How to accurately predict distillation tower flooding?

[ghamsa]

How we can SAFELY increase the distillation tower throughput and, at the same time, don’t flood it. There are exiting pressure drop readings across the tower but the high-high alarm limits seem very conservative.

Other than performing actal tower tests, how can we check if the tower pressure drop alarms are really very conservative?

Thanks for stopping by.

 

[StoneCold]

ghamsa
You can do simulations on your column and set the simulation up to give you the same pressure drop as you are getting at your current opperating conditions. Then increase the flow rate until the unit floods in the simulation. That will give you an idea of what your max flow rate is. However the only way to really know is to increase the feed rate until the column floods. You might be able to fool around with the reflux ratio as well to try to get a feeling about how far the top of the column is away from flooding. The exact flooding point can be hard to track down because it can occur in a localized spot in the column. Maybe if we new exactly what your design conditions are someone here could do a simulation for you, assuming you don't have that capability where you are.

Good Luck

StoneCold

 

[katmar]

Ghamsa,

It is tricky to do this in a live test because as you increase the flowrate the pressure drop increases relatively slowly until the tower gets close to flooding (called "loading"). And then as it starts to flood the pressure drop can increase rapidly. At that stage it is hard to tap off and recover stable operation without badly affecting your product purities. In other words, rather start off by calculating the conditions - this will show you if you are a long way from flooding, but don't expect better than 10 to 15% accuracy.

You don't say whether it is a trayed or packed tower? The tray or packing supplier should be able to do calculations for you. If you don't have the software its a pain to do it by hand, although we used to do it that way 20 years ago because we didn't have an easy way out!

regards
Katmar

 

[steveen]

ghamsa,

Optimum tower performance is time dependent as energy and feedstock costs as well as product revenues move as a function of supply and demand. Most distillation columns built into the mid 80's were based on "ROT" (rule of thumb) -- Cheap energy drove most designs, while steel was viewed as expensive. Widespread use of computer programs allowed designers to move toward more "cost effective" designs with less design error allowances. Smaller diameters = less cost.
SO, increased throughput depends on a couple of factors.
1. Date of design of column.
2. Product purity. Can the targets be relaxed so a reduction in boilup/feed can be achieved.
3. Tower pressure. Does the relative volitility of the key components increase with decreasing pressure? If so, careful reduction in tower pressure can achieve higher throughputs. This is dependent on overhead condenser capacity and the rate at which pressure is lowered. Reflux rates must be cut concurrently or tower flooding can result. See Shinsky for a discussion on tower operating envelopes.
4. Optimization of feed temperature and quality can debottleneck a rectifying or stripping section. Rarely are both balanced.
5. How good is your control system?
6. Can the feedrate to the tower be reduced? Don't be myopic in debottlenecking. Lean oil systems and saturated gas plants can develop nasty external recycle rates.
7. Check out your pressure drop instrumentaion installation. Do the taps come off the tower directly or a overhead or reboiler outlet vapor line? Are the taps self draining?

Enough to keep you busy for a couple of weeks.
Good luck, If you want to discuss 713-374-7974.
Steve

 

[sshep]

Suggestion: Test your tower to find the flood point by increasing reflux rather than feed. This can be done gradually, is less risk to upset the rest of the plant, and can usually be quickly recovered from if you go too far. It is also something which you can usually just ask the panel operator to do without it being a big deal. Use the calculated tower traffic and plant data to estimate what sort of feed increase is possible. -sshep

p.s. By the same token there are many over-refluxed towers out there which are percieved to be feed limited. They could easily handle more feed but which are instead simply wasting energy. Most of these cases are just years of habit operating at the same point, because (as ghamsa points out) energy used to be of less concern.