Method for Purifying Biodiesel Fuel

Abstract

A method for purifying a biodiesel fuel includes the use of subjecting a biodiesel fuel to an electric field. The electric field forms a precipitate in the fuel that removes the impurities of excess catalysts and soap that are byproducts of the reaction that forms the biodiesel. This electric field assisted washing process can be applied to a biodiesel fuel in a batch process or, alternatively, in a continuous process.

Description

This application claims the benefit of U.S. Provisional Application No. 61/099,654, filed Sep. 24, 2008, entitled “An Electric Field Assisted Method for Biodiesel Purification”.

ORIGIN OF THE INVENTION

The invention was made in part by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND

The present invention is directed to the use of an electric field for biodiesel fuel purification during the manufacture of biodiesel fuel.

Biodiesel fuel is commercially produced by homogeneous catalysis of transesterification from oil to methyl esters. The homogeneous production method has some commercial inefficiencies in it. One of the most expensive parts of the homogeneous biodiesel production process is the washing step. Crude biodiesel is washed with distilled water once it has been transesterified in order to remove excess catalyst and soap. This is a critical step in the production process because the leftover catalyst, generally sodium hydroxide, may induce clogging or have caustic effects upon the engine. Distilled water, however, is expensive and is used profusely during the washing process. Apart from the extensive use of distilled water, the catalyst cannot be recovered once it has been used to facilitate the reason.

The biodiesel research community has looked into heterogeneous catalysis as an alternative, anticipating that it will be cheaper and faster than the current homogeneous process. Currently however, it is more viable to use the homogeneous catalysis method since it takes less time. Due to existing infrastructure, many companies will be unwilling to dramatically change their process. Sodium hydroxide, the preferred catalyst used in commercial plants, is also fairly cheap, easy to access, and produces a high conversion rate, making it the ideal catalyst to use.

SUMMARY

Accordingly, it is an object of the present invention to facilitate the washing of biodiesel fuel during the synthesis of that fuel. Specifically, the biodiesel fuel is subjected to an electric field that will precipitate out unwanted byproducts of the synthesis reaction that formed the biodiesel fuel.

In one example, a method for purifying a biodiesel fuel comprises the steps of providing a triglyceride, methanol, and homogeneous catalyst. A mixture of the triglyceride, methanol, and catalyst is reacted to form a reaction product comprising a glycerol fraction and a crude biodiesel fuel fraction. The glycerol and crude biodiesel fuel fractions are separated. The biodiesel fuel fraction is subjected to an electric field to form a precipitate in the crude biodiesel fuel fraction. The precipitate is then removed from the biodiesel fuel fraction, whereby the remaining biodiesel fuel fraction after removal of the precipitate is more purified than the crude fraction.

In another example, a method of purifying crude biodiesel fuel comprises the steps of providing a crude biodiesel fuel formed by a process comprising homogeneous catalysis. The crude biodiesel fuel is placed in an electric field to form a precipitate. The precipitate is then removed from the crude biodiesel fuel.

The triglycerides with long chain carbon and hydrogen atoms typically consisting of 11 to 18 carbons that may be used during the synthesis of the biodiesel are selected from the group consisting of vegetable oil, soybean oil, corn oil, rapeseed oil, canola oil, peanut oil, cottonseed oil safflower oil, linseed oil, coconut oil, animal fat, lard, tallow and mixtures thereof. The homogeneous catalyst used during the synthesis of the biodiesel fuel may be selected from the group consisting of sodium hydroxide, potassium hydroxide, and sodium methoxide. The methods may be performed in a batch process or, alternatively, a continuous process. The electric field may have a strength of at least about 20 V/cm, or in the range of about 10 to 200 V/cm.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 displays the exemplary transesterification reaction of triolein to produce a biodiesel fuel.

FIG. 2a is a circuit diagram of an experimental set up, and FIG. 2b is an illustration of the experimental set up.

FIGS. 3a and 3b are graphs showing the current vs. time performance of the carbon 5 mm electrode and carbon 5 mm electrode at 24 hrs.

FIG. 4 is a graph of the current vs. time for the carbon 15 mm electrode.

FIG. 5 is a graphic overlay of FTIR spectra for vegetable oil and biodiesels.

FIG. 6 is a graphic overlay of FTIR spectra for precipitate and biodiesel fuel purified for 24 hours.

FIGS. 7 and 8 are graphs of NMR results of the biodiesel fuel in various phases.

FIG. 9 is the kinematic viscosity results for purified biodiesel fuels.

DETAILED DESCRIPTION

The most common way of producing biodiesel fuel is transesterification. In a transesterification reaction, methanol and the catalyst are mixed together to produce a methoxide. The methoxide is then mixed with a triglyceride and left to rest. During this step, the glyceride chains are cut off and then replaced with the alcohol, yielding glycerol as a viscous by-product and methyl esters (biodiesel) which are less viscous than the triglyceride used as the starting material. The example reaction shown in FIG. 1 is for the triglyceride known as triolein, one of the main components in soybean oil.

After the reaction is completed, biodiesel rises to the top of the mixture, while the glycerol (by-product) settles to the bottom. The biodiesel is removed and then washed with distilled water to remove the excess catalyst and any soap formed in the reaction. Once the purified biodiesel is heated and dried, it is ready for commercial use.

In the present invention, it was determined that the washing process should be considered for modification. Instead of washing the as-produced biodiesel fraction with distilled water, electric field assisted purification of the unwashed product was explored. An electric field was generated between two electrodes immersed in unwashed biodiesel to assist purification by replacing the normal washing process in biodiesel production. Since sodium ions are positively charged, they are attracted to the negative electrode, while the negative component of the soap impurity is attracted to the positive electrode, therefore removing the excess catalyst and the soap without using distilled water.

Independent variables tested include the electric field strength and the length of time that the electric field was being applied. The dependent variable was the amount of sodium remaining in the biodiesel. This dependent variable was measured by tracking the current readings as a function of the applied voltage, spectroscopy to provide qualitative information, viscosity and elemental analysis to determine residual sodium. The controls were commercial grade biodiesel (B 100) and washed biodiesel from the stock biodiesel produced. The constants of the examples were the environment in which the example was conducted, the materials and method used to construct the electrodes, the voltage used in the electric field assisted “washing” examples, the area of the electrodes immersed in the sample, the source of unwashed biodiesel and the instruments that were used. As demonstrated, electric field assisted “washing” has the same effect as washing raw biodiesel with distilled water, and electric field assisted “washing” will clean the biodiesel faster when a stronger electrical field is applied by shortening the distance between the electrodes while keeping the voltage constant.

Producing Biodiesel

Biodiesel was made by reacting soybean oil with base catalyst dissolved in methanol following a procedure based on the one published in Make Journal—Elam, R., Making Biodiesel, Make: Technology on Your Side 2005, 3 68-75. The stock biodiesel was produced by combining 500 mLs of Food Lion brand soybean oil with methoxide made from 2.5 g of Roebic® Crystal Drain Opener (100% NaOH) dissolved in 110 mLs of Heet® (100% methanol). The oil was warmed to 54.4° C. and mixed with the methoxide in a large plastic (PET) bottle. The mixture was shaken vigorously for five minutes and then allowed to settle on its side overnight. Glycerol settled out at the bottom and the as-made biodiesel separated out on top. All samples used in the examples described herein were taken from this batch of unwashed stock biodiesel.

Conventional Washing With Distilled Water

Approximately 50 mL of the biodiesel produced was transferred into a separatory funnel. The biodiesel was washed by gently mixing warm distilled water with the biodiesel. Cloudy wash water was drained from the separatory funnel and more warm water was added. This process was repeated until the water separated out quickly and was clear. Once washing was completed, the biodiesel was transferred to a beaker and gently heated until it dried and was clear enough to read a newspaper through it. After sitting overnight to dry more thoroughly, it was transferred into a glass storage container.

Electric Field Assisted Washing

Electric field assisted purification of biodiesel was carried out using two types of electrodes: copper and carbon. The copper electrodes were constructed by stripping 5 cm of insulation off a bell wire. The stripped wires were used as electrodes in the experimental set-up displayed in FIG. 2b.

The carbon electrodes were constructed using fifteen sticks of Pentel® Super Hi-Polymer® 0.9 mm thick mechanical pencil lead held together with Loctite® Metal/Concrete Epoxy. In order to assure uniform electrodes for each experiment, a jig was constructed by gluing two pieces of wood on a larger piece of wood with a gap equal to the width of the electrodes. To construct the electrodes, a piece of aluminum foil was placed near the top part of the jig. The fifteen sticks of pencil lead were placed in the jig and covered with another piece of wood to insure that the sticks were flat and even. A small amount of epoxy was then applied using toothpicks to the top part of the sticks with the aluminum foil underneath.

The tops of the electrodes were wrapped in aluminum foil to ensure that the electrodes were completely conductive. An ohmmeter was used to measure resistance to insure that the epoxy had not made any breaks in the electrodes. Once the electrodes were constructed, they were glued to a piece of wood that kept them 15 millimeters apart. The holder was modified with a thinner shim for the 5 millimeter electrodes.

Circuit Configuration of Experimental Set-Up

The electrodes were connected to a Pasco Scientific Model SF·9585 High Voltage Power Supply and an ammeter used to measure the amount of current flowing between the electrodes. The circuit diagram for the experimental set-up is shown in FIG. 2a. The electrodes were placed in 25 milliliters of unwashed biodiesel as shown in FIG. 3b and the power supply was set to 50 volts. A stir plate and Teflon coated stir bar were also used to insure that the biodiesel circulated between the electrodes. Current was recorded for experiments that lasted for 1.5, 4, 8, and 24 hours in increments of 15 minutes for the first four hours and every half hour after that.

Characterization of Biodiesel

Fourier Transform Infrared Spectroscopy

Fourier Transform Infrared Spectroscopy (FTIR) was run on a Thermo Nicolet model number IR300 spectrometer. The sample collection parameters were: 64 scans with a resolution of 1 cm⁻¹ from 550-4000 cm⁻¹.

Proton Nuclear Magnetic Resonance Spectroscopy

Proton Nuclear Magnetic Resonance Spectroscopy (¹H NMR) was run on a Bruker 300 MHz spectrometer. Each of the samples was prepared by diluting a few drops of the specimen in deuterated chloroform and then shaking in an NMR sample tube.

Viscosity

To obtain the kinematic viscosity of the samples, a constant temperature water bath was set at a temperature of 40° C. A Cannon 100 Fenske viscometer was filled with 10 mL of sample using a syringe and a 0.5 μtm Teflon filter. Once the water bath was up to the correct temperature, the viscometer was placed into the water bath and was held by a rubber stopper for at least 10 minutes to let the sample come to temperature. A pipet pump was used to suck up the sample above the upper line on the lower bulb on the right arm of the viscometer. The flow time of the sample was measured by marking the time it took for the sample to move from the line at the top to the line at the bottom of the lower bulb. The time was measured by a stopwatch and recorded. The process was repeated five times for each sample.

Elemental Analysis

Elemental analysis for sodium was performed by Midwest Laboratories. The tests were performed using the inductively coupled plasma method according to the DIN EN 14538:2006-09 standard.

Results

The experiment of electric field assisted wash of crude biodiesel was first conducted using copper electrodes because they were easy to construct and incorporate into the experimental set-up. However, when the experiment was being conducted, the current readings kept increasing instead of decreasing to zero as was hypothesized. The decrease in current was expected because it was thought that the sodium ions in the unwashed biodiesel would be attracted to the negative electrode and the carboxylate ions of the soap would be attracted to the positive electrode. Once there were no conductive species in the biodiesel, current should not be able to now flow between the electrodes. Copper tends to be reactive, so the electrode material had to be changed to something else. To be sure that the electric field was indeed the only thing cleaning the biodiesel, the copper electrodes were switched to carbon electrodes. Carbon is an inert material and will not affect the reaction already happening with the electric field, so the results from the experiments can be correctly interpreted.

In order to look at the effect of electric field strength on the process of washing biodiesel, two electrode distances were utilized. At constant voltage, the smaller the gap between the electrodes, the stronger the electric field. The general equation to calculate electric field strength is E=V/l, where E is the electric field strength, V is the applied voltage, and l is the distance between the electrodes. One set of experiments had the electrodes 15 mm apart and the other set had the electrodes 5 mm apart. Therefore, the electric field strengths were calculated to be 33 V/cm and 100 V/cm respectively.

Data was initially collected at 15 and 30-minute intervals. FIG. 3a is the overlay of current vs. time for 1.5, 4, 8 and 24 hours for the first 8 hours. It shows that current decreased for the first 3 hours and that at around 4 hours, the current reading fluctuated between 0 and 0.1 μA. During this period, the biodiesel also became cloudy beginning around 1.5 hours. The 1.5-hour samples stayed cloudy even after they were left to rest. The samples processed for longer than 1.5 hours did clear up after being left to rest; a gel precipitated out and settled at the bottom of the container. Between 6 and 7 hours, the current readings suddenly spiked, decreased, then continued to rise. Although the cause of the spike is not understood at this time, it was noted that at the end of the 24-hour experiment, some of the gel fell off the electrodes when the electrodes were removed from the sample.

The current recorded for the 15 mm carbon electrode behaved similarly as the 5 mm carbon electrode as shown in FIG. 4. Current decreased to 0 μA in the first 3 hours then spiked up between 6 and 7 hours and continued to increase up to 24 hours. Gel was observed to form during the process as well.

Fourier Transform Infrared Spectroscopy

The results for FTIR are shown in FIGS. 5 and 6. FIG. 5 is an overlay of the spectra of vegetable oil, washed biodiesel and commercial B100 and shows the conversion of vegetable oil to biodiesel. The peak at 1008 cm⁻¹ characteristics of the asymmetric O—CH₂-—C group found in the triglyceride disappeared in the biodiesel spectrum. The two peaks present for biodiesel but not for the triglyceride were 1433 cm⁻¹ for the CH₃-asymmetric bend and 1200 cm⁻¹ for the O—CH₃ stretch. FTIR was useful for confirming the transesterification reaction. However, it cannot distinguish impurity levels since the unwashed biodiesel's spectrum matched the commercial B100's spectrum.

One spectrum in FIG. 6 represents the 24-hour carbon electrode (5 mm) experiment and the second spectrum represents the precipitate. The peaks on the second spectrum, 1560 cm⁻¹ and 1400 cm⁻¹, represent a carboxylic acid converted into its inorganic salt. The peak at 3364 cm⁻¹ represents bonded OH, which also implies the presence of a carboxylic acid. This suggests that the precipitate was the soap formed from the transesterification side reaction. It was also observed that suds were formed when the containers with the residue present were rinsed with water. Therefore, it may be concluded that electric field assisted purification removes soap.

The spectra in FIG. 7 show the conversion of soybean oil to biodiesel. The signals at 4.1-4.4 ppm represent the glyceryl groups of the soybean oil and the singlet at 3.7 ppm represents the methyl ester which is biodiesel. This data agrees with the FTIR data that biodiesel was produced. The 3.7 ppm peak is smaller for the unwashed biodiesel than it is for the commercial B100 and washed biodiesel, indicating the lower purity of the unwashed biodiesel. In contrast to FTIR, NMR was able to show the presence of impurities in the unwashed biodiesel. This suggests that NMR should be coupled with FTIR to determine the identity of the impurities in the commercial biodiesel production process.

The spectra in FIG. 8 are for the samples purified with the 15 mm carbon electrodes. The spectra are arranged to show the effect of the length of time an electric field is applied to unwashed biodiesel (bottom). At 1.5 hours, a singlet at 3.5 ppm appears, suggesting that the carboxylic acid was present at the same time the samples turned cloudy. In the presence of sodium ions, carboxylic acid can be converted to carboxylase salt. As processing time progresses, this singlet slowly disappeared until the spectrum of the 24-hour sample matched the washed biodiesel spectrum. This suggests that electric field assisted washing has the same effect as washing biodiesel with distilled water, removing the soap by precipitation.

Viscosity

The kinematic viscosity was calculated by converting the time of the viscosity trials to seconds. The time was then multiplied by the calibration constant for the viscometer 40° C.-0.01470 mm²/s². The results for kinematic viscosity are shown in FIG. 9.

Kinematic viscosities were only taken for the 24-hour samples because the other samples would not have produced accurate readings since the samples had to be filtered before being deposited into the viscometer. The filtration would have removed the source of the cloudiness from the samples and not give the kinematic viscosity of the impure samples. Filtration was necessary to prevent the clogging of the capillary in the viscometer. The standard deviations for these runs ranged from 0.01-0.11% over five trials for each sample. The kinematic viscosities of each of the samples are within the specified range of 1.9-6.0 mm²/s for commercial biodiesel. The samples with kinematic viscosity closest to the commercial biodiesel were those purified using the carbon electrodes. Based on this specification, electric field assisted purification yielded high quality biodiesel.

Elemental Analysis

Sodium analysis was required in order to determine whether or not the electric field assisted washing removed the sodium and the soap from the biodiesel. The results in Table 1 show that an applied electric field removes sodium from the biodiesel.

TABLE 1
Sodium Analysis Results
	SODIUM		SODIUM
	CONTENT		CONTENT
SAMPLE	(PPM)	SAMPLE	(PPM)
Soybean oil	1.01	1.5 Hours 15 mm	8.97
		Carbon
B100 Commercial	Not Detected	4 Hours 15 mm	3.39
Biodiesel		Carbon
Unwashed	28.5	8 Hours 15 mm	2.94
biodiesel		Carbon
Washed biodiesel	Not Detected	24 Hours 15 mm	1.64
		Carbon
1.5 Hours Copper	3.13	1.5 Hours 5 mm	12.0
		Carbon
3 Hours Copper	6.31	4 Hours 5 mm Carbon	6.14
24 Hours Copper	Not Detected	8 Hours 5 mm Carbon	1.82
		24 Hours 5 mm	1.69
		Carbon

Each set of carbon samples show a decrease of sodium as a function of time. There was a dramatic reduction in sodium content after only 1.5 hours of processing with an electric field. The results also imply that the field strength was not directly related to rate of removal of sodium since the rates at which sodium were reduced using 15 mm or 5 mm electrodes were similar. The data for samples purified with copper electrodes were consistent with the current readings and the NMR data. The sodium content increased as time increased, but the sodium was eventually removed after 24 hours. This behavior was not anticipated and suggests that the copper electrodes may have been involved in a chemical reaction. Electric field assisted washing may be more complicated for the reactive copper electrodes than for the inert carbon electrodes.

Summary And Conclusion

This study showed that it is possible to remove soap from biodiesel without using water by applying an electric field using inert carbon electrodes. The electrodes provided a means to remove soap by attracting sodium and carboxylate ions to the oppositely charged electrodes. Once the ions have been attracted. they clump together and precipitate out of the biodiesel, leaving behind clean biodiesel. This process was made possible with field strengths in the range of 33-100 V/cm, which is much lower than the field strengths required to transesterify oil by electro-catalysis. Alternatively, the field strength is at least about 20 V/cm, or still further, in the range of about 10 V/cm to 200 V/cm.

It is believed that the DC (and by inference sufficiently low frequency AC) conductivities are bounded by 1.8×10⁻⁸ to 2.3×10⁻⁶ Mhos/m. The above conductivities relate to the ionic byproducts of the synthesis reaction, presumably the sodium and carboxylate ions. Of course, different conductivities would apply to other reaction products including ionic, polar and nonpolar components. Depending on the biodiesel reaction ingredients, different conductivities of reaction products will exist and must be factored in the design and operation of an appropriate electric field.

As is evident from the foregoing, the reaction rate in this purification process is such that substantial electrode area will be required to make the process useful at any large scale. The foregoing examples are all batch type examples. It is also possible to have a continuous process. Potential constructions that could be used in a continuous process include a trough or tube for crude biodiesel to flow through with multiple sets of electrodes running parallel or perpendicular to the tube/trough. Alternatively, a trough/tube constructed from electrode material (e.g., graphite, or other chemically inert material) which acts as one electrode and another electrode through the center of the trough/tube to act as a second electrode. This allows the continuous application of an electric field as crude biodiesel flows through.

Processing volume can be adjusted by having multiple troughs/tubes in parallel branching from the crude biodiesel source or in series from the source.

Mechanisms for moving gelled precipitate from an electric field assisted purification system include the use of a scrape or squeegee to clean soap residue off of the electrodes at periodic times. Automated wipers could be used. The precipitate could also be removed by applying an electrical switching sequence to the electrodes. Filters could also be used in one or more locations in the process to otherwise filter out the precipitate byproduct.

The triglycerides that may be used in connection with this biodiesel synthesis and subsequent purification process include the following: triglycerides with long chain carbon and hydrogen atoms typically consisting of 11 to 18 carbon atoms, vegetable oil, soybean oil, rapeseed oil, corn oil, canola oil, peanut oil, cottonseed oil, safflower oil, linseed oil, coconut oil, animal fat. lard, tallow, and mixtures thereof.

Also, sodium hydroxide is identified herein for use as a homogeneous catalyst. Other catalysts that may be used include potassium hydroxide, and sodium methoxide.

Although the present method of electric field assisted purification is discussed alone, it will be apparent to those of skill in the art that an electric field may be used with traditional distilled water rinsing to purify crude biodiesel. The use of an electric field in parallel or in series with distilled water rinsing could reduce presumably significantly, the amount of distilled water necessary to end up with acceptably purified biodiesel fuel.

The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The experiments were chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.

Claims

1. A method for purifying a biodiesel fuel comprising the steps of:

providing a triglyceride, methanol, and homogeneous catalyst;

reacting a mixture of the triglyceride, methanol and catalyst to form a reaction product comprising a glycerol fraction and a crude biodiesel fuel fraction;

separating the glycerol and crude biodiesel fuel fractions;

subjecting the biodiesel fuel fraction to an electric field to form a precipitate in the crude biodiesel fuel fraction; and

removing the precipitate from the remaining biodiesel fuel fraction;

whereby the remaining biodiesel fuel fraction after removal of the precipitate is more purified than the crude fraction.

2. A method for purifying a biodiesel fuel as described in claim 1, wherein the triglyceride is selected from the group consisting of triglycerides with long chain carbon and hydrogen atoms consisting of 11 to 18 carbon atoms, vegetable oil, soybean oil, corn oil, rapeseed oil, canola oil, peanut oil, cottonseed oil, safflower oil, linseed oil, coconut oil, animal fat, lard, tallow and mixtures thereof.

3. A method for purifying a biodiesel fuel as described in claim 1, wherein the homogeneous catalyst is selected from the group consisting of sodium hydroxide, potassium hydroxide, and sodium methoxide.

4. A method for purifying a biodiesel fuel as described in claim 1, wherein the crude biodiesel fuel fraction comprises methyl esters.

5. A method for purifying a biodiesel fuel as described in claim 1, wherein the step of subjecting the crude biodiesel fraction to an electric field is performed in a batch process.

6. A method for purifying a biodiesel fuel as described in claim 1, wherein the step of subjecting the crude biodiesel fraction to an electric field is performed in a continuous process.

7. A method for purifying a biodiesel fuel as described in claim 1, wherein the electric field has a strength of at least about 20 V/cm.

8. A method for purifying a biodiesel fuel as described in claim 1, wherein the electric field has a strength in the range of about 10 to 200 V/cm.

9. A method of purifying a crude biodiesel fuel comprising the steps of providing a crude biodiesel fuel formed by a process comprising homogeneous catalysis;

placing the crude biodiesel fuel in an electric field to form a precipitate;

removing the precipitate from the crude biodiesel fuel.

10. A method for purifying a biodiesel fuel as described in claim 9, wherein the crude biodiesel fuel fraction comprises methyl esters.

11. A method for purifying a biodiesel fuel as described in claim 9, wherein the step of subjecting the crude biodiesel fraction to an electric field is performed in a batch process.

12. A method for purifying a biodiesel fuel as described in claim 9, wherein the step of subjecting the crude biodiesel fraction to an electric field is performed in a continuous process.

13. A method for purifying a biodiesel fuel as described in claim 9, wherein the electric field has a strength of at least about 20 V/cm.

14. A method for purifying a biodiesel fuel as described in claim 9, wherein the electric field has a strength in the range of about 10 and 200 V/cm.