Tar sands oil upgrading technology
Technology for processing heavy crudes and bitumen-derived materials such as synthetic crude oil are examined, along with the challenges they present to primary and secondary refinery conversion units
Donald B Ackelson
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It is now clear that the reduced availability of light conventional crudes will create a demand for new crude sources. This demand, combined with economic incentives, is projected to result in a very significant increase in the production of heavy crudes, including a large percentage of synthetic crudes and bitumen blends from tar sands. The utilisation of these heavy materials coupled with increasing demands for high-quality distillate fuels will present difficult challenges for upgraders and refiners. There will be a need for increased conversion of the bottom of the crude barrel and, in many cases, refinery configuration changes to primary and catalyst-based secondary upgraders will be necessary to accomplish this with the difficult hydrocarbon-type mix presented by heavy crudes.
The volume of non-OPEC heavy crude supplied to the market increased by 23% between 2000 and 2004, while the volume of light crude oil produced by non-OPEC countries dropped 10% over that same period.1 This trend toward heavy and more difficult crude sources will continue well into the future. Among the heavy sources playing prominent roles will be the very heavy crudes from the Orinoco belt and, most significantly, bitumen-derived materials from Canadian tar sands.
Given the increasing demand for oil and the declining production of conventional crudes, Canadian oil sands will become a key source of future crude supply. The potential of this resource is only beginning to be realised. The volume of oil in place in the various deposits is estimated at 1.6 trillion barrels, of which 174 billion barrels is recoverable with existing technology. This places the size of the established reserves second only to Saudi Arabia. As recovery technologies improve, the size of those recoverable reserves could increase significantly. The proximity of the source, security of supply and competitive pricing will drive the refinery investment needed to accommodate these materials. It is estimated that about 20% of the proven bitumen reserves can be surface mined, while the remaining portion are deeper than 75m and necessitate some kind of in-situ recovery method. Most of the initial production was surface mined, but from the mid-1980s in-situ production began to increase.
The level of bitumen production from tar sands now exceeds one million barrels per day (bpd), and is expected to increase to almost three million bpd by 2015.2 Oil sands crudes are expected to represent more than 75% of the crude produced in Western Canada. This will be about 16% of the 19 million bpd overall demand for the Canadian and US market, and will help to control the increase in crude oil import requirements.3
While most of the imports into the US are in PADDs II and IV (north-central US), further pipeline expansions will increase southern penetration in PADD II and include the Pacific Northwest (Northern PADD V). New pipeline systems are expected to reach Texas (PADD III) and a new seaport in British Columbia. From this new port, marine shipments to both California (Southern PADD V) and the Far East are expected.
As will be discussed further, the composition and contaminant levels of bitumen-derived crudes do not make them an easy replacement for conventional crudes, especially since most existing refineries have limited capacity to accept poorer-quality feedstocks. These crudes are funda-mentally different, so refiners will need to understand them and be prepared for the changes needed to process them.
Origin and characterisation of tar sands oils
The characteristics of tar sands oils are somewhat unique. In order to understand the nature of these oils, it is interesting to consider their origin. The bitumen source materials were originally formed as light oils in deeper sections of the Western Canada sedimentary basin. The combined forces of high pressures and temperatures to which the source rock was subjected caused the expulsion and migration of the light oil. Through the course of migration, the oil was subjected to bio-degradation through microbial action, and lighter components escaped to the surface. Over time, the oil was transformed into heavy oils and then into bitumen that is immobile at reservoir conditions.4
The deposits are mixtures of bitumen, water and sand. A typical composition for the oil sands includes grains of sand (around 75% of total) contained within an envelope of water (3–5%), with the water surrounded by a film of bitumen (10–12%).
The key to the somewhat unusual nature of bitumens is the severe biodegradation of the oil during migration and the type of bacterial action that occurs. Table 1 is an interesting comparison of the “diets” of bacteria and diesel engines compiled by Murray Gray of the University of Alberta.5 Bacteria like to consume normal paraffins. Aromatics are fair food for the bacteria, while naphthenes are avoided because they are toxic. Unfortunately, the hydrocarbon types that the bacteria like to consume are the same types that are high-quality fuels for diesel engines. So, the resulting composition of the bitumen is not ideal for producing high-quality distillate fuels.
Another set of data from the same source is shown in Table 2.5 It is clear that as the extent of biodegradation increases, the concentration of asphaltenes and polar compounds increases significantly, while the levels of saturates decrease. These characteristics are common to bitumens and present very significant challenges for upgrading to fuels.
As shown in Figure 1, compared to West Texas Intermediate (WTI) or Arab Light crude, the fraction boiling heavier than diesel can be almost doubled when comparing to heavier crudes.6 There is also a significant increase in the VGO portion of heavy crudes that will require conversion to transportation fuels.
The majority of the heavy crudes discussed are derived from unconventional crude sources. The term “unconventional crudes” is typically defined by an API of <10° and also some reference to viscosity, where extra-heavy oil has a viscosity of 1000–10 000 centipoise. Bitumen is traditionally defined as oil requiring the assistance of steam or diluent to be extracted from the reservoir and has a typical viscosity of >10 000 centipoise. Significant extra-heavy oil crude reserves are found in the Orinoco Belt of eastern Venezuela as well as Columbia and other nearby regions. The most extensive bitumen reserves are found in the Alberta Oil Sands of Canada.4
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