Distillation Column Sump Residence Time Basis

[jamesbanda]

I'm looking for advice on Rules of thumb on column sump design (and making sure i've not missing anything key)

I am looking at the basis to reduce the liquid volume in a column sump for safety reasons my system is a packed column without reboiler feeding another column which is trayed. These are large stable columns which dont change feed rate often. For planning permission showing a smaller sump on a toxic chemical production plant is very attractive..but equally putting something in that does'nt work is not acceptable too!

So, I can only seem to find stuff in Kisters book (or books that reference Kisters book!), anyone seen other guidelines/checks to be done..

Speed / control considerations (My guidelines)
> is the time from NLL to high level Trip allow the operator to start the spare pump. (plus margin)
> Is the time from NLL to dry runing allow the operator time to act and stop the pump (plus margin)


Operating conditions (Kister book)
1.Liquid is withdrawn by level control and feeds another column directly by pressur ~ 3.0
2.Liquid is withdrawn by level control and pumped away. Spare pump starts manuall ~ 3.0
3. Liquid is withdrawn by level control and pumped away. Spare pump starts automaticall ~ 1.0
4. Liquid is withdrawn by level control and feeds a unit that is some distance away or that has its instruments on different control board ~ 5 to 7
5. Liquid is withdrawn by flow control ~ 3 to 5
6. Liquid flows through a thermo siphon reboiler without a level controller to maintain a level in the sump ~ 1.0
7. Self venting critera( Kister has a chart for this)

 

[25362]

My suggestion: read the following threads and, in particular, follow Mr Montemayor's advice:

thread798-179623
thread124-159905
thread124-155681

 

[sshep]

Hey James,

I worked in a plant with some thermally sensitive products (color) which had reduced diameter sumps. This worked fine. There was a LC->FC cascade to compensate for disturbances and give better level control. This could work for toxics also.

It is always wise to have sufficient volume above the normal liquid level to account for a slump of the column. A couple of times I have worked in units which did not provide volume for liquid slump, and had small bottoms pumps- these were disasters. If the column tripped or lost heat and slumped, it would take an extended period (hours in one case) to pump the level down enough to allow a restart.

Kister's times seem optimistic for outside operator action. The ROT I used for a long time was 10 minutes. Anything less than 5 minutes was always a source of operator comment in our design reviews.

best wishes,
sshep