Desulfurization of Heavy Oil–Oxidative Desulfurization (ODS) As

ARTICLE pubs.acs.org/EF

Desulfurization of Heavy Oil
Oxidative Desulfurization (ODS) As Potential Upgrading Pathway for Oil Sands Derived Bitumen Rashad Javadli† and Arno de Klerk* Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada.

bS Supporting Information ABSTRACT: Heavy oil usually contains percentage levels of sulfur. Most of the sulfur in heavy oil is found in bulky thiophenic structures. Thiophenic sulfur is difficult to remove by catalytic hydrodesulfurization, but it can readily be oxidized. The sulfoxides and sulfones produced from sulfur oxidation can be solvent extracted from the heavy oil as a result of their increased polarity. Oxidative desulfurization of heavy oil was studied using Canadian Cold Lake bitumen (5% S, 1100 kg/m3), with air as oxidant. At the conditions investigated, namely, autoxidation at 145
175 °C followed by water washing, 46
47% of the sulfur in the bitumen could be removed. This is equivalent to >20 kg sulfur per ton oil. Part of the sulfur was removed as SO2 and part as water extracted sulfurcontaining compounds. Lower autoxidation temperatures led to better desulfurization. The main challenge was to prevent free radical addition reactions that cause a viscosity increase and bitumen hardening. Autoxidation of undiluted bitumen and bitumen
water mixtures resulted in hardening. Hardening was prevented when bitumen was diluted with naphtha (n-heptane). However, the oxidized sulfur compounds could not be extracted with water from the bitumen
heptane phase, and some material was precipitated as a result of solvent deasphalting. Oxidation selectivity was studied using a model dibenzothiophene and n-heptane mixture. Some precipitation was also observed, and the chemistry was analogous to the precipitation chemistry (gum formation) that undermines storage stability of transportation fuels.

’ INTRODUCTION Transportation fuels and petrochemicals produced as final products from crude oil refining, must contain little sulfur. Heavy oils (875
1000 kg 3 m
3) and extra-heavy oils (>1000 kg 3 m
3), such as Maya crude oil and Athabasca bitumen, generally have a high sulfur content. The desulfurization of such heavy oils or bitumens is one of the challenges faced during upgrading and refining to produce transportation fuels and petrochemicals. Three strategies are industrially employed to lower the sulfur content from such heavy oils. The first desulfurization strategy is to chemically remove the sulfur by hydrodesulfurization (HDS). When sulfur-containing oil is hydrotreated over an appropriate catalyst, the sulfur can be removed as hydrogen sulfide (H2S). Various factors affect the HDS performance of catalysts in relation to the feed material.1
3 The ease of sulfur removal depends on the chemical nature of the sulfur, with aliphatic sulfur being more easily removed than aromatic sulfur. The ease of sulfur removal is also dependent on the structure and bulkiness of the sulfur-containing compounds, because HDS requires adsorption of the compound on a catalytic surface in such a way that the sulfur can be hydrogenated. To remove sulfur from the alkyl dibenzothiophene class of compounds, severe hydroprocessing is required and hydrogen is not only consumed by HDS but also increasingly by other reactions. For example, during the HDS of thiophenic compounds in Athabasca bitumen over NiMo/Al2O3, hydrogenation of the thiophene to a cyclic sulfide is a meaningful pathway.4 As the feed becomes heavier, molecular accessibility to the catalyst decreases and HDS becomes less efficient. Deposit formation and fouling of the catalyst precludes the use of standard fixed bed HDS technology with heavy oil fractions.5 Catalytic processes that are r 2011 American Chemical Society

used for heavy oil hydroconversion employ slurry phase and ebullated bed reactor technology,5
8 such as H-oil and LCfining. Although these processes are often referred to as hydrocracking technologies, they are primarily thermal cracking processes with hydrogenation; catalytic bifunctional hydrocracking plays a minor role. The second desulfurization strategy is to remove sulfur during carbon rejection in processes such as coking. Coking involves thermal cracking under conditions that allow the free radicals to disproportionate between lighter hydrogen enriched fragments and heavier carbon enriched coke. Sulfur that is difficult to remove by HDS is aromatic sulfur, and some of it can be removed as part of the coke. In addition to the sulfur removed in the form of coke, sulfur is also removed from aliphatic compounds in the form of H2S. Sulfur removal by carbon rejection is effective, albeit not very selective, for the desulfurization of heavy oil. The third desulfurization strategy is to physically remove the sulfur with the heaviest material by solvent deasphalting. Solvent deasphalting removes the asphaltene fraction from the oil by precipitation from a light hydrocarbon. The “asphaltene” fraction is a solubility class, and in terms of molecular composition, the heavier and more polar compounds are precipitated.9 Because heteroatoms (O, N, S) are responsible for an increase in polarity and the sulfur content increases with the boiling point of the oil fraction, solvent deasphalting results in a net reduction in sulfur. As a desulfurization process, it is not very selective for sulfur Received: September 24, 2011 Revised: November 13, 2011 Published: December 05, 2011 594

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Table 1. Selected Properties of the Cold Lake Bitumen Used during the Experimental Investigation property

Cold Lake bitumen
3

Figure 1. Progressive oxidation of sulfur compounds by oxygen in air, as illustrated by the oxidation of dibenzothiophene.

removal, and desulfurization is not the primary objective of solvent deasphalting. For the desulfurization of lighter oil fractions, more options are available, only some of which are industrial processes.10
13 Unfortunately, many of these concepts cannot be applied to heavy oil. For example, in the olefin alkylation of thiophenic sulfur (OATS) process, the boiling point of thiophenic compounds in naphtha is selectively increased, which enables the removal of thiophenic compounds by distillation. A separation based on change in boiling point becomes untenable when dealing with heavy oil where a substantial fraction of the feed material has the boiling range of a vacuum residue. A review of heavy oil desulfurization literature indicates that oxidative desulfurization (ODS) has the potential to improve on current heavy oil desulfurization practice.14
19 This work contributes to a growing understanding of heavy oil ODS. The oxidation by air (autoxidation) of oil sands derived bitumen is studied to determine the extent of sulfur removal that is possible, as well as to highlight operating issues that must be resolved in order to develop an industrially viable process. It reports proof of concept work. The sequence of experiments was determined by our efforts to address the effect of free radical addition reactions, first autoxidation of undiluted bitumen, then a bitumen
water mixture, and last a bitumen
naphtha mixture.

’ OXIDATIVE DESULFURIZATION OF HEAVY OIL ODS processes consist of two steps: oxidation and sulfur removal. In the first step, the sulfur containing compounds are oxidized, which then facilitates the sulfur removal step, which can be by adsorption, extraction, thermal decomposition, catalytic conversion, or biodesulfurization.20
22 The application of oxidation to upgrade heavy oil by ODS is conceptually simple. The heavy oil is brought into contact with an oxidant, preferably air (for practical and economic reasons), under mild conditions, typically near atmospheric pressure and below 200 °C. When doing so, the oxidant converts the sulfur species into sulfoxides and sulfones (Figure 1). The sulfoxides and sulfones are more polar than the original sulfur species and can be extracted with an appropriate solvent. The
(SO2)
group in the sulfones can also be selectively removed by thermal bond dissociation, because the C
S bonds in sulfones are weakened by around 40 kJ 3 mol
1, compared to that in the original sulfur species.23,24 In fact, the liberation of SO2 during pyrolysis is one of the most sensitive indicators that heavy oil was subjected to low temperature oxidation.25 Autoxidation and thermal bond dissociation both proceed by a free radical mechanism. Under mild conditions, free radical addition reactions are thermodynamically favored over free radical cracking, which is exactly the same principle that is applied in light olefin polymerization to produce plastics. This is likely to be a problem during ODS of heavy oil. It is therefore not surprising to find reports documenting the negative impact of oxidation on heavy

liquid density (kg 3 m )

1100

initial boiling point (°C) sulfur content (mass % S)

168 5.1

Figure 2. Generalized reactor configuration employed for the autoxidation experiments at low pressure. High-pressure experiments employed a similar configuration but without motorized stirrer.

oil and oil sands derived bitumen. Aging, which is ambient temperature autoxidation, reduces bitumen recovery from oil sands.26 Oxidation over the temperature range 137
207 °C shows an increase in the asphaltene content of the oxidized product compared to the product exposed to an inert atmosphere.27 An increase in asphaltene content was also reported for autoxidation at 125 °C,28 at 150
250 °C,29 and at 260
320 °C.30 In addition to these changes, a marked increase in viscosity was observed after oxidation.31
35 All of these constitute clear challenges that must be overcome in the development of ODS as desulfurization strategy for heavy oil.

’ EXPERIMENTAL SECTION Materials. The oil sands derived bitumen employed for this study was obtained from Cold Lake, Alberta, Canada. The bitumen can be classified as extra-heavy oil on the basis of its liquid density, and selected properties of the feed are given in Table 1. Bitumen, when used in this paper, specifically refers to the organic fraction obtained from oil sands (i.e. after mineral removal by hot water extraction). Air was employed as the oxidant, and compressed air was commercially obtained in cylinders. The n-heptane (99%), dibenzothiophene (98%), and dibenzothiophene-1,1-dioxide (97%) that were employed in some of the work were obtained commercially and were used without further purification. Distilled water was used for water extraction. Equipment and Procedure. Two types of batch reactors (Figure 2) were employed in this study. The first type of reactor was a 1000 mL jacketed glass Buchi Autoclave with air sparger and mechanical stirrer. During each experiment the autoclave was charged with the bitumen (600
700 mL) and heated to reaction temperature by circulating a hot heating fluid. This decreased the viscosity of the bitumen sufficiently to allow stirring. Air was then introduced, and the flow rate was controlled by a Supelco rotameter equipped with needle valve. Experiments were conducted at near atmospheric pressure, and the off-gas was scrubbed before discharging it into the fumehood vent system. At the end of the experiment, the oxidized bitumen was removed from the reactor for analysis. A small portion of the bitumen (5
7 mL) was water washed at 60 °C for 1 h and the extracted bitumen was analyzed. The second type of reactor was a 100 mL stainless steel Swagelok vessel. This reactor was operated under pressure sufficient to keep bitumen
solvent mixtures (70
80 mL) in the liquid phase. 595

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Table 2. Autoxidation of Bitumen at 145 and 175 °C at near Atmospheric Pressure for 6 h in a Batch Reactor with Continuous Air Flow of 1.5 cm3 3 min
1 per cm3 Bitumena

extraction (Table 2). Yet, the net mass loss was low (20 kg S per ton oil (sulfur removal of >20 000 μg 3 g
1).

’ CONCLUSIONS Oxidative desulfurization (ODS) using air as the oxidant was investigated for desulfurization of heavy oil, using oil sands derived bitumen as feed material. Autoxidation was conducted in the temperature range 145
175 °C and was followed by water washing to remove oxidized sulfur compounds from the bitumen. Under the conditions investigated, 46
47% sulfur removal could 600

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Energy & Fuels be obtained, which is equivalent to the removal of >20 kg S per ton oil. The experimental investigation highlighted the following issues: (a) The study confirmed the existence of a change in oxidation kinetics and product selectivity with temperature. Autoxidation at 145 °C was markedly different from autoxidation at 170
175 °C. In all instances, autoxidation at 145 °C resulted in better desulfurization of bitumen. (b) Autoxidation is selective for the oxidation of thiophenic sulfur to sulfoxides and sulfones in a hydrocarbon matrix. The hydrocarbon matrix is also oxidized and selectivity for sulfur oxidation deteriorates with an increase in temperature. Autoxidation at 145 °C resulted in 2
3% hydrocarbon oxidation to mainly alcohols and ketones. (c) The prevention of free radical addition reactions is very important. When free radical addition reactions are not prevented, it results in a significant viscosity increase and bitumen hardening. Autoxidation of undiluted bitumen and bitumen diluted with water was unable to prevent this. Autoxidation of bitumen diluted with n-heptane (as representative of naphtha) was seemingly able to prevent free radical addition reactions resulting in a viscosity increase and resulted in some solvent deasphalting (4
5% of the bitumen precipitated). (d) Although n-heptane prevented bitumen hardening during autoxidation, it almost completely suppressed sulfur removal during water extraction after autoxidation. This is likely due to the higher solubility of the oxidized sulfur compounds in n-heptane than in water, although the contributions of solvent oxidation and aggregation could not be ruled out. (e) Autoxidation of model oil consisting of a dibenzothiophene and n-heptane mixture in a 1:225 ratio also yielded a precipitate. Reaction chemistry that explains the formation of the precipitate was proposed. Such precipitation (gum formation) has also been reported during storage of transportation fuels.

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’ ASSOCIATED CONTENT

bS

Supporting Information. Infrared analysis of the sediment that resulted from the autoxidation of the bitumen
heptane mixture (Figure S1); gas chromatogram (Figure S2) and electron impact mass spectra (Figures S3
S4) of the oxidized dibenzothiophene compounds after autoxidation of the model oil at 145 °C. This material is available free of charge via the Internet at http:// pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +1 780-248-1903. Fax: +1 780-492-2881. E-mail: deklerk@ ualberta.ca. Present Addresses †

Presently at ConocoPhillips, Canada.

’ ACKNOWLEDGMENT This investigation was funded by the Centre for Oil Sands Innovation (COSI-2010-07) and permission to publish the results is appreciated. The reviewers are thanked for many thoughtful suggestions. 601

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