Understanding the Adsorption Process
Adsorption is the process of adhesion used by atoms and molecules to attach themselves to a surface. It is a process that has been capitalized on by many industries and has become essential to producing many of the different everyday products people use. Below is an, “Industrial Adsorption Course 101,” for people looking to learn more about adsorption and some of the common types of adsorption processes.
Below are some helpful definitions in bold:
Capacity – is defined by temperature and pressure.
Working capacity – in a regenerative process is defined by the difference in the adsorption state (Ta, Pa) and the desorption state (Tr, Pr).
The adsorption and desorption of any material in liquid of vapor phase is a state function. Adsorption/desorption works the following way, while imaging a Carnot cycle. The higher the temperature is the lower the adsorption capacity will be; the lower the temperature is, the higher the adsorption capacity will be. The higher the pressure is, the higher the adsorption capacity will be; the lower the pressure is, the lower the adsorption capacity will be.
There must be a change between two states for adsorption and regeneration to work – State 1. The states can be in the vapor or liquid, but there will be no regeneration of the sieve without a state difference. The working capacity of the sieve is defined as the difference in partial vapor pressure of the two states of the material being adsorbed. Ethanol/water combinations (Daltons Law Pt = p ethanol + p water) are fairly well defined, but other systems with multiple constituents can get a bit more messy.
Liquid phase adsorption can be done, but the vapor pressure variations of most liquids over narrow temperature and pressure ranges are small. The narrow temperature and pressure ranges decrease the working capacity, rendering the system inefficient. Most liquid phase adsorption processes will fit the description of sacrificial adsorption process (listed below) because it is cheaper to replace the sieve, compared to the cost of energy that will be needed to regenerate it.
There are three basic adsorption processes as well as hybrid systems used for adsorption. These are listed below:
1) Sacrificial – you simply dispose of the sieve material after one adsorption cycle: Ta=Tr and Pa = Pr. Expensive solvent recovery would be one example of this.
2) Pressure Swing Application “PSA” – Isothermal process where Ta=Tr and Pa >>>>>>Pr –Isotherms for the particular adsorbent and process material is used to define the process. Ethanol would be one example of this.
3) Temperature Swing Application “TSA” – Isobaric process where Ta<<<<<<<<<<<<<<<<Tr and Pa=Pr – Isotherms for the particular adsorbent and process material is used to define the process. Natural gas would be one example of this.
Note One: There do exist hybrid systems which utilizes changes in temperature and pressure with or without vacuum.
Note Two: There are proprietary computer programs written by myself and others to calculate partial pressures for SOME multiple component systems. As you probably remember from thermodynamics fugacities, Van der Waal interactions, viscositys, density changes in multi component systems can get very tricky.