Process for producing cation exchangers

Abstract

Strongly acidic cation exchangers with high mechanical, osmotic and oxidation stability can be prepared by sulfonating bead polymers formed from one or more vinylaromatic monomer(s), one or more crosslinker(s) and from 0.2 to 20% by weight of one or more vinyl ethers and / or vinyl esters.

Description

[0001]The invention relates to a process for producing strongly acidic cation exchangers with high mechanical, osmotic and oxidation stability by sulfonating bead polymers formed from one or more vinylaromatic monomer(s), one or more crosslinker(s) and one or more ether(s) and / or ester(s) of vinyl alcohol.BACKGROUND OF THE INVENTION[0002]Strongly acidic cation exchangers can be obtained by functionalizing crosslinked styrene bead polymers. The functionalization generates covalently bonded sulfonic acid groups through reaction of aromatic units of the polymer skeleton with a sulfonating agent, for example sulfuric acid.[0003]One problem with the known strongly acidic cation exchangers is that of their stability under stress, which is not always sufficient. For instance, cation exchanger beads can break up as a result of mechanical or osmotic forces. For all applications of cation exchangers, the exchangers present in bead form must maintain their habit and must not be degraded partly...

AI Technical Summary

Benefits of technology

[0014]Surprisingly, cation exchangers which are obtained by the process according to the invention have a significantly higher oxidation stability as compared with the prior art, coupled with equal or higher mechanical and osmotic stability. It has additionally been found that, surprisingly, the improvement in the oxidation stability of the strongly acidic cation exchangers obtained from the inventive bead polymers is attributable solely to the incorporation of the comonomer in the bead polymer, irrespective of in what manner and at what time the comonomer is added and polymerized in the course of formation of the bead polymer.

[0044]It is advantageous to establish the necessary acid concentration by mixing sulfuric acid of a relatively high concentration and of a relatively low concentration, in which case the sulfuric acid with a relatively low concentration used may be recovered sulfuric acid from earlier sulfonation reactions. The mixing of the sulfuric acid can be effected in the sulfonation reactor in the presence of the bead polymer to be sulfonated, such that the resulting heat of mixing leads to a temperature increase in the reaction mixture.

[0053]After the conversion, the cation exchangers can be purified further by treating with deionized water or aqueous salt solutions, for example with sodium chloride or sodium sulfate solutions. It has been found that the treatment at 70-150° C., preferably 120-135° C., is particularly effective and does not bring about any reduction in the capacity of the cation exchanger.

[0060]The strongly acidic cation exchangers obtained by the process according to the invention are notable for a particularly high mechanical stability, osmotic stability and oxidation stability. Even after prolonged use and multiple regeneration, they exhibit barely any defects in the ion exchanger spheres.

Problems solved by technology

One problem with the known strongly acidic cation exchangers is that of their stability under stress, which is not always sufficient.

Moreover, the presence of damaged bead polymers is unfavorable for the functioning of the cation exchangers themselves which are used in column processes.

Splinters lead to an elevated pressure drop of the column system and hence reduce the throughput of the liquid to be purified through the column.

A further problem of the known strongly acidic cation exchangers is their tendency to release sulfonated, water-soluble fragments from water in use as a result of the action of a wide variety of different oxidizing agents dissolved in water (atmospheric oxygen, hydrogen peroxide, vanadyl salts, chromates).

This phenomenon, known in general to those skilled in the art by the term “leaching” leads to the enrichment of sulfonated organic constituents in the water to be treated, which can lead to various problems in the downstream systems which are reliant on the supply of fully demineralized water.

For example, the water-soluble fragments lead, for example, to corrosion problems in the cooling circuit of power plants, to defects in the microchips produced in the electronics industry, to the failure of the system owing to excessively high electrical conductivity of the water in eroding machines.

There has been no lack of attempts to provide strongly acidic cation exchangers which have an improved mechanical stability, osmotic stability and / or an improved oxidation stability.

However, the presence of acrylic acid units in the polymer structure leads to a higher oxidation susceptibility of the resins.

Bachmann et al. describe in EP-A 868 444, a process for producing mechanically and osmotically stable cation exchangers by sulfonating without addition of chlorinated swelling agents at temperatures between 125 and 180° C. However, dispensing with the chlorinated swelling agent in the sulfonation does not improve the oxidation stability of the resins.

Furthermore, they are spent after a relatively short time in use.

However, the incorporation of major amounts of crosslinker makes the polymers more brittle, which leads to a significant reduction in the mechanical stability of the beads.

Moreover, the kinetics of the cation exchange decrease significantly with increasing crosslinking density, which leads to insufficient absorption capacities in many applications.

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