How to Regenerate Adsorption Tower Effectively?

In this Tip of the Month, we will explore the regeneration of molecular sieve dehydrators. Can you save energy by ending the heating cycle when the regeneration outlet temperature reaches approximately 90% of the regeneration inlet heating temperature? Frequent readers of the Tip of the Month surely know the answer: it depends!

But on what does it depend? There are many factors to consider, but to simplify this discussion we shall assume a molecular sieve unit designed to do the following:

• Dehydrate natural gas to less than 1 ppmv
• Heating and cooling are done countercurrent to adsorption and the regeneration medium is bone dry
• The regeneration inlet temperature to the molecular sieves is 288˚C (550˚F)

First, let’s take a brief review of the entire cycle.  Molecular sieve dehydrators are typically two or three tower units. While one vessel is being regenerated, the remaining vessels are adsorbing water from the flowing natural gas.

Below is a simplified cycle schedule showing a three bed dehydrator where A1=First Half of Adsorption Cycle Time, A2=Second Half of Adsorption Cycle Time, C= Cooling Cycle Time and H=Heating Cycle Time:

Not shown in this cycle schedule are the times required to depressurize and re-pressurize the system. The time required for these steps are included in the regeneration cycle and must be done in such a manner to avoid lifting the molecular sieves and/or causing a mechanical bed support failure.

The astute reader will notice the total regeneration time per vessel (heating plus cooling time) must be equal to or less than the total adsorption time per vessel (A1+A2). Failure to accomplish this will result in early water breakthrough of the vessel on adsorption and will ruin someone’s day.

Let’s now explore the mechanisms that lie behind the regeneration cycle of a molecular sieve bed.  During regeneration sufficient sensible heat must be provided to heat everything in the vessel up to 260˚C (500˚F) or so. This includes the molecular sieves, the inert support media, the metal, etc.  The heat of desorption of the water must also be supplied. The sum of the sensible heat plus heat of desorption is the required regeneration heat and helps set the regeneration heating cycle. The heating time is larger than the cooling time because the heating cycle must provide the heat of desorption of water while the cooling cycle is concerned only with sensible heat.

There is more to regeneration than simply waiting until the temperature of the spent regeneration gas reaches a plateau before switching to cooling.  The heating cycle is not complete until the water is swept out of the system and the molecular sieves have reached their design residual water loading. During low pressure regeneration, the limiting step is simply getting the total required heat into the bed of molecular sieves. In this instance, once the outlet temperature reaches approximately 90% of the inlet temperature [262˚C (500˚F)], the heating cycle is completed. In some situations, the heating cycle is actually stopped before the outlet temperature reaches 262˚C (500˚F). Such a cycle is called a thermal pulse.

During high pressure regeneration, the limiting step is getting the water away from the molecular sieves. Despite the fact that the outlet temperature has reached a plateau, the regeneration may not be completed because the high pressure gas cannot carry away the water molecules. In this situation, you have to continue heating the molecular sieves for a period of time until the residual moisture left in the bed is equal to or less than what it was designed for.

This should lead the reader to ask “How can I tell what is limiting my regeneration?” The answer to this is not so simple. One could run field trials; however, this is time consuming and provisions must be taken to avoid premature water breakthrough. It would be much simpler to contact your molecular sieve vendor and ask them. They have design techniques available to determine what the limiting mechanism is.

A conservative rule of thumb is if the regeneration pressure is greater than 4 MPa [600 psia], be careful. Sweeping the water from the molecular sieves may be your limiting step. This discussion assumes you are operating between a minimum pressure drop during the regeneration heating cycle of 0.23 kPa/m [0.01 psi/ft] to ensure good flow distribution and a maximum pressure drop of 5.4 kPa/m [0.24 psi/ft] to avoid bed lifting.

Harvey M. Malino