(210a) Energy Efficient Light Olefin Recovery: Absorption Versus Cryogenic Distillation

The energy consumption of the light olefins recovery section of olefin production processes is enormous due to the low temperatures needed to produce high purity ethylene and propylene. During the coming years olefin production will be expanding fast in the USA due to the construction of steam crackers using cheap shale gas as feed. Methanol To Olefins (MTO) is an alternative process developed to produce olefins from a variety of carbon feedstocks, ranging from shale gas over crude oil fractions to coal. Several intermediates are identified in the process including syngas and DME. The light ends content (CH₄, CO, H₂, N₂) of the MTO process is generally smaller compared to traditional steam cracker effluents. Therefore, alternative setups for the olefin separation section can be investigated which differ in the separation of the C1- components from the C2+ components.

The traditional cryogenic schemes include the demethanizer first configuration, the deethanizer first configuration and the depropanizer first configuration which are identified by a different sequence of the distillation columns. A common feature in all these separation setups is the need for very low temperatures (–100 °C and below) to avoid excessive losses of light olefins in the fuel gas stream. These low temperatures are generated in a refrigeration section by subsequent expansion and compression steps of ethylene and propylene. Since compression is very energy-intensive, generating these low temperatures is quite costly, both in terms of equipment cost and operating cost. Therefore, Wison Engineering has developed and is successfully operating an MTO plant with an alternative separation section in Nanjing, China. The main feature is the replacement of the traditional demethanizer column for the separation of the C1- fraction and the C2+ fraction by a combination of a pre-cutting column and an absorption column. In this setup the C2+ fraction is separated from the C1- fraction partly by traditional distillation and partly by absorption in an adequate solvent. The minimum temperature is limited to around -40 °C to eliminate the need for an ethylene refrigeration cycle and thus only use the propylene refrigeration cycle. This should make the absorption separation scheme favorable over the traditional cryogenic scheme in terms of both capital expenditure and operational expenditure.

Therefore a comparison is made between a demethanizer first scheme as representative of the traditional cryogenic separation configuration and an absorption configuration with propylene as solvent. Pinch analysis is applied to maximize heat integration, while sensitivity analyses on important parameters allow gaining additionoal insight in which recovery section is truly optimal. An estimate is made of the capital expenditure for both cases. An estimate of the operational expenditure is also made for both cases.