Ciência/Science MODELING OF PARTIAL OXIDATION OF METHANE IN A MEMBRANE REACTOR F. A. N. Fernandesa, ABSTRACT b C. P. Souza Partial oxidation of methane is one of the most important chemical processes b for the production of syngas. In recent years, the abundant availability of and J. F. Sousa natural gas and the increasing demand of hydrogen have led to high interest to further develop this process increasing the yield of syngas. In this work the a partial oxidation of methane was studied from a modeling point of view in a Universidade Federal do Ceará membrane reactor and in a conventional reactor. A mathematical model of a Departamento de Engenharia Química membrane reactor used for partial oxidation of methane, assuming steady- Campus do Pici, Bloco 709 state conditions, was developed to simulate and compare the maximum yields and operating conditions in the reactor with that in a conventional reactor. 60455-760 Fortaleza, CE, Brasil Simulation results show that different parameters affect methane conversion [email protected] and H2/CO ratio, such as temperature, operating conditions, and membrane bUniversidade Federal do Rio Grande do Norte parameters such as membrane permeance. In a membrane reactor an increase Departamento de Engenharia Química in the operating pressure corresponds to an increase in methane conversion, since allows for a greater partial pressure gradient between the reaction and Campus Universitário permeate zone, thus contributing to shift the equilibrium towards the products. Natal, RN, Brasil As such, the membrane reactors are a good alternative to produce syngas especially for GTL processes. Operating conditions can be set to control the H2/CO ratio to a desired value, and high conversions at mild temperatures can be achieved reducing capital and operational costs. Keywords: membrane reactor, partial oxidation, methane, modeling NOMENCLATURE studied for the production of syngas (Ashcroft et al., 1990; Disanayake et al., 1991; Rajaput & Prabhakar, 1992; A tube sectional area [m2] Hickman & Schmidt, 1993). Partial oxidation of methane CP heat capacity [J/g.K] involves one non-reversible reaction (methane Fi flow rate of component i [mol/h] combustion) and two reversible reactions (steam reforming and carbon dioxide reforming). Methane ki rate constant of reaction i combustion is a very fast and highly exothermic reaction, Ki equilibrium constant of reaction i or adsorption coefficient of component i which releases a great quantity of energy demanding a P partial pressure [atm] good temperature control as not to compromise the reactor i and reaction tubes structure. So a membrane reactor offers P total pressure in the reaction zone [atm] T the possibility of supplying oxygen for the reaction R rate of reaction i [mol/h.g ] i cat throughout the entire reactor length controlling the r radius of the membrane [m] m combustion reaction in the reactor and spreading the heat rT radius of the reaction tube [m] release throughout the reactor not only at its beginning. T temperature [K] The development of oxygen permeable membranes uS superficial velocity [m/h] has opened up a new possibility to enhance the partial z distance from the inlet of the reaction tube [m] oxidation of methane process. New improvements are been done in membrane materials and structures, which Greek symbols supports the selective compound in porous alumina, porous ceramic substrate, and in nanostructured carbides, 3 b catalyst density [g/m ] and several works suggests the oxygen-permeating dense membranes have potential applications in partial oxidation Superscripts of methane (Balachandran et al., 1997; Tsai et al., 1997; Kao et al., 1997). AZ air flow zone The modeling and simulation of catalytic RZ reaction zone membrane reactors for methane conversion to syngas has been done by some authors, specially on steam reforming of methane and oxidative coupling of methane (Shu et al., 1994; Wang and Lin, 1995; Kao et al., 1997; Assaf et al., INTRODUCTION 1998; Chen et al., 2003; Lin et al., 2003; Gallucci et al., 2004). Very few works have dealt with partial oxidation of Partial oxidation of methane has attracted methane in membrane reactors (Tsai et al., 1997; Jin et al., substantial interest over the years for the production of 2000) and none dealing with the production of syngas for syngas and hydrogen. In recent years, the abundant gas-to-liquid (GTL) processes. availability of natural gas and the increasing demand of This work presents the mathematical modeling of a hydrogen have led to high interest to further develop this one-dimensional, isothermal membrane reactor operating process increasing the yield of syngas. at steady-state, comparing and discussing reactor and yield The partial oxidation reaction has been extensively improvements. Engenharia Térmica (Thermal Engineering), Vol. 5 - Nº 01 - July 2006 40 Ciência/Science Fernandes, Souza and Sousa et al. Modeling of Partial ... MEMBRANE REACTOR CH 4 + O2 ® CO2 + 2H 2O (1) CH + H O « CO + 3H (2) In partial oxidation of methane, the catalytic fixed- 4 2 2 CH + CO « 2CO + 2H (3) bed reactor is fed with a gas mixture of CH42 and O . 4 2 2 Commercial catalyst is composed of zirconium and platinum supported in alumina or nickel supported in The kinetic model for the reaction on a Ni/Al23O alumina and the reactor is composed of vertical tubes catalyst is based on a Langmuir-Hinshelwood reaction (between 10 and 900 tubes inside the reactor) with internal mechanism which rate expressions for reactions (1) to (3) diameters from 7 to 16 cm and lengths from 6 to 12 m, are given by: inserted in a radiant furnace chamber. The feed temperature is about 800C. The maximum temperature 0.5 k1 × PCH 4 × PO2 that the reactor can support is limited by the metallurgical R1 = 2 (4) limitations of the tubes, since at higher temperatures the (1+ KCH 4 × PCH 4 + KO2 × PO2 ) metal tubes can creep under stress. The membrane reactor configuration is quite æ P × P3 ö simple and consists of an external steel tube (shell) with an ç CO H 2 ÷ (5) R2 = k2 × PCH 4 × PH 2O ×ç1- ÷ inner membrane wall tube which contains the catalyst. Air è K2 × PCH 4 × PH 2O ø flows in the outer tube and oxygen permeates to the inner tube through the membrane. Methane and optionally O2 are continuously fed into the catalytic zone. A scheme of the æ P2 × P2 ö ç CO2 H 2 ÷ (6) reactor and mathematical model is shown in Fig. 1. R3 = k3 × PCH 4 × PCO2 ×ç1- ÷ è K3 × PCH 4 × PCO2 ø The changes on the components CH42, CO, H , CO2 and H2O along the reactor length are given by: dFi = rb × A×ån ij × R j (7) dz j The changes on the component O2 along the reactor length are given by: dF O2 = r × A×(- 2× R )+ 2p ×r × F (8) dz b 1 M M Figure 1. Scheme of the membrane reactor. Oxygen permeation was calculated based on the permeation of a La0.6Sr0.3Co0.2Fe0.8O3 membrane described by Jin et al. (2000) who correlated the permeation to MATHEMATICAL MODEL oxygen partial pressure and temperature. The basic assumptions made for the membrane F = (9.77×10-6 ×T +1.37×10-8 ×T 2 )× reactor were: M -(2.26×10-3 +1.62×10-5 ×T ) · Steady-state operation; é(P RZ ×10-5 ) - ù · Isothermal conditions; ê O2 ú (9) -3 -5 · Plug-flow on both reaction and air zones; ê AZ -5 -(2.26×10 +1.62×10 ×T ) ú ë(PO2 ×10 ) - 0.044û · No boundary layer on membrane surfaces; · Intrinsic kinetics for methane combustion, carbon dioxide reforming and steam reforming reactions; The kinetic parameters for the reaction are · Adsorption is the determinant step of the kinetic presented in Table 1. The simulated operating conditions process (Xu & Froment, 1989); are presented in Table 2. · The catalyst active sites have different adsorption capabilities for CH , O , H O and CO ; Table 1. Kinetic parameters for partial oxidation of 422 2 methane. Since the partial oxidation of methane is a mildly exothermic reaction and diluted feed is used, the Parameter Pre-Exponential Factor Ea or DH assumption of an isothermal reactor is reasonable (Jin et [J/mol] al., 2000). 1.5 k1 1.10 [mol/s.gcat.Pa ] 166000 Partial oxidation of methane involves one non- k -9 2 2 4.19 x10 [mol/s.gcat.Pa ] 29000 reversible reaction and two reversible reactions, which -9 2 k3 2.42 x10 [mol/s.g .Pa ] 23700 were thoroughly studied by Arai, Yamada and Eguchi cat K -4 -1 (1986), Ross & Steel (1973), Richardson and Paripatydar CH4 6.65 x10 [Pa ] 103500 K 5 -0.5 (1990) and Xu and Froment (1989). O2 1.77 x10 [Pa ] 66200 Engenharia Térmica (Thermal Engineering), Vol. 5 - Nº 01 - July 2006 41 Ciência/Science Fernandes, Souza and Sousa et al. Modeling of Partial ... Table 2. Operating conditions and reactor parameters. The production of carbon dioxide is considerable due to the combustion of methane. Steam and carbon Methane flow rate [mol/h] 5200.0 dioxide reforming are responsible for the formation of Total Pressure [atm] 1.0 syngas, which occurs only to a certain extent, and the 3 Catalyst Density [gcat/m ] 2355.2 reaction selectivity to carbon monoxide is low. Bed Porosity 0.30 A maximum production of hydrogen is obtained Reactor Length [m] 20.0 around a conversion of methane of 40 to 60%, depending Tube internal radius [m] 0.1016 on reaction temperature, after which the equilibrium of the Tube external radius [m] 0.1322 reactions (2) and (3) take a major shift towards the Membrane external radius [m] 0.1500 reactants reducing the amount of hydrogen and carbon Membrane thickness [m] 0.0150 monoxide in the reactor.