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Q: We have remote-handled, (relatively high activity), 35% solids, radioactive sludges in Oak Ridge National Laboratory's Melton Valley storage tanks from which we're going to extract (at 2%-11% solids concentration in the slurry), dilute to <5% solids in a hold tank, transfer to the process facility and solidify/dispose as radioactive waste at a pre-approved federal disposal site.
This sludge is potentially stratified and definitely heterogeneous in each tank. Its supernatant phase varies both across the tanks and across the extraction process. We're to measure flow, percentage solids and density in real time, having considered Coriolis and other meters. Also:
- Bulk density of the sludge is 1.4-1.6
- Density of the dry solids is 2.75-3 g/cc
- Density of the supernatant is 1.05-1.1 g/cc
- pH of the supernatant is 9-12 (most likely 11-12)
At the risk of sounding flippant, I believe the answer lies somewhere between "You can't get there from here," and "Other than that, Mrs. Lincoln, how was the play?" We're not optimistic regarding on-line techniques. We can design the extraction system with a recirculation loop (both for the monitoring instrumentation and maintaining particles in suspension), but that doesn't get us to the point of measuring percentage of suspended solids in a variable-density solution. Some level of transmissivity measurement might be possible, but this sludge has all the opacity of chocolate milk (once mobilized).
In the near term, we're looking to put a mock-up of the tanks and extraction system in place (non-radioactive) here, and will be in a position to test candidate measurement systems. So, we may end up with something contrived, but that also works. Any thoughts or suggestions on hardware would be appreciated.
A: For mass flow detection, both Coriolis and ultrasonic flowmeters can be considered if the volumetric flow reading of the ultrasonic meter is multiplied with sludge density. If the pipe is large, ultrasonic, sonar and fiberoptic designs are good candidates. Sonar flow detection is used in downhole offshore oil and gas production.
A sonar meter has an array of sensors clamped onto the pipe, and detects the speed of eddies as they pass these sensors. All-fiberoptic flowmeters also have been deployed in applications for both flow and phase fraction measurement.
As to continuously detecting percentage of solids in applications where you know the density of the solids and of the supernatant, I usually calculate percent of solids by: % = (SP1-SP3)/(SP2-SP3), where SP1 is the specific gravity of the sludge, SP2 is the dry solids, and SP3 is the supernatant. Therefore, you need three density measurements and the corresponding sampling systems.
For the details of automatic sludge sampling designs, see Chapter 8.2 in Volume 1 of the 4th edition of my handbook.
You should keep in mind that measurements that work in a mock-up might not work when you apply them to the full-scale process. Also, I will have an article (probably the Sepember issue) on the state of the art of nuclear power plant waste disposal, which you might be interested in reading.
A: Have you had a look at the sonic meters from Cidra? They are being used in the oil sands to measure the four-phase slurry flows in 27-in. lines from the mine face to the extraction plants and then again in tailings. They are non-intrusive, and should do a good part of the job. I have copied John Viega of Cidra on this note, so he can respond directly as well.
Also, a company called Caltron, (part of Emerson now, I think) had a non-invasive capacitance probe installed between a pair of flanges that could provide water content.
A: When I was at Oak Ridge, I developed a probe that measured density and temperature along the probe. This probe was based on ultrasonics; it used the torsional mode. The probe was non-circular; we used a flattened probe about the size of a thermocouple probe used in reactors. Any non-circular probe would work. I switched between extensional mode to measure temperature and torsional mode to measure the density of the fluid surrounding the probe. There is an equation describing the velocity of propagation that includes density of the fluid surrounding the probe. It sounds to me like this probe might have application in this project.
By the way, the probe was originally developed after the Three Mile Island accident to measure coolant level in a nuclear reactor. In that measurement, one can't measure coolant level because the coolant is always at the top. Where coolant is lost in a reactor is at the hot spot, i.e., the core. You would have, from the bottom of the reactor vessel, water, void fraction and then water. This probe was designed for that environment, and would have taken the place of one of the thermocouples in the reactor.
George N. Miller, P.E.
Q: Could you suggest to me some MPC (multivariable predictive control) literature (books, articles, white papers, lectures, etc.) that I can study in order to fully understand how to design, implement and maintain an MPC project?