(64a) An Ethylene Plant Start-up with Total Recycles: Dynamic Simulation of De-Ethanizer Transient Behaviors

Flaring emissions during plant turnarounds have always been a major environmental and economical concern in the chemical process industry. A significant case is during plant start-ups. Traditionally, off-spec products are directed to flare system as wastes until operating units are in specifications. One of the recent flare minimization practices is the start-up with total recycles, in which off-spec streams from downstream are directed to system input until the process operates normally. Dynamic simulation is employed as a virtual tool to generate a new strategy and provide guiding procedure to operators.

In this study, an operation procedure of an ethylene plant start-up is investigated via dynamic simulations. Three case studies are presented to demonstrate the efficacy of dynamic simulations.  The base case is the conventional method of flaring off-spec products during the first three and a half hours to fire and stabilize three cracking furnaces before starting charged-gas compressor (CGC) system. Off-spec C₂-hydrocarbons are withdrawn from DeC₂ overhead and recycled back to feed CGC as an alternate to either one (Case One) or two furnaces (Case Two). This recycled stream is also mixed with another off-spec one from C3 splitter and additional natural gas to make up to achieve an equivalent molecular weight as CGC normal feed. Dynamic simulation contributes a great deal in evaluating new strategies as well as predicting process dynamic behaviors. It also helps achieving more precise timing to control the recycle loop; hence, flaring is potentially minimized while the start-up time is shortened. One observation is the difference of the recycle stream temperature and the expected value entering the first-stage compressor, a solution of which is confirmed by dynamic simulations. Following this representative, utilizing dynamic simulation as a driving force for operation during start-ups, shutdowns and malfunctions (SSM) conditions effectively provides an in-depth understanding and provides more detailed planning and expectations for all related personnel before the real action is proceeded.