4 replies to this topic

[nivedita12]

I have a problem in deciding the number of beds used in dehydration for pre-treatment of LNG

 

One of the vendors has suggested a 5 bed for LNG pretreatment a 5 bed system is that you can reduce the regeneration rate by 66% by using a semi closed loop system . The downside is the additional vessel and associated costs. The other downside is that if water gets into the treater section it is harder to strip off the water.

 

 

A 3 bed system is capex friendly but opex wise costly 

 

 

Peak CO2 demand is 0.8 mole% 

 

Please suggest what should i go with 

[Mahdi1980]

Hi,

 

Please let me know the moisture content of your feed and the maximum allowable water in your feed as well as the dimensions of your beds recommended by vendor.

 

Please also be informed that you can just negotiate with vendors and you should trust them to enjoy their warranty on your dehydration package. 

The best technical action on the selection of moisture removal beds is the gathering the technical quotations from several vendors and comprise them with each other and choose the best economical one.

 

Regards

[nivedita12]

Moisture content is 4.5 lb/mmcf is the limit and it should be less than 0.15 lb/mmcf

 

i have not even gone ahead to go to the vendor but wanted a general idea of comparing a 3 bed versus 5 bed.

 

Is it only economical advantage or  saving regeneration gas containment in the LNG plant which again goes to tail gas if regeneration  gas is contaminated due to a 3 bed adsorption

Edited by nivedita12, 28 June 2017 - 07:21 AM.

[Bobby Strain]

All the plants I am familiar with use the minimum number of beds, typically 3 for capacities up to 5 million tonnes/annum. More beds will reduce reliability somewhat. If you are working for a client, that client should direct you.

 

Bobby

[Art Montemayor]

nivedita12:

 

I have designed, built, and operated adsorption systems for many gases destined for subsequent liquefaction.  This has involved a product moisture in some cases as low as less than -100 ^oF.

 

I agree with Bobby's experience.  I never used more than 3 towers in any one system.  After careful study and field experience, I found that more towers only complicated the process and increased the risk of process upsets due to multiplicity of the only moving parts involved in the process - the critical block valves that separate the drying towers from the regenerating towers.  Additionally, I found that more towers cost more.   Unlike what you state, I always found that the two-tower system was a lot less expensive to operate as well.  I do not agree with your statement that more towers mean less regeneration requirements.  Theoretically, the amount of adsorbent (and regeneration needs) is the same for a given gas capacity regardless of number of towers.  In fact, in the real application, you have less heat efficiency because you have to heat and cool more vessel parts and components in more than the 2-tower design.

 

There is an advantage to more than 2 towers in an adsorption drying system, but it isn't what you state.  Don't forget: a system supplier, if left to his/her own wishes, will always try to sell you the most expensive system.

 

Bobby's stress on process reliability is a very important point.  And in that respect, the basic 2-tower adsorption design is not only the simplest, but inherently the most reliable.  I highly recommend that you give particularly detailed attention and importance to the design, make, operating history, and reliability of the block valves and their actuators.   These items, together with the required instrumentation and programming, will determine the success of the process more than any other items.