US2686796A - Process for alkali-refining crude natural oils - Google Patents

Description

Patented Aug. 17, v 1954 QUJUNITED STATES PATENT OFFICE PROCESS FOR ALKALI-REFINING CRUDE NATURAL OILS Klare S. Markley and Reuben 0. Feuge, New rleans, La., assignors tothe United States of j America as represented by the Secretary of Agriculture No Drawing. Application March 23, 1951,

1 Serial No. 217,282

5 clai s. (01. 260-425) (Granted under 1 The invention herein described may be manufactured and used byor forthe Government of theUnited States of America for governmental purposes throughout the world without the payment to us of any royalty thereon.

, This invention relates toprocesses for removing color bodies from crude natural oils by refining them with an aqueous alkali. I

More particularly, this invention provides an improvement in the step of bringing the aqueous alkali into contact with the color bodies. This improved contact results in a more complete con version of the color bodies into removable sub.-

stances and in a reduction of the undesirable side and physical phenomena. When contacted with the alkali, thefatty acids present form oil-insolublc soaps and pass into the aqueous phase where most of them remain suspended, rather than dissolved, due to the salting out effect of the .high ion concentration of the aqueous phase.

The aqueous phase tends to become an enormous number of minute particles of a soft gelatinous nature. At the same timethe phosphatides, proteins "or protein fragments, and gums are hydrated and forced out of solution or at least into coarse suspension in the oil. At the same time a portion of the oil issaponified by the alkali into soaps and glycerol.

Since the particles of the aqueous phase coalesce, and since during their growth or coalescence the color bodies along with other impurities in the oil phase are isolated from the oil phase and adsorbed into the water phase, refiningprocedures have heretofore been directed toward quickly mixing the aqueous alkali with the oil to form an emulsion, and, from that point on, avoiding' undue agitation while increasing the rate of growth of the particles oftheaqueous phase. For. example in a typical commercial refining operation in whichcrude cottonseed oil is refined with aqueous sodium hydroxideinlarge open kettles, the kettles have a capacity of 60,000 pounds of oil each, are cylindrical in shape, and

Title 35, U. S. Code (1952),

havea conicalbottoma Each kettle is provided l with a steam coil for heating the charge and all trapped air bubbles have escaped from the oil,

.2 the agitator is set for rapid mixing, which means that the agitator is made to revolveat a rate of 30 to 35 revolutions per minute. The correct amount of lye is added, and a milky emulsion forms immediately. As mixing continues small particles of soapstock appear and commence to grow in size. After a mixing period which may range anywhere between 10 and 45 minutes, the agitator is slowed to 8 to 10 revolutions per minute, steam is passed into the heating coil, and the temperatureiis raised to the point where relatively large flakes of soft soap form (about 140 F.). At this point heating and agitation is discontinued, and the mixture is allowed to separate into an upper layer of clear, refined oil tand a lower layer of semi-solid soapstock or While numerous improvements in the mixing of aqueous alkali with the oil have been proposed,

and while better results in re-refining processes (where the oil treated contains practically no free fatty acids and emulsifies more slowly) are known to be obtained when high rates of agitation are used; the improvements in crude oil refining have allbeen directed toward rapidly mixing the aqueous alkali with the oil so that the naturally resulting milky-appearing emulsion is quickly formed and avoiding, as far as is practically possible, mixing further, so as to allow the rapid breaking of the emulsion due to the growth of the particles of the aqueous phase. Since crude natural oil-aqueous alkali mixtures rapidly form a relatively stable emulsion of a milk-like consistency, and since the removal of nonglyceride components (fatty acids, etc.) with the least possible loss of glyceride components (oil) is a principal object of the refining; it was heretofore believed that since the soap and other surface active agents are good emulsifying agents, as soon as the milky emulsion was formed, any further vigorous agitation defeated an objective of the refining process by mechanically forcing more and more oil to remain as fine droplets in the aqueous phase. Such additional agitation is avoided evenin continuous processes where for example, proportioned oil and alkali streams at from about '75 to F. are combined in a mixing chamber where the milky emulsion forms, quickly raised to about F. until the emulsion breaks and quickly separated by high speed centrifuges, with only about 3 minutes being allowed to elapse between the time the oil leaves the proportionometer until it emerges as a refined oil. It being strongly believed that longer periods of mixing merely increased refining losses due to the reten- 3 tion of more oil droplets in the aqueous phase and to an increased period in which 'saponification was occurring, the improvements proposed for such continuous process either increase the rate of mixing prior to the formation of the emulsion, or shorten the time that the phases are aqueous alkali; if this agitation is continued for from 0.5 to 40 minutes, the refining process can be conducted, employing 'the usual apparatus and conditions for all but the mixing step, without an increase in the refining loss, but with a color reduction far above any heretofore obtained; particularly in the refining of dark crude oils containing a high percentage of color bodies which are reactive only with strong alkali.

As employed throughout the specification and. claims, the term shearing-together refers mixing at a rate equal to or greater than the minimum rate at which we have found that a crude oil-lye mixture must be mixed in the practice of the present invention.

Recognizing that rate of agitation is very difficult to define we have devised a simple test whereby one skilled in the art can tell whether a given increase in rate of agitation of an oil-aqueous alkali mixture amounts to a mere change in degree or crosses the threshold into the realm of shearing-together the oil and aqueous alkali wherein entirely unobvious and advantageously different effects are produced. We have found that the approximately minimum rate of agitationwhich amounts to shearing-together is one which emulsifiies the oil-aqueous alkali mixture in a fraction of a second and which, continued at the same rate under substantially isothermal conditions after the heat due to the reaction of the aqueous alkali with the oilhas been dissipated, increases the temperature of the mixture at the rate of at least 05 -F. per minute. In general, rates so increasing the temperature of the mixture of from 0.5 to 3 F. have been found to be sufficient in the practice of the present inven- .tion.

The shearing-together of the oil-aqueous alkali mixture can be accomplished in any mixing apparatus capable of providing the above rate of agitation. The agitation need not be continuous in the sense of a paddle continuously turning in a liquid but can be intermittent (for example as in the forcing and reforcing of the mixture through very small or sharply curving passages) so long as the overall agitation causes the temperature to rise at the above rate. The stirring of an oil-aqueous alkali mixture in a vessel having sufficientbafiles to insure a thorough mixing,

.oil-aqueous alkali mixture to the point of shearing-together thecoinponents makes a marked improvement in the amount of color removal'in arable phases.

. 4 1n the refining process, further increases in the rate of agitation have been found to produce but little effect upon the color removal.

The mere application of a substantially instan taneous shearing-together agitating action upon the oil-aqueous alkali mixture followed by an immediate separation of the aqueous and oil phases does not accomplish the advantages of the present process. Particularly in the early stages, the length of time for which the oil-aqueous alkali mixture is sheared-together effects the color of the refined oil. We have found that in refining crude cottonseed oil in accordance with the process of our invention, at least one-half minute is required. However, the improvement in color with increased shearing-together time becomes .decreasingly small as the time becomes longer,

and in general, the improvements obtained by shearing-together for more than about 40 minutes are relatively small. We have found the preferred shearing-together times to be from about 1 to 15 minutes with the exact time to be used for a particular oil varying with the character of the oil and the economics of the application of longer times.

i The temperature at Which the oil-aqueous alkali mixture is sheared-together should be as low as is economically feasible (the power requirements increase rapidly with the increase in viscosity). We have found that the color of the refined oil from crude cottonseed oil increases rapidly as the temperature at which the shearing-together operation is increased beyond 160 F. In general this operation is preferably conducted at a temperature of from about 60 to F. with the exact temperature best suited for a particular oil varying with the characteristics of the oil and the economics of using the lowest temperature.

The process of this invention has been found to vastly improve the alkali refining of glyceridic mixtures, such as those of the natural oils, containing color bodies which react with strong alkali to form oil-insoluble substances. Crude cottonseed oil is a particularly preferred natural oilto be refined by the process of this invention. In this connection, the step of shearingtogether the oil and aqueous alkali in accordance with this invention provides the entirely unobvious and highly advantageous result of forming an aqueous phase-oil phase mixture from which the fatty acids in the form of soaps can be readily separated from the oil Without necessary additional treatments or careful control to insure the breaking of the emulsion into two sep- I This result is particularly surprising since while mixing a comparable aqueous alkali with a'refined cottonseed oil at a comparable rate under comparable conditions provides an improvement in the amount of color so removed as compared to that removed by a rerefining process employing a, conventional mixing step; in order to remove as much soapstock as would be removed in the conventional process, it is necessary to add. water and slowly and carefully stir the mixture produced by the rapid m ix- -of the refined oil and aqueous alkali. 1n contrast, when the cottonseed oil'in the crude'state'is sheared 'together with the aqueous alkali in accordance with the process 'of'this invention, the color removal is not only much greater than that obtainable by a conventional refiningof the same oil, but the soapstcck removal is as easy'andas complete, and the application of special steps or stantially less color than those containing from about m 36%. Aqueous solutions containing between about 14 and 26% sodiumhydroxide are particularly preferred for employment the r se prbcesa w :m 131.; H: t ,As inthe conventional alkali refining processes, the amount of aqueous? alkali usedshouldbe sufne nrw; provide the stoichiometric equivalent of alkali based upon theiatty acid content of the oil andaboutaol to 1.0% excess. As-in theconventlonal processes,increasing the amount-of ex cess alkali? increases the amount "of color removal butv also increase the amount of refining loss, and the amount of excess to be used depends upon the balance; desired between most completely attainablecolorremovaland the refining loss. The process or the present invention can be employed in acontinuous or batch-wise manner.

Inflgeneral, withthe exception of the conditions specified "above forthe mixing step, it can be suitably accomplished under conventional condi ens lemployingthevarious types of apparatus new'oomrnonly used. 'I 'he following example aresubmitted to illustrate certain phases of the present invention in detail." Howeverfsincenumerous other materials and processing conditions can suitably be used, the scope of theinvention is definedby the claims and is not to be construed as being limited to the particular substances, steps, or conditions recited in the examples. 1

The efiect of shearing-together the ml and aqueous alkali Four samples of crude cottonseed oils obtained by conventional extraction processes from cottonseed produced in areas scattered throughout the cotton belt were refined by the oflicial method of the American Oil Chemists Society, Method Ca9a41, modified only in that 20 B. lye was used in an amount providing a 0.5% excess of sodium hydroxide.

The following refined 011 colors and refining losses were obtained in the above refining processes.

Refining Refined oil color Sample loss Lovibond umts permit 35 yellow, 5.5 red 5. 7 35 yellow, 5.6 red 4.4 35 yellow, 32 red-.-.. 12. 6 35 yellow, M5 red"..- 6. 2

"meta oil are; ggg

Lovibond units percent 35 yellow, 5.4 red 5. 7 35 yellow, 5.2 red 4. 8 35 yellow, 9.5 red 14.7 35 yellow, 14.8 red.- r 5;!

It is apparent that even without adapting the operating conditions to the particular oil being refined, a markedly greater (upto' nearly 4 times greater) reduction in color is-obtained with substantially the same refining loss by merely,- shearing-together, rather than simply mixing together, the oilandaqueousalkali H The died of temperature -1 Sample 4 was refined by shearing together (16.8 inches of cutting edges per pound of oil and the cutting edges traveling at 1125 feet per minute) for 10 minutes the crude oil andanamount of 20 Be. sodium hydroxide providing a 0.5% excess of sodium hydroxide over that required to react with the free fatty acids; The following relationshipbetween the temperature at which the shearing-together was performed and the Lovibond color of; the. refinedoil was observed.

Color of refined oil,

35 yellow, 10.7 red. 35 yellow, 12.8 red. 35 yellow, l5.1red.:, 35 yellow, 17.7 red,

e EXAMPLEII IHE I Theefiect of alkali concentration Sample 4 was refined by shearing-together (16.8 inches of cutting edges per pound of oil and the cutting edges traveling at 1125 feet per minute) for ten minutes the crude oil with various strengths of sodium hydroxide added in an amount providing a 0.5% excess of sodium hydroxide over that required to react with the free fatty acids. The following relationship between the strength of the sodium hydroxide solution used in the refining and the Lovibond color of the refined oil was observed.

Color of refined oil,

Strength of sodium hydroxide, Be. Lovibond units 320 redl. 320 red. 35 yellow, 15.1 red. 35 yellow" 11.5 red. 35 yellow, 11.3 red.

EXAMPLE IV The efiect of excess alkali tween excess alkali over that needed to react with the free fatty acids and the Lovibond color of the refined oil was observed.

Color of refined oil, Lovlbond units Excess sodium hydroxide, percent 324) red.

35 yellow, 22.0 red. 35 yellow, 15.1 red. 35 yellow, 12.5 red.

Color of refined oil,

. ,Tiuie oi sheaiingetogether, niin. Lqvmond mm 35 yellow, 15.1 red. 35 yellow, 15. 1 red. 35 yellow, 14.7 red. 35. y ow. 13.7 red.

Having thus described our invention, we claim:

1. A process for refining a crude natural oil containing color bodies which react with a strong alkali to form oil-insoluble substances, comprising mixing the said oil with an aqueous alkali containing an amount of alkali equivalent to the amount contained in a sodium hydroxide solution containing about from to 36% of sodium hydroxide, agitating the resulting mixture for about.

from .5 to 40 minutes at a temperature below about 160 F. in a vessel containing battles and fitted with an agitator having 18 inches of cutting edges per pound of mixture and moving at a rate causing the cutting edges to pass through the mixture-at a. rateoi at least 800 feet per minut so that the agitation emulsifies the mixturein a fraction, of a second. and. when continu under substantially isothermal conditions-sitar theheat due to the reaction of the oil and the aqueous alkali has been'dissipated, in reases the. temperature of the mixture. at the rate of at least 0:15". F. p r. minute. and separating the resulting aqueous andoil phases. I 2. The process of claim 1 wherein the crud neturaloil is a crude vegetable oil. 5

3. The process of claim 1 wherein to crude natural iserude cottonseed oil. 1

,4. The process of claim 1 wherein the aqueous alkali is an aqueous solution containing about irom 14. to 26% of: sodium hydroxide.

..5. A process for refining crude cottonseed 911 containing color bodieswhioh. react with odium hydroxide to form oil-insoluble subs ances, com: nrisingmixing the said crude cottonseed oilwith an aqueous solution containing about from 1,4. to 26% of sodium hydroxide, agitating the he sulting mixture for about from 1 to 15 minutes at a temperature of about from 6 o 120? in a vessel containing baffles and fitted with an a ta r a ing 18 inches of cutting edges per pound of mixture and moving at a rate caus the cutting edges to pass through the nurture at a rate of at least 800 .feet per minute so that the agitation emulsifies the mixture in a traction or a second, and, when tontinuedv under substantially isothermal conditions after the heat ,ldue to the reaction of the oil and thelaquegus sodium hydroxide has been dissipated. increases the temperature of the mixtur at the rate of at least 0.5 F per minute. and separating the resulting aqueous and oil phases.

References Cited in th his of this paten

Claims (1)

1. A PROCESS FOR REFINING A CRUDE NATURAL OIL CONTAINING COLOR BODIES WHICH REACTS WITH A STRONG ALKALI TO FROM OIL-INSOLUBLE SUBSTANCES, COMPRISING MIXING THE SAID OIL WITH AN AQUEOUS ALKALI CONTAINING AN AMOUNT OF ALKALI EQUIVALENT TO THE AMOUNT CONTAINED IN A SODIUM HYDROXIDE SOLUTION CONTAINING ABOUT FROM 10 TO 36% OF SODIUM HYDROXIDE, AGITATING THE RESULTING MIXTURE FOR ABOUT FROM .5 TO 40 MINUTES AT A TEMPERATURE BELOW ABOUT 160* F. IN A VESSEL CONTAINING BAFFLES AND FITTED WITH AN AGITATOR HAVING 18 INCHES OF CUTTING EDGES PER POUND OF MIXTURE AND MOVING AT A RATE CAUSING THE CUTTING EDGES TO PASS THROUGH THE MIXTURE AT A RATE OF AT LEAST 800 FEET PER MINUTE SO THAT THE AGITATION EMULSIFIES THE MIXTURE IN A FRACTION OF A SECOND, AND, WHEN CONTINUED UNDER SUBSTANTIALLY ISOTHERMAL CONDITIONS AFTER THE HEAT DUE TO THE REACTION OF THE OIL AND THE AQUEOUS ALKALI HAS BEEN DISSIPATED, INCREASES THE TEMPERATURE OF THE MIXTURE AT THE RATE OF AT LEAST 0.5* F. PER MINUTE, AND SEPARATING THE RESULTING AQUEOUS AND OIL PHASES.