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

 

 

   

6 Precautions to Consider While Dehydrating Ethanol

 

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.

8 Useful Guidelines Before Doing a Complete Molecular Sieve Change Out

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.

8 Steps to Prepare Your Vessel for Unloading Molecular Sieve 

 

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.

The Guide To Supporting Your Molecular Sieve Beds

The use of molecular sieve supports such as screens and ceramic balls are very important for the industrial use of molecular sieve.  Ceramic balls prevent adsorbent from leaking on the screens that have been placed below them, and the screens keep the molecular sieve separate from the ceramic balls.

In an industrial setting molecular sieve is under a lot of pressure.  The large dehydration vessels that molecular sieves are used in subject the molecular sieve to a great deal of weight and temperature related stress.

In order to counteract these stresses, supports and screens are needed in the sieve vessel at the top and at the bottom.

Supporting the Bottom:

At the bottom of the vessel a 6 inch layer of ceramic balls is needed for support.  The two layers of ceramic balls will each need 3 inches of space each adjacent layer starting from the molecular sieve should double in size.

For example, if you are using a 4×8 mesh size molecular sieve the first ceramic ball layer beneath it should consist of ¼” balls, and second layer of ceramic balls beneath the first layer should consist of ½” balls.  If you are using an 8×12 mesh size, the first layer of ceramic balls should be 1/8” and the second layer should be ¼”.

Loading the support balls on the bottom is usually done (depends on the plant design) through a side manway that should be located on the bottom of the vessel.  Ceramic balls should be leveled once they are placed, this can be done using a rake.

Supporting the Top:

At the top of the vessel a 6 inch layer of ceramic balls is also needed.  Only one layer of ceramic balls is needed for the top.  Ceramic ball size should be either ½” (for 8×12 mesh) or 1” (for 4×8 mesh).

Loading the top is done with either by using small buckets or lifting the boxes/bags of ceramic balls to the top of the vessel.  They are then placed evenly on the screen that is covering the molecular sieve.  In order to prevent ceramic balls from breaking do not let them fall over 6 feet.

Setting the Screens:

Most support structures designed for molecular sieve can endure a 50-100 PSI pressure drop.  The most common designs use I-beams supports that are attached to the vessel walls, the I-beams should be able to move around freely during heating or cooling processes.

When setting up screens at the bottom of the vessel a metal grating is cut so that it is smaller than the vessel diameter and it is placed on top of the I-beam.  The gap needs to be covered by rope packing that can withstand temperatures up to 400 degrees Fahrenheit.

Multiple different screens are needed when setting them up in a molecular sieve vessel.  On top of the grating a 3-5 mesh screen (¼” to ½” openings) is used first, after that a second screen 10-20 mesh is placed on top of the 3-5 mesh screen.

When setting up screens at the top of the vessel a 10-20 mesh screen is needed.  The screen should overlap the vessel wall by 4 inches, this prevents any of the ceramic balls from falling into the molecular sieve.

The use of screens and ceramic balls is very important in a molecular sieve vessel.  It can prevent gas surges that move your sieve, an unleveled bed, flow channeling, and a possible early breakthrough.