Separation of Liquid-Gas Mixtures

be the mass concentration of solvent, and piQ - mass concentrations of components in the gaseous phase. Assume the process to be isothermal. Then the continuity equation for components in the solution is 

Assume that there is no mass exchange between components in the solution and no condensation. Then Jjn = 0 and Jci = 0 and Eq. (23.1) simplifies to 

The gaseous phase consists of small-sized spherical bubbles with low volume concentration, so possible interaction between them can be neglected due to their compactness. Bubble radii can range from the minimum radius R of a germ to the maximum radius that can be attained by a bubble that was bom at the bottom of the liquid and has risen to the surface. 

The state of the gaseous phase at each moment of time t at any point P of the volume can be characterized by a continuous distribution of bubbles over the mass of i-th component t, P). This distribution must obey the kinetic equa- 

Multiply both parts of Eq. (23.3) by m and integrate over m from 0 to oo. The outcome is 

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Fig. 24.1 Three regions arising during separation of liquid-gas mixture I - pure liquid II - intermediate region III - zone of constant gas concentration.

Section VII is devoted to liquid-gas (oil-gas) mixtures. The topics discussed are the dynamics of gas bubbles in multi-component solutions the separation of liquid-gas mixtures in oil separators both neglecting and taking into account the hindrance due to the floating-up of bubbles and the coagulation of bubbles in liquids. 

The mass transfer coefficients considered so far - namely, kQ,kj, KQ,andKj - are defined with respect to known interfacial areas. However, the interfacial areas in equipment such as the packed column and bubble column are indefinite, and vary with operating conditions such as fluid velocities. It is for this reason that the volumetric coefficients defined with respect to the unit volume of the equipment are used, or more strictly, the unit packed volume in the packed column or the unit volume of liquid containing bubbles in the bubble column. Corresponding to /cg, Kq, and we define k a, k, a, K, /i, and K a, all of which have units of (kmol h m )/(kmol m ) - that is, (h ). Although the volumetric coefficients are often regarded as single coefficients, it is more reasonable to consider a separately from the Ar-terms, because the effective interfacial area per unit packed volume or unit volume of liquid-gas mixture a (m m ) varies not only with operating conditions such as fluid velocities but also with the types of operation, such as physical absorption, chemical absorption, and vaporization. 

Consider the process of liquid-gas mixture separation in a gravitational horizontal separator. The mixture enters the separator from the supply pipeline equipped with a device for preliminary separation of free gas. A study of dispersivity of liquid-gas flow at the entrance is presented in [7]. As is shown in this work, the preliminary separation of large bubbles results in that the bubbles entering the separator have diameters ranging within a narrow interval from Di to D2, with D2 3Di, and with the variance 0.003. The distribution of bubbles over diameters D at the entrance can be approximated in the interval (Di D2) by a uniform (in the simplest case) distribution,... 

Pervaporation is a relatively new process with elements in common with reverse osmosis and gas separation. In pervaporation, a liquid mixture contacts one side of a membrane, and the permeate is removed as a vapor from the other. Currendy, the only industrial application of pervaporation is the dehydration of organic solvents, in particular, the dehydration of 90—95% ethanol solutions, a difficult separation problem because an ethanol—water azeotrope forms at 95% ethanol. However, pervaporation processes are also being developed for the removal of dissolved organics from water and the separation of organic solvent mixtures. These applications are likely to become commercial after the year 2000. 

As mentioned earlier, the physical properties of a liquid mixture near a UCST have many similarities to those of a (liquid + gas) mixture at the critical point. For example, the coefficient of expansion and the compressibility of the mixture become infinite at the UCST. If one has a solution with a composition near that of the UCEP, at a temperature above the UCST, and cools it, critical opalescence occurs. This is followed, upon further cooling, by a cloudy mixture that does not settle into two phases because the densities of the two liquids are the same at the UCEP. Further cooling results in a density difference and separation into two phases occurs. Examples are known of systems in which the densities of the two phases change in such a way that at a temperature well below the UCST. the solutions connected by the tie-line again have the same density.bb When this occurs, one of the phases separates into a shapeless mass or blob that remains suspended in the second phase. The tie-lines connecting these phases have been called isopycnics (constant density). Isopycnics usually occur only at a specific temperature. Either heating or cooling the mixture results in density differences between the two equilibrium phases, and separation into layers occurs. 

Temperature Two modes of temperature parametric-pumping cycles have been defined—direct and recuperative. In direct mode, an adsorbent column is heated and cooled while the fluid feed is pumped forward and backward through the bed from reservoirs at each end. When the feed is a binary fluid, one component will concentrate in one reservoir and one in the other. In recuperative mode, the heating and cooling takes place outside the adsorbent column. Parametric pumping, thermal and pH modes, have been widely studied for separation of liquid mixtures. However, the primary success for separating gas mixtures in thermal mode has been the separation of propane/ethane on activated carbon [Jencziewski and Myers, Ind. Eng. Chem. Fundam., 9, 216-221 (1970)] and of air/S02 on silica gel... 

EXAMPLE 13-1 Calculate the producing gas-oil ratio, stock-tank oil gravity, and oil formation volume factor which will result from a two-stage separation of the hydrocarbon mixture below. Use separator conditions of75°F and 100 psig and a stock-tank temperature qf75°F. The mixture is a liquid at its bubble point at reservoir conditions of 2620 psig and 220°F. Use K-factors from Appendix A. Use decane K-factors for heptanes plus. 

The calculations of Example 13-1 could be repeated for different separator pressures to determine the separator pressure which produces the largest amount of stock-tank liquid. Results of a laboratory separation of the hydrocarbon mixture of Example 13-1 are given in Figure 10-5. Optimum separator pressure is about 100 psig. This corresponds to minima in gas-to-oil ratio and formation volume factor and maxima in quantity of stock-tank oil and stock-tank oil gravity. 

The gas-chromatographic separation of plasticizers can be effected directly or after conversion to low boiling point compounds. This is achieved by a transesterification reaction with methanol or diazomethane. After separation of the plasticizers mixtures with liquid chromatography, identification by spectroscopic methods is possible. 

In the discussion of concentration polarization to this point, the assumption is made that the volume flux through the membrane is large, so the concentration on the permeate side of the membrane is determined by the ratio of the component fluxes. This assumption is almost always true for liquid separation processes, such as ultrafiltration or reverse osmosis, but must be modified in a few gas separation and pervaporation processes. In these processes, a lateral flow of gas is sometimes used to change the composition of the gas on the permeate side of the membrane. Figure 4.14 illustrates a laboratory gas permeation experiment using this effect. As the pressurized feed gas mixture is passed over the membrane surface, certain components permeate the membrane. On the permeate side of the membrane, a lateral flow of helium or other inert gas sweeps the permeate from the membrane surface. In the absence of the sweep gas, the composition of the gas mixture on the permeate side of the membrane is determined by the flow of components from the feed. If a large flow of sweep gas is used, the partial... 

Another type of gas exchange process, developed to the pilot plant stage, is separation of gaseous olefin/paraffin mixtures by absorption of the olefin into silver nitrate solution. This process is related to the separation of olefin/paraffin mixtures by facilitated transport membranes described in Chapter 11. A membrane contactor provides a gas-liquid interface for gas absorption to take place a flow schematic of the process is shown in Figure 13.11 [28,29], The olefin/paraffin gas mixture is circulated on the outside of a hollow fiber membrane contactor, while a 1-5 M silver nitrate solution is circulated countercurrently down the fiber bores. Hydrophilic hollow fiber membranes, which are wetted by the aqueous silver nitrate solution, are used. 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

Chromatography can be defined as the separation of mixtures by distribution between two or more immiscible phases. Some of these immiscible phases can be gas-liquid, gas-solid, liquid-liquid, liquid-solid, gas-liquid-solid and liquid-liquid-solid. Strictly speaking, a simple liquid-liquid extraction is in fact a chromatographic process. Similarly, distillation is a chromatographic process that involves separation of liquids by condensation of their respective vapours at different points in a column. 

Membranes are used to separate gaseous mixtures or liquid mixtures. Membrane modules can be tubular, spiral-wound, or plate and frame configurations. Membrane materials are usually proprietary plastic films, ceramic or metal tubes, or gels with hole size, thickness, chemical properties, ion potential, and so on appropriate for the separation. Examples of the kinds of separation that can be accomplished are separation of one gas from a gas mixture, separation of proteins from a solution, dialysis of blood of patients with kidney disease, and separation of electrolytes from non electrolytes. 

Absorption, or gas absorption, is a unit operation used in the chemical industry to separate gases by washing or scrubbing a gas mixture with a suitable liquid. One or more of the constituents of the gas mixture dissolves or is absorbed in the liquid and can thus be removed from the mixture. In some systems, this gaseous constituent forms a physical solution with the liquid or the solvent, and in other cases, it reacts with the liquid chemically. 

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