6 replies to this topic

[bobbyfresh2703]

Hi everyone. 

I am having a "big exercise" of designing a distillation column and got stuck at some points. I have been searching and reading for some days but there has not been promising result yet.

 

Here's my condition:

 

Isobutane - n-Butane, continuous system

Pmax = 6 bar

Tmax = 56 oC

Vapor Vol. flow at top tray = 1933 m3/h

Liquid Vol. flow at bottom tray = 56.7 m3/h

Feed liquid flow = 11.47 m3/h

Overhead liquid outlet = 4.16 m3/h

Bottom liquid outlet = 7.3 m3/h

Reflux flow = 41 m3/h

Reboiled flow: not cal. yet

Tray spacing = 0.55 m for stripping part and 0.5 m for rectitying part

Diameter = 1.4 m

 

Please, anyone can provide me with some info. about these:

 

1/ I am choosing the nozzle size for feed, overhead vapor, reflux, bottom and reboiled vapor flow, but I really have no idea about how to choose it, whether I should specify the size of it and then calculate the velocity, for choose a rule-of-thumb-velocity and then calculate the size and choose the "nearest size" one (if it's right for the second choice, then what is the R-o-Thumb-velocity should we use?). And I wonder that the thickness of the nozzle should be less or more than the shell or " the most approximate" ( I think it should be appro. or less, esp. for the vapor nozzle b/c the velo. increasing leads to the pressure decreasing. Sounds reasonable? ).

 

2/ The materials for fabricating the shell and the internals for my binary system - the isobutane and n-butane. Suppose there is little dirty duty and little corrosion. Should we use high quality carbon steel (which contains <0.03% P and S each) or use the low alloy steel. It's really "darkening" and confusing reading books about material selection. Please, give me 1 or 2 specific suggests.

 

3/ My pro. told me not to put manways on the column because it will weaken the shell it's hydrocarbon fractionator at high pressure. But I think with my internal condition I chose, the temp. and press. are not too high for not using manway. If I'm right, can you suggest me how many manways and where to put them.

 

4/ The space to contain liquid at bottom and vapor at top head should be chosen by R-o-Thumb or we have a specification for that? They told me to choose but I don't think it's not that facile. i think it depends on the liquid and vapor flow at the bottom and top tray respectively.

 

Thank you very much, for at least reading my queries and I really appreciate your opinions.

Edited by bobbyfresh2703, 01 November 2014 - 11:50 AM.

[PingPong]

1/ Feed nozzle and reboiler return nozzle are sized on basis of a maximum ρ^.v² (rho v square). As one learns best the hard way, google to find maximum recommended values.

Overhead vapor outlet nozzle size  is same as connected vapor outlet line.

Bottom liquid outlet nozzle(s) are usually sized for max 1.0 - 1.5 m/s, or made larger if process piping diameter is bigger due to pressure drop reasons.

The nozzle is just a piece of pipe welded to the column. If the attached process pipe is X inch Schedule Y then the nozzle can also be X inch Schedule Y. In cases where the nozzle has to withstand a lot of stress there will be a reinforcing pad around the nozzle and it may be that a higher schedule than Y is chosen for the nozzle itself.

 

2/ Shell will be Carbon Steel with 3 mm corrosion allowance. Some clients may require fine grain CS (killed CS) for LPG.

For trays best use 12Cr (AISI 410), although some cheap contractors and/or cheap clients would use CS.

 

3/ Your pro exaggerates, 6 bar is not exactly high pressure. Even columns at 30 bar will have manways.

Normally one puts manways above the top tray, at each feed tray, at the column bottom liquid compartment, add additional manways in between such that there are nowhere more than 10 - 15 trays (number depends on client requirements) between two manways.

 

4/ Use (at least) 1 m between top tangent line and top tray. Use (at least) 1 m tray spacing at every manway location between trays.

Design of bottom compartment depends very much on the type of reboiler and the type of baffles used. Also depends on the liquid hold-up requirement (depends on whether there is a pump, on destination of liquid bottoms product, and on client requirements).

With a column diameter of only 1.4 m it is very likely that the liquid bottoms compartment best be designed with a bigger diameter.

Edited by PingPong, 01 November 2014 - 10:26 AM.

[bobbyfresh2703]

Thanks for your kind reply, PingPong. I'd like to have some other questions

 

1/ As you said, then I can freely choose the nozzle size if my nozzle satisfies the (rho*v^2) criteria? Can you give me an example for real plant velocity of vapor? (let's make it a reference).

 

2/ Can we just increase the height of bottom compartment instead of resizing the D, b/c I think it would be harder to fabricate and recalculate for this part?

 

3/ I am going to choose kettle type reboiler. So, I think I wouldn't need a pump for it b/c it would work as I set the level of liquid in the column equals the baffle height in the reboiler. Is it practical?

 

4/ Ahh, one more thing Do you know (or have) any valve tray dimension (or simply valve diameter) of Koch-Glitsch (the Ballast) or any supplier. I'm assigned that kind of internal, but I'v searched up-and-down the doctor Google, yet haven't found a single detailed size (maybe I must train my googling skill again )

 

Thank you again. I really appreciate your reply!

Edited by bobbyfresh2703, 01 November 2014 - 10:33 PM.

[breizh]

Hi ,

Probably good if you can get a copy of GPSA handbook ( chapter 19 ) .

Another resource could be Perry's engineer chemical handbook .

 

Hope this helps

.

Breizh

Edited by breizh, 01 November 2014 - 06:49 PM.

[bobbyfresh2703]

Thanks for your help, BREIZH. I'll read these carefully and in case I can't find answer by my own, maybe I'll ask you guys.

See ya soon, I hope

[PingPong]

1/ You can freely choose nozzle size as long as it is not too small. Obviously.

Different sources will give you somewhat different criteria for maximum ρ^.v² or maximum v. Obviously. This is not an exact science.Moreover the max values depend on the internal that may be attached to the nozzle at the inside of the column.

You could use ρ^.v² of maximum 2500 kg/m.s² for vapor inlets and maximum 5000 kg/m.s² for mixed phase inlets.

For liquid bottoms product outlet nozzle use max 1.0 - 1.5 m/s, for liquid oulet nozzle to reboiler use max 0.5 m/s.

In no case shall the nozzles be smaller than the connecting process piping.

 

2/ As you use kettle reboiler you would not need to increase bottom diameter.

However using different column diameters for different parts of a column is quite normal. Nothing special, no matter what your pro says.

 

3/ You don't need a pump for the reboiler, but maybe for the bottoms product, depending on the destination pressure for that liquid.

 

4/ I am not sure that I understand your question, but conventional valve trays have circular openings of 1.5 inch (38 mm).

[bobbyfresh2703]

Thanks for your kind, useful answer PingPong! 

I think I kinda get something

And that 38 mm is what I really mean.

Thanks again

Edited by bobbyfresh2703, 04 November 2014 - 04:54 AM.