Summary
The phase behavior of Athabasca vacuum bottoms (ABVB), a 798.15+ K (525+°C)
boiling fraction comprising 32 wt% pentane asphaltenes + pentane mixtures, is
elucidated using x-ray transmission tomography. These pseudobinary mixtures are
models for the development of novel heavy-oil/bitumen production and refining
processes. Depending on their overall composition, these mixtures are shown to
exhibit three- and four-phase equilibria, including both expected (L1L2V) and
unexpected (L2L3V) phase behavior separated by a small L1L2L3V zone. These
multiphase equilibria provide challenges in production environments, where
miscible flooding is typically desired, but afford new opportunities for the
development of separation technologies in refining. Example phase diagrams are
presented (pressure-temperature at constant composition and
pressure-composition at constant temperature), which focus on the bubble
pressure region where multiphase regions arise. Limited phase equilibria data
sets for ABVB + heptane, decane, and dodecane are also presented so that trends
for ABVB + alkane mixtures with the size of the alkane diluent can be explored.
It is expected that these phase diagrams will provide useful insights to
heavy-oil producers and refiners, in addition to providing a benchmark for
testing phase-behavior models for these and related complex hydrocarbon
mixtures.
Introduction
Understanding the phase behavior of asphaltene-rich heavy oil + alkane
mixtures is important to the successful development and operation of processes
for heavy oil recovery and refining. In contrast to conventional crude oils,
heavy oils and bitumen contain high concentrations of asphaltenes, heavy
metals, and heteroatoms and possess high densities and viscosities (Speight
1999; Gray 1994; Reynolds 1999). These characteristics present challenges for
production, transport, and refining. Upstream, steam-assisted gravity drainage
(SAGD), vapor extraction (VAPEX) (Das and Bulter 1995; Bulter 1991),and other
hybrid technologies have been developed or are being developed for heavy
oil/bitumen production. Diluents, such as light hydrocarbons, are often used,
with or without steam (heat), to reduce viscosity and/or to achieve in-situ
upgrading by rejecting part of the oil in the reservoir. Downstream, refiners
are attempting to maximize yields of value-added middle distillates such as
gasoline and diesel from heavy feedstocks. However, high concentrations of
heavy metals, heteroatoms, and inorganic solids in heavy oils cause a host of
problems in refining, such as catalyst poisoning and line plugging, to name but
two (Gray 1994; Das and Bulter 1995). Undesirable constituents
concentrate in asphaltenes and are removed along with them using de-asphalting
technologies, for example, when the residual oil supercritical extraction
(ROSE) process is applied to conventional oils (Wilson et al. 1936; McHugh and
Krukonis 1986) with low asphaltene concentrations.
N-alkanes, from propane to decane, offer potential for both upstream and
downstream applications. The identification of those compositions and operating
conditions which offer the best overall yield and selectivity for separating
undesirable constituents should be based on understanding the phase behavior of
heavy oil + n-alkane mixtures under reservoir and refining conditions. Yet the
phase behavior of such mixtures has not been investigated systematically to
date. Meanwhile, it is clear that in conventional oil production,
asphaltene-related multiphase behavior, such as asphaltene precipitation and
deposition, creates production problems that are costly to rectify. Valuable
hydrocarbons distributed among various condensed phases or trapped by solid
plugs are difficult to recover, and consequently the utility of processes
based, for example, on the injection of driving gases is limited when complex
phase equilibria arise.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
29 August 2005
- Revised manuscript received:
26 May 2006
- Manuscript approved:
1 June 2006
- Version of record:
20 May 2007
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