Vapor-liquid flash vaporizations

This problem is similar in some ways to a vapor-liquid flash and here is referred to as a liquid-liquid flash calculation. 

For an isothermal vapor-liquid flash, T is icnown and Equation (7-14) is used only to find Q after the other equations have... 

The same fundamental development as presented here for vapor-liquid flash calculations can be applied to liquid-liquid equilibrium separations. In this case, the feed splits into an extract at rate E and a raffinate at rate R, which are in equilibrium with each other. The compositions of these phases are... 

Equation (7-8). However, for liquid-liquid equilibria, the equilibrium ratios are strong functions of both phase compositions. The system is thus far more difficult to solve than the superficially similar system of equations for the isothermal vapor-liquid flash. In fact, some of the arguments leading to the selection of the Rachford-Rice form for Equation (7-17) do not apply strictly in the case of two liquid phases. Nevertheless, this form does avoid spurious roots at a = 0 or 1 and has been shown, by extensive experience, to be marltedly superior to alternatives. 

It is important to stress that unnecessary thermodynamic function evaluations must be avoided in equilibrium separation calculations. Thus, for example, in an adiabatic vapor-liquid flash, no attempt should be made iteratively to correct compositions (and K s) at current estimates of T and a before proceeding with the Newton-Raphson iteration. Similarly, in liquid-liquid separations, iterations on phase compositions at the current estimate of phase ratio (a)r or at some estimate of the conjugate phase composition, are almost always counterproductive. Each thermodynamic function evaluation (set of K ) should be used to improve estimates of all variables in the system. 

Both vapor-liquid flash calculations are implemented by the FORTRAN IV subroutine FLASH, which is described and listed in Appendix F. This subroutine can accept vapor and liquid feed streams simultaneously. It provides for input of estimates of vaporization, vapor and liquid compositions, and, for the adiabatic calculation, temperature, but makes its own initial estimates as specified above in the absence (0 values) of the external estimates. No cases have been encountered in which convergence is not achieved from internal initial estimates. 

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. )... 

Chlorine/ 7782-50-5 2,200 lb/ liquid Plan 1234 Location BB12 0.25 inch thick steel vessel Ambient 100 psi Inhalation None needed NA TQ 1500 tb/ X cssel rupture -liquid flashes to vapor to injure workers by inhalation... 

Fluid in a container is a combination of hquid and vapor. Before container mpture, the contained liquid is usually in equilibrium with the saturated vapor. If a container mptures, vapor is vented and the pressure in the liquid drops sharply. Upon loss of equilibrium, liquid flashes at the liquid-vapor interface, the liquid-container-wall interface, and, depending on temperature, throughout the liquid. 

Thus, the BLEVE theory predicts that, when the temperature of a superheated liquid is below T, liquid flashing cannot give rise to a blast wave. This theory is based on the solid foundations of kinetic gas theory and experimental observations of homogeneous nucleation boiling. It is also supported by the experiments of BASF and British Gas. However, because no systematic study has been conducted, there is no proof that the process described actually governs the type of flashing that causes strong blast waves. Furthermore, rapid vaporization of a superheated liquid below its superheat limit temperature can also produce a blast wave, albeit a weak... 

Assuming that the blasts from vapor expansion and liquid flashing are simultaneous, the total energy of the surface explosion is ... 

Figure 8-10. Schematic liquid flash tank. Note Feed can be preheated to vaporize feed partially.

This is the vessel where the refrigerant rejects its heat to waste or reclaim, turning back to liquid in the process. Sub-cooling is practiced by the removal of further heat. This prevents liquid flashing back to vapor on return to the evaporator. 

A tank containing liquid butane (C4H10) is located 500 ft from an electrical substation. One of the scenarios we are considering is the breaking of a 1-in schedule 40 pipe (internal diameter = 1.049 in) with discharge of the liquid butane. We are concerned that this leak will cause flammable vapor concentrations at the substation. Assume that all the liquid flashes to vapor. 

The conditions that promote a BLEVE are external fires heating up the tank walls above the liquid surface. This heating weakens the tank wall surrounding the vapor phase leading to a rupture. The released liquid flashes and ignites, resulting in a huge fireball. 

The vapor volume is the total volume of the separator minus the volume of the liquid collected and the volume of liquid is the volume collected (separated) from the feed minus the volume of liquid flashed when the static pressure decreases as the flow rate declines. The maximum volume of liquid collected is also a function of pressure—a greater volume is collected at higher pressure, and time. 

The vast majority of fractionators have top reflux. Cold liquid from the reflux drum is pumped onto the top tray of the tower. The cold liquid flashes to a hotter vapor. For example, let s say 1500 lb/h of liquid butane, at 100°F, flashes to 1500 lb/h of vapor at 260°F. 

The refrigerant liquid partially flashes to a vapor as it flows through the letdown valve. The flashing represents the conversion of the sensible heat of the refrigerant to latent heat of vaporization. In Fig. 22.1, the refrigerant is chilled from 100 to 40°F. Approximately 25 percent of the liquid flashes to a vapor to provide this autorefrigeration. 

On being pumped to the next stage, the liquid flashes through the nozzle and the mixture travels down the tube bundle as it did in the first stage. Vapor from the first-effect separator provides the heat for evaporation in the second stage. 

Applying these equations repeatedly until code line 920 is satisfied for an absolute SK2 value less than 0.00001, the program thereby finds the sum of the flash Xt and sum of the Y, mole fractions equal to 1.0000. The program then proceeds to display the results, giving both molar and mass flow rates of the component vapor-liquid flash. 

The cause of cavitation is that the pumped liquid flashes to vapor at one point inside the pump, where pressure is below the vapor pressure, and as the spinning impeller throws the liquid and vapor outward, the vapor bubbles collapse as the pressure rises above the vapor pressure. When the col-... 

The low exergetic efficiency is typical for distillation systems with close boiling mixtures and with high energy requirements in the reboiler. An alternative is to use reboiler-liquid flashing. A compressor is used to return the reboiled vapor to the bottom of the column. The required reboiler duty is somewhat larger than the required condenser duty, and so an auxiliary steam-heated reboiler is needed. Thus, a trade-off is made between the power used in the compressor and the large reduction in reboiler steam. 

The normal practice in the design of forced-convection reboilers is to calculate the heat transfer coefficient assuming that the heat is transferred by forced convection only. This will give conservative (safe) values, as any boiling that occurs will invariably increase the rate of heat transfer. In many designs the pressure is controlled to prevent any appreciable vaporization in the exchanger. A throttle value is installed in the exchanger outlet line, and the liquid flashes as the pressure is let down into the vapor-liquid separation vessel. 

The entrance of a liquid-flashing vapor mixture into the distillation column feed location requires a specially designed distribution tray to separate the vapors from the liquid, which must drop onto the packing bed for that section in a uniform pattern and rate. 

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