JP3942833B2 - Method for producing fatty acid lower alkyl ester - Google Patents

Description

【０００９】
（式中、ｎは粗脂肪酸低級アルキルエステル中モル比で最も多く含まれる構成脂肪酸の炭素数を示す。）
【００１０】
【発明の実施の形態】
本発明に使用される油脂としては、例えばヤシ油、パーム油、パーム核油、大豆油、ナタネ油、牛脂、豚脂、若しくは魚油等の動植物油脂又はこれらの硬化油が挙げられ、特に主構成脂肪酸の炭素数が６〜１８のものが好ましい。これら油脂は、そのままあるいは精製処理したものが用いられるが、遊離脂肪酸や水分がエステル交換反応を阻害する場合には、これを精製処理によって除去しておくことが望ましい。
【００１１】
本発明において、炭素数１〜４のアルコール（以下低級アルコールという）としては、例えばメタノール、エタノール、１−プロパノール、２−プロパノール、１−ブタノール、２−ブタノール等の１価アルコールが好適に用いられるが、反応性や価格、得られる脂肪酸低級アルキルエステルの物性等の観点から、メタノールが特に好ましい。
【００１２】
エステル交換反応に用いられる触媒としては、アルカリ金属、アルカリ土類金属及びこれら金属のアルコキシド、水酸化物、炭酸塩等の化合物や、固体酸触媒等が挙げられる。
【００１３】
本発明においては、まず原料油脂と低級アルコールとを、触媒の存在下に、エステル交換反応させる。低級アルコールは、油脂１モルに対して、好ましくは３〜２０モル、更に好ましくは３〜１０モル用いる。触媒の量は、種類にもよるが、油脂に対して、０．１〜１．０重量％が好ましく、０．２〜０．５重量％が更に好ましい。反応温度は、原料にもよるが、一般に４０〜２００℃が好ましく、５０〜１００℃が更に好ましい。
【００１４】
このエステル交換反応においては、原料油脂の主成分である脂肪酸トリグリセリドと、低級アルコールとの反応によって、脂肪酸低級アルキルエステルの他に、脂肪酸ジグリセリド、脂肪酸モノグリセリド及びグリセリンが生じる。反応後の混合物からグリセリン及び低級アルコールを主成分とする相を分相除去し、粗脂肪酸低級アルキルエステルを得る。
【００１５】
エステル交換反応により得られた粗脂肪酸低級アルキルエステルは、残留するエステル交換触媒やグリセリン、及び未反応の低級アルコール等を除去するために、必要に応じて水洗や濾過を行う。
【００１６】
９０％を越えるエステル交換反応の収率が得られるような条件では、エステル交換反応工程を経た粗脂肪酸低級アルキルエステル中に脂肪酸トリグリセリドはほとんど含まれない。しかしエステル交換反応が可逆過程であることから、粗脂肪酸低級アルキルエステル中には、通常脂肪酸モノグリセリド及び脂肪酸ジグリセリドが含まれる。これらは脂肪酸低級アルキルエステルと逆反応して脂肪酸トリグリセリドになるので、脂肪酸低級アルキルエステルを消費してしまうこのような反応は極力避けねばならない。従って、本発明においては、粗脂肪酸低級アルキルエステル中の上記式（Ｉ）で表されるＴＧ比は１以下になることが必要である。エステル交換反応工程を経た後にＴＧ比が１を越えているような粗脂肪酸低級アルキルエステルは、既に脂肪酸低級アルキルエステルを逆反応により相当量失っており、かつこれを蒸留精製した場合、沸点の高い脂肪酸トリグリセリドが多いために蒸留塔底部の温度が上昇しやすく、硫黄分の塔頂側への留出を抑えるために蒸留収率を下げなければならない。
【００１７】
尚、ＴＧ比は、脂肪酸トリグリセリド（ＴＧ）及び脂肪酸モノグリセリド（ＭＧ）の含量を正しく測定できる方法により算出することができるが、例えば、予め標準試薬によって校正されたガスクロマトグラフィーにより、これらの含量を測定し、下記▲１▼〜▲４▼の操作によりＴＧ比を得ることができる。
▲１▼ ガスクロマトグラフィーにエステル交換反応終了後の粗脂肪酸低級アルキルエステルを注入する。
▲２▼ 出てきたピークにより、ＭＧ中の最大含量を示す炭素鎖長のＭＧを選び出す。
▲３▼ その炭素鎖長からのみなるＴＧピークを選ぶ。
▲４▼ 上記ＴＧと上記ＭＧのピーク比に標準試薬から割り出した係数を乗じてモル比（ＴＧ比）を求める。
【００１８】
本発明において、ＴＧ比が１以下の粗脂肪酸低級アルキルエステルは、蒸留精製工程によって硫黄分を低減し、高級アルコール製造用原料として好適な脂肪酸低級アルキルエステルとする。脂肪酸低級アルキルエステル中の硫黄分の量は、例えばRosemount Analytical, Inc.製Dohrmann型低濃度硫黄分析器等により測定される。蒸留塔への粗脂肪酸低級アルキルエステルの供給はバッチ式でも連続式でもよいが、生産効率の観点から連続式が好ましい。粗脂肪酸低級アルキルエステルの組成及び硫黄分に応じて蒸留の条件を制御することで、蒸留して得られる脂肪酸低級アルキルエステル中の硫黄分濃度は、数mg／kg以下にまで低減することができる。塔底に残った残渣中には残りの硫黄分が濃縮される。
【００１９】
蒸留精製工程の塔底部においては、上述の脂肪酸モノグリセリド及び脂肪酸ジグリセリドの逆反応が進行する。蒸留精製前にＴＧ比を１以下に抑えられた粗脂肪酸低級アルキルエステルであっても、蒸留精製工程において上記逆反応による脂肪酸低級アルキルエステルのある程度のロスは避けられない。かくして蒸留残渣中には未反応の脂肪酸モノグリセリド、脂肪酸ジグリセリド及び脂肪酸トリグリセリドもまた濃縮されることになる。
【００２０】
本発明においては、このような蒸留残渣の一部を回収し、先のエステル交換反応工程に添加することで、原料油脂と同時に残渣中未反応分をエステル交換反応させ、脂肪酸低級アルキルエステルの収量を増加させることができる。残渣中に除去されたはずの硫黄分もまた回収されてしまうので、これは一見無駄な行為のようで、事実粗脂肪酸低級アルキルエステル中の硫黄分濃度も上昇する。しかし驚くべきことに、これを蒸留精製した脂肪酸低級アルキルエステルは硫黄分濃度を残渣回収前同様に低く抑えることができる。
【００２１】
蒸留残渣を回収してエステル交換反応工程に添加すると、蒸留される脂肪酸低級アルキルエステルの量が増えるので、回収前と同じ量の脂肪酸低級アルキルエステルを留出させる場合、残渣中に残す脂肪酸低級アルキルエステルの量が増え、塔底部温度が下がって留出する硫黄分は低減される。したがって、硫黄分濃度が同じになる蒸留条件では脂肪酸低級アルキルエステルの収量が増えるのである。
【００２２】
蒸留残渣回収の効果としては、回収前と同じ硫黄分濃度の脂肪酸低級アルキルエステルを得る蒸留条件では、回収前に比べて塔底部温度が低くなる。これは、脂肪酸低級アルキルエステル相当の蒸留物性を有する硫黄分が、残渣を回収することで粗脂肪酸低級アルキルエステル中にある程度増えたためである。塔底部温度が下がると、留出分に対する塔底残渣分の量が増加するが、連続で蒸留を行う場合、残渣が増加することで塔底部の滞留時間が短くなる。先に述べた残渣中未反応分の脂肪酸トリグリセリドへの逆反応は、塔底部の温度及び滞留時間によって促進される。残渣の回収を行った時の蒸留条件は、これらをいずれも緩和する方向に変化し、逆反応による脂肪酸低級アルキルエステルのロスが低減されるのである。
【００２３】
油脂を原料として低級アルコールとのエステル交換反応により得た粗脂肪酸低級アルキルエステルには、脂肪酸低級アルキルエステルや未反応グリセリド、残留グリセリン及び低級アルコールの他に、脂肪酸低級アルキルエステルとしては回収不可能な微量成分が含まれている。例えばパーム油中にはカロテノイド、スクアレン、ステロール類、トコフェロール、トコトリエノール等が知られている（加藤秋男編著「パーム油・パーム核油の利用」幸書房刊）。これらの中には、蒸留残渣としてエステル交換を経ても殆ど損失せずに濃縮されていくものがある。したがって蒸留残渣の全部を回収し続けると、エステル交換反応工程における処理量が増え続けることになる。定常的に安定した運転を行うためには、蒸留残渣のうちの一部を回収せずに系外に抜き出し続けることが望ましい。精製後の脂肪酸低級アルキルエステル中の硫黄分濃度を一定にするような条件での、定常状態における残渣の回収量及び系外への抜き出し量の関係を、一例として図１に示す。蒸留残渣を回収することで系外への蒸留残渣の抜き出し量が減る、すなわち脂肪酸低級アルキルエステルの収量が増えるが、回収する割合を増やすに従い、抜き出し量の減り方は小さくなっていくことがわかる。蒸留残渣の回収量はエステル交換反応工程における処理量の増加を意味する。エステル交換反応工程への容量負荷を抑えながら効果的に脂肪酸低級アルキルエステルの収量を増やすには、効率の観点から、蒸留残渣中、回収してエステル交換反応工程に添加する割合が９５質量％以下（但し、０質量％を除く）であることが望ましく、２０〜９０質量％であることが更に望ましい。
【００２４】
蒸留精製工程において、留出する硫黄分を抑えて硫黄分の少ない脂肪酸低級アルキルエステルを得るためには、蒸留残渣中にある程度脂肪酸低級アルキルエステルを残すことが好ましい。脂肪酸低級アルキルエステルをほとんど全部留出させると、塔底部には脂肪酸トリグリセリドを始めとする沸点の高い成分のみが濃縮されることになり、塔底部温度は大きく上昇する。その結果、塔頂側へ留出する硫黄分も大幅に増え、目的に適った脂肪酸低級アルキルエステルは得られ難い。従って、蒸留残渣中に１５質量％以上の脂肪酸低級アルキルエステルが含まれることが望ましく、２０質量％以上含まれることがさらに好ましい。尚、ある程度の脂肪酸低級アルキルエステルが蒸留残渣中に残っていても、残渣の一部又は全部が回収されてエステル交換反応工程に添加されるので、残渣として系外に抜き出されてしまう脂肪酸低級アルキルエステルの量は低く抑えることができる。
【００２５】
本発明の方法により製造される脂肪酸低級アルキルエステルは、硫黄分濃度が数mg／kg以下と低く、エステル還元触媒の存在下に水素で接触還元して高級アルコールを製造するための原料として好適に用いることができる。また上記高級アルコールの製造にあたり、脱硫触媒によって脂肪酸低級アルキルエステルの硫黄分を低減する場合にも、本発明の方法により製造された脂肪酸低級アルキルエステルを用いれば、上記エステル還元触媒の活性寿命を維持する上で効果がある。
【００２６】
【実施例】
実施例中では、脂肪酸メチルエステルを化合物としてとらえる場合には、（純分）という表記を行う。
【００２７】
実施例１
原料油脂として精製ヤシ油４５６２ｇ、触媒として水酸化ナトリウム１０％を含むメタノール溶液３５６ｇ及び低級アルコールとしてメタノール２１３０ｇを用い、５０℃で１時間エステル交換反応を行った。その後、４５６ｇの水により水洗し、６６７Ｐａ、４５℃の条件で低沸点分の蒸発除去を行って粗脂肪酸メチルエステルを得た。この粗脂肪酸メチルエステル中のＴＧ比は０であった。
【００２８】
この粗脂肪酸メチルエステルを、塔頂圧４００Ｐａ、精留塔段数５、還流比０．５の条件で蒸留精製を行い、９６．５質量％を塔頂側へ留出させ、蒸留残渣（初回は１６１ｇで、２回目以降は１６５ｇ）のうち１００ｇを回収しエステル交換反応工程に添加した。その後エステル交換反応及び蒸留精製を同様に繰り返して、１回の反応及び精製につき脂肪酸メチルエステル（純分）４５００ｇを得た。回収５回目の脂肪酸メチルエステル中の硫黄分を、アンテック社製トレースレベル全硫黄分析計7000TSにて測定した。結果を表１に示す。
【００２９】
実施例２
蒸留精製工程における留出分の割合が９７．２質量％である以外は実施例１と同様の操作により、１回の反応及び精製につき脂肪酸メチルエステル（純分）４５２７ｇを得た。
【００３０】
比較例１
蒸留精製工程における留出分の割合が９８．７質量％であり、かつ蒸留精製工程において発生する蒸留残渣を回収せず抜き出した点を除いて、実施例１と同様の操作により、１回の反応及び精製につき脂肪酸メチルエステル（純分）４４９９ｇを得た。
【００３１】
比較例２
エステル交換反応工程の後に、水酸化ナトリウム１０％を含むメタノール溶液４８ｇを加え、メタノールの蒸発除去を行いながら反応させて未反応の脂肪酸モノグリセリドをジグリセリド及びトリグリセリドとし、かつ蒸留精製工程における留出分の割合が９６．０質量％である以外は実施例１と同様の操作により、１回の反応及び精製につき脂肪酸メチルエステル（純分）４４９５ｇを得た。
【００３２】
実施例１〜２及び比較例１〜２の結果を表１にまとめて示す。
【００３３】
【表１】

【００３４】
上記結果から明らかなように、得られる脂肪酸低級アルキルエステルの量が同じ場合で比べると、本発明の実施例では硫黄分濃度の低い脂肪酸低級アルキルエステルが得られる。また留分の脂肪酸低級アルキルエステル中の硫黄分濃度が同じ場合で比べると、本発明の実施例では脂肪酸低級アルキルエステルの収量が増える。
【図面の簡単な説明】
【図１】 蒸留精製後の脂肪酸低級アルキルエステル中の硫黄分濃度が一定の条件での、蒸留残渣のエステル交換反応工程への回収量と系外への抜き出し量の関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing a fatty acid lower alkyl ester, which is suitable as a raw material for producing alcohol by an ester reduction catalytic reaction, in a high yield.
[0002]
[Prior art and problems to be solved by the invention]
In order to produce higher alcohols using fats and oils mainly composed of fatty acid triglycerides, in general, fats and lower alcohols are transesterified into fatty acid lower alkyl esters, which are then hydrogenated in the presence of an ester reduction catalyst. A catalytic reduction method is used.
[0003]
In the step of performing the transesterification reaction, the crude fatty acid lower alkyl ester obtained by separating the glycerin-containing phase after the reaction and washing or filtering as necessary is usually at least several mg / kg to several tens mg / kg. Contains sulfur. When the crude fatty acid lower alkyl ester is subjected to catalytic reduction with hydrogen in the presence of an ester reduction catalyst as it is, a trace amount of sulfur contained in the raw material ester acts as a catalyst poison, and the active life of the ester reduction catalyst is significantly reduced. Therefore, it is necessary to purify the crude fatty acid lower alkyl ester to remove sulfur as much as possible.
[0004]
As a purification method of the crude fatty acid lower alkyl ester, a method using a normal distillation (for example, Kreutzer, UR, JAOCS, 61, 343 (1984)) or a desulfurization catalyst (for example, JP-A-5-978) is employed. However, when the crude fatty acid lower alkyl ester is purified by distillation, there is a problem in that the reaction between the unreacted fatty acid monoglyceride or fatty acid diglyceride and the fatty acid lower alkyl ester occurs at the bottom of the column, and the fatty acid lower alkyl ester is consumed. In the case of purification using a desulfurization catalyst, the same problem occurred when distillation was used in the preceding stage in order to maintain the active life of the desulfurization catalyst.
[0005]
As a method of producing a fatty acid lower alkyl ester having a high purity, glycerin, fatty acid monoglyceride and fatty acid diglyceride contained in the crude fatty acid lower alkyl ester are converted to fatty acid triglyceride and then purified by distillation, and the distillation residue is recovered in the transesterification step. A method of adding is disclosed in JP-A-3-200743. However, according to this method, the fatty acid lower alkyl ester is consumed by the conversion of the unreacted product to the fatty acid triglyceride, and the amount of sulfur compounds distilled to the top of the column increases due to the increase in the bottom temperature of the distillation column. Became more serious and unsuitable for producing fatty acid lower alkyl esters with low sulfur content.
[0006]
An object of the present invention is to provide a method for producing a fatty acid lower alkyl ester having a low sulfur content in a high yield.
[0007]
[Means for Solving the Problems]
In the present invention, an oil and fat and an alcohol having 1 to 4 carbon atoms are subjected to a transesterification reaction in the presence of a catalyst to obtain a crude fatty acid lower alkyl ester, followed by distillation purification to produce a fatty acid lower alkyl ester. In the crude fatty acid lower alkyl ester, the crude fatty acid lower alcohol was distilled so that the TG ratio represented by the formula (I) was 1 or less and the distillation residue contained 10.5% by mass or more of the fatty acid lower alkyl ester. This is a method for producing a fatty acid lower alkyl ester in which a part of the alkyl ester distillation residue is recovered and added to the transesterification reaction step.
[0008]
[Expression 2]

[0009]
(In the formula, n represents the number of carbon atoms of the constituent fatty acid contained most in the molar ratio in the crude fatty acid lower alkyl ester.)
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Examples of the fats and oils used in the present invention include animal and vegetable fats and oils such as coconut oil, palm oil, palm kernel oil, soybean oil, rapeseed oil, beef tallow, pork fat and fish oil, and hardened oils thereof. A fatty acid having 6 to 18 carbon atoms is preferred. These fats and oils are used as they are or after being purified, but when free fatty acids or moisture inhibit the transesterification reaction, it is desirable to remove them by purification.
[0011]
In the present invention, monohydric alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol and 2-butanol are preferably used as the alcohol having 1 to 4 carbon atoms (hereinafter referred to as lower alcohol). However, methanol is particularly preferred from the viewpoints of reactivity, price, and physical properties of the obtained fatty acid lower alkyl ester.
[0012]
Examples of the catalyst used in the transesterification reaction include alkali metals, alkaline earth metals, alkoxides of these metals, hydroxides, carbonates, and other compounds, solid acid catalysts, and the like.
[0013]
In the present invention, first, the raw oil and fat and the lower alcohol are subjected to a transesterification reaction in the presence of a catalyst. The lower alcohol is preferably used in an amount of 3 to 20 mol, more preferably 3 to 10 mol, per 1 mol of the oil or fat. The amount of the catalyst is preferably 0.1 to 1.0% by weight, more preferably 0.2 to 0.5% by weight based on the fat or oil, although it depends on the type. Although the reaction temperature depends on the raw material, it is generally preferably 40 to 200 ° C, more preferably 50 to 100 ° C.
[0014]
In this transesterification reaction, fatty acid diglyceride, fatty acid monoglyceride and glycerin are produced in addition to the fatty acid lower alkyl ester by the reaction of the fatty acid triglyceride which is the main component of the raw oil and fat with the lower alcohol. A phase mainly composed of glycerin and lower alcohol is removed from the mixture after the reaction to obtain a crude fatty acid lower alkyl ester.
[0015]
The crude fatty acid lower alkyl ester obtained by the transesterification is washed with water or filtered as necessary in order to remove the remaining transesterification catalyst, glycerin, unreacted lower alcohol and the like.
[0016]
Under conditions such that a yield of transesterification exceeding 90% is obtained, the fatty acid triglyceride is hardly contained in the crude fatty acid lower alkyl ester that has undergone the transesterification step. However, since the transesterification reaction is a reversible process, the fatty acid monoalkyl glyceride and the fatty acid diglyceride are usually contained in the crude fatty acid lower alkyl ester. Since these react reversely with the fatty acid lower alkyl ester to become a fatty acid triglyceride, such a reaction that consumes the fatty acid lower alkyl ester must be avoided as much as possible. Therefore, in the present invention, the TG ratio represented by the above formula (I) in the crude fatty acid lower alkyl ester needs to be 1 or less. A crude fatty acid lower alkyl ester having a TG ratio exceeding 1 after the transesterification reaction step has already lost a considerable amount of the fatty acid lower alkyl ester by a reverse reaction, and has a high boiling point when purified by distillation. Since the amount of fatty acid triglycerides is large, the temperature at the bottom of the distillation column tends to rise, and the distillation yield must be lowered to suppress the distillation of sulfur to the column top side.
[0017]
The TG ratio can be calculated by a method capable of correctly measuring the content of fatty acid triglyceride (TG) and fatty acid monoglyceride (MG). For example, these contents can be calculated by gas chromatography calibrated in advance with a standard reagent. The TG ratio can be obtained by measurement and the following operations (1) to (4).
(1) The crude fatty acid lower alkyl ester after the transesterification reaction is injected into the gas chromatography.
{Circle around (2)} The carbon chain length MG showing the maximum content in the MG is selected based on the peak that appears.
(3) Select a TG peak consisting only of the carbon chain length.
(4) The molar ratio (TG ratio) is determined by multiplying the peak ratio of TG and MG by the coefficient determined from the standard reagent.
[0018]
In the present invention, a crude fatty acid lower alkyl ester having a TG ratio of 1 or less reduces the sulfur content by a distillation purification step, and is a fatty acid lower alkyl ester suitable as a raw material for producing a higher alcohol. The amount of sulfur in the fatty acid lower alkyl ester is measured by, for example, a Dohrmann type low concentration sulfur analyzer manufactured by Rosemount Analytical, Inc. The supply of the crude fatty acid lower alkyl ester to the distillation column may be either a batch type or a continuous type, but a continuous type is preferred from the viewpoint of production efficiency. By controlling the distillation conditions according to the composition and sulfur content of the crude fatty acid lower alkyl ester, the concentration of sulfur in the fatty acid lower alkyl ester obtained by distillation can be reduced to several mg / kg or less. . The remaining sulfur content is concentrated in the residue remaining at the bottom of the column.
[0019]
The reverse reaction of the above-mentioned fatty acid monoglyceride and fatty acid diglyceride proceeds at the bottom of the distillation purification step. Even if it is a crude fatty acid lower alkyl ester in which the TG ratio is suppressed to 1 or less before distillation purification, some loss of the fatty acid lower alkyl ester due to the reverse reaction is unavoidable in the distillation purification step. Thus, unreacted fatty acid monoglyceride, fatty acid diglyceride and fatty acid triglyceride are also concentrated in the distillation residue.
[0020]
In the present invention, a part of such distillation residue is recovered and added to the previous transesterification reaction step, so that the unreacted portion in the residue is transesterified simultaneously with the raw oil and fat, and the yield of fatty acid lower alkyl ester is increased. Can be increased. Since the sulfur content that should have been removed in the residue is also recovered, this seems to be a seemingly useless action, and in fact the sulfur content in the crude fatty acid lower alkyl ester also increases. Surprisingly, however, the fatty acid lower alkyl ester obtained by distillation purification can keep the sulfur concentration as low as before the residue recovery.
[0021]
When the distillation residue is recovered and added to the transesterification reaction step, the amount of fatty acid lower alkyl ester to be distilled increases, so when distilling the same amount of fatty acid lower alkyl ester as before recovery, the fatty acid lower alkyl remaining in the residue The amount of ester is increased, and the sulfur content distilled off at the bottom temperature is reduced. Therefore, the yield of fatty acid lower alkyl esters increases under distillation conditions where the sulfur concentration is the same.
[0022]
As an effect of collecting the distillation residue, the column bottom temperature is lower than that before the recovery under the distillation conditions for obtaining the fatty acid lower alkyl ester having the same sulfur concentration as that before the recovery. This is because the sulfur content having distillative properties corresponding to the fatty acid lower alkyl ester increased to some extent in the crude fatty acid lower alkyl ester by recovering the residue. When the column bottom temperature decreases, the amount of the column bottom residue with respect to the distillate increases. However, when distillation is continuously performed, the residue increases and the residence time at the column bottom decreases. The reverse reaction to the unreacted fatty acid triglyceride in the residue described above is promoted by the temperature at the bottom of the column and the residence time. The distillation conditions at the time of collecting the residue are changed to alleviate these, and the loss of the fatty acid lower alkyl ester due to the reverse reaction is reduced.
[0023]
Crude fatty acid lower alkyl esters obtained by transesterification with lower alcohols starting from fats and oils cannot be recovered as fatty acid lower alkyl esters in addition to fatty acid lower alkyl esters, unreacted glycerides, residual glycerin and lower alcohols. Contains trace components. For example, carotenoids, squalene, sterols, tocopherols, tocotrienols, etc. are known in palm oil (edited by Akio Kato, “Use of Palm Oil / Palm Core Oil”, published by Koshobo). Some of these are concentrated as a distillation residue with little loss even after transesterification. Therefore, if the entire distillation residue is continuously recovered, the amount of treatment in the transesterification reaction step will continue to increase. In order to perform a stable operation in a steady manner, it is desirable to continue to extract out of the system without collecting a part of the distillation residue. FIG. 1 shows an example of the relationship between the amount of residue recovered and the amount withdrawn out of the system in a steady state under the condition that the concentration of sulfur in the fatty acid lower alkyl ester after purification is constant. By collecting the distillation residue, the extraction amount of the distillation residue to the outside of the system is reduced, that is, the yield of the fatty acid lower alkyl ester is increased, but it can be seen that the method of reducing the extraction amount decreases as the recovery rate increases. . The recovered amount of distillation residue means an increase in the throughput in the transesterification reaction step. In order to effectively increase the yield of the fatty acid lower alkyl ester while suppressing the capacity load on the transesterification reaction step, from the viewpoint of efficiency, the proportion of 95% by mass or less recovered and added to the transesterification step in the distillation residue (However, 0% by mass is excluded), preferably 20 to 90% by mass.
[0024]
In the distillation purification step, it is preferable to leave the fatty acid lower alkyl ester to some extent in the distillation residue in order to obtain a fatty acid lower alkyl ester having a low sulfur content while suppressing the sulfur content to be distilled. When almost all the fatty acid lower alkyl ester is distilled, only the components having a high boiling point such as fatty acid triglyceride are concentrated at the bottom of the column, and the temperature at the bottom of the column is greatly increased. As a result, the sulfur content distilled off to the tower top side is greatly increased, and it is difficult to obtain a fatty acid lower alkyl ester suitable for the purpose. Therefore, it is desirable that 15% by mass or more of the fatty acid lower alkyl ester is contained in the distillation residue, and more preferably 20% by mass or more. Even if some fatty acid lower alkyl ester remains in the distillation residue, since a part or all of the residue is recovered and added to the transesterification reaction step, the fatty acid lower alkyl ester that is extracted out of the system as a residue. The amount of alkyl ester can be kept low.
[0025]
The fatty acid lower alkyl ester produced by the method of the present invention has a low sulfur content concentration of several mg / kg or less, and is suitable as a raw material for producing a higher alcohol by catalytic reduction with hydrogen in the presence of an ester reduction catalyst. Can be used. In addition, in the production of the above higher alcohols, even when the sulfur content of the fatty acid lower alkyl ester is reduced by a desulfurization catalyst, if the fatty acid lower alkyl ester produced by the method of the present invention is used, the active life of the ester reduction catalyst is maintained. It is effective in doing.
[0026]
【Example】
In the examples, when the fatty acid methyl ester is regarded as a compound, the notation (pure) is used.
[0027]
Example 1
A transesterification reaction was carried out at 50 ° C. for 1 hour using 4562 g of refined coconut oil as a raw oil and fat, 356 g of a methanol solution containing 10% sodium hydroxide as a catalyst, and 2130 g of methanol as a lower alcohol. Thereafter, it was washed with 456 g of water, and low boiling point components were removed by evaporation under conditions of 667 Pa and 45 ° C. to obtain a crude fatty acid methyl ester. The TG ratio in this crude fatty acid methyl ester was 0.
[0028]
This crude fatty acid methyl ester was distilled and purified under the conditions of a tower top pressure of 400 Pa, a rectifying column stage number of 5, and a reflux ratio of 0.5, and 96.5% by mass was distilled off to the tower top side. 161 g, 100 g of the second and subsequent 165 g) was recovered and added to the transesterification step. Thereafter, the transesterification and distillation purification were similarly repeated to obtain 4500 g of fatty acid methyl ester (pure) per reaction and purification. The sulfur content in the fifth recovery fatty acid methyl ester was measured with a trace level total sulfur analyzer 7000TS manufactured by Antec. The results are shown in Table 1.
[0029]
Example 2
Fatty acid methyl ester (pure) 4527 g was obtained per reaction and purification by the same operation as in Example 1 except that the proportion of the distillate in the distillation purification step was 97.2% by mass.
[0030]
Comparative Example 1
Except that the fraction of the distillate in the distillation purification process was 98.7% by mass and the distillation residue generated in the distillation purification process was extracted without being recovered, the same operation as in Example 1 was performed once. For the reaction and purification, 4499 g of fatty acid methyl ester (pure component) was obtained.
[0031]
Comparative Example 2
After the transesterification reaction step, 48 g of a methanol solution containing 10% sodium hydroxide is added, and the reaction is performed while evaporating and removing methanol to convert unreacted fatty acid monoglycerides to diglycerides and triglycerides. Except for the ratio being 96.0% by mass, 4495 g of fatty acid methyl ester (pure) was obtained per reaction and purification in the same manner as in Example 1.
[0032]
The results of Examples 1-2 and Comparative Examples 1-2 are summarized in Table 1.
[0033]
[Table 1]

[0034]
As is apparent from the above results, when the amount of the obtained fatty acid lower alkyl ester is the same, the fatty acid lower alkyl ester having a low sulfur content is obtained in the examples of the present invention. Moreover, compared with the case where the sulfur content concentration in the fatty acid lower alkyl ester of the fraction is the same, the yield of the fatty acid lower alkyl ester is increased in the examples of the present invention.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the amount of distillation residue recovered in a transesterification step and the amount extracted out of the system under the condition that the concentration of sulfur in the fatty acid lower alkyl ester after distillation purification is constant.

Claims (3)

In the presence of a catalyst, an oil and fat and an alcohol having 1 to 4 carbon atoms are subjected to a transesterification reaction to obtain a crude fatty acid lower alkyl ester, followed by distillation purification to produce a fatty acid lower alkyl ester. Distillation of crude fatty acid lower alkyl ester distilled so that the TG ratio represented by formula (I) in alkyl ester is 1 or less and the distillation residue contains 10.5% by mass or more of fatty acid lower alkyl ester A method for producing a fatty acid lower alkyl ester in which a part of the residue is recovered and added to the transesterification reaction step.

(In the formula, n represents the number of carbon atoms of the constituent fatty acid contained in the largest amount by molar ratio in the crude fatty acid lower alkyl ester.)   The method according to claim 1, wherein the proportion of the distillation residue to be recovered is 95% by mass or less (excluding 0% by mass) of the total distillation residue.   The process according to claim 1 or 2, wherein the fatty acid lower alkyl ester is a raw material for alcohol production by an ester reduction catalytic reaction.

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