Equilibrium flash calculation

In an equilibrium flash process a feed stream is separated into liquid and vapour streams at equilibrium. The composition of the streams will depend on the quantity of the feed vaporised (flashed). The equations used for equilibrium flash calculations are developed below and a typical calculation is shown in Example 11.1. 

Flash calculations are often needed to determine the condition of the feed to a distillation column and, occasionally, to determine the flow of vapour from the reboiler, or condenser if a partial condenser is used. 

Single-stage flash distillation processes are used to make a coarse separation of the light components in a feed often as a preliminary step before a multicomponent distillation column, as in the distillation of crude oil. 

If the vapour-liquid equilibrium relationship is expressed in terms of equilibrium constants, equation 11.7 can be written in a more useful form  

The groups incorporating the liquid and vapour flow-rates and the equilibrium constants have a general significance in separation process calculations. 

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The system is specified by the algebraic equations listed above. This is just a traditional steadystate equilibrium-flash calculation. 

An equilibrium-flash calculation (using the same equations as in case A above) is made at each point in time to find the vapor and liquid flow rates and properties immediately after the pressure letdown valve (the variables with the primes F , F l, y], x j,.. . shown in Fig. 3.8). These two streams are then fed into the vapor and liquid phases. The equations describing the two phases will be similar to Eqs. (3.40) to (3.42) and (3.44) to (3.46) with the addition of (1) a multi-component vapor-liquid equilibrium equation to calculate Pi and (2) NC — 1 component continuity equations for each phase. Controller equations relating 1 to Fi and P to F complete the model. 

Equilibrium Flash Calculations in Systems Containing Gas Hydrates. Fluid Phase Equilibria, 53,97-104. 

Since K-vslue is defined as the ratio of mol fraction of a component in the vapor to mol fraction of that component in the equilibrium liquid, when K-values arc applied in a conventional equilibrium flash calculation the condition which will give the greatest proportion of a component in the liquid phase is the condition where its K-value is lowest. 

The emphasis on vapour-liquid equilibria (including vapour pressure) is inherant in the petroleum industry due to the importance of distillation in separations. If separations by extraction are to be undertaken, then liquid-liquid equilibrium is equally important. Fugacities for thermodynamic equilibrium (flash calculations) are probably one of the most sought-after properties. This is because fugacities and enthalpies often provide sufficient information to calculate a mass and energy balance. 

Bishnoi, P.R. Gupta, A.K. Englezos, P. Kalogerakis, N. Multiphase equilibrium flash calculations for systems containing gas hydrates. Fluid Phase Equilibria 1989, 53, 97-104. 

This chapter considers the vapor-liquid equilibrium of mixtures, conditions for bubble and dew points of gaseous mixtures, isothermal equilibrium flash calculations, the design of distillation towers with valve trays, packed tower design. Smoker s equation for estimating the number of plates in a binary mixture, and finally, the computation of multi-component recovery and minimum trays in distillation columns. 

The following equations are used for the multicomponent equilibrium flash calculations. 

The computer program PROG72 performs equilibrium flash calculations for an ideal multi-component mixture. The program listed determines the moles of each component in the liquid and vapor phases... 

Input Data and Computer Output for Multicomponent Equilibrium Flash Calculations... 

FORMAT(2X, MULTICOMPONENT EQUILIBRIUM FLASH CALCULATION AT ) WRITE(, 140) T, P... 

The calculation of liquid-liquid equilibrium using equations of state proceeds as in the equilibrium flash calculation described in Sec. 10.3 and illustrated in Fig.. 10.3-6, with two changes. First, since both phases are liquids, the liquid root for the compressibility (which will differ in the two liquid phases since the compositions are different) must be found in each case and used in the fugacity calculation. Second, the initial guesses for K = afe not made using the pure component vapor pressures as... 

A vapor-liquid equilibrium flash calculation is again carried out to obtain new fugacity values using Equation 15. The process of comparing fugacities and adjusting mixture composition is repeated until Equation 1 is satisfied and the SLV equilibrium condition is considered to have been found. 

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