METHOD FOR REFINING GLYCERIDE OILS USING AMORPHOUS SILICA

Abstract: Adsorbents comprising amorphous silicas with effective average pore diameters of about 60 to about 5000 Ang­ troms are useful in processes for the removal of trace contaminants, specifically phospholipids and associated metal ions, from glyceride oils.

Description: This invention relates to a method for refining glyceride oils by contacting the oils with an adsorbent capable of selectively removing trace contaminants. More specifically, it has been found that amorphous silicas of suitable porosity are quite effective in adsorbing phospholipids and associated metal containing species from glyceride oils, to produce oil products with substantially lowered concentrations of these trace contaminants. The term "glyceride oils" as used herein is intended to encompass both vegetable and animal oils. The term is primarily intended to describe the so-called edible oils, i.e., oils derived from fruits or seeds of plants and used chiefly in foodstuffs, but it is understood that oils whose end use is as non-edibles are to be included as well. Crude glyceride oils, particularly vegetable oils, are refined by a multi-stage process, the first step of which is degumming by treatment with water or with a chemical such as phosphoric acid, citric acid or acetic anhydride. After degumming, the oil may be refined by a chemical process including neutralization, bleaching and deodorizing steps. Alternatively, a physical process may be used, including a pretreating and bleaching step and a steam refining and deodorizing step. Physical refining processes do not include a caustic refining step. State-of-the-art processes for both physical and chemical refining are described by Tandy et al. in "Physical Refining of Edible Oil," J. Am. Oil Chem. Soc., Vol. 61, pp. 1253-58 (July 1984). One object of either refining process is to reduce the levels of phospholipids, which can lend off colors, odors and flavors to the finished oil product. In addition, ionic forms of the metals calcium, magnesium, iron and copper are thought to be chemically associated with phospholipids and to negatively effect the quality of the final oil product. The removal of phospholipids from edible oils has been the abject of a number of previously proposed physical process steps in addition to the conventional chemical processes. For example, Gutfinger et al., "Pretreatment of Soybean Oil for Physical Refining: Evaluation of Efficiency of Various Adsorbents in Removing Phospholipids and Pigments," J. Amer. Oil Chem. Soc., Vol. 55, pp. 865-59 (1978), describes a study of several adsorbents, including Tonsil L80 (TM) and Tonsil ACC (TM) (Sud Chemie, A.G.), Fuller's earth, Celite (TM) (Johns-Manville Products Corp.), Kaoline (sic), silicic acid and Florosil (sic) (TM) (Floridin Co.), for removing phospholipids and color bodies from phosphoric acid degummed soybean oil. U.S. 3,284,213 (Van Akkeren) discloses a process using acid bleaching clay for removing phosphoric acid material from cooking oil. U.S. 3,955,004 (Strauss) discloses improvement of the storage properties of edible oils by contacting the oil, in solution in a non-polar solvent, with an adsorbent such as silica gel or alumina and subsequently bleaching with a bleaching earth. On page 2, lines 35 to 37 it is explicitly stated that it is essential that the oil is in solution in a non-polar solvent. This publication therefore teaches that the storage properties of edible oils can only be improved by the treatment with adsorbents like silica gel or alumina, if the oil is in solution in a non-polar solvent. U.S. 4,298,622 (Singh et al.) discloses bleaching degummed wheat germ oil by treating it with up to 10% by weight of an adsorbent such as Filtrol (TM) (Filtrol Corp.), Tonsil (TM), silica gel, activated charcoal or fuller's earth, at 90°-110°C under strong vacuum. The technical problem solved by this publication is particularly related to the characteristics of naturally occurring wheat germ oil which is a very dark product unsuitable for many end uses. To remove the colour bodies it is therefore recommended to use at least 5% by weight of the adsorbent, such amount being indicated as often not sufficient to produce an oil which is light enough, while 10% by weight are stated to produce a very light oil. Finally GB-A-612,169 discloses a process for bleaching highly discoloured oils and fat stocks. To reduce the colour content it is proposed to intimately mix the respective oil with an acidic phosphoric compound and oxygen in the presence of an inorganic silicon compound consisting of hydrated silica gel, hydrated amorphous silica, a silicic acid or fluo-silicic acid. The oxygen is provided as commercial oxygen, gas or air or is furnished by an oxygen-liberating compound, such as hydrogenperoxide. Consequently in GB-A-612 169 a chemical reaction is utilized for the particular purpose of reducing the colour content of highly discoloured oils and fat stocks. Up to now no simple and economic process for reducing the phospholipid and associated metal ions content of glyceride oils has been described, in which the use of solvents or chemical reactions is completely avoided. It is therefore the primary object of this invention to make feasible a physical refining process by providing a method for reducing the phospholipid and associated metal ions content of oils from a level of up to 230 ppm to acceptable levels. According to the invention this technical problem is solved by using amorphous silica having an effective average por diameter of greater than 6 nm (60 Å ) for the removal of phospholipids and metal ions, from glyceride oils having a phosphorus content from about 230 to about 1.0 ppm in the absence of any solvent and without the addition of oxygen/phosphoric acid at temperatures, at which the respective oils are liquid and at a concentration of the amorphous silica calculated on a dry weight basis after ignition at 954°C of 0.01 to 1.0 weight% based on the weight of the oil processed. Adsorption of phospholipids and associated contaminants onto amorphous silica in the manner described can eliminate any need to use caustic refining, thus eliminating one unit operation, as well as the need for wastewater treatment from that operation. Over and above the cost savings realized from simplification of the oil processing, the overall value of the product is increased since a significant by-product of caustic refining is aqueous soapstock, which is of very low value. It is also intended that use of the method of this invention may reduce or potentially eliminate the need for bleaching earth steps. Reduction or elimination of the bleaching earth step will result in substantial oil conservation as this step typically results in significant oil loss. Moreover, since spent bleaching earth has a tendency to undergo spontaneous combustion, reduction or elimination of this step will yield an occupationally and environmentally safer process. DETAILED DESCRIPTION OF THE INVENTION It has been found that certain amorphous silicas are particularly well suited for removing trace contaminants, specifically phospholipids and associated metal ions, from glyceride oils. The process for the removal of these trace contaminants, as described in detail herein, essentially comprises the steps of selecting a glyceride oil with a phosphorous content from about 230 to about 1.0 ppm, selecting an adsorbent comprising a suitable amorphous silica, contacting the glyceride oil and the adsorbent, allowing the phospholipids and associated metal ions to be adsorbed, and separating the resulting phospholipid- and metal ion-depleted oil from the adsorbent. Suitable amorphous silicas for this process are those with pore diameters greater than 6nm (60Å ).In addition, silicas with a moisture content of greater than about 30% by weight exhibit improved filterability from the oil and are therefore preferred. The process described herein can be used for the removal of phospholipids from any glyceride oil, for example, oils of soybean, peanut, rapeseed, corn, sunflower, palm, coconut, olive, cottonseed, etc. Removal of phospholipids from these edible oils is a significant step in the oil refining process because residual phosphorous can cause off colors, odors and flavors in the finished oil. Typically, the acceptable concentration of phosphorous in the finished oil product should be less than about 15.0 ppm, preferably less than about 5.0 ppm, according to general industry practice. As an illustration of the refining goals with respect to trace contaminants, typical phosphorous levels in soybean oil at various stages of chemical refining are shown in Table I. Phosphorous levels at corresponding stages in physical refining processes will be comparable. In addition to phospholipid removal, the process of this invention also removes from edible oils ionic forms of the metals calcium, magnesium, iron and copper, which are believed to be chemically associated with phospholipids. These metal ions themselves have a deleterious effect on the refined oil products. Calcium and magnesium ions can result in the formation of precipitates. The presence of iron and copper ions promote oxidative instability. Moreover, each of these metal ions is associated with catalyst poisoning where the refined oil is catalytically hydrogenated. Typical concentrations of these metals in soybean oil at various stages of chemical refining are shown in Table I. Metal ion levels at corresponding stages of physical refining processes will be comparable. Throughout the description of this invention, unless otherwise indicated, reference to the removal of phospholipids is meant to encompass the removal of associated trace contaminants as well. The term "amorphous silica" as used herein is intended to embrace silica gels, precipitated silicas, dialytic silicas and fumed silicas in their various prepared or activated forms. Both silica gels and precipitated silicas are prepared by the destabilization of aqueous silicate solutions by acid neutralization. In the preparation of silica gel, a silica hydrogel is formed which then typically is washed to low salt content. The washed hydrogel may be milled, or it may be dried, ultimately to the point where its structure no longer changes as a result of shrinkage. The dried, stable silica is termed a xerogel. In the preparation of precipitated silicas, the destabilization is carried out in the presence of polymerization inhibitors, such as inorganic salts, which cause precipitation of hydrated silica. The precipitate typically is filtered, washed and dried. For preparation of gels or precipitates useful in this invention, it is preferred to dry them and then to add water to reach the desired water content before use. However, it is possible to initially dry the gel or precipitate to the desired water content. Dialytic silica is prepared by precipitation of silica from a soluble silicate solution containing electrolyte salts (e.g., NaNO₃, Na₂SO₄, KNO₃) while electrodialyzing, as described in pending U.S. patent application Serial No. 533,206 (Winyall), "Particulate Dialytic Silica," corresponding to EP-A-83110145.6. Fumed silicas (or pyrogenic silicas) are prepared from silicon tetrachloride by high-temperature hydrolysis, or other convenient methods. The specific manufacturing process used to prepare the amorphous silica is not expected to affect its utility in this method. In the preferred embodiment of this invention, the silica adsorbent will have the highest possible surface area in pores which are large enough to permit access to the phospholipid molecules, while being capable of maintaining good structural integrity upon contact with an aqueous media. The requirement of structural integrity is particularly important where the silica adsorbents are used in continuous flow systems, which are susceptible to disruption and plugging. Amorphous silicas suitable for use in this process have surface areas of up to about 1200 square meters per gram, preferably between 100 and 1200 square meters per gram. The method of this invention utilizes amorphous silicas having an effective average pore diameter greater than 6 nm (60 Å ), calculated on the basis of the following equation: as defined herein, after appropriate activation where necessary for the measurement of surface area and pore volume. Activation typically is by heating to temperatures of about 232 to 371°C (450 to 700°F) in vacuum. One convention which describes silicas is average pore diameter ("APD"), typically defined as that pore diameter at which 50% of the surface area or pore volume is contained in pores with diameters greater than the stated APD and 50% is contained in pores with diameters less than the stated APD. This value is approximated by the aforementioned equation (1). Thus, in amorphous silicas suitable for use in the method of this invention, at least 50% of the pore volume will be in pores of at least 6 nm (60 Å ) diameter. Silicas with a higher proportion of pores with diameters greater than 6 nm (60 Å) will be preferred, as these will contain a greater number of potential adsorption sites. The practical upper APD limit is about 500 nm (5000 Å ). Silicas which have measured intraparticle APDs within the stated range will be suitable for use in this process. Alternatively, the required porosity may be achieved by the creation of an artificial pore network of interparticle voids in the 6 to 500 nm (60 to 5000 Å ) range. For example, non-porous silicas (i.e., fumed silica) can be used as aggregated particles. Silicas, with or without the required porosity, may be used under conditions which create this artificial pore network. Thus the criterion for selecting suitable amorphous silicas for use in this process is the presence of an "effective average pore diameter" greater than 6 nm (60 Å ). This term includes both measured intraparticle APD and interparticle APD, designating the pores created by aggregation or packing of silica particles. The APD value (in Angstroms) can be measured by several methods or can be approximated by the following equation, which assumes model pores of cylindrical geometry: where PV is pore volume (measured in cubic centimeters per gram) and SA is surface area (measured in square meters per gram). Both nitrogen and mercury porosimetry may be used to measure pore volume in xerogels, precipitated silicas and dialytic silicas. Pore volume may be measured by the nitrogen Brunauer-Emmett-Teller ("B-E-T") method described in Brunauer et al., J. Am. Chem. Soc., Vol 60, p. 309 (1938). This method depends on the condensation of nitrogen into the pores of activated silica and is useful for measuring pores with diameters up to about 60 nm (600 Å ). If the sample contains pores with diameters greater than about 60 nm (600 Å ), the pore size distribution, at least of the larger pores, is determined by mercury porosimetry as described in Ritter et al., Ind. Eng. Chem. Anal. Ed. 17,787 (1945). This method is based on determining the pressure required to force mercury into the pores of the sample. Mercury porosimetry, which is useful from about 3 to about 1000 nm (30 to about 10,000 A), may be used alone for measuring pore volumes in silicas having pores with diameters both above and below 60 nm (600 Å ). Alternatively, nitrogen porosimetry can be used in conjunction with mercury porosimetry for these silicas. For measurement of ADPs below 60 nm (600Å ),it may be desired to compare the results obtained by both methods. The calculated PV volume is used in Equation (1). For determining pore volume of hydrogels, a different procedure, which assumes a direct relationship between pore volume and water content, is used. A sample of the hydrogel is weighed into a container and all water is removed from the sample by vacuum at low temperatures (i.e., about room temperature). The sample is then heated to about 232 to 371 o C (450 to 700°F)to activate. After activation, the sample is re-weighed to determine the weight of the silica on a dry basis, and the pore volume is calculated by the equation: where TV is total volatiles, determined by the wet and dry weight differential. The PV value calculated in this manner is then used in Equation (1). The surface area measurement in the APD equation is measured by the nitrogen B-E-T surface area method, described in the Brunauer et al., article, supra . The surface area of all types of appropriately activated amorphous silicas can be measured by this method. The measured SA is used in Equation (1) with the measured PV to calculate the APD of the silica. In the preferred embodiment of this invention, the amorphous silica selected for use will be a hydrogel. The characteristics of hydrogels are such that they effectively adsorb trace contaminants from glyceride oils and that they exhibit superior filterability as compared with other forms of silica. The selection of hydrogels therefore will facilitate the overall refining process. The purity of the amorphous silica used in this invention is not believed to be critical in terms of the adsorption of phospholipids. However, where the finished products are intended to be food grade oils care should be taken to ensure that the silica used does not contain leachable impurities which could compromise the desired purity of the product(s). It is preferred, therefore, to use a substantially pure amorphous silica, although minor amounts, i.e., less than about 10%, of other inorganic constituents may be present. For example, suitable silicas may comprise iron as Fe₂O₃, aluminum as Al₂O₃, titanium as TiO₂, calcium as CaO, sodium as Na₂O, zirconium as ZrO₂, and/or trace elements. It has been found that the moisture or water content of the silica has an important effect on the filterability of the silica from the oil, although it does not necessarily affect phospholipid adsorption itself. The presence of greater than 30% by weight of water in the pores of the silica (measured as weight loss on ignition at 954 o C (1750 o F) is preferred for improved filterability. This improvement in filterability is observed even at elevated oil temperatures which would tend to cause the water content of the silica to be substantially lost by evaporation during the treatment step. The adsorption step itself is accomplished by conventional methods in which the amorphous silica and the oil are contacted, preferably in a manner which facilitates the adsorption. The adsorption step may be by any convenient batch or continuous process. In any case, agitation or other mixing will enhance the adsorption efficiency of the silica. The adsorption can be conducted at any convenient temperature at which the oil is a liquid. The glyceride oil and amorphous silica are contacted as described above for a period sufficient to achieve the desired phospholipid content in the treated oil. The specific contact time will vary somewhat with the selected process, i.e., batch or continuous. In addition, the adsorbent usage, that is, the relative quantity of adsorbent brought into contact with the oil, will affect the amount of phospholipids removed. The adsorbent usage is quantified as the weight percent of amorphous silica (on a dry weight basis after ignition at 954 o C (1750°F)), calculated on the weight of the oil processed. The preferred adsorbent usage is about 0.01 to about 1.0%. As seen in the Examples, significant reduction in phospholipid content is achieved by the method of this invention. The specific phosphorous content of the treated oil will depend primarily on the oil itself, as well as on the silica, usage, process, etc. However, phosphorous levels of less than 15 ppm, preferably less than 5.0 ppm, can be achieved. Following adsorption, the phospholipid-enriched silica is filtered from the phospholipid-depleted oil by any convenient filtration means. The oil may be subjected to additional finishing processes, such as steam refining, heat bleaching and/or deodorizing. The method described herein may reduce the phosphorous levels sufficiently to eliminate the need for bleaching earth steps. With low phosphorous levels, it may be feasible to use heat bleaching instead. Even where bleaching earth operations are to be employed for decoloring the oil, the sequential treatment with amorphous silica and bleaching earth provides an extremely efficient overall process. By first using the method of this invention to decrease the phospholipid content, and then treating with bleaching earth, the latter step is made to be more effective. Therefore, either the quantity of bleaching earth required can be significantly reduced, or the bleaching earth will operate more effectively per unit weight. It may be feasible to elute the adsorbed contaminants from the spent silica in order to re-cycle the silica for further oil treatment. The examples which follow are given for illustrative purposes and are not meant to limit the invention described herein. The following abbreviations have been used throughout in describing the invention: A - Angstrom(s) APD - average pore diameter B-E-T - Brunauer-Emmett-Teller Ca - calcium cc - cubic centimeter(s) cm - centimeter Cu - copper °C - degrees Centigrade °F - degrees Fahrenheit Fe - iron gm - gram(s) ICP - Inductively Coupled Plasma m - meter Mg - magnesium min - minutes ml - milliliter(s) P - phosphorus ppm - parts per million % - percent PV - pore volume RH - relative humidity SA - surface area sec - seconds TV - total volatiles wt - weight EXAMPLE I (Amorphous Silicas Used) The silicas used in the following Examples are listed in Table II, together with their relevant properties. Four samples of typical degummed soybean oil were analyzed by inductively coupled plasma ("ICP") emission spectroscopy for trace contaminants. The results are shown in Table III. EXAMPLE I (Treatment of Oil A with Various Silicas) Oil A (Table III) was treated with several of the silicas listed in Table II. For each test, a volume of Oil A was heated to 100°C and the test silica was added in the amount indicated in the second column of Table IV. The mixture was maintained at 100°C with vigorous stirring for 0.5 hours. The silica was separated from the oil by filtration. The treated, filtered oil samples were analyzed for trace contaminant levels (in ppm) by ICP emission spectroscopy. The results, shown in Table IV, demonstrate that the effectiveness of the silica samples in removing phospholipids from this oil is correlated to average pore diameter as well as to the type and amount of silica used. EXAMPLE II (Treatment of Oil B with Various Silicas) Oil B (Table III) was treated with several of the silicas listed in Table II according to the procedure described in Example I. Samples 13-17 were all a uniform particle size of 0,074-0.149mm (100-200 mesh (U.S.)). The results, shown in Table V, demonstrate that the effectiveness of the silica samples in removing phospholipids from this oil was correlated to average pore diameter as well as to the type and amount of silica used. EXAMPLE III (Treatment of Oil C with Various Silicas) Oil C (Table III) was treated with several of the silicas listed in Table II according to the procedures described in Example I. The results, shown in Table VI, demonstrate that the effectiveness of the silica samples in removing phospholipids from this oil is correlated to average pore diameter as well as to the type of silica used. EXAMPLE IV (Filtration Rate Studies in Soybean Oil) The practical application of the adsorption of phospholipids onto amorphous silicas as described herein includes the process step in which the silica is separated from the oil, permitting recovery of the oil product. The procedures of Example I were repeated, using Oils B or D (Table III) with various silicas (Table II), as indicated in Table VII. Silicas 5A and 9A (Table VII) are wetted versions of silicas 5 and 9 (Table II), respectively, and were prepared by wetting the silicas to incipient wetness and drying to the % total volatiles indicated in Table VII. The filtration was conducted by filtering 50.0 gm oil containing either 0.4 wt.% (dry basis silica) (for the 25°C oil samples) or 0.3 wt.% (dry basis silica) (for the 100°C oil samples) through a 5.5 cm diameter Whatman #1 paper at constant pressure. The results, shown in Table VII, demonstrate that silicas with total volatiles levels of over 30 wt.% exhibited significantly improved filterability, in terms of decreased time required for the filtration. EXAMPLE VII (Treatment of Oil C at Various Temperatures) The procedures of Example II were repeated, using Oil C (Table III) and Silicas 5 and 7 (Table II), and heating the oil samples to the temperatures indicated in Table VIII. The results, shown in Table VIII, demonstrate the effectiveness of the process of this invention at temperatures of 25 to 100°C.

Claims: Use of amorphous silica having an effective average pore diameter of greater than 6 nm (60 Å) in which the average pore diamter (APD) is calculated from the measured pore volume and surface area using the equation for the removal of phospholipids and metal ions from glyceride oils having a phosphorus content of about 230 to about 1 ppm in the absence of any solvent and without the addition of oxygen/phosphoric acid at temperatures at which the respective oils are liquid and at a concentration of the amorphous silica (calculated on a dry weight basis after ignition at 954°C) of 0.01 to 1.0% by weight, based on the weight of the oil processed. Use according to claim 1 in which said glyceride oil is degummed oil. Use according to claims 1 or 2 in which said glyceride oil is soybean oil. Use according to claims 1 to 3 in which said average pore diameter is between 6 and 500 nm (60 and 5000 Å). Use according to claims 1 to 4 in which an amorphous silica is utilized wherein the effective average pore diameter is provided by an artificial pore network of interparticle voids having diameters of 6 to 500 pm (60 to 5000 Å). Use according to claim 5 in which said amorphous silica is provided by aggregated particles of non-porous silica. Use according to claims 1 to 6 in which said amorphous silica is selected from the group consisting of silica gels, precipitated silicas, dialytic silicas, and fumed silicas. Use according to claim 7 in which said silica gel is a hydrogel. Use according to claims 1 to 8 in which the water content of said amorphous silica is greater than 30% by weight. Use according to claims 1 to 9 in which said amorphous silica has a surface area of up to about 1200 m²/g. Use according to claims 1 to 10 in which said amorphous silica comprises minor amounts of inorganic constituents. Use according to claims 1 to 11 in which the phospholipid-depleted oil is subsequently treated with bleaching earth.

Inventors: PARENT, YVES OVILA, WELSH, WILLIAM ALAN
Applicants: W.R. GRACE & CO.-CONN. (A CONNECTICUT CORP.)
Application year: 1985
Main classification: C11B3
IPC codes: B01D15, B01J20, C11B3