Vapor sweeping effect

To study the down stream vapor pressure effects that exist when permeating into non-zero water vapor pressure, nitrogen sweep gas of desired water vapor pressure was used on the permeate side. The humidity of the sweep and feed gas exiting the membrane device was measured to estimate the water permeability. 

Process Description Pervaporation is a separation process in which a liquid mixture contacts a nonporous permselective membrane. One component is transported through the membrane preferentially. It evaporates on the downstream side of the membrane leaving as a vapor. The name is a contraction of permeation and evaporation. Permeation is induced by lowering partial pressure of the permeating component, usually by vacuum or occasionally with a sweep gas. The permeate is then condensed or recovered. Thus, three steps are necessary Sorption of the permeating components into the membrane, diffusive transport across the nonporous membrane, then desorption into the permeate space, with a heat effect. Pervaporation membranes are chosen for high selectivity, and the permeate is often highly purified. 

Discontinuance of vapor flow and removal of residual vapors by combined effect of a steam sweep and vacuum produced by steam ejectors... 

In the United States, nearly all bromine is derived from natural brines. The Arkansas brines which coiiLain a minimum of 4000 ppm bromide account for over half of this production. Recovery is effected by a steaming-out process. After heating fresh brine, the solution is fed to the top of a tower. Chlorine and steam are injected at the bottom of the tower. The chlorine oxidizes the bromide and displaces one resultant bromine from solution. For brines of lower concentration, air instead of steam is used to sweep out the bromine vapors after chlonnation,... 

Figure 4.19 The effect of a small permeate-side, counter-flow sweep on the water vapor concentration on the permeate side of a membrane. In this example calculation, the sweep flow reduces the membrane area by two-thirds...

Pervaporation is a concentration-driven membrane process for liquid feeds. It is based on selective sorption of feed compounds into the membrane phase, as a result of differences in membrane-solvent compatibility, often referred to as solubility in the membrane matrix. The concentration difference (or, in fact, the difference in chemical potential) is obtained by applying a vacuum at the permeate side, so that transport through the membrane matrix occurs by diffusion in a transition from liquid to vapor conditions (Figure 3.1). Alternatively, a sweep gas can be used to obtain low vapor pressures at the permeate side with the same effect of a chemical potential gradient. 

The evaporation velocity at ambient pressure and, say, 60 °C, which corresponds to a mole fraction in the sweep gas of about 30 % water vapor, is about uuq = IO-5 m s-i. This results in Kj , = 0.904 which is rather close to 1, so that the effect of the liquid phase mass transfer resistance on the selectivity of an open distillation process with a free gas-liquid interface in most cases can be ignored. If, however, kuq becomes very small (as in the pervaporation process described in the next example), Kuq might become very small and thus reduce the selectivity of the open distillation process practically down to zero. 

Figure 4.24 shows the reactive arheotrope trajectories according to Eq. (83) for various amounts of the liquid phase mass transfer resistance - that is, for various values of Kiiq and a low sweep gas flow rate G (at large NTt/ -values). As a result, the reactive arheotropic composition X, 02 is shifted to larger values as the liquid phase mass transfer resistance becomes more important - that is, as the value of Kuq decreases. Note that the interface liquid concentrations are in equilibrium with the vapor phase bulk concentrations. Therefore, gas phase mass transfer resistances cannot have any influence on the position of the reactive arheotrope compositions. On the other hand, liquid phase mass transfer resistances do have an effect, though the value of all binary hiq have been set equal. Again, this effect results from the competition between the diffusion fluxes and the Stefan flux in the liquid phase. 

The SGMD is a temperature driven process, and it involves (a) evaporation of water at the hot feed side, (b) transport of water vapor through the pores of hydrophobic membrane, (c) collection of the permeating water vapor into an inert cold sweeping gas, and (d) condensation outside the membrane module. A decrease in driving force has been observed due to polarization effects of both temperature and concentration [80,82]. To calculate both heat and mass transfer through microporous hydrophobic membrane as well as the temperature and concentration polarization layer, the theoretical model suggested by Khayet et al. [58] can be written as... 

In convective vaporization, the same boiling regimes are encountered, but modified by the net motion of the two-phase fluid past the surface. At low velocities or high heat fluxes, the convection effect is small, and nucleate boiling dominates. At higher velocities, the heat-transfer rate is dominated by the two-phase mixture sweeping across the surface. It is still important to avoid transition and film boiling, but the onset of these phenomena is complicated by many factors. (See [1, 34].)... 

This Section Ls restricted to a description of some of the work of Ander-gon, 8-a> who has ably applied the quantitative analysis of vapors by infrared spectroscopy to analytical problems in carbohydrate chemistry, principally to the Zeisel alkoxyl determination. In this particular application, the usual Zeisel apparatus was used, and the volatile iodide liberated was carried by a flow of nitrogen into a cold trap where it was collected quantitatively Anhydrone (magnesium perchlorate) was used for removing water vapor which would otherwise interfere in the spectrum. The contents of the trap were allowed to vaporize into an evacuated gas-cell, and air was then admitted through the trap to sweep all the vapor into the gas-cell. Double-beam compensation of atmospheric water vapor and carbon dioxide was not upset by this procedure, which also served the purpose of increasing the sensitivity of the infrared method by the well known pressure-broadening effect. The complete spectrum of the vapor... 

If a process gas is supplied to the cathode with an H,S level of 2000 ppm, a CO, level of 1%, and an H,0 level of 12% (a saturated natural gas composition), it is assumed that 99% of the H,S is removed by reaction (5), and if the process and sweep gas flowrates are equal, then there exist an activity ratio of a oJa of 665 in the anolyte before significant (e.g. 1%) of the carbonate is oxidized. This assiunes equivalent electrode kinetics for the cathodic and anodic reactions. When compared to the activity ratio of Ocos a of 26.9, this shows the thermodynamic preference for the oxidation of to elemental sulfur by equation (8) when there is an absence of reductant at the anode. This mode of operation is preferable for commercial application, with direct production of elemental sulfur vapor, eliminating this need for a Claus reactor for sulfur production. The net effect, under these conditions, is continuous removal of H,S from the process gas accompanied by enrichment of the process gas with H, and direct generation of elemental sulfrur. ITie only reagent required is electric power at a potentially attractive rate, which will be shown. 

Film boiling. When boiling inside a tube, the fluid velocity helps sweep the vapor bubbles from the tube wall, thus retarding vapor film formation. In kettle reboilers, the effect of fluid velocity is much smaller, and film boiling may be a severe problem. It is therefore important to ensure that vapor can escape faster than it is generated. Some quantitative criteria are presented elsewhere (187, 253, 314). 

With pervaporation membranes the water can be removed during the condensation reaction. In this case, a tubular microporous ceramic membrane supplied by ECN [124] was used. The separating layer of this membrane consists of a less than 0.5 mm film of microporous amorphous silica on the outside of a multilayer alumina support. The average pore size of this layer is 0.3-0.4 nm. After addition of the reactants, the reactor is heated to the desired temperature, the recyde of the mixture over the outside of the membrane tubes is started and a vacuum is apphed at the permeate side. In some cases a sweep gas can also be used. The pressure inside the reactor is a function of the partial vapor pressures and the reaction mixture is non-boiling. Although it can be anticipated that concentration polarization will play an important role in these systems, computational fluid dynamics calculations have shown that the membrane surface is effectively refreshed as a result of buoyancy effects [125]. 

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