N-d-M method

The n-d-M correlation is an ASTM (D-3238) method that uses refractive index (n), density (d), average molecular weight (MW), and sulfur (S) to estimate the percentage of total carbon distribution in the aromatic ring structure (% C ), naphthenic ring structure (Cj,), and paraffin chains (% Cp). Both refractive index and density are either measured or estimated at 20°C (68°F). Appendix 4 shows formulas used to calculate carbon distribution. Note that the n-d-M method calculates, for example, the percent of carbon in the aromatic ring... 

Structure. For instance, if there was a toluene molecule in the feed, the n-d-M method predicts six aromatic carbons (86%) versus the actual seven carbons. 

The n-d-M method is very sensitive to both refractive index and density. It calls for measurement or estimation of the feed refractive index at 20°C (68°F). The problem is that the majority of FCC feeds are virtually solid at 20°C and the refractometer is unable to measure... 

Using the feed property data in Example 2-1. determine MW, C, Cj,, and Cp using the n-d-M method. 

Figure 12.6 shows the percentage of aromatic carbon content estimated from H-NMR spectra versus observed Ca (n-d-M method). Although the coefficient of determination, R, for the Ca model prediction is 0.97, the relative error for Ca prediction can be substantial approaching 23% for low Ca values. Interestingly, the Ca correlates the most with protons in di-aromatics and tetra-aromatics, HIO region,... 

The coefficient of determination, K, of the Cp model is 0.95. The maximnm error for Cp prediction can be as high as 7%. Figure 12.8 shows the percentage of paraf-hnic carbon content Cp estimated from H-NMR spectra versns observed (n-d-M method). 

Standard Test Method for Calculation of Carbon Distribution and Structural Group Analysis of Petroleum Oils by the n-d-M Method. ASTM D3238-95 (2005). 

This equation has been obtained by applying the method of least squares to 81 saturated mineral oil fractions, of which the number of rings had been determined by the n-d-M method (cf. p. 24). 

In the n-d-M method use is made of three physical constants, namely the refractive index n (measured for the sodium D-line), the density d in g/ml and the molecular weight M, for the determination of the composition of mineral oil fractions. 

The n-d-M method for determining the composition of olefin-free mineral oil fractions has proved to be reliable if %Cr < 75 with the further limitation that %Ca < %Cjv. For the number of rings the limitation is Rt < 4 with Ra < i Rt<... 

Nowhere is the contribution of spectroscopic studies more emphatic than in application to the delineation of structural types in the heavier feedstocks. This has been necessary because of the unknown nature of these feedstocks by refiners. One particular example is the n.d.M. method (ASTM D-3238) which is designed for the carbon distribution and structural group analysis of petroleum oils. Later investigators have taken structural group analysis several steps further than the n.d.M. method. 

Structural Bulk Analysis of Heavy Crude Oil Fractions n-d-M Method... 

The refraction index, density and average molecular weight of the sample must be measured in order to use the n-d-M method. Evaluating the results of this method begins with calculating four factors v, X, W and Y - according to equations (2.23), (2.24), (2.25) and (2.26). 

It should be noted that the same analysis results could be obtained, for example, for a mixture of different derivates with different lengths of the paraffinic chain or different number of aromatic rings. Now it is obvious that the information obtained from the n-d-M method describes the average structure of the molecules in the sample mixture and the amount of carbon in the different groups. This is why this method is called structural bulk analysis. 

The evaluation method described in this chapter was developed by Van-Ness [85]. More about the n-d-M method and similar methods for the analysis of heavy crude oil fraction can be found in many references at the end of this chapter. 

Early compositional analyses on petroleum (and lubricating oil base stocks) were focused on quantifying the three major hydrocarbon types present, namely paraffins, naphthenes or cycloparaffins, and aromatics. In that period (the 1920s to the 1950s), the availability of instrumental techniques was essentially nil in terms of our viewpoint today, since spectroscopic methods were in their infancy, as was electronics technology. Accordingly, research workers used the limited tools available at that time—density, refractive index, molecular weight, and elemental analyses. Based on work with model compounds, these led to compositional relationships between structure and these measurements and development of the concepts of VGC, refractivity intercept, and the n-d-M method. 

The n-d-M method is an empirical method for determining the carbon type distribution (%CP %CN, %CA) by simple measurement of the refractive index ( ), density (d), and molecular weight (M) of the sample. It also provides the mean number of naphthenic (RN) and aromatic (RA) rings per molecule. The method was developed by researchers at Koninklijke/Shell in Holland after World War II. Its application includes lube feedstocks and raffinates.2 Nearly all applications have been to solvent refined stocks. The current American Society for Testing and Materials (ASTM) method is D3238. ASTM D2140 is applicable to insulating oils. 

Accuracy of the n-d-M Method Based on Comparison with Direct Method Data... 

Development of the n-d-M method was the consequence of much preceding work relating composition to density, refractive index, and molecular weight. One of the intermediate steps involved development of the VGC. 

The VGC is a measure of petroleum composition that connects two physical properties—specific gravity and viscosity—for distillation fractions. It was developed by Hill and Coates in 19286 to be an index of the paraffinic or naphthenic character. The objective was to fill in the gap between clearly paraffinic and clearly naphthenic samples. It is still reported for base stocks and ranges from approximately 0.78 (paraffinic base stocks) to 1.0 (highly aromatic base stocks) and its value provides some guidance for the solvency properties of the oil. Like the results of the n-d-M method, the VGC is usually reported for naphthenic products, but not for paraffinic ones. 

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