Interbed cooling

Adl b tic Converters. The adiabatic converter system employs heat exchangers rather than quench gas for interbed cooling (Fig. 7b). Because the beds are adiabatic, the temperature profile stiU exhibits the same sawtooth approach to the maximum reaction rate, but catalyst productivity is somewhat improved because all of the gas passes through the entire catalyst volume. Costs for vessels and exchangers are generally higher than for quench converter systems. 

Hydrogenation of benzene to cyclohexane was effected in a fixed bed reactor at 210-230°C, but a fall in conversion was apparent. Increasing the bed temperature by 10°C and the hydrogen flow led to a large increase in reaction rate which the interbed cooling coils could not handle, and an exotherm to 280°C developed, with a hot spot of around 600° C which bulged the reactor wall. 

The desulfurized feedstock is then mixed with superheated steam and passed over a nickel catalyst (730 to 845°C 1350 to 1550°F 400 psi) to produce a mixture of hydrogen, carbon monoxide, and carbon dioxide as well as excess steam. The effluent gases are cooled (to about 370°C 700°F) and passed through a shift converter which promotes reaction of the carbon monoxide with stream to yield carbon dioxide and more hydrogen. The shift converter may contain two beds of catalyst with interbed cooling the combination of the two catalyst beds promotes maximum conversion of the carbon monoxide. This is essential in the event that a high-purity product is required. 

Production of cumene from benzene and propylene using a phosphoric acid on quartz catalyst (Fig. 19-22c). There are four reactor beds with interbed cooling with cold feed. The reactor operates at 260°C. 

In most applications of trickle-flow reactors, the conversions generate heat that causes a temperature rise of the reactants, since the industrial reactors are generally operated adiabatically. In the cocurrent mode of operation, both the gas and the liquid rise in temperature as they accumulate heat, so there is a significant temperature profile in the axial direction, with the highest temperature at the exit end. When the total adiabatic temperature rise exceeds the allowable temperature span for the reaction, the total catalyst volume is generally split up between several adiabatic beds, with interbed cooling of the reactants. In the countercurrent mode of operation, heat is transported by gas and liquid in both directions, rather than in one direction only, and this may increase the possibility of obtaining a more desirable temperature profile over the reactor. 

In a similar way, the mechnical reactor configuration can be specified, or built in specifications for various types of reactors can be sued. With the sued of optional data groups it is possible to specify interbed cooling, to include calculation of lower (feed effluent) heat exchanger, to carry out an optimization of inlet temperature(s) to the catalyst bed(s), to calculate required catalyst volume to obtain a specified conversion, etc. As a further important feature it is possible to include -by specifying an optional data group- calculation of diffusion restrictions in the catalyst particles and to determine the effectiveness factor at each point in the catalyst bed. 

After integration of the catalyst bed(s) - if so desired after interbed cooling indirectly or by quench and calculation of lower heat exchanger - the sequence may be repeated in order to determine optimum operating temperatures, when the desired calculation has been performed, the results are edited and printed in a report. 

Adiabatic beds One or more packed beds in series or parallel—gas or gas/liquid feed. Adiabatic temperature increase with interbed cooling to control selectivity. 

Operating problems with palladium eatalysts have been associated with increasingly high volumes of acetylene in the process gas, whieh is a result of increased steam cracking severity to improve ethylene yields. Both front-end and tail-end reactors now include several adiabatic beds, with interbed cooling, to control reaction and remove the excessive heat evolved as acetylene is hydrogenated. 

The catalyst is normally divided into several beds, with interbed cooling, to limit the temperatnre rise in each bed to less than 15-25°C and to control the selectivity of the reaction. Temperatnre increases by 40°C and 75°C for every 1.0% of acetylene converted to ethylene or ethane, respectively, and the increase for converting 1.0% MAPD to propylene is 40°C. The number of catalyst beds needed in a reactor can easily be estimated. Reaction is readily controlled by variation of the bed inlet temperature. 

Note Steam added to propylene/air mixture with interbed cooling. 

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