Top-Feed Procedure

Top-Feed Procedure The sequence of operations with a top-feed leaf test is the same as in a bottom-feed test, except that the leaf is not immersed in the sluriy. The best method for transferring the slurry to the top-feed leaf is, of course, a function of the characteristics of the sluriy. If the particles in the sluriy do not settle rapidly, the feed can usually be transferred to the leaf from a beaker. If, however, the particles settle veiy rapidly, it is virtually impossible to pour the slurry out of a beaker satisfactorily. In this case, the best method is to make use of an Erlenmeyer flask, preferably one made of plastic. The slurry is swirled in the flask until it is completely suspended and then abruptly inverted over the leaf. This technique will ensure that all of the sohds are transferred to the leaf. 

All stage-to-stage methods that work from both ends of the column toward the middle suffer from two other disadvantages. First, the top-down and the bottom-up calculations must me somewhere in the column. Usually the mesh is made at a feed stage, and if more than one feed stage exists, a choice of mesh point must be made for each component. When the components vary widely in volatility, the same mesh point cannot be used for all components if serious numerical difficulties are to be avoided. Second, arbitrary procedures must be set up to handle nondlstrihuted components. (A nondistributed component is one whose concentration in one of the end-product streams is smaller than the smallest number carried by the computer.) In the LM and TG equations, the concentrations for these components do not natur ly take on nonzero values at the proper point as the calculations proceed through the column. 

The complexity of multicomponent distillation calculations can be appreciated by considering a typical problem. The normal procedure is to solve the MESH equations (Section 11.3.1) stage-by-stage, from the top and bottom of the column toward the feed point. For such a calculation to be exact, the compositions obtained from both the bottom-up and top-down calculations must mesh at the feed point and match the feed composition. But the calculated compositions will depend on the compositions assumed for the top and bottom products at the commencement of the calculations. Though it is possible to... 

The usual procedure is to start the calculation at the top and bottom of the column and proceed toward the feed point. The initial estimates of the component distributions in the products are then revised and the calculations repeated until the compositions calculated from the top and bottom starts mesh, and match the feed at the feed point. 

The topping operation differs from normal distillation procedures insofar as the majority of the heat is directed to the feed stream rather than by reboiling the material in the base of the tower. In addition, products of volatility intermediate between that of the overhead fractions and bottoms (residua) are withdrawn... 

The start-up procedure for the experiments shown in Figure 11 involved filling the reactor with distilled, deionized water prior to starting the feed stream pumps. In these runs the conversion seems much more stable. The top curve, however, demonstrates that these apparently stable steady states may be subject to rapid change perhaps the seeking of a new, more stable, steady state or the beginning of oscillations. ... 

Follow the calculation procedure outlined in Table 6.28 using Equations listed in Table 6.27. The light key (LK) is ethane and the heavy key (HK) is propylene. First, calculate the composition of the top and bottom products. Then, determine the optimum reflux ratio. Next, calculate the number of equilibrium stages. Finally, calculate the location of the feed tray. 

To obtain the composition of the top and bottom products, first calculate the relative volatility of each component using the conditions of the feed as a first guess. The relative volatility depends on temperature and pressure. The bubble point of the feed at 400 psia (27.6 bar) and at the feed composition, calculated using ASPEN [57], is 86.5 °F (130 °C). The K-values of the feed are listed in Table 6.7.1. Bubble and dew points could also be calculated using K-values from the DePriester charts [31] and by using the calculation procedures given in Chapter 3. Next, calculate the relative volatility of the feed stream, defined by Equation 6.27.18, for each component relative to the heavy key component. 

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