Fractionating columns feed plate

Stripping or desorption is the transfer of gas, dissolved in a liquid, into a gas stream. The term is also applied to that section of a Fractionating column below the feed plate. 

A fractionating column is required to distill a liquid containing 25 per cent benzene and 75 per cent toluene by mass, to give a product of 90 per cent benzene. A reflux ratio of 3.5 is to be used, and the feed will enter at its boiling point. If the plates used are 100 per cent efficient, calculate by the Lewis-Sorel method the composition of liquid on the third plate, and estimate the number of plates required using the McCabe-Thiele method. 

In the previous sections conditions have been considered in which there has been a continuous feed to the still and a continuous withdrawal of products from the top and bottom. In many instances processes are carried out in batches, and it is more convenient to distil each batch separately. In these cases the whole of a batch is run into the boiler of the still and, on heating, the vapour is passed into a fractionation column, as shown in Figure 11.33. As with continuous distillation, the composition of the top product depends on the still composition, the number of plates in the column and on the reflux ratio used. When the still is operating, since the top product will be relatively rich in the more volatile component, the liquid remaining in the still will become steadily weaker in this component. As a result, the purity of the top product will steadily fall. Thus, the still may be charged with S mols of a mixture containing a mole fraction xsl of the more volatile component. Initially, with a reflux ratio Ri, the top product has a composition... 

The portion of the column that lies below the feed plate is called the stripping section. The function of this section is to obtain a nearly pure bottoms stream, B, of the less volatile component of the feed. As with the rectifying section, we can analyze in terms of a mass balance to obtain an operating line relating the vapor-phase mole fraction to the liquid-phase mole fraction exiting a stage. Again, it is assumed that the molar flowrates B, L and V are constant and that constant molar overflow applies. The overall mass balance around the bottom of the column, then, becomes ... 

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. 

Williams et al. (Wl), describe the results of studies of the automatic control of continuous fractional distillation. These studies were made on an analog computer which could simulate a five-plate tower. The effects of column design, varying feed rate, imperfect sampling, and quality of feed and reflux on controllability were evaluated. An earlier article by Rose and Williams (R2) on the same system compares various schemes for controlling fractionation columns. One interesting conclusion is that derivative control action cannot improve the control for any of the various combinations of measurement and regulation that were studied. 

A mixture of xylenes plus other aromatics is separated in a large fractionating column operating at atmospheric pressure. Calculate the minimum number of plates and the minimum reflux ratio for the conditions in Table 19.4. Use the Gilliland correlation to estimate the reflux ratio that will permit the separation to occur in 100 ideal stages. The relative volatilities are calculated for 18 psia and 150°C, the estimated conditions near the feed tray. 

Consider the section of the column at the tray where the feed is introduced. The quantities of the liquid and vapor streams change abruptly at this tray since the feed may consist of liquid, vapor, or a mixture of both (fraction vaporized = VjJF). If, for example, the feed is a saturated liquid, Lst will exceed L by the amount of the added feed liquid. To establish a general relationship, an overall material balance around the feed plate is... 

H2. [VBA required] Either write your own program or use the program in Appendix A of Chapter 5 to solve the following problem, A feed of 100 mol/h of a saturated liquid that is 25 mol% A = benzene, 35 mol% B = toluene, and 40 mol% C = cumene is fed on the optimum feed plate to a distillation column that has a total condenser and a partial reboiler. Fractional recoveries of B (toluene) in the distillate of 0.9 and of C in the bottoms of 0.97 are desired. The relative volatilities are 2.25, Ogg = 1.0, and = 0.21. Use an external reflux ratio of L/D = 0.3. Find the optimum feed stage, the total number of stages, the fractional recovery of A (benzene) in the distillate, D and B. After solving the problem, try What if simulations to explore the effects of changing the feed concentrations, the fractional recoveries, L/D, and the relative volatility a B-... 

X = mol fraction of most volatile component This relationship is similar to Eq. (7-62) for small values of Xw-Above the feed plate, the corresponding relationship for a column and total condenser is obtained by using a similar analysis for the least volatile component,... 

It is interesting to consider what would happen if the feed had not been introduced on the thirteenth plate. This calculation has been carried out and the results plotted in Fig. 9-11. Up to the thirteenth plate, the results are obviously identical with those given in Fig. 9-10 but above this plate, the change of concentration per plate is much less in Fig. 9-11. By the twenty-sixth plate, all the components have become almost asymptotic, and increasing the plates to an infinite number would make little difference in the concentrations from those for the twenty-sixth plate. Thus it is impossible to obtain the desired separation without having plates above the feed plate, since the asymptotic ratio of phenol to o-cresol is less than the desired ratio in the distillate. The limit to this asymptotic ratio is obvious from Fig. 9-11 since the o-cresol, m-cresol, xylol, and residue must all flow down the column, their concentrations cannot decrease below the value necessitated by material balance for their removal from the still. Although the concentration of the phenol is not limited by the same factor as the heavier components, it is limited by the fact that its value cannot exceed 1 minus the sum of the concentration of the heavier fractions and since a minimum limit for the heavier components is fixed, a maximum for the phenol is likewise fixed. The condition illustrated in Fig. 9-11 around twentieth to twenty-sixth plate is termed pinched in i.c., conditions are so pinched that effective rectification is not obtained. As soon as the feed plate is passed, this pinched-in condition would be relieved, since the heavier components would decrease rapidly, as in Fig. 9-10, thereby allowing the phenol to increase. 

Actually there is no sharp line of demarcation between these five sections, but this division serves as a useful picture for considering the case of the minimum reflux ratio. The feed to the fractionating column would be introduced on some plate in intermediate section 3, and the true criterion for the minimum reflux ratio should be based on matching the ratio of the concentrations of the key components above and below the feed plate under conditions such that a pinched-in section occurs both above and below the feed plate. For mixtures of normal volatility, a pinched-in region in only one section does not necessarily mean that an infinite number of plates would be required to perform the desired separation at the reflux ratio under consideration, since by relocating the feed plate, such as to shift the ratio of the concentrations of the key components at this plate, the section that was not limited could be made to do more separation and thereby relieve the load on the pinched-in section. In other words, for mixtures of normal volatilities the condition of the minimum reflux ratio is not determined by either the fractionation above or below the feed plate alone, but is determined such that the separation is limited both above and below the feed. The conditions in the intermediate feed section lead to... 

Fig. 29 Estimation of the optimal feed plate the CPF column for the fiactionation according the molecular mass if the original polymer is descrihed by (16). The stars represent the values of the sol haction and the crosses the values from the gel fractions...

Since (H y- Htf) represents the amount of heat needed to vaporize one mole of saturated liquid under feed plate conditions, q also represents the fraction of that heat required to vaporize completely the feed as it is introduced into the column. If the feed introduced into the column is a saturated liquid at feed plate conditions, Le. Hf= H, (/ = 1. If the feed is introduced as saturated vapor at feed plate condition, then Hf= = 0. If the feed introduced is a mixture of liquid and vapor, then 0 < < 1. If the feed is introduced as a subcooled liquid, then Hf < the molar enthalpy of the saturated liquid at the temperature and pressure of the feed plate correspondingly, q >1. If the feed is introduced as a superheated vapor vis-a-vis the feed plate conditions, then Hf > H aaA q <0. Note at this time the overall material and component i balance for the column ... 

The absorber from object palette is used for this purpose. Inlet air stream is connected to the bottom of the column while liquid stream is connected to the top of the column. Gas stream released from the column is connected to top of the column and liquid stream released from the bottom of the column. Feed streams are fully specified. The required number of trays is set so that the exit SO2 mole fraction is 0.003 as required. In this example, the number of theoretical plates needed to achieve 0.003 mole fraction of SO2 in the exit air is greater than 4 and less than 5 since the mole fraction in the air-out stream is 0.0038 (Figure 7.35). The... 

Complete Fractionation Columns. A complete fractionation column, as shown in Fig. 6.11, may also be analyzed using the Ponchon-Savarit technique. In the McCabe-Thiele analysis, equations for two operating lines were found. These correspond to the A points in the Ponchon-Savarit analysis, where one A point represents the difference between passing streams in the column above the feed plate and the other A point represents the difference between passing streams below the feed plate. Assume that the reflux ratio, along with the composition and enthalpy of the feed, overhead distillate, and bottom product are known. Then points F, B, D, and A i may be located on... 

Theoiy of Fractionating Columns. The derivation of formulas describing two-component fractionation involves the following assumptions (1) the molal latent heats of all materials are equal (2) no reflux for cooling the overhead product from the feed-plate temperature is provided. 

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