WO2007113530A2

WO2007113530A2 - Biodiesel production

Info

Publication number: WO2007113530A2
Application number: PCT/GB2007/001192
Authority: WO
Inventors: Colin Horton
Original assignee: Greenergy Biofuels Limited
Priority date: 2006-03-31
Filing date: 2007-03-30
Publication date: 2007-10-11
Also published as: GB2436836A; GB0606533D0; WO2007113530A3

Abstract

The disclosure elates to the field of biodiesel production to provide fuel for diesel internal combustion engines. In particular, the disclosure seeks to provide an improved process for manufacturing diesel fuels having at least a component derived from waste natural (non- synthetic) oils which are contaminated with free fatty- acids, especially used cooking oil. Accordingly, there is provided a process for the production of biodiesel from natural oils comprising: providing waste natural oil having a relatively high free fatty acid content, providing virgin natural oil having a relatively low, or zero, free fatty acid content, blending the waste oil and virgin oil in appropriate proportions to produce an oil blend having a target reduced free fatty acid content, stripping the oil blend to produce distilled fatty acids and separating the fatty acids from the oil blend, feeding the oil blend to a transesterification stage thereby to form biodiesel end product. Also provided is apparatus for conducting this process.

Description

Biodiesel Production

The present invention relates to the field of biodiesel production to provide fuel for diesel compression ignition internal combustion engines. In particular, the present invention seeks to provide an improved process for manufacturing diesel fuels having at least a component derived from waste natural (non-synthetic) oils which are contaminated with free fatty acids.

Biodiesel is a renewable transport fuel made mainly from plant/vegetable oils such as soya oil or rape seed oil. It has long been known that diesel engines may be operated on fuel oil derived from plants. The original diesel engine was demonstrated in 1895 operating on fuel derived from peanut oil. Biodiesels are particularly attractive as they have a neutral environmental impact, in that the oils are derived from plants which grow by absorbing carbon dioxide from the atmosphere . Clean burning of diesels fuels releases carbon dioxide into the atmosphere, continuing a cycle in which further oils may be grown for fuel use. Vegetable/plant oils may be converted into biodiesel by a simple transesterification process. The resulting biodiesel (or methyl ester) can be used in any diesel engine, either alone or blended with conventional mineral (petroleum) oil derived diesels.

Although raw vegetable oils may be used as fuels for certain compression ignition internal combustion engines, biodiesel generally refers to oil which has been processed to provide a clean-burning alternative fuel, produced from domestic, renewable resources. Biodiesel can be used on its own or blended in any proportions with petroleum diesel to create a biodiesel blend. Biodiesel, or blends thereof, may be used be used in diesel engines with little or no engine modification. Biodiesel is biodegradable, non-toxic, and has very low levels of sulphur and harmful aromatic compounds. In other words it is a clean and environmentally friendly fuel.

Biodiesel is derived from natural oil by an esterification process in which raw vegetable oil or animal fat is converted into methyl esters (biodiesel) and glycerin. Glycerin is itself a useful raw material that may be used to make soaps and other products . The term 'biodiesel' basically means an animal or vegetable oil processed to provide fuel grade material. One formal standard definition of biodiesel is set out in European standard ENBS 14214. Thus, 'biodiesel' may be formally defined as mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats which conform to ENBS 14214 specifications for use in diesel engines.

With mineral oil supplies having finite reserves and being prone to large price variations, there has arisen in recent years renewed interest in the use of plant oils for fuel. These fuels have clear environmental benefits in reducing net carbon dioxide emissions.

There are three established routes by which alkyl esters may be produced from oils and fats. The first is transesterification of the oil by alcohol in the presence of an alkali catalyst such as sodium or potassium hydroxide. Second, acid esterification of the oil by methanol in the presence of an acid catalyst, and third, conversion of the oil into fatty acids and thence alkyl esters by acid catalysis. Of these, the first method dominates current industrial processes because the plant required is simple - low temperatures and pressures are used, conversion rates are high (about 98%) and methyl ester is derived directly with no significant intermediate steps.

PCT publication WO 95/02661 discloses a process for preparing fatty acid alkyl esters by catalytic transesterification of triglycerides. The reaction product comprises an ester phase and a glycerine phase that contains fatty acids, fatty acid salts and other fatty acid compounds . The ester and glycerine phases are separated and the fatty acids separated from the glycerine. The fatty acids are pre-esterified using an alcohol and then transesterified to produce a high yield of fatty acid alkyl esters. WO 02/28811 discloses a similar multistage process in which transesterifications and direct esterifications are carried out to give a high alkyl ester yield.

US2004/0254387 discloses a single step method for making biodiesel from vegetable oil, in which free fatty acids and glycerides of the vegetable oil are directly esterified using methanol and sulphuric acid catalyst.

GB-A-812474 discloses a method and apparatus for refining fats by the use of a stripping vapour to remove free fatty acids.

In recent years, developments have been made in the recycling of waste cooking oils by conversion into biodiesel. Vast quantities of cooking oils are used in both the domestic and commercial sectors. Waste recycling initiatives have made available sources of such waste cooking oils for processing into fuel oil. Substantial volumes of this used cooking oil (UCO) , also known as recovered vegetable oil (RVO) or yellow grease, are generated from food manufacturers, restaurants and convenience food outlets. UCO may be vegetable oil or animal fat-based, but vegetable oils dominate the domestic and commercial food industries. The UCO material is a cheaper alternative than virgin vegetable oils (WO) for the production of biodiesel. Thus, there are potential economic and environmental benefits in using UCOs in biodiesel production. However, UCO' s require additional processing due to the higher levels of moisture and free fatty acids (FFA' s) typically contained in UCO as compared to WO. These FFA' s interfere with the transesterification process, as they can neutralise the base catalyst in the transesterification reaction as well as forming unwanted side products which may be difficult to separate.

JP 2005-350631 (Electric Power) discloses a method for producing biodiesel from acidic oils and fats, which involves removing free fatty acid from the feedstock oil.

The FFA' s present in UCO typically arise due to the partial decomposition of triglycerides present in the WO under the high temperatures utilised in the various cooking processes. Thus, in order to reduce the level of FFA present in the UCO, conventional treatment involves a preliminary esterification reaction stage prior to the main transesterification stage required to convert WO 's to biodiesel. This preliminary pre-esterification stage is required to reduce the free acidity of the UCO to a level that allows the principle transesterification reaction to proceed effectively.

One pre-esterification route typically also requires the dilution of the high FFA UCO stream with virgin oil to reduce the overall percentage of FFA' s in the combined stream. The diluted oil stream is then subject to a direct acid esterification process in which concentrated sulphuric acid is added to the reactor in the presence of methanol under an inert (nitrogen) gas pressure blanket. The reaction is virtually instantaneous and the resultant mixture of crude methyl ester, methanol and residual fatty acids are pumped to a separation vessel where the 'heavy' methyl ester is separated from the 'light' methanol/glycerine phase. The 'heavy' phase is pumped to the main transesterification reactor to be reacted to form biodiesel and the 'light' phase is recirculated to the esterification reactor to be re-reacted.

Failure to adopt these processes to convert the FFA' s to crude esters in a preliminary treatment stage causes the FFA' s acidity to neutralise the alkaline solution in the subsequent transesterification reaction, thereby producing unwanted soaps instead of biodiesel. The consequential presence of soaps in the biodiesel / glycerine transesterification reactor effluent makes their efficient separation into their two respective product grades very difficult.

A notional preliminary esterification process is shown schematically in figure 1. A reservoir (10) of UCO provides a feed of UCO to a heater (11) . The heater heats the oil to a temperature of about 105 degrees centigrade. The feed is then passed through a vacuum dryer (12) to remove excess moisture from the oil. The UCO is then pumped (17) into a mixing vessel (13) which is charged with concentrated sulphuric acid and methanol, along with a supply of uncontaminated virgin vegetable oil. This mixture is then passed into a reactor (14) and heated to about 65 degrees centigrade in a nitrogen atmosphere. The acid catalyses the reaction between the fatty acid in the oil and added methanol which produces a methyl ester. The dual phase product is then passed to a separation vessel (15) in which the components are allowed to settle, with the excess methanol and sulphuric acid settling to the bottom and crude methyl ester at the top. The methanol and acid are recycled via a feedback loop (16) into the reactor, whereas the crude methyl ester is fed to a transesterification stage 18.

The present invention seeks to provide an efficient and economically beneficial process for converting used oils into biodiesel .

According to one aspect of the present invention there is provided a process for the production of biodiesel from natural oils comprising: providing waste natural oil having a relatively high free fatty acid content, providing virgin natural oil having a relatively low, or zero, free fatty acid content, blending the waste oil and virgin oil in appropriate proportions to produce an oil blend having a target reduced free fatty acid content, stripping the oil blend to produce distilled fatty acids and separating the fatty acids from the oil blend, and feeding the stripped oil blend to a transesterification stage thereby to form biodiesel end product .

According to another aspect of the invention there is provided apparatus for the production of biodiesel from natural oils of comprising: a feedstock of waste natural oil having a relatively high free fatty acid content, a feedstock of virgin natural oil having a relatively low, or zero, free fatty acid content, means for blending the waste oil and virgin oil in appropriate proportions to produce an oil blend having a target reduced free fatty acid content, a stripping stage for stripping the oil blend to produce distilled fatty acids and means for separating the fatty acids from the oil blend, and means for feeding the stripped oil blend to a transesterification stage in which biodiesel end product is formed. The apparatus may be modified by the provision of means or stages for carrying out any of the optional process steps mentioned in the following description or the claims.

In one preferred embodiment wherein the waste natural oil is a used cooking oil, such as vegetable oil.

It is possible to use the method or apparatus of the invention in modes in which only waste natural oil, e.g. UCO is used as the source, or only virgin natural oil. In the case of waste oil only, this may be possible where the FFA content is relatively low.

The waste oil may have significant free fatty acid content which content comprises degradation products due to heating of the oil during cooking of foodstuffs. The degradation products typically derive from triglycerides present in the original virgin oil before use. Typical free fatty acid content of the waste oil is from 1.0 to 7.0 wt% or slightly more.

The target reduced free fatty acid content of the oil blend is typically within 0.5 wt% to 2.0 wt%, preferably 0.5 to 1.5 wt%.

The virgin natural oil is typically a vegetable oil. one preferred embodiment process the virgin oil is c gummed oil provided from a virgin oil de-gumming stage. The virgin natural oil typically has a free fatty acid content that is specific to the oil source. For example Soya derived oil may have a free fatty acid content of about 0.5 wt% whereas palm oil typically has a free fatty acid content of about 5 wt% . In order to achieve a target level of free fatty acid content in the waste/virgin oil blend, the virgin oil may itself need to have a preliminary blend step so as to dilute the free fatty acid content arising from a high fatty acid oil such as palm oil .

Unlike prior art industrial biodiesel processes involving the use of waste cooking oils, the process is conducted in the absence of a preliminary esterification process conducted on the waste oil to remove free fatty acids before blending. This step is regarded in the prior art as an essential step in treating the waste oil to make it suitable for biodiesel production. However, the present inventor has realised that the preliminary esterification step, and associated plant machinery, may be omitted. This provides significant economic benefits, both in terms of process simplification and reduced use of machinery and materials. The process of the invention is more efficient than the prior art preliminary esterification phase, eliminating the use of highly corrosive concentrated sulphuric acid involved in preliminary esterification, thereby reducing both capital costs and revenue costs in the production of biodiesel . The oil stripping step then serves a dual purpose in removing fatty acids from both the virgin oil feed and the waste oil feed. The stripping step also provides a significant secondary product stream of graded fatty acids which may be exploited as commercial grade raw material in many chemical processes.

In a preferred arrangement the oil blend is cleaned in a cleaning stage to remove impurities after the blending step and before the stripping step. The cleaning step may beneficially be adapted to remove fine particulate debris, un-reacted phosphoric acid from a virgin oil de- gumming stage and residual hydratable gums which may be present in the oil blend.

The cleaning step may comprise mixing the oil with hot water, such as by introducing a hot water feed into the oil blend feed stream. The cleaning step preferably then further comprises centrifuging the oil and hot water mixture to wash the oil blend and remove the impurities. The wash water from the centrifuge may then be fed to an effluent treatment stage.

In a preferred embodiment of the process, before the stripping step, and after any cleaning step, the blended oil is heated in a heating stage to between 85 to 95 degrees centigrade. The heating stage conveniently comprises a steam heat exchanger. Typically, after heating the oil blend is dried to remove air and moisture from the oil blend. The drying may be carried out in a drying vessel operating in a partial vacuum whereby moisture and air is removed.

The stripping step is preferably a steam stripping conducted in steam stripping stage in which free fatty acids contained in the oil are distilled from the oil by a steam flow. Beneficially, the free fatty acids are condensed and collected for later use and preferably commercial exploitation in further chemical processes.

In a preferred arrangement, the steam stripping stage comprises at least one counter-current heat exchanger which serves to heat the oil blend before steam stripping. Thus the stripped oil blend may flow away from the stripping stage through the counter current heat exchanger, whereby heat from the stripped oil blend is transferred to the pre-stripped oil blend.

In yet another aspect of the invention the proportion of waste and virgin oils is maintained at a pre-determined target ratio by a ratio controller stage. The ratio controller stage may comprise a modulating control valve apparatus in which appropriate proportions of waste oil is blended with a feed of virgin oil. The target free fatty acid content in the oil blend may be 0.5 to 1.5 wt%.

Following is a description by way of example only and with reference to certain of the figures, of a method of putting the present invention into effect.

In the drawings : -

Figure 1 is a schematic representation of a prior art process and apparatus for the preliminary treatment of used cooking oil in a biodiesel manufacturing process.

Figure 2 is a schematic representation of a process and apparatus of the present invention. Figure 3 is a schematic representation of a process according to an optional aspect of the present invention.

Figure 1 is a prior art process described in the introductory portion of this description. The process and apparatus of one embodiment of the present invention are shown in figures 2 and 3. Figure 3 shows the essential elements of a modulating control valve arrangement. In figure 3 a stored reservoir of used cooking oil methyl ester is indicated at (20) . The UCO includes impurities such as water, particulate debris such as carbonised food particles and about 5 wt% free fatty acids. A feed of UCO is drawn from the reservoir (20) through a solids filter (21) by a first pump (22) . The pump (22) feeds a flow controller (25) via a first mass flow meter (26) . The meter has an output feed line (27) which is provided with an adjustable flow control valve (28) . The feed line merges at (29) with corresponding feed lines from virgin vegetable oil reservoirs (30 and 31) . The first reservoir (30) contains virgin soya oil which has a free fatty acid content of 0.5 wt % . The second reservoir contains virgin palm oil having a free fatty acid content of 3.0 wt %. Each of these further reservoirs is provided with a corresponding pump (32), a mass flow meter (33) and an adjustable control valve (34). Output feed lines (35, 36) merge with line (27) and are directed to the flow controller (25) . Flow data from each of the meters (26, 33) is fed into a data processor (not shown) in the controller by data sample lines (40) . This data provides the ability to calculate the ratio of UCO to WO fed from the reservoirs, as well as the absolute oil feed from the reservoirs. Thus a pre-determined feed blend ratio and oil flux may be set and controlled by adjustment of the valves (28, 34) and feedback control sampling via lines (40) . In addition the proportions of soya oil and palm oil may be controlled to provide a virgin oil pre-blend having a desired free fatty acid content of, for example, 1.0 wt %. This is then combined with the UCO to provide a target free fatty acid in the combined blend of waste and virgin oils . The blended oil feed is thus provided in the required ratio and rates to the processing plant (50) shown in more detail in figure 2. The virgin vegetable oil is preferably de-gummed by use of a phosphoric acid de-gumming process with a centrifuge (not shown) , as is common in the art, and before blending with the UCO.

The blended oil stream receives a hot water feed in mixer (51) . The water oil mixture is then fed into a centrifuge (52) in which the oil is washed to remove fine particles still present in the UCO (typically of about 100-200 micron size) , and to remove both unreacted phosphoric acid from the deguπiming stage and small quantities of hydratable gums not completely removed by the up stream degumming centrifuge.

The effluent wash water from the centrifuge (52) is fed to a decanting tank (not shown) under level control and then pumped to the effluent treatment system (not shown) . The blended, combined oil stream from the centrifuge (52) is then fed to a steam heat exchanger (53) where the oil temperature is raised to between 85 to 95 degrees centigrade.

The hot oil from the steam heat exchanger is then fed to vacuum dryer (54) comprising a de-aerating and drying vessel operating at approximately 0.9 bar so that any air and moisture is drawn off to the vacuum system and subsequently condensed or vented to atmosphere. The oil post drying has a water content of about 100 ppm.

The dried and de-aerated oil from the drying vessel is then pumped (55) via a series of counter current heat exchangers (56) to a steam stripping column (57) . The temperature of the oil is raised to 210 degrees centigrade in the heat exchangers .

The oil is then heated in the steam stripping column up to a maximum temperature of about 260 degrees centigrade via a series of coiled heat exchangers (not shown) immersed in the oil by means of a hot oil heating system. The heated oil then flows down the column through a series of trays (not shown) in which live steam is sparged into the oil. The steam stripping column operates at a vacuum of between 3 to 5 mbar.

The free fatty acids contained in the oil are distilled out of the oil by the steam flow and are subsequently condensed and pumped away to storage (59) from where it can be sold as a commercial product. The use of the steam-stripping column produces a stream of distilled fatty acids at a 80-90 wt% concentration that are readily marketable at high values, typically 50-60% of virgin vegetable oil price.

The free fatty acids may then be separated by fractional distillation in a further step in the process of the invention, or after delivery to a customer.

The bottoms of the steam stripping column are FFA-free vegetable oil. The oil flows through the counter current heaters mentioned above to recover its latent heat. Once cooled (60) the vegetable oil will be stored in a reservoir (61) and then processed to biodiesel in a normal transesterification process suitable for de-gummed virgin vegetable oil.

The present invention avoids the use of a complex and expensive preliminary acid esterification process to remove unwanted FFA' s. In fact the FFA' s are removed during steam stripping and thereby provide an additional income stream for the biodiesel plant operator. In the prior art UCO biodiesel processes, the use of concentrated sulphuric acid as an esterification catalyst requires substantial safety precautions and the use of specially specified materials of construction. The use of corrosive materials inevitably increases maintenance costs over the life of the plant, as well as requiring additional revenue for the purchase of the sulphuric acid. The present invention eliminates the aforementioned problems and provides more economically viable biodiesel manufacture process.

Claims

1. A process for the production of biodiesel from natural oils comprising: providing waste natural oil having a relatively high free fatty acid content, providing virgin natural oil having a relatively low, or zero, free fatty acid content, blending the waste oil and virgin oil in appropriate proportions to produce an oil blend having a target reduced free fatty acid content, stripping the oil blend to produce distilled fatty acids and separating the fatty acids from the oil blend, feeding the oil blend to a transesterification stage thereby to form biodiesel end product .

2. A process as claimed in claim 1 wherein the waste natural oil is a used cooking oil.

3. A process as claimed in claim 1 or claim 2 wherein the at least one of the waste and virgin oils is a vegetable oil.

4. A process as claimed in any preceding claim wherein the virgin oil is degummed oil provided from a virgin oil degumming stage.

5. A process as claimed in any preceding claim wherein the process is conducted in the absence of a preliminary esterification process conducted on the waste oil to remove free fatty acids before blending.

6. A process as claimed in any preceding claim further comprising cleaning the oil blend to remove impurities after the blending step and before the stripping step.

7. A process as claimed in any preceding claim wherein the cleaning step is adapted to remove fine particulate debris, un-reacted phosphoric acid from a virgin oil degumming stage and residual hydratable gums which may be present in the oil blend.

8. A process as claimed in claim 6 or claim 7 wherein the cleaning step comprises mixing the oil with hot water.

9. A process as claimed in claim 8 wherein the cleaning step further comprises centrifuging the oil and hot water mixture to wash the oil blend and remove impurities.

10. A process as claimed in claim 9 wherein the wash water from the centrifuge is passed to an effluent treatment stage.

11. A process as claimed in any preceding claim wherein before the stripping step, and after any cleaning step, the blended oil is heated in a heating stage to between 85 to 95 degrees centigrade.

12. A process as claimed in claim 11 wherein the heating stage comprises a steam heat exchanger.

13. A process as claimed in any preceding claim wherein the before the stripping step, and after any cleaning step, the oil blend is dried to remove air and moisture from the oil blend.

14. A process as claimed in any preceding claim wherein the stripping step is steam stripping conducted in steam stripping stage in which free fatty acids contained in the oil are distilled from the oil by a steam flow.

15. A process as claimed in any preceding claim wherein the free fatty acids are condensed and collected for later use.

16. A process as claimed in claim 14 or claim 15 wherein the steam stripping stage comprises at least one counter current heat exchangers which serve to heat the oil blend before steam stripping.

17. A process as claimed in claim 16 wherein the stripped oil blend flows away from the stripping stage through the counter current heat exchanger, whereby heat from the stripped oil blend is transferred to the pre-stripped oil blend.

18. A process as claimed in any preceding claim wherein the proportion of waste and virgin oils is maintained at a pre-determined target ratio by a ratio controller stage.

19. A process as claimed in claim 18 wherein the ratio controller stage comprises a modulating control valve apparatus in which appropriate proportions of waste oil is blended with a feed of virgin oil.

20. A process as claimed in any preceding claim wherein the virgin natural oil is a pre-blended combination of two or more types of virgin natural oil so as to produce the relatively low free fatty acid content virgin oil .

21. A process as claimed in claim 20 wherein the said combination of two or more virgin natural oils comprises at least one virgin oil having a relatively high free fatty acid content and at least one diluting virgin oil having a lower fatty acid content which ensures that the pre-blend free fatty acid content is less than that of the high fatty acid content virgin oil before blending.

22. A process as claimed in any preceding claim wherein said target reduced free fatty acid content of the oil blend is between 0.5 wt% and 1.5 wt%.

23. Apparatus for conducting the process for the production of biodiesel from natural oils of comprising: a feedstock of waste natural oil having a relatively high free fatty acid content, a feedstock of virgin natural oil having a relatively low, or zero, free fatty acid content, means for blending the waste oil and virgin oil in appropriate proportions to produce an oil blend having a target reduced free fatty acid content, a stripping stage for stripping the oil blend to produce distilled fatty acids and separating the fatty acids from the oil blend, and means for feeding the stripped oil blend to a transesterification stage in which biodiesel end product is formed.

24. Apparatus as claimed in claim 23 and adapted to conduct a process according to any of claims 2 to 22.

25. Apparatus as claimed in any of claims 23 to 24 and provided with feedstock flow rate controller means adapted to control the relative proportions of virgin and waste oils blended to produce the target reduced free fatty acid content.

26. Apparatus as claimed in claim 25 wherein the controller means comprises a data processor operating in a feedback loop with valve means in one or more of the feed stocks, thereby permitting adjustment of the relative volumetric flow rates of feed stocks required to achieve the desired proportions of waste oil to virgin oil in the oil blend.

27. Apparatus as claimed in any of claims 23 to 26 wherein the virgin oil feedstock comprises a first virgin oil source and a second virgin oil source.

28. Apparatus as claimed in claim 27 and provided with means for blending the first and second virgin oils to form the virgin oil feedstock.

29. Apparatus as claimed in claim 25 or claim 26 wherein the first virgin oil source has a higher free fatty acid content than the second.

30. Apparatus as claimed in claim 25 and either of claims 28 and 29, wherein the controller means comprises means adapted to control the relative proportions of first and second virgin oils which are pre-blended to produce the virgin oil feedstock.

31. Apparatus as claimed in claim 30 wherein the data processor further includes a feedback loop with valve means in at least one of the first and second virgin oil sources, whereby the relative flow rates of the respective virgin oils may be adjusted to give a desired pre-blend free fatty acid content.