Molecular-Level Structural Insight into Clarified Oil by Nuclear

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Molecular Level Structural Insight into Clarified Oil by NMR Spectroscopy: Estimation of Hydrocarbon Types and Average Structural Parameters Sujit Mondal, Anil Yadav, Ravindra Kumar, Veena Bansal, S.K. Das, Jayaraj Christopher, and G.S. Kapur Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 30 May 2017 Downloaded from http://pubs.acs.org on June 5, 2017

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A simple and easy to grasp method for the estimation of hydrocarbon types and average structural parameters for clarified oil (CLO) has been developed. The method exploits the concept of group molecular weight for the estimation of aromatics and saturates by quantitative 1H NMR spectroscopy. Combination of quantitative 1H and 13C NMR aided with above three empirical relationships allows developing several NMR based equations which finally provide average structural parameters useful in feed (CLO) quality monitoring for needle coke production. 639x384mm (96 x 96 DPI)

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Molecular Level Structural Insight into Clarified Oil by NMR Spectroscopy: Estimation of Hydrocarbon Types and Average Structural Parameters Sujit Mondal*, Anil Yadav, Ravindra Kumar, Veena Bansal, S. K. Das, J. Christopher & G. S. Kapur Indian Oil Corporation Limited, R&D Center, Sector-13, Faridabad-121007 *Corresponding author: [email protected]

Abstract A direct and easy to grasp methodology based on combination of quantitative 1H and 13

C-NMR has been developed for the estimation of total aromatics, saturates and

several important structural parameters like aromaticity, average number of aromatic rings per molecule, average number of aromatic carbon atoms per molecule, average molecular weight, degree of aromatic substitution, degree of aromatic condensation, nature of condensation and substitution to aromatic ring etc. for clarified oil (CLO) from Indian Oil refineries. These parameters along with HPLC analysis data for di- to pentaring aromatics provide molecular level understanding of this potentially valuable feedstock, which can thus be correlated with process parameters for needle coke production from CLO. The method exploits the concept of group molecular weight (GMWt) and uses three empirical equations governing the nature of aromatic condensation. The various kinds of CLO originated from Indian Oil refineries have been classified into three major classes by virtue of their differential nature and composition. 2D HSQC NMR has been extensively studied for accurate assignment of different classes of protons in 1H NMR spectra of CLOs. The method was validated by SARA analysis using TLC-FID (IP-469) (R2 = 0.9698) and by open column chromatography (ASTM D-2549) (R2 = 0.9887) for hydrocarbon types. Key Words: Quantitative 1H and

13

C NMR, clarified oil, group molecular weight,

average structural parameters.

1. Introduction Clarified Oil (CLO), bottom of fractionators from FCC unit, is a byproduct during refinery processing.1 Due to its overwhelmingly aromatic nature this complex mixture cannot be used as fuels.1 However, it has the potential to offer value added products in case their


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structural information could be ascertained and is considered to be a good feed for needle coke production.2,3 Production of needle coke requires feedstock with characteristics coke generating groups, coking conditions and calcination conditions. It can be argued that each feed stock differs in terms of their composition and knowing the insight of the composition and relating it to optimized processing parameters could enable one to offer high quality value added products. Aromatic and saturated compounds in CLO play an important role. Aromatic compounds determine the mesophase formation and presence of small quantity of saturates are favorable for gas evolution in the solidification stage.4–6 Detailed hydrocarbon type analysis of various heavier end petroleum fractions such as vacuum residues, reduced crude oil, residual fuel oil, clarified oil, coker fuel oil, coker residue, recycle oil etc. have well been studied by various analytical techniques, such as NMR,7–15 Liquid Chromatography (LC) -high temperature comprehensive 2D Gas Chromatography (GC),16,17 Mass Spectrometry (MS) etc.18,19 Two recent reviews, one on application of NMR spectroscopy in the petroleum industry by John C. Edwards,20 published by ASTM international and second on analytical methods for the characterization of high mass hydrocarbon mixture by Harod et al. in Chemical Reviews21 covers most of the important works till 2012. Detailed structural information such as aromaticity, aromatic fused ring-configuration and degree of alkyl substitution for heavier petroleum samples by NMR have also been reported in literature.7–15 Kapur and Berger22 have done a phenomenal job by resolving α-methyl and α-methylene protons unambiguously in

1

H NMR spectra for heavy petroleum fractions by 1D

HSQC/1D-HMQC. In spite of a substantial number of studies on heavier end feed stocks by analytical techniques there has been no specific report on CLO analysis by NMR spectroscopy. Moreover, the methods for average structural parameters described in the literature, including one of our previous reports15, mostly devoted to coal tar and asphaltene characterization.10–15 So a simple method for the estimation of hydrocarbon types and average structural parameters like aromaticity (fa), average number of aromatic ring carbon atoms (Ca*) per molecule, average number of aromatics rings (Ra) per molecule, degree of aromatic substitution (σ), degree of aromatic condensation (λ), average


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molecular weight (AMWt), nature of condensation and substitution to aromatic ring for CLO would be of great significance and would provide vital inputs in modeling and simulation of various processing parameters for coke production. As part of our continuous efforts3,15,22-31 to develop NMR based analytical method for the analysis of various kind of petroleum products, in the present work a direct and easy to grasp methodology based on combination of quantitative 1H (Q1H) and

13

C (Q13C)-NMR

has been developed for estimating total aromatics, saturates and other important structural parameters mentioned above, thus provides molecular level understanding of this potentially valuable feed stock. The method exploits the concept of group molecular weight (GMWt) for hydrocarbon type estimation and uses three empirical equations governing the nature of aromatic condensation along with few NMR based equations for the estimation of average structural parameters, which does not require the use of elemental composition (%C, %H).15 2D HSQC NMR has been extensively studied for accurate assignment of different classes of protons in a

1

H NMR spectrum.

Hydrocarbon types-the aromatics and saturates have been validated by SARA analysis using TLC-FID (IP-469) and by open column chromatography (ASTM D-2549). Also the average parameters were correlated with mono-, di- , tri-, tetra- and penta- ring aromatics data as obtained through HPLC analysis.

2. Experimental 2.1. Samples 15 CLO samples with variable composition have been analyzed by quantitative 1H,

13

C

and 2D HSQC NMR, open column chromatography following ASTM D-2549, TLC-FID (IP-469) and HPLC.

2.2. NMR Method 1

H NMR: The sample solutions were prepared in chloroform-D-carbon disulfide (9:1 v/v,

composite solvent) and tetra methyl silane (TMS) was used as an internal standard. The concentration of the sample used was 60-80 mg in 0.6-0.7 ml of composite solvent. All proton NMR spectra were recorded on a Jeol ECA-500 NMR spectrometer operating at the proton frequency of 500 MHz, spectral width 7512 Hz (-2.5-12.5 ppm), 90° pulse =


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10.7µs, relaxation delay = 20s, digital resolution 0.57 Hz/point. 32 repetitions were averaged with 32K data point and 12.0 min experimental time in ECA machine. 13

C NMR:

13

C NMR spectra were acquired on a Agilent DD2 500 MHz spectrometer

equipped with a 10 mm broadband observe probe with enhanced operating at spectrometer frequency 125.7 MHz for

13

C sensitivity

13

C, acquisition time = 1.049s,

relaxation delay = 15s, 900 pulse = 14.10µs at 58dB, line broadening = 3-5 Hz, spectral width = 31250 Hz (-10 to 235 ppm), scans = ~1500. The concentration of the sample used 1.0-1.1 g in 2.0-2.5 ml of the said composite solvent. Quantitative

13

C appearance

was ensured by adding the relaxation agent chromium triacetylacetonate [Cr(acac)3] in the concentration range of 10 mg/mL in inverse gated condition. All the 1H and 13C NMR spectra were integrated after baseline correction and a mean of minimum three integral values has been taken for each calculation. DEPT-135: Distortionless Enhancement by Polarization Transfer (DEPT-1350), the

13

C-

DEPT NMR spectra were acquired on a Agilent DD2 500 MHz spectrometer equipped with the same 10 mm broadband probe, experiments for editing CH, CH2, and CH3 carbons, the 900 proton decoupler pulse 81.0µs at 48dB, the carbon 900 observation pulse 14.35µs at 58dB and 1800 observation pulse 125.0µs at 57dB, respectively. Relaxation delay of 10s, t = 1/4 J = 3.425ms (JCH = 146 Hz) and 500 scans were used. 2D HSQC & Edited HSQC: Two-dimensional phase-sensitive gradient-selected (GS) edited

heteronuclear

single

quantum

coherence

(Edited-HSQC)

or

normal

heteronuclear single quantum coherence (HSQC) spectra were recorded on a Jeol ECA-500 NMR spectrometer operating at the proton frequency of 500 MHz. For the GS HSQC experiment, the 900 and 1800 pulse widths were 10.0 and 8.7 µs, respectively, with 16 numbers of scans and 512 numbers of experiments. The total experimental time was 3 h with a 3s delay (d1) for a sweep width of 2705.6 Hz.

2.3. SARA Analysis by TLC-FID: Instrumentation and Analytical Conditions: The hydrocarbon type analysis in terms of saturates, aromatics, resins and asphaltenes (SARA) was accomplished by using Iatroscan MK-6 instrument, equipped with a flame ionization detector (FID) and interfaced with computerized data acquisition system. The pure H2 gas was used as


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fuel. The flow of hydrogen gas was kept at 160ml/min and that of air was kept at 2000ml/min. The chromrods used were silica coated SIII type with pore diameter 60Å and particle size 5µm. Feed sample solutions at the level of 2% were prepared by dissolution in dichloromethane. Before sample application, chromrods were passed twice through the FID to remove contaminants, and to obtain constant activity of the silica layer. Each rod was spotted by 1µl of sample solution. The IP-469 method of SARA analysis has been adopted for the chromrod developments. Accordingly three stage developmental schemes were used. The first stage development was carried in hexane (10cm), the second stage in toluene (5-6cm) and third stage in 95:5 chloroformmethanol (2.5cm). Each set of rods was dried for 3min and then, pyrolized over FID at a constant scan speed of 0.30cm/sec. In order to compare with NMR data, aromatics, resins and asphaltenes were combined to provide total aromatics.

2.4. HPLC Method: HPLC Instrumentation: The high performance liquid chromatography analysis was carried on Shimadzu LC-2010 CHT HPLC instrument. It is comprised of a degasser unit (DGU-20A) for extracting any dissolved air from the solvents, quaternary pump (LC30AD) for isocratic and gradient solvent programme, an auto sampler (SIL-30AC) for sample injection, column oven (CTO-20AC) and combination of two detectors photo diode array (PDA) (SPD-M20A) and electrospray light scattering detector (ELSD) (Waters, model no. ELS) connected in series. Optimized HPLC Analytical Conditions Chromatographic separation was obtained using diammine column (ES industries make) and mobile phase gradient comprising of hexane, dichloromethane and isopropyl alcohol as solvents. A 1µl of 5%(w/V) of sample solution prepared in dichloromethane and filtered through 0.45µl was used for injection. The ELSD and PDA were used as detectors.

2.5. Open Column Chromatography: Column chromatographic separation of aromatics from the saturates has been carried out as per the ASTM-2549/85 method and discussed in several of our previous articles.24,30


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2.6. Safety Considerations: As the CLOs are overwhemly populated with polynuclear aromatics with significant amount of bay region protons, indicated that they are potential carcinogens.32 Proper personal safety equipments including mask, hand-gloves, fume hoods etc. must be used while handling the samples.

3. Results & Discussions 3.1. Hydrocarbon Type: Estimation of hydrocarbon types, viz. the aromatics, saturates are based on group molecular weight (GMWt) method and determination of average substitution chain length for aromatic substitution. The philosophy of group molecular weight has well been discussed in our previous publications while quantitative estimation of hydrocarbon types has been described for middle distillates24,25, gasolines26–28, ATF29, VGO30 etc. The most important aspect of GMWt method is the assignment of a 1H NMR spectrum in terms of CHn (n = 0,1,2 and 3) groups in order to estimate the relative number of carbon and hydrogen atoms corresponding to, first for whole of the spectrum providing the total GMWt of the sample and secondly assigning the same corresponding to any particular class, e.g., aromatics, olefins, naphthenes etc. providing the group molecular weight for those, viz. aromatics (GMWt-Ar), of olefins (GMWt-Ole) etc..

3.1.1. Nature and Classification of CLO: The nature of aromatics in CLO has been found to be completely different than similar boiling range other refinery streams, viz. vacuum gas oil (VGO). In CLO the aromatic rings are predominantly methyl (or ethyl) substituted3,22 rather than long alkyl chain substitutions. Moreover, due to its nature of origin1 the CLOs are of more medium sized polynuclear aromatics with short substitutions. Thus accurate assignment of methyl and methylene regions in the 1H NMR spectrum is crucial for correct estimation of GMWt and was ensured by way of analysis of HSQC-NMR. It was observed that while in general the aromatic methyl groups appears between 2.1-2.5 ppm24,30 the CLO aromatic methyls appeared in a much more extended region of 2.0-2.9 ppm, sometime even beyond 2.9 ppm. It is worthy to mention that total aromatics will be the contribution from aromatic ring


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structures as well as various substitutions, may be acyclic substitutions, benzonaphathenic type components etc., associated to them.30 In addition, the CLO samples are overwhelmly rich in angularly fused polycyclic rings with significant amount of bayregion32 aromatic protons. To be more accurate the CLO samples as received from FCC units of Indian Oil refineries have been divided into three different classes according to their origin and composition. Type-I has been defined as those which are exclusively aromatic in nature consisting of 90-100% aromatic compounds, overwhelmly methyl substituted, almost free from any saturates. Type-II is predominantly aromatics in nature consisting of more than 75% aromatics, mostly methyl substituted and Type-III is having 60-75% aromatics, also have relatively higher %age of unconverted VGO with long chain substitutions in aromatics.1 Less than 60% aromatics have not been considered as CLO; rather they could be termed as equivalent to unconverted VGO feed of the FCC process. It is needless to mention that type-I has been originated from a refinery FCC process where highest conversion occurred, type-II from an efficient FCC process whereas type-III originated either from an incomplete catalytic process or has been collected before complete conversion. 3.1.2. Assignment in the 1H NMR Spectra of CLO and 2D HSQC NMR Analysis: A 500 MHz 1H NMR spectrum of a representative CLO sample of type-I (a), along with expanded spectra of aromatic fraction in the regions of 2.0–4.5 ppm (b) representing the protons directly attached to the alpha carbons to aromatic ring and 6.0–9.5 ppm (c) representing the aromatic protons have been shown in Figure 1. The hydrogen type distribution as shown in these spectra included the chemical shift regions of aromatic rings (c, 6.0–9.5 ppm), hydrogens on carbon alpha to aromatics rings (b, 2.0–4.5 ppm). In expanded spectrum c, integral Cb has been assigned to the presence of di-plus aromatic rings in total aromatics designated by integral A. In spectrum b, the integral B has been assigned to α-CH2 (4.5–2.9 ppm) and integral C due to α-CH3 (2.9–2.05 ppm) to aromatic rings. The accurate assignment has been done by HSQC and DEPT-135 experiments as shown in Figure 2. It is noteworthy while few contours for α-CH2 have appeared beyond 2.9 ppm, at the same time few contours for α-CH3 have also been observed beyond 2.9 ppm. Thus an average 2.9 ppm division between α-CH2 and α-


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CH3 is reasonable and is further supported by final estimation of hydrocarbon types and comparing the data obtained from other techniques (vide infra). Hydrogens on βposition (CH/CH2) to aromatic rings and –CH/CH2 hydrogens of isoalkanes and cycloalkanes appeared in at 2.05–1.40 ppm (integral region D), methylene hydrogens (CH2) of longer alkyl chains at 1.40–1.00 ppm (integral region E), and methyl hydrogens (CH3) on γ- and δ- to aromatic rings and of alkanes and cyclo-alkanes (integral region F; 1.00– 0.30 ppm). Overlapped –CH/CH2 groups in the region D has been assumed to be due to average CH1.5 groups as evident by 13C NMR spectral analysis and literature study.30 Highly intense aromatic signals beyond 7.4 ppm are indicative of the presence of polyaromatic rings in excess quantity. 1H NMR spectrum of representative CLO samples of type-II and type-III along with expanded spectra of aromatic fraction has been shown in Figure 3 & 4 respectively. It is clearly evident from the spectra that the difference is mainly in the extent of methyl and methylene substitution in the aromatic rings. Complete proton assignment for all three types of CLO has been shown in Table-1. These assignments were corroborated not only by 2D HSQC analysis (see supporting documents for HSQC spectra for type-II & III CLOs) but also by comparing the hydrocarbon type estimation data for all three types of CLOs with that of by TLC-FID and open column chromatography. Comparison of hydrogen content data for these CLO samples as estimated by using eq-15 (vide infra) vis. á vis. by an independent method31 was also found to be in close agreement and hence support our basis of classification and assignments.

3.1.3. Estimation of Aromatics, Olefin, Saturates and Hydrogen Content Following Group Molecular Weight: As complete assignment of the 1H NMR spectrum has been achieved, the first step towards hydrocarbon type estimation involves the estimation of the relative total number of carbons (TC) and the total group molecular weight (GMWtT) of the sample. The TC for the sample is calculated by dividing the individual integral regions in the 1H NMR spectrum by the number of protons causing the signal.24 GMWtT is then obtained by multiplying these numbers with the respective molecular weight of the groups (12 for the C group, 13 for the CH group, 14 for the CH2 group, and 15 for the CH3 group). Therefore, TC and GMWtT are given as-


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Tc = A/1 + B/2 + C/3 + D/1.5 + E/2 + F/3 + X - - - - - - (1) GMWtT = A/1*13 + B/2*14 + C/3*15 + D/1.5*13.5 + E/2*14 + F/3*15 + X*12 - - - - - - (2) where A, B, C, D, E and F are the integral areas of 1H NMR spectrum as given in Figure 1, 3, 4 and summarized in table-1; the quantity ‘X’ in above eq-1 & 2 is the relative contribution of aromatic quaternary carbons substituted by aliphatic as well as bridgedhead aromatic carbons given as. X = Csub + Cbrs - - - - - - (3) where Csub is the relative contribution of aromatic carbons substituted by aliphatics and Cbrs is relative contribution due to bridgehead aromatic carbons in the

1

H NMR

spectrum. However, these carbons (Csub and Cbrs) cannot be estimated directly from the 1

H NMR spectrum due to their non-protonated nature. Therefore, the following indirect

approaches have been adopted to estimate Csub and Cbrs Csub = [B/2 + C/3]

- - - - - - (4) &

Cbrs = Cb*0.7 - - - - - - (5) Genesis of the eq-5, considering an assumption that di-ring and tri-ring aromatics are present in the ration of 4:1 (mol fraction), has well been discussed in our previous publications.24,30 The equation that was used for the estimation of total relative number of polynuclear aromatic carbons (CPNA), an important parameter towards eq-5, is as followsCPNA = (2Cb*K*1.25*0.8)+(2Cb*K*1.4*0.2) - - - - - - (6)24 where K, a relative parameter directly related to degree of substitution and is estimated as K = (A/1+B/2+C/3)/A, factors 1.25 and 1.4 come from the ratio of total and nonbridgehead

aromatic

carbons

[(ArH+Arsub+Arb)/(ArH+Arsub)]

in

standard

di-ring

(naphthalene) and tri-ring (phenanthrene) aromatics, factors 0.8 and 0.2 are relative mol %age of di-ring and tri-ring aromatics. However, as the quantity and nature of aromatics in CLO differs significantly from the middle distillates24 the equations-5 & 6 need to be rationalized. Contribution from the bridgehead carbon has thus been re-visited. Keeping in mind that CLO are significantly rich in higher aromatics, the above equations were accordingly modified. It was observed that the contribution for aromatic protons below 7.4 ppm is comparatively less and thus the integral contribution for di- aromatic onward cannot simply be considered


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as 2Cb. The equations thus re-formulated for all three different classes of CLOs are given as followsCPNA (type-I) = (1.55Cb*K*1.25*fd)+(1.55Cb*K*1.4*ft)+(1.55Cb*K*1.55*ftr)+(1.55Cb*K*1.65*fp) - - - - (7) CPNA (type-II) = (1.7Cb*K*1.25*fd)+(1.7Cb*K*1.4*ft)+(1.7Cb*K*1.55*ftr)+(1.7Cb*K*1.65*fp) - - - - (8) CPNA (type-III) = (1.8Cb*K*1.25*fd)+(1.8Cb*K*1.4*ft)+(1.8Cb*K*1.55*ftr)+(1.8Cb*K*1.65*fp) - - - - (9) where factors 1.25, 1.4, 1.55 and 1.65 come from the ratio of total and non-bridgehead aromatic carbons [(ArH+Arsub+Arb)/(ArH+Arsub)] in standard di-ring (naphthalene), tri-ring (phenanthrene), tetra-ring (contribution of 50:50 angular pyrene and linear tetracene) and penta-ring (contribution of 50:50 angular benzo[a]pyrene and pentacene) aromatics. Factors fd, ft, ftr and fp are relative mol %age of di-ring, tri-ring, tetra-ring and penta-ring aromatics respectively and have been obtained with the help of area %age of di- to penta- ring aromatics (including polars) from the HPLC analysis (vide infra). As described by Bansal et al.,24 it has been observed that one and half times the integral intensity in the region 7.4-9.5 ppm (Cb) should give contribution due to protons of diplus aromatic rings (1.55*Cb) i.e., all di-, tri-, tetra- and penta- ring aromatics for CLOs of type-I (see supporting information). For type-II CLO it has been found to be 1.7*Cb and for type-III CLO was 1.80*Cb (see supporting information). Considering the theoretical percentage of bridgehead carbons in unsubstituted di-, tri-, tetra- and penta- ring aromatics is 20.0%, 28.6%, 35.4% and 39.2% respectively and following the similar procedure as describe by Bansal et al.24 all these parameters along with the relative contribution from the bridgehead carbon (Cbrs) have been estimated for all three types of CLOs (Table-2). At least three samples for each type have been considered for the estimation. Eq-5 has thus been modified and found to beCbrs = Cb*1.1 - - - - - - (10) It is interesting to mention that though the average K values, mol fractions (f) and CPNA values come significantly different for these three classes of CLOs, the combination of these parameters providing relative contribution from bridgehead carbon (Cbrs) estimated to be almost same (Table-2).


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In order to estimate, e.g., the %age of aromatics in the CLO, another requirement would be the realization of complete contributions made by the aromatic class in the spectrum; requires in addition to quaternary carbons estimation of the average chain length (R1) of the substituents attached to aromatic rings.30 The chromatographic separation of saturates from the aromatics was found to be very helpful for the estimation of average substitution chain length to aromatics, especially for type-II and III CLOs which have substantial amount of saturates. For type-I CLO and the aromatic fraction for type-II and III CLOs, the average aromatic substituent chain length (other than CH3) would thus be give byR1 = (B/2+D/1.5+E/2+F/3)/(B/2) - - - - - - (11) So, GMWtT = 13(A+B) + 9(C+D) + 7E + 5F + 13.8Cb - - - - - - (12) GMWtAr = A/1*13 +R1*B/2*14 + C/3*15 + 12(B/2+C/3) + 12*Cb*1.15 = 13A + (7R1+6)B + 9C + 13.8Cb - - - - - - (13) The total aromatic content in the CLO sample could then be estimated as Weight %age of Aromatics = (GMWtAr/ GMWtT)*100 - - - - - - (14) Weight %age Hydrogen = (I T/ GMWtT)*100 - - - - - - (15) (IT = A+B+C+D+E+F, total integral value in a 1H NMR spectrum) The aromatic, saturate and hydrogen contents thus estimated from the above equations have been tabulated in Table-3. There has been found to be a good correlation in the estimated hydrocarbon types between the 1H NMR and open column ASTM D-2549 (R2 = 0.9887) and with IP-469 TLC-FID (R2 = 0.9698) methods. The hydrogen content values as estimated by GMWt method using eq-15 and by a recently published independent method31 for all the CLOs have been tabulated in Table-3.

3.1.4. Class-wise Distribution of Hydrocarbon upto Tetra-ring Aromatics and Polars by HPLC: The hydrocarbon group-type separation to saturates and mono- di-, tri-, tetra- and polar (5+ ring plus heteroaromatics, if any) aromatics was carried out using a normal phase diammine column and following the analytical conditions described in section-2.4. The cut points are based on eicosane (saturates), nonadecylbenzene and o-xylene (1- ring),


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isopropylnaphthalene and naphthalene (2-ring), phenanthrene (3-ring) and pyrene (4ring). The valve switching time was selected by the retention time of benzo[a]pyrene in order to estimate the penta+ ring aromatics and polars. The aromatic distribution profile in terms of HPLC chromatograms of three representative CLOs has been shown in Figure-5 (others are given in supporting documents). CLO-1 (type-I) reveals its richness in tri-, tetra- and penta- aromatics and this was further weighted towards tetraaromatics class, suggesting it to be potential candidate to form quality coke; however its saturates content is too low. It is also evident from the chromatogram that the sample CLO-14, representative of type 3, is rich in saturates and lower aromatics, while the sample CLO-9 (type-2) has intermediate composition. The area under each peak of different aromatic classes was measured. The conversion of area %age in HPLC to wt %age can theoretically be done with the help of reference standard or TLC-FID data. However as response for different classes in ELSD vary a lot the estimation may not be accurate. So the area %age obtained for aromatic classes, normalized to 100%, was considered to arrive at the mol fractions of different aromatic classes. The area %age for di-, tri-, tetra- and penta+ aromatic as obtained by HPLC analysis for few samples belonging to type-I, type-II and type-III are tabulated in Table-4.

3.2. Average Structural Parameters: There have been a numerous number of articles7-13 describing the method for the estimation of average structural parameters on heavier end petroleum feed stocks starting from R. B. Williams phenomenal work33, however covered mostly ashphaltenes and coal tars. Before going to describe the actual method for the estimation of average structural parameters for the aromatic fraction in CLOs, let us understand three empirical equations and their implication in our studies. Ca* = 7 x (No. Ca/No. Cper)2 − 1

- - - - - - (16)

Ra = (No. Ca − No. Cper)/2 + 1 = Cbr*/2+1

- - - - - - (17)

Cib = (6 − No. Ca + 2xNo. Cbr) = (6-Ca*+2xCbr*)

- - - - - - (18)

where, Ca* = absolute no. of aromatic carbons; Ra = no. of aromatic rings in a condensed polycyclic system and Cib = absolute no. of internal bridged aromatic carbon


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Page 14 of 25

in a peri-condensed system; No. Ca = relative total no. of Ar-carbon (same with Ca* for a standard unsubstituted structure); No. Cper = relative total no. of Ar-carbon at the periphery of the system; No. Cbr = relative total no. of bridgehead Ar-carbon and Cbr* = absolute total no. of bridgehead aromatic carbons. All the structures shown in Table-5 follow these three empirical equations by virtue of natural laws. In our quest to estimate the average structural parameters of CLO these equations have been exploited with several other equations (vide infra) developed from quantitative 1H and

13

C NMR spectra and few assumptions based on scientific values

were also taken into consideration. Eq-14 could easily be modified as follows:15 Absolute averages no. of aromatic carbons per average structure in a given sample = Ca*= 7x (No. Ca/No. Cper)2 -1 = 7x (%Ca/%Cper)2 -1 - - - - - - (19). %Ca can be estimated by Q13C NMR [ASTM-5292-99(2014) and supporting information]. %Cper can be estimated as follows%Cper = (Cah+Csub)*100/CT - - - - - - (20) where, Cah = relative total no. of hydrogenated Ar-carbon; Csub = relative total no. of substituted Ar-carbon, are given asCah = A/1; Csub = B/2+C/3 - - - - - - (21) and CT = relative total no. of carbon = Ct+Cbr; Ct = relative total no. of carbon excluding bridgehead carbon. Ct = (A/1)+(B/2)+(B/2)+(C/3)+(C/3)+(D/1.5)+(E/2)+(F/3) - - - - - - (22) where, A-F signify the same integral regions (A to F) of a 1H NMR spectrum. Due to noprotonated nature, Cbr cannot be obtained from 1H NMR spectrum; severe overlap limited the estimation of Cbr from

13

C NMR as well. However a combined use of 1H and

13

C NMR can provide Cbr and various other parameters thereafter. The relative total

number of bridgehead aromatic carbon, Cbr, is estimated as followsfa = %Ca/100 = (Cah+Csub+Cbr)/CT = (Cah+Csub+Cbr)/(Ct+Cbr) - - - - - - (23) So, Cbr = [(fa*Ct)−(Cah+Csub)]/(1−fa) - - - - - - - - (24) The fa was obtained from Q13C NMR spectra [ASTM-5292-99(2014)] and other unknowns in eq-22 could be obtained from Q1H NMR spectrum as described previously.


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Now the C*(absolute average no of total carbon), %Cbr, Cbr*, %Csub, Csub* (absolute average no of total aromatic carbons substituted by aliphatics), Cper* (absolute average no of total aromatic carbons at the periphery of the aromatic sheet), Cah* (absolute average no of total protonated aromatic carbons), H* (absolute average no of hydrogen per molecule) and average molecular weight (AMWt) can be estimated as followsC* = Ca*/fa - - - - - - - - (25) %Cbr = (Cbr/CT)x100 - - - - - - - - (26) Cbr* = (%Cbr/100)xC* - - - - - - - - (27) %Csub = (Csub/CT)x100 - - - - - - - - (28) Csub* = (%Csub/100)xC* - - - - - - - - (29) Cper* = (%Cper/100)xC* - - - - - - - - (30) Cah* =Ca*−(Cb*+Csub*) - - - - - - - - (31) H* = (Cah*/IA)xIT - - - - - - - - (32) AMWt = C*x12 + H*x1 - - - - - - - - (33) σ = (Csub*/ Cper*)x100 - - - - - - - - (34) λ = (Cbr*/ Ca*)x100 - - - - - - - - (35) Using the above equations average structural parameters for these CLO samples have been estimated and tabulated in Table-6. There was a clear difference in several structural parameters among the three classes of CLO. For example, the Ca*, Ra values revolves around 16 and 4 respectively for type-I CLO whereas they were found to be ~14 and 3.1 for type-II; ~12 and 2.0 for type-III CLO. The average substitution to aromatic ring was also found to be varying from less than 4 for type-I CLO to ~5.0 for type-II and ~6.0 to type-III CLO. The nature of aromatic structures, viz. Cbr*, Cib, σ, λ vary significantly for a given class as well. For example the aromatics in CLO-2 are more peri condensed in nature (Cib = 2.4) than CLO-1 (Cib =1.7) though they have other parameters comparable. Similarly, CLO-7 is least in aromatic substitution (σ = 24.3) while more in condensation (λ = 34.8 & Cib = 1.3) among all of the type-II CLOs. It is necessary to mention that Cbrs and Cbr (Eq. 10 & Eq. 24 respectively), though represent same parameter, however estimation of Cbr for each CLO sample by Eq. 24 using Q13C NMR is more representative and desirable for the estimation of average parameters as Cbrs obtained from average factors for a class(from 1H NMR). A list of


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Page 16 of 25

Cbrs and Cbr values estimated using Eq. 10 and Eq. 24 have been given in supporting documents (Table-1S). The parameters, thus estimated, represent whole sample of a CLO, not only aromatic fraction of it. As the three empirical equations-16, 17 &18 are valid for aromatic only structures, the average parameters may not be accurate for aromatic only fraction of the sample, especially for type-III CLO, e.g, the AMWt shows an increasing trend in its values while estimated for samples with relatively higher amount of saturates. An extended method for the estimation of average structural parameters for aromatic only fraction without column separation of saturates and correlation of analytical data with needle coke potential of various kinds of feeds will be discussed in our upcoming article.

3.3. Validation CLO samples are neither homogeneous nor regular in nature. So the data generated considered few assumptions and optimization, especially to classify CLO into three different classes and aimed at to development of a method by which insight of this class of feed can be understood. The 1H NMR method for the estimation of hydrocarbon types (aromatic and saturates) was validated by comparing the NMR data with open column chromatography (ASTM D-2549) (R2 = 0.9887) and TLC-FID (IP-469) (R2 = 0.9698) data. The correlation coefficients derived from regression analysis of data sets obtained by NMR and those by other methods have been given in supporting documents.

3.4. Repeatability and Reproducibility The accuracy of the NMR method is heavily dependent on the optimization of acquisition parameters, especially the recycle delay- minimum of 20s for 1H NMR and of 15s with relaxation agent for 13C NMR (10 mm broadband observe probe with enhanced 13

C sensitivity) are essential to obtain desirable quantitative representation. Number of

scan ~1500 for

13

C acquisition was found to be good with RSD ~0.5 in repetitions. The

repeatability of the estimation of aromatics and saturates by NMR method have found to be excellent for type-I (RSD ~0.25-0.5) to good for type-II (RSD 0.8-1.1) and type-III (RSD 1.5-1.7) CLOs.


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The reproducibility of the developed proton NMR method has been established and found to be satisfactory when some of the samples were recorded by two different operators

following

same

experimental

condition,

sometime

in

two

different

spectrometers as well.

4. Conclusion A simple, direct and easy to grasp method based on Q1H and Q13C NMR spectroscopy has been developed for estimating the total aromatics and saturates content (weight percentage) as well as several average structural parameters of CLO. The estimation of hydrocarbon type agrees well with the results obtained using standard ASTM D-2549 and thin-layer chromatography- flame ionization detection (TLC- FID) methods (IP-469). 2D HSQC-NMR helped assigning various kinds of protons in a 1H NMR spectrum, which in turn provided basis for hydrocarbon type analysis. Combination of Q1H and Q13C NMR offers estimation of average structural parameters, which when combined with HPLC analysis offers in-depth understanding of CLO. The proposed NMR-based method would be very useful in quality monitoring for needle coke production from CLO. During the course of the study it has been observed that the effect of aromaticity and assignment of methyls and methylenes to aromatic substitutions play most important and dramatic role in the estimation of both hydrocarbon types and average structural parameters. Acknowledgements The authors wish to acknowledge the management of IOCL R&D Centre for proving necessary facilities to carry out the work and granting permission to publish this paper.

Corresponding Author *[email protected] The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Page 18 of 25

13

Supporting information includes Q C NMR spectra, 2D HSQC spectra for type-II and III CLOs, overlaid

1

H NMR spectra for several aromatic fractions of a CLO and few HPLC-ELSD

chromatograms, correlation diagrams etc..

References 1.

Srivastava, M.; Kumar, M.; Singh, R,; Agarwal, U.C.; Garg, M.O. Petroleum Science and Technology

2011, 29(2), 172. 2.

Srivastava, M.; Gupta, S.; Thapliyal, M.; Garg, M.O. Petroleum Science and Technology 2003,

5&6 (5-6), 699. 3.

Bansal, V.; Kumar, R.; Krishna, G.J.; Patel, M.B.; Sarpal, A.S.; Basu, B. Fuel 2014, 118, 148 and

the references thereof. 4.

Eser, S.; Jenkins, R.G. ; Carbon 1989, 27 (6), 877.

5.

Wang, G.; Eser, S. Energy Fuels 2007; 21, 3563 and the reference thereof.

6.

Marsh, H. The Chemistry of Mesophase Formation. Petroleum Derived Carbons. ACS

Symposium Series. 1986, 303. 7.

Clutter, D.R.; Petrakis, L.; Stenger, Jr. R.L.; Jensen, R.K. Anal. Chem. 1972, 44, 1395.

8.

Dickinson, E.M. Fuel 1980, 59, 290.

9.

Netzel, D.A. Anal. Chem. 1987, 59, 1775.

10.

Diaz, C.; Blanco, C.G. Energy Fuels 2003, 17, 907.

11.

Rongbao, L.; Zengmin, S.; Bailing, L. Fuel 1988, 67, 565.

12.

Guillen M, Diaz C Blanco. Fuel Process Technol 1998, 58, 1.

13.

Álvarez, P.; Díez, N.; Blanco, C.; Santamaría, R.; Menéndez, R.; Granda, M. Fuel 2013, 105,

471. 14.

Poveda, J.C.; Molina, D.R. Journal of Petroleum Science and Engineering 2012, 84-85, 1.

15.

Christopher, J.; Sarpal, A.S.; Kapur, G.S.; Krishna, A.; Tyagi, B.R.; Jain, M.C.; Jain, S.K.;

Bhatnagar, A.K. Fuel 1996, 75, 999 and the reference thereof. 16.

Netzel, D.A. Guffey, F.D. Energy Fuels 1989, 3, 455.

17.

Dutriez, T.; Courtiade, M.; Thiebaut, D.; Dulot, H.; Borras, J. Energy Fuels 2010, 24(8), 4430.

18.

Rodgers, R.P.; Schaub, T.M.; Marshall, A.G. Anal. Chem. 2005, 77, 20A.

19.

Hsu, C.S.; Hendrickson, C.L.; Rodgers, R.P.; McKenna, A.M.; Marshall, A.G. J. Mass. Spectrom.

2011, 46(4), 337. 20.

Edwards, J.C. A Review of Applications of NMR Spectroscopy in the Petroleum Industry-Chapter

16 in "Spectroscopic Analysis of Petroleum Products and Lubricants" © ASTM International, 2011. 21.

Herod, A.A.; Bartle, K.D.; Morgan, T.J.; Kandiyoti, R. Chem. Rev. 2012, 112, 3892.

22.

Kapur, G.S.; Berger, S. Energy Fuels 2005, 19, 508.

23.

Bansal, V.; Krishna, G.J.; Chopra, A.; Sarpal, A.S. Energy Fuels 2007, 21(2), 1024.


17

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Energy & Fuels

24.

Bansal, V.; Kapur, G.S.; Sarpal, A.S.; Kagdiyal, V.; Jain, S.K.; Srivastava, S.P.; Bhatnagar, A.K.

Energy Fuels 1998,12, 1223. 25.

Bansal, V.; Vatsala, S.; Kapur, G.S.; Basu, B.; Sarpal, A.S. Energy Fuels 2004, 18(5), 1505.

26.

Kapur, G,S.; Singh, A.P.; Sarpal, A.S. Fuel 2000, 79, 1023.

27.

Sarpal, A.S.; Kapur, G.S.; Mukherjee, S. Tiwari, A.K. Fuel 2001, 80, 521.

28.

Bansal, V.; Krishna, G.J.; Singh, A.P.; Gupta, A.K. Sarpal, A.S. Energy Fuels 2008, 22, 410.

29.

Mukherjee, S,; Kapur, G.S.; Chopra, A.; Sarpal, A.S. Energy Fuels 2004, 18, 30.

30.

Kapur, G.S.; Chopra, A,; Sarpal, A.S. Energy Fuels 2005, 19, 1065.

31.

Mondal, S.; Kumar, R.; Bansal, V.; Patel, M.B. J Analyt Sci Technol 2015, 6, 24.

32.

ISO 21461:2012(E), Third edition 2012-06-0, Case postale 56, CH-1211, Geneva.

33.

Williams, R.B. Symposium on Composition of Petroleum Oils, Determination and Evaluation.

ASTM Spec. Tech. Publ. 1958, 224, 168.

Figure 1. Representative 1H NMR spectrum of type-I CLO (a), substituted alkyl region of aromatic fraction (b) and aromatic region (c).


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Page 20 of 25

Figure 2. A representative HSQC spectrum of a type-I CLO.


19

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Energy & Fuels

Figure 3. A representative 1H NMR spectrum of type-II CLO (a), substituted alkyl region of aromatic fraction (b) and aromatic region (c).

Figure 4. A representative 1H NMR spectrum of type-III CLO (a), substituted alkyl region of aromatic fraction (b) and aromatic region (c).


20

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Page 22 of 25

Figure 5. Overlaid HPLC-ELSD chromatogram for CLO-1 (type-I, blue), CLO-9 (type-II, red) and CLO-14 (type-III, green) Table 1. Assignment of CHn protons in 1H NMR spectra of type-I, II and III CLO (integral intensities as per Figure-1, 3 & 4). Assignments CH of aromatic rings CH of diplus aromatic rings CH of mono-aromatic rings α-CH2 to aromatic rings α-CH3 to aromatic rings CH, CH2 of naphthenes, and iso-paraffins (average CH1.5 groups) CH2 CH3 CH of Bay region in poly-aromatic rings

Region (ppm) Type-I Type-II Type-III 9.50-6.50 9.50-6.50 9.50-6.50 9.50-7.40 9.50-7.40 9.50-7.40 7.40-6.50 7.40-6.50 7.40-6.50 4.80-2.90 4.50-2.70 4.50-2.50 2.90-2.05 2.70-2.05 2.50-2.05

Integral Intensity A Cb A - Cb B C

2.05-1.40 2.05-1.40 2.05-1.40

D

1.40-1.00 1.40-1.00 1.40-1.00 1.00-0.30 1.00-0.30 1.00-0.30 9.50-8.30 9.50-8.30 9.50-8.30

E F BH


21

Page 23 of 25

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Energy & Fuels

Table 2. Estimation of average K, CPNA and Cbrs values for all three types of CLO. CLO

Average K

Relative Mol Fraction by HPLC Di-(fd) Tri-(ft) Tetra-(ftr) Penta-(fp)

CPNA

Cbrs

Type-I

1.30

0.02

0.26

0.30

0.42

3.12 34.92*CPNA/100=1.09Cb

Type-II

1.40

0.10

0.36

0.30

0.24

3.55 32.32*CPNA/100=1.15Cb

Type-III

1.55

0.34

0.30

0.14

0.22

3.81 28.96*CPNA/100=1.10Cb

Table 3. Estimation of hydrocarbon types: NMR method vs. ASTM-2549 vs. TLC-FID (IP-469). Hydrocarbon Types-Aromatics (wt%)a

Type

CLO

CLO-1 CLO-2 Type-I CLO-3 CLO-4b CLO-5 CLO-6 CLO-7b CLO-8 Type-II CLO-9 CLO-10b CLO-11b CLO-12b CLO-13b Type-III CLO-14 CLO-15 a

Hydrogen Content Internal Ref NMR Method ASTM D-2549 TLC-FID GMWt Method Method30 100.00 99.20 98.00 6.73 6.91 99.90 99.10 98.80 6.62 6.88 98.20 97.70 97.50 7.33 7.51 96.10 94.20 92.80 7.31 7.58 90.10 89.80 88.10 8.91 9.18 87.80 85.10 83.70 9.10 9.22 87.30 84.20 85.10 8.88 9.08 82.30 80.20 78.00 9.78 9.96 79.50 77.60 74.50 9.61 9.82 78.20 76.50 74.10 9.84 10.20 77.90 74.30 74.00 9.92 10.17 77.00 78.10 75.90 9.72 9.98 72.80 70.40 73.00 10.42 10.55 66.30 65.10 68.20 11.25 11.37 63.70 61.10 60.00 11.77 11.70 b

The rest (100-%age Ar) is wt %age of saturates; These samples showed detectable level of olefinic signals.


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Table 4. Class-wise Distribution (area %) of Mono- to Penta- Ring Aromatics by HPLC. Type Type-I

Type-II

Type-III

Sample No CLO-1 CLO-3 CLO-6 CLO-7 CLO-8 CLO-9 CLO-11 CLO-13 CLO-14 CLO-15

Area %age Normalized to 100 DiTri- Tetra- Penta0.81

24.25

30.14

44.80

2.58

26.98

30.14

40.30

8.10

31.48

38.40

22.02

5.86

34.58

33.03

26.53

14.15

44.75

22.81

18.29

6.82

32.83

28.28

32.07

8.30

37.65

28.58

25.47

34.24

27.95

15.73

22.08

42.56

16.20

5.50

35.74

24.86

45.33

20.80

9.01

Table 5. Standard Aromatic Structures and Empirical Equations 14-16. Ca* (Eq.14)

Structure

Ra (Eq.15)

Cib (Eq.16)

Actual

Estimated

Actual

Estimated

Actual

Estimated

6

6

1

1

0

0

10

9.94

2

2

0

0

13

13.6

3

3

1

1

16

16.9

4

4

2

2

19

19.8

5

5

3

3

20

18.5

5

5

2

2


23

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Energy & Fuels

Table 6. Average Structural Parameters for Various Kind of CLO. Type

CLO

CLO-1 CLO-2 Type-I CLO-3 CLO-4b CLO-5 CLO-6 CLO-7b CLO-8 Type-II CLO-9 CLO-10b CLO-11b CLO-12b CLO-13b Type-III CLO-14 CLO-15 a

1

Average Structural Parameters for Whole Sample fa=%Ca/100)

Ca*

Ra

σ

λ

Cbr*

Cib

AMWt

0.82 0.79 0.76 0.77 0.60 0.57 0.59 0.51 0.52 0.49 0.48 0.50 0.45 0.30 0.37

16.4 18.5 15.6 15.2 13.0 13.2 15.4 14.4 13.7 12.7 12.4 12.6 12.2 10.1 12.0

4.0 4.7 3.8 3.6 2.9 3.0 3.7 3.4 3.1 2.8 2.7 2.8 2.7 2.0 2.5

23.0 23.6 25.4 19.2 29.8 30.5 24.3 30.2 27.9 25.2 29.7 27.2 28.7 38.7 33.6

36.7 40.1 35.0 34.3 29.2 29.8 34.8 32.6 30.1 28.5 27.6 28.4 27.3 20.5 26.6

6.0 7.4 5.2 5.5 3.8 4.0 5.4 4.7 4.2 3.6 3.4 3.6 3.3 2.1 3.2

1.7 2.4 1.2 1.4 0.7 0.7 1.3 1.0 0.8 0.6 0.5 0.5 0.4 0.1 0.4

258.8 299.7 266.1 257.6 287.3 306.8 342.9 377.3 353.0 336.6 345.3 345.7 364.9 445.5 436.5

R1a 3.5 3.4 4.0 4.4 4.8 4.7 5.1 4.4 4.3 4.9 4.8 4.6 5.1 5.5 5.8

b

Estimated by H NMR spectra of pure aromatic fraction; These samples showed detectable level of olefinic signals


24

Molecular-Level Structural Insight into Clarified Oil by Nuclear

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