Feed stage, optimum

Change the feed temperature. This may make a nonoptimum feed stage optimum... 

Maas,J. H., Optimum-Feed-Stage Location in Multicomponent Distillations, Chem. Eng, Apr. 16, (1973) p. 96. 

In Sec. 2.2.4, it was assumed that the feed enters the column at the optimum feed stage, which is located at the intersection of the component balance lines. At that point, the construction was switched from the rectifying section component balance line to the stripping section component balance line,... 

The feed point should give the most equal slopes on both sides of the feed stage in a key ratio plot (Fig. 2,20). Too high a feed causes excessive retrograde distillation below the feed, and too low a feed causes excessive retrograde distillation above it. In order to reach the optimum feed point, the feed stage should be moved from the sharp maximum (or minimum) toward the flat one (Fig. 2.20). 

Light nonkeys raise the optimum key ratio at the feed stage, while heavy nonkeys lower it. Rules 1 and 2 therefore become less reliable when there are a lot more light than heavy nonkeys or vice versa, or when the amount of nonkeys exceeds the amount of keys,... 

Nonkeys whose volatility is either close to the keys or far removed from the keys tend to shift the optimum key ratio (or light key concentration) at the feed stage to a lesser degree than nonkeys whose volatility is moderately removed from the keys,... 

Alternatively, results from a computer simulation can be plotted to determine the optimum feed stage. Simulation runs ere performed at several different feed points, keeping the material balance, reflux ratio, and total number of stages constant. Ksy component concentrations in the product streams are plotted against the feed stage number (Fig. 3.7). The minimum is at the optimum feed stage. 

Application. Previous methods (Secs. 3.2.1 to 3.2.6) produce a design. They take product compositions and deliver the number of stages, reflux, and optimum feed stage. The Smith-Brinkley method rates a column using the reverse sequence of steps. It takes the number of stages, reflux ratio, and actual feed location, and yields the product compositions. 

Feed stage location Locating the feed stage far above or below the optimum will cause a composition pinch. The pinch represents a column that as specified cannot be solved. A pinch can best be detected with an x-y diagram (Sec. 2.4.1). 

The easiest way to use a shortcut distillation model is to start by estimating the minimum reflux and number of stages. The optimum reflux ratio is usually between 1.05 and 1.25 times the minimum reflux ratio, Rmm, so 1.15 x Rmin is often used as an initial estimate. Once the reflux ratio is specified, the number of stages and optimum feed stage can be determined. The shortcut model results can then be used to set up and initialize a rigorous distillation simulation. 

To improve the stage distribution for each section in the crude tower, the tower is decomposed into a sequence of simple columns as a first step. After that, the ideal number of stages and the optimum feed stage for each simple column are found. Finally, the simple columns are merged back to the complex column with new numbers of stages for each section. 

Kister says that d/b plots are primarily used when there is a tight spec, on a nonkey component or a concern about the distribution of an intermediate key component. His book shows d/b curves for various feed stage locations on a plot of the mole ratio of a reference component in the distillate to the bottom product, versus the relative volatility of each component to this reference component. This plot is made on log-log paper. The optimum feed produced a curve closest to linear. The d/b plot is suggested as a troubleshooting tool in the subsection of the Troubleshooting section, Fractionation Operating Problems. ... 

A column is to be designed to separate 1000 moles/hr of a binary mixture of benzene (QH ) and toluene ( 7 ). The feed will contain 40% benzene and 60% toluene. A distillate that is 99% benzene and a bottoms that is 1% benzene are desired at a reflux ratio of 3 to 1. For this mixture, the average value of relative volatility (a) is 2.50. Estimate the number of equilibrium stages at this reflux ratio and the optimum feed stage location. 

Figure 4.5 Feed stage location is (a) optimum, and (b) above optimum.

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. 

Distillate composition Bottoms composition Feed tale Feed composition Feed enthalpy Desiga/minimum reflux ratio Optimum feed stage Pressure Number of stages Feed stage Reflux ratio Distillate rate... 

The optimum feed stage local ion is the one (hat gives the fewest numbers of stages for the required separation. On this basis, Gilliland19 developed criteria for evalneting ihe opt)mam feed location. He dealt with liquid, liquid-vapor, and vapor fend cases, For a liquid feed, the criterion is... 

Figure 6.10 Location of feed stage (a) optimum location (b) location below optimum stage (c) location above optimum stage.

Use the Gilliland correlation to estimate the actual number of stages in a multicomponent column and the optimum feed stage location. 

Estimate the total number of equilibrium stages and the optimum feed-stage location for the distillation problem presented in Examples 6.10 and 6.11 if the actual reflux ratio is set at R = 1.0. 

Use the Gilliland correlation to estimate the number of ideal stages required for the separation of Example 6.4. Assume that, for the system benzene-toluene at atmospheric pressure, the relative volatility is constant at a = 2.5. Use the results of Problems 6.15 and 6.16. Use the Kirkbride equation to estimate the optimum feed-stage location. 

---