Process Technology: An Introduction - Haan A.B. 2015

4 Chemical reactors and their industrial applications
4.4 Loop reactors

A loop reactor consists of a reaction vessel through which process fluids are recirculated. Often the reactor is combined with a pump for recirculation of the liquid, a heat exchanger, an injection device for reactants, and a separation device. Loop reactors can be operated in either batch or continuous configuration

4.4.1 Continuous loop reactors

A continuous loop reactor has feed streams entering the loop and product streams leaving it. The rate of recirculation around the loop can generally be set independently of the throughput rate. This flexibility can be used to adapt the residence-time distribution of the overall reactor to the requirements of the reaction. A good example is the rearrangement of cyclohexanone oxime into caprolactam which includes the opening of the cyclohexyl ring and takes place according to the following equations:

Since this so called Beckmann rearrangement is very rapid and highly exothermic, the reaction is carried out in a loop reactor configuration with heat exchanger outside the reaction vessel (Fig. 4.16). The rate of reaction is so high that insufficient time is available to achieve complete mixing on a microscale. This problem is increased because a low viscous reactant (cyclohexanone oxime) has to be mixed with a high viscous product mixture of caprolactam in concentrated sulfuric acid. Because of the incomplete mixing, many side products are formed which must be removed in the subsequent purification train. To minimize side product formation the molten cyclohexanone oxime and concentrated oleum are introduced at carefully selected places into a relatively large amount of already rearranged product to provide an optimal distribution of the feed streams over the reaction mixture. For optimal heat removal and minimization of temperature differences in the reaction mixture a high circulation rate is chosen. In this way the circulation time is small compared to the residence time in the reactor system, and the overall behavior is close to that of an ideal stirred tank.

4.4.2 Buss loop reactor

A Buss loop reactor combines a reaction autoclave with a specially designed centrifugal pump, a shell and tube heat exchanger of variable size, and a top-mounted ejector in the autoclave (Fig. 4.17). In a batch loop reactor the reaction vessel is initially charged with the reactants. For multiphase reactions the choice of the proper recirculation rate may also be influenced by the need to disperse a second phase (gaseous or immiscible liquid reactant) in the reacting liquid. This is especially valuable in gas-liquid systems, where the energy of a fast recirculating liquid stream can be used to draw in a gas reactant by means of an ejector. The Buss loop reactor is based on this principle and is used for reactions where intensive contact between gas, liquid, and/or catalyst is needed, such as hydrogenation, hydroformylation, amination, ethoxylation, oxidation, epoxidation, and carbonylation. The draft tube of the ejector dips into the liquid level of the reactor, and the gas bubbles are thus distributed over the entire reactor contents, generating turbulence and thus good mixing. It has the advantage of rapid gas-liquid mass transfer in the initial reaction zone, combined with a high heat removal capability in the tubular heat exchanger. Because the heat exchanger is separated from the reactor, it has no influence on the mass-transfer and mixing mechanisms inside the reactor. This results in easy scaling up from laboratory size to industrial production, because the dimensions of the reactor scale linearly with capacity.

Fig. 4.16: Loop reactor for the production of caprolactam from cyclohexanone oxime.

Fig. 4.17: Buss loop reactor.