11 replies to this topic

[timiaz]

Hi,

 

I currently designing a C3 distillation column for my design project, however it is not pure propane-propylene, and has H2, C1, C2s etc.

 

I was wondering what was the best way to calculate the K-Values and thus the relative volatilities of the system? I am working at 16 bar and 40C.

 

Is there any point taking the H2, C1 and C2s into account when designing the column, or should I just take into account of them when designing the condenser?

 

Thanks for any help given

 

Regards

[marchem]

probably a EOS such as Peng Robinson with specific parameters for H2 (and some other components if original alpha doesn't provide good results) can provide reasonable accuracy,

for incondensables you may adopt a partial condenser to avoid very low condensation temperatures.

[PingPong]

I currently designing a C3 distillation column for my design project, however it is not pure propane-propylene, and has H2, C1, C2s etc.

What is the purpose of the column?

Is it a propylene propane splitter?

Or is it a de-ethanizer to separate H2, C1, C2s from the C3's ?

[timiaz]

Pingpong:

 

Hi, I was thinking it could do both.  My supervisor suggested that the H₂, C₁, C₂ and High grade Propylene go through the condenser, where the H₂, C₁ and C₂ come off in the top whilst the propylene goes out in the side stream?

 

I don't know if that's actually feasible?

 

Thanks

Edited by Art Montemayor, 23 February 2014 - 12:51 PM.
Grammar, symbology

[PingPong]

Some distillation columnms have a so called pasteurization section to separate any lights from the distillate.

 

In the sketch below you see a C3 splitter (consisting of two columns only because of the large amount of trays required) with a pasteurization section between the lights in the vent gas, and the high grade propylene product drawn from tray 12

 

 

It depends on the amount of lights in your column feed whether this will work in your case.

 

The relative volatilty (alfa) of Propylene versus Propane is very small and moreover depends on the pressure and liquid composition on each tray, so alfa varies from top to bottom in the column. You need a reliable source for alfa values, or use a process simulator with proper thermo data for this separation.

 

For the pasteurization section the alfa between C2 and Propylene matters.

[timiaz]

Some distillation columnms have a so called pasteurization section to separate any lights from the distillate.

 

In the sketch below you see a C3 splitter (consisting of two columns only because of the large amount of trays required) with a pasteurization section between the lights in the vent gas, and the high grade propylene product drawn from tray 12

 

 

It depends on the amount of lights in your column feed whether this will work in your case.

 

The relative volatilty (alfa) of Propylene versus Propane is very small and moreover depends on the pressure and liquid composition on each tray, so alfa varies from top to bottom in the column. You need a reliable source for alfa values, or use a process simulator with proper thermo data for this separation.

 

For the pasteurization section the alfa between C2 and Propylene matters.

 

That's been very informative thank you,

 

Okay, well with my rough design, i have 50 mol% h2,  to separate out of the feed.

 

So for this it's not really possible to do the design by hand? I am being told and wanting to design this using a mass balance tray by tray. However is this possible?, and also on top your diagram has 325 trays, but  at my T&P (40c, 16 Bar) my rough design gets only to 120? which should only be a single column?

 

THanks for help so far

[PingPong]

your diagram has 325 trays, but  at my T&P (40c, 16 Bar) my rough design gets only to 120? which should only be a single column?

Those 325 trays were actual trays, not theoretical, to produce polymer grade propylene. Theoretical trays were 214 in this particular case, including the pasteurization section, so about 196 for C3 splitting.

 

How many theoretical stages you need depends on your feed composition and your propylene specification. How many actual trays that are depends on the type of trays you use. Whether that number will fit in one column depends on the diameter of the column, or better: on the L/D ratio.

 

i have 50 mol% h2,  to separate out of the feed.

If the feed to the column contains as much as 50 mol% H2 you should probably remove that in a separate higher pressure column upstream the C3 splitter, otherwise you will lose a lot of propylene with the vent gas. In any case you should include a vent condensor (cooled by a refrigerant) like in the drawing.

[PingPong]

A well known correlation for relative volatility of Propylene/Propane is that of Howat and Swift.

I don't have the correlation but one can use the graph below, which should be based on that:

 

 

Attached Files

•  Propylene-Propane Relative Volatility by Howat-Swift.jpg   210.76KB   8 downloads

[timiaz]

A well known correlation for relative volatility of Propylene/Propane is that of Howat and Swift.

I don't have the correlation but one can use the graph below, which should be based on that:

 

 

I have all the correlations from Howat Swift, but there is no way of getting a TXY diagram from that?

[PingPong]

What would be the point of a T-x-y diagram?

 

Use McCabe-Thiele in an x-y diagram.

[timiaz]

Thanks, having gone through all my calculations is a 12m diameter column feasible?

[PingPong]

It is feasible for shorter columns, like vacuum distillation columns, but I dare not say for sure whether it is also feasible for a very very long propylene splitter.

 

Strange though that for a student design project you would choose such a large column feed quantity that you get such huge column diameter. Who not simply half the design capacity of the unit in your project so that all equipment becomes smaller? The unit capacity that you chose is just an arbitrairy number anyway, right?