Prior to the invention of Hydrotransport technology, Syncrude and Suncor relied on large rotating drums (tumblers and rotary breakers) to produce oil sands slurry. The mined oil sands was conveyed from the crushing plant into rotating drums and mixed with copious amounts of hot water. The rotating action of the vessels provided the mixing and agitation required to break down the lumps of oil sands and created a pumpable slurry for the Extraction plant. Although very effective, these large vessels have a few shortcomings:
- Short residence times, providing only a few minutes of mixing, and
- High water and energy intensity, requiring large volumes of hot water to break down the mined oil sands.
As part of Syncrude’s efforts to reduce hot water consumption, the company began experimenting with the use of long pipelines and large slurry pumps in order to breakdown the lumps of oil sands. Commercial scale testing of Hydrotransport technology began in the early 1990s. The dry oil sands was mixed with lukewarm water in a simple pumpbox, then pumped a considerable distance to the main Extraction plant. The Hydrotransport line was several kilometres long, providing 10 to 20 minutes of residence time, much longer than the tumblers were capable of providing. The turbulent flow in the pipeline was also found to be very effective in breaking down the lumps of oil sands, improving the recovery of bitumen in the main Extraction plant.
Apart from allowing for lower temperature water to be used, Syncrude was able to replace the costly tumblers with simpler Mix Boxes, which were considerably cheaper to build and install. Since Hydrotransport pipelines are typically several kilometres long, the technology provides several other benefits:
- It enabled the Slurry Preparation Plant (SPP) to be closer to the mine, reducing the required hauling distance for the mining trucks. This translates into less diesel consumption and fewer trucks required.
- It greatly improved the flexibility in laying out the overall processing plant, allowing the owner to trade-off shorter conveyors with longer pipelines, which are a cheaper and more reliable form of transportation.
Hydrotransport technology was patented by Syncrude in the late 1990s and is now a standard feature in all mined oil sands processing plants, regardless of operating temperature and slurry preparation technology used.
PURPOSE: WHY HYDROTRANSPORT LINES ARE SO IMPORTANT
Hydrotransport pipelines serve 3 basic purposes:
- They transport the oil sands slurry from the SPP to the main Extraction plant: At its essence, a Hydrotransport line is basically a large diameter slurry pipeline equipped with a number of pumps in series. The line begins at the exit of the Slurry Preparation Plant within OPP and terminates at the feedwell of the Primary Separation Cell (PSC) or Vessel in Extraction.
- They provide sufficient mechanical shear to break down the lumps of oil sands and liberate the bitumen from the sand: The oil sands slurry produced within the Slurry Preparation Plant is non-homogenous, lumpy and poorly mixed. Clumps of bitumen, clay, ice and sand can be bound together in large chunks, as large as 5” in diameter. As the slurry travels down the Hydrotransport pipeline at relatively high velocities (normally 3 to 5 meters per second), the large pumps and turbulent flow in the pipeline impart a serious amount of mechanical shear onto the slurry, breaking down the lumps and liberating the bitumen.
- They provide much needed additional residence time for the bitumen to attach to air bubbles within the slurry mixture, allowing for improved bitumen recovery rates within Extraction: Since the recovery of bitumen is largely a gravity separation process, the liberated bitumen particles must attach themselves to free air bubbles. Once the oil sands slurry reaches the Extraction plant, the sand sinks to the bottom of the PSC and the aerated bitumen floats to top, enabling it to be recovered. Since bitumen attachment to air bubbles is a time-sensitive process, the lengthly Hydrotransport line provides an additional 10-20 minutes for this bitumen/air attachment to occur, therefore improving bitumen recovery rates.
LINE LENGTH: HOW LONG IS LONG ENOUGH?
Given the need for sufficient residence time, Hydrotransport lines can be quite lengthy, spanning a distance of 1 to 5 km, depending on process conditions. The required length of pipeline is a function of the following factors:
- Slurry Preparation Plant Design: The technology used in SPP is an important factor in determining the amount of residence time required during Hydrotransport. Plants that use the Mix Box design (such as Horizon and the Aurora Mine) produce a slurry with very large chunks, up to 5” in diameter. These larger chunks require more time and energy to break down and would benefit from a longer pipeline. Facilities which use rotary breakers to produce oil sands slurry (found at all Suncor and Shell facilities), produce a slurry with only 2” lumps and therefore do not require as much Hydrotransport length.
- Process Temperature: System operating temperature is adjusted by controlling the temperature of the hot/warm water used in the SPP. At higher temperatures, bitumen viscosity is greatly reduced, making it easier to break down the lumps of oil sands. If lower temperature process water is used, bitumen viscosity remains high, making the oil sands lumps more difficult to break down. Lower temperature processes (such as the Aurora and the Muskeg River) therefore directionally require longer Hydrotransport pipelines to make sure all the bitumen is liberated from the solids. Plants that use hotter process water can get away with a short line length.
- Slurry Density and Velocity: Higher density slurries which flow at faster velocities down the pipeline are subject to more abrasion and turbulence, improving the breakdown of bitumen lumps. Therefore, Hydrotransport pipelines should be long enough to provide enough bitumen ablation during turndown periods, when the slurry density is relatively low and flowrates are below normal.
- Plant Layout: The most obvious factor which determines the length of the Hydrotransport pipelines is the distance between the SPP and the main Extraction plant. SPPs are normally located closer to the mine in order to reduce conveyor lengths and shorten hauling distances for the mine dump trucks. In cases where the SPP is too close to the main plant (i.e., shorter than the minimum Hydrotransport distance desired), some operators have added “extension loops” on their Hydrotransport lines to provide the minimum required residence time for complete oil sands ablation.
Still confused? The University of Alberta Department of Chemical and Materials Engineering offers assistance in predicting the residence time required for complete lump digestion. Sometimes referred to as the “Masliyah and Bara oil sand lump ablation model”, this proprietary model was first developed in 2008 and results are only made available through the University of Alberta (click here for link).
COMPRESSED AIR INJECTION: PROCESS AID OR PLACEBO EFFECT?
In an effort to improve bitumen recovery at lower temperatures, some oil sands operators have borrowed a page from conventional mineral processing plants and inject compressed air into the front-end of the Hydrotransport pipeline. Despite little evidence of improved recovery rates, here’s a look at why more bubbles should indeed be a good thing.
TEMPERATURE: THE MOST IMPORTANT PROCESS VARIABLE
Process temperature is the single most important operating variable affecting performance of the Extraction plant. At low temperatures (below 40°C), bitumen viscosity is high and attachment to air bubbles is very weak. However, as the temperature increases above 50ºC, bitumen viscosity is reduced and approaches the viscosity of water. This enables the bitumen to envelope the air bubble and provides a far more stable attachment. Bitumen recovery rates are therefore greatly improved at higher operating temperatures.
So why not run every process at higher temperatures? Mines that are integrated with an adjacent upgrader (such Millennium, Mildred Lake and Horizon) have an abundant supply of hot water since they capture the waste heat produced during final product cooling at the upgrader. These mining facilities normally operate at temperatures higher than 50°C. Mines that do not have an on-site upgrader (such as Muskeg River, Jackpine, Aurora North and the upcoming Fort Hills Mine) do not have access to large amounts of waste heat and must therefore burn natural gas to heat the process water. In order to save on energy costs and reduce greenhouse gas emissions, these bitumen extraction plants operate at lower temperatures, typically closer to 40°C.
HYDROTRANSPORT LINES: MAINTENANCE, MAINTENANCE AND MORE MAINTENANCE
Hydrotransport lines are some of the most maintenance intensive components of a Bitumen Production facility. Abrasive silica sand and entrained oxygen in the process water cause both erosion and corrosion of both the pumps and pipelines. Aside from good preventative maintenance planning, the overall reliability of the system can be greatly improved through intelligent design and good operating practices.
BASIC DESIGN PRINCIPLES & OPERATING BEST PRACTICES
Hydrotransport system design is a trade-off between cost and bitumen recovery. Long lines ensure the lumps of oil sands are broken down before reaching Extraction, but additional length proportionally increases installation and operating costs.
The most critical factors affecting Hydrotransport design include:
- the line length, as dictated by the distance from SPP to Extraction
- the volume of oil sands to be sent through the plant
- elevation gains from the SPP pumpbox to the top of the Primary Separation Cell in Extraction, and
- coarseness of the mined feed.
THE NEXT STEP: BITUMEN EXTRACTION
Once the well mixed oil sands slurry is pumped to the Extraction plant, coarse solids are first removed through a water-based gravity separation process. This produces an intermediate bitumen froth product with about 50-60% bitumen, 10-15% fine solids and up to 40% water.