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.

Molecular Sieve 3A, 4A, 5A, and 13X…

What’s the difference?

So what’s the difference between all of these molecular sieve types?  – The difference is the size of the pores that come on each molecular sieve bead.  The A in 3A stands for Angstrom,a unit of measurement named after Swedish scientist Anders Jonas Angstrom who was looking for a unit of measurement small enough to measure spectral lines (beams of light).  An angstrom is equal 1/10 of a nanometer, or 1/10,000,000,000 of a meter, so when speaking of 3A sieve it refers to the size of the pore on the bead which is 3 angstroms or 3/10,000,000,000 of a meter.  (On a side note here 13X equals 10A).

So why would someone chose 3A over 4A? – The answer depends on what you are trying to accomplish with your molecular sieve.  For example 3A vs 4A, ethanol producers try to make ethanol that is over 99% pure ethanol.  Traditional distillation methods only give them a 95% ethanol purity rate, while the remaining 5% of the substance  is mostly water.  In short they need to separate the final 5% of the water from the ethanol.  Ultimately the choose a 3A molecular sieve here is why.

For this example 3A sieve works best because the size of water molecule is approximately 2.8 angstrom and the size of an ethanol molecule is 3.8 angstrom.  The 3A sieve adsorbs all of the water molecules because they are small enough to fit inside the pores.   The ethanol molecules, which are too large to fit in the pores, are free to pass by thus separating water from ethanol.  If this person were to use 4A, 5A, or 13X sieve it would not work because the pore sizes are large enough to adsorb both the ethanol and water molecules, and thus no separation would occur.

Generally speaking 3A sieve is used for purifying methanol and ethanol.  4A is used for removing C02  and ammonia from natural gas streams as well as being a desiccant for refrigerants, medicines, and electrical components.  5A is used for sweetening natural gas and purifying hydrocarbon gas and liquid streams.  13X (which is really 10 Angstrom) is a multipurpose sieve, it can adsorb the all the particles that previous 3 sieves can adsorb, but it is usually used to sweeten natural gas streams and purify petrochemical liquids and gases.  Ultimately the pore size of sieve can have a very specific use like 3A or it can have wide range of uses like 13X, it all depends on what you wish to accomplish.