Liquid phases subroutines

Figure 7-2. Conjugate liquid phase compositions for water-acrylonitrile-acetonitrile system calculated with subroutine ELIPS for feeds shown by . ...

Subroutine ACTIV2. This subroutine calculates the activity coefficients using one of the liquid-phase equations discussed in the previous section. 

The computer subroutines for calculation of vapor-phase and liquid-phase fugacity (activity) coefficients, reference fugac-ities, and molar enthalpies, as well as vapor-liquid and liquid-liquid equilibrium ratios, are described and listed in this Appendix. These are source routines written in American National Standard FORTRAN (FORTRAN IV), ANSI X3.9-1978, and, as such, should be compatible with most computer systems with FORTRAN IV compilers. Approximate storage requirements and CDC 6400 execution times for these subroutines are given in Appendix J. 

Block 7 integrates the differential equations of the component material balances. A standard differential equation solver subroutine should be used. The output of this block is values for the liquid phase composition at a new time levels. 

The calculation of vapor and liquid fugacities in multi-component systems has been implemented by a set of computer programs in the form of FORTRAN IV subroutines. These are applicable to systems of up to twenty components, and operate on a thermodynamic data base including parameters for 92 compounds. The set includes subroutines for evaluation of vapor-phase fugacity... 

The computation of pure-component and mixture enthalpies is implemented by FORTRAN IV subroutine ENTH, which evaluates the liquid- or vapor-phase molar enthalpy for a system of up to 20 components at specified temperature, pressure, and composition. The enthalpies calculated are in J/mol referred to the ideal gas at 300°K. Liquid enthalpies can be determined either with... 

The subroutine is well suited to the typical problems of liquid-liquid separation calculations wehre good estimates of equilibrium phase compositions are not available. However, if very good initial estimates of conjugate-phase compositions are available h. priori, more effective procedures, with second-order convergence, can probably be developed for special applications such as tracing the entire boundary of a two-phase region. 

Subroutine PRDTA2. This subroutine reads the pure-component and binary parameters required for the various correlations describing the liquid and vapor phases. All input parameters are printed for verification. 

THE SUBROUTINE ACCEPTS BOTH A LIQUID FEED OF COMPOSITION XF AT TEMPERATURE TL(K) AND A VAPOR FEED OF COMPOSITION YF AT TVVAPOR FRACTION OF THE FEED BEING VF (MOL BASIS). FDR AN ISOTHERMAL FLASH THE TEMPERATURE T(K) MUST ALSO BE SUPPLIED. THE SUBROUTINE DETERMINES THE V/F RATIO A, THE LIQUID AND VAPOR PHASE COMPOSITIONS X ANO Y, AND FOR AN ADIABATIC FLASHf THE TEMPERATURE T(K). THE EQUILIBRIUM RATIOS K ARE ALSO PROVIDED. IT NORMALLY RETURNS ERF=0 BUT IF COMPONENT COMBINATIONS LACKING DATA ARE INVOLVED IT RETURNS ERF=lf ANO IF NO SOLUTION IS FOUND IT RETURNS ERF -2. FOR FLASH T.LT.TB OR T.GT.TD FLASH RETURNS ERF=3 OR 4 RESPECTIVELY, AND FOR BAD INPUT DATA IT RETURNS ERF=5. 

These subroutines are used to calculate respectively, the bubble temperature of a liquid composition, dew temperature of a vapor composition, equilibrium temperature and composition of the phases produced by the flash of a particular stream at known values of total vapor and liquid, and the composition and amount of the phases produced by the flash of a particular stream at a known temperature. All are done at the column pressure which applies. 

Subroutine isotfl calculates in similar fashion, except that the smaller phase is changed by a fraction of itself, the fraction being 10% at the start and diminishing by a factor of 10 each time the calculation crosses the correct solution. A flash calculation at a set temperature can result in all vapor or all liquid, if the temperature is above the dew point or below the bubble point of the feed this possibility is provided for by setting the small phase to zero if it falls below one millionth of the feed. 

The snapshot approach for gas-liquid flows was implemented using a commercial CFD code, FLUENT (Fluent Inc., USA). User-defined subroutines were used for this purpose. Half of the vessel was considered as a solution domain. The solution domain and details of the finite volume grid used was similar to those used for singlephase flows discussed earlier (however, the number of cells in the 6 direction were half of that used in single-phase simulations). A QUICK discretization scheme with SUPERBEE limiter function was used to integrate all the equations (Fluent User Guide, 1997). Simulations were carried out for three values of dimensionless gas flow rates (Qc/ND ), 0.01, 0.02 and 0.03. 

Another consideration in the case of gas-liquid mixtures is the impact of the impeller on gas bubble size. In an actual stirred tank, the momentum of the rotating impeller often acts to break up gas bubbles as they pass through the region. This reduces the bubble size and can lead to an increase in the gas hold-up as well as a change in the momentum exchange term (drag) between the phases. When experimental data are used, this phenomenon is missing from the formulation but can often be incorporated into the calculation if subroutines, written by the user, are available to modify the model in the commercial software. 

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