Mass transfer strippers

For the liquid-phase mass-transfer coefficient /cl, the effects of total system pressure can be ignored for all practical purposes. Thus, when using Kq and /cl for the design of gas absorbers or strippers, the primary pressure effects to consider will be those which affect the equilibrium curves and the values of m. If the pressure changes affect the hydrodynamics, then Icq, and a can all change significantly. 

As shown in Fig. 13-92, methods of providing column reflux include (a) conventional top-tray reflux, (b) pump-back reflux from side-cut strippers, and (c) pump-around reflux. The latter two methods essentially function as intercondenser schemes that reduce the top-tray-refliix requirement. As shown in Fig. 13-93 for the example being considered, the internal-reflux flow rate decreases rapidly from the top tray to the feed-flash zone for case a. The other two cases, particularly case c, result in better balancing of the column-refliix traffic. Because of this and the opportunity provided to recover energy at a moderate- to high-temperature level, pump-around reflirx is the most commonly used technique. However, not indicated in Fig. 13-93 is the fact that in cases h and c the smaller quantity of reflux present in the upper portion of the column increases the tray requirements. Furthermore, the pump-around circuits, which extend over three trays each, are believed to be equivalent for mass-transfer purposes to only one tray each. Bepresentative tray requirements for the three cases are included in Fig. 13-92. In case c heat-transfer rates associated with the two pump-around circuits account for approximately 40 percent of the total heat removed in the overhead condenser and from the two pump-around circuits combined. 

More often than not the rate at which residual absorbed gas can be driven from the liqmd in a stripping tower is limited by the rate of a chemical reaction, in which case the liquid-phase residence time (and hence, the tower liquid holdup) becomes the most important design factor. Thus, many stripper-regenerators are designed on the basis of liquid holdup rather than on the basis of mass transfer rate. 

Tray Efficiencies in Plate Absorbers and Strippers Compn-tations of the nnmber of theoretical plates N assnme that the hqnia on each plate is completely mixed and that the vapor leaving the plate is in eqnihbrinm with the liqnid. In actnal practice a condition of complete eqnihbrinm cannot exist since interphase mass transfer reqnires a finite driving-force difference. This leads to the definition of an overall plate efficiency... 

Mass Transfer Relationships for calculating rates of mass transfer between gas and liquid in packed absorbers, strippers, and distillation columns may be found in Sec. 5 and are summarized in Table, 5-28. The two-resistance approach is used, with rates expressed as transfer units ... 

Some performance data of plants with DEA are shown in Table 23-11. Both the absorbers and strippers have trays or packing. Vessel diameters and allowable gas and liquid flow rates are estabhshed by the same correlations as for physical absorptions. The calciilation of tower heights utilizes data of equilibria and enhanced mass-transfer coeffi-... 

Prepare an outline design of the reactor and carry out the chemical engineering design of the stripper, specifying the interfacial contact area which will need to be provided between the carbon dioxide stream and the product stream to enable the necessary mass transfer to take place. 

Applying Henry s law at the point that air leaves the stripper (i.e., the contaminated water enters stripper) and assuming that equilibrium for mass transfer holds between air and water at that point, Equation 18.9 becomes... 

Use of HTU and K a Data In estimating the size of a commercial gas absorber or liquid stripper it is desirable to have data on the overall mass-transfer coefficients (or heights of transfer units) for the system of interest, and at the desired conditions of temperature, pressure, solute concentration, and fluid velocities. Such data should best be obtained in an apparatus of pilot-plant or semiworks size to avoid the abnormalities of scale-up. Within the packing category, there are both random and ordered (structured) packing elements. Physical characteristics of these devices will be described later. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

For the liquid-phase mass-transfer coefficient the effects of total system pressure can be iraored for all practical purposes. Thus, when using Kq and for the design of gas absorbers or strippers, the... 

Structured packings have replaced trays and random packings as their cost has decreased and more is known of their performance behavior. Initially thought to be appropriate only for high vacuum distillations, they are now used for absorbers, strippers, and pressure distillations. Because of their open structure (over 90% voids) and large specific surface areas, their mass transfer efficiency is high when proper distribution of liquid and gas over the cross section can be maintained. Table 13.15 shows a comparison of features of several commercial makes of structured packings. 

The general form of the equation for the rate of mass transfer across the gas/liquid interface in a gas stripper is (4)... 

The function of the packing material in an air stripper is to provide a large wetted surface area for mass transfer of contaminants to the gas phase, or ambient air. Several shapes and sizes are available, such as rings, saddles, and spheres. The packing material is commonly manufactured from polypropylene, PVC, or ceramic. 

Volatile compounds exhibit high activity coefficients in water and are easily evaporated. The countercurrent air stripper provides a large wetted surface area for mass transfer in a compact unit. Although routine maintenance is required, the components of the air stripper should have long service lives. The air stripper is capable of removing large numbers of volatile compounds at relatively low cost. 

J. E. Thom and W. D. Byers, Limitations and practical use of a mass transfer model for predicting air stripper performance. Enviorn. Prog. 12(1), 61 (1993). 

The component separation in absorbers or strippers depends both on the number of stages in the column and on the ratio of liquid-to-vapor feed rates. This ratio is bracketed on the basis of key component selection (Section 8.2). Most of the mass transfer of different components from one phase to the other usually takes place at different stages in the column. The transfer of the key component generally takes place over a larger number of stages than the other components. This implies that the number of stages affects mostly the key component recovery. 

The mass transfer equations discussed above are now combined with a material balance on the transferred component to calculate the column or packing height required for a given separation. The column cross-sectional area A is assumed known at this point although in a complete column design A must be determined based on pressure drop considerations. The column, which is in countercurrent flow with only liquid feed and vapor product at the top, and vapor feed and liquid product at the bottom (absorber, stripper, column section), is deflned as follows ... 

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