EP0200982B1 - Process for the catalytic interesterification of fatty-acid glycerides with lower alkanols - Google Patents

Abstract

Solid sodium carbonate and/or sodium hydrogen carbonate is/are used as heterogeneous solid catalyst(s) in the transesterification of de-acidified and anhydrous fatty acid glycerides, more especially from fats and/or oils of natural origin, with lower monofunctional alcohols to form fatty acid alkylesters and glycerol.

Description

The invention relates to an improved process for the preparation of esters of fatty acids or fatty acid mixtures and lower monofunctional alcohols by catalytic transesterification of fatty acid glycerides with the lower monofunctional alcohols in the presence of basic catalysts. The corresponding C, _-4- alcohols are suitable as lower monofunctional alcohols, methanol being of decisive technical importance in practice. Fatty acid glycerides in the sense of the teaching according to the invention are corresponding triglycerides or any partial esters of fatty acids or fatty acid mixtures and glycerol. Of particular importance here are the triglycerides and, above all, fats and / or oils of native origin which can be converted into fatty acid methyl esters in a simple manner by the process according to the invention.

Fatty acid methyl esters are of great technical importance as a starting material for the production of fatty alcohols and other oleochemical products such as ester sulfates, fatty acid alkanolamides and soaps. The industrial production of the fatty acid methyl esters takes place predominantly by catalytic transesterification (alcoholysis) of fatty acid triglyceride mixtures, as are present in the fats and oils of vegetable and / or animal origin.

In practice, various tried and tested methods are available for this transesterification. The choice of the particular process conditions depends to a considerable extent on the amount of fatty acids present in the triglycerides.

Native fats and oils almost always contain considerable amounts of free fatty acids, whereby the respective value - depending on the origin of the material and its history - can vary within a wide range and is almost always above 3% by weight.

It is known to transesterify fats and oils with a higher content of free fatty acids in the presence of alkali or zinc catalysts at 240 ° C. under a pressure of about 100 bar with a 7- to 8-fold molar excess of methanol to give the corresponding fatty acid methyl esters (Ullmann, Encyclopedia of Technical Chemistry, 4th Edition, Volume 11, 1976, page 432).

It is also known to convert fats and oils at the lower temperatures of 25 to 100 ° C. and under normal pressure with a limited excess of monofunctional alcohols in the presence of alkaline catalysts to give the corresponding fatty acid alkyl esters and free glycerol. A corresponding method is described as the first stage of soap production in US Pat. No. 2,360,844. Alcoholic solutions, in particular methanolic solutions, of NaOH or KOH (caustic soda or potash) or corresponding alcoholic solutions of sodium or potassium methylate are used here as basic catalysts. The basic catalyst is therefore homogeneously dissolved in the reaction mixture and is consumed in the course of the further process to form the corresponding sodium or potassium fatty acid soaps.

However, this alkali-catalyzed pressure-free transesterification of fatty acid glycerides to the corresponding fatty acid alkyl esters requires the use of fats or oils which are practically or largely anhydrous and whose free fatty acid content is below 0.5% by weight (corresponding to an acid number of about 1) . Since there are almost always larger amounts of water and fatty acids in technical fats and oils, the unpressurized transesterification requires drying and almost always a pretreatment to reduce the acid number. B. by converting the free fatty acids present into the corresponding alkyl or glycerol esters, the acid number of the feed is reduced to the required extent.

Such pre-esterification of acidic fatty acid glycerides can be carried out in several ways. For example, it is possible to work in the presence of alkaline catalysts from 240 ° C. and 20 bar (Ullmann, Encyclopedia of Industrial Chemistry, 4th edition, volume 11, 1976, page 432). However, this type of pre-esterification with methanol in turn requires the use of expensive reactors. It is also known to esterify the free fatty acids in the oil with added monofunctional lower alcohols, especially methanol, in a homogeneous phase by means of acid catalysis, for. B. using p-toluenesulfonic acid as a catalyst. However, the subsequent removal of the catalyst with simultaneous removal of water by washing the pre-esterified oil with methanol is then required. For example, the teaching of DE-OS 3319 590 deals with the problems that arise here. An improved method for reducing the content of free acids in fats and / or oils by treating them with a lower monoalcohol in the presence of acidic esterification catalysts is the subject of Aug. 27, 1986 published European patent application EP-A-192 035. The process is characterized in that solid cation exchange resins in acidic form are used as catalysts. Here, the catalyst is present as a heterogeneous solid phase, the separation of which from the reaction mixture can be carried out without difficulty.

From BE-A-506 421 a transesterification process is known in which anhydrous fatty acid glycerides are treated with an alcohol at elevated temperatures and in the presence of a transesterification catalyst, then the catalyst is removed in the cold, the excess alcohol is distilled off and finally the fatty acid ester obtained from the glycerol separates. The Ver used catalysts, sodium alkyl carbonates and sodium alkyl sulfites, are almost neutral and practically insoluble in the cold.

The teaching of the present invention is based on the fact that fatty acid glycerides are reacted with lower monofunctional alcohols, in particular with methanol, with basic catalysis without problems under mild reaction conditions, in particular at ambient pressure, low temperatures and low use of methanol to give the corresponding alkyl esters and free glycerol can and that for the preconditions set here - low contents of the feed material in free fatty acids and water - there are sufficient practical possibilities of implementation. The inventors have set themselves the task of modifying this base-catalyzed transesterification of the fatty acid glycerides with monofunctional alcohols, in particular methanol, in such a way that solids which are practically insoluble in the feedstock or reaction mixture are used as catalysts - and thus with heterogeneous catalysis. It is obvious that considerable simplifications are obtained in this way, in particular when the catalyst is separated from the reaction mixture.

The solution to the problem according to the invention is based on the surprising finding that solid sodium carbonate Na ₂ CO ₃ and / or sodium hydrogen carbonate NaHCO ₃ are suitable as heterogeneous solid catalysts, the desired alcoholic cleavage of fatty acid glycerides under the mild conditions known per se, in particular of low pressures and temperatures catalyze effectively.

Accordingly, the invention relates in a first embodiment to the use of solid sodium carbonate and / or sodium bicarbonate as heterogeneous solid catalysts in the transesterification of deacidified to an acid number of at most 1 and to a water content of at most 0.8% by weight of anhydrous fatty acid glycerides with im essential anhydrous monofunctional C _'-4 alcohols to fatty acid alkyl esters and glycerol.

In a further embodiment, the invention relates to a process for the catalytic transesterification of fatty acid glycerides, in particular fats and / or oils of natural origin, with monofunctional Cl-4-alkanols by reacting the acidified to an acid number of at most 1 and to a water content of at most 0.8 % By weight of anhydrous glyceride starting material with the essentially anhydrous alkanol in the presence of a catalyst which is practically insoluble in the cold, in the range from atmospheric pressure and moderately elevated temperatures above 60 ° C., preferably in the range of the boiling point of the alkanol, with subsequent separation of the glycerol released. The new process is characterized in that solid sodium carbonate and / or solid sodium bicarbonate is used as a heterogeneous solid catalyst.

In the preferred embodiment of the process according to the invention, the process is carried out under energy-saving and cost-saving conditions in such a way that the fatty acid glyceride feedstock is reacted in the range of normal pressure and at only moderately elevated temperatures which do not substantially exceed the range of about 100.degree. The preferred measures to keep the excess of the monofunctional alcohol low and to effect the separation of the reaction mixture by means of physical phase separation, and to use distillation separation steps preferably only where they are unavoidable, serve in particular the goal of saving costs.

The process temperature in the transesterification is preferably in the range of the boiling point of the alkanol used. Since work is carried out at normal pressure or only slightly elevated pressures, the reaction temperature for the transesterification of the glycerides with methanol is usually in the range from about 60 to 75 ° C., preferably in the range from about 65 to 70 ° C.

Suitable fatty acid glycerides are in particular appropriately pretreated (deacidified) fats and / or oils of vegetable and / or animal origin. They are subjected to the transesterification with monofunctional alkanols with 1 to 4 carbon atoms. Methanol is preferably used as the lower alkanol. The glyceride feed material should have an acid number of at most 1, preferably acid number of at most 0.7, it being particularly expedient to work with acid numbers in the range of about 0.5 or less. The water content of the glyceride feed should be as low as possible; it is not more than about 0.8% by weight, it is preferred to work under practically anhydrous conditions. Deacidified and anhydrous glyceride feed material can easily be obtained as a reaction product of the upstream process stages described in the introduction - in particular as a product of a pre-esterification of the starting material.

The excess of monofunctional alcohol introduced into the transesterification stage of the glycerides is also restricted as far as possible in the sense of the invention. In the preferred embodiment, the reaction mixture uses weight ratios of alkanol - preferably methanol - to fatty acid glyceride in the range from 0.2 to 1: 1 and particularly preferably in the range from 0.2 to 0.5: 1.

All known process modifications can be used for the practical use of the sodium carbonate and / or sodium bicarbonate present in the heterogeneous solid phase. The heterogeneously catalyzed transesterification can be carried out batchwise or continuously. The catalyst can be either a finer or coarser powder, in Form of chips or tablets or as impregnation catalyst applied to a support material. The catalyst material can be arranged as a fixed bed, it is also possible to work with a moving bed of catalyst material, for example in stirred tanks, in a fluidized bed or fluidized bed in pulsation devices and the like. It is particularly expedient to carry out the heterogeneously catalyzed transesterification at normal pressure with low boiling point of the alcohol.

The released glycerol is preferably separated from the reaction mixture by phase separation. For this purpose, it may be expedient to cool the reaction mixture or a partial stream branched off from the reaction mixture. It is advantageous to evaporate part of the monofunctional alcohol from the reaction mixture or from the branched-off partial stream of the reaction mixture before this cooling. As a result, the solubility of the released glycerol in the fatty acid ester / alcohol / oil phase is reduced and at the same time the density of the glycerol phase is increased, so that the resulting glycerol can be easily removed by phase separation. The alkanol drawn off from the reaction mixture in this way is preferably returned to the reactor in a circuit. In this procedure, the partial removal of the glycerol from the reaction mixture is carried out in such a way that when a partial stream, in particular continuously branched out of the reactor, is freed from the methanol and then separated from free glycerol, the methyl ester / oil phase returned to the reactor is a homogeneous liquid phase is, d. H. that neither a separate methanol phase nor a phase consisting of free glycerol occurs in the recirculated liquid phase.

Accordingly, a particularly preferred embodiment of the invention consists in first passing at least a partial stream of the reactant mixture through an evaporator in which the free alkanol present is evaporated at least partially in the continuous process. The liquid phase is then cooled to temperatures below 50 ° C., in particular to temperatures in the range from 30 to 40 ° C., whereupon the heavier glycerol phase is separated and discharged by phase separation, while part of the lighter ester phase is recycled into the transesterification as a recycle stream the evaporated alkanol portion and fresh reactants are fed in simultaneously.

It is possible to facilitate the separation of the glycerol phase from the circulation stream by means of additional separation aids, for example by using coalescence separators. Together with the glycerol phase, any water or soaps present or obtained are removed from the reactor.

In a preferred embodiment of the process according to the invention, the transesterification is carried out in several stages in a reactor cascade.

The process according to the invention is particularly suitable as an important process step in the processing of natural in particular unpurified fats and / or oils such as coconut oil, palm kernel oil, soybean oil, tallow and the like. It is not necessary to clean these raw materials from the naturally contained sludge and mucilage. The natural starting material is first subjected to pre-esterification as described at the beginning. The process carried out with solid cation exchange resins according to the applicant's older EP-A-192 035 is particularly suitable here. The material obtained in this preliminary stage with acid numbers preferably below 0.7 contains, in addition to methanol, the excess from the pre-esterification also water components which can be partially or completely expelled as a mixture. By eliminating the water of reaction from the pre-esterification, there is a feedstock for the process according to the invention which can be subjected directly to the transesterification in the sense of the invention. The prior separation of the sludge and mucilage is not necessary. It is brought about in the anyway necessary distillative treatment of the fatty acid methyl esters according to the invention.

Examples example 1

1,000 g of pre-esterified coconut oil (acid number of the treated material 0.57) and 500 g of methanol were in the presence of 10 g of Na ₂ CO ₃ powder in a stirred tank (stirrer speed n = 350 rpm) under normal pressure and gentle boiling under reflux of the Condensate (reaction temperature 69 ° C) implemented within a period of one hour. After cooling and settling of the glycerol phase, the quantitative ratio of the fatty acid methyl ester phase: glycerol phase was 3.1: 1. In the fatty acid methyl ester phase, the proportion of bound glycerol was from 13% by weight to 0.2% by weight. -% lowered.

Example 2

In a stirred kettle with a capacity of 2.5 l (stirrer speed 800 rpm), 2 kg of pre-esterified and anhydrous coconut oil together with 1 kg of methanol were reacted discontinuously for 2 hours with gentle boiling. 400 g of dried soda chips (average particle size 1 to 5 mm) were swirled as a catalyst in the reaction mixture. Then 0.5 l / h of pre-esterified coconut oil (acid number = 0.43; 13% by weight bound glycerol) and 0.24 l / h of methanol were fed to the reactor in a continuous process. A recycle stream of 5.6 l / h was withdrawn from the reactor gene and largely freed of methanol in an evaporator. After cooling to 35 ° C., a heavier glycerine / methyl ester phase was withdrawn from this recycle stream from a separator in a controlled manner via a control valve so that the liquid level in the reactor remained constant. The undrawn portion, a low-glycerol fatty acid methyl ester phase, was returned to the reactor as a recycle stream. The methanol separated in the evaporator was also returned to the reactor in a circuit. The reaction in the reactor was carried out with gentle boiling of the methanol and at normal pressure.

The heavier glycerol / methyl ester phase drawn off from the recycle stream in the first separator separated into a glycerol and a fatty acid methyl ester phase in a separating vessel. In the case of stationary operation, the proportion of bound glycerol in the methyl ester phase after the glycerol separation was approximately 0.5% by weight. The values were reproducible over several days without changing the catalyst.

Claims (8)

1. The use of solid sodium carbonate and/or sodium hydrogen carbonate as heterogeneous solid catalysts in the transesterification of fatty acid glycerides deacidified to an acid value of at most 1 and anhydrous to a water content of at most 0.8 % by weight, more especially from fats and/or oils of natural origin, with substantially anhydrous monofunctional C₁-C₄ alcohols to fatty acid alkyl esters and glycerol.

2. A process for the catalytic transesterification of fatty acid glycerides, more especially of fats and/or oils of natural origin, with monofunctional C,-C₄ alkanols by reaction of the glyceride starting material deacidified to an acid value of at most 1 and anhydrous to a water content of at most 0.8 % by weight with the substantially anhydrous alkanol in the presence of a catalyst substantially insoluble in the cold, under normal pressure and at moderately elevated temperatures above 60 °C, preferably in the range of the boiling point of the alkanol, followed by separation of the glycerol released, characterized in that solid -sodium carbonate and/or sodium hydrogen carbonate is/are used as heterogeneous solid catalyst (s).

3. A process as claimed in claim 2, characterized in that the solid catalysts are used in the form of a powder, more especially dispersed in the mixture of reactants, or in the form of a particulate material optionally applied to a support, more especially in the form of fixed-bed catalysts.

4. A process as claimed in claims 2 and 3, characterized in that de-acidified, substantially anhydrous fatty acid glycerides having an acid value of at most 0.7 are used.

5. A process as claimed in claims 2 to 4, characterized in that the ratio by weight of alkanol to fatty acid glyceride in the reaction mixture is in the range from 0.2 :1 to 1 : 1 and preferably in the range from 0.2 1 to 0.5 : 1 and in that methanol is preferably used as the alkanol.

6. A process as claimed in claims 2 to 5, characterized in that the glycerol released is removed from the reaction mixture by cooling and phase separation, alkanol initially being evaporated and a phase containing free glycerol then being separated off.

7. A process as claimed in claims 2 to 6, characterized in that, where it is carried out continuously, at least one sidestream of the reaction mixture is passed through an evaporator in which any free alkanol present is at least partly evaporated, after which the liquid phase is cooled to temperatures below 50 °C and more especially to temperatures of the order of 30 to 40 °C, the heavier glycerol phase is then separated off by phase separation and removed from the circuit while part of the lighter ester phase is returned as a recycle stream to the transesterification stage into which the evaporated alkanol and fresh reactants are simultaneously introduced.

8. A process as claimed in claims 2 to 7, characterized in that the transesterification is carried out in several stages in a cascade of reactors.