What is Adsorbtion?
The following definition from International Adsorbtion Society website is pertinent.http://ias.vub.ac.be/General/Adsorption.html#anchor844743 Quote” The adsorbing phase is the adsorbent, and the material concentrated or adsorbed at the surface of that phase is the adsorbate. “ Unquote. For natural gas drying the adsorbate is water. The dryness specification achieved using Molecular Sieves is below 1 ppmv ( 1 part per million by volume). This stringent specification is necessary when the gas is processed further downstream in cryogenic processes where temperatures range from –() to - () and presence of water will result in ice formation and blockage of flow.
*The adsorbent is zeolite.
What is Zeolite?
The brief narrative from British Zeolite Association at their website provides a good introduction. http://www.bza.org/ ( click ZEOLITE icon ). In layman terms, natural gas containing water makes contact with the zeolite and the water molecules are preferentially trapped within the zeolite crystalline structure. Gas molecules are too large and flow past without hindrance. The term zeolite in Greek literally means “boiling stones” as explained at the Zeolyst International website: http://www.zeolyst.com/html/faq.htmlWhat is Temperature Swing Adsorbtion?
It is clear form the material presented so far, that zeolites can adsorb water but obviously there is a limit to how much it can adsorb. A good metaphor would be parking lots at a shopping mall. If there is a capacity for 10,000 cars, then car number 10,0001 will simply have to exit. Likewise, if all the adsorbent is saturated with water, newly arriving water molecules will not be retained within the adsorbent. Heating the zeolites will cause release of water, didn’t we say boling stones earlier !!
Why does heating free the water molecule from the zeolite?
Heat input causes vibration of water molecule within the zeolite structure and this vigorous movement is sufficient to overcome the weak forces that are holding the water molecule within the crystalline structure.
Shifting Gears and understanding Natural Gas Drying in real process plants.
Now that we have understood the amazing property of zeolites or more commonly known as molecular sieves in the Oil & Gas / NGL / LNG & Petrochemical industry, lets now look at how an actual plant processing say 1 billion cubic feet of Natural Gas per day is actually built. First and foremost the zeolites have to be housed in a proper home or physical structure. Typically this takes the form of a cylindrical vessel as shown below:
In the discussion below we will assume there are six beds. The number of beds is an equally interesting subject that will be addressed later. Let’s now delve deeply into the subject of molecular bed sequencing.
With six beds operating in parallel, at any given time, four beds will be drying the gas. In other words each bed on duty has a flowrate of approximately 250 MMSCFD.. The fifth bed will be under going a cooling cycle. The sixth bed will be undergoing a heating or regeneration cycle.
What do you mean when a vessel is in duty or cooling or regeneration?
Imagine each of the vessel is having 3 inlet valves and 3 outlet valves. We will label them valve 1ID ( 1_Inlet_Duty), 2IR, 3IC & 1OD, 2OR & 3OC. When we say a vessel is on duty, then the 1ID and 1OD valves are open and all other valves are closed. Likewise a vessel is in regeneration mode when 2IR & 2OR valves are open and all other valves are closed. Finally, when 3IC Valve and 3OC valves are fully open while the rest are closed, then the vessel is in a cooling mode.
Understanding the time control aspect of bed sequencing.
The supplier of the zeolite will advise the plant designer on the maximum duration a bed can remain in a duty mode. Lets say in our example that Vendor X says that this duration is 8 hours. Vendor X will also advise us that the bed must be regenerated say at a temperature of 550 deg F for 2 hours. In other words, heating the zeolite for 2 hours, will eliminate most of the water molecules that were trapped in the bed during the previous 8 hours of duty cycle. Having heated the bed to such a high temperature, there is a need for a third process operation namely cooling of the bed to bring down the core bed temperature to say about 120 deg F.
The 2 hours needed for regeneration now becomes a critical parameter for us to control the bed sequencing.
D = DUTY R = REGENERATION C = COOLING
Clock |
Bed 1 |
Bed 2 |
Bed 3 |
Bed 4 |
Bed 5 |
Bed 6 |
|
12.00 |
14.00 |
D |
D |
D |
D |
C |
R |
SEQ 1 |
14.00 |
16.00 |
R |
D |
D |
D |
D |
C |
SEQ 2 |
16.00 |
18.00 |
C |
R |
D |
D |
D |
D |
SEQ 3 |
18.00 |
20.00 |
D |
C |
R |
D |
D |
D |
SEQ 4 |
20.00 |
22.00 |
D |
D |
C |
R |
D |
D |
SEQ 5 |
22.00 |
24.00 |
D |
D |
D |
C |
R |
D |
SEQ 6 |
*So after 12 hours the sequence will repeat itself.
It is to be noted that every bed will only experience a total duty cycle not exceeding four multiplied by the regeneration duty cycle of 2 hours i.e. the 8 hours maximum given by the zeolite vendor.
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