Mathematical Modeling and Optimization of Heat Exchanger Design in Process Flowsheets

In the last two decades, optimization methods and algorithms have improved immensely in both criterion: number of problems solved and their solution times. Unfortunately, this has not translated in the more accurate modeling of chemical engineering process models.

Current flowsheet models use simple mass and energy balance to represent process equipments and do not incorporate effect of its design in the model. In this thesis, the heat exchanger design is studied from the viewpoint of embedding it in the flowsheet models. A novel design model based on differential heat equation and the geometric structure of the exchanger is proposed. The model is capable of capturing the effects of phase change

and variable thermophysical properties on the exchanger design. This is demonstrated by solving multiple examples of shell and tube heat exchangers and flooded phase change

heat exchangers. In the subsequent chapter, the exchanger design model is integrated into the heat exchanger

network synthesis model using two separate methods: two-step hybrid strategy and trust region filter approach. Examples from previous studies are used to show the significance effect of including detailed exchanger design in the synthesis of heat exchanger networks. Afterwards, the design model for multi-stream heat exchanger using a discretized system of equations and complementarity constraints is developed. The design equations relating the heat exchanger area to the design variables and degrees of freedom are derived. This design model is solved simultaneously with a natural gas liquefaction model using a step-by-step initialization procedure. The optimization results for both the exchanger design and flowsheet variables are presented along with plots of temperature variation and liquid fraction for each stream inside the exchanger.