Distillation flash calculations

Figure 1.8. Information flow, binary flash distillation calculation (a) Information recycle (b) Information flow...

BUBBLE-POINT AND DEW-POINT CALCULATION. Determination of the bubble point (initial boiling point of a liquid mixture) or the dew point (initial condensation temperature) is required for a flash-distillation calculation and for each stage of a multicomponent distillation. The basic equations are, for the bubble point,... 

Integral condensation in which the liquid remains in equilibrium with the uncondensed vapour. The condensation curve can be determined using procedures similar to those for multicomponent flash distillation given in Chapter 11. This will be a relatively simple calculation for a binary mixture, but complex and tedious for mixtures of more than two components. 

Distillation Calculations, Work done with flash calculations and sparse matrix methods was extended to distillation calculations. Holland and Gallun (51) explored the use of Broyden s method coupled with sparse updating procedures to distillation calculations with highly non-ideal solutions. Shah and Boston (52), and Ross and Seider (53j discuss the case of multiple liquids phases on a tray. 

The pressure in the flash distillation unit is 1.121 psia and the temperature is 100°F. Calculate the pounds of pure water obtained in the vapor stream per 100 lb of feed solution and the weight percent of the dissolved nonvolatile solid leaving in the liquid stream. 

Column pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude oil feed temperature at flash-zone pressure must be sufficient to vaporize the total distillates plus the overflash, which is necessary to provide reflux between the lowest sidestream-product drawoff tray and the flash zone. Calculations are made by using the crude oil EFV curve corrected for pressure. For the example being considered, percent vaporized at the flash zone must be 53.1 percent of the feed. 

In this chapter, the fundamental principles and relationships involved in making multicomponent distillation calculations are developed from first principles. To enhance the visualization of the application of the fundamental principles to this separation process, a variety of special cases are considered which include the determination of bubble-point and dew-point temperatures, single-stage flash separations, multiple-stage separation of binary mixtures, and multiple-stage separation of multicomponent mixtures at the operating conditions of total reflux. 

A liquid containing 25 mole percent toluene, 40 mole percent ethylbenzene, and 35 mole percent water is subjected to a continuous flash distillation at a total pressure of 0.5 atm. Vapor-pressure data for these substances are given in Table 18.5. Assuming that mixtures of ethylbenzene and toluene obey Raoult s law and that the hydrocarbons are completely immiscible in water, calculate the temperature and... 

In another study, Grayson examined the effect of K-values on bubble-point, dew-point, equilibrium flash, distillation, and tray efficiency calculations. He noted a wide range of sensitivity of design calculations to variations in K-values. 

In the table the second, third, and fourth problems each result from a permutation of the known and unknown quantities that occur in the bubble-T calculation. We refer to these as P-problems, because each problem is well-posed when values are specified for P independent intensive properties, where the value of T is given by the phase rule (9.1.14). However, the flash problem in Table 11.1 differs from the others in that it is an P -problem it is well-posed when values are specified for T independent intensive properties, with the value of T given by (9.1.12). Flash calculations pertain to separations by flash distillation in which a known amount N of one-phase fluid, having known composition z, is fed to a flash chamber. When T and P of the chamber are properly set, the feed partially flashes, producing a vapor phase of composition xP in equilibrium with a liquid of composition x ). The problem is to determine these compositions, as well as the fraction of feed that flashes NP/N. Unlike the other problems in Table 11.1, the flash problem involves the relative amounts in the phases and therefore a solution procedure must invoke not only the equilibrium conditions (11.1.1) but also material balances. 

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. 

This chapter discussed VLE and the calculation procedures for binary and multiconponent flash distillation. At this point you should be able to satisfy the following objectives ... 

A13. Calculations are simpler for multiconponent flash distillation if the feed flow rate and mole fractions of the feed are specified plus... 

C9. For a vapor-liquid-liquid flash distillation, derive Eqs. (2-62) and (2-63) and the equations that allow calculation of all the mole fractions once V/F and i/F are known. 

McCabe-Thiele calculations are easiest to do on spreadsheets if the y versus x VLE data are expressed in an equation. The form y = f(x) is most convenient for flash distillation and for distillation columns (see Chapter 4) if stepping off stages from the bottom of the column up. The form x = g(y) is most convenient for distillation columns if stepping off stages from the top down. Built-in functions in Excel will determine polynomials that fit the data, aldiough the fit will usually not be perfect. This will be illustrated for fitting the ethanol-water equilibrium data in Table 2-1 in the form y = f (x ). (Note An additional data point Xg = 0.5079, = 0.6564 was added to the numbers in the table.) Enter the data in the... 

If VLE data are available in equation form, spreadsheet calculations can also be used for multiconponent flash distillation. These calculations are illustrated for a chemical mixture that follows Eq. 12-161 for Problem 2.D16. First, the spreadsheet is shown in Figure 2-B3 with the equations in each cell. Cells B3 to B6, C3 to C6, D3 to D6, E3 to E6, F3 to F6, and G3 to G6 are the constants for Eq. (2-161 from Table 2-3. Conditions for the operation are input in cells B7, D7, and F7, and the feed mole fractions are in cells B8, C8, F8, and G9. Eq. 12-161 is programmed for each conponent in cells BIO, Bll, B12, and B13. Then the liquid mole fractions are determined fromEq. 12-381 in cells B15 to B18. These four numbers are summed in cell B19. The Rachford-Rice terms from Eq. 12-421 for each conponent are calculated in cells B20 to B23, and the sum is in B24. 

This result is analogous to the use of q in flash distillation. Since the liquid and vapor enthalpies can be estimated, we can calculate q fromEq. (4-17). Then... 

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. 

Absorbers, like flash distillation, are equivalent to very wide boiling feeds. Thus, in contrast with distillation, a wide-boiling feed (sum rates) flowchart such as Figure 2-13 should be used. The flow rate loop is now solved first, since flow rates are never constant in absorbers. The energy balance, which requires the most information, is used to calculate new temperatures, since this is done last. Figure 12-13 shows the sum-rates flow diagram for absorbers and strippers when K = K (T, p). If K = Kj (T, p, Xj, X2,. .. Xj,) a concentration correction loop is added. The initial steps are very similar to those for distillation, and usually the same physical properties package is used. 

F. Generalize. This procedure is similar to the one we used for binary flash distillation in Figure 2-9. Thus, there is an analogy between distillation calculations on enthalpy-conposition diagrams (Ponchon-Savarit diagrams) and extraction calculations on triangular diagrams. 

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