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.

Fukushima Nuclear Disaster Overview

On March 11, 2011 a 9.0 Earthquake, the fourth largest on record since 1900, occurred in the Pacific Ocean causing a series of large tsunamis to strike the eastern coast of Japan.  Now called the Tohoku earthquake, it (and the subsequent tsunamis) caused one of the worst natural disasters on record with over 15,361 people killed, a million buildings damaged or destroyed, and a financial cost of $235 billion dollars (the most expensive natural disaster on record).

In addition to the large amount of damage caused by this natural disaster, it also caused one of the greatest man made disasters on record at Fukushima.  Japan, a country that heavily relies on nuclear power, had a number of nuclear plants that were built near the eastern coast when the tsunamis struck.  Fortunately Japan, a country that sits near two fault lines and has a history with experiencing earthquakes, had containment measures set in place for just such an occurrence.  Once powerful enough earthquake tremors are recorded near Japan’s nuclear plants, they begin to shutdown and cool off.  For the most part this worked, except at Fukushima.

At Fukushima the plants power failed during the earthquake, so the emergency power system along with the emergency cooling condensers had to be used.  Less than half an hour after the emergency systems turned on, tsunamis began to strike the coast near the Fukushima power plant.  A seawall 19 feet high was put in place around the plant to protect it against tsunamis.  The waves from the tsunamis that hit Fukushima were over 46 feet tall, rendering the seawall worthless.  After crashing over the seawall, this wave not only destroyed backup power to the plant but also wiped out key equipment that was part of the emergency core cooling system.

The destruction of key cooling equipment by the tsunami triggered full meltdowns of reactors 1, 2, and 3, thus beginning what we now call the Fukushima Nuclear Disaster.  After the earthquakes subsided, cleanup began and radioactive damage was assessed.  High levels of  Caesium-134, Caesium-137, and some other radioactive isotopes were detected around the nuclear plant and in the ocean.   Currently efforts are on-going to clean up the radioactive waste as a result from the meltdowns.

Molecular Sieve and Zeolites Role in Clean Up

The Fukushima disaster is only the second nuclear disaster (the first was Chernobyl)  to receive a 7 on the International Nuclear Event Scale (INES), the highest disaster rating a nuclear event can be rated.  The clean up process will see new technological developments as well tried and true methods during the clean up of radioactive waste.

One method that’s currently being used, and was used in the past, is using zeolites.  Shortly after the disaster, the Japanese government began to order the dropping of zeolites in the oceans surrounding the disaster site.  The Japanese government is hoping that zeolites (the one’s that the Japanese government are using have specialized in nuclear waste processing), will help to slow down radioactive contamination of the ocean.  Zeolites had previously been used in the clean up of the 1979 Three Mile Island Nuclear Disaster in the United States.

Although dropping zeolite in the ocean seems like a desperate attempt to contain the disaster, Japan is also utilizing  molecular sieve in the clean up process, too.  The molecular sieve in use was specifically designed to capture Caesium, and is being used to treat radioactive wastewater that is on the disaster site. Since the disaster began over 43 million gallons of wastewater have been treated with this molecular sieve at Fukushima.

Experts expect the Fukushima disaster clean up to last decades.  As the clean up continues adsorption technology will continue to play an important role in cleaning up the oceans, environment, and reducing the amount of damage that will be done to our atmosphere.

Sources:

https://share.sandia.gov/news/resources/news_releases/fukushima_cleanup/

http://www.japannewstoday.com/?tag=fukushima-zeolite-absorbs-radiation

http://www.emfnews.org/Fukushima-Decontamination-and-Zeolite.html

http://www.world-nuclear.org/info/fukushima_accident_inf129.html

http://earthquake.usgs.gov/earthquakes/eqarchives/poster/2011/20110311.php

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