Les réacteurs à membranes : possibilités d'application dans l'industrie pétrolière et pétrochimique

Membrane Reactors: Possibilities of Application in the Petroleum and Petrochemical Industry

Ecole Polytechnique

Résumé

Cet article fait le point sur l'état de la recherche dans le domaine des réacteurs chimiques avec séparation par membrane intégrée et de leur applications dans le domaine du raffinage et de la pétrochimie. Trois applications potentiellement intéressantes sont identifiées et, pour chacune, les avantages de l'utilisation d'un réacteur à membrane sont discutés. Ce sont : la déshydrogénation du propane en propylène, la déshydrogénation d'un naphtène cyclohexanique et le vaporéformage du gaz naturel. Pour ces réactions, les membranes à base de palladium apparaissent les plus performantes compte tenu de leur tenue en température, de leur sélectivité et de leur perméabilité à l'hydrogène. Quelques éléments relatifs à leur développement sont présentés en conclusion.

Abstract

Recently, the use of membrane in reaction engineering has been more and more advocated. The selective separation of the products from the reaction mixture allows to achieve higher conversion or better selectivity or to operate under less severe conditions or with smaller units. This paper presents an update on the recent advances in the field of chemical membrane reactors and on their applications in refining and petrochemistry. Previous work. Most of the possible applications of membrane reactors in petroleum and petrochemical industry concern gaseous catalytic reactions. For this reason, gas permeation membranes are the primary component of membrane reactors. Gas permeation membranes present different types of physical structure : dense, microporous or asymmetric which is a combination of the two. Separating properties of dense membranes are function of the solubility and diffusivity of each gaseous component in the membrane material. For microporous membranes, they follow four mechanisms : Knudsen diffusion, surface diffusion, capillary condensation or molecular sieving. Although organic polymers are the common constituent of gas permeation membrane, their use is very limited in membrane reactors as they cannot withstand temperatures higher than 150°C. Metal, ceramic or glass membrane are preferred. Published work on membrane reactors is mainly concerned with dehydrogenation reactions and the in-situ separation of hydrogen. Dense palladium membranes or microporous inorganic membranes are used. A typical membrane reactor is presented in Fig. 1. The catalyst constitutes a fixed bed in the inside tube where dehydrogenation of cyclohexane into benzene takes place. Hydrogen produced by the reaction, permeates through the palladium wall. Carrier argon is used on the permeate side to lower the partial pressure of hydrogen and therefore increase the permeation rate. The main factors enhancing the equilibrium shift and therefore the conversion are presented in Table 1. Potential applications in the petroleum and petrochemical industry. Three potentially interesting applications are identified and the advantages of using a membrane reactor are discussed. They are : propane dehydrogenation into propylene, cyclohexanic naphthene dehydrogenation and natural gas steam reforming. For these chemical reactions, palladium based membranes show the best performance in terms of temperature resistance, hydrogen selectivity and permeability. The conversion of the dehydrogenation reaction of propane is increased by a higher temperature or a lower pressure as presented in Table 2. Selective draw-off of hydrogen from the reactor through a permeable wall increases the conversion from 48. 6% to 75. 5% (Table 3) or decreases the reaction temperature from 600 to 500°C (Table 4). Table 5 presents the effect of the selective draw-off of hydrogen on the conversion or the operating temperature for conditions found in industrial propane dehydrogenation processes. For a specified conversion, the use of a membrane reactor results in a lower operating temperature which reduces considerably catalyst coking. It allows also the use of common materials for the reactor walls which otherwise would facilitate secondary unwanted reactions. The membrane area necessary for a typical propylene production of 100 000 t/year is reasonable (from 50 to 312 m² as presented in Table 6) in the case of a dense palladium membrane but much too high in the case of a carbon molecular sieve. The discussion of the application of membrane reactors to the dehydrogenation of cyclohexane shows that the operating pressure in the reactor is critical. It should not be increased in order to build a high driving force across the membrane because it would then displace the reaction equilibrium towards the reactants, lowering therefore the equilibrium conversion. For this reason, palladium membranes are all the more attractive as the permeation driving force is proportional to the difference of the square roots of the partial pressures. Steam reforming of natural gas to produce hydrogen is a catalytic endothermic reaction of which conversion is increased by high temperature (Table 7). The traditional process includes the reformer itself, a second reactor to finish converting carbon monoxide and a pressure swing adsorption hydrogen separator. Fig. 2 presents the process if a membrane reactor is used. The effect of the in-situ membrane separation is quantified in Table 8 : for characteristic operating conditions, conversion increases from 0. 678 to 0. 916. In conclusion, some aspects of the development of membrane reactors are discussed. The necessity to develop resistant asymmetric composite membranes with a palladium deposit less than 1 µm is stressed out.