Recovery of olefins from refinery offgases
The Lummus process eliminates the multi-stage feed gas compressor yet maintains over 90 per cent recovery of ethylene while cutting capital cost and reducing energy consumption
Margaret M Shreehan, ABB Lummus Global
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Refinery offgases, typically from fluid catalytic cracker units and coker units, are normally disposed of as feed into the refinery’s fuel gas system. However, these streams contain significant amounts of olefins which can be economically recovered and/or converted. In fact, many of the projects have simple payout times of less than two years.
Offgas recovery units can be integrated with existing olefins units or, if the flows are large enough, stand-alone units can be realised. In addition, if ethylene is not a desired product, an olefins conversion unit (OCU) can be installed to react the recovered ethylene with FCC or imported butenes to produce propylene. ABB Lummus Global has developed and patented a low pressure offgas recovery sequence and has also acquired Olefins Conversion Technology, which was developed by Phillips.
Offgas recovery units can be broken down into several sections including feed contaminant removal, ethylene recovery and, with an OCU, the reaction section and propylene recovery. Feed contaminants including acid gases, arsine, mercury, ammonia, nitriles, COS, acetylenes and water, must be removed. It is critical that the designer of the unit be experienced with feedstock pretreatment since the many trace components that accompany the offgas feeds can have an impact on ultimate product purity, catalyst performance and operational safety.
The ethylene recovery section can be a stand-alone unit where polymer grade ethylene is produced, or an integrated unit where partially recovered streams suitable for integration into an ethylene unit recovery section are produced.
Because of the high quantity of methane and lighter components in the feed gas, the typical offgas recovery unit compresses the feed gas to pressures as high as 35kg/cm2g in a multi-stage feed gas compressor before beginning fractionation. This allows for C2 and heavier component recoveries greater than 90 per cent. The fractionation steps are then carried out with a combination of mechanical refrigeration and expansion of the methane and lighter portion of the feed gas in an expander/recompressor unit.
The Lummus process operates at low pressure, eliminating the multi-stage feed gas compressor and the expander/ recompressor system while maintaining ethylene recovery above 90 per cent. This process sequence has resulted in sharply reduced capital cost and energy consumption, significantly enhancing the already attractive economics associated with olefin recovery from refinery offgases. Other benefits of the low pressure sequence include increased reliability and flexibility when compared to high pressure configurations.
As previously mentioned, an OCU can also be incorporated to react ethylene with butenes to produce propylene via a metathesis reaction. This fixed bed process features greater than 90 per cent selectivity to propylene. In this application, the ethylene in the FCC offgas is upgraded from fuel value to ethylene/propylene values.
If butenes are not available, another option is to convert some of the ethylene to butene via a dimerisation reaction. This butene can then be reacted with the remaining ethylene to produce propylene.
In this article several topics concerning refinery offgas processing are discussed:
- Feed gas contaminants, including the kind of contaminants to be expected and how they can be removed or processed. (Contaminants to both the ethylene recovery and the OCU are considered)
- A comparison of a low pressure recovery sequence with a high pressure recovery scheme
- Integration of an OCU with low pressure offgas recovery to produce propylene.
There are a number of potential impurities contained in refinery offgases which must be removed to prevent contamination of the final products or for safety considerations.
Typical impurities include: acid gas – including CO2 and H2S – acetylene, oxygen, ammonia, nitrogen oxide, nitriles, water, carbonyl sulphide, mercury and arsine.
Acid gases (carbon dioxide and hydrogen sulphide)
Carbon dioxide and hydrogen sulphide are temporary poisons for the metathesis reactor catalyst. They are removed from the FCC offgas by scrubbing in a caustic/water wash tower with caustic solution. If caustic consumption is very large due to a high concentration of CO2, it may be economical to first have an amine wash followed by caustic scrubbing.
Acetylene promotes coking in the metathesis reactor. It is hydrogenated to ethane in an acetylene converter, using a sulphided nickel catalyst. A palladium catalyst is not recommended and this is explained below.
Oxygen is present in FCC offgas because it enters the FCC system during catalyst regeneration. The concentration of oxygen is dependent upon the excess oxygen content of the regenerator flue gas. Oxygen poses a safety hazard when it undergoes reaction (at cryogenic temperatures) with the nitrous oxides in FCC offgas. These can form explosive gums that can accumulate in the plate fin exchanger.
Oxygen is also a poison for the palladium catalyst used for acetylene hydrogenation. Ppm levels of oxygen have been found to drastically reduce hydrogenation catalyst activity. Oxygen is also a temporary poison to the metathesis reactor catalyst.
Oxygen can be removed by a reduced bed of copper, or by sulphided nickel catalyst. However, if acetylenes are present, removal of oxygen using a reduced copper bed is not feasible. When acetylene comes into contact with copper it forms copper acetylides, which are explosive. Therefore, from a safety point of view, it is usually recommended to remove oxygen using sulphided nickel catalyst.
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