Effect of support rings on separation efficiency of random packing 1

[DSulmon]

Hello,

We have recently modified existing distillation columns by replacing the sieve trays they contained by random packing.
Each column contained 16 sieve trays. In each column, the plates were replaced by 5,6 m in height of random packing (Raschig Super Ring 1.5). The HETP of this packing is according to vendors lower than 590 mm.
The rings supporting the trays were left in the columns to reduce modification costs. Although we did not expect the rings to interfere with the separation, we accounted for a 10% loss in efficiency due to their presence, and used thus for our calculation a HETP of 650 mm.
Based on these numbers we expected between 8 and 9 theoretical separation stages. Separation was thus expected to be comparable to previous operation since the existing 16 sieve trays gave us between 6 and 10 theoretical stages depending on the tray efficiency (40 to 60%).
We now however discover that the separation efficiency is such that the light concentration in the bottom is much worse than expected (1000 ppm level vs 100 ppm expected). According to Aspen calculations, only 4 theoretical trays are present, i.e. 50% of the expected amount. Scans of the columns and visual observation show everything to be fine.
We are currently investigating different possible causes.
One possible explanation is that the support rings left inside the column cause by-pass of a small amount of the liquid feed which ends up in the bottom effluent, thus increasing significantly the light concentration in this stream. Do any of you have experience with such phenomena? I assume that this is not the first time that sieve trays are replaced by random packing.

Thank you for the help!

 

[sshep]

Failing to remove the support rings is in general a mistake. Not only does it reduce the effective tower diameter and capacity, but it can cause several distribution problems from initial packing loading to actual operation.

At this stage your troubleshooting has not positively identified the column section itself as the problem. We have all seen cases where leaking exchangers, controls, bad levels, etc gave the illusion that something is wrong in the column. You should continue to chase down these things.

Before jumping to any conclusions the tower should be tested at higher (and lower if you dare) reflux to insure column seperation behavior is normal. A straight back mixing problem is usually insensitive to reflux. At this point your simulation tool can only used to identify how the column should be operating if it is operating normally. If your 4 Aspen stages include the reboiler and condenser, then the performance sounds unrealistically poor because your packing in that case represents only 2 stages of seperation- barely more than a flash. Trying to force fit a simulation to match a broken process rarely provides useful troubleshooting information- better to compare actual temp profiles vs simulation, ect.

Best wishes,
sshep

 

[25362]

Packing efficiency may decrease by a factor as high as 2 to 3 due to maldistribution (Kister).

Some questions:

1. What is the service(s), corrosion products, coking ?
2. Diameters of towers ?
3. Support plates or grid types, how many sections ?
4. Liquid distributors ?
5. Bed limiters ?
6. Reflux load, gpm/ft²?
7. Boil up: reboiler, stripping steam ?
8. Pressure drops ?
9. Filling procedure ?
10. Towers' verticality ?
11. Super-rings plastic or metal ?

Thanks.

 

[unclesyd]

Where is the feed point on the new packing?

 

[katmar]

A general problem in packed columns is that any liquid that migrates to the column wall remains there, and sometimes redistribution rings are deliberately introduced to take the liquid off the wall and distribute it back into the body of the packing. In this respect leaving the rings behind could be regarded as a good thing.

But you need to look at the actual numbers. If the 16 trays were spread over the same height, the tray spacing would have been about 350 mm. This is much closer together than you would normally intstall redistribution rings. You also need to consider the relative sizes of the rings and the column ID. If the rings are 40 mm wide in a 3000 mm ID column it is a very different scenario from an 80 mm wide ring in a 600 mm column.

The liquid flowing down the wall will be brought back into the packing by the rings, and then it will gradually migrate back to the wall. This means the packing in the area directly under the ring is not wetted, and will have a lower pressure drop than the bulk packing. This will result in gas bypassing the central wetted section and will definitely cause a drop in efficiency. The extent of this loss of efficiency depends on the relative sizes of the rings and the column.

regards
Katmar

 

[121202]

HETP is not a fixed quantity soley related to the type and size of packing used. It is a function of the physical properties of the fluids and their flowrates\velocities through the packing. Unless you used estimates based on towers in the same service operating under similar conditions then the values could be meaningless.

 

[DougJ]

I find it difficult to believe that leaving behind support rings is a significant contributor to this problem. Sounds like you have a major disconnect between actual and expected. For a packed tower, that frequently has to do with liquid distribution problems. Suggest looking there.
Good Luck.

 

[mrtangent]

Hi,

There is plenty of sound advice above. One think just to check is have you performed a 4 chord scan. We found to our determent that scanning companys dont always scan what you need they scan what you ask (not all just some).

For packed columns and for mal-distribution you must scan like Noughts and crosses grid. The attached article explains this.

http://www.eptq.com/Pages/Articles/Sp0212.htm

Another thought. I had is how do you control pressure , top/bottom. Sometimes with packing you have less DP per meter. Therefore you think you have not changed your feed profile but actually you have. So, because of the packing and the pressure at the feed stage are you now vapourising / subcooling your feed. An unexpected vapour feed can blow the contents of a liquid distrubtor tray and cause loss of efficiency.

Good Luck

James

 

[siretb]

You are dealing with trace amounts, and not much of maldistribution can account for poor results. How sure are you that distribution is even on the top of each bed?
Do you have a collector under the top bed?
I would certainly consider as a mistake to have left the support rings.
Please answer the points that 25362 is requesting.
Also, did you use the same column for the same separation whaen it had trays. you mention that you have 1000ppm instead of 100ppm expected, but do you have actual separation data with trays.
I have one experience with much the same situation, we were expecting also about 100 ppm lights in bottoms, we got much more, but the problem came from inadequate modeling of the ELV.