Novel retrofit designs using a modified coordinate descent methodology for improving energy efficiency of natural gas liquid fractionation process

• An energy efficient hybrid technology was proposed for retrofit.
• Optimal design is efficiently done by modified coordinate descent methodology.
• Operating cost saving of 44.55% could be achieved.
• A TCDS can increase the energy efficiency of a heat pump.
• The proposed sequence is an attractive option for industrial implementation.

Tough environmental regulations, intense competition, expensive fossil energy use, and the strong growth predictions of the natural gas market have prompted efforts to retrofit the existing purification processes to reduce their energy requirements. The important goals of retrofit design are to analyze, evaluate and propose suitable technologies to improve the energy efficiency and/or increase the capacity. This paper reports the results of a techno-economic feasibility study to retrofit a natural gas liquid (NGL) fractionation process. A novel hybrid system, side reboiler and heat pump-assisted, thermally-coupled distillation sequence to maximize the energy efficiency, was proposed. Fractional utilization of the area was used as a hydraulic performance indicator to determine if a bottleneck occurs in a retrofit design. A modified coordinate descent methodology was employed to solve the optimization problem. As a result, the modified coordinate descent methodology was successful in finding the optimal proposed sequence structure and the operating variables, which resulted in operating cost savings of 44.55% compared to the representative base case. The short payback period of 14 months and reduced CO2 emissions of up to 42.05% showed that the proposed sequence is an attractive option for retrofitting in industrial implementation. This sequence can be employed for both grass-root and retrofit designs. This study also showed that even the heat pump can reduce the energy requirements significantly, and may have higher exergy loss than the existing conventional distillation columns.

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