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

An Overview of How Desiccants Are Involved in Sulfur Removal

Sulfur Stinks…

Literally, in nature some naturally occurring sulfur and sulfur compound smells include skunk spray and rotting eggs.  Sulfur’s reputation as a horrible smelling element has led the natural gas industry to calling any natural gas with sulfur in it, sour gas.  In addition to its horrible smell sulfur can also be deadly to humans when it is potent enough, and it is also corrosive.  Thus the removal of it from natural gas streams is an essential process, which is called natural gas sweetening.

In order for natural gas to be considered sour, hydrogen sulfide must  exceed 5.7 milligrams per cubic meter of natural gas.  Natural gas sweetening usually uses a process called amine treatment in order to remove sulfur from gas streams.  Sulfur has an affinity for amine and when it passes through the tower that contains amine, the sulfur sticks to the amine and is removed from the stream.  Amine treatment works similarly to glycol treatment because both use a liquid solution to remove unwanted elements and compounds from natural gas streams.

Despite my sulfur bashing earlier, and yes it does smell, it can also be useful.  For example sulfur is frequently used in fertilizer, matches, and insecticides and it is used to make sulfuric acid which has many industrial applications.  The sulfur that is removed from natural gas streams can be recovered using the Claus Process and sold as a separate product from natural gas.

The Claus Process has two steps: the thermal step and the catalytic step.

The thermal step is designed to turn the majority of the hydrogen sulfide removed from the gas stream into regular sulfur.  This is done by oxidizing hydrogen sulfide with air at high temperatures.  Approximately 60-70% of the sulfur is produced during this step.

The catalytic step is designed to take the remaining hydrogen sulfide and the newly created sulfur to make even more sulfur by heating them together over a catalyst, usually activated alumina and a specialty titania.  Specialty titania helps to convert the remaining hydrogen sulfide into sulfur and activated alumina helps to protect the titania from sulfation poisoning due to oxygen breaking through.

Approximately 97% of sulfur that is removed from natural gas streams is recovered using the Claus Process.  In the United States approximately 15% of all produced sulfur is extracted from natural gas streams.

Sources:

http://www.naturalgas.org/naturalgas/processing_ng.asp#sulphur

http://minerals.er.usgs.gov/minerals/pubs/commodity/sulfur/

What Is It and How Does It Separate Oxygen from Nitrogen

What is carbon molecular sieve?

Carbon molecular sieve is an adsorbent that fuses the ideas behind both activated carbon and zeolites into one product.  Activated carbon is known for its high porosity and zeolites are known for their ability to be crafted into highly specialized adsorbents called molecular sieve.  Carbon molecular sieve is a product that brings the benefits of both of these products together.

Carbon molecular sieve is made out of coal (the same material most activated carbon is made out of) and it specializes in adsorbing material under 10 angstroms, something activated carbon can not do accurately.  The smallest pore size created for carbon molecular sieve is 4A, but it exists in a 5A, and 10A (or 13X) as well.

Carbon molecular sieve specializes in separating oxygen from nitrogen, an important part in natural gas processing. This process is done with a PSA (Pressure Swing Adsorption) device in two phases.  The first phase sees the gas enter the PSA generator and the oxygen is adsorbed while the nitrogen passes through because the nitrogen molecules are too large and are used as a separate product.  The second phase sees the oxygen slowly released from the sieve at low pressures and thereby regenerating it so that the separation process can be repeated.

Carbon molecular sieve is used in this situation as opposed to activated carbon because the physical size between oxygen (0.28nm×0.40nm) and nitrogen (0.30nm×0.41nm) molecules are so close.  The pore sizes on carbon molecular sieve are able to accommodate these small size differences, where as activated carbon would just end up adsorbing both of them.

Molecular sieve isn’t used because it is a polar adsorbent, meaning its surface area attracts other polar molecules.  Oxygen is a non-polar molecule and would be attracted to other non polar surfaces.  Carbon molecular sieve is one of the few non-polar adsorbents out there which is why it is chosen over molecular sieve for this application.

In addition to separating nitrogen from oxygen carbon molecular sieve can be used for metal heat treatment, electron production, and as preservative in food products.

Differences Between Glycol Treatment and Desiccant Treatment

Water removal from a natural gas stream (drying) is an important step in processing natural gas, it prevents corrosion in pipelines and also prevents plugging of pipelines by removing free water and hydrates.  Natural gas drying is preceded by the removal of oil and condensate from the natural gas stream.  Currently there are two common processes which see to the removal of water from gas streams: glycol treatment and desiccant treatment.

Glycol treatment primarily uses triethylene glycol , diethylene glycol, or tetraethylene glycol to adsorb and remove the water from the natural gas stream.  Glycol will adsorb water from liquid gas streams in a dehydrator.  As glycol adsorbs water it becomes heavier and sinks to the bottom of the dehydrator.  After the glycol has adsorbed the water it is boiled out of the dehydrator leaving behind liquid natural gas.

Desiccant dehydration requires the use of adsorption towers, which contain desiccant usually molecular sieve, activated alumina, or silica gel.  Wet natural gas is passed through the top of the tower which contains thousands of pounds of sieve or alumina beads and by the time it reaches the bottom of the tower the water will be removed from the gas stream.  Multiple adsorption towers are used during this process to allow over saturated desiccant to be regenerated.  In other words while one tower has gas running through it another tower is regenerating the previously used (and now over-saturated) desiccant.

The advantage of using dry desiccants is their ability to adsorb and reduce the water from natural gas streams to lower concentrations than glycol.  Pipelines require that water content in gas streams not exceed 7lb/MMSCF (million standard cubic feet) and dry desiccants can achieve this level easily (up to 2lb/MMSCF).  Glycol dehydrators can achieve this level but usually at the bare minimum and sometimes they don’t make the requirement and have to go through the treatment process again.  Although glycol treatment is more popular right now, dry desiccants appear to be more effective at drying natural gas.

http://www.kwintl.com/glycol-dehydrators.html