Upgrading and extraction of bitumen from Nigerian tar sand by supercritical carbon dioxide

Highlights

• Bitumen extraction from tar sand by liquid carbon dioxide method was investigated.

• Greatest recovery of 29.6% was obtained at 50 MPa with the addition of salty water.

• Upgraded bitumen was clean of asphaltenes and petcokes.

Abstract

The current experimental study describes the recovery of bitumen from Nigerian tar sand by the supercritical carbon dioxide method, which has the advantages of a substantial reduction in water consumption, an absence of chemicals, and an upgrading of the bitumen through the rejection of solids, petcokes and asphaltenes. The bitumen extraction from 50 g piece of tar sand was carried out at 50, 60 and 65 MPa and 110 °C by pure carbon dioxide and with the addition of 10 mL of fresh or salty water. The recovery decreased in the following order: with salty water, with fresh water, pure CO₂. The maximum recovery of 29.6% was obtained at 50 MPa with the addition of salty water declining at higher pressures. The heaviest fractions were extracted by pure carbon dioxide at 60 and 65 MPa. The presence of bisnorhopane, a biomarker typical for source rocks formed in marine depositional environment, was detected in crude bitumen and all extracts by GC–MS TIC analysis.

Introduction

Currently, crude oil produced from light oil deposits comprises only one third of all world oil reserves. The other two thirds belong to heavy and extra heavy oil, of which only one per cent is being exploited [1]. With declining reserves of conventional crude oil and increasing oil prices, production from nonconventional sources, including tar/oil sands will become increasingly profitable and the scale of their exploitation will increase.

Nowadays, the biggest producer of synthetic oil from tar sands is Canada. In 2010, 55% of its tar sands production was from mining operations with a maximum burial depth of 75 m and in situ operations produced the other 45% at deeper depths [2]. The proportion of non-upgraded bitumen exports is projected to rise from 42% of total production in 2009, to 52% by 2019 [3]. In addition to the existing export to the USA, Canada would like to develop new markets and export to the other areas of the world. One of the future markets is the EU, where imported oil from tar sands currently accounts for just 0.01% of EU fuels but this could increase significantly [4]. However, the EU parliament is discussing a proposal to label fuel from tar sands as particularly polluting and more carbon-intensive than oil from other crude sources [5]. If this legislation becomes valid, it will erect some barriers to the export of fuels from tar sand. This will be of concern, not just to Canada but also to some other potential exporting candidates with their own deposits of tar sands. Many countries, including Venezuela, Kazakhstan, Russia, Nigeria, Trinidad, Madagascar, Egypt, and etc., have shallow deposits of tar sand, which could be mined [6] and some of these countries are trying to follow the Canadian processing model.

Energy production from tar sands is very promising for the future because the resources are vast. However, some serious environmental problems have emerged, which must be overcome in the face of more stringent regulatory standards and public expectations. In the case of open pit mining in Canada, the environmental issues facing the development of tar sands are primarily: land reclamation; the protection, conservation and remediation of potable water resources; and the reduction of gas emissions. Mining and processing disturbs large amounts of land. Approximately 2 tons of tar sand must be extracted and processed to make one 159-l barrel of crude oil [7]. In addition, intensive energy is required to process the sands into crude oil. Oil sands operations currently use about 2 billion cubic feet of natural gas a day.

In the first step of tar sand processing, the bonds holding the ore components (bitumen, sand, water film and clay) together have to be disintegrated. Tar sand is conditioned by mixing it in a rotating drum with hot water, sodium hydroxide and steam to produce a slurry of ca. 65% solids at 79–93 °C [8]. By using NaOH, the asphaltic acids present in bitumen become water soluble and act as surfactants. The reduction of surface and interfacial tensions by increasing the pH of the suspension results in the disintegration of the oil sands ore structure and recovery of the bitumen [9].

In the second step, the bitumen (froth) containing ca. 8 wt% solids and 40 wt% water is diluted with naphtha and treated by centrifuging at ca. 76 °C to reduce solids and water to match the requirements for bitumen upgrading units [10].

The resultant tar sands process water is stored in on-site tailings ponds in an area covering 530 km² where tailings ponds take up approximately 20% of the area [11]. The tailings consist of sand, clay, water and discarded bitumen from the bitumen production operation, where the solids are allowed to settle out by gravitational forces and the clarified water is recycled to the extraction plant. When the tailings are discharged, the coarse sand particles segregate quickly and form a beach, the remaining fine tails accumulate in the tailing ponds.

Fine tails form a stable intractable gel-like slurry structure with toxic characteristics [12], which remains in a fluid state for decades because of its very slow consolidation rate [13]. Problem of settling the dispersed solids is solved by adding gypsum (CaSO₄) as source of ${\text{Ca}}_2^{+}$ (about 1 kg CaSO₄/m³ tailings). However, the accumulation of ${\text{Ca}}_2^{+}$ in the recycled water would cause problems such as low bitumen recovery efficiency and the elimination of its effect would require an increased NaOH consumption rate in the hot water process [13]. In the initial years of operation, when recycled water is not yet available owing to the very slow fine tailings dewatering rates, the tailings storage is required to hold three times the volume of the oil sands [14]. The very slow release of water from the fine tailings is the focus for much of the ongoing research into tailings activities and final reclamation planning and these present the greatest challenges for the mineable tar sands sector.

Currently, two to four barrels of water are required to extract one barrel of oil and four cubic meters of production water are produced for each cubic meter of tar sands processed [15]. Although recycling of production water reduces the demand for freshwater, the process affects water quality by concentrating the organic and inorganic constituents within the recycled production water [8]. The need for large quantities of freshwater in tar sands recovery schemes has required the maximization of water recycling and in areas of Alberta experiencing freshwater shortages; it has required the development of alternative sources, such as brackish water, fresh water from adjacent water basins or seawater. These are feasible but it was found that the supply of seawater was costly owing to the necessity of constructing delivery pipelines.

With so many issues under consideration and taking into account the seriousness of the associated environmental problems, any improvements to the Canadian process model could make the technology more environmentally friendly.

Substantial amounts of money are being spent worldwide on research related to CO₂ capture and the construction of CO₂ storage as a part of worldwide efforts to curb global warming. Technologies utilizing and commercializing CO₂ might return the financial expenditure considering that existing processes allow over 90% recycling of CO₂. Carbon dioxide has been chosen for many supercritical extraction processes because of its low critical pressure and temperature, above which CO₂ acquires gas-like mass-transfer properties and the solvation characteristics of liquids. Its high diffusivity and very low viscosity with almost no surface tension allows it to penetrate low-porosity materials and its liquid-like density enables it to dissolve analytes from a solid matrix providing excellent extraction efficiency and speed [16]. Although it is a non-polar solvent, carbon dioxide’s large quadrupole moment leads to an affinity for polar solutes and many large organic molecules. CO₂ is an excellent extraction medium for non-polar compounds such as alkanes. It is also relatively good for moderately polar polyaromatic hydrocarbons (PAHs) but is less useful for more polar compounds at typical working pressures (8–60 MPa) [17]. Significant changes in CO₂ density and viscosity and correspondingly, in its ability to solubilize compounds and extract the fractions of various hydrocarbon compositions, could be achieved by manipulating pressure and temperature [18]. The recovery of crude oil improves as pressure rises and worsens with temperature increase [19]. Lighter hydrocarbons are extracted at lower pressures; however, extraction of many other hydrocarbon mixtures, especially from the porous matrix, might exhibit different dependencies on pressure and temperature [20].

The application of CO₂ to the extraction of bitumen from tar sands was reported in several patents and publications. Williams and Martin [21] patented the extraction of hydrocarbons from tar sands by using various solvents, including carbon dioxide, under supercritical conditions at a pressure of 10 MPa and temperatures of up to 550 °C.

The current experimental study describes the recovery of bitumen from Nigerian tar sand by the pure supercritical carbon dioxide and with the addition of water at high pressures. The upgrading of the bitumen through the rejection of solids, petcokes and asphaltenes was obtained, and the extracted fractions were analyzed by GC–MS TIC chromatography.