Molecular sieves beds insulation 4

[thalathb]

We have two different plants one to produce sales gas and the other for LNG, Any how both of the two plant have the dehydration unit by molecular sieves. For the LNG plant molecular beds have an internal insulation and for the sales gas plant is an external insulation. Of course the inside area for the LNG plant is smaller than the sales gas plant due to the internal insulation.

So, what do you think the observation will be and differences are ?

 

[thalathb]

No body has replied yet......

 

[Montemayor]

thalathb:

First of all, like me, I think most readers are trying to figure out just what exactly you are trying to state or ask.

If I have to guess, I would say you are new to the Natural Gas business and have come upon an adsorption unit used as a dehydrator in both an LNG plant and a sales gas pipeline operation. You also are unfamiliar with what an adsorption unit is and how it works. If you did, you would know that it employs a van der Waals type of attraction force to maintain selective impurities on its surface and requires regeneration energy (in the form of heat) to regenerate the adsorbent (usually Activated Aluminia or molecular sieves) in a batch type operation. Because of the regeneration energy requirements, insulation is need for heat conservation. This insulation can be applied either on the outside of the adsorber vessels or on their inside surface. There are trade-offs involved either way and usually in the case of molecular sieves (used where extremely low dew points are called for) very high regeneration temperatures are involved - 600 to 700 ^oF. This is the case for LNG production and internal insulation is used to maximize the heat conservation and maintain vessel shell temperatures low.

I hope the above addresses what I suspect is your intended question.

 

[thalathb]

Montemayor,

Thank you for telling that the post wasn't clear and thanks for the formative and usfull information.

Anyway,I have kind of study on both type of molecular sieves vessel: 1- one with internal insulation
2- other one with external insulation.
There are some diffrencies between both of vessels in operating wise i.e total sulphur slippage, temperature profile and regen gas temp.

so all the issue is just study on both kind of internal/external vessels, taking into account the sulphure slippage for vessels with internal insulation is going up faster more than the vessel with external insulation

 

[dcasto]

Most mole sieves were internally insulated in the begining for metalurgical reasons, by the time I got on the scene in the late 70's most people were specifying external insulation.

 

[thalathb]

dcasto,

regardless when people starts using internal or external insulation, there are a lot of diffrencies in operation point of view.

Regards

 

[Montemayor]

thalathb:

I believe dcasto's point has been mis-interpreted. I believe I clearly understand that what he is pointing out are the engineering and technical improvements since the 1970's that have allowed adsorber vessel walls to withstand higher temperatures. This is an important point in that internal insulation has always presented practical operating and maintenance problems. What I believe he recognizes and is inferring is that were it not for that, we would have to put up with higher superficial velocities, much heavier vessels, and probably a lot of impurities slipping or "channeling" behind the internal insulation and by-passing the main adsorbent bed. I failed to point this out but he has helped out in that regard.

I don't know where you obtained or read your study, but channeling and by-passing of impurities behind internal insulation has been a detrimental effect and trade-off that we "old timers" have long known about. And I'm not even going to discuss the major differences in expansion and stresses that exist between internal insulation and vessel walls when one imposes temperatures of 500 ^oF or more to the adsorbent bed.

I disagree with your description of "there are a lot of diffrencies in operation point of view". On the contrary, there are few, if any operational differences. There are a lot of operational RESULTS differences between the internal and external insulated vessels - and most of them are from by-passing of the beds and higher pressure drops.

I believe dcasto's point is an important one and should be noted by less-experienced engineers.

 

[dcasto]

so i hope readers understand that you can't take an internally insulated vessel and make it external without a lot of engineering, metalurgical, not just ChemE engineering.

 

[thalathb]

dear dcasto,

I didn't say I want to take an internally insulated to make it an externally insulated, I already have them both on my plant and no need to do any changing.

Hint: I just discovered the velocity in the internal one is faster than the externally insulated.

Regards

 

[thalathb]

All,

I've found some information say there are three zones in each MS bed as mass transfer wise. the zones are equilibrium zone, mass transfer zone and safe zone at the bottom of the bed.

Can any one help me in this and give more explaination about those three zones.

I really apprcieate you help in advance.


Regards

 

[pipehead]

Ththe equalibrium zone is the area in a bed where the adsorbent has be saturated (to equlibrium)) of the contaminant. The safe zone id a the region in the bed where the adsorbant has not been in contact with the contaninate at a concentration greater than it's regenertaed (of fresh) cocentration. The mass transfer zone is the area in the bed where at one end it is in equalibrium and at the other it is at it regenerated concentration.
The goal for well engineered adsorbant is to get one that has a minimum mass transfer zone. break through occurs when the contaminate has filled the equalibrium zone. A bed size required is the eqaulibrium zone plus the mass transfer zone.

 

[CECherry]

Any adsorbent bed system will have the three zones mentioned. I am not sure if you are just trying to understand how your dryers work or if there is some underlying problem you are trying to research. The earlier issue of internal versus external insulation is really a question of optimization during the design phase. Internally insulated beds reduce the amount of energy needed to heat up the steel during regeneration but either: increase the pressure drop by decreasing the effective bed diameter, or; increase the overall vessel cost by making the overall vessel diameter larger to accomodate the insulation (typically ~3" thich, so making the vessel ~6" larger). Internally insulated beds are usually seen in high pressure applications.