Ethene is an important commodity chemical and monomer which has traditionally been produced industrially by the steam or catalytic cracking of hydrocarbons derived from crude oil.  However as reserves of crude oil diminish and prices rise there becomes an increasing need to find alternative economically viable methods of making this product.  By virtue of its ready availability from the fermentation of biomass and synthesis gas based technologies, ethanol is emerging as an important potential feedstock from which ethene can be made in the future. 
The production of ethene by the vapour phase chemical dehydration of ethanol is a well known chemical reaction which has been operated industrially for many years (see for example Kirk Othmer Encyclopaedia of Chemical Technology (third edition), Volume 9, pages 411-413).  Traditionally this reaction has been carried out at elevated temperature and low pressure in the presence of an acid catalyst such as activated alumina or supported phosphoric acid. 
In recent years attention has turned to finding alternative new catalysts having improved performance.  Our co-pending European patent application 06255980.2 discloses such an approach by providing a process involving the use of a supported heteropolyacid catalyst satisfying the relationship:PV>0.6−0.3×[HPA loading/Surface Area of Catalyst]wherein PV is the pore volume of the dried supported heteropolyacid catalyst (measured in ml/g catalyst); HPA loading is the amount of heteropolyacid present in the dried supported heteropolyacid catalyst (measured in micro moles/g catalyst) and Surface Area of Catalyst is the surface area of the dried supported heteropolyacid catalyst (measured in m2/g catalyst).  The process described in this patent application is carried out at elevated temperature typically between 180 and 250° C. The catalysts described show higher ethene productivity and lower ethane make than those of the prior art.  This is desirable because firstly ethane is an undesirable by-product and secondly its separation from ethene on a large scale is both difficult and energy intensive.  Our European patent application EP 1792885 and PCT application WO 2007/03899 disclose preferred modes of carrying out the process. 
In processes of the type described in this patent application a feed comprising ethanol, optionally water and other components are continuously fed to a reactor containing a bed of heteropolyacid catalyst and the products continuously removed.  Under steady state conditions the feed entering the reactor is rapidly converted near the inlet into an equilibrium mixture of water, ethanol and ethoxyethane (the product of a rapid first stage dehydration of the ethanol).  The catalyst therefore works in the presence of significant amounts of water the exact amount of which will depend on the equilibrium concentration described above (which itself is dependent upon the overall temperature and pressure of the reactor).  In fact the situation is even more complicated because as conversion of the feed occurs in the catalyst bed further water is generated.  So depending on the geometry of the reactor the water concentration in the catalyst bed can vary from significant (for example in a tubular fixed bed) to very little (for example in a fluid bed).  This variation in water concentration also depends on the temperature of the reactor and the contact time of the feed with the catalyst.