Diffusional distillation

Ordinary diffusion involves molecular mixing caused by the random motion of molecules. It is much more pronounced in gases and Hquids than in soHds. The effects of diffusion in fluids are also greatly affected by convection or turbulence. These phenomena are involved in mass-transfer processes, and therefore in separation processes (see Mass transfer Separation systems synthesis). In chemical engineering, the term diffusional unit operations normally refers to the separation processes in which mass is transferred from one phase to another, often across a fluid interface, and in which diffusion is considered to be the rate-controlling mechanism. Thus, the standard unit operations such as distillation (qv), drying (qv), and the sorption processes, as well as the less conventional separation processes, are usually classified under this heading (see Absorption Adsorption Adsorption, gas separation Adsorption, liquid separation). 

As a result of the diffusional process, there is no net overall molecular flux arising from diffusion in a binary mixture, the two components being transferred at equal and opposite rates. In the process of equimolecular counterdiffusion which occurs, for example, in a distillation column when the two components have equal molar latent heats, the diffusional velocities are the same as the velocities of the molecular species relative to the walls of the equipment or the phase boundary. 

The problems relating to mass transfer may be elucidated out by two clear-cut yet different methods one using the concept of equilibrium stages, and the other built on diffusional rate processes. The selection of a method depends on the type of device in which the operation is performed. Distillation (and sometimes also liquid extraction) are carried out in equipment such as mixer settler trains, diffusion batteries, or plate towers which contain a series of discrete processing units, and problems in these spheres are usually solved by equilibrium-stage calculation. Gas absorption and other operations which are performed in packed towers and similar devices are usually dealt with utilizing the concept of a diffusional process. All mass transfer calculations, however, involve a knowledge of the equilibrium relationships between phases. 

From the analysis given already of the diffusional nature of absorption, one of the outstanding requirements is to provide as large an interfacial area of contact as possible between the phases. For this purpose, columns similar to those used for distillation are suitable. However, whereas distillation columns are usually tall and thin absorption columns are more likely to be short and fat. In addition, equipment may be used in which gas is passed into a liquid which is agitated by a stirrer. A few special forms of units have also been used, although it is the packed column which is most frequently used for gas absorption applications. 

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. 

Soo et al. (2002) studied tliB vitro release of hydrophobic Luorescent probes from PEO-b PCL micelles. Micelle solutions were placed in dialysis bags (MWCO 50,000) in a stirred water bath with a constant overLow of distilled water. This maintained the release environment at near perfect sink conditions, so the limited solubility ofthe probes in the medium did not affect release kinetics. Release was determined by removing aliquots ofthe dialysis bag contents and measuring Luorescently. Soo et al. found an initial burst release of probe followed by slow diffusional release. For the probes studies, benzopyrene and Cell-Tracker-CM-Dil, diffusion constants were ofthe order 10"15 cnnP/s. 

The modeling of RD processes is illustrated with the heterogenously catalyzed synthesis of methyl acetate and MTBE. The complex character of reactive distillation processes requires a detailed mathematical description of the interaction of mass transfer and chemical reaction and the dynamic column behavior. The most detailed model is based on a rigorous dynamic rate-based approach that takes into account diffusional interactions via the Maxwell-Stefan equations and overall reaction kinetics for the determination of the total conversion. All major influences of the column internals and the periphery can be considered by this approach. 

In the following, the principles of mass-transfer separation processes will be outlined first. Details of mass-transfer calculations will be introduced next and examples will be given of both equilibrium-stage processes and diffusional rate processes. The chapter will then conclude with a detailed discussion of the two single most applied mass-transfer processes in the chemical industries, namely distillation and absorption. 

From a kinetic standpoint (4), mass transfer per unit volume in distillation is limited only by the diffusional resistances on either side of the vapor-liquid interface in turbulent phases, with no inerts present, In almost every other separation process, there are inert solvents or... 

Vogelpohl (193) and Medina et al. (203) applied the diffusional interaction method for predicting ternary distillation composition profiles using binary data. They achieved this by eliminating the first two steps and assuming that all the mass transfer resistance is in the vapor. This procedure was shown to give excellent agreement with experimental data for dissimilar components. Biddulph and Kalbassi (194), however, report some discrepancies between prediction and experiment due to this assumption. 

From a kinetic view, in distillation, mass transfer is limited only by the diffusional resistances on either... 

Fullarton and Schlunder (1983) investigated the process of diffusional distillation for separating liquid mixtures of azeotropic composition. The process is shown schematically in Figure 8.8. A liquid mixture is evaporated at a temperature below its boiling point, diffuses through a vapor space filled with inert gas and condenses at a lower temperature. The inert gas functions as a selective filter that allows preferential passage of those components that diffuse more quickly. Thus, the condensed liquid has a composition different from that of the original mixture. 

In Example 8.3.2 we determined the rates of mass transfer in diffusional distillation, a process described by Fullarton and Schliinder (1983) for separating liquid mixtures of azeotropic composition. Estimate the heat flux through the gas/vapor mixture under the conditions prevailing in the experiment described in Example 8.3.2. 

Using an entirely different approach to the modeling of multicomponent mass transfer in distillation (an approach that we describe in Chapter 14), Krishnamurthy and Taylor (1985c) found significant differences in design calculations involving nonideal systems. For an almost ideal system (a hydrocarbon mixture), pseudobinary methods were found to be essentially equivalent to a more rigorous model that accounted for diffusional interaction effects. 

Repeat Example 8.3.2 (diffusional distillation) using the linearized equations for determining the fluxes and composition profiles. Compare your results to those given in Example 8.3.2. 

Burghardt, A., Warmuzinski, K., Buzek, J., and Pytlik, A., Diffusional Methods of Multicomponent Distillation and their Experimental Verification, Chem. Eng. J., 26, 71-84 (1983). 

This chapter deals with the diffusional transfer of mass to and across a phase boundary. In particular, gas-liquid, gas-solid, and liquid-liquid phase combinations have been considered. Process applications include absorption, stripping, distillation, extraction, adsorption, and the diffusional aspects of chemical reactions on a solid surface. For steady-state transfer operations, the rates of mass transfer can be correlated by variations of Pick s first law, which states that the rate is directly proportional to the concentration driving force and the extent of interfacial area, and inversely proportional to the distance of movement of the mass to the interface. 

The operations which include humidification and dehumidification, gas absorption and desorption, and distillation all have in common the requirement that a gas and a liquid phase be brought into contact for the purpose of diffusional interchange between them. The equipment for gas-liquid contact can be broadly classified according to whether its principal action is to disperse the gas or the liquid, although in many devices both phases become dispersed. In principle, at least, any type of equipment satisfactory for one of these operations is suitable for the others, and the major types are indeed used for all. For this reason, the main emphasis of this chapter is on equipment for gas-liquid operations. 

The advantages of distillation as a separation method are clear. In distillation the new phases differ from the original by their heat content, but heat is readily added or removed, although, of course, the cost of doing this must inevitably be considered. Absorption or desorption operations, on the other hand, which depend upon the introduction of a foreign substance, result in a new solution which in turn may have to be separated by one of the diffusional operations unless it happens that the new solution is useful directly. 

Using Fig. 1.17 with data from Appendix I, plot logo (relative volatility) of C5 to C12 normal paraffins as referred to n-Cs against the carbon number (5 to 12) at a reasonable temperature and pressure. On the same plot show log a for Cs to Cio aromatics (benzene, toluene, ethylbenzene, etc.), also with reference to n-Cs. What separations are possible by ordinary distillation, assuming a mixture of normal paraffins and aromatic compounds (E. D. Oliver, Diffusional Separation Processes Theory, Design Evaluation, J. Wiley Sons, New York, 1966, Chapter 13. 

A direct conclusion from this is that by increasing the ratio of external to internal surface area, it should be possible to increase the activity of a given zeolite Y. Meanwhile by decreasing the crystallite size, the diffusional problems of the middle distillate molecules should be lowered, decreasing consecutive reactions leading to recracking and hydrogen transfer. 

Reactive distillation occurs in multiphase fluid systems, with an important role of the interfacial transport phenomena. It is an inherently multicomponent process with much more complexity than similar binary processes. Multi-component thermodynamic and diffusional coupling in the phases and at the interface is accompanied by complex hydrodynamics and chemical reactions [4, 42, 43]. As a consequence, an adequate process description has to be based on specially developed mathematical models. However, sophisticated RD models are hardly applicable for plant design, model-based control and online process optimization. For such cases, a reasonable model reduction should be applied [44],... 

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