(612b) Ethylene Plant Start-Up With Total Recycles for Flare Minimization: Scenario Study On High-Pressure and Low-Pressure Depropanizer System

In recent years, CPI has put more and more efforts on improving the sustainability for manufacturing processes due to stricter environmental regulations and more competitive market, which makes FM a necessary approach to optimize the plant operations at minimum environmental emissions. Flaring generates a large amount of volatile organic compounds (VOC) and a portion of them are categorized as highly reactive VOCs (HRVOC) which are hazardous to the wellbeing of local communities, especially those around chemical plants. Therefore, it is of utmost importance to limit the amount of flaring. Also, flaring is usually accompanied with undesirable material waste and excessive energy consumptions. Thus, from the economical standpoint, flaring minimization practices are helpful to reduce the burned-off materials and improve energy efficiency. Ethylene production is one of the most complicated and integrated  processes in chemical industry which provides feed stock to a wide variety of important products such as ethylene oxide, vinyl acetate, chlorinated ethane and high/low density polyethylene. During plant start-up, both flow rate and composition are abnormal, which leads to a large amount of flare emission. For example, a typical ethylene plant at an annual capacity of 1.2 billion pounds may send 5 million pounds of ethylene to flaring system during start-up.

From literature review, it is known there are not many efforts devoted to proactive flare minimization strategies plant wide. In this paper, a front-end DeC₃ ethylene plant is investigated. There are mainly four subsections composing the whole process: CGC (Charging Gas Compressors), LP/HP DeC₃ coupled with acetylene reactor, cryogenic separation system (chilling train and DeC₁), and recovering system including DeC₂ through DeC₄, as well as C₂S and C₃S. The dynamic model for plant start-up is built in a systematic way through comprehensive work in data validation, model troubleshooting and interactions among steady state simulation, normal operation and start-up dynamic simulations. A zero-flare approach with total recycles is implemented in our study which successfully identifies the potential flare sources and associated material waste, transition time and resulting flaring amount. A case study is demonstrated extensively to show the capability of dynamic simulation to find the root causes of flaring and also propose feasible solutions.  The case study is focused on the start-up of HP/LP DeC₃. In the proposed start-up plan, LP DeC₃ is already built in self recycle before CGC is started. And after CGC is started, LP DeC₃ will send out feeds to HP DeC₃ to help its top temperature to drop to the desired level as soon as possible, which prompts the top C₄ composition to meet specification and facilitates liquid accumulation for HP DeC₃. Afterwards, the acetylene reactor can be gradually started and DeC₃ can send out feeds to the downstream DeC₁ system.