Reflux ratio, binary separation column

This paper provides a framework for the application of Second Law based design methodology to separation systems. A relationship is derived for the available-energy destruction in a binary separation column as a function of the reflux ratio and the feed and product mass fractions. This derivation is limited to separations in which the entropy production is predominately due to mass transfers. 

In the example, the minimum reflux ratio and minimum number of theoretical plates decreased 14- to 33-fold, respectively, when the relative volatiHty increased from 1.1 to 4. Other distillation systems would have different specific reflux ratios and numbers of theoretical plates, but the trend would be the same. As the relative volatiHty approaches unity, distillation separations rapidly become more cosdy in terms of both capital and operating costs. The relative volatiHty can sometimes be improved through the use of an extraneous solvent that modifies the VLE. Binary azeotropic systems are impossible to separate into pure components in a single column, but the azeotrope can often be broken by an extraneous entrainer (see Distillation, A7EOTROPTC AND EXTRACTIVE). 

This illustrative example is taken from the recent work on interaction of design and control by Luyben and Floudas (1994a) and considers the design of a binary distillation column which separates a saturated liquid feed mixture into distillate and bottoms products of specified purity. The objectives are the determination of the number of trays, reflux ratio, flow rates, and compositions in the distillation column that minimize the total annual cost. Figure (1.1) shows a superstructure for the binary distillation column. 

Rose et al. (1950) and Rose and O Brien (1952) studied the effect of holdup for binary and ternary mixtures in a laboratory batch column. They qualitatively defined the term sharpness of separation as the sharpness in the break between successive components in the graph of instantaneous distillate composition against percentage distilled. They showed that an increase in column holdup enhanced the sharpness of separation at low reflux ratio but did not have any effect at a very high reflux ratio. 

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. 

This example is taken from Mujtaba and Macchietto (1996). The problem is to design a column for 2 binary separation duties. One of the separations is very easy compared to the other one. The fraction of production time for each duty is specified together with the still capacity (B0) and the vapour load (V). Each binary mixture produces only one main distillate product and a bottom residue (states MPf= Dl, Bfl] and MP2=[D2, Bf2]) from feed states EFt= Fl and EF2= F2], respectively, with only one distillation task in each separation duty. Desired purities are specified for the two main-cuts (x Di and xID2). Also obtain the optimal operating policies in terms of reflux ratio for the separations. 

Two binary mixtures are being processed in a batch distillation column with 15 plates and vapour boilup rate of 250 moles/hr following the operation sequence given in Figure 7.7. The amount of distillate, batch time and profit of the operation are shown in Table 7.6 (base case). The optimal reflux ratio profiles are shown in Figure 7.8. It is desired to simultaneously optimise the design (number of plates) and operation (reflux ratio and batch time) for this multiple separation duties. The column operates with the same boil up rate as the base case and the sales values of different products are given in Table 7.6. 

Figure 11.5 illustrates the operating sequence for a binary mixture. Successive passes (multi-pass) are used sequentially using the same column to separate only component. The purity of the distillate product remains the same for each pass but the distillate rate varies (this in turn varies the reflux ratio). This strategy is similar to a time sequenced reflux ratio operation for individual cuts in CBD operation. The total recovery of a component (say A) is calculated from the accumulated amount of distillate from all the passes. 

Also Mujtaba (1997) considered the separation of binary mixtures into one distillate product of specified purity. The objectives were to find out whether it was possible to replace conventional dynamic operation of batch columns by steady state operation using continuous columns for a comparable recovery, energy consumption, operation time, productivity, etc. and to obtain optimal operating policy in terms of reflux ratio. The following strategy was considered to compare the performances of the two types of operations ... 

The concentration profiles are displayed in Figure 3.15 (right-hand). The first column has 50 theoretical trays with feed on 20, entrainer/feed ratio 2 and reflux ratio 3.5. A bound in the concentration profile takes place around the feed. In the stripping zone the toluene carries out preferentially the chloroform. The rectification part separates mainly the binary acetone-chloroform, the entrainer concentration being negligible. The profile of the second column... 

Once one has proposed alternative configurations for systems of separation devices to effect a desired separation, one must then design these devices so the various alternatives may be compared. For a distillation column, the first set of design decisions is to choose the number of trays, the feed tray location, and the reflux ratio at which to operate it. For a binary separation, the McCabe-Thiele diagram (or the concepts behind it) is an indispensable aid in making these decisions. 

The ternary, tetradecene-TEA-Chlorex azeotrope consists of approximately 2 parts of tetradecene, 1 part of TEA, and 1 part of Chlorex. The boiling point of this azeotrope is several degrees below the boiling point of the binary azeotrope. An essentially complete separation between the ternary and binary azeotropes has been achieved in a column of approximately five theoretical plates at a reflux ratio of 1 to 1. 

A column is to be designed to separate 1000 moles/hr of a binary mixture of benzene (QH ) and toluene ( 7 ). The feed will contain 40% benzene and 60% toluene. A distillate that is 99% benzene and a bottoms that is 1% benzene are desired at a reflux ratio of 3 to 1. For this mixture, the average value of relative volatility (a) is 2.50. Estimate the number of equilibrium stages at this reflux ratio and the optimum feed stage location. 

Two binary streams each containing component 1 (the more volatile) and component 2 are to be separated in a single column equipped with a total condenser and a reboiler. Feed Fl enters the column as saturated vapor and feed F2 enters the column as saturated liquid. A vapor side draw Sl and a liquid side draw S2 are taken from the column. Using the data below, determine the correct relative locations of the feeds and products, the distillate and bottoms flow rates, and the L/V ratio in each column section. The column uses a reflux ratio of 1.8. 

As a learning tool, the binary model is useful for qualitatively studying the characteristics of multistage separation. The model is used in this chapter to answer such questions as what effect the reflux ratio or product rate or number of trays has on separation or what is the minimum number of trays or minimum reflux ratio required to achieve a given separation, or over what ranges column performance specifications are feasible. 

Since the system is binary, the two recovery specifications determine the total product rates by overall material balance. With the total number of stages fixed, the reflux ratio required to achieve the desired separation depends on the feed locations. It is desirable to operate the column at the lowest possible reflux ratio in order to minimize the condenser and reboiler duties. 

A feed stream at the rate of 100 kmol/h contains 50% mole acetone and 50% mole chloroform. The two components form a maximum boiling azeotrope which prevents their separation by conventional distillation. It is proposed to separate them by extractive distillation using benzene as a solvent, at a rate of 800 kmol/h. Both the main feed and the solvent are at 75 C and 110 kPa, and the column pressure is assumed uniform, also at 110 kPa. A total condenser is used, with a reflux ratio of 4. The distillate composition is specified at 95% mole acetone and the bottoms at 5% mole acetone on a solvent-free basis. Using the pseudo-binary... 

MINIMUM REFLUX RATIO. The minimum reflux ratio for a multicomponent distillation has the same significance as for binary distillation at this reflux ratio, the desired separation is just barely possible, but an infinite number of plates is required. The minimum reflux ratio is a guide in choosing a reasonable reflux ratio for an operating column and in estimating the number of plates needed for a given separation at certain values of the reflux ratio. 

Although the separation achieved in a column depends to some extent on all components in the feed, an approximate value of the minimum reflux ratio can be obtained by treating the mixture as a pseudobinary. Taking only the moles of light key and heavy key to make a new pseudofeed, product compositions could be calculated along with a vapor-liquid equilibrium curve based on aLK-mt-Then could be obtained using Eq. (18.43) as illustrated in Fig. 18.19. An alternate equation for a saturated liquid feed gives the minimum ratio of liquid rate to feed rate for a binary mixture of A and B ... 

The separation in the distillation column is binary between A and C, so the design of the column is straightforward. Typically, the reflux ratio is set at 1.2 times the minimum, and tray-to-tray calculations give the total number of trays Nt and the optimal feed tray Np. 

The binary separation of methanol and water is used as an example column. A feed of 82 mol% methanol and 18 mol% water is fed to a column with 40 trays (42 stages in Aspen terminology with feed on Stage 27 and the condenser labeled as Stage 1). Condenser pressure is 1 bar, condenser pressure drop is 0.1 bar, and tray pressiue drop is 0.01 bar per tray (giving a base pressure of 1.5 bar). Product purities are 99.9 mol% methanol in the distillate and 99.9 mol% water in the bottoms. The required reflux ratio is 0.8569. Column diameter is 5.61 m. Reboiler heat input is 64.1 MW. Condenser heat removal is 60.0MW. The NRTL physical property package is used. 

In design problems, the desired separation is set, and a column is designed that will achieve this separation. For binary distillation we would usually specify the mole fraction of the more volatile component in the distillate and bottoms products. In addition, the external reflux ratio, L D in Figure 4-6. 

Recycle Example 3, Two-column binary separation with heterogeneous azeotrope. First, see Note 2 above. Then set up the system with total condensers and modest reflux ratios (say 0.5 or 1.0). Run the system and reduce L7D in both columns in steps to very low values that approximate all of the reflux coming from the decanter. Specify the value of B for one column, but not the other (try boilup ratio in the stripping column). 

The separation power base in the classic McCabe-Thiele graphical model of a binary distillation column is established by the reflux ratio, R/D, which is the ratio of the reflux flow rate divided by the distillate flow rate. For example, with a distillation column that is fed 1,000 kg/h of feed that produces 85 kg/h of distillate with 425 kg/h of reflux, the reflux ratio is 425/85 = 5. A minimum reflux ratio is required to achieve the desired separation with an infinite number of theoretical stages. The maximum reflux ratio, called total reflux, with zero distillate flow rate can be used in design calculations to determine the minimum number of theoretical stages required to achieve a desired separation. 

The distillation column used as a numerical example is shown in Figme 4.47. A binary mixture of methanol and water are separated in a 16-stage column operating a 14.7 psia in the reflux drum. The feed is 40 mol% methanol. Distillate purity is 99 mol% methanol. Bottoms purity is 99.5 mol% water. A reflux ratio of 1.27 is required to achieve these specifications. Reboiler heat input is 152 x 10 Btu/h. Column diameter is 0.474 ft. The reflux drum and column base are sized for 5 min holdup when half full. Valve pressure drops are 30 psi. 

Sxs = solvent added to system at top of column In many cases, the mixture to be separated is a binary, since it is usually desirable to separate all but two components by regular distillation and then subject them to the extractive operation. Generally, the amount of solvent added at the top of the tower is varied with the reflux ratio in order to maintain a constant mol fraction of solvent in the total liquid returned to the top region of the column. An alternative method of operation is to employ a given solvent rate independent... 

As a basis for the estimate, it is assumed that the feed is a binary mixture containing 20 mol per cent cyclopentadiene and 80 mol per cent C7. The overhead product is to contain 98 mol per cent cyclopentadiene, and the cyclopentadiene content of the bottoms is to be 0.5 mol per cent. The column will operate with a still and total condenser at atmospheric pressure. A reflux ratio, 0/D, three times the minimum reflux ratio for the separation of the binary mixture with no polymerization will be used. 

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