|The styrene monomer (SM) market is mature, with average global growth rates of only 3-3.5% pa, with world demand over 25 mln tpa, supplying feedstock for many important polymers and elastomers. For several years, the styrenics market has been faced with problems of poor profitability. A recently developed cheaper manufacturing process could help rescue the sector, as per ICIS. Styrene's largest derivative, polystyrene (PS), has been faced with torrid times, mainly due to higher prices, leading to shrinking demand and consumption. PS prices have been impacted by the high prices of feedstock benzene and styrene, resulting in users switching to other polymers and materials. As a result, margins in the styrenics chain have been thin, and even nonexistent.
Conventional styrene technology, a two step process, has been in use for some 70 years. First, benzene is alkylated with ethylene over a solid acid catalyst to make ethylbenzene (EB). In the next step, the EB is mixed with high-temperature steam at about 900�C (1,652�F) and passed over an iron oxide catalyst at a temperature in excess of 600�C. The catalyst dehydrogenates the EB to styrene. A problem with the conventional process is that it consumes a large amount of energy in the second EB dehydrogenation step. The reaction is thermodynamically limited and highly endothermic, resulting in a significant input of energy. Most developments in EB and styrene technologies in recent years have centered on process optimization, catalyst upgrades and equipment improvements. A more recent problem for conventional styrene manufacturers has been the high price of feedstocks. In particular, benzene prices in the past couple of years have been volatile and have reached record highs, making life difficult for styrene producers. Styrene can also be manufactured as a co-product with propylene oxide (PO) by the PO/SM process. In this process, EB is oxidized to its hydroperoxide, which is next reacted with propylene to produce PO and methyl benzyl alcohol. The latter product is then dehydrated to styrene.
A cheaper manufacturing process - ExSyM, is under development by a research organization Exelus. Adoption of this technology depends upon the successful testing and commercial demonstration of the pilot plant. The process employs toluene and methanol feedstocks to reduce operating costs while the one-step process with relatively mild operating conditions leads to lower capital costs. The economic driver is the lower cost of toluene and methanol compared to benzene and ethylene in conventional technology. An alternative styrene route is the side-chain alkylation of toluene, with methanol to yield styrene, hydrogen and water. Researchers have been attempting to develop this route for the past 30 years, but it has been difficult to devise a catalyst with the yield and selectivity to make a commercially viable process. One problem was that the methanol decomposed easily to hydrogen and carbon monoxide, leading to a low selectivity. In addition, the hydrogen could convert the styrene to EB, leading to low styrene yields. This by-product formation also made the styrene purification difficult. As a result, the maximum styrene yield achieved was 10%. A breakthrough to the higher selectivity of styrene has been achieved through a combination of catalyst science, reaction engineering and process design. The catalyst is a modified zeolite material containing basic active sites in a highly optimized pore structure. The active sites selectively adsorb toluene over methanol to limit methanol decomposition. The pore structure facilitates diffusion and residence time of the reactants to enhance toluene alkylation. Improvements to the reactor design have also concentrated on reducing methanol decomposition and enhancing conversion. Process enhancements have aimed to increase the SM/EB ratio of the product and maintain energy efficiency. The combined styrene and EB selectivity of more than 90% has been achieved at high methanol conversion with a SM/EB distribution of 85/15%. This gives a total styrene yield in excess of 60%, based on methanol. On a production scale, the EB produced could be sold to a conventional styrene producer or dehydrogenated on-site to increase styrene yields. The hydrogen co product is easily recovered and burned to provide much of the energy required to operate the process. These breakthroughs have allowed for the development of a simple, fixed-bed process. The reaction occurs at around 400-425�C at atmospheric pressure and there is no need to generate large amounts of steam, leading to claimed energy savings of up to 40%, compared with the conventional route. The milder reaction conditions and the elimination of the EB dehydrogenation unit are claimed to reduce capital costs significantly.
The investment costs for a 250,000 tpa styrene plant, based on the ExSyM process, is estimated at US$63 mln (�42.4 mln), compared with US$125 mln for a conventional unit. Using long-term average prices of US$750/ton for benzene, US$820/ton for ethylene, US$580/ton for toluene and US$315/ton for methanol, variable operating costs could be reduced by US$200/ton. The ExSyM process is that it is configured to resemble a conventional styrene plant, enabling existing units to be retrofitted. Revamping a 250,000 tpa plant with the new process could cost in the region of US$10-15 mln, giving a quick payback time. The new process also reduces the amount of greenhouse gas emissions, in particular methane and carbon dioxide.