column steam load

[wfontjr]

I have a distillation column with a thermosiphon reboiler, the steam to the reboiler is controlled by the level in the column. Prior to a shutdown and inspection of the column if you decreased the feed rate the steam rate would decrease, after the shutdown regardless of feed rate (250 gpm vs 400 gpm) the steam rate is about the same. The steam valve output remains about the same so its not an indication problem. Temperatures on the column increase at the lower feed rate. I have verified the level in the column by a sight glass.

How is this possible?

 

[10815L]

Hi,
With out P&ID Difficult to trouble shoot this problem.

 

[CH5OH]

to me,the temperature probe is no longer measuring the feed out from your boiler, it might hang loose next to his pocket. so steam will be at max flow, regardless the feed temperature...a lower feed rate of will result in a higher temp in the column

 

[wfontjr]

My question is more how can you cut the feed rate by 40% and the steam rate not cut back on level control. Is there a physical problem with the column, anyone have any similiar experiences.

 

[SeanB]

The level in your column should be nearly constant - regardless of your feed rate. What you are stating for control does not make any sense to me. Your reboiler should be controlled on temperature. Normally it is controlled by a tray temperature, your bottoms temperature, etc. to meet a product specification.

 

[wfontjr]

this column is doing a rough split of C4's and C6's, not in a refinery, the level is controlled by a cascade loop to the steam to the reboiler.

level is kept constant by varying the steam rate

pressure control is a hot vapor bypass of the overhead condenser

reflux rate cascaded to feed rate

bottoms drawoff cascaded to feed rate

 

[SeanB]

Can you attach a P&ID?

 

[wfontjr]

sorry no

 

[FMJalink]

wfontjr,
With the little information available, the risk to give a wrong reply is substantial. You state that this effect happens at start-ups and not at shut-downs. Also you mention an increased temperature. You do not mention the time this effect continuous.
Unless you know the feed composition exactly, it appears to me, as SeanB already pointed out, that you will require temperature or pressure information for control.
A shift of the C6 concentration in the feed during start-up would explain the increase in temperature and continued liquid level.
With the reduced feed rate (40%) at start-up and not cut back steam rate (assumed for 100% feed rate) the heat will have to go somewhere. Unless required during pressurizing the column, this will most probably be removed by the condenser leading to an increased reflux rate.
An increased reflux rate at start-up could have been used in the design to achieve stabile operation of the column prior to start of draw-off.

 

[chemebabak]

wfontjr, I understand that you want to keep the P&ID confidential.
Draw a sketch of the column with level controls to give engineers enough information to help you troubleshoot.