Adiabatic reactors with periodic flow reversal

4 Adiabatic Reactors with Periodic Flow Reversal 

Exothermic equilibrium reactions like methanol or ammonia synthesis have the disadvantage that a low temperature is needed to reach a favorable high equUibrium conversion of the reactants. Conversely, a sufficiently high temperature is required with respect to kinetics to carry out the reaction at an acceptable rate. Unfortunately, the temperature increases towards the exit of the fixed bed due to the exothermicity of the reaction (if we do not use intensive cooling), which additionally lowers the obtainable equilibrium conversion. Thus, the temperature profile is exactly the wrong way round, and the feed has to be preheated and the product stream has to be cooled, usually by feed-effluent heat exchangers. In addition, heat has to be removed between reaction stages, if the reaction temperature increases too much. 

Feeding a hot catalyst bed with relatively cold gas will cool the inlet side of the bed on the other hand, the temperature at the exit of the bed will increase due to the heat produced by the reaction. By reversing the direction of flow the heat contained in the catalyst bed will bring the cold inlet stream to reaction temperature. The part of the bed that has now become the ouflet zone is relatively cold, which is favorable for the reaction equiUbrium. After some time the inlet has cooled again, while the outlet has become warmer. Then the flow is reversed again and a new 

As only a sufficiently high temperature is maintained in the middle part of the reactor, part of the bed can consist of inert material, which should have a high heat capacity with regard to heat storage and heat release and a large particle diameter to lower the pressure drop. 

The adiabatic fixed-bed reactor with periodic flow reversal has three commercial applications, oxidation of SO2 for sulfuric acid production, oxidation of volatile organic compounds (VOCs) for purification of industrial exhaust gases, and NO, reduction by ammonia in industrial exhaust gases. Other possible future applications are steam reforming and partial oxidation of methane for syngas production, synthesis of methanol and ammonia, and catalytic dehydrogenations (Matros and Bunimovich, 1996). 

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Novel developments in reactor technology are multifunctional reactors, which couple different processes such as reaction and separation by membranes, adsorption, or distillation, catalytic or reactive distillation, monolithic reactors, microreactors, and adiabatic reactors with periodic flow reversal. 

Beld, L. Van de, Wagenaar, B.M., Prins, W., Cleaning of hot producer gas in a catalytic adiabatic packed bed reactor with periodic flow reversal, in Developments in thermochemical biomass conversion, eds A.V. Bridgwater and D.G.B. Boocock, Banff 1996, pp. 907 - 920,... 

Beld, L. Van de, Air purification by catalytic oxidation in an adiabatic packed bed reactor with periodic flow reversal, PhD thesis. University Twente, 1995. 

L. van de Beld. Air Purification by Catalytic Osidation in an Adiabatic Packed Bed Reactor with Periodic Flow Reversal. Ph. D. Thesis. University Twente 1995)... 

An alternative is the adiabatic fixed-bed reactor with periodic flow reversal (Borekov, Matros, and Kiselev, 1979 Matros, 1985, 1989 Matros and Bunimovich, 1996). Figure 4.10.82 shows the principle of such a system. The main idea is to utilize the heat of reaction within the catalyst bed itself. 

Figure4.10.82 Principle of an adiabatic fixed-bed reactor with periodic flow reversal (a) first half of the cycle (b) second half of the cycle. Adapted from Moulijn, Makkee, and Van Diepen (2004).

A completely different design of an autothermal reactor has been proposed and developed by Boreskov and Matros [41-43], Fig. 24A gives a sketch of the basic concept, showing an adiabatic fixed bed reactor with fecd/cxit tubes and two valves for periodic flow reversal. Before start of operation the fixed bed has again to be heated above the ignition temperature of the catalytic reaction. If the reactor is then fed with cold feed from one side the cold feed gas is heated up... 

Figure 27. Methanol synthesis under autothermal operation (reverse flow reactor) A. B) Temperature and conversion pr< period C) Reaction path for reverse flow operation and for a two-stage adiabatic reactor with interstage cooling [46]...

Fig. 13. Comparison of simulated and experimental temperature profiles in a 2-m, near-adiabatic, packed-bed S02 reactor using a Chinese S101 catalyst and operating under periodic reversal of flow direction with r = 180 min, SV = 477 h"1, and inlet S02 = 3.89 vol% and T = 25°C. (Figure adapted from Wu et at., 1996, with permission of the authors.)...

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