3 replies to this topic

[Art Montemayor]

karthik:

I’ve done a lot of design and operation on both systems, so I’ll be as specific as I can.

Adsorption dryers are one type within the generic class of “desiccants”. Adsorption is unique and very effective because it uses the effect of van der Waal forces to capture and retain selective molecules on the surface of an adsorbent (usually activated carbon, silica gel, activated Alumina, and Molecular Sieves). When the retained molecules (sorbate) are water, the operation is one of drying the parent fluid. This adsorption unit operation can be done in the liquid or gaseous phase. Drying is usually done on gaseous phase. It is very important to note the difference between ADsorption and ABsorption. The two are entirely different unit operations and are designed differently.

Refrigeration “dryers” do no more than what Jeff H clearly describes. They merely cool a compressed, humid gas phase and subsequently separate the liquid condensate from the gas, leaving a water-saturated gas at the operating pressure & temperature of the cooler. On subsequent pressure reduction and gas heat up, the relative saturation is reduced. It is obvious that you cannot obtain an actual, presurized dew point at a temperature lower than approximately 35 oF; otherwise the unit would freeze up with water ice.

Please take careful note of the following: To judge the absolute differences between both unit operations you should measure the amount of minimum water content that each process can accomplish at the same, process pressure. And the absolute measurement should be in PPM (mass) not in dew point or PPM (volume). The reason for this is that if you have a critical service the dried gas is going into, you want a definite mass measurement of the quantity of water you are introducing into the system.

In these regards, the adsorption dryer has all other types of dryers beat – and beaten to a pulp! The refrigeration dryer or deliquescent-type lab dryers (like CaCl2) can’t come close to the product quality levels that the adsorption dryer can achieve with ease and simplicity: product gas with less than 1.0 PPM(mass).

It is not true that adsorption (fixed bed) dryers account for large loss in pressure and free air delivery! What is probably being used as a “typical” adsorption dryer is one of the cheap, “down-and-dirty” commercial adsorption dryers that are designed on a “heat-less” cycle. These are not the industrial grade type of adsorption dryers used in serious process plants and where an engineered profit must be made. Adsorption dryers come in two generic types: TSA (Temperature Swing Adsorbers) where heat is used to reactivate the spent bed, and PSA (Pressure Swing Adsorbers) where pressure is use to reactivate the spent bed. PSA has a process fit in some unique applications, but generally cannot be justified where there is no useful application for the reactivation gas stream. TSA is the process of choice if you want good, clean, ultra-dry compressed air for critical services and instruments.

Refrigeration dryers require a much larger pressure drop through their system than do TSA, fixed bed adsorption dryers. This is common knowledge because of the many mechanical constraints imposed on the mechanical type. I design TSA adsorption dryers to operate with 20-25 ft/min of superficial velocity and only have a pressure drop of 0.5 psi (maximum) across the entire unit – something a refrigeration type can’t do.

I’m not selling adsorption dryers. I only want to make the real facts and empirical knowledge known to everyone so that you can make a sound, rational, engineering decision in your applications. There are trade-offs (as always) to consider, but you haven't specified any interest in these so I'll stop at this point.

I hope the above addresses your questions and comments in a positive and understandable manner and is helpful to you.

Art Montemayor
Spring, Texas