Enhancement of Methanol Production in a Membrane Dual-Type Reactor

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  • Research Article

    Enhancement of Methanol Production in aMembrane Dual-Type Reactor

    In this study, a dynamic model for a membrane dual-type methanol reactor wasdeveloped in the presence of long term catalyst deactivation. The proposed modelis used to compare the performance of a membrane dual-type methanol reactorwith a conventional dual-type methanol reactor. A conventional dual-typemethanol reactor is a shell and tube heat exchanger reactor in which the first reac-tor is cooled with cooling water and the second one is cooled with synthesis gas.In a membrane dual-type reactor, the wall of the tubes in the gas-cooled conven-tional reactor is covered with a palladium-silver membrane, which is only perme-able to hydrogen. Hydrogen can penetrate from the feed synthesis gas side intothe reaction side due to the hydrogen partial pressure driving force. Hydrogenpermeation through the membrane shifts the reaction towards the product sideaccording to the thermodynamic equilibrium. The proposed dynamic model wasvalidated against measured daily process data of a methanol plant recorded for aperiod of four years and a good agreement was achieved. The simulation resultsshow that there is a favorable profile of temperature and activity of the membranedual-type reactor relative to single and conventional dual-type reactor systems.Therefore, the performance of methanol reactor systems improves when a mem-brane is used in a conventional dual-type methanol reactor.

    Keywords: Catalysts, Dynamic models, Membrane reactors

    Received: March 21; 2007; revised: April 22; 2007 accepted: April 27, 2007

    DOI: 10.1002/ceat.200700114

    1 Introduction

    In the methanol synthesis process, synthesis gas (CO2, CO andH2) converts to the methanol in a tubular reactor. The factorsaffecting the production rate in an industrial methanol reactorare parameters such as thermodynamic equilibrium limita-tions and catalyst deactivation. In the case of reversibleexothermic reactions, such as methanol synthesis, selection ofa relatively low temperature permits higher conversion, butthis must be balanced against a slower rate of reaction, whichleads to the requirement of a large amount of catalyst. Up tothe maximum production rate point, increasing temperatureimproves the rate of reaction, which leads to more methanolproduction. Nevertheless, as the temperature increases beyondthis point, the deteriorating effect of equilibrium conversionemerges and decreases methanol production [1]. Thereforeone of the important key issues in methanol reactor configura-

    tion is implementing a higher temperature at the entrance ofthe reactor for a higher reaction rate, and then reducing tem-perature gradually towards the exit from of reactor for increas-ing thermodynamic equilibrium conversion.Recently, a dual-type reactor system instead of a single-type

    reactor was developed for methanol synthesis. The dual-typemethanol reactor is an advanced technology for convertingnatural gas to methanol at low cost and in large quantities.This reactor configuration permits a high temperature in thefirst reactor and a low temperature in the second reactor. Inthis system, the water-cooled reactor is combined in series witha synthesis gas-cooled reactor. The first reactor, the isothermalreactor, accomplishes partial conversion of the synthesis gas tomethanol at higher space velocities and higher temperaturescompared with the single-type reactor. In this dual-type sys-tem, hydrogen is withdrawn from the methanol synthesispurge stream by a pressure swing adsorption (PSA) unit and isrecycled to the reactor in order to control the stoichiometricnumber and to prevent the wasting of hydrogen. In the reac-tion system, the addition of hydrogen to the reacting gas selec-tively leads to a shift of the chemical equilibrium towards theproduct side, resulting in a higher conversion of synthesis gasto methanol [2].

    2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com

    Mohammad Reza

    Rahimpour1

    Mansooreh Lotfinejad1

    1Department of Chemical andPetroleum Engineering, Schoolof Engineering, ShirazUniversity, Shiraz, Iran.

    Correspondence: Associate Professor Dr. M. R. Rahimpour (rahimpor@shirazu.ac.ir), Department of Chemical and Petroleum Engineering,School of Engineering, Shiraz University, P.O. Box 71345, Shiraz, Iran.

    1062 Chem. Eng. Technol. 2007, 30, No. 8, 10621076

  • One of the key issues of the dual-type methanol reactor con-figuration is the addition of H2 to the reacting gas using amembrane reactor [2]. This leads to higher CO conversion rel-ative to CO2 conversion so that a little water is produced dur-ing the methanol synthesis [2]. It should be noted that waterproduced during methanol synthesis by CO2 hydrogenationgreatly reduces the methanol synthesis rate by suppressing thegas shift reaction [3]. Water produced during methanol syn-thesis from CO2 conversion accelerated the crystallization ofCu and ZnO contained in a CuO/ZnO/Al2O3 catalyst, leadingto its deactivation [3, 4].The main advantages of a membrane dual-type methanol re-

    actor are: simultaneous methanol synthesis reaction and diffu-sion of reactant, the possibility of overcoming the limitationimposed by thermodynamic equilibrium [2], enhancement ofkinetics-limited reactions in the first reactor due to the higherfeed temperature, enhancement of equilibrium-limited reac-tions in the second reactor due to a lower temperature, andstoichiometric control of reacting gases in the reactor. A mem-brane reactor is a system or device which combines the chemi-cal reaction and membrane in one system [5].The application of membrane reaction technology in chemi-

    cal reaction processes are now mainly focused on reaction sys-tems containing hydrogen and oxygen, and are based on inor-ganic membranes such as Pd and ceramic membranes [5]. Inmany hydrogen-related reaction systems, Pd-alloy membraneson a stainless steel support were used as the hydrogen-perme-able membrane [6]. It is also well known that the use of purepalladium membranes is hindered by the fact that palladiumshows a transition from the a-phase (hydrogen-poor) to the b-phase (hydrogen-rich) at temperatures below 300 C and pres-sures below 2 MPa, depending on the hydrogen concentrationin the metal. Since the lattice constant of the b-phase is 3%larger than that of the a-phase, this transition leads to latticestrain and, consequently, after a few cycles, to a distortion ofthe metal lattice [7]. Alloying the palladium, especially withsilver, reduces the critical temperature for this embittermentand leads to an increase in the hydrogen permeability. Thehighest hydrogen permeability was observed at an alloy com-position of 23 wt% silver [8]. Palladium-based membraneshave been used for decades in hydrogen extraction because oftheir high permeability and good surface properties and be-cause palladium, like all metals, is 100% selective for hydrogentransport [9]. These membranes combine excellent hydrogentransport and discrimination properties with resistance to hightemperatures, corrosion, and solvents. Key requirements forthe successful development of palladium-based membranes arelow costs as well as permselectivity combined with good me-chanical, thermal and long-term stability [10]. These proper-ties make palladium-based membranes such as Pd-Ag mem-branes very attractive for use with petrochemical gases.A thin palladium or palladium-based alloy layer is prepared

    on the surface or inside the pores of porous supports. Manyresearchers have developed supporting structures for palla-dium or palladium-based alloy membranes. The materials incommercial use for porous supports are: ceramics, stainlesssteel and glass. The membrane support should be porous,smooth-faced, highly permeable, thermally stable and metaladhesive [11].

    There are a few investigations on methanol synthesis in Pd-Ag membrane-type methanol reactors [2, 8]. However, there isno information available in the literature regarding the use ofa Pd-membrane in an industrial methanol reactor. Therefore,it was decided to first study this system. The main goals of thiswork are the improvement of methanol production and the re-duction of catalyst deactivation in dual-type methanol reac-tors. In this new system, the walls of tubes in the second reac-tor are coated with a hydrogen permselective membrane. Thehydrogen partial pressure gradient is the driving force for hy-drogen permeation from feed synthesis gas to the reacting gas.The advantages of this concept will be discussed based on tem-perature and concentration profiles as well as catalyst activityprofiles along the reactors. The results are compared with theperformance of single and conventional dual-type type metha-nol reactors. This comparison shows that the production rateof membrane dual-type methanol reactors is greater than sin-gle and conventional dual-type methanol reactors. Also, theprofile of catalyst activity along the membrane dual-type reac-tor system shows that the catalyst activity along the second re-actor of the membrane system is maintained at a higher levelrelative to the second reactor of the conventional system andthis leads to a longer catalyst lifetime in membrane dual-typereactor.

    2 Process Description

    2.1 Single-Type Methanol Reactor

    Fig. 1 shows the schematic diagram of a single-type methanolreactor. A single-type methanol reactor is basically a verticalshell and tube heat exchanger. The catalyst is packed in verticaltubes and surrounded by the boiling water. The methanol syn-thesis reactions are carried out over a commercial CuO/ZnO/Al2O3 catalyst. The heat of exothermic reactions is transferredto the boiling water and steam is produced.The technical design data of the catalyst pellet and the input

    data of the single-type methanol reactor have been summa-rized in Tabs. 1 and 2.

    2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com

    Table 1. Catalyst and reactor specifications for the single-typemethanol reactor.

    Parameter Value Unit

    qs 1770 [kgm3]

    dp 5.47 103 [m]

    cps 5.0 [kJ kg1 K1]

    kc 0.004 [Wm1K1]

    av 626.98 [m2m3]

    ess

    0.123 []

    Number of tubes 2962 []

    Tube length 7.022 [m]

    Chem. Eng. Technol. 2007, 30, No. 8, 10621076 Membrane reactors 1063

  • 2.2 Conventional Dual-Type Methanol Reactor

    Fig. 2 shows the schematic diagram of a conventionaldual-type methanol reactor. This system is mainlybased on the two-stage reactor system consisting of awater-cooled and a gas-cooled reactor. The synthesisgas is fed to the tube of the gas-cooled reactor (secondreactor). The cold feed synthesis gas for the first reac-tor is routed through tubes in the second reactor in acounter-current flow with the reacting gas and thenheated by the heat of reaction produced in the shell.Therefore, the reacting gas temperature is continuouslyreduced over the reaction path in the second reactor.The outlet synthesis gas from the second reactor is fedto tubes of the first reactor (water-cooled) and thechemical reaction is initiated by the catalyst. The heatof reaction is transferred to the cooling water insidethe shell of reactor. In this stage, the synthesis gas ispartly converted to methanol in a water-cooled single-type reactor.The methanol-containing gas leaving the first reac-

    tor is directed into the shell of the second reactor. Fi-nally, the product is removed from the downstream ofthe second reactor. The large inlet gas preheater nor-mally required for synthesis by a single-type water-cooled reactor is replaced by a relatively small trimpreheater. As fresh synthesis gas is only fed to the firstreactor, no catalyst poisons reach the second reactor.The poison-free operation and the low operating tem-perature results in a virtually unlimited catalyst servicelife for the gas-cooled reactor. In addition, reactioncontrol also prolongs the service life of the catalyst inthe water-cooled reactor. If the methanol yields in thewater-cooled reactor decrease as a result of decliningcatalyst activity, the temperature in the inlet section ofthe gas-cooled reactor will rise with a resulting im-provement in the reaction kinetics and, hence, an in-creased yield in the second reactor.The technical design data of the catalyst pellet and in-

    put data of the conventional dual-typemethanol reactorhave been summarized in Tabs. 3 and 4.

    2.3 Membrane Dual-Type Methanol Reactor

    Fig. 3 shows the schematic diagram of a membrane dual-typereactor configuration for methanol synthesis. The methanolsynthesis process in the membrane dual-type methanol reactoris similar to that in the conventional dual-type methanol reac-tor, with the exception that in the membrane system the wallsof tubes in the second reactor (gas-cooled) consist of hydrogenpermselective membrane. The pressure difference between theshell (71.2 bars) and tube (76.98 bars) in conventional dual-type reactor permits diffusion of hydrogen through the Pd-Agmembrane layer. On the other hand, in the new system, themass and heat transfer process simultaneously occurs betweenshell and tube, while in the conventional reactor only a heattransfer process occurs between them.

    2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com

    Figure 1. A schematic diagram of a single-type methanol reactor of plant I.

    Figure 2. Schematic diagram of a conventional dual-type methanol reactor.

    Table 2. Input data for the single-type methanol reactor.

    Feed conditions Value Unit

    Composition [mol.-%]:

    CH3OH 0.50 []

    CO2 9.40 []

    CO 4.60 []

    H2O 0.04 []

    H2 65.90 []

    N2 9.30 []

    CH4 10.26 []

    Total molar flow rate per tube 0.64 [mol s1]

    Inlet temperature 503 [K]

    Pressure 76.98 [bar]

    1064 M. R. Rahimpour et al. Chem. Eng. Technol. 2007, 30, No. 8, 10621076

  • The present simulation study was based on a Pd/Aglayer thickness of 0.8 lm. In this study all specifica-tions for the first and second reactors in the membranedual-type system are the same as those of the industrialmethanol reactor listed in Tabs. 3 and 4.

    3 Mathematical Model

    The mathematical model for the simulation of mem-brane dual-type methanol reactors was developedbased on the following assumptions: (1) one-dimen-sional plug flow in shell and tube sides; (2) axial dis-persion of heat is negligible compared to convection;(3) gases are ideal. An element of length Dz as depictedin Fig. 4 was considered. The differential equations de-scribing mass and heat transfer in the axial directionare described in the following subsections.

    3.1 Water-Cooled Reactor (First Reactor)

    The mass and energy balance for the solid phase (catalyst sur-face) are expressed by:

    esctyist kgiyi yis g riqBa i = 1,2,...,N1 (1)

    qBcpsdTsdt

    avhf T Ts qBaNi1

    g riDHf i (2)

    where, yis and Ts are, respectively, the mole fraction and tem-perature of the solid-phase (catalyst surface), and i representsH2, CO2, CO, CH3OH, and H2O. Argon and methane are inertcomponents. The following two conservation equations arewritten for the fluid phase:

    eBctyit

    FtAc

    yiz avctkgiyis yi i = 1,2,...,N-1 (3)

    2007 WILEY-VCH Verlag GmbH & C...

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