GB2148897A - Catalytic esterification of carboxylic acid/glyceride mixtures - Google Patents

Abstract

A process for the esterification of carboxylic acid/glyceride mixtures such as palm oil or palm oil fractions comprises refluxing the mixture with an alcohol in the presence of a solid alkali metal bisulphate, especially sodium bisulphate, which is then removable by filtration. The glyceride content is then transesterified preferably using a sodium hydroxide as base catalyst.

Description

SPECIFICATION Esterification of carboxylic acid/glyceride mixtures The present invention relates to a process for esterification of free fatty acids in admixture with glycerides.

It is known, see for example British Patent Specifications Nos. 2051786 and 2072167, to transesterify the glyceride components of such mixtures by reaction with an excess of lower alcohol in the presence of a basic catalyst such as sodium hydroxide after having first esterified the free fatty acid content of the mixture. The esterification serves to prevent the free fatty acids forming soaps with the base added as the transesterification catalyst. Conventionally, the free fatty acids are esterified by reaction with an alcohol in the presence of a mineral acid catalyst such as sulphuric acid or hydrochloric acid. However, the use of such liquid acids presents a separation problem.

Alkali bisulphates, especially sodium or potassium bisulphate, have been used as catalysts in the esterification of carboxylic acids (see, for example, UK Patent Nos. 465983, 957679 and 957979). However, it has not previously been proposed to employ said catalysts in the esterification of carboxylic acid/glyceride mixtures and it was not previously appreciated that such use would be of particular assistance in facilitating subsequent base-catalysed transesterification of the glyceride content.

It has now been found that carboxylic acid/glyceride mixtures can be efficiently esterified by first esterifying the carboxylic acid content using a solid alkali metal bisulphite and subsequently base-catalysting the glyceride content.

The invention has particular applicability to the esterification and subsequent transesterification of acid rich fractions derived from vegetable oil milling and refining procedures, e.g. the refining of palm oil. In the physicai refining of palm oil, free fatty acids are distilled off under vacuum e.g. at 260 to 270to, so producing a fraction known as "palm fatty acid distillate" (PFAD). This fraction contains typically 80 to 90% free fatty acid, the bulk of the remainder being glycerides. A second acid rich palm oil product or fraction obtained from palm oil mills is sludge oil which typically contains 70 to 80% free fatty acid. These fractions have in the past been regarded essentially as waste products, but by the esterification process of the present invention can be converted into valuable esters.After separation of the solid catalyst employed in the present invention, the glyceride component of the mixture may be transesterified under basic catalysts.

The present invention provides a process for the esterification of a mixture of at least one carboxylic acid and at least one glyceride, which comprises reacting said mixture with at least one alcohol in the presence of a solid alkali metal bisulphate as a catalyst to esterify the carboxylic acid content, and subsequently transesterifying the glyceride content.

Preferably, the esterification step is carried out with heating, e.g. to reflux the alcohol.

The major part of the bisulphate remains solid through the esterification step and hence can readily be separated by filtration.

The process is particularly applicable to the treatment of long chain carboxylic acids such as occur naturally in vegetable oils. Preferably therefore the carboxylic acid is a C,0 to C25 carboxylic acid or a mixture of such acids, more preferably a C16 to C18 acid or mixture thereof.

The carboxylic acids may be saturated or unsaturated e.g. with one or two double bonds.

The process has particular applicability to naturally occurring mixtures of free fatty acids and glycerides and to fractions obtained therefrom.

The process has especial applicability to fatty acid distallates obtained from vegetable oils such as palm oil and to sludge oils derived from the refining and milling of such vegetable oils.

As explained above these fractions are very rich in fatty acids, for instance containing above 70% by weight free fatty acid. The remainder of these fractions is chiefly made up by triglycerides but these fractions also contain some minor ingredients of considerable intrinsic value such as sterols, tocotrienols, carotenes and tocopherols.

Preferably, the esterification of the acid is with one or more C, to C8, more preferably C, to C3 alcohols, especially methanol. Esters of vegetable oils with such lower alcohols are known to be useful as fuels for diesel engines either alone or in admixture with conventional diesel mineral oil fuel.

Preferably, there is employed from 0.5% to 30% by weight of the bisulphate based upon the weight of carboxylic acid, more preferably 1 to 20% by weight e.g. about 10% by weight.

The catalyst is an alkali metal bisulphate e.g. sodium or potassium bisulphate; sodium bisulphate being most preferred.

Alkyl esters of carboxylic acids, e.g. the lower alkyl esters described above, can be obtained from glyceride mixtures containing free fatty acid both by the esterification of the free fatty acid and by transesterification of the glyceride component.

The transesterification step is preferably carried out using a basic catalyst such as sodium hydroxide. Method of this general kind for transesterifying glycerides are described in British Patent Specifications Nos. 2051786 and 2072167.

In particular, the transesterification step may be carried out following removal of the bisulphate catalyst, e.g. by filtration, by adding to the glyceride/ester mixture a catalyst such as an alkali metal hydroxide together with alcohol if not already present in sufficient quantity and preferably heating, e.g. to reflux the alcohol. The alkali catalyst level is preferably from 0.05 to 2.0% by weight of the glyceride/free fatty acid mixture originally employed, more preferably from 0.1 to 1.0%, e.g. 0.3 to 0.7% by weight. Sodium hydroxide is the preferred catalyst. The process is conveniently carried out using excess alcohol as a solvent, e.g. using a molar excess of methanol over glyceride of more than 6:1, more preferably more than 8:1, e.g. more than 15:1.

The transesterification step is generally essentially quantitative and the choice of catalyst level and temperature is largely governed by the desired reaction time.

Apart from the value of the lower alkyl esters of fatty acids obtained by the above combination of an esterification and a transesterification reaction of fractions of vegetable oil, transforming the glycerides and the free fatty acids into lower alkyl esters provides a convenient method for enabling the ready separation of these components, e.g. by vacuum distillation, from the minor components of high intrinsic value such as tocopherols mentioned above.

Accordingly, the present invention includes a method of separating such intrinsically valuable minor constituents of a vegetable oil or vegetable oil fraction which comprises esterifying free fatty acids in the vegetable oil or vegetable oil fraction, transesterifying glyceride components of the vegetable oil or vegetable oil fraction and separating the fatty acid esters produced from the said minor components, e.g. by vacuum distillation, wherein the esterification reaction is carried out using a solid alkali metal bisulphate as a catalyst.

The invention includes sterols, tocopherols, tocotrienols and carotenes separated from vegetable oils or vegetable oil fractions in this way.

The invention will be illustrated by the following Examples provided for the sake of illustration.

EXAMPLE 1 Sludge oil (30 g) obtained from the refining and milling of palm oil was heated under reflux with stirring with methanol (60 ml) and sodium bisulphate (5 9). After 30 minutes, the methyl ester content of the reaction mixture was greater than 70% based on the starting oil content.

The original free fatty acid content of the sludge oil was 76% by weight.

EXAMPLE 2 A sample of palm oil (30 9) was heated under reflux with methanol (60 ml) and sodium bisulphate (1 g). The original free fatty acid content of the palm oil was 17%. At the completion of the reaction the mixture contained 15% by weight of methyl ester based on the starting oil content.

EXAMPLE 3 Crude palm oil (30 9) was heated under reflux with methanol (20 ml) and sodium bisulphate (0.2 g). The original free fatty acid content of the crude palm oil was 2% by weight. After about one hour, the reaction mixture contained about 2% by weight of methyl ester based on the starting oil content.

In each of Examples 1 to 3 above, negligible transesterification of the triglyceride component during the esterification reaction was observed. However, after filtering off the solid bisulphate catalyst, the triglyceride content was readily transesterified by refluxing with methanol in the presence of sodium hydroxide as base catalyst (see Example 7).

EXAMPLE 4 Palm fatty acid distillate (containing 80 to 90% free fatty acid) was heated under reflux with stirring with methanol and sodium bisulphate in the proportions shown in Table 1 below.

Methyl esters were produced in the amounts indicated in the Table. Once again, transesterification was not observed at this stage. However, after filtering off the solid bisulphate catalyst, the triglyceride content was readily transesterified by refluxing with methanol in the presence of sodium hydroxide as base catalyst (see Example 7).

TABLE 1

Palm Fatty % Ester in Reaction Acid I NaHS04 CH30H Mixture after Distillate ~ time (t) 1) 80 g 5 g 30 ml 80% (5 min) 2) 100 g 1 g 75 ml 80% (5 min) 90% (60 min) 3) 30 g 0.15 g 20 ml 60% (5 min) 80% (60 min) 90% (120 min) EXAMPLE 5 A mixture of crude palm oil (15 9) and sludge oil (15 9) was heated under reflux with stirring with methanol (60 ml) in the presence of 5 g of sodium bisulphate. The initial free fatty acid content of the oil mixture was about 40%.The ester content of the reaction mixture, based on the original weight of the oil, was about 50% at the end of the reaction. After filtering off the solid bisulphate catalyst, the triglyceride content was readily transesterified by refluxing with methanol in the presence of sodium hydroxide as base catalyst (see Example 7).

EXAMPLE 6 Example 3 was repeated except that the catalyst level was increased so that 5 9 of sodium bisulphate were employed. The amount of methanol present was increased to 60 ml. The reaction proceeded in the same way as in Example 3. After filtering off the solid bisulphate catalyst, the triglyceride content was readily transesterified by refluxing with methanol in the presence of sodium hydroxide as base catalyst (see Example 7).

COMPARATIVE EXAMPLE 1 Sludge oil containing 76% free fatty acid obtained from palm oil refining and milling was refluxed with methanol (60 ml) in the presence of, in a first case, Amberlite (Trade Mark) resin (acid form) (5 9) and, in a second case, sodium dihydrogen phosphate (5 g). In each case, no esters were formed after one hour of reaction.

COMPARATIVE EXAMPLE 2 Example 4 (Run 3) was repeated using only 0.1 g of catalyst. The reaction did not proceed to completion.

EXAMPLE 7 Crude palm oil and palm fatty acid distillate respectively were treated with sodium bisulphate and methanol under reflux in the manner described above to produce methyl esters from the free fatty acid content. In each case, the reaction mixture was filtered to remove the solid catalyst and treated with 1 % by weight sodium hydroxide and refluxed for a further 4 hours to produce methyl esters by transesterification.

EXAMPLE 8 From the reaction mixture obtained in each run of Example 7, the methyl ester fraction was separated from the glycerol produced by the transesterification reaction in a separating funnel, the glycerol forming a separate layer. Methanol and methyl esters of fatty acids were removed by distillation under vacuum. The residue was found to contain substantial quantities of tocopherol, sterol and carotene as shown in Table II below: TABLE II Palm fatty acid Source Crude Palm OI1 distillate Tocopherol 2% 6% Sterol 8.9% Carotene 1.5% EXAMPLE 9 Vegetable oils and tallow were subjected to an esterification process according to the invention as shown in Table Ill below. In each case esters were formed from the free fatty acid content of the starting material.The reaction products were filtered to remove the solid catalyst and transesterified using 0.3 9 NaOH and 20 ml methanol under reflux with stirring. In each case the transesterification reaction was complete within 5 minutes.

TABLE III

Oils Sunflower Soyabean Tallow FFA of oil used 2.46% 0.8% 2.8% Wt of oil used 30 g 30 g 30 g NaHSO4 0.3 g 0.3 g 0.3 g Reaction time 1 hr. 1 hr. 1 hr.

Methanol 20 ml 20 ml 20 ml EXAMPLE 10 a) Vegetable oils as shown in Table IV below (30 9) were heated with sodium bisulphate (0.3 g) and methanol (30 9) to the reflux temperature of the methanol and the reaction mixture was stirred. The procedure was continued for 1 hour but esters of the free fatty acids present in the oils were formed after only 5 minutes.

TABLE IV

Vegetable Corn Rapeseed Palm Kernel Oil Oil Oil Oil (1) (2) FFA present in oil 3.7% 2.35% 1.55% 3.17% Wt. of NaHSO4 with respect to oil 0.3 g 0.3 g 0.3 g 0.3 g The reaction mixture was filtered to remove the catalyst and subjected to the following further procedure.

b) The filtered reaction mixture was neutralised with an appropriate quantity of sodium hydroxide/methanol. The triglyceride content of the oil was then transesterfied with methanol under reflux using sodium hydroxide (0.1 5 9) as catalyst in 10 ml methanol.

In each case the reaction appeared to be complete in 5 minutes from the addition of catalyst.

Ester and glycerol layers were produced. When the methanol was distilled off the two layers became more pronounced with the glycerol forming a jelly like lower layer.

EXAMPLE 11 5 g of potassium hydrogen sulphate and 5 9 of sodium hydrogen sulphate were separately packed into two small chromatography columns (0.8 cm internal diameter, 230 cm length). In each case the 5 9 of acid salt occupied about 10 cm. The columns were eluted with methanol to remove air. A separate sample of palm fatty acid distillate (PFAD) (1 g) dissolved in 5 ml methanol was passed through each column. In each case the retention time was about 10 minutes.

The eluted PFAD from each column was analysed by thin layer chromatography and it was found that the initial free fatty acid content (about 50%) had been converted to methyl esters.

The triglyceride content was readily transesterified by refluxing with methanol in the presence of sodium hydroxide as base catalyst (see Example 7).

EXAMPLE 12 Crude palm stearin (30.5 9) (4% free fatty acid) was heated under reflux with methanol (20 ml) and sodium bisulphate (0.3 9) for 1 hour. T.L.C. indicated that about 4% of methyl ester had been formed. The reaction mixture was filtered and the ester/methanol/oil layer was neutralized (pH7) with sodium hydroxide/methanol. The layer was then heated for 10 mins with 0.11 33 g sodium hydroxide and 10 ml methanol to effect transesterification. Substantially complete transesterification occurred yielding an upper ester layer and a lower glycerol layer.

The procedure above was repeated except for the use of 30.5 9 crude palm stearin and 0.0932 9 sodium hydroxide. Transesterification was complete in about 1 5 minutes.

Claims (14)

1. A process for the esterification of a mixture of at least one carboxylic acid and at least one glyceride, which comprises reacting said mixture with one or more alcohols in the presence of a solid alkali metal bisulphate as a catalyst, to esterify the carboxlic acid content, and subsequently transesterifying the glyceride content.

2. A process as claimed in Claim 1, wherein following said esterification, the solid bisulphate catalyst is separated from the reaction mixture and a transesterification catalyst is added thereto and the mixture is heated in the presence of an alcohol to carry out the said transesterification.

3. A process as claimed in Claim 1 or Claim 2, wherein the carboxylic acid is a C,O to C25 carboxylic acid or a mixture of such acids.

4. A process as claimed in any preceding claim, wherein a naturally occurring mixture or a fraction derived from such a mixture is subjected to the esterification.

5. A process as claimed in Claim 4 wherein the mixture is palm oil, a treated palm oil or a palm oil fraction.

6. A process as claimed in Claim 4 or Claim 5 wherein the mixture is a fatty acid distillate obtained from a vegetable oil or a sludge oil derived from a vegetable oil.

7. A process as claimed in any preceding claim wherein the carboxylic acid is esterified with one or more C, to C3 alcohols.

8. A process as claimed in Claim 7 wherein the carboxylic acid is esterified with methanol.

9. A process as claimed in any preceding claim, wherein there is employed from 0.5% to 30% by weight of the bisulphate based upon the weight of carboxylic acid.

10. A process as claimed in Claim 9, wherein there is employed from 1 to 20% by weight of catalyst based upon the weight of carboxylic acid.

11. A process as claimed in any preceding claim, wherein the catalyst is sodium bisulphate.

1 2. A process as claimed in any preceding claim, wherein the transesterification reaction is carried out using an alkaline transesterification catalyst.

1 3. A process as claimed in Claim 12, wherein the transesterification catalyst is sodium hydroxide.

14. A process for the esterification of a fatty acid/glyceride mixture substantially as hereinbefore described in any one of Examples 1 to 1 2.

1 5. A carboxylic acid ester produced by a process as claimed in any preceding claim.

1 6. A liquid fuel comprising one or more esters as claimed in Claim 1 5 in admixture with other liquid fuel constitutents.