This type of operation has been applied to the distillation of materials that have very low vapor pressures at the maximum operating temperature. The available pressure drops in such cases would be too low to obtain practical production rates in conventional equipment, but by operating such that the rate of distillation is approximately equal to the absolute evaporation rate of the liquid reasonable capacities can be obtained. The most common method of obtaining the molecular distillation conditions is to carry out the operation at a high vacuum (0.01 mm. Hg or less) and to place the condensing surface so that it is parallel to the evaporating surface and in close proximity to it. The condenser is operated at a low temperature to limit the reevaporation. In order to obtain satisfactory absolute rates of evaporation, it has been found that as an approximate rule the temperature should not be lower than 100°C. below the temperature at which the vapor pressure of the substance being evaporated is 1 to 5 mm. Hg abs.
Even with molecular distillation, the rates of evaporation obtained are low when the vapor pressure is less than 0.01 mm. Hg abs. Thus for a material having a molecular weight of 400 and a vapor pressure of 0.01 mm. Hg at 100°C., the absolute evaporation rate by Eq. (15-3)
would be only 0.02 X 10~s g. per sec. per sq. cm. This is much lower than the rate estimated on page 394 for normal vapor-liquid interchange. High-molecular-weight polymers would have low evaporation rates regardless of the vacuum.
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 the high-vacuum method of operating, it is usually suggested that the distance between the condenser and the evaporating surface should be of the order of the mean free path of the molecules in the vapor. Jeans (Ref. 4) gives the following equation for the mean free path (M.F.P.) of a molecule:
« 1.75 X 1019(P/T) for a perfect gas d ® diameter of molecule, cm. (As an approximate rule use cube root of 6/r times the liquid volume per molecule) P = absolute pressure, mm. Hg T = absolute temperature, °R.
Thus for a material that has a molecular weight of 500 and a liquid density of 0.9, the mean free path at a pressure of 10-* mm. Hg and 400°F. would be
However, it does not appear to be necessary to make the mean free path as large as the distance between the condenser and the evaporating surface to obtain molecular distillation conditions. Brônsted and Hevesy (Réf. 1) obtained separations of mercury isotopes that corresponded closely to molecular distillation rates under conditions where the condenser was separated from the evaporating mercury surface by a distance approximately 100 times the mean free path. Taylor (Ref. 7) distilling petroleum fractions found the rate of evaporation to be independent of the total pressure over a range corresponding to mean free paths of 0.01 to 10 times the clearance between the condenser and the evaporating surface. He also found that noncondensable residual gas at pressures up to the vapor pressure of the liquid being distilled did not materially lower the distillation rate. Higher residual gas pressure can cause appreciable lowering of the rate.
The most common type of high vacuum molecular distillation still is the vertical-tube falling-film unit, a schematic diagram of which is shown in Fig. 15-1. The liquid to be distilled is first degassed. This is essential if splashing in the distillation unit is to be avoided. This liquid then flows down in a film on the outside of the inner tube which is internally heated. The inner surface of the outer tube is the condenser which can be air- or water-cooled. For a high rate of distillation, the clearance between the two surfaces should be relatively small, but if they are too close, any noncondensable gas released at the bottom of the still will have difficulty flowing out of the unit. A clearance of 0.4 to 1.0 in. appears to be about optimum for a unit 2 to 4 ft. long.
In such a falling-film unit a molecule moving from the evaporating liquid to the condenser encounters a number of resistances: (1) diffu-sional resistance from the interior to the surface of the liquid, (2) evaporational resistance, (3) resistance to transfer in the vapor, (4) resistances in condensation.
The resistance to condensation is small, and the resistance to vapor transfer is made small by the use of low pressure and by keeping the condenser close to the evaporating surface. The evaporating rate of the molecules in the surface is chiefly a function of temperature which should be kept as high as possible without thermal degradation or bubbling of the liquid which throws unvaporized material over to the condenser. In most cases, the limiting rate is diffusion in the liquid phase. Owing to the large size of the molecules and the viscosity of the liquid, the mass-transfer rate is very low. The outer surface of the liquid is depleted of the more rapidly evaporating molecules a short distance from the top of the unit, and the surface then has a higher concentration of the less volatile molecules than the average composition of the liquid. This reduces both the evaporation rate and the degree of separation obtained. A small amount of large-size, essentially nonvolatile, material in the liquid can give a serious blocking of the surface. Owing to this effect, increasing the length of the apparatus does not give a proportional increase in the amount of evaporation. For this reason, the falling-film units are seldom made over 2 to 4 ft. high. Several methods can be used to reduce the effect of this surface blocking: (1) Various mechanical devices have been proposed to cause mix-
Fig. 15-1. Diagram of falling-film unit.
ing of the falling film. (2) The liquid circulation rate can be increased, resulting in a lower percentage evaporation per pass through the unit. A high percentage evaporation can be obtained either by recirculating the liquid or by using several units in series with mixing between each unit. (3) A high liquid flow rate can be used to cause the outer portion of the falling film to be in turbulent rather than laminar flow. This type of operation may also necessitate recirculation or series operation to obtain a high percentage of the liquid distilled. (4) The unit may be modified to give a continually increasing surface. For example, the inner tube can be made conical with the small end at the top. As the liquid flows down, it must increase in area which will present fresh surface for evaporation. Hickman (Ref. 3) has developed a spinning-plate type of film unit in which the liquid is fed at the center and flows across the spinning plate due to centrifugal force. The plate is heated, and the condenser is placed parallel to it. Because of the high centrifugal force, it is possible to obtain thin films which mean large surface area per unit volume of liquid, and the increasing diameter of the plate requires the formation of new surface as the liquid flows outward. The spinning-plate unit is effective for the purpose of increasing evaporation rate, but other methods would appear to be simpler for large-scale units.
Besides keeping the temperature at a low level, molecular distillation holds only a small volume of the liquid at the evaporation temperature and thereby reduces the thermal degradation. The spinningplate type of still is particularly effective in this respect because of the thin film obtained.
The thermal efficiency of a molecular distillation is low. Fawcett (Ref. 2) has given a heat balance on a unit distilling triolein at 240°C. with the condenser at 25°C. The data are summarized in the following table:
Only 17 per cent of the total heat is usefully employed, the other 83 per cent is lost by heat transfer. The loss by radiation could be reduced somewhat by increasing the temperature of the condenser, but this might reduce the effectiveness of the distillation.
Another drawback to molecular distillation is the fact that an effective rectification system has not been developed. Greater separations than are equivalent to a single distillation stage have been obtained by repeated distillations in the manner described on page 102, but they are tedious and difficult to perform.
Schaffner, Bowman, and Coull (Ref. 6) have described a vertical falling-film multiple distillation column that can be employed for fractionation under vacuum. The wall is made up of short sections with
Preheat of liquid.. Radiation. ..
Latent heat of evaporation Conduction..
Per Cent 8
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