5 replies to this topic

[juan198]

I am currently trying to size a desulfurization unit, I am using zinc oxide to react with H2S. So far I have not found kinetic data that I can use in order to size a bed reactor.

 

My question is, how is a unit like this one usually sized in industry? and are there any sources that talk about this topic?

[breizh]

Hi ,

let you try the search engine in this forum, Art Montemayor posted a few documents related to this matter .

Good luck

 

Breizh

[juan198]

l have checked threads related to this topic where Art Montemayor had a mayor input, some information is very useful. I found the following statement related to sizing in one of the documents provided by Art;

 

"Typically, the size of the zinc oxide beds is calculated by applying a 15% (by weight) capacity for sulfur in the bed charge." I am still unsure what is meant by "15% capacity for sulfur in the bed charge". In addition he states that "a space velocity of 200 can be used", I would like to know how is this number calculated?, because for different temperatures the rate of reaction will be different, and this will cause a different space velocity. Or maybe those 2 statements are some kind of heuristic rules?

Edited by juan198, 24 April 2016 - 05:13 PM.

[gegio1960]

space velocity has nothing to share with rate of reaction.

15% is the margin, quantity of ZnO to be charged above the calculated weight.

good luck!

[juan198]

ok that makes more sense, and more people have told me to size it this way. My question becomes, why choose 15%? and I can infer that this extra amount of ZnO will ensure that the reaction comes to completion. But why not 20% or any other number. Are there any sources that explain the underlying reason. Because sure I can add an extra 15% of ZnO, but how do I ensure that the reaction comes to completion?. There might be some kinetic argument associated with it.

[Art Montemayor]

Juan,

 

I'll take you up on your word that you are trying to size a desulfurization unit using hot zinc oxide.  If that is a correct interpretation, then my comments follow:

• Sizing the "theoretical" size of the bed is not as important as employing techniques and proven capacity information that has been obtained empirically out in the field - not in a test lab or pilot plant, and much less in a theoretical calculation.
• The first rule in sizing process equipment - especially equipment that serves to safe-guard your process and serves as the underpinning of the entire operation (in other words, it has to work consistently, 100% of the time) is that the design must be proven and incorporate common sense for practical operation in the field.
• For a steady state flow process, you must ensure that you are removing almost 100% of all H₂S in the feed stream - even if the process selected is a fixed bed, semi-batch process.  For such a process, like the hot zinc oxide one, your common sense tells you that you must employ the beds in SERIES and remove the spent bed while only the one down stream is working.  For that, you must resort to factual field data that employs human factors, time requirements, etc. and employ proven, successful values for the Space Velocity and the size (quantity of Zinc Oxide) in each bed.  That is where the figures that I've given in my workbook come from.
• To design a hot Zinc Oxide reactor using analytical calculated methods is ludicrous.  It just doesn't work.  As engineers we have to use common sense related to the application which is over and above the theory employed to understand the process.
• There are no such Kinetic arguments or theories employed here.  You must allow an excess of theoretical amounts of Zinc Oxide in order to keep the process going.  The Space Velocity is related to the residence time of the reaction taking place and to the reaction's completion, attrition, channeling, bed movement, and distribution taking place.

Of course you can use 20% capacity - or even try to get away with 90 -100% capacity.  But before doing that, ask yourself some common sense questions: How do you ensure 100% removal of the H₂S?  How do you change out the Zinc Oxide and keep the process going?  How do you calculate an acceptable and workable fixed bed diameter?  Unless you have the capability (and money) to design, fabricate, install, and operate your own system and accept the consequences, I would go with proven, recommended field data that has been gathered by experts and furnished with confidence.

 

 Desulfurization by Hot Zinc Oxide.docx   44.74KB   58 downloads

 Hot Zinc Oxide DeSulfurizer.xls   124.5KB   51 downloads