Sniffing
out the
sulphur
Richard Shehab,
Vicki Luckie and Jim Keasbey, AMETEK –
Process & Analytical Instruments Division,
USA, look at online sulphur analysis in crude
oil and petrochemical products using X‑Ray
technology.
C
urrently, there is a growing need to determine the sulphur content of crude oil and hydrocarbon‑based
fuels and oils with an online analyser that is able to operate at the pressures and temperatures typical
of process pipelines and that requires little or no sample conditioning. It is also desirable for the tested
sample to be returned to the process pipeline, thereby eliminating the need for effluent sumps or
storage systems.
This paper addresses the two types of X‑Ray analysers available on the market for online sulphur analysis:
X‑Ray transmission/absorption (XRT) and X‑Ray fluorescence (XRF), and offers an explanation of the advantages
and proper application of each type.
july 2012 | Reprinted from World pipelines
X‑Ray transmission/absorption sulphur analyser
In XRT, X‑Rays are emitted from an X‑Ray source at high
enough energy to pass through the volume of oil to a
detector on the opposite side of the sample flow cell. The
presence of sulphur in the oil absorbs these source X‑Rays.
Calibration to yield concentration is obtained by relating
concentration to X‑Ray count rate. This calibration is an
inverse relationship: as the sulphur concentration increases,
the X‑Ray intensity reaching the detector decreases. In this
way, the sulphur is indirectly measured by measuring the
amount of X‑Ray radiation transmitted through the oil.
The transmission (or absorption) of X‑Rays is given by the
following equation:1
Figure 1. XRT block diagram.
I/Io = exp ‑ dt[μm(1‑Cs) + μsCs] Table 1. Selected mass absorption coefficients at X‑Ray energy 21 keV
(1)
2
Element
Mass absorption coefficient, μ(g/cm2)
H
0.37
C
0.41
O
0.79
S
5.82
Where,
I = final X‑Ray intensity, after absorber (counts).
Io = initial X‑Ray intensity, before absorber (counts).
d = density of absorbing material (g/cm3).
t = thickness of absorber (cm).
μm = mass absorption coefficient for the matrix (cm2/g).
μs = mass absorption coefficient for sulphur (cm2/g).
Cs = weight fraction of sulphur (%wt/wt).
Table 1 shows the mass absorption coefficients (μ) for the
hydrogen, carbon and oxygen components of petroleum oil, as
well as μ for sulphur. Because the sulphur (μ) is on the order of
ten times greater than those for hydrogen, oxygen and carbon,
the transmission of X‑Rays is primarily proportional to the
sulphur content.
Possible interferences include the presence of chlorine
in the oil and coating of the flow cell’s beryllium windows.
Chlorine may interfere with the sulphur measurement if it
is present in a concentration ratio to the sulphur of greater
Figure 2. Model 682T‑HP component diagram.
Figure 3. Model 682T‑HP component diagram.
Reprinted from World pipelines | july 2012
Figure 4. Model 682T‑HP online sulphur analyser.
than 1:10 and the chlorine concentration varies independently
of the sulphur. Flow cell window coating has negligible
effect on the sulphur measurement in XRT because a coating
build‑up of a few hundred microns is a minor part of the
entire X‑Ray path length.
impact the sulphur measurement. The AMETEK 682T‑HP
employs a special Parylene C conformal coating on the
beryllium windows to mitigate build‑up of material and
protect the window.
Description of the XRT analyser
The X‑Ray source is an X‑Ray tube. The source power is tuned
such that X‑Rays pass through the volume of oil at an energy
between 20 ‑ 25 keV. This allows optimum sensitivity to
changes in sulphur concentration.
The AMETEK ASOMA Model 682T‑HP online XRT analyser
was used to obtain the data presented in this paper. All
measurements and data were obtained with the sample in a
static condition, completely filling the sampling pipes (flow
cell and densitometer paths). X‑Rays and X‑Ray transmission
are not affected by sample flow.
Figure 2 shows a drawing of the analyser. The essential
components are: sample flow cell; X‑Ray source; X‑Ray detector;
and densitometer.
X‑Ray source
X‑Ray detector
The detector is a gas‑filled proportional counter. The bias
voltage is set such that the detector operates in the normal
proportional counter mode, detecting all X‑Ray energies
between 3 keV and the tube voltage setting.
Sample flow cell
Densitometer
The oil sample enters the bottom of the flow cell tube and
exits at the top. The X‑Ray measurement is taken as the
sample flows through the tube. Beryllium windows in the
tube allow the X‑Rays to enter and exit the analysis volume.
Because of the relatively long X‑Ray path length compared to
the thickness of window build‑up, fowling of the window due
to build‑up of wax or other materials does not statistically
Oil from the process stream simultaneously flows through
the X‑Ray sampling flow cell and a densitometer. The
densitometer measures the oil density and temperature and
inputs these values into the data processing software for
corrections to the calibration and final analysis results.
Table 2. Bunker/residual fuel calibration results
Element: S
Standard error of estimate = 0.031
Sample
Assay value (%)
Measured value (%)
B1
0.90
0.89
B2
1.00
1.00
B3
1.30
1.31
Data output
Total X‑Ray counts from the detector are simply accumulated
and output using a programmable logic controller (PLC). The
total counts are then expressed as count rate (total counts
divided by analysis count time) and input to the software for
use in calibration and analysis algorithms.
Figure 5 shows the sample flow and data output of the
analyser.
B4
2.50
2.45
Performance
B5
3.00
3.02
B6
4.00
4.04
B7
4.40
4.43
B8
5.50
5.46
This section describes current results obtained for
calibration, precision, sensitivity, and long‑term stability of
the AMETEK Model 682T‑HP XRT analyser. A comparison
of repeatability is also made with ASTM D4294. All
measurements were obtained using an analysis
count time of 100 seconds and results are
quoted as %wt/wt.
Calibration
Each calibration was performed by analysing
a suite of certified standards.3 A broad range
calibration was built for bunker fuel in the
range of sulphur from 0.9 ‑ 5.5% (Table 2), with
the density increasing with sulphur content.
For sweet crude and crude oil blending
operations, a second calibration was built over
the range of 0.27 ‑ 1.44% sulphur (Table 3), with
independently varying densities.
Precision and sensitivity
Figure 5. Sample flow and data output.
The precision results for selected samples
shown in Table 4 were obtained by making
100 repeat analyses of each sample using a
count time of 100 secs/analysis.
july 2012 | Reprinted from World pipelines
Working range
Table 3. Crude oil calibration results
The data shows performance over the typical working range
of 0.1 ‑ 5.5% S. The limit of quantification (LOQ) is estimated
to be when relative precision is on the order of 10 ‑ 15%
relative, and for this XRT instrument the LOQ is on the order
of 0.02% sulphur.
Long‑term stability
Long‑term stability data were gathered over 3000 continuous
measurements of a static sample over a 92 hr time period.
Sample C6 averaged 1.438% with a standard deviation of
0.007%.
Element: S
Standard error of estimate = 0.031
Sample
Assay value (%)
Measured value (%)
C1
0.27
0.28
C2
0.42
0.41
C3
0.60
0.59
C4
0.92
0.90
C5
1.18
1.21
C6
1.44
1.43
Table 4. Precision based on bunker/residual fuel calibration – element: S
Sample
Assay value (%)
Mean value (%)
1σ std. dev.
% relative
Comparison to ASTM D4294‑08a
B3
1.30
1.311
0.005
0.4
Section 16.1.1 (Repeatability) of ASTM D4294‑08a expresses the
r‑value (repeatability) for the measurement of sulphur in oils
using EDXRF to be:
B7
4.40
4.442
0.005
0.1
C3
0.60
0.605
0.004
0.7
C6
1.44
1.440
0.007
0.5
r = 0.4347 x X^0.6446 (2)
Where,
X is the sulphur concentration in mg/kg.
Table 5 shows a comparison of the precision results of the
XRT method and the r‑value given in D4294. The table shows
the 95% confidence value (2σ) for the precision results using
XRT as compared to the maximum 95% confidence r‑value
listed in D4294.
These results indicate that the XRT online method is
equivalent to the repeatability performance quoted in
ASTM D4294‑08a (for EDXRF) for sulphur concentrations above
0.6%.
Table 5. XRT results compared to ASTM D4294‑08a
Sample
XRT mean value (% sulphur)
XRT 2.77σ std. dev.
r‑value from D4294
B3
1.311
0.014
0.020
B7
4.442
0.014
0.043
C3
0.605
0.011
0.012
C6
1.440
0.019
0.021
Advantages/limitations of XRT online analysers
There are several distinct advantages associated with the use
of XRT online analysers to determine the sulphur content of
crude oil and other hydrocarbon‑based fuels. Among them,
minimal, if any, sample conditioning is required, nor is a sample
recovery system necessary. It is a non‑contact, non‑destructive
measurement technique with no waste products produced
and no consumables required. The technology is well suited
for high pressure and high temperature applications. The large
X‑Ray path eliminates the effect of window fouling on results.
No routine maintenance is required other than calibration.
Among the limitations of XRT online analysers is that only
one element is detectable and the limit of detection is
200 ppm.
Figure 6. Characteristic X‑Ray generation.
X‑Ray fluorescence sulphur analyser
Energy‑dispersive X‑Ray fluorescence (EDXRF) is a common
technique for measuring sulphur in hydrocarbon oils. At‑line
and laboratory EDXRF analysis is covered in the ASTM
standard test method for sulphur in petroleum products:
ASTM Standard Method D4294‑08a.1
XRF measures the fluorescent X‑Ray emission of sulphur,
which occurs at an energy of 2.308 keV (K‑alpha emission
line), while XRT measures total X‑Ray transmission. Thus,
X‑Ray absorption effects are much greater in XRF than in the
XRT method. Matrix effects that occur in XRF are minimised
Reprinted from World pipelines | july 2012
Figure 7. Polarised EDXRF block diagram.
using XRT. Also, because the sulphur fluorescent X‑Rays are
low energy, the path length that they travel before being
completely absorbed is a tiny fraction of the path length of
the higher energy X‑Rays used in the XRT technique. Coatings
that may build up on the flow cell window will greatly
affect the XRF measurement. Furthermore, the thin windows
necessary to efficiently transmit the soft fluorescent X‑Rays
cannot sustain pressure differentials as high as 800 psig. For
these reasons, XRT is a more desirable technique for the online
analysis of sulphur in crude oil. EDXRF is also well suited for
the online measurement of sulphur content for less dense oils
that are pumped at low pressures, typically at a maximum of
30 psig, such as diesel fuel.
Refineries, pipelines and blending operations, sometimes
require online analysis of thick, dense, viscous petroleum
oils pumped at stream pressure. The XRT mechanical design
eliminates the need for a sample recovery system and
minimises the need for sample conditioning.
Polarised EDXRF is also used for the determination of
ultra‑low sulphur content (< 15 mg/kg) in diesel and gasoline,
using ASTM D7220‑06.4 EDXRF is commonly used as a
crosscheck method at‑line to ensure online XRT measurements
check against an ASTM or other international methods.
Advantages/limitations of XRF online analysers
The advantages of XRF online analysers include low limits
of detection in the tens of ppm; however, analysis time is
an order of magnitude longer than using an XRT analyser.
XRF analysers are also able to detect elements other than
sulphur, if there are no matrix interference issues. Among the
disadvantages of XRF analysers is the thin beryllium windows
it requires. This limits use of an XRF analyser to low‑pressure
applications. In addition, the coatings that build up on
beryllium windows have a significant effect on the sensitivity
and detection capabilities of an XRF analyser due to the weak
characteristics of the X‑Rays being detected.
Conclusion
The X‑Ray transmission technique is a viable method for
the determination of sulphur content in crude oil, bunker
fuels, residual oil, diesel, and other hydrocarbon oils in the
range of 0.02 ‑ 6%. XRT gives performance comparable to
the ASTM EDXRF standard method. The technique is robust,
requires little or no sample conditioning and no special
recovery system. It is ideal for the demanding requirements
of monitoring sulphur content throughout the petroleum
industry.
References
1.
2.
3.
4.
‘Standard Test Method for Sulphur in Petroleum Products by Energy‑Dispersive
X‑Ray Fluorescence Spectroscopy’, D4294‑08a, ASTM International.
Robinson, J.W., CRC Handbook of Spectroscopy, Volume 1 (CRC Press, Inc., 1974),
p. 208.
Standards certified and assayed using ASTM D2622 by Analytical Services, Inc.,
USA.
‘Standard Test Method for Sulphur in Automotive Fuels by Polarization
X‑Ray Fluorescence Spectrometry’, D7220‑06, ASTM International.
july 2012 | Reprinted from World pipelines