Naphtha Recovery Units
A Naphthenic Froth Treatment (NFT) facility produces 2 streams: (1) a relatively clean diluted bitumen product and (2) a fine tailings stream, comprised of water, fine solids, residual bitumen and trace volumes of naphtha (or diluent). Before the tailings can be sent to a storage pond, the naphtha is removed and recycled back into the NFT process.
Tailings produced by cyclones or centrifuges contain about 2 to 4% naphtha (by weight). This naphtha needs to be recovered for both economical and environmental reasons. Naphtha Recovery Units (NRU) used in the oil sands are modified versions of traditional flash columns, using temperature and pressure to recover the naphtha. The naphtha-free tailings can then be discharged into the tailings pond.
DESIGN & THEORY
The NRU circuit consists of an NRU flash column, an overhead condenser and an overhead separator (or 3-phase decanter) all used to recover and recycle the naphtha diluent.
NRU columns are modified flash vessels that use steam-stripping to vapourize the naphtha. Froth Treatment tailings are pumped into the upper end of the column, where flow is distributed over a row of shed decks or trays. Steam is injected into the bottom of the column, using spargers or a ring header located just above the liquid level. As the steam rises up through the column, naphtha is stripped or vapourized out of the tailings. The tailings stream flows counter-currently, spread out over the shed decks which increases the contact area between the steam and the tailings.
Demister pads are normally installed within the overhead vapour space of the column to knock-out the fine solids that sometimes get carried up with the steam and vapour. These pads are typically constructed of bent stainless steel sheets and help prevent erosion of the overhead system.
Naphtha vapours and uncondensed steam reports to the overhead vapour space. Non-condensables such as natural gas or nitrogen (used for pressure control) may also be present in the overhead system, resulting in a 3-phase environment (steam, naphtha and non-condensables). The mixture is cooled and condensed using cooling water within a heat exchanger. The cooled vapour then reports to a separator or drum, where the 3 streams are separated. Water is recycled back into the NRU. Naphtha is recycled back to the front end of the NFT process. Any non-condensables are redirected to the Vapour Recovery Unit (VRU).
NRU columns typically operate close to 100ºC at near-atmospheric pressures (around 100 kPa), although exact operating conditions can vary among the operators. Naphtha recovery rates achieved in a single column can vary greatly depending on the amount of naphtha reporting to the vessel. Under normal operating conditions, a single column typically recovers anywhere from 60 to 80% of the naphtha. Some operators have added a tailings pre-heat step where steam is injected directly into the tailings and allowed to flash in a separate vessel. Direct steam injection has been found to significantly improve recovery rates. Another option to improve naphtha recovery rates would be to operate 2 columns in series, typically one under pressure and the second under vacuum. This increases naphtha recovery rates to over 95%.
INSTRUMENTATION AND CONTROL
The liquid level in the NRU flash column is controlled by the underflow tailings pump(s), typically maintained below the steam injection header. Liquid level in the NRU column is measured by differential pressure. Steam injection is often added at a fixed flow or as a ratio of the feed.
The water/naphtha interface level in the overhead separator is typically measured using differential pressure. The interface is controlled by adjusting the flow of the water outlet, using a control valve or pump. Water is normally recycled back into the NRU column.
The naphtha liquid level in the overhead separator is also measured using differential pressure and is controlled using a control valve or pump, which directs the naphtha back to the front end of the NFT process.
Both the NRU 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 the non-condesables to the VRU. Alternately, pressure can be increased by adding nitrogen or natural gas 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 NFT facility use NRUs to remove naphtha from Froth Treatment tailings, specifically Mildred Lake, Suncor's Base Plant operations and CNRL Horizon. The number, operating conditions and configuration of units (series vs parallel operation) vary between the operators.