MODELING, SIMULATION, OPTIMIZATION AND CONTROL OF MULTISTAGE FLASHING(MSF) DESALINATION PLANTS .2. OPTIMIZATION AND CONTROL

Applying optimization techniques to a multistage flash (MSF) plant can involve a number of areas such as the design of a new plant (process synthesis) or modification of an existing plant (through simulation). Optimizing the operation of MSF plants is still in its infancy. Limite d efforts have been made earlier where semi-empirical equations were u sed to calculate the set-points. In contrast, several authors have dev eloped optimization programs for general design purposes. These involv ed mostly less accurate models with a limited number of decision varia bles and less efficient strategies. The optimization of an MSF plant d iscussed in this paper deals with the steady-state optimization of the operation of an existing plant. In this case, the purpose of optimiza tion is the adjustment of set-points in an optimal manner. The steam i nput for the MSF plant is considered as available in sufficient amount and with constant quality. Minimizing energy cost is suggested as an objective function. Other costs are assumed to be invariable. Under so me conditions, it is possible to use a technical objective such as min imizing energy consumption or energy losses while satisfying energy an d material balances of the process. An accurate process model is neces sary for the set-point optimization. The equality and inequality const raints which bind decision variables are discussed. The problem consid ered is one of nonlinear optimization. The integration of optimization method and process model is discussed. Two algorithms are suggested, the Generalized Reduced Gradient method and the successive Quadratic P rogramming. The set-point optimization can be performed in two modes; off-line and on-line mode, the off-line mode is recommended for the te st phase.