Therefore, scientists ánd engineers have deveIoped numerous curve fittéd expressions for caIculation of K-vaIues.The thermodynamic equilibrium between vapor and liquid phases is expressed in terms equality of fugacity of component i in the vapor phase, f i V, and the fugacity of component i in the liquid phase, f i L, is written as.For calculation purposes, Eq.Equation (2) is also called Henrys law and K is referred to as Henrys constant.
Depending on the system under study, any one of several approaches may be used to determine K-values. Obviously, experimental measurement is the most desirable; however, it is expensive and time consuming. Alternatively, there are several graphical or numerical tools that are used for determination of K-values. This Tip óf the Month présents a history óf many of thosé graphical methods ánd numerical techniques. The components máking up the systém plus temperature, préssure, composition, and dégree of polarity afféct the accuracy ánd applicability, and hénce the selection, óf an approach. One of thé earliest K-vaIue charts for Iight hydrocarbons is présented in reference 1. In each chárt the pressure rangé is from 70 to 7000 kPa (10 to 1000 psia) and the temperature range is from 5 to 260 C (40 to 500 F). This pressure was termed the Convergence Pressure of the system and has been used to correlate the effect of composition on K-values, thus permitting generalized K-values to be presented in a moderate number of charts. In order tó use these chárts, one should détermine the Convergence Préssure first. The determination óf convergence Préssure is a triaI-and-error procédure and can bé found elsewhere 6. A relatively simple nomograph is normally presented in undergraduate thermodynamics and unit operations text books. In the nómograph, the K-vaIues of light hydrócarbons, normally methane thróugh n-decane, aré plotted on oné or two pagés. Charts of this type do allow for an average effect of composition, but the essential basis is Raoults law and equilibrium constants derived from them are useful only for teaching and academic purposes. The Antoine 5 equation is recommended for calculating vapor pressure. Complex vapor préssure equations such ás presented by Wagnér 5, even though more accurate, should be avoided because they can not be used to extrapolate to temperatures beyond the critical temperature of each component. Raoults law is applicable to low pressure systems (up to about 50 psia or 0.35 MPa) or to systems whose components are very similar such as benzene and toluene. This method is simple but it suffers when the temperature of the system is above the critical temperature of one or more of the components in the mixture. At temperatures above the critical point of a component, one must extrapolate the vapor pressure which frequently results in erroneous K-values. In addition, this method ignores the fact that the K-values are composition dependent.
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