Design calculations absorption

The simplified design calculations presented in this section are intended to reveal the Key features of gas absorption involving multi-component systems. It is expected that rigorous computations, based upon the methods presented in Sec. 13, will be used in design practice. Nevertheless, it is valuable to study these simplified design methods and examples since they provide insight into the key elements of multicomponent absorption. 

This section discusses the design of a suitable absorption column for the production of nitric acid by the single-pressure process. A comprehensive design study determined the column mechanical details and physical characteristics, together with its anticipated performance. This chapter contains the operating criteria and product specifications, the solution method, and finally the results of the design calculations. Details of the design calculations and all associated data are included in Appendix G. 

Determine the relevant vapor-pressure data. Design calculations involving vapor-liquid equilibrium (VLE), such as distillation, absorption, or stripping, are usually based on vapor-liquid equilibrium ratios, or K values. For the tth species, K, is defined as K, = y, /x, where y, is the mole fraction of that species in the vapor phase and x, is its mole fraction in the liquid phase. Sometimes the design calculations are based on relative volatility c/u], which equals K,/Kj, the subscripts i and j referring to two different species. In general, K values depend on temperature and pressure and the compositions of both phases. 

Surface tension data of liquids are important in many process design calculations for situations where these is a two-phase interface, e.g., two-phase flow, distillation, absorption and condensation. Surface tension can be expressed as ... 

Actual partial condensers usually operate somewhere between Cases 1 and 2. For an absorption naphtha stabilizer, Gunness (Ref. 6) (see page 116) found good agreement with Case 2. In actual design calculation, the conservative assumption is to assume operation as in Case 1, and any fractionation that does occur will act as a factor of safety with ordinary condenser design, with the most optimistic assumption,... 

They performed laboratoiy tests to measure the rate of COS absorption in MDEA and used a model based on the penetration theory to calculate the kinetic rate constant for equation 2-46. The results should be of value in rigorous design calculations to predict the fiac-tion of COS in a feed stream that will be absorbed in a commercial MDEA contactor. 

In general, the design of hydrocarbon absorption systems is straightforward. Since mass U ansfer is not complicated by the occurrence of chemical reactions, conventional absorption coefficient, theoretical plate, and absorption factor concepts can be used for design calculations (see Chapter I). Basic data for such calculations, including the thermodynamic properties of compounds found in coke-oven gas and equilibrium data for several gas-coal liquid systems, are given in the U.S. DOE Coal Conversion Systems Data Book (1982) and other hydrocarbon data compendia. 

Units. The unit of sound absorption is the metric sabin, which is equivalent to one square meter of "perfect" absorption, eg, one square meter of a material with a = 1.0. The Knglish unit of sound absorption is the sabin, which is equivalent to one square foot of perfect absorption. In order to avoid confusion, the designation metric should always be used when referring to metric sabins. The number of metric sabins of absorption provided by an area of material is calculated by multiplying its area by its sound-absorption coefficient. For example, 10 m of material having a sound-absorption coefficient of 0.75 provides 7.5 metric sabins of absorption. 

The proper design of distillation and absorption columns depends on knowledge of vapor—Hquid equiHbrium, as do flash calculations used to determine the physical state of streams at given conditions of temperature, pressure, and composition. Detailed treatments of vapor—Hquid equiHbria are available (6,7). 

When it is known that Hqg varies appreciably within the tower, this term must be placed inside the integr in Eqs. (5-277) and (5-278) for accurate calculations of hf. For example, the packed-tower design equation in terms of the overall gas-phase mass-transfer coefficient for absorption would be expressed as follows ... 

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

In gas absorption, the new phase consists of an inert nonvolatile solvent (absorption) or an inert nonsoluble gas (stripping), and normally no reflux is involved. The following paragraphs discuss some of the considerations peculiar to gas-absorption calculations for plate towers and some of the approximate design methods that can be employed when simplifying assumptions are vahd. 

In concentrated wstems the change in gas aud liquid flow rates within the tower and the heat effects accompanying the absorption of all the components must be considered. A trial-aud-error calculation from one theoretical stage to the next usually is required if accurate results are to be obtained, aud in such cases calculation procedures similar to those described in Sec. 13 normally are employed. A computer procedure for multicomponent adiabatic absorber design has been described by Feiutnch aud Treybal [Jnd. Eng. Chem. Process Des. Dev., 17, 505 (1978)]. Also see Holland, Fundamentals and Modeling of Separation Processes, Prentice Hall, Englewood Cliffs, N.J., 1975. 

When two or more gases are absorbed in systems involving chemical reac tions, the situation is much more complex. This topic is discussed later in the subsection Absorption with Chemical Reac tion. Graphical Design Method for Dilute Systems The following notation for multicomponent absorption calculations has been adapted from Sherwood, Pigford, and Wilke (Mass Transfer, McGraw-Hill, New York 1975, p. 415) ... 

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