Thermal diffusion separation

Thermal diffusion separation can be used to characterize the differences in composition and quality obtained by RHC. Fig. 8.3 is a schematic description of the thermal diffusion separation apparatus. In thermal diffusion separation, two concentric tubular walls are separated by a small gap and each wall is maintained at a different temperature. The difference in temperature creates thermal convection current, which allows the separation of the liquid consistent by their density. At equilibrium, the low density molecules are concentrated at the top of the separation device and the high density molecules accumulate at the bottom. Using a collection of 10 ports distributed along the length of the external tube, samples are collected and analyzed for VI and composition. As expected, low density molecules are essentially paraffinic whereas high density molecules are more aromatic in nature. 

The thermal diffusion factor a is proportional to the mass difference, (mi — mo)/(mi + m2). The thermal diffusion process depends on the transport of momentum in collisions between unlike molecules. The momentum transport vanishes for Maxwellian molecules, particles which repel one another with a force which falls off as the inverse fifth power of the distance between them. If the repulsive force between the molecules falls off more rapidly than the fifth power of the distance, then the light molecule will concentrate in the high temperature region of the space, while the heavy molecule concentrates in the cold temperature region. When the force law falls off less rapidly than the fifth power of the distance, then the thermal diffusion separation occurs in the opposite sense. The theory of the thermal diffusion factor a is as yet incomplete even for classical molecules. A summary of the theory has been given by Jones and Furry 15) and by Hirschfelder, Curtiss, and Bird 14), Since the thermal diffusion factor a for isotope mixtures is small, of the order of 10", it remained for Clusius and Dickel (8) to develop an elegant countercurrent system which could multiply the elementary effect. 

The Clusius-Dickel column is shown schematically in Figure 2. A wire is mounted at the axis of a cylinder. The wire is heated electrically and the outer wall is cooled. This sets up a radial thermal gradient which leads to a thermal diffusion separation in the x direction. As a result of the radial temperature gradient, a convection current is established in the gas, which causes the gas adjacent to the hot wire to move up the tube with respect to the gas near the cold wall. The countercurrent flow leads to a multiplication of the elementary separation factor. For gas consisting of elastic spheres, the light molecules will then concentrate at the top of the column, while the heavy molecules concentrate at the bottom. The transport theory of the column has been developed in detail (3, iS, 18) and will not be presented here. In a later section we shall discuss the general aspects of the multiplication of elementary separation processes by countercurrent flow. 

Here v and d represent dimensionless convection velocity and dimensionless thermal dif-fusivity, resp. (axial mass dispersion is neglected) B is dimensionless reaction enthalpy. Da is the Damkohler number and Le is the Lewis number. We have avoided the conventional introduction of the Peclet number since we would like to examine the effects of convection and thermal diffusion separately. Otherwise, the scaling of the variables and definitions of the dimensionless quantities are conventional, see e.g. (Nekhamkina et al. (2000)). 

Example 1.5.3 Consider the thermal diffusion separation of a gas mixture of H2 and N2 initially present with a hydrogen mole fraction Xif in a two-bulb cell. The volumes of the two bulbs are Vi and V2. Let the uniform temperature of the two-bulb cell be changed so that the bulb of volume V] now has a temperature Ti while the other one is at T2 (Ti > T2) (Figure 1.1.3, example 111). Assuming that this is a case of close separation such that Xn Xj/ we obtain... 

For the thermal diffusion separation of solute 2 present in solvent 1, located between two flat plates as shown in Figure l.P.l, it is known that the solute will concentrate near the cold plate (region 2) while the solution near the hot plate at the top gets depleted in solute. With y coordinate values of 0 at the top plate and i at the bottom plate, it is known that the mole fraction distribution of the solute X2(y) is given by the equation... 

Many challenging industrial and military applications utilize polychlorotriduoroethylene [9002-83-9] (PCTFE) where, ia addition to thermal and chemical resistance, other unique properties are requited ia a thermoplastic polymer. Such has been the destiny of the polymer siace PCTFE was initially synthesized and disclosed ia 1937 (1). The synthesis and characterization of this high molecular weight thermoplastic were researched and utilized duting the Manhattan Project (2). The unique comhination of chemical iaertness, radiation resistance, low vapor permeabiUty, electrical iasulation properties, and thermal stabiUty of this polymer filled an urgent need for a thermoplastic material for use ia the gaseous UF diffusion process for the separation of uranium isotopes (see Diffusion separation methods). 

The foremnner of the modern methods of asphalt fractionation was first described in 1916 (50) and the procedure was later modified by use of fuller s earth (attapulgite [1337-76-4]) to remove the resinous components (51). Further modifications and preferences led to the development of a variety of fractionation methods (52—58). Thus, because of the nature and varieties of fractions possible and the large number of precipitants or adsorbents, a great number of methods can be devised to determine the composition of asphalts (5,6,44,45). Fractions have also been separated by thermal diffusion (59), by dialysis (60), by electrolytic methods (61), and by repeated solvent fractionations (62,63). 

A number of special processes have been developed for difficult separations, such as the separation of the stable isotopes of uranium and those of other elements (see Nuclear reactors Uraniumand uranium compounds). Two of these processes, gaseous diffusion and gas centrifugation, are used by several nations on a multibillion doUar scale to separate partially the uranium isotopes and to produce a much more valuable fuel for nuclear power reactors. Because separation in these special processes depends upon the different rates of diffusion of the components, the processes are often referred to collectively as diffusion separation methods. There is also a thermal diffusion process used on a modest scale for the separation of heflum-group gases (qv) and on a laboratory scale for the separation of various other materials. Thermal diffusion is not discussed herein. 

Thus the addition of an inert gas which does not intervene chemically in the uansport reaction but adds to the density of die gas, reduces the segregation due to thermal diffusion. An example of this is the reduction of tlrermal separation in a mixture of H2 and H2O by the addition of Hg vapour (Dastur and Chipman, 1948). 

Grady and Asay [49] estimate the actual local heating that may occur in shocked 6061-T6 Al. In the work of Hayes and Grady [50], slip planes are assumed to be separated by the characteristic distance d. Plastic deformation in the shock front is assumed to dissipate heat (per unit area) at a constant rate S.QdJt, where AQ is the dissipative component of internal energy change and is the shock risetime. The local slip-band temperature behind the shock front, 7), is obtained as a solution to the heat conduction equation with y as the thermal diffusivity... 

There exist a number of other methods for the separation of diamondoids from petroleum fluids or natural gas streams (1) a gradient thermal diffusion process [54] is proposed for separation of diamondoids (2) a number of extraction and absorption methods [53,83] have been recommended for removing diamondoid compounds from natural gas streams and (3) separation of certain diamondoids from petroleum fluids has been achieved using zeolites [56, 84] and a number of other solid adsorbents. 

A steady-state method is disadvantageous in measurements on a mixture because for a long time the temperature gradient is likely to generate separation of the mixture due to thermal diffusion. Accurate measurement itself seems to be still one of the most pressing concerns for thermal diffusion of high-temperature melts. 

Detailed analysis of the Oad positions following dissociation at room temperature shows that most Oad are found one lattice constant (in the [0 0 1] direction) away from the Ob-vac that is filled, the rest being immediately adjacent and two lattice constants away [50]. As there is little thermal diffusion of Oad on Ti02(l 1 0), the separation of Oad from the closest positions to the reacting Ob-vacs was attributed to the energy released during the exothermic dissociation of 02 (calculated at 3.5 eV [18]) in a similar way to that observed, for example, for Cl2 dissociation on Ti02(l 1 0) [51],... 

Ironically, our current plans call for the reverse linkage of the above enrichment procedures. That is, we shall use an electromagnetic isotope separator to enrich argon isotopes for a mass spectrometry experiment, and we shall enrich radiocarbon via thermal diffusion for improved mini-gas proportional counting. 

Clusius A process for separating isotopes by a combination of thermal diffusion and thermal siphoning. Invented in 1938 by K. Clusius and G. Dickel. 

The counterpart to the photo-induced electron transfer is the corresponding thermal transformation of the electron donor-acceptor complex the barrier to such an adiabatic electron transfer is included in Fig. 18 as T, with the implicit understanding that solvation is an intrinsic part of the activation process (Fukuzumi and Kochi, 1983). When the rate of back electron transfer is diminished (e.g. by a reduced driving force), the dynamics for the contact ion pair must also include diffusive separation to solvent-separated ion pairs and to free D+- and A-- (Masnovi and Kochi, 1985a,b Yabe et al., 1991). 

As detailed earlier, Aston had been faced with this problem for the case of neon where he was not convinced by Thomsons s conclusion that there are two neon isotopes of masses 20 and 22. He attempted to show that indeed neon consists of two isotopes by trying to separate the two isotopes using thermal diffusion. The result proved unsatisfactory and he then proceeded to invent the mass spectrograph. 

Fig. 8.9 (a) A four stage thermal diffusion cascade for argon isotope separation (Modified from Spindel, W. ACS Symp. Ser. 11, 82 (1975)). (b) The thermal diffusion cascade operated by K. Clusius and collaborators at the University of Zurich during the 1950s (Photo credit Archives of the Institute of Physical Chemistry, University of Zurich)... 

Table 8.2 Isotopes separated by K. Clusius and coworkers by thermal diffusion (Clusius, K. and Dickel, G.,Naturwissenschaften 26,546 (1938) Clusius, K. and Starke, K., Z Naturforsch. 4A, 549 (1949))...

Many other methods for separating isotopes have been described. A partial list includes membrane and membrane pervaporation, thermal diffusion of liquids, mass diffusion, electrolysis and electro-migration, differential precipitation, solvent extraction, biological microbial enrichment, and more. Although not discussed in... 

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