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.

Carbon in Pulp Helps to Purify Gold

Gold is one of the most historically significant, valuable, and precious naturally occurring metals that exists here on Earth.  Its history with humans has been both beautiful and tumultuous:a lot of people personally choose to wear gold as rings or other jewelry to symbolize marriage or wealth, while wars have been started over lands that contain vast amounts of gold to mine.

Gold’s most significant part of human history may have been its use as a monetary policy that was called the Gold Standard, which was one of the most widespread forms of monetary policy until the  Flat Standard started to be implemented in the early 20th century.

Gold’s value as currency and as a form of jewelry requires gold to be pure and this is where the adsorbent activated carbon plays an important role.   Since freshly mined gold ore is rarely ever found in a purified form it must be purified, through a gold extraction process.  Although purifying gold had been done for centuries, beginning in the back half of the 20th century activated carbon began to be used to purify gold because of its strong adsorption capabilities.

Activated carbon is used in a gold extraction method called The Gold Cyanidation Process (which is sometimes referred as Gold Leaching).  Gold Cyanidation is currently the most commonly used gold extraction method.

Gold Cyanidation works by crushing ore with gold in it into small pieces.  Water is then added along with cyanide which breaks the gold down into a pulpy type of liquid.  After this is done the gold must be harvested from the cyanide solution.  Two processes are commonly used after the Gold Cyanidation Process, they are: The Carbon in Pulp Process (CIP) or The Carbon Column Process.

Both of these processes are very similar in that they use activated carbon to remove the gold from the leaching solution, however the equipment used is different.  Carbon in Pulp is used when gold leaching occurs in tank with activated carbon, while the Carbon Column Process removes gold in large stacked columns that are loaded with activated carbon.

Both processes work by utilizing activated carbon which is charged so it can specifically adsorb gold.  The gold from the leaching solution is adsorbed by the activated carbon in either the columns or the tanks, leaving behind the water and cyanide.

As activated carbon reaches its adsorption capacity it is removed and sent to a stripping circuit where it is heated to high temperatures and mixed with cyanide and sodium hydroxide.  After the gold is removed from the activated carbon the gold is placed in vessel where it is attached to steel cathodes where it is usually removed with a pressure water spray and placed in Dore bars.

Activated carbon can help to purify gold over 99% making its role in modern gold extraction very important.

Sources:

http://www.goldminded.com/carbon_in_pulp.html

http://mine-engineer.com/mining/minproc/carbon_ad.htm

http://www.e-goldprospecting.com/html/carbon_in_pulp_.html

http://www.responsiblegold.org/gold_production.asp

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

Molecular Sieve Basics: Crystals Help Determine the Pore Size of Molecular Sieve

This article is a kind of a continuation on an article we wrote in 2011 that discusses the pores sizes of molecular sieve.

Molecular sieve are crystalline metal aluminosilicates that belong to the zeolite family.  That means that the molecules and atoms that make up a molecular sieve are made out of alumina, silicon, and oxygen and because they are crystalline they have a strong degree of order in the way they are laid out.

Molecular sieves specialize in separating very small molecules and atoms apart from one another.  Being part of the zeolite family, molecular sieve has a three dimensional network of pores which can adsorb molecules of a specific size.  The pores on a molecular sieve is what makes sieve special, this is because they can separate any substance down to the 1/10,000,000,000th of a meter, or an Angstrom.  There are four standard pore sizes that a molecular sieve can have:

• 3A, 3 Angstrom pore size
• 4A, 4 Angstrom pore size
• 5A, 5 Angstrom pore size
• 13X, 10 Angstrom pore size (depending on the manufacturer the pore size may be either 8 or 9 Angstrom)

The pores on molecular sieve could have one of two structure types: A structure or X structure.  3A, 4A, and 5A are made from an A structure while 13X is made from an X structure.  The A structure is smaller and more square-shaped than the X structure which is larger and circle shaped.

Aluminum Hydroxide, Sodium Hydroxide, Sodium Bicarbonate, and clay are used in the sieve manufacturing process, when the process is created this combination of material will make 4A molecular sieve when created with a type A structure or 13X molecular sieve when created with a type X structure.

3A and 5A molecular sieve are made once they are ion exchanged with the originally cre

ated 4A sieve.  4A molecular sieve is ion exchanged with potassium to create 3A sieve, the potassium molecules are larger than the sodium molecules they were exchanged with shrink the pore size.  5A sieve is created when 4A sieve is ion exchanged with calcium, calcium molecules are exchanged in a 1:2 ratio.  Every calcium molecule removes two sodium molecules thus increasing  the size of the pore.

The various pore sizes of molecular sieve offer a great variety of services to anyone looking to separate different combinations of molecules from one another.

The Far Reaching Effect Zeolite Has On Everyday Human Lives

Some of the natural occurring forms of zeolite that can be seen on Earth.

Before reading this article here are four questions to consider:

1. Have you ever drank a glass of tap water?
2. Have you ever been to a hospital and seen someone receive medical oxygen?
3. Do you heat up your home with gas?
4. Have you ever washed your clothes with laundry detergent?

The majority of people in Western society would have to answer yes to at least one of these questions, if not all of them.  These are just but a few of the many diverse  medical, practical, and  luxury based purposes and products that zeolites have made possible for humans in everyday life.  This broad spectrum of uses makes zeolite one of the most widely used minerals on Earth, yet most people have never heard of it before.  So the question is, what is zeolite and where does it come from?

Zeolite is a natural occurring group of microporous aluminosilicates that are found here, naturally on Earth. Their widespread use amounted to just under 3,000,000 tons of Zeolite being mined around the world in 2010.

Zeolites are naturally formed under low grade metamorphic conditions.  Low grade metamorphism occurs naturally in the cavities of volcanic rocks, where at temperatures between 200 -320 degrees Celsius, and while under low pressure, zeolites can be formed.  However they have been synthetically formed by humans as well, allowing the creation of a wide variety of different zeolites with many different uses.

Note: Some of the most recently created zeolite was made on-board the Columbia Space Shuttle.The reason for creating zeolite in space is to minimize nucleation effects and eliminate sedimentation.

There are 45 different minerals that are classified as zeolites but they only have three different structure types.  These three structures include chain structure, sheet structure, and framework structure.  Chain structure has crystal pores that form prism shaped crystals, sheet structure has crystal pores that are flat, and framework crystal pores have relatively equal sized pore dimensions.

As of November 2011 there are 201 different frameworks (pore classifications) for each of the three different structure types that have been discovered or synthesized by humans.  This combination of having variable structures and having many different pore (framework) sizes and shapes give zeolite the ability to perform many different tasks because of all of the different variations it can be produced in.

How does zeolite work?

Zeolite is microporous.  On its surface are millions of tiny pores that adsorb different materials which is based on the size and shape of the pore and what type of mineral the zeolite is.  Zeolite is also used to make other adsorbents like molecular sieve which is very effective at separating and purify chemicals.  These tiny pores can filter out material that is not needed for a specific application.

Molecular sieve (pictured above) is one of the products that is created and designed from the structure of zeolite.

Referring to the questions asked at the beginning:

Have you ever drank a glass of tap water?

In the case of tap water, zeolite or molecular sieve collects contaminants in water and removes them so you can drink it.

Have you ever been to a hospital and seen someone receive medical oxygen?

Medical oxygen requires pure 100% oxygen before it can be used.  This pure oxygen is frequently made by removing the other elements like nitrogen and argon from the air that occurs naturally here on Earth.  In this case a 13X molecular sieve is used to remove all other components (that are not oxygen) in our atmosphere so that pure oxygen can be made and administered to patients.

Do you heat up your home with gas?

When natural gas (which is turned into the gas that heats your home) is first harvested from the Earth, it is harvested with a lot of other different elements that could be dangerous for human consumption.  Water also needs to be removed from natural gas streams and again these processes require the use of zeolite based molecular sieve.

Have you ever washed your clothes with laundry detergent?

Laundry detergent uses zeolite as a water softener by removing calcium and magnesium from water.  These elements can interfere with the cleaning benefits that the soaps in the laundry detergent provide.

These are only a few of the many different functions zeolites can provide a person, but there importance in the development of technology and in our everyday lives is undeniable.

Sources:

Metamorphism: http://www.tulane.edu/~sanelson/geol111/metamorphic.htm

Zeolite grown in space: http://www.tubitak.gov.tr/tubitak_content_files//spaceworkshop/presentations/Bac.Nurcan.pdf

Zeolite structures: http://www.galleries.com/Zeolite_Group

Zeolite production: http://minerals.usgs.gov/minerals/pubs/commodity/zeolites/mcs-2011-zeoli.pdf

Amount of Zeolite mined: http://minerals.usgs.gov/minerals/pubs/commodity/zeolites/mcs-2011-zeoli.pdf

More Structures: http://www.iza-structure.org/