(486a) Dynamic Simulation of Ethylene Refrigeration and Cryogenic Separation Systems

Refrigeration and cryogenic separation systems are very important and complex sections in an ethylene plant. In these systems, the temperature of cracking gas from ethylene furnaces is decreased to -160 °C, and components of cracking gas are separated by distillation. Due to its complexity, dynamic simulation on shutdown and startup procedures is not considered very well. This paper focused on process shutdown dynamic simulation and optimization of ethylene refrigeration and cryogenic separation systems.

The cryogenic separation system includes cold box facility that separates hydrogen and methane, demethanizer that separates methane, deethanizer and ethylene distillator that separate ethane and ethylene, as well as depropanizer and propylene distillator that separate propane and propylene. The refrigeration system includes propylene refrigeration cycle, ethylene refrigeration cycle, and methane refrigeration cycle. Other than process sequence based simulation model, temperature based simulation was introduced in the model. In a temperature based simulation model, the cryogenic separation process and refrigeration cycles are divided into several temperature interval stages. Distillations worked at valid temperature interval can produce qualified products. During a shutdown procedure, when distillation columns work at invalid temperature interval, their products will be sent to off-spec product drums in stead of downstream equipments. The refrigeration and cryogenic systems have four temperature stages: -105 ~ -160 °C, -60 ~ -105 °C, -22 ~ -60 °C and 16 ~ -22 °C. Each stage includes its separation process and refrigeration process.

The dynamic simulation model was built by Aspen Plus Dynamics. A shutdown plan was simulated as base case to analysis its performance. After that, several new cases were studied to optimize component recovery and reduce waste gas flaring. In these new cases, several temporary pipelines were added. Liquid-phase and vapor-phase refrigerants were recovered through temporary pipelines and deethanizer column to storage tanks. The optimized shutdown procedure recovered 5% of more useful components than original procedure. Such that flaring emissions was also reduced.