AU2005201140A1 - Method of producing alkyl esters - Google Patents

Description

AUSTRALIA

Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Applicant: BIODIESEL AUSTRALIA LTD Invention Title: METHOD OF PRODUCING ALKYL ESTERS The following statement is a full description of this invention, including the best method of performing it known to me/us: 2- METHOD OF PRODUCING ALKYL ESTERS Field of the Invention The present invention relates to method of producing alkyl esters from mixtures of glycerides and free fatty acids, in particular mixtures of glycerides and free fatty acids from grease trap waste, tallows, animal fats and other high fatty acid content lipids.

Background of the Invention Biodiesel is comprised of alkyl esters of fatty acids derived from naturally occurring lipids. Biodiesel is becoming increasingly recognized as a viable fuel alternative for diesel engines. Its use as a fuel is known to reduce the output of particulate hydrocarbon pollutants and improve the lubrication of diesel engines. Biodiesel may also be successfully used as a lubricant, a hydraulic fluid and as a multi-purpose solvent.

Furthermore, biodiesel is attractive from a commercial and environmental viewpoint because the raw materials for biodiesel production may be sourced from renewable oilseed feedstocks, used vegetable oils, palm oil, coconut oil, soapstock materials as a lipid-rich byproduct of vegetable oil refining, tallows, animal fats, or from grease trap waste which would normally be disposed of in land fill.

The widespread adoption of biodiesel as an alternative to petroleum-based diesel fuel is hampered by inefficient methods for the production of biodiesel. Known methods for Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 3 the production of biodiesel are limited by incomplete esterification of all fatty acids in the starting material, lengthy purification methods such as water washing, relatively long reaction times, contamination and separation difficulties associated with co-production of glycerol, and saponification of the starting material under certain reaction conditions.

The Boocock method for formation of methyl esters from vegetable oils has sought to accelerate reaction times for a two-step acid- then base-catalysed methanolysis of vegetable oils by forming a single phase reaction mixture using a cosolvent such as tetrahydrofuran (THF) or methyltetrabutylether (MTBE). Boocock reports that a 99% recovery of methyl ester is achieved with this method.

However, glycerol is a byproduct and the resultant alkyl ester normally requires extensive purification including water washing of the alkyl ester.

The Haas et al method for synthesis of fatty acid methyl esters from soapstock involves another two-step process wherein the soapstock is first saponified and then esterified in an acidic alcohol solution. Haas et al reports that a 60% recovery of fatty acid methyl ester is achieved with this method. The method is characterized, however, by high consumption of the reagents.

The present invention seeks to overcome at least some of the aforementioned disadvantages.

It is to be understood that, although prior art use and publications may be referred to herein, such reference does Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 4 not constitute an admission that any of these form a part of the common general knowledge in the art, in Australia or any other country.

Summary of the Invention According to a first aspect of the invention there is provided a method for producing alkyl esters from a mixture of glycerides and free fatty acids, comprising: a) providing an alcoholic solution of the mixture of glycerides and free fatty acids; b) treating the solution with an acid catalyst and effecting acid-catalysed alkyl esterification; c) treating the solution from step b) with a base catalyst and effecting base-catalysed transesterification; d) treating the solution from step c) with the acid catalyst and effecting acid-catalysed esterification; e) distilling the solution from step e) and removing alcohol; and f) separating the resulting alkyl esters.

Preferably, the mixture of glycerides and free fatty acids is sourced from grease trap waste, tallows, animal fats, palm oil, coconut oil, and other naturally occurring lipids, including waste or fresh unused vegetable oils and tallows, renewable oilseed feedstocks, and soapstock materials as a lipid-rich byproduct of vegetable oil refining.

In the preferred embodiment of the invention, the alcoholic solution of the mixture is formed by dissolving the mixture of glycerides and free fatty acids in a similar volume of Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 5 alcohol to afford about 50% v/v alcoholic solution of the mixture.

Preferably, the alcohol has a low-boiling point.

Typically, the alcohol is selected from a group comprising methanol, ethanol, propanol, and butanol, including npropyl alcohol, n-butyl alcohol, isopropyl alcohol, isobutyl alcohol, sec-butyl alcohol, t-butyl alcohol.

In the preferred embodiment the alcoholic solution of the mixture is a single phase homogenous solution.

In an alternative embodiment of the invention the alcoholic solution contains an immiscible suspension of lipids in the mixture. Preferably, the alcoholic solution of this particular embodiment is agitated during steps c) and d) to discourage separation of the alcoholic solution into two immiscible phases.

The method is conducted wherein any one of steps b) or d) are performed under reflux conditions.

In the preferred embodiment, the acid catalyst is a strong inorganic acid including but not limited to sulphuric acid, hydrochloric acid, phosphoric acid, perchloric acid and mixtures thereof.

Typically, in step b) of the method, the pH of the solution is decreased to a low pH, preferably to about 1 to 2.

Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 6 Typically, in step d) of the method, the pH of the solution is decreased to about 2.

In one embodiment of the invention the base catalyst comprises a concentrated alkoxide solution. Preferably, the concentrated alkoxide solution comprises sodium and/or potassium hydroxide dissolved in the alcohol used in step or sodium or potassium dissolved in the alcohol used in step Typically, the concentrated alkoxide solution is a 10 %w/w alkali metal alkoxide solution.

Typically, in step c) of the method, the pH of the solution is raised to an elevated pH, preferably up to about 12.

In the preferred embodiment, any residual acid remaining in the solution resulting from step e) or the alkyl esters resulting from step f) is neutralized to about pH 6-7, preferably by treating the solution or the alkyl esters with a weak base or elution thereof through an ion exchange column.

In the preferred embodiment, base-catalysed transesterification of glycerides contained in the mixture is effected in step c).

In the preferred embodiment, acid-catalysed esterification of residual saponified by-products from step c) is effected in step d).

In accordance with a second aspect of the invention there are provided alkyl esters as produced by a method for Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 7 producing alkyl esters from a mixture of glycerides and free fatty acids, comprising: a) providing an alcoholic solution of the mixture of glycerides and free fatty acids; b) treating the solution with an acid catalyst and effecting acid-catalysed alkyl esterification; c) treating the solution from step b) with a base catalyst and effecting base-catalysed transesterification; d) treating the solution from step c) with the acid catalyst and effecting acid-catalysed esterification; e) distilling the solution from step e) and removing alcohol; and f) separating the resulting alkyl esters.

In the description of the invention, except where the context requires otherwise due to express language or necessary implication, the words "comprise" or variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to specify the presence of the stated features, but not to preclude the presence or addition of further features in various embodiments of the invention.

Detailed Description of the Preferred Embodiments of the Invention Before the preferred embodiment of the present apparatus is described, it is understood that this invention is not limited to the particular materials described, as these may vary. It is also to be understood that the terminology used herein is for the purpose of describing the particular embodiment only, and is not intended to limit the scope of the present invention in any way. It must be noted that as Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 8 used herein, the singular forms and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs.

The present invention describes a method for reacting a mixture of free fatty acids and glycerides, in particular those found in grease trap waste, tallows, animal fats, palm oil, coconut oil, and other naturally occurring lipids, including waste or fresh unused vegetable oils and tallows, renewable oilseed feedstocks, and soapstock materials as a lipid-rich byproduct of vegetable oil refining, with short-chain alcohols such as methanol, ethanol, propanol, or butanol to produce corresponding alkyl esters.

It will be understood that grease trap waste encompasses a highly variable mixture of organic fats, greases and vegetable oils, comprising a high free fatty acid and mono- Sdi-, and tri-glyceride content, optionally contaminated by detergents, soaps, inorganic and particulate matter.

The term "total free fatty acid content (FFA)" as used herein refers to the total content of free fatty acids and mono-, di-, and triglycerides comprised in grease trap waste, tallow, animal fats, and other naturally occurring lipids, which are available for alkyl esterification.

Typically, the grease trap waste is between 80 to 100% total free fatty acid content.

Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 9 The method for producing alkyl esters from a mixture of glycerides and free fatty acids comprises the following steps: a) providing an alcoholic solution of the mixture of glycerides and free fatty acids; b) treating the solution with an acid catalyst and effecting acid-catalysed alkyl esterification; c) treating the solution from step b) with a base catalyst and effecting base-catalysed transesterification; d) treating the solution from step c) with the acid catalyst and effecting acid-catalysed esterification; e) distilling the solution from step e) and removing alcohol; and f) separating the resulting alkyl esters.

The alcoholic solution of the mixture is formed by dissolving the mixture of glycerides and free fatty acids, in the form of grease trap waste, tallows, animal fats, palm oil, coconut oil, and other naturally occurring lipids, including waste or fresh unused vegetable oils and tallows, renewable oilseed feedstocks, and soapstock materials as a lipid-rich byproduct of vegetable oil refining, in alcohol.

Preferably, the mixture of glycerides and free fatty acids is dissolved in a similar volume of alcohol to afford about v/v alcoholic solution of the mixture. In this way, the alcohol is in molar excess of the total free fatty acid content, preferably in a range of 1:10 to 1:25.

Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 10 The reaction proceeds satisfactorily with a lesser proportion of alcohol to mixture of glycerides and free fatty acids, however the reaction proceeds optimally at about 50% v/v.

The alcohol is selected from a group comprising methanol, ethanol, propanol, and butanol, in their single-chain and isomerised forms including n-propyl alcohol, n-butyl alcohol, isopropyl alcohol, isobutyl alcohol, sec-butyl alcohol, t-butyl alcohol. Preferably, the alcohol has a low boiling point to facilitate ready removal by distillation in step e) Dissolution of the mixture of glycerides and free fatty acids is afforded by mixing and agitating the mixture in alcohol. Under these conditions the alcoholic solution of the mixture of glycerides and free fatty acids will generally form a single phase homogenous solution. Grease trap waste, for example, has been shown to readily dissolve in alcohol.

However, in some instances and depending on the nature and content of the glycerides and free fatty acids, the alcoholic solution will contain an immiscible suspension of lipids in the mixture. For example, tallow and some vegetable oils may not form a single phase alcoholic solution. Preferably, in instances where the alcoholic solution contains an immiscible suspension of lipids, the alcoholic solution is continually agitated by vigorous stirring during steps c) and d) to discourage separation of the alcoholic solution into two immiscible phases. For the purposes of the invention, the agitated Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 11 suspension is to be regarded and treated as an alcoholic solution of the mixture of free fatty acids and glycerides.

Regardless of the content of the mixture of glycerides and free fatty acids, the process is not dependent on the presence of a co-solvent. However, use of such co-solvents will not impair the reaction or its final alky ester product. Co-solvent usage may marginally increase the reaction rate of the alkyl ester conversion process but the reactions of steps and d) are quite rapid and go to completion in the absence of a co-solvent.

The alcoholic solution of the mixture is first treated with an acid catalyst to effect acid-catalysed alkyl esterification, then treated with a base catalyst to effect base-catalysed transesterification of glycerides contained in the mixture. The resulting reaction mixture is finally treated with an acid catalyst to effect acid-catalysed alkyl esterification of residual saponified by-products resulting from the previous treatment steps.

Preferably, both of the acid-catalysed esterification steps and the base-catalysed transesterification step are performed under reflux conditions, although said steps may also be performed at ambient temperatures, albeit with longer reaction times.

The method of the present invention is preferably performed at ambient atmospheric pressure. However, it is envisaged that said method can be successfully performed under higher pressures with use of a pressure vessel, or under autoclave conditions.

Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 12 Typically, the acid catalyst is a strong inorganic acid including but not limited to sulphuric acid, hydrochloric acid, phosphoric acid, perchloric acid and mixtures thereof. Preferably, the ratio of acid to total fatty acid content in the mixture is about 1:20 in the first acidcatalysed alkyl esterification reaction step, and about in the second acid-catalysed alkyl esterification reaction step.

In the first acid-catalysed alkyl esterification reaction step, the pH of the solution is decreased to a low pH, preferably to about 1 to 2.

In the second acid-catalysed alkyl esterification reaction step, the pH of the solution is decreased to a low pH, preferably to about 2.

Typically, the base catalyst includes but is not limited to alcoholic solutions of sodium and/or potassium hydroxide dissolved in the alcohol used in step or alcoholic solutions of sodium or potassium dissolved in the alcohol used in step In one embodiment, the base catalyst is a concentrated alkoxide solution comprising a 10% w/w alkali metal alkoxide solution, such that the ratio of added base to total fatty acid content in the mixture is about In the base-catalysed alkyl transesterification reaction step, the pH of the solution is raised to an elevated pH, preferably up to about 12.

Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 13 The alcohol is removed from the resulting reaction mixture by distillation, with or without vacuum assistance, leaving a two-phase heterogeneous mixture of alkyl esters and a solid phase. The alkyl esters are separated from the solid phase by decantation, and further purified by filtration or separated by filtration alone.

Once the alcohol has been removed from the reaction mixture by distillation, or the alky ester phase has been separated from the solid phase by decantation, the alkyl ester phase can be treated with a weak base to effectively neutralize residual acid remaining in the alkyl ester phase. It is envisaged that the pH of the alkyl ester phase will be neutralized to about pH 6-7.

The neutralization step is achieved by treating the alkyl ester phase with a weak base, preferably by addition of small amounts of sodium bicarbonate to the alkyl ester phase. It is envisaged that other well known means for neutralizing pH such as elution of the liquid phase through an ion exchange resin, can be successfully employed to effect neutralization of residual acid within the alkyl ester phase.

Alternatively, any residual acid remaining in the solution resulting from step e) can be neutralized to about pH 6-7, preferably by treating the solution or the alkyl esters with a weak base or elution thereof through an ion exchange column.

Residual water can then be removed by conventional means such as passing the alkyl ester phase over a hygroscopic Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 14 desiccant or by heating the alkyl ester sufficiently to remove steam at atmospheric pressures or under vacuum.

Depending on the final intended use of the alkyl ester, a small amount of entrained water may be acceptable and its removal may not be necessary.

Surprisingly, the inventors have found that the alkyl esters produced by the method of the present invention remain uncontaminated by glycerol byproducts from the acidcatalysed esterification and base-catalysed transesterification reactions. An exception was found from the treatment of fresh unused vegetable oil as a free fatty acids and glyceride mixture feedstock whereby a small quantity of good quality glycerol was readily separated.

The amount of glycerol byproduct was at less than 8% of the starting oil and noticeably less than the amount generated by other esterification methods. No noticeable glycerol production occurred when treating other lipids such as palm oil, coconut oil, animal tallow or grease trap waste.

The present invention will now be illustrated according to the following example.

Example A single phase 1:1 v/v methanolic solution of grease trap waste (800 ml) in methanol (800 ml) was prepared. A GC/MS analysis of the grease trap waste indicated that the major components of the grease trap waste were octadecanoic acid and n-hexadecanoic acid. The pH of the methanolic solution Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 15 of grease trap waste was 4, reflecting a high fatty acid content.

Concentrated sulphuric acid (8.5 g) was added with stirring to the methanolic solution resulting in a solution pH of between 1 and 2. The methanolic solution was refluxed for 30 minutes.

A sodium methoxide solution of 10% w/w NaOH in methanol (160 ml) was then added dropwise to the stirred refluxing methanolic solution resulting in a final pH of 12.

Concentrated sulphuric acid (21 ml) was added dropwise to the stirred refluxing methanolic solution resulting in a final pH of 2.

Excess methanol was removed by distillation. The resulting methyl esters (825 ml) were separated from a semi solid basal residue (64.5 g) by decantation, and finally filtered to remove fine sediment. It was estimated that there was a better than 91% conversion of waste trap grease to methyl esters.

A similar amount of grease trap waste was treated according to the present invention as described in the abovementioned Example wherein ethanol, rather than methanol, was used as the solvent. It was found that a similar degree of conversion of the grease trap waste to corresponding ethyl esters was achieved.

Furthermore, similar experiments using a potassium alkoxide solution rather than a sodium hydroxide solution produced Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 16 similar results as those exemplified in the abovementioned Example.

Although the residual sediment formed from the reaction mixture using ethanol and potassium hydroxide was less crustiform than the sediment described in the Example, it was readily separated from the ethyl esters by filtration.

On testing, the crustiform residue was found to comprise mostly sodium sulphate with minor entrained methyl ester that totaled 9% by weight of the starting grease trap waste. The conversion of grease trap waste to methyl ester therefore exceeded 91%. The entire processing time to final product alkyl ester is less than 1 hour.

It is a rapid and efficient method whose reaction kinetics may be increased by many well known methods including increasing reaction parameters, such as temperature, pressure, and agitation of the reaction mixture.

The final product alkyl ester does not require water washing thereby eliminating a major processing step common to other methods that is both time consuming and polluting.

Glycerol byproduction is eliminated in most lipid feedstocks tested. The method and process permits design of commercial plants that should be low in capital cost, small in size, with high production rates, and portable as truck-mounted, trailer-coupled units, or as a static plant.

Numerous variations and modifications will suggest themselves to persons skilled in the relevant art, in addition to those already described, without departing from Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 17 the basic inventive concepts. All such variations and modifications are to be considered within the scope of the present invention, the nature of which is to be determined from the foregoing description.

Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005

Claims (23)

1. A method for producing alkyl esters from a mixture of glycerides and free fatty acids, comprising: a) providing an alcoholic solution of the mixture of glycerides and free fatty acids; b) treating the solution with an acid catalyst and effecting acid-catalysed alkyl esterification; c) treating the solution from step b) with a base catalyst and effecting base-catalysed transesterification; d) treating the solution from step c) with the acid catalyst and effecting acid-catalysed esterification; e) distilling the solution from step e) and removing alcohol; and f) separating the resulting alkyl esters.

2. The method according to claim 1, wherein the mixture of glycerides and free fatty acids is in the form of grease trap waste, tallows, animal fats, palm oil, coconut oil, and other naturally occurring lipids, including waste or fresh unused vegetable oils and tallows, renewable oilseed feedstocks, and soapstock materials as a lipid-rich byproduct of vegetable oil refining.

3. The method according to claim 1 or claim 2, wherein alcohol is in molar excess of the total free fatty acid content of the mixture.

4. The method according to any one of the preceding claims, wherein the alcoholic solution of the mixture is formed by dissolving the mixture of glycerides and free Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 19 fatty acids in a similar volume of alcohol to afford about v/v alcoholic solution of the mixture. The method according to any one of claims 2 to 4, wherein the alcohol has a low-boiling point.

6. The method according to any one of claims 2 to wherein the alcohol is selected from a group comprising methanol, ethanol, propanol, and butanol, including n- propyl alcohol, n-butyl alcohol, isopropyl alcohol, isobutyl alcohol, sec-butyl alcohol, t-butyl alcohol.

7. The method according to any one of claims 1 to 6, wherein the alcoholic solution is a single phase homogenous solution.

8. The method according to any one of claims 1 to 6, wherein the alcoholic solution contains an immiscible suspension of lipids in the mixture.

9. The method according to claim 8, wherein the alcoholic solution is agitated during steps c) and d) to discourage separation of the alcoholic solution into two immiscible phases. The method according to any one of the preceding claims, wherein any one of steps b) or d) are performed under reflux conditions.

11. The method according to any one of the preceding claims, wherein the acid catalyst is a strong inorganic acid including but not limited to sulphuric acid, Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 20 hydrochloric acid, phosphoric acid, perchloric acid and mixtures thereof.

12. The method according to any one of the preceding claims, wherein the base catalyst comprises a concentrated alkoxide solution.

13. The method according to claim 12, wherein the concentrated alkoxide solution comprises sodium and/or potassium hydroxide dissolved in the alcohol used in step or sodium or potassium dissolved in the alcohol used in step a)

14. The method according to claim 12 or claim 13, wherein the concentrated alkoxide solution is a 10% w/w alkali metal alkoxide solution. The method according to any one of the preceding claims, wherein the pH of the solution is decreased to a low pH in step b).

16. The method according to claim 15, wherein the pH of the solution is decreased to about 1 to 2.

17. The method according to any one of the preceding claims, wherein the pH of the solution is raised to an elevated pH in step c)

18. The method according claim 17, wherein the pH is raised to about 12. Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 21

19. The method according to any one of the preceding claims, wherein the pH of the solution is decreased to about 2 in step d).

20. The method according to any one of the preceding claims, wherein any residual acid remaining in the solution resulting from step e) or the alkyl esters resulting from step f) is neutralized to about pH 6-7.

21. The method according to claim 20, whereby neutralization to about pH 6-7 is effected by treating the solution resulting from step e) or the alkyl esters with a weak base.

22. The method according to claim 20, whereby the solution resulting from step e) or the alkyl esters are treated with sodium bicarbonate.

23. The method according to any one of the preceding claims, wherein base-catalysed transesterification of glycerides contained in the mixture is effected in step c)

24. The method according to any one of the preceding claims, wherein acid-catalysed esterification of residual saponified by-products from step c) is effected in step d). Alkyl esters as produced from the method as claimed in any one of claims 1 to

26. A method for producing alkyl esters from a mixture of glycerides and free fatty acids substantially as Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005 22 hereinbefore described with reference to the accompanying example.

27. Alkyl esters as produced from the method as hereinbefore described with reference to the accompanying example. Dated this 17th day of March 2005 BIODIESEL AUSTRALIA PTY LTD By Its Patent Attorneys GRIFFITH HACK Fellows Institute of Patent and Trade Mark Attorneys of Australia. Kirstie/keep/speci/P56320 Method of Producing Alkyl Esters 17/03/2005