Fractional Distillation under Nonequilibrium Conditions

A number of experts in the distillation field have examined this manuscript. Their general reaction was that, although un- able to compete commercially, this i s an interesting idea that may be adaptable to small-scale use.

Because this article aroused so much interesting comment, I&EC editors present a shortened version for readers' attention and possible development.

MODERN distillation columns function as the result of near-equilibrium between rising vapor and descending liquid. Success with bubble-plate, packed, and other columns, oriented toward equilib- rium operation, has tended to obscure the fact that simple distillation is not an equilibrium action.

With a simple open coil arrangement having proper heat exchange, it has been possible to pack, within a small space, an efficient fractionating tower based on nonequilibrium principles.

The new column is made by winding 20 turns of 1-inch copper tubing around a 6-inch brass cylinder 22 inches long. Copper to copper and copper to brass contacts are soldered to assist heat exchange. A 2-inch jacket of rock wool insulates the outer areas of the helical coil. Axis of the helix is vertical.

Water circulated within the 6-inch central cylinder, and insulation over the outer areas of the coil combine to provide a temperature gradient across each increment of the spiral vapor path. As mixed vapors travel upward, con- densation occurs on the colder areas of the tube, first as a film, then as an ac- cumulated stream of reflux liquid that flows down toward the pot. Uncon- densed fractions continue upward and out into the condenser.

With colder areas of the tube promot- ing condensation from vapors and hotter areas promoting evaporation from de- scending liquid, a greatly extended but closely coupled continuous evaporator- condenser system occurs within a com- pact apparatus. The open spiral vapor and opposed liquid reflux paths com- bined with an across-the-flow tempera-

ture gradient apparently give rise to a number of actions whose individual values are difficult to assess, but which taken together may account for the high efficiency attained.

Continuous vapor removal is effective throughout the whole area of liquid- vapor contact and velocity of the vapors ensures their removal from any point of production. Continuous reboiling occurs through heat exchange from hot vapors in the upper reaches of each cross section of each turn of the coil through the metal fillets into reflux liquid in the turn next above. Con- tinuous countercurrent reflux caused by contact of upward-traveling vapors with downward-flowing liquid results in recti- fying action over a large, extended area of liquid and volume of vapor.

Selective condengation occurs on the inner surfaces of the coil because of differences in vapor pressures, molecular weights, and average molecular veloci- ties. Materials of higher molecular weights, higher boiling points, and lower molecular velocities tend to condense on the inner or colder walls of the coil. Those with lower molecular weights, lower boiling points, and higher molec- ular velocities tend to respond to the higher temperature of the outer wall elements of the coil, and travel upward and out into the condenser.

Selective vaporizing action occurs over each increment of the path of descending liquid, The hot edge of the liquid reflux stream tends to inhibit condensa- tion and promote vaporization, while the cold edge tends to promote condensa- tion and inhibit vaporization. A con- tinuous selective evaporation of low- boiling and condensation of high-boiling compounds follows.

Temperature gradient migration comes about in mixed vapors because lighter molecules migrate to the hot walls and heavier molecules to the cold walls of any closed space. Although vapors move rapidly through the coil, molecular velocities are so much greater than stream velocity, that the effect is nearly that of no stream movement.

Proof of this action cannot be demon- strated; nevertheless, there is powerful, readily demonstrable, fractionating ac- tion requiring some explanation. In binary mixtures, for example, methanol- water is completely separated. In in-

dustrial experience, such separation re- quires 14 theoretical plates which defines the upper height equivalent of the theoretical plate limit. Benzene- carbon tetrachloride gives 58 theoret- ical plates; and benzene-ethylene chlo- ride figures 62 theoretical plates.

These results reduce to an HETP value between 0.82 and 3.9 cm. which, though attained a t relatively rapid rates of dis- tillation, is lower than most laboratory packed columns under total reflux.

The most frequent criticism of this method is that it is confined to small applications of low capacities. How- ever, hot vapors can be driven through open tubes at high rates. Computation shows that a vapor rate of only 70 feet per second in the coil gives a throughput equal to that of a bubble-plate column of equal over-all diameter. Another objection is that such a system must be thermally inefficient. Certainly, small models thus far tested have been wasteful of heat, but so are small bubble-cap and packed columns. What is not so apparent is that heat normally extracted from vapors by overhead or reflux con- densers in conventional equipment is here extracted within the coil.

There are supplementary considera- tions. Pressure drop in the coil is smaller than for either bubble-plate or packed columns and there is no static holdup because the upright helical coil drains freely, The operating holdup, considerably smaller than in a bubble- plate still, subject to direct control, is a function of reflux ratio.

Amount of material distilled always exceeds static holdup and usually ex- ceeds operating holdup. Construction and maintenance are simple; the column is easily altered to operate under either pressure or vacuum and free space and vapor velocity are relatively large. The liquid-vapor ratio is subject to wide variation through reflux control. Ver- tical temperature gradient assists batch operation and makes possible fraction cutting in a column fitted with side- draw traps.

Little can be said of the relative im- portance of these principles because of difficulties in separating one from the other. However, the most important concept of all is the greatly extended, closely coupled, evaporator-condenser system through which pass each incre- ment of vapor on the way up, and each increment of reflux on the way down, as through a series of successive incre- mental stills. Therefore, summation of rectifying action throughout the column length, increment by increment, each of which contributes to the separatory effect, gives a measure of fractionating power.

F. V. ATKESON 532 North James St. Springdale, Pa,

VOL. 49:'NO. 2 0 FEBRUARY 1957 239