6 replies to this topic

[Arvind Iyer]

Dear Friends,

I recently saw a distillation column that had two reflux condensers on the ground floor and the reflux receiver drum on the first floor(9.0 m). Please see attached the rough sketch.

I am used to seeing a condenser above the reflux drum.
Please help me understand how the scheme works. I am wondering how the liquid flows from the ground floor up to 9m into the receiver drum.

What are the design considerations behind this scheme?

Thanks for your kind help.

Additinal info: Conderser duty is 10 MWatts. The normal flow of fuel gas from the reflux drum is 500 kg/hr.

Best Regards,
Arvind

Attached Files

•  column.JPG   106.31KB   76 downloads

[Art Montemayor]

Arvind:

I don't understand the purpose of your query or the way you present it to us. You state that you have "seen" this arrangement, but yet are able to give us:

• The height of the reflux receiver;
• The condenser duty;
• The identity of the fluid;
• The normal fuel gas flow rate;

However, you don't tell us:

• If there is a reflux pump (you don't even mention this, or the fact that one is normally used);
• The vapor pressure at the top of the distillation column;
• The pressure at the top of the reflux receiver.

Is this a test for us? Or is this a subject that you would like to form a discussion group on and analyze those components that you don't understand? The reason I ask this is to form an idea of what you are seeking. If you were able to gather the fuel gas flow rate (whatever that flow is), the condenser duty, and the identity of the fluid, you certainly could gather much more basic data that would be helpful in analyzing what is taking place in this this Unit Operation and how it is probably controlled - if at all. Can you furnish us with some background information - such as site, process, size, and more basic data?

I think this subject has been discussed before on this Forum, but it still is an interesting application of Fluid Mechanics and probably of interest to some of our members - or maybe already known to them. But it would help if we had more basic data - if this is a real-life application and not an academic study. Await your reply with more data.

[Arvind Iyer]

Dear Art,

I had seen this column a few days back and the information I provided were based on my
interaction with the area operator and the P and ID of the column.

To your questions:-
•If there is a reflux pump (you don't even mention this, or the fact that one is normally used);
--Yes. There is a reflux pump. The usual set-up, nothing different.
•The vapor pressure at the top of the distillation column;
--about 868 kPa(g)
•The pressure at the top of the reflux receiver
--I do not know this sir.

Addiitional details from PFD:-

Column Feed:-
Feed Rate: 12 ton/hr
Temp.: 42 C
Pressure: 1200 Kpa(g)
Phase: Liquid

Column Top:-
Flow Rate: 80 ton/hr
Temp. : 69 C
Pressure : 868 kPa(g)
Phase: Vapor

Outlet of Reflux Condenser:-
Flow Rate: 80 ton/hr
Temp. : 46 C
Pressure: 806 kPa(g)
Phase: Liquid

Reflux Flow to Column:-
Flow Rate: 62 ton/hr
Temp. : 46 C
Pressure: 1206 kPa(g)
Phase: Liquid

Overhead product:-
Flow Rate: 7 ton/hr
Temp. : 46 C
Pressure: 1206 kPa(g)
Phase: Liquid

Vapor to fuel gas header:-
Flow Rate: NNF as per PFD
(Note: I made a mistake. I had mentioned this figure as 500 kg/hr which is what I gathered from the
field operator)

Pressure control is based on the two PVs: one on the condenser bypass line from column to the
receiver and another PV on the line to fuel gas header

System consists of a feed of butene-1 , lighters and Butene-2. and this column separates
butene-1/lighters as the top product from Butene-2 at the bottom.

Thanks for your kind help.
Arvind

[djack77494]

Arvind,
This is not an uncommon configuration and things are arranged this way for very practical reasons. While it is possible to locate condensers "way up in the air", it is expensive to do so. Furthermore, this practice makes it more difficult to maintain these exchangers, and is viewed as "unacceptable" by some maintenance departments. Better practice calls for the exchangers to be located at grade where maintenance is easier and costs for supporting the exchanger are minimized. Of course, the accumulator or reflux drum must still be located above grade since it contains a flashing liquid and sufficient elevation is needed to feed a presumed reflux pump. Obviously, when you configure your system this way, you give up the convenience of gravity flow of condensed liquid to the accumulator. This is not "a killer" however; it just means you need to pay more attention tot he system's hydraulics.

[Arvind Iyer]

Dear Sir,

Is there any process constraint/reason that could favor such a configuration?

Best Regards,
Arvind

[SSWBoy]

Judging by the size of the bypass line i'd say this is probably an example of hot-vapour bypass.

For the sake of argument imagine the pressure at the split between the line to the condenser is 10 Bar, the bypass line could then (again plucking reasonable numbers) have a pressure drop across the CV of 0.3 bar and across the rest of the line 0.05 bar (in any case you'll want to have the majority across the CV to get a more stable process). Therefore the pressure of the reflux drum would be 9.65 bar.

As we can see we now have a differential pressure between the tie-in to the bypass line and the reflux drum itself. Now let us move along the main route to the condenser, imagine we have a pressure drop across the condenser of 0.15 bar, this leaves us another 0.2 bar to get to the reflux drum (ignoring line loss's for the sake of simplicity). This 0.2 bar is what we lose in terms of static head.

If column pressure goes up then the control valve will close, raising liquid level in the drum and exposing more tubes in the condenser ==> higher duty. Conversely if the column pressure drops the control valve will open, forcing down liquid level in the drum and conversely submerging more tubes in the condenser ==> more sub-cooling and less condensing.

This is as noted, a tricky system to design. The condenser itself must be designed for sub-cooling else the liquid will flash as the pressure drops. If we get the elevation of the drum too high then there is a chance that there is not enough sub-cooling and the inlet pipe may choke. If the elevation of the drum is too low then there will not be enough pressure drop across the bypass line to control the process adequately.

As you note you have a 10MW condenser, I expect this to be fairly large, therefore for ease of maintenance, construction etc. it is easier to mount it at grade.

Now a question for you, does this drum have insulation?

[Arvind Iyer]

Yes. IH/25 type insulation is provided on the reflux drum.

Best Regards,
Arvind