Self-sustaining process of softening – desalination in a simple two-component system. Experimental verification of adequacy of the mathematical model and the calculation of a stationary process under experimental conditions

Abstract

Self-sustaining softening – the desalination process (SSP) includes alternating stages of water softening in a column with a cation exchanger in the sodium form, deep desalination of softened water by distillation, and the reverse conversion of the cation exchanger to the sodium form during counter-current regeneration with brine formed during desalination. Almost no additional reagents were required for regeneration. Two columns with a cation exchanger operating in antiphase allowed organizing continuous softening – regeneration cycles. In this work, the SSP was studied in laboratory conditions in a simple system – a KU-2x8 cation exchanger in the sodium form and a solution of calcium and sodium chloride salts. The objective of the study was to verify the adequacy of the mathematical model of the SSP and calculate all its cycles in order to verify the possibility of achieving a stationary mode. Cycle “0” of the SSP was experimentally reproduced. The initial solution was passed through a column with a cation exchanger in the sodium form. The sorption stage was stopped when the concentration of calcium ion at the column outlet was 0.005 mol-eq/l. We obtained the output curve of calcium ion sorption. The content of components was determined in the entire volume of softened water. We set the concentration of sodium ion in the regeneration solution at 1 mol-eq/l, calculated the concentration coefficient during desalination, and prepared the solution for regeneration. Counter-current regeneration was performed using the entire volume of the resulting regeneration solution. We obtained the output curve of calcium ion regeneration. The concentrations of components in the filtrate were determined. The numerical experiment was conducted using the Createscheme program. The program simulates the complete technological scheme of the SSP and calculates its ion-exchange stages. The adequacy of the mathematical model was checked by comparing the experimental and calculated output curves of calcium ions in the course of sorption and regeneration. The calculated and experimental compositions of technological solutions after the ion-exchange stages of cycle «0» of the SSP were also compared. We experimentally confirmed the correctness of the mathematical model. The SSP was calculated before the beginning of the stationary mode, starting from the 10th cycle. It was calculated that only 2/3 of the exchange capacity of the cation exchanger was used in the stationary mode. The rest of the exchange capacity was blocked by calcium ions. The decreased efficiency of ion-exchange stages is a disadvantage of self-sustaining processes. It was shown that mathematical modelling and a numerical experiment were necessary when studying and organizing a self-sustaining process.