Bias and Imprecision in the Determination of Free Glycerin in Biodiesel: The Unexpected Role of Limitations in Solubility

Abstract

Abbreviations 0%DFAME saturated with moisture at 40 °C 20% D 0%D FAME diluted to 20% v/v DDMF (100%D) 40%D0%D FAME diluted to 40% v/v DDMF (100%D) 60%D0%D FAME diluted to 60% v/v to DDMF (100%D) 80%D 0%D FAME diluted with 80% v/v to DDMF100%D 100% DDMF, moisture <50 ppm 95CI 95% Confidence interval 95CI1 95% Confidence interval, n = 1 95CI3 95% Confidence interval, n = 3 AG Concentration of FG after agitation AGQM Arbeitsgemeinschaft Qualitätsmanagement Biodiesel, e.V. ASTM American Society for Testing and Materials ASUB An approximately 100.0 mg subsample of B100 ASUBINT Time interval in seconds between ~100.0 mg subsamples B100 Commercial Grade of FAME biodiesel BSTFA Bis-(trimethylsilyl)trifluoroacetamide CDC US Center for Disease Control CEN Comité Européen de Normalisation-European Committee for Standardization CV Coefficient of variation DDMF Distilled, dry, methanol free EI Electron ionization EN European normalization EU European Union FAME Fatty acid methyl esters: here refers to aAbstract Residual free glycerin (FG) is a critical marker of fatty acid methyl ester (biodiesel) fuel quality. Yet, routine determinations by standard methods display excessive imprecision, and the uncertainties undermine confidence in the data. Various degrees of agitation are used here to evaluate the possibility of heterophase formation as a contributor to imprecision and potential bias. Statistical markers from the analysis of 13 biodiesel samples reveal that seven contain unexpected heterophases, which, due to settling, cause underestimates of 15-68%. Agitation alone resuspends heterophases for brief periods, easing potential biases, but also increases imprecision. A promising new sample pretreatment is presented, which reduces the deleterious effects of heterophases. Solubility limitations are explored as possible contributing factors, and measurements made at 23 °C in purified soy FAME reveal an equilibrium solubility which is more than 80% below previously published estimates. Strong interactions of liquid FG with small amounts of moisture reduce solubility to below international quality limits, while interactions of initially dissolved forms of moisture and FG produce bias-inducing heterophases at 0.02% FG. The unexpected low solubility of FG, exacerbated by interactions with impurities, is seen as an important contributor to underestimates and imprecision, and a strong influencing factor on the fate of residual FG in commercial biodiesel fuels.

Solubility limitations of residual steryl glucosides, saturated monoglycerides and glycerol in commercial biodiesel fuels as determinants of filter blockages

“…Along with numerous successful introductions to the commercial fuel market, certain impurities derived from FAME synthesis suffer from severe solubility limitations, and are implicated in incidents involving obstructions of flow through fuel filters (Cardeno et al, 2020;Chupka et al, 2014;Heiden, 2008;Heiden et al, 2017;Lacoste et al, 2009;Lee et al, 2007;Mittelbach, 1996;Moreau et al, 2008;Pfalzgraf et al, 2007). Solubility limitations, rooted in equilibrium thermodynamics, are expectedly exacerbated by declines in temperature, resulting in the formation of heterophases (HPs) that remain suspended in fuel until settled or caught in the pores of a filter.…”

“…Most impurities in B100 or HD remain soluble and have little or no discernible influence on engine operability. However, among common impurity residues originating in FAME, glycerol (GLY), saturated monoglycerides (SMG) and steryl glucosides (SG), all have documented histories of deleterious effects from deposits that impede flow through filters, accumulate on surfaces or in storage tanks (Chupka et al, 2014;Heiden et al, 2017;Lacoste et al, 2009;Lee et al, 2007;Mittelbach, 1996;Moreau et al, 2008). Also, saturated diglycerides originating from animal fat feedstocks or elevated due to high total diglyceride residues accumulated on filters (Heiden, 2008).…”

“…The SG and SMG impurities in B100 are derived from triacylglycerol (TAG) feedstock impurities (SGs and SG precursors-acylated steryl glucosides [ASGs]), incomplete conversions by transesterification (SMGs), the most widely used reaction for the production of B100, and purification processes that are less than complete. GLY residues are produced as a byproduct of transesterification, and as discussed in an earlier publication (Heiden et al, 2017), are left behind in commercial grades of B100 due to incomplete post reaction purification processes. The SMGs, if unaltered by purification, inherit their molecular makeup from the unique saturated fatty acid (SFA) signature of TAG feedstocks.…”

“…Solubility limitations of GLY in reference fresh, dry, distilled FAME (RFAME) and B100 were studied and published recently (Heiden et al, 2017) in connection with degraded laboratory accuracy and precision in standard methods (ASTM D6584-17, 2017b). The reported solubility determined for dry GLY in fresh, distilled FAME was an order of magnitude lower than previously published data suggested.…”

“…The concentration of GLY in blends was determined by GC-FID as described earlier by Heiden et al, 2017, after isolation by solid phase extraction (SPE) fractionation (Yang et al, 2011). Solvent was evaporated with a gentle stream of nitrogen and GLY derivatized as described below (see Derivatization section).…”