3 replies to this topic

[JamieG]

Hello - I am a process engineer working in specialty chemicals. I am installing two new distillation towers with associated equipment. The towers will both run under vacuum. One will run at 100 mmHg and the other at 20 mmHg. I have gone to Graham for condenser design as I understand that they are well suited for vacuum condenser design...they are also the most expensive. They have suggested that I consider using a hot-well or boot on the condensers in lieu of accumulators. They claim that it eliminates the need for sloping the piping to the accumulators (sloped to prevent inerts from accumulating) and also saves the cost of the accumulators. They also claim it to be a common practice.

My question is, have any of you encountered this design? Did it work well? I anticipate control issues due to size limitations of the hot-wells. One other point...reflux ratios are fairly small for both towers: ~1 - 2.

Thanks in advance for any help

[PingPong]

If you are absolutely sure that there will always (also during start-up) be only one liquid phase present, then you can simply have a boot on the condensor, like proposed by Graham. You then need to specify to Graham what hold-up time (hold-up volume) you want in that boot, depending the control systems, on whether the reflux is pumped or flows back on gravity, and on the destination of the pumped distillate product (storage, or another column, or a reactor).

 

If you can have two liquid phases (two immiscible hydrocarbon liquids, or one hydrocarbon and water), even if that is only at start-up or during an upset, then it may be too complicated to accomodate proper liquid phases separation in just a boot.

 

If have seen condensors with boots as compressor interstage and aftercoolers where a small amount of condensed liquid has to be separated from the vapor.

Also steam turbines with vacuum condensors at outlet use a hotwell instead of a separate vessel.

[xavio]

Jamie G,

 

Yes, the boot is a feasible design.

The key consideration is the amount of condensate.

The boot should provide sufficient hold-up volume for the chosen control system.

For example, you may consider 5-10 minutes of hold-up time in case of reflux/top product pump trouble.

Translate the required time to volume (diameter and length) and you will see if your boot size is suitable for your condenser dimension or not.

 

Good luck.

 

xavio

[JamieG]

Thank you both for your responses. In addition to any controls issues, I also envision the operators saying "WHAT?! No accumulator?". And from that point on, any operational problems will be blamed on this