Feed point location

The precise location of the feed point will affect the number of stages required for a specified separation and the subsequent operation of the column. As a general rule, the feed should enter the column at the point that gives the best match between the feed composition (vapour and liquid if two phases) and the vapour and liquid streams in the column. In practice, it is wise to provide two or three feed-point nozzles located round the predicted feed point to allow for uncertainties in the design calculations and data, and possible changes in the feed composition after start-up. 

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Hz and decreases at 33.4Hz. By comparing data of different feed point positions, it was found that the nucleation rates for the experiments with the feed point located near the impeller were higher than those for feed points outside the draft tube. 

A limitation of the Erbar-Maddox, and similar empirical methods, is that they do not give the feed-point location. An estimate can be made by using the Fenske equation to calculate the number of stages in the rectifying and stripping sections separately, but this requires an estimate of the feed-point temperature. An alternative approach is to use the empirical equation given by Kirkbride (1944) ... 

Some distillation columns must handle two or more feed streams simultaneously. Furthermore, alternative feed nozzles are often provided to allow the actual feed-point locations to be altered. By optimizing the feed-point locations, energy consumption in the reboiler can often be minimized. 

Figure 3-10 Nonsptunum feed points, (a) Feed peint located too high (b) feed point located too low (c) increasing reflux and reboil in order to accommodate for a feed point located too high,...

Sometimes, column design has a bearing on which spec is most difficult to achieve. For instance, in some C3 splitters, the feed point location has a greater effect on MAPD than on the propane concentration in the top product. With a low feed point, the propane spec may be the most difficult to achieve with a high feed point, the MAPD spec may be more difficult to achieve. The nonkey content of the feed also has an effect. For instance, if the methane plus ethane in the product is 500 ppmv, then up to 4500 ppm propane can be tolerated. This may be easier to achieve than the MAPD spec. On the other hand, if methane plus ethane is 3000 ppm, then no more than 2000 ppm of propane can be tolerated. This may be more difficult to achieve than the MAPD spec. 

For refluxed columns this involves determining the best feed point location, and the best division between reflux and overhead recycle to the reactor section. In some cases it has been found that isobutane concentration in the reactor could be increased by moving the feed point up, reducing the reflux and increasing the recycle to the maximum that the reactor loop could handle. 

The feed point location is calculated from the Kirkbride equation. Equation 6.27.12. 

Feed point location should be such that the settled sludge has minimum disturbance—to be informed by vendor. 

Particle diffusion was studied with a separate code written in MS Excel utilizing the step-by-step routine and gradient-based optimization (Eq. 15.13). The particles were assumed to be spherical. The calculations were made with the highest reaction rate present in the reactor. In practice this meant a place close to feed point location in the beginning of the experiment Results of calculations are shown in Figure 15.3, where the dimensionless concentrations (C,/Qs) of stearic add and heptadecane are illustrated. 

Figure 3.6a shows that at R = oo and iV = oo for the type of azeotropic mixtures under consideration, there is only one sharp split 2 1,3 regardless of the feed point location. However, if the point xp lies to the left of straight line 2-13, then the bottom product point appears at the segment 1-13, otherwise, at the segment 13-3 (Fig. 3.7a). Correspondingly, in the second column, the bottom product will be component 1 or 3. Thus, at sharp separation of such azeotropic mixture in each column, the set of column sequence products depends only on the feed point location relative to the straight line 2-13.

In circulating fluidized bed combustors, the volatile release and combustion are important with regard to pollutants emissions, heat release patterns and the subsequent reactivity of the char particles. For coals with high volatiles content, the release of volatiles is important in the selection of the feed point location, since the time constant for volatiles release and combustion is much shorter than the char burnout time. In addition, solid mixing and rate of devolatilization determine the region of volatile release. 

Improper feed point location may require more theoretical stages than calculated. For a binary system, the feed point is located where the feed liquid composition matches the downflowing liquid composition in the column. With a multicomponent feed, the theoretical stage that matches the light-key concentration in the feed probably will not match the heavy-key concentration. In such a case, the feed stage should be selected so that the ratio of light key to heavy key in the liquid feed is the same as the ratio in the downflowing liquid within the column. 

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