Elsevier

Fuel

Volume 87, Issues 15–16, November 2008, Pages 3369-3372
Fuel

Thin layer chromatography and image analysis to detect glycerol in biodiesel

https://doi.org/10.1016/j.fuel.2008.04.033Get rights and content

Abstract

Glycerol is a by-product of biodiesel production and is also considered a contaminant in the final product. Current US and EU standards place limits on the permissible levels of free glycerol and bound (unconverted) mono, di and triglycerides in biodiesel, which can be quantified using gas or high pressure liquid chromatography.

We report a method based on thin layer chromatography that can be effectively used to determine the presence of glycerol in biodiesel. Visual and machine (image analysis) detection limits of 0.1% and 0.2% v/v of free glycerol, respectively, were established using this procedure. In conjunction with a suitable sample enrichment procedure, this procedure could provide a cost effective alternative to glycerol detection.

Introduction

The method widely used to produce biodiesel consists of a transesterification reaction of a vegetable oil with methanol in the presence of a base catalyst. This reaction yields mono-alkyl esters of the fatty acids in the vegetable oil and glycerol as a by-product. While purification procedures exist, the final fuel product can be contaminated by partial glycerols, unreacted triacylglycerols, unseparated glycerol, free fatty acids, residual alcohol, and catalyst [1]. These contaminants can create severe problems in the engine, such as engine deposits, corrosion, and failures. The unconverted mono, di, and triglycerides increase the fuel viscosity, causing an increase in pressure within the fuel system and poor fuel atomization in the combustion chamber. Glycerol also settles in the fuel tank and can clog fuel filters, accelerating the wear and tear of the entire combustion system [2].

Other contaminants, not within the scope of this study, also pose serious concerns in its utilization in combustion engines: The presence of free fatty acids in the finished fuel indicates the fuel is being oxidized and is decomposing; alcohol lowers the flash point of the fuel and can pose a serious safety risk when handling the fuel and can also break down rubber components in the fuel system; and catalyst leads to corrosion in the fuel system [2]. The hygroscopic nature of glycerol may contribute synergistically to processes such as corrosion and fuel oxidation as well.

Biodiesel fuel quality is of great importance as production and consumption levels rapidly increase around the world. Fuel quality and quality standards were the second most important research priority identified in Annual Biodiesel Technical Workshop in January 2005 organized by the National Biodiesel Board. Continued research in biodiesel quality is especially necessary, as the variability in final fuel quality tracks the variability in the lipid composition of the various feedstocks, of both plant and animal origin [3].

The ASTM standard D6751-07a applied in the US sets forth the specifications that must be met for a fatty acid ester product to carry the designation “biodiesel fuel” or “B100” [4]. Products that meet the specification, by implication, will perform properly as a compression ignition fuel either as B100 or in blends with any petroleum-derived diesel fuel defined by ASTM specification D 975 Grades 1-D, 2-D, and low sulfur 1-D and 2-D [5]. The European standard EN 14214 includes an additional specification, the oxidative stability test, over the ASTM standard [6]. Both standards specify limits for contaminants, including glycerol at 0.2 wt% and 0.02wt% for total and free glycerol, respectively. Total glycerol includes free and unconverted glycerol in the form of mono, di and triglycerides.

The measurement of glycerol concentrations in various solvents is not a new problem. It is an indicator in several metabolic processes and beverages, and has been measured using multiple approaches. High pressure liquid chromatography, gas chromatography and enzymatic methods coupled with amperometric measurements constitute the main methods for determining glycerol concentrations in aqueous solutions. These methods, especially chromatographic methods have also been adapted to measure glycerol in biodiesel successfully, and may be reviewed in [7], [8], [9]. However, these methods require expensive equipment and trained professionals. Compared to the ease with which biodiesel lends itself to small scale manufacture, the availability of simple, effective, and inexpensive quality testing methods is severely lacking. This creates a barrier to entry for the local, small scale production of biodiesel where the capability to produce biodiesel may not be sufficiently complemented with the availability of analytical testing facilities.

The ninth most important research priority listed at the Annual Biodiesel Technical Workshop in 2005 was developing faster and simpler test methods. In cognizance of this need our main objective in developing this assay was cost effectiveness. Thin layer chromatography (TLC) has been frequently used as a simple, inexpensive method for analyzing or visually separating components of different heterogeneous solutions. TLC, and its even simpler counterpart, paper chromatography, has been used to determine the glycerol concentration in several different solutions as well. However, it has never been tested for feasibility in the measurement of glycerol in biodiesel.

Our secondary objective was to develop a framework for quantifying the amount of glycerol in a given solution. We therefore developed an image analysis procedure involving acquiring the image of the TLC plate using an easily available document scanner and a computer. An image analysis algorithm serves to analyze and quantify the size of the glycerol spots with respect to the amount of glycerol in the biodiesel.

Section snippets

Materials and methods

The following reagents were obtained: Potassium permanganate and 1-butanol from Sigma Inc. (St. Louis, MO). Glycerol, sodium hydroxide, and acetone from Fisher Scientific Inc. (Hampton, NH), and TLC plastic sheets (Silica Gel 60 F254) from Merck Inc. (Whitehouse Station, NJ).

The biodiesel sample was obtained from Growmark FS (Bloomington, IL). Its properties as measured by Phoenix Chemical Laboratory Inc. (Chicago, IL) are as shown in Table 1. The free and total glycerol contents were within

Image analysis

Fig. 1 clearly shows that the area of the spot varies in proportion to the amount of glycerol in the sample. We explored the possibility of performing image analysis on the scanned images to quantify the amount of glycerol seen on the TLC plates. The image analysis was performed using Matlab® (MathWorks Inc., Natick, MA).

The algorithm for image analysis used was as follows: the images were first cropped to include only the glycerol spots to remove irrelevant image features. They were converted

Discussion

Thin layer chromatography provides a simple, cost effective way of rapidly determining glycerol contamination in a biodiesel sample. Using a few easily available materials, a commercial scanner and some image analysis software, we have demonstrated the ability to detect concentrations of free glycerol as low as 0.1% by volume. While the prescribed maximum limit on glycerol in biodiesel is placed at 0.02%, it should be noted that this procedure does not incorporate any method for enriching or

Conclusion

Thin layer chromatography provides a feasible, economical route to detecting glycerol contamination levels in biodiesel. The procedure to detect glycerol described here is robust and easily executed. The ability to quantify detection levels also allows the computation of assay reliability. To be truly useful however, the assay needs to gain an order of magnitude in sensitivity to detect the prescribed 0.02% free glycerol contamination in biodiesel. We believe that a suitable sample preparation

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