Contact:
(203) 744-5905
Frequently
Asked Questions:
Q: What is NMR? What does it do?
Q: Is NMR technology new?
Q: What is unique about NMR technology?
Q: Does the NMR measure the stream property directly, or is it a correlation?
A: NMR uses correlative methodologies similar to other spectroscopic measurements (i.e. NIR – near infra-red spectroscopy), however, certain measurements can be made by measuring the stream properties directly. Typically, NMR analyzes the hydrogen nuclei in the sample to produce a spectrum of chemical information that describes the chemistry of the stream. In practice, Partial Least Squares regression algorithms are used for "chemometric" correlation of NMR spectra to chemical/physical properties of interest. Simply, the spectral data is used to develop models that predict the property based on a known set of laboratory data. Consequently, a wide range of analysis is recommended to ensure the predicted measurement for the on-line application is within the valid range of the model. However, outlying data is easily incorporated into the on-line model once a sample is captured and analyzed.
Q: What is chemometrics?
A: Chemometrics is a term used to describe an advanced statistical approach to chemical analysis. Using chemometrics, the NMR spectra of large data sets of representative samples, with known measurement values, are obtained and the results mathematically compared against the known values to produce a characterization “model”.
Q: How does it differ from MRI (magnetic resonance imaging)?
A: In NMR, the signal is converted to spectra, whereas in MRI, the signal is converted to an image of concentration distribution. In NMR the analysis is performed in a homogeneous magnetic field that yields a consistent spectral position for given proton chemistries. In MRI the analysis is performed by imposing magnetic field gradients at the sample thus yielding spectral positions that are related to spatial location for the same proton chemistry.
Q: Why is the term “nuclear” used?
A: The term ‘nuclear’ indicates that the magnetic moments of atomic nuclei are being observed. The analysis is performed at a nuclear level because the nuclei are spinning, creating a small magnetic dipole, similar to a bar magnet. The NMR experiment involves the movement of the net nuclear magnetic moment off the axis of alignment with the external magnetic field. NMR analyzers can be “atom specific” in that they can be tuned for hydrogen, fluorine, or phosphorous nuclei. This is possible as different nuclei have different observation frequencies in the presence of the same magnetic field.
Q: Does the NMR instrument provide on-line control?
A: The NMR provides information, much like a flow or temperature measurement, that complements advanced process control or optimization to enhance both process performance and plant profitability.
Q: What is the expected NMR analyzer availability (uptime)?
A: What sets NMR apart from competing analyzers is its long-term reliability and reduced life cycle (maintenance) costs. Another significant advantage of the analyzer is its relatively simple construction. It utilizes highly dependable electronic circuits, contains very few moving parts, requires very little field maintenance, and supports remote calibration and system diagnostics. These attributes contribute to system availability that typically exceeds 95 percent.
Q: Which technologies are in competition with NMR?
A: Near infrared, Mid-IR, Raman, gas chromatography, and specific analysis such a boiling point, flashpoint, etc.
Q: How much floor space is required?
A: Currently, unit dimensions are 135.7 cm H (53.4”) x 191.0 cm W (75.2”) x 99.4 cm D (39.1”). Clearance of 107 cm (40”) is recommended on all sides. 198 cm (80”) is the recommended headroom.
Q: What utilities must the user provide for NMR installation?
A: Currently, the electrical power supply requirement is 230/240 VAC, 50/60 Hz, single phase. The NMR enclosure requires instrument air at 80 psig @ 10cfm. Cooling water may be required for sample cooling.
Q: What type of enclosure is required to house NMR?
A: It depends on where it is going to be located. In general, it is recommended that it be placed in an instrument house to minimize exposure to contaminants, dust, foul weather, and extreme temperature gradients. Where climatic conditions remain relatively mild, an open three-sided shelter may suffice. NMR magnet and associated electronics are enclosed in a NEMA4X/IP56 stainless steel box equipped with air purging and climate control.
Q: What happens if there is a flammable gas leak inside the NMR analyzer enclosure?
A: If needed, gas monitoring can be installed that will shut down the power to system at the first sign of a leak.
Q: Can the powerful NMR magnet cause any health hazards?
A: No, there is no health risk. Fringe field is less than 1 gauss 0.5 m from the magnet housing inside a typical enclosure.
Q: Is it safe for someone with a pacemaker to be near the NMR unit (open or closed)?
A: The unit has an extremely low fringe field that will cause no ill effect to anyone with a pacemaker.
Applications
Q: Why is NMR technology good for advanced process control (APC)?
A: Advanced process control, utilizing technology ranging from simple multivariable control to model-based predictive control and rigorous on-line modeling, generally requires process stream quality information. The exceptional availability of the NMR analyzer enables this information to be supplied reliably for process control while the technology secures accuracy and repeatability. Because this analyzer can be applied to numerous component quality measurements, a single analyzer can often alleviate the need for multiple analyzers to satisfy an APC application.
Q: What are good applications for the NMR analyzer?
A: Applications providing the best return on investment are those where NMR-enhanced APC or optimization is applied. These are provided as solutions rather than as a list of products and services. Solutions that have been recognized as having best ROI are those with: complex processes, high energy costs, or, significant downstream impact, if process control is not tightly maintained.
Q: What is the solution for a given application?
A: The solution will vary depending upon the complexity of the process stream and the complexity of the process control algorithms. NMR can be applied where other approaches fail due to its near real-time sampling, multiple stream and multiple component capability. Processes with high degrees of interaction, long process leads or lags will lend themselves to advanced process control. Simpler processes may use NMR with advanced regulatory control, e.g. self-tuning PID with feed forward and feedback, or cascaded self-tuning PID loops.
Q: What happens to the NMR application if a significant change to the process is made?
A: Re-verification and possible recalibration may be required for any new condition.
Q: How do you know when there is a need to recalibrate?
A: If process change falls outside of the range of the model, then the model will have to be adjusted. The documentation of the initial sensor calibration will define the range of the sensor model.
Q: Is NMR magnet shimming an automatic function? How frequently is shimming done/required?
A: The shimming process for the magnet can be done automatically or manually. In normal operation, the unit will automatically re-shim itself if the unit drifts to a predetermined degree. There are times when our service technician may desire to manually re-shim if a change is made to hardware. But it is not a function the user needs to be concerned with since it is handled by the instrument in 90% of all cases. A standard reference sample will be provided for determining analyzer drift.
Samples
Q: What is the lag time involved between the sample and the analysis?
A: The lag time is composed of two elements. The first is the time for the sample to travel from the process to the NMR and the second is the time for the NMR to update analysis. NMR update time varies from once every two minutes for one stream to once every 20-30 minutes for five streams.
Q: What are the temperature and pressure limits of the NMR?
A: Maximum process sample conditions at the NMR are 120 °C and 350 psig.
Measurements
Q: What are known limitations of measurement?
A: Reliable measurements generally require that concentration of chemical species be greater than 1000 ppm. Sulfur, nitrogen, oxygen are qualitative, secondary measurements if they are associated with hydrogen bonding. NMR cannot detect color.
Q: What is the minimum detection level?
A: NMR measures typically down to concentration levels of 1 part / 1000
Q: What is the differentiation between high resolution NMR and low resolution NMR?
A: Higher resolution provides more information about the sample properties.
Q: How close should the streams/tanks be to each other for multi-streaming?
A: This is a function of desired cycle time, and on the impact of long sample runs on sample conditioning. The closer to the tanks, the faster the results. Normally we figure that 2 streams will generate results every 4 minutes. There is a dead time while sample switching occurs. It is dependent on how fast you need an answer to control the feed. The tanks can be located far from the instrument, but this will extend your cycle time multiplied by the amount of streams coming into the NMR unit. Samples that become viscous over long transport distances may require heating.
Q: What is the meaning of a secondary correlation for sulfur, pour point, etc.?
A: The NMR analyzer can serve as a secondary reference for some parameters. However, accuracy and precision levels are not quantifiable in many cases, e.g. sulfur compounds bound to hydrogen as in high sulfur crude oil.
Certification
Q: Does the process NMR analyzer have Cenelec certification?
A: Yes, Zone 1, Gr. IIA & IIB, T3
Q: Does the process NMR analyzer have ETL certification?
A: Yes, Class 1, Div. 1, Gr. C&D
NMR vs NIR
Q: How does NIR / FTIR modelling compare with NMR?
A: For NIR, one model per property, per stream is necessary. In many cases NMR needs only one model per property irrespective of number of streams. This greatly simplifies NMR modelling.
Since NMR also includes a lot of process variation in its models, the models themselves are more robust and will not require the maintenance that NIR/FTIR needs. Model build time is also greatly reduced, meaning that NMR can get the analyzer on line and producing faster. As an indication, NMR can be on line in less than half the time it takes to implement an FTIR analyzer, even if using calibrations developed at other sites. The process NMR also requires less maintenance and has higher analyzer availability than many NIR devices.
Q: How is the NIR different from the NMR analyzer?
A: The NIR analyzer is very sensitive to changes in sample temperature and uses the 3rd harmonic overtone as the basis of the NIR analysis. This means all of the spectra appear similar to a large extent, and extensive mathematical data manipulation are needed to find a difference in NIR analyzer output before any models can be built. Spectrometer repeatability and stability is critical if the calibrations are to going to produce meaningful and repeatable results.
Q: Is sample conditioning critical with NIR?
A: Yes. Water in the sample will cause major problems, including total absorbency of all the IR spectra, as water absorbs at over 10 times more than any hydrocarbon. There will also be a back scattering interaction of the water with the IR energy. ASTM E1252-88 states that all water must be removed from the samples before measurement. Water can be removed using either molecular sieves or swirl clean filters with hydrophilic membranes. However, neither are 100% effective and both require maintenance to regenerate or replace.
Water vapor in the atmosphere surrounding the NIR spectrometer is also important. This requires an "optical purge" to remove water and hydrocarbon vapors from the sample cell. A molecular sieve may be used. The hydrocarbon dew point of the purge air needs to be below -48 deg C, and, of course, it needs to be oil free. NMR does not have any similar requirements for its sampling probe area.
Particulates are another problem for NIR, as the sample cell is only 2 – 500 micron thick (versus >5mm spacing in the NMR). Very good sample filtration is needed to remove particulates. Sample cells also need to be removed to clean on a 3 - 6 month basis, as the optics will become discolored, even with regular (daily) solvent washing. This is not a trivial task and most technicians have 2 cells and do an exchange when maintenance of the NIR optics are required. Special care needs to be taken when introducing a new cell to ensure that the sample path length remains the same and that the windows are correctly aligned to prevent fringing. The models then need to be "tuned or revalidated" before the analyzer can go back on line. An experienced technician can do this in a day. This procedure needs to be carried out per measurement cell/probe. NMR has either a 1/4" or 1/2" flow-though tube. It is not an optical technique, so particulates are not a concern. With such a large "path length", fouling is not an issue.
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