27 replies to this topic

hi all

iam designing hydrogen production plant i want to design adsorber for desulfurization unit in which H2S adsorbed into ZnO catalyst
as following reaction shows
H2S+ZnO=ZnS+H2O
i have operating conditions and flow rate of H2S
i need rate of reaction and its parameter(Keq) so i can calculate catalyst weight

please help me

[Art Montemayor]

Ameer:

I think that you are a student at Sultan Qaboos University doing a homework assignment that involves the design of a ZnO reactor – not an adsorber. I also believe this thread belongs in the Student Forum and not here, in the Professional Petroleum Refinery Forum. That is why I am moving it to the Student Forum where you can get more constructive help for your level of knowledge and training. Here, you are in very deep water and you don’t have an environment conducive to tolerating your lack of understanding certain engineering principles.

For example, what you are describing is simply a ZnO desulfurization unit used conventionally in front of main processing units such as steam reformers in order to protect the expensive catalyst used in the latter. The ZnO acts as a consumable REACTANT and is NOT AN ADSORBENT. Therefore, you are wrong in your description, even before you start. You also make the mistake of calling the ZnO a catalyst – which it obviously is not, since it participates in the chemical reaction you have shown us in your equation!

Since I also had to go through a learning lesson with respect to this process many years ago, I am attaching a workbook that explains how the process is designed and how it works in the real-life industrial world. I applied the process to steam reformers for the production of pure hydrogen. I also have a specification sheet for the vessels, but I can’t find it right at this moment. If I find it in the near future, I’ll attach it to the workbook because that is where it rightfully belongs.

I hope this helps you understand how the chemistry, the logic, the reasoning, and the engineering all work together to bring about the desired results.

 Desulfurization_by_Hot_Zinc_Oxide.xls   903.5KB   1383 downloads

Sir Art Montemayor

Thank you for passing
I know that this should be a REACTOR and I'm dealing with it as a packed bed reactor. I wanted to find the weight of the bed but I do not have rate of reaction. I need also the whole sizing procedure for the reactor if possible.

Why did I call it adsorption unit?
I have read in many sources that this unit is called adsorption unit and that hydrogen sulfide is adsorbed over ZnO catalyst! Can you comment on this.

[Art Montemayor]

Ameer:

As a future Chemical Engineer, it is your duty to master all the chemistry courses you took in high school and those that you have to take in university. You have not filled in your personal profile with your background information so I can only speculate that you are a university student at Sultan Qaboos University. If I am correct, then you must complete approximately 2 courses of General Chemistry, 2 courses of Physical Chemistry, 2 courses of Organic Chemistry, a course in Qualitative Analysis, and another course in Quantitative Analysis. These, I believe, are a minimum requirement. Therefore, you should already know that what you are dealing with is nothing more than a common, inorganic reaction that can be assumed to take place to completion. My comment on your sources of information is that they are WRONG. For the truth on this simple reaction refer to a convenient, elementary chemistry book or reference. There, you will find proof that you are dealing with an inorganic reaction that goes to a solid state completion and whose properties you can find out.

I am surprised that the excellent and modern scholastic preparation that the great state of Oman offers you is not being understood. As an engineering student, you must challenge any assertion that is not based on scientific grounds or on facts. Any General Chemistry textbook will have a reference to the Zinc Oxide reaction with Hydrogen Sulfide. I know you must have excellent reference libraries as well as universities in Oman. You should make the maximum use of these great facilities that have been instituted for your generation and your self-improvement.

I have given you “the whole sizing procedure for the reactor” in my workbook. All you need to do is to make a stoichiometric balance of the equation and find out how much ZnO is needed to react with the H₂S for a duration of 3 months. Follow the explanation I gave you and the vessel specification sheet that I attached. Note that the reaction takes place at an elevated temperature. It is normally called the “Hot Zinc Oxide Reaction”. Make sure you understand why the two vessels are piped and valved the way that I show them. You haven’t even mentioned the workbook that I have sent you, so I don’t even know if you have read it. The strongest point I can make here is that it should be YOU who takes the time and effort to create the means and tools of communicating and resolving your problem. I would expect you, the student, to generate the workbook - not others.

Good Luck.

[ruibernardes]

Sir Art Montemayor

"Normally H2S absorption (I think not adsorption) by ZnO is controlled by the
following reaction:

ZnO + H2S ↔ ZnS + H2O

This is an exothermic reaction, and the equilibrium H2S concentration is determined by the structure of the ZnS product, the temperature, and the H2O partial pressure.

However the presence of CO greatly decreases the H2S breakthrough capacity of ZnO. After the H2S absorption tests where CO was present in the feed, the ZnO changed to a grey color, indicating carbon formation occurred on the ZnO surface, presumably via the Boudouard reaction:

2CO (g) ↔ CO2 (g) + C (s)"

Comments!?
My questions are:
ZnO guard beds are indicated for syngas production (downstream, p.e. a FT reactor)??
ZnO is regenerable, like CuO??

Best Regards,
Rui Bernardes

[Art Montemayor]

Rui:

First of all, since this is the Student Forum I am forced to assume that you are a student and not a field-experienced professional Chemical Engineer. But even if you were, the Basic Chemistry Facts show that you are wrong when you state "Normally H₂S absorption (I think not adsorption) by ZnO is controlled by the following reaction". The following is a formal definition of "absorption" as per Wikipedia, not me:

"Absorption, in chemistry, is a physical or chemical phenomenon or a process in which atoms, molecules, or ions enter some bulk phase - gas, liquid or solid material. Absorption is basically where something takes in another substance."

Therefore your statement is totally wrong and misleading. At the risk of being redundant, I repeat once again (with emphasis): The ZnO acts as a consumable REACTANT and is NOT AN ADSORBENT (nor ABSORBENT).

I don't understand your use of quotation marks. If you are quoting someone or a text, then please so state. Otherwise, I have to take it as your personal comment(s).

If you have never designed this type of reactor or operated one, then you are going to have to believe me. I have done both and I am speaking from personal experience in the field.

Answers to your 2 questions are:

• Hot Zinc Oxide beds are used to protect downstream processes from trace H₂S contaminants.
• When used in the method and type of installation I have described, the reaction product (or Zinc sulfde) is never "regenerated" - it is simply discarded or thrown away (unless you find a useful application for it.

[ruibernardes]

The document I used for your comments stay in attachment.

I believe in you, but I think in this articles some people are confused between Ad and Absorption.
In the past I read "Absorption is an imbibing or reception by molecular or chemical action". So, in this moment I'm confused. But my main question is: How I can remove traces of H2S after carbon activated adsorption (I think the correct term, and you can regenerate carbon) and before copper oxide guard beds (absoprtion, right?)???

In accordance with your comments, ZnO is an adsorption process (ok, no problem) but I don't loose Carbon monoxide?? (see attachment). My question is, chemical reaction happening!! Ok, but can regenerate ZnO. Ok no problem, I need make-up ZnO.

Regards,
Rui

Attached Files

•  H2S_removal_with_ZnO_during_fuel_processing_for_PEMFC_applications.pdf   158.94KB   548 downloads

[Art Montemayor]

Rui:

I hate to keep repeating the same things over and over, but this thread has to have an end. The subject matter is one of the most basic in first-year university Chemistry classes.

You are absolutely correct. The people who wrote the article you submitted don't know what is going on. They are so lost, they fail to identify and detail exactly what they are trying to accomplish. When you introduce H2S into a packed bed of very hot Zinc Oxide, you will get an immediate reaction. Believe me. I have not only designed these systems, I have had them built and operated them in a continuous, 24 hr/day, 330 days/year Hydrogen steam reforming plant. And it worked exactly as designed.

Before citing any articles or your thoughts, at least read the workbook that I submitted and study exactly what I wrote and sketched out. The process works, It is working all around the world as we speak and write - and it will continue to do so until something better comes along. A Space Velocity of 200 works if you design the system well.

Read my post carefully and you will see that I wrote: "The ZnO acts as a consumable REACTANT and is NOT AN ADSORBENT (nor ABSORBENT). In plain, simple English this means that you don't regenerate the produced ZnS - you throw it away and replace it with fresh ZnO. THAT IS WHY I USE TWO (2) REACTION VESSELS. ONE VESSEL CONTINUES TO WORK WHILE THE OTHER IS REPLENISHED.

If you are looking for PARTS PER BILLION as a product gas contamination level, you probably need another process. But you fail to give us any Basic Data or information about what you want to do.
I don't know where you come up with the Carbon Monoxide.

[ruibernardes]

Sir Art Montemayor

Thanks for your explanations.

Could you send me some info about your design calculations? If you prefer send directly to rui.bernardes@gmail.com .

[Qalander (Chem)]

Dear All

Respect Our Art and Honor his Request/Comments please!
Just by the way;

I'v considered 'Adsorption' nothing but "Surface Absorption"

In simple terms as such no deep diffusion of any sort.

[djack77494]

I think it's essentially consistent with the more formal definitions and is easier to think of absorption to be basically a physical process such as absorbing acid gases into a solution of amines. Adsorption could be physical or chemical and would be the process used in, for example, pressure swing adsorption processes. Here one a compound in a fluid selectively attaches itself to the solid adsorbant. Both of the above processes are reversible in that the application of heat or pressure reduction will reverse the process.

As Art has tried so hard to get across, the use of a Zinc Oxide bed to remove H2S from a gas stream is neither absorption nor adsorption. It isn't a physical process. It is a chemical reaction process that fully consumes the zinc oxide during the removal of the H2S.

[Zauberberg]

This is how I got the subject from Don Pfennig, the master of Mol Sieve design and operation. It's quite simple, straightforward, and 100% clear.

 Adsorption_vs._Absorption.pdf   132.69KB   387 downloads

[gvdlans]

This is how I got the subject from Don Pfennig, the master of Mol Sieve design and operation. It's quite simple, straightforward, and 100% clear.

... a picture tells a thousand words...

[djack77494]

:lol: ... a picture tells a thousand words... :lol:

Thank you Zauberberg. Now it should be clear to all.

[zull]

dear ART MONTEMAYOR

shall u email me the file that u attached above..i've download the file but cannot open it..
this is my email: mr.zull@yahoo.com
thanks...

[husna]

dear ART MONTEMAYOR

I also want the file attached above.i cannot open it too.
please send it to my email.this is my email: una_hz@yahoo.com
thank you.

[syaitan]

Hi,

Could anyone please email me a working copy of the above design calculation file 'Desulfurization_by_Hot_Zinc_Oxide.xls' to syaitan@msn.com for reference? I'm currently working on a similar project and the file seems very useful.

Many thanks in advance.

[CristianG]

Excellent explanation, Art!

You taught me that engineering is just common sense and simple reasoning. Sometimes we fail because we try to understand things in a sofisticated manner.

Thank you very much!

[syaitan]

Hi.

Please help. Anyone who is kind enough to provide me with a design calculation spreadsheet on the above topic?

Many thanks.

[Art Montemayor]

It has become apparent that my original Excel Workbook somehow is being interrpeted as containing an undesirable Macro - when in reality, it doesn't. Therefore, new versions of Excel refuse to load it.

To remedy the situation and to quckly furnish the material that I submitted, I am uploading the material in Word for Windows - something I abhor, but am forced to do in order to get the information into the right hands.

My apologies to all concerned for the inconvenience and tardiness.

Attached Files

•  Desulfurization by Hot Zinc Oxide.doc   29KB   539 downloads

[cada2001]

hi Art and everyone else,

i am also designing a Zinc oxide reactor and it would be grateful if you could please send the sizing procedure for the reactor and the design calculations to my email address. cada2001@hotmail.com This is a message coming from a person who has less than a week to design a HydroDesulphurisation system for his dissertation. I only just discovered this site yesterday and already some of my concerns with designing a Zinc oxide have been resolved. Thank you.

[Art Montemayor]

Cada:

Please read this entire Thread. I feel that I have supplied ALL the necessary information to allow you to process design a hot zinc application.

I emphatize with your situation, but I will not do YOUR calculations for YOUR dissertation - and your grade. Simply follow what I have expounded throughout this thread and you will get your results.

[cada2001]

HI Art,
i do have a concern with the k constant k=4.079x10^-4. Is this rate constant to quick for this type of reaction. Only reason i ask is because i have been going through literature online because the books in my library doesn't actually cover this reaction.

[adamgrinat]

Hi Art,

If possible, will you be able to e. mail me the procedures for the desulfulrization reactor design.

adamgrinat@yahoo.com
Thank you in adavnce

[Abraham Joseph]

Hi Art,

If possible, could you please email me the procedures for the desulfulrization reactor design.

akulahakulahakulah@gmail.com

Thank you in adavnce