8 replies to this topic

[Mathi]

I am supposed to do a preliminary sizing of molecular sieve bed column for removal of CO2 and moisture. I have selected the 13X Molecular sieve which is good for the removal of both. How to proceed the preliminary sizing using the attached data?

 

I tried it roughly. The size came out to be 7  m (ID) x 21 m (H) and Bed Height came to be 15 m for 25 MMSCFD operating at 12 barg and 50 deg C.

Attached Files

•  13X Molecular Sieve.PNG   49.52KB   12 downloads

[thorium90]

Perhaps it might be better if you show how you got 15m?

[Propacket]

GPSA Chapter 20 provides detailed explanation for sizing of dehydration mol sieve beds. Not heard of mol sieves removing CO2.

[thorium90]

I'm not sure if the OP realized it, but in order to do any sizing, the concentration of the component to be adsorbed in the inlet gas and the required concentration in the outlet gas is an important parameter. The question has incomplete information and therefore no additional help is possible for the OP. 25 MMSCFD means nothing if you have no idea how much of that 25 MMSCFD is actually CO2 and moisture.

Edited by thorium90, 29 March 2015 - 01:00 PM.

[Mathi]

Thorium90

 

The input CO2 and H2O concentration is 4%  and 1% respectively. The output concentration required is <3% and <.0000001% respectively.

[Supriyo Mukherjee]

First of all I would like say Mole sieves 13 X is OK for both moisture and Co2 removal.

 

This design can be divided into two parts.
 

1. Amount of molecular sieves required : that you can calculate by knowing adsorption capacity(dynamic) of the sieves for moisture and

 

CO2 adsorption. 

The capacity mentioned in the vendor data sheet is not the one you should use. As dynamic adsorption capacity depends on the

 

following :

 

Operating pressure

 

Operating temperature

Regeneration temperature/flow

Type of regeneration gas(Dry or wet).

 

So dynamic adsorption capacity should be chosen based on above parameters and then should be used to find out the amount

 

required. 

Take 10% margin(general) you got your amount of Sieves.

2. Next thing : Diameter and height

Calculate minimum diameter by Ergun equation. 

Use around 60-70% of fluidisation velocity to arrive at diameter. Calculate height based on that.

Also calculate pressure drop across the bed using the selected diameter.

If it is within limit then you use the diameter u got else reduce dia and repeat.

[clarkr3000]

I have just built a pilot plant to run a series of experiment on 5A, 13x, Silica gel, and R2030 activated carbon for the removal of CO2 from Syngas.13X material was by far the best adsorbent when it came to the uptake of CO2 however this choice will depend upon how you plan to regenerate the material.  In my case we are using vacuum regeneration.  So in a sense we are boiling off the CO2 that is adsorbed. Since you are looking to remove H2O and CO2 the best choice is to use a multi layered bed system where you remove the H2O with either Activated Alumina or Silica gel prior to the 13 X bed. H2O has a higher heat of adsorption on 13X than CO2 does and thus will be harder to remove. This is due to the fact that the 13 X is a polar adsorbent with a net positive charge on it's surface which is greater than that of either activated alumina and silica gel. You also cannot use the static adsorption reported in the specifications as they are for the pure component isotherm at very high pressure where the isotherm is essentially asymptotic and are generally reported for 25 C. Adsorption is very sensitive to temperature so at 50 C the uptake will be less.  Since you have a mixed gas and are operating at a medium pressures then the amount adsorbed will be less than that value reported at the breakthrough point you wish to operate with.

 

If I had to guess you are trying to remove CO2 and H2O from NG? How long do you want the bed to run before regeneration? If you are trying to remove H2O and water from NG then 13X is not the right choice as it will also adsorb CH4 look for a modified 4A material that will exclude CH4 based on its Size. This will reduce your losses.

 

As mentioned above the column you designed has a large depth, that's ok if you can live with the pressure drop and are getting the pressure for the NG supplier. If however you are compressing this gas yourself then you need multiple vessels with shallower beds so that you are not wasting compression energy. There really is a lot to the design of these systems if you wish to optimize their performance

.

Edited by clarkr3000, 07 April 2015 - 01:19 AM.

[furtoconscasso]

Is it possible to estimate in liquid lpg sweetening (by sieve adsorber) the peak load of Sulfur compound (like mercaptan) in Regen Gas?

[Mathi]

furtoconscasso

 

The vendor usually gives the adsorption capacity of the particular molecular sieve from which it can be roughly calculated. Since most adsorbents absorb moisture and acid gases, the estimate will be pretty rough.