Number of plates —

Another variable that needs to be set for distillation is refiux ratio. For a stand-alone distillation column, there is a capital-energy tradeoff, as illustrated in Fig. 3.7. As the refiux ratio is increased from its minimum, the capital cost decreases initially as the number of plates reduces from infinity, but the utility costs increase as more reboiling and condensation are required (see Fig. 3.7). If the capital... 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

This is the one case where the overall column efficiency can be related analytically to the Murphree plate efficiency, so that the actual number of plates is calculable by dividing the number of theoretical plates through equation 86 ... 

Nonisothermal Gas Absorption. The computation of nonisothermal gas absorption processes is difficult because of all the interactions involved as described for packed columns. A computer is normally required for the enormous number of plate calculations necessary to estabUsh the correct concentration and temperature profiles through the tower. Suitable algorithms have been developed (46,105) and nonisothermal gas absorption in plate columns has been studied experimentally and the measured profiles compared to the calculated results (47,106). Figure 27 shows a typical Hquid temperature profile observed in an adiabatic bubble plate absorber (107). The close agreement between the calculated and observed profiles was obtained without adjusting parameters. The plate efficiencies required for the calculations were measured independendy on a single exact copy of the bubble cap plates installed in the five-tray absorber. 

The membrane system consists of multiple plate and frame stacks holding the thin-fHm composite membranes clamped together. The system capacity is iacreased by increasing the number of plates. 

Favorable Vapoi Liquid Equilibria. The suitabiHty of distiUation as a separation method is strongly dependent on favorable vapor—Hquid equiHbria. The absolute value of the key relative volatiHties direcdy determines the ease and economics of a distillation. The energy requirements and the number of plates required for any given separation increase rapidly as the relative volatiHty becomes lower and approaches unity. For example given an ideal binary mixture having a 50 mol % feed and a distillate and bottoms requirement of 99.8% purity each, the minimum reflux and minimum number of theoretical plates for assumed relative volatiHties of 1.1,1.5, and 4, are... 

Feed Composition. Feed composition has a substantial effect on the economics of a distillation. Distillations tend to become uneconomical as the feed becomes dilute. There are two types of dilute feed cases, one in which the valuable recovered component is a low boiler and the second when it is a high boiler. When the recovered component is the low boiler, the absolute distillate rate is low but the reflux ratio and the number of plates is high. An example is the recovery of methanol from a dilute solution in water. When the valuable recovered component is a high boiler, the distillate rate, the reflux relative to the high boiler, and the number of plates all are high. An example for this case is the recovery of acetic acid from a dilute solution in water. For the general case of dilute feeds, alternative recovery methods are usually more economical than distillation. 

L/V) are drawn from the distillate composition, and the appropriate number of plates is stepped off to find the corresponding bottoms composition. 

The more volatile (i.e., less soluble) components will be only partially absorbed even though the effluent liquid becomes completely saturated with respecd to these lighter substances. When a condition of saturation exists, the value of will remain finite even for an infinite number of plates or transfer units. This can be seen in Fig. 14-9, in which the asymptotes become vertical for values of greater... 

The number of plates, Np, and the height equivalent to a theoretical plate, HETP, are defined as measures of dispersion effects as ... 

A high number of plates and a low HETP indicate a high column efficiency. 

For strongly retained components (Ic 1), the number of plates required to obtain a given resolution with a finite feed injection is approximated by ... 

Since the term (1 -i- k i)/k approaches unity for large /c -value, the number of plates is equal to one half the number of transfer units for a strongly retained component. For these conditions, when Np = N/2, Eq. (16-174) and Eq. (16-161) produce the same peak retention time, peak spreading, ana predict essentially the same peak profile. 

Determination of pressure drop through the column (for packed columns, correlations dependent of packing type, column-operating data, and physical properties of the constituents involved are available to estimate the pressure drop through the packing for plate columns, the pressure drop per plate is obtained and multiplied by the number of plates)... 

The volume of mobile phase per plate (vm), multiplied by the number of plates (n), will give the total volume of mobile phase in the column (Vm)- Similarly, the volume of stationary phase per plate (vs), multiplied by the number of plates (n), will give the total volume of stationary phase in the column (Vs). 

The peak width at the points of inflexion of the elution curve is twice the standard deviation of the Poisson or Gaussian curve and thus, from equation (8), the variance (the square of the standard deviation) will be equal to (n), the total number of plates in the column. 

Knowing the number of theoretical plates that are required, the length of the column (L) is defined as the product of the number of plates and the variance per unit length of the column (H), i.e.. 

In light ends fractionation it is usually just as important to remove light material from the heavier cut as it is to keep heavy material out of the lighter cut sidestreams are seldom withdrawn. The desired purity (expressed as per cent of impurity) of the overhead and bottoms is determined by product specifications or by the requirements of subsequent processing units. To meet these purity requirements, higher reflux ratios and greater numbers of plates between cuts are required than in crude distillation units. 

To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

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