Specifying

In vapor-liquid equilibria, if one phase composition is given, there are basically four types of problems, characterized by those variables which are specified and those which are to be calculated. Let T stand for temperature, P for total pressure, for the mole fraction of component i in the liquid phase, and y for the mole fraction of component i in the vapor phase. For a mixture containing m components, the four types can be organized in this way ... 

When only the total system composition, pressure, and temperature (or enthalpy) are specified, the problem becomes a flash calculation. This type of problem requires simultaneous solution of the material balance as well as the phase-equilibrium relations. 

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... 

If the equilibrium ratios were known or specified. Equation (7-8) could be substituted in Equation (7-6) or Equation (7-9) in (7-7) to give implicit relations for a ... 

The equilibrium ratios are not fixed in a separation calculation and, even for an isothermal system, they are functions of the phase compositions. Further, the enthalpy balance. Equation (7-3), must be simultaneously satisfied and, unless specified, the flash temperature simultaneously determined. 

Both vapor-liquid flash calculations are implemented by the FORTRAN IV subroutine FLASH, which is described and listed in Appendix F. This subroutine can accept vapor and liquid feed streams simultaneously. It provides for input of estimates of vaporization, vapor and liquid compositions, and, for the adiabatic calculation, temperature, but makes its own initial estimates as specified above in the absence (0 values) of the external estimates. No cases have been encountered in which convergence is not achieved from internal initial estimates. 

CALCULATE PURE COMPONENT LIQUID FUGACITY AT SPECIFIED TEMP AND ZERO PRESSURE IF IVAP.LE.2 C PURE CCMPDNENT VAPOR PRESSURE IF IVAP.EQ.3... 

VALIK calculates vapor-liquid vaporization equilibrium ratios, K(I), for each component in a mixture of N components (N 20) at specified liquid composition, vapor composition, temperature, and pressure. 

All of the above calculations are done at the specified system temperature. 

The temperature and composition of each feed stream and the stream ratios are specified along with a common feed pressure (significant only for the vapor stream) and the flash pressure. For an isothermal flash the flash temperature is also specified. Resulting vapor and liquid compositions, phase ratios, vaporization equilibrium ratios, and, for an adiabatic flash, flash temperature are returned. 

ERF error flag, integer variable normally zero ERF= 1 indicates parameters are not available for one or more binary pairs in the mixture ERF = 2 indicates no solution was obtained ERF = 3 or 4 indicates the specified flash temperature is less than the bubble-point temperature or greater than the dew-point temperature respectively ERF = 5 indicates bad input arguments. 

BUDET calculates the bubble-point temperature or dew-point temperature for a mixture of N components (N < 20) at specified pressure and liquid or vapor composition. The subroutine also furnishes the composition of the incipient vapor or liquid and the vaporization equilibrium ratios. 

The temperature, T, and overall mole fractions, Z(I), of the system must be specified. 

FOR NEW SYSTEMS WITHOUT IR. IE SPECIFIED. FIND IR. IE AS LEAST SOLUBLE... 

At this stage, how great the excess of chlorine should be for Fig. 4.7c to be feasible cannot be specified. Experimental work on the reaction chemistry would be required in order to establish this. However, the size of the excess does not change the basic structure. 

Specifying the hot utility or cold utility or AT m fixes the relative position of the two curves. As with the simple problem in Fig. 6.2, the relative position of the two curves is a degree of freedom at our disposal. Again, the relative position of the two curves can be changed by moving them horizontally relative to each other. Clearly, to consider heat recovery from hot streams into cold, the hot composite must be in a position such that everywhere it is above the cold composite for feasible heat transfer. Thereafter, the relative position of the curves can be chosen. Figure 6.56 shows the curves set to ATn,in = 20°C. The hot and cold utility targets are now increased to 11.5 and 14 MW, respectively. 

These expressions define Pn-2n for number of 1-2 shells in series in terms of R and Xp in each shell. The expressions can be used to define the number of 1-2 shells in series required to satisfy a specified value of Xp in each shell for a given R and Pjv 2n- Hence the relationship can be inverted to find the value of N which satisfies Xp exactly in each 1-2 shell in the series ... 

British thermal unit (Btu) The most commonly used industrial heal unit the amount of heat required to raise 1 lb of water through UF under specified conditions. Since the specific heat of water varies, particularly with temperature, the actual value of Btu is dependent on the conditions chosen as stan-... 

U is essential to specify the physical states of the reactants and products, since there may t>e additional heat changes associated with changes in state. 

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