Process Synthesis and Optimization of Heat Pump Assisted Distillation for Ethylene-Ethane Separation Matthew Bryan Leo, Arnab Dutta, and Shamsuzzaman Farooq* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585 * S Supporting Information ABSTRACT: Ethylene is a platform chemical, and its production process includes energy intensive ethylene-ethane separation via cryogenic distillation. As potential avenues for energy reduction, in this study we have explored four distillation process configurations based on the concepts of mechanical vapor recompression (MVR) of the top product and throttling of the bottom product i.e., bottom flashing (BF). Each of these configurations is optimized in MATLAB using the genetic algorithm to minimize the total annualized cost (TAC) of separation. Our results show that both MVR and BF reduce energy as well as cost of separation compared to the conventional distillation configuration. However, BF outperforms MVR, and together with a prethrottling cooler it provides the minimum TAC configuration for ethylene-ethane separation. The proposed heat pump assisted distillation configuration has the potential to reduce the total energy requirement by 57% and the separation cost by 44% over the conventional distillation process. 1. INTRODUCTION Ethylene is one of the most versatile and widely used petrochemicals in the world. Owing to the highly reactive double bond in ethylene, it is a precursor for many useful derivatives such as polyethylene, poly(chloroethene), ethylene oxide, ethylene dichloride, ethylbenzene, and linear alcohols. These derivatives are used in the production of plastics, solvents, cosmetics, pharmaceuticals, paints, and packaging materials. 1 In 2016, the annual production capacity of ethylene was 146 million metric tons and is expected to increase to about 200 million tons by 2025 at a growth rate of ca. 3.6% per year. 2 Ethylene is predominantly produced via steam cracking of petroleum hydrocarbons. 3 An ethylene-ethane separator, commonly called a C2 splitter, is the last in a series of separation processes where an ethylene-ethane mixture is fed to produce 99.9 wt % pure polymer grade ethylene. 4 The C2 splitter is an energy intensive cryogenic distillation process, operated at a high pressure of about 17-28 bar to minimize the refrigeration cost 4 incurred to condense the top product. Propylene is the most commonly used refrigerant for the C2 splitter. 4 Owing to this high operating pressure, the relative volatility of ethylene over ethane for an ethylene rich feed decreases to 1.13-1.20. 5,6 Although the separation becomes easier at lower pressures due to higher relative volatility, it also lowers the required refrigerant temperature, thus increasing the cost of refrigeration. From an economic perspective, a high- pressure system is preferred. Energy efficient techniques for ethylene-ethane separation is an active area of research. Park et al. 7 presented a vacuum swing adsorption process for separating the C2 mixture, where 99.9 wt % ethylene purity was achieved with over 90% recovery. MOF (metallic-organic framework) adsorbents have also been reported for ethylene-ethane separation. 8,9 Hybrid separation processes, combining membrane separation with conventional distillation, have also shown some promise. 10,11 Despite the advances made in these alternative separation technologies, conventional distillation remains the major player for ethylene-ethane separation in petrochemical industries. 12 Therefore, in this study we investigate various distillation techniques to obtain an energy efficient and a cost-effective cryogenic C2 splitter. Kiss et al. 13 and Jana 14 have provided excellent reviews on the various heat pump assisted distillation (HPAD) techniques for binary as well as multicomponent systems. Among them, the ones suitable for the C2 splitter include mechanical vapor recompression (MVR), bottom flashing (BF), and heat integrated distillation (HIDiC). MVR has been applied to several industrial processes. 15,16 Besides petroleum refining, MVR is also utilized in a variety of processes such as pulp manufacturing, beer brewing, dairy products and soft drink manufacturing, desalination of seawater, and nuclear power generation. 17 MVR is generally suitable for close boiling point systems. 18,19 MVR has been studied for C2 splitting by Sivanantha and Maffia. 6 The authors simulated the distillation process without and with MVR in Aspen HYSYS and compared the two configurations based on their utility requirements. Including MVR proved to be a better option in terms of utility consumption as it was able to eliminate a significant proportion of the condenser load and the entire reboiler load. However, the authors did not perform any Received: June 4, 2018 Revised: August 2, 2018 Accepted: August 6, 2018 Article pubs.acs.org/IECR Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.iecr.8b02496 Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX Downloaded via SAINT LOUIS UNIV on August 23, 2018 at 20:50:21 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.