Detailed Hydrocarbon Characterization of RFCC Feed Stocks by NMR

Feb 21, 2007 - Detailed Hydrocarbon Characterization of RFCC Feed Stocks by NMR Spectroscopic Techniques. V. Bansal,*G. J. Krishna,A. Chopra, andA. S...
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Energy & Fuels 2007, 21, 1024-1029

Detailed Hydrocarbon Characterization of RFCC Feed Stocks by NMR Spectroscopic Techniques V. Bansal,* G. J. Krishna, A. Chopra, and A. S. Sarpal Indian Oil Corporation Limited, Research & DeVelopment Centre, Sector-13, Faridabad-121007, India ReceiVed June 12, 2006. ReVised Manuscript ReceiVed January 9, 2007

The paper discusses in detail the hydrocarbon characterization of resid fluid catalytic cracking (RFCC) feed stocks by 1D/2D NMR spectroscopic techniques. A direct, fast, and convenient method based on NMR spectroscopic techniques has been developed for the estimation of total aromatics and the ratio of normal to isoparaffin content in the feedstocks for the RFCC process. The realization of three essential parameters, (a) estimation of the average alkyl chain length of substituents on aromatics, (b) substituted aromatic carbons, and (c) bridged-head carbons of fused polyaromatic rings from the 1H NMR spectral analyses, forms the basis of derivation of mathematical equations used for the estimation of total aromatics, degree of substitution, and condensation. The normal and isoparaffinic contents of saturate and aromatic fractions have been estimated by the method on the basis of 13C NMR spectroscopic techniques. The exact assignments of signals due to R-CHn (n ) 1 to 3) groups in the 1H NMR spectral regions of 2.0-4.0 ppm and the 13C NMR spectral region of 5-50 ppm of aromatics and saturate fractions have been achieved by the applications of 1D and 2D multipulse NMR techniques such as 13C-DEPT and phase-sensitive gradient-selected edited 2D heteronuclear single quantum coherence. The results of aromatics obtained by the developed method for samples of RFCC feeds from different sources have shown good correlation with the standard methods of open column chromatographic separation (ASTM D-2549, R2 ) 0.992) and saturates, aromatics, resins, and asphaltenes analysis by thinlayer chromatography-flame ionization detection (IP-469, R2 ) 0.993). The nature of the alkyl chain length of aromatic substituents and saturate fractions has also been discussed. The detailed hydrocarbon data have been utilized for the development of a process model for predicting the product profile and yield pattern.

Introduction Catalytic cracking, especially fluid catalytic cracking (FCC) and resid FCC (RFCC), has been used as an important petroleum refining process for the past several decades. The main purpose is to convert low-value heavier petroleum fractions into lighter high-value products to utilize the bottom of the barrel. Various useful products obtained from FCC/RFCC processes are liquefied petroleum gas, light-cycle naphtha, heavy-cycle naphtha, and total cycle oil, which is a diesel-blending component. In general, vacuum gas oil (VGO) is used as a major feedstock for catalytic cracking units, whereas, in RFCC processes, feed is comprised of a combination of VGO/hydrocracked bottom (HCB) and vacuum residues (570 °C plus). However, other combinations of residual oils available in a refinery can also be used in the RFCC process for upgradation. RFCC feed is operationally defined as a mixture of vacuum residue (VR) at 550 °C plus, VGO at 370 °C plus, and HCB. The hydrocarbon composition of the feedstock plays an important role in the catalytic cracking due to the difference in the reactivity of different hydrocarbon types and their cokeforming tendencies. Much of the work on the cracking of pure hydrocarbons and petroleum feed stocks suggests the dependability of cracking relies on hydrocarbon types and the boiling range. This also influences the selectivity of cracked products. As the feed characteristics are changing day by day, frequent optimization of process parameters including the design of suitable catalysts is becoming very essential. Various empirical correlations such as K-factor, the n-d-M method, and API * Corresponding author e-mail: [email protected].

gravity are normally used in refineries to understand relationships between feed characteristics and product profile/properties. However, these empirical correlations do have their limitations, as they do not completely define the feed compositional characteristics. Generally, feedstock are characterized in terms of hydrocarbon types and their composition by using various sophisticated analytical techniques, such as mass spectrometry (MS), highperformance liquid chromatography (HPLC), and nuclear magnetic resonance (NMR), to derive correlations among feed properties, yields, and product properties.1-9 These correlations can be used for the upgradation of residual feeds using the FCC process. The hydrocarbon group type classification cannot explain completely the crackability and coke-forming tendencies of feedstocks used in FCC. This behavioral complexity in heavier feedstock arises because each individual component in (1) Speight, J. G. Handbook on Petroleum Products Analysis; Publisher: Location, 2002. (2) Dave, R. A. Modern Petroleum Technology; Publisher: Location, 2000; Vols. 1 and 2. (3) Sarpal, A. S.; Kapur, G. S.; Chopra, A.; Jain, S. K.; Srivastava, S. P. Fuel 1996, 75 (4) 483-90. (4) Sarpal, A. S.; Kapur, G. S.; Mukherhee, S.; Jain, S. K. Fuel, 1997, 76, 931-37. (5) Bansal, V.; Kapur, G. S.; Sarpal, A. S.; Kagdiyal, V.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Energy Fuels 1998, 12, 1223-27. (6) Lappas, A. A.; Patiaka, D.; Ikonomou, D.; Vasalos, I. A. Ind. Eng. Chem. Res. 1997, 36, 3110-16. (7) Liang, Z.; Hsu, C. S. Energy Fuels 1998, 12, 637-43. (8) Boduszynskl, M. M.; Hurtublse, R. J.; Allen, T. W.; Silver, H. F. Anal. Chem. 1983, 55, 232-41. (9) Mukherjee, S.; Kapur, G. S.; Chopra, A.; Sarpal, A. S. Energy Fuels 2004, 18, 30-36.

10.1021/ef060268x CCC: $37.00 © 2007 American Chemical Society Published on Web 02/21/2007

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a molecule behaves in a different way. For example, the aromatic substituent groups form lighter ends and coke much faster than naphthenic heavy compounds. Also, the cracking rates of most paraffins and naphthenes increase with increasing molecular weight. The side chains on aromatic rings crack quite readily, but aromatic rings are very stable and are extremely resistant to cracking.10 Therefore, the characterization in terms of the types and nature of a hydrocarbon is of the utmost importance to establish a correlation between the nature, type, and composition of feed molecules with properties such as crackability and coking tendency in the FCC process. The total aromatics and their distribution as mono-, di-, and polyaromatics in middle distillates are estimated by the standard methods (IP-391, ASTM D-6591, and ASTM-5682) based on HPLC techniques. Though the qualitative separations of aromatics components of heavier ends could be achieved by HPLC methods, the quantitative analysis was limited by the nonuniform response factors of various classes of aromatics. The estimation of the saturates, aromatics, resins, and asphaltenes (SARA) content of heavier boiling fractions is carried out by the standard procedure based on thin-layer chromatography-flame ionization detection TLC-FID (IP-469). The detailed hydrocarbon type analysis of petroleum fractions in terms of total aromatics and paraffins and their types are more conveniently carried out by methods based on NMR spectroscopic techniques because of the directness, ease, rapidness, freedom of boiling point restriction, and strong interpretation skills offered by 1D/2D NMR techniques.3-5,9 The aim of the present investigation is to develop an integrated approach for the detailed hydrocarbon characterization of RFCC feeds (blends of VGO-370 °C plus, VR-550 °C plus, and HCB) with respect to aromatics and paraffins by the applications of multipulse 1D and 2D NMR spectroscopic techniques. A direct rapid and convenient method based on the 1H NMR spectroscopic techniques has been developed for the estimation of total aromatics without prior separation. The aromatics and saturate fractions have been analyzed for the determination of degree of substitution, degree of condensation, and normal to isoparaffinic content. Experimental Section Samples. The RFCC feed samples were obtained from pilot plant studies conducted at one of the Indian refineries. The compositions of three batches of feed samples comprised of VR-550 °C plus, VGO-370 °C plus, and HCB have been given in the table below. Feed numbers 4-8 have been received from an Indian refinery, and their compositions have not been revealed. % w/w feed

VGO

VR

HCB

feed-1 feed-2 feed-3

80.0 82.0 83.8

5.0 7.5 10.5

15.0 10.5 5.7

NMR Spectroscopic Studies. All of the 1H/13C NMR spectra were recorded on a Bruker ACP-300 MHz NMR spectrometer. 1H NMR. The concentration of the sample used was 5-10% w/w in CDCl3 for 1H NMR containing tetramethylsilane as an internal reference as per the following experimental conditions described in our previous work:3-4 spectral width ) 5000 Hz (0.012.0 ppm), spectral size ) 16 000, digital resolution ) 0.49 Hz/ point, 90° pulse ) 18 µs, relaxation delay ) 10 s, and number of scans ) 64. (10) Solomon, M. J.; Benjamin, G.; Sterling, E. V.; Weekman, V. W., Jr. AlChE J. 1976, 22 (4) 701-12.

13C

NMR. The quantitative 13C NMR spectra were obtained for a solution of approximately 30% w/w in CdCl3. The quantitative 13C NMR spectra were obtained in the inverse gated mode using 0.1 M chromium acetylacetonate [Cr(acac)3] as a relaxation agent. Under these conditions, reproducible quantitative spectra were obtained. The spectral parameters were as follows: spectral width ) 20 000 Hz (0.0-200.0 ppm), spectral size ) 16 000, digital resolution ) 0.49 Hz/point, 90° pulse ) 8.7 µs, relaxation delay ) 5.0 s, and number of scans ) 10 000. All the 1H and 13C NMR spectra were integrated after baseline correction, and a mean of three integration values has been taken for each calculation. Two-dimensional phase-sensitive gradient-selected (GS) edited heteronuclear single quantum coherence (HSQC) and DEPT-135° experiments for editing CH, CH2, and CH3 carbons were carried out using the pulse sequence of Davis11 and Doddrel et al.12 as per the experimental conditions described in our previous work.9 For the DEPT 135° experiment, the 90° proton decoupler pulse and the carbon 90° and 180° pulse widths used were 12.55, 6.0, and 12.0 µs, respectively. The delays D1 ) 3 s and D2 ) 0.00345 s (JCH ) 135 Hz) were used with 400 scans. For the GS edited HSQC experiment, the 90° and 180° pulse widths were 10.0 and 8.7 µs, respectively. The number of scans was 16; the number of experiments was 512. The total experimental time was 3 h, with a 3 s delay (D1) for a sweep width of 2705.6 Hz.

Open-Column Chromatography The RFCC feed samples were fractionated into saturated and aromatic fractions as per the ASTM-2549/91 (reapproved 1995) method. SARA Analysis. A SARA analysis has been carried out as per the IP-469 method, using an Iatroscan TLC-FID instrument equipped with an FID. The rods used for the analysis were silicacoated quartz rods of the type S-II manufactured by Iatran laboratories, Japan. About 1 µL of the 2.0% sample solution is spotted on the chromrods and developed sequentially in hexane, toluene, and tetrahydrofuran solvents. The first development is carried out up to 10 cm in hexane, the second up to 5 cm in toluene, and the third up to 2 cm in tetrahydrofuran. The rods are dried in an oven at 70 °C for about 2 min after each development. The peaks are detected by scanning the rods in an oxygen-hydrogen flame at a scan rate of 35 s per scan. The hydrogen and air flow rates are 160 mL/min and 2000 mL/min, respectively. The integrator used for the quantitative purposes is the Iatro Recorder TC-21, and the averages of three integral values are taken for the further analysis.

Results and Discussions Estimation of Total Aromatic Content by the 1H NMR Method. Estimation of the total aromatic content in the RFCC sample using a 1H NMR spectrum is based on the group molecular weight method and the determination of the relative number of carbon atoms from the 1H NMR spectrum. The philosophy of the group molecular weight method is to first assign the 1H NMR spectrum in terms of CHn groups (n ) 1, 2, and 3) in order to calculate the relative number of carbon atoms. Another requirement would be the realization of complete contributions made by the hydrocarbon class under the estimation (say aromatics) in the spectrum. The latter part requires estimation of the quaternary carbons and the average chain length of the substituents attached to aromatic rings. Besides saturates, RFCC feed samples are rich in aromatics; especially, the di-ring plus aromatics are present in an ap(11) Davis, D. G. J. Magn. Reson. 1991, 91, 665-672. (12) Dodderal, D. M.; Peg, D. T.; Bendall, M. R. J. Magn. Reson. 1982, 48, 323.

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Figure 1. 300 MHz 1H NMR spectrum of a representative RFCC feed sample, along with division of various regions. Table 1. Divisions of 1H NMR Spectrum of RFCC Feed (for Measuring Integral Intensity as per Figure 1) region (ppm) 9.0-6.5 4.0-2.65 2.65-2.00 2.00-1.40 1.0-1.4 0.5-1.0 7.4-9.0

assignment CH of aromatics rings R-CH2 to aromatic rings R-CH3 to aromatic rings CH, CH2 of naphthenes, and isoparaffins (average CH1.25 groups) CH2 CH3 di-plus aromatic rings

integral intensity A B C D E F Id

preciable amount. The degree of substituents (σ) and the chain length of the alkyl substituents on the aromatics depend on the boiling range of the sample. The total aromatic content, therefore, will be the contribution of the ring and the substituents. Knowledge about the average chain length attached to aromatics (other than R-methyl groups) denoted as “n” is a must for estimation of the total aromatics using the present methodology. The nature and average length of the alkyl substituents greater than CH3 groups cannot be estimated directly, as their signals are merged with the signals of paraffins (0.5-2.0 ppm) in the 1H NMR spectra. Besides this, knowledge about the contribution of substituted aromatic ring (ARs) and bridge-head aromatic carbons (ARb) is also required. However, these carbons cannot be estimated directly from a 1H NMR spectrum, because they are nonprotonated and, hence, invisible to 1H NMR spectroscopy. In the present method, an indirect approach has been adopted to obtain information about the parameters: ARs, ARb, and “n”. The relative ARs carbons have been realized from the signals in the region 2.0-4.0 ppm assigned to R-CH3 and R-CH2 groups. ARb carbons have been realized by the developed equations. An average value of “n” has been estimated from the 1H NMR spectral analysis of the pure aromatic fractions collected from open-column chromatographic separation of a few feed samples. The 300 MHz 1H NMR spectra of a representative RFCC feed sample are shown in Figure 1, along with the expanded spectrum in the region 2-9 ppm. Various resonance signals assigned to different groups have been marked in the spectrum. The complete assignment of the 1H NMR spectrum in terms of various hydrocarbon groups is included in Table 1. The assignments with regard to R-CHn (n ) 1-3) have been made

Figure 2. GS edit 2D-HSQC contour plot of the aromatic fraction of a RFCC feed.

by the application of 2D edited HSQC and 13C-DEPT-135 NMR experiments. 2D Edited HSQC. Proton-carbon chemical shift correlation experiments (HSQC) provide both carbon and proton chemical shifts at the same time and establish a one-to-one correspondence for each pair of directly bonded carbon and hydrogen atoms. Figure 2 shows the edited HSQC contour spectrum of the aromatic fraction of a RFCC feed sample. The edited 2D-HSQC multipulse experiment correlates, edits, and separates the signals due to R-CHn multiplicity in the 1H NMR spectral region of 2.0-4.0 ppm with the corresponding signals in the 13C NMR from 15 to 50 ppm. It is clear from the spectrum that signals between 18 and 22 ppm in the 13C NMR spectrum are due to the R-methyl substituents of the aromatic carbons, as also confirmed by the DEPT-135 spectral analyses (Figure 3). These signals and their corresponding correlations with the 1H NMR signals in the spectrum have been used to assign the 1H NMR region from 2.0 to 4.0 ppm. Careful examination of the edited HSQC spectrum (contour “X”) shows that the 1H NMR spectral region 2.0-2.65 ppm (region C, Figure 1) is due to R-CH3 groups with marginal extension of low-intensity signals up to 3.0 ppm. The R-CH groups of very low intensity appear between 3.4 and 4.0 ppm in the 1H NMR spectrum, showing connectivity with the corresponding signals in the 13C NMR spectral region of 32-55 ppm. The CH2 contours (“Y”) of the edited HSQC spectrum shows the correlation between the R-CH2 carbons (25-40 ppm) and the R-CH2 protons in the broad region 2.654.0 ppm. This shows that the 2.65-4.0 ppm region contains signals due to mostly R-CH2 protons. The interference from R-CH3 and R-CH groups is marginal in the region of 2.65-4.0 ppm. The 2D edited HSQC heteronuclear correlation NMR spectra analysis has resolved the complexity of the region 2.04.0 ppm of the 1H NMR spectra and provided useful and reliable

Detailed Hydrocarbon Characterization

Energy & Fuels, Vol. 21, No. 2, 2007 1027

aromatic fractions of eight samples separated by silica gel column chromatography. The value of “n” has been calculated using the following equation:

n ) [I4.0-2.65/2) + (I2.0-1.0/2) + (I1.0-0.5/3)]/(I4.0-2.65/2) where I4.0-2.65 and so forth are the integral intensities of the, in this example, 4.0-2.65 ppm region. The R-CH3 groups have been excluded as their contribution can be directly realized from the integral intensity of the region 2.05-2.65 ppm. The average value of “n” has been estimated to be 6.0. Estimation of Total Aromatics by 1H NMR Method. After the complete assignment of the 1H NMR spectrum has been made (Table 1), and the relative contribution of ARs and ARb has been realized, the total aromatic content of a sample can be estimated by the following equations. The procedure has been described in our earlier published work.5,9 Figure 3. (A) 13C NMR spectra of a representative RFCC feed, (B) the aromatic fraction, and (C) DEPT-135 of the aromatic fraction. Table 2. Hydrocarbon Type Analysis of RFCC Feeds

sample

aromatics 1H NMR

aromatics ASTM D-2549

aromatics IP-469 SARA analysis

feed-1 feed-2 feed-3 feed-4 feed-5 feed-6 feed-7 feed-8

13.4 19.9 10.4 21.0 40.8 17.0 41.0 16.0

15.0 19.7 12.0 20.0 38.0 21.0 39.0 17.6

14.4 20.0 13.5 22.4 38.9 18.3 44.0 16.5

Tc ) A/1 + B/2 + C/3 + D/1.25 + E/2 + F/3 + ARs/1 + ARb/1 where Tc, Tw, Aw, and n are the relative total carbons, total

Tw ) 13(A + B) + 9C + 13.25D/1.25 + 7E + 5F + 12 × 0.7Id saturates

Ca

Cs

86.6 80.3 89.6 79.0 59.2 83.0 30.0 84.0

7.5 8.7 6.7 8.6 16.3 8.2 23.8 7.8

92.5 91.3 93.3 91.4 83.7 91.8 76.2 92.2

information regarding the identity of the R-CHn (n ) 1-3) group. Thus, the regions of 2.05-2.65 and 2.65-4.0 ppm in the 1H NMR spectra are used for the estimation of R-CH3 and R-CH2 groups, respectively. Estimation of Substituted Aromatic Ring Carbons (ARs). On the basis of the above discussion, the substituted carbons can be estimated by the following equation:

ARs ) B/2 + C/3 where B and C are integral areas as shown in Figure 1. Contribution of Bridge-Head Carbons (ARb). The contribution of bridge-head carbons is required to be known, in samples where polyaromatics are also present (7.4-9.0 ppm region). The following equation will be applicable for feed samples also, where appreciable amounts of di-plus aromatics are present as discussed in our earlier work.5 The genesis of the equation and error involved in this estimation have also been discussed in that work.

ARb ) 0.7Id where Id is the integral intensity of the signals in the region 7.4-9.0 ppm. Determination of Average Alkyl Chain Length (n). The value of “n” is required to be known in order to realize the contribution of longer alkyl chains (β, γ, and δ carbons) attached to aromatic rings whose signals overlap with those of paraffin chains in the region 0.5-2.0 ppm. The average alkyl chain length (n) of aromatic ring substituents (other than R-CH3) has been estimated from the 1H NMR spectral analysis of pure

Aw ) 13A + n × 7B + 5C + 12ARs + 12Arb group molecular weight of the sample, total group molecular weight of aromatics, and average alkyl chain length of the alkyl substituents (other than methyl groups) on the aromatic rings. Substituting n ) 6.0 and Arb, Aw becomes

Aw ) 13A + 48B + 9C + 12 × 0.7Id The total aromatic content (A) of the sample (weight %) can then be estimated as

A ) 100[Aw/Tw] The total aromatic content for the number of RFCC feed samples has been estimated from the developed equation, and the data are given in Table 2. The aromatic content for few samples has been found to be in the range of 10.4-70.0% w/w. There is a good correlation between the 1H NMR and ASTM D-2549 methods (R2 ) 0.992) and the 1H NMR and IP-469 methods (R2 ) 0.993). Structural Characterization of Aromatics by NMR. The total aromatic carbons (Ca) has been estimated from 13C NMR spectral analysis as per the equation given below:

Ca ) 100(I120.0-160.0/IT) where I120.0-160.0 and IT correspond to the integral intensities of the aromatic region and the total integral intensity in the region 5-160 ppm.

Cs ) 100 - Ca The Ca content estimated from 13C NMR compared well with that obtained by 1H NMR, as given in our earlier work.13 The Ca content has been found to be in the range 6.5-25.0% w/w (Table 2) in the feed samples, whereas in column-separated aromatic fractions, Ca has been estimated between 28.0 and 43.0% w/w (Table 3). (13) Sarpal, A. S.; Kapur, G. S.; Bansal, V.; Jain, S. K.; Srivastava, S. P.; Bhatnagar, A. K. Pet. Sci. Technol. 1998, 16 (7,8), 851-68.

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Table 3. Carbon Type Analysis for Aromatic Fractions of RFCC Feedsa

Table 4. Carbon Type Analysis for Saturate Fractions of RFCC Feedsa

sample

Ca

Np/Ip

Cnp

Cip

Cn

s

sample

Np/Ip

Cnp

Cip

Cn

aromatic frac. feed-1 aromatic frac. feed-2 aromatic frac. feed-3 aromatic frac. feed-4 aromatic frac. feed-5 aromatic frac. feed-6 aromatic frac. feed-7

32.3

0.8

28.3

35.7

3.7

47.5

28.6

1.1

27.4

24.1

19.9

49.8

41.6

1.7

27.2

15.5

15.7

46.2

34.5

1.5

26.1

17.4

22.0

47.7

saturate frac. feed-1 saturate frac. feed-2 saturate frac. feed-3 saturate frac. feed-4 saturate frac. feed-5 saturate frac. feed-6 saturate frac. feed-7

1.1 1.3 1.2 1.2 1.7 1.6 1.6

52.3 50.9 47.4 47.9 52.1 49.2 49.9

45.6 39.1 40.9 40.8 30.1 30.3 30.5

2.1 10.0 11.7 11.3 17.8 20.5 19.6

43.1

1.2

24.8

20.0

12.1

43.6

35.5

0.9

23.8

25.1

15.6

45.0

38.6

0.9

22.7

24.3

14.4

48.4

a Ca ) aromatic carbons, Np/Ip ) ratio of normal to isoparaffins, Cnp ) normal paraffinic carbons, Cip ) isoparaffinic carbons, Cn ) naphthenic carbons, σ ) degree of substitution.

The degree of substitution (σ) calculated from the 1H NMR spectra of the aromatic fraction ranges from 40 to 50% (Table 3).

%(σ) ) Csub/A × 100 where A, B, and C are integral areas as shown in Figure 1.

A ) A/1 + B/2 + C/3 Csub ) B/2 + C/3 Nature of the Alkyl Chain of Aromatics by NMR. The average alkyl chain length of substituents, other than R-CH3, of the aromatics has been estimated to be in the range of 5.56.5 carbons. The R-CH3 signals are due to the presence of both mono- and polyaromatic ring systems. The chain length is an average of the naphthenic ring attached to aromatics and longer alkyl chains either on aromatic rings or naphthenic rings.9 The evidence of the presence of a longer alkyl carbon chain has been provided by the 13C NMR spectral analysis of different samples of feeds for RFCC. The appearance of signals at 13.9, 22.1, 31.9, and 29-29.9 ppm are indicative of longer alkyl chains in the aromatic ring system (Figure 3). The intense signals in the region of 29.0-29.9 ppm due to -(CH2)n carbons clearly indicate the existence of a normal carbon chain length of more than four carbon atoms. On the basis of the integral intensity of the γ-carbon at 31.9 ppm, the average alkyl chain length of longer chains in the aromatic ring system has been estimated for a number of samples obtained from different sources. Since the 2D edited HSQC NMR analyses of the aromatic fractions have shown R-CH2 signals in the region of 2.65-4.0 ppm, it is most likely that a longer alkyl chain is attached to the naphthenic ring of the aromatic ring system. The chemical shift range of R-CH2 on aromatic rings falls in the region of 2.4-2.7 ppm. The chemical shift region of 15-22 ppm is composed of signals due to the R-CH3 of aromatic substituents and CH3’s of the branched chain, that is, isoparaffinic carbons. The 2D HSQC NMR contour plots of different samples of RFCC have provided sufficient evidence of the presence of branched CH3 in this region. The correlation between the protons of CH3 at 0.5-1.5 ppm in the 1H NMR spectrum and corresponding carbon in the 13C NMR spectrum 15.0-22.0 ppm is established in the 2D edited HSQC contour plot (Figure 2). The supporting signals of CH2 and CH of the isoparaffinic chain are present in the regions of 10-12.0 ppm, 26-29 ppm, and 32.5-50 ppm in the 13C NMR spectra. The complexity of the spectra indicate

a Np/Ip ) ratio of normal to isoparaffins, Cnp ) normal paraffinic carbons, Cip ) isoparaffinic carbons, Cn ) naphthenic carbons.

branching at the terminal end and center of the chain similar to isopropyl, ethyl, propyl, butyl, and so forth branchings along the carbon chain. It is clear from the above discussion that both normal and branched alkyl chains are present in the aromatic ring system (Table 3). Estimation of Total Aromatics by SARA Analysis (IP469). The total aromatics by SARA analysis as per the IP-469 method using the TLC-FID technique have been obtained and used for comparison purposes with those obtained from the 1H NMR method. The TLC-FID technique, which combines the advantages of TLC with the quantification using a FID can be potentially used to assess the compositional analysis of hydrocarbons with the advantage of speed, a small quantity of the sample, and solvent requirements. In the case of RFCC feed stocks, the sample produces four peaks in the TLC-FID chromatogram corresponding to saturates, aromatics, resins, and asphaltenes. The separation is achieved using a sequence of three successive developments in hexane, toluene, and THF in which hexane separates the saturate part, toluene is used to separate aromatics from resinous material, and THF is used to separate resins and asphaltenes. The saturate peak appears at 0.153 min in the chromatogram, aromatics at 0.324 min, resins at 0.45 min, and asphaltenes at 0.54 min. The area percent is directly taken as the percentage of components since the response factor is considered to be uniform using a FID detector. The area corresponding to aromatics, resins, and asphaltenes is added up to give the total aromatics as reported in Table 2. Nature of Saturate Fractions. The assignment of the signals corresponding to normal and various isoparaffinic structures has been achieved by the 13C and DEPT-135 NMR spectra. The percentage of normal paraffinic (Cnp) and isoparaffinic carbons (Cip) has been estimated from 13C NMR, as given below:

Cnp ) 100[(3I31.9 + I29.5-30.7)/IT] where I31.9, I29.5-30.7, and IT are the integral intensities in the regions 31.9 ppm and 29.5-30.7 ppm and the total integral intensity in the region 5.0-160.0 ppm, respectively. The ratio of Cnp to Cip has been used for estimating Cip, as given in the following equation:

Cnp/Cip ) I31.9/I10.0-15.0 - I31.9 The normal and isoparaffinic content of the saturate fractions

Cn ) 100 - (Ca + Cnp + Cip) of various samples has been given in the Table 4. Repeatability of the NMR Method. The repeatability of the developed NMR method has been estimated on three samples having different average values of the total aromatic content. The values of the standard deviation (at the 95% confidence level) and repeatability have been found to be 1.4 and 5.2, respectively.

Detailed Hydrocarbon Characterization

Applications to the Refinery Process. The structural parameters such as the total aromatics and their nature; the degree of substitution and condensation; the nature of the average alkyl chain length of the substituents on aromatic, normal, and isoparaffinic contents of the saturates; and so forth are very important for the prediction of the product profile, yield pattern, coking tendency of the feed on the catalyst, and establishing a structure-property correlation. These parameters are being used routinely for optimization of the refining parameters and the selection of an appropriate catalyst.

Energy & Fuels, Vol. 21, No. 2, 2007 1029

normal and isoparaffinic hydrocarbons (Np/Ip ∼ 1.2-1.4). The aromatics ring system is highly condensed and substituted, the average degrees of substitution and condensation being 47.0% and 25.0%, respectively. The total aromatics content estimated by the developed method is in the range of 10.4-70.0% w/w. There has been good correlation between the 1H NMR and ASTM 2549 methods (R2 ) 0.992) as well as the 1H NMR and IP-469 methods (R2 ) 0.993). The method is fast and convenient and requires no prior separation. These structural parameters have been used to build models for the prediction of product profiles and their physicochemical properties.

Conclusions The NMR spectroscopic analyses of samples of RFCC feedstocks provide in depth structural information regarding the nature of aromatics and saturate fractions. The alkyl chain length of both aromatics and saturate fractions is comprised of both

Acknowledgment. The authors wish to acknowledge the Management of IOC, R&D Faridabad, for allowing them to publish the present work. EF060268X