ADIABATIC GAS-LIQUID CONTACT OPERATIONS

Your objectives in studying this section are to be able to  

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

Develop the design equation for water-cooling towers and dehumidifiers using air. 

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

Cooling a liquid. The cooling occurs by transfer of sensible heat and also by evaporation. The principal application is cooling of water by contact with atmospheric air. 

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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. 

Fig. 5. Adiabatic gas—liquid contacting, graphical representation where point A is an arbitrary point along the column line CAB is the operating line having slope of av(g) CL / V point E represents the interface conditions corresponding to point A and the tie line AE has slope of hL-a/(kY-a). The bold line defines the equilibrium curve, H vs T. Conditions shown are those of a water-cooling process. To convert J to Btu, divide by 1054.

Figure 8.6 Operating diagram for adiabatic gas-liquid countercurrent contact.

Consider the operation indicated schematically in Figure 8.3. Here the entering gas is contacted with liquid, for example, in a spray. As a result of heat and mass transfer between gas and liquid, the gas leaves at conditions of humidity and temperature different from those at the entrance. The operation is adiabatic since no heat is exchanged with the surroundings. A mass balance for substance A gives... 

Also plotted on the psychrometric chart are a family of adiabatic saturation curves. The operation of adiabatic saturation is indicated schematically in Figure 1.5. The entering gas is contacted with a liquid and as a result of mass and heat transfer between the gas and liquid the gas leaves at conditions of humidity and... 

In contacting operations of this sort, where one phase approaches equilibrium with the other under conditions such that the characteristics of the latter do not change, the maximum change in the first phase corresponds to the operation of one theoretical stage (see Chap. 5). Since the humidity in adiabatic equilibrium with the liquid is Y, the Murphree gas-phase stage efficiency is... 

Problem statement. Ammonia(l) is to be removed from air(2) by contacting with water(3) in an absorption column. The column is to be operated adiabatically and at steady state it can be represented schematically as in Figure 12.17, with superscripts a-d denoting the following streams a => gas inlet, b gas outlet, c liquid inlet, and d => liquid outlet. The gas phase enters the column at 20°C and has yf = 0.416. The column removes 99 mole % of the ammonia from the gas, and the gas phase leaves the column at 20°C. Pure water x = 1) is fed to the column at 20°C. 

It is worthwhile pointing out here that, although the adiabatic saturation curve equation does not reveal anything of the enthalpy humidity path of either the liquid phase or gas phase at various points in the contacting device (except for the air water vapor system), each point within the system must conform with the wet bulb relation, which requires that the heat transferred be exactly consumed as latent heat of vaporization of the mass of liquid evaporated. The identity of hcJK with Cpy was first found empirically by Lewis and hence is called the Lewis relation. The treatment given here on the wet bulb temperature applies only in the limit of very mild drying conditions when the vapor flux becomes directly proportional to the humidity potential A Y. This is the case in most drying operations. 

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