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Carbon
Aromaticity and Related Parameters ......Details
of Calculation
For years research has
shown that carbon aromaticity is a superior parameter when used to define the
aromatic nature of hydrocarbon streams within the refinery. Its use would be
extremely useful in characterization of FCC feed-streams, coker feedstreams,
reformer/platformer feed, hydro-desulfurization units, etc. In fact,
anywhere the process requires knowledge of a hydrogen balance the carbon
aromaticity number is invaluable. Unfortunately, the only way to truly determine
this value is by liquid-state 13C NMR spectroscopy. This technique
has always proved to expensive for the refinery to use on a regular basis.
Instead, refiners have turned to easier test methods that equate roughly to the
aromaticity - they use refractive index, aniline point, Conradson Carbon, or
Ramsbottom Carbon to deduce the aromaticity. Below is a correlation of
refractive index to carbon aromaticity obtained by 13C NMR ... the
correlation is poor!
Recently, PNA has
developed another correlative technique that yields far greater accuracy with
respect to being able to define a samples carbon aromaticity rapidly through 1H
NMR spectroscopy. 13C NMR data is obtained on a 300 MHz NMR
spectrometer, the aromaticity is calculated and these values are regressed
(using PLS) against 1H NMR spectra obtained on a process
NMR instrument. Below are the resulting models obtained on a dataset of base
oils and on a dataset of base oils plus a large number of vacuum gas oils (VGO).
These models are currently
being improved by addressing signal to noise issues in the original 13C
NMR data. All spectra are being acquired with a 10mm NMR probe increasing the
signal to noise by an order of magnitude. Standard error of prediction is
currently being reduced from about 0.7 atomic%C to 0.3 atomic%C by simply
improving the accuracy of the primary method.
From a specialized
processing routine it is also possible to determine the carbon paraffinicity and
carbon naphthenicity from the same 13C NMR data. These values can
also be regressed against the 1H NMR spectra using PLS to yield
calibrations that will improve online process control for units processing heavy
distillates.
PNA has developed a large
database of aromaticity/paraffinicity/naphthenicity for a large range of
hydrocarbons from light naphtha to HVGO (heavy vacuum gas oils). This data will
be invaluable in defining hydrocarbon chemistry to advanced process control
systems and process optimizers. The use of these parameters is being undertaken
in several projects such as FCC feed characterization and lube oil
manufacturing.
It has been proven in the
course of these studies that ASTM D2140 (which correlates aromaticity by an
empirical relationship with refractive index and density) does not accurately
predict aromaticity and is awful at predicting naphthenicity and paraffinicity.
The relationship between D2140 values (CA, CP, and CN)
and the actual values calculated from 13C NMR (FA, FP,
and FN) are shown below. One can see that a correlation, though poor,
does exist between CA and FA. However a poor correlation
exists between CP and FP (D2140 consistently too high),
and CN and FN (D2140 consistently too low).
Correlations are also shown for a small dataset of base oils.
Clearly, when accurate hydrocarbon
chemistry is required for process control the NMR derived parameters are far
superior to what is available through empirical correlation. No other technique
can yield this information.
For
more information on this topic please contact:
John
Edwards.
Manager,
Process and Analytical NMR Services
Process
NMR Associates LLC, 87A
Sand Pit Rd, Danbury,
CT 06810, USA, Tel:
(203) 744-5905
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