Production of biodiesel from triglycerides via a thermal route

Abstract

A method is presented for producing biodiesel from a triglyceride feedstock. The feedstock is first thermally cracked to remove contaminants and 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 centrifugation, filtering or a combination thereof. A method is also provided for producing a biodiesel/naphtha mixture, in which a triglyceride feedstock is thermally cracked 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. The basic salts are removed by known means.

Description

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 from 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 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 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 biodieseunaphtha stream. The mixed biodieseunaphtha 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 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 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 hydrocarbon 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.

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, 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 bio-diesel. 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), potassium hydroxide (KOH), 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.

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), potassium hydroxide (KOH), 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 (H2SO4(l)), sulphamic acid (H2NSO3H(l)), formic acid (HCO2H(l)), acetic acid (CH3CO2H(l)), propionic acid (CH3CH2CO2H(l)), hydrochloric acid (HCl(l)), phosphoric acid (H3PO4(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

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