Single-Stage Operation—Flash Vaporization

Flash vaporization, or equilibrium distillation as it is sometimes called, is a single-stage operation wherein a liquid mixture is partially vaporized, the vapor allowed to come to equilibrium with the residual liquid, and the resulting vapor and liquid phases are separated and removed from the apparatus. It may be batchwise or continuous. 

On the Hxy diagram, this represents a straight line through points of coordinates (Hiy representing Z , x ) representing PF, and ( Hp + 

D and V would be on a tie line in the upper figure and on the equilibrium curve at N on the lower figure. The richest vapor, but infinitesimal in amount, is that corresponding to P at the bubble point of the feed and the leanest liquid, but also infinitesimal in amount, is that corresponding to T at the dew point of the feed. The compositions of the actual products will be between these limits, depending upon the extent of vaporization of the feed and the stage efficiency. 

All the equations apply equally well to the case where the feed is a vapor and Qy the heat removed in the heat exchanger to produce incomplete condensation, is taken as negative. On the upper part of Fig. 9.14, point F is then either a saturated or superheated vapor. 

The equilibrium data were determined in Illustration 9.1 and are plotted in Fig. 9.15. The point representing the feed composition is plotted at P, and the operating line is drawn with a slope - 0.667 to intersect the equilibrium curve at T, where y% 0.575 mole fraction heptane and Xk, = 0.387 mole fraction heptane. The temperature at T is 113 C, 

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Successive flash vaporizations can be made on the residual liquids in a series of single-stage operations, whereupon the separation will be better than that obtained if the same amount of vapor were formed in a single operation. As the amount of vapor formed in each stage becbmes smaller and the total number of vaporizations larger, the operation approaches differential distillation in the limit. 

Flash vaporization is yet another single-stage operation in which a liquid mixture is first heated xmder pressure and then allowed to flash into a chamber maintained at a lower pressure, causing partial vaporization (Figure 7.32a). Its purpose is to achieve a first crude separation of the mixture into volatile and high boiling fractions. The two equilibrated phases are then withdrawn separately for further processing. 

The calculation of single-stage equilibrium separations in multicomponent systems is implemented by a series of FORTRAN IV subroutines described in Chapter 7. These treat bubble and dewpoint calculations, isothermal and adiabatic equilibrium flash vaporizations, and liquid-liquid equilibrium "flash" separations. The treatment of multistage separation operations, which involves many additional considerations, is not considered in this monograph. 

Single contact. This involves the use of a single stage, where solution to be separated and extracting solvent are contacted once and extract and raffinate phases separated. Operation may be batch wise or continuous. The distillation analog is flash vaporization or equilibrium distillation. 

The initial starting point rests on the proposition that an absolute value for V, the molar vapor rate, can be established by a single-stage flash-type calculation on the feed or feedstream to the operation. As developed in Chapter 3 and Appendix 3, this determination is trial and error in V that is, V is the permeate phase arising from the feed stage designated both by + 1 and m -i- 1 (or by f). 

Extraction Extrusion n An extrusion operation in which a volatile component present in the feedstock is removed by flash vaporization through a vent coimected to a vacuum pump. The volatile component is typically a small amount of water, but may be monomer or solvent. In a two-stage, single-screw extruder, the vent is located over the deep extraction section that begins the second stage of the screw. A few double-vented (three-stage) machines have been made. Some twin-screw machines have greater capacity for removal of volatiles. 

Flash Units. In simulators, the term flash refers to the module that performs a single-stage vapor-liquid equilibrium calculation. Material, energy, and phase equilibrium equations are solved for a variety of input parameter specifications. In order to specify completely the condition of the two output streams (liquid and vapor), two parameters must be input. Many combinations are possible—for exanple, temperature and pressure, temperature and heat load, or pressure and mole ratio of vapor to liquid in exit streams. Often, the flash module is a combination of two pieces of physical equipment, that is, a phase separator and a heat exchanger. These should appear as separate equipment on the PFD. Note that a flash unit can also be specified for batch operation, in which case the unit can serve as a surge or storage vessel. 

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