Vapor-liquid stripping operations

The engineering analysis and design of these operations addresses questions which are different than those addressed in connection with the shaping operations. This is illustrated in Fig. 1 which is a flow sheet, cited by Nichols and Kheradi (1982), for the continuous conversion of latex in the manufacture of acrylonitrile-butadiene-styrene (ABS). In this process three of the nonshaping operations are shown (1) a chemical reaction (coagulation) (2) a liquid-liquid extraction operation which involves a molten polymer and water and (3) a vapor-liquid stripping operation which involves the removal of a volatile component from the molten polymer. The analysis and design around the devolatilization section, for example, would deal with such questions as how the exit concentration of... 

Data on the gas-liquid or vapor-liquid equilibrium for the system at hand. If absorption, stripping, and distillation operations are considered equilibrium-limited processes, which is the usual approach, these data are critical for determining the maximum possible separation. In some cases, the operations are are considerea rate-based (see Sec. 13) but require knowledge of eqmlibrium at the phase interface. Other data required include physical properties such as viscosity and density and thermodynamic properties such as enthalpy. Section 2 deals with sources of such data. 

A vapor-liquid equilibrium calculation shows that a good separation is obtained but the required product purity of butadiene <0.5 wt% and sulfur dioxide <0.3 wt% is not obtained. Further separation of the liquid is needed. Distillation of the liquid is difficult because of the narrow temperature limits between which the distillation must operate. However, the liquid can be stripped using nitrogen (Figure 14.21c). 

None of the experimental techniques described by Bonner, however, has been capable of providing reliable vapor-liquid equilibrium data at the combined extremes of elevated temperature and reduced pressure, conditions applicable to most commercial polymer-stripping operations. This problem has been addressed by Meyer and Blanks (1982), who developed a modified isopiestic technique that could be used when solubilities are low. Although the success of this new technique was demonstrated using just polyethylene with isobutane and propane, the idea shows considerable promise for obtaining data at unusual conditions of temperature and pressure. 

C. D. Prater, J. Wei, V. W. Weekman, Jr., and B. Gross, A Reaction Engineering Case History Coke Burning in Thermofor Catalytic Cracking Regenerators Costei D. Denson, Stripping Operations in Polymer Processing Robert C. Reid, Rapid Phase Transitions from Liquid to Vapor John H. Seinfeld, Atmospheric Diffusion Theory... 

The design of a tray tower for gas absorption and gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical trays needed to achieve a specified composition change (see Sec. 13). Distillation differs from absorption because it involves the separation of components based upon tne distribution of the various substances between a vapor phase and a liquid phase when all components are present in both phases. In distillation, the new phase is generated from the original phase by the vaporization or condensation of the volatile components, and the separation is achieved by introducing reflux to the top of the tower. 

First, the effects of gas and liquid flows, co-current versus countercurrent operation, pressure and temperature were checked. As expected, based on the influence of these parameters on the vapor pressure or the vapor-liquid equilibrium (VLE), the fraction of water stripped by the nitrogen increases with increasing gas flows, decreasing liquid flows, lower pressures and higher temperatures. Countercurrent operation is more efficient than co-current operation, because the liquid phase at the inlet was already enriched with the compound which was to be separated. 

Costel D. Denson, Stripping Operations in Polymer Processing Robert C. Reid, Rapid Phase Transitions from Liquid to Vapor John H. Seinfeld, Atmospheric Diffusion Theory... 

Vapor-liquid mass-transfer operations, such as absorption, stripping and distillation, are carried out in packed and plate columns. The key difference is that counterflowing vapor and liquid are contacted continuously with packings, and discretely with plates. The equilibrium and operating lines of packed and plate columns are identical under the same operating conditions—feed and product flowrates and compositions, temperature and pressure. Models for the design and analysis of packed columns are based on their close analogy to plate devices. 

The following information is requested as part of a wastewater treatment stripping operation for ethyl alcohol (EtOH). Generate a Txy diagram given the vapor-liquid equilibrium (VLE) data below. What is the vapor mole fraction of the ethanol if the liquid mixture contains 3.6mol % ethanol ... 

EPAR ATION and purification processes account for a large portion of the design, equipment, and operating costs of a chemical plant. Further, whether or not a mixture forms an azeotrope or two liquid phases may determine the process flowsheet for the separations section of a chemical plant. Most separation processes are contact operations such as distillation, gas absorption, gas stripping, and the like, the design of which requires the use of accurate vapor-liquid equilibrium data and correlating models or, in the absence of experimental data, of accurate predictive methods. Phase behavior, especially vapor-Uquid equilibria, is important in the design, development, and operation of chemical processes. 

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