Rotor-stator spinning disc reactor

The chemical industry is continuously working to make the production more efficient and safer. Process intensification is the trend in which new equipment and processing methods are developed, which require less energy, are safer and produce less waste products. The main improvements can be made by increasing the mass and heat transfer rates significantly. The mass transfer from the gas phase to the liquid phase, and from the liquid phase to the solid phase are often the rate limiting steps in multiphase reactors. Increasing the mass transfer rates therefore leads to higher reaction rates, and thus more productivity or a lower reactor volume, thus leading to more efficient and safer reactors. This thesis describes the hydrodynamics and mass transfer in a novel type of multiphase reactor, the rotor-stator spinning disc reactor, which shows high mass transfer rates compared to conventional reactor equipment. The rotor-stator spinning disc reactor consists of a rotating disc in a cylindrical housing. The distance between the rotor and the reactor wall, the stator, is small, in the order of 1 mm. Two reactor configurations are studied. In the first configuration, the liquid, which is the continuous phase, is injected to the reactor from the top. The gas phase is injected through a small gas inlet in the bottom stator, near the rim of the rotor. Gas bubbles are sheared off at the gas inlet, due to the high velocity gradient, and thus shear force, between the rotor and the stator. The gas bubble size decreases with increasing rotational disc speed. The centrifugal force causes the gas bubbles to move radially inward; the gas holdup in the rotor-stator spinning disc reactor with a single gas inlet in the bottom stator is only a few percent. The inward radial velocity decreases with decreasing bubble size; the residence time, and thus the gas holdup, increases with increasing rotational disc speed. The decrease in gas bubble size, combined with the increase in gas holdup, leads to an increase in gas-liquid interfacial area (aGL) with increasing rotational disc speed. Two types of gas bubbles are distinguished in the rotor-stator spinning disc reactor. At low rotational disc speeds, the gas bubbles are larger than the rotor-stator distance. The main gas-liquid interfacial area, and thus the main part of the mass transfer occurring, is between the gas bubble and the liquid films on the rotor and the stator. At higher rotational disc speeds, the gas bubbles are smaller than the rotor-stator distance. The mass transfer coefficient (kGL) in the latter case is determined by the size and the velocity of the turbulent eddies in the liquid, and is therefore a function of the energy dissipation rate in the reactor. The volumetric gas-liquid mass transfer coefficient (kGLaGL), is the product of the interfacial area and the mass transfer coefficient, and it therefore also increases with increasing rotational disc speed. It increases with increasing gas flow rate, increasing rotor radius and decreasing rotor-stator distance. The maximum value obtained in this study, measured using the desorption of oxygen from water, is 2.5 m3L m-3R s-1, at a gas flow rate of 1.5*10-5 m3 s-1 and a rotational disc speed of 209 rad s-1, using a rotor with 0.135 m radius and 1 mm rotor-statordistance. This is one order of magnitude higher than in conventional reactor systems, such asstirred tank reactors or bubble columns. The energy dissipation rate in the rotor-stator spinning disc reactor is up to 3 orders of magnitude higher than in conventional reactors, such as bubble columns or stirred tank reactors. The increase in gas-liquid mass transfer is only a factor of 20. The rate of gas-liquid mass transfer per unit of energy dissipation ( kGLaGL/Ed) is only 1.1 m3L MJ-1, while it is 80 m3L MJ-1 for stirred tank reactors. Increasing the rotor radius from 0.066 m to 0.135 m increases the volumetric mass transfer coefficient by a factor 3, while the energy input increases with a factor of 15. It is therefore, from energetic point of view, preferred to scale up the reactor by using multiple rotor-stator stages in series, instead of scaling up by increasing the rotor size. For the scale up by numbering up, of the reactor configuration with a single gas inlet, however, a gas redistribution system may be needed, which may prove to be inconvenient in practice. In an alternative reactor configuration gas and liquid are fed together to the top of the reactor. The liquid forms a thin film, which flows outwards, on the rotor. The gas phase fills up the space between this liquid film and the top stator. Near the rim of the rotor, small gas bubbles are sheared off from the gas phase. The region surrounding the rim of the disc, and the region between the rotor and the bottom stator, is filled with gas bubbles dispersed in the liquid, which is the continuous phase. The gas-liquid mass transfer rate in the latter part, the dispersed flow region, is higher than in the film flow region. The mass transfer performance (kGLaGLVR, i.e. the mass transfer rate divided by the driving force), of the configuration where gas and liquid are fed together to the reactor, is twice as high as in the case of the single gas inlet in the bottom stator. The scale up of the configuration where gas and liquid are fed together is relatively easy. Multiple rotor-stator units can be added in series, preferably with the rotors on a common axis. It is shown that the mass transfer per stage is equal to the mass transfer in the single stage reactor, it is therefore highly probable that the same flow behaviour (film flow and dispersed flow) is obtained in the multistage reactor as in the single stage reactor. The pressure drop in the reactor is mainly caused by the centrifugal pressure; an increase in liquid flow rate leads to an increase in pressure drop. Additionally, in the case of gas and liquid flow, the pressure drop is up to a factor 2.5 higher than in the case of liquid flow only; the highest value measured is 0.64 bar pressure drop per stage, at a rotational disc speed of 459 rad s-1. A heterogeneously catalyzed reaction, the oxidation of glucose on platinum, is performed in the rotor-stator spinning disc reactor. The catalyst is supported on the rotor using a Nafion coating, the reaction is performed with a liquid phase only. The reaction rate is, under the conditions used, influence by the kinetics of the reaction as well as by the mass transfer from the bulk of the liquid to the rotor. The liquid-solid mass transfer coefficient increases with increasing rotational disc speed. At low rotational disc speeds the flow is laminar, with an increasing rotational disc speed a transition towards turbulent flow will take place. The maximum liquid-solid mass transfer coefficient (kLS) obtained is 8*10-4 m3L m-2i s-1 at 157 rad s-1, which is one order of magnitude higher than in conventional reactors like a fixed bed reactor. The liquid-solid interfacial area (aLS) that can be obtained, if the stators and the rotor are used to deposit catalyst, is 2000 m2i m-3R , which is comparable to a fixed bed reactor; the volumetric liquid-solid mass transfer coefficient (kLSaLS) is thus an order of magnitude higher. The high gas-liquid and liquid-solid mass transfer rates give the rotor-stator spinning disc reactor a large potential for multiphase reactions. The energy input is relatively high, but the mass transfer coefficients are an order of magnitude higher than in conventional reactor systems. The rotor-stator spinning disc reactor is therefore mainly suitable for reactions where the energy costs play a minor role, and conversion and selectivity are more important. Also reactions with dangerous reactants or products and reactions at high pressure can benefit from the spinning disc reactor, since a large decrease in volume can be achieved. Additionally, the high heat transfer coefficients expected make the spinning disc reactor a promising alternative for exothermic reactions. The high mass transfer coefficients can also have a significant effect on the selectivity of a reaction, thereby giving scope for improved processes.