Ion Exchange Resin for Chromatographic Separation

Chromatographic Separation Process in Corn Sweetener Refining

What is Chromatographic Separation? 
Different sugars passing through a bed of strong acid cation resin in the calcium or sodium form will separate from one another chromatographically due either to a difference in affinity for the resin or to different rates of diffusion into and out of the resin beads. This separation technique can be used to create solutions which have sugar profiles that provide the desired sweetness, taste or physical properties for a consumer product. 

A 55% fructose solution will match the sweetness of sucrose when used in soft drinks. This sweetener is produced by passing a 42% fructose solution through a calcium form strong acid cation resin to effect a separation and create a 75-90% fructose solution which can be blended back with additional 42% fructose to produce a 55% fructose purity.

Another class of sweeteners produced utilizing chromatographic separation is the sugar alcohols. Hydrogenation of sugars to produce sugar alcohols such as sorbitol, mannitol, maltitol, erythritol, xylitol or polyols requires high purity feedstocks in order to avoid unwanted byproduct sugar alcohols. As a common example, 95% dextrose is enriched to a 99.4%+ dextrose purity on a sodium or potassium form strong acid cation resin prior to hydrogenation to sorbitol.

Fructose Enrichment
In the production of 55 HFCS from dextrose, an economical limit of 42-46% fructose is achieved using isomerase enzyme. To obtain a higher purity fructose solution, the dextrose and fructose must be separated to produce two fractions, both of which are enriched in either sweetener.

This is accomplished via chromatographic separation on a fractionation ion exchange resin. The fructose-rich fraction can be blended into a 55% fructose solution while the dextrose-rich fraction is recycled in the HFCS refining process.

Owing to a greater number of sites available for hydrogen bonding, the fructose molecule will form a coordination complex with calcium ions fixed onto a strong acid cation resin. This results in a preferential affinity of the resin for the fructose molecule over the glucose molecule and hence a chromatographic separation of the two sweeteners as they pass through the resin bed. From a feed solution containing a purity of 42% fructose by weight, the fructose in the product fraction can reach in excess of 99% purity.

When producing 55% fructose, the optimum productivity and efficiency of the system is achieved by enriching to an 85-90% fructose concentration. Blending this product with a 42% fructose solution to produce the 55% fructose will match the sweetness of sucrose in soft drinks. When enriching to produce crystalline fructose, a product purity in excess of 95% fructose is desired prior to the crystallization step.

When enriching fructose to produce 55 HFCS with a simulated moving bed chromatographic separation system, a product purity of 90% fructose can be achieved at a 90% recovery of the fructose in the feed steam and a desorbent consumption of 1.1-1.25 lbs. water per lb. of 55 HFCS dry solids.

The production achieved will be approximately 200 lbs. of 55 HFCS dry solids/cu. ft. resin per day depending on the type of system and resin utilized. At constant production and desorbent consumption the purity and recovery will vary inversely with each other.

Chromatographic separation of dextrose is also commercially practiced to separate the dextrose from the oligosaccharides and produce a dextrose purity in excess of 99% in order to minimize unwanted byproducts in the subsequent hydrogenation to sorbitol or in fermentation. Dextrose separation from oligosaccharides occurs due to a difference in the rate of diffusion into and out of the monovalent form cation fractionation resin.

fructose enrichment material balance

55 HFCS Chromatographic Separation Material Balance

Zones in a Fructose Enrichment SMB Profile

Chromatographic Separation System
Fructose and dextrose enrichment had initially been accomplished in batch separation systems where the introduction of a pulsed volume of feed was both preceded and followed by recycle fractions and then eluted with desorbent water. In the 1970s, the technology was developed for continuous chromatographic separation of fructose utilizing a simulated moving bed (SMB) separation system.

The SMB technology employs continuous feed and desorbent introduction and continuous product and byproduct withdrawal into and out of a recirculating fluid flow. The recirculating fluid flow can range up to six times greater than the feed flow rate.

In the SMB system, fructose purity in the resin bed upstream from the feed introduction point increases as the distance away from the feed point increases. From a feed inlet reference point, it appears that the adsorbent media is moving upstream and carrying with it the fructose molecules which it has adsorbed, thus the terminology, "simulated moving bed.”

The SMB separation system is divided into four process zones with each zone having a unique flow rate. A fifth zone is added when the withdrawal of a polysaccharide rich stream is desired.

Once a stable concentration profile has been established across the entire length of the separation system it moves slowly down the system with the aid of the recirculation flow. The sweetener concentrations are maintained constant by moving the location of the feed, desorbent, product and byproduct inlet and outlet points down the system at the same rate as the concentration profile moves.

Movement of the introduction and withdrawal points is accomplished using either a multiport valve or with multiple manifolds of two-position valves.

Auxiliary systems complement the chromatographic separation system. Degasification of feed and desorbent streams prior to introduction to the resin bed ensures minimal oxidation of the resin will occur. Calcium salt addition to the separator feed is sometimes utilized to maintain a high calcium ion content on the resin, but this increases the ash load to the polishing mixed beds by a small amount.

corn time diagrams

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