vacuum problems in stripping column

[pbrod]

Dear all,

We have a stripping colum for stripping alcohol and water out of phthalates. It's a two section column with a steam ejector (500kg/u) between the two sections to reach a vacuum of 10mbara in the bottom and a serie of three steam ejectors using 8bar steam care for the vacuum. The top condensor is a lammela heat exchanger using 15°C water (can go up to 20°C). In the input is preheated to about 130°C and is fed in the top of the stripping column.

The problems.
We encounter pulsating vacum with a 150-200mbar amplitude on the colum. And when the vacuum reaches the 200mbara then it can be suddenly broken.
The strange thing is that the vacuum comes back faster when it pulsates than when it is buildup from start.

I know this is a very broad question but I hope that someone can give a good hint to solve the problem.
We already did some tests and found some minor leaks, If ok I won't post this information just to keep everything open.

 

[ash9144]

have you checked your pressure taps and lines to the pressure gauge for liquid or pluggage. Are you sure the pressure is swinging in the column?

 

[pbrod]

This should be ok. we have different pressure gauges all indicating the same trend. We also see from the product quality that we do not have enoug vacuum.
thanks for the tip

 

[TD2K]

Has this problem just started or is this a new column you are starting up?

Are the pressure taps on different areas of the column (overhead, mid, bottoms)?

If everything on the colume shows the same loss of vacuum, I'd start putting some gauges on the ejectors and checking them out. Ejectors do not have much additional capacity on the upside above their design discharge pressure so piping hydraulic losses are important. Alternatively, have you checked out the steam traps to be sure you don't have wet steam to the ejectors or for the stripper steam?

 

[Fzob]

Have you checked the steam eductor nozzles? Once they start to wear you can lose diff P and or overwelm a condenser.

Also see if large amounts of inerts may be entering the column.

 

[pbrod]

- the column is an old column and was out of service for about 2 years. We recently restarted for a 10 day campaign. Then we had troubles reaching our 60mbar in the top and saw the swings.

- we see the vacuum drop on all the gauges and measurements.

- the nozzles were checed for damage and were measured. I will check this before we start testing this afternoon.
there are some taps on the ejectors I will try to have gauges installed.

- the steam is made from 10 bar steam in a local heat exchanger so it should be OK but I will recheck temperatures

 

[pbrod]

we found a leak on a corroded tube between the ejectors and the top condensor.
Now this leak is isolated from the column and if the column is set on a pressure of 400mbarg it drops to 318mbarg in 15 minutes.
With the leak the pressure dropped from 400mbarg to 180mbarg in 15 minutes.

the problem is that a few days ago we isolated the part with the leak from the column and then we had a good vacuum on the part with the leak.
So we are not sure that we found the problem yet.

 

[pbrod]

just for everyones info, biggest problems were leaks and condensate draining of the ejector system.

thanks for the help all!

 

[TD2K]

Glad it's working now