Liquid gas systems

Corresponding equations for the two special cases of gas-film mass-transfer control and surface-reaction-rate control may be obtained from these results (they may also be derived individually). The results for the latter case are of the same form as those for reaction-rate control in the SCM (see Table 9.1, for a sphere) with R0 replacing (constant) R (and (variable) R replacing rc in the development). The footnote in Example 9-2 does not apply here (explain why). 

The over-all behavior of a gas-liquid agitated system will depend on basic phenomena of mass transfer, bubble dynamics, and the fluid-dynamic regime in the mixing vessel. A general discussion of mass transfer is beyond the scope of this review, and the book by Sherwood and Pigford (S5) is recommended as a guide to that subject. 

A growing body of published information on bubbles has been concerned with such areas as the flow field around bubbles, and of bubble formation at orifices. However, this information has not yet been related to the problem of gas-liquid contacting, and hence will also be excluded from this review. The interested reader is referred to the recent annual reviews on Fluid Dynamics in Industrial and Engineering Chemistry for summaries of the work in this field. 

With more specific reference to agitated systems, information is lacking on the effects of gas bubbles on basic properties like mean flow pattern, impeller discharge rate, or turbulence characteristics. The observations presented in Section II, based mainly on one-liquid-phase data, must therefore be considered as the best available approximations for the flow regimes in gas-liquid agitated systems. There have been a few papers of somewhat basic nature with direct application to these systems and these will be discussed in the remainder of this section. 

Foust et al. (F8) studied the contacting of air and water in cylindrical baffled vessels agitated with Mixing Equipment Company dispersers, 

The experimental data in this study were correlated in the dimensional form  

We can understand this result from the following simplistic explanation based on Henry s law (relation (3.3.60b))  

Typical units of Hi are atmospberes/mole fraction. If H2 Hi, then, for the same partial pressure pig = p2g) or mole fraction in the gas phase, the mole fraction of gas species 2 in the liquid phase is less than that of gas species 1. Species 1 is thus more soluble in the liquid phase and therefore may be separated from species 2 by absorption in a suitable liquid. Gas absorption based separation processes utilize this preferential solubility of some gases in selected liquid absorbents. The values of Henry s constant H,- in units of atmospheres/mole fraction for a variety of gases in water are provided in the handbook by Perry and Green (1984). For some species, the values are given at a number of temperatures and values of Pig. The latter does indicate a weak dependence of Hi on p,-g. 

A common example involves the removal of acid gases, e.g. SO2, H2S, CO2, COS, etc., from a gas stream by absorption in a solvent. Table 4.1.1 identifies the values of Hi for a number of gaseous species in a few absorbents for 

The solution contains n ionic species species i has a valence of Z,- at a molar concentration of Q gmol/cm. Contributions of positive ions, negative ions and dissolved free gas species h+, h- and ho, respectively) have been tabulated by Danckwerts (1970) for calculating the quantity h in definition (4.1.9a) as 

In general, the solubilities of most gases in an absorbent decrease with an increase in temperature. This implies, from Henry s law, that ffr increases with temperature. In fact, Henry s constant for species i changes with temperature in the manner of 

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Gas Absorption and Gas-Liquid System Design James R. Fair, D. E. Steinmeyer,... 

B. B. Crocker, S.M., P.E., Consulting Chemical Engineer Fellow, American Institute of Chemical Engineers Member, Air Pollution Control Association (Section 14, Gas Absorption and Gas-Liquid System Design)... 

Based on area of contact accorcing to inside or outside (iameter of tubes depencing on location of interface between aqueous and organic phases. Can also be applied to gas-liquid systems with liquid on shell side. 

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