Tailings Solvent Recovery Units
A Paraffinic Froth Treatment (PFT) facility produces a tailings stream that contains water, asphaltenes, fines solids, bitumen and residual solvent. Solvent content in the tailings can vary between 5 and 10%, although the exact composition of the tailings stream varies according to operating conditions.
For both economic and environmental reasons, this solvent must be recovered before the tailings can be stored in the tailings pond. Tailings Solvent Recovery Units (TSRU) used in the oil sands are modified versions of traditional flash columns, using temperature and pressure to recover the solvent. The solvent-free tailings can then be stored in the tailings pond for future reclamation.
DESIGN & THEORY
Solvent recovery is achieved using modified steam-stripping flash columns, where the solvent is vapourized, condensed using cooling water and recycled back to the front end of the PFT process.
Due to the presence of asphaltenes, there are a few variations on the TSRU flash-column design used in the oil sands. There are 2 types of columns currently in service:
- a flash column with an agitator and little internals, patented by Shell, and
- a flash column with internal shed decks and no moving parts, patented by Imperial Oil and ExxonMobil.
The Imperial TSRU column is almost identical to the Naphtha Recovery Unit (NRU) flash-column used in Naphthenic Froth Treatment (NFT) facilities. Feed from the second stage Froth Settling Unit (FSU) underflow is diluted with hot water and pumped to the top of the column, where the slurry is distributed over a series of shed decks. Steam is introduced into the bottom of the vessel, just above the liquid level. Solvent is stripped out of the tailings as steam rises up the column and contacts the tailings counter-currently. The shed-decks help spread out the tailings and improves contact between the tailings and the steam. Demister pads at the top of the column capture any fines that might get carried up into the overhead system.
The Shell TSRU design in contrast has no internal shed-decks but instead relies on agitation to improve steam recovery. Steam is injected directly into the tailings prior to being fed into the vessel. This induces a pressure drop and atomizes the tailings as it enters the column. The liquid pool is simultaneously agitated using an impeller, adding more shear to the system which is thought to improve solvent recovery. The agitator stirs up the asphaltenes, breaking up agglomerations and reducing foaming. If foam is detected in the TSRU column, that foam can be redirected to a foam-breaking vessel (not shown), which operates at a lower pressure than the columns. The foam is further agitated and sprayed with hot water and defoamer chemicals. The de-foamed liquid is then pumped back to the main TSRU flash columns.
To improve solvent recovery, TSRU columns are typically arranged in pairs, normally operated in series. Some operators choose to operate one vessel at near-atmospheric pressure, and the second column either at a lower pressure or under vacuum to improve solvent recovery.
High-shear environments have been found to greatly improve the liberation of solvent from the asphaltene agglomerates. TSRU columns are normally equipped with bottoms recycle pumps that can recirculate the underflow back into the vessel. This recirculation is thought to improve solvent recovery, reduce foaming, prevent the formation of asphaltene mats and prevent plugging of the column during upset conditions.
Asphaltenes contain natural surfactants which can cause severe foaming within the TSRU vessels. Foaming can cause fine solids to be carried-over into the overhead system and can also cavitate the underflow pump. Foaming within the TSRU column can be mitigated by:
- adding hot water or steam
- applying shear to the slurry through mixing, spraying or pumping
- adding chemical defoamers and/or asphaltene dispersants.
Hot dilution water is commonly added to TSRU vessels in order to reduce foaming, however this has been met with very limited success. The problem is that very large volumes of very hot water is required to suppress the foam, which is not always practical. Defoamers and other chemicals are usually added to the TSRU circuit to more effectively mitigate foaming.
INSTRUMENTATION AND CONTROL
The liquid level in the TSRU flash column is usually measured by differential pressure and controlled by the underflow tailings pump(s). The level in the column is normally maintained below the steam injection header. Steam injection is generally added at a fixed flowrate or as a ratio of the feed stream. Nuclear density meters (or densitometers) can be installed at various intervals to detect foaming in the column. Defoamers and other chemicals are added on an as-needed basis.
The water/solvent interface level in the overhead separator is typically measured using differential pressure. The interface is controlled by adjusting the outflow of the water, using a control valve or pump. Water is commonly recycled back into the TSRU column.
The solvent liquid level in the overhead separator is also normally measured using differential pressure and is controlled using a control valve or pump, which directs the solvent back to the front end of the PFT process.
Both the TSRU flash column and overhead separator operate at the same pressure since they are physically connected. Pressure of the system is controlled by a pressure control valve which relieves non-condesables to the Vapour Recovery Unit (VRU). Alternately, pressure can be increased by adding natural gas or nitrogen to the separator.
Note that some facilities operate the system under vacuum (instead of near-atmospheric pressure). In this case, a vacuum pump is required to maintain the negative pressure.
Every PFT facility use TSRUs to remove solvent from Froth Treatment tailings, specifically Muskeg River, Jackpine, Kearl and the Fort Hills PFT facilities. The number of TSRU columns installed, operating conditions and configuration of units (series versus parallel) vary between the operators.