Kinematic viscosity of biodiesel fuel components and related compounds. Influence of compound structure and comparison to petrodiesel fuel components

Abstract

Biodiesel, defined as the mono-alkyl esters of vegetable oils and animal fats is an alternative diesel fuel that is steadily gaining attention and significance. One of the most important fuel properties of biodiesel and conventional diesel fuel derived from petroleum is viscosity, which is also an important property of lubricants. Ranges of acceptable kinematic viscosity are specified in various biodiesel and petrodiesel standards. In this work, the kinematic viscosity of numerous fatty compounds as well as components of petrodiesel were determined at 40 °C (ASTM D445) as this is the temperature prescribed in biodiesel and petrodiesel standards. The objective is to obtain a database on kinematic viscosity under identical conditions that can be used to define the influence of compound structure on kinematic viscosity. Kinematic viscosity increases with chain length of either the fatty acid or alcohol moiety in a fatty ester or in an aliphatic hydrocarbon. The increase in kinematic viscosity over a certain number of carbons is smaller in aliphatic hydrocarbons than in fatty compounds. The kinematic viscosity of unsaturated fatty compounds strongly depends on the nature and number of double bonds with double bond position affecting viscosity less. Terminal double bonds in aliphatic hydrocarbons have a comparatively small viscosity-reducing effect. Branching in the alcohol moiety does not significantly affect viscosity compared to straight-chain analogues. Free fatty acids or compounds with hydroxy groups possess significantly higher viscosity. The viscosity range of fatty compounds is greater than that of various hydrocarbons comprising petrodiesel. The effect of dibenzothiophene, a sulfur-containing compound found in petrodiesel fuel, on viscosity of toluene is less than that of fatty esters or long-chain aliphatic hydrocarbons. To further assess the influence of the nature of oxygenated moieties on kinematic viscosity, compounds with 10 carbons and varying oxygenated moieties were investigated. A reversal in the effect on viscosity of the carboxylic acid moiety vs. the alcohol moiety is noted for the C10 compounds compared to unsaturated C18 compounds. Overall, the sequence of influence on kinematic viscosity of oxygenated moieties is COOH≈C–OH>COOCH₃≈CO>C–O–C> no oxygen.

Introduction

Biodiesel is an alternative diesel fuel obtained by transesterifying vegetable oils or other materials largely comprised of triacylglycerols, such as animal fats or used frying oils, with monohydric alcohols to give the corresponding mono-alkyl esters [1], [2]. Production and use of biodiesel has increased significantly in many countries around the world and it is in nascent status in numerous others. While biodiesel faces some technical challenges such as reducing of NO_x exhaust emissions, improving oxidative stability and cold flow properties, advantages of biodiesel compared to petrodiesel include reduction of most exhaust emissions, improved biodegradability, higher flash point and domestic origin. Biodiesel is also largely compatible with the existing fuel distribution infrastructure.

Reducing viscosity is the major reason why vegetable oils or fats are transesterified to biodiesel because the high viscosity of neat vegetable oils or fats ultimately leads to operational problems such as engine deposits. It is also a property any ‘designer’ fuel modified for fatty acid composition or alcohol used for transesterification would need to meet. Table 1 lists the kinematic viscosity specifications contained in standards for biodiesel and conventional, petroleum-derived diesel fuel (petrodiesel) in the United States and Europe [3], [4], [5], [6]. The viscosity of biodiesel is slightly greater than that of petrodiesel but approximately an order of magnitude less than that of the parent vegetable oil or fat [1], [2]; see also data in this paper. Biodiesel and its blends with petrodiesel display temperature-dependent viscosity behavior similar to that of neat petrodiesel [7]. An application of the viscosity difference of biodiesel and its parent oil or fat is monitoring of the transesterification reaction [8].

Viscosity data of pure fatty esters can be used for predicting the viscosity of the mixture of fatty esters comprising biodiesel [9]. Dynamic [9], [10], [11], [12], [13], [14] and kinematic [15], [16], [17] viscosity data (which are related by density as a factor) of some individual fatty compounds are available in the literature and some data have been compiled [1], [2], [18]. Kinematic viscosity (at 40 °C), however, is the parameter required by biodiesel and petrodiesel standards. However, the data in the literature vary not only by dynamic vs. kinematic viscosity but also by temperature with most data not obtained at 40 °C with some data being redundant. On the other hand, data on the influence of some structural features on viscosity of fatty compounds are available only with difficulty or not at all, although some of these aspects were discussed in a general fashion in more dated literature [19], [20]. This paper, therefore, reports the kinematic viscosity of a range of fatty compounds determined under identical conditions at 40 °C covering structural features such as chain length, varying the acid and alcohol moieties of esters as well as number, nature and configuration of double bonds. Some free fatty acids and fatty alcohols are also included. These data are discussed with respect to biodiesel fuel standards. The data are also compared to some compounds occurring prominently in petrodiesel fuels. Some literature references for kinematic viscosity data of some branched and straight-chain aliphatic as well as aromatic hydrocarbons as they can occur in petrodiesel are [21], [22], [23].