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/

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

How are they different?

Activated alumina and molecular sieve, they look similar (they both come in a small spherical beaded shape),  they perform the same process (they adsorb material at a molecular level), and they both have regenerative properties (you can re-use the material once the desiccant has reached capacity) yet they are two completely different products.  So the question is, with so many similarities, how are they different?

What they are made of is a good starting point.  Activated alumina is made out of aluminum oxide that is highly porous, while molecular sieve is made out crystalline metal alumino-silicates.  What this means is the pores on molecular sieve can be shaped into specific sizes most commonly seen as 3A, 4A, 5A, and 13X, where as activated alumina’s pores do not have specifically measured sizes.  This means molecular sieve can be used to separate certain molecules of specific sizes from one another, for example removing ammonia from natural gas streams.

From an application standpoint, here is how they differ.  Activated alumina has a real strong water adsorption capacity, it can adsorb a lot more water than molecular sieve, this makes it a very useful material in air compressors or for certain natural gas processing applications. The durability of the material allows it withstand a lot of pressure along with high levels of humidity.

Activated alumina can’t adsorb the large variety of materials or separate certain molecules from one another like molecular sieves can, making it ineffective in a process like ethanol dehydration.  This is because activated alumina would be able to adsorb both ethanol and water molecules and thus no separation would occur.

Molecular sieve may not be able to adsorb as much water but if you needed to reduce water to very low amount, up to 0.1ppm, molecular sieve would be your absorbent of choice because this is something other adsorbents besides molecular sieve have been incapable of doing.

Molecular sieve can also be used to separate specific molecules from one another, due to the customization of their pore sizes.  For example you can separate water from ethanol, and carbon dioxide, ammonia, and larger hydrocarbons from natural gas streams, which is something activated alumina can’t do, or won’t do with same efficiency.