WO2003042343A1 - Esterification of fatty acids in oil - Google Patents

Abstract

The invention relates to a method for acid catalyzed esterification of fatty acids in oils and fats, i.e. esters of glycerin with fatty acids, by introducing low alcohols, especially methyl alcohol, into the liquid starting product. The invention is characterized in that the methyl alcohol or another alcohol is fed into the reaction mixture in a gaseous form. The invention also relates to embodiments of the above method.

Description

 Esterification of fatty acids in oils

The invention relates to a process for the esterification of fatty acids in oils and fats by introducing methyl alcohol or other lower alcohols into the liquid starting product.

Depending on their origin and quality, oils and fats contain a certain percentage (from approx. 1 to over 40%) of free fatty acids (FFA).

When refining fats / oils in oil refineries, FFA-containing material is usually treated with lye (mostly NaOH or KOH), whereby the FFA is saponified. This procedure is unsuitable in a biodiesel plant for various reasons.

Firstly, for economic and operational reasons, the failure of FFA using lye only works at FFA concentrations up to max. 10%. This means that fats / oils of poor quality, which are relatively inexpensive and pose a disposal problem, which is why they would be of particular interest for biodiesel production, cannot be treated satisfactorily using this method. Furthermore, the treatment with the lye (especially with high FFA contents) causes an additional hydrolysis of the fat / oil, whereby new FFA quantities are created. Finally, the treatment of the FFA precipitated in the form of soap necessitates the presence of additional processing steps (e.g. separation, distillation at high temperatures and under vacuum). The formation of inorganic salts caused by soap splitting can cause additional disposal problems.

When producing biodiesel, the FFA react with the basic transesterification catalysts, producing the soaps (saponification). The effects of this reaction are very unfavorable: high alkali consumption, increased neutralization effort, emulsion formation, lower yield.

When using FFA-containing oils and fats for the production of biodiesel, the FFA are usually esterified in a stirred tank. This reaction is usually before the transesterification reaction of the subsequently purified oil, using acidic catalysts (eg sulfuric acid). Since the esterification reaction is an equilibrium reaction, an almost complete esterification only takes place if one of the reaction products (for example water) is drawn off continuously or if one of the reactants (for example methanol) is present in excess.

It is also possible to use another lower alcohol, for example ethanol, propanol, butanol and, in special cases, even pentanol instead of the methyl alcohol and thus to obtain the corresponding esters, but methyl alcohol is mostly used for reasons of cost and because of the simpler reaction.

The reaction of the esterification differs fundamentally from the transesterification reaction. During the esterification, 1 mol of acid (organic or inorganic) reacts with 1 mol of alcohol, producing 1 mol of ester and 1 mol of water. This reaction is an equilibrium reaction, whereby the equilibrium is usually only slightly shifted towards products.

While the transesterification can be catalyzed both basic and acidic, only acidic catalysts are used in the esterification in the prior art. Sulfuric acid is mostly used for technical purposes.

The basis of the method according to the invention lies in the fact that the base-catalyzed transesterification reaction or the acid-catalyzed esterification reaction proceed much faster and more completely when the transesterification / esterification agent (methanol) is added to the fat / oil / FFA in a vaporous state. It is possible to catalyze acidic, basic and in some cases not at all, as will be explained below.

An embodiment of the process according to the invention for the esterification of the FFA with vaporous methanol can proceed as follows: The fat / oil to be treated is extracted in e.g. 80 ° C preheated column filled. At the bottom of the column

Methanol introduced in the vapor state. After the mixture is saturated with The methanol vapors begin to leave the mixture after the condensed methanol, the gas bubbles also having a stripping effect. This stripping effect appears to be of particular importance for the course of the reaction, since after the addition of catalyst (sulfuric acid) free fatty acids and methanol form fatty acid methyl esters and water, water being able to be stripped out. By stripping water, the esterification reaction is constantly shifted towards products, so that high degrees of conversion of fatty acids are possible.

The process can also be carried out continuously, e.g. constantly add the fat / oil and the catalyst to the top of the column and e.g. the fully reacted mixture is continuously removed at the bottom of the column.

The fully reacted mixture consists essentially of fat / oil, fatty methyl ester, methanol and catalyst (sulfuric acid). Since the fat / oil is immiscible with the methanol, the mixture is divided into a fat phase that also contains the fatty methyl ester and a methanol phase that contains the catalyst. The fat / oil now freed from the free fatty acids can then be easily transesterified to biodiesel.

The constant removal of one of the reaction products (in particular, e.g. water) caused by stripping contributes to the fact that the esterification of free fatty acids can be carried out in a relatively short time and with a high yield. With this method it is possible to reduce the content of fatty acids in used cooking fat from 8-10% in only 30 minutes to approx. 0.5%. In comparison with other common processes (esterification in a stirred tank at 120 ° C., esterification under high pressure), these facts bring decisive advantages to the process with vaporous methanol.

It is also important that, for example, a column without internals (thus a kind of simple tubular reactor) can be used, in which the flow through is not hindered. This promotes the discharge of the water formed during the esterification, which in turn shifts the equilibrium towards a reaction that is as complete as possible. The process according to the invention thus consists in the presence in the presence of the fats or oils which are present in liquid form and which are optionally contaminated with free fatty acids with gaseous, for example technically pure methyl alcohol which has a temperature above its boiling point at the pressure prevailing in the reactor an acidic catalyst.

A preferred variant of the invention, which relates to a continuous process, provides for e.g. at least substantially vertically arranged tubular reactor to supply the oil and the catalyst from above, while gaseous methyl alcohol is introduced from below and rises in the tubular reactor, the liquid reaction mixture which has reacted at the bottom and the excess methyl alcohol and water formed during the reaction at the top of the tubular reactor are withdrawn continuously in appropriate collecting sections. After condensing, the excess methyl alcohol, optionally cleaned and brought to the appropriate temperature and pressure, can be fed back to the tube reactor in the foot region thereof. The fully reacted reaction mixture is fed to a settling tank or a continuously operating separator, in which it essentially separates into an oil phase lying on the bottom and a floating methanol phase. When using settling tanks, there are preferably so many available that the process in the tubular reactor can be operated continuously throughout.

Sulfuric acid is preferably used as the catalyst; sulfonic acids or any other acid which can be used as an esterification catalyst can also be used. The catalyst itself is not the subject of the patent, therefore any suitable catalyst can be used, including basic and neutral catalysts.

The reaction is preferably carried out at ambient pressure, the methyl alcohol to be blown in having a temperature of about 65 ° C. (boiling point of the methyl alcohol under normal conditions) or above. In the course of the slightly exothermic reaction, the reaction mixture heats up with cooling and partial condensation of the methyl alcohol. Particularly if fats which have solidified or solidified at ambient temperature are to be treated according to the invention, it is recommended, for oils, to have a temperature To increase the starting of these starting materials so that the condensation of the methyl alcohol in the reaction mixture is negligible or even prevented.

Since the essence of the invention is essentially the gaseous nature of the alcohol, it is immaterial whether the alcohol is vaporized outside or inside the reactor. It is therefore also possible to bring the alcohol as a liquid into the reactor, where the alcohol is then e.g. the temperature of the oil / fat / FFA / etc. verdämpft. This can be particularly advantageous for designs that operate at ambient pressure (i.e. unpressurized) but at temperatures of the oil / grease / FFA / etc. work above the boiling point of the alcohol.

The invention is explained in more detail on the laboratory scale using the following exemplary embodiments:

Experiment 1: 200 g of used cooking oil (FFA content 8.86%) was blown through with methanol vapor in the stripping column until the first drops condensed on the cooler. Then 2.5 ml of 96% sulfuric acid was added while blowing in steam for the next 10 minutes. Subsequent measurement of the residual FFA content was 1.85% (i.e. the reaction yield was 79.1%).

Experiment 2: The procedure was as in experiment 1, but the reaction time after the addition of catalyst (sulfuric acid) was extended to 20 minutes. Subsequent measurement of the residual FFA content was 0.89% (i.e. the reaction yield was 89.9%).

Experiment 3: The procedure was as in experiment 2, but the reaction time after the addition of catalyst (sulfuric acid) was extended to 30 minutes. Subsequent measurement of the residual FFA content gave 0.54% (i.e. the reaction yield was 93.9%).

Experiment 4: In this experiment, the so-called FFA phase, which is produced as a by-product in the production of fatty methyl esters during the working up of the glycerol phase obtained, was used. This FFA phase contained 26.96% FFA. 200 g of this phase were used in the usual column and stripped with methanol vapor. After the After 2 g of 96% sulfuric acid, the residual FFA contents were measured after 10, 20, 30 and 45 minutes. The measurements gave the following results:

After 5 minutes: residual FFA: 7.40% yield: 72.55% After 10 minutes: residual FFA: 5.73% yield: 78.75%

After 20 minutes: residual FFA content: 1.77% yield: 93.43%

After 30 minutes: residual FFA content: 0.91%. Yield: 96.62%

After 45 minutes: residual FFA content: 0.21% yield: 99.22%

Experiment 5: In this experiment, the FFA phase was used, which contained 48.86%) FFA. Otherwise the test conditions remained unchanged. The residual FFA contents were measured after 25, 30 and 40 minutes with the following results:

After 25 minutes: residual FFA: 4.28% yield: 91.20% After 30 minutes: residual FFA: 1.86% yield: 96.20%

After 40 minutes: residual FFA content: 0.85% yield: 98.27%

The invention is not restricted to the examples described. For example, different catalysts can be used, because of the gaseous alcohol and the temperatures used, "slower ones", which only become fully effective at higher temperatures. At particularly high temperatures, it is possible to dispense with the catalyst entirely or in large part Mixtures of alcohols and mixtures of alcohol with inert gas can be used, in particular azeotropic ones, in which the transition to the gas phase is energetically favorable and the preparation is simple.

The question of pressure is not critical, but it can have an impact on the profitability of the process, the yield and the course of the reaction: the volumes become smaller, the reactivity increases, the stripping effect decreases. The reactor can have internals, fixed or movable (stirrer), tubular, standing or lying, if the appropriate flow is ensured.

In what form the alcohols are present, e.g. n-Butanol, iso-butanol etc. is of no great importance, so it is not dealt with in the description.

Claims

claims: 1. Process for the esterification of free fatty acids (FF As) in oils and fats, namely the esters of glycerol with fatty acids, by introducing lower alcohols, in particular methyl alcohol, into the liquid starting product, characterized in that the free, if appropriate Fats or oils contaminated with fatty acids are mixed with a technically pure, gaseous, low alcohol which has a temperature which is above its boiling point at the pressure prevailing in the reactor, preferably in the presence of a catalyst. 2. The method according to claim 1, characterized in that the alcohol, as known per se, is methyl alcohol. 3. The method according to claim 1 or 2, characterized in that the method is carried out continuously in an at least substantially vertical tubular reactor, that the oil and optionally the catalyst are supplied from above, that gaseous alcohol is introduced at the bottom, and that the reaction mixture which has reacted to the end and the excess alcohol and water at the top of the tube reactor are continuously withdrawn from the tube reactor in corresponding collecting sections. 4. The method according to claim 3, characterized in that the excess alcohol drawn off at the top of the tube reactor, after condensation, optionally cleaned and brought to temperature and pressure accordingly, is fed back to the tube reactor in the foot region thereof. 5. The method according to any one of claims 2 to 4; characterized in that it is carried out at ambient pressure and that the methyl alcohol is blown in at a temperature of about 65 ° C or above. 6. The method according to any one of the preceding claims, characterized in that the catalyst used, as known per se, sulfuric acid or another acid suitable as a catalyst for the esterification reaction.