Absorption theoretical trays

Kremser-Brown-Sherwood Method — No Heat of Absorption, 108 Absorption — Determine Component Absorption in Fixed Tray Tower, 108 Absorption — Determine Number of Trays for Specified Product Absorption, 109 Stripping — Determine Theoretical Trays and Stripping or Gas Rate for a Component Recovery, 110 Stripping — Determine Stripping-Medium Rate for Fixed Recovery, 111 Absorption — Edmlster Method, 112 Example 8-33 Absorption of Hydrocarbons with Lean Oil, 114 Inter-cooling for Absorbers, 116 Absorption and Stripping Efficiency, 118 Example 8-34 Determine Number of Trays for Specified Product Absorption, 118 Example 8-35 Determine Component Absorption in Fixed-Tray Tower, 119 Nomenclature for Part 2, 121... 

A presaturator to provide lean oil/gas contact prior to feeding the lean oil into the tower can be a good way to get more out of an older tower. Absorber tray efficiences run notoriously low. A presaturator that achieves equilibrium can provide the equivalent of a theoretical tray. This can easily equal 3-4 actual trays. Some modem canned computer distilla-tion/absorption programs provide a presaturator option. 

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. 

Four theoretical trays have been stepped off for the key component (butane) on Fig. 14-11, and are seen to give a recovery of 75 percent of the butane. The operating lines for the other components have been drawn with the same slope and placed so as to give approximately the same number of theoretical trays. Figure 14-11 shows that equilibrium is easily achieved in fewer than four theoretical trays and that for the heavier components nearly complete recovery is obtained in four theoretical trays. The diagram also shows that absorption of the light components takes place in the upper part of the tower, and the final recovery of the heavier components takes place in the lower section of the tower. 

The left side of Eq. (14-55) represents the efficiency of absorption of any one component of the feed gas mixture. If the solvent is solute-free so that X2 = 0, the left side is equal to the fractional absorption of the component from the rich feed gas. When the number of theoretical trays N and the liquid and gas feed rates Lh and GjJ, have been fixed, the fractional absorption of each component may be computed directly, and the operating lines need not be placed by trial and error as in the graphical method described above. 

Multicomponent distillation, 393 absorption factor method, 398 azeotropic, 420-426 bubblepoint (BP) method, 406-409 computer program references. 404 concentration profiles, 394 distribution of non-kevs. 395 Edmister method, 398,399 extractive, 412, 417-422 feed tray location, 397 free variables, number of 395 Lewis-Matheson method 404 MESH eauations. 405-407 molecular, 425-427 nomenclature, 405 number of theoretical trays, 397 packed towers, 433-439 petroleum, 411-415 reflux, minimum, 397 reflux, operating, 397 SC (simultaneous correction) method, 408-411... 

The initial solvent and feed concentrahons, the desired hnal concentrations, and equilibrium behavior determine the direction of mass transfer, the minimum solvent-to-feed ratio, and the minimum theoretical tray requirements. These theoretical trays (or stages) are analogous in many respects to theoretical plates of a distillation colnmn, and absorption (or stripping) columns discussed by Fair in another chapter. For any particnlar solvent-to-feed ratio, equilibrium relationships and the operating line determine theoretical stage reqnirements. 

It is desired lo recover 75% of the propane (called the key component) from a gas stream of the composiliou givan in Table 6.2-1 using oil absorption. The absorber is equivalent to six theoretical trays nad operatra at 1000 psig (7 X ICh kPa). Il is assumed that the entering lean oil is stripped completely of rich gas components and that the absorber operates at a constant temperature of I04 F (40°C). What oil cjioslalion rate is required, and what will be the composiliou of the residue gas leaving the absorber ... 

Equations (6.2-6) and (6.2-8) are plotted in Fig. 6.2-3, which can be used for the quick estimation of the number of theoretical trays required for absorbers or strippers handling dilute gas and liquid mixtures where the absorption (or stripping) factor is relatively constant over the colurtm. 

EXAMPLE 10.6-J. Absorption of SO 2 in a Tray Tower A tray tower is to be designed to absorb SOj from an air stream by using pure water at 293 K (68°F). The entering gas contains 20 mol % SO and that leaving 2 mol % at a total pressure of 101.3 kPa. The inert air flow rate is 150 kg air/h m, and the entering water flow rate is 6000 kg water/h m. Assuming an overall tray efficiency of 25%, how many theoretical trays and actual trays are needed Assume that the tower operates at 293 K (20 C). 

Analytical Method for Number of Trays in Absorption. Use the analytical equations in Section 10.3 for countercurrent stage contact to calculate the number of theoretical trays needed for Example 10.6-1. 

Absorption of Ammonia in a Tray Tower. A tray tower is to be used to remove 99% of the ammonia from an entering air stream containing 6 mol % ammonia at 293 K and 1,013 x 10 Pa. The entering pure water flow rate is 188 kg H20/h m and the inert air flow is 128 kg air/h m. Calculate the number of theoretical trays needed. Use equilibrium data from Appendix A.3. For the dilute end of the tower, plot an expanded diagram to step off the number of trays more accurately. 

In all the previous discussions on theoretical trays or stages in distillation, we assumed that the vapor leaving a tray was in equilibrium with the liquid leaving. However, if the time of contact and the degree of mixing on the tray is insufficient, the streams will not be in equilibrium. As a result the efficiency of the stage or tray is not 100%. This means that we must use more actual trays for a given separation than the theoretical number of trays determined by calculation. The discussions in this section apply to both absorption and distillation tray towers. 

The amount of BTEX absorbed in the contactor is a function of its solubility in the glycol used, concentration in the feed gas, absorption pressure and temperature, number of theoretical trays, and glycol circulation rate. The Henry s law constant for benzene in TEG at 1,000 psia is plotted in Figure 11 37, which presents values calculated by Fitz and Hubbard (1987)... 

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. 

Determining the number of theoretical and actual trays in a distillation column is only part of the design necessary to ensure system performance. The interpretation of distillation, absorption, or stripping requirements into a mechanical vessel with internal components (trays or packing, see Chapter 9) to carry out the function requires use of theoretical and empirical data. The costs of this equipment are markedly influenced by the column diameter and the intricacies of the trays, such as caps, risers, weirs, downcomers, perforations, etc. Calcvdated tray efficiencies for determination of actual trays can be lost by any unbalanced and improperly designed tray. 

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