanic acids contributiny to titratable

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acidity; and (c) passing the effluent -frorn said microporous adsorbent t~roug~ a becl containing ar-anion exchange resin.

BRIEF DESCRIPTIûN OF THE DRAWINGS
FIG. I describes a converltional process scheme For generating ~lg~ fructose corn syrup (HFCS). Sections A-D represent t~e dextrose side and Sections E-I represent t~ fructose side.
FIG. II details one embodiment of t~e present inventi.on, namely t~e placement of a microporous adsorbent (MPA) between t~e cation-anion exchange resin pair used to purify sweetener solutions. The solid lines represent conventional solution flow directions, t~e broken lines show alternative flow directions.
FIG. III is a conductance breakt~roug~ curve s~owing improved ion exc~ange resin system performance for carbon treated dextrose syrup (curve 8) over untreated dextrose syrup (curve A).
FIG. IV is a conductance breakt~roug~ curve s~owing improved ion exc~ange resin system performance for regenerated carbon treated dextrose syrup (curve C) over virgin carbon treated syrup (curve B) over untreated syrup (curve A).
FIG. V is a conductance breakt~roug~ curve s~owing improved ion exc~ange resin sys-tem performance for carbon treated fructose syrup (curve E) over untreated fructose syrup (curve D).

DETAILED DESCRIPTIûN ûF THE INVENTIûN
-T~e present invention involves t~e use of a microporous adsorbent in a sweetener solution refining process to protect t~e anion resin o-f t~e

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cation-anion exchange resin system. The terms "sweetener solution" are intended to inc1ur~e t~ose crude sugar solutions recognized in the art as common "sweeteners", for example, corn SyIup, cane syrup, beet syrup, ~ig~l fructose syrup, h19h dextrose syrup, sorbitol and the like. The terms "rr;lcroporous adsorbent" are intended to include those comrnon adsorbents having microporous intersticies (pore volume) of more t~an 0.1 cc/gram in pores of lOOA or less, including activated carbon, alumina, silica, Ambersorb type macrorecticular resins, zeolites, microporous clays and the like. The preferred microporous adsorbent is activated carbon. Activated carbon may be employed in any convenient form, granular, powder, or a mixture of both.
It has been discovered t~at the acids w~ich contribute to titratable acidity in crude sweetener solutions are effectively adsorbed by microporous adsorbents such as activated carbon w~en in the undissociated form. To obtain a hig~ degree of undissociated acids, a pH of from about 2 to 2.5 is necessary. Two locations in a sweetener solution purification process where pH is this low are (1) wit~ln the cation exchange resin bed and (2) directly following t~e cation exc~ange resin bed, including for a short time within the anion exchange resin bed. Ot~er sweetener solution impurities, some of which are generated in t~e cation resin bed, such as proteins, hydroxy methyl furfuryl, fragments of the cation resin and unidentified organics not contributing to titratable acidity, are rendered less soluble at low prl and will also be adsorbed by a properly positioned microporous bed.

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A conventional high ~ructose co:rn syrup purification process is described in FIG. I. The process is generally divided into two areas; dextrose purification and fruc-tose puri,fication. '~Ihi],e this general description is directed to a colurmn operation, it is anticipated -that a batch processing operation will derive a similar benefit frorn -the present invention.
Referring to Figure I in detail, raw materials are ground into a pulp, are washed and starches are separated (Section A). This is followed by conversion of the crude materials either by chemical conversion (acid~ or by enzymatic conversion of the starch slurry to a high dextrose equivalent syrup (Section B).
Th~ dextrose syrup is passed through a column of activated carbon (Section C) for decolorization and removal of some impurities. The syrup pH at this poin-t is about 3.5-5.0 and it has been found that many weak organic acids contributing to titratable acidity are not effectively adsorbed by this activated carbon.
The carbon treated syrup passes next to tbe cation-anion exchange system (Sec-tion D) wherein first a cation and second an anion exchange resin remove salts and ot~er impurities. The syrup exiting the cation exchange resin has a pH below 3.0 (preferably 2.5 or less). The syrup exiting the anion exchange resin has a pH of frorn 3.5 -to 5Ø
T~e use of a microporous adsorbent in this ion exchange system would delay the organic acid rollover from the anion exchange resin and thus improve the overall process efficiency.

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The purified dextrose syrup is tben converted into big~ fructose corn syrup (IIFCS) by enzymatic conversion (Section E).
Activated carbon is again ernployed to decolorize t~e syrup and adsorb impurities (Section F). T~e syrup pl-l is between 4.0 and 5.0 enterirlg anrJ
exiting t~is carbon bed and it has been found that any remaining weak organic acids are not adsorbed.
T~e fructose syrup is t~en passed t~rough an ion-exchange system (Section G~ to remove salts and ot~er impurities remaining in the syrup. Tbe use of a microporous adsorbent in t~is ion-exc~ange system will agaln delay t~e rollover from t~e anion exchange resin and t~us improve t~e overall process efficiency.
The purified syrup is t~en eit~er concentrated by evaporation (Section H) or again passed t~rougb a cation excbange resin to upgrade the syrup from 42% HFCS to 55% HFCS (Section I).
The instant invention, wbile described in t~e examples tbat follow by a preferred embodiment, is not intended to be limited to t~e use of activated carbon. For example, using adsorbents otber t~an activated carbon at the low pH ion-exc~ange sites may also prevent t~e premature need for regenerating t~e anion excbange resin. Examples of other microporous adsorbents capable of adsorbing -tbe acids responsible for titratable acidity and ot~er solution impurities include: Ambersorb Macrorecticular Resins (Ro~m &
Haas Co.), Fuller's Eart~ and other microporous clays, zeolite sieves, silica gel, alumina~ and tbe like.

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a5~7~ -A two inc~ diameter bed containing 12 9 of' acid washed PCB activated carbon (available From Calgon Corporation, Pi-ttsburgh, Pa.) was inserted between a cation and anion exchange resin pair (see FIG. II).
Untreated dextrose syrup was run t~rough the carbon containing system and t~le conductance breakthroug~ of t~e system was measured. Conductance of the solution was measured in lOO ml increments on a Cole-Parmer Model 1481-OO conductivity meter. The unit of conductance generally used in the sweetener industry are microm~os. Thus the condu'ctance breakt~rough plots s~ow when (in terms of volurrle treated) an ion exc~ange resin system has ceased adsorbing ionic species contributing to solution conductivity. This conductance breakt~roug~ was plotted against t~e conductance breakthroug~ of the same ion-exc~ange resin system without the carbon bed inserted. FIG. III s~ows t~ese two curves. Curve A
represents the untreated (no carbon) syrup w~ile curve B represents the treated syrup. Assuming an arbitrary breakt~roug~ point of lOO micromhos, an extension of anion resin lifetime of about 30 percent was shown as measured by t~e increased volume of syrup treated by the carbon containing system.

_~MPLE 2 The carbon bed employed in Example 1 was regenerated using conditions conventionally used for regeneration of the anion exchange resin, i.e., caustic (NaOH) was~ (about 2 wt%) followed by water rinse (to a pH of about 9.O). T~e conduct3nce ~ `
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breakt~rough test was repeated, as in Exarnple I, using the regenerated PCB activated carbon and fresh supplies of cation and anion excharlge resins. T~e conductance breakthrough curves shown in FIG. IV s~ow an extension of anion resin of about ~19% lb;s lmprovement over t~e virgin carbon is probably due to better wetting of t~e carbon bed following t~e caustic regeneration. Curve A represents an untreated (no carbon) system, curve B represents a virgin carbon system and curve C represents a regenerated carbon breakthroug~ cycle.
In addition to t~e two weigbt percent caustic was~ employed in this Example, ot~er caustic solutions such as NH40H, KOH, Na2C03, NaHCû3, Ca(OH)2, Mg(OH)2 and the like, may be employed so long as a high pH is attained. In addition to the water rinse described above, an acid rinse (neutralization) could be employed or a series of acid-water rinses or water-acid rinses could be employed.

Following the successful extensions of anion exchange resin lifetime on t~e dextrose side of tbe HFCS purification system, a similar experiment was run on the fructose side. T~e same conditions as used in the preceeding examples were used. FIG. V
shows t~e breakthroug~ curves D, for the untreated anion effluent and E, for the carbon treated effluent. T~a extension factor ~ere is smaller t~an t~at seen on t~e dextrose side of the process because most of t~e organic acids likely to in-terfere with the fructose side anion exc~ange resin bave already been removed earlier in t~e processing.

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Tbe examples dernonstrate tbe userulness of -the present inventlon. Uslng tbe flgures showrl on t~e conductance breakt~rough curves as guides, expected extension of anion excbange resin lifetime for tbe dextrose side of a HFCS proc:ess woulcl be on t~e order of about 25 to 50% or ~igher.
As described above, adsorption of t~le weak organic acids contributing to titratable acidity by microporous adsorbents will occur best at low pH.
Low pH conditions exist wit~in t~e cation excbange resin bed. Tberefore a mixture of t~e cation exchange resin and ac-tivated carbon or another microporous adsorbent would also be expected to delay prema-ture exhaustion of the anion exchange resin as does carbon treat~ent of t~e cation exc~ange resin effluent. Finally, a low pH condition also exists initially ln t~e anion exchange bed. After some time passes, ~owever, the anion excbange process raises the pH of the syrup within tbis anion bed. However, one could mix a microporous adsorbent wit~ tbe anion exc~ange resin for additional protection of t~e anion bed. T~ls placement would be tbe least desirable of t~ose described, but, t~e concept would still apply.
Mixtures of ion exc~ange resin (cation or anion) witb the microporous adsorbent may vary from 1 percent by volume microporous adsorbent to 99 percent by volume microporous adsorbent. T~e quality of t~e adsorben-t, - tbat is, its adsorptive capacity will be a determinatlve factor. For activated carbons a preferred amount in a mixture wit~ an ion exchange resin would be from 10 percent to 50 percent by volume carbon. T~e addition of acids (e.g., HCl, H2S04, etc.) to t~e syrup to acbieve a low pH

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condition is not generally a desirable alternative because the acid counter--ions would necessarily have to be rernoved by the anion exchange resin. This would reduce or eliminate the at-tempted ex-tension of that resin's useful lifetime. HoweverJ in -those cases where the impurities to be removed are of the highly pH ,ensitive type, the addition of an aqueous mineral acid (e.g. HCl, H25û4, H3P0~, HN03 and the like) mlght be advanteagous. Such a case would be the irnproved purification of the sweetener solution in which the pH shift through a cation resin would be insuf~icient to achieve significantly enhanced adsorption of pH sensitive species, or when practical engineering considerations preclude the use of a cation bed to lower the pH. This improved sweetener purification process ernploying the addition of acid is illustrated in Table I.

Table I

25~ nm U.V. impurity Removal Efficiency from Dextrose Syrup Column Brea~t~rough at 0.25 lbs. of carbon/100 lbs of sugar.

pH Percentage of Impurity removed 4.6 26%
*3.0 39~0 *2.5 51%
*2.0 60%

-~Ajusted with (12.0 molar) HCl.

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A microporous adsorben-t placed followlny a cation exchange resin could also precede a In.ixed cation/anion resln bed and serve as a useful adsorber of undesirable contaMinants. For example, sweetener solution refineries may errlploy rnixed cation/anion resin beds in tbeir refining process. T~le use of a cation resin bed, fol].owed by, for exarnple, an activated carbon bed, would improve t~e sweetener solution purification by removing organics not adsorbed earlier in t~e processing, tbus extending the lifetime of t~e mixed cation/anion resin system.
8ased on t~e improved anion resin performance for sweetener applications, it is anticipated t~at under ot~er conditions, a properly positioned microporous adsorbent could be used to enhance t~e performance oF a cation exc~ange resin.
For example, ion exchange resins used in t~e treatment of condensate water systems (boiler corrosion prevention) rely on a cation resin to remove weak organic bases from aqueous solutions and retain them as -tbe positively charged species.
Examples of typical weak organic bases include aniline, purine, pyridine, toluidine, quinollne and ot~er common organic amines wi-t~ PKa of greater than 7. T~ese weak organic bases become less water soluble at ~ig~ pH. T~us, at a systern pH of at least lû, these bases could easily be removed by a microporous adsorbent, tbus reserving the cation exchange resin for t~ose materials not adsorbed, t~ereby extending its useful lifetime. To accomplis~
this extension, t~e solution being purified would initially enter an anion exc~ange resin, exiti.ng at a ~igb pH and t~en pass to t~e cation exc~ange resin.
As described above, the microporous adsorbent could be (l) rnixed witb eitber of t~e ion-excbange beds or (2) placed between the beds as a separate adsorber.
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Claims (7)

1. A fructose syrup purification process consisting essentially of:

(a) passing impure fructose syrup through a bed containing a cation exchange resin to lower the pH of said syrup to a range of from 2.0 to 2.5, (b) passing the effluent of said cation exchange resin through a bed containing activated carbon, thereby adsorbing weak undissociated organic acids and impurities not adsorbed by the cation exchange resin, and (c) passing the effluent from said activated carbon bed through a bed containing an anion or a mixed cation/anion exchange resin.

2. The process of Claim 1 wherein said activated carbon is granular.

3. The process of Claim 1 wherein said activated carbon is powdered.

4. The process of Claim 1 wherein said activated carbon is acid washed.

5. The process of Claim 1 wherein said activated carbon has been regenerated using a caustic wash.

6. The process of Claim 1 wherein the separate beds of cation exchange resin and activated carbon are eliminated and replaced by a single bed comprising a mixture of from 10 to 50 percent by volume activated carbon and 90 to 50 percent by volume cation exchange resin.

7. The process of Claim 1 wherein the separate beds of anion exchange resin and activated carbon are eliminated and replaced by a single bed comprising a mixture of from 10 to 50 percent by volume activated carbon and 90 to 50 percent by volume anion exchange resin.