US2224925A - Fractional distillation of fatty acid materials - Google Patents

Description

Dec. 17, 1940- R. H. POTTS ETAL FRACTIONAL DISTILLATION OF FATTY ACID MATERIALS Filed Oct. 13, 1959 2 Sheets-Sheet 1 INVENTORS Ea/pfi H P0275 BY Jo/m 5. M kes 66 11 AToR Y Patented D-' 72 FRACTIONAL DISTILLATION F FATTY v 3 MATERIAL non) s Ralph n. Potts, La Grange, and John E. McKee, Western Springs, 11]., assignors to Armour and Company, Chicago, 111., a corporation of Illiois Application October 13, 1939, Serial No. 299,248

7 Claims.

This invention relates to the fractional distillation of fatty acid materials and particularly those materials containing also'a quantity of neutral oil. The invention is concerned principally with an improvement involving the amount -of steam introduced into the fractional distillation zones, a feature set forth in our copending application,- Serial 96,732, of which the present application isa continuation in part.

In the treatment of oleaginous fatty acid materials it is necessary to separate the volatile fatty acids-from the non-volatile glycerides, and to accomplish this in an eflicient manner it has been found that the material must not only be heated to a temperature at which the fatty acids will vaporize at the vacuum employed, but some sort of mechanical stripping agent must be used. Since steam is economical and an excellent stripping agent, the art has employed it universally in the distillation of such materials. By passing superheated steam through the fatty acid material heat is added and also the vaporized fatty acids are carried away from the non-volatile material; thus the steam serves a double purpose, and prior fatty acid distillation processes have utilized both of these functions.

In this description the term straight distillation signifies the separation of volatile fatty acids from glycerides, impurities or other nonvolatile matter, while the term fractional dis-- tillation sign fies the separation of the volatile fatty acids themselves'into separate fractions having different boiling points. Thus when ole- In the practice of the process set forth in our Patent No. 2,054,096, it was realized that the aginous fatty acid materials are fractlonally diswas desired. It is this problem to which this invention is particularly directed.

For sharper fractionation, those skilled in the art turn to adjustment of the trays in the fractionating tower. The addition of more trays serves to give sharper fractionation. It was found however, that this expedient was not the solution to the present problem, since by adding more trays a greater vacuum differential was created between the top of the fractionating column and the feed stock inlet. While with other materials this might have been immaterial, it had the effect of rendering the present process practically inoperative, since at e higher pressure near the inlet a higher mperature was required for vaporization, and this proved very injurious to the heat-sensitive 'unsaturated'fatty acids.

We have discovered that by reducing the total amount of steam in the fractionating' procedure to below one-half of the amount of feedstock, the sharpness of fractionation is greatly improved and the stripping function of the steam is retained to a satisfactory degree. When" the amount of steam used is so reduced, the heat which it carries becomes insufflcient to reboil the reflux liquid, and we have provided other means for supplying the necessary heat, as will be described later. With still more heat supplied otherwise than by steam, we have also found fractionation can be further improved greatly if the steam which is passed through the fractionation .zones is reduced to an amount less than 25% of the amount of the feed stock.

A clear understanding of the advantages and benefits of the invention will be had after the following'detailed explanation, taken in connection with the accompanying drawings in which:

Figure 1 is a schematic representation of an apparatus suitable for carrying out the improved- 40 process.

Figure 2 is a view showing another means of supplying heat to the fractionating column which might be substituted for that shown in'Figure 1,

and

Figure 3 is a view of still another means for supplying heat to the fractionating column which might be used instead of that shown in Figure 1.

The improved process is useful for the treatment of fatty acid stocks obtained from a great variety of oils of animal, marine or vegetable origin; but for cleamess of understanding this detailed explanation will deal specifically with a feed stock known as black acid foots, obtained from cottonseed oil and having a fatty acid content of about 70 percent, the remainder being predominantly triglycerides.

, material, which is a by-product of cottonseed l oil refining by the alkali process, is acidulated with sulfuric acid to break the snap into fatty acid and the sulfate salt of the positive radical combined with the fatty acid. The mass thenseparates into an oil layer and a water layer, these layers being separately recovered. The fatty material thusobtained is called black acid foots," and is the material which is specifically referred to hereinafter. The distillation of this type of raw materialinvolves problems the so- 16 lution of which is mademore difficult because of the tendency of the fatty acids to decompose at the higher temperatures, the, decomposition products rendering the materials viscous and of poor color and odor.

Referring particularly to Figure 1 of the drawings, the black acid foots, for example, are fed into the tank It from which the material. is transferred by a pump it through line It and a preheater iii to the bottom of a tube heater indicated generally at l4. The preheater l3 may be any type of a heat exchange app ratus for conserving heat and raises the temperature of the fatty acid and neutral oil mixture to the .40 a temperature of about 600 F. These vapors pass downwardly through column I 5 and about the tubes l8. The fatty acids rising in the tubes I8 cool the tubes to a temperature below the liquefaction temperature of the vapors in column 45 I5 and the latter condense into a liquid upon the tubes, thereby imparting to the tubes their latent heat of liquefaction. The liquefied diphenyl and diphenyl oxide trickle down into the bottom of the fatty 'acid heater and are withdrawn through 60 line 2| to a suitable heater, not shown, for revaporizing these materials.

This type of feed stock heating means is very satisfactory since the highest temperature to which thefatty acids are subjected is the temperature at which the-diphenyl and diphenyl oxidemixture is liquefied. However, other heating agents may be employed and the heating 'unit for heating the continuously moving input o feed stock may take other forms.

The fresh feed stock from tank. l0 enters the chamber l9 at a temperature of about 300 to 400 F... and is then passed through tubes l8 toward the outlet of the heater. Advantageously, erheated steam at a temperature of about 0 F.. for example, may be introduced into the chamber l9 by the injector 26, in an amount of about 0.05 to 0.25 pound of steam per pound of fe d stock. The int ction of steam at this point serves to ipc efi the speed of travel of t e feed stock through tubes l8, and sweep clean the interior surfaces I the tubes. However, the use of steam at this point is not essential to the process. and the invention can be practiced very satisfactorily without the use of steam in the tube heater.

The tubes u connecting with pipe which n of greater cross sectional area than the combined In the preparation of the feed stock the fools cross sectional areas of the tubes l8, and the pipe 23, connects in turn with an intermediate portion of the fractionating column 24 beneath the baffle plate 21. The column 24 is maintained under a high vacuum and preferably as in the illustrated apparatus this vacuum is allowed'to extend through pipe 23 and into the tubes It. This feature gives an advantage in that by having the vacuum extend .back into the tube heater vaporization may take place within the tubes thus enabling the latent heat of vaporization to be supplied as vaporization takes place, and eliminating the need for raising the materials to a temperature above their boiling points under As the material passes upwardly through the I heater tubes, th'estatic pressure diminishes, the fatty acids are vaporized at least in part and the speed of flow becomes greater, being of the order of 150 to 155 feet per second when-steam is employed in the tube heater. At the outlet of the tubes the partly vaporized material slows its movement upon entering the pipe 23 of larger cross section, and upon entering the fractionating column 24 the movement is again slowed down.

The vaporized portion of the feed stock is composed almost entirely of fatty acids having different boiling points. This volatile mixture of fatty acids is passed upwardly through the bubble caps 33, of the trays 32 positioned above the vapor inlet, while the liquid from the upper trays passes downwardly through the overflow lines 3|. Thus there is maintained a series of pools or zones having progressively lower boiling points toward the top of the column.

The unvaporized portion of the stock entering column 24 and consisting mainly of neutral oil passes downwardly in the column through a series of zones a function of which is to strip this material of the entrained volatile fatty acids. This unvaporized portion passes through pipes 29 of the trays 30 toward the bottom of the column, while steam injected at the bottom of the column through line 23 passes upwardly through bubble caps 3|, the steam serving to strip the volatile fatty acids from the glycerides The amount of been the practice. The amount of steam how- :from the tank 40 by the line 53 and returned in part to the column 25 by means ofthe pump mean in ways such as will be described in connection with the modifications illustrated in F1g-- ures 1, 2, and 3 of the drawings, but not through direct contact of a gaseous medium such as with steam or nitrogen gas.

In the apparatus illustrated in Figure ladditional heat is indirectly supplied to the fractionating column by recirculating at,least a part of the oleaginous bottoms withdrawn atthe bottom of the column through line 25, through the tube heater I4 and back again through pipe 23 into the column. The recycled stock is taken from the bottom of the column 24 at a temperature 'of about 500 F. and during its passage through the tube heater is raised to the order of 550 F., so that through this circulation additional heat is imparted to the zones within the column, and the amount of heat so supplied may be regulated by controlling the amount of material recirculated through the heater H. The part of the bottom materials from the column which is not recirculated through the tube heater is withdrawn in a residue tank 35 which may be emptied through the outlet line 35. If desired this resi- 25 due may be again hydrolyzed and subjected to repeated treatment by the same process.

The volatile fatty acid mixture passing upwardly through trays 32 is met with a downwardly flowing stream of reflux liquid which preferably initially is composed of a condensate of a lighter fraction of fatty acid taken from adjacent the top of the fractionating column. This condensate flows downwardly acrossthe bubble plates, and in travelling upward through the cuts of the fatty acid vapors are condensed by the reflux and the material on each succeeding higher tray is of a lower boiling point. Thefractionating column reaches the state of equilibrium '40 in which the composition of the material on each plate remains substantially constant and the boiling point of the material on each higher tray is lower than the liquid on the next lower tray. The vapors pass upwardly through a series of 45 pools of liquid having the different constituents of the vapors in varying amounts, and having progressively lower boiling points.

From the upper bubble plate of the fractionating column the vapors, consisting principally 50 of steam and'low boiling acids, pass through line 31 into a condenser 38 which reduces the temperature of the vaporized material to -about 150 F., thereby liquefying the fatty acids. As the entire system is maintained under a high vacuum 55 the steam does not liquefy at this temperature, but passes onwardly leaving the condensed fatty acids behind. If desired to recover from the steam the entrained fatty acids which it contains, it may be introduced into apparatus such 0 as the tank through the line 39. The vapor line 39 extends downwardly in the tank 40 and terminates at an intermediate point therein so that the steam is caused to change its direction sharply thereby sweeping the entrained materials 65 downwardly into the liquid pool at the lower portion of tank 40. The steam passing upwardly within tank 40 may be caused to proceed through a bubble plate indicated at and through baflle plates 49 and 50 and through line 5! to vacuum apparatus indicated at 52. The apparatus 52 may include a barometric condenser connected to suitable vacuum pumps not shown.

The temperature of the liquid within tank 40 may be maintained at about 150 F. by the closed 75 steam coil 44. This liquid may be withdrawn fractionating column the higher boiling constitu-' 54 and lines 55 and 55. Also a part of this liquid may be returned through pipe 41 to the bubble plate '45 and a part withdrawn through line 51 an overhead product.-

referably at least 80'percent of the vapors withdrawn from the column should be returned as condensed liquid to serve as a reflux material. For example where about 3000 pounds of vaporous fatty acids are withdrawn and condensed, about 2500 pounds of condensed acids may be returned as reflux. The amount of reflux returnedis a quantity which will vary somewhat according to the type of raw stock being treated.

A second fraction may be taken from the column at any desired point depending upon the character of the product required. For example, liquid may be withdrawn from the selected tray through line 58 and passed into a stripper 59 which contains a series of bubble plates 60. Steam may be introduced into the bottom of the stripper 59 through the line 5|. The stripped portion of the material entering the stripper through line 58 is returned to the fractionating column through the outlet line 52. The higher boiling portion of the material within stripper '59 is withdrawn through line 63 and cooled in the cooler 64. Thereafter the product material may be disposed of as desired.

In the fractionation of fatty acids, the column should be operated within rather narrow vacuum limits, and it has been found satisfactory to maintain the upperportion of the column at a pressure of about 5 mm. of mercury, or as low as can practicably be maintained. The pressure at the vapor inlet will be substantially higher than that maintained at the top of the column, and with a pressure of 5 mm. at the top of the column the pressure at the feed stock inlet may be, for example, about 43 mm. We have found that there is a limit to the pressure which can be permitted at the inlet, and if the pressure at this point is allowed to exceed about 50 mm. of mercury, there will usually be polymerization and poor operation to the detriment of the products obtained.

It is now apparent that when a greater number of trays are introduced into the fractionating column and the differential between the pressure at the top and inlet portions of the column thereby increased, that no possible vacuum at the top of the column would be suiflcient to maintain the vapor portion at a pressure below the minimum required.

However, by the solution given by this invention in the reduction of the amount of steam introduced into the fractionating zones, the number of trays need not be increased, the pressure differential between the top and inlet portions of the column may remain the same yet the sharpness of fractionation is found to be greatly improved. By reducing the amount of steam to a substantial extent and supplying the necessary heating 'for reboiling reflux by other means than the steam introduced, we are able to produce greater separation and less overlapping of the high and lowboiling fractions along the height of the fractionating tower. To accomplish this purpose we have found that the total amount of steam introduced into the zones of fractionation must be less than one-half and preferably less than one fourth of the amount of the feed stock treated. This total amount includes the steam introduced through 1ine28 at the bottom of the column, the amount'introducedthrough line 28 at the tube'heater, and the: steam introduced 1 through line Si in the stripper 59. There may be also other strippers besides 59, and the total amount of steam is intended to include the steam supplied at these or from any other source to the fractionating zones. We have found that the amount of steam introduced at the bottom of the fractionating column may advantageously be less 1 than 20 percent of the feed stock. For example,

assuming that 3000' pounds of feed stock are treated per hour, 500 pounds of steam per hour may be introduced into the bottom of the column,- 300 pounds per hour in the-tube heater,

and 250 pounds per hour into each of two side strippers, making a total of 1300 pounds per hour.

To takea further specific example, we have foundthat under conditions where 3000 pounds of feed stock per hour was treated extremely eflicient operation was obtained where only 300 poundsof steam per hour was introduced into the bottom of the column and 100 pounds of steam per hour in each of two side strippers, and with no steam introduced into the tube heater.

vReferring more particularly to Figure 20f the drawings, there.is' here'shown a second type of means forindirectly supplying to the column the .necessary heat forreboiling reflux liquid. As

here shown the glycerides at the bottom of the column are withdrawn through line 25 and a part .ings the material is shown a's being withdrawn from the bottom of the column, reheated, and introduced through line "on the third tray from the bottom, but the material may be withdrawn from other desired zones in the lower portion of the column and introduced at other zones including the zone from which it was withdrawn. In principle this means of supplying heat is substantially the same as .that shown in Figure 1, except that here the glycerides by means of 50 which the heat is added, are recirculated independently of the fatty acid feed stock. It will be understood that the glyceride materials cause less trouble in reheating than do .the fatty acids themselves. The heated glyceri'de materials reintroduced-within the lower portion of the column provide heat-for the reboiling of the reflux liquid descending within the column.

- Still another means of indirectly supplying heat for reboiling reflux is illustrated in Figure 3 of the drawings. Here the tray heaters 14 which may contain electrical elements, pass heat directly to the glycerides on the trays in the lower portion of the column, and these in turn serve as the means for supplying heat to the zones 55 throughout the tower for reboiling the descending reflux liquid. The heaters 14 may be spaced along the tower's height and though only two are here shown, as many may be used as desired.

The foregoing detailed description, and the 11- 7o lustrations given are intended for explanation only, and it will be recognized that many changes may be made in the process and apparatus for carrying it out without departing from the scope of the invention as defined in the, appended 75 claims.

fatty acids within said zones to supply the heat We claim:

1. A' process for the fractional distillation of fatty acids comprising heating a feed stock containing fatty acids of different boiling points to produce a vaporized fatty acid mixture, passing I said vaporized mixture upwardly throughv zones of condensed fatty acid mixtures having progressively lower boiling temperatures against a countercurrent of reflux liquid, passing steam upwardly through said zones with said fatty 0 acids, the total amount of said steam-beingless than one-half the amount of said feed stock, and indirectly heating the fatty acids within said zones to supply the heat which is necessary in addition to the heat supplied by said steam for 15 reboiling said reflux liquid.

2. A process for the fractional distillation of fatty acids comprising heating a feed stock containing fatty acids of different boiling points to produce a vaporized fatty acid mixture, passing 20 said vaporous mixture upwardly through zones of 'condensed fatty acid mixtures of progressively lower boilingpoints, condensing fatty acid vapors from an upper of said zones returning at least 80-percent of the condensate to said zones as re-' flux liquid, passing steam upwardly through said zones with said fatty acids, the. total amount of said steam being less than one-half the amount of said feed stock, and indirectly heating the so which isnecessary in addition to the heat supplied by said steam fpr reboiling said reflux liquid.

3. A process for fractionally distilling fatty a acids comprising heating a f stock material containing glycerides and fatt acids to produce a vaporous fatty acid mixture and a glyceride portion; passing said mixture upwardly in a fractionation enclosure through zones of, condensed fatty acid mixtures having progressively lower boiling points against a countercurrent of reflux liquid, passing said glyceride portion downwardly in said enclosure through successive zones in which entrained volatile matter is separated from the glycerides, passing steam upwardly through said zones containing glycerides and fatty acid mixtures whereby to effect the separation of volatile matter from said glycerides and to reduce the fatty acid vapor pressure, the total amount of said steam being less than one-half the amount ofsaid feed stock, and indirectly heating said glycerides within said enclosure to supply the heat which is necessary in addition to the heat supplied by saidsteam for reboiling said reflux liquid. I

4. A'process as set forth in claim 3 wherein said glycerides are withdrawn from said enclosure, reheated, and returned to said enclosure for supplying heat to reboil said reflux liquid. v

5. A process for the fractional distillation of fatty acids comprising heating a feed stock containing fatty acids of different boiling points to produce a vaporized fatty acid mixture, passing said vaporized mixture upwardly through zones 5 of condensed fatty acid mixtures having progressively lower boiling temperatures against a countercurrent of reflux liquid, and passing steam upwardly through said zones with said fatty acids, the total, amount of said steam being less than one-half the amount of said feed stock, a part of said steam being introduced into said zones through said heater with said feed stock, and indirectly heating the fatty-acids within said zones to supply the heat which is necessary in 7 addition to the heat supplied by said steam for reboiling said reflux liquid.

6. A process for fractionally distilling fatty acids comprising heating a feed stock containing glycerides and fatty acids to produce a vaporous fatty acid mixture and a portion containing glycerides, passing said mixture upwardly within a fractionating enclosure through zones of condensed fatty acid mixtures having progressively lower boiling points against a countercurrent of reflux liquid, passing said portion containing glycerides downwardly within said enclosure through a series of stripping zones, passing steam upwardly through said stripping zones and said zones of condensed fatty acid mixtures, the amount of said steam being less than 20 percent of said feed stock, and indirectly heating the fatty acids within said zones to supply the heat which is necessary in addition to the heat supplied by said steam for reboiling said reflux liquid.

7. A process for the fractional distillation of fatty acids comprising heating a feed stock containing fatty acids of different boiling points to produce a vaporized fatty acid mixture, passing said vaporized mixture upwardly through zones of condensed fatty acid mixtures having progressively lower boiling temperatures against a countercurrent of reflux liquid, passing steam upwardly through said zones with said fatty acids, the total amount of said steam being less than 25 percent the amount of said feed stock, and indirectly heating the fatty acids within said zones to supply the heat which is necessary in addition to the heat supplied by said steam for reboiling said reflux liquid.

RALPH H. PO'I'IS. JOHN E. McKEE.

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