Mobil cracks propylene shortage
15 Jan 2000
Catalyst researchers at Mobil Technology believe they have found the key to meeting the fast-rising demand for propylene. Based on a proprietary zeolite, the new system enhances the propylene output of both refineries and steamcrackers.
Currently, the main sources of propylene are ethylene crackers and fluid catalytic crackers (FCCs) in refineries. However, explained Mobil researcher Robert Ware at a recent CMAI petrochemicals conference in Houston, the yield from crackers is extremely low only 40 600kg per tonne of ethylene. But demand for propylene is expected to double over the next 20 years, outstripping ethylene, and new cracker construction will not be able to meet this. Therefore, said Ware, the most logical way to produce more propylene is to adapt the FCC process.
FCC works by converting gas oils and residuals into gasoline, distillates and liquefied petroleum gas over a zeolite catalyst at high temperatures and moderate pressure. The zeolite usually used is faujasite, which has a high density of acid sites (which donate hydrogen to the hydrocarbon chains) and a structure which consists of pores around 7.5A across, opening out into a 'supercage' structure about 15A in diameter. This is a very good structure for cracking gas oils, says Ware; in particular, it converts reactive olefins into stable paraffins very efficiently. However, it's a rather in efficient way to make propylene, because the supercage is too large.
Ware's team proposes replacing the faujasite with a Mobil proprietary zeolite, known as ZSM-5. This has a much more constrained structure, with pores of only 5.5A and no supercage. This makes it more 'shape-selective' than faujasite, Ware explained.
Dubbed Mobil Olefin Interconversion (MOI), the process contacts vapourised, pre-heated feed which could be gas oil from a refinery or the C4 feed from a steamcracker with ZSM-5 in a single fluid-bed reactor, at similar pressures and temperatures to FCCs. Coke tends to form on the catalyst, which is withdrawn from the reactor continually and regenerated (see diagram).
Propylene yield from a light (C5-7)naphtha stream is typically about 26 per cent, says Ware; yields from steamcrackers are similar. An MOI unit to produce 150000tpa of propylene from steamcracker C4 is $30million, Ware claimed, including reactors, catalyst handling, feed pretreatment, byproduct recycling and product recovery. Mobil is currently seeking a partner to help commercialise the process.
