Gas-Liquid Contact Operations

Direct contact of a gas with a pure liquid may have any of several purposes  

Although operations of this sort are simple in the sense that mass transfer is confined to the gas phase (there can be no mass transfer within the pure liquid), they are nevertheless complex owing to the large heat effects which accompany evaporation or condensation. 

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In Vermeulen s work, a paddle impeller stirred fixed amounts of gas and liquid in a closed vessel. When the impeller was brought to the proper speed (240-360 rpm), the liquid and the gas that had been above it were dispersed together and completely filled the vessel. It is impossible to extrapolate from this experimental set-up to the usual type of gas-liquid contacting operation. 

Condensation Scrubbing The collection efficiency of scrubbing can be increased by the simultaneous condensation of water vapor from the gas stream. Water-vapor condensation assists in particle removal by two entirely different mechanisms. One is the deposition of particles on cold-water droplets or other surfaces as the result of Stefan flow. The other is the condensation of water vapor on particles as nuclei, which enlarges the particles and makes them more readily collected by inertial deposition on droplets. Both mechanisms can operate simultaneously. However, for the buildup of particles by condensation to be effective, there must be adequate time for the particles to grow substantially before the principal gas-liquid-contacting operation takes... 

Curve B of Figure 2 is typical of gas-liquid contacting operations. Here the rate pf mass transfer between phases increases to a maximum at small impeller diameter and then decreases as impeller diameter is increased. The significance is that more turbulence is available with the small impeller and that turbulence is more important than flow in this operation. 

A gas-liquid contact operation is illustrated in Figure 3.8. Gas is contacted with a liquid from a spray, resulting in both diffusion and heat transfer between the gas and liquid. The gas exits the system at conditions of humidity and temperature quite different from the entrance conditions. Assume the operation to be adiabatic. Perform a material and energy balance for the system. 

T he solubility data of gases in aqueous mixed-salt solutions are funda-mentally important in research on liquid-gas mass transfer and in designing gas-liquid contacting operations, especially gas absorption accompanied by chemical reaction. 

Gas-liquid contacting operations, which transfer one or more components between a gas phase and a liquid phase, are important to numerous industrial chemical processes. Their significance is reflected in the abundance of different contactor designs and review articles. The importance of these operations to the chemical industry is affirmed by their global prevalence and involvement in annually producing hundreds of millions of tons of basic chemicals. The various gas-liquid contactor designs attempt to optimize controlling parameters or such specific domains as the gas-liquid interface or continuous-phase residence time. 

When warm liquid is brought into contact with unsaturated gas, part of the liquid is vaporized and the liquid temperature drops. This cooling of the liquid is the purpose behind many gas-liquid contact operations, especially air-water contacts. Water is cooled in large quantities in spray ponds or more commonly in tall towers through which air passes by natural draft or by the action of a fan. 

Combine material and energy balances to develop the equation for the operating line for a countercurrent adiabatic gas-liquid contact operation. 

Many examples of gas-liquid contacting operations are found in the process industries, often involving gas incorporation or absorption into liquid, perhaps with chemical reaction in the liquid, washing or humidifying a gas stream, removal of gas from liquid, and so forth. 

Membrane gas absorption (MGA) is a gas-liquid contacting operation [1,2,24,25]. The key element in the process is a microporous hydrophobic HFM. The process is illustrated in Figure 4.3 for removal of component X from a gas stream. The gas stream is fed along one side of the membrane where an absorption liquid is flowing at the other side of the membrane. The hydrophobic membrane wall keeps gas phase and absorption liquid separated from each other. The absorption liquid is chosen in such a way that it has a high affinity for component X. Component X will now diffuse through the gas-filled pores of the membrane to the other side of the membrane where it is absorbed in a liquid phase. Absorption in the liquid phase takes place either by physical absorption or by a chemical reaction. This determines the selectivity of the process. The membrane used... 

Table 11-1 lists many of the process considerations that will influence the selection of equipment for gas-liquid contacting operations. The equipment possibilities are... 

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