US20070144060A1

US20070144060A1 - Production of biodiesel from triglycerides via a thermal route

Info

Publication number: US20070144060A1
Application number: US11/638,348
Authority: US
Inventors: Michio Ikura
Original assignee: Canada Minister of Natural Resources
Current assignee: Canada Minister of Natural Resources
Priority date: 2005-12-16
Filing date: 2006-12-14
Publication date: 2007-06-28
Also published as: US20070137097A1; WO2007068097A1; EP1976959A1; EP1976959A4; CA2633632C; CA2633632A1

Abstract

A method is presented for producing biodiesel from a triglyceride feedstock. The feedstock is pretreated by thermal cracking or rapid pyrolysis to convert triglycerides to form a middle distillate fraction rich in fatty acids. The middle distillate fraction is then esterified the in the presence of an alcohol and a catalyst to produce a biodiesel stream. The biodiesel stream can be treated with a basic solution to convert unesterified free fatty acids to non-foaming metallic soaps, which can be removed by known means. A method is also provided for producing a biodiesel/naphtha mixture, in which a triglyceride feedstock is pretreated by thermal cracking or rapid pyrolysis to produce a middle distillate fraction, a naphtha stream and a gas stream. The naphtha stream and the middle distillate fraction are then esterified to produce a mixed biodiesel/naphtha stream, which can be treated with a basic solution to convert unesterified free fatty acids to non-foaming metallic soaps, which are then removed by known means.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation-in-part of U.S. patent application Ser. No. 11/304,658 filed Dec. 16, 2005.

FIELD OF THE INVENTION

The present invention relates to a method of producing biodiesel from triglycerides using thermal cracking. The present invention specifically relates to the production of biodiesel from waste triglycerides.

BACKGROUND OF THE INVENTION

In recent years, the area of biodiesels has drawn a great deal of attention. Biodiesels are plant and animal based fuels produced from the esterification of biomass-derived oils with alcohol. Biodiesel can be produced from such sources as canola, corn, soybean etc. Biodiesels are generally considered less environmentally damaging than traditional fossil fuels.
Another potential source for biodiesels is the waste triglycerides of animal rendering facilities and waste cooking oils, such as those found as restaurant trap greases. However, this potential is presently still under-explored and waste triglycerides are most commonly dumped into landfills (“Biodiesel Production Technology, August 2002-January 2004”; Van Gerpen, J. et al., July 2004, NREL/SR-510-36244). Waste triglycerides often have a high contaminants content that must effectively be removed before processing. Furthermore, waste triglycerides tend to have a high content of free fatty acid (FFA), anywhere in the range of from 50% to 100%. Mixtures of free fatty acids and triglycerides have been found to be very difficult to convert to useful fuels by any traditional methods.
Traditional methods of producing biodiesels include transesterification and esterification with alcohol using either an acid or base catalyst. However, the high FFA content in waste triglycerides causes undesirable soap formation in base catalyzed esterification processes, rendering this process inoperable.
Waste triglycerides are also often heavily contaminated by contaminants, such as bacteria, detergents, silts and pesticides. These contaminants must be removed before esterification can take place, without adding significant additional cost to the overall processes.
One known method of processing high FFA feedstocks involves adding glycerol to the feedstock to convert FFA's to mono- and diglycerides, followed by conventional alkali-catalyzed esterification. This method addresses the high FFA content of waste triglycerides, but does not treat or remove contaminants. A second method involves performing both esterification and transesterification of triglycerides using a strong acid such as H₂SO₄. However, water formation by FFA esterification prevents this process from going to completion. A third method involves pre-treating an FFA-rich triglyceride feedstock with an acid catalyst to convert FFA to alkyl-esters and reduce FFA concentrations to less than about 0.5%, followed by traditional base-catalyzed esterification. This method again, only deals with the FFA content of waste triglycerides, and not the high contaminant levels.
Thermal cracking of clean triglycerides under typical cracking conditions with and without catalyst has been attempted, but this process was found to yield mainly naphtha, not diesel fuels. Furthermore, in typical thermal cracking of clean or waste triglycerides in the presence of a catalyst, there is a tendency for coke formation to occur on the catalyst, resulting in rapid deactivation.
It is therefore greatly desirable to find a method of converting waste triglycerides feedstocks to biodiesel that is both efficient and economical. It is also desirable to find ways of dealing with contaminants and high FFA content in waste triglyceride feedstocks so that they can be converted into usable fuels.

SUMMARY OF THE INVENTION

The present invention thus provides a method of producing biodiesel from a triglyceride feedstock, comprising pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to remove contaminants and convert triglycerides, to form a middle distillate fraction rich in free fatty acids. The middle distillate fraction can then be esterified in the presence of an alcohol and a catalyst to produce a mixed biodiesel/diesel stream. The mixed biodiesel/diesel stream can then be treated with a basic solution to convert unesterified free fatty acids to non-foaming metallic soaps, which non-foaming metallic soaps can be removed by centrifugation, filtering or a combination thereof.
The present invention also provides a method of producing a biodiesel/naphtha mixture from a triglyceride feedstock. The method involves first pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to remove contaminants and convert triglycerides, to produce a middle distillate fraction rich in free fatty acids, a naphtha stream and a gas stream. Next, the naphtha stream and middle distillate fraction are esterified in the presence of an alcohol and a catalyst to produce a mixed biodiesel/naphtha stream. The mixed biodiesel/naphtha stream can then be treated with a basic solution to convert unesterified free fatty acids to non-foaming metallic soaps, which non-foaming metallic soaps are removed by centrifugation, filtering or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail with reference to the following drawings, in which:
FIG. 1 is a flow sheet of a first preferred process for carrying out the present invention; and
FIG. 2 is a flow sheet of a second preferred process for carrying out the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present process employs a novel combination of thermal cracking followed by esterification to convert low quality triglycerides feedstock into usable biodiesel. In the present process, thermal cracking is used as a pre-treatment step to break down the triglycerides into a broad range of free fatty acids and lower molecular weight components. Thermal cracking also serves to remove contaminants found in waste triglycerides, which can cause problems downstream. The resulting product from the cracking step can then be esterified to convert fatty acids into alkyl esters (biodiesel).
For the purposes of the present invention, thermal cracking is considered to loosely cover the process of breaking down large molecules into smaller molecules at a predetermined temperature and pressure. Rapid pyrolysis of waste triglycerides can also be used in the present process and is considered to be encompassed by the term thermal cracking. Details of rapid pyrolysis are given below.
A flow diagram of the process steps and streams of one embodiment of the present invention is shown in FIG. 1. A feedstock 12 of low quality or waste triglycerides is fed to a thermal cracking unit 10. The feedstock 12 can be any variety of waste triglyceride, including restaurant trap greases, waste greases from animal rendering facilities and other forms of waste oils and greases and low-quality vegetable oils. The feedstock stream 12 can be heterogeneous in nature and can contain water and other contaminants. Waste triglycerides used as the feedstock stream 12 can also have free fatty acid (FFA) content as high as 50 to 100 wt. %. In an optional embodiment (not shown), the triglyceride feedstock 12 may be filtered to remove any macroscopic contaminant particles prior to thermal cracking.
In the thermal cracking unit 10, triglycerides in the feedstock stream 12 are significantly reduced since they are converted into free fatty acids, thus forming a mixture of free fatty acids and conventional hydrocarbons, such as paraffins, olefins and aromatics. Thermal cracking is preferably carried out at mild cracking conditions which, for the purposes of the present invention, are described as an operating temperature preferably in the range of from 390 to 460° C., more preferably from 410 to 430° C., and preferably at an operating pressure of from 0 to 60 psig (6.9 to 515 kPa), more preferably from 30 to 40 psig (308 to 377 kPa). Thermal cracking produces various fractions including gases 14, naphtha 16, middle distillate 22, and residue 18. Contaminants from the feedstock 12 end up in the residue stream 18.
It was noted that the mild thermal cracking conditions used in the present invention to crack waste triglycerides produces mainly diesel, having a boiling range of between 165° C. and 345° C., rather than naphtha (IBP to 165° C.), as was produced from thermal cracking of triglycerides at higher temperatures and pressures.
The middle distillate fraction 22 makes up more than half of the thermally cracked product and has been found to have suitable characteristics for further treatment by esterification. The middle distillate fraction 22 comprises free fatty acids formed from thermal cracking of triglycerides, the original free fatty acids present in the feedstock and conventional hydrocarbons. Middle distillates typically encompass a range of petroleum equivalent fractions from kerosene to lubricating oil and include light fuel oils and diesel fuel. In one embodiment of the present invention the middle distillate fraction 22 was found to have a boiling point range of from 150 to 360° C., and more preferably from 165 to 345° C. The middle distillate fraction 22 still has some fuel quality issues such as high viscosity, high acid number, high cloud point and high concentrations of nitrogen and/or sulphur.
The middle distillate fraction 22 is next fed to an esterification unit 20, where it is reacted with an alcohol stream 24 in the presence of a catalyst to produce alkyl esters (biodiesel). The esterification process is carried out at a temperature preferably ranging from 70 to 120° C., more preferably in the range of from 90 to 110° C., and preferably at atmospheric pressure. The alcohol stream 24 can be any suitable alcohol known in the art, or mixtures thereof. The alcohol stream 24 is preferably methanol.
It is surprisingly noted that esterification could be carried out well above the boiling temperature of the reacting alcohol, despite low alcohol concentration in the liquid phase of the reaction mixture. The ability to conduct the esterification at higher temperatures is advantageous since this allows continuous water stripping by the flashing alcohol stream. Since water is a co-product of acid esterification, it can detrimentally quench the esterification reaction if not removed continuously.
The catalyst can be either an acidic solid or liquid catalyst. Preferably, the acid catalyst is chosen from sulphuric acid (H₂SO_4(l)), sulphamic acid (H₂NSO₃H_(l)), formic acid (HCO₂H_(l)), acetic acid (CH₃CO₂H_(l)), propionic acid (CH₃CH₂CO₂H_(l)), hydrochloric acid (HCl_(l)), phosphoric acid (H₃PO_4(l)), sulphated metal oxides such as sulphated zirconia, and styrene divinylbenzene copolymers having SO₃H functional groups, such as Amberlyst 36™. Amberlyst 36 is most preferred for the esterification reaction, as this does not leave any trace in the esterification product, and further washing of the esterification product is thus not required.
Free fatty acids can be acid esterified by the following reaction, here shown with the alcohol optionally being methanol:
The water byproduct can inhibit the reaction, and may prevent esterification from going to completion. As mentioned above, esterification at temperatures above the boiling temperature of the alcohol has been surprisingly found to alleviate this problem in the present invention.
Esterification produces a raw diesel stream 26 of approximately 50% alkyl esters (biodiesel) and 50% hydrocarbons. These hydrocarbons can include tetradecane, pentadecane, 1-hexadecene, hexadecane, heptadecane, 1-octadecene, octadecane, nonadecane, 1-eicosene, eicosane, heneicosane, 1-docosene, docosane, tricosane, tetracosane, pentacosane, hexacosane, heptacosane, octacosane, nonacosane, triacontane, untriacontane, dotriacontane, tritriacontane, tetratriacontane, pentatriacontane, hexatriacontane, heptatriacontane, and octatriacontane.
It should be noted that, in addition to esterifying only the middle distillates fraction 22 from thermal cracking, it is also possible to esterify both the naphtha stream 16 and middle distillates fraction 22 from the thermal cracking step. This optional method circumvents an extra step of separating naphtha 16 from the middle distillates 22.
Depending on the type of catalyst used and the degree of esterification achieved, the raw diesel stream 26 may exceed acidity limits allowed by ASTM specifications for biodiesel, namely 0.8 mg KOH/g. To reduce acidity, the raw diesel stream 26 can optionally be fed to a base treatment unit 30, together with a basic solution 28. The basic solution 28 reacts with any unreacted fatty acids in the raw diesel stream 26 to produce non-foaming metallic soaps with low solubility in biodiesel. These non-foaming metallic soaps can then be removed by either centrifugation or filtering or a combination thereof. Base treatment is preferably carried out at temperatures of from 30 to 60° C., and more preferably at temperatures of from 40 to 50° C. and preferably at atmospheric pressure. The basic solution is preferably chosen from lithium hydroxide (LiOH), magnesium hydroxide (Mg(OH)₂), and calcium hydroxide (Ca(OH)₂). Most preferred are LiOH and Ca(OH)₂.
The base treatment step results in a mixed biodiesel/diesel product 32 that has been found to have excellent fuel properties. The boiling point of the resultant biodiesel/diesel product 32 is found to be lighter and the boiling point distribution broader than that of biodiesel produced by conventional transesterification alone. The mixed biodiesel/diesel product 32 can be used both neat or can optionally be further blended with regular diesel.
The naphtha stream 16 from the thermal cracking unit 10 contains oxygenates and can optionally be sold as a valuable by-product such as octane improver. The residue stream 18 can be discarded by well known means in the art.
As mentioned earlier, the step of thermal cracking can optionally be replaced by a step of rapid pyrolysis. This process is shown in FIG. 2. Rapid pyrolysis is a process of decomposing a chemical at very high temperatures and in the absence of an oxidizing agent. Rapid pyrolysis has very short residence times when compared to thermal cracking.
In the present invention, rapid pyrolysis of triglycerides, more specifically trap grease, was conducted at temperatures ranging from 480° C. to 600° C. for approximately 2 seconds. The triglycerides 12 are fed to a fluidized bed reactor 34 which is preferably fluidized with steam 36, although other suitable fluidizing media known in the art can also be used and are encompassed by the present invention. Steam 36 may be fed to the reactor at a ratio ranging from 0.5 to 1.5, relative to the triglyceride feed stream. The preferred steam to triglyceride feed ratio is 0.9.
Any known inert gas 38 can optionally be added to the reactor to purge the reactor of free oxygen during pyrolysis. The inert gas 38 is preferably nitrogen. A catalyst may also be added, and suitable catalysts include, but are not limited to acid washed activated carbon, calcined sewage sludge solids and silica sand, such as silica alumina. The catalyst acts to enhance the selective cracking of triglyceride molecules to largely free fatty acid molecules.

Sample data of rapid pyrolysis conducted by the inventor on a trap grease feedstock is listed in Table 1 below. The resultant pyrolysis products are shown in Table 2.

TABLE 1


Rapid pyrolysis conditions

Run ID

	261	265	253

Temperature (° C.)	511	575	580
Fluidizing media	Steam	Steam	Steam
Steam/Feed ratio	˜0.9	˜0.9	˜0.9
by weight
N₂purge/Feed	˜7	˜7	˜7
ratio by weight
Catalyst	Acid washed	Sewage sludge	Silica sand
	activated	solids,
	carbon, 35	calcined at
	mesh minus	750° C.
Gas phase	˜2	˜2	˜2
contact time (s)

TABLE 2


Rapid Pyrolysis Products

	261	265	253

Gas	28.2	11.3	7.6
Liquid	50.3	89.4	90.7
Solids (coke)	9.0	Trace	1.4
Total above	87.5	100.7	99.7

The liquid fraction identified in Table 2 above contains middle distillates 22 as well as naphtha 16 and some residue 18. The boiling point distribution of the liquid fraction was determined by thermogravimetric analysis (TGA) and is given in Table 3 below. The middle distillates yield is given in Table 4. These tables indicate that rapid pyrolysis of triglycerides produces an even larger proportion of desirable middle distillates than thermal cracking.

TABLE 3

Boiling point distribution of the liquid fraction (from TGA)

261 265 253

Naphtha (IBP˜165° C.) 86% 10% 8%

Middle distillate (165˜345° C.) 12% 75% 64%

Residue (345° C. plus) 2% 15% 28%

TABLE 4


Middle distillate yield with respect to feed

	261	265	253

	Middle distillate (wt % of feed)	6%	67%	58%

The middle distillate fraction 22 produced by rapid pyrolysis was found to have varying free fatty acids (FFA) content, depending on the pyrolysis conditions. These details are shown in Table 5 below:

TABLE 5

Fatty acids in the middle distillate fraction

Run ID

261 265A 265B 253

Pyrolysis Temperature (° C.) 511 575 575 580

Total FFA wt % 0.63 45.70 45.50 33.17
The present inventor noted that the largest middle distillates fraction was produced by rapid pyrolysis at a temperature of 575° C. As well, FFA content was highest for this temperature range. A preferred temperature range for rapid pyrolysis of the present process is therefore from 550° C. to 600° C. and a most preferred range is from 565° C. to 585° C.
The difference in middle distillates yield between the run at 575° C. and the run at 580° C. is thought to be due to the difference in catalysts rather than the small difference in temperature. Catalyst derived from sewage sludge is less acidic than silica sand. Thus, although the run with silica sand produced a slightly larger liquids fraction by deoxygenation, this was accompanied by higher coke and residue formation, resulting in an overall lower level of middle distillates. Thus the sewage sludge appears to provide a preferred balance between higher middle distillate yield and lower coke formation.
It has also been noted that the middle distillate stream produced by rapid pyrolysis comprises practically no nitrogen. Nitrogen content in the middle distillate obtained by mild thermal cracking was in the order of 5200 ppm whereas that in the middle distillate obtained by rapid pyrolysis was 0.3 ppm. This is particularly advantageous since the presence of nitrogen diminishes the quality of the final biodiesel product.
As well, total sulphur in the middle distillate obtained by mild thermal cracking was in the order of 500 ppm whereas that in the middle distillate obtained by rapid pyrolysis was 150 ppm. Both pre-treatment steps produce free fatty acids and other components containing sulphur and nitrogen. However, it is thought that products from rapid pyrolysis leave the reactor before the sulphur and nitrogen-containing components start to react with each other and become an integral part of the middle distillates fraction. Once nitrogen and sulphur enter the middle distillate stream, it can be very difficult to remove them from the final alkyl ester (biodiesel) product.
The following examples serve to better illustrate the process of the present invention, without limiting the scope thereof:

EXAMPLE 1

Conversion of Restaurant Trap Grease into Mixed Biodiesel/Diesel Product

Restaurant trap grease having an average density of 0.925 g/mL was fed to a thermal cracking unit where it was cracked at a temperature of 418.5° C. and a pressure of 29 psig (301 kPa) for 40 minutes. Thermal cracking produced a gas stream, a naphtha stream, a middle distillate stream with a maximum boiling point of approximately 343° C., as well as water and residue. The middle distillates stream made up 63.0 wt % of the total cracked product and had an acid number of 83.93 mg KOH/g.
The middle distillate stream was then fed to an acid esterification unit, where it was contacted with methanol in the presence of an Amberlyst 36 catalyst. Esterification was conducted at a temperature of 90° C. and at atmospheric pressure for 20 hours.
Esterification produced a raw diesel stream which was then treated with a calcium hydroxide solution, Ca(OH)_2(aq), to produce a final mixed biodiesel/diesel product having an acid number of 0.45 mg KOH/g. The final product was found to have 0.22 wt. % nitrogen, 136 ppm sulphur and a viscosity of 5.02 cSt; the sulphur content and viscosity being well within ASTM 6751 standards for biodiesel

EXAMPLE 2

Conversion of Rendered Animal Fat into Mixed Biodiesel/Diesel Product

Rendered animal fat, having an average density of 0.918 g/mL was fed to a thermal cracking unit in which it was cracked at 411° C. and atmospheric pressure for 40 minutes. The thermally cracked product contained 68.6 wt % middle distillates having a maximum boiling point of 345° C., naphtha and the remainder gas, water and residues.
The middle distillate stream, having a viscosity of 8.50 cSt, and an acid number of 146.96 mg KOH/g, was then fed to an acid esterification unit, where it was contacted with methanol in the presence of an Amberlyst 36 catalyst. Esterification was conducted at a temperature of 90° C. and at atmospheric pressure for 20 hours.
The resultant raw diesel stream was then treated with a calcium hydroxide solution, Ca(OH)_2(aq), to produce a final mixed biodiesel/diesel product having an acid number of 0.75 mg KOH/g. The final product was found to have 18 ppm sulphur and 158 ppm nitrogen, and a viscosity of 4.84 cSt.
This detailed description of the process and methods is used to illustrate certain embodiments of the present invention. It will be apparent to those skilled in the art that various modifications can be made in the present process and methods and that various alternative embodiments can be utilized. Therefore, it will be recognized that various modifications can also be made to the applications to which the method and processes are applied without departing from the scope of the invention, which is limited only by the appended claims.

Claims

1. A method of producing biodiesel from a triglyceride feedstock, the method comprising:

a. pretreating the triglyceride feedstock by thermal cracking to remove contaminants and convert triglycerides to form a middle distillate fraction rich in free fatty acids;

b. esterifying the middle distillate fraction in the presence of an alcohol and a catalyst to produce a biodiesel stream;

c. treating the biodiesel stream with a basic solution to convert unesterified free fatty acids to non-foaming metallic soaps; and

d. removing the non-foaming metallic soaps by centrifugation, filtering or a combination thereof.

2. The method of claim 1 wherein the basic solution is an aqueous solution of a compound selected from the group consisting of lithium hydroxide (LiOH), magnesium hydroxide (Mg(OH)₂), and calcium hydroxide (Ca(OH)₂).

3. The method of claim 2 wherein the basic solution is an aqueous solution of calcium hydroxide or lithium hydroxide.

4. The method of claim 1 wherein the triglyceride feedstock is selected from the group consisting of restaurant trap grease, rendered animal fats, waste greases, low-quality vegetable oils and combinations thereof.

5. The method of claim 1 wherein thermal cracking is conducted at a temperature of from 390° C. to 460° C.

6. The method of claim 1 wherein thermal cracking is conducted at a temperature of from 410° C. to 430° C.

7. The method of claim 1 wherein the middle distillate fraction is esterified in the presence of methanol as the alcohol.

8. The method of claim 7 wherein esterifying is conducted at a temperature of from 70° C. to 120° C.

9. The method of claim 8 wherein esterifying is conducted at a temperature of from 85° C. to 110° C.

10. The method of claim 1, wherein the middle distillate fraction is esterified in the presence of an acid catalyst.

11. The method of claim 10 wherein the acid catalyst is selected from the group consisting of sulphuric acid (H₂SO_4(l)), sulphamic acid (H₂NSO₃H_(l)), formic acid (HCO₂H_(l)), acetic acid (CH₃CO₂H_(l)), propionic acid (CH₃CH₂CO₂H_(l)), hydrochloric acid (HCl_(l)), phosphoric acid (H₃PO_4(l)), sulphated metal oxides such as sulphated zirconia, and styrene divinylbenzne copolymers having SO₃H functional groups.

12. The method of claim 11 wherein the acid catalyst is a styrene divinylbenzene copolymer having an SO₃H functional group.

13. The method of claim 1, further comprising filtering the triglyceride feedstock before thermal cracking to remove macroscopic contaminant particles.

14. A method of producing a biodiesel/naphtha mixture from a triglyceride feedstock, the method comprising:

a. pretreating the triglyceride feedstock by thermal cracking to remove contaminants and convert triglycerides to produce a middle distillate fraction rich in free fatty acids, a naphtha stream and a gas stream;

b. esterifying the naphtha stream and middle distillate fraction in the presence of an alcohol and a catalyst to produce a mixed biodiesel/naphtha stream;

c. treating the mixed biodiesel/naphtha stream with a basic solution to convert unesterified free fatty acids to non-foaming metallic soaps; and

15. A method of producing biodiesel from a triglyceride feedstock, the method comprising:

a. pretreating the triglyceride feedstock by rapid pyrolysis to remove contaminants and convert triglycerides to form a middle distillate fraction rich in free fatty acids;

16. The method of claim 15 wherein the basic solution is an aqueous solution of a compound selected from the group consisting of lithium hydroxide (LiOH), magnesium hydroxide (Mg(OH)₂), and calcium hydroxide (Ca(OH)₂).

17. The method of claim 16 wherein the basic solution is an aqueous solution of calcium hydroxide or lithium hydroxide.

18. The method of claim 15 wherein the triglyceride feedstock is selected from the group consisting of restaurant trap grease, rendered animal fats, waste greases, low-quality vegetable oils and combinations thereof.

19. The method of claim 15 wherein rapid pyrolysis is conducted at a temperature of from 480° C. to 600° C.

20. The method of claim 15 wherein rapid pyrolysis is conducted at a temperature of from 550° C. to 600° C.

21. The method of claim 15 wherein rapid pyrolysis is conducted at a temperature of from 565° C. to 585° C.

22. The method of claim 15 wherein the triglyceride feedstock is fluidized with steam.

23. The method of claim 22 wherein the steam to triglyceride feedstock ratio ranges from 0.5 to 1.5.

24. The method of claim 23 wherein the steam to triglyceride feedstock ratio is 0.9.

25. The method of claim 15 wherein an inert gas is used to purge any oxidizing agents during rapid pyrolysis.

26. The method of claim 25 wherein the inert gas is nitrogen.

27. The method of claim 15 wherein a catalyst is used during rapid pyrolysis to enhance the cracking of triglycerides to largely free fatty acids.

28. The method of claim 27 wherein the catalyst is selected from the group consisting of acid washed activated carbon, calcined sewage sludge solids and silica sand.

29. The method of claim 15 wherein the middle distillate fraction is esterified in the presence of methanol as the alcohol.

30. The method of claim 29 wherein esterifying is conducted at a temperature of from 70° C. to 120° C.

31. The method of claim 30 wherein esterifying is conducted at a temperature of from 85° C. to 110° C.

32. The method of claim 15, wherein the middle distillate fraction is esterified in the presence of an acid catalyst.

33. The method of claim 32 wherein the acid catalyst is selected from the group consisting of sulphuric acid (H₂SO_4(l)), sulphamic acid (H₂NSO₃H_(l)), formic acid (HCO₂H_(l)), acetic acid (CH₃CO₂H_(l)), propionic acid (CH₃CH₂CO₂H_(l)), hydrochloric acid (HCl_(l)), phosphoric acid (H₃PO_4(l)), sulphated metal oxides such as sulphated zirconia, and styrene divinylbenzne copolymers having SO₃H functional groups.

34. The method of claim 33 wherein the acid catalyst is a styrene divinylbenzene copolymer having an SO₃H functional group.

35. The method of claim 15, further comprising filtering the triglyceride feedstock before thermal cracking to remove macroscopic contaminant particles.

36. A method of producing a biodiesel/naphtha mixture from a triglyceride feedstock, the method comprising:

a. pretreating the triglyceride feedstock by rapid pyrolysis to remove contaminants and convert triglycerides to produce a middle distillate fraction rich in free fatty acids, a naphtha stream and a gas stream;