7 replies to this topic

[Kahirn]

Regarding the following essay question:

 

"Discuss the various choices that an engineer must make in designing an adsorber for a specific duty.  In particular, you should consider the issues governing the choice of adsorber and adsorbent, including, for example, forms of adsorbent, factors affecting materials selection, etc.  Each factor discussed should detail the relevant physics and/or chemistry governing the process considered."

 

Does the “specific duty” mean that we have to specify what does adsorber do (e.g., if I attempt to cover gas phase adsorbers, should we need to specify purification of H2 from water-gas shift off-gas or sulphur removal of natural gas etc.)?

 

Then, we name a few adsorbents in particular to the above application - e.g., activated carbon, molecular sieve zeolites, etc. and select the best option out of the nominees by comparing one to another with physio-chemical reason?

 

Appreciate if you can confirm my approach in answering the question is relevant or otherwise.

 

Looking forward to your reply.  Thanks.

Edited by Kahirn, 10 May 2019 - 01:12 PM.

[Art Montemayor]

Kahirn:

 

These are valid Student questions that can be addressed and answered by our experienced members.

 

HOWEVER, PLEASE FIRST edit your post, removing all the unnecessary highlighted text (ugly purple) which you have furnished.  We don't operate a Chemical Engineering Forum for Kinder garden kids.  In order to address serious and important questions, our members should have clear, specific, and LEGIBLE text.  With experience comes age and well worn eyesight, so please use conventional and readable text  so that our members can fully read and understand your questions.  Out text editor is meant for highlighting specific words in message contents - not the entire post.  Excessive use of this feature is very distracting and difficult to read.  Lets make it easy for our members to read and understand the query so that they can furnish the important information sought by you.  Why make it difficult?

 

Await your response and legible writings.

[Kahirn]

Hi Art Montemayor:

 

Apologies to displaying murky highlighted text. Kindly refer to the modified post above. 

 

Let me know if the question is still not concise and hard to understand/read. 

 

Looking forward to your reply. Thanks.

 

Best Regards,

Kahirn

[Pilesar]

I will paraphrase how I understand the question:

"What are some of the design choices an engineer makes when specifying an adsorber for a particular service? What criteria does the engineer use to select among the design options? Include in your explanation how an engineer should decide which adsorbent to use and what materials to select. Write enough in this essay to convince the reader that you understand what you are talking about."

[Art Montemayor]

Kahrim:

 

O.K., thank you for the editing.  Allow me to contribute my comments to your specific questions:

 

Since this is an essay type of response to the given instructions, it is important to identify the main subject, adsorption, as a Unit Operations procedure (a physical change occurs - the separation of components from a mixture.  No chemical change takes place).  Before addressing the “choices” as requested, the important features and characteristics of the adsorption process should be outlined, specifying how they differ and why.

 

For example, adsorption is meant to separate components from fluid mixtures.  This means:

• it applies to gas mixtures as well as to liquid mixtures; each type of fluid is handled differently - state how;
• it is inherently a batch process since once depleted, an adsorption bed must be regenerated (or disposed, in some cases);
• the need for spent adsorbent regeneration necessitates the use of one (or more) stand-by, fresh adsorbent beds while the spent bed(s) are being regenerated.  This means the process is cyclic: the beds are switched between regeneration and application;
• the type of mixture and its makeup makes a difference on how the adsorbent operation is designed and operated.  Liquid fluids require special procedures for draining, venting and regeneration while toxic or hazardous gases being separated require safe handling and disposal operational methods.
• Regeneration methods for a spent bed can be based on temperature or pressure - or a combination of both.
• The temperature required for regeneration depends on the type adsorbent employed as well as the type of cycle and its cyclic timing.  Once regenerated with heat, the heated bed must be cooled prior to its subsequent process operation.

The specific characteristics and properties of the adsorbent selected in the application must be incorporated in the actual and final design of the adsorption unit in question:

• The method of applying any adsorbent involves its selectivity to the fluid mixture component intended for removal;
• The means by which an adsorbent attracts and captures a specific component must be known and identified by its molecular size and this must be in accordance with the physical size of the adsorbent’s physical structure and its micro pore sorptivity.
• Many adsorbent materials, such as carbons, silica gels and aluminas, are amorphous and contain complex networks of inter-connected micropores, mesopores and macropores.  In contrast, pores in zeolitic adsorbents have precise dimensions.
• Physical properties of the involved adsorbents such as the ruggedness and stability of the material also play a part in the proper application.  The ability of an adsorbent to withstand temperature and pressure applications - as well as bed movement - play a large part in avoiding “dusting” or physical breakdown and depletion.  The engineering design of the respective beds, their size, dimensions, stability, regeneration, and switching methods are vitally important.

The “specific duty” of an adsorbent is how the adsorbent works and how it is important to apply its specific properties to different components desired for separation.  How is it possible to continue to use the adsorbent if it captures certain components and reaches saturation?  If it is regenerated, how can the process continue in a steady state?  Or is the separation process only a batch operation, not a continuous one?  Can adsorption be carried out in a steady state process?  If so, how?  Describing a specific component’s removal is not what is called for.  The term “specific” here applies to the process - not to the components that may be subjected to it.

 

There are a variety of adsorbent materials - some natural, others man-designed and man-made.  Explain the differences between them as well as their individual advantages and disadvantages (“trade-offs”).  The proper engineering selection of the correct or optimum adsorbent depends on the evaluation of its operation under the actual local application and its safe operability as well as its economic impact.  Explain what important factors must be taken into account to recommend the proper adsorbent - based on a complete understanding of how the adsorption process is expected to work and critical characteristics of the operation.

 

Explain the different design aspects when dealing with relatively benign gas or liquid components as opposed to hazardous or toxic ones.  Explain why pressure assists in adsorption and the attraction (and trade-off) of its opposite PSA (Pressure Swing Adsorption) operation.  What are the upside and downside factors to consider?  What are the critical physical components of an adsorption unit?  Are the block/switching valves separating the regeneration vessel from the adsorbing vessel critical and “special”?  Why?  When should adsorption vessels be switched?  How is the proper switching time determined?  Does an adsorbent bed last forever?  Why not?  What affects the effective and efficient operation lifetime of an adsorbent and how can this be ensured?

 

I hope these comments help.  However, you should do extensive reading and studying of the Adsorption process in the literature and the internet in order to write authoritatively.

 

 

[Kahirn]

Hi Admin,

 

Thank you for the constructive feedback and additional insights to the addressed question.

 

However,may I know if I need to justify relevant factors governing adsorbent selection [e.g. selectivity, regenerability, compatibility to operating conditions [temperature, pressure, pH, resistance to chemical attack etc.]:

 

1) which is common to all applications [General terms],

2) Or specifically to one application [Case-by-case basis] of liquid/gas phase adsorption e.g. CO₂ capture for addressing climate change? 

 

Looking forward to your reply. Thanks.

 

Best Regards,

Koay

Edited by Kahirn, 10 May 2019 - 06:08 PM.

[Art Montemayor]

Kahirm:

 

Please read carefully what Pilesar has posted as his interpretation of the basic question given to you.  Pilesar has written an excellent reply to your query in that he has expressed what we both interpret as what your instructor/professor is seeking to find out from you:

• Do you fully and thoroughly understand what the Adsorption Process is?
• Do you fully understand why it is important and where and how it can be applied?
• And most importantly, can you convince your reader that you know what you are writing about?

In my opinion, you should discuss the process and its design requirements - not to a specific mixture, but to all the possible mixtures.  He/she wants to know if you understand the important data you must know when applying adsorption to any application.  In my opinion, you are not carefully reading our recommendations in full nor carefully thinking about what we are trying to tell you.

 

For example, when Pilesar advises you to fully convince your reader that you know your subject, he is telling you what I also tell you when I advised you to read, study, and understand as much research material on the subject as you can.  You have to dedicate a lot of time and effort in studying this subject.  You can't just expect to get all the information you need for the essay from this Forum or elsewhere.  You must work hard at it.

[breizh]

Hi ,

 a few documents to support your study .

Good luck

Breizh

Edited by breizh, 10 May 2019 - 07:35 PM.