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Design Of Molecular Sieve Tower

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Posted 02 April 2011 - 04:47 AM

I want to dehydrate IPA (isopropanol) by removing the azeotropic qty of water from IPA-Water mixture.

I want to know few things (I am new to this area):

1. Is it true that vapor phase inlet can be good compared to liquid phase?
2. What is the role of L/D ratio (other than pressure drop)?
3. It is said that Molecular sieves can handle as much as 21.7% moisture. What is the practical truth?

Can anybody guide me in this regard?

Thanking in advance...

#2 Zauberberg


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Posted 02 April 2011 - 07:18 AM

21.7% can be considered only as theoretical static adsorption capacity of Molecular Sieve, when it is brand new. Static capacity refers to the amount of moisture adsorbed by desiccant in a perfectly sealed system, when equilibrium conditions are reached. In practice, you are not interested in static adsorption capacity but rather in the dynamic adsorption capacity, and at EOR (end of run) conditions which is usually somewhere between 6-8% (=6-8kg water adsorbed by 100kg of Mol Sieve). This is the most common design approach.

Not sure what do you mean by your first question. Apart from the sequence issue (draining and filling of liquid adsorber), both processes are very similar. As for the L/D ratio, it comes out as the result of at least two important considerations:

- Pressure drop in the direction of flow (adsorption);
- The minimum required flow of regeneration gas for proper distribution. This amount decreases with increased L/D ratio.

We've had a lot of topics dedicated to Mol Sieves on these forums. I'd suggest you to use the "Search" option and you will find a lot of quality materials and spreadsheets. In addition, check a few pages in GPSA Databook regarding liquid-solid phase adsorption.


#3 Art Montemayor

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Posted 02 April 2011 - 02:05 PM


This is a strange query to be posted in the Industrial Professionals Forum. If you are a professional engineer is should be safe to assume that you are qualified to undertake what you propose: the design (or specification) of an adsorption unit. It is analogous to a professional Chemical Engineer undertaking the design and specification for a distillation tower. It is taken for granted that you have the training, experience, and tools to undertake such an endeavor. Otherwise, you have no business getting involved in something you know little about or lack the experience. If this is just an exercise or a learning trip, that is perfectly OK. I make this blunt statement because of what is discussed in the first paper submitted by Zauberberg, “A How-to guide for Adsorber Design”. To date, adsorption still remains a semi-arcane Unit Operation and research is still being carried out in this area. It is a complex science. Consequently, if you have not followed it and practiced it in the last 40 to 50 years (when most of the original basic design work was done), you have a lot of catching up to do. The above paper will explain the highlights of what this Unit Operation can – and can’t – do, as far as the state-of-the art is today.

To address your 3 questions:

  • I believe what you intend to state in your first question is: adsorption in the gas or vapor phase is easier than that in the liquid phase. If so, the answer is YES. Handling a liquid on a redundant batch process can be complicated and requires some empirical know-how. The regeneration phase is much more complicated than that for a gas phase regeneration. As Zauberberg points out, you inherit the problem of the draining and filling of liquid as well as the increased regeneration energy required to evacuated residual liquid. You never will be able to drain or blow-out the last remaining residual liquid in an adsorber. You just don’t have the available time to do so. Additionally, complete regeneration is more difficult to establish for liquids than for gaseous systems. The adsorption portion is very similar for both liquid and gaseous feeds.
  • The L/D plays no role – other than aesthetics and available space. It is the SUPERFICIAL VELOCITY that sets the L/D ratio and that is the parameter to keep an eye on. It is important because it fixes the residence time of the adsorbate within the adsorbent bed. This is fully explained in words and mathematics in the referenced paper (read the section on Fixed Bed Dynamics). To comply with a maximum superficial velocity, you have to accept trade offs with respect to the adsorption and regeneration pressure drops and the resulting bed distribution as well as fabrication economics. All these factors are tied together in a complex manner. But it starts with the set maximum superficial velocity allowed in the specific bed. Often, this value is empirical and based on experience.
  • The important “capacity” when dealing with any adsorbent is exactly as Zauberberg has described it: it is the Dynamic Adsorption capacity that is of interest to the design engineer. Any other “capacity” is of academic and research interest only. The 6 - 8% figure involves some assumed basis for application. Aging, poisoning, attrition, regeneration efficiency, and plugging are just some factual and real operational problems that can occur from application-to-application. The designer should always take these into consideration. Any adsorbent, regardless of the approach to perfect regeneration, will undergo aging and be subject to a complete removal and replacement after a certain amount of operational time. The time for an adsorbent to be spent can be as short as 2 years or as long as 4-5 years. It is dependent on the process, operation, and feed/regeneration gas purity.
When discussing and debating the issue of adsorbent type, capacity, size, quality, results, and design parameters on an actual, real-life application, the adsorbent manufacturer should be considered as the EXPERT – rather than any academic text or research paper. This is not to imply that the study and research being done at the academic and study level isn’t important. It is VERY important. However, as I previously stated, this Unit Operation still labors under on-going study and is still being developed and is not as elegantly defined in design as distillation, heat transfer, fluid flow, and other Unit Operations. As Dr. Knaebel astutely notes, “Adsorption is considered complicated, compared with distillation, absorption, and extraction.”

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