Ion Exchange Operations

The typical cycle of operations involving ion exchange resins include pretreatment of the resin and possibly of the feed solution; loading the resin with the solutes to be adsorbed by contacting the resin with the feed solution; and elution of the desired material from the resin. The scale of operation has ranged from analytical applications with a few milligrams of resin and microgram quantities of material to commercial production units containing several cubic meters of resin to produce metric tons of material. The loading may be applied batchwise, to a semi-continuous batch slurry, or to a column filled with resin which may be operated in a semi-continuous, continuous, or chromatographic manner.

A list of designers and manufacturers of ion exchange equipment is given in Table 18. The list is weighted toward those companies that operate in the United States.

Table 18. Ion Exchange Process Designers and Manufacturers

Company

Location

Belco Pollution Control Corp.

Parsippany, NJ

Chem Nuclear Systems

Columbia, SC

Cochrane Divison of Crane Corp.

King of Prussia, PA

Downey Welding & Manufacturing Co.

Downey, CA

Envirex Corp.

Waukesha, Wl

Epicor

Linden, NJ

Ermco Water Conditioners

St. Louis, MO

Hittman Nuclear & Development

Columbia, MD

Hungerford & Terry, Inc.

Clayton, NJ

Industrial Filter and Pump

Cicero, IL

Infilco Degremont, Inc.

Richmond, VA

Intensa

Mexico City, Mexico

Kinetico, Inc.

Newbury, OH

Kurita

Japan

Liquitech, Div. of Thermotics, Inc.

Houston, TX

Mannesmann

Germany

Mitco Water Labs, Inc.

Winter Haven, FL

Permutit, Division of Zurn

Warren, NJ

Rock Valley Water Conditioning

Rockford, IL

Techni-Chem, Inc.

Belvidere, IL

Unitech, Divsion of Ecodyne

Union, NJ

United States Filter, Fluid System Corp.

Whittier, CA

U. S. Filter/IWT

Rockford, IL

Wynhausen Water Softener Company

Los Angeles, CA

6.1 Pretreatment

Filtration, oil flotation and chemical clarification are the pretreatment operations commonly employed with the feed stream. This pretreatment is primarily concerned with the removal of excess amounts of suspended solids, oils, greases and oxidative compounds. Suspended solids, including bacteria and yeasts, in amounts exceeding approximately 50 mg/L should be removed prior to applying the fluid to the column to prevent excessive pressure buildup and short operating cycle times. The presence of oils and greases in excess amounts would coat the resin particles, thereby dramatically reducing their effectiveness. Oils and greases in concentrations above 10 mg/L should not be applied to resins in either column or batch operations. Synthetic resins are subject to de-cross-linking if oxidative materials are present in the feed solution or eluant.

For many biochemical recovery applications, it is necessary to pretreat the resin to ensure that the extractable level of the resin complies with Food Additive Regulation 21 CFR173.25 of the Federal Food, Drug and Cosmetic Act. The pretreatment recommended for a column of resin in the backwashed, settled and drained condition is:

1. Add three bed volumes of 4% NaOH at a rate sufficient to allow 45 minutes of contact time.

2. Rinse with five bed volumes of potable water at the same flow rate.

3. Add three bed volumes of 10% H2S 04 or 5 % HC1 at a flow rate that allows 45 minutes of contact time.

4. Rinse with five bed volumes of potable water.

5. Convert the resin to the desired ionic form by applying the regenerant that will be used in subsequent cycles.

If the column equipment has not been designed to handle acid solutions, a 0.5% CaCl2 solution or tap water may be used in place of H2S04 or HC1 for cation resins. Similarly, for anion resins, a 10% NaCl solution could be used in place of acids.

6.2 Batch Operations

The batch contactor is essentially a single stage stirred reactor with a strainer or filter separating the resin from the reaction mass once the reaction is complete. This type of contactor has an advantage in some fermentation operations because of its ability to handle slurries. Additional advantages include the low capital cost and simplicity of operation.

In a batch operation, an ion exchange resin in the desired ionic form is placed into a stirred reaction vessel containing the solution to be treated. The mixture is stirred until equilibrium is reached (about 0.5 to 3 hours). Then the resin is separated from the liquid phase by rinsing with the eluting solution. An additional step may be required to reconvert the resin to the regenerated form if this is not done by the eluting solvent. The cycle may then be repeated.

The batch system is basically inefficient since the establishment of a single equilibrium will give incomplete removal of the solute in the feed solution. When the affinity of the resin for this solute is very high, it is possible that the removal is sufficiently complete in one stage. The batch process has the advantage over fixed bed processes that solutions containing suspended solids may be treated. In these cases, the resin particles, loaded with the adsórbate and rinsed from the suspended solids, may be placed in a column for recovery of the adsórbate and for regeneration.

The batch contactor is limited to use with reactions that go to completion in a single stage or in relatively few stages. Difficulties may also arise with batch contactors if resin regeneration requires a greater number of equilibrium stages than the service portion of the cycle.

6.3 Column Operations

Fixed Bed Columns. Column contactors allow multiple equilibrium stages to be obtained in a single unit. This contactor provides for reactions to be driven to the desired level of completion in a single pass by adjusting the resin bed depth and the flow conditions. The main components of a column contactor are shown in Fig. 28. At the end of its useful work cycle, the resin is backwashed, regenerated and rinsed for subsequent repetition of the work cycle. Typically, this nonproductive portion of the cycle is a small fraction of the total operating cycle.

Column contactors may be operated in cocurrent, countercurrent or fluidized bed modes of operation. The cocurrent mode means that the regenerant solution flows through the column in the same direction as the feed solution. The countercurrent mode has the regenerant flowing in the opposite direction as the feed solution.

Countercurrent operation of a column may be preferred to reduce the ion leakage from a column. Ion leakage is defined as the amount of ion being removed from solution which appears in the column effluent during the course of the subsequent exhaustion phase. The leakage caused by re-exchange of non-regenerated ions during the working phase of cocurrently regenerated resin is substantially reduced with countercurrent regeneration.

Figure 28. Ion exchange column contactor.

The fixed bed column is essentially a simple pressure vessel. Each vessel requires a complexity of ancillary equipment. Each column in a cascade will require several automatic control valves and associated equipment involving process computer controls to sequence the proper flow of different influent streams to the resin bed.

Combinations of column reactors may sometimes be necessary to carry out subsequent exchange processes, such as in the case of demineralization (Fig. 29). As this figure shows, a column of cation exchange resin in the hydrogen form is followed by a column of anion exchange resin in the hydroxide form. A mixed bed, such as shown in Fig. 30, may also be used for demineralization. Mixed bed operation has the advantage of producing a significantly higher quality effluent than the cocurrently regenerated beds of Fig. 29, but has the added difficulty of requiring separation of the two types of resin prior to regeneration.

Inlet Distributor and n Backwash Outlet

Effluent Distributor

Raganarant Distributor

Effluent Distributor

Mix Bed Ion Exchanger
Figure 29. Demineralization ion exchange column scheme.

Anion Anion

Ion Exchange And Backwash

Service Backwash Simultaneous Mixing Cycle Regeneration

Figure 30. Operation of a mixed bed demineralization ion exchange column.

An important requirement for the successful operation of a mixed bed is the careful separation of the strong base anion resin from the strong acid cation resin by backwash fluidization. This is followed by contact of each type of resin with its respective regenerant in a manner which minimizes the cross contamination of the resins with the alternate régénérant. This requires that the quantity of resin, particularly the cation resin, be precisely maintained so that the anion-cation interface will always be at the effluent distributor level. Typically, matched pairs of resins are used so that an ideal separation can be repeatedly achieved during this process. Inert resins are marketed which enhance the distance between the anion-cation interface and allow less cross contamination during regeneration.[86J

The air mixing of the anion and cation must also be performed in such a manner that complete mixing of the resin and minimum air entrapment are obtained at the end of the regeneration cycle.

Ion exchange is usually in a fixed bed process. However, a fixed bed process has the disadvantage that it is cyclic in operation, that at any one instant only a relatively small part of the resin in the bed is doing useful work and that it cannot process fluids with suspended solids. Continuous ion exchange processes and fluid bed systems have been designed to overcome these shortcomings.

Continuous Column Operations. When the ionic load of the feed solution is such that the regeneration/elution portion of the operating cycle is nearly as great or greater than the working portion, continuous contactors are recommended instead of column contactors.

Continuous contactors operate as intermittently moving packed beds, as illustrated by the Higgins contactor'871 (Fig. 31), or as fluidized staged (compartmented) columns, as shown in Fig. 32, by the Himsley contactor. I881'891'901

In the Higgins contactor, the resin is moved hydraulically up through the contacting zone. The movement of resin is intermittent and opposite the direction of solution flow except for the brief period of resin advancement when both flows are cocurrent. This type of operation results in a close approach to steady-state operations within the contactor.

Elegant slide valves are used to separate the adsorption, regeneration and resin backwash stages. The contactor operates in predetermined cycles and is an ideal process for feedstreams with no suspended solids.

The Higgins type of contactor is able to handle a certain amount of slurry due to the continued introduction of fresh resin material to act as a filter media during the operation. A lower resin inventory should result with continuous contactors than with column reactors handling the same ionic load feedstream.

Higgins Contactor Adsorber

Figure 31. The Higgins contactor for continuous operation.[87i

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