Modeling and simulation of multiple effect evaporator system

Abstract

In the present work a mathematical model based on set of nonlinear equations has been developed for synthesis of multiple effect evaporator (MEE) systems. As evaporator house is one of the most energy intensive units of pulp and paper industries, different configurations are considered in the model to reduce the energy consumption. These are condensate, feed and product flashing, vapor bleeding, steam splitting, etc. Along with these the present model also accounts the complexities of real MEE system such as variable physical properties, boiling point rise. Along with complexities discussed above, the present model also accounts the fouling resistance. For this purpose a linear correlation is developed to predict fouling resistance based on velocity as well as temperature difference. The fouling resistance observed by this correlation is within the limit shown in the literature (Muller-Steinhagen and Branch, 1997). It reduces overall heat transfer coefficient by 11.5% on average.For the present study two MEE systems of typical Indian pulp and paper industries are considered. First MEE system selected for modeling and simulation is seven effect evaporator system located in north India which is being operated in a nearby Indian Kraft Paper Mill for concentrating weak black liquor using plate falling film evaporators. This system employs steam splitting in first two effects, feed and product flashing along with primary and secondary condensate flashing to generate auxiliary vapor, which are then used in vapor bodies of appropriate effects to improve overall steam economy of the system. The second system used for present study is located in south India. It is ten effect evaporator system used for concentrating black liquor and being operated in mixed flow sequence. In this system feed and steam splitting as well as vapor bleeding is employed.For seven effect evaporator system total fourteen models are developed. Initially, a simplest model without any variation is derived based on mass and energy balance. Further, it is improved by incorporating different configurations such as variation in physical properties, BPR, steam splitting, feed, product and condensate flashing and vapor bleeding. These models are developed with and without fouling resistance.The governing equations of these models are nonlinear in nature. Further, it is observed that for these models the number of equations as well as the number of variables are equal and hence unique solution exist for all cases. The set of nonlinear algebraic equations are solved using software called ‘system of non linear equations’. However, in the present work to incorporate the complex interactions of variables during solution of model an iterative procedure is used.For seven effect evaporator system total 14 models are proposed to visualize that how individual configuration is affecting the steam economy of the MEE system. The comparison shows that maximum steam economy is observed for the model where flashing as well as vapor bleeding are used. In comparison to the simplest system the improvement in steam economy through best model is found as 27.3%. The modified seven effect evaporator system, obtained using best model, requires four shell and tube heat exchangers and five pumps. This modification has total capital investment as Rs 29.3 lakh. However, saving in steam consumption is found as Rs 21.8 lakh/year thus, total payback period for the modified seven effect evaporator system is 1.3 years. For ten effect evaporator system improvement in steam economy is observed by 12.8% in comparison to existing system. It incorporates three preheaters which use bled vapor from the system. Based on the comparison with published model as well as industrial data it is found that the present model can be effectively applied to simulate the real MEE system and improve the steam economy of MEE system by 15%.