I am a student of chemical engineering that is making the project and had doubts on this.

I want to design the control for a system of triple effect evaporators in parallel in each step. My doubt is about:
- i am not sure if is better instrumentation for the measurement of viscosity or the density.
-where i could receive information about specifics on the material to choose.
thanks,sergio

 

If you want to measure the composition of a pure (binary) water/ethylene glycol system both measurements should work in theory. However, on-line density measurement is a well known and established technique, many instruments on the market (both Coreolis/flow through and tuning fork types/flow past). Viscosity is more of a specialised technique. Also, the temperature correction is a thing to consider. Viscosity varies more with temperature than density, so if you will have fluctuations in temp. you have to consider this. So, I would go for density measurement.

If you have bubbles in the liquid, density measurement might give false readings. In this case you might consider on-line measurement of the refractive index, which is not affected by bubbles. See http://www.kpatents.com

Regarding cost I would hint at
Density: 6-7k$
Refractive: 10k$
Viscosity: ?

 

Measure density using a differential pressure transmitter in a fixed level tank or stillwell. Your control system is configured to condition the signal to relate density (specific gravity) to concentration for the measured process temperature.

For in-line measurements, either use a mass flow meter configured to output proportional density (based upon vibration theory - more density/less vibration) or one of several nuclear density meters. The mass flow measurement accuracy may be effected by high flow rates.

 

If the ethylene glycol is a binary mixture with water, the concentration measurement with an ultrasonic sound velocity measurement is perfectly suitable. The advantage of this is that it is not sensitive to fouling, as refractive index measurement is and can detect gas bubbles, thus avoiding problems.
Due to the large change in sound velocity as function of concentration, this measurement is well suitable for inline measurement, even at temperatures far below 0 degree C.
Check out the website of Rhosonics, who have various inline sensors for your application: www.rhosonics.nl

Regards,
Willem

 

Both density and viscosity show a good variation with concentration.
Temperature will be an issue with both as both vary significantly with temperature as well as with concentration.
Gas bubbles have been mentioned.
Many technologies suffer from the effects of bubbles, not just density and viscosity technologies, so be sure to investigate if you will have gas bubbles and if the technology you are considering is susceptible. Vibrational density systems can be signiciantly affected by gas bubbles but not all. Some operate at a different harmonic to the standard instruments and can handle very high volumes of gas if necessary. Some viscosity measurement systems are also quite capable of being installed in such a way as to handle gas bubbles. In fact many applications involve entraimned gas that shouldn't. Installations are often far from perfect. Then again, not all applications as a matter of course involve entrained gas. In a refrigeration unit in a pharmaceutical plant, for example, if you measure the density of the return flow from the hotwell then you will need an instrument capable of handling a lot of gas bubbles, however, in evaporators entrained gas is rarely a problem if the measurement point is on the discharge of the product outflow pump.
You will need to conduct an internet search and approach several manufacturers of each tecnology to give yourself a good choice.
Don't be sidetracked into instrument accuracy discussions. None of the instruments measures concentration directly but may depend on measurement of other parameters such as temperature and on calculation. You want the accuracy with which they can measure the concentraion based on your operating conditions. Be sure you know what accuracy you need.