5 replies to this topic

[imtiaz]

Hello Good day everyone,
I am currently looking at the benefits of converting Molecular Sieve Dehydrators from the Johnson Screen Bed Support to Ceramic balls support. This entails basically removing the Fixed Johnson Screen Support and filling the bgottom dome with Ceramic balls. Two potential issues are Flow distribution during regeneration and Energy requirements as a result of the increased volume of ceramic balls. Two independent reviews indicated that the current fired heater design will be adequate at the present plant rates, since we would be able to increase the volume of molecular sieve added and thus havbe a higher adsorptive capacity. all that needs to be done is extend the adsorption cycle so that the regeneration cycle can be effectively done. The other potential issue is flow distribution during regeneration. CFD modelling has infact came up with a conceptual model which we are currently looking at. Is there anyone with experience in the above. The main problem we are trying to solve is extending molecular sieve life- so this design gives us the potential to add more molecular sieves- of course it also eliminatesany DP issues that in the past has hurt us on collapse of the Johnson Screen.

Many Thanks,
imtiazE

[Zauberberg]

Pressure drop issues will not be eliminated as the Mol Sieve beads have their own crush strength as well - they will crush and collapse once when certain DP/velocity are exceeded. High velocities, as consequence of high DP, will also accelerate desiccant attrition. This is not something you want.

Johnson screens are usually designed to withstand approximately 2 bar pressure drop across the adsorber (excluding weight of the desiccant itself). This is more than sufficient margin for prolonged, 3-year operation of the Dehydration unit without shutdown. Something else is an issue.

What you are proposing is focused on treating the consequence of a problem, rather than treating the cause. You need to identify the root cause that is causing Johnson screens to collapse, and fix that root cause - it could be improper mechanical design, high carryover from the KO drum upstream, undersized beds (high velocities) etc. That is the best way to attack the problem - find why the screens collapsed.

Just as a side note - I don't think that regeneration is possible with the vessel fully loaded with ceramic balls on the bottom side. Regardless of what the CFD analysis is saying, the regeneration gas would see different resistance to flow as it enters through the bottom basket, and channeling would occur leading to incomplete regeneration. In my opinion, Johnson screens are absolutely mandatory.

[Art Montemayor]

I agree 100% with what Doctor Zauberberg is stating. I use the title because he writes with a thorough knowledge of the adsorption process. My aim here is to reinforce his comments because they coincide exactly with my past experience.

I wish I had had 100% of all my adsorption towers outfitted with Johnson Screens. Unfortunately, in some of the years I designed and built my own towers, there were no Johnson screens. But when I had the opportunity to use them, I did so with remarkable success and never had any operational problems. I wish I had these screens on every installation I operated. I have never heard of anyone collapsing one. Notice that I use the phrase as the operator causing the collapse. If the screen is selected and installed correctly, I cannot see how it would collapse on its own. Like Zauberberg, I emphasize the action of going after THE CAUSE - and not the effect. Something definitely is going on that is very wrong with an operation that collapses a Johnson screen. I consider this a major process defect and the consequences can be hazardous in some cases.

I high suspect that there is an opertional "story" behind this occurance and I would not proceed forward until I found out the exact cause of the failure. I believe that when that occurs, the Johnson screens will be installed again.

[imtiaz]

Mr. Zauberberq/ Montemayor,
Thank you very much for your responses. Indeed you are correct. Historically, we are having issues with liquid HC contamination in the Molecular Sieves. We have added another upstream Separator with good results thus far. I have done some research indicating that the ceramic balls support at the bottom dome has had some success. I agree with your comments and i am cognizant that one of the major issue will be the flow distribution during regeneration. Do you have any performance data (or can you point me in a direction)supporting your position that can further assist me?
Mr. Montemayor- is it possible for you to indicate the type of operational problems you faced in the past?

Gentlement, thank yuou again for your responses- you have been very helpful.

Regards,
ImtiazE

[Art Montemayor]

Intiaz:

I can make a listing of operational adsorption problems I have had, seen, and remedied in the past. However, as you can appreciate, this is not a short list and not one that I can simply sit down and do as "texting". It will be long and, therefore, take time.

I'll do the best I can with what time I have available.

[Zauberberg]

One of the things you can do to alleviate carryover problems, is to install a layer of Sorbead on top of the Mol Sieve - if the additional height is available inside the vessel (usually it is). It will extend the life of desiccant.

Ideal process solution - and the one I would be looking for - is to find the true reason for liquid carryover and adsorbent degradation, leading to high DP and failure of the Johnson screens: whether it is in undersized separators, improper piping arrangement, or something else. Eliminating the root cause of malfunction is the most effective and, looking on a long term basis, the cheapest solution.