Mass diffusion stage

In mass diffusion, separation of isotopes occurs throu diffusion of the li t isotope of a gas mixture into a condensible vapor at higher rate than diffusion of the heavy isotope. Mass diffusion separation has been canied out in a cascade of individual mass diffusion stages and in a mass diffusion column. 

The stage type of mass diffusion was patented by Hertz [H4], who used this method to separate the isotopes of neon [HS, H6]. The means by which separation is effected in a mass diffusion stage are shown in Fig. 14.33, which illustrates the type of equipment used by Maier [Ml] to separate hydrogen from other gases. 

Table 14.21. Isotopes separated by cascade of mass diffusion stages...

Because the separation obtainable in a mass diffusion stage is even smaller than in a gaseous diffusion stage, a practical degree of separation requires either a multistage cascade, such as the 48-stage cascade used by Hertz [H3] to separate neon isotopes, or a mass diffusion column. 

Separation of Uranium Isotopes by Cascade of Mass Diffusion Stages... 

Physical separation. Figure 14.37 is a schematic flow sheet for the system used for the physical separation stage that prepares feed for each mass diffusion stage. The mixture of UF and N43... 

Equations for sqtaiation performance of mass diffusion stage. At the bottom of Fig. 14.35, the stage conditions found to lead to minimum unit cost of separative work are tabulated. The condition that the UP6 content of the two effiuent streams be equal,... 

N43 through the holes of a mass diffusion screen and for material balances in countercurrent flow of the two streams in the mass diffusion stage of Fig. 14.35 to obtain Eq. (14.321) for the stage separation factor a. When there is no net flow through the screen. 

The rate of production of separative work in this mass diffusion stage, with a cut of 5, is... 

This equation predicts that the height of a theoretical diffusion stage increases, ie, mass-transfer resistance increases, both with bed height and bed diameter. The diffusion resistance for Group B particles where the maximum stable bubble size and the bed height are critical parameters may also be calculated (21). 

The film diffusion stages (i) and (v) are similar to the external mass transfer stages in leaching and may be dealt similarly (see Section 2.1.3). Solution flow conditions for resin touching the fixed beds are likely to be in the laminar flow range. 

The form of equation (21) is interesting. It shows that the uptake curve for a system controlled by heat transfer within the adsorbent mass has an equivalent mathematical form to that of the isothermal uptake by the Fickian diffusion model for mass transfer [26]. The isothermal model hag mass diffusivity (D/R ) instead of thermal diffusivity (a/R ) in the exponential terms of equation (21). According to equation (21), uptake will be proportional to at the early stages of the process which is usually accepted as evidence of intraparticle diffusion [27]. This study shows that such behavior may also be caused by heat transfer resistance inside the adsorbent mass. Equation (22) shows that the surface temperature of the adsorbent particle will remain at T at all t and the maximum temperature rise of the adsorbent is T at the center of the particle at t = 0. The magnitude of T depends on (n -n ), q, c and (3, and can be very small in a differential test. 

Figure 8.8. Entropy production rate profile for a 15 stage RD column for MTBE synthesis. Legend 5m entropy produced by interfacial mass diffusion S entropy produced by interfacial energy transfer Srx. entropy produced by chemical reaction Remarks stages are numbered from top to bottom reboiler and condenser drum are not depicted MeOH feed stream is fed at 9 tray and iCt stream at 10 tray. The operational and design variables are listed in table 8.4.

Affinity at reaction stage r Acceleration Activity of species a Left Cauchy-Green tensor Body force per unit mass Diffusive body force Right Cauchy-Green tensor Mass-molar concentration Mass concentration of species a Mass-energy flux of the system... 

The SD control of (kt) in the initial stage of a polymerization is often called plateau-regime, since (kt) remains more or less constant with increasing conversion. The plateau level depends on (mostly the viscosity of) monomer and solvent. SD control is characterized by fast centre-of-mass diffusion of macroradicals through the environment of mostly monomer and solvent and subsequent segmental re-orientation, which also occurs against the friction of... 

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