Desiccant - Dynamic Adsorption Capacity

You could do what your competition is doing and go to competitive websites and see how they do it....or you could Google on your title and follow the descant manufacturers' recommendations, there's about 11,800 hits besides this one...

http://www.dynamicadsorbents.com/products2.htm

"What is the adsorption capacity?

Adsorption capacity is defined as the accumulation of the solute molecules at the surface of a solid. This capacity is directly proportional to the area of the surface exposed and is dependent on the solute partial pressure and the temperature. An increase in temperature reduces the adsorption capacity of activated alumina as the adsorption of water on alumina is exothermic.

Adsorption capacity depends on the surface site reactivity and is measured by the volume of adsorbed water per unit of surface area. Water in the air actually sticks to the alumina itself in between the tiny passages as the air passes through them. The water molecules become trapped so that the air is dried out as it passes through the filter. This process is reversible, and when the alumina desiccant is heated to between 350-600 F (177-316 C) it releases all of the water stored inside it. The process of heating the activated alumina is called regenerating the desiccant.

When a gas is compressed the partial pressure of the water present increases. At a constant temperature the adsorptive capacity for water increases with increasing water partial pressure (and relative humidity).

Desiccants are manufactured to meet standards such as "Standard Methods of Testing Sorbent Performance of Absorbents" published by ASTM International, as well as other US government standards."

Please note that this articles touches on process conditions that you did not state in your question thereby limiting the amount of information we can provide.

There cannot be a basis for property of material. 'adsorption capacity' is a property of a particular desiccant, it can be different for even different makes of it. Only thing I can say for sure is that 'dynamic adsorption capacity' can be proportional to 'static adsorption capacity'.

Adsorption capacity greatly varies with temp and partial pressure of water in air to be dried. For instance a particular desiccant "molecular sieves" has static adsorption capacity of 22-23% at 25 degC and 11.88 mmHg water partial pressure. But for use its dynamic adsorption capacity is taken as 11-13% by the supplier for similar conditions. As per this analogy dynamic adsorption capacity is about half of the static, but this also may vary manufacturer to manufacturer.

"As per this analogy dynamic adsorption capacity is about half of the static, but this also may vary manufacturer to manufacturer"

I think above sentence of my last comment is OK. You have not given the SAC of activated alumina. You agreed that static adsorption capacity is a property and I said dynamic adsorption capacity depends on static or a fraction of it. For the case I narrated, it is around 0.5. Now you want the basis of this fraction. Deciding the basis for this fraction is always a complicated matter because it depends on several factors like temp, RH, pressure, velocity, contact area etc. The most important factor besides all above is reliability factor. I will explain it taking a general case as follows.

Suppose considering all the factors like temp, RH, pressure, velocity, contact area etc. at rated condition a drier bottle filled with desiccant of bed hight 'L' is fully saturated in 'T' hours. From this information you can get DAC of the desiccant (as air flow rate, RH and weight of desiccant are measurable). But then comes reliability factor to find practical DAC. To take care of eventualities of wide variation in the factors affecting it, to take care life of desiccant (deterioration of quality) and also to take care any untoward problem in regeneration cycle (like failure of heating etc.) value of DAC is taken about 60 to 75% of measured DAC.

So, in your case, suppose this reliability factor is 70%, in 8 hours only 70% of activated alumina bed hight will be saturated. So bed will be OK even you prolong changeover up to 11 hrs.