Reflux infinite

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). 

Residue Curve Maps. Residue curve maps are useful for representing the infinite reflux behavior of continuous distillation columns and for getting quick estimates of the feasibiHty of carrying out a desired separation. In a heterogeneous simple distillation process, a multicomponent partially miscible Hquid mixture is vaporized ia a stiH and the vapor that is boiled off is treated as being ia phase equiHbrium with all the coexistiag Hquid phases. 

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,... 

Petlyuk FB and Avetyan VS (1971) Investigation of Three Component Distillation at Infinite Reflux, Theor Found Chem Eng, 5 499. 

Similarly, a distillation line map (DLM) shows the distribution of liquid composition on the stages of a continuous distillation column at infinite reflux and for infinite number of stages. DLM is obtained even simpler by computing successive dew and bubble points as described by the relation ... 

Although RCM and DLM are not identical, the assessment of a distillation process gives similar results in both diagrams. The representation is often called °°/°° analysis because it assumes infinite reflux and an infinite number of stages. The... 

Note that the last condition implies infinite reflux and infinite number of stages. The assessment of feasibility of a design for zeotropic mixtures is fully correct, the only problem left being the sizing. 

A similar representation is based on distillation tines [1], which describe the composition on successive trays of a distillation column with an infinite number of stages at infinite reflux (°°/°° analysis). In contrast with relation (A.8) the distillation lines may be obtained much easier by algebraic computations involving a series of bubble and dew points, as follows ... 

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. 

The definition of the separation factor, Equation 7, when combined with infinite reflux condition. Equation 11, gives... 

There are two limits at which we can examine the behavior of a distillation column. The first is at total reflux (i.e., with an infinite reflux ratio, which is often called infinite reflux conditions). The other extreme is to operate at minimum reflux. In this section we shall limit our discussion to the total reflux case in later sections we shall look at operating columns at finite reflux (ratio) conditions. Intuitively, we tend to expect that a column will give its maximum separation when run at infinite reflux. While this is true for ideally behaving species, it does not have to be true when separating nonideally behaving species. Thus, we need to look carefully at running colunons all the way from minimum to total reflux conditions. 

The second extreme we can imagine is to maintain finite flows for the feed and products but increase the internal flows for L and V to infinite values. This second case cannot really occur, as we would need a column with an infinite diameter. It is a limiting case. Both ways to think of infinite reflux are useful. In the latter case the column is still thought of as producing its products. 

At infinite reflux we can add a second constraint that must hold for the compositions of the two products ... 

Infinite Reflux Constraint The compositions for the distillate and bottoms... 

Fig. 48. Examples of reachable products for a column operating at infinite reflux.

Fig. 52. All compositions that can be reached for both finite and infinite reflux conditions from the distillate D and bottoms B compositions by stepping away from them using a tray-by-tray...

We see that we have both an upper bound and a lower bound on the column reflux ratio. Does having an upper bound make intuitive sense We have set the. solvent flow proportional to the distillate product flow i.e., S = RsD. As we increase the reflux ratio R, the ratio of solvent feed flow. RsD, to reflux flow, RD, decreases. This decreases the solvent concentration throughout the column, thus reducing its impact on the liquid activity coefficients that we are using to separate A from B. With an infinite reflux ratio, the. solvent flow reduces to zero, and we have a normal column operating at total reflux which we know cannot separate A from B. 

For this example, you choose the hrst column in the process shown in Figure 5.4. In Aspen Plus you use the module DSTWU for the shortcut method you also use RK-Soave as the physical property method, because it is a good one for hydrocarbons. The feed is 100 lb mol/h propane, 300 lb mol/h /-butane, and the other chemicals as listed in Table 6.1, at 138 psia and 75°F. The column operates at 138 psia with a reflux ratio of 10 (a wild guess initially, confirmed because the column worked). Remember, the minimum number of stages goes together with infinite reflux, so if your column does not work, increase the reflux ratio. 

In the rest of this chapter these methods are sampled in the following way the estimates of the minimum number of plates at infinite reflux and of minimum reflux with an infinite number of plates are made by group methods on the assumption of constant relative volatilities and are based on a design point of view. An empirical relation for the number of plates at an operating reflux is described. 

MINIMUM NUMBER OF PLATES. The Fenske equation (18.41) applies to any two components, i and j, in a conventional plant at infinite reflux ratio. In this case, the equation has the form... 

Example 19.3. A mixture with 33 percent -hexane, 37 percent -heptane, and 30 percent n-octane to be distilled to give a distillate product with 0,01 mole fraction n-heptane and a bottoms product with 0.01 mole fraction n-hexane. The column will operate at 1.2 atm with 60 percent vaporized feed. Calculate the complete product compositions and the minimum number of ideal plates at infinite reflux. 

A five-component mixture is to be distilled with 99 percent recovery of the light and heavy keys in the distillate and bottoms (Table 19-3). Calculate the product compositions for the case of infinite reflux. Explain how these concentrations would shift as the reflux ratio was decreased using the compositions at minimum reflux as a guide. 

Figure 10.1-12 Stage-to-stage calculation for a distillation column operating at the infinite reflux ratio.

For a given specification, a reflux ratio can be selected anywhere from the minimum, Rmin, to an infinite value (total reflux) where all the overhead vapor is condensed and returned to the top stage (thus, no distillate is withdrawn ). The minimum reflux corresponds to an infinite number of stages, while an infinite reflux ratio corresponds to the minimum number of stages. 

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