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

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


Zhang, S. - Presenter, Dan F. Smith Department of Chemical Engineering, Lamar University
Dinh, H., Lamar University
Xu, Q., Lamar University

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.