Absorption tray efficiency

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. 

Absorption Efficiency, or fraction absorbed Overall tray efficiency, fraction Stripping efficiency, or fraction stripped Fraction of v + li absorbed by the liquid Fraction of loi stripped out of the liquid Mols individual components stripped per hour Total heat of absorption of absorbed components, thousand Btu/day... 

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. 

Tray efficiencies for distillation of light hydrocarbons and aqueous solutions are 60-90% for gas absorption and stripping, 10-20%. 

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. 

W. Witt and H. Knapp, Tray efficiencies in high pressure absorption columns experiments and correlation, German Chem. Engng., 8 (1985) 158-164. 

The mathematical model developed to describe the nitric acid absorption process isdiscussed in detail in Section G.2. It uses a tray-by-tray approach that incorporates reaction-mass balance corrections and heat balance calculations. Tray efficiency calculations are also included in the model, the efficiency being a function of the tray geometry and gas velocity. Rate equations and other data specific to the nitric acid/nitrous gas system are applied. 

The number of trays is determined by dividing the theoretical number of stages, which is obtained from the relationships in Section III, by the appropriate tray efficiency. It is best to use experimental efficiency data for the system when available, but caution is required when extending such data to column design, because tray efficiency depends on tray geometry, liquid and gas loads, and physical properties, and these may vary from one contactor to another. In the absence of data, absorption efficiency can be estimated using O Connell s empirical correlation. This correlation should not be used outside its intended range of application. 

In real reactive absorption processes, the thermodynamic equilibrium can seldom be reached. Therefore, some correlation parameters such as tray efficiencies or HETP-values (Height Equivalent to One Theoretical Plate) are introduced to adjust the equilibrium-based theoretical description to the reality. However, reactive absorption always occurs in multicomponent mixtures, for which this simplified concept often fails [16, 23, 24]. 

For low-viscosity absorption systems, one set of data (186) shows that in the froth regime tray efficiency increases as surface tension is reduced, while for high-viscosity absorption systems, surface tension had little effect on mass transfer (166). 

O Connell derived his correlation from binary systems in distillation service with bubble-cap trays. Calculated values are slightly conservative for sieve and valve trays. Credit for the slight improvement in valve and sieve tray efficiency should be ignored and counted as a design margin. A separate correlation was developed for absorption services. 

The absorber and stripper shown in Fig. 9 can be combined into a single reboiled absorption column however water condensed internally will cause bottom column corrosion and plugging, whereas in the scheme of Fig. 9, the water is removed at the feed drum. Tray efficiencies in absorption columns are much lower than in distillation columns because of the presence of noncondensible gases. Furthermore, there is a tendency to... 

Although liquid and gas streams for absorption or stripping could be contacted using a tray column (like that used in distillation), tray columns are seldom used. The reason is that tray efficiencies are generally much lower for absorption and stripping than for distillation (perhaps only 5% instead of 50%). 

Two different approaches have evolved for the simulation and design of multicomponent distillation columns. The conventional approach is through the use of an equilibrium stage model together with methods for estimating the tray efficiency. This approach is discussed in Chapter 13. An alternative approach based on direct use of matrix models of multicomponent mass transfer is developed in Chapter 14. This nonequilibrium stage model is also applicable, with only minor modification, to gas absorption and liquid-liquid extraction and to operations in trayed or packed columns. 

Chapter 12 presents models of mass transfer on distillation trays. This material is used to develop procedures for the estimation of point and tray efficiencies in multicomponent distillation in Chapter 13. Chapter 14 uses the material of Chapter 12 in quite a different way in an alternative approach to the simulation and design of distillation and absorption columns that has been termed the nonequilibrium stage model. This model is applicable to liquid-liquid extraction with very little modification. Chapter 15 considers the design of mixed vapor condensers. 

Extraction involves the transfer of components between two liquid phases, much as absorption or stripping involves the transfer of components from liquid to vapor phase or vice versa. As in vapor-liquid multistage separation processes, the device employed to carry out liquid-liquid extraction is usually a counterflow column that performs the function of a number of equilibrium stages interconnected in counterflow configuration. In each stage, two inlet liquid streams mix, reach equilibrium, and separate into two outlet liquid streams. As in vapor-liquid columns, the lack of complete equilibrium in liquid-liquid extractors is accounted for by some form of tray efficiency. Liquid-liquid extraction may also be carried out in a cascade of mixing vessels connected in series in counterflow. 

Based on preliminary studies, it was determined that a 10-tray column is appropriate for an absorption operation. The overall tray efficiency is estimated at 60%. For the indicated feed gas and absorbent, determine the expected flow rates and compositions of the products using the Kremser method. 

To determine the required size of an absorption or stripping nrtl, it is necessary to know not only the equilibrium soluhility of the solute in the solvent and the material balance atound the column bas also the rate at which solute is transferred from one phase to the other within the tower. This rale directly affects the volume of packing needed in a packed tower, the degree of dispersion requited in a spray contactor, and (somewhat less directly) the number of trays required in a nay tower. The last effect occurs as a result of the influence of mass transfer rms on tray efficiency which is discussed in a later section. Because of its direct effect ou packed tower design and the importance of this type of contactor in absoiption. this discussion of mass transfer is aimed primarily at the packed tower case. A more detailed review of mass transfer theoty is given in Chapter 2. 

FIGURE 6,4 13 Correlation of overall tray efficiency for absorption columns. H = Henry s Law coeffi-ciem, atm/flb-moie/ft3) or (N/m /tmol/m3), P = total pressure (atm or N/m1), and p = viscosity (cP or N-s/m1, (From O Connell,13 reprinted with permission from Trans. AlChE.)... 

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