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Vacuum Col


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#1

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Posted 13 May 2008 - 05:24 AM

What is the typical tray efficiency to be considered for simulation of vacuum column? 50-60% is OK? Thanks.

#2 Zauberberg

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Posted 14 May 2008 - 09:15 AM

Hello,

My recommendation is that you should not think about "general efficiency" in vacuum towers - it is something you will not find anywhere in practice. Besides that, there are plenty of different vacuum applications: crude distiller, solvent fractionation, petrochemical towers etc. Each of these services is different, design/operating parameters are different, column internals are different, K-values and physical parameters are also different, everything is different from case to case. And resulting tray/packing efficiency is different.

What you can do, is to try to calculate the number of theoretical plates based on your feed and product qualities, and existing operating parameters. This is an easy thing to be done. If it is a new design, you have to think about the type of internals, V/L profiles across the tower etc. - something that it is much more difficult. There are some published correlations in ChE literature (O'Connell's is the most famous one), but - if I was at your place - I would wait to see response from the vendor. As you are probably aware, even if you design a perfect fractionation system, there is no vendor on this planet who will guarantee your results if they are not based on their (proprietary) design calculations and manuals.

However, some general rules of thumb are available in process engineering literature, and Karl Kolmetz website is the place which can offer you more detailed information.

Best regards,

#3 riven

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Posted 15 May 2008 - 03:23 AM

I remember the design and built of a pilot unit and yes I certainly did not use efficiency factors. The advice above is what I would say.

The best additional advice I can give is to use Kister and ask the vendors when you have a design ready.

#4

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Posted 16 May 2008 - 05:12 AM

QUOTE (Zauberberg @ May 14 2008, 10:15 AM) <{POST_SNAPBACK}>
Hello,

My recommendation is that you should not think about "general efficiency" in vacuum towers - it is something you will not find anywhere in practice. Besides that, there are plenty of different vacuum applications: crude distiller, solvent fractionation, petrochemical towers etc. Each of these services is different, design/operating parameters are different, column internals are different, K-values and physical parameters are also different, everything is different from case to case. And resulting tray/packing efficiency is different.

What you can do, is to try to calculate the number of theoretical plates based on your feed and product qualities, and existing operating parameters. This is an easy thing to be done. If it is a new design, you have to think about the type of internals, V/L profiles across the tower etc. - something that it is much more difficult. There are some published correlations in ChE literature (O'Connell's is the most famous one), but - if I was at your place - I would wait to see response from the vendor. As you are probably aware, even if you design a perfect fractionation system, there is no vendor on this planet who will guarantee your results if they are not based on their (proprietary) design calculations and manuals.

However, some general rules of thumb are available in process engineering literature, and Karl Kolmetz website is the place which can offer you more detailed information.

Best regards,


Thank you. The purpose of simulation is to check the adequacy of existing crude vacuum column for higher t’put(130%). In the existing column, only top zone (up to gas oil) is packed section, and rest 45 nos of tray internals, operating at 20-40 mmHg A.

#5 djack77494

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Posted 16 May 2008 - 02:25 PM

spse,
I've found that you can't beat the advise offered by Zauberberg, espeically when it comes to Crude and Vacuum Units. I endorse his opinion that the number of theoretical trays is best set to match the measured or expected performance. Having worked on the design (revamps) of 3 Vacuum Units, I can state that a small number of theoretical trays (2 or max of 3) were used to model the performance of each important section in the Vacuum Column (i.e. the LVGO section, the HVGO section, and VTB stripping). Even with this, I suspect that 2 trays is often chosen, and this is done strictly for convenience. It lets you have a tray to "pull off" and to return pumparound liquids, and also provides for product draws and heat removal.

Simulators are great, but the tendency I've seen is to overuse them. They are General Purpose process simulators, and do NOT do a wonderful job at all things. I'd recommend that you use software that your tray vendor would be able to provide for the highest quality tray calculations.

45 trays in a Vacuum Column. WOW! I guess that's how they used to do things. Imagine the reduction in flash zone pressure you'd be able to achieve if the trays were replaced with packing.

#6 Zauberberg

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Posted 18 May 2008 - 09:14 AM

Having 44 trays in this service indicates lube-oil type of vacuum distillation with (usually) 4 side-cuts, if I got your statement correctly. However, for the purpose of tray efficiency evaluation this is not the primary concern.

I'd suggest you to go backwards, and based on avilable product yields and quality parameters (D1160 and viscosity curve, plus specific gravity - that should be minimum input data), reblend your products in HYSYS oil environment and simulate the tower untill you obtain the same yields and qualities. Independent parameter used: tray efficiency. You can model tray efficiency on a section-basis (between adjacent cuts), or on a single tray basis. For most distillation towers this should be a relatively easy task. However, for vacuum distillation of heavy petroleum fractions this type of simulation can become a nightmare.

Using PR package is not recommended for heavy vacuum applications (Braun or Esso are more preferred), but this strongly depends on the way how process streams (feed) is characterized. I found PR to be more reliable if heavy end of crude oil is defined properly - which requires serious and rather wide set of laboratory analyses which can be incorporated in simulation environment.

And now, the bottom line: you say that purpose of simulation is evaluating the possibility of increased column throughput. This is far more serious task and it calls for full feed characterization, in order to obtain reasonable physical parameters and vapor/liquid loadings of tower internals. Any incorrect asumption in this work will give you completely misleading results - so better prepare yourself for a hard job. Here are some references:

http://home.earthlin...nti/GVMFeed.pdf
http://www.ascenteng...0Incentives.pdf

#7 Zauberberg

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Posted 18 May 2008 - 09:31 AM

And look at these guys' excellent work as well:

http://www.revamps.c...cuments/199.pdf
http://www.revamps.c...cuments/173.pdf
http://www.revamps.c...cuments/167.pdf
http://www.revamps.c...cuments/169.pdf
http://www.revamps.c...cuments/197.pdf

Best regards,




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