Process Synthesis and Optimization of Energy Integrated Distillation Sequences for Natural Gas Liquids Separation

Natural gas liquids (NGL) fractionation is one of the most energy-demanding process in the oil and gas industry. Due to the massive scale of operations, energy savings for the NGL fractionation are important and would contribute to less greenhouse emissions. The synthesis of energy efficient NGL distillation sequences is challenging due to many interconnected degrees of freedom associated with the large number of structural options and operating conditions. The synthesis requires to determine the basic separation configuration, including thermally coupled complex columns, as well as the column pressures, reflux ratios, feed conditions, condenser types and heat integration arrangements. For all but the simplest separation problems an exhaustive search is not feasible, and it is also not clear whether short-cut distillation models provide the necessary accuracy for reliable screening, and whether heat integration must be included simultaneously. To solve these problems, this study explores the basis on which reliable screening can be carried out. A screening algorithm has been developed using a superstructure optimization approach equipped with short cut models for the sequence synthesis, in conjunction with a transportation algorithm for the synthesis of the heat integration arrangement. Different approaches for the inclusion of heat integration are explored and compared. Then the best few NGL fractionation designs from this screening are evaluated using rigorous simulation. It has been found that for separation problems of the type explored can be screened reliably using shortcut distillation models in conjunction with the synthesis of heat exchanger network designs. Unintegrated designs using thermally coupled complex columns show much better performance than the corresponding designs using simple columns. However, once heat integration is included the difference between designs using complex columns and simple columns narrows significantly. Finally, the most techno-economically sequences for the NGL fractionation process are identified and their operating conditions are detail designed.