Study On Concentrating Dilute Chlorhydric Acid By Combined Method

Chemical and pharmaceutical production processes commonly use hydrochloric acid as raw materials and pickling agent, and emit large quantities of waste dilute hydrochloric acid as well. Since hydrochloric acid forms the highest boiling point azeotrope with water, which contains HCl 20.2 wt.%, and the azeotropic point is 108.6℃. It can not be separated by common distillation to obtain the hydrochloric acid whose concentration is higher than the azeotropic composition 20.2 wt.%.To get concentrated hydrochloric acid with mass concentration larger than 30% and water discharged with extremely little hydrochloric acid, a combined method consisting of salt desorption– gas absorption– salt distillation was proposed and studied for concentration of dilute hydrochloric acid.Firstly the combined method considering salt desorption– gas absorption was studied to concentrate the dilute hydrochloric acid. In the designed procedure, the the dilute hydrochloric acid was divided into two parts.one half was heated in the column bottom, which released the desorbed HCl rising upward; the other flew down in the fill layer,as sorbent to absorb the HCl. The effects of salt concentration, liquid to gas ratio,and feeding amount and temperature in the column reactor on the yield were investigated respectively. Below is the result: the higher the concentration of salt(MgCl2) was,the deeper extent of HCl desorption was, the more HCl was absorbed in the sorbent. When the MgCl2 was 0.3g/mL, the HCl concentration in the bottom declined to 6.50%, and rose to 32.80% in the sorbent after the equilibrium was achieved. The efficiency of absorption shifted up significantly as the liquid gas ratioincreased. When the liquid to gas ratio for a certain value, the yield of HCl was 75.12%. Compared with feeding with the column temperature at 50℃, it took much less time to achieve equilibrium when the solution in the bottum was boiling.Then, to investigate the effect of salt concentration and reflux ratio on the concentration of hydrochloric acid at the top and bottom of the tower, experiment of salt distillation, which used the residual from desorption of hydrochloric acid as the raw material was carried out in this paper. The results showed that the average concentration of hydrochloric acid and the amount of 20% hydrochloric acid at the top increased with the concentration of the salt in the still. When the concentration of MgCl2 was 0.3g/ml, the average product concentration at the top was 17.25% and the amount of 20% hydrochloric acid was 16.4%.