Constant total molar overflow

However, the total number of equilibrium stages N, N/N,n, or the external-reflux ratio can be substituted for one of these three specifications. It should be noted that the feed location is automatically specified as the optimum one this is assumed in the Underwood equations. The assumption of saturated reflux is also inherent in the Fenske and Underwood equations. An important limitation on the Underwood equations is the assumption of constant molar overflow. As discussed by Henley and Seader (op. cit.), this assumption can lead to a prediction of the minimum reflux that is considerably lower than the actual value. No such assumption is inherent in the Fenske equation. An exact calculational technique for minimum reflux is given by Tavana and Hansen [Jnd. E/ig. Chem. Process Des. Dev., 18, 154 (1979)]. A computer program for the FUG method is given by Chang [Hydrocarbon Process., 60(8), 79 (1980)]. The method is best applied to mixtures that form ideal or nearly ideal solutions. 

A batch still corresponding to a total separation capacity equivalent to eight theoretical plates (seven plates plus the still) is used to separate a hydrocarbon charge containing four (A, B, C, D) simple-hydrocarbon components. Both the liquid and vapour dynamics of the column plates are neglected. Equilibrium data for the system is represented by constant relative volatility values. Constant molar overflow conditions again apply, as in BSTILL. The problem was originally formulated by Robinson (1975). 

Our aim is to estimate the duration of the processes and the amount of products. A simplified model was applied based on the following assumptions maximal separation, negligible hold-up on the trays and in the decanter, constant molar overflow, the flow rates do not vary with the time, one-phase liquid streams leave the decanter, negligible duration of pumping between the operation steps (BR), no entrainer loss (in the case of the ternary mixture). The total and component material balances for one column and the decanter are analytically solved. For the DCS we assume that both products reach the prescribed purity at the same time, that is, the duration is nrinimal. The process time (x) for both configurations and for the DCS the optimal division (v ) of total vapour flow rate (V) between the two reboilers and that of the charge (Ub /Uch) are shown. 

We desire to use a distillation column to separate an ethanol-water mixture. The column has a total condenser, a partial reboiler, and a saturated liquid reflux. The feed is a saturated liquid of composition 0.10 mole fraction ethanol and a flow rate of 250 mol/hr. A bottoms mole fraction of 0.005 and a distillate mole fraction of 0.75 ethanol is desired. The external reflux ratio is 2.0. Assuming constant molar overflow, find the flowrates, the number of equilibrium stages, optimum feed plate location, and the liquid and vapor compositions leaving the fourth stage from the top of the column. Pressure is 1 atm. 

A distillation column with a partial reboiler and a total condenser is being used to separate a mixture of benzene, toluene, and 1,2,3-trimethylbenzene. The feed, 40 mol% benzene, 30 mol% toluene, and 30 mol% 1,2,3-trimethylbenzene, enters the column as a saturated vapor. We desire 95% recovery of the toluene in the distillate and 95% of the 1,2,3-trimethylbenzene in the bottoms. The reflux is returned as a saturated liquid, and constant molar overflow can be assumed. The column operates at a pressure of 1 atm. Find the number of equilibrium stages required at total reflux, and the recovery fraction of benzene in the distillate. Solutions of benzene, toluene, and 1,2,3-trimethylbenzene are ideal. 

If the reflux ratio R or distillate rate D is fixed, instantaneous distillate and bottoms compositions vary with time. For a total condenser, negligible holdup of vapor and liquid in the condenser and the column, equilibrium stages, and constant molar overflow, the Rayleigh equation can now be written as... 

If this process is carried out in a distillation column, the minimum energy required may be determined from the heat Qk supplied in the reboiler/gmol of feed at Tg if we may assume that the total heat supplied at the reboiler is equal to that withdrawn in the condenser (i.e. Qc) at Tc-Further, this minimum will occur at the minimum reflux ratio, which means that there will be an infinite number of plates. Following Humphrey and Keller (1997), we aissume the fallowing complete separation of feed into two pure products constant relative volatility i2 constant molar overflow feed at bubble point minimum reflux ratio single reboiler and condenser liquid feed at bubble point. Consider now the distillation column shown in Figure 10.1.5(a). The overall and component material balance equations are ... 

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