Near-infrared spectroscopy to determine cold-flow improver concentrations in diesel fuel

Highlights

• Near-infrared spectroscopy determine cold-flow improver concentrations in diesel fuel.

• Fast and accurate control of cold-flow improver concentrations.

• Avoiding overdose of cold-flow improvers, which has economic benefits.

Abstract

The properties of diesel fuels must be controlled during industrial production. Significant diesel additives are cold-flow improvers that affect fuel properties at low temperature properties of diesel fuels. Improvers are added to fuels during processing in order to lower their cold filter plugging point (CFPP). In order to meet the European standard (EN 590), large quantities of cold-flow improvers may be required, increasing the cost of processing.

In this study, near-infrared (NIR) spectroscopy was used for the rapid determination of cold-flow improver concentration. Our predictive models included 150 calibration standards and predictions were verified using 15 validation standards. The concentration of cold-flow improvers ranged from 41 to 325 mg/kg in the partial least squares (PLS) and from 60 to 325 mg/kg in the principal component regression (PCR) algorithm. The RSD of repeatability was 5.89% for the PLS algorithm and the RSD of 3.51% for the PCR algorithm of the model. Thus, NIR spectroscopy provides a fast and accurate tool for the determination of cold-flow improver concentrations in diesel fuel.

Introduction

Additives are essential components of diesel fuels and include a wide range of compounds, such as detergents, corrosion inhibitors, lubricity enhancers, cetane improvers, and cold-flow improvers. Additives are often mixed with fuel and packaged as formulations for specific uses. For example, winter diesel fuel contains cold-flow improvers whereas summer fuel does not. Because cold-flow improvers are quite expensive, winter diesel fuel is less economical for both the producer and consumer [1], [2].

Cold-flow improvers are polymeric compounds; they are categorized into three groups: Middle Distillate Flow Improvers (MDFIs), Wax Anti-Settling Additives (WASAs), or Wax Anti-Settling Flow Improvers (WAFIs). Each type of cold-flow improver performs a specific function determined by its chemical composition. Regardless of the group, the composition of each cold-flow improver is optimized for the low-temperature parameters of a diesel fuel [3], [4]. The low-temperature parameters of diesel fuel are its cold filter plugging point (CFPP), cloud point (CP), and pour point (PP). These parameters indicate diesel fuel quality and evaluate the degree of potential damage to an engine. The low-temperature parameters for diesel fuel are influenced by admixing cold-flow improvers during fuel production [5].

A major limitation to the efficient use of cold-flow improvers is the lack of a rapid method to control improver concentration after diesel processing. Time- and labor-intensive laboratory tests must be carried out to evaluate the low-temperature parameters of the fuel. Because evaluation is based solely on the CFPP value, cold-flow improvers are often added in excess [6]. While this allows the parameters of the resulting fuel to comply with EU standards, the overuse of improvers increases their price and the overall cost of production. Thus, it is imperative to find a rapid and precise method to identify the optimal levels of such additives [7].

Near-infrared (NIR) spectroscopy is a spectral analytical method that is becoming increasingly popular in the chemical industry, especially for its instrumentation capabilities. In diesel fuel monitoring, NIR spectroscopy was used to determine the oxidation stability of diesel, biodiesel and their mixtures [8], or other important utilization of NIR spectroscopy was for a determination of diesel quality and other properties of diesel fuels (cetane number, viscosity, freezing temperature, CFPP) [9], [10], [11].

NIR spectroscopy has potential for the rapid and accurate determination of cold-flow improver concentration in winter diesel. This technique enables analysis in less than one minute while leaving samples intact. The results obtained are more accurate than those achieved using the reference method, additivation [12]. In addition to its speed, NIR spectroscopy offers the possibility of monitoring the production process online and, thus, of immediate composition regulation. On this basis, it could ensure proper dosing, thereby minimizing additive overuse to reduce costs [13], [14], [15].

In this study, an NIR spectroscopic model was developed to predict the optimal concentration of cold-flow improvers (MDFI, WASA, and WAFI) for winter and intermediate grade diesel fuels from two refineries. The optimized model was based on a chemometric diagnostic using partial least squares (PLS) regression and principal component regression (PCR).