Flash distillation mass balance

Flash calculations and the application of the lever rule (overall mass balance relating the feed, distillate and bottoms product streams) to predict feasible sharp splits for a given feed condition. 

Note that calculating the flash trajectories at (f> = 0.5 does not provide the entire feasible product regions for continuous RD, but instead generates a subset of the feasible products. Selecting an iterate on the stripping cascade trajectory as a potential bottoms and an iterate on the rectifying cascade trajectory as a potential distillate does not guarantee that these products can also be obtained simultaneously from a RD column. This is simply because these product compositions may not simultaneously satisfy the overall mass balance for a reactive column. However, when the flash trajectories are used in conjunction with the lever rule for a continuous reactive column, the feasible splits for continuous RD can be quickly predicted. 

For binary flash distillation, the simultaneous procedure can be conveniently carried out on an enthalpy-composition diagram First calculate the feed enthalpy, hp, from Eq. t2-81 or Eq. (2=9b) then plot the feed point as shown on Figure 2-9 (see Problem 2-All. In the flash drum the feed separates into liquid and vapor in equilibrium Thus the isotherm through the feed point, which must be the T nun isotherm, gives the correct values for x and y. The flow rates, L and V, can be determined from the mass balances, Eqs. f2-51 and 2-61. or from a graphical mass balance. 

With three phases, the conponent mass balance for a flash distillation system is... 

In Section 2.7 we looked at solution methods for multiconponent flash distillation. The questions asked in that section are again pertinent for multiconponent distillation. First, what trial variables should we use As noted, because N and Np are required to set up the matrices, in design problems we choose these and solve a number of simulation problems to find the best design. We select the tenperature on every stage Tj because tenperature is needed to calculate K values and enthalpies. We also estimate the overall liquid Lj and vapor Vj flow rates on every stage because these flow rates are needed to solve the conponent mass balances. 

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. 

---