Canada is the fifth largest producer of natural gas in the world. With advancements in technology, accessing Canada’s vast natural gas reserves is easier using a combination of horizontal drilling and hydraulic fracturing. But there’s still a challenge removing produced water from the gas.
Glycol dehydration is used as a drying agent to remove water from natural gas and natural gas liquids (NGL). It is the most common and economical way to remove water from these streams and is similar to the glycol variants of the same green liquid used in your car radiator.
Dehydration and gas transmission
Dehydration is necessary to ensure efficient operation of gas transmission lines. Without dehydration or water removal, hydrates or solid blocks of ice can form in the gas lines. Even if hydrates don’t form in the pipeline, liquids might accumulate in low spots on the pipeline. These liquids can form slugs. Slugs can sit in a low spot until enough liquid collects to block the full diameter of the pipe. If enough pressure builds up the slug of liquid gets pushed down the pipe and causes damage as it hammers its way through valves, meters and other equipment.
Obviously, it’s important to remove water from the gas stream prior to delivering it to a pipeline. One of the more common ways to do this is using a glycol dehydrator. Glycol is a favorite because it really likes water, and it is easy to recover water back out of it.
At first glance, a dehydration skid can be quite imposing looking, but imagine a 30 foot tall tower about 24” diameter standing in the corner of the building.
What is a glycol dehydrator?
Glycol dehydrators are steel towers, typically 12” to as big as 72” in diameter, and range in height anywhere from 12’ to roughly 34’ seam to seam.
For the purpose of this article an over-simplified version of a glycol dehydrator will be used as an example. A typical basic dehydrator used in Canada is packaged on a single skid with a house, a reboiler and accumulator (typically with an coil-in-shell exchanger built-in), a trayed or packed dehydrator tower, and one or two pumps. Other items found on the skid might include a flash separator, electrical pumps, still column to name a few.
What is a dehydration skid?
At first glance, a dehydration skid can be quite imposing looking, but imagine a 30 foot tall tower about 24” diameter standing in the corner of the building. Near the bottom of the tower (in an integral separator, and sometimes in a separate vessel), wet gas enters a separator section.
Free liquids are collected below this inlet, and the gas is allowed to move further up the tower. As gas travels up, it is forced through a packed section (common in large diameter towers) or a number of trays (In Canada, typically eight to ten) fixed with special bubble caps or valve tray. These trays are designed to maximize glycol to gas contact.
At the top of the tower, dry glycol enters. The glycol and the gas run counter-current, with the driest glycol contacting the driest gas at the top, and the wettest glycol and wettest gas finding each other at the bottom. Process engineers calculate the flow rates of the glycol and match it with the gas flow rate, and can provide a reasonable estimate of the amount of water left in the gas stream exiting the tower. (OilPro uses Sivalls’ “Glycol Dehydration Design Manual” by C. Richard Sivalls to get an indication of likely requirements) In Canada, a typical target acceptable amount is no more than four lbs. of water per one MCF, warmer climates can deal with 6-8 lbs/ MMSCF.
The reboiler boils the “wet” glycol to the point that the water is evaporated
Wet glycol collected at the bottom of the tower is directed through a pump to a reboiler. The reboiler boils the “wet” glycol to the point that the water is evaporated. This water is released from the reboiler through the stripping still column. The stripping still column uses fins or packing to try to condense the glycol that is trying to escape out the vapor vent, but still allows water vapor to escape to atmosphere. The process also uses glycol filters, accumulator drums, and gas/glycol and glycol/glycol heat exchangers, but the simplified process is as described. Some producers use different varieties of glycol to suit their type of reboiler. (Di-Ethylene and Tri-Ethylene Glycol). Reboilers are often heated with a firetube, but sometimes indirect heat from hot oil heaters or other sources are used.
The desiccant dehydrator is most economical when used for small volume applications
A note about packing is that it is typically less expensive than trays, but more prone to getting plugged and not as efficient as bubble caps and doesn’t have as good a turn-down ratio. Another process which manufacturers such as Westerman and Natco are re-pioneering, is called desiccant dehydration. This uses a tablet made up of a special type of salt (Calcium Chloride) that attracts water. The tablets need to be replaced as they turn into briny water. This process releases no emissions but does require appropriate disposal of salt water. Smaller volume applications, such as low volume fuel gas solutions offered by Lo-Tech, are sold by OilPro.The desiccant dehydrator is most economical when used for small volume applications such as the drying of casing gas and tank vapors that are used as fuel gas for motors and burners.
Additional trays allow for a slower glycol recirculation rate and help reduce BTEX emissions
A glycol dehydrator not only captures water, but also Benzene, Toluene, Ethyl Benzene, and Xylene. These chemicals, known as “BTEX” emissions, are a byproduct of glycol dehydrators. One popular method used to mitigate BTEX uptake is to add trays: additional trays (i.e.10 instead of 8 or 6) allow for a slower glycol recirculation rate and help reduce BTEX uptake and thus reduce BTEX emissions from the still column.
Still column vapours
Incineration of still column vapours, offered by companies such as Total Combustion Inc. is proven method of BTEX emission reduction. A rule of thumb is that still column incineration typically starts to make economic sense in dehydration packages with towers of 24″ and up. Companies such as Sivalls, Inc. also offer the RBTEX unit, which condenses and separates these vapours. The desiccant dehydrator has made a comeback in areas where emissions are a concern also, as Ca Cl does not pick up BTEX.
Summary
Currently, larger, centrally located units are gaining favour over smaller wellsite-based dehydrators. Using larger centrally-located units avoids having to run a complicated, and high-maintenance dehydrator. Local hydrate reduction, achieved by injecting glycol or methanol in the smaller flow lines is often sufficient. These products are later recovered in the bottom of the larger dehydrator tower, or in a separator located upstream.
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