Separating the Fractions of Cottonseed

E. F. Pollard, H. L. E. Vix, J. J. Spadaro.

A cotton seed that has been ginned is egg-shaped. It measures about one-half inch by one-fourth inch. It has a fuzzy appearance because of the short linters that remain even after ginning. The linters grow out of the hard, dark-brown seed shell, or hull, which contains the meat part of the seed. The meats consist of oil, meal, and glands.

For the first 80 years of the crushing industry in this country, crude cottonseed oil was obtained by hydraulic and screw pressing of cooked cottonseed flakes. The heat, moisture, and pressure required for this mechanical extraction rupture the pigment glands, so that the meal is highly discolored and suitable only for use in feeding certain kinds of livestock.

Since 1945 several American mills have used solvent extraction to obtain cottonseed oil. The meal fraction obtained by the present commercial solvent-extraction process is dark because of the cooking before or after extraction which is necessary to produce a meal suitable for animals.

Two methods have been developed for the fractionation, or separation, of cottonseed-flake components. The first, the flotation process, takes advantage of the difference in densities of the pigment glands, meal, and hulls the solid parts of cottonseed. Either defatted or undefatted flakes (that is, the flakes before or after the extraction of the oil) can be used in the process.

In the Southern Regional Research Laboratory, cottonseed flakes were disintegrated violently in a slurry containing inert solvents, such as commercial hexane and tetrachloroethylene, mixed in a proportion to give a resulting specific gravity of 1.378. Separation of the detached pigment glands is effected because the specific gravity of the glands (1.36), lower than that of the mixed solvents, causes the glands to float to the top. The meal and hulls have a higher specific gravity (1.41 and 1.46, respectively) than the mixed solvents and go to the bottom. The oil is left in the solvent solution.

The hulls can be further separated from the essentially pigment-free meal by raising the specific gravity of the slurry to 1.45 (to induce flotation of the meal and the settling of the hulls).

After preliminary investigation in the laboratory, chemists and engineers developed this method on a small scale in runs in which up to 225 pounds of flakes were handled in each run. More than 550 pounds of meal practically free of pigment glands and more than 40 pounds of pigment glands were obtained for utilization studies. Meal was produced with as little as 0.5 percent oil; the content of gossypol, the pigment present in the largest quantity within the gland, was 0.06 percent.

We obtained extensive quantitative data on cottonseed meal-solvent slurries during the development of each of the chemical engineering unit operations, namely, material preparation, size reduction of flakes (by pulverization and disintegration), separation (flotation and centrifugation), filtration, desolventization, and distillation.

Small-scale experiments showed that the process had inherent disadvantages for commercial adaptation. Size reduction of the flake was the immediate hindrance in the operation.

Microscopic tests showed that in order to separate the pigment glands completely from the meal, the flake particles had to be disintegrated in a solvent slurry fine enough to pass an 80-mesh screen. Hence, the percentage of meal through a screen of that size was a standard of efficiency in disintegration.

A high-speed, dissolver-type impeller (3 1/4 -inch diameter) gave the best disintegration. More than 90 percent of 80-mesh material was obtained under the best conditions. Flakes that had a moisture content of more than 5 percent reduced the efficiency of disintegration noticeably and increased both the power consumption and the slurry viscosity. Peripheral speeds of up to 6,000 feet a minute gave the best disintegration. The use of whole flakes gave a better disintegration than did the use of flakes prepulverized in a dry state. The presence of hulls slightly in-, creased power consumption and viscosity, but also increased disintegration efficiency. The effect of using different solvents was negligible.

Screening tests of disintegrated slurries revealed that when 90 percent through-80-mesh meal was produced, 70 to 75 percent of the meal was disintegrated sufficiently to pass a 300-mesh screen. More than 90 percent of the pigment glands were of a size between the openings of an 80- and a 300-mesh screen. This high percentage of fine meal caused interference and entrapment in the separation operation, particularly because of the fluffiness of the fraction. Moreover, the apparent density of the fraction tended to overlap the density of the heavier pigment glands.

Results of centrifugal tests for separation showed no improvement in the process because of the small differences in specific gravity of the components to be separated.

Those factors added to the difficulty of quantitatively controlling the procedure.

Another factor that discouraged commercial adaptation of the flotation process was that the heavier solvents (perchloroethylene, trichloroethylene, and carbon tetrachloride) required to obtain the proper specific gravity cost five times more than hexane. The heavier solvents also are toxic.

There were four other disadvantages. The miscella (when using undefatted flakes) was a three-component system, which complicated and increased the cost of the evaporation, stripping, and fractionation operations. The higher temperature required in the stripping operation increased the possibility of darkening the oil, which is sold primarily on the basis of its color. The higher temperature required for desolventizing the meal had a denaturing effect on the protein in the meal--perchloroethylene boils at 121 C., as compared to 66 C. for hexane. Use of the mixed solvents virtually prevents any possible adaptation of the flotation process directly to industrial solvent-extraction processes.

Collectively, the disadvantages led to the discovery and development of the differential-settling fractionation process, which overcame the difficulties. Primarily on the basis of the data obtained on the reduction of flake size and on the settling of the solid components, the successful differential-settling process was conceived.

THE DIFFERENTIAL-SETTLING PROCESS depends principally on the force of frictional resistance between the solvent and the solid components in the slurry. The method was suggested by the slow settling characteristic of the fluffy, fine meal particles noted during the flotation experiments.

Several characteristics of the solid components bring about the play of frictional resistance. The hulls are dense, solid particles with relatively smooth surfaces and have a specific gravity of 1.45. Compared to the pigment glands, which are compact particles with a granular-appearing surface and have a specific gravity of 1.36, the fine meal particles (2 to 40 microns) are a fluffy, feathery mass of no definite shape, with relatively large surface area per unit weight, and have a specific gravity of 1.42.

A preliminary experiment in which flakes were disintegrated in commercial hexane (specific gravity, 0.68) showed that the hulls and coarse meal settled rapidly. The whole pigment glands settled a little more slowly than the hulls and coarse meal. The fine meal particles, although they have a higher specific gravity than the glands, settled more slowly than any of the other fractions.

The time required for complete settling of hulls and practically complete settling of pigment glands can be established for various conditions.

Laboratory experiments further showed that in order to detach 90 to 95 percent of the meal tissue in a slurry containing a ratio of 1 gram of flakes (solid basis) to 1.5 to 1.8 milliliters of solvent (commercial hexane), a disintegration which reduces 70 percent of the meal tissue to 2 to 40 microns is necessary.

More than 90 percent of the fine meal particles will be in suspension at the end of the settling time, but the amount of pigment-gland fragments remaining in suspension is negligible. Meal particles of more than 40 microns settle at rates intermediate to those of pigment glands and hulls. This coarser meal fraction can be redisintegrated and resettled to increase the yield of fine meal.

With undefatted flakes, the oil content of the slurry may go up to at least 30 percent by weight without noticeably affecting the yield of fine meal by the increase of viscosity.

An excellent solvent for fractionation is one in the low specific-gravity range of 0.67 to 0.98 typically, commercial hexane.

The first results obtained with differential settling were so promising that a full-scale pilot plant was designed, constructed, and installed in the Southern Laboratory, where it occupies 550 square feet of floor area. The equipment for the principal unit operations was specially designed and constructed or purchased ready-made, after consultations and trials with manufacturers. The principal pieces of equipment are a 10-horsepower, highspeed, dissolver-type disintegrator; a 300-gallon specially designed tank; an 18- by 28-inch continuous horizontal centrifuge; two pressure-type rotating leaf filters for filtering various meal fractions, including effluent from centrifugal operation; and one 3-stage evaporator for concentration of oil miscella and recovery of solvent.

The pilot-plant procedure has eight consecutive operations:

1. Preparation of the flakes.

2. Disintegration of flakes in hexane with concentrations of up to 50 percent solids by weight.

3. Dilution of disintegrated slurry to 12 to 15 percent solids.

4. Differential settling to separate the fine meal from the pigment glands and coarse meal tank differential settling in which the slurry is settled for a predetermined time of at least 10 minutes prior to decanting, or centrifugal differential settling, employing relative centrifugal forces of about 60 times gravity, in which fine meal (8 to 10 percent solids) is discharged with effluent at one end of the centrifuge and the coarse meal and pigment glands (60 to 80 percent solids) are discharged at the other end.

5. Recovery of fine meal from slurry from either tank or centrifugal settling operations by centrifugation at about 1,500 times gravity, which gives a fine-meal-cake discharge of 60 to 80 percent solids and a solvent- effluent discharge at about 0.6 percent solids, or by filtration of the fine-meal slurry in pressure filters to obtain a fine-meal cake of about 70 percent solids and a clear filtrate of solvent and oil (miscella).

6. Clarification by filtration of the effluent obtained when fine meal is recovered by centrifugation.

7. Removal of solvent from solvent-damp fine- and coarse-meal fractions.

8. Evaporation for recovery of oil and solvent.

This procedure was followed in a series of 15, runs, with the following results : Feed-meal preparation studies showed that either defatted or undefatted flakes could be used; low moisture content was necessary for efficient distintegration; high temperatures had a denaturing effect on the protein of the meal. A procedure was developed and maintained for reducing the moisture content to 3.5 percent and for keeping the meal temperature below 145 F.