Abstract On-line Nuclear Magnetic Resonance (NMR) analyzers are beneficial in chemical and petroleum industries for qualitative and quantitative analyses of physical properties of process streams. NMR selectively specifies and quantifies 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 […]
On-line Nuclear Magnetic Resonance (NMR) analyzers are beneficial in chemical and petroleum industries for qualitative and quantitative analyses of physical properties of process streams. NMR selectively specifies and quantifies 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 analyzers are applicable to opaque and transparent solutions alike. This is highly beneficial for its application in process control in chemical process and petroleum industries.
Stability, reliability and accuracy are basic requirements for successful implementation of process analyzers. Previous generations of NMR process analyzers had significant issues moving from the lab to hostile production environments. This was predominately caused by a high sensitivity towards temperature fluctuations in production locations and input streams.
Process streams are characterized by different temperatures and flow properties. It is up to the analyzer and the sampling system to eliminate any interference of these on analytical results.
A thorough failure analysis of the first and second generation production units was the foundation on which the third generation on-line NMR process analyzer was developed. The challenge was to reduce its sensitivity towards the influence 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, and improved sensitivity has been achieved.
The third generation is distinct from its previous generations by its low susceptibility towards temperature fluctuations, 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 benefit the chemical plants and refineries in their efforts to effectively monitor and control their entire processes. It prevents production of off-spec and borderline products and avoids the need for reprocessing. This in turn will definitely have its output on the plant economics.
At present, refinery streams are predominately monitored by discrete process analyzers, based on standard ASTM methods, and/or optical spectrometry based process analyzers, such as NIR/FTIR. Standard method analyzers are not dependent on crude quality and other factors. However, their response time is longer and their maintenance is expensive. Various optical spectroscopy analyzers require close attention to the modeling efforts and are restricted to measuring transparent fluids 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 fluctuating concentrations depending of the crude oil origin.
NMR technology is based on the influence of differences in alignment of nuclei in the presence of a magnetic field. When a group of spinning nuclei with an odd number of protons, is placed in a static magnetic field, each nucleus aligns with the magnetic field. By its spinning, small magnetic fields are formed that oppose the externally applied field, which reduce the effective magnetic field at the nucleus. Neighboring protons, atoms and chemical bonds influence the magnetic field differently for each proton. This phenomenon results in a shifting of the spectral signal differently for each proton (chemical shift). The chemical structure of different species in molecules can be identified, while its spectral response correlates linearly with the proton concentrations. NMR spectrometry is a fundamental method. It focuses on molecular structures of the substances in the mixture. In combination with the proper chemometrics accurate quantification of physical properties is achieved.
This is in contrast to optical spectroscopic methods, which are based on the determination of “fingerprints” of substances. Its accuracy is affected by the impact of weak analytical signal variance of overtone and combination band vibrations, heavy overlapping spectral bands, of non carbon or hydrogen atoms and a lack of linear spectral response.
The introduction of Fourier transform (FT) in NMR spectrometry increased its sensitivity to measure low concentrations. Multiple scans of the spectrum reduce the signal to noise ratio. Spectral resolution improved as compared to continuous wave NMR spectrometers of similar magnetic strength.
The concept of NMR process analyzers is based on the assignment and quantification of the different types of hydrogen atoms of organic molecules or water, which are present in distillates or in crude oils. The linear spectral response correlates accurately with the hydrogen atom assigned to molecular species of the substances which are present in a composition.
NMR spectral peaks are influenced by the nature of neighboring chemical carbon-carbon bonds and neighboring non-carbons in the molecular structure. It enables the assessment of the chemical character of the substance, present crude oil or distillates. Assignment can be made to indentify whether these molecules are linear or branched paraffins, olefins, and mono-aromatics, poly-aromatics, hetero-cyclic, naphthenic, acids, oxygenates and water, as shown in Figure 1 . This is the principle on which the first NMR analyzer has been developed.
Introduction of the third generation NMR process analyzers opens a new road to accurate and reliable process control of chemical processes, refinery streams and blending processes. In contrast to other optical spectrometry based technologies, such as NIR analyzers, NMR analyzers are not dictated by the need for transparency of the process streams, such as refinery streams, to be analyzed. NMR technology can be implemented without any restriction to transparent, dense and opaque process streams alike. However, due to the lack of stability, accuracy and reliability of the first and second generation NMR analyzers, many end users, predominately in refineries, were skeptical about moving these systems out of the lab and integrating these analyzers in their process control schemes. Elimination of these obstacles has been accomplished in the third generation NMR analyzers, which is required for successful implementation in the refining and process industries.
NMR technology is applicable to any process stream where organic molecules are involved. It enables the correlation between physical properties of process feed and product streams. It provides an effective tool to improve the ability to make real-time adjustment of the process conditions, which is a major requirement or optimized utilization of a process unit.
Physical properties of a process streams are an accumulation of physical properties and concentrations of each individual component present.
Optical methods, such as NIR/FT-IR spectrometry, have the advantage of being able to measure at locations far from the analyzer, through installed fixed field probes or flow cells which are connected to the analyzer by fiber optics. Sample switching, by an optical multiplexer, occurs almost instantaneous. However its application is restricted by the transparency of the process stream. Furthermore, NIR/FT-IR spectrometry is based on “fingerprints” of chemical compositions, without specification to molecular structures. Overlapping and weak spectral bands and the lack of linear spectral response dictate chemometric models to include a wide range of expectable compositions in the model to enhance its accuracy.
In contrast, NMR precisely distinguishes between of molecular structures and the nature of chemical bonds involved. Quantification of hydrogen atoms according to the spectral signal enables quantitative and qualitative assessment of molecular structures with a high certainty. In combination with its linear response chemometrics accurately correlates between spectral data and physical properties.
A basis on “fingerprints” and a lack of linear response in optical spectrometric methods requires the chemometric model to include all possible variation in order to correlate between spectral data and quantification of physical properties. The linear response of NMR enables extrapolation to quantify physical properties also of compositions, are not included in the calibration curve of the chemometric model.
Various hetero-atomic substances are present at different levels in crude oils, depending on its origin. These substances partially distill alongside distillation products. If they are not included in the chemometric model, crude oil switching will affect the accuracy the analytical results due to non-consistent light absorption in the NIR/FT-IR region. However, as a fundamental method, the accuracy in NMR spectrometry is not affected by the presence of these hetero-atomic molecules. Assessment can be made specifically for hydrocarbon molecules only. This is extremely important for its application in crude oil distillation to analyze crude oil, naphtha, diesel, kerosene and heavy distillates, vacuum distillates and bottom products.
Figure 4 illustrates the application of NMR process analyzers in a crude distillation unit of a refinery. Light distillates can be measured by NIR/FT-IR or NMR analyzers. Monitoring of kerosene and diesel can also be performed by both, but under the restrictions that process streams are transparent, and that crude switching is omitted. Otherwise, NMR is the method of preference. Heavy distillates, vacuum distillates and bottom product can only be monitored by NMR.
Full monitoring of all refinery streams is essential to the most efficient performance of the crude unit. However, temperature differences between distillate streams prevented previous generation of NMR analyzers to switch between the distillate streams without losing accuracy. The enhanced temperature insulation between magnet and probe in the third generation eliminated these drawbacks.
Analysis of feed and product streams by the same analytical method and the same analyzer is the preferred strategy to accurately correlate between physical properties of process streams. Boiling ranges of different distillates partially overlap. Efficient and stringent adjustment of the temperature profile in the distillation to optimize cut points between distillates increases its production capacity of the most required distillates. It enables the optimization of its capacity towards the distillates that will have the highest profit on the market.
Crude switching is a common practice in many refineries. Implementation of NMR process analyzers reduces the impact of the transition period, until optimized process conditions are restored.
Beside petroleum industries, chemical process industries can benefit from NMR process analyzers. NMR technology can provide accurate information about the substances available in the process stream. As a molecular determining method, NMR distinguishes between and enables quantification of raw materials, intermediates and final products. It provides an efficient tool for analyses of reaction proceedings and failure analyses in chemical processes. NMR spectra can be analyzed by chemist throughout the entire production process.
Other applications of NMR analyzers can be found in the pharmaceutical industries, the food industries, fermentation processes in biotechnology industries, in all other processes, where organic substances are available with NMR distinguishable chemical compositions.
Process NMR can be applied for at many locations within refinery processes. Its ability to not be restricted to transparent streams provide an effected tool to continuously monitor the feed and product streams of refinery units, especially those units where optical spectrometry methods fail.
In other process chemical industries, NMR technology can provide accurate information about the substances available in the process stream. As a molecular determining method, NMR enables quantification of raw materials, intermediates and final products.
The incorporation of NMR analyzer in process stream monitoring prevents the production of border-line and off-spec materials and avoids the investment of time and money to upgrade these products.
A comparison between the characteristics of NIR and third generation NMR technology is summarized in table I.
Table I – Comparisons between NIR and NMR Technology
Laboratory analyses are time consuming and expensive. In many cases, lab analytical results come too late, to establish adequate process control. Integration of the new third generation NMR process analyzers in the process control of refinery units, such as crude distillation units, CCR, FCC, and in the process chemical industries gives the opportunity to effectively monitor the quality of feed and process streams. It provides an effective tool for ongoing optimization of process conditions to produce most valuable products with at maximum quality and at minimum cost. It minimizes the production of borderline and off-spec material, and reduces time to be invested in reprocessing.
The successful application of on-line process analyses is dictated by its stability, its feasibility to provide reliable analytical results process streams and its capability to switch without any impact between different process streams. Previously, its high sensitivity towards temperature fluctuations, its lack of stability of the magnetic systems and its deficient reliability harmed the reputation of NMR process analyzers in refineries and process industries. The conclusions of a thorough and all including failure analyses of first and second generation on-line NMR process analyzers were implemented in the entirely new design of the third generation. Incorporation of innovative hardware and software has eliminated the drawbacks of previous generations and increased its stability accuracy and reliability. The cost of human resources required for calibration and maintenance is reduced. It will benefit chemical industries and refineries to effectively monitor and control its entire process. It enables the entire production unit to run at maximum efficiency and profitability. Stability, Reliability and accuracy of the third generation of NMR process analyzers is the major challenge in restoring the reputation of NMR technology for process control in the chemical and petroleum industries.
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