Separation liquid mixtures, nonideal

Malone, M. F. and Doherty, M. F. (1995). Separation system synthesis for nonideal liquid mixtures. AlChE Symp. Ser., 91(304), 9-18. 

Critical currents, 23 821-823 in superconducting, 23 819-825 Critical event (CE), 15 462 Critical failure, 26 982 Critical features, in separating nonideal liquid mixtures, 22 307 Critical fields, thermodynamic, 23 809-811 Critical flocculation concentration, 11 631 Critical item evaluation, for reliability, 26 991... 

Direct mode of operation, in separating nonideal liquid mixtures, 22 303, 306-307... 

Low-antimony lead alloys, 14 770 Low-birefringence polycarbonates, 19 822 Low-blush copolymers, 26 538 Low boiling node, in separating nonideal liquid mixtures, 22 303 Low-calorie beer, 3 577 Low calorie sweeteners, 12 38 Low calorific value (LCV) gas, 26 575—576... 

Nonideal liquid mixtures, separations process synthesis for, 22 301-329,... 

Unsaturation, in oils, 10 826 Unsensitization phenomenon, 19 237 Unshaped refractories, 6 491 Unslaked lime, 15 29 Unstabilized liquid sulfur trioxide, 23 517 Unstable angina, 5 109 Unstable flows, 11 761-765 Unstable node, in separating nonideal liquid mixtures, 22 303 Unstable nodes, residue curve maps, 8 790 Unstable reagents, measurement strategies for, 14 621... 

It has been long established that 3He/4He liquid mixtures phase separate at temperature below 0.9 K. (see Fig. 5.9), and the theoretical explanation for this, first advanced by Prigogine, has been outlined above. Similarly, mixtures of solid 3He/4He (formed at elevated pressure) and mixtures of solid H2/D2 both phase separate, but liquid mixtures of H2 and D2 do not, although they do show appreciable nonideality. No other small molecule isotopomer mixtures phase separate, but... 

Early in the synthesis of separation schemes for nonideal liquid mixtures, it may not be known exactly where in the flow sheet a strategic separation is going to be, only that it is required some place in some form in order to overcome or exploit a particular critical feature. Thus, strategic separations often... 

Calculations of the relations between the input and output amounts and compositions and the number of extraction stages are based on material balances and equilibrium relations. Knowledge of efficiencies and capacities of the equipment then is applied to find its actual size and configuration. Since extraction processes usually are performed under adiabatic and isothermal conditions, in this respect the design problem is simpler than for thermal separations where enthalpy balances also are involved. On the other hand, the design is complicated by the fact that extraction is feasible only of nonideal liquid mixtures. Consequently, the activity coefficient behaviors of two liquid phases must be taken into account or direct equilibrium data must be available. 

Determination of T y. In the formulation of the phase equilibrium problem presented earlier, component chemical potentials were separated into three terms (1) 0, which expresses the primary temperature dependence, (2) solution mole fractions, which represent the primary composition dependence (ideal entropic contribution), and (3) 1, which accounts for relative mixture nonidealities. Because little data about the experimental properties of solutions exist, Tg is usually evaluated by imposing a model to describe the behavior of the liquid and solid mixtures and estimating model parameters by semiempirical methods or fitting limited segments of the phase diagram. Various solution models used to describe the liquid and solid mixtures are discussed in the following sections, and the behavior of T % is presented. 

We shall now look at the synthesis of separation systems for liquid mixtures of species that display highly nonideal behavior. We reference a small sampling... 

We now consider the separation of a nonideal mixture where the species do not display liquid/liquid behavior. Almost certainly, the technology of choice... 

H -I- HD is the only mixture of compounds of hydrogen that has a separation factor as favorable as in conventional industrial distillation. In this case, however, the true separation factor is less favorable than here calculated from the vapor-pressure ratio, because of nonidealities in gaseous and liquid mixtures of hydrogen and HD. Moreover, it is desirable to operate above atmospheric pressure, to preclude in-leakage of air. Under practical conditions, at... 

The BP and SR methods for vapor-liquid contacting converge only with difficulty or not at all for separations involving very nonideal liquid mixtures (e.g., in extractive distillation) or for cases where the separator is like an absorber or stripper in one section and a fractionator in another section (e.g., a reboiled absorber). Furthermore, BP and SR methods are generally restricted to the very limited specifications stated above. More general procedures capable of solving ail types of multicomponent, multistage separation problems are based on the solution of all the MESH equations, or combinations thereof, by simultaneous correction (SC) techniques. 

Example 15.5. The separation of benzene B from n-heptane H by ordinary distillation is difficult. At atmospheric pressure, the boiling points differ by 18.3°C. However, because of liquid-phase nonideality, the relative volatility decreases to a value less than 1.15 at high benzene concentrations. An alternative method of separation is liquid-liquid extraction with a mixture of dimethylformamide (DMF) and water. The solvent is much more selective for benzene than for n-heptane at 20°C. For two different solvent compositions, calculate interstage flow rates and compositions by the rigorous ISR method for the countercurrent liquid-liquid extraction cascade, which contains five equilibrium stages and is shown schematically in Fig. 15.22. 

In 8.4.5 we described the stability conditions that, when violated, can cause a one-phase liquid mixture to separate into two liquid phases. We also showed in Figure 8.20 an isobaric, liquid-liquid, Txx diagram on which one-phase states divide into stable, metastable, and unstable states. Liquid-liquid separations occur in nonideal mixtures that have strong positive deviations from ideal-solution behavior in such mixtures the activity coefficients become much greater than unity. This occurs when attractive forces between molecules of the same species are stronger than those between molecules of different species. Liquid-liquid separations have never been observed in mixtures that are negative deviants over the entire composition range. 

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