Application of silica membranes in separations and hybrid reactor systems

Abstract

In this Chapter the integration of separation and reaction in a membrane reactor is discussed. The use of a membrane reactor based on a silica pervaporation membrane has been studied for esterification reactions. The main purpose of the pervaporation membrane is to remove the water formed during the reaction, so that the equilibrium conversion can be exceeded. A general introduction on membranes is given with emphasis on separation performance and the integration with reactions. Furthermore, an outline of this thesis is presented. * A part of this chapter has been published in Separation and Purification Technology 22-23 (2001) 689-695, “Properties of high flux ceramic pervaporation membranes for dehydration of alcohol/water mixtures” by A.W. Verkerk, P. van Male, M.A.G. Vorstman and J.T.F. Keurentjes 2 Membrane reactors for pervaporation-assisted esterifications In this Chapter the main principles of combining a membrane separation process and an esterification reaction in a membrane reactor are discussed. In such reactions high temperatures and high pressures are often desirable to improve reaction kinetics. This implies that membranes have to be stable at these reaction conditions. Current membrane reactors predominantly use polymeric membranes, which have a limited stability at high temperatures. A promising alternative for these polymer membranes is the use of inorganic membranes. A number of different inorganic microporous membranes are currently available, including zeolite membranes, carbon molecular sieve membranes and silica membranes [Bein, 1996; Drioli and Romano, 2001]. Membrane reactors are being used for various purposes, ranging from controlled addition of reactants [Coronas and Santamaria, 1999], and localization of homogeneous catalysts [Nair et al., 2001], to the selective removal of one of the products. In this thesis the application of a membrane reactor in pervaporation-assisted esterification reactions is studied. The membrane reactor is used to exceed the equilibrium conversion by the selective removal of one of the reaction products. An inorganic membrane is used in combination with a mono-esterification and a di-esterification reaction, where the water produced during the reaction is selectively removed by a supported silica pervaporation membrane. Esterification reactions A class of industrially relevant equilibrium reactions are esterification reactions in which water is one of the products. Esters have various applications, ranging from plasticizers, surface-active agents, flavor and perfume materials, to solvents for the production of various chemicals. The annual production of these esters in the USA exceeded 5·10 tons in the year 1990 [Kirk-Othmer, 1994]. One of the main disadvantages of esterification reactions is that they suffer from a low conversion. In addition to the low conversion, the presence of a possible azeotrope between reactants and products also makes an esterification process more difficult to operate. A simplified reaction equation is given by: R(COOH)n + R(OH)m ! ester + H2O In practice, there are two methods to improve the conversion of equilibrium reactions. In the first approach, a large excess of one of the starting reagents is used. However, this results in a 3 Alcohols + carboxylic acids H2O dist. column relatively inefficient use of reactor space, and an efficient separation is required afterwards. In the second approach, the equilibrium conversion can be exceeded by removing one of the reaction products. For esterification reactions water is the most appropriate component that can be removed. There are a number of ways to remove one of the reaction products. Distillation is an appropriate technique for the removal of water from alcohols (see Figure 2a). However, for these systems the formation of an azeotrope is a potential drawback, which limits the (process) selectivity. Furthermore, in the case of the production of temperature-sensitive products or for biocatalytic conversions, the application of distillation is not feasible as a result of temperature constraints. Membrane separations can be considered a viable alternative for a number of cases. For the removal of water from organic streams, pervaporation seems to be the appropriate membrane technique. The main purpose of the pervaporation membrane is to remove the water from the reaction mixture in order the increase the product yield. In Figure 2b-d the various operation methods are shown. In this thesis the set-up depicted in Figure 2d has been chosen for the pervaporation-assisted reactions. Figure 2a. Conventional set-up for the production of esters.