9 replies to this topic

[A mukherjee]

Hi, currently I am designing a cryo distillation colmn of air flow around 12,000 Nm3/hr. The provisional diameter of the column is coming 1.38 m,but we had to decrease it to 1.35 m to avoid weeping in 50% turndown of air flow.Now with the reduced dia,the problem of flooding is arising in the column.so we had to increase the tray spacing from 100 to 150 mm .This is unnecessarily increasing the height of colmn by 2.5 m,as the no. of actual trays is 50.Can anyone suggest me any other way of sloving these flooding & weeping problem?Is there any good book regarding cryogenic distillation column design?

[Zauberberg]

Cryogenic towers follow the same rules and principles like other distillation columns do: there is absolutely nothing that makes them special except different material selection philosophy due to extremely low temperatures.

I would question your design because of at least two issues:

1. Tray spacing of 100mm or even 150mm is unreasonably low and impractical - you'll never see such tower in reality (at least I didn't). Common tray spacings are 540, 600, or 800mm, depending on several factors, and with 600mm being the most common.

2. There is something that should be suspected if only 3cm difference in tower diameter in both cases causes significant deviations of the overall tower performance. Also, unless it is very foaming service there is no justification for increasing tray spacing - the same effect (reduced weeping %) can be obtained by reducing tray open area, for example.

As for the literature, I'd suggest you to consult some of the following: GPSA Databook, Koch-Glitsch Design manual, Ludwig's "Applied process design", or some of Henry Kister's distillation books. In addition, there is a freeware package from KG that can assist you in tower sizing: http://www.koch-glit...re/default.aspx
If I were at your place, I would make sure to verify my design with the distillation column internals vendor, because it will come to that point in any case.

Regards,

Edited by Zauberberg, 11 July 2010 - 11:27 AM.

[Propacket]

A mukherjee,

• You have not described the type of tray. I assume that it is Sieve Tray because weeping problem is mostly associated with sieve tray.

• I agree with Zauberberg that your tray spacing is not practical. I have reviewed dozens of quotations from tray vendors and i have never seen such low tray spacing recommended by any vendor. The most common tray spacing is 24 in. Increasing tray spacing will help solve your problem of flooding. I don’t understand why you are concerned about the height of tower. You are not increasing the height of tower luxuriously. If design asks for increasing the tower height then you should do it.

• Weeping/flooding problems are also controlled by varying tower pressure. I remember a design where bottleneck was flooding. Increasing the tower pressure reduced velocity and eliminated flooding. Similarly, you can eliminate the problem of weeping by reducing tower pressure (increasing velocity). But you also need to consider several other factors before you vary the tower pressure. For example, reducing the tower pressure will also reduce reflux rate and at some point, your reflux drum may go dry.

Thanks

[katmar]

Cryogenic distillation column design does not follow the normal economics of column design. Heat gain from the atmosphere is the over-riding consideration. Columns are kept as short as possible to minimize the surface area for heat transfer to the atmosphere. This means having small tray spacings that would not be considered economic in petrochemical service. This is not a design you should be doing in-house if you do not have specialist knowledge and experience.

[Zauberberg]

I have seen, inspected and/or operated several cryogenic towers, mostly De-C2 towers and the tray spacing was 600mm. To what kind of cryogenic columns are you referring, Harvey?

[katmar]

I was referring to air separation columns. See the reference to typical tray spacings in the following patent
http://www.patentves...t/04871382.html

[Art Montemayor]

"Is there any good book regarding cryogenic distillation column design?"

Well, in 1966 McGraw-Hill published "Cryogenic Systems", a book by Randall Barron - at the time an associate professor at Lousiana Polytechnic Institute. This was, for me, the one true book I could use and refer to when I was working in the Compressed Gas Industry and Cryogenics. You may still be able to find it or borrow it.

It won't tell you any recommendations on tower tray spacings or other mechanical details such as weir heights, downcomer areas, tray efficiencies, etc. But it does discuss cryogenic process design and how it is carried out (even though it was designed for undergraduate mechanical engineering students). It discusses all the major cryogenic processes in detail that were known at that time - pretty much of what we have today.

[katmar]

My previous response was not very helpful to the OP, so let me try to give a bit more detail. Although tray spacings of 24" (600mm) and more are typical in petrochem work, there are applications other than air separation that use low tray spacings. Especially in years gone by when monster hydraulic cranes were not readily available it was not easy to build and erect tall columns. In small potable ethanol distilleries it was not at all uncommon to have 5 or 6" (125-150mm) tray spacings. In applications where the liquid to vapor ratio is high and the vapor velocities are low you can get away with these close spaced trays.

It is also not essential to jump from 100mm all the way to 150mm tray spacing just because those are the two lines shown on the design graph. If you are reducing the diameter from 1.38m to 1.35m that is only a 4.5% increase in vapor velocity and you should not need a 50% increase in tray spacing. If you can get away with a 110mm tray spacing it will have a much smaller impact on your column height.

You should get specialist advice from the tray suppliers. I suggest trying suppliers of fixed valve trays such as Kuhni or Koch as these trays can induce less entrainment in some applications. But I still believe that air separation is such a specialized application requiring empirical knowledge that I would leave it to the established suppliers.

[Propacket]

Six months back, i got quotation from a renowned tray vendor for a cryogenic distillation column where minimum temperature was around -90°F. It was a large column with 4 ft diameter and 70 ft length, so large surface area exposed to atmosphere. The vendor offered 24 in tray spacing. Heat loss to atmosphere was mitigated by insulation.

[A mukherjee]

Thnxxx all for replys...The column I was talking about have arnd 12000Nm3/hr of air flow...the pressure drop I was getting arnd 3.8 mbar in each tray.WE use sieve trays of hole spacing arnd 1mm...I know u'll be surprised by knowing such small hole dia. & tray spacing...butwe areusing these kind of trays & column for atleast last 7 years & never faced any problem..I would be grateful to u guys if u can kindly provide the name or link of any source where I can get to know about the detailed design of Cryogenic distillation column..Thnxx in advance..
Arpan