Study on Thermal Cracking of Kuwaiti Heavy Oil (Vacuum Residue

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Study on Thermal Cracking of Kuwaiti Heavy Oil (Vacuum Residue) and its SARA Fractions by NMR Spectroscopy Andre Hauser, Faisal S AlHumaidan, Hassan Ali Al-Rabiah, and Mamun Absi Halabi Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 13 Jun 2014 Downloaded from http://pubs.acs.org on June 16, 2014

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Study on Thermal Cracking of Kuwaiti Heavy Oil (Vacuum Residue) and its SARA Fractions by NMR Spectroscopy By Andre´ Hauser*†, Faisal AlHumaidan**, Hassan Al-Rabiah**, Mamun Absi Halabi** * **

Central Analytical Laboratory Petroleum Refining Department, Petroleum Research and Studies Center Kuwait Institute for Scientific Research, P.O. Box 24885, Safat 13109, Kuwait

†

Corresponding author. Tel.: + 965 24989053

E-mail address: [email protected]

ABSTRACT Three vacuum residual oils (VR) derived from Ratawi Burgan (RB), Lower Fares (LF) and Eocene (EOC) crude oils were subjected to thermal cracking in a pilot plant, which simulates the Eureka process, to produce cracked distillate petroleum products and residual pitch. The cracking reaction was performed at 430°C for 50 min. The chemical composition of the produced cracked petroleum products and by-product pitch was studied to determine its relationship to the variations in the properties of the feedstock. SARAanalysis of the vacuum residues, cracked oils and pitch show that the residues and pitch consist mainly of aromatic hydrocarbons (VR: 94wt%; Pitch: 99wt%) while the oils themselves contain about 42wt% saturated hydrocarbons (OilRB: 46wt%; OilEOC: 44wt%; OilLF: 36wt%). 1H and 13C NMR revealed that the VRs consist predominantly of alkyl aromatics with di-, tri- (aromatics, resins) and polyaromatic rings (asphaltenes) that thermally decompose splitting the molecules into saturated lower molecular weight hydrocarbons and aromatics having lower aliphatic carbon attached to it. Regardless of the feed, all oils contain more aliphatic (≈ 62wt%) than aromatic carbon (≈21wt%). The cata-condensed aromatic moiety in the oil is tri-aromatic. The effect of feedstock on the chemical composition of the oil and pitch is most prominent for the aromatic and asphaltenic fractions. Keywords: Thermal Cracking, Vacuum Residue, Product Analysis, SARA-Fractions, NMR

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1. Introduction

The increasing dependence on heavy crude oils to meet the world increasing demand for transportation fuel during the past decade have drawn back the attention of the scientific community towards conventional conversion processes. Thermal cracking processes, such as Delayed Coking, Flexicoking, Visbreaking and others, are among the most commonly used conversion technologies by the refining industry. The popularity of these processes stems from their lower capital and operating costs, and greater tolerance to feedstocks with high metal and asphaltenes content as compared with hydroconversion processes. The quality of the products of thermal conversion processes depends primarily on the properties of the feedstocks, particularly those related to its composition, such as their saturates, aromatics, and asphaltenes contents, and to a lesser extent on process conditions and the processing technology. Traditionally the cracked oils produced by thermal cracking, such as naphtha, kerosene, and gas oil are mixed with straight run streams and other similar petroleum cuts from other processing units for further treatment to produce final petroleum products with the desired specifications. However, thermally cracked oils are characterized by being unstable and refractory. Hence, there is also a growing interest in understanding the thermal cracking reactions. A number of studies have been reported in the literature focusing attention on process kinetics and the relationship between process condistions, such as temperature, pressure, and residence time, on product yields 1-5. The literature related to studies aimed at defining the relationship between the composition of the residual oil used as feedstock and the properties of the produced cracked liquid products of thermal conversion processes is more limited

6-9

. These studies used as feedstocks vacuum residual oil derived

from different crude oils. The residual oil were subjected to thermal cracking under conditions resembling those of the delayed coking process, with temperatures ranging between 450-500°C, at atmospheric pressure, and varied residence time. Shishavan et al.7 observed that the properties of feedstocks had little effect on the quality of the respective produced distillate oils. However, with increasing temperature and residence time, the C/H ratio and aromaticity increased. The authors assumed that with increasing severity, the aliphatic and cyclic saturates are increasingly dehydrogenated, leading to higher C/H ratio and aromatization. In his studies, Alvarez et al.8 concluded that asphaltenes are the main contributor to coke formation, while the maltenes are nearly completely thermally decomposed into saturates, aromatics, and resins. This decomposition is assumed to take place through scission of alky groups as the prevailing cracking reaction. Nearly similar conclusion was reached by Torregrosa-Rodriguez et al.6, who observed


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that aliphatic side chains are lost rapidly during the thermal processing of petroleum residues between 420° to 480°C for 12 hours. Similar observations were made by Akmaz et al.9 while investigating the thermal decomposition of asphaltenes from Raman crude oil. At low residence time, the cracked oil yield from peripheral alkyl groups was observed to increase as the temperature was increased; however, when the residence time was raised to 120 min., increasing amounts of coke were observed to form at higher temperatures. This paper is a more in-depth study of our previously published research10 on thermal cracking of VRs. In this paper, an attempt is made to provide an insight into the fate of the SARA-fractions of highly aromatized heavy oils (VRs) during the thermal cracking. In order to reduce the multiple cracking, nitrogen stripping has been applied to speed-up the separation of the cracked oil after its formation. The cracking experiments were carried out in a semi-batch pilot-scale reactor at 430°C and a residence time of 50 min. The SARA-fractions of the feeds and reaction products were subjected to a thorough analysis by GC and 1H, 13C NMR. To consolidate the results, three VRs with API gravity between 1.2° and 3.2° were examined.

2. Materials and methods 2.1Bulk properties of vacuum residues The thermal cracking process uses vacuum residues as feedstock. The vacuum residues (VR) feedstocks were prepared by true boiling point and pot still distillation processes. A 30L Gecil MiniDis true boiling point distillation unit following ASTM D 289211 was used to produce the atmospheric residues (AR). The AR were then vacuum distilled with 5L Gecil PotStill vacuum distillation unit following ASTM D 5236-0312 to produce the VR. Vacuum residues that were used in this study were obtained from three Kuwaiti crude oils. Two heavy crudes, Eocene (EOC) and Lower-Fares (LF) with API of 16 and 18 respectively and one conventional crude Ratawi-Burgan (RB) with an API gravity of 27. The bulk properties of the three vacuum residues are shown in Table 1. 2.2 Thermal cracking The thermal cracking of the vacuum residues was performed in a 2 L semi-batch pilot-scale reactor that emulate the commercial Eureka process9. A schematic diagram of the thermal cracking pilot plant is shown in Figure 1. 500 g of vacuum residue was loaded in the reactor that is equipped with a mixer and a nitrogen-injector. The mixer (300 rpm) was used to ensure a uniform reaction temperature within the reactor while the preheated nitrogen stream was injected (1 Nl/min.) to strip the products immediately


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after formation to prevent over cracking. The continuous stripping by nitrogen also shifts the reaction equilibrium toward product formation by reducing the hydrocarbon-partial pressure in the reactor. The thermal cracking experiments produce three products: off-gases, cracked oil and pitch (Table 2). The cracked oil, in vapour state, was separated from the non-condensable gas using two condensers in series. The first condenser operates at 50ºC while the second one operates at 2ºC. The gas, which consisted mainly of light hydrocarbons and inert carrier gas, was passed through a mist trap and a dry gas counter before being collected in a 150 L gas-sampling bag. The collected gases were immediately analysed after the experiment by a refinery gas analyzer (Varian CP-3800). The cracked oil samples, on the other hand, were collected in amber, sealed sampling bottles and stored in the refrigerator for further analysis. The pitch samples were recovered from the reactor and stored in sampling cans. Material balance was carried out using the measured quantities of the cracked oil, pitch and gas, with reasonably high accuracy in the range of 97–99%. More details about the thermal cracking experiments are reported in AlHumaidan et al.13.The nongaseous products, cracked oils and pitch, were separated in their SARAfractions and thoroughly analysed.

2.3 Preparative SARA analysis To obtain enough material for further analysis, a sample (VR, oil or pitch) of about 10g was processed. The asphaltene (As) separation was performed according to the ASTM standard procedure D6560-0014. The separation of the deasphalted samples (maltenes) into saturates (S), aromatics (A) and resins (R) was performed according to a modified ASTM standard procedure D2007-1115. The following modifications have been made: activated alumina gel (mesh size 200) as column filling and solvents charged to the column - 300ml of petroleum spirit (saturated), 300ml of a 50:50 mixture of petroleum spirit and dichloromethane (aromatics) and 260ml of methanol (resins). All fractions were evaporated to dryness, weighed and stored in vials at ambient temperature for further analysis. 2.4 Elemental analysis (C,H,N,S) A Thermo Fisher Flash 2000 elemental analyzer using a chromatographic separation followed by thermal conductivity detector was used to quantify the amount of C, H, N, S elements in the samples. The analytical test was performed twice on each sample to verify the quality of the analyses. 2.5. 1H and 13C NMR analysis A Bruker Avance300 spectrometer (7.05T) was used to measure the 1H and 13C NMR spectra. The samples were dissolved in CDCl3 (99.8%) and doped with TMS, both chemicals from MERCK. The chemical shift values are reported relative to TMS for 1H and the central signal of the CDCl3 at 77.7 for 13

C NMR. The sample concentrations were approximately 15%wt for 1H and 60%wt for 13C NMR.


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The acquisition parameters have been for 1H NMR: spectral width of 5.5 kHz, pulse angle of 90° (20 µs) and delay time of 2 s; and for

13

C NMR: inverse gated decoupling as pulse program, spectral

width of 25 kHz, pulse angle of 30° (3.2 µs) and delay time of 180 s. 2.6 Thermo-gravimetrical analysis The thermo-gravimetric analysis of approximately 10 mg of sample placed in a macro platinum crucible was carried out on a SHIMADZU TGA-50. The VR-samples were heated at 50oC/min up to 430oC and then kept at a constant temperature for 50 min followed by a heating ramp of 20oC/min up to 800oC. All TG-experiments with SARA-fractions were carried out with a constant heating rate of 15oC/min starting at room temperature up to 800oC. The weight loss versus time was recorded. During the experiment, the reaction chamber was purged with nitrogen to avoid oxidation and to remove volatile reaction products from the chamber. The flow rate of the gas was 50 ml/min. 2.7 Gas chromatography A modified Varian CP-3800 gas chromatography system with five columns distributed between 3 channels (two TCD and one FID), Refinery Gas Analyzer was used to analyze and quantify the noncondensable gas fraction collected throughout VRs thermal cracking process (Table 3). 2.8 Simulated distillation Simulated distillation (SD) gas chromatography system from Analytical Control using Agilent GC 7890 according to ASTM 7169-1116 was used to conduct the SD analysis for both VRs and the cracked oils.

3. Results The three vacuum residues that were used in this study are characterized by relatively high sulfur, microcarbon residue, and asphaltene contents and low hydrogen to carbon ratio (Table 1). Thermal cracking as it takes place during the Eureka process minimizes the gasification of the feedstock and produces a carbon-rich by-product in form of pitch, which can be utilized in some other industries. The oil yields from the three feeds is noted to be inversely proportional to micro-carbon residue percentage in the vacuum residue (Table 1, 2). 3.1 Average molecular weight In this study, the average molecular weight (AMW) of the VR and cracked oil were predicted using two parameter (Tb.p., SG) correlations17


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VR: AMW = 42.965[exp(2.097*10-4Tb.p. – 7.78712* SG + 2.08476 * 10-3Tb.p. SG)] Tb.p.1.26007SG4.98308 ln[SG] = {ln[ρ20/(0.983719*Tb.p.0.002016)]}/1.0055 ρ20

= ρ15 – 10-3(2.34 – 1.9 ρ15) * 5

Cracked Oil: AMW = 1.6607 * 10-4Tb.p. 2.1962 * SG-1.0164 where Tb.p. is the boiling point in K, which were determined by simulated distillation, SG is the specific gravity at 15.5oC, and ρ the density at 15oC or 20oC. Figures 2 and 3 show the relative abundance of computed molecular weights in the boiling range of VRs (480o -750oC) and cracked oils (140o -660oC). The predicted AMWs are listed in Table 4. Though the average molecular weight values of VRs vary depending on the origin of the crude and the technique applied for determination, however, the predicted AMWs fall into the range from 695Da18 to 1062Da19 reported for VRs in the literature. DeCanio et al.19 showed using mass spectrometry (MS) that for RB-VR the upper bound of the AMW is 712Da compared to 1062Da obtained by vapour pressure osmometry (VPO). Takegami et al.20 reported for a Kuwaiti VR an AMW of 1000Da (VPO). Moreover, for the latter VR with a similar SARA mass distribution (S:5.9wt%, A:53.2wt%, R:31.0wt%, A:9.9wt%) as our VRs the authors reported the following AMWs: S:680Da, A:810Da, R:1200Da and As: 3200Da. Taking into consideration that asphaltenes tend to aggregate and VPO usually overestimates the AMW of asphaltenes a lower value is more realistic (≈1000Da (MS)19). By using gel permeation chromatography (GPC) Leon and Parra21 obtained for a VR with low API gravity (7.36°) and a SARA mass distribution of S:6.3wt%, A:53.3wt%, R:29.0wt% and As:11.4wt%

the following AMWs: S: 604Da, A:687Da,

R:1026Da and As: 1266Da. Table 5 compiles the assumed AMWs and molecular formulas of an average VR and its SARA-fractions representing the VRs studied. According to the bulk characteristics, the cracked oil falls into the category of gas oils (GO). In the literature, the AMW of straight run GO with a boiling range from 148o to 404oC is 243Da22, and of vacuum GO with a boiling range from 235o to 568oC is 352Da 23. For GOs with a similar boiling range from 295o to 543oC Fu at al.24 reported AMWs ranging from 216Da (low boiling) and 256Da (middle boiling) to 336Da (high boiling) depending on the cut. Ali et al.25 reported for heavy GO, with a boiling range from 350o to 550oC, an AMW of 462Da. For the latter heavy GO, the authors reported for the SARA-fractions the following AMWs: 469Da (S/49.7wt%), 466Da (A/22.0wt%) and 447Da (R/28.3wt%). Woods et al.26 found for a coker GO with a more narrow boiling range (343o to >524oC)


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and less saturates the following AMWs: 460Da (S/27wt%), 315Da (A/40wt%) and 269Da (R/33%wt). Taking into consideration that our cracked oils show a wider boiling range and that the predicted molecular weights range between 110 and 890Da (Fig. 3) we assumed the AMWs of the SARA-fractions as shown in Table 5. 3.2 Carbon and sulfur analysis and mass balance of thermal cracking The results of elemental analysis for carbon and sulfur and the calculated mass balance for both elements for the VRs and their corresponding thermally cracked products (gas: G, oil: O, pitch: P) are listed in Table 6. The mass balance for C is 99%, while that for sulfur ranges between 94-96%. 3.3 Effect of thermal decomposition on SARA type hydrocarbons Table 7 lists the percentages of maltenes and asphaltenes obtained by SARA analysis for the three VRs, and their corresponding cracked oil and pitch. The SARA separation is purely based on the polarizability and polarity of the feed components. The affiliation of a particular hydrocarbon compound with one of the SARA type fractions depends on the fact which structural moiety in the hydrocarbon prevails in terms of polarity. In particular for molecules consisting of large aliphatic and small aromatic substructures there is an overlapping between the SARA type fractions possible. The content of aromatic carbon in the saturate fractions, however, was less than 0.3% of the total carbon (by NMR). Consequently, the alkylated aromatics in the VRs and nongaseous products are part of the ARA pool. SARA-fractionation showed that the three Kuwaiti VRs contain a very low amount of saturates (54wt%) and the percentages of resins and asphaltenes are almost equal (15-22wt%). To further understand the nature of structural changes that take place during thermal cracking of the VRs, the SARA analysis results were used to calculate the yield of each fraction in both the cracked oil and pitch. Basically, the percentage of the SARA-fraction from Table 7 was multiplied by the product yield from Table 2. The results (Fig. 4) show an obvious change in the mass distribution among the SARA-type hydrocarbons of the nongaseous products (oil and pitch) compared with their corresponding VRs. For instance, the saturate fraction in both products together (oil+pitch) grows up to ≈21wt% while the saturates in the VRs accounts for only ≈6wt% (Fig. 4a). Most of the newly formed saturates are found in the oil (≈97%). The growth in weight for saturates is accompanied by a reduction in weight for aromatics. The aromatic fraction shrinks from ≈61wt% in VRs to ≈38wt% in oil and pitch together (Fig. 4b). For the resins, the products have slightly lower percentages than the VRs (Fig. 4c), while for the asphaltenes, the products of VR-RB and VR-LF have higher percentages than their corresponding VRs,


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while for VR-EOC the products contain nearly the same as its corresponding VR (Fig. 4d). In all the products, it is observed that the asphaltenes occur exclusively in the pitch. The thermal stability of the three VRs (Fig. 5a) and their SARA-fractions (Fig. 5b-d) were further studied by TGA under nitrogen. It was found that the propensity of the VRs towards thermal decomposition increases in the following order (Fig. 5a)

EOC-VR > RB-VR > LF-VR. Furthermore, the saturates are almost completely volatilized at around 450oC for all three feeds. (residue at 450oC < 10wt%). Aromatics as well as resins show two steps of weight loss, one between RT and 350oC (≈10wt%) and another one between 350oC and 550oC (≈65wt%). Alvarez et al.8 observed the same behaviour while pyrolyzing atmospheric residues. They suggested that evaporation of light components occur at an initial step of heating at 100o-320oC, followed by cracking of heavier fractions at 320o-480oC. Asphaltenes of all three VRs show a significantly lower degree of weight loss (40wt% only) compared to the SAR-fractions. 3.4 Structural parameters of VRs and their products of thermal cracking To develop further understanding of the molecular structures of VR, the cracked oil and pitch for the three feeds, 1H and 13C-NMR was carried out on all SARA-fractions. It is important to note first that the molecules of all petroleum fractions including those investigated in this study are made of common hydrocarbon groups e.g., CHx groups with x = 0, 1, 2 or 3, that resemble building blocks. These groups show characteristic signals in 1H and 13C NMR27-37 depending on their chemical environment. Through quantification of these building blocks in any petroleum fraction by integrating their 1H and

13

C NMR

bands (see Appendix Tables A1 and A2), and using elemental analysis data, a set of average structural parameters (ASP)38 can be generated which describes the general structural features of a petroleum fraction (Table 8).

4. Discussion 4.1 Average molecular structure Using the ASP (Tables 9–12), the carbon to hydrogen ratio, sulfur and nitrogen analysis and the AMW (Table 5), it is possible to construct tentatively molecular structures that match the analytical results and illustrate an average hydrocarbon molecule for each SARA-fraction. The discussion on deriving the average molecular structure of the petroleum fractions investigated in this study will be based


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on three parameters: the ASPs (in wt%); n*, the number of saturate carbon atoms in aromatics; and n, the number of paraffinic carbon atoms existing in either paraffines or as alkyl substituent of aromatic entities. As shown in Table 9 for the VR’s saturates, the alkyl carbons are split almost equally between two categories: straight chain paraffins (Cal;n-alkyl) and iso-parafins/naphthenes (Cal;ip+naph). Around 10wt% of the carbon is in the form of CH3 groups. For the products, the cracked oil and pitch, it is observed that the straight paraffinic chains are higher in percentage than the iso-paraffinic and naphthenic carbon. The percentage of CH3 group is in the range of 12-15wt% for the cracked oils, and 7-8wt% for the pitches. The saturate fractions did not show any aromatic carbons. Furthermore, elemental analysis has also shown that the saturates contain no sulfur indicating that mercaptans or thioethers are rather found in the resin fraction due to their polarity. Hence, the saturate molecules are most likely either straight chain paraffins with carbon numbers of n≈26 (VR) or n≈17 (oil) or iso-paraffins and naphthenes as shown in Figure 6. The saturates for the cracked oils and pitch show similar structural characteristics as those of the VRs; however, the paraffinic entities vary in length with average numbers of carbons/molecule (n) for the VRs, cracked oil, and pitch being at around 25, 17, and 28, respectively. For the aromatic and resin fractions of the VRs, the average molecular structural (Figs. 7 and 8) is significantly more complex than that of the saturates. The carbon in these fractions, which is around 83wt% for all three VRs, is split into 56wt% as Cal and 28wt% as Car (Tables 10 and 11). The Cal is distributed nearly equally between straight chain alkyl groups (Cal;n-alkyl) and iso-paraffinic/ naphthenic carbon (Cal;ip+naph). The Car is split into nearly 8wt% as Car;H and 20wt% as Car;q. The Car;q are further split between external (Car;b2) and internal (Car;b3) carbon in bridge head position, alkyl substituted carbon (Car;alk) as well as carbon attached to a hetero atom (Car;X) . The aromatic fractions contain also substantial amounts of sulfur, which are most likely thiophenic in nature. Hence, the aromatic molecules are most likely cata-condensed aromatic compounds with side n-alkyl chains as shown in Figures 7 and 8. The aromatic molecules in the cracked oils are similar to those in the VRs, but with shorter aliphatic side chains. For the pitches and oils, the aliphatic side chains in aromatic molecules have about the same length. The data in Table 12 for the asphaltene fractions indicate that the average molecular structure (Fig. 9) is significantly richer in aromatic carbon. The structure is similar to those of the aromatics and resins, but with higher percentages of carbon in bridge head position of peri-condensed aromatic rings (Cal,b≈26). Table 13 compares the number of C, H and S in hydrocarbon building blocks as calculated from ASP-averages (experimental data) with those in the average molecular structures (models) shown in Figures 6-9. Despite taking the ASP-averages (last columns in Table 9-12), the constructed average


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molecular structures (Figs. 6-9) are in agreement with the experimental values within the range of the experimental error. 4.2 Proposed reactions during thermal cracking Based on the detailed SARA analysis, the possible reactions during the thermal conversion of the SARA-fractions can be assumed to take place as follows:

(Oil)S (VR)S

(Oil)S

Gas (Pitch)S

Gas

(Oil)A,R

(VR)A,R (Pitch)S

(Oil)S (VR)As

(Pitch)A,R,As

Gas (Pitch)As

For the saturates, it is most likely that the prevailing reactions are the cracking of long chain parafines into lighter saturated hydrocarbons that end up in the gas and the cracked products. A small percentage of heavy saturated molecules remain in the pitch. Concerning the aromatic ARA-fractions that contain hydrocarbons with aliphatic groups attached to aromatic cores, it is very likely that these molecules undergo alkyl group scission resulting in a total weight reduction of the ARA-fractions in oil and pitch by around 23 wt% (Fig. 4). Additionally, any inter- or intramolecular condensation will most likely lead to an increase of asphaltenes in the pitch (Fig. 4) whereas peripheral aliphatic substituents are either converted to gas or they contribute to the saturates in the oil. In aromatics with long unbranched alkylgroups, cracking occurs mainly in the alkyl portion of the molecule because of the weakness of the β-C,C-bond (C,C-bond in β–position to the aromatic ring) of the alkyl group while the aromatic radical (e.g., benzyl radical) is stabilized by resonance and hydrogen transfer terminates the reaction causing the aromatic moiety to resist thermal decomposition39,40. βscission of n-alkylaromatics produces n-paraffins that are shorter in chain length (n) than the n-alkyl substituent in the parent alkylaromatics. The n-paraffins that are formed as a result of cracking are part of the saturate fraction in the cracked oil. The 13C NMR spectra of this fraction prove that n-paraffins have S S shorter chain length than those present in the VRs ( nOil ≈ 17, nVR ≈ 28 ). On an average, the n-paraffin,

however, are somewhat longer than the n-paraffinic side chain of the aromatic molecules in the ARAARA fractions of the VRs ( nVR ≈ 11 ) indicating that also recombination of two n-paraffinic radicals have

taken place. This is also proven by the fact that the paraffin of the pitches has an average chain length


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S nPitch of ≈28. Beside β–scission, 1,2-scission takes place shortening the chain length of the n-alkyl group

in the alkylaromatics. The reaction products, 1,2-fragment and n-alkylaromatics, are on one hand a part of the gas fraction and on the other hand a component of the ARA fractions in oil and pitch. As an average, AR ARA the chain length of the n-alkylaromatics in the cracked oil and pitch ( nOil ≈ nPitch ≈ 7 ) is shorter by three ARA CH2 units compared with their parent molecules in the VRs ( nVR ≈ 11 ).

The ARA-fractions of the VRs are also made up of a significant amount of iso-parafins and naphthenes that are attached to aromatic rings (24≤ Cal;ip+naph ≤ 28). Thermal decomposition of these hydrocarbons leads to molecules with less carbon in the iso-parafinic and naphthenic sub-structures, as can be seen from the average number of carbon atoms (n*) in all saturated moieties of aromatics entities ARA AR ARA (ARA). This number falls from n *VR ≈ 6 in VRs to n *Oil ≈ n *Pitch ≈ 2 in oil and pitch (Table 14).

The comparison of the ASP of the ARA-fractions of the parent VRs with those of the cracking product, oil and pitch, demonstrates that there is an increase of the aromatic carbon in both (Tables 1012). As a result of the secession of alkyl groups from the aromatic molecules, the content of aromatic carbon in the ARA-fractions grows from ≈31wt% in VRs to ≈43wt% in the oils and to ≈57wt% in the pitch. The aromatic moieties in the oils are mainly cata-condensed di- or tri-aromatic rings. The percentage of aromatic carbon in internal bridge head position of polycyclic aromatic hydrocarbons (PAHs) in the ARA-fractions (Car;b3) is about ≈ 3wt% in the VRs, ≈ 1wt% in the oils, and about ≈ 5wt% in pitch. The sum of Car;b3 from both cracking products, oil and pitch, was found to be nearly equal to the Car;b3 in the VRs, which indicates that condensation reactions of aromatic rings do not play a major role at 4300 C. Taking into consideration the bond dissociation energy of the most common bonds in hydrocarbons41-43, the thermal conversion of alkyl sulfides into hydrogen sulfide and paraffins is much more likely than the cracking of aromatic thiophenes44-46. Hence, under cracking conditions as applied in our expereiments, it is most likely that aliphatic sulfides, either in saturates or in the side chains of the ARA-fractions, crack forming paraffins and H2S. The high content of sulfur (14wt%) in the gas proves that the non-thiophenic sulfur is removed from the VRs and converted to H2S at 430°C. The refractory sulfur in thiophenic entities are more resistant to cracking; hence, they are mostly concentrated in the residual pitch (9wt% sulfur), and to a lesser extent in the cracked oil (4wt% sulfur).


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5. Summary and conclusions In this study, we examined the thermal cracking of VRs originating from three Kuwaiti heavy crude oils, namely, Ratawi Burgan (API: 22.8o), Lower Fares (API: 12.8 o), and Eocene (API: 18.7 o). The results indicate that the cracking behaviour of all three feeds were very similar, except for the cracking of the aromatic fractions. It was observed that the cracking of RB aromatics resulted in aromatic fractions that are richer in alkyl-carbons, as compared with the products of the cracking of aromatics from LF and EOC. Thermal cracking at 430°C for 50 min. produces 7wt% gas, 48wt% cracked oil and 44wt% pitch. The VRs and the nongaseous products were thoroughly studied using SARA separation, elemental analysis (C,H,N,S), and 1H and 13C NMR. Investigations on the composition of the VRs (C50.21H69.12S1.53N0.31) showed that it contains 94.0wt% of aromatic hydrocarbons (ARA), and that half of the total carbon is found in alkyl groups. Thermal cracking of these alkyl-aromatics produces gas 6.4wt% (C1-C5 and H2S), 48.5wt% cracked oil (C21.88H36.66S0.37 N0.13) with 42.2wt% saturates and 44.0wt% pitch with 1.4wt% saturates. The cracking results also in formation of aromatic hydrocarbons (71.4wt%) that make up the ARA-fractions in oil and pitch, distributed between the cracked oil (39wt%), and the pitch (61wt%). In detail, the VRs consisting mainly of a cata-condensed aromatic core with large naphthenic and paraffinic substituents undergo ß- and 1,2-scission resulting in saturated hydrocarbons that are mainly found in the saturate fraction of the cracked oil (C16H32) and to a lesser extent in the gas (C1-C5). The aliphatic carbon in cracked oil is equally distributed between n-paraffins (C17H36) and naphthens (C10H17) as well as isoparaffins (C5H11). On an average, the n-paraffin are somewhat longer than the n-paraffinic side chain of the aromatic molecules in the VRs indicating that also recombination of n-paraffinic radicals have taken place. As a result of the secession of alkyl groups from the aromatic molecules, the content of aromatic carbon in the aromatic fractions of oil (≈43wt%) and pitch (≈57wt%) grows compared with the VRs (≈31wt%). The aromatic moieties in the oils are mainly cata-condensed di-or tri-aromatic rings. The amount of internally bridged aromatic carbon (Car;b3) in both cracking products, oil and pitch, was found to be nearly the same as in the VRs, which indicates that condensation reactions of aromatic rings do not play a major role at 430o C.

Acknowledgements The authors would like to thank Kuwait Institute for Scientific Research (KISR) and Japan Cooperation Center–Petroleum (JCCP) for funding this research. The technical contributions of Arabian Oil Company,


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Chiyoda Corporation (Eureka process licensor), and Fuji Oil Company is acknowledged with appreciation. The authors also gratefully acknowledge the assistance of A. Boota and B. Al-Dosary in carrying out sample analyses.

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Study on Thermal Cracking of Kuwaiti Heavy Oil (Vacuum Residue

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