Comparison of diester waste treatment by conventional and bipolar electrodialysis

Diester manufacturing plants produce large quantities of glycerol solutions. Their economic development requires sodium sulfate elimination to obtain glycerol rates over 80% of the initial content. Conventional electrodialysis (ED) and bipolar electrodialysis (BED) were investigated for their performances in the demineralisation of effluents. Results indicate that both ED and BED achieve the demineralisation objective with a global loss below 2.5% of the initial glycerol. Salt fluxes are twice as high with a bipolar membrane, bipolar membranes having however a lower efficiency. Hence energetic consumption is higher for BED. This technique also leads to the production of alkali and acid solutions which are useful to the diester process. The glycerol content of the solutions acts, via its viscosity, on the energetic cost of fluid pumping and on energetic efficiency. Therefore, ED or BED has to be used before the preconcentration of the solution.

Trials showed ED, with either conventional or bipolar membranes, was an efficient process for the demineralisation of diester waste. In both cases ED achieved a minimum of an 80% demineralisation rate with a global loss of about 2.5% of the initial glycerol. Salts fluxes were twice as high with bipolar membranes than with conventional membranes, but with lower efficiency — 48% and 70%, respectively. Hence, energy consumption was 0.65 kWh/kg of glycerol solution for ED compared to 1.3 kWh/kg with BED. During BED, the selectivity loss of the anionic membrane led to the production of acidified glycerol, with a maximum acidity of 0.15 mol/L. When the demineralisation rate was increased, the electric current transport was realised by the protons, which induced a deacidification up to a concentration of 0.025 mol/L at 95% demineralisation. BED is therefore the best technique for diester waste demineralisation before the concentration stage. It should be carried out before concentration, i.e., when glycerol concentration is low. The higher the glycerol concentration, the higher the pumping costs and the lower the demineralisation speed. According to the sodium fluxes obtained with the apparatus, the membrane area needed to perform the treatment of 6150 t/y of glycerol solution is 370 m2 with a continuous treatment. When including maintenance, the membrane area needs to be about 500 m2, which is a standard industrial membrane area. ED was initially studied for the treatment of issues containing soap which allow the production of glycerol and sodium hydroxide solutions, both solutions of great interest for industrial purposes. Soap precipitation by the addition of sulphuric acid induces the production of glycerol and sulphate sodium solutions. As the latter solution is a waste, a more attractive ED treatment could be realised with bipolar membranes. An adequate disposition of these membranes could produce sulphuric acid, sodium hydroxide and glycerol solutions. The two former solutions could be recycled in the plant at Compiègne.