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Hello All,
I am trying to define the optimum control scheme for a distillation tower at my plant. The tower is 92 tray (bubble cap) with the feed at tray 50. The top of the tower runs at 50 mmHg absolute pressure and 300 oF and the bottom runs at 465 oF. The temperature profile across the whole column is pretty linear (i.e. there aren't any really sensitive trays compared to others). The feed rate to the column is about 3000 lb/hr with the overhead flow being only 200 lb/hr and a reflux of about 10,000 lb/hr. The way the control is set up now is the hot oil valve to the reboiler is in manual, essentially fixing the heat input at the reboiler (I know it should really be set up with a flow/deltaT calculator block but we lack the instrumentation). The reflux is on level control and the overhead flow rate is taken in manual --> it is set up to be on temperature control with one of the trays, but since the temperature profile is so flat it causes the tower to swing wildly. We get a good split and don't really lose much material with this set-up, but I know we are really over-doing the reflux and reboiler heat duty.
Does anyone out there have a similar column -- what kind of control scheme do you use? How about any good references for literature regarding distillation control?
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Distillation Tower Control
Started by dehn0045, May 11 2007 09:55 PM
2 replies to this topic
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#2
Guest_yolofi_*
Guest_yolofi_*
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Posted 31 May 2007 - 02:31 PM
Since your heat input is being ran in manual, not much you can do about the reflux. About all you can do in the short term is ask your operators to cut back on the heat valve to get a reflux you think you need. A reflux ratio of 50 sounds a little high but best to refer to an ASPEN simulation or other data source. If your simulation shows a ratio of 25, I'd first have the operators cut back to a reflux of 7500. You should be safe and still get a high quality. Better to take small steps and prove to the operators you know what you are talking about than to make a big change and have it fail.
As far as where to control your top takeoff, if you still plan to go with temperature control I'd look at an upper mid thermocouple. They tend to be more sensitive than using the top column temperature. Your simulation or plant data should give you a good starting point.
If youf feed composition is consistent and if your temp profile is 300°-465°F, you should do pretty well. Make sure to tune your reflux drum level tight. You'll probably want to run a lower setpoint in the reflux drum as well so it speeds up the response. If you can calculate it, the speed of the temperature loop needs to be at least 3 times slower than your level loop.
On a really long term basis, you could look at automating the heat input. Could setup a really slow PI that would use the reflux flow or reflux ratio as the PV and slowly increase/decrease the heat input. You could consider column DP, but doubt you have much in a vacuum distillation.
Wish you luck.
As far as where to control your top takeoff, if you still plan to go with temperature control I'd look at an upper mid thermocouple. They tend to be more sensitive than using the top column temperature. Your simulation or plant data should give you a good starting point.
If youf feed composition is consistent and if your temp profile is 300°-465°F, you should do pretty well. Make sure to tune your reflux drum level tight. You'll probably want to run a lower setpoint in the reflux drum as well so it speeds up the response. If you can calculate it, the speed of the temperature loop needs to be at least 3 times slower than your level loop.
On a really long term basis, you could look at automating the heat input. Could setup a really slow PI that would use the reflux flow or reflux ratio as the PV and slowly increase/decrease the heat input. You could consider column DP, but doubt you have much in a vacuum distillation.
Wish you luck.
#3
aluma
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Posted 05 June 2007 - 12:50 PM
there is very good text I once came across:
Design of distillation column control systems; Buckley,Page S
Design of distillation column control systems; Buckley,Page S
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