Large Surface Area is Key to a Valuable Adsorbent

Why is surface area key to a quality adsorbent?

Before we talk about surface area it’s helpful  to understand how adsorption works.

Adsorbents work by adsorbing liquids or vapors into pores on their surface.  The adsorption process doesn’t truly absorb the vapor or liquid that’s running through it (meaning the the liquid or vapor isn’t turned into a solid with the adsorbent),  rather molecules from the vapor or liquid are adsorbed and thus they get stuck on to the adsorbent.  In short an adsorbent acts like a magnet.

The pores on an adsorbent are where adsorbed molecules are kept.  The pores can have diameters between a couple of nanometers to hundreds of nanometers.  The purpose of the pores is to not only store molecules but sometimes to separate certain molecules by size.  The pore sizes can differ by nanometers or Angstroms (1 Angstrom = 1/10,000,000,000th of a meter) so you can separate liquids and gases at a molecular level.

For example if you wanted to separated methane from water you would use a 3A molecular sieve because the pore size on 3A is 3 Angstrom.  Water molecules have diameters up to 2.9 Angstrom and methane molecules have diameters up to 3.8 Angstrom.   The molecular sieve adsorbs the water and doesn’t adsorb the methanol thus separating the two molecules from one another.

Surface area measures how much exposed area there is on solid objects.  It’s important to distinguish that surface area and volume are not the same.  As long as the width, length, and height of an object remain the same the volume will never change.  Surface area, on the other had, can change if you break the object into smaller pieces.  See the example with the cube below.

Surface Area

Surface Area of a Cube = l*w*6

Volume of a Cube = l*w*h

 

 

Cube Length: 10mm

 Cube Width: 10mm

 Cube Height: 10mm

 

 

Cube Volume = 10*10*10=1,000mm³

 Cube Surface Area = 10*10*6=600mm²

The volume of an object will remain the same, but surface area can expand.  For example if you break the cube above into 5 parts you would find the following.

 

 

 Length: 10mm

  Height: 10mm

  Width: 2mm

Number of Cube Shaped Boxes: 5

 

Cube Surface Area:

 (2*10*10) + ( 4*2*10)*5=1,400mm²

 Cube Volume: (2*5)*10*10=1000mm³

 

By breaking the cube up into smaller sections, the surface area of the cube increases while the volume remains constant.

Surface area in adsorbents can be large.  1 gram of activated carbon for example has a surface that’s usually around 500m²

The pores on most adsorbents go only a few molecules deep so what you need is a lot of pores if you want to adsorb a lot of material.  Since pores are on the surface that is why you need a lot of surface area.  More surface area means more pores which means more liquid/gas is adsorbed.

 

Sources:

Size of methane molecule,  Slide 16 http://www.epa.gov/lmop/documents/pdfs/conf/12th/gladstone.pdf

Size of water molecule http://www.mc3cb.com/pdf_chemistry/What%20is%20the%20diameter%20of%20a%20water%20molecule.pdf

 

 

   

Carbon Molecular Sieve and Producing Nitrogen Lasers

 

First invented in 1960 the laser was first called the solution looking for a problem.  More than fifty years later lasers have become an invaluable part of human technology. Today uses for lasers range from being used in material processing endeavors such as laser cutting, welding, and bending, to reading bar-codes when you purchase something at a store, to being used by the military as a  targeting sight, and even being used to do surgery (laser eye surgery being the one of the most common).

As technology has improved many different types of lasers have been developed.  One of the more common types of lasers developed was the nitrogen laser.  This laser uses nitrogen as a medium and an electrical discharge to create its beam.

Nitrogen lasers are particular useful in handling material processing functions for example they are good at cutting metal.  However material processing functions require that lasers be efficient and cost effective and that is where nitrogen generation systems play an important role.

In order for the laser to function it needs pure nitrogen (between 97%-99.99%).  The most common type of technology used in purifying nitrogen is membrane technology.  This method is able to produce nitrogen up to 99%.  However if that amount of nitrogen purity is not enough to generate a laser.  Depending on what you are using the laser for the nitrogen may not be purified enough.

In order to get the purest form of nitrogen a PSA system and a carbon molecular sieve is needed.  The PSA system, air compressor, and carbon molecular sieve work  when the air compressor forces compressed air into the PSA system.  Naturally compressed air is composed of 78% nitrogen, 21% oxygen, and less than 1% of various other gases, the same air that makes up the air in our atmosphere.

Once this air enters the PSA system the carbon molecular sieve adsorbs all of the oxygen and other gases, the nitrogen is able to pass by because it is not attracted to the carbon molecular sieve and it is then guided into a storage tank (See our earlier article on adsorption with carbon molecular sieve).  Once the carbon molecular sieve reaches its adsorption capacity it can be regenerated so that it can be used over and over again.

The end result of this is process is that you have now produced nitrogen that is between 99%-99.99% pure.  This highly pure form of nitrogen is useful for cutting through tougher and thicker metals.

 

Sources:

http://www.thefabricator.com/article/lasercutting/a-case-of-the-gas

http://inventors.about.com/od/lstartinventions/a/laser.htm

http://www.megacarbon.com/techlit/carmolsiv.pdf

The Steam-Methane Reforming Process Purifies Hydrogen

Hydrogen, the most abundant element in the Universe (also the lightest) is actually rare to find in a pure form here on Earth.  This is due to hydrogen’s willingness to bond with other atoms and molecules.  Despite its abundance it needs to be separated from these other atoms and molecules in order to be available in a pure form.

Hydrogen is useful to humans and is useful in some important industries.   Pure hydrogen is primarily used to make ammonia (which is in turn used to make fertilizer) and methanol (which is usually turned into fuel).  However it needs to be separate from all of the atoms and molecules it likes to bond to in order to be of any industrial use to humans.

95% of purified hydrogen produced today is made from the Steam-Methane Reforming Process.  This process produces hydrogen from a hydrogen generating source, this is usually natural gas or oil, however other sources can be used.

Molecular sieve’s role in producing hydrogen doesn’t occur until the end of the steam-methane reforming process.  Before molecular sieve gets used the feed stock(most likely natural gas) must go through a hydrodesulfurization process, a steam reforming process, a heat recovery process and a CO conversion process.  These processes further breakdown the complex molecular structure of the feedstock preparing it for the final stage for hydrogen purification.

The final stage in purifying hydrogen is to use a Pressure Swing Adsorption (PSA) process.The PSA process will use either a 5A molecular sieve, which is usually used to create high purity hydrogen or a 13X molecular sieve to adsorb larger hydrocarbons and other impurities if they are there.

5A  specializes in separating straight and branch chained hydrocarbons from one another.  13X molecular sieve will specialize in removing any additional C02 or NH3 if there is any remaining at this point, it will depend on what you used as a feed stock.

There are over 200 Hydrogen producing plants in the world, most of them should be listed in the link below.  Hydrogen plays an important role in various industrial and scientific applications and molecular sieve plays an important role in making it pure.

 

List of Hydrogen Plants: http://bit.ly/wsYzKM

 

Sources:

http://www1.eere.energy.gov/hydrogenandfuelcells/production/natural_gas.html

 

A Guide to Determine the Value of Sieve in Ethanol Dehydration

All molecular sieves are not the same.  They are not a commodity and the quality varies from manufacturer to manufacturer, therefore it is important to take the time to examine not only the price of molecular sieve but the value.

 Virtually all molecular sieve manufacturers measure the same characteristics and properties in molecular sieve, and it is the various measurements of these characteristics that allow you to determine the value of your sieve.

Although this list focuses on determining the value of ethanol grade sieve a lot of these measurements can help determine the suitability of a sieve product for any particular application.  Ultimately knowing what makes sieve valuable can make a difficult buying decision less complicated.  Listed below are the sieve properties that can help you determine its value.

1. Density – Knowing the density (when coupled with water adsorption) allows you to figure out the overall water capacity of a vessel in terms of volume or mass. Higher capacity = more water adsorbed.  A more valuable sieve has a higher volumetric capacity.
2. Particle size and distribution – Allows for the calculation of pressure drop, fluidization parameters, and critical velocity through the bed which ultimately effects flow rate.  A higher quality sieve has a tight distribution with less “tails.”
3. Static water adsorption – This refers to the overall capacity of the sieve to adsorb water.  (Do not confuse with working capacity which is much less than static capacity and varies with the operation as well as the sieve).  For more information on working capacity see my previous article on calculating working capacity.  A sieve with a higher static water adsorption capacity is always better.
4. CO2 adsorption – This measures how much ethanol is being adsorbed with the water in your dehydration beds.   Water and (sometimes ethanol) can be adsorbed by 3A sieve because 3A is made from 4A sieve and as a result the sieve bed will not entirely be made up of 3A.  Some of the left over 4A sieve adsorbs CO2 and ethanol therefore the higher the CO2 adsorption rate is the higher the ethanol co-adsorption rate in the bed is.  This ultimately reduces the overall working capacity per cycle in an ethanol plant, look for low CO2 adsorption rates.
5. Crush strength – This one’s simple, the higher the crush strength the higher the durability of the molecular sieve beads in operation.  A higher number here means a higher quality sieve.
6. Attrition – This refers to fryability, which is the tendency of the sieve beads to grind up, which produces dust, thus lowering the overall capacity of the bed.  A lower attrition number is better.
7. Ethanol ΔT (Methanol Delta T) – This is a measurement of the ability of sieve to adsorb ethanol, or a measurement of the co-adsorption characteristics of water and ethanol.  If capacity is being taken up by ethanol then the water capacity suffers, which is why a lower number is better.

Feel free to use this list  as a guide to determine if the sieve you are currently using or or wish to buy is going to be a quality product.  You can find most of, or all of this, information about your sieve  by asking your supplier or manufacturer for a certificate of analysis from their quality control department.

Is the sieve you’re buying valuable?

View Molecular Sieve Comparison Chart By Clicking the Link Below

Molecular Sieve Comparison Chart

…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.