Distillation column temperature inversion 1

[mjpetrag]

I currently have a column that uses free 10# saturated steam injected in the column bottom as the boilup force. There are 22 trays in the bottom of the column, and two packed sections in the top. The column runs under vacuum at around 300 mmHg. Tray spacing is 12". Above tray 22 there is a collector and distributor from the packed section above.


The column gets a vapor feed of about 70 degrees C on tray 7 (tray 1 is the bottom and 22 the top). The steam is controlled by a temperature probe on tray 10 cascaded with the steam flow meter. There are 2 trays in the top above the packed sections with a 3rd chimney tray below which is used as the product draw off. The overheads product is controlled by a setpoint for the total column dP.

The problem we are currently seeing is that the bottom tray dP starts to rise from a normal value around 115 mmHg to about 200 mmHg. During this time, the packing dP drops off significantly. The control temp on tray 10 starts to climb, while the temperature probe on tray 4 and the bottoms temperature start dropping. The overheads product then goes off spec from stripping heavies. The steam control valve starts closing off since the temperature is above setpoint. It will keep pinching back and seems like it can never get to setpoint no matter how far it walks back. Then the whole column dumps. Even when we run the steam is local, eventually the dP in the bottom of the column will rise. However, the column has periods of steady operation where all products are in spec and dP is fairly constant.

It seems as if we are entraining up the bottom section since we are well above the dP/tray limit. However, no operating conditions have changed. We flushed the column with condensate for a few hours thinking we have some pluggage in the bottom, but it did nothing. Column scan is scheduled, however in the event nothing is found I want to poll some opinions of what could be going wrong

-Mike

 

[ash9144]

Yep you either have plugging/scaling or tray damage (maybe collapsed). Often doing a flush won't clear up pluggages since the flush material will just run around the scale build up.

 

[786392]

I also suspect that some mechanical problem of plugging or dislodging of trays exists.
basically

any under vacuum distillation system operation may be risky if under high system vacuum conditions;

If somehow wet (saturated with condensed water droplets)steam is injected without exercising due caution to make it dry prior to injection. The water expands to around 17000 times changing into steam.Sufficient force to dislodge trays or disorient

This condition may also have resulted from lower than usual ambient temperatures along with inadequate insulation of steam injection piping!

Another Very remote possibility could be some explosion with air ingress if volatile flammables liquids handled inside and air ingress completed so called fire triangle"

Both of these situations may render internal trays dislodged,disoriented thus higher dp!

Hopefully this helps in problem diagnostics& resolution!

Best Regards
Qalander(Chem)

 

[mjpetrag]

Column scan revealed the level was actually right at the steam inlet to the column, and this was after we lowered the level about 30%.

I'm thinking that when it was higher, we were sending slugs of steam/bottoms product up to tray 1, causing that tray dP to get high. Then liquid hold up would propagate up the column and we would jet flood the column

All trays were intact and showed good vapor/liquid disengagement and no signs of fouling.

-Mike

 

[786392]

Thanks for the real update!

Best Regards
Qalander(Chem)