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Hi there.
I am currently designing a bubble column reactor for my third year project. Its purpose is to chlorinate acetic acid to monochloroacetic acid. Due to a lack of information, my supervisor has suggested that I assume the reaction is instantaneous therefore mass transfer controlled. I have determined that the reaction is controlled almost entirely by the gas phase mass transfer rate, with the reaction taking place at the gas-liquid interface.
I have calculated a value for my gas side mass transfer coefficient (1.9027m3/s) and now need to determine the volumetric MTC using the interfacial area of the bubble surface.
My reactor volume was calculated to be 11.12m3. As a starting point I took a diameter of 1m and 2m to identify the difference in hydrodynamic properties so I can refine my final dimensions to minimise gas dispersion and back flow and maximise the interfacial area available for mass transfer. I am using a static gas sparger with 3mm perforations; d = 1m sparger has approximately 93000 perforation, d = 2m sparger has approximately 37600 perforations. I have determined preliminary values for the gas hold up for both cases and bubble diameter (below) which area required to calculate the interfacial area. the bubble diameter is constant as apparently gas velocity has little effect when a static gas sparger is used. This is where my problem lies. The interfacial area is defined as (6*gas holdup)/2. As is possible to see below, the gas hold up is lower for a larger diameter column (slower velocity)(which is descirbed in texts) therefore when calculating the interfacial area the value obtained is lower for a larger diameter column i.e. there is less bubble surface area available for a wider diameter column which has 4 times as many perforations in its gas dispersion plate! Can this be true?? Both columns operate in the heterogeneous regime so this could be due to variations in bubble diameter. Yet with a slower velocity (larger diameter column) the regime approaches the homogeneous area suggesting that the variation in bubble size is reduced. Is there and explanation for this or is it likely that my calculation is wrong?? I have checked several times and each of the values i have calculated seem to be correct as they follow trends that occur in other similar systems!
Gas hold up:
d = 1 m - 0.3724
d = 2 m - 0.2016
Bubble diameter - 0.003m
Interfacial area:
d = 1 m - 744.7419m2/m3
d = 2 m - 421.3144
Also, I have one other problem. There are a number of different definitions which require variations in velocity; velocity, superficial gas velocity, mean gas velocity etc. No text that I have come across defines any of these with regards to the system involved. Where do I base my values for these i.e. is the velocity the inlet velocity to the column, velocity through the gas sparger, through the column itself??Where is the mean velocity taken from?? And WHAT is superficial gas velocity?? I rave read numerous definitions and am still unclear which applies to this system!
PLEASE HELP!
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Bubble Column - Gas Hold Up/ Interfacial Area/ Superficial Gas Velocit
Started by Guest_pm06pb_*, Apr 01 2009 12:36 PM
3 replies to this topic
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#2
Guest_pm06pb_*
Guest_pm06pb_*
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Posted 01 April 2009 - 01:37 PM
Appologies, interfacial area = (6*gas hold up)/bubble diameter
#3
asanka124
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Posted 05 April 2012 - 08:40 PM
hi,It's sound like a problem for me also when I read your problem.But be clear that the inter-facial area you get is per unit volume.No matter how many perforations are there since the gas flow rate is same for both cases, the larger diameter one will has a lower superficial gas velocity.
I can barely help with your next problem,superficial gas velocity =gas flow rate/column cross sectional area , and you can take the mean velocity as terminal velocity.
I can barely help with your next problem,superficial gas velocity =gas flow rate/column cross sectional area , and you can take the mean velocity as terminal velocity.
Edited by asanka124, 05 April 2012 - 08:42 PM.
#4
Art Montemayor
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Posted 05 April 2012 - 10:27 PM
Pm06pb:
Our Forums - especially the Student Forum - have confronted and resolved this type of problem(s) many, many times. All you have to do is use our SEARCH machine, keying in the right key words - such as Brown-Souders equation. Or look up Souders and vapor-liquid separators.
Your professors are delinquent if they haven't told you about Mott Souders and George Granger Brown. These persons are the cornerstone of distillation column / bubble cap design involving the tower diameter. If you haven't heard of them to date, you have wasted your time.
Our Forums - especially the Student Forum - have confronted and resolved this type of problem(s) many, many times. All you have to do is use our SEARCH machine, keying in the right key words - such as Brown-Souders equation. Or look up Souders and vapor-liquid separators.
Your professors are delinquent if they haven't told you about Mott Souders and George Granger Brown. These persons are the cornerstone of distillation column / bubble cap design involving the tower diameter. If you haven't heard of them to date, you have wasted your time.
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