Coldbox, exchanger dew point

[Gulmi]

Hello everyone,

I am a Process engineer of Cryogenic N2 production unit.
More than a year it is beaing operating <60% of total capacity.
We conducted several maintanances, repaired and replaced turbines, replaced adsorbents, check valves, purged inside exchangers in Cold box, however again production dropped.

Currently pressure drop via exchanger is high 2.8 bar, and consequently P at inlet to turbine is lower than needs to be. When we increase production, P drop increases.

During maintanance we found carried over adsorbents in E1 exchanger (inside a coldbox), and blowed them with hot air. With borescope inspected after puff test section of pipe in E1, some dust left. We cannot see internal of Coldbox, including E1, as it is sealed for insulation.

please help me to troubleshoot. Did someone had similat problems in Cryo plants? what you can recommend to us? Did anyone opened Coldbox& is it possible to get it welded after opening and operating? other guess is hydrands, but, dew point before start up is achieved -53C.

will very much appreciate any ideas or suggestions

 

[EmmanuelTop]

Have you tried to derime ("defrost") the unit?

Based on your description it seems like the problem is definitely related to something that causes an obstruction to flow - you are seeing rise in pressure drop when you increase the flow. Deriming can tell you if this obstruction is a chunk of frozen water/hydrocarbons, or it is something else (mechanical obstruction).

The procedure for deriming is normally a part of vendor supplied documentation. Heating of the unit needs to be done in a very controlled manner - typically not more than several degrees Centigrade per hour.



Dejan IVANOVIC
Process Engineer, MSChE

 

[Gulmi]

Thanks for feedback Dejan, we did deriming before start up, achiving -53C dew point. Straight after start up pressure drop over column was 0.6 bar, after few days increased to 2.6 bar.
pressure at inlet to turbine initially was low 2.5-3 bar vs required 4.5 bar.

does this mean that there are both, hydrates and mechanical obstruction?

is it realistic, to tear up coldbox and weld it again? or solution is its replacement?

 

[EmmanuelTop]

I wouldn't do anything before I am 100% sure what is the cause of the problem. Replacing the cold box is definitely a massive (and extremely expensive) overkill.

Are you saying that, post-deriming, the plant operates with normal/acceptable pressure drop, and then pressure drop builds up in a few days? There lies the answer to your problem.

Dejan IVANOVIC
Process Engineer, MSChE

 

[Gulmi]

through one exchanger P drop increased, through second - condenser, was initially high -

 

[georgeverghese]

Check the operation of the mole sieves unit. You've obviously got water vapor and or CO2 carryover from the dryers to the cold box main cryogenic HX. There may be many reasons for the mole sieves unit overload / failure:
a) Upstream refrigeration unit is feeding wet air to the mole sieves unit with liquid water carryover-the refrig unit final KOD water level control may be faulty.
b) Mole sieves may not be adequately regenerated - final temp on completion of regeneration for waste gas leaving the bed in regeneration should be at least 220degC or so - check the operating manual for specifics. Waste gas flow should also be adequate. Heating and cooling steps during regeneration are usually 3-4 hours each.

In either case, beware of the risk of CO2 solids getting into the expander inlet. With the expander runnning at >10 000 rpm in many cases, it could be ruined in a matter of seconds.

 

[EmmanuelTop]

The key to understanding what is happening is during the startup sequence, when throughput is increased. You are apparently getting either a carryover of desiccant particles from upstream or, as George says, a carryover of components that are freezing in the downstream equipment due to under-performance of the upstream dryers. And then pressure drop starts building up.

If the deriming process restores the unit performance (at least temporarily), then it is almost certain that the unit gets blocked by the components that melt off at higher temperatures.

You need to investigate what causes the carryover/breakthrough, and there lies the remedy to (probably) all your problems with the unit. It could be many things - damaged adsorbent beds, mechanical damage in the adsorbers, damaged downstream filters, etc. We don't know the configuration of the unit neither we have access to process parameters, so it is the plant crew who has to do the troubleshooting.


Dejan IVANOVIC
Process Engineer, MSChE

 

[georgeverghese]

It is unusual to see mole sieves dust carried over into the cold box main cryo HX in my experience, and these are the barriers that prevent this solids carryover:

a) The ceramic fibre blankets on the top of each mole sieves bed which are held down by a mesh type grating
b) A 40 or 80mesh Y type strainer downstream of the mole sieves unit common dry gas feed line to the cold box.

When the mole sieves beads have disintegrated over time, the blankets dont hold and fine particles break past these blankets - the OPM would suggest that that each of the mole sieves bed should be inspected yearly or so and the grating should be readjusted lower to hold the bed down firmly as necessary as the mole sieves bed level drops.
Pls follow the OPM to the letter on how the ceramic blankets are to be cut into circular segments and installed on the top of each bed. Face masks are compulsory when entering the mole sieves bed vessels through the top manhole.
Premature disintegration of mole sieves beads is a sign of liquid water carryover in the feedgas from upstream. If the whole bed shows signs of disintegration, the entire inventory may need to be replaced as bed dp may become unacceptable.