Gas-liquid-suspended solid reactors

The pulsating three-phase reactor has been examined only at the laboratory level. The pulsation gives good mixing and l)eat- and mass-transfer characteristics in the column. The first three types of gas-liquid-suspended-solid reactor are the most commonly used in practice. Schematic diagrams for these reactors are shown in Fig. l-3fn), (b), and (c), respectively. The agitated and nonagitated slurry... 

Figure 1-3 Schematic diagrams of some gas-liquid-suspended-solid reactors, (a) Agitated slurry reactor, ( >) nonagitated slurry reactor, (c) fluidized-bed reactor.

A large number of gas-liquid-suspended-solid operations are operated under no liquid flow (i.e., batch) conditions. A transient model for this type of isothermal reactor is given by Govindarao.10 Here, we briefly describe his model and the important results obtained from it. 

There are several industrially important gas-liquid-solid reactions such as carbonation of lime,coal liquefaction etc.,where solids take part in the reaction.The common type of reactor for these reactions is normally a gas-liquid-suspended solid column with or without mechanical agitation.An extensive literature is already available on the hydrodynamic,mixing,mass transfer,and heat transfer characteristics of these reactors,and critical reviews have been published in recent years(1-3). 

In many important cases of reactions involving gas, liquid and solid phases, the solid phase is a porous catalyst. It may be in a fixed bed or it may be suspended in the fluid mixture. In general the reaction occurs either in the liquid phase or at the liquid-solid interface. In fixed bed reactors the particles have diameters of about 3 mm and occupy about 50% of the vessel volume. Diameters of suspended particles are limited to 0.1-0.2 mm minimum by requirements of filterability and occupy 1-10% of the volume in stirred vessels. 

Slurry Bubble Column Reactors As in the case of gas-liquid slurry agitated reactors, bubble column reactors may also be used when solids are present. Most issues associated with multiphase bubble columns are analogous to the gas-liquid bubble columns. In addition, the gas flow and/or the liquid flow have to be sufficient to maintain the solid phase suspended. In the case of a bubble column fermenter, the sparged oxygen is partly used to grow biomass that serves as the catalyst in the system. Many bubble columns operate in semibatch mode with gas sparged continuously and liquid and catalyst in batch mode. 

With the above relations for three phase reactors with gas, liquid and solid catalyst phases we can determine the mass and heat transfer coefficients for the transport to and from the catalyst particle, as it is suspended in the liquid phase. The same holds for transfer in the liquid phase surrounding catalyst particles through which gas and liquid flow. 

Only one publication on gas-liquid mass transfer in bubble-column slurry reactors has come to the author s attention. However, a relatively large volume of information regarding mass transfer between single bubbles or bubble swarms and pure liquid containing no suspended solids is available, and this information is probably of some relevance to the analysis of systems... 

Mass transfer across the liquid-solid interface in mechanically agitated liquids containing suspended solid particles has been the subject of much research, and the data obtained for these systems are probably to some extent applicable to systems containing, in addition, a dispersed gas phase. Liquid-solid mass transfer in such systems has apparently not been studied separately. Recently published studies include papers by Calderbank and Jones (C3), Barker and Treybal (B5), Harriott (H4), and Marangozis and Johnson (M3, M4). Satterfield and Sherwood (S2) have reviewed this subject with specific reference to applications in slurry-reactor analysis and design. 

Johnson et al. (J4) investigated the hydrogenation of a-methylstyrene catalyzed by a palladium-alumina catalyst suspended in a stirred reactor. The experimental data have recently been reinterpreted in a paper by Polejes and Hougen (P4), in which the original treatment is extended to take account of variations in catalyst loading, variations in impeller type, and variations of gas-phase composition. Empirical correlations for liquid-side resistance to gas-liquid and liquid-solid mass transfer are presented. 

The expression gas-liquid fluidization, as defined in Section III,B,3, is used for operations in which momentum is transferred to suspended solid particles by cocurrent gas and liquid flow. It may be noted that the expression gas-liquid-solid fluidization has been used for bubble-column slurry reactors (K3) with zero net liquid flow (of the type described in Sections III,B,1 and 1II,V,C). The expression gas-liquid fluidization has also been used for dispersed gas-liquid systems with no solid particles present. 

These reactors contain suspended solid particles. A discontinuous gas phase is sparged into the reactor. Coal liquefaction is an example where the solid is consumed by the reaction. The three phases are hydrogen, a hydrocarbon-solvent/ product mixture, and solid coal. Microbial cells immobilized on a particulate substrate are an example of a three-phase system where the slurried phase is catalytic. The liquid phase is water that contains the organic substrate. The gas phase supplies oxygen and removes carbon dioxide. The solid phase consists of microbial cells grown on the surface of a nonconsumable solid such as activated carbon. 

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

Other reactor types are also used for gas-liquid reactions, but they are not very common in fine chemicals manufacture. Spray towers and jet reactors are used when the liquid phase is to be dispersed. In spray towers the liquid is sprayed at the top of the reactor while the gas is flowing upward. The spray reactor is useful when a solid product, possibly suspended in the liquid, is formed, or if the gas-phase pressure drop must be minimized. In a jet reactor, the liquid is introduced to the reaction zone through a nozzle. The gas flows in, being sucked by the liquid. 

Stirred tank reactors are employed when it is necessary to handle gas bubbles, solids, or a second liquid suspended in a continuous liquid phase. One often finds that the rates of such reactions are strongly dependent on the degree of dispersion of the second phase, which in turn depends on the level of agitation. 

Anaerobic Filter. An anaerobic filter consists of packed support media that traps biomass as well as facilitates attached growth of biomass as a biofilm (Fig. 8). Such a reactor configuration helps in the retention of suspended biomass as well as gas-liquid-solid separation. The flow of liquid can be upward or downward, and treatment occurs due to attached and suspended biomass. Treated effluent is collected at the bottom or top of the reactor for discharge and recycling. Gas produced in the media is collected underneath the bioreactor cover and transported for storage or use. Volumetric loading rates vary from 5 to 20 kg COD/m day with HRT values of 0.5-4 days. 

Agitated slurry reactor (ASR) This is a mechanically agitated gas-liquid-solid reactor (Figure 3.13). The liquid is agitated by a mechanical apparatus (impeller). The fine solid particles are suspended in the liquid phase by means of agitation. Gas is sparged into the liquid phase, entering at the bottom of the tank, normally just under the impeller. This reactor can also be of continuous type or of semibatch type. This type is used only in catalysis. 

Foam Production This is important in froth-flotation separations in the manufacture of cellular elastomers, plastics, and glass and in certain special applications (e.g., food products, fire extinguishers). Unwanted foam can occur in process columns, in agitated vessels, and in reactors in which a gaseous product is formed it must be avoided, destroyed, or controlled. Berkman and Egloff (Emulsions and Foams, Reinhold, New York, 1941, pp. 112—152) have mentioned that foam is produced only in systems possessing the proper combination of interfacial tension, viscosity, volatility, and concentration of solute or suspended solids. From the standpoint of gas comminution, foam production requires the creation of small bubbles in a liquid capable of sustaining foam. 

The trickle bed reactors that operate in the downflow configuration and have a number of operational problems, including poor distribution of liquid and pulsing operation at high liquid and gas loading. Scaleup of these liquid-gas-solid reactors is much more difficult than a gas-solid or gas-liquid reactor. Nevertheless, the downflow system is convenient when the bed is filled with small catalyst particles. And, because the catalyst particles are small, these reactors are quite effective as filters of the incoming feed. Any suspended fine solids, such as fine clays from production operations, accumulate at the front end of the bed. Eventually, this will lead to high pressure differentials between the inlet and outlet end of the reactor. 

Power or energy dissipated in the aerated suspension has to be large enough (a) to suspend all solid particles and (b) to disperse the gas phase into small enough bubbles. It is essential to determine the power consumption of the stirrer in agitated slurry reactors, as this quantity is required in the prediction of parameters such as gas holdup, gas-liquid interfacial area, and mass- and heat-transfer coefficients. In the absence of gas bubbling, the power number Po, is defined as... 

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