6 replies to this topic

[curtin]

Hi,

Does anybody know about, or does anybody have any experience in monitoring the density of a lime/water slurry online?

We would either like to be able to measure it online in a pipe and/or online in a transfer tank.

Regards,

Chad Burrows

[ChemE1969]

Chad,
Another engineer friend of mine and I discussed on line density measurement of a slurry about a year ago. Although the application is different, I think our discussion may apply.

If the liquid carrier (I assume water) is saturated, that is its density is fairly constant, you "might" be able to monitor the solution density using a coriolis effect flow meter like a Micro Motion or Endress-Huaser unit. Theoretically, if the water density is considered constant the slurry density will change with solids concentration. Neither of us have tried this concept yet, but I currently monitor the concentration of a nitric acid solution fed to a reactor using the density reading from a Micro Motion unit with good results. Not a solid/liquid stream like you have, but I would suggest you contact your local Micro Motion or Endress Hauser representative to discuss. They may have already have a similar application.

I would think you would need to generate yourself a density/concentration curve also so it would be easy to look up or have your control system (if you have one) display the slurry density and % solids.

Hope this helps some.

[JMW]

Vibrating tube (sinle straight tube) density meters are extensively used for china clay slurries and, more recently, tuning fork sensors in the same applications which have the advantage of being installed into the side of pipe or into tanks without the need for a bypass.
A bypass is usually an arrangement with a pump, flushing ports, isolation valves and frost

However, calcium carbonate slurries may be more problematic as they are more abrasive than china clays but the tuning fork sensors can be used because they install in the side of the full bore pipe and by careful installation erosion is avoided or limited.

Fork sensors are used with chalk slurries (from the quarry to the cement works and which may contain everything from sand and flints to bits broken off machinery.... they won't like being hit by large heavy objects but can be installed so they don't see them).
Other applications include underflow pump control in clarifiers and separators, wash tanks etc for sands, granite wash slurries and so on.

The concern about coriolis is that though they may have adequate density measurement accuracy, their twin path design and the complex flow paths may lead to either blockages or erosion or both so take care this route to talk very carefully to the manufacturers.

On the whole, if using vibrating element sensors I would favour a tuning fork design as there are no thin walls to perforate, there are no flow paths to block and they can be installed side of pipe or in tank without any special installation arrangements.

Your choice is not limited to vibrating element; you might also want to consider gamma ray sensors (often popular in the sugar industry) or ultrasonic (used extensively in water treatment/effluent systems).

For more information on density applications, visit www.viscoanalyser.com and go to the applications pages. You should also find there an illustration of a fork sensor used for many years for chalk slurries. Badly installed, it has been seriously eroded but every so often they simply applied an offset correction o bring the calibration back in spec. Its final demise came when it was evidently hit by something large enough and fast enough to bend the fork tines and stem (the tines are spark eroded from bar stock and the stem is bar stock gun drilled for the sensor wiring).
Fork sensors are quite competitively priced against tube and coriolis and are low cost to install and maintain. I suggest you obtain some prices, applications histories and compare to the other technologies or manufacturers on offer.
For more density measurement information look for companies such as Mobrey, Sarasota (Peek), Controlotron, Dynatrol, and of course, the mass meter manufacturers. Gamma Ray try Berthold.

[JMW]

PS to CHEME1969's comments:
most density meter systems include the necessary software for an output in percent solids (mass or volume) and, where it matters, such as in caustic dilution control where temperature is a problem, the means to determine the density at a reference temperature so you can then determine the percent solids from the reference temperature density.
Coriolis meters are less good where temperature is a factor due to the changes in tube stress which affects the resonant frequency.

[Sharry]

Hi, this post is very informative; however I would like some specific information. If someone can help me then please send me a private message. Best Regards,

[ChemE1969]

JMW,
Thank you for your informative response. My thoughts were definately around a straight tube design if a coriolis effect unit were considered due to the plugging and erosion issues you mentioned. Also, I did forget to mention and you picked up on the fact that most instruments currently in manufacture today are capable of %solids, density, mass flow, volume flow and a multitude of other outputs directly from the instrument.

I was not aware of the tuning fork application for density/slurry concentration. Very good information that I will have to look into further. My practice is around coriolis effect meters to mass load batch reactors manufacturing primary explosives, particularly in a pilot plant setting where the fluid density varies from batch to batch and even the fluid being fed varying depending upon the current recipe in use. This application requires accuracy and repeatability which the coriolis supplies. However, there is no concerns around plugging or erosion of the tubes.

[JMW]

The tuning fork sensors are not quite as accurate as tube types being a round 1kg/m3 accurate but have other advantages such as tolerance for entrained air and lower cost (unless you want top entry into a tank in which case you can have upto 4 meter extension versions), tube types will give you around 0.15kg.m3 accuracy. If your temperatures are near ambient the temperature effect on density won't be that much of a problem for coriolis Dedicated ensity meters are usually much more temperature tolerant).

In china clay processing they use Mag meters with the density meters and use a simple math block to determine the net mass rate ratios when mixing clays of different types.

If mass flow accuracy is key then I guess the mass meters will be the best option for slurries.