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

Purifying ethanol requires running your distilled ethanol through molecular sieve beds in order to produce over 99% pure ethanol.  In order to dehydrate ethanol thoroughly most plants require that you have ten’s if not hundred’s of thousands of pounds of sieve installed in your vessels.

Making a significant mistake here could be hazardous to your co-workers and it could cost your plant a lot of money if you end up rolling your beds or if you have to shut down the vessels for awhile so here are six precautions to be aware of when running your sieve beds.

1. Watch the temperature – The adsorption process creates a lot of heat energy; do not let temperatures exceed 600 degrees Fahrenheit at any time.
2. Start the dehydration procedure with 200 proof ethanol, if you do not have 200 proof ethanol available, use extra caution until a stream with low water content is available for recirculation.
3. Avoid massive slugs of liquid, these can stir the bed.  Liquids may need to be drained while you are adding the wet feed.
4. Avoid rapid pressure fluctuations, these can cause bumping or lifting in the bed.  Pressure is normally released in order to control temperature.  Be aware that as sieve and ethanol/water streams are in contact with one another intermolecular frictional heat can occur.  Heat releases of up to 1,800 BTUs/lb of adsorbed water and 700 BTUs/lb  of adsorbed ethanol can occur.
5. Watch out for hot spots on the bed.  This can be avoided by having a recirculating feed rate that is high enough to maintain a vigorous flow throughout the sieve beds.
6. Make sure you purge the air.  Ethanol is a flammable vapor and it is running through your beds at high temperatures and in the presence of oxygen.  Purging the air can prevent fire hazards.

You have decided to replace your molecular sieve, and now it’s time to load in what may be thousands upon thousands of pounds of molecular sieve in your vessel (of course the amount of sieve you load in depends on the size of your vessel).  What can you do to prepare your vessel before unloading all of this sieve?

In order to help you with that question we have prepared eight useful guidelines that could help prepare your vessel for sieve unloading.

Note: These guidelines are to be carried out before you load the sieve into your vessel.

1)      Before unloading the sieve you should regenerate your vessel by heating and cooling with process gas. Use the same operating conditions that you would normally use when regenerating your bed.

2)       If process gas is not available use nitrogen or another non-toxic gas instead.  Do not use any gas that contains any toxic components at hazardous levels to regenerate your vessel.

3)      After heating the sieve beds, cool them with gas by de-pressurizing the bed to flare.

4)      After using process gas you can start purging the vessel with inert gas at ambient temperature to flare.  It is important that the gas flow rate be sufficient enough to have good distribution inside the bed.

5)      It’s recommended, if you want to be very thorough in the purging process, to pressure up the bed and de-pressure to flare 2 to 3 times.

6)      When outlet gas is 50% below the L. E. L. and free of toxic materials the purging process should be complete.  Once purged the bed is ready to have the molecular sieve dumped inside.

7)      Unloading the sieve is done from the bottom dump port (or manway) with the flow of gravity guiding the sieve to the bottom.

8)      If you decide not to unload the sieve through the bottom dump port then you can unload the sieve with a vacuum hose from the top port.  Bins containers or dumpsters can be used to aid you.

Here are some additional things to consider…

Never enter a vessel that contains used molecular sieve.

During the unloading process the molecular sieve may have adsorbed chemical compounds.  These adsorbed chemicals may be desorbed again when the molecular sieve is exposed to open air, especially if humidity is high or the air is very moist.

These desorbed chemical compounds can create hazards if the desorbed chemical compounds are toxic.   The plant manager or operator has the responsibility to know what chemicals may have be desorbed in this manner and to know what precautions may be necessary to ensure everyone’s safety.

You have decided to replace your molecular sieve, and now it’s time to load in what may be thousands upon thousands of pounds of molecular sieve in your vessel (of course the amount of sieve you load in depends on the size of your vessel).  What can you do to prepare your vessel before unloading all of this sieve?

In order to help you with that question we have prepared 8 useful guidelines that could help prepare your vessel for sieve unloading.

Note: These guidelines are to be carried out before you load your sieve into your vessel. 

1)      Before unloading the sieve you should regenerate your vessel by heating and cooling with process gas. Use the same operating conditions that you would normally use when regenerating your bed.

2)       If process gas is not available use nitrogen or another non-toxic gas instead.  Do not use any gas that contains any toxic components at hazardous levels to regenerate your vessel.

3)      After heating the sieve beds, cool them with gas by de-pressurizing the bed to flare.

4)      After using process gas you can start purging the vessel with inert gas at ambient temperature to flare.  It is important that the gas flow rate be sufficient enough to have good distribution inside the bed.

5)      It’s recommended, if you want to be very thorough in the purging process, to pressure up the bed and de-pressure to flare 2 to 3 times.

6)      When outlet gas is 50% below the L. E. L. and free of toxic materials the purging process should be complete.  Once purged the bed is ready to have the molecular sieve dumped inside.

7)      Unloading the sieve is done from the bottom dump port (or manway) with the flow of gravity guiding the sieve to the bottom.

8)      If you decide not to unload the sieve through the bottom dump port then you can unload the sieve with a vacuum hose from the top port.  Bins containers or dumpsters can be used to aid you.

Here are some additional things to consider…

Never enter a vessel that contains used molecular sieve.

During the unloading process the molecular sieve may have adsorbed chemical compounds.  These adsorbed chemicals may be desorbed again when the molecular sieve is exposed to open air, especially if humidity is high or the air is very moist.  

These desorbed chemical compounds can create hazards if the desorbed chemical compounds are toxic.   The plant manager or operator has the responsibility to know what chemicals may have be desorbed in this manner and to know what precautions may be necessary to ensure everyone’s safety.

What is Arundo Donax?

Arundo Donax is a large cane plant that is native to Asia and parts of Africa.  It is currently showing great potential as a biofuel producing plant, and as a feedstock for producing cellulosic ethanol.  Despite being native to these portions of the world humans have brought it to Europe and North America, showing that it can thrive in versatile climates.

The stem of the Arundo Donax plant is very durable and sturdy and has been used throughout human history to make fishing poles, walking sticks, and many different types of flutes.  Currently they are used to make the reeds for woodwind instruments like the clarinet, saxophone, oboe, and bassoon, but recent studies are showing the potential this plant has to be converted into biofuel.

Arundo Donax has great biofuel potential because of how large the plant is and how fast it can grow.  Arundo Donax grows to heights between 20 and 33 feet tall on average, and can be harvested twice a year per field it is grown on.  Large amounts of fertilizer are NOT needed to grow this plant, and additionally it is also resistant to biotic and abiotic stresses.   This means it does not require a lot of pesticide thus saving farmers or growers of this plant a considerable amount of money.

Arundo Donax has also shown to offer protection against soil erosion and land degradation, and it even has even adapted to grow in saline (salt) land and water.  This ability to grow in harsher conditions and on harsher lands means that Arundo Donax will not need fertile land that is required to grow food crops, another major benefit.

States in GREEN are locations where Arundo Donax grows in the U.S.

Arundo Donax yields approximately 8,000-8,400 BTU’s of energy per pound, and about 20-25 tons of the plant can be produced per acre.  These energy yields plus its ability to grow in difficult areas makes this plant a great choice for producing biofuel.

Arundo Donax is already beginning to be applied to biofuel production.  Midway through 2012, construction on the largest cellulosic ethanol facility in the world will be completed in Italy.  The Crescentino Plant will be able to produce over 13 million gallons of cellulosic ethanol a year.  The primary feedstock for this plant will be Arundo Donax.

As the world continues to look towards alternative forms of energy, Arundo Donax looks to be another potential and realistic source of alternative energy.

Sources:

http://www.chemicals-technology.com/projects/mg-ethanol/

http://plants.usda.gov/java/profile?symbol=ardo4

http://www.biggreenenergy.com/default.aspx?tabid=4269