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| Home | Glycol,
the un-Gatorade
Canada is the second largest producer of natural gas in the world, but ranks something like fifteenth in terms of reserves in the ground. Getting all this hard-to-find gas to market can be a challenge. One of the challenges is taking water out of the gas. Glycol dehydration is common in the Canadian Petroleum Industry. Glycol variants of the same green liquid in your radiator are used to take water out of gas streams all over Canada. Dehydration is necessary to ensure efficient operation of gas transmission lines. The reason is simple; without dehydration, (read: water-removal) hydrates or solid blocks of ice would 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 what are called slugs. Slugs can quietly sit and sit in a low spot until enough liquid collects to block the full diameter of the pipe. If enough pressure builds up this "slug" of liquid will get pushed down the pipe and cause unthinkable amounts of damage as it "hammers" its way through valves, meters and other equipment. More on slugs next month. 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. Glycol dehydrators are steel towers, 12" to as big as 48" in diameter, and range in height anywhere from 12’ to roughly 32’ seam to seam. For the purpose of this month’s article, I will provide an over-simplified version of a glycol dehydrator. If you want more detail, get your hands on a Sivalls, Inc. design manual which provides every possible iteration in full Technicolor®. A typical Canadian dehydrator will be on a single skid with a house, a boiler, a dehydrator tower, and one or two pumps. "How works the beast?" you ask. When you see a dehydration skid it can be quite intimidating trying to make sense of the process, with pipes of all sizes going every which way, but it works essentially like this: Picture a 30 foot tall tower about 24" diameter standing in the corner of the building. Near the bottom of the tower, wet gas enters. Free liquids are collected below this inlet, and the gas is allowed to go further up the tower. As it travels up, it is forced through a number of trays with special "bubble caps" on them. At the top of the tower, dry glycol enters. The glycol and the gas run in counter-current fashion, with the driest glycol seeing the driest gas at the top, and the wettest glycol seeing the wettest gas at the bottom. By calculating the flow rates of the glycol and matching it with the gas flow rate, an engineer can provide a reasonable estimate of the amount of water left in the gas stream exiting the tower. Our friends at TCPL typically prefer to see no more than four lbs. of water per 1 Million Standard Cubic Feet. The wet glycol that is collected at the bottom of the tower and is directed through a pump to what is called a reboiler. The reboiler boils the "wet" glycol to the point that the water, but not the glycol is evaporated. This water is let out of the reboiler through what is called the stripping still column. The stripping still column uses some fins to try to condense whatever glycol is trying to escape out the vapor vent, but still allows water vapor to escape to atmosphere. There’s more to it, but this describes the essentials. What are left out are intermediate steps such as glycol filters, accumulator drums, and gas/glycol and glycol/glycol heat exchangers, but essentially the process is as described. Some producers use different varieties of glycol to suit their type of reboiler. (Di-Ethylene and Tri-Ethylene Glycol if you really want to know). Reboilers are often heated with a firetube, but sometimes indirect heat, from hot oil heaters are used also.
There are two variations to the above process worth noting. One older
process is a packed column, sometimes used for very small volumes, where
instead of trays, a packing medium of some sort is used. The flow is the
same, but it is a lot less expensive, more prone to getting plugged and
not as efficient as bubble caps. The second process is also as old as the
hills, but is making comeback. Outfits like Westerman and Natco are
re-pioneering this process called desiccant dehydration. This uses a
medium, usually a tablet made up of a special type of salt (Calcium
Chloride) which, as you know from your youthful slug/snail-experiments,
also has an affinity for water. The catch: the tablets need to be replaced
as they turn into briny water, and some poor fellow is going to have to
put the salt into the tower at inconvenient intervals in -40°
C weather. There is a plus to the second process though: no emissions
other than salty water. You see, the part I left out in the Glycol Dehydrator section above is that glycol not only likes water, but also Benzene, Toluene, Ethyl Benzene, and Xylene, none of which most people care to inhale daily. These chemicals often abbreviated as "BTEX" emissions are almost inevitable as a byproduct of Glycol Dehydrators. The Desiccant Dehydrator has thus made a comeback in areas where emissions are a concern, and where they show promising alternatives to incineration. The desiccant dehydrator is already seeing widespread use for small volume applications such as the drying of casing gas and tank vapors that are used as fuel gas for motors and burners. Such small dehydrators are made by LoTech manufacturing in Edmonton in 6" and 8" diameters. We are seeing a trend away from smaller wellsite-based dehydrators to larger centrally located units. This is because one way to avoid having to run a complicated, and thus high-maintenance, dehydrator, is by injecting glycol or methanol in the smaller flow lines. These products are later recovered in the bottom of the larger dehydrator tower, or in a separator located upstream. Each process works, and every producer decides differently. There will be lots of dehydration going on as gas pricing and pipeline capacities keep going up the way they have been, and that is good for producers, fabricators, and all involved in bringing what’s in the ground to the consumer whose air conditioner, heater, police cruiser, taxi, barbecue just won’t work the same unless we take the water out. |
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