ANALYSIS AND SIMULATION OF AN EXISTING MEE SYSTEM WITH REHEATERS

Abstract

The present investigation deals with modelling, analysis and simulation of multiple effect evaporator (MEE) system used to concentrate weak Kraft black liquor in the pulp and paper mills for the recovery of cooking chemicals. As the evaporator house in pulp and paper industry consumes about 24-30% of the total energy consumption in the plant so optimum feed flow sequence has to be selected which provides high steam economy and low steam consumption (SC). To improve the steam economy (SE) inclusion of product flashing, condensate flashing, steam splitting has been used. In addition to the above reheaters are used at appropriate places to improve the steam economy as it takes vapor from the vapors generated in the evaporator section. The investigation considers the variation of input parameters i.e. liquor feed Temperature (TF), liquor feed rate (F), steam temperature (To) and last effect temperature (TL) to evaluate the influence on SC and SE. The effect of choice of liquor sequence has also been investigated on steam energy requirements. Equations based on material balances, energy balances and heat transfer rate have been formulated for this purpose.Emperical equations have been included. For the solution of these equations globally convergent method has been employed. A computer program was developed in 'C' and executed using DevCpp software. This investigation uses the plant data from nearby paper mill for the validation of the results obtained from simulation. Different liquor flow sequences like mixed feed flow sequence (MFFS) and backward feed flow sequence (BFFS) have been studied without including reheaters and with reheaters. The conclusion which can be drawn from this investigation is backward feed flow sequence (BFFS) with reheaters is the optimum feed flow sequence (OFFS) providing highest steam economy (SE) and lowest steam consumption (SC). iii