Triglyceride Refining

Abstract

A new use for a centrifuge having an axial screw conveyor, heretofore used only for liquid-solids separation, is proposed for liquid-liquid separation in the refining of triglycerides.

Description

This invention relates to triglyceride refining, and more particularly to the refining of various animal and vegetable materials for the production of edible and inedible fats and oils.

The satisfactory refining of raw triglyceride materials usually involves adding sodium hydroxide or another suitable alkaline reagent to a filtered triglyceride. In the case of crude triglyceride oils from vegetable oil seeds, the latter may have been previously dehulled, cooked, pressed, and separated from the solids cake. In the case of crude triglyceride oils from animal sources, such oils may have been derived from conventional wet or dry rendering processes. In either case, the addition and mixing of the alkaline reagent with the crude oil further involves elevating the temperature of the liquid mixture and separating the resultant heavier liquid soapstock from the lighter refined oil. Although in the early days of such refining the foregoing steps were done in a batch kettle, in recent times continuous processing has been carried out by injecting the reagent into a conduit for the raw oil and effecting heating in one or more heat exchangers after which separation is done continuously by centrifugation. The separated liquid soapstock may be delivered as feed to any one of a variety of other processes, depending upon the desired end product, while the separated oil is usually mixed with water and again centrifuged in order to further reduce the soap content of the separated oil. The oil from this water washing treatment is forwarded as feed to other processes for conversion into the desired end product.

The foregoing conventional process suffers from a number of disadvantages. These include the necessity of filtration prior to centrifuging in order to remove undesirable solids, usually protein or fibrous solids comprising up to 3 percent of the feed, partly because such solids contaminate the end product but more importantly because they plug up disc type centrifuges used for separation. A disadvantage involved in the prior filtration of feed is the requirement of manual disassembly of the filter for cleaning and the occasional replacement of filter media. Even with filtration efficiency of 99 percent and upwards, the periodic disassembly and cleaning of the centrifuge apparatus, required to remove the accumulated solids which escape the filter, is a further disadvantage of such conventional refining plants. When a centrifuge is cleaned, it not only entails labor and interruption of the process, but also the cost of replacement parts such as seals, discs and the like.

The aforesaid conventional process carries out the liquid-liquid separation step by means of a relatively high speed centrifuge having an internal disc stack to promote separation, and which subjects the feed to about 8,000 times the force of gravity. The separated solids accumulated on the disc stack and especially on the inner surface of the centrifuge bowl require manual removal as aforesaid. Efforts to retard solids deposition by introducing a water or reagent wash are only partially effective, as is back-flushing the machine, the latter being also disadvantageous from the standpoint of process continuity.

The present invention seeks to obviate the foregoing disadvantages of conventional triglyceride refining in a manner to be described more fully hereinafter.

Briefly stated, the present invention retains the conventional steps of adding an alkaline reagent such as aqueous sodium hydroxide to the crude oil being refined, for example a stream of rendered fat or vegetable oil, either of which contains triglycerides. Optionally the crude oil is in hexane solution, forming a miscella. The present invention also retains the step of heating by means of a heat exchanger, the mixture just described and which may also contain traces of other lipids, fatty acid in solution with the triglycerides, and protein and/or fibrous material in suspension. But the present invention overcomes the aforementioned disadvantages by employing for liquid-liquid separation centrifuge apparatus of a type heretofore used only for the separation of solids and liquid from a mixture thereof. The essence of the invention, therefore, is a new use for a known machine. Such known machine is a decanter type centrifuge having an imperforate and elongated solid bowl disposed within a housing. A major portion of the bowl is of cylindrical shape and a minor portion thereof is of conical configuration at one end of the bowl and tapering, or decreasing in diameter, toward that end. The bowl is adapted to be motor-driven for rotation about its longitudinal axis at speeds applying to the feed up to 3,000 times the force of gravity and beyond. A helical screw conveyor is mounted on a hollow shaft or hub extending coaxially of the bowl. The bowl and screw conveyor are arranged to rotate on a common axis, preferably with a speed differential of from 5 to 60 r.p.m. in order to move the viscous heavy phase liquid soapstock and other matter heavier than oil to the discharge port at the tapered end of the bowl, while the lighter liquid oil is discharged from the other end of the bowl. A detailed description of the construction of the general type of centrifuge which may be employed in practicing the present invention is omitted herefrom for the sake of brevity. Instead, the centrifuge constructions disclosed in U.S. Pat. Nos. 2,679,974 and 2,703,676 of F. P. Gooch; also U.S. Pat. No. 3,148,145 of K. G. Reed; and U.S. Pat. No. 3,172,851 of C. M. Ambler, are incorporated herein by reference. The cited patents are assigned to the assignee of the present invention.

The construction of the cited Ambler patent is believed to be preferred, and it is distinguishable from the other cited constructions in that the discharge port in the tapered end of the bowl is radially more distant from the rotational axis than is the discharge port for the lighter phase material, the refined triglycerides -- either vegetable or animal fat. Of course it is to be understood that according to the present invention heavy phase material, comprising sodium soaps, water, traces of other lipids, and protein and/or fibrous material in suspension, is to be discharged through the discharge port used for the discharge of solids prior to the present invention. It is theorized that the heavier and more viscous liquids separated in accordance with the present invention are just as effective as are the solids described in the conventional use of the Ambler centrifuge for forming, as if by cresting, an annular dam about the discharge port in the tapered end of the bowl in order to prevent the flow of the lighter liquid phase from the same discharge port with the heavy phase.

With the more commonly used centrifuges having an axial screw conveyor, such as in the cited Gooch and Reed centrifuges, the discharge port for the heavier phase separated material is radially closer to the rotational axis than is the discharge port for the light phase material, so that there is even less chance of the two phases being discharged together from the tapered end of the bowl. It would seem that neither material would be discharged from the tapered end of the bowl unless conveyed by the axial screw conveyor. It is theorized that the more viscous heavy phase material is more easily conveyed up the inclined beach or slope by the axial screw conveyor even though the heavy phase material must pass through the lighter phase liquid on its way to the discharge port.

The centrifuge of Ambler is preferred over the Gooch and Reed centrifuges because it is believed that the hydrostatic pressure of the blanketing layer of lighter phase material on the outwardly disposed layer of heavy phase material promotes the movement of the heavy phase material toward its discharge port. In other words, the layer of light phase material extends to the heavy phase discharge port and helps to support the heavy phase material as it flow toward the discharge port with the urging of the screw conveyor.

The disclosure of the present invention assumes that the heavier phase liquid material is more viscous, i.e. possesses a high viscosity, than the lighter phase liquid material. The present invention is not operative where the light phase liquid material is more viscous than the heavy phase liquid material. Furthermore, the term "liquid" as used herein is applicable to nongaseous flowable or fluid materials having a broad range of viscosities.

An important advantage of the present invention over the prior practice is that centrifuging according to the present invention eliminates the need for prior filtration since there is no longer a danger of the protein and fibrous material plugging the centrifuge for the flow passages therein. Rather, the centrifuge is truly self-cleaning and therefore may be operated continuously in the refining of triglycerides, without the need for shut-down to perform manual cleaning.

It is essential to an appreciation of the present invention to understand that centrifuges of the type described have heretofore been used only for the separation of solids from liquids and not for a liquid-liquid separation.

The present invention is especially applicable to refining edible triglycerides derived from cotton, rape, sesame, soy, coco, lard and tallow; also inedible fats, tallow and grease.

Claims

What is claimed is:

1. A process employing a centrifuge, said centrifuge including an elongated bowl mounted for rotation about a longitudinal axis thereof and having at least a portion of its inner surface decreasing in diameter approaching the end of said bowl, said bowl having a first discharge opening at said one end of said bowl, and second discharge means communicating with the interior of said bowl, a feed tube for delivering feed comprising heavy phase material and light phase material to the interior of said bowl, a helical conveyor disposed within the bowl rotatable in coaxial relationship, said conveyor extending axially for substantially the entire length of the bowl and having screw flights with distal edges generally complementing the inside contour of the bowl, the conveyor being adapted when turning at a different speed relative to the bowl to more settled heavy phase material along said surface portion of decreasing diameter to said first discharge opening, and means for driving the bowl and the conveyor at slightly different speeds sufficient to convey heavy phase material to said first discharge opening, said second discharge means conducting separated light phase material out of said bowl, said process comprising:

a. introducing to the bowl, through said feed tube, feed comprising a liquid light phase material and a liquid heavy phase material which is substantially more viscous than said light phase material, said feed being derived from the reaction of aqueous sodium hydroxide with a stream of fat or oil containing triglycerides and traces of other lipids, fatty acid in solution with said triglycerides, and protein in suspension, whereby the heavy phase material primarily comprises sodium soaps, water, traces of other lipids and protein in suspension, and said light phase material comprises the refined triglycerides,

b. centrifugally separating said feed, by rotation of said bowl, into respective inner and outer layers of light phase material and heavy phase material,

c. conveying said settled liquid heavy phase material along said surface portion of decreasing diameter to said first discharge opening, while discharging liquid light phase material from said bowl through said second discharge means.

2. The invention according to claim 1 wherein the second discharge means has an inlet opening at a smaller radial distance from said axis than is said first discharge opening.

3. The invention according to claim 1 wherein the second discharge means has an inlet opening at a greater radial distance from said axis than is said first discharge opening.

4. The invention according to claim 1 wherein the heavy phase material is a liquid that is substantially more viscous than the light phase liquid material.

5. The invention according to claim 1 wherein the feed is unfiltered prior to introduction to said centrifuge bowl.

6. The invention according to claim 1 wherein the heavy phase liquid in the feed is a triglyceride material.

7. The invention according to claim 1 wherein the feed is derived from the reaction of aqueous sodium hydroxide with a stream of rendered fat.

8. The invention according to claim 7 wherein the stream of rendered fat is in hexane solution.

9. The invention according to claim 1 wherein the feed is derived from the reaction of aqueous sodium hydroxide with a stream of vegetable oil.

10. The invention according to claim 9 wherein the stream of vegetable oil is in hexane solution.