Mass transfer molecular distillation

Since the molecular weight of benzene (78) is close to that of thiophene (84), the mass transfer in distillation do not change substantially the amount of liquid phase in the process, we may let... 

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). 

Most of the properties change somewhat from one end to the other of industrial columns for effecting separations, so that the mass transfer coefficients likewise vary. Perhaps the property that has the most effect is the mass rate of flow which appears in the Reynolds number. Certainly it changes when there is a substantial transfer of material between the two phases in absorption or stripping and even under conditions of constant molal overflow in distillation processes, the mass rate of flow changes because of differences of the molecular weights of the substances being separated. As a practical expedient, however, mass transfer coefficients are evaluated at mean conditions in a column. 

Since water is the byproduct, and also has an undesired inhibitory effect on catalyst activity, it must be separated efficiently from the reaction mixture. To achieve this, both conventional reactive distillation and reactive membrane separation are considered as process alternatives. In the latter process, a Knudsen-membrane is applied. Consequently, the mass transfer matrix [/c] has a diagonal structure and the diagonal elements are the Knudsen-selectivities - that is, the square-roots of the ratios of the molecular weights Mr. 

Nonequilibrium molecular dynamics simulations show that the assumption of local equilibrium in a column with heat and mass transfer is acceptable. The dissipation function in a binary distillation is (Ratkje et al., 1995 Sauar etal., 1997)... 

Fast and satisfactory mass transfer calculations are necessary since we may have to repeat such calculations many times for a rate-based distillation column model or two-phase flow with mass transfer between the phases in the design and simulation process. The generalized matrix method may be used for multicomponent mass transfer calculations. The generalized matrix method utilizes the Maxwell-Stefan model with the linearized film model for diffusion flux, assuming a constant diffusion coefficient matrix and total concentration in the diffusion region. In an isotropic medium, Fick s law may describe the multicomponent molecular mass transfer at a specified temperature and pressure, assuming independent diffusion of the species in a fluid mixture. Such independent diffusion, however, is only an approximation in the following cases (i) diffusion of a dilute component in a solvent, (ii) diffusion of various components with identical diffusion properties, and (iii) diffusion in a binary mixture. 

Diffusion is the main mechanism involved in the mass transfer during osmotic distillation and the resistance to mass transfer comes from both membrane stmcture and presence of air trapped within the membrane pores. While the former resistance can be described by Knudsen diffusion Equation 19.26, the latter is described by molecular diffusion Equation 19.27. 

The above mass transfer equations, although based on sound molecular diffusion principles, are limited in their applicability in a number of ways. A basic condition for their validity is the assumption of equimolar or dilute unimolar mass transfer. This limits the NTU and HTU approach to processes that are essentially either binary (distillation) or ternary (absorption or stripping) with only one component crossing the phase boundary. Another shortcoming of the transfer units technique is its exclusion of energy balances or temperature calculations. 

A similar sinplification occurs for the mass-transfer analysis in Section 15.4 and for the analysis of distillation in Section 16.1. If the molecular weights are not equal, then v gf 355 v f moi and solution of... 

Absorption. When the two contacting phases are a gas and a liquid, the unit operation is called absorption. A solute A or several solutes are absorbed from the gas phase into a liquid phase in absorption. This process involves molecular and turbulent diffusion or mass transfer of solute A through a stagnant nondiffusing gas B into a stagnant liquid C. An example is absorption of ammonia A from air B by the liquid water C. Usually, the exit ammonia-water solution is distilled to recover relatively pure ammonia. 

The balance (C.25) has the obvious form of a conservation law with source term. With the minus sign at the integral it means that the increase in equals the quantity transported into across the boundary plus that produced by chemical reactions, per unit time. The jk-term can be relevant at a permeable boundary imagine as liquid phase in a distillation column, thus j .n represents total mass transferred per unit area and j. n is due to molecular diffusion of species. ... 

It should be possible to obtain results similar to molecular distillation at higher total pressures by a high degree of turbulence in the space between the condenser and the evaporating liquid in order to obtain rapid mass transfer. 

In atmospheric distillations, the gas film many times offers a resistance to mass transfer similar to that of the liquid film. The vapor density is low, providing a high molecular diffusion rate in that phase. The diffusion coefficient in the vapor phase at atmospheric pressure may be several orders of magnitude times that in the liquid phase. However, the liquid-phase resistance becomes increasingly important as the liquid viscosity increases, which reduces the diffusion rate in that phase. Also, there is a tendency for the liquid-film resistance to increase as the liquid molecular weight increases. 

The introductory Section 3.1.2.5 in Chapter 3 identifies the negative chemical potential gradient as the driver of targeted separation, and the relevant species flux expression is developed in Section 3.1.3.2 (see Example 3.1.9 also). Section 3.1.4 introduces molecular diffusion and convection and basic mass-transfer coefficient based flux expressions essential to studies of distillation and other phase equilibrium based separation processes. Section 3.1-5.1 introduces the Maxwell-Stefan equations forming the basis of the rate based approach of analyzing distillation column operation. After these fundamental transport considerations (which are also valid for other phase equilibrium based separation processes), we encounter Section 3.3.1, where the equality of chemical potential of a species in all phases at equilibrium is illustrated as the thermodynamic basis for phase equilibrium (Le. = /z ). Direct treatment of distillation then begins in Section 3.3.7.1, where Raouit s law is introduced. It is followed by Section 3.4.1.1, where individual phase based mass-transfer coefficients are reiated to an overall mass-transfer coefficient based on either the vapor or liquid phase. 

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