Determination of Hydrogen in Fuel Content Using NMR

By AZoM

Table of Contents

Introduction

The hydrogen content of aviation fuel is a significant parameter as it determines its combustion properties. Conventional methods such as smoke point, smoke volatility index and luminometer number are difficult, time-consuming and usually require skilled analysts. Nuclear Magnetic Resonance (NMR) enables monitoring the hydrogen content of fuels quickly, non-destructively and with minimal sample preparation.

Method

For 20 years, Oxford Instruments led the way with the Oxford 4000 Continuous Wave (CW) NMR Analyser, an American Society for Testing and Materials (ASTM) compliant instrument, for quick and effective measurement of hydrogen content in fuels (Fig. 1). Since CW instruments are no longer available commercially, the previous ASTM standard method has been updated for the use of Pulsed NMR. In this technique, the fuel samples are carefully transferred into glass tubes using a pipette, weighed and conditioned at 35 °C or 40 °C for 30 minutes before NMR analysis.

Even though this method was designed for aviation fuel, it can be adapted to suit distillates covered by other methods such as D3701-01 and D4808-01, as well as those which are more volatile or have a high wax content.

Figure 1. Hydrogen content calibration using hydrocarbons at 40 °C.

Benefits of Benchtop NMR

The benefits of benchtop NMR are:

• NMR is a very stable technique that can be reliably used for a long period of time and therefore requires little re-calibration.
• Minimal sample preparation is required.
• The NMR technique is non-destructive, so repeatability measurements can be performed easily.
• Sample measurement time is considerably reduced.

Calibration and Results

The instrument can be calibrated using real samples of known hydrogen content which span the range of interest. A list of chemicals is recommended in the standard method. In this example, the calibration was done by using known masses of dodecane, diethyl malonate, cyclohexyl acetate, ethyl heptanoate, octyl acetate, ethyl caprate, 2-nonanone and pentadecane providing a correlation coefficient of 1.00 and standard deviation of 0.03. The predicted NMR results from this calibration are compared against the reference values in Table 1.

Table 1. Accuracy for the hydrogen in fuel method which is primarily dependent on sample preparation.

Recommended Instrument Configuration

The MQC-23 with 0.55 Tesla magnet, fixed with an 18-mm diameter (8-ml) sample probe is ideal for this application. The Hydrogen-in-Fuel package comprises the following:

• The MQC-23 which can be controlled using its own built-in computer using Microsoft Windows or via a stand alone PC.
• MultiQuant software including RI Calibration, RI Analysis and the EasyCal ‘Hydrogen in Fuel’ application.
• 18-mm glass tubes.
• PTFE stoppers (to seal the test cells).
• Stopper insertion/removal rod.
• Installation manual.
• Method sheet.
• A dry heater and aluminium block with holes for conditioning the sample at 35 °C or 40 °C.
• A precision balance.

Benefits of the MQC-23

The instrument offers a number of benefits over other instruments on the market:

• High-signal sensitivity.
• Small benchtop footprint.
• Low maintenance.
• Minimal sample preparation.

Table 2. Precision of the hydrogen in fuel method.

Results

The precision of the experiment was checked by measuring a sample of 2-nonanone (12.756 % wt H) against this calibration curve, the results of which are shown in Table 2. The results show that the method gives accurate and reproducible measurement of hydrogen content in fuels.

About Oxford Instruments Magnetic Resonance

Oxford Instruments Magnetic Resonance are committed to the development and manufacture of information-driven solutions for novel applications in life sciences, research and industrial process control. The company knowledge was founded in the field of Nuclear Magnetic Resonance (NMR) instrumentation, receiving high recognition for its innovation dating over forty years. Today Oxford Instruments Magnetic Resonance are focused on solving complex and often unique technology problems for the understanding of biomolecular structure and function across industrial, life science and drug discovery applications. In the industrial process area, the low field benchtop instruments offer fast, accurate and simple measurement solutions to routine Quality Control problems.

This information has been sourced, reviewed and adapted from materials provided by Oxford Instruments Magnetic Resonance.

For more information on this source, please visit Oxford Instruments Magnetic Resonance.