Hydrocarbon column preps for a shutdown - water wash or purge? 1

[mrtangent]

Dear All,

As far as I've been aware we have some hydrocarbon columns which we normally prep for general inspection by emptying and pressure cyclicing. (NO HOT WORKS)

The hydrocarbons are liquid at normal conditions BUT FLAMMABLE (FLASH POINT BELOW 55DEG C). Having reviewed the risks we are considering water washing - even though there is no CONFIDEND SPACE ENTRY OR HOT WORKS ON THE COLUMN.

Does anyone else water wash or just purge and empty.

The column has sieve trys which DO (SHOULD) SELF DRAIN.
The top product is a gas at normal pressure/temps. But forms a hydrate with water (ie we prefer not to water wash).

So, I'm keen to learn how other people prep these columns.

We think we should water wash drain. To ensure we dont have a flammable atomsphere is this over the top or normal pratice in say refineries etc.

 

[jalvarez]

So, you are handling gases and liquids. If your column handle only gases, normally it's enough to purge with nitrogen and later with air.
As you process liquids, it's very difficult to eliminate the vapors, and water wash is commonly not sufficient. Nasty odors remain, and inspection wont be possible, not mention to open internal pathways or doing maintenance work. Further, it's difficult to eliminate the explosivity.
To deal with this, it's preferable to steam out the tower, filling it with water and adding Low Pressure Steam in the bottom. The top of the tower must be opened to the atmosphere, in order to avoid overpressures (be sure that the design pressure of the column is not exceeded, if there is the necessity to blind it, or remove safety valves for tuning or whatever). The temperature will increase slowly with the steam addition, and after some hours (depending on size of the tower), you may drain the hot water, which will then drag and remove the hydrocarbons (including heavier ones). Steaming out this way frequently takes one whole day or more (depending on the hydrocarbons that are being handled.
All this is valid and I used to do this steam out procedure with streams like C5+/C6+ and heavier.
Again, with C4 and lighter, nitrogen is preferable, and sweep with air at the end is a must, except you are going to "mouthball" the column for an extense period.
Hope this helps, have a safe day.
J.Alvarez

 

[epoisses]

we did exactly the same, jalvarez

 

[25362]

Beware of pyrophoric iron.

 

[mrtangent]

jalvarez,

Thanks for your responce- much appreciated.

Our column is a MeOH/MeCl split col. MeOH is water soluble. Would you still steam up to atmosphere, then drain ? or do you think water wash is adequate ? the service is very clean (ie no solids) do you think water wash is over the top ?

Is the steaming mainly for the the hydrocarbon removal or the deposited material on the trays ?.

Our trays do self drain easily. We've tested this several times.

Sorry if my inital question was a little unclear.

 

[jalvarez]

James:
Steaming is for Hydrocarbons. Pirophoric materials precuations also aplies to refineries, it has been the cause of several accidents and deaths in the past.
But yours is another issue:
Methanol is completely soluble and Methil chloride highly soluble in water, so water should be enough, from top to bottom.
Consider if you can have another contaminant. Do you? Anyway, methanol itself is enough toxic to take it very seriously.
Maybe to use warm water at the final step is recommendable, to eliminate odors. If the odors still are present, maybe forced ventilation will finish the job, after opening the manholes.
If still you have problems, consider to use steam, verifying the design temperature of the tower.
Only one last comment, be sure the basement of the tower is prepared to stand the vessel full of water. This is the normal design assumption (hydraulic test), but sometimes you can reduce the civil work, testing the vessel at the shop. I'm saying this because in an operation out of the routine, valves usually are not in the right position at the right time, you know.
Good luck; have a safe day
J.Alvarez