Flare minimization under normal and abnormal operating conditions in large-scale industrial processes, especially for refineries and petrochemical plants, is a double-win practice, which simultaneously benefits industrial and environmental sustainability. Unfortunately, proactive and cost-effective flare minimization (PCFM) approaches under abnormal situations are still lacking. In this study, a PCFM approach for an ethylene plant is presented for its start-up operations. This approach employs rigorous steady-state and dynamic models of a front-end deethanizer ethylene plant to serve as a foundation to explore flare root causes during plant start-ups and subsequently a test bed to support both design and operational strategies for start-up flare minimizations. It has been demonstrated that the charge gas compressor (CGC) start-up is the most critical operation, which results in the largest amount of flaring sources. Several start-up strategies at different CGC feed rates and compositions, including scenarios recycling off-spec C 2s and C 3s from downstream recovery units to the CGC inlet as a substitute of a portion of charge gas, are virtually examined to identify the least flaring one. This work contributes to evaluating new start-up strategies, predicting abnormal process dynamic behaviors, and acquiring more precise estimation of flare emission sources. The study shows that the plant flaring can be significantly reduced, while the start-up time is shortened.

Refrigeration system holds an important role in chemical/petrochemical processes. The traditional cascade refrigeration system (CRS) used in ethylene plants contains multiple refrigerants working at multiple temperature/pressure levels. In this study, a general methodology is developed for the optimal process synthesis of a CRS based on exergy analysis. This procedure involves four stages: (1) refrigeration system exergetic analysis; (2) optimization model development for simultaneous synthesis of refrigeration system and heat exchanger network (HEN); (3) HEN configuration; and (4) final solution validation. The exergy-temperature chart is used to comprehensively analyze a CRS. A mathematical model is presented to minimize total compressor shaft work of the HEN-considered CRS, where multiple recycling loops satisfying all cooling/heating demands are simultaneously addressed. The optimal solution is examined by rigorous simulations to verify its feasibility and consistency. The efficacy of the developed methodology is demonstrated by a case study of a propylene CRS in an ethylene plant.

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The refrigeration system is one of the most important and critical utility systems in the chemical and petrochemical industries. The traditional cascade refrigeration system (CRS) has been used for decades in ethylene plants. However, the mixed refrigerant system (MRS) with higher energy efficiency and less capital expenses than CRS has not widely been applied in ethylene plants. In this paper, a general methodology has been proposed for the synthesis of a tertiary refrigerant of MRS containing a mixture of methane, ethylene and propylene that can satisfy all the cooling/heating refrigeration that the traditional ethylene−propylene CRS serves in an ethylene plant. The exergytemperature chart combined with the exergy analysis is presented to analyze comprehensively the thermodynamic nature of both CRS and MRS. Furthermore, the comparison of economic performances of these two different systems is also conducted. The efficacy of the developed MRS methodology has been demonstrated by a case study, where the capital and utility costs are respectively saved by 21.0% and 12.1%; and the compressor shaft work is also reduced by 11.2% compared with the CRS.

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