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At-Line 31P Low Field NMR Application
New Flash: Chevron purchases high resolution 31P
NMR system for at-line analysis of a phosphoric acid catalyzed alkylation
process.
The Chevron Richmond Refinery needed a simple and quick
measurement of phosphoric acid strength and quality.
Prior analysis procedures included transporting samples several miles
from the refinery process to the technical center in order to perform the
31P NMR analysis on a high field superconducting NMR system. This
process was also complicated by the fact that the process operators were not
able to perform the analyses on the conventional high field NMR systems (400
MHz for 1H, 161 MHz for 31P) themselves, but had to wait
for NMR technician and instrument availability.
If sample analyses was required after research facility working hours
the NMR lab technicians were required to come in and run the NMR for the
process operators.
A simple permanent magnet bench-top NMR system
has now been placed in the existing process lab adjacent to the process
unit control room. The NMR
samples are placed in specially designed 9 mm (diameter) x 50 mm (length)
sample tubes in order to accommodate sample handling issues (high viscosity
sample with no solvents being added) and are place easily into the NMR probe.
Sample preparation is essentially non-existent and sample waste is
minimal. The 31P NMR analysis itself takes around 3 minutes to
perform.
The “at-line” NMR system is a high resolution Qualion
permanent magnet system operating at 58.3 MHz for 1H and 24.2 MHz
for 31P. The system is
located on a lab bench in the process lab building.
Some examples of the system performance are shown along with the
comparison with high field NMR results. It can be seen that the lower field
instrument provides more than adequate resolution to perform the acid strength
analysis.
Figure 1: 31P NMR of 85% Phosphoric Acid
Figures 1 and 2 show quantitative 31P NMR
spectra of different phosphoric acid concentrations dissolved in water. The
NMR analysis is found to be fully quantitative.
Figure 2: 31P NMR signal acquired at 24.2 MHz on several
concentrations of phosphoric acid in water.
Figure 3 shows the 31P NMR data obtained at 7
Tesla (121 MHz) and 1.4 Tesla (24.2 MHz) of actual process samples.
The peak at 0 ppm corresponds to orthophosphoric acid, the peak at -15
ppm corresponds to the terminal P atoms of a polyphosphoric acid chain, and
the peak at -30 ppm corresponds to internal P atoms in the middle of
polyphosphoric acid chains. Relative areas of these peaks are used to
calculate the acid strength.
Figure 3: High field and low field 31P NMR spectra of
Used Phospohoric Acid from Alkylation Process
Figure 4: 31P NMR spectra
and calculated acid strengths obtained on 3 different used phosphoric acid
samples
Figure 5: Another comparison of 7 Tesla laboratory acquired NMR
data compared to rapid at-line 1.4 Tesla NMR assay of phosphoric acid strength.
The NMR system being utilized at the Chevron Richmond refinery is a Qualion
60 MHz NMR system utilizing a 31P probe.
For further details or to discuss your own NMR applications
please contact Paul Giammatteo (Tel:
+1 203-744-5905)
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