Metathesis for maximum propylene
Using metathesis technology to process refinery-based C4 feedstocks can maximise propylene. The economic advantages of flow schemes using alpha-olefins in the metathesis reaction with C4 olefins are discussed
Robert J Gartside and Marvin I Greene, ABB Lummus Global
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The past few years have seen a dramatic increase in the demand for propylene to feed the growing markets for polypropylene, propylene oxide and acrylic acid. Traditionally, propylene has been produced as a by-product of steam cracking and fluid catalytic cracking (FCC). However, propylene demand growth has exceeded ethylene and gasoline/distillate demand growth, and propylene supply has not kept pace with this increase in demand. In addition, considerable ethylene capacity added in the Middle East has been based upon ethane, which produces only a small amount of propylene, further exacerbating the tight supply situation. The limited availability of propylene has resulted in a dramatic increase in propylene pricing, as shown in Figure 1.1
This extremely tight supply situation has resulted in producers looking for alternate sources of propylene. CMAI has projected that, based upon current capacity and growth rates, an additional 19.8 million tons of new propylene capacity will be required by 2010.1 A large fraction of this capacity will come from new steam crackers and/or FCC units.
There have been significant advances in FCC catalysts and additives to increase propylene production. Yields from existing FCC units can be increased from a nominal 3–5% on feed to 7–9%, and investment in increased propylene recovery is occurring. However, a major fraction of the propylene shortfall will have to be supplied by on-purpose propylene projects. On-purpose propylene can be produced in standalone systems via propane dehydrogenation, high-severity FCC or FCC olefins units and gas-to-olefins projects utilising methanol reaction to mixed olefins or propylene. These processes are characterised by high capital costs and, in some cases, high by-product volumes. Alternatively, a number of technologies have been or are being developed based upon by-product transformation techniques using C4–C6 olefins as feed. These lower-valued olefin by-products from steam crackers or FCC units are reacted to produce propylene. There are two technology approaches to this transformation. Olefin metathesis is an equimolar olefin interconversion technology, where two olefins are reacted with each other to produce two different olefins. Olefin cracking utilises zeolitic cracking catalyst technology to crack the olefins to a mixture of lighter olefins and aromatics.
The term “metathesis” is derived from the Greek meta (change) and tithemi (place). It refers to the changing of positions of the “R” groups around a pair of double bonds. Thus:
This equilibrium reaction is carried out over Group VIA or VIIA metal oxide catalysts. The predominant technology was first developed by Phillips in the 1960s and is now licensed by ABB Lummus Global under the proprietary name Olefins Conversion Technology (OCT). The most well-known metathesis reaction is between ethylene and 2-butene to form two propylene molecules. However, metathesis can occur between any two olefins and/or dienes as long as there is a pair of double bonds.
The equilibrium metathesis reactions for the C4 fraction are shown in Table 1. Paraffins do not react, which is also true for the olefins cracking technologies. Thus, for any upgrading technology, the fact that refinery-based C4 streams contain considerably more paraffins than thermal cracking C4 feeds is an issue. Any butadiene in the feed should preferentially be removed. For refinery-based feedstocks, the butadiene content is nominally below 5000ppm. Butadiene must be selectively hydrogenated to a lower level and the location of that hydrogenation is a function of unit design.
While it is commonly believed that ethylene is required for the metathesis reaction and that isobutylene should be minimised, this is not correct. Excess ethylene is utilised in the reactor not by requirement, but in order to maximise the selectivity of the normal C4 olefin fraction to propylene.
As shown in Table 1, by operating with excess ethylene, the equilibrium reactions between the various butenes (iso and normal) are minimised either by concentration or reverse equilibrium. However, the ability of butenes to react with themselves (the side reactions) can be exploited for product flexibility or for operation at lower ethylene/butene ratios.
In the operating system with ethylene (ie, conventional metathesis), if there is 1-butene present the catalyst system typically employs a co-catalyst that provides double bond isomerisation activity. This shifts the 1-butene to 2-butene, as the 2-butene is reacted away with ethylene and maximises selectivity to propylene as well as propylene production. When using ethylene, isobutylene is typically removed prior to metathesis to minimise recycle, since isobutylene does not react with ethylene, as shown in Table 1. However, isobutylene can be processed through the reaction system similarly to the normal C4 olefins.
It is important to understand the nature of the metathesis reactions and their influence on the selectivity to propylene. As shown in Table 1, in order to make propylene, a reaction must occur between an alpha olefin and a secondary olefin. If the alpha olefin is ethylene and the secondary olefin is 2-butene, two propylene molecules are formed. If the reaction is between an alpha olefin (1-butene or isobutylene) and a secondary olefin (2-butene), one propylene and one C5 olefin are formed. The structure of the C5 olefin depends upon the structure of the alpha olefin (normal or iso). When two alpha olefins are present and one of those alpha olefins is ethylene, there is no reaction, since there is no potential for shifting of the appropriate R groups around the double bond. When both alpha olefins contain R groups (for example, 1-butene and isobutylene), reaction is possible but at a lower rate because of the steric influence of the double bond shifting. These reactions occurring between two higher alpha olefins are termed “half productive” reactions. Further, the reaction between two alpha olefins produces ethylene and another hydrocarbon, not propylene and another hydrocarbon.
For the reaction between ethylene and 2-butene, the selectivity of the ethylene and/or butenes to propylene is theoretically 100%. In this reaction, there is one associated propylene for each feed molecule. Thus:
([C3H6 formed]/2) / [C2H4 used] = 1 or ([C3H6 formed]/2) / [2-C4H8 used] = 1
If 1-butene is isomerised to 2-butene and the reaction between 1- and 2-butene avoided, 1-butene will also have a 100% theoretical selectivity to propylene. By operating with excess ethylene to minimise the side reactions between C4s, the selectivity of both normal butenes (1-butene and 2-butene) to propylene can be maximised.
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