Adiabatic bed

Fig. 9. Tube-cooled converter temperature profile. A, adiabatic bed B, tube-cooled bed C, equiUbrium line and D, maximum rate line.

Bj A series of adiabatic beds with a decreasing temperature profile if exothermic... 

A low-pressure process has been developed by ICl operating at about 50 atm (700 psi) using a new active copper-based catalyst at 240°C. The synthesis reaction occurs over a bed of heterogeneous catalyst arranged in either sequential adiabatic beds or placed within heat transfer tubes. The reaction is limited by equilibrium, and methanol concentration at the converter s exit rarely exceeds 7%. The converter effluent is cooled to 40°C to condense product methanol, and the unreacted gases are recycled. Crude methanol from the separator contains water and low levels of by-products, which are removed using a two-column distillation system. Figure 5-5 shows the ICl methanol synthesis process. 

Nonisothermal reactors with adiabatic beds. Optimization of the temperature profile described above assumes that heat can be added or removed wherever required and at whatever rate required so that the optimal temperature profile can be achieved. A superstructure can be set up to examine design options involving adiabatic reaction sections. Figure 7.12 shows a superstructure for a reactor with adiabatic sections912 that allows heat to be transferred indirectly or directly through intermediate feed injection. 

Optimal adiabatic bed reactors for sulfur dioxide with cold shot cooling (with K.-Y. Lee). Ind. Eng. Chem. Proc. Des.-Dev. 2, 300-306 (1963). 

The resulting equations are linear in the unknown variables, so an analytical solution is easily obtained. For example, for the case with three adiabatic beds (NR = 3) in the reactor vessel, there are six equations and six unknowns (T ci r(2, 7 c3> Fj, Fos,2 and Fes,3) ... 

In ammonia synthesis, high temperatures correspond to small reactor volumes. For exothermic reactions, the equilibrium conversion Xe decreases as the temperature increases. Therefore, these reactions are often carried out in a series of adiabatic beds with either intermediate heat exchangers to cool the gases or bypass the cold feed to decrease the temperatures between the beds. Some compromise can be achieved between high temperatures involving small reactor volumes and high equilibrium conversions. 

Fig. 14A). A reasonable reaction pathway for a multistage adiabatic reaction can therefore be composed of the straight lines of an adiabatic bed followed by the vertical lines of indirect intermediate cooling (Fig. 14C).

The choice of a model to describe heat transfer in packed beds is one which has often been dictated by the requirement that the resulting model equations should be relatively easy to solve for the bed temperature profile. This consideration has led to the widespread use of the pseudo-homogeneous two-dimensional model, in which the tubular bed is modelled as though it consisted of one phase only. This phase is assumed to move in plug-flow, with superimposed axial and radial effective thermal conductivities, which are usually taken to be independent of the axial and radial spatial coordinates. In non-adiabatic beds, heat transfer from the wall is governed by an apparent wall heat transfer coefficient. ... 

The converter of the Japanese Consulting Institute [867], [1495] combines a catalyst bed with co-current cooling tubes and adiabatic beds (Section 4.5.3.3). An older Uhde design uses U-tubes for cooling a single bed, having co-current and counter-current flow [853]. 

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. 

Leitenberger s work (1939) is concerned with the oxidation of sulfur dioxide in adiabatic beds and tubular reactors. Using the Boresskow-Slinko kinetic equation he calculated the optimal tem-6... 

The broken lines are adiabatic paths given by Eq. (2). The equilibrium line Fe, on which r = 0, is shown and also the locus F , of maximum reaction rate within an adiabatic bed. If Cm,Tm is a point on this curve, it provides a natural origin on the adiabatic path through it. This path can then be graduated in the holding time... 

Proceeding now to consider two adiabatic beds we have to solve the equation... 

Design difficulties Catalyst separation sometimes difficult possible problems in pumps due to risks of deposits or erosion Fairly simple for a downward cocurrent adiabatic bed... 

The catalyst bed temperature increases in the direction of gas flow due to the WGS reaction exotherm. Typical temperature gradients in the bed are about 20-30°C. The lifetime and state of activity of the catalyst is conveniently monitored by the temperature profile through the adiabatic bed. As the reaction front moves through the bed when the catalyst ages, so does the temperature rise from the reaction (Fig. 3). 

This suggests a useful graphical presentation of the design of an adiabatic tubular reactor. In Fig. 8.8 the equilibrium line Fe and the curve of maximum reaction rate in an adiabatic bed, are shown. The broken lines are adiabatic paths. Let and be the extent and temperature at a point on Then the adiabatic path through such a point is... 

Fig. 8.17 Possible steady states for a catalyst particle in the adiabatic bed.

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