How can I reliably measure sodium hydroxide (50%) density online.

 

Some of the modern models of Coriollis flowmeters will give you this information.
see Endress&Hauser website

 

DP Cell.

Bob

 

Probably the best thing to do is use a Coriolis Mass Flow meter and get the inferred density measurement out.

I've done this application long ago with nuclear densitometers, but I wouldn't use one on this now.

You can also try the non-coriolis density meters from people like Anton Paar and Solartron (IIRC).

But if I were doing that app right now, I'd use a Hastelloy C body Coriolis meter. Unless I could get better corrosion resistance in another material.

Good luck,

Walt

Walt Boyes
Editor in Chief
Control and ControlGlobal.com
555 W. Pierce Rd Suite 301
Itasca, IL 60143

[email protected]
www.controlglobal.com

 

I've recently replaced two coriolis meters with inline density meters on caustic duty at a Nuclear fuels processing plant. The original mass meters were getting a bit long in the tooth and thus their density performance isn't what we'd expect today from coriolis.

However, accuracy is more than the declared measurement accuracy which is usually defined at 20degC, you also have to consider other aspects. Most usually caustic quality control is when diluting of raw or concentrated caustic with water.

Thus we have two problems to contend with.
The first is the possibility of crystals in the solution which can be abrasive, and temperature which, if you are diluting raw caustic, will be an issue. If crystals are a problem at your operating temperature (and it is not a nascent dilution) then coriolis may be problematic unless you opt for straight tube.

Coriolis meter performance can suffer more from temperature effects than dedicated density meters.

Design is intended to optimise mass flow performance, not density. It means some coriolis meters will suffer from more significant temperature effects and it will depend on how well the density function is calibrated and how well modeled the temperature effect is.

This is the effect of temperature on the measurement of density at the operating temperature, not the easy bit of taking that measurement and converting it to density at reference temperature and from the base density, calculating the concentration. Most sensors will have the software to manage that.

At 25% the 316 stainless steel sensors were chosen. Hastelloy may be necessary at higher concentrations, I'd check with Haynes before commenting.

The problem with dedicated density meters is that you don't have a lot of choice on line size so whereas you can choose a mass meter to suit line size if you choose a Solartron (Micromotion these days) you get a 1" straight tube sensor and if you are dealing with larger pipelines you may have to install in a bypass. Then it is a question of which is more cost effective and more practical.

Today I'd consider instead the Lemis density meters. These can be inline upto about 3" or insertion for larger bores. These are more compact and give equivalent performance (and materials) to the Solartron.

These have an advantage over Solartron in that the sensing tube (much shorter and resonated in hoop mode) is fully floating so there are even fewer temperature effects than the Solartron, and there are no pressure effects.

Realistically, your choices are quite open. Coriolis meters or dedicated density meters. You need to define your expected accuracy and then run the application by the various manufacturers to see what they can offer.

With a relaxed accuracy specification you could also consider the Micromotion insertion fork density meters. Visit the Micromotion site and take a look at both Micromotion coriolis and Solartron density meters or go to www.viscoanalyser.com to look at the Lemis sensors and Solartron sensors.

You could also search for other coriolis and dedicated density meters. There is no real benefit to choosing gamma ray sensors that I can see. You might consider microwave density meters (Toshiba?) but I have no experience of those so couldn't comment.

 

If your solution is in a tank you can get a good density reading using a DP cell and differential bubble tubes or a flange mounted DP with a flooded wet leg with water purge.

Otherwise Coreolis as others suggest.
Roy

 

If you want very high precision/accuracy in determining concentration then measure sound velocity.

Vince Dooley

 

If your solution is on a pipe you may want to look at mounting a DP cell on a stand pipe to similar effect of the tank solution.

Bob

 

Thanks All.

In our process we are diluting raw caustic from 48% to 23%.

 

Funny how things work. I just started up a caustic plant this week that makes down 50% sodium h to 10%. I used a Micromotion mass meter and it has been great. I take a makedown tank level and using the 50% SPg and % solids to calculate ratio to make to 10% has been very good.

Dave Ferguson
Control Systems Engineer

Sent from my iPhone

 

refractive index is used to measure flowing concentration of NaOH. See K-Patents' app note at

http://www.kpatents.com/pdf/applications/apn-4-08-01.pdf

 

"In our process we are diluting raw caustic from 48% to 23%."

then this is exactly the application I described.
I have supplied density meters for these applications over the years and put together an FAQ which is here:
http://eng-tips.com/faqs.cfm?fid=1007

Today's Coriolis will be OK but do ask the questions on temperature effects and accuracy.
I'd suggest you put an inquiry to some of the main coriolis meter and density meter suppliers and see what comes back.

 

Another useful link is to this FAQ supplied by M J Cronin:

http://eng-tips.co m/faqs.cfm?fid=1096
There are useful links to temperature density data in that FAQ.