(64b) Development of Different Strategies of Refrigeration for Fixed Bed Catalytic Reactor: Mixed Coolant Flow as Optimal Factor
AIChE Spring Meeting and Global Congress on Process Safety
Monday, April 24, 2006 - 2:45pm to 3:25pm
Fixed Bed Catalytic Reactor have served as the workhorse of the chemical and petrochemical industries for many decades, and are still one of the most used gas-solid reactors in industry, in which heat-transfer plays an important role. The relatively simple construction and behaviour of this reactor make is still favored over other types of more advanced and complicated reactors. Therefore is a large incentive for the design of high performance reactors since this may lead to large production rate, almost total yield and selectivity while ensuring safe operation. The design of cooled tubular reactors, involves complex tradeoffs between tube geometry, pressure drop, and heat-transfer area. Thus the behaviour of chemical reactors depends on variations in the inlet conditions, as well as in other physical and chemical parameters of the system. With a suitable process representation it is possible to propose alternative configurations so that high performance can be achieved. This is the scope of this paper, which explores the benefits of both conventional cooling modes, namely co and counter-current, to design a new fixed bed catalytic reactor using both conventional modes of refrigeration simultaneously, so that it is possible to take advantage of the positive features of each cooling mode. As case studies, the catalytic oxidation of the ethanol to acetaldehyde, the catalytic oxidation of benzene to maleic anhydride and the Ethylene oxidation, were considered. These are strongly exothermic reactions, representatives of important classes of industrial processes. The proposal is to evaluate the potential of different cooling modes to obtain temperatures profiles with lower hot spot and minimum difference between the inlet and outlet reactor temperatures, allowing to keep high conversions and safe operation.
Coolant Configurations This work considers an alternate configuration for the circulation of the coolant fluid combining both concurrent and countercurrent cooling. The coolant flow is divided in two streams: one flowing co-currently while other flowing counter-currently (Figures 1 and 2). The objective is to evaluate the effect of this alternative design in the reactor performance and to compare it with the conventional cooling approaches.
1)The coolant stream flows in co-current mode in the interval from z = 0 to z = L1 and the flow is counter-current from z = L1 to z = L. This configuration is illustrated in Figure 1 and is called ?Alternative Configuration 1?.
2)The coolant stream flows in counter-current mode in the interval from z = 0 to z = L1 and the flow is co-current from z = L1 to z = L. This configuration is described in Figure 2 and is called ?Alternative Configuration 2?.
It should be noticed that at z = L1 the coolant stream changes direction in both schemes. Thus, the position of L1 varies for each analysis with the temperature profiles being obtained for each of such situations in both configurations. These two alternative designs have different effects on the reactor operation according to the selected L1 value. So the L1 parameter should be carefully selected. It should be pointed out that while the proposed Alternative Configuration 2 tends to be less efficient than Alternative Configuration 1, it may be useful for the case of process with side and consecutive reactions. The use of alternative coolant configurations increases the operational flexibility and it makes possible a more effective control of the reactor temperature profile. The proposed configurations take advantage of co-current cooling, near to the reactor entrance, and countercurrent operation towards the reactor exit.