Tray absorption

Derive the design equations for multi-tray absorption towers taking into account the change of the liquid and vapor flow rates L and V due to absorption. 

Part II contains the design of a major item of equipment (in this case study, it is a sieve-tray absorption column (Chapter 9) ), including the mechanical design, fabrication, materials specification, detailed engineering drawing, HAZOP study, control scheme and associated instrumentation. In summary, as complete and professional a design as... 

Load HPLC vials into tray Absorption (skin) <5 min Negligible Nitrile gloves, disposable laboratory coat... 

Height Equivalent to a Theoretical Plate. Provided both the equilibrium and operating lines are straight, HETP values may be estimated by combining the HG and HL values predicted by the above correlations and by translating the resulting HQG into HETP by combining equations 47, 51, and 56 with equation 85, which is discussed under bubble tray absorption columns ... 

In a paper entitled A new simulation model for a real trays absorption column, Grottoli et al. (1991) present their version of a nonequilibrium model for computer simulation of absorption columns. Using the material presented in Chapter 14 as a basis, critically examine any novel aspects of their paper. 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

Fig. 3. Flow diagram for a chemical absorption process where the horizontal lines within the towers represent trays or packing.

When it is desired to compute, with rigorous methods, actual rather than equilibrium stages, Eqs. (13-69) and (13-94) can be modified to include the Murphree vapor-phase efficiency T ij, defined by Eq. (13-29). This is particularly desirable for multistage operations involving feeds containing components of a wide range ol volatility and/or concentration, in which only a rectification (absorption) or stripping action is provided and all components are not sharply separated. In those cases, the use of a different Murphree efficiency for each component and each tray may be necessary to compute recovery accurately. 

Computation of Tower Height The required height of a gas-absorption or stripping tower depends on (1) the phase equilibria involved, (2) the specified degree of removal of the solute from the gas, and (3) the mass-transfer efficiency of the apparatus. These same considerations apply both to plate towers and to packed towers. Items 1 and 2 dictate the required number of theoretic stages (plate tower) or transfer units (packed tower). Item 3 is derived from the tray efficiency and spacing (plate tower) or from the height of one transfer unit (packed tower). Solute-removal specifications normally are derived from economic considerations. 

Mass-transfer theory indicates that for trays of a given design the factors most hkely to inflnence E in absorption and stripping towers are the physical properties of the flnids and the dimensionless ratio Systems in which the mass transfer is gas-film-controlled may be expected to have plate efficiencies as high as 50 to 100 percent, whereas plate efficiencies as low as 1 percent have been reported for the absorption of gases of low sohibility (large m) into solvents of relatively high viscosity. 

Plate Towers Plate (tray) towers are countercurrent gas-atomized spray scrubbers using one or more plates for gas-liquid contacting. They are essentially the same as, if not identical to, the devices used for gas absorption and are frequently employed in apphcations in which gases are to be absorbed simultaneously with the removal of dust. Except possibly in cases in which condensation effects are involved, countercurrent operation is not significantly beneficial in dust collection. 

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-... 

Methods for quick sizing trayed fractionation and absorption column diameter have been reduced here to equations to facilitate programming for calculators or computers. Three methods are discussed and it is not a bad idea to compare results with all three. 

Zanker gives this method for distillation and absorption column trays, which is stated to agree well with published data ... 

Why not put new lyrics to an old tune This is an excellent idea, and many have done this very thing. Rice" started w ith the Smith-Brinkley raethod" used to calculate distillation, absorption, extraction, etc., overhead and bottoms compositions, and developed distillation equations for determining the liquid composition on any tray. This together with bubble point calculations yield a column temperature profile useful for column analysis. 

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