Reflux constant ratio

Batch distillation under constant reflux ratio is analyzed mathematically by considering that the moles lost from the still represent moles of distillate collected in the product receiver. Thus, 

Example 5.3 A mixture containing 50 moles each of benzene and toluene is to be distilled under conditions of constant reflux ratio until mole fraction of the residual benzene is less than 0.20. The column contains three theoretical stages. Calculate the material balance for this separation. 

NOTE The xw values are divided into eegments of equal size for application of Simpson s rule. 

by returning reflux to the column at a constant rate, the mole fraction of benzene was increased to 0,84 in the receiver and decreased to 0.13 in the still pot. (Remember, the simple still only concentrated the product to 0.58 mole fraction while reducing still concentration to 0.10 mole fraction.) 

Other useful information can be developed by further consideration 

Rgura 5.2 Batch distillation of benzene-toluene at constant reflux ratio, Example 6.3. te-e) McCabe-Thiele diagram for prt essively reducing still concentration to 0.13 mole fraction benzene. 

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FIG. 13-98 Typical variation in distillate and reboiler compositions with amount distilled in binary batch distillation at a constant-reflux ratio. 

Figure 8-32. Batch operations constant reflux ratio and variable overhead composition for fixed number of theoreticai stages/trays. Used and modified by permission, TreybaJi, R. E., Chem. Eng. Oct. 5 (1970), p. 95.

Fixed Number Theoretical Trays Constant Reflux Ratio and Variable Overhead Comporations... 

Batch with Constant Reflux Ratio, Fixed Number Theoretical Plates in Column, Overhead Comporation Varies... 

For a constant reflux ratio, the value can be almost any ratio however, this ratio affects the number of theoretical plates and, consequently, actual trays installed in the rectification section to achieve the desired separation. Control of batch distillation is examined in Reference 134. 

Figure 8-34. Minimum reflux for abnormal equilibrium curve for Batch Operation, constant reflux ratio.

Figure 8-35. Batch distiilation constant reflux ratio after McCabe-Thieie diagram. Revised/adapted and used by pennission, Schweitzer, PA Handbook of Separation Techniques for Chemical Engineers, McGraw-Hiil Book Co. (1979) aiso reprinted by special permission, Chem. Eng. Jan. 23 (1961), p. 134., 1961 by McGraw-Hili, Inc., New York.

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

Distillation at constant reflux ratio but varying top product composition. 

Figure 14.8 Integration of the Rayleigh Equation for constant reflux ratio. 

Operation at constant reflux ratio is better than operation with constant distillate composition for high-yield batch separations. However, operation with constant distillate composition might be necessary if high product purity is required. In fact, it is not necessary to operate in one of these two special cases of constant reflux ratio or constant distillate composition. Given the appropriate control scheme, the reflux ratio can be varied through the batch... 

An alternative method of operation is to work with a constant reflux ratio and allow the composition of the top product to fall. For example, if a product of composition 0.9 with respect to the more volatile component is required, the composition initially obtained may be 0.95, and distillation is allowed to continue until the composition has fallen to some value below 0.9, say 0.82. The total product obtained will then have the required composition, provided the amounts of a given purity are correctly chosen. 

These equations enable the final reflux ratio to be determined for any desired end concentration in the still, and they also give the total quantity of distillate obtained. What is important, in comparing the operation at constant reflux ratio with that at constant product composition, is the difference in the total amount of steam used in the distillation, for a given quantity of product, Db. 

If the same column is operated at a constant reflux ratio R, the concentration of the more volatile component in the top product will continuously fall. Over a small interval of time At, the top-product composition with respect to the more volatile component will change from xd to Xd + Axd, where Axd is negative for the more volatile component. If in this time the amount of product obtained is ADb, then a material balance on the more volatile component gives ... 

If the same batch as in Example 11.12 is distilled with a constant reflux ratio of R = 2.1, what will be the heat required and the average composition of the distillate if the distillation is stopped when the composition in the still has fallen to 0.105 mole fraction of ethanol ... 

Figure 11.37. Batch distillation-constant reflux ratio (Example 11.13)...

As the name implies, ratio control involves keeping constant the ratio of two or more flow rates. The flow rate of the "wild or uncontrolled stream is measured and the flow rate of the manipulated stream is changed to keep the two streams at a constant ratio with each other. Common examples include (1) holding a constant reflux ratio on a distillation column, (2) keeping stoichiometric amounts... 

Hence the reflux ratio, the amount of distillate, and the bottoms composition can be related to the fractional distillation time. This is done in Example 13.4, which studies batch distillations at constant overhead composition and also finds the suitable constant reflux ratio that enables meeting required overhead and residue specifications. Although the variable reflux operation is slightly more difficult to control, this example shows that it is substantially more efficient thermally—the average reflux ratio is much lower—than the other type of operation. 

This mode of operation demands constant rate of distillate throughout. This means that, for constant reflux ratio operation, the vapour load to the condenser is also constant. Boston et al. (1981) and Holland and Liapis (1983) considered this type of operation. 

Using binary mixtures, Luyben (1971) studied the effects of holdup, number of plates, relative volatility, etc. on the capacity (total products/hr). For an arbitrarily assumed constant reflux ratio the author observed both positive and negative effects of tray holdup on the capacity for columns with larger number of plates, while only negative effects were observed for columns with smaller number of plates. It is apparent that these observations are related to the degree of difficulty of separation. 

Kerkhof and Vissers showed that for difficult separations an optimal reflux control policy yields up to 5% more distillate, corresponding to 20-40% higher profit, than either constant distillate composition or constant reflux ratio policies. 

Robinson (1970) considered an industrial 10-component batch distillation operation. The feed condition is shown in Table 5.3. The distillation column was currently producing the desired product using constant reflux ratio scheme. Table 5.4 summarises the results of the application of minimum time problem using simple model with and without column holdup. 

Bonny et al. (1996) considered separation of 3 multicomponent mixtures composed of same components (Cyclohexane, n-Heptane, Toluene) but with different compositions in 3 batches. The input data for the problem is given in Table 7.10. A single constant reflux ratio is considered in each batch which is optimised together with the batch time. The objective of the operation is to maximise the productivity. The results are summarised in Table 7.11 and Figure 7.10. 

The column compositions are initialised to the composition of the mixed reboiler charge and a total of 2% of the fresh feed is used as column holdup. Half of the column holdup is assumed to be in the condenser and the rest is distributed equally over the plates. Piecewise constant reflux ratio was used, with 3 time intervals for the main-cut separation and 1 interval for the off-cut. 

The maximum conversion, the corresponding amount of product, optimal constant reflux ratio and heat load profiles for different batch times are shown in Figures 9.3-9.6. The maximum conversion profile achieved under total reflux operation (where no product is withdrawn) is also shown in Figure 9.3. The latter approximates the conversion which would be achieved in the absence of distillation. Note that if there is a large column holdup, the conversion under total reflux will not approximate the conversion achieved in the absence of distillation. 

Open loop strategy A simple control strategy is adopted operate the column at constant reflux ratio (r), reboiler heat duty (QR) and terminate when the reboiler liquid level falls below a threshold level (H,hreshoid) to avoid reboiler running dry. This gives three operating parameters to be chosen. 

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