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What’s the Difference Between Absorption and Adsorption…

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Posted on : 08-09-2011 | By : Mr. Green | In : Activated Alumina, Activated Carbon, Silica Gel, Zeolites
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…and Oxygen and Carbon Compound Adsorbents?

Absorption and adsorption are two natural occurring processes that are similar, but are not the same.  Here is a basic breakdown of how they are different:  absorption occurs when one material’s physical state is absorbed into another material’s physical state, while adsorption occurs when one material physically sticks to another material without changing it’s physical state.

Absorption occurs when a gas turns into a liquid, or a liquid into a solid, etc.  This is what separates it from adsorption, the physical state of the molecules have changed.  For example if you were to drink a glass of milk, your body would absorb it into your digestive system and eventually into your bloodstream.  The earth absorbs the suns rays and has converted its energy into the life sustaining planet we live on today.  The roots of plants absorb water when it rains converting into the energy it needs to survive.  All of these examples feature one material’s phase being turned into another.

Adsorption occurs when liquid or gas molecules stick to the side of surface, preserving their physical state.  This is useful for separating certain molecules from one another.  Adsorbents are most commonly found as carbon compounds or oxygen compounds.

Oxygen compound adsorbents are used to make products like silica gel which works to absorb moisture and reduce humidity levels or zeolites which can be tailored to specifically remove certain molecules from the air like carbon dioxide.

Carbon compound adsorbents like activated carbon can be effectively used to treat waste water and gas.  Contaminates will get stuck to the pores that are found all over the surface area of activated carbon while the water filters through.

Absorption and adsorption are both sorption processes, they both take in a substance or hold it in place and that is how they are related and why the are so similar, the process, however, is different.

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Activated Alumina and Molecular Sieve

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Posted on : 16-08-2011 | By : Mr. Green | In : Activated Alumina, Molecular-Sieve-Mavens
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How are they different?

 

Activated alumina and molecular sieve, they look similar (they both come in a small spherical beaded shape),  they perform the same process (they adsorb material at a molecular level), and they both have regenerative properties (you can re-use the material once the desiccant has reached capacity) yet they are two completely different products.  So the question is, with so many similarities, how are they different?

What they are made of is a good starting point.  Activated alumina is made out of aluminum oxide that is highly porous, while molecular sieve is made out crystalline metal alumino-silicates.  What this means is the pores on molecular sieve can be shaped into specific sizes most commonly seen as 3A, 4A, 5A, and 13X, where as activated alumina’s pores do not have specifically measured sizes.  This means molecular sieve can be used to separate certain molecules of specific sizes from one another, for example removing ammonia from natural gas streams.

From an application standpoint, here is how they differ.  Activated alumina has a real strong water adsorption capacity, it can adsorb a lot more water than molecular sieve, this makes it a very useful material in air compressors or for certain natural gas processing applications. The durability of the material allows it withstand a lot of pressure along with high levels of humidity.

Activated alumina can’t adsorb the large variety of materials or separate certain molecules from one another like molecular sieves can, making it ineffective in a process like ethanol dehydration.  This is because activated alumina would be able to adsorb both ethanol and water molecules and thus no separation would occur.

Molecular sieve may not be able to adsorb as much water but if you needed to reduce water to very low amount, up to 0.1ppm, molecular sieve would be your absorbent of choice because this is something other adsorbents besides molecular sieve have been incapable of doing.

Molecular sieve can also be used to separate specific molecules from one another, due to the customization of their pore sizes.  For example you can separate water from ethanol, and carbon dioxide, ammonia, and larger hydrocarbons from natural gas streams, which is something activated alumina can’t do, or won’t do with same efficiency.

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