“12 th Romanian International Conference on Chemistry and Chemical Engineering” OPTIMISATION OF THE CRYSTALLISATION BASED SEPARATION SCHEMES Raluca Isopescu and Laurentiu Filipescu Faculty of Industrial Chemistry, University POLITEHNICA of Bucharest, ROMANIA Keywords: fractional crystallisation, separation sequence design, optimisation Abstract Crystallisation-based separation has a wide variety of industrial applications. Because of the significant role the separation processes play in the operating cost of a chemical plant, the optimal design of separation scheme is of great importance. The crystallisation based separation schemes are described using a network flow model. The construction of the network flow is based on the identification of feasible thermodynamic states. The overall network is formulated as an optimisation problem whose objective is to minimise the operation cost that constrains the mass balance, flow-rate specification, and composition specification. In the present work a system with three solutes was investigated. The optimisation problem is solved using non-linear programming implemented in GAMS. Introduction Multicomponent separation sequences are an important part of many chemical- processing systems. The large variety of industrial applications consists of sequences of heating, vaporisation, cooling, diluting and solid phase separation units. The problem is that even for relatively simple systems several separating sequences can be designed and it is difficult to choose the best solution. Most of the research on the crystallisation based separation schemes synthesis relies on the identification and generation of separation schemes on the phase diagrams [1,2]. Recently, Cisternas and Swany [3,4] developed a flow sheet network based methodology to optimise the sequencing design of fractional crystallisation. In this method the liquid-solid equilibrium is analysed as an evaluation of the relative solubility variation at different possible working points. The feasible pathways in the separation process are represented in a network. The network is generated considering the general rules of separation sequences synthesis applied to fractional crystallisation. These rules are: (1) The n-components system is divided in all (n-1)- components systems, which are again divided in all (n-2)-components systems and so on until two solutes systems are reached. A network is derived for each system based on thermodynamic considerations. Each network contains unknown stream connections that lead to different