Infinite Reflux Columns

From a geometric perspective, the mass balance is a straight fine with p lying between Xd and Xg. Furthermore, since the liquid profile is following a residue curve, then both the distillate composition (x ) and bottoms composition (Xg) have to lie on the same residue curve. 

In short, when designing an infinite reflux column, two criteria have to be met 

FKvURE 2.12 RCM for the benzene/p xylene/toluene system at P = 1 atm widi a possible feasible design for an infinite reflux column. 

With this example, it should be obvious that there are other combinations of and Xjj that can be sought for a fixed Xjri these can be achieved by either altering the length of the mass balance line and/or by rotating the line, by pivoting about x. Of course, there are limits as to how much the line can be lengthened and rotated. The line has to remain within the MBT for the products to be real, and its direction has to be such that the products still remain on the same residue curve. With this, one can identify the following two important mass balance lines  

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If one further assumes that the internal flows of the column are infinitely large, then drawing off finite amounts of overheads and bottoms products (as done in normal operation of a simple column, refer to Figure 1.1) will have a negligible effect on the internal flows. Of course, doing this then warrants the need for a feed stream to the column in order to obey mass balance. However, it can still be assumed that the internal flows are much larger than either the feed or products, and thus the same material balances put forward for total reflux operation apply, as shown in Equations 2.15 2.17. Thus, it can be stated that the path followed by the liquid in an infinite reflux column will follow a residue curve as well, with the temperatures in the column dictated by the profile. 

Although an infinite reflux column is still impractical, it does have feed and product streams associated with it. Thus, unlike the total reflux column, the infinite reflux column has an external mass balance. The mass balance is for the column shown in Figure 1.1 is very simple, and can be summarized as... 

The reader should also be aware that infinite reflux columns are the smallest attainable columns in terms of column height however, they do require infinitely large internal flows and hence have operating expenses that are infinitely large to ensure continuous vaporization and condensation. This is the one extreme mode of operation, the other mode being minimum reflux which requires an infinitely high column to be built (and therefore have an initial capital investment that is infinitely large), but the smallest internal flows necessary for a desired separation. This is discussed in more detail in Chapter 4. 

Having established that there are topological similarities between CPMs and RCMs, it is of interest to determine what the exact relationship between the two is. In Section 2.6.2, it was shown that a liquid profile in a total reflux column follows a residue curve exactly, and that the liquid composition in an infinite reflux column will sufficiently approximate a residue curve too. Thus, it should come as no surprise then that when taking the limit in the DPE such that we observe that the DPE... 

Equation 9.12 implies that both flows and compositions are the same at the top of the MCS. This is equivalent to describing a total reflux column, and also sufficiently expresses an infinite reflux column, as discussed by Peters et al. [13]. Refer to Chapter 2 (Section 2.6) for the equivalent discussions on total and infinite reflux in simple distillation columns. 

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. 

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 ... 

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 ... 

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. 

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. 

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

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. 

The number of trays in an actual column is, of course, never infinite, and columns will never achieve the specified separation if operated at minimum reflux ratio. What the estimation of minimum reflux does provide is a limiting condition or a reference point. The operating reflux ratio is commonly expressed as a factor multiplied by the minimum reflux ratio. This factor must be greater than one in order to make the specified separation possible. The minimum reflux ratio is also used for correlating the required number of trays and reflux ratio as described in Section 12.3. 

Because ab intersects the equilibrium line, no further separation can occur. In other words, the equilibrium concentration of the solute in the solvent cannot be exceeded. This is the minimum liquid flowrate. Just as with the minimum reflux ratio, this situation is hypothetical because an infinite number of equilibrium stages (or an infinitely tall column) would be required. 

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. 

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

As before, tracing the DCM of a ternary mixture involving azeotropes may lead to one of more distillation regions. Numerical investigation demonstrates that distillation and residue curves are, in general, close to each other. In fact, both are related with the variation of concentration in a distillation column operated at infinite reflux, RCM for a packed column, and DCM for a frayed column. 

Fig, 135). Recently, test distillation apparatus (Fig. 237) for amounts of 200 to 500 ml has increasingly been used. It contains a short packed column and allows initial operation at an infinite reflux ratio in order to determine the bubble point exactly. 

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