The Mixed Bed Polishing Process
Color, hydroxymethylfurfural (HMF) and residual ash are removed with ion exchange resins from the fructose-rich product before or after blending to 55 HFCS.
The fractionation system product fraction contains 85-90% fructose and can also contain ppm quantities of impurities such as color bodies, weak acids, HMF (hydroxymethylfurfural), residual ash and protein. Impurity levels can vary because of the fractionation system operating conditions or performance of upstream refining unit operations.
To produce a heat-stable product of consistently high quality, the syrup can be polished by passage through a single bed of strong base anion resin or a mixed bed of strong acid cation/strong base anion resin. The syrup can be treated as a 90% fructose solution or, after blending, as a 55% fructose solution.
A salt form strong base anion resin will remove color bodies to produce a heat-stable product. These strong base anion polishers can be operated with a single bed in service while the second bed of the pair is in regeneration or on standby. Salt regeneration with occasional caustic cleanup is enough to remove color. For complete removal of weak acids, HMF, ash and protein in addition to color, a mixed bed of hydrogen form strong acid cation resin and hydroxide form strong base anion resin (Type II) is utilized.
In order to more closely match the capacity of the cation and anion resins, a 50% excess of strong base anion resin volume is required to approximately equalize the number of anion exchange groups with the number of cation exchange groups in the mixed bed column since the capacity per unit volume of the strong base anion resin may be 40-50% lower than the cation resin.
Thus, the mixed bed resin volume will typically be 60% anion and 40% cation.
Some degradation of the HFCS can occur in the mixed bed due to contact with the high pH strong base anion resin. Fructose will convert to psichose at elevated pH, so to limit the amount of fructose lost in the mixed beds, the velocity of the syrup is kept high enough to minimize contact time with the anion without affecting the kinetics of the mixed bed exchange.
In service, syrup passes down through one bed of homogeneously mixed cation and anion resin while the second column of the pair is in regeneration or on standby. The primary function of a polisher is color removal, and service is terminated based on color leakage. The conductivity of the mixed bed effluent at breakthrough will be on the order of 1-2 µS/cm. Upon exhaustion of a polisher, the service unit is sweetened off and regenerated while the standby unit is sweetened on and put into service.
To regenerate an exhausted mixed bed unit, syrup is displaced from the resin bed with water, the intermixed resins are separated from each other due to density differences with a backwash and each is then chemically regenerated. The cation resin is stripped of protein and ash with a dilute hydrochloric acid solution while the strong base anion resin is stripped of color bodies, weak acids, ash and HMF with a dilute caustic solution.
Mixed Bed Polishing Regeneration Sequence in Corn Sweetener Refining
Syrup is displaced from the bed of intermixed cation/anion resin by introduction of demineralized water into the feed distributor which pushes the syrup down through the bed and out to sweetwater collection. This step is terminated when the effluent sweetener concentration has decreased to 0.1-0.5% dry solids.
The water in the freeboard space between the feed distributor and backwash collector is displaced by pushing the water down through the bed with pressurized air until the liquid level has reached the feed distributor point. Evacuation of the freeboard space results in less back pressure and hence a more rapid rise of the bed of mixed resin during the initial minutes of the backwash step. This provides for a better separation of the more dense cation resin from the lighter anion resin.
Process water is passed upflow through the bed of mixed resins to achieve a 100% fluidized expansion of the resin bed and accomplish a separation of the cation from the anion resin in addition to removing resin fines and particulates from the bed. Owing to specific gravity differences, the anion resin will be fluidized to a greater height in the vessel and settle at a slower velocity than the heavier cation resin. Thus, the cation and anion resin separate into two discrete beds with the resin interface occurring at the same level in the vessel as the interface takeoff distributor.
The dilute caustic solution continues to enter the feed distributor located above the separated resin bed and passes down through the anion bed and out the interface collector while a simultaneous flow of deionized water enters the bottom distributor and passes up through the cation resin and out through the interface collector with the spent caustic solution. The 50% excess volume of strong base anion resin requires a regenerant chemical volume in excess of the cation regenerant volume. In order to ensure equal contact times per unit volume of resin, regeneration of the anion resin begins prior to the cation resin.
The dilute caustic solution continues to enter the feed distributor located above the separated resin bed and passes down through the strong base anion resin and out the interface collector while a simultaneous flow of dilute hydrochloric acid enters the bottom distributor and passes up through the cation bed and out the interface collector with the spent caustic solution.
Slow Rinse Cation/Anion:
Demineralized water streams simultaneously enter the feed and bottom distributors and pass down through the anion and up through the cation respectively and mix as they exit the column through the interface collector. The demineralized water displaces the regenerant chemicals from the resins at a rate that ensures that all of the resin receives an adequate regeneration contact time.
Fast Rinse Cation/Anion:
Demineralized water streams simultaneously enter the feed and bottom distributors and pass down through the anion bed and up through the cation bed respectively and out the interface collector to rinse out the residual chemicals from the bed. Due to the excess volume of strong base anion resin, and its higher rinse requirement per cubic foot, it is desirable to rinse the anion resin at a higher rate than the cation resin.
Pressurized air enters the top head of the vessel and pushes the water in the freeboard space down through the resin until the liquid level reaches the feed distributor point. Lowering the liquid level prevents water and resin from being carried out the vent nozzle during the subsequent resin mixing steps.
An air/water mixture enters the bottom distributor and flows up through the bed producing a churning action which mixes the resin. The air escapes through a vent while the added water raises the liquid level in the column slowly. The water provides a strong initial hydraulic force to slightly fluidize the resin, so the churning air bubbles will easily affect mixing of the cation and anion resins.
After the churning action is initiated with the air/water combination, the water flow rate is terminated, and the resins continue mixing through the action of the air bubbling up from the bottom distributor and escaping out through the vent.
In order to settle the well-mixed resins without incurring a separation due to differences in terminal settling velocity, air continues to be introduced into the bottom or interface distributor with the vent valve closed while water is withdrawn from either the bottom or interface distributor.
Demineralized water enters the top distributor to completely fill the mixed bed vessel with water.
Demineralized water enters the top distributor and passes down through the freeboard space and through the resin until the effluent conductivity decreases to less than 1 microsiemen/cm. The speed at which the conductivity declines, and the value attained serves as a check against the quality of the regeneration.
Syrup enters the feed distributor under a demineralized water dome and passes down through the intermixed resin bed and out the bottom collector until the effluent dry solids concentration equals 95% of the feed solids concentration.