Abstract: A high pressure evaporation vessel which is operated at a predeterminate pressure above atmospheric pressure, is connected to a low pressure expansion vessel which is operated at a predeterminate pressure below the pressure prevailing in the high pressure evaporation vessel, through a U-shaped siphon tube. The U-shaped siphon tube contains a liquid column absorbing the pressure difference existing between the high pressure evaporation vessel and the low pressure expansion vessel.

Abstract: A caustic alkali solution is concentrated to high density or high caustic alkali concentration by evaporation under high pressure at elevated temperature and expansion of the thus obtained concentrated caustic alkali solution to low pressure, preferably below atmospheric pressure. The pressure difference between the high pressure evaporator and the low pressure expansion vessel is absorbed gradually by a pressure difference absorber, preferably comprising a liquid column in a U-shaped siphon tube, so that corrosion and erosion are reduced considerably.