Previously, in the Oil & Gas industry, waste and surplus gas has been disposed of by flaring to atmosphere. Today this process is becoming increasingly unacceptable as the industry progresses towards eliminating the emission of greenhouse gases into the atmosphere, whilst simultaneously conserving energy.
To recover, purge gas after collecting from main header, pass through a flare gas recovery package which is called Ethylene Recovery Package for this project and usually includes compressor or ejector if pressure is not enough for further process. This unit should be isolated by means of isolation valves from flare system. The main objective of this package is to remove impurities from purge gas and to separate Ethylene with 99.9% volume fraction for further use into the plant.
Petrochemical Fuel Systems are generally a mixture of Gases coming from several different units within the petrochemical plants. The quantity and quality of gases in the fuel system depends on petrochemical capacity, severity of unit’s process and the quality of raw material into the process.
Despite being flared, this fuel gas contains several valuable components like hydrogen, ethylene, propylene, propane and higher olefins. These components can be recovered economically from the fuel gas. By not flaring these components, chemical plants reduce their emissions and contribute to the margins.
Different processes are known for Ethylene Recovery such as membrane technology and distillation. Herein after KASRAVAND’s technologies are briefly described.
Distillation:
In the first step, purge gas is passed through dehydration absorber or suction drum prior to compressor to eliminate the probable liquid particles. Purge gas pressure is escalated via compressor and then routed to filters. They are coalescing filters and filters with activated carbon, and should be designed to remove any oil droplets present in the gas stream. The gas will be dried by passing through the dehydration package which is packed molecular sieve.
In the subsequent stages, “ethylene distributor” column which separates methane from ethane in the presence of hydrogen while distributing ethylene between the overhead and bottoms products is utilized. The ethylene distributer column overhead is processed in a downstream de-methanizer column, and the ethylene distributor bottom is processed in a downstream C2 splitter column.
The pure Ethylene from top of the C2 Splitter and from bottom of the De-methanizer will mix to create the on-spec recovered product.
Membrane:
The purge gas stream, typically pressurized and containing the ethylene monomer and disturbing inert gas, is fed into the membrane system, while the permeate side of the membrane separation stage is fed back to the existing process gas compressor’s suction side. By means of the resulting pressure difference and the hydrocarbon selective membrane material, the purge gas stream is separated into the inert gas enriched off gas and the recycled ethylene-rich permeate stream. Typically no other equipment or machinery is required, which results in a very simple and reliable process solution.