Zeotropic mixture

The 400-series are zeotropic mixtures. T is based on bubble point. 

Estimates based on limited data. f The 400 -series are zeotropic mixtures. gThe 500 -series are azeotropic mixtures. 

Solutions of type 1. Mixtures which shows neither a maximum nor a minimum on the vapour pressure-composition curve or boiling temperature-composition curve are known as zeotropic mixtures. 

For zeotropic mixtures the problem is not the feasibility of separation, because a suitable column can always be designed, but the optimal sequencing of splits. The optimality criterion should be the total cost of separations, in term of investment and operation. 

For sequencing the separation of liquid mixtures, there are general heuristics, as shown in Table 3.11. More specific rules for the separations of zeotropic mixtures by distillation will be discussed later in this chapter. 

The procedure known as the list-processing method allows the designer to identify feasible sequences for separating ideal or slightly nonideal zeotropic mixtures by means of simple distillation columns. Table 3.13 presents a list of more specific heuristics for sequencing, supplementary to those in Table 3.7. The first one shows... 

Table 3.13 Heuristics for separation sequencing of zeotropic mixture.

By using computer simulation accurate near-optimal solutions can be found with relatively reduced effort quickly by the sequencing of zeotropic mixtures. The following strategy can be applied ... 

Enhanced distillation designates special techniques for separating nonideal azeotropic mixtures, or zeotropic mixtures with very low relative volatility, based on the use of a dedicated mass-separation agent that has to be recycled. Sometimes, a component already present in the mixture can play this role. 

The extractive distillation profits from the capacity of an entrainer (solvent) to modify selectively the relative volatility of species. Normally, the entrainer is the highest boiler, while the component to be separated becomes heavier, being carried out in bottoms. For this reason, this operation may be regarded as an extractive absorption. Extractive distillation can be used for separating both zeo-tropic and azeotropic mixtures. The entrainer is fed near the top for a zeotropic mixture or a minimum-boiling azeotrope, or mixed with the feed for a maximumboiling azeotrope. The separation sequence normally has two columns, for extraction and solvent recovery [5]. 

RCM is a powerful graphical tool for assessing the feasibility of separations by distillation. As a simple example, Figure 3.11 presents the generation of alternatives for separating a zeotropic mixture with A, B, C from lowest to highest boiler. The initial feed is the point F. Let us consider the segment d,b, as a representation of a simple distillation process (one feed and two products). This obeys the rules ... 

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. 

In a first case study, the zeotropic mixture of pentane, hexane, heptane and octane is supposed to be separated into the pure components. Five different sequences are possible (Fig. 1), which have ten individual separation tasks. They are referenced as sequence one to five according to the labeling in the figure. Assuming a pressure of 1.013 bar, an equimolar feed and a total flowrate of 10 mol/s, the energy and exergy demands are calculated for the ten tasks using the RBM shortcut. Sequence five, which corresponds to the direct sequence, is foimd to be optimal. Obviously, the optimal sequence depends on... 

Zeotropic mixture A mixture whose components have different mass fractions in the liquid phase than in the vapor phase at equilibrium conditions. 

Selector analysis. In each selector, a logical diagram will guide the identification of the suitable separation method. A split becomes potential if it can be accomplished by at least one separation method. The selection of a separation method is supported by a ranked list of characteristic properties. For example, the relative volatility is a characteristic property for the separation of a mixture by simple distillation. However, it can be used only in for zeotropic mixtures, but not for azeotropic mixtures. 

In a bulk separation, the products to be separated are in significant amounts in the initial mixture, more than 5%. Distillation is the most interesting separation method, but the vapour/liquid phase behaviour of the mixture is essential. The logic diagrams for the separation method selection can follow two patterns zeotropic mixtures, and azeotropic mixtures. The first is relative easy to handle, the second is much more difficult. Chapter 9 will be entirely devoted to this topic. 

Separation Method Dilute Separations Zeotropic mixtures Azeotropic Mixtures Temperature sensitive... 

The progress in simulation allows, however, a rapid and accurate solution of the sequencing problem for zeotropic mixtures. We recommend the following procedure ... 

The assembly of residue curves having as origin and terminus the same points forms a distillation region. Hence, a zeotropic mixture displays always a single distillation region. On the contrary, if azeotropes are present several distillation regions can exist. 

Figure 9.4 displays RCMs for some typical azeotropic mixtures. Figure 9.4a presents the mixture acetone (56.2 °C) / benzene (80.1 °C) / heptane (98.4 °C). Acetone and heptane form a minimum boiling azeotrope with nbp of 55.1 °C, which is the lowest boiler. It may be observed that the residue curves emanate from the azeotropic point, take the direction to the benzene/heptane edge, and then deflect to the heptane vertex. Binary azeotrope and heptane are unstable and stable nodes, respectively. Acetone and benzene are saddles. In this case there is a single distillation region, as for zeotropic mixtures, but the shape of the residue curves is peculiar. 

Hence, the RCM representation allows the designer to identify the feasible separations. In the case of zeotropic mixtures if the separation is possible at total reflux, then it is certainly feasible at finite reflux, the problem being only finding a... 

Figure 9.9 Separation regions for a ternary zeotropic mixture...

The two important design elements of a distillation column are the number of theoretical stages N, and the reflux R ratio. If the mixture is zeotropic, they are related, as illustrated in Fig. 9.35-a. There is a minimum number of theoretical stages as well as a minimum reflux both depending on the sharpness of separation. For zeotropic mixtures with n-components, the shortcut design procedure known as Fenske-Underwood-Gilliland (FUG) method is well-established (Perry s handbook, 1997). 

The target of most commercial SSCs has focused mainly on the separation of zeotropic mixtures. The criteria for the use of side streams for the separation of ideal mixtures with the expected feed composition regions of SSC arrangements are (Tedder and Rudd, 1978) ... 

Fig. 5.2-22 Regions of feasible products of a zeotropic mixture at very high reflux ratio. The regions of feasible products are limited by a distillation line passing through the feed F and by straight lines emerging from the endpoints of this distillation line...

Fractionation of multicomponent zeotropic mixtures (i.e., mixtures without azeotropes) into all constituents can be performed by a sequence of distillation columns. Generally, different sequences of columns are feasible to achieve the same products. However, investment costs and energy demand of the feasible processes may... 

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