Flash vaporization calculation

Calculations of gas-liquid equilibria using either Equation 12-17 or Equation 12-18 often are called flash vaporization calculations. Usually the term is reduced to flash calculations. 

Flash vaporization calculations involving multicomponent mixtures are essential to the design and successful operation of many processes. These calculations are often required to determine the condition of the feed to a fractionating column or to determine the flow of vapor from a reboiler or condenser. 

For the isothermal flash vaporization calculation, we proceed as in Illustration 8.1-3. First, we calculate the K factors, i.e. 

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... 

Figure 6.3 Flash vaporization calculations with Mathcad.

Consider now the partial reboiler shown schematically in Figure 6.17. Saturated liquid leaving the last equilibrium stage in the tower enters the reboiler at a rate of 271.4 kmol/h (75.4 mol/s). Saturated vapor leaves the reboiler and returns to the column at the rate of 193.6 kmol/h (53.8 mol/s), while the liquid residue is withdrawn as the bottoms product at the rate of 77.8 kmol/h (21.6 mol/s). The bottoms product is a saturated liquid with a composition of 5 mol% benzene. A flash-vaporization calculation is done in which the fraction vaporized is known (53.8/75.4 = 0.714) and the concentration of the liquid residue is fixed at xw = 0.05. The calculations yield the following results TR = 381.6 K, xl2 = 0.093, and y]3 = 0.111. The liquid entering the reboiler is at its bubble point, which is Tn = 319.7 K. An energy balance around the reboiler is... 

The single-stage membrane unit becomes equivalent to a so-called flash vaporization. The flash vaporization calculation itself is straightforward, with the vapor and liquid phases assumed at equilibrium, and is presented in a number of references." " The limits correspond to the dew-point and bubble-point calculations for vapor-liquid equilibrium, which are special or limiting cases for the flash vaporization calculation. It is the object, therefore, to adapt the membrane calculation to the techniques for the flash vaporization calculation and thereby take advantage of the relative simplicity of the latter. 

Techniques used in steady-state flash vaporization calculations and multistage distillation calculations can be utilized to show that membrane separations are enhanced by the use of cascade or multistage operations. This is of importance particularly in the use of membrane materials showing low selectivity between the components to be separated. 

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 ... 

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. 

It is then easy to calculate what the proportions must be so that d x overhead composition -l- r x flash liquid composition = flash vapor composition. Thus ... 

Calculate individually the orifice area required to pass the flashed vapor component, using Equation (5a), (3b), (4), (5), or (6), as appropriate, according to service, type of valve and whether the back pressure is greater or less than the critical flow pressure. 

The quantity of fuel in a cloud is calculated by use of release and (flash) vaporization models that have been extensively described by Hanna and Drivas (1987). To account for aerosol formation during vaporization, the flash fraction should be doubled up to, but not exceeding, a value of unity. Pool vaporization is also considered. 

The amount of liquid that will evaporate can be calculated if it is assumed that all heated liquid will be exposed to air (see Section 6.3.3.3). Results of calculations can then be compared with experimental results. When the calculated percentage of flash evaporation exceeded 36%, all fuel became an aerosol for fireball formation. At lower percentages, a portion of the fuel formed the fireball, and the remainder former a pool fire on the ground. Thus, these results imply that, when calculated flash evaporation is less than 36% of the available fuel, fuel in the fireball can be expected to amount to approximately three times the amount of flashed vapor. 

Hasegawa and Sato (1977) showed that, when the calculated amount of flash vaporization equals 36% or more, all released fuel contributes to the BLEVE and eventually to the fireball. For lower flash-vaporization ratios, part of the fuel forms the BLEVE, and the remainder forms a pool. It is assumed that, if flash vaporization is below 36%, three times the calculated quantity of the flash vaporization contributes to the BLEVE. 

Thus, is 20% of the energy calculated for nonideal gases or for flash-vaporization situations. For scaled energies ( ) larger than about 0.8 as calculated by Eq. (9.3.5), the calculated velocity is too high, so method 3 should be applied. 

Because flashing steam-condensate lines represent two-phase flow, with the quantity of liquid phase depending on die system conditions, these can be designed following the previously described two-phase flow methods. An alternate by Ruskin [28] uses the concept but assumes a single homogeneous phase of fine liquid droplets dispersed in the flashed vapor. Pressure drop was calculated by the Darcy equation ... 

In either procedure, the enthalpy of the streams resulting from the first flash is calculated as qout. At statement 80, the enthalpy of the streams resulting from the second flash is calculated as toth. In addition the enthalpy of the streams of the first flash at the temperature of the second flash is calculated as sensh. The temperature of the first flash is called T, the temperature of the second flash is called ti. In the same way the vapor phases resulting from the two flashes are flv and flvi. In the ordinary case flvi would not equal flv, nor would ti equal t. These differences are then used to predict temperature and flow amounts at the solution to the isenthalpic flash, totcp is the amount of energy which must be added to the system to raise the temperature by 1°F. and is calculated by (toth — qout) /(t1 — t). Similarly the amount of energy added to produce one more mole of vapor (and hence one less mole of liquid) is TOTMCP = (toth — qout) /(flvi — flv). SENSCP is the amount of sensible heat which would have to be added to raise the temperature 1°F. if no vaporization or condensation occurred in changing from t to ti. 

At pressures above the bubble point, fluid properties are calculated by combining the data from the flash vaporization and a separator test. 

At pressures above bubble-point pressure, oil formation volume factors are calculated from a combination of flash vaporization data and separator test data. 

Equation 8-11 may be used with the flash vaporization data to calculate oil compressibility at pressures above the bubble point. 

Calculate the relative volume at 2900 psig for the flash vaporization of Example 10—1. 

A flash vaporization process is described in Chapter 10 as a part of the discussion of black-oil laboratory procedures. The results of this process can be calculated by performing calculations like Example 12-4 for a... 

However, in order to calculate the results of the differential vaporization of the black-oil laboratory analysis, one should calculate a series of flash vaporizations with the gas removed at the end of each step. 

Compositions and Quantities of the Equilibrium Gas and Liquid Phases of a Real Solution — Calculation of the Bubble-Point Pressure of a Real Liquid—Calculation of the Dew-Point Pressure of a Real Gas Flash Vaporization 362... 

The VF factor is the flashed vapor to feed molar ratio, which has been calculated in the program in the previous lines. The VF factor is calculated in a unique method using a convergence technique shown in code lines 810 and 890 ... 

Feed analyses in terms of component compositions are usually not available for complex hydrocarbon mixtures with a final normal boiling point above about 38°C (100°F) (n-pentane). One method of handling such a feed is to break it down into pseudocomponents (narrow-boiling fractions) and then estimate the mole fraction and K value for each such component. Edmister [Ind. Eng. Chem., 47,1685 (1955)] and Maxwell (Data Book on Hydrocarbons, Van Nostrand, Princeton, N.J., 1958) give charts that are useful for this estimation. Once K values are available, the calculation proceeds as described above for multicomponent mixtures. Another approach to complex mixtures is to obtain an American Society for Testing and Materials (ASTM) or true-boiling point (TBP) curve for the mixture and then use empirical correlations to construct the atmospheric-pressure equihbrium flash vaporization (EFV) curve, which can then be corrected to the desired operating pressure. A discussion of this method and the necessaiy charts is presented in a later subsection Petroleum and Complex-Mixture Distillation. 

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. 

To calculate the amount of the respective vapor and liquid phases that evolve at equilibrium when a liquid of known composition flashes (flash vaporization) at a known temperature and pressure, you must use Eq. (3.37) together with a material balance. Figure 3.16 illustrates the steady-state process. A mole balance for component i gives... 

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