Thermal diffusion columns

Thermal diffusion column. Thermal diffusion remained a scientific curiosity until 1938, when Clusius and Dickel [C5] developed their thermal diffusion column, which made possible useful separations in simple equipment. In the Qusius-Dickel column the mixture to be separated is... 

Because the complete theoretical description of the operation of a thermal diffusion column is quite intricate, a simplified theory due to Jones and Furry (J12) is summarized here a more extensive discussion along with a survey of column operation from the phenomenological standpoint is given by Grew and Ibbs (Gil). 

When a closed thermal diffusion column (i.e., a column with no throughput) has been running a long time a steady state is ultimately reached, for which there is no net transport through the column. Then the expression in Eq. (222) is zero and HxA B = —K(d /dz), where K = Kc + Kd. This equation can be integrated from the bottom of the column (z = 0) to the top (z = L) to give ... 

Figure 19.14. Construction and performance of thermal diffusion columns, (a) Basic construction of a thermal diffusion cell, (b) Action in a thermogravitationai column, (c) A commercial column with 10 takeoff points at 6 in. intervals the mean dia of the annulus is 16 mm, width 0.3 mm, volume 22.5 mL (Jones and Brown, 1960). (d) Concentration gradients in the separation of cis and trans isomers of 1,2-dimethylcyclohexane (Jones and Brown, 1960). (e) Terminal compositions as a function of charge composition of mixtures of cetane and cumene time 48 hr, 50°C hot wall, 29°C cold wall (Jones and Brown, 1960).

G. Vasaru et al., The Thermal Diffusion Column, VEB Deutscher Verlag der Wissenschaften, Berlin, 1969. 

In 1946, the problem was demonstrating that the most fundamental assumptions did in fact hold. Initially, this meant obtaining measurements of the natural radiocarbon concentrations in living organics to see if it occurred in the amount expected and if the worldwide distribution of radiocarbon was essentially constant. An experiment was devised whereby biological methane gas derived from the sewage disposal plant at Baltimore, MD and petroleum methane from the Sun Oil Co. refinery were each enriched by a similar factor in a thermal diffusion column. It was assumed that the petroleum methane contained no because of its age in excess of many tens of millions of years whereas the biological methane contained about 17-18 dpm radiocarbon per g of carbon. The experiment was conducted, and the results confirmed the calculations (26). 

Unfortunately, the use of a thermal diffusion column to measure samples routinely was obviously impractical since each sample would cost several thousand dollars to process. A method to measure directly... 

The thermal diffusion method requires large quantities of power and is therefore primarily of interest for preparation of laboratory scale samples. As such, it has been developed by Clusius among others, and is a very effective separation process. Overall separations as high as 10,000,000 have been achieved by the Clusius group. A summary of the evolution of the thermal diffusion column in Clusius laboratory is given in Table III (JO). Of particular note is the enrichment of Ar, a middle isotope, from a natural abundance of 0.064% to a final isotopic purity of 99.984%. 

Samples of biological methane ( biomethane ) obtained from a sewage disposal plant in Baltimore, Md., and of petroleum methane ( pe-tromethane ) from an oil refinery were enriched by equal amounts of in a thermal diffusion column. Whereas the activity of the biomethane increased in direct proportion to the amount of enrichment, there was no significant increase in the counting rate of the petro-methane with enrichment (4). The confirmation of Libby s prediction initiated an extensive study of the distribution of in nature. However,... 

The degree of separation obtainable in thermal diffusion (the difference in composition between hot and cold walls) is much less than in other diffusion processes, so that use of a column to multiply the composition difference is practically essential. The stage type of thermal diffusion has been used only to measure the thermal diffusion coefficient and is never used for practical separations. In some thermal diffusion columns, htu s are as low as 1.5 cm, and as many as 800 stages of separation have been obtained from a sin e column. Even with such a great increase in separation, it is often necessary to use a tapered cascade of thermal diffusion columns for isotopic mixtures, to minimize hold-up of partially enriched isotopes and to reduce equilibrium time. 

Isotopes separated. Table 14.24 gives examples of some of the highest reported concentrations of separated isotopes that have been obtained by thermal diffusion. Most of these separations were on a small laboratory scale. The high purity to which scarce isotopes such as C, N, and 0 have been concentrated is a notable feature of these examples of thermal diffusion. The feasibility of concentrating rare isotopes of intermediate mass, such as Ne and A, by thermal diffusion is also noteworthy. These separations are facilitated by the large number of stages obtainable from a single thermal diffusion column. 

Their separation performance was characterized by two parameters. F is In ypjyp, the overall separation between top and bottom when equilibrium is attained at total reflux. 0 is a parameter that was inferred from the rate at which product composition at total reflux approached equilibrium. The theory of the time dependent separation performance of a thermal diffusion column developed by Cohen [C6] and others shows that 0 is given by... 

Table 14.25 Separative capacity and power consumption of UF thermal diffusion column ... 

The maximum separative capacity, A , and the power consumed per unit separative capacity, G/ max, given in the last two columns of Table 14.25 have been calculated from Abelson s parameters Y and 0 to permit comparison with the other processes for enriching uranium treated in this chapter. Because the thermal diffusion column operates with constant reflux ratio, its steady-state separation performance as an enricher is given by Eq. (14.237), expressed here in the form... 

At /yC] around 1.8, 0.80 is the maximum value of the ideality efficiency Ej for this thermal diffusion column considered as a square enriching cascade. 

Theoretical prediction of the constants Cj and Cs of the UF thermal diffusion column would be very difficult because of the great difference in properties of UF between the liquid at the cold wall and the dense gas at the hot wall. For other gases at pressures around atmospheric, at temperature differences between hot and cold walls small enough so that separation performance can be characterized by gas properties at a mean temperature, closed expressions can be given for the separation parameters Ci and C5. Quantities involved are... 

Equations for thermal diffusion column. Equations for the separation performance of a thermal diffusion colunm can be derived in somewhat similar fashion to the countercunent gas centrifuge of Sec. 5.5. The results will be summarized for the simplest case to treat theoretically, that of an aimular column in which the spacing d between the heated and cooled... 

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