Flash vaporization, curve equilibrium

Experimental Vaporization Curves. When such complex materials as gasoline and petroleum fractions are dealt with, the application of the aforementioned equilibrium laws is cumbersome. Furthermore, the component analyses of these heavy oils cannot be easily obtained and even if such analyses are-availalUe, accurate vapor-pressure or equilibrium data for the compounds or fractions contained in them are not always available. At present most equilibrium relations are obtained by determining experimental flash-vaporization curves or by computing such curves from the empirical relationships discussed in Chap. 4. Empirical flash curves can be estimated from true-boiling-point or ASTM curves, and with less accuracy from Hempel or Saybolt distillation curves. 

Fig. 19-10. Estimates of distillation and atmospheric-flash cu rves of furnace feed and effluent of Example 19-8, and the equilibrium-flash-vaporization curve of the effluent at 415 psia.

A third fundamental type of laboratory distillation, which is the most tedious to perform of the three types of laboratory distillations, is equilibrium flash vaporization (EFV), for which no standard test exists. The sample is heated in such a manner that the total vapor produced remains in contact with the total remaining liquid until the desired temperature is reached at a set pressure. The volume percent vaporized at these conditions is recorded. To determine the complete flash curve, a series of runs at a fixed pressure is conducted over a range of temperatures sufficient to cover the range of vaporization from 0 to 100 percent. As seen in Fig. 13-104, the component separation achieved by an EFV distillation is much less than that by the ASTM or TBP distillation tests. The initial and final EFV points are the bubble point and the dew point, respectively, of the sample. If desired, EFV curves can be established at a series of pressures. 

For the small concentrations of interest in flashing furfural residues (5 % by weight of furfural in water corresponds to a mere 0.977 % by mole), this ratio can be well approximated by the initial slope of the vapor/liquid equilibrium curve. Referred to mass fractions, this slope is known as the amplification factor k . Its dependence on pressure is illustrated in Figure 124. As can be seen, k increases strongly with decreasing pressure. 

Example 18.1. A mixture of 50 mole percent benzene and 50 mole percent toluene is subjected to flash distillation at a separator pressure of 1 atm. The vapor-liquid equilibrium curve and boiling-point diagram are shown in Figs. 18.2 and 18.3. Plot the following quantities, all as functions of f, the fractional vaporization (n) the temperature in the separator, b) the composition of the liquid leaving the separator, and (c) the composition of the vapor leaving the separator. 

A vertical vessel (drum), as shown in Fig. 7.1, can be used to separate vapor from liquid following equilibrium flash vaporization or partial condensation. A reasonable estimate of the minimum drum diameter Dr, to prevent liquid carryover by entrainment can be made by using (13-7) in conjunction with the curve for 24-in. tray spacing in Fig. 13.3 and a value of Fha = 10 in (13-5). To absorb process upsets and fluctuations and otherwise facilitate control, vessel volume Vv is determined on the basis of liquid residence time t, which should be at least 5 min with the vessel half full of liquid. Thus... 

The temperatures at which the concentrations of solvent vapor in equilibrium with the liquid reach the lower or upper ignition limit are obtained from the vapor pressure curves of the solvents these temperatures are termed the lower and upper explosion points. The lower explosion points roughly correspond to the flash points of the solvents [14.94]-[14.98]. 

The use of a y-x curve in the McCabe-Thiele method for binary distillation calculations brings up the matter of a flash-vaporization representation, in case the feedstream mixture is at saturation. An inspection of the y-x curve relative to a given feed composition shows that the equilibrium mixture varies along the curve over a range from the bubble point (where the liquid phase composition x is equal to that of the feed mixture x ) to the dew point (where the vapor composition y is equal to that of the feed mixture Xj.). Between the two is the region of flash vaporization, where the equilibrium compositions (y, x) respectively of phases V and L must satisfy the flash material balance relation F = V + L, where... 

Specifying p (or T) eq 9.38 permits the calculation of the equilibrium temperature (or p) as function of ij. The curve T 1 ) at constant p or p l ) at constant T is called Equilibrium-Flash-Vaporization (EFV) curve. Knowing T, p and with eqs 9.36 and 9.37 the distribution functions for the phases L and Vmay be calculated.An analytical solution of the integral of eq 9.38 is only possible for 1 = 0, that is when the feed and liquid phases are equal, when the following assumptions hold (1), the distribution function of the given liquid phase has to be Gaussian and (2), the vapour pressure function p x, T) has to be calculated with a combination of Clausius-Clapeyron s equation and Trouton s rule. 

If the pressure or the temperature is specified the Equilibrium-Flash-Vaporization (EFV) curve T ) or p ) is obtained. 

The third type of laboratory distillation which is used in discussing petroleum fractionation is the equilibrium flash vaporization (distillation) commonly called the BFV. The procedure is also discussed in Chapter 4 of Nelson. This distillation can be run at pressures above atmospheric as well as under vacuum, whereas the TBP and ASTM distillations are run either at atmospheric pressure or under vacuum. EFV curves are seldom run because of the time and expense involved and are almost exclusively limited either to crude oil or to reduced crude samples (atmospher-... 

Fig. 4-22. Correction for the effect of superatmospheric pressure on the slope of equilibrium-flash-vaporization (E.F.V.) curves. Edmister, Pet. Eng.)...

In solving these equations it is necessary to assume a value of L (or F) and by substituting this value, a value of L can be computed. If the computed value is not the same as the assumed value, other assumptions must be made until finally the assumption checks the computed value. By solving the equation for several temperatures an equilibrium-flash-vaporization (E.F.V.) curve may be drawn, but such curves are usually not so accurate as the empirical curves described on pages 112 to 119 unless precise equilibrium ratios for the system are available. The partition between liquid and vapor will occur at or near the component whose vapor pressure is equal to the pressure of the system (or K = 1.0). 

Fio. 15-14. Typical tnie-boiling-point distillation curves of vapor and liquid products from three equilibrium flash vaporizations of a petroleum fraction. (JPel. Refiner.)... 

Equilibrium separation involves the two-phase region between the two curves. Equihbrium calculations, often referred to as "flash calculations," are based upon a combination of the vapor— liquid equilibrium relationship and material balance equations. 

The flash curve of a petroleum cut is defined as the curve that represents the temperature as a function of the volume fraction of vaporised liquid, the residual liquid being in equilibrium with the total vapor, at constant pressure. 

From Table 13-5 it can be seen that the variables subject to the designer s control are C -H 3 in number. The most common way to utilize these is to specify the feed rate, composition, and pressure (C -H 1 variables) plus the drum temperature T2 and pressure P2. This operation will give one point on the equilibrium-flash curve shown in Fig. 13-26. This curve shows the relation at constant pressure between the fraction V/F of the feed flashed and the drum temperature. The temperature at V/F = 0.0 when the first bubble of vapor is about to form (saturated liquid) is the bubble-point temperature of the feed mixture, and the value at V/F = 1.0 when the first droplet of liquid is about to form (saturated liquid) is the dew-point temperature. 

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

The liquid and vapor compositions as a result of the flash are found from the intersection of the operating line and the equilibrium curve ... 

The computer-aided procedure, unless automated by the program, requires running a series of liquid-liquid equilibrium calculations (the equivalent of vapor-hquid flash calculations) at constant temperature and pressure. The composition is varied around the equilibrium curve, and the transition points from one phase to two, or vice versa, are noted. As many points as needed are obtained this way to generate the entire equilibrium curve. Also, each time an equilibrium calculation is done in the two-phase region, the compositions of the two phases are recorded. Each pair of data points thus obtained defines a tie line. The data obtained at one temperature and pressure generate one triangular diagram. If so desired, the procedure is repeated at other temperatures and pressures to determine the effect of these variables. 

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