Hydrochloric acid level measurement

[tona2008]

Hi,

I'm struggling to control the level on a hydrochloric acid tank that is part of an acid recovery system. The problem is controlling the level of the acid. The contaminated acid is heated to 236 F and is boiling. This creates turbulence, foaming and vapors. We are currently using a bubbler type DP system to measure the level. This transmitters sends a signal to the computer which in turns controls a feed valve to add acid as required. Due to all the vapor created, the pressure on the upper portion of hte tank keeps changing constantly which affects the accuracy of the level transmitter. We compensate by injectin nitrogen to try to maintain the pressure constant. My question is: Is there any other type of level measurement system that I can use that will not be affected by the foaming and boiling of the acid. We already tried non-contact ultrasonic level transmitters, radar type transmitteres and all this don't work due to the foaming and the constant agitation of the surface of the acid.

Any recommendation is really appreciated.

Thanks,

 

[jamesbanda]

With Hydrochloric acid is a corrisive service and if you have a strong solution you any change will be expensive.

Are you sure the vapour is making that much difference ? the density of vapour is much lower than liquid it may be other symptions you are seeing. Typically i've used N2 bubblers to keep debris and junk out the tappings of a DP transmitter.

I'm not aware of any easy answers to this.

 

[MJCronin]

Tona,

Consider the installation of a stilling well to mimic the level in the tank.

Measurement can be made from the well by a variety of methods.

Would the installation of such a well disrupt the process ?

-MJC

 

[Compositepro]

The issue is probably due to the fact that in boiling conditions you can experience constant, but small, variations in vessel pressure. The differential pressure measured by a bubbler tube is small, also. When vessel pressure increases slightly the gas in the sensor tubes must compress also.But one is in vapor and the other is in liquid. The vapor tube will respond more quickly so the differential pressure reading changes. You should be able to filter the output signal of the sensor to get a good reading.

Another possibility is that you do not have identical lengths of tubing for the high and low pressure sides so the flow resistance and volume of the tubes is not the same. You can the adjust the purge flows through the tubes so the the sensor is zeroed at one pressure but if the pressure changes the zero will shift.

 

[tona2008]

MJCronin,

Are you referring to a stand pipe to avoid turbulence? We've tried a stand pipe but the acid produces so many vapors that need to be released from the pipe and most of the meters either don't resist the acid and the ultrasonic transmitters don't work well with the vapor. may be a parallel well where we can have cool acid that mimics the level of the tank. Most transmitters work well when the acid is cool. What do you think?

 

[moltenmetal]

What are you interested in detecting: continuous liquid level, or top of froth/foam/splashing zone?

Tantalum diaphragm true differential pressure level units are available and will compensate for the variable headspace pressure. They will of course not be able to handle the variable void fraction due to boiling, nor will they be helpful in detecting the top of froth or foam if you care about that.

 

[WhiskeySierra81]

To add on to moltenmetal's comments, we had good results with Rosemount's (part of emerson) dP cells with welded tantalum diaphragms.. we were working with boiling phosphoric acid/hydriodic acid at 150-200 psi. While it's true that they won't see the froth, they still gave us a good indication of the liquid level, even with the variable void fraction due to the boiling. Maybe it's because there was enough liquid to form a continuous section where pgh applies?

Note: they are expensive!

 

[tona2008]

WhiskeySierra81

Do you have a model number for the dP cell you are using. I'm having trouble finding the right one.

Thanks,

 

[moltenmetal]

What you want is a 3051 dP transmitter with external diaphragm seals, at least 2" flanged. You'll need to got to Emerson's website to figure out model numbers, or work with their distributor.

 

[unclesyd]

We too use the dip tube type instrument to measure level in our process that involves HCl.

Here are two other level instruments as shown on the Vega Site you might want to consider, nuclear and radar. We only have two installations of the radar variety, but around 25 nuclear level and density controls.

http://www.vegacontrols.co.uk/ohmart_nucleonic_index.asp

http://www.vegacontrols.co.uk/vegapuls_radar_index.asp