Pressure Measurement in a Challenging Medium

[z4nzaa]

Dear all,

In a torrefaction system (oxygen-free thermal conversion of biological material) I am looking for a applicable pressure measurement. However, there is an interesting combination of process factors, that make this measurement challenging:

Operating Range: roughly -50...+50 mbar (pressure needs to be maintained at minimum overpressure, like +1 mbar)
Operating temperature: 300-350 degC
Medium: Gaseous mixture, consisting of steam, CO2, CO, condensables (with boiling points of up to the actual process temperature)

The main challenge I'm facing is the fact that the applicable pressure transmitters in this pressure range cannot withstand the process temperature. However, cooling the medium (and/or sensor) triggers condensation and therefore contamination of the sensor or the tubing leading up to the sensor.

I am currently looking into a measuring device with a continuous, small flow of nitrogen through it, but I am not convinced this is appicable.

Looking forward to hear thoughts on the matter! Thanks in advance!

Best regards,
Jan Oltvoort

 

[TiCl4]

I am most familiar with Endress Hauser, so this is what I have found:

The E&H PMP55 appears to fit your needed criteria. It uses a filling oil between the diaphragm and the sensor to achieve a higher temp rating - much like a remote diaphragm dP sensor. If you use Rosemount or other manufacturer normally, perhaps they offer a comparable item. You can just give them the E&H model and specs and ask if they have an offset.

PMP55:
Range: -400 to 400 mbar, 100:1 turndown typcial, accuracy up to 0.10%.
Temp Range: -70 C to 400 C.

Tech Sheet

https://portal.endress.com/wa001/dla/5000498/6753/000/04/TI00436PEN_2920.pdf

Edit: Is there a chance of an impulse line plugging up and messing the reading up if you let it stay liquid full? It is common in steam applications to use a "pigtail" impulse line to measure pressure - the pigtail traps condensate so the sensor sees a fluid that is much cooler than actual process temp. If your process liquid can plug up the line with solids, though, then that's a no-go.

 

[georgeverghese]

At these pressures, air ingress into the reactor through instrument tubing lines is often a problem. Avoid compression fittings and screwed connections; ask your field instrument engineer if he can manage with seal welded connections.

 

[moltenmetal]

The traditional solution to protecting a pressure instrument from excessive temperature in a condensing environment is pretty simple- it's an impulse leg full of condensate. I see no reason you couldn't use that here.

The other solution is a sweep purge of inert gas through the impulse leg at a constant rate. That is frequently done in systems such as fluid beds. The constant gas flow induces a constant pressure drop in the impulse line which can be calibrated out.