Study of the design parameters for evaporation of dilute caustic soda for application in the textiles mills in Bangladesh

Abstract

An experimental investigation was undertaken that involved the design, construction and operation of an evaporation unit which would concentrate dilute caustic soda from the mercerizing unit wastewater of a textile mill. The concentration of caustic soda in the wastewater was to be raised to 20 w/w % by evaporating the water from the dilute solution containing caustic soda 4 w/w % using waste heat from flue gases. The process unit designed for this purpose consists of an evaporator, flash chamber, preheater, natural gas fuelled furnace, feed tank, storage tank and circulating pump. Parameters such as the amount of water evaporated, volume of natural gas burnt, pressure drop, circulation rate and concentration change of caustic soda solution were measured. From these, the rate of evaporation, LMTD, total heat supplied, overall heat transfer coefficient etc. were determined. From the experiments, it was has been found that the rate of evaporation ranged from 0.08 liters/hr to 19.00 liters/hr with an average of 6.91 liters/hr. Overall heat transfer coefficient in the evaporator varied from 20.70 W/m2 K to 52.58 W/m2 K. Cost analysis carried out showed the process is economically operable in a textile mill with mercerizing operation. The experimental results suggest that this process can be implemented in the industry.