Flash distillation energy balance

Try the following problem to sharpen your skills in working with material and energy balances. Crude oil is heated to 525° K and then charged at a rate of 0.06 m /hr to the flash zone of a pilot-scale distillation tower. The flash zone is maintained at an absolute temperature of 115 kPa. Calculate the percent vaporized and the amounts of the overhead and bottoms streams. Assume that the vapor and liquid are in equilibrium. 

It is often possible to make a material balance round a unit independently of the heat balance. The process temperatures may be set by other process considerations, and the energy balance can then be made separately to determine the energy requirements to maintain the specified temperatures. For other processes the energy input will determine the process stream flows and compositions, and the two balances must be made simultaneously for instance, in flash distillation or partial condensation see also Example 4.1. 

The emphasis on vapour-liquid equilibria (including vapour pressure) is inherant in the petroleum industry due to the importance of distillation in separations. If separations by extraction are to be undertaken, then liquid-liquid equilibrium is equally important. Fugacities for thermodynamic equilibrium (flash calculations) are probably one of the most sought-after properties. This is because fugacities and enthalpies often provide sufficient information to calculate a mass and energy balance. 

Return to the design of the toluene hydrodealkylation process, as it is presented in Section 4.3. In the reactor section, after heuristics are utilized to set (1) the large excess of H2 in the hydrodealkylation reactor, (2) the temperature level of the quenched gases that enter the feed-product heat exchanger, and (3) the temperature in the flash vessel, the simulator is used to complete the material and energy balances and to examine the effects of these heuristics on the performance of the reactor section. In the distillation section, after heuristics are used to set (1) the quahty of the feed, (2) the use of partial or total condensers, (3) the use of cool-... 

The Newtonian procedure oudined in Eqs. 12-511 to 12-571 for the energy balance for flash distillation can be considered a simplified version of the method used by Naphtali and SandholnL I recommend that students reread that material before proceeding. 

Absorbers, like flash distillation, are equivalent to very wide boiling feeds. Thus, in contrast with distillation, a wide-boiling feed (sum rates) flowchart such as Figure 2-13 should be used. The flow rate loop is now solved first, since flow rates are never constant in absorbers. The energy balance, which requires the most information, is used to calculate new temperatures, since this is done last. Figure 12-13 shows the sum-rates flow diagram for absorbers and strippers when K = K (T, p). If K = Kj (T, p, Xj, X2,. .. Xj,) a concentration correction loop is added. The initial steps are very similar to those for distillation, and usually the same physical properties package is used. 

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