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wilmers (Chemical) (OP)
13 Feb 05 1:42
Dear friends:
I would like to know if anyone has some information about de procedure of deriming a Air Separation Unit, I mean which are de aprox. operation conditions (temperatures, pressures , etc).

thanks a lot for your help

Wilmers
Helpful Member!  cryotechnic (Chemical)
17 Feb 05 7:30
The purpose of deriming an AirSeparationUnit is to remove parts of Carbondioxide, Water, and Hydrocarbons out of the whole unit.
Therefore, it's necesarry that the temperature of the unit is brought above zero centigrade.

To do so, first all the liquid has to be dumped. Then, without producing cold by using expansionturbine's etc. air is run through the unit.
It's very important that the deriming of the plant is done slowly. 10degrees per hour is the maximum.
During the deriming, the pressure in the unit will rise, and the process will ask less air due to this. So it's important to open as many way's out to the atmosphere, this helps to control the pressure, and it allows you the derim all the parts of the plant.
Most important thing in deriming is that the whole deriming process is done slowly and evenly throughout the whole ASU.

Normaly the manufacterure of the ASU supplies reggulations how to derim the ASU.

What kind of ASU are you talking about? What are the capacities, which products, who's the manufacturer of the plant?

Cryotechnic
NTCIINC (Mechanical)
3 Mar 05 19:32
Dear Wilmers,

Deriming an ASU is performed in three steps:

1. Warm up the cold box

2. Dry the cold box

3. cool down the cold box for re-start

1. WARMING THE COLD BOX:
Reduce your air flow and shut down the expander
Dump your liquids from the columns
Open all the liquid and gas purges on the cold box. Open the non-condensible purge fully on the main condenser.
You will see the purges ice up. Wait for the purges to thaw out, this will give you a good indication that the cold box temperature is above freezing.

Nonetheless, check your temperature, especially in the sub-cooler area, there might be some trapped liquid that does not allow air circulation.

2. DERIMING THE COLD BOX
Take a set of P&ID's, a highlighter and study the cicuits. Some circuits may require that air is passed in one direction and then in the other to make sure that you have circulation.

Your main condenser should be the first item to be derimed and the last to be buttoned-up. It is the most critical part of the plant.

The expander should be derimed in counter flow to the normal flow path (outlet to inlet). Make sure the turbine does not spin and make sure the seal gas and oil skid are turned on.

Adjust the derime gas for maximum flow but also for minimum pressure. I always limit my derime flow to a whisper, like a small breeze on my hand. The lower the pressure of the derime gas, the more moisture you will pick up. Screaming derime flows out of the purges and vents are an exercise of futility, a waste of power and time.

I personally never use a heater, the risk to damage the aluminum exchangers is too great. There are exceptions to this rule when dealing with newer plants (falling film vaporizers especially). Check the plant manufacturer's instructions.

Start checking the purges and vents for a dew point below -90F (-70C). As you reach such a dewpoint at a given purge or vent, close it (again, except the non-condensible vent). If a dew point takes for ever to come down, be patient, this means that liquid water is slowly evaporating from somewhere. If a purge does not have any flow when it should, it means that it has an ice block (which is a whole other ball of wax), do not ignore it.

As you close a purge or a vent, mark the date and dewpoint on the P&ID and check the new flow path. Make sure you do not send a wet stream back into a dry area inadvertently.

3. COOL DOWN THE COLD BOX FOR RE-START
When all dewpoints are good, fire up the turbine, watch the temperatures go down in the exchangers.

As you see the purges and vents frosting up, close them so that the cold gas goes to the warmer areas. However do not close the non-condensible vent or the lower column sump purge. The lower (or high pressure) cloumn sump purge is where the last hot gas devils are chased out.

Do not inject liquid until all the temperatures are at design and the plant is ready to make liquid.

I hope this helps.

Sincerely,

Nils
cryotechnic (Chemical)
9 Mar 05 14:29
A few things I would like to add to the post of NTCIINC. (which is allready allmost complete)
Sometimes with large structured packed columns it's usefull to change the air flow during the warming up stage. We have the experience that this give's a much better result. So for a few hour from bottom to top and then a few hours top to bottom. (in our plant, the warmup lines are build in a way that makes it possible to do all this.

In the deriming stage. It's very usefull to open also ALL the vent's at the transmitters in the transmitterroom. Also take a dewpoint at these vent's
Same thing here, if one vent had no flow, there is a block.

Cryotechnic
NTCIINC (Mechanical)
28 Mar 05 20:45
Dear Cryotechnic,

Many thanks for the valuable tip on deriming the falling film vaporizers.

NTCI, Inc.
www.ntciinc.com

asukid99 (Chemical)
27 Sep 05 23:08
Good suggestions above.  I will add the following:

Eliminate dead legs.  They take FOREVER to clean up.
Don't use heated derime gas unless your specific exchanger is designed for it.
As certain circuits clean up, pinch those off which will direct more flow to the other circuits that still have higher dewpoints.
Get a good dewpoint meter such as a Cosa.
Dont forget to open the vents on your transmitters.
Keep derime pressures within their normal operating ranges.
Derime flow should be generally be reverse from the normal flow.  (Blow up from the bottom or your columns.)
-90deg F dewpoints are the required. (OK I admit I cheat sometime if I am still in the 80's.)
Many ASU operators paste E-size P&Is on the wall and highlight their derime circuits to keep track of what is going on.
Have fun.  Deriming is a pain in the butt!

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