Separation Factors in Distillation

When only two isotopic compounds are present in the mixture being separated, such as a mixture of CH4 and CH3D or a mixture of H2 0 and H2 0, the separation factor in distillation may be estimated with sufficient accuracy for survey purposes from the ratio of the vapor pressures it of the two compounds. 

Table 13.3 Separation factors in distillation estimated from vapor-pressure ratios...

These results for water and aimnonia suggest that Eq. (13.4) can be used to predict separation factors in distillation with an enor in In a no greater than 10 percent. 

Table 13.6 Effect of deuterium content on separation factor in distillation of ammonia...

Separation factor in distillation of Isotopic mixtures Isotopic mixtures are often separated by distillation. The production of heavy water, D2O, is a prime example. In the distillation of natural water, which contains variable amounts of deuterium depending on the source (see Table 13.1 in Benedict et al. (1981)), deuterium is invariably concentrated in the liquid phase. Water in the tropical oceans contains about 156 ppm deuterium. Multistage distillation (see Section 8.1.3) has to be carried out to obtain a liquid fraction substantially enriched in deuterium. Here we will consider only the... 

Relative volatility is the volatility separation factor in a vapor-liquid system, i.e., the volatility of one component divided by the volatility of the other. It is the tendency for one component in a liquid mixture to separate upon distillation from the other. The term is expressed as fhe ratio of vapor pressure of the more volatile to the less volatile in the liquid mixture, and therefore g is always equal to 1.0 or greater, g means the relationship of the more volatile or low boiler to the less volatile or high boiler at a constant specific temperature. The greater the value of a, the easier will be the desired separation. Relative volatility can be calculated between any two components in a mixture, binary or multicomponent. One of the substances is chosen as the reference to which the other component is compared. 

Separation Factor The separation factor in extraction is analogous to relative volatility in distillation. It is a dimensionless factor that measures the relative enrichment of a given component in the extract phase after one theoretical stage of extraction. For cosolutes i andj,... 

Derivation of Eq. (13.3). The following derivation of Eq. (13.3) relating the deuterium separation factor in the distillation of water to the vapor pressure rr of H2O and D2O is similar to that given by Urey [Ul]. It is assumed that ... 

Distillation of water. Combs et al. [Cll] have determined the deuterium separation factor in the distillation of water by measuring the H/D ratio in water liquid and vapor in equilibrium. The third and fourth columns of Table 13.4 compare their measured separation factors with values predicted by Eq. (133) from their values for the vapor pressures of pure H2O and DjO. The agreement in the two sets of values of In a is within 6 percent. The agreement with Kirdienbaum s vapor-pressure ratios [K2] is somewhat poorer. Rolston et al. [R8] have proposed the equation In a = 03592 — 803/T -I- 25,490/r to correlate all data to... 

Distillation of ammonia. Petersen and Benedict [P2] have made similar direct measurements of the deuterium separation factor in the distillation of ammonia. Table 13.5 compares their values for ammonia containing 24 percent deuterium with those predicted by Eq. (13.9) from vapor pressures of NH3 and ND3 measured by Kirshenbaum and Urey [K3], Groth et al. [G4], and Taylor and Jimgers [Tl]. 

The separation factor in the Kremser method is an effective absorption factor, for absorption and a stripping factor, 5, for stripping, rather than a relative volatility as in the FUG method for distillation. These two factors, which are different for each component, are defined by ... 

In most cases, special distillation processes such as azeotropic or extractive distillation are applied to separate azeotropic systems by distillation, where a suitable solvent is added. While in the case of azeotropic distillation a solvent is required which forms a lower boiling azeotropic point, in the case of extractive distillation a high-boiling selective solvent is used which changes the separation factor in a way that it becomes distinctly different from unity. Both processes are shown in Figure 11.16 together with the column configuration. 

Distillation is a countercurrent process in which equihbrium is maintained between Uquid and vapor, normally in a vertical column containing packing, thin wetted walls, or bubble cap plates. The single-stage separation factor for distillation is given by the vapor pressure isotope effect. Because mixtures of isotopic molecules show little or no nonideality in either liquid or vapor phase (Jancso et al. 1993), liquid vapor separation factors should be virtually independent of isotope enrichment (plate number), and the analysis of the distillation process is... 

The component C in the separated extract from the stage contact shown in Eigure 1 may be separated from the solvent B by distillation (qv), evaporation (qv), or other means, allowing solvent B to be reused for further extraction. Alternatively, the extract can be subjected to back-extraction (stripping) with solvent A under different conditions, eg, a different temperature again, the stripped solvent B can be reused for further extraction. Solvent recovery (qv) is an important factor in the economics of industrial extraction processes. 

Irreversible processes are mainly appHed for the separation of heavy stable isotopes, where the separation factors of the more reversible methods, eg, distillation, absorption, or chemical exchange, are so low that the diffusion separation methods become economically more attractive. Although appHcation of these processes is presented in terms of isotope separation, the results are equally vaUd for the description of separation processes for any ideal mixture of very similar constituents such as close-cut petroleum fractions, members of a homologous series of organic compounds, isomeric chemical compounds, or biological materials. 

Corrosivity. Corrosivity is an important factor in the economics of distillation. Corrosion rates increase rapidly with temperature, and in distillation the separation is made at boiling temperatures. The boiling temperatures may require distillation equipment of expensive materials of constmction however, some of these corrosion-resistant materials are difficult to fabricate. For some materials, eg, ceramics (qv), random packings may be specified, and this has been a classical appHcation of packings for highly corrosive services. On the other hand, the extensive surface areas of metal packings may make these more susceptible to corrosion than plates. Again, cost may be the final arbiter (see Corrosion and corrosion control). 

In distillation towers, entrainment lowers the tray efficiency, and 1 pound of entrainment per 10 pounds of liquid is sometimes taken as the hmit for acceptable performance. However, the impact of entrainment on distiUation efficiency depends on the relative volatility of the component being considered. Entrainment has a minor impact on close separations when the difference between vapor and liquid concentration is smaU, but this factor can be dominant for systems where the liquid concentration is much higher than the vapor in equilibrium with it (i.e., when a component of the liquid has a very lowvolatiUty, as in an absorber). 

Selectivity. The relative separation, or selectivity, Ot of a solvent is the ratio of two components in the extraction-solvent phase divided by the ratio of the same components in the feed-solvent phase. The separation power of a hquid-liquid system is governed by the deviation of Ot from unity, analogous to relative volatility in distillation. A relative separation Ot of 1.0 gives no separation of the components between the two liquid phases. Dilute solute concentrations generally give the highest relative separation factors. 

The Smith-Brinkley Method uses two sets of separation factors for the top and bottom parts of the column, in contrast to a single relative volatility for the Underwood Method. The Underwood Method requires knowing the distillate and bottoms compositions to determine the required reflux. The Smith-Brinkley Method starts with the column parameters and calculates the product compositions. This is a great advantage in building a model for hand or small computer calculations. Starting with a base case, the Smith-Brinkley Method can be used to calculate the effect of parameter changes on the product compositions. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

By taking the ratio of the distribution coefficients for the two Components i and j, the separation factor can be defined, which is analogous to relative volatility in distillation ... 

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