Process and Apparatus for Liquid-Liquid Counterflow Extraction

This invention relates to a process and apparatus for liquid-liquid counterflow extraction based on the mixer-separator principle, in which a dispersion of two liquid phases is fed to a separator and the clear phases are withdrawn from said separator.

It is known to use liquid-liquid counterflow extractors for a separation of two phases which are immiscible or difficultly miscible with each other. Such counterflow extractor consists usually of several stages, which are superimposed in a tower. For economical reasons, a small volumetric proportion of solvent, a short phase-separating time, and a large exchange surface area are desired (Verfahrenstechnik, 6th Year, No. 4, 1972, pages 123-128).

In these known extractors the dispersion to be separated is fed to a point at the periphery of the relatively low, circular separator and the separate phases are withdrawn on the opposite side. Where this procedure is adopted, the throughput rate per unit of bottom area, measured in m.sup.3 /m.sup.2 h, decreases as the diameter of the apparatus increases.

This invention avois these and other disadvantages of the state of the art and provides a process and apparatus which permit a high throughput rate per unit of bottom area, which can be attained even in apparatus which is large in diameter.

This is accomplished according to the invention by feeding the dispersion to be separated to the separator in two partial streams which are equal in rates of flow, the partial streams flow through the separator in mutually opposite directions, and each partial stream flows along a path which has a length equal to one-half of the diameter of the separator.

According to a preferred feature of the invention, the separator chambers are filled with sharp-edged turnings of high-grade stainless steel.

Apparatus of the invention for carrying out the process has separator chambers designed so that the inlets for the dispersion are disposed at two opposite points of the periphery and the outlets for the separate clear phases lie on the intervening diametral chord, or the inlet for the disperion is disposed along the diametral chord and the outlets for the clear phases are disposed at two opposite points of the periphery and laterally of said diametral chord.

The advantages afforded by the process and apparatus according to the invention reside particularly in that a high throughput rate per unit of bottom area can now be attained even with separators which are large in diameter. For instance, in accordance with the invention, the throughput rate of a dispersion to be separated in a separator which is 6 meters in diameter can be increased from the previous value of 660 m.sup.3 /h to 924 m.sup. 3 /h, i.e., by 40%.

The invention will be explained more fully in the following example and with reference to the accompanying drawings, in which

FIG. 1 is a vertical transverse sectional view showing a separator chamber according to the invention provided with inlets disposed at two opposite points of the periphery,

FIG. 2 is a horizontal transverse sectional view showing the arrangement of FIG. 1,

FIG. 3 is a vertical transverse sectional view showing a known separator chamber of the prior art,

FIG. 4 is a horizontal transverse sectional view showing the arrangement of FIG. 3,

FIG. 5 is a vertical transverse sectional view showing a separator chamber according to the invention with an inlet disposed on the diametral chord,

FIG. 6 is a horizontal transverse sectional view showing the arrangement of FIG. 5,

FIG. 7 shows apparatus for measuring the throughput rate of a separator, and

FIG. 8 is a graph showing the throughput rate of the separator chamber per unit of area plotted against the horizontal velocity of the dispersion.

In the drawing 1, 1' designate inlets for the dispersion, 2, 2' outlets for the heavy phase, and 3, 3' outlets for the light phase. 4 designates the diametral chord of the separator chamber. The line 4 can also be described as diameter line mid way between and at right angles to the inlets 1, 1' (or the outlets as the case may be). 5, 5' designate the periphery of the separator chamber, 6,6' are distributor chambers, 7,7' are separator chambers, 8,8' are internal weirs, and 9, 9' are collecting chambers. In FIG. 7, 1o designates a pump for withdrawing the heavy phases, 11 a pump for withdrawing the light phase, 12 a flow meter for the heavy phase, 13 a flow meter for the light phase, 14 an electric heater, 15 a stirrer (200 r.p.m.), and 16 a thermometer.

A dispersion consisting of 27 % N-methylpyrrolidone, 14% ethylene glycol, 36% toluene, and 23% n-heptane by weight is fed through inlets 1, 1' into the separator chamber of a counterflow extractor shown in FIGS. 1 and 2. The separator chamber is 6 m in diameter and has a height of 0.8 m. The dispersion flows through the distributor chambers 6, 6' into the separator chambers 7 and 7', which are filled with turnings of high-grade stainless steel. The dispersion flows through these separator chambers to the weirs 8, 8' in two partial streams, which are at the same rate and have opposite directions. The heavy and light phases, respectively, flow under and over the weirs 8, 8' into the collecting chamber 9, which is disposed on the diametral chord 4, and are withdrawn from chamber 9 has clear phases through outlets 2 and 3.

In this experiment, a dispersion throughput rate of 924 m.sup.3 /h was obtained, which corresponds to a throughput rate per unit of area of 42 m.sup.3 /m.sup.2 h.

Because the dispersions are divided in accordance with the invention into two partial streams flowing at the same rate, the horizontal velocity of the dispersion in the separator chambers 7 and 7' is reduced by approximately one-half. This reduction of the horizontal velocity results in a decisive increase of the throughput rate of a separator chamber.

In the comparison tests to be described hereinafter, the dependence of the throughput rate per area of bottom surface on the horizontal velocity of the dispersion in the separator chambers was investigated.

In the set-up shown in FIG. 7, five different known separators were tested. The dispersion described in connection with the example was used also in the tests.

The separators had a rectangular bottom surface of 1 m.sup.2 and of different configurations and had a height of 0.8 m. The separator chambers 7 were filled with high-grade steel turnings. A dispersion consisting of light and heavy phases in equal parts was fed to the separator chamber. The clear phases were withdrawn through 2 and 3. In each clear phase, the content of the respective other phase was determined. The feed rate of the dispersion was increased until in one phase the content of the other phase exceeded 1%. The resulting feed rate of the dispersion is referred to as throughput rate. Because the separator had a bottom surface area of 1 m.sup.2, said throughput rate was numerically equal to the throughput rate per unit of area.

The tests had the following result:

The results are plotted in the chart of FIG. 8.

In the chart of FIG. 8, the horizontal velocity of the dispersion in mm/sec is plotted along the axis of abscissae.

The dispersion throughput rate per unit of area, in m.sup.3 /m.sup.2 h, is plotted along the axes of ordinates. The curve, which is based on the measured values compiled in the preceding Table, shows clearly how the attainable throughput rate per unit of the area decreases strongly as the horizontal velocity of the dispersion in the separator chamber increases.

A known separator as shown in FIGS. 3 and 4, which was 6 m in diameter and had a height of 0.8 m, and in which the dispersion inlet was disposed at one point of the periphery and the dispersion outlets 2 and 3 were disposed on the opposite side of the periphery, was used to separate a dispersion such as had been described in Example 1. A throughput rate of 660 m.sup.3 /h was attained, which corresponds to a throughput rate of 30 m.sup.3 /m.sup.2 h per unit of area.

From the result of this test it is clearly apparent that the throughput rate is much lower than in the process according to the invention.