JP5058459B2 - Method for producing fatty acid alkyl ester - Google Patents

Description

  The present invention relates to a method for producing a fatty acid alkyl ester (also referred to as “fatty acid ester”). More specifically, the present invention relates to a method for producing a fatty acid alkyl ester used as a biodiesel fuel without a catalyst.

  Biodiesel fuel generally abbreviated as “BDF” is a transesterification of fatty acid triglyceride (also referred to as “fatty acid glyceride”), which is a main component of vegetable oil, animal oil or waste oil (for example, waste cooking oil). Or it is the fuel which consists of fatty acid ester obtained by esterifying the fatty acid obtained by hydrolysis, and can be used for the vehicle which has a diesel engine, a ship, an agricultural industrial machine, a generator, etc.

  Compared to light oil, this biodiesel fuel has less sulfur oxides that cause graphite and acid rain in its exhaust gas, and generates less suspended particulate matter (PM). Since it is a fuel derived from biomass resources, it has already been used as an alternative to fossil fuels because it has the advantage of not disrupting the global carbon balance. In addition, several methods for industrially producing fatty acid esters from fats and oils have been developed, and this method can be roughly divided into an alkali catalyst method, an acid catalyst method, and a lipase enzyme method.

  The alkali catalyst method is a method for obtaining a target fatty acid methyl ester by adding a methanol and a basic catalyst to an oil and fat and conducting a transesterification reaction. This method allows the reaction to proceed under relatively mild temperature and pressure conditions, but requires a step of removing the alkali catalyst in the purification step. In addition, the free fatty acid in the raw oil and fat reacts with the alkali catalyst to produce alkali soap, and the water in the raw oil and fat reduces the catalytic function, resulting in a decrease in ester yield. Yes.

  The acid catalyst method does not generate alkali soap like the alkali catalyst method, but, like the alkali catalyst method, the catalytic function is lowered by the moisture in the raw oil and fat, and the reaction rate is also slow. Therefore, it is difficult to make an industrial process.

  The lipase enzyme method is a method of converting raw fats and oils into biodiesel fuel by the catalytic action of the lipase enzyme, and there is an advantage that neutralization of the product is unnecessary and it is not affected by free fatty acids in the raw material. However, control of the amount of methanol added is essential, and there are problems such as slow reaction rate and high cost.

  For these production methods, the present inventor has proposed a technique for producing a fatty acid ester under non-catalytic conditions. For example, in Patent Document 1, a fatty acid ester composition is produced without a catalyst by performing a transesterification reaction and an esterification reaction using raw material fats and oils in a supercritical or subcritical alcohol at a high temperature and high pressure as a solvent. The technology to do is proposed. In this technology, there is a reverse reaction in which the fatty acid alkyl ester and glycerin react to return to the fatty acid monoglyceride. Therefore, it is necessary to use a large excess of alcohol to tilt the reaction in the direction of fatty acid alkyl ester formation, Pressure conditions were severe and there was room for improvement.

  Patent Document 2 and Non-Patent Document 1 disclose techniques for improving the technique shown in Patent Document 1. More specifically, hydrolysis is performed by coexisting raw material fats and oils containing fatty acid triglycerides and water to obtain fatty acids and glycerin from the fatty acid triglycerides, and alcohol is added to the product of the first step, and a predetermined temperature is obtained. A second process (that is, an esterification process) for converting the fatty acid in the product into a fatty acid alkyl ester under pressure conditions (hereinafter referred to as “non-catalytic / two-stage process”). Proposed.

In this non-catalytic two-step method, after the first step, glycerin is separated and removed to effectively prevent the reverse reaction in the second step, and water in the fatty acid obtained from the first step is removed. By setting it, the esterification reaction in the second step can proceed more preferentially. For this reason, fatty acid alkyl ester can be manufactured efficiently. This method is particularly useful as an industrial process for producing fatty acid alkyl esters using raw oils and fats such as waste oil containing water and free fatty acids.
JP 2000-204392 A. PCT International Publication WO 03/106604. Journal of the Japan Institute of Energy, Vol. 84, 413-419 (2005).

  In order to improve the overall production efficiency of the non-catalytic and two-stage process for fatty acid alkyl esters disclosed in Patent Document 2 and Non-Patent Document 1, it is necessary to increase the reaction rate in the first step (hydrolysis step). There is. For this purpose, a method of adding an acid catalyst such as sulfuric acid or hydrochloric acid to the reaction system of the first step can be considered, but in this method, a step of removing the acid catalyst is added after the first step, and the process becomes complicated. Above all, the essential advantages of the present production process without catalyst are lost.

  Therefore, the main object of the present invention is to increase the reaction rate and improve the production efficiency of fatty acid alkyl esters while maintaining the catalyst-free idea of the non-catalytic and two-stage production method of fatty acid alkyl esters. .

  The inventors of the present application further improve the reaction rate of the step of obtaining fatty acid and glycerin by hydrolyzing fatty acid triglyceride contained in the raw oil and fat with supercritical water or subcritical water with increased water ion product. We have earnestly researched to achieve this. As a result, it was newly found that the fatty acid acts as a catalyst (acid catalyst) in this step by using a predetermined amount of the fatty acid contained in the product of the above step once again in this step. Thus, it was found that the fatty acid alkyl ester can be continuously produced while maintaining the reaction rate of the first step improved under supercritical water or subcritical water conditions at a higher level. That is, the present inventors have newly found that the reaction rate of the step of hydrolyzing fatty acid triglyceride to obtain fatty acid and glycerin can be further increased without adding an acid catalyst from the outside.

Therefore, in the method for producing a fatty acid alkyl ester of the present invention, the fatty acid triglyceride contained in the raw oil and fat is hydrolyzed with supercritical water or subcritical water under a predetermined temperature and pressure condition to obtain a fatty acid and glycerin; A phase separation step in which a reaction solution containing the product obtained from the first step is phase-separated into an aqueous phase and an oil phase, and glycerin is separated into an aqueous phase and a fatty acid is separated into an oil phase from the products; Adding alcohol to the oil phase recovered by the phase separation step , and performing at least a second step of converting a fatty acid contained in the oil phase into a fatty acid alkyl ester under a predetermined temperature and pressure condition, From the oil phase obtained by the phase separation step, 3 to 70% by weight of the total fatty acid contained in the product of the first step is separated and returned to the first step for use .

In the present invention, after the first step, the phase separation process of the product of the step, ie the product becomes rows separating into an oil phase and an aqueous phase, the fatty acid to remove the glycerin in the aqueous phase It isolate | separates into an oil phase and devise so that glycerin may not be introduced into a subsequent 2nd process. By this device, for example, the occurrence of a reverse reaction in which glycerin and the fatty acid alkyl ester which is the target product of the second step react to return to the fatty acid monoglyceride can be effectively prevented. The production reaction can be effectively advanced.

On the other hand, in the production method of the present invention, the first step can be performed, for example, at a temperature of 150 to 300 ° C. and a pressure of 5 to 25 MPa. In the present invention, the phase separation step is devised so as to be performed while maintaining the temperature and pressure conditions of the first step. That is, by separating the phase as it is without cooling the product after the first step, there is no need to reheat and repressurize the oil phase (fatty acid) before the second step. Improvement can be achieved.

Further, in the present invention, the amount of alcohol added pressure Te second step smell, in volume ratio fatty, preferably to 0.3 to 10, more it is 0.3-2.0 Konomashikui . For example, the pressure in the reaction system was lower than the critical pressure by vaporizing a portion of said alcohol, the reaction was carried out in the concentration of fatty acids relatively high condition to alcohol than supercritical conditions, fatty acids To improve the acid catalyst effect. If such a device is implemented, the acid catalyst effect can be enhanced while relaxing the pressure condition in the second step (esterification step), which is preferable.

  In the present invention, “oil and fat” includes at least one of fatty acid triglyceride, fatty acid diglyceride, fatty acid monoglyceride and fatty acid. “Fatty acid alkyl ester” is a fatty acid that was originally contained in raw material fats and oils and fatty acid triglycerides in raw material fats and oils that were hydrolyzed and esterified with added alkyl alcohol, and the alkyl alcohol It means a fatty acid ester having an alkyl group derived therefrom.

  According to the present invention, it is possible to increase the reaction rate while maintaining the idea of using no catalyst in the non-catalytic and two-stage production method of fatty acid alkyl ester, and to improve the production efficiency of fatty acid alkyl ester which is the final object. it can. In addition, since the fatty acid serves as an internal catalyst, unlike the case where a catalyst is added separately, there is no need to separate and purify the product after completion of production, which is very advantageous in terms of the production process.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, preferred embodiments for carrying out the invention will be described with reference to the accompanying drawings. Each embodiment shown in the accompanying drawings shows an example of a typical embodiment of the method according to the present invention, and the scope of the present invention is not interpreted narrowly.

  First, FIG. 1 is a diagram showing the concept of basic steps constituting the method for producing a fatty acid alkyl ester according to the present invention.

As shown in FIG. 1, the basic process of this production method includes at least a two-stage process of a first process P ₁ and a second process P ₂ , and a catalyst is separately added in any process. The basic flow of all the steps is performed in the order of the first step P ₁ → the phase separation step → the second step P ₂ .

Figure 2 is a diagram showing a basic reaction scheme of the first step P ₁ of the production method. The first step P _1, as shown in Figure 2, a fatty acid triglyceride included in the raw material oil or fat is hydrolyzed to give compound liberated fatty acids and glycerol.

In the first step _{P 1,} for example, a temperature 150 to 300 ° C., particularly 250 ° C. to 300 ° C., a pressure 5～25MPa, especially in subcritical water condition of 7～20MPa, 15 to 60 minutes, particularly preferably 20 to Run for 40 minutes. Such the first step the product obtained from P _1, there is an advantage that the fatty acid triglyceride unreacted hardly remains.

  In FIG. 2, fatty acid triglyceride is shown as a representative example, but this is not intended to limit the present invention. Fatty acid diglyceride and fatty acid monoglyceride that can be included in the raw oil and fat also undergo hydrolysis reaction as in FIG. Is generated. In addition, the “pressurized hot water” in the reaction formula shown in FIG. 2 means the subcritical water, but is not limited to it, but is supercritical water or subcritical water at low temperature and low pressure. Widely includes water.

Subsequently, the reaction solution containing the product obtained from the first step P ₁ and allowed to stand, to achieve a phase separation into an oil phase and an aqueous phase (phase separation step, see FIG. 1). The oil phase separated by this step contains fatty acids and one aqueous phase contains glycerin, a by-product. By separating and recovering the oil phase, to recover the fatty acids to be used in the subsequent second step P _2. Note that this fatty acid, which contains free fatty acid that was initially contained in the fatty acid and the raw material oil and fat which had been produced by the hydrolysis first step P _1.

Here, the water in the oil phase obtained by the phase separation process remains follows from a part of the second step fatty acid ester in P ₂ returns to fatty acids undergo hydrolysis, as far as possible from the oil phase It is preferable to remove the water. In consideration of the energy efficiency of the entire manufacturing process, it is not preferable to cool the product after hydrolysis (first step P ₁ ) to room temperature. Therefore, in the present invention, the phase separation step is devised to be performed near the hydrolysis temperature (for example, 250 to 300 ° C.) (see Example 2 described later). This eliminates the need to cool the product and provides the advantage that the energy for heating again for the esterification reaction (second step P ₂ ) is then unnecessary.

Next, alcohol was added to the oil phase recovered by the phase separation step, the second step P ₂ (see FIG. 1). Figure 3 shows the basic reaction scheme of the second step P ₂ of the manufacturing method. The second step P ₂ is a step of esterifying the fatty acid with methanolysis (solvolysis) ability of supercritical or subcritical alcohol. The “pressurized hot alcohol” shown in FIG. 3 means a supercritical alcohol or a subcritical alcohol.

More particularly, the second step P _2, the the first step the oil phase containing the free fatty acid contained originally in the fatty acid and the raw material oils and fats in the product of P ₁ alcohol (e.g., such as methanol An esterification step of converting a fatty acid into a fatty acid alkyl ester under a temperature / pressure condition that is a supercritical or subcritical condition, for example, a temperature of 200 to 300 ° C. and a pressure of 7 to 20 MPa. It is. The process time is, for example, 10 to 60 minutes, more preferably 15 to 25 minutes.

  Here, alcohol means linear alcohols such as methanol, ethanol, 1-propanol, 2-propanol, and 1-butanol, and branched alcohols such as isobutyl alcohol, 2-butanol, t-butanol, and allyl alcohol. And represented by the general formula ROH (R is a hydrocarbon group containing 1 to about 24 carbon atoms).

  Here, the supercritical state of alcohol refers to a state in which the temperature in the reaction system is equal to or higher than the critical temperature (Tc) of alcohol and the pressure is equal to or higher than the critical pressure (Pc) of alcohol. The subcritical state is a state in which the temperature in the reaction system is not less than the boiling point of the alcohol and generally not less than 150 ° C., and the pressure is not less than the vapor pressure of the alcohol at the reaction temperature and approximately not less than 2.0 MPa. To tell.

Further, in this manufacturing method, as shown in FIG. 1, after the first step P ₁ by interposing the phase separation step, most of the glycerin is removed into the aqueous phase from the product, glycerin second step It is devised so as not to be introduced almost to P _2. Thus, in the second step P ₂ to perform the esterification reaction, it can be effectively prevented progression of the reverse reaction and the fatty acid alkyl esters of glycerin and the target product of the second step P ₂ is returned to the reaction to the fatty acid monoglyceride. As a result, the fatty acid alkyl ester production reaction can be effectively advanced, and the fatty acid alkyl ester can be produced in a high yield.

  Here, it is also possible to produce a fatty acid alkyl ester from a fatty acid triglyceride by a one-step reaction by transesterification. However, in this conventional method, it is necessary to add a large excess of alcohol to the reaction system in order to prevail the formation reaction of the target fatty acid alkyl ester.

On the other hand, in the case of the esterification reaction of a fatty acid in the present invention (second step P ₂ ), the fatty acid itself obtained from the hydrolysis is an acid as in the hydrolysis reaction of a fatty acid triglyceride (first step P ₁ ). Acts as a catalyst. For this reason, conversely, the esterification reaction can proceed efficiently by lowering the alcohol concentration and relatively increasing the fatty acid concentration.

  For example, the concentration of fatty acid is relatively decreased by a method in which the amount of alcohol added is reduced or a method in which the pressure of the reaction system is lowered and a part of the low boiling alcohol is vaporized to lower the concentration of the alcohol. Thus, the esterification reaction can proceed more efficiently.

FIG. 4 is a diagram showing a preferred embodiment example of the first step P ₁ employed in the manufacturing method according to the present invention.

The first step P ₁ suitable in the present invention, the use of a portion of the fatty acids contained in the product of the first step P ₁ and return to the first step P ₁ (feedback to) It is a feature. The returned fatty acids, acts as an acid catalyst in the first step P _1, functions to promote the hydrolysis reaction first step P _1.

For example, 3 to 70 wt% of the total fatty acid content of the first step _{P 1} is in the product, by more preferably to return the 10 to 50% by weight of a fatty acid, and more preferably, 30 to the total fatty acid content by returning 40% by weight of a fatty acid, the reaction time was shortened by increasing the reaction rate of the hydrolysis reaction the first step P _1, and can increase the fatty acid yield.

In the test according to Example 1, in the case of performing the return of fatty If you did not return (Example) and fatty acids (comparative example), is what effect the first step P ₁ fatty acid yield It was verified whether it occurred. FIG. 5 is a drawing substitute graph showing the results of this test.

Here, the plot of “no fatty acid addition” in the graph of FIG. 5 represents a change in the amount of fatty acid produced when rapeseed oil (fatty acid triglyceride) was treated in pressurized hot water at 270 ° C./20 MPa ( The volume ratio of water: oil is 1: 1). In this case, the increase in the fatty acid yield was moderate at the beginning of the reaction (when the reaction time was 20 minutes or less), but when the reaction time was 20 minutes or more, the fatty acid yield increased rapidly. As a result, the fatty acid itself produced by the hydrolysis reaction first step P ₁ is, functions as an acid catalyst, can be considered because it promotes the hydrolysis of rapeseed oil (fatty acid triglyceride).

  Therefore, when 10% by weight of fatty acid is added to rapeseed oil, which is a raw material fat and oil in advance (see the plot of “Addition of 10% by weight of fatty acid” in FIG. 5), the fatty acid yield is reduced even in the reaction time zone of 20 minutes or less. A rapid increase in rate could be confirmed. In this experiment, oleic acid is added.

  In addition, based on these experimental results, the reaction equation when it is assumed that the fatty acid functions as an acid catalyst can be expressed as follows.

_{C FA} is in Equation 1 above, the molar concentration of fatty acid _{^{(mol / m 3), C}} TG, C DG and _{C MG} are fatty acid triglycerides, respectively, the molar concentration of fatty acid diglycerides and fatty acid monoglycerides (mol ^/ m 3), C _W is the molar concentration ^(mol / m 3) of water, _{C G} represents the molar concentration of glycerin (mol / ^{m 3),} respectively. In addition, based on this Formula 1 (equation), the result of having carried out the simulation calculation about the change of the fatty acid yield at the time of adding 40 weight% of a fatty acid was shown in FIG. 5 (refer the uppermost curve in FIG. 5).

From the above examination results, it is possible to propose a reaction process of “feeding back the fatty acid itself produced in fat hydrolysis (first process P ₁ ) to the first process P ₁ to function as an acid catalyst”. .

  Next, FIG. 6 is a diagram illustrating an example of reaction conditions when the fatty acid is not returned, and FIG. 7 is a diagram illustrating an example of reaction conditions when the fatty acid is returned. However, all have unified the internal volume of the reaction tube to 288 ml.

  In the case where the fatty acid is not returned (comparative example), the temperature and pressure conditions are 290 ° C./15 MPa and a residence time (reaction time) of about 40 minutes is required for the hydrolysis reaction to reach equilibrium (see FIG. 6). At this time, the feed rates of water and fat become 178 g / h and 162 g / h, respectively. As a result, 158 g of fatty acid having a purity of 91% as a reaction product and 13 g of glycerin as a by-product are produced. (See FIG. 6).

Return contrast, as shown in FIG. 7, when used in the first step P ₁ and return 30% of the generated fatty acids (Example), temperature and pressure conditions of 270 ° C. / 15 MPa (fatty The residence time (reaction time) for the hydrolysis reaction to reach equilibrium was shortened to 25 minutes even when the temperature condition was lower by 20 ° C. than when not. At this time, the feed rates of water and fat were 300 g / h and 189 g / h, respectively. As a result, 183 g of fatty acid having a purity of 94% was produced as a reaction product per hour and 17 g of glycerin as a by-product was produced. .

  That is, when compared in plants of the same scale (same reaction tube volume), in this example, (1) the purity of the fatty acid produced is improved by returning (feedback) the fatty acid, and (2) unit time. The per-fatty acid production increases, and (3) relaxation of the reaction temperature and reduction of the reaction time can be achieved.

  In addition, in this invention, although the method of using a fatty acid as an acid catalyst in the two-stage reaction of hydrolyzing raw material fats and then esterifying is proposed, fatty acid alkyl ester is converted from raw material fats and oils by one step by transesterification. In the obtaining step, it is also possible to add a predetermined amount of fatty acid to the reaction system, to cause the fatty acid to act as an acid catalyst, and to improve the reaction rate of the step.

Experiments to verify the separation behavior of the water and the fatty acid in the phase separation step performed between the first step P ₁ and the second step P ₂ was carried out. Specifically, using a high-pressure cell with a sapphire visible window (pressure 20 MPa), the separation behavior of water and fatty acids in the phase separation step was directly observed. The result is shown in the drawing substitute photograph of FIG.

As can be seen from the results shown in FIG. 8, water and fatty acids are well phase separated in the range from room temperature to 295 ° C. On the other hand, under the condition of 305 ° C., water and fatty acid were not phase separated. Thus, it became clear that water and fatty acids can be phase-separated even at temperatures around 295 ° C. Therefore, without cooling the product after the first step P _1, it was possible to verify that can be directly phase separation. That is, in the manufacturing method according to the present invention, since the second step P ₂ is not necessary to reheat the oil phase (fatty acid) before, it is possible to improve the energy efficiency of the entire manufacturing process.

An experiment was conducted to verify the relationship between "ester yield" and "volume ratio of methanol: fatty acid" when fatty acid was esterified in supercritical methanol at a reaction temperature of 270 ° C and a pressure of 20 MPa (second step P ₂ ). . FIG. 9 is a drawing substitute graph showing the results of this experiment.

As shown in FIG. 9, when the reaction time was the same, the ester yield increased as the amount of methanol added to the fatty acid decreased. This is because the lower the methanol, the higher the fatty acid concentration and the higher the acid catalyst effect of the fatty acid. Therefore, the volume ratio of methanol to fatty acid is possible in the range of 0.3 to 10, more esterification reaction volume methanol is small proceeds quickly, particularly preferably in the range of 0.3 to 2.0 . That is, in the esterification reaction (second step P ₂ ) of the production method according to the present invention, it was found that the amount of alcohol used is small, so that the production process is energy efficient.

  Note that when the fatty acid functions as an acid catalyst in the esterification reaction, the theoretical reaction rate equation is expressed by the following Equation 2.

In Formula 2 described above, C _FAME , C _FA , C _M and C _W represent the molar concentrations (mol / m ³ ) of fatty acid ester, fatty acid, methanol and water, respectively. The change in fatty acid ester yield calculated based on Equation 2 is shown by a broken line in FIG.

  Next, an experiment was conducted to verify the relationship between the reaction pressure and the ester yield in the esterification reaction. However, the reaction temperature is 270 ° C., and the volume ratio of methanol: fatty acid is 1.8: 1. FIG. 10 is a drawing substitute graph showing the results of this experiment.

In the reaction of transesterifying fatty acid triglycerides to obtain a fatty acid alkyl ester, the reaction is more efficient when the reaction pressure is higher. However, in the esterification reaction (second step P ₂ ) employed in the present invention, as shown in FIG. 10, the ester yield does not change much at a pressure exceeding 20 MPa, but in a pressure region of 20 MPa or less, The lower the pressure, the higher the ester yield.

Thus, in the esterification reaction of fatty acids according to the present invention, it is advantageous that the pressure is lower in the range of 7 to 20 MPa. For this reason, it became clear that the reaction pressure of esterification can be lowered in the present invention. More specifically, in the case of producing a fatty acid alkyl ester by employing a transesterification reaction, the ionic product of alcohol is increased under supercritical conditions to develop alcoholysis ability, but this effect is more effective as the pressure is higher. On the other hand, in the esterification reaction (second step P ₂ ) employed in the present invention, although the alcoholysis ability works as in the transesterification reaction, the action of the fatty acid as an acid catalyst is greater, and the lower the pressure, the more methanol It can be explained that this is because the fatty acid concentration is relatively increased due to the relative decrease in the volume ratio of the acid, and therefore the acid catalyst effect is improved. Therefore, according to the method for producing a fatty acid alkyl ester according to the present invention, it is possible to reduce the equipment cost of the production plant, relax the pressure condition, and the like, and achieve an improvement in the energy efficiency of the entire process.

  INDUSTRIAL APPLICABILITY The present invention can be used as a technique for efficiently producing a fatty acid alkyl ester used as a biodiesel fuel without using a catalyst.

It is a figure which shows the concept of the basic process which comprises the manufacturing method of the fatty-acid alkylester which concerns on this invention. It is a diagram showing a basic reaction scheme of the first step P ₁ in the same manufacturing process. It is a diagram showing a basic reaction scheme of the second step P ₂ in the same manufacturing process. The more preferred embodiment of the first step P ₁ employed in the manufacturing method according to the present invention. FIG. 3 is a drawing substitute graph showing the results of a test according to Example 1. FIG. It is a figure which shows an example of reaction conditions in case fatty acid is not returned. It is a figure which shows an example of reaction conditions at the time of returning fatty acid. It is a drawing substitute photograph which shows the result of the experiment (Example 2) which verifies the separation behavior of water and a fatty acid in a phase separation process. 10 is a drawing substitute graph showing the results of an experiment related to Example 3. 10 is a drawing substitute graph showing the results of an experiment according to Example 4. FIG.

Explanation of symbols

P ₁ 1st process P ₂ 2nd process

Claims (5)

A first step of obtaining fatty acid and glycerin by hydrolyzing fatty acid triglyceride contained in the raw oil and fat with supercritical water or subcritical water under predetermined temperature and pressure conditions;
A phase separation step in which a reaction solution containing the product obtained from the first step is phase-separated into an aqueous phase and an oil phase, and glycerin is separated into an aqueous phase and fatty acid is separated into an oil phase in the product;
A second step of adding an alcohol to the oil phase recovered by the phase separation step, and converting a fatty acid contained in the oil phase into a fatty acid alkyl ester under a predetermined temperature and pressure condition;
Having at least
From the oil phase obtained by the phase separation step, the fatty acid of 3 to 70% by weight of the total amount of fatty acid contained in the product of the first step is separated and returned to the first step for use. A method for producing a fatty acid alkyl ester. The method for producing a fatty acid alkyl ester according to claim 1, wherein the first step is performed at a temperature of 150 to 300 ° C and a pressure of 5 to 25 MPa. The method for producing a fatty acid alkyl ester according to claim 1 or 2 , wherein the phase separation step is performed while maintaining the temperature and pressure conditions of the first step. The method for producing a fatty acid alkyl ester according to any one of claims 1 to 3, wherein an amount of the alcohol added in the second step is 0.3 to 10 in terms of a volume ratio with respect to the fatty acid. In the second step, the pressure in the reaction system is made lower than the critical pressure of the alcohol, whereby a part of the alcohol in the reaction system is vaporized so that the fatty acid concentration relative to the alcohol is higher than in the supercritical condition. The method for producing a fatty acid alkyl ester according to any one of claims 1 to 4 , wherein the fatty acid alkyl ester is produced.