T E C H N O L O G Y

Ion Exchange Dries Organic Liquids Ion exchange resins often have higher capacities and lower regenerating temperatures than do conventional desiccants

140TH ACS NATIONAL M E E T I N G

Industrial and Engineering Chemistry

Ion exchange resins are on the verge of what could be a new major use— desiccants for drying nonpolar organic liquids. Dow is actively promoting them for this application, has tried them out in several pilot plants and is now engineering them for use in pro­duction plants.

With dryness of solvents and re-actants in processes such as polymeri­zation growing in importance, ion ex­change resins can offer a number of advantages over conventional solid desiccants. With many organic liquids they have higher capacities. They also have a lower regeneration tempera-ture-115° to 140° C , compared to 205° to 315° C , the recommended temperature for synthetic zeolite ma­terials. And pilot plant data show that the economics compare favorably with conventional desiccants, accord­ing to Dow.

The resins used are sulfonated cat­ion types, and they show up best in drying nonpolar organic liquids, often drying them to less than 1 p.p.m. water, Dow's Dr. C. E. Wymore ex­plains. Their mode of dehydration is very similar to that of anhydrous salts. The resin matrix serves to insolubilize the drying agent (the charged ions) and keep it in place so that regenera­tion is possible.

Of the various alkali metal forms of resin that can be used, the lithium form sorbs more water at all relative humidities than does the sodium form; and the sodium form, in turn, sorbs more water than does the potassium form. Dr. Wymore explains that this is probably due to lithium's smaller size. It thus coordinates more tightly with the water than do sodium or potassium.

Rates of water sorption, on the other hand, are just the reverse, potassium having the fastest kinetics. This is,

again, likely due to size. Since the potassium ion is larger, it doesn't hold the water so tightly. The water can thus pass more easily from one ion to another on the inside of the resin matrix.

The hydrogen form is an anomaly. It has the highest capacity, as ex­pected. But it also appears to have unexpectedly fast kinetics. This, Dr. Wymore points out, is probably a re­sult of its very small size and very high mobility, or perhaps of a different mechanism.

Potassium Best. All in all, Dr. Wymore says, the potassium form is generally the best form to use. The sodium form, while sometimes best and thus usable in these cases, is gen­erally more sensitive to flow rate. The hydrogen form degrades at lower tem­peratures than do the metal ion forms and requires more precise temperature control during regeneration. It also tends to swell in polar materials. So, while it has a very high capacity in some cases and can be used, it isn't generally recommended.

As for cross-linking, a resin with 4 to Sc/c divinylbenzene usually has the highest capacity. Dr. Wymore favors the 8% resin because it swells less as it picks up water and because it's a standard water conditioning resin.

Liquids that have been successfully dried include those with almost any organic functional group—ester, acid, phenyl, ketone, and the like. Some specific examples are benzene, Dow-therm A, carbon tetrachloride, tri-ethylene glycol, ethanol, diisobutyl-ketone, acetic acid, and air. Exact conditions and contact time needed depend on the nature of the specific material being dried.

Ion exchange resins offer still other advantages in addition to higher ca­pacity and lower regeneration tem­perature. The metal salt forms don't introduce impurities into the liquid being dried. Nor do they react with most organic liquids or polymerize monomers. And the alkali metal salt

forms generally remove only water from most liquids—important, Dr. Wymore points out, when inhibitor-containing solvents are being dried.

Analyzed Effluent. In making lab­oratory tests to determine the per­formance of ion exchange resins as desiccants, Dr. Wymore periodically analyzed effluent from a resin bed. A Karl Fischer titration was used for the relatively polar materials. Hydro­carbons and chlorinated hydrocarbons, the relatively nonpolar materials, were stripped of water with dry nitro­gen. An electrolytic hydrometer was then used to analyze for the water content.

The Karl Fischer method, Dr. Wy­more feels, could detect water down to 5 p.p.m., while the stripping proce­dure made it possible to detect as low as 1 p.p.m. The resins used in the study had been dried at 110° C , and were then redried overnight at 110° C. in a vacuum oven just prior to use.

In one of the systems studied, methyl chloroform containing about 200 p.p.m. water was dried in a 30-in. resin bed at a flow rate of 5 gal. per min. per sq. ft. Both sodium and po­tassium forms of resin dried methyl chloroform to less than 1 p.p.m. water, and at the point of breakthrough both had sorbed about 20 lb. of water per 100 lb. of dry resin.

Dr. Wymore also tried mixtures of resins—one with good capacity, such as lithium, and one with good kinetics, such as potassium. However, results fell between the two, and perform­ance tended to be like that of the poorer resin.

In regenerating the resins, purge gas can be heated to provide regenera­tion energy requirements, or it can be used just to help sweep the water va­por out of the column. The amount of energy required, Dr. Wymore ex­plains, varies with the amount of water on the resin and the degree of regen­eration wanted. But a good average is about 1800 B.t.u. per lb. of water sorbed on the resin.

74 C&EN SEPT. 11, 1961