Contact:
(203) 744-5905
Fuel Gas BTU Analysis and Control
This application marketed by NMR Process Systems Inc.
For approximately two years, on-line nuclear magnetic resonance (NMR) has been determining BTU, specific gravity, and composition on a fuel gas feed stream to a refinery’s power co-generation plant. Why NMR? Compared to other analytical techniques NMR is non-invasive, nondestructive and independent of the sample’s physical (solid, liquid, gas) or environmental (temperature, pressure) condition. With design improvements in the magnet, electronic, and data processing components of an NMR spectrometer, “portable” NMRs are being utilized in several at line and/or near line applications. By using process line slipstreams, non-invasive NMR analyses overcomes some of the optical path and contamination restrictions experienced with on-line infrared systems and eliminates the need for solvents, columns, carrier gases, and/or separations required with on-line chromatographic systems. NMR analysis is also real-time with typical detection of any protonated analyte at the 0.1-100 percent level.
NMR Fuel Gas Analysis at Texaco’s Los
Angeles Refinery:
The unit was installed October, 1995, on the refinery's co-generation
unit’s fuel gas stream as a beta test to assess process NMR's performance,
reliability, accuracy, support and maintenance requirements. Measuring BTU,
specific gravity, methane content, hydrogen content, and, hydrocarbon content,
the NMR compares favorably with the GC. Unlike
the GC, process NMR does not require component speciation to determine, BTU and
specific gravity. With respect to
maintenance and reliability, the process NMR requires no column changes, minimal
calibration and minimal manpower in either operating or maintaining the
analyzer. The refinery is currently
using 2 4-20 mA output signals from the process NMR, one for BTU and one for
specific gravity, in their operations. Further, the analyzer has been registered with the Southern
California Air Quality Monitoring District, an environmental agency responsible
for monitoring and permitting air emission sources in the greater Los Angeles
region.
Fuel Gas Analysis:
The NMR measures BTU directly, without speciation, by measuring the
carbon-hydrogen bond distribution directly, and determining the CH bond quantity
by solving a variant of the ideal gas equation, PV=nRT. Initially, pure methane is used to set a calibration curve.
In the calibration experiment, the NMR probe volume and methane
temperature are known. The
integrated intensity of the methane NMR signal is then measured as a function of
methane pressure. The calibration
curve below gives an example of the NMR response to the change in methane
concentration.
Once
the calibration curve is established, a
fuel gas spectrum is obtained. Each
analysis of the fuel gas includes: the
NMR data, stream pressure and stream temperature.
Since the NMR probe volume is known, a gas equation is used to determine
the total concentration of gas in the probe.
Since methane is always present in this fuel gas stream, it is used as an
internal reference. The integrated
intensity of methane in the fuel gas spectrum is compared to the calibration
curve to determine the concentration of methane in the fuel gas.
This value is then used to obtain the additional carbon-hydrogen bond
distribution which is then used to determine the BTU, specific gravity and
composition of the fuel gas. The
NMR does not observe any species that does not have a hydrogen in its molecular
structure. Non-hydrogen containing
species are determined by difference.
A
typical fuel gas spectrum is shown below. Typical
stream conditions are 50 - 55 psia and 20 - 25 degrees C resulting in a total
concentration of less than 6 micro moles of gas in the NMR probe.
To improve sensitivity, the NMR averages 32 scans per analysis.
However, before each scan, a new sample of gas is purged into the probe.
Each analysis therefore is the average of 32 samples. Currently, the refinery is performing a fuel gas analysis
every half hour. Further, as
required by its environmental permit, a methane spectrum is obtained daily, and
the measured methane value is compared to the predicted methane value from the
calibration curve at the pressures and temperatures obtained during the daily
calibration run. Predicted verses
measured methane values must be within +/- 2.5% for the NMR to be within daily
calibration. Examples of the daily
calibration and fuel gas analysis reports are also shown below.
Daily
Methane
Fuel Gas Analysis
Calibration
Report Run Report
Sample
No.
c092096.0
Sample No.
a092096.3
Date
09/20/96
Date
09/20/96
Time
04:36 AM
Time
03:54 AM
Pressure
in PSIA 49.0
Pressure in PSIA
50.6
Temperature
in C 23.6
Temperature in C
24.0
RMS
339
RMS
194
Predicted
Sum 151.1 BTU/Cu. Ft.
1301
Measured
Sum 151.6 Specific Gravity
0.873
%
Error
0.34
Vol% Methane
43.9
Peak
Max Location
158.0
Vol% Hydrogen
17.7
SumMation
of Noise
5.7
Vol% C2 - C5+
25.0 11.7
% Olefin
Vol%
Non-Detected
13.4
ch4
2.0932
ch3
2.9747
ch2
1.7702
ch
2.2866
dch2
0.4014
dch
0.5336
h2
0.8464
sum
noise
9.19
ch4
peak
158
NMR Performance:
The graph below shows the comparison BTU values between the NMR and the
on-line Bendix 9000 GC. Both
systems sample the same fuel gas line, with sampling points separated by
approximately one hundred feet. Operationally,
the NMR has required less than 3 man weeks of support by plant personnel since
its installation. Recalculation of
the methane calibration curve has been performed twice and have been related to
hardware upgrades from the vendor. Since
its installation, the system has experienced no NMR related failure.
Pressure transducers have failed twice.
Loss of water flow to the enclosure air conditioning units occurred three
times.
Comparison
of NMR BTU vs. Bendix 9000
On-Line
GC BTU
Final Design:
The purpose of the Los Angeles installation was to demonstrate that
process NMR meets and/or exceeds the requirements for reliability, accuracy,
on-stream performance, low maintenance/manpower support,
etc., when compared to other analyzer technology.
In its current configuation, the NMR cannot quantify hydrogen sulfide
which is in the fuel gas in the 5 - 100 part per million concentration level.
Detection of H2S is necessary for the refinery’s environmental permit, and
therefore, the refinery is unable to utilize the NMR as the primary
environmental analyzer on the fuel gas. Alternate detection of H2S within NMR
system is being investigated. However,
the refinery is using the NMR for on-line validation of the primary GC as well
as using BTU and specific gravity
from the NMR for process control. Finally,
with upgrades in software, the NMR
will be fully automated for both process analysis and control as shown in the
flow scheme below.
For more information on this topic please contact:
Manager, Process and Analytical NMR Services
Process NMR Associates LLC,
87A Sand Pit Rd
Danbury, CT 06810, USA
Tel: (203) 744-5905