Mass transfer gas-to-liquid

Clearly, for a very slow reaction (Ha<03), the gas to liquid mass transfer is not limiting, which translates into CH L CfJjL (see above), thus making the contribution of mass transfer negligible. Equation (29) can be discarded as Ch,l = CfJjL can be used in Eq. (28). 

TABLE 3 Comparison of Gas-to-Liquid Mass Transfer in Three Common Three-Phase Reactor Types... 

The first role of agitation is to keep the catalyst particles uniformly suspended in the reaction medium. When gas and liquid reactants are simultaneously used, agitation plays an essential role in facilitating the gas to liquid mass transfer.1201 Moreover, an efficient stirring is needed to avoid external (i.e. from the organic phase to the external surface of the catalyst particles) mass and heat transfer limitations.1113-151... 

In the above-described set-up (Fig. 8.16), the physical absorption of oxygen in water was used to measure the gas to liquid mass transfer. Thin-film fluorescence quenching-based sensors were installed to determine the oxygen concentration at the inlet and exit of the reactor. From the measured oxygen concentrations, the mass transfer coefficient was calculated based on the reactor-based liquid velocity ... 

With a reduced fused magnetite catalyst a substantial gas-to-liquid mass transfer resistance can be encountered, which causes the paraffin-to-olefin ratio of the hydrocarbon products to decrease. 

The term Z-aws represents the ratio of the actively wetted external catalyst area per unit volume of reactor,while ag represents the gas-to-liquid mass transfer interfacial area per unit volume of reactor. 

Reactors used to obtain fundamental data on intrinsic chemical rates free of mass-transfer resistances or other complications. Some of the gas-liquid lab reactors, for instance, employ known interfacial areas, thus avoiding the uncertainty regarding the area for gas to liquid mass transfer. When ideal behavior cannot be achieved, intrinsic kinetic estimates need to account for mass- and heat-transfer effects. 

Gas-to-liquid mass transfer is a transport phenomenon that involves the transfer of a component (or multiple components) between gas and liquid phases. Gas-liquid contactors, such as gas-liquid absorption/ stripping columns, gas-liquid-solid fluidized beds, airlift reactors, gas bubble reactors, and trickle-bed reactors (TBRs) are frequently encountered in chemical industry. Gas-to-liquid mass transfer is also applied in environmental control systems, e.g., aeration in wastewater treatment where oxygen is transferred from air to water, trickle-bed filters, and scrubbers for the removal of volatile organic compounds. In addition, gas-to-liquid mass transfer is an important factor in gas-liquid emulsion polymerization, and the rate of polymerization could, thus, be enhanced significantly by mechanical agitation. 

The objective of this entry is to introduce the readers to the fundamental principles of gas-to-liquid mass transfer, as well as its major applications. Therefore, the first section of the entry is on the three fundamental mechanisms of gas-to-liquid mass transfer the film theory, the penetration theory, and the surface renewal theory followed by the applications of gas-to-liquid mass transfer in unit operations that are widely used in various chemical processes. There is a vast pool of reported literature on different aspects of gas-to-liquid mass transfer processes, all of which is impossible to be included in this entry. Therefore, only typical gas-to-liquid mass transfer processes are presented here. 

Gas-to-liquid mass transfer can take place from a gas phase to a liquid phase (and vice versa) with or without chemical reactions. The concentration gradient of a transferred component in the bulk fluid and in the fluid at the interface is the driving force for mass transfer. When the mass transport occurs in a phase that is moving, the transport of the component is known as convective mass transfer. Convective mass transfer... 

Distillation is also a common transport process involving gas-to-liquid mass transfer. 

The fundamental principles of the gas-to-liquid mass transfer were concisely presented. The basic mass transfer mechanisms described in the three major mass transfer models the film theory, the penetration theory, and the surface renewal theory are of help in explaining the mass transport process between the gas phase and the liquid phase. Using these theories, the controlling factors of the mass transfer process can be identified and manipulated to improve the performance of the unit operations utilizing the gas-to-liquid mass transfer process. The relevant unit operations, namely gas absorption column, three-phase fluidized bed reactor, airlift reactor, liquid-gas bubble reactor, and trickled bed reactor were reviewed in this entry. 

The use of these criteria requires an experimentally measured point value for the reaction rate, the solubility of gas phase reactant and an estimation of gas to liquid mass transfer coefficient k,a. Some correlations for calculating k,a values in different multiphase reactor systems are presented in Table 3. 

Proper agitator selection is very important to provide good contact between the phases. Firstly, the catalyst particles must be kept uniformly suspended in the reaction medium. When a gaseous reactant is used, agitation is of primary importance in facilitating the gas-to-liquid mass transfer. In addition, good agitation is needed for optimum temperature control. 

The rate of the gas-to-liquid mass transfer can be calculated from the following equation ... 

Plot of substrate concentration vs normalised catalyst mass in cinnamaldehyde hydrogenation (Figure 9.26) indicates that gas-to-liquid mass transfer resistance does not play a significant role at the tested experimental conditions. 

Conventional trickle bed reactors work under steady state conditions, whereby components in the. liquid, mostly via gas-to-liquid mass transfer are converted by the solid catalyst. Such a process is highly non-linear and thus it is questionable whether steady operation will provide an optimal conversion and selectivity in particular. Operation in the dynamic mode will provide an extra parameter to optimise the production. Trickle beds happen to show just naturally a dynamic flow regime in which gas/liquid discontinuities occur the pulsing flow regime. 

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