Simulation of the Condensate Treatment Process in a Kraft Pulp Mill

Abstract

A simulation program has been developed and used to study the condensate treatment process in a kraft pulp mill consisting of a stripper, a methanol column and a decanter. The simulations require rigorous models for vapor-liquid and liquid-liquid equilibrium calculations. The Wilson equation and the NRTL equation (non random two liquids) have been used for these calculations. New data were included in the flowsheeting program Aspen Plus for seven volatile compounds, to perform accurate calculations of both vapor-liquid and liquid-liquid equilibria. Simulations were compared with measurements on the industrial condensate treatment process and the results of the simulations were used to improve the process. By changing the position of the decanter in the process, the COD (chemical oxygen demand) content in the stripped condensate was decreased from 700 to 300 mg O2/l. The effect of various process parameters, such as the vapor fraction after the stripper and the flow rate of secondary steam to both the stripper and the methanol column, on the quality of clean condensate were investigated. The design of the stripper and the methanol column was studied by calculating the optimal feed location for various numbers of trays in the columns. The results showed that the feed location in the methanol column would be more optimal if it were located higher up in the column (above tray 5–7) instead of above tray 10, which was the case in the industrial process investigated.