Theoretical Investigation of a Pd-Membrane Reactor for Methanol Synthesis By M.R.Rahimpour* and S.Ghader A numerical study is performed in order to evaluate the performance and optimal operating conditions of a palladium membranereactorformethanolsynthesis.AnovelreactorconfigurationwithaPdwall,whichisperm-selectivetohydrogen,has beenproposed.Inthisconfigurationthereactantsareaddedtothetubesidewhilepurehydrogenisaddedtotheshellside,asa result, the hydrogen diffuses across the membrane from the shell side to the tube side. In this membrane reactor, hydrogen penetrates to the reaction side in order to maintain a suitable hydrogen level in the whole length of the reactor and shift the equilibrium reaction. The effects of different parameters on the methanol output mole fraction were investigated in the co- currentmode.Theseparametersweremembranethickness,reactionsideflowrate,reactionsidepressure,shellsidepressureand H 2 =CO 2 ratio in the feed. 1 Introduction Membranes have been used in reactive applications to enhancetheyield/selectivityofreactions[1].Palladium-based membranes have been used for decades in hydrogen extrac- tion because of their high permeability and good surface properties and because palladium, like all metals, is 100% selective for hydrogen transport [2]. Palladium combines excellent hydrogen transport and discrimination properties with resistance to high temperatures, corrosion, and solvents. Further, palladium is easily formed into tubes that are simply fabricated into hydrogen extraction units [3,4] and palladium surfacesarenotreadilypoisonedbycarbonmonoxide,steam, and hydrocarbons [1,5,6]. These properties would make palladiummembranesveryattractiveforusewithpetrochem- ical gases. Methanolisanimportantchemicalproductthatisproduced from synthesis gas on a large scale worldwide. Synthesis gas consists of H 2 , CO 2 , CO and some inert components. Methanol conversion in a conventional fixed bed methanol reactor is low due to the equilibrium nature of the reaction. Therefore,mostofthesynthesisgasmustbecirculatedaround the loop and this poses problems in operating costs. The methanolsynthesisreactionisexothermicandthetotalmoles reduce as the reaction proceeds (see Appendix). The temperature and pressure of reaction are 495±535 K and 5±8 MPa, respectively. Many efforts have been made to increase methanol conversion by removal of products over a perm-selective perfluorinated cation exchange membrane [7,8]. Chemical reactants may be shifted to products due to thermodynamic equilibrium by removal of reactants from product gases. The synthesis gas composition from a steam-reforming reactorisessentiallypoorinH 2 ,suchthatadditionalH 2 could enhancethemethanolproduction.Onepotentiallyinteresting application of Pd-membrane in methanol synthesis involves the addition of pure hydrogen to the reacting gases, which is the subject of this work. However, there is no information available in the literature regarding the use of a Pd-membrane reactor for methanol synthesis. Therefore, it was decided to first study more thoroughly the phenomenon of methanol synthesis by numerical simulation. It is clear that there are several areas that require additional research on this subject. Key areas requiring investigation include the current system where product gases pass through the shell side counter-currently and hydrogen diffuses across the membrane to the reaction side. In this way the separation costs of the product gases are also avoided. Thereforethispaperfocusesonthenumericalinvestigation ofashellandtubePd-membranereactor,withpurehydrogen fed into the shell side and the reactant gases fed to the tube side. This reactor configuration differentially provides a suitable H 2 /CO 2 ratio along the reactor. In what follows, a novelmembranereactorsystemformethanolsynthesisisfirst briefly described. Subsequently, the reactor models are presentedanddiscussed.Thesemodelscanbeusedtoidentify membrane and process conditions under which the desired product yields can be significantly enhanced. For the sake of conciseness only a limited discussion of the basic model features is presented here. Numerical simulation is utilized to explore potential configurations for experimental methanol synthesis as well as to quantify the effects of key design parameters such as membrane thickness, reaction side feed flowrate,tubesideflowrate,reactionsidepressure,shellside pressure, and H 2 /CO 2 ratio in the feed. 2 The Reactor Configuration Fig. 1 shows the schematic diagram of a Pd-membrane reactor. The reactor is considered to be two concentric pipes 15 cm in length. The inner pipe is filled at the reaction side with 902 Ó 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/ceat.200301717 Chem. Eng. Technol. 26 (2003) 8 ± [*] Dr. M. R. Rahimpour (corresponding author, e-mail: rahimpor@shir- azu.ac.ir), S. Ghader, Department of Chemical Engineering, School of Engineering, Shiraz University, P.O.Box 71345, Shiraz, Iran. Full Paper