Process for the production of fats and oils with reduced acidity

Abstract

A process for reducing the acid number of fats and oils, which have an acid number in the range of 1 to 250, to acid numbers below 1, characterized by feeding the fats and oils to be reduced with respect to their acid number, as well as a metal salt of fatty acids with 14 to 20 carbon atoms and a metal of group VIIIB, IB, or IIB of the periodic table of elements as a homogeneous esterification catalyst, into the upper part of a countercurrent esterification column, wherein the fats and oils have a temperature of 180 to 300 °C upon entering the column, while the column is charged from below with a mono- or polyhydric alcohol with 1 to 18 carbon atoms, wherein this alcohol evaporates upon entering the column and is additionally superheated to temperatures in the range of 150 to 300 °C, and wherein the fats and oils are discharged downwards via the internals present in the column.While the alcohol flows upwards as vapor in countercurrent flow, and the reaction water formed as well as excess alcohol are removed as vapors at the top of the column, an operating pressure in the column is set in the range of 1 to 16 bar, the mass ratio of alcohol to fat or oil is set in the range of 0.01 to 0.3, and the residence time of the liquid phase within the column is between 10 minutes and 180 minutes.

Description

Field of invention The invention relates to a method for reducing the acid number of fats and oils. State of the art Raw, untreated fats and oils of animal and vegetable origin, as well as fats and oils from recycled sources, generally contain a certain amount of free fatty acids (FFA), which is indicated by the acid value. The acid value is determined by titration: A pre-weighed quantity of the oil to be analyzed is mixed with the indicator phenolphthalein and diluted with ethanol. The mixture is then titrated with KOH solution until the indicator changes color. The acid value (AC) is calculated from the amount of KOH solution consumed and expressed in mg KOH / g sample. For the transesterification processes described in the literature, which are based on basic catalysts such as sodium or potassium methylate, oils with a low acid number must generally be used. In practice, this means that the acid number of such oils must be below 1; oils with an acid number < 0.7 are frequently used. This is due to the fact that basic catalysts, when used with oils with a higher acid number, initially form soaps with the fatty acids and are thus deactivated. Only when all the free fatty acid molecules have been saponified can the reaction be accelerated by adding more catalyst.Due to soap formation, the use of low-temperature processes based on basic catalysts for the transesterification of fats and oils is therefore economically uninteresting for feedstocks with high saturation levels for two reasons: • The catalyst consumption is very high compared to the amount required for the transesterification of oils with low saturation levels. This leads to higher production costs. • The soaps formed from the basic catalyst and the free fatty acids act as emulsifiers. These substances severely hinder the phase interaction between glycerol and fatty acid methyl esters that is necessary after the transesterification process; in some cases, phase separation may even fail completely, so that the products cannot be separated at all. Both of these circumstances mean that processes using basic catalysts generally require oils that have been deacidified through special pretreatments, and are therefore more expensive than raw, untreated oils. Methods for reducing the acid number of fats and oils to an acid number below 1 are known from German patent DE 198 03 053 C1 and from German patent application DE 25 03 195 A1. German patent application DE 39 32 514 A1 discloses a method for producing fatty acid alkyl esters by transesterification of fats and oils with methanol, characterized in that fats and oils with a free fatty acid content of less than 1% are used as raw materials. Description of the invention The object of the present invention was to provide a method that allows fats and oils with acid numbers in the range of 1 to 250 to be converted into fats and oils whose acid numbers (AN) are below 1 and preferably below 0.7. The present invention relates to a process for reducing the acid number of fats and oils having an acid number in the range of 1 to 250 to acid numbers below 1, characterized in that the fats and oils to be reduced with respect to their acid number, as well as a metal salt of fatty acids with 14 to 20 carbon atoms and a metal of group VIIIB, IB or IIB of the periodic table of elements as a homogeneous esterification catalyst, are fed into the upper part of a countercurrent esterification column (catalytic distillation column), wherein the fats and oils have a temperature of 180 to 300 °C upon entering the column, while the column is charged from below with a mono- or polyhydric alcohol with 1 to 18 carbon atoms – in particular methanol – wherein this alcohol evaporates upon entering the column and is additionally superheated to temperatures in the range of 150 to 300 °C.and wherein the fats and oils are discharged downwards via the internals present in the column, while the alcohol flows upwards in countercurrent flow as vapor, and the reaction water formed as well as excess alcohol are removed as vapors at the top of the column, wherein an operating pressure in the column is set in the range of 1 to 16 bar, the mass ratio of alcohol and fat or oil is set in the range of 0.01 to 0.3, and the residence time of the liquid phase within the column is between 10 minutes and 180 minutes. The fats and oils to be reduced with regard to the SZ can be fats and oils of animal or vegetable origin as well as fats and oils from recycling sources. Particular embodiments of the process according to the invention are: • The fats and oils are mixed with the homogeneous esterification catalyst before being fed into the countercurrent esterification column. • The homogeneous esterification catalyst is fed into the countercurrent esterification column in the form of an aqueous solution. • The acid number of the fats and oils used is in the range of 8 to 35. • The temperature at which the mixture of fat or oil and esterification catalyst is fed into the countercurrent esterification column is in the range of 210 to 225 °C. • A mass ratio of alcohol to fat or oil is set in the range of 0.01 to 0.3. • An operating pressure in the column is set in the range of 1 to 16 bar. • A metal salt of fatty acids with 14 to 20 carbon atoms and a metal from group VIIIB, IB, or IIB of the periodic table is used as the esterification catalyst.• A tray column is used as the countercurrent esterification column. • The trays of the column are selected from the group of sieve trays, valve trays, tunnel trays, bubble-cap trays and double bubble-cap trays. For the avoidance of doubt, it should be expressly stated that the pressure specification “bar” within the scope of the present invention is always to be understood as “absolute bar”, which means the absolute pressure in the apparatus, not an overpressure relative to the ambient pressure. The process according to the invention reliably reduces the acid value in fats and oils from relatively high values ​​to values ​​< 1 and, in particular, < 0.7 by reacting alcohols with 1 to 18 carbon atoms, especially methanol, with the fatty acids that cause the acidity of fats and oils, using a homogeneous catalyst. This deacidification occurs chemically through the esterification of the free fatty acids contained in the fats and oils with the countercurrently circulated alcohol, forming the fatty acid ester and water (reaction water). Since the water formed during the reaction is carried away with the alcohol phase by the countercurrent flow, chemical equilibrium is never established within the countercurrent column, and the conversion of the reactants is almost complete.The oil, thus deacidified, contains only small to no mass fractions of free fatty acids and therefore has a correspondingly low acid number. The acid-reduced fat and oil produced by the process according to the invention can be subjected to conventional transesterification processes – which involve the reaction of fats / oils with alcohols (especially methanol) to form fatty acid alkyl esters (especially fatty acid methyl esters). If these transesterification processes are high-pressure processes, the catalyst used in the process according to the invention can remain in the product. However, it may also be desirable to remove the catalyst from the product obtained by the process according to the invention, for example by washing – such a catalyst-free and acid-reduced fat or oil can readily be used in transesterification processes carried out as low-pressure processes. Important advantages of the process according to the invention are: • The acid value of crude fats and oils is reduced from relatively high values ​​to very low values ​​(acid value < 1, and in particular acid value < 0.7, and most preferably acid value < 0.5). • The free fatty acids present in the fats and oils used are preferably esterified with methanol in the process according to the invention, yielding fatty acid methyl esters. This has the advantage that the acid-value-reduced fats and oils, which now contain fatty acid methyl esters, are ideally suited for use in transesterification processes; because then fatty acid methyl esters represent a valuable product and not an undesirable byproduct. The process according to the invention is based on the use of a counterflow esterification column, wherein the fat or oil with a relatively high acid number and at a relatively high temperature is initially fed into the column in the upper part. The fat / oil feedstock has a temperature of 180-300°C upon entering the column, preferably a temperature of 200 to 250°C. An operating temperature of 210 to 225°C is particularly preferred. The acid number of the fat / oil is in the range of 1 to 250, preferably between 1 and 50, and particularly between 8 and 35. The process according to the invention provides for two feed streams: • Firstly, the fat or oil to be reduced with respect to its acid number, as well as the catalyst, are fed into the countercurrent esterification column from above. The fat / oil and catalyst can be added together or separately. Preferably, the fat / oil is mixed with the homogeneous esterification catalyst before being fed into the countercurrent esterification column, this mixture is heated, and then fed into the column. • Secondly, the alcohol, in particular methanol, is fed into the countercurrent esterification column from below. In the upper feed stream, the homogeneous catalyst is added, accelerating the esterification reaction. In principle, all esterification catalysts mentioned in the literature, such as organic or inorganic acids, are suitable. However, compounds containing elements from groups VIIIB, IB, and IIB of the periodic table are preferred. Compounds of these elements with carboxylic acids having a chain length of 2 to 24, and especially 14 to 20, carbon atoms are particularly preferred, and these carboxylic acids can be branched or unbranched. Zinc acetate and zinc salts of fatty acids with 14 to 20, and especially 16 to 18, carbon atoms are particularly preferred catalysts, as are mixtures of these salts. The amount of catalyst added depends on the catalytic activity of the chosen compound and is generally between 0.5 and 10 wt% based on the amount of crude fats / oils used. For economic reasons, the smallest possible amount of catalyst is always used. The lower feed stream consists of alcohol, in particular methanol. The alcohol is vaporized and, if desired, additionally superheated to temperatures between 150°C and 300°C, but particularly to temperatures between 230°C and 270°C. Operating temperatures between 240°C and 260°C are especially preferred. These values ​​must be adjusted depending on the catalyst used. The mass ratio between the alcohol stream (in particular: methanol stream) and the fat / oil stream used in the process according to the invention is between 0.01 and 0.3; however, a mass ratio of 0.02 to 0.15 is preferred. The specific mass ratio of the feed streams depends on the specific gravity of the fats / oils used and the desired specific gravity in the resulting fat / oil. The countercurrent esterification column is operated such that the hot fat / oil is discharged downwards via the column internals, while alcohol (especially methanol) flows upwards as vapor in the countercurrent flow. By appropriately selecting the column operating pressure in the range of 1 to 16 bar, preferably in the range of 7 to 10 bar, the alcohol (especially methanol, MeOH) reacts at the liquid-gas interface with fatty acids (FAs) using the catalyst to form fatty acid alkyl esters (especially fatty acid methyl esters, FSMe) and water (H₂O). The reaction equation for fatty acids and methanol is: FAs + MeOH → FSMe + H₂O The water released by the reaction evaporates under the conditions present in the reaction column and flows as vapors to the column head. Therefore, it is not available for a reverse reaction, meaning that chemical equilibrium between reactants and products can never be reached. Consequently, with a suitable column design, almost complete conversion is achieved, and the acid number is reliably reduced to values ​​below 1. The design of the countercurrent esterification column largely corresponds to the parameters typical for a distillation column, but in some aspects is preferably specifically adapted to the fact that a chemical reaction takes place in parallel with the separation process. In particular, the residence time of the liquid phase within the column is adjusted to the kinetics of the ongoing chemical reaction; otherwise, if a certain residence time is not reached, only incomplete conversion could be achieved. Residence times between 10 and 180 minutes are preferably targeted. Particularly preferred residence times are in the range of 40 to 100 minutes. Furthermore, the hydrodynamics of the column are preferably adapted to the vapor loads, which change with the column height, since the concurrent chemical reaction can influence the amount and composition of vapor present.To achieve a nearly constant vapor load, the column internals are preferably modified in a manner familiar to those skilled in the art from distillation technology. Compared to conventional distillation columns, this column can optionally be designed without an evaporator, since methanol is introduced into the column as vapor below the lowest column section. However, the option of adding a separate evaporator is not excluded. The process according to the invention is not limited with regard to the type of countercurrent esterification column. Tray columns are preferably used, in particular those with a number of trays in the range of 2 to 100. In one embodiment, sieve trays, valve trays, and tunnel trays are used. So-called bubble-cap trays and trays of comparable design are particularly suitable, as they allow for a particularly high liquid level on the tray. Double bubble-cap trays, as known, for example, from EP 0 332 971 B1 or DE 196 00 025 C2, are especially suitable. In the process according to the invention, the bottoms product consists of the fat / oil used, the fatty acid ester formed by the esterification, and a small amount of methanol, which dissolves in the mixture of fat / oil and fatty acid ester at the temperatures and pressures present in the column. The distillate consists of excess alcohol that was not consumed by the reaction within the countercurrent esterification column and water (which is partly reaction water and partly originates from the catalyst solution – provided that it was added in the form of an aqueous solution, see above). Depending on its composition, this distillate stream can be subjected to wastewater treatment or processed separately. Corresponding processes for the processing of methanol / water mixtures are known to those skilled in the art. Examples General Acid values ​​(AV) are given in mg KOH / g sample, as is customary. They were determined according to DGF method CV 2. Example 1 A countercurrent esterification column DN80 with 17 bubble-cap trays was charged with 5.0 kg / h of coconut oil with an acid number of 8.9. A catalyst consisting of zinc acetate and C18 fatty acids was added to the oil at a mass fraction of 2.0 wt%. The temperature of the oil and catalyst feed was 220°C. Countercurrently, the column was charged with 0.85 kg / h of methanol vapor at a temperature of 245°C. After establishing a steady-state column profile, an acid number of 0.50 was measured in the bottom stream exiting the column. The experiment was conducted at a pressure of 9.5 bar within the reaction column. Example 2 A countercurrent esterification column DN80 with 17 bubble-cap trays was charged with 2.5 kg / h of coconut oil with an acid number of 15.8. A catalyst consisting of zinc acetate and C18 fatty acids was added to the oil at a mass fraction of 2.0 wt%. The temperature of the oil and catalyst feed was 220°C. Countercurrently, the column was charged with 0.85 kg / h of methanol vapor at a temperature of 245°C. After establishing a steady-state column profile, an acid number of 0.63 was measured in the bottom stream exiting the column. The experiment was conducted at a pressure of 9.5 bar within the reaction column.

Claims

A process for reducing the acid number of fats and oils, which have an acid number in the range of 1 to 250, to acid numbers below 1, characterized by feeding the fats and oils to be reduced with respect to their acid number, as well as a metal salt of fatty acids with 14 to 20 carbon atoms and a metal of group VIIIB, IB, or IIB of the periodic table of elements as a homogeneous esterification catalyst, into the upper part of a countercurrent esterification column, wherein the fats and oils have a temperature of 180 to 300 °C upon entering the column, while the column is charged from below with a mono- or polyhydric alcohol with 1 to 18 carbon atoms, wherein this alcohol evaporates upon entering the column and is additionally superheated to temperatures in the range of 150 to 300 °C, and wherein the fats and oils are discharged downwards via the internals present in the column.While the alcohol flows upwards as vapor in countercurrent flow, and the reaction water formed as well as excess alcohol are removed as vapors at the top of the column, an operating pressure in the column is set in the range of 1 to 16 bar, the mass ratio of alcohol to fat or oil is set in the range of 0.01 to 0.3, and the residence time of the liquid phase within the column is between 10 minutes and 180 minutes. The method according to claim 1, wherein the fats and oils are mixed with the homogeneous esterification catalyst before being fed into the countercurrent esterification column. The method according to claim 1, wherein the homogeneous esterification catalyst is fed into the countercurrent esterification column in the form of an aqueous solution. Method according to any one of claims 1 to 3, wherein the acid number of the fats and oils used is in the range of 8 to 35. Method according to any one of claims 1 to 4, wherein the temperature at which the mixture of fat or oil used and esterification catalyst is fed into the countercurrent esterification column is in the range of 210 and 225 °C. Method according to any one of claims 1 to 5, wherein zinc acetate and zinc salts of fatty acids with 14 to 20 and in particular 16 to 18 carbon atoms are used as esterification catalysts. Method according to any one of claims 1 to 6, wherein a tray column is used as the countercurrent esterification column. Method according to claim 7, wherein the trays of the column are selected from the group consisting of screen trays, valve trays, tunnel trays, bubble cap trays and double bubble cap trays. Method according to any one of claims 1 to 8, wherein methanol is selected as the alcohol.