High performance catalyst for liquid phase EB technology
The EBZ-500 catalyst has raised catalyst peformance, achieving not only greater purity in the ethylbenzene produced and, consequently, in styrene monomer, but also in extended reactor runs and favourable investment costs
Frederick Narsolis, Guy Woodle and Gregory Gajda, UOP
Dinesh Gandhi, ABB Lummus Global Inc
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In 1996 UOP and Lummus successfully commercialised a new zeolitic EBZ-500 catalyst for the alkylation of benzene with ethylene to produce ethylbenzene (EB). Ethylbenzene is the raw material used mainly to produce styrene monomer (SM). The new catalyst represents a major milestone in the two companies’ efforts to develop advanced process technology in order to provide customers with a clear advantage in the competitive EB-SM market.
The first liquid-phase EB unit, which used a zeolitic catalyst, was commercialised in 1990. This technology, which was developed through the joint efforts of Lummus and Unocal over a period of more than seven years, is now licensed as the Lummus/UOP liquid-phase EB process. Currently, four commercial plants using the process are in operation. Three more units with EBZ-500 catalyst are scheduled for startup in 1997.
Since the startup of the first plant, the EB technology has been further optimised and improved. The most significant improvements have been the introduction of new catalysts. The first-generation catalyst was UOC-4120, which was used in both the alkylation and transalkylation sections.
Second-generation designs included a new EBZ-100 transalkylation catalyst, which was commercialised in 1993. Latest designs incorporate the recently commercialised EBZ-500 catalyst, with which the EB process sets a new industry standard in yield, product quality, and catalyst stability.
The demand for EB is primarily driven by styrene demand, which itself is led by polystyrene, which represents 55–60 per cent of total consumption. The balance is used for the production of unsaturated polyester resin, styrene-butadiene rubber (SBR), and acrylonitrile-butadiene-styrene (ABS) resin.
The 1995 demand for styrene was 16.5 million metric tons/year (mta). This amount translates to an equivalent EB demand of 17.5 million mta. Overall, projected demand growth is about 4 per cent per year.
The United States, western Europe and the Asia-Pacific region dominate the world demand for styrene. At 9 per cent per year, demand in the Asia-Pacific region is projected to have the highest rate of growth – which is not surprising as most of the fastest-growing economies are in that region. In fact, most of the recent project awards have been in Asia-Pacific.
Most EB units are part of a styrene monomer complex, so EB has no significant merchant market. Much of the EB produced is based on AlCl3 catalyst, although many customers concede that they would choose a zeolitic catalyst for future plants in view of long-term problems with AlCl3.
The Lummus/UOP process produces EB from ethylene and benzene. Propylene and heavier olefinic material should be minimised in the feed because they produce impurities such as cumene and heavy aromatics in the product.
The reaction takes place in a liquid phase. The process has alkylation and transalkylation reactor sections (Figure 1). Polyethylbenzenes (PEBs) produced from minor side reactions in the alkylation section are recycled to the transalkylation section and reacted with benzene to produce more EB.
The alkylation and transalkylation effluents are fractionated into recycle benzene and PEB, EB product, and by-product flux oil streams using three distillation columns. A fourth column is used to remove a small amount of light ends, light nonaromatics, and water from the recycle stream.
Ethylbenzene is made by alkylating benzene with ethylene in the presence of zeolite catalyst (Figure 2). Successive alkylations occur, producing diethylbenzenes (DEB) and higher ethylated benzenes (Figure 3).
Other coupling reactions, which occur to a minor extent, yield materials such as butylbenzene, diphenylethane (DPE), and other high-boiling compounds. All the alkylation reactions are highly exothermic. The PEBs formed in the alkylation reactions are recovered in the PEB column of the fractionation section and recycled to the transalkylation reactor. The typical reaction is shown in Figure 4.
The benzene column recovers excess benzene from the reactor effluents. The recycle benzene stream for alkylation and transalkylation comes from the benzene column overhead. Bottoms from the benzene column is then fed to the EB column, where EB product is recovered overhead. The EB product is sent to an adjoining EB dehydrogenation unit or to storage.
Bottoms from the EB column is fed to the PEB column, where PEB is recovered overhead and recycled to the transalkylation reactor. The high boiling bottoms, flux oil, is used as a high-quality burner fuel.
Both the alkylation and transalkylation catalysts are regenerable. The performance of the regenerated catalyst equals that of the fresh catalyst. The regeneration cycle ranges from two to four years, based on customer needs.
A liquid-phase reaction has inherent advantages over a vapour-phase reaction. The liquid-phase process requires lower temperatures, resulting in lower xylenes production. The fluid content in liquid-phase reactors can absorb heat release fluctuations arising from feed or process variations. This heat-absorption ability minimises the possibility of rapid catalyst coking and deactivation. Additionally, a run-away reaction is unlikely to occur.
With EBZ-500 catalyst a new level of performance has been introduced to the market. The superior selectivity of the catalyst has led to higher EB purity and consequently higher SM purity. The EB purity is about 99.95wt% (Figure 5). Product impurities have been minimised. Dramatic reductions in cumene, xylenes, toluene, and nonaromatics have been observed in both pilot plant data and a commercial unit.
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