Froth Settling Units
Froth Treatment is a gravity separation process by which fine solids and water are removed from bitumen froth produced in Extraction through the addition of a light hydrocarbon. In a Paraffinic Froth Treatment (PFT) process, a paraffinic solvent is added to precipitate asphaltenes, allowing for an almost complete removal of solids and water from the bitumen product.
Froth settling is the first step within the PFT process, where solids, water and precipitated asphaltenes are allowed to settle in large gravity separation vessels commonly referred to as a Froth Settling Unit (FSU).
There are two types of froth settling vessels used in PFT facilities:
- large modified clarifiers with a shallow-sloped bottom and internal rakes, and
- smaller vessels with a steep cone bottom and no internal moving parts.
The choice of technology depends on the operating temperature of the process.
HISTORY OF FROTH SETTLING IN THE OIL SANDS
The very first PFT facility built in the oil sands was the Muskeg River Mine (MRM). MRM was also the first mining operation to be built without a nearby upgrader.
Upgraders generate copious amounts of waste heat, which is normally used to produce hot water for the adjacent mining facility. In the absence of waste heat from the upgrader, mining facilities need to burn natural gas to produce hot water. Since MRM had no access to heat from a nearby upgrader, the facility was designed to be a low-energy process, aiming to minimize the consumption of natural gas. The site was the first to implement Low Temperature Paraffinic Froth Treatment (LTPFT).
Since the process operates at near-ambient conditions (around 35ºC), the retention time required to settle all the asphaltenes is relatively long. The use of inclined plate separators (IPS), normally the technology of choice for existing naphthenic operations, could not be used for PFT since the asphaltenes would plug-up the internal plates. MRM therefore chose to use a conventional thickener or clarifier as a froth settling unit. Since paraffinic solvent normally has a very low boiling point (near 35ºC), the settlers can be operated at near-atmospheric pressures while still keeping the solvent in the liquid phase.
LOW TEMPERATURE FROTH SETTLERS/CLARIFIERS
Clarifiers or thickeners are cylindrical tanks with a shallow-sloped bottom. Feed is pumped into the centre of the vessel via a feedwell, which discharges below the liquid level. This minimizes turbulence in clarifier/thickener. The solids and asphaltenes settle to the bottom and are moved towards the centre underflow of the vessel using rakes that rotate very slowly. The clarified liquid overflows into a launder, where it is pumped to the downstream unit.
MRM uses a 3-stage Counter-Current Decantation (CCD) process. Fresh solvent is added to the feed of the last (third stage) settler, where the overflow cascades back to the feed of the second stage. Warm bitumen froth from Extraction is diluted with chilled second-stage overflow, and allowed to settle in the first stage settler. The first stage settler produces a clean overflow product, containing very little fines and water. The first stage settler is slightly larger that the second and third stages (70 versus 50 meters) to provide adequate retention times.
Propane refrigeration units are used to chill the solvent, keeping the process temperature below 35ºC. The quality of the product produced from low temperature PFT meets all pipeline specifications and is identical to high temperature PFT.
HIGH TEMPERATURE FROTH SETTLING UNITS (FSU)
The next generation of PFT utilized the many learnings of LTPFT to improve the design. Increasing the operating pressure of the settling vessels enabled the whole process to operate at a higher temperature, which greatly reduced the residence time required. This makes for a process with a much smaller footprint. High Temperature Paraffinic Froth Treatment (HTPFT) requires much smaller settling vessels, which can now be elevated and designed with a generous slope, eliminating the need for internal rakes. This makes the units far more reliable. FSUs used in HTPFT are approximately 10 meters in diameter, and require only 2 stages of settling. The settling area for HTPFT is therefore almost 10 times smaller than the settlers used in the original LTPFT process.
FSUs used in HTPFT have the exact same function as the original low-temperature settlers: to separate the clean diluted bitumen from the solids, water and asphaltenes. Like any other conventional gravity separation vessel, a slurry stream is fed into the vessel, allowing the heavy solids to settle and the lighter, clarified liquid to overflow from the top.
Once the slurry enters the vessel, the very fine solids will settle more slowly, whereas the larger/heavier asphaltene agglomerates tend to quickly sink to the bottom, allowing them to be removed from the underflow. The upward flux (or superficial velocity) is the speed at which the clarified liquid flows to the top of the FSU. If the settling rate of the fines is slower than the upward flux, then fine solids and clays will report to the overflow, contaminating the diluted bitumen product. The diameter of the FSU must therefore be large enough to ensure that the fine solids do not get carried-over into the final diluted bitumen product.
Another important design feature is how the diluted feed is introduced into the FSU. Gravity separation vessels require a very quiescent environment, with minimal disturbance to ensure that the different phases separate out efficiently. Feed must therefore be introduced very slowly and evenly around the perimeter of the FSU. Gravity separation units normally use a vertically-installed feedwell with some form of deflector plate installed at the feedwell outlet. This acts to reduce the velocity of the slurry as it enters into the liquid phase of the vessel. Alternately, feed can be distributed radially around the outer wall through a series of inlet nozzles. Two FSU designs commonly used in the oil sands are shown below.
The side inlet feed-nozzle design is an Imperial Oil/ExxonMobil patent and claims to reduce turbulence as compared to the conventional design with an internal feedwell fed from the top. By introducing the slurry around the perimeter of the vessel, the solids and asphaltenes concentrate on the outer edge, encouraging the finer solids to settle along with the asphaltenes. Clean diluted bitumen circulates to the centre of the vessel, flowing up and over the launder.
Although the exact design of each FSU varies among the operators, there are some common design features. All FSUs have a steep cone angle, typically 60°. This ensures no solids or asphaltenes build-up on the side wall of the lower cone section, which could cause plugging of the underflow. All FSUs are pressurized vessels, with design pressures of about 1,000 kPa. The need for pressure containment is why all FSUs have a domed roof design.
All HTPFT facilities employ 2 stages of FSUs installed in series. The overflow from the first stage is the bitumen/solvent product, which is directed to the Solvent Recovery Unit (SRU). Fresh solvent is added to the underflow of the first stage FSU, which is then mixed and fed to the second stage FSU. Overflow from the second stage FSU has a higher S:B ratio and is pumped back to the feed of the first stage. Underflow of the second stage is the tailings product, which contains water, solids, asphaltenes and residual solvent. This stream is directed to the Tailings Solvent Recovery Unit (TSRU), where the solvent is stripped out and recycled back into the process.
INSTRUMENTATION & CONTROL
Regardless of type of settler used, there are a number of important operating variables that need to be controlled in order to meet overflow product specifications. The most important variables include:
- Bitumen froth flowrate: A higher flowrate reduces the residence time in the vessel, increases the upward flux velocity and increases the risk of solids carry-over into the product launder. In the cases where the bitumen froth is of lower quality, it may be necessary to reduce flow to the FSU to allow for more settling time. Froth flowrate can be measured directly with a flowmeter, which can be challenging due to the air content of the bitumen froth. An alternative (or back-up measurement) can be achieved by subtracting the solvent flow from the total feed (bitumen froth + solvent) to the FSU.
- Solvent addition or Solvent:Bitumen (S:B) ratio: A higher S:B ratio reduces the viscosity of the bitumen/solvent mixture, generally resulting in better separation and more asphaltene precipitation. However, a high S:B ratio also lowers the residence time in the FSU and can result in more solvent reporting to the underflow (which can translate to greater solvent losses if not fully recovered). It is therefore important to use only as much solvent as is required by the process. Solvent addition is measured directly by flowmeter. The S:B ratio can be calculated using flowmeters, densitometers and lab samples. Direct measurement can also be made using a refractometer.
- Interface level: The interface between the product and FSU underflow (sometimes referred to as the hindered settling zone) is primarily controlled by adjusting the speed of the underflow pump. If the interface is too high, product quality can be compromised due to solids carry-over into the launder. If the interface is too low, this increases bitumen/solvent losses to the underflow, which can disrupt the process. Vessel interface can be measured using nuclear profilers, segmented capacitance probes or pressure transmitters. In most cases, operators will use a combination of instruments to provide a full profile of the vessel and ensure adequate redundancy.
Although temperature is an important operating variable, it is typically kept constant. The operating temperature of the vessel can be controlled by adjusting the temperature of the bitumen froth and/or the added solvent.
Underflow density is also monitored to ensure that solvent losses to the underflow are minimized. Under normal operating conditions, underflow density does not vary by much. However, a sudden drop in density can be indicative of high solvent losses to the underflow. High underflow density can warn of potential plugging issues.
Overflow density is also monitored in order to calculate the solvent content (or S:B ratio) of the overflow product. It is important to monitor the various S:B ratios in each step in order to ensure product specifications are met. Sudden changes in S:B ratios are indicative of a process disruption.
The original low temperature/pressure froth settlers are still in operation at Shell's Muskeg River Mine. However, new generation PFT facilities have adopted the use of FSUs, which have the ability to operate at relatively higher temperatures and pressures. This includes Jackpine, Kearl and the Fort Hills PFT facilities.