Analysis of the operational characterisics and stablilty of multitubular reactors

Consideration has been given to various aspects of the behaviour of multitubular fixed bed catalytic reactors supporting highly exothermic reactions. In particular, the problem of adequately representing the detailed characteristics of such reactors has been investigated, and mathematical models describing the steady state and dynamic behaviour of both co- and counter-currently cooled systems have been introduced as a basis for design and control studies. Valuable insight into the operational characteristics of large multitubular reactors has been obtained by investigating the phenomena related to the interactive heat transfer within the system, and such results provide a very useful assessment of how flexible the final unit might be in its ability to accomodate new operating conditions. The role of the coolant, and the manner in which it is presented to the tubes, has been shown to be especially important. Not only is the routing of the coolant through the bundle significant, but also the flowrate. Relatively small variations in the rate can cause very large changes in the temperature profiles within the tubes. This is not primarily a result of the heat transfer coefficient being modified, but because of the change in the residence time of the cooling fluid and the consequent effect on the coolant temperature rise. The results of an investigation into the configuration and coolant flow direction have shown that there can be considerable economic advantages in using co-currently cooled reactors with more than two shell-side passes. Such an arrangement is significantly more stable than a counter-current configuration for a wide range of coolant flow conditions. Furthermore, the effective feed forward of the heat in the coolant gives more even temperature profiles inside the reactor tubes which can result in a greater overall conversion. A steady state method has been developed which is capable of representing the ranges of coolant and reactant conditions under which the reactor is stable. It is shown how this may be extended to enable the local stability of the tubeside to be related to the overall stability of the unit. Thus, the stability of the system is related to easily obtainable variables, namely the coolant and reactant inlet temperatures and coolant flowrate. Dynamic models of the reactor have been formulated and used to demonstrate that the initial transient response of the system can lead to temperature runaway even though both the initial and final stationary states are stable. This behaviöur, which would not be predicted by normal frequency response techniques, clearly has significant effects on the design of the system and its control strategy.