Flash vaporization computations

Figure 6.3 shows a Mathcad computer program to perform flash-vaporization calculations. Data supplied to the program include the parameters of the Wagner equation for both components, the separator pressure, feed concentration, and the desired fractional vaporization. Initial estimates are given to the program of the separator temperature (320 K) and the vapor and liquid concentrations (yD = 0.6, xw = 0.4). The... 

Kalb, C. E., and J. D. Seader, Equilibrium Flash Vaporization by the Newton-Raphson Method, in Computer Programs for Chemical Engineering, Volume V, Thermodynamics, R. Jelinek, Ed., Aztec Publishing Co., P. O. Box 5574, Austin, Texas 78763 (1972). 

A computer simulation of a thermal cracker fractionator pumparound section based on equilibrium flash vaporization calculations shows that the heat-transfer coefficient for a theoretical separation stage was 1,600 BTU/hr/ft /°F. On this basis, the height equivalent to a theoretical stage of packing, such as the Flexipac type 4 in section 3 (see Table 8-3), is ... 

IHustnitioii 9.5 A liquid containing 50 mol % benzene (A), 25 mol % toluene (B), and 25 mol % 0-xyIene (C) is flash-vaporized at 1 std atm pressure and lOO C. Compute the amounts of liquid and vapor products and their composition. 

Note The bubble point of the feed is 76.0 C. Had ty as computed above exceeded the bubble point, the above enthalpy balance would have been discarded and made in accordance with flash-vaporization methods,) For the feed, = - 902,5 kJ/kmol. Enthalpy of feed at 58,3 C is... 

Example 4-6. Curvature of Flash-vaporization Curve. The experimental flash-vaporization curve and the ASTM distillation of a pressure distillate are shown in Fig. 4-24. The computed straight-line curve is shown dotted, the triangular points are computed points, and the circular points are experimental ones. 

Edmister, Reidel, and Mervin have determined flash-vaporization curves on three oils up to pressures of 200 psia. They find that the higher the pressure the flatter the vaporization curve, and hence the curves at high pressures should not be drawn parallel to the atmospheric-pressure curve. Obviously, the flash curve should be horizontal at the critical pressure, and hence the slope of the flash curve will be flatter and flatter as the pressure is increased. They find that the 50 per cent atmospheric boiling point should be corrected to the new pressure by using the vapor-pressure relationship of the paraffin hydrocarbons (Fig. 5-27). In practical design computations it is common practice to convert to the new pressure by using any convenient point on the atmospheric flash-vaporization curve. Refer also to Figs. 4-22 and 15-13. 

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). 

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. 

Application of Flash Vaporization. With the possible exception of heat-transfer and fluid-flow computations, no design fundamental is more widely useful than equilibrium vaporization. These many applications will be discussed at appropriate times in succeeding chapters. 

The distillation and flash vaporization curves were computed for the furnace feed, the effluent, and for the material when it was half cracked, as indicated in Fig. 19-10. In estimating the distillation curves, the mid boiling points of Table 19-16 were used, along with a terminal temperature of 770 F for the recycle stock (see Fig. 19-9), and an initial boiling point of 590 F for the gas oil. It was judged that materi boiling up to 1000°F could be vaporized (into the gas oil) by exposure of the reduced crude oil in vessel A of Fig. 19-7. 

The flash-vaporization curve (Fig. 24-1) was computed as follows (Figs. 4-18 and 4-19) ... 

In modern separation design, a significant part of many phase-equilibrium calculations is the mathematical representation of pure-component and mixture enthalpies. Enthalpy estimates are important not only for determination of heat loads, but also for adiabatic flash and distillation computations. Further, mixture enthalpy data, when available, are useful for extending vapor-liquid equilibria to higher (or lower) temperatures, through the Gibbs-Helmholtz equation. ... 

Liquid-liquid equilibrium separation calculations are superficially similar to isothermal vapor-liquid flash calculations. They also use the objective function. Equation (7-13), in a step-limited Newton-Raphson iteration for a, which is here E/F. However, because of the very strong dependence of equilibrium ratios on phase compositions, a computation as described for isothermal flash processes can converge very slowly, especially near the plait point. (Sometimes 50 or more iterations are required. )... 

Binary minimum reflux so calculated implies feed enthalpy just equal to the above started vapor V and liquid L. Any increase or decrease in that enthalpy must be matehed by inerease or decrease in total heat content of overhead reflux. Note that the Underwood" binary reflux equation essentially computes the flash versus specifi-eation composition relationship along with enthalpy correction. 

Radiation effects from a flash fire are now fully determined if vapor cloud composition, as well as the geometry of the flame front (dependent on time), is known. Vapor cloud composition is, of course, place- and time-dependent, and the shape of flame front will greatly depend on cloud shape and ignition site within the cloud. The total radiation intercepted by an object equals the surmnation of contributions by all successive flame positions during flame propagation. This is an impossible value to compute with the simplified approach just described. Because there are many uncertainties (e.g., cloud composition, location of ignition site) which greatly influence the final result, a conservative approach is recommended for practical applications ... 

Raoult s law is used to compute the vapor pressure (Psat) of pure methanol, based on the partial pressure required to flash ... 

To use the ECES Program Builder, the user creates computer input as shown in Figure 1. The input consists of three parts The names of the components in the inlet stream to the flash unit, the names of the species present in all the outlet streams - vapor, aqueous, or solids, and the chemical and/or ionic equilibria of interest. From this input the Program Builder Block using the imbedded thermodynamic framework described earlier writes the model description as given in Figure 2. 

For reasonable accuracy, this answer should be valid for almost any case. The example problem is an actual case in which the debutanizer feed heat exchanger specification will be set at 203°F accordingly. The debutanizer fractionator is designed and specified to receive a bubble point feed. To find such an answer, please note that the computer program simply ran a flash, finding a very small vapor flashing. 

A computer algorithm has been developed for making multi-component mixture calculations to predict (a) thermodynamic properties of liquid and vapor phases (b) bubble point, dew point, and flash conditions (c) multiple flashes, condensations, compression, and expansion operations and (d) separations by distillation and absorption. 

Compute the temperature rise for the operating NPSH. An NPSH of 18.8 ft is equivalent to a pressure of 18.8(0.433)(0.995) = 7.78 psia at 220°F, where the factor 0.433 converts feet of water to pounds per square inch. At 220°F, the vapor pressure of the water is 17.19 psia, from the steam tables. Thus the total vapor pressure the water can develop before flashing occurs equals NPSH pressure + vapor pressure at operating temperature = 7.78 + 17.19 = 24.97 psia. Enter the steam tables at this pressure and read the corresponding temperature as 240°F. The allowable temperature rise of the water is then 240 — 220 = 20°F. Using the safe-flow relation of step 2, the minimum safe flow is 62.9 gal/min (0.00397 m3/s). 

Multicomponentflash calculations based on the ganuiia/plii formulation are leaddy carried out by computer as outlined in Fig. 14.5. Table 14.2 shows the results of a P, T-flash calculation for the system n-hexane(l)/ethanoI(2)/methyIcyclopentane 3)/benzene(4). This is the same system for which results of a BUBL T calculation are presented in Table 14.1, and the same correlations and parameter values are used here. The given P and T are here 1 atm and 334.15 K (61°C). The given overall mole fractionsfor the system z, are listed in Table 14.2 along with the calculated values of the hquid-phase and vapor-phase mole fractions and the K-values. The molarfraction of the system that is vapor is found to be V = 0.8166. 

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