2 replies to this topic

[Eprocess]

Dear all,

I do have two zeolite beds (4000mm*300mm), in which ethanol is dehydrated in 120 C as superheated vapor. I do need to know a rough estimate (order of magnitude rough) of the amount of hot air (200- 230 C) that is required to regenerate each kg of Zeolite . The beds are saturated every 10 hrs (considering 20% water absorption capacity for Zeolite. I would appreciate if any one could give me a clue based on relevant experience or practical data. The Zeolite`s specification is attached.

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[Art Montemayor]

Eprocess:

This is a good query and one I have responded to in many prior threads in our Forums. If you are a Chemical Engineer, you know that you are dealing with an ADSORPTION process – one that is operated on a batch-wise basis. One bed is on dehydrating service while the other, spent bed is under a regeneration cycle. The regeneration cycle involves heating the adsorbent (molecular sieve, in this case) together with all components related to the bed (steel vessel, internals, piping, valves, insulation, etc.) to a temperature that liberates the captured water molecules. After heating, the bed (and related components) is cooled down prior to being put back into dehydration service and replacing the other bed.

I mention all the basics of the process because they are important in attempting to define and understand the steps and principles under which the adsorbent regeneration is done. The adsorption batch-operated process requires a detailed, step-by-step regeneration process in order to secure a consistent and reliable performance. And in order to undertake the regeneration you must have a complete description and identification of your adsorbent’s characteristics. You must know:

• The maximum, recommended regeneration BED temperature that your adsorbent can tolerate safely without undergoing attrition, dusting, sorptivity loss, or damage. Your adsorbent supplier should furnish you with this vital information.
• The maximum temperature that your adsorber vessel and all other related components can safely withstand. You should know this basic information.
• The recommended BED temperature for correctly heating your adsorbent in order to meet the required subsequent alcohol drying specifications. Your adsorbent supplier should furnish you with this vital information. You mention you are going to use 200 ^oC air. I consider this temperature as THE MINIMUM BED temperature for a molecular sieve, and therefore a hotter air input should be used. Please confirm this data. Molecular Sieve beds usually can withstand 250 to 260 ^oC and that ensures a well-regenerated bed with a secure, dry product. I always try to regenerate Mol Sieve beds to 260 ^oC – if possible, and if needed for product purity. Usually, the use of Mol Sieves indicates that a high product purity is sought – and therefore, a relatively high regeneration temperature is advantageous. BUT the adsorbent vessel and related hardware must be rated for the high temperatures – and temperature cycles.
• The recommended fluids (gases) that can be employed as the heating and cooling medium for your adsorbent beds’ regeneration cycle. Your adsorbent supplier should furnish you with this vital information and, presumably has approved hot air.

In order to estimate the amount of regeneration air required for your specific adsorbent bed you have to calculate a heat and mass balance of the regeneration process:

• Knowing the ultimate temperature that you are going to heat the beds to, calculate the amount of steel and insulation in the system that has to be heated;
• Calculate the amount of heat to liberate all the water captured (heat of desorption);
• Calculate the amount of heat to convert the captured water to exiting water vapor;
• Calculate the amount of heat to heat up the bed;
• Add up all the heating requirements and add a contingency factor – I recommend 20%.
• You now know all the heat energy you have to transfer into the system, but bear in mind that you have an unsteady state heat process, so the figures are only averages and not exact.
• You know the time increment you have reserved for heating, so you obtain the average heat input figure, Q.
• Q = W Cp (T₂-T₁), and you can fix all variables except W – which is what you want. Here is where you have to pick a credible and practical hot air temperature exiting your heater – and why I stress that you fix the maximum bed temperature acceptable. You must design for a practical temperature driving force, so you know that your air heater outlet temperature has to be higher than your ultimate bed temperature – otherwise there is no heat transfer at that point.

That is how to estimate the air flow required to carry out the desired bed regeneration in the specified time.

You don't tell us WHY you need to identify the air flow rate when you could easily resort to asking the contractor or designer who supplied you with the adsorption unit (or resort to your operating manual), but I presume that you don't have any literature or information and you find yourself in a jam because you need a new air blower or source. Am I correct?

[Eprocess]

Dear Art,

Thank you so much, your description was more than complete.
For regeneration temperature my reference is the zeolite vendor's spec sheets, in which it is stated that this type of zeolite has to be regenerated with 200-230 C Air. For mechanical design of the vessels I am going to inform the designer after the temperature is finalized.
Actually this is because my client is going to build a pilot plant and does not want to refer to professional package suppliers for financial reasons. The initial design was based on pressure swing process but we had to change it for some reasons. And frankly I do think this job has to be done by professional dehydration package providers instead of me with having no experimental data in hand. But I am going to indicate the blower capacity in the way you taught me.

Edited by Eprocess, 18 February 2011 - 02:39 AM.