MODEL OF THE PARTIAL OXIDATION OF METHANE TO METHANOL IN A GAS-SOLID-SOLID REACTOR
Multi-scale and/or multi-disciplinary approach to process-product innovation
CFD & Multiscale Modelling in Chemical Engineering (T3-4P)
Keywords: Methane, Methanol, multifunctional reactor, cfd, partial oxidation
The partial oxidation of methane to methanol has considerable potential for the utilization of vast natural gas fields in remote areas of the world. This process would not only be easier to of transport, but also increases the range of subsequent applications for further processing. The number of recent publications in this area reflects renewed interest in this reaction as an alternative to the two-stage steam reforming route to methanol [1-4]. The partial oxidation reaction is, potentially, a simpler and more energy-efficient process than the steam-reforming route, but in spite of the effort of investigators, no catalyst has been obtained to achieve desired levels of selectivity and conversion to take this process at industrial practice.
Bearing this in mind, this work focuses in the way to increase the conversion of reversible reactions in a special kind of multifunctional reactors [5-7]: the gas-solid-solid reactors. In this reactor, a mixture of gaseous reactants is fed at the bottom of the packed column. The solid packing contains catalyst pellets. Another solid material, a selective product adsorbent, is fed at the top of the column and trickles down over the packing. The solids stream adsorbs the product immediately after it has been released from the catalyst surface. The reaction product therefore leaves the reactor in the adsorbed state at the lower end of the reactor. The unconverted reactants leave the reactor at the top, together with the non-adsorbed fraction of the product formed. With such design the reaction rates are not hampered by a reversed reaction and remain high. In this way, higher conversions and reaction rates can be expected. It may be possible even to find out operation conditions so that almost complete conversion can be achieved, so that the recycling is no longer necessary. This necessarily will result in considerable investment and operating costs savings.
In this work this concept of reactor design is explored. A two dimensional steady-state model for the reactor is developed based on the mass and energy balance. The model was solved numerically through the finite difference method with software written in FORTRAN 90 with the NAG routines. The influence of the process parameters on the behavior of the reactor is discussed. The model was applied to the case of methanol synthesis directly from methane over ferric molybdate [8-9], with an amorphous silica-alumina powder as the methanol adsorbent [10]. The concentration of the adsorbed methanol was optimized at the exit of the reactor using as decision variables the feed relationship CH4/O2 and the supply rate of adsorbent. The results show that this reactor, in principle, must be cable to lead to high conversions of the feed gas to product in spite of unfavorable equilibrium.
On the basis of the results of this study, we conclude that a gas-solid-solid reactor offers a very attractive alternative to today's processes based on catalytic equilibrium reactions and deserves further evaluation in a pilot plant.
References
1 K. Otsuka, Y. Wang, Applied Catalysis A: General 222 (2001) 145
2 Q. Zhang et al, Fuel 81 (2002) 1599
3 T. Takemoto et al., Journal of Molecular Catalysis A: Chemical 179 (2002) 279
4 X. Wang et al., Journal of Catalysis 217 (2003) 457
5 K.R. Westerterp, Chemical Engineering Science 47 (1992) 2195
6 L.V. Barysheva et al., Chemical Engineering Journal 91 (2003) 219
7 N. Nikacevic and A. Dudukovic, Ind. Eng. Chem. Res. 44 (2005) 6509
8 A. Chellappa and D. Viswanath, Ind. Eng. Chem. Res. 34 (1995) 1933
9 A. Chellappa University of Missouri-Columbia, Doctoral Thesis (1997)
10 M. Kuczynski, A. van Ooteghem and K. R. Westerterp, Colloid Polymer Sci. 264 (1986) 362
Presented Tuesday 18, 13:30 to 15:00, in session CFD & Mutliscale Modelling in Chemical Engineering (T3-4P).
