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

Most people don’t know what they are looking for when looking to purchase molecular sieve and are at the mercy of their vendor.

Molecular sieves are prevalent throughout the refining and chemical processing industry.  Equally prevalent is a myth that all molecular sieves are the same both from a physical property and performance standpoint. Nothing could be further from the truth.  All molecular sieve manufacturers have various options on the purchase of raw material as well as the way they manufacture these material.

The sources for the silicates, caustic, potassium, calcium etc must be considered as well as the type of clay being used in beaded products.

Processing conditions and methods must also be considered, the methods utilized for crystal growth, ion exchange, blending, calcification and particle distribution control must all be considered.

Even though all of these details are important to understand,  it is not always practical to do so. Really, the person responsible for purchasing needs to only look at the end properties and compare these between vendors.

When doing so it is important to get actual C of A’s from the vendors and not depend only on spec sheets. The purchaser should create a spread sheet listing each pertinent property along with the names of the various vendors. This enables a quick and easy analysis and comparison to facilitate the decision making process.

I’ve included an example of a spread sheet I use for comparing molecular sieves targeted towards the Bio refinery market.I always include an explanation of the relevance of each property in the spread sheet to make the value decision more evident.