Reflux ratio generalized

T.eflux Tatio. Generally, the optimum reflux ratio is below 1.15 and often below 1.05 minimum. At this point, excess reflux is a minor contributor to column inefficiency. When designing for this tolerance, correct vapor—Hquid equiUbrium (VLE) and adequate controls are essential. 

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. 

Optimum Reflux Ratio The general effecl of the operating reflux ratio on fixed costs, operating costs, and the sum of these is shown in Fig. 13-39. In ordinary situations, the minimum on the total-cost cui ve wih geueraUy occur at an operating reflux ratio of from 1.1 to 1.5 times the minimum R = Lv + i/D value, with the lower value corresponding to a value of the relative volatility close to 1. 

Thus, while it is possible in theory to cross a curved distillation boundary as shown in Figure 12.35, it is generally more straightforward to follow designs that will be feasible over a wide range of reflux ratios and in the presence of uncertainties. Such designs can be readily developed using distillation line and residue curve maps. 

Various rules of thumb and empirical correlations exist to assist in making initial guesses for the values of the independent variables. All the values of the feeds here can be assumed to be equal initially. If the reflux ratio is selected as an independent variable, a value of 1 to 1.5 times the minimum reflux ratio is generally appropriate. 

In this analysis, a is taken as the volatility of A relative to B. There is, in general, therefore a different value of R , for each plate. In order to produce any separation of the feed, the minimum relevant value of Rm is that for the feed plate, so that the minimum reflux ratio for the desired separation is given by ... 

Optimum Reflux Ratio. The reflux ratio affects the cost of the tower, both in the number of trays and the diameter, as well as the cost of operation which consists of costs of heat and cooling supply and power for the reflux pump. Accordingly, the proper basis for choice of an optimum reflux ratio is an economic balance. The sizing and economic factors are considered in a later section, but reference may be made now to the results of such balances summarized in Table 13.3. The general conclusion may be drawn that the optimum reflux ratio is about 1.2 times the minimum, and also that the number of trays is about 2.0 times the minimum. Although these conclusions are based on studies of systems with nearly ideal vapor-liquid equilibria near atmospheric pressure, they often are applied more generally, sometimes as a starting basis for more detailed analysis of reflux and tray requirements. 

Equation (13.97) can be used to find the still composition—x in that equation—at a particular reflux ratio in a column-reboiler combination with n stages. Example 13.4 employs instead a computer program with Equations (13.104) and (13.105). That procedure is more general in that a constant relative volatility need not be assumed, although that is done in this particular example. 

We stress that this equation dictates the minimum reflux ratio based purely on thermodynamic arguments. As a1 3 = 1.11 in our case, the value of rmin = 12.28. In general, the mixture will not be equimolar, and if the products are not pure but satisfy a less strict specification, the value of rmin will be smaller. Now, a column operated under these conditions will have an efficiency of 100% since it is using the minimum amount of work necessary to separate the components. 

Of these, the feed mixture may or may not vary, but is generally taken as given. The column pressure and the degree of subcooling are normally fairly constant. The main operational variables are the reflux ratio R and the heat input to the reboiler QR and once these are set, the amount of product withdrawal at the bottom or at the top will also be given by the product specifications. An optimum exists for the reflux ratio in terms of operating costs, and normally a number of ratios are tested, and the economics of each scenario is investigated, before a decision is reached. 

For single separation duty, Mujtaba and Macchietto (1993) proposed a method, based on extensions of the techniques of Mujtaba (1989) and Mujtaba and Macchietto (1988, 1989, 1991, 1992), to determine the optimal multiperiod operation policies for binary and general multicomponent batch distillation of a given feed mixture, with several main-cuts and off-cuts. A two level dynamic optimisation formulation was presented so as to maximise a general profit function for the multiperiod operation, subject to general constraints. The solution of this problem determines the optimal amount of each main and off cut, the optimal duration of each distillation task and the optimal reflux ratio profiles during each production period. The outer level optimisation maximises the profit function by... 

In Greaves et al. (2001), a hybrid model for an actual pilot plant batch distillation column is developed. However, taking advantage of some of the inherent properties of batch distillation processes a simpler version (new algorithm) of the general optimisation framework is developed to find optimal reflux ratio policies which minimises the batch time for a given separation task. 

The reliability or stability of a method covers its ability to reach a solution for a wide group of problems in a general range of mixtures, such as wide or narrow boiling, and if it can solve columns across the whole spectrum of boiling point ranges. It also covers the ability of the method to solve the same column with variations in some of the specifications such as number of trays, reflux ratio, or feed conditions. 

As indicated in Fig. 11-7, the optimum reflux ratio occurs at the point where the sum of fixed charges and operating costs is a minimum. As a rough approximation, the optimum reflux mho usually falls in the range of 1.1 to 1.3 times the minimum reflux ratio. The following example illustrates the general method for determining the optimum reflux ratio in distillation operations. 

The ethylbenzene recovery rate is usually over 95 per cent, and its purity greater than 99.8 per cent. The quality of the product obtained conditions that of its derivative, the styrene monomer, and its aptitude for polymeiizatioiL This depends on the presence of toluene or other aromatics in the feed, whose content must generally not exceed 0.3 per cent This fractionation can only be calculated conveniently on a computer. The theoretic cal number of trays is as high as 330 for 95 per cent recovery. Since the efiicieocy of these trays approaches 85 per Ccnu abotH 390 real trays must be used with reflux ratios up to 80 to 90. 

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