Separation ratio plates required

Otherwise expressed, the number of theoretical plates required for a given separation increases when the reflux ratio is decreased, i.e., when the amount of condensed vapour returned to the colunm is decreased and the amount distilled off becomes greater. 

Two heat-sensitive organic liquids of an average molecular mass of 155 kg/kmol are to be separated by vacuum distillation in a 100 mm diameter column packed with 6 mm stoneware Raschig rings. The number of theoretical plates required is 16 and it has been found that the HETP is 150 mm. If the product rate is 5 g/s at a reflux ratio of 8, calculate the pressure in the condenser so that the temperature in the still does not exceed 395 K (equivalent to a pressure of 8 kN/m2). It may be assumed that a = 800 m2/m3, /x = 0.02 mN s/m2, e = 0.72 and that the temperature changes and the correction for liquid flow may be neglected. 

Altering the slope of the <7-line will alter the liquid concentration at which the two operating lines cut each other for a given reflux ratio. This will mean a slight alteration in the number of plates required for the given separation. Whilst the change in the number of plates is usually rather small, if the feed is cold, there will be an increase in reflux flow... 

Any change in the reflux ratio R will therefore modify the slope of the operating line and, as may be seen from Figure 11.15, this will alter the number of plates required for a given separation. If R is known, the top line is most easily drawn by joining point A (xd, Xd) to B (0,Xd/(R + 1)) as shown in Figure 11.17. This method avoids the calculation of the actual flow rates L and Vn, when the number of plates only is to be estimated. 

Figure 11.17. Influence of reflux ratio on the number of plates required for a given separation...

Theoretical Plate In a distillation column, it is a plate onto which perfect liquid-vapor contact occurs so that the two streams leaving are in equilibrium. It is used to measure and rate the efficiency of a column at separating compounds. The ratio of the number of theoretical plates to the actual number of plates required to perform a separation is used to rate the efficiency of a distillation column. Actual separation trays in refinery distillation units are usually less effective than theoretical plates. 

Otherwise expressed, the number of theoretical plates required for a given separation increases when the reflux ratio is decreased, i.e. when the amount of condensed vapour returned to the column is decreased and the amount distilled off becomes greater. The variation in the reflux ratio is achieved by the use of a suitable take-off head (or still-head), usually of the total condensation variable take-off type. In use, all the vapour is condensed and the bulk of the condensate is returned to the fractionating column, small fractions of the condensate being allowed to collect in a suitable receiver. The design may be appreciated from the line diagram shown in Fig. 2.107 in which the controlled collection of distillate is by the socket-cone screw-operated valve sited just below the condenser drip end. 

With the aid of a suitable computer software program, it is possible to investigate the effect of a number of reflux ratios on the number of stages that will be required to obtain the desired separation between ethylene and ethane. This approach is outlined on the computer software output which plots the effect of decreased reflux on the number of theoretical plates required for the separation. 

Example 1-6 It is desired to find the minimum number of perfect plates required to separate an equal molar mixture of benzene and toluene into a distillate product containing 96 percent benzene (XD= 0.96) and a bottom product containing no more than 5 percent benzene (xB = 0.05) at the following operating conditions (1) the column pressure is 1 atm, and a total condenser is to be used (D is a liquid), (2) the thermal condition of the feed is such that the rate Ls at which liquid leaves the feed plate is given by Ls = Lr- 0.6F, and (3) a reflux ratio L1/D = 2.2 is to be employed. The equilibrium sets x, yA of benzene used to construct the equilibrium curve shown in Fig. 1-9 were found by solving Prob. 1-1. 

As the specified value of the reflux ratio (Ll/D) is decreased, the intersection of the two operating lines moves closer to the equilibrium curve and the minimum number of plates required to effect the specified separation (xB = 0.05, XD = 0.96) increases. On the other hand, as L /D is decreased, the condenser and reboiler duties decrease. The minimum reflux ratio is the smallest one which can be used to effect the specified separation. This reflux ratio requires infinitely many plates in each section as demonstrated in Fig. 1-10. It should be noted that for this case, the plates at and adjacent to the feed plate have the same composition. (In the case of multicomponent systems, these limiting conditions do not necessarily occur at and adjacent to the feed plate as discussed in Chap. 11). From the standpoint of construction costs, this reflux ratio is unacceptable because infinitely many plates are required, which demands a column of infinite height. 

The feed given in Table 9-1 is to be separated into two fractions by use of a conventional distillation column. In particular, it is desired to find the smallest number of plates required to effect the following separation between ethyl benzene (the light key) and styrene (the heavy key) at a reflux ratio LlfD = 5.44. 

The boiling point-equilibrium data for the system acetone-methanol at 760 mm Hg are given in Table 18.7. A column is to be designed to separate a feed analyzing 25 mole percent acetone and 75 mole percent methanol into an overhead product containing 78 mole percent acetone and a bottom product containing 1.0 mole percent acetone. The feed enters as an equilibrium mixture of 30 percent liquid and 70 percent vapor. A reflux ratio equal to twice the minimum is to be used. An external reboiler is to be used. Bottom product is removed from the reboiler. The condensate (reflux and overhead product) leaves the condenser at 25°C, and the reflux enters the column at this temperature. The molal latent heats of both components are 7700 g cai/g mol. The Murphree plate efficiency is 70 percent. Calculate (a) the number of plates required above and below the feed (b) the heat required at the reboiler, in Btu per pound mole of overhead product (c) the heat removed in the condenser, in Btu per pound mole of overhead product. 

Since typical biological mixtures are exceedingly complex, adequate chromatographic resolution is imperative for both identification and quantitation purposes. Improved resolution is feasible through either increasing the column efficiency (number of theoretical plates), or phase selectivity. Alternatively, a combination of both can be practiced. The number of theoretical plates, required for adequate resolution of two adjacent peaks (98% separation of the peak areas) is related to the column selectivity (relative retention, a) and to the capacity ratio, k, according to the well-known equation derived by Purnell [70] ... 

In addition to the practical signiflcance of being ablb to calculate the number of plates required for a continuous reactor, other important information can be obtained from the method. (1) Attainment of mass-action equilibrium is not necessary in addition, long contact times, obtained by large holdup volumes and numerous plates, are not necessary to obtain high over-all conversions. (2) Similar calculations show that, for a given plate and composition, the rate of reaction increases with the reflux ratio. (3) The effects of catalyst concentration, whether alcohol and acid should be added separately, or mixed, the temperature of the feed, etc., may be evaluated. 

The plate equivalent is the minimum number of plates required at infinite reflux ratio to attain the same enrichment (xB- -Xg) as in a countercurrent distillation with a finite reflux ratio. All distillation conditions except the reflux ratio remain the same. Thus, in the McCabe-Thiele diagram the separating stages are drawn between the diagonal and the equilibrium curve v = oo). 

One of the most useful forms of plate efficiency is due to Lewis [13], the overall plate efficiency,. It is defined as the ratio between the number of theoretical plates necessary to effect a given separation and the number of actual plates required for the same separation. This overall plate efficiency,, has, in itself, no fundamental mass transfer basis, but is still quite useful since only terminal conditions are required for its application. 

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