Influence of Cooling Design on Fixed-Bed Reactors Dynamics

BUCALA, VERONICA; BORIO, DANIEL; ROMAGNOLI, JOSÉ; PORRAS, JOSE

AICHE JOURNAL

JOHN WILEY & SONS INC

Lugar: New York; Año: 1992 vol. 38 p. 1990 - 1994

0001-1541

Tubular fixed-bed reactors of the heat-exchanger type are selected commonly in industrial plants to carry out highly exothermic reactions. Due to the important heat effects in- volved, these units usually exhibit the well-known problems of a pronounced maximum in the axial temperature profile (hot spot), combined with high parametric sensitivity. Ray (1972) and Jorgensen (1986) reviewed publications re- lated to the dynamics and control of these types of reactors. Except for autothermal reactors, which will not be included in the present discussion, all their references assumed constant- coolant temperature. However, this is a strong hypothesis, valid only when boiling or perfectly mixed fluids are used at the shell side. The effect of different cooling designs on the dynamic behavior of a wall-cooled catalytic reactor has been analyzed only by Gatica et al. (1989). These authors analyzed the response of axial temperature and concentration profiles to changes in the reactant?s inlet conditions for the cocurrent design. Nevertheless, more critical disturbances are changes at the inlet coolant temperature which is often used as an indirect manipulated variable for control purposes. Therefore, the in- fluence of this variable on the reactor dynamics will be studied in the present work. Recent steady-state simulation results have shown that the behavior of this type of reactors depends strongly on the mutual direction of the reacting fluid and coolant streams. In a recent article, three basic cooling schemes (countercurrent, cocurrent, and perfectly-mixed coolant) were analyzed (Borio et al., 1989a). Their conclusion was that for equivalent production rates, the steady-state cocurrent operation yields the lowest values for both maximum temperature and parametric sensi- tivity. In a second contribution, the authors defined the optimal cocurrent coolant flow rate leading to conditions of maximum attainable safety and developed a simple expression which en- ables to predict its value (Borio et al., 1989b). As optimal operation conditions cannot be defined only on the basis of steady-state analysis, this work complements the above-mentioned works by means of a similar development, performed under nonsteady-state conditions. The underlying idea is that to be confirmed as the best choice, the optimal design found from steady-state analysis should demonstrate also to exhibit an acceptable dynamic performance