Reflux ratio infinite

Unequal Molal Flow, 63 Ponchon-Savarit Method, 63 Example 8-21 Ponchon Unequal Molal Overflow, 65 Multicomponent Distillation, 68 Minimum Reflux Ratio — Infinite Plates, 68 Example 8-22 Multicomponent Distillation by Yaw s Method, 70 Algebraic Plate-to-Plate Method,... 

The two most frequently used empirical methods for estimating the stage requirements for multicomponent distillations are the correlations published by Gilliland (1940) and by Erbar and Maddox (1961). These relate the number of ideal stages required for a given separation, at a given reflux ratio, to the number at total reflux (minimum possible) and the minimum reflux ratio (infinite number of stages). 

Consider the case of minimum reflux ratio (infinite stages). As the amount of solvent is reduced, point M (equal to S + F) in Fig. 11.9 moves towards F, and P (equal to D + So) moves towards D. Point P" (equal to B - S) moves away from the equilibrium curve. The maximum distance that points M, P, and P" can be moved is determined by the slope of the tie lines. The minimum solvent ratio, which corresponds to the minimum reflux ratio, is reached when a tie line and an operating line coincide. A pinch point can occur either in the enriching or in the stripping section of the column, so it is necessary to seek the highest value of the minimum reflux ratio by trial and error. In this example, it occurs at the feed stage. The minimum reflux ratio is 0.58 and the corresponding minimum solvent ratio is 0.74. 

Fig. 2.3.2-9 Balance lines and staircase construction, a) Minimum reflux ratio, infinite plate number, b) Finite reflux ratio, resp. finite plate number, c) Total reflux, minimum plate number.

As an example of the brief design of an air separation unit, we take a closer look at the lower distillation column (prs6bar). The minimum reflux ratio (infinite number of trays) is 0.5 (Figure 6.2.8, details on the method in Section 3.3.2.2). Thereby, notably, the high-pressure column consists only of a rectification section, and thus a line for the stripping section is not needed. 

Figure 6.2.8 High-pressure column of air distillation unit minimum reflux ratio (infinite number of trays in the stripping section, rectification section consists only of reboiler).

The dominance of distiHation-based methods for the separation of Hquid mixtures makes a number of points about RCM and DRD significant. Residue curves trace the Hquid-phase composition of a simple single-stage batch stiHpot as a function of time. Residue curves also approximate the Hquid composition profiles in continuous staged or packed distillation columns operating at infinite reflux and reboil ratios, and are also indicative of many aspects of the behavior of continuous columns operating at practical reflux ratios (12). 

The limiting condition occurs at minimum reflux ration, when an infinite number of trays will be required to effect separation. Most columns are designed to operate between 1.2 to 1.5 times the minimum reflux ratio because this is approximately the region of minimum operating costs (more reflux means higher reboiler duty). 

As the reflux ratio is decreased from infinity for the total reflux condition, more theoretical steps or trays are required to complete a given separation, until the limiting condition of Figure 8-23 is reached where the operating line touches the equilibrium line and the number of steps to go from the rectifying to stripping sections becomes infinite. 

This graphical representation is easier to use for nonideal systems than the calculation method. This is another limiting condition for column operation, i.e., below this ratio the specified separation cannot be made even with infinite plates. This minimum reflux ratio can be determined graphically from Figure 8-23, as the line with smallest slope from xp intersecting the equilibrium line at the same point as the q line for mixture following Raoul t s Law. 

Using Figure 8-33 the separation from Xq, initial kettle volatile material to X3 as the distillate of more volatile overhead requires three theoretical plates/stages at total reflux. Using finite reflux R4, and four theoretical plates the same separation can be achieved with infinite theoretical plates and the minimum reflux ratio, Rmin- The values of reflux ratio, R, can be determined from the graph with the operating line equation as,... 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

As the reflux ratio is reduced a pinch point will occur at which the separation can only be achieved with an infinite number of stages. This sets the minimum possible reflux ratio for the specified separation. 

The advantages that may justify the additional costs of reflux at the bottom of a solvent extraction tower are illustrated in Figure 6. Minimum reflux is represented by the tie line from / to fe. Maximum reflux would be represented by the line / to the extract layer which would exist for infinite solvent to feed ratio. Practical operation of the equipment will fall between the limits of minimum and maximum reflux, as represented by fy. The operating point for the enriching section of the extraction column is located by an intersection between the lines erE and fy, and the reflux ratio is the ratio of the distances k e /k e (22). 

Any practical separation requires that the component balance lines intersect below the equilibrium curve, as for a reflux ratio of 3.0 in Fig. 2,11c. The McCabe-Thiele construction corresponding to this ratio is shown in Fig. 2.9c, If insufficient reflux is provided, the component balance lines intersect above the equilibrium curve, as for a reflux ratio of 1,0 in Fig. 2.11c, The McCabe-Thiele construction (Fig. 2,116) for these conditions shows that even with an infinite number of stages, the separation cannot be achieved. 

Too low a reflux ratio cannot produce the required product specification no matter how many trays are installed. Conversely, even infinite reflux will not be sufficient if an inadequate number of equilibrium stages has been provided. 

This effect is best explained by a simple illustration. Suppose we feed a column with 50 mol/h of A and 50 moVh of B, and A is the more volatile component. Suppose the distillate contains 49 mol/h of A and 1 mol of B, and the bottoms contains 1 mol/h of A and 49 mol/h of B, Thus the distillate flowrate is D = 50 mol/h and the purity of the distillate is xDA = 0.98. Now we attempt to fix the distillate flowrate at 50 mol/h and also hold the distillate composition at 0.98 mole fraction A. Suppose the feed composition changes to 40 mol/h of A and 60 mol/ h of B. The distillate will now contain almost all of the A in the feed (40 mol/h), but the rest of it (10 mol/h) must be components. Therefore the purity of the distillate can never be greater than xD A = 40/50 = 0.80 mole fraction A. The overall component balance makes it impossible to maintain the desired distillate composition of 0.98. We can go to infinite reflux ratio and add an infinite number of trays, and distillate composition will never be better than 0.80. 

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