A continuing trend in propylene oxide production has been to develop and commercialise process routes that do not produce sizeable co-products or do not use chlorine-based chemistry, according to consultants Nexant ChemSystems. Propylene oxide (PO) is made traditionally by chlorohydrin and epoxidation routes but new technologies based on hydrogen peroxide or cumene hydroperoxide have been commercialised.
A significant amount of PO capacity is based on the older chlorohydrin process but this route suffers from environmental liabilities and has high capital costs. Propylene and chlorine in the presence of water are reacted to form propylene chlorohydrin, which is further reacted with sodium hydroxide or calcium hydroxide to obtain PO. These plants are often integrated with chlor-akali plants which consume a large amount of power to make the chlorine and caustic soda. Extensive effluent treatment is needed to handle the large dilute calcium chloride brine waste stream.
An epoxidation process that had been gaining in popularity due to its superior economics was the propylene oxide/styrene monomer (PO/SM) route. However, it has the potential disadvantage of coproducing 2.25 tonnes of styrene for every tonne of PO. This can present difficulties in balancing the markets for PO and styrene, leading to volatility in economic performance of the operations over time. Capital costs can also be relatively high. A number of PO/SM plants have been built by companies such as Repsol, Ellba (Shell/BASF) and LyondellBasell.
In the PO/SM process, ethylbenzene is first reacted with oxygen to make ethylbenzene hydroperoxide and then with propylene to form PO. The phenylmethylcarbinol co-product is dehydrated to styrene.
An alternative epoxidation route uses isobutane which makes a tertiary-butyl alcohol co-product that can be converted to methyl tertiary butyl ether (MTBE). LyondellBasell employs this epoxidation technology.
New PO technologies without co-products have now been developed. The first to be commercialised was a cumene hydroperoxidation technology employed by Sumitomo at a 200,000 tonne/year plant in Chiba, Japan. The process has later been used in a 200,000 tonne/year plant at its PetroRabigh joint venture with Saudi Aramco in Saudi Arabia.
The cumene hydroperoxide is obtained by the oxidation of cumene (made from benzene and propylene) with air. According to Nexant ChemSystems, on giving up oxygen to propylene, the cumene hydroperoxide is converted to cumyl alcohol (also referred to as dimethylbenzyl alcohol). The cumyl alcohol can be dehydrated to alphamethyl styrene which in turn can be hydrogenated back to cumene for recycle. According to a Sumitomo patent, the dehydration and hydrogenation steps can be combined into a single hydrogenolysis step.
A number of companies have been developing technologies to make PO from propylene and hydrogen peroxide. Nexant ChemSystems says the greatest attraction of the HPPO process is its apparent simplicity with a relatively straightforward reaction and few by-products. The co-product water can be treated normally and discharged. Although reaction conditions are mild, the presence of water and possibly alcohol as a solvent can result in product loss to secondary ring-opening reactions.
Evonik (formerly Degussa) and the German engineering contractor Uhde developed a HPPO process which was then licensed to SKC of South Korea. The first commercial-scale plant with 100,000 tonne/year capacity using the HPPO process started up at Ulsan in 2008.
Dow Chemical acquired its hydrogen peroxide technology when it purchased EniChem’s polyurethane business in 2001. Meanwhile, BASF had been exploring hydrogen peroxide-based routes to PO since the mid-1990s. The companies began collaborating in 2003 to develop a joint process which has been employed in a 300,000 tonne/year joint venture plant in Antwerp, Belgium, that started up in 2009. Dow with Siam Cement will also use the HPPO process in Thailand.
Source: ChemSystems PERP Program, Propylene Oxide, January 2009
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Propylene oxide (PO) is a highly reactive chemical used as an intermediate for the production of numerous commercial materials. The largest derivative of PO is polyether polyols, taking about 60-70% of PO demand. Polyether polyols are one of the main components used in the manufacture of polyurethanes (PUs), which are used in applications such as rigid foam installation and flexible foam seat cushions. World demand for polyols is growing about 5%/year.
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