Metathesis: refinery and ethylene plant applications
With the demand for propylene now exceeding that for C4s, the authors see the integration of the metathesis process in FCC units and steam crackers as a low capital cost way of increasing propylene production and reducing the C4s product
Ronald M Venner and Stephen I Kantorowicz, ABB Lummus Global
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The future of the olefins industry will be shaped by product flexibility, low capital investment and energy efficiency of larger capacity plants. As propylene demand continues to grow, the impact of integrating proprietary olefins conversion technology (OCT) in existing or grassroots ethylene and/or refinery applications becomes evident. OCT implementation results in improved overall economics through low capital investment, excellent energy efficiency and improved gross margins.
OCT provides an excellent vehicle for varying product slate to cope with fluctuating demand of downstream operations. For example, in ethylene applications, propylene-to-ethylene ratios can increase from the typical 0.5 ratio to greater than 1.0. In refinery applications, FCC fuel gas and excess butylenes are converted to propylene.
The demand for ethylene and propylene light olefins will increase at different rates. Global propylene demand is forecast to increase 6 to 8 per cent per year, which exceeds the global ethylene demand growth forecast of 4 to 6 per cent per year. The four commercially proven routes to propylene production are:
- Steam cracking
- Fluid catalytic cracking (FCC)
- Propane dehydrogenation
- Metathesis of ethylene and butylenes.
Some of the reasons for the dynamic and constantly fluctuating propylene and ethylene demand and pricing seen throughout various regions of the world include:
- NGL and crude prices
- Downstream product prices (particularly polyethylene and polypropylene)
- Butadiene demand
- Gasoline prices and plant operating rates.
As shown in Figure 1, the majority of propylene is currently produced in steam crackers and FCC units. In these processes, propylene is produced as a byproduct of ethylene production or transportation fuels. Historically, FCC units balanced the propylene demand fluctuations by varying severity. The other technology routes will start playing a greater role in fulfilling the increasing demand for propylene. Propylene via propane dehydrogenation is typically considered in areas where there is ample low-cost propane available as feedstock.
Metathesis is emerging as a low capital/low energy production option that can stand alone or be integrated with FCC units or steam crackers for improved flexibility and performance. With the demand for propylene outpacing the demand for C4s, the metathesis process offers the potential for significant improvement in a steam cracker’s or FCC unit’s operating margins by reducing C4 product and increasing propylene production.
Olefins conversion technology
Proprietary OCT converts normal butylenes and ethylene to polymer grade propylene via metathesis. The two main equilibrium reactions taking place are metathesis and isomerisation. Propylene is formed by the metathesis of ethylene and butene-2, and butene-1 is isomerised to butene-2 as butene-2 is consumed in the metathesis reaction.
In addition to the main reactions, numerous side reactions between olefins also occur.
Ethylene feed can be polymer grade ethylene or a dilute ethylene stream. Any saturated hydrocarbons, such as ethane and methane, do not react. The technology can be used with a variety of C4 streams, including mixed C4s produced by FCC or steam cracking or C4 raffinate from butadiene extraction or MTBE production. Based on the reaction stoichiometry, three tons of propylene are produced from two tons of butylene and one ton of ethylene.
Figure 2 is a simple process flow diagram of the Lummus OCT process. Fresh C4s (plus C4 recycle) are mixed with ethylene feed (plus recycle ethylene) and sent through a guard bed to remove trace impurities from the mixed feed. The feed is heated prior to entering the vapour phase fixed-bed metathesis reactor where the equilibrium reaction takes place. The reactor is regenerated in-situ on a regular basis.
As mentioned previously, the catalyst promotes the reaction of ethylene and butene-2 to form propylene and simultaneously isomerises butene-1 to butene-2. The per-pass conversion of butylene is greater than 60 per cent, with overall selectivity to propylene exceeding 90 per cent. The product from the metathesis reactor is primarily propylene and unreacted feed.
Reactor effluent is sent to the ethylene recovery tower where the unreacted ethylene is recovered and recycled to the reactor. The C2 tower bottoms is processed in the C3 tower to produce propylene product and a C4 recycle stream. Purge streams containing non-reactive light material and C4s and heavier are also produced.
Depending on the quantity of isobutylene in the C4 feed, the unit design may include a deisobutaniser to extend reactor run-length between regenerations and reduce OCT unit throughput, resulting in an overall lower capital cost plant. The deisobutaniser is a catalytic distillation tower that isomerises butene-1 to butene-2 (CDIsom) to maximise recovery of OCT feed. The deisobutaniser option is evaluated on a case-by-case basis. Ultra-high purity propylene exceeding polymer grade specification is produced without a propylene fractionation system, since the only source of propane is that contained in the C4 and ethylene feeds.
OCT was originally developed by Phillips Petroleum and was first commercialised in 1965. Due to the propylene demand at that time, this unit processed propylene to produce ethylene and butylenes. A second unit, which is still operating at Lyondell Petrochemical in the USA, was commissioned in 1985 to produce propylene.
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