Chemically enhanced distillation

Chemically enhanced distillation is similar to extractive distillation, but this time the solvent is an ionic salt or a complex organic molecule. As an example, acetone separates easier from a solution with methanol in the presence of a concentrated solution of calcium chloride. More modern solvents are based on ionic liquids. 

---

Jeffrey J SiirolO/ Ph D / Research Fellow, Eastman Chemical Company Member, National Academy of Engineering Fellow, Ametican Institute of Chemical Engineers, American Chemical Society, Amet ican Association for Artificial Intelligence, Amet ican Society for Engineering Education. (Enhanced Distillation)... 

The variation of efficiencies is due to interaction phenomena caused by the simultaneous diffusional transport of several components. From a fundamental point of view one should therefore take these interaction phenomena explicitly into account in the description of the elementary processes (i.e. mass and heat transfer with chemical reaction). In literature this approach has been used within the non-equilibrium stage model (Sivasubramanian and Boston, 1990). Sawistowski (1983) and Sawistowski and Pilavakis (1979) have developed a model describing reactive distillation in a packed column. Their model incorporates a simple representation of the prevailing mass and heat transfer processes supplemented with a rate equation for chemical reaction, allowing chemical enhancement of mass transfer. They assumed elementary reaction kinetics, equal binary diffusion coefficients and equal molar latent heat of evaporation for each component. 

In the vinyl-chloride process, because of the significant differences in the volatilities of the three principal chemical species, distillation, absorption, and stripping are prime candidates for the separators, especially at the high production rates specified. For other processes, liquid-liquid extraction, enhanced distillation, adsorption, and membrane separators might become more attractive, in which case the design team would need to assemble data that describe the effect of solvents on species phase equilibrium, species adsorption isotherms, and the permeabilities of the species through various membranes. 

When the reaction products are in the liquid phase, Douglas recommends that a liquid-separation system be inserted in the flowsheet, as shown in Figure 5.7. The liquid-separation system involves one or more of the following separators distillation and enhanced distillation, stripping, liquid-liquid extraction, and so on, with the unreacted chemicals recovered in a liquid phase and recycled to the reaction operation. 

When a multicomponent fluid mixture is nonideal, its separation by a sequence of ordinaiy distillation columns will not be technically and/or economically feasible if relative volatiK-ties between key components drop below 1.05 and, particularly, if azeotropes are formed. For such mixtures, separation is most commonly achieved by sequences comprised of ordinary distillation columns, enhanced distillation columns, and/or liquid-liquid extraction equipment. Membrane and adsorption separations can also be incorporated into separation sequences, but their use is much less common. Enhanced distillation operations include extractive distillation, homogeneous azeotropic distillation, heterogeneous azeotropic distillation, pressure-swing distillation, and reactive distillation. These operations are considered in detail in Perry s Chemical Engineers Handbook (Perry and Green, 1997) and by Seader... 

Sections 7.4 and 7.5 deal primarily with the synthesis of separation trains for liquid-mixture feeds. The primary separation techniques are ordinary and enhanced distillation. If the feed consists of a vapor mixture in equilibrium with a liquid mixture, the same techniques and synthesis procedures can often be employed. However, if the feed is a gas mixture and a wide gap in volatility exists between two groups of chemicals in the mixture, it is often preferable, as discussed in Section 7.1, to partially condense the mixture, separate the phases, and send the liquid and gas phases to separate separation systems as discussed by Dou (1988) and shown in Figure 7.44. Note that if a liquid phase is produced in the gas separation system, it is routed to the liquid separation system and vice versa. 

Soluble impurities can be extracted by washing with deionized or distilled water foUowed by filtration (1,12,26). Powders prepared by wet chemical synthesis are often washed and filtered for purification prior to use. The dewatering (qv) process can be enhanced by pressure filtration. Organic solvents can be used to remove water-insoluble impurities and wash-water sensitive materials. 

Reactive distillation. Methods that use chemical reaciion to modify the composition of the mixture or, alternatively, use existing vapor-hquid differences between reaction products and reaciants to enhance the performance of a reaction. 

There are no available data to establish whether nonconductive, low viscosity chemical products such as ethyl ether similarly display hyperbolic relaxation below about 2 pS/m, or even whether this phenomenon is a practical reality for such liquids. Should Ohmic relaxation behavior continue to much less than 0.5 pS/m the risk of static accumulation would be enhanced compared with petroleum distillates. 

Removal of thermodynamic restriction through reactive distillation and enhancement of hydrogen generation reactivity due to this concept made it possible to utilize organic chemical hydrides in the field of hydrogen storage from a novel standpoint. 

MTBE is a well known enhancer of the number of octanes in gasoline and as excellent oxygentated fuel additives that decrease carbon monoxide emissions. Therefore, MTBE has been one of the fastest growing chemicals of the past decade. MTBE is produced by reacting methanol with isobutylene from mixed-C4 stream liquid phase over a strong acid ion-exchange resin as catalyst. An excess of methanol is used in order to improve the reaction conversion. This excess has to be separated from the final product. The pervaporation technique, more energy efficient and with lower cost process, has been proposed as alternative to distillation [74],... 

Chemical/Physical. Ozonolysis of acetic acid in distilled water at 25 °C yielded glyoxylic acid which oxidized readily to oxalic acid before undergoing additional oxidation producing carbon dioxide. Ozonolysis accompanied by UV irradiation enhanced the removal of acetic acid (Kuo et al, 1977). 

Distilled rather than natural water is often used as the solvent for determination of quantum yields for two major reasons. First, the total absorbance of the solution at the wavelength of irradiation should not exceed 0.02. Second, and more important, the presence of natural water constituents (e.g., humic material, nitrate) could enhance the total photolytic transformation rate by indirect photolytic processes as described in Chapter 16. Zepp and Baughman (1978) have argued that for many chemicals d>,r obtained in distilled water is nearly the same as that observed in natural waters (at least in uncontaminated freshwaters), because concentrations of natural water constituents that could undergo reactions with or quench photolysis of excited pollutants are generally very low. Furthermore, the effects of molecular oxygen, which may act as a quencher, can also be studied in distilled water. 

---