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Topics Covered
Introduction
Advantages of Using NMR to Determine Hydrogen Content in Fuels
Determining Hydrogen Content in Fuels using NMR
Calibration and Results
Advantages of the Oxford Instruments MQC-23 over Other Instruments
Introduction
The hydrogen content of aviation fuel is an important parameter
as it determines the combustion properties of the fuel. Traditional
methods such as smoke point, smoke volatility index and
luminometer number are tedious, time-consuming and usually
require skilled analysts. Nuclear
Magnetic Resonance (NMR)
offers the opportunity to monitor the hydrogen content of fuels
rapidly, non-destructively and with minimal sample preparation.
Advantages of Using NMR to Determine Hydrogen Content
in Fuels
Advantages of Benchtop NMR
include:
- NMR
is a very stable technique over the long-term and
therefore requires little re-calibration
- Minimal sample preparation is required
- The NMR
technique is non-destructive, so repeatability
measurements can be made conveniently
- Sample measurement time is relatively short
Determining Hydrogen Content in Fuels using NMR
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 rapid
and efficient measurement of hydrogen content in fuels.
Since CW instruments are no longer commercially available the
previous ASTM standard method has been updated for the use of
Pulsed NMR.
In this method, the fuel samples are carefully transferred into glass
tubes using a pipette, weighed and conditioned at 35°C or 40°C
for 30 minutes prior to NMR
analysis.
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Figure 1: Hydrogen content calibration using
hydrocarbons at 40°C
Although this method was designed for aviation fuel, it can be adapted to
suit distillates covered by other methods (e.g. D3701-01 and D4808-01) as well
as those which are more volatile or have a high wax content.
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 are recommended in the standard method. In
this example, the calibration was produced by using known
masses of dodecane, diethyl malonate, cyclohexyl acetate,
ethyl heptanoate, octyl acetate, ethyl caprate, 2-nonanone
and pentadecane giving 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
| Sample |
Ref. %wt H |
NMR %wt H |
Difference |
| Dodecane |
7.552 |
7.553 |
-0.001 |
| Diethyl malonate |
9.924 |
9.946 |
-0.022 |
| Cyclohexyl acetate |
11.466 |
11.510 |
-0.044 |
| Ethyl heptanoate |
11.703 |
11.709 |
-0.006 |
| Octyl acetate |
12.077 |
12.103 |
-0.026 |
| Ethyl caprate |
12.756 |
12.749 |
+0.007 |
| 2-nonanone |
15.185 |
15.227 |
-0.042 |
| Pentadecane |
15.386 |
15.385 |
+0.001 |
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.
Table 2. Precision of the
hydrogen in fuel method
|
Repeat |
H Content (%) |
|
1 |
12.757 |
|
2 |
12.757 |
|
3 |
12.752 |
|
4 |
12.739 |
|
5 |
12.730 |
|
6 |
12.737 |
|
7 |
12.745 |
|
Average |
12.745 ± 0.010 |
The results show that the method gives
accurate and reproducible measurement
of hydrogen content in fuels.
Advantages of the Oxford Instruments MQC-23 over Other Instruments
The MQC-23
with 0.55 Tesla magnet, fitted
with an 18mm diameter (8 ml) sample probe
is ideal for this application. The Hydrogen in
Fuel package comprises:
- 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
In addition you may also wish to purchase:
- A dry heater and aluminium block with
holes for conditioning the sample at
35°C or 40°C
- A precision balance
MQC-23
offers multiple advantages over other instruments on the market:
- High signal sensitivity
- Small benchtop footprint
- Low maintenance
- Minimal sample preparation
Source: Oxford
Instruments Magnetic Resonance
For more information on this source, please visit Oxford Instruments
Magnetic Resonance