(553f) Resin Separation and Transfer Studies In Mixed Bed Ion Exchange for Ultrapure Water Production

1) Introduction: Mixed bed ion exchange finds extensive use in ultrapure water production. The most important step in mixed-bed regeneration with regard to water purity is resin separation. Cross contamination of beads during resin separation results in of sodium and sulfur/chlorine ionic leakage into the treated water. The influence of backwash flowrate and bed expansion on the cross contamination of cation resin in the anion layer and anion resin in the cation layer is investigated experimentally. Cross contamination in both layers were estimated for different bed conditions 2) Experimental: The effectiveness of conventional backwash was evaluated. All experiments were performed in the water purification unit shown in fig. 1. The service column consists of cation and anion resin in the 2:1volume ratio. Cation and anion resins are separated by backwashing and gravity settling. An initial air mix homogenizes the bed prior to backwashing. DI water enters from the bottom of the service vessel thereby fluidizing the bed. The denser cation resin beads settle more quickly than the less dense anion resin beads resulting in the formation of upper anion layer and a lower cation layer. Five backwashing steps were performed in this evaluation. For each trial a backwash flow rate and volume expansion percentage was selected. After backwash, water was completely drained from the column so that samples could be collected through the ports immediately above and below the interface. Cross contamination was determined by analyzing each of these samples. 3) Results and Discussion: Data for eight sets of experiments are presented. In the first two experiments, the bed was backwashed to 100% volume expansion at flowrates of 13.5gpm (50% rotameter) and 6.5 gpm (25% rotameter) respectively. Higher flowrate resulted in more cross contamination due to the increased mixing of resin layers during backwash. Subsequent experiments were carried out for 150% and 50% volume expansions at flowrates of 13.5 gpm, 6.5 gpm and 4 gpm (15% rotameter). Variation of cross contamination with each backwash is shown graphically for all bed conditions. Three types of graphical comparisons are shown to visualize the collected data more easily. The three types of comparison shown are: high flow (13.5 gpm) with different backwash expansion; low flow (6.5 gpm and 4 gpm) with different backwash expansion; and, different bed expansion at constant flow. The effects of variation in flowrates and percentage expansions were then discussed based on these graphs. The flowrates used were also compared with the backwash expansion data given by Dowex for the both resins. Cross contamination levels of 0.1% was routinely obtained with low flowrate and maximum expansion. The parameters to achieve the best possible performance are high backwash at low flowrates.