[b] The liquid methanol blow out from stripper [/b]

[hazel]

Dear all,
we have the CO2 Stripper which stripes the CO2 from the Methanol by N2.
Recently its differential pressure is increasing gradually from 0.35 to 0.45 Kg/cm2 in past 2 months. Sometimes as the differential pressure increased quickly and reached 0.5 Kg/cm2, the methanol will spill out in the CO2 stream from the stack to outside area.
There are not any abnormal operator variables as the situation occurred, only the differential pressure increase and the leaving stripper flow rate(CO2 and N2)decrease.
Does anyone advice us how to solve the problem.
Thanks in advanced.

 

[Milutin]

Hazel,

Probably you have tray fouling and plugging, which caused tray flooding and liquid spilling from column top. Did you experienced tray fouling in column history?

Regards,

 

[hazel]

This stripper has 89 valve trays divided into two sections.
The overhead temperature is -58 degC and bottom temperature is -23 degC. From my experience on cleaning the stripper,the internal of the stripper is very clean. what reasons will cause fouling at such operated variable?
There has been an event occurred in this stripper, the downcomer was leaned to center side. If such damages took place again, will it cause the problem as I described?
how do I to fixed the problem?

 

[Milutin]

If downcomer, as you say is leaned to column center it can lose liquid seal and upward flowing vapors from tray below can enter downcomer and cause flooding from damaged tray to trays above.

Reason mentioning fouling is gradual increase of column dP during past two months. You should check process/equipment upstream of stripper is there everything is ok, because they can have some problems which cause problems to your stripper.
What is with your throughput is it increased or same as before?
How often you have sudden increase in dP to 0.5kp/cm2.

Regards,

 

[hazel]

Dear Milutin,
You definitely have good experiences on the column operation. The situation was exactly what you mentioned. One more problem is such flooding occurred in the lower section column will cause the liquid methanol blow out throuth the demister?
Why does it only occurred two times in past two months if the downcomer have damaged? Is it possible that the more downcomers continuously damaged?

I have checked the feed and overhead pressure control valve which are normal and the throughout kept the same as before. In the past two months, the dp reached 0.5 Kg/cm2 two times. To solve the problem, we usually decresed the methanol flow rate by the raised the level of the drum, operated at 18.5kg/CM2, before the stripper.
The stripper has a pressure control valve set at 0.7 kg/cm2.

thanks for your help.

 

[Milutin]

Hazel,


Trays are strong piece of equipment and you need upset to demage it, if you had upsets it is possible to destroy more trays. What upset caused earlier downcomer demage and did you have similar upsets?

You possibly operate stripper near flooding, (when you have problem you decrease throughput and solve problem), and slight increase in throughput can lead stripper to flooding.
Do you have NaOH and water upstream of stripper?


Regards,

 

[hazel]

In fact, I had no ideas in which upset to cause the downcmoer problem. In two years ago, there was an upset in this stripper when started up the oter unit. An operator opened 1",50kg/cm2 valve connected to stripping N2 line too much.

There are no NaOH and water upstream of stripper. Why did you ask for?

Best regards,

 

[STI666]

If you are having problems with the operation of the tower as described you may want to conduct a Gamma Scan of the tower to confirm or reject possibilities. From your initial question, yes it did appear that the trays were fouling off, after you mentioned the damaged downcomers and the upsets it may be mechanical damage affecting the flow of vapour and fluid through the tower.

If you have a Gamma Scan performed, the scan will help you determine exactly where the problem is and whether it is an operational issue or damage to internals. Gamma Scans are relatively inexpensive when compared to the lost time, production and off spec product produced trying to determine what and where the problem is. You should call a service provider or check out their websites to see what kind of information can be obtained.

Depending on your location, the response time may be very short and the pricing relatively inexpensive.

For more information on this troubleshooting technique see this website.

http://www.scanningtech.com

 

[Milutin]

Hazel,


It is good practice to keep records about column history, like such tray damage.
Tray position, bottom, feed or top tray, damage description, picture of damaged tray
And it is necessary to fid out what caused damage.
It can give you some clue in such situations.

I asked for NaOH because it can in some services cause fouling.


Regards,

 

[hazel]

The stripper bottom temperature decreased when the methanol spilled out. It is an endothermic reaction as the methanol contained CO2 flush into the stripper.

If there is a flooding in the stripper that it should accumulate more methanol in tower. Since the inlet flow rate is fixed and then the amount of heat taken away by CO2 should be fixed, then, the bottom temperature should be higher.
The other interesting phenomenon is the overhead temperature also lower a little.

Could someone help me to figure out a reasonable description?

Thanks!

 

[Milutin]

Hazel,

what is feed temperature?

Regards,

 

[hazel]

Hi Milutin,
There are three different feeds to the stripper.
The top one temperature rich co2 methanol is -58 DegC,the the middle one temperature riched H2S methanol is no indicator and the bottom one temperature lean co2 methanol is -30 DegC.
The top and bottom feed in upper section stripper.

From my openion, I thought the lower section stripper must have some problems to lower the efficiency and the CO2 did not completely flush out, therefore, it need to more energy to strip. When the stripping N2 temperture is low, the gas decreased volume can not penetrate the tray valve.
That is why the differential pressure is increased.
Now, I decrease the lower section feed to reduce the loading.