Feed-Stage Location

Pressures can be specified at any level below the safe working pressure of the column. The condenser pressure will be set at 275.8 kPa (40 psia), and all pressure drops within the column will be neglected. The eqnihbrinm curve in Fig. 13-35 represents data at that pressure. AU heat leaks will be assumed to be zero. The feed composition is 40 mole percent of the more volatile component 1, and the feed rate is 0.126 (kg-mol)/s [1000 (lb-mol)/h] of saturated liquid (q = 1). The feed-stage location is fixed at stage 4 and the total number of stages at eight. 

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

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). 

Number of stages Feed stage location Component flows in feed Feed pressure Feed vapor fraction Pressure at the top of the column Pressure drop per stage Heat duty on each stage except reboilers and condensers Reflux ratio (replaces heat duty of condenser)... 

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. ... 

Most of the quantities normally specified in an equilibrium stage simulation (number of stages, feed stage location, feed flow rates and composition, reflux ratio, distillate or bottoms rate, and so on) must be specified for a comparable nonequilibrium simulation. By including the hydraulic or pressure drop equations in the model it is not necessary to specify the pressure of each stage for a nonequilibrium simulation only the pressure of the top stage and of the condenser need be specified. 

A number of simulations were carried out for different column configurations (number of stages and feed stage location) and for differing operation specifications (reflux ratio and bottom product rate). Only the specifications and results of our final simulation are reported here. 

A number of simulations were needed to design this column. The most important parameters were the overall height of packing, the feed stage location, and the reflux ratio. The purity specifications were not used in any simulation since nonstandard specifications of that kind can make the simulations harder to converge. 

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.

Number of Equilibrium Stages and Feed-Stage Location... 

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