4
Measurement and Testing
COMPARISON BETWEEN NIR/FT-IR AND 3RD GENERATION OP-NMR TECHNOLOGY
The on-line process analysers market is evolving quickly from discrete analysers into corelative ones. Correlative analysers provide an inherent advantage that multiple properties can be derived from a single measurement. Well known on-line process correlative analysers are the NIR /FT-IR/RAMAN, based optical methodologies. Lately, a new concept of an on- line process correlative analyser has emerged, the On-line Process NMR (OP-NMR) analysers
On-line Process Nuclear Magnetic Resonance (OP-NMR) analysers are benefi cial in chemical and petroleum industries for qualitative and quantitative analyses of physical properties of process streams. NMR selectively specifi es and quantifi es hydrogen atoms with regard to the molecular structure of substances and their presence in mixtures. Its linear spectral response enables chemometrics to easily perform accurate linear correlation between provided spectral data and the physical properties to be determined. NMR process analysers are applicable to opaque and transparent solutions alike. This is highly benefi cial for its application in process control in chemical process and petroleum industries.
Stability, reliability and accuracy are basic requirements for successful implementation of process analysers. Previous generations of NMR process analysers had signifi cant issues moving from the lab to hostile production environments. This was predominately caused by a high sensitivity towards temperature fl uctuations in production locations and input streams.
Process streams are characterised by different temperatures and fl ow properties. It is up to the analyser and the sampling system to eliminate any interference of these on analytical results.
A thorough engineering analysis of the fi rst and second generation production units based on real-like deployments feedback was the foundation on which the 3rd generation on-line OP-NMR process analyser was developed. The challenge was to reduce its sensitivity towards the infl uence of temperature variations of process streams and to improve its reliability. This was achieved by entirely re-innovating its hardware and software. The third generation includes a new design of the magnet with an increase of the bore size to 30 mm and a newly developed measuring probe. Manually wrapped shim coils were replaced by new state of the art PCB cards. As a result of these improvements, enhanced long and short term stability, reduced sensitivity towards temperature variations, smaller footprint, improved SNR ratio, improved sensitivity and signifi cant conditioning simplifi cation have been achieved.
The 3rd generation is distinct from its previous generations by its low susceptibility towards temperature fl uctuations, its high stability and its reduced cost of maintenance. Reliable measurements of transparent, dense and opaque process streams can be conducted without any impact from temperature differences between different streams. It can therefore benefi t the chemical plants and refi neries in their efforts to effectively monitor and control their entire processes.
PIN OCTOBER / NOVEMBER 2023
It prevents production of off-spec and borderline products and avoids the need for reprocessing. This in turn will defi nitely have its output on the plant economics.
Introduction
At present, refi nery streams are predominately monitored by discrete process analysers, based on standard ASTM methods, and/or optical spectrometry based process analysers, such as NIR/FTIR. Standard method analysers are not dependent on crude quality and other factors. However, their response time is longer and their maintenance is expensive. Various optical spectroscopy analysers require close attention to the modeling efforts and are restricted to measuring transparent fl uids only. Their reliability depends of the accuracy of the chemometric model, as well as the impact of the presence of hetero-atomic molecules which are present at fl uctuating concentrations depending of the crude oil origin.
Principle of OP-NMR Analysers
NMR (Nuclear Magnetic Resonance) technology relies on the alignment of nuclei in a magnetic fi eld. When spinning nuclei with an odd number of protons are placed in a magnetic fi eld, they align with it, and their spinning generates small opposing magnetic fi elds, affecting the effective magnetic fi eld at the nucleus. Neighboring protons, atoms, and chemical bonds have varying effects on this magnetic fi eld, resulting in a unique shifting of the spectral signal for each proton, known as chemical shift. NMR spectrometry is a fundamental method that focuses on identifying the molecular structures of substances in a mixture, with spectral response correlating linearly with proton concentrations. This is different from optical spectroscopy, which relies on substance “fi ngerprints” and is impacted by signal variance, overlapping spectral bands, and a lack of linear spectral response.
The introduction of Fourier transform (FT) in NMR spectrometry has enhanced its sensitivity for measuring low concentrations. Multiple scans of the spectrum improve the signal-to-noise ratio and spectral resolution compared to continuous wave NMR spectrometers with similar magnetic strength.
NMR process analysers are designed to assign and quantify different types of hydrogen atoms in organic molecules or water, commonly found in distillates or crude oils. The linear spectral response accurately correlates
with the hydrogen atom assignment to molecular species within the composition. NMR spectral peaks are infl uenced by neighboring chemical carbon-carbon bonds and non-carbon atoms in the molecular structure, allowing for the assessment of the chemical character of substances in crude oil or distillates. This enables identifi cation of whether molecules are linear or branched paraffi ns, olefi ns, mono-aromatics, poly-aromatics, hetero-cyclic compounds, naphthenic compounds, acids, oxygenates, or water. This principle underlies the development of the fi rst NMR analysers.
The On-line Process NMR (OP-NMR) technology was developed to enable expanding the scope NMR technology from the lab environment into the on-line process environment taking into consideration the special conditions and attributes of on-line streams, mainly in terms of temperature range, viscosity and opacity.
Applications of 3rd Generation OP-NMR Process Analyser in Process Control
The introduction of third-generation On-line Process NMR (OP- NMR) process analysers represents a signifi cant advancement in the fi eld of process control for chemical processes, refi nery streams, and blending processes. Unlike optical spectrometry- based technologies like NIR analysers, OP-NMR analysers do not require process streams to be transparent for analysis. This makes OP-NMR technology applicable to a wider range of process streams, including those that are dense and opaque.
In the past, the fi rst and second-generation NMR analysers faced issues related to stability, accuracy, and reliability, which led to skepticism among end users, particularly in refi neries, about integrating these analysers into their process control systems. However, the third generation, the OP-NMR analysers has overcome these obstacles, making them suitable for
Figure 1: Clarity of various hydrogen types in gasoline NMR spectra [1].
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