Sand can settle in a pipeline if oil sands are not flowing at or above terminal velocity

BP, a major energy company is engaged in a new project transportating oil sands. Oil sands are a mixture of sand, water and bitumen. Bitumen is a very dense and viscous liquid. Since bitumen can make oil sands too viscous to transport in pipelines at normal pipeline temperatures, heated water is added to the sands so pipeline transport is possible. The oil sands slurry is then transported through the pipeline to the extraction plant at high speeds.

A potential problem is if the flow is not fast enough in the pipelines, the sand component could begin to settle in the pipeline and cause blockage. Marco deRegt, a process engineer working on the pipeline project, is assigned the task of determining the minimum velocity at which the sand component of the slurry will not begin to settle (commonly referred to as the terminal or deposition velocity.)

The pipeline under consideration has an inner diameter of 0.711 m. After hot water has been added, the volume concentration of sand is 0.25 and the mole fraction of the liquid bitumen/water mixture is 0.7 bitumen and 0.3 water. Research performed on liquid bitumen and its interaction with the water added has shown that it has a density of 978 kg/m3 at 90˚C. Bitumen has an API (American Petroleum Institute gravity) of 8, water has a viscosity of 3.165x10-4 Pa*s at 90˚C. Assuming that the drag coefficient of sand is 0.45 and that the sand particles have a uniform diameter of 0.5 mm, Marco is ready to find a solution.

Finding the terminal velocity of the slurry

Marco types ‘terminal velocity’ into Knovel’s basic search to find information on calculating this value

He chooses the text Slurry Systems Handbook and finds several formulas needed for calculating the terminal velocity under the given conditions in section '4.3.2 The Transitional Velocity V3 or Speed for Minimum Pressure Gradient’.

For an accurate calculation based on viscosity, the Froude number, F, in terms of the Archimedean number, must first be calculated:

Then the terminal velocity can be calculated using the following formula since particle diameter is assumed to be uniform:

Now he's ready to find the Archimedean number for the viscosity of the liquid mixture and the density of the solid (sand.) Marco decides to calculate the viscosity of the liquid mixture of bitumen and water searching ‘viscosity liquid mixtures’:

and sees a useful and accurate way of calculating this mixture’s viscosity in Perry’s Chemical Engineers’ Handbook (7th Edition) under the section ‘Vapor and Liquid Thermal Conductivity’.

Since bitumen and water are of different chemical families, this equation will yield a liquid mixture viscosity with only 10-15% error.

Marco finds the viscosity of bitumen at 90˚C searching ‘bitumen viscosity temperature’ and using the Handbook of Petroleum Analysis. In section 4.5, ‘viscosity’, he finds a plot in figure 4.5 which will determine the density of bitumen at the given temperature.

Remembering that the API of bitumen is 8, the viscosity appears to be about 1000 cP or 1 Pa*s according to the figure. Marco can now complete all the required calculations by going back to the equation for calculating liquid mixture viscosity and using the given information to find the liquid viscosity. SEE HIS WORK

A proper design of the pipeline should ensure that the slurry flows above 5.56 m/s.

These calculations show a very close approximation to the terminal velocity of the oil sands in the pipeline under these conditions. As long as the slurry is maintained at a velocity higher than 5.56 m/s then sand accumulation should not be a problem. Safety precautions should always be built into the process in the event that pipeline velocity drops below this value.

Without having to travel with a collection of handbooks or wait for colleagues to find the information for him, Marco was able to save time out in the field.

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“Using Knovel, I found critical information early in the design phase, so I didn’t waste time wandering down the wrong path. I’d estimate that checking Knovel first to optimize the process design saved me at least a month of rework time, significantly condensing the project timeline and improving process quality.”

Associate Process/Specialty Engineer
Fluor Corporation