Keep cool when designing batch reactors

The stirred batch reactor is a workhorse at many plants. In designing such a unit, focus on effective temperature control to achieve optimum performance.

By John Edwards

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Stirred batch reactors, with coils or external jackets, have inherent thermal lags due to the heat capacities of the masses associated with the reactor, reaction mix, jacket contents and jacket services [4]. To minimize these lags, reduce, wherever possible, jacket service volumes and thermal masses associated with external equipment and also install good thermal insulation.

A study of the heatup and cooldown curves or responses to setpoint step changes can provide an estimate of time constant. For instance, the typical overall value for heating 1,000 kg of toluene in a 1,600-L Hastelloy C reactor with Dowtherm J fluid is 21.1 minutes; breaking down this estimate into the contributions for the different interfaces gives the inside contributing 15.4 minutes, the wall 3.1 minutes and the outside 2.6 minutes.

Endothermic reactions exhibit a marked degree of self-regulation in regard to thermal stability and do not need further consideration.

Exothermic reactions, however, require a detailed understanding of kinetics to obtain rate and heat of reaction. The heat removal capability is a function of the resistances to heat transfer, the temperature difference and the heat-transfer area. Reaction temperature increases lead to a higher rate of reaction and pose a risk of thermal runaway if heat cannot be removed fast enough; any reduction in heat-transfer area due to a decrease in reactor contents adds to the problem. Design cannot always provide stability where not enough heat-transfer area is available for the temperature difference. However, removing heat by boiling one or more of the components can ensure stability because this tends to create an isothermal system.

When reactions are carried out with all the reactants charged, carefully consider the implications of cooling failure, taking into account common mode failures. It is preferable to limit the reaction rate by adding the reactant continuously at a controlled rate to ensure that the heat generated does not exceed the system’s heat-removal capability.

Tempered reactions, i.e., those operating at the boiling point, remove heat using the latent heat of vaporization. This procedure is self-regulating, provided the sizing of the overhead condenser ensures material is not removed from the reaction (which would decrease the heat-transfer area). In this case, the reactor cooling system only needs to remove any excess heat from the reaction.

Gassy systems generate a permanent gas and require the total heat evolved to be absorbed by the jacket/coil cooling system.

It has been empirically established that a velocity of 1 m/s across the service-side heat-transfer surface will provide optimal economic heat transfer. Achieving this necessitates the use of circulating pumps and jacket-inlet circulating nozzles, which induce a rotational movement similar to spiral baffles and significantly reduce the circulation flow required for efficient heat transfer. The combined reactor and nozzle pressure drop determines the number and size of mixing nozzles. Vendors provide curves to establish the optimal circulation rate and pressure drop [5].

When using HTFs that might have high viscosities within the operating temperature range, circulating nozzle pressure drops might become limiting and half pipe coil constructions might be required.

Minimize the heat load on the refrigeration system by first using a higher-temperature cooling service and then switching to a lower temperature medium only when necessary. Excessive evaporator temperature in the refrigeration loop can result in compressor shutdown and, ultimately, failure. Any water present in the system will accumulate and freeze at the compressor suction.

Boilup and wall temperature might be excessive with direct steam and might require pressure control. Boilup can be limiting with indirect HTF systems and only can be boosted by raising the jacket temperature, which is subject to maximum operating-temperature constraints.

External heat exchangers 
Batch operation can involve rapid temperature cycling that leads to severe thermal stresses. Therefore, use a fully welded shell-and-plate heat exchanger rather than a less expensive brazed unit, which might experience stress failure. The thermal fluid is usually on the plate side and the service fluid on the shell side. For cryogenic applications, opt for a coiled-tube heat exchanger with liquid nitrogen on the tube side [6].

Base the heat duty for exchanger sizing on the reactor heat-transfer area available at maximum operating level.

Inlet and outlet temperature differences are determined from the services’ supply and return temperatures and by selecting reasonable HTF inlet and exit temperatures at the approach to the services’ inlet temperatures.

For heating with steam, the inlet and outlet temperature differences are unlikely to be critical at the approach to maximum HTF temperature. 

For cooling, the inlet and outlet temperature differences can be critical at the approach to minimum HTF temperature, particularly for low-temperature applications. Select a design temperature difference that gives an economic design while providing a heat-transfer capability that exceeds the reactor’s by a reasonable margin.

The liquid service flow is established by setting an acceptable temperature difference across the heat exchanger, typically 10°C (18°F). The type of cooling system and its operation determine the allowable return temperature.

A characteristic of plate heat exchangers is that the cross-sectional area for flow is small and the pressure drop, particularly at low temperatures, usually sets the number of plates and their geometric arrangement.

The heat-transfer area is estimated thermally, and the configuration then is adjusted to give an acceptable pressure drop. The plate area determined by pressure drop, usually on the circulating heat-transfer-fluid side, normally results in an increased design margin for heat-transfer area.

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