(105c) Modeling and Simulation for Flow Assurance in FCC-Cracked Gas Processing

The Ethylene Recovery Unit (ERU) processes a refinery offgas (ROG) stream from the unsaturated gas plant (USGP), and plays a critical role in separation of high-value olefinic material. Due to the cryogenic nature of the process, moisture present in the ROG needs to be eliminated using molecular sieve driers. Otherwise, upon moisture slippage to the cold regions, gas hydrate formation, accumulation and subsequent plugging of pipelines and equipment is very likely to occur. Operational problems have been experienced in this unit due to this phenomenon. Therefore, it has been imperative to analyze - using modeling and simulation tools - hydrate formation and accumulation risks in the circuit at different levels of moisture in the process streams, and methods to mitigate such risks.

Thermodynamics-based analysis of gas hydrate formation in the unit under design conditions has been conducted using the simulation model of the plant. It has been concluded that even at moisture content lower than that allowed by design, i.e. 0.1 ppm H₂O, hydrate formation is favored by the operating temperatures and pressures in the overhead circuit. Since it may not be feasible to operate at conditions far from those favoring hydrate formation, it is necessary to minimize the moisture upstream, at the feed driers. If moisture breakthrough is an issue, the other industrial best practice is to inject inhibitors such as methanol, ethanol or ethylene glycol. The simulation model has been utilized to determine the minimum amount of each inhibitor that is required to suppress hydrate formation. At these minimum levels, hydrate formation anywhere in the circuit, whether it is a pipeline or an equipment, can be suppressed. Attempt is also made to incorporate the use of n-propanol as an inhibitor, which is otherwise not available in widely employed process simulators.