Process for Separating Fatty Acids

1. Field of the Invention

The field of art to which this invention pertains is the solid bed molecular sieve separation of fatty acids. More specifically, the invention relates to a process for separating saturated fatty acids from unsaturated fatty acids which process employs a molecular sieve comprising crystalline silica.

2. Background Information

It is known in the separation art that certain crystalline aluminosilicates can be used to separate certain esters of fatty acids from mixtures thereof. For example, in U.S. Pat. Nos. 4,048,205; 4,049,688 and 4,066,677 there are claimed processes for the separation of esters of fatty acids of various degrees of unsaturation from mixtures of esters of saturated and unsaturated fatty acids. These processes use adsorbents comprising an X or a Y zeolite containing a selected cation at the exchangeable cationic sites.

In contrast, this invention relates to the separation of certain fatty acids rather than fatty acid esters. We have discovered that a specific molecular sieve that exhibits selectivity for a saturated fatty acid with respect to an unsaturated fatty acid thereby making separation of such fatty acids by solid bed selective retention possible. Furthermore, we have discovered the enhanced effectiveness of specific displacement fluids at certain displacement conditions. Substantial uses of fatty acids are in the plasticizer and surface active agent fields. Derivatives of fatty acids are of value in compounding lubricating oil, as a lubricant for the textile and molding trade, in special lacquers, as a waterproofing agent, in the cosmetic and pharmaceutical fields, and in biodegradable detergents.

We have discovered that crystalline silica is uniquely suitable for the separation process of this invention in that it exhibits acceptance for a saturated fatty acid with respect to an unsaturated fatty acid when used with a specific displacement fluid, at specific displacement conditions, and does not exhibit reactivity with the free acids.

In brief summary, the invention is, in one embodiment, a process for separating a saturated fatty acid from an unsaturated fatty acid contained in a feed mixture comprising the acids, the process comprising contacting the feed mixture at separation conditions with a molecular sieve comprising a crystalline silica having a silica to alumina mole ratio of at least 12, thereby selectively retaining the saturated fatty acid, removing the remainder of the feed mixture from the molecular sieve, and recovering the saturated fatty acid from the molecular sieve by displacement at displacement conditions with a displacement fluid comprising a diluent soluble in the feed mixture and having a polarity index of at least 3.5.

Other embodiments of our invention encompass details about feed mixtures, molecular sieves, displacement fluids, process flow schemes and operating conditions, all of which are hereinafter disclosed in the following discussion of each of the facets of the present invention.

At the outset the definitions of various terms used throughout the specification will be useful in making clear the operation, objects and advantages of our process.

A "feed mixture" is a mixture containing one or more extract components and one or more raffinate components to be separated by our process. The term "feed stream" indicates a stream of a feed mixture which passes to the molecular sieve used in the process.

An "extract component" is a compound or type of compound that is retained by the molecular sieve while a "raffinate component" is a compound or type of compound that is not retained. In this process, saturated fatty acid is an extract component and unsaturated fatty acid is a raffinate component. The term "displacement fluid" shall mean generally a fluid capable of displacing an extract component. The term "displacement fluid stream" or "displacement fluid input stream" indicates the stream through which displacement fluid passes to the molecular sieve. The term "diluent" or "diluent stream" indicates the stream through which diluent passes to the molecular sieve. The term "raffinate stream" or "raffinate output stream" means a stream through which a raffinate component is removed from the molecular sieve. The composition of the raffinate stream can vary from essentially a 100% displacement fluid to essentially 100% raffinate components. The term "extract stream" or "extract output stream" shall mean a stream through which an extract material which has been displaced by a displacement fluid is removed from the molecular sieve. The composition of the extract stream, likewise, can vary from essentially 100% displacement fluid to essentially 100% extract components. At least a portion of the extract stream and preferably at least a portion of the raffinate stream from the separation process are passed to separation means, typically fractionators, where at least a portion of displacement fluid and diluent is separated to produce an extract product and a raffinate product. The terms "extract product" and "raffinate product" mean products produced by the process containing, respectively, an extract component and a raffinate component in higher concentrations than those found in the extract stream and the raffinate stream. Although it is possible by the process of this invention to produce a high purity, saturated or unsaturated, fatty acid product at high recoveries, it will be appreciated that an extract component is never completely retained by the molecular sieve, nor is a raffinate component completely not retained by the molecular sieve. Therefore, varying amounts of a raffinate component can appear in the extract stream and, likewise, varying amounts of an extract component can appear in the raffinate stream. The extract and raffinate streams then are further distinguished from each other and from the feed mixture by the ratio of the concentrations of an extract component and a raffinate component appearing in the particular stream. More specifically, the ratio of the concentration of a saturated fatty acid to that of non-retained unsaturated fatty acid will be lowest in the raffinate stream, next highest in the feed mixture, and the highest in the extract stream. Likewise, the ratio of the concentration of unsaturated fatty acid to that of the retained saturated fatty acid will be highest in the raffinate stream, next highest in the feed mixture, and the lowest in the extract stream.

The term "selective pore volume" of the molecular sieve is defined as the volume of the molecular sieve which selectively retains an extract component from the feed mixture. The term "non-selective void volume" of the molecular sieve is the volume of the molecular sieve which does not selectively retain an extract component from the feed mixture. This volume includes the cavities of the molecular sieve which admit raffinate components and the interstitial void spaces between molecular sieve particles. The selective pore volume and the non-selective void volume are generally expressed in volumetric quantities and are of importance in determining the proper flow rates of fluid required to be passed into an operational zone for efficient operations to take place for a given quantity of molecular sieve. When molecular sieve "passes" into an operational zone (hereinafter defined and described) employed in one embodiment of this process its non-selective void volume together with its selective pore volume carries fluid into that zone. The non-selective void volume is utilized in determining the amount of fluid which should pass into the same zone in a countercurrent direction to the molecular sieve to displace the fluid present in the non-selective void volume. If the fluid flow rate passing into a zone is smaller than the non-selective void volume rate of molecular sieve material passing into that zone, there is a net entrainment of liquid into the zone by the molecular sieve. Since this net entrainment is a fluid present in the non-selective void volume of the molecular sieve, it in most instances comprises non-retained feed components.

Before considering feed mixtures which can be charged to the process of this invention, brief reference is first made to the terminology and to the general production of fatty acids. The fatty acids are a large group of aliphatic monocarboxylic acids, many of which occur as glycerides (esters of glycerol) in natural fats and oils. Although the term "fatty acids" has been restricted by some to the saturated acids of the acetic acid series, both normal and branched chain, it is now generally used, and is so used herein, to include also related unsaturated acids, certain substituted acids, and even aliphatic acids containing alicyclic substituents. The naturally occurring fatty acids with a few exceptions are higher straight chain unsubstituted acids containing an even number of carbon atoms. The unsaturated fatty acids can be divided, on the basis of the number of double bonds in the hydrocarbon chain, into monoethanoid, diethanoid, triethanoid, etc. (or monoethylenic, etc.). Thus the term "unsaturated fatty acid" is a generic term for a fatty acid having at least one double bond, and the term "polyethanoid fatty acid" means a fatty acid having more than one double bond per molecule. Fatty acids are typically prepared from glyceride fats or oils by one of several "splitting" or hydrolytic processes. In all cases, the hydrolysis reaction may be summarized as the reaction of a fat or oil with water to yield fatty acids plus glycerol. In modern fatty acid plants this process is carried out by continuous high pressure, high temperature hydrolysis of the fat. Starting materials commonly used for the production of fatty acids include coconut oil, palm oil, inedible animal fats, and the commonly used vegetable oils, soybean oil, cottonseed oil and corn oil.

The source of fatty acids with which the present invention is primarily concerned is tallow. In North America, tallow is understood to designate the fat from the fatty tissue of bovine cattle and sheep. The fatty acid content of tallow is typically as follows: oleic acid (C.sub.18, unsaturated, one double bond) 37-43 wt. %; palmitic acid (C.sub.16, saturated) 24-32 wt. %; stearic acid (C.sub.18, saturated) 20-25 wt. %; myristic acid (C.sub.14, saturated) 3-6 wt. %; and the remainder linoleic acid (C.sub.18, unsaturated, two double bonds).

Feed mixtures which can be charged to our process may contain, in addition to the components of tallow, a diluent material that is not adsorbed by the adsorbent and which is preferably separable from the extract and raffinate output streams by fractional distillation. When a diluent is employed, the concentration of diluent in the mixture of diluent and acids will preferably be from a few vol. % up to about 75 vol. %.

Displacement fluids used in various prior art adsorptive and molecular sieve separation processes vary depending upon such factors as the type of operation employed. In separation processes which are generally operated continuously at substantially constant pressures and temperatures to ensure liquid phase, and which employ a molecular sieve, the displacement material must be judiciously selected to satisfy many criteria. First, the displacement material should displace an extract component from the molecular sieve with reasonable mass flow rates but yet allow access of an extract component into the molecular sieve so as not to unduly prevent an extract component from displacing the displacement material in a following separation cycle. Displacement fluids should additionally be substances which are easily separable from the feed mixture that is passed into the process. Both the raffinate stream and the extract stream are removed from the molecular sieve in admixture with displacement fluid and without a method of separating at least a portion of the displacement fluid, the purity of the extract product and the raffinate product would not be very high nor would the displacement fluid be available for reuse in the process. It is therefore contemplated that any displacement fluid material used in this process will preferably have a substantially different average boiling point than that of the feed mixture to allow separation of at least a portion of displacement fluid from feed components in the extract and raffinate streams by simple fractional distillation, thereby permitting reuse of displacement fluid in the process. The term "substantially different" as used herein shall mean that the difference between the average boiling points between the displacement fluid and the feed mixture shall be at least about 5.degree. C. The boiling range of the displacement fluid may be higher or lower than that of the feed mixture. Finally, displacement fluids should also be materials which are readily available and therefore reasonable in cost. In the preferred isothermal, isobaric, liquid-phase operation of the process of our invention, we have found, as will be discussed at length hereinbelow, displacement fluids comprising a diluent soluble in the feed mixture and having a polarity index of at least 3.5 to be effective when the conditions at which the retention and displacement is carried out is from about 20.degree. C. to about 200.degree. C. with pressure sufficient to maintain liquid phase. When the feedstock is tallow, the preferred conditions are about 120.degree. C. to about 150.degree. C. with pressure sufficient to maintain liquid phase.

It has been observed that even crystalline silica may be ineffective in separating fatty acids from each other. It is hypothesized that hydrogen-bonded dimerization reactions occur in which there is an alignment between the molecules of the fatty acids. These dimerization reactions may be represented by the formula:

where FA stands for fatty acids. The dimers would preclude separation of the fatty acids by blocking access into the pores of the molecular sieve. This hindrance to separation caused by the presence of dimers does not appear to be a significant problem in the aforementioned process for separation of esters of fatty acids.

We have discovered that the above dimerization reactions may be minimized if the displacement fluid comprises a properly selected diluent. There are diluents which exhibit the property of minimizing dimerization. The measure of this property was found to be the polarity index of the liquid. Polarity index is as described in the article, "Classification of the Solvent Properties of Common Liquids"; Snyder, L. J. Chromatography, 92, 223 (1974), incorporated herein by reference. The minimum polarity index of the displacement fluid-diluent required for the process of the present invention is 3.5. Polarity indexes for certain selected diluents are as follows:

The molecular sieve to be used in the process of this invention comprises crystalline silica having a silica/alumina mole ratio of at least 12. One such crystalline silica is known as silicalite which has a silica/alumina mole ratio of infinity, i.e., it contains no alumina. Silicalite is a hydrophobic crystalline silica molecular sieve. Silicalite is disclosed and claimed in U.S. Pat. Nos. 4,061,724 and 4,104,294 to Grose et al., incorporated herein by reference. Due to its aluminum-free structure, silicalite does not show ion-exchange behavior, and is hydrophobic and organophilic. Silicalite is uniquely suitable for the separation process of this invention for the presumed reason that its pores are of a size and shape that enable the silicalite to function as a molecular sieve, i.e., accept the molecules of saturated fatty acids (which are relatively flexible) into its channels or internal structure, while rejecting the molecules of the unsaturated fatty acids (which are relatively rigid). A more detailed discussion of silicalite may be found in the article, "Silicalite, A New Hydrophobic Crystalline Silica Molecular Sieve"; Nature, Vol. 271, Feb. 9, 1978, incorporated herein by reference.

Examples of other crystalline silicas suitable for use in the present invention are those having the trademark designation "ZSM" and silica/alumina mole ratios of at least 12. The ZSM adsorbents are as described in U.S. Pat. No. 4,309,281 to Dessau, incorporated herein by reference.

Typically, adsorbents used in separative processes contain the crystalline material dispersed in an amorphous material or inorganic matrix, particularly an amorphous material having channels and cavities therein which enable liquid access to the crystalline silica. The binder aids in forming or agglomerating the crystalline particles of the crystalline silica which otherwise would comprise a fine powder. The silica molecular sieve may thus be in the form of particles such as extrudates, aggregates, tablets, macrospheres or granules having a desired particle range, preferably from about 16 to 60 mesh (Standard U.S. Mesh). Colloidal amorphous silica is an ideal binder for crystalline silica in that like the crystalline silica itself this binder exhibits no reactivity for the free fatty acids. The preferred silica is marketed by DuPont Company under the trademark "Ludox". The crystalline silica powder is dispersed in the Ludox which is then gelled and treated so as to substantially eliminate hydroxyl groups, such as by thermal treatment in the presence of oxygen at a temperature from about 450.degree. C. to about 1000.degree. C. for a minimum period from about 3 hours to about 48 hours. The crystalline silica should be present in the silica matrix in amounts ranging from about 75 wt. % to about 98 wt. % crystalline silica based on volatile free composition.

The molecular sieve may be employed in the form of a dense compact fixed bed which is alternatively contacted with the feed mixture and displacement fluid. In the simplest embodiment of the invention, the molecular sieve is employed in the form of a single static bed in which case the process is only semi-continuous. In another embodiment, a set of two or more static beds may be employed in fixed bed contacting with appropriate valving so that the feed mixture is passed through one or more molecular sieve beds, while the displacement fluid can be passed through one or more of the other beds in the set. The flow of feed mixture and displacement fluid may be either up or down through the molecular sieve. Any of the conventional apparatus employed in static bed fluid-solid contacting may be used.

Moving bed or simulated moving bed flow systems, however, have a much greater separation efficiency than fixed bed systems and are therefore preferred. In the moving bed or simulated moving bed processes, the retention and displacement operations are continuously taking place which allows both continuous production of an extract and a raffinate stream and the continual use of feed and displacement fluid streams. One preferred embodiment of this process utilizes what is known in the art as the simulated moving bed countercurrent flow system. The operating principles and sequence of such a flow system are described in U.S. Pat. No. 2,985,589 incorporated herein by reference. In such a system, it is the progressive movement of multiple liquid access points down a molecular sieve chamber that simulates the upward movement of molecular sieve contained in the chamber. Reference can also be made to D. B. Broughton U.S. Pat. No. 2,985,589 and to a paper entitled, "Continuous Adsorptive Processing--A New Separation Technique" by D. B. Broughton presented at the 34th Annual Meeting of the Society of Chemical Engineers at Tokyo, Japan on Apr. 2, 1969, both references incorporated herein by reference, for further explanation of the simulated moving bed countercurrent process flow scheme.

Another embodiment of a simulated moving bed flow system suitable for use in the process of the present invention is the co-current high efficiency simulated moving bed process disclosed in our assignee's U.S. Pat. No. 4,402,832, incorporated by reference herein in its entirety.

It is contemplated with any flow scheme used to carry out the present invention that at least a portion of the extract output stream will pass into a separation means wherein at least a portion of the displacement fluid can be separated to produce an extract product containing a reduced concentration of displacement fluid. Preferably, but not necessary to the operation of the process, at least a portion of the raffinate output stream will also be passed to a separation means wherein at least a portion of the diluent can be separated to produce a diluent stream which can be reused in the process and a raffinate product containing a reduced concentration of diluent. The separation means will typically be a fractionation column, the design and operation of which is well known to the separation art.

Although both liquid and vapor phase operations can be used in many adsorptive separation processes, liquid-phase operation is preferred for this process because of the lower temperature requirements and because of the higher yields of extract product that can be obtained with liquid-phase operation over those obtained with vapor-phase operation. Displacement conditions will thus include, as hereinbefore mentioned, a pressure sufficient to maintain liquid phase. Separation conditions may include, as a matter of convenience, the same range of temperatures and pressures as used for displacement conditions.

A dynamic testing apparatus is employed to test various molecular sieves with a particular feed mixture and displacement fluid to measure the molecular sieve characteristics of retention capacity and exchange rate. The apparatus consists of a helical molecular sieve chamber of approximately 70 cc volume having inlet and outlet portions at opposite ends of the chamber. The chamber is contained within a temperature control means and, in addition, pressure control equipment is used to operate the chamber at a constant predetermined pressure. Quantitative and qualitative analytical equipment such as refractometers, polarimeters and chromatographs can be attached to the outlet line of the chamber and used to detect quantitatively or determine qualitatively one or more components in the effluent stream leaving the molecular sieve chamber. A pulse test, performed using this apparatus and the following general procedure, is used to determine data for various molecular sieve systems. The molecular sieve is filled to equilibrium with a particular displacement fluid material by passing the displacement fluid through the molecular sieve chamber. At a convenient time, a pulse of feed containing known concentrations of a tracer and of a particular extract component or of a raffinate component or both, all diluted in displacement fluid is injected for a duration of several minutes. Displacement fluid flow is resumed, and the tracer and the extract component or the raffinate component (or both) are eluted as in a liquid-solid chromatographic operation. The effluent can be analyzed on-stream or alternatively, effluent samples can be collected periodically and later analyzed separately by analytical equipment and traces of the envelopes or corresponding component peaks developed.

From information derived from the test, molecular sieve performance can be rated in terms of void volume, retention volume for an extract or a raffinate component, and the rate of displacement of an extract component from the molecular sieve. The retention volume of an extract or a raffinate component may be characterized by the distance between the center of the peak envelope of the tracer component or some other known reference point. It is expressed in terms of the volume in cubic centimeters of displacement fluid pumped during this time interval represented by the distance between the peak envelopes. The rate of exchange of an extract component with the displacement fluid can generally be characterized by the width of the peak envelopes at half intensity. The narrower the peak width, the faster the displacement rate. The displacement rate can also be characterized by the distance between the center of the tracer peak envelope and the disappearance of an extract component which has just been displaced. This distance is again the volume of displacement fluid pumped during this time interval.

The following non-limiting working example is presented to illustrate the process of the present invention and is not intended to unduly restrict the scope of the claims attached hereto.

The above described pulse test apparatus was used to obtain data for this example. The liquid temperature was 120.degree. C. and the flow was down the column at the rate of 1.2 ml/min. The feed stream comprised 10 wt. % fatty acid mixture and 90 wt. % displacement fluid and was introduced into the column in 5 ml pulses. The fatty acid mixture comprised 25.6 wt. % palmitic acid, 17.5 wt. % stearic acid, 41.6 wt. % oleic acid and the remainder comprising a mixture of various short and long carbon chain organic compounds, each of insufficient concentration to be detected on the pulse test apparatus. The column was packed with 23 wt. % Ludox bound silicalite (77 wt. % silicalite) of 40-60 mesh. The displacement fluid used was pure acetone.

The results of this example are shown on the accompanying FIGURE. It is apparent from the FIGURE that the separation of the saturated fatty acids (palmitic and stearic) from the unsaturated fatty acid (oleic) is clear and distinct.