Cost optimum reflux ratio

Figure 3.3.17 Principle of the method used to determine the cost optimum reflux ratio of a distillation.

The cost optimum reflux ratio is typically in the range 1.1-1.5 Here we use a value of 1.5 and thus the reflux ratio is about 0.8 (Figure 6.2.9). 

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. 

The effect of utilities costs on optimum operation was noted by Kiguchi and Ridgway [Pet. Refiner,. 35(12), 179 (1956)], who indicated that in petroleum-distillation columns the optimum reflux ratio varies between 1.1 and 1.5 times the minimum reflux ratio. When refrigeration is involved, 1. IRmm < flopt < 1 is used in the condensers, 1.2Rrniii < fLpt < 1 -4Rrn... 

The iterative program incorporating the quadratic interpolation search yielded the results in Table E12.4B. The optimum reflux ratio was 17.06 and the cost,/, was 3870/day. Table E12.4C shows the variation in/ for 10 percent change in R. The profit function changes 100/day or more. 

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. 

At the optimum reflux ratio, the reboiler and condenser represent 21% and 12% of the total capital investment. Similarly, at optimum design, the fuel costs, steam and cooling water, represent 64% and 8% of total system costs. 

Flgurt 3.6 Optimum reflux ratio. (a) Capital and operating cost curves (6) effect of using expensive materials of construction on the optimum (c) effect of high energy costs on the optimum (optimum reflux, toluene-benzene separation, show a flat total cost curve near the optimum. (Part d reprinted with permission from W. R. Fisher, M. F. Doherty, and J. M. Douglas, bid. Eng. and Chem. Proc. Dee. and DeveL, Vol. 24. p. 955, Copyright (1985) American Chemical Society). 

Hgw U Continued) Optimum reflux ratio, id) optimum reflux, tolueoe-beniene separation.showing a flat total cost curve near the optimum. (Part d re-fainted with permission from W. R. Fisher, M. F. 

In this study it was shown that propane or mixtures of propane and butane may be used as supercritical solvents to fractionate mixtures of high molecular weight alkanes. By using an optimum reflux ratio, selectivities better than that of molecular distillation can be obtained. It was also found that for the system in this study, the operating costs of a supercritical extraction unit may be marginally lower than that of a molecular distillation unit. 

Supercritical fractionation of high molecular weight alkane mixtures with propane or LPG may be used to produce products with lower polydispersity that that of molecular distillation. Operating temperatures just above the cloud point of the mixtures can be used compared to the high temperatures needed in molecular distillation. It was also shown that an optimum reflux ratio exists for every set of operating conditions. For this system it was also found that the operating costs of a supercritical fraction unit is marginally less than that of a molecular distillation unit. 

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. 

Solution. The variable costs involved are cost of column, cost of reboiler, cost of condenser, cost of steam, and cost of cooling water. Each of these costs is a function of the reflux ratio, and the optimum reflux ratio occurs at the point where the sum of the annual variable costs is a minimum. The total variable cost will be determined at various reflux ratios, and the optimum reflux ratio will be found by the graphical method. 

There are other mles-of-thumb based on economic experience, which the reader will recognize, such as the optimum reflux ratio in distillation and the optimum liquid to gas ratio in gas absorption. You may also specify recoveries of key conq)onents or their concentrations in an exit stream for separators. When we use any of these rules, the assim tion is that the calculated separator size will be of reasonable cost, approximating the optimum-size separator. Similarly, for chemical reactors we may specify conversion of a desirable con jound, its exit composition or an approach temperature difference. For chemical reactors, the approach temperature difference is the difference between the actual temperature and the chemical-equilibrium ten5)erature. Again, we assume that a reactor that approximates the optimum-size reactor will result when using this rule. 

Distillation towers feed-tray location for, 10 optimum reflux ratio for, 371-376 specifications for, 16 (See also Bubble-cap contactors, Packed towers. Sieve trays, and Valve trays) Distribution costs, 194, 196, 207, 211 Distribution in statistical analyses, 745-746 Dividends, tax exemptions for, 259 Documentation, 137-149,452-476 Double-entry bookkeeping, 143-144 Downcomers in tray columns, 684-686 Drives, cost of 532-533 Dryers, cost of 713-716... 

OPTIMUM REFLUX RATIO. As the reflux ratio is increased from the minimum the number of plates decreases, rapidly at first and then more and more slowly until, at total reflux, the number of plates is a minimum. It will be shown later that the cross-sectional area of a column usually is approximately proportional to the flow rate of vapor. As the reflux ratio increases, both V and L increase for a given production, and a point is reached where the increase in column diameter is more rapid than the decrease in the number of plates. The cost of the unit is... 

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