Non-azeotropic mixtures

Non-azeotropic mixtures have been utilized in refrigeration systems for several direct and indirect advantages like, enhanced coefficient of performance, lower power consumption, reduced thermal irreversibility, increased chemical stability, improved oil miscibility, varying condensation temperatures and variable capacity refrigeration systems. All these merits offer rich prospects for the use of mixed component working fluids in heat pumps, power cycles and refrigeration systems. 

In the mid 1980s, a new thermodynamic power cycle using a multicomponent working fluid as ammonia-water with a different composition in the boiler and condenser was proposed (known as the Kalina cycle). The use of a non-azeotropic mixture decreases the loss of availability in a heat recovery boiler when the heat source is a sensible heat source, and in a condenser when the temperature decreases with heat exchange. Most heat input to a plant s working fluid is from variable temperature heat sources. 

The algorithm for the generation of operational sequences for the case of batch distillation has been discussed in Papaeconomou et al., 2003 for the separation of multicomponent non-azeotropic mixtures. In this paper, a case study of binary azeotropic mixture is presented. The algorithm is applied, in order to obtain an operational route that will remove the azeotrope in minimum time and/or cost, so that the desired high purity product will remain in the vessel. 

The simplest form of ternary RCM, as exemplified for the ideal normal-paraffin system of pentane-hexane-heptane, is illustrated in Fig. 13-58 7, using a right-triangle diagram. Maps for all other non-azeotropic ternary mixtures are qiiahtatively similar. Each of the infinite number of possible residue curves originates at the pentane vertex, travels toward and then away from the hexane vertex, and terminates at the heptane vertex. 

Propane tute for use in low azeotropic mixture temperature refrig- temperature refrig- ture used in the low (CHF5/C2F6) A non-... 

If, for example, a mixture of ethanol and water is distilled, the concentration of the alcohol steadily increases until it reaches 96 per cent by mass, when the composition of the vapour equals that of the liquid, and no further enrichment occurs. This mixture is called an azeotrope, and it cannot be separated by straightforward distillation. Such a condition is shown in the y — x curves of Fig. 11.4 where it is seen that the equilibrium curve crosses the diagonal, indicating the existence of an azeotrope. A large number of azeotropic mixtures have been found, some of which are of great industrial importance, such as water-nitric acid, water-hydrochloric acid, and water-alcohols. The problem of non-ideality is discussed in Section 11.2.4 where the determination of the equilibrium data is considered. When the activity coefficient is greater than unity, giving a positive deviation from Raoult s law, the molecules of the components in the system repel each... 

THF well dissolves oxygen from the air and the unwanted peaks are observed in the area of high retention volumes (Section 16.4.5). THF is highly hydroscopic and it readily absorbs large amounts of moisture. As a result, even the well stored THF eluents may contain the non-negligible amount of water, which may affect retention volumes of polymers both in the SEC and in coupled modes of polymer HPLC [28,267,268]. Azeotropic mixture of THF with water contains about 4.5wt.% of water and its boiling point differs less than 3°C from the boiling point of dry THF at the atmospheric pressure. 

Process synthesis and design of these non-conventional distillation processes proceed in two steps. The first step—process synthesis—is the selection of one or more candidate entrainers along with the computation of thermodynamic properties like residue curve maps that help assess many column features such as the adequate column configuration and the corresponding product cuts sequence. The second step—process design—involves the search for optimal values of batch distillation parameters such as the entrainer amount, reflux ratio, boiler duty and number of stages. The complexity of the second step depends on the solutions obtained at the previous level, because efficiency in azeotropic and extractive distillation is largely determined by the mixture thermodynamic properties that are closely linked to the nature of the entrainer. Hence, we have established a complete set of rules for the selection of feasible entrainers for the separation of non ideal mixtures... 

Pervaporation Liquid Non-porous membrane with pressure gradient Separation of azeotropic mixtures... 

Although reverse osmosis can be used to separate organic and aqueous-organic liquid mixtures, very high pressures are required. Alternatively, pervaporation can be used in which the species being absorbed by, and transported through, the non-porous membrane are evaporated. This method, which uses much lower pressures than reverse osmosis, but where the heat of vapourisation must be supplied, is used to separate azeotropic mixtures. 

Dussel and Stichlmair (1995), Ahmad and Barton (1996), Safrit and Westerberg (1997) discussed the theoretical aspects of the separation of azeotropic mixtures in conventional and non-conventional BED column configurations using simulation techniques. Refer to the original references for further details. 

The feasibility of separations of non-ideal mixtures, as well as the screening of mass separation agents for breaking azeotropes can be rationalised by means of thermodynamic methods based on Residue Curve Maps (Chapter 9). 

Simple Distillation. In this category, we include the separation of ideal or slightly non-ideal mixtures that do not form azeotropes, based on the differences in the relative volatilities of components. A simple column designates a device that separates one or several feeds in only two products top distillate and bottoms. Complex columns are can deliver more than two products. In this category we include columns with side-streams, columns equipped with auxiliary devices, as prefractionators, side-strippers and side-rectifiers, as well as thermally integrated columns. 

The above sequencing methods valid for zeotropic systems cannot be applied in the case of mixture with strong non-ideal character and displaying distillation boundaries, as those in the case of breaking azeotropes. Fortunately, the sequencing problem in this case has a different character. Most of the separations of multi-component non-ideal mixtures can be reduced by appropriate splits to the treatment of ternary mixtures, for which two or three columns are normally sufficient. The separation sequence follows direct or indirect sequence. The energetic consumption due to the recycle of entrainer dominates the economics. From this viewpoint preferred is that sequence in which the entrainer is recycled as bottoms. Hence, in azeotropic distillation the main problem is the solvent selection and not columns sequencing. 

This term designates a class of distillation techniques used to separate non-ideal azeotropic mixtures or zeotropic systems with very low relative volatility. In most cases the presence of a mass separation agent (MSA) is necessary. 

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