This is a common question that is a function of a couple of variables. The refill rate of the vessel is a function of the volume of desiccant contained in each vessel and the total quantity of water that is removed from the gas stream to meet the customerʼs outlet gas specification. Please note that the consumption of the various types of desiccants are different; therefore the refill rates in a multistage system will all be different. Also in a multistage system the first vessel will remove approximately 70% of the water and will be the vessel that will need to be serviced the most. The NATCO engineering design program can be used to design a system to meet the customerʼs needs. It is important to remember that for the vast majority of situations, refill intervals are not a critical issue. Most production sites are visited every day or two and the refill process, except for very large systems, takes no more that 30 minutes. Longer refill intervals require larger or more vessels and consequently higher equipment costs. Stress the ease of the refill operation unless the customer has a legitimate reason for extended refills, such as a remote site location or sour gas, etc.

Yes the desiccant dehydration technology will work in sour gas production and is a definite benefit because it eliminates odor problems caused by H2S and unwanted stack gas emissions such as SO2. There are desiccant dryers in operation today that are dehydrating sour gas with H2S concentrations of 30,000ppm. Additionally, dry desiccant units are successfully operating on gas streams containing 20% by volume of CO2. These desiccants will operate on pure H2S and CO2 gas streams.

The NATCO engineering program is used to calculate the life bed of a vessel for a given set of process conditions. After each unit is installed at the customers production site it is recommended that the vessel be reopened after a few days of operation to measure how far the top of the desiccant bed had dropped inside the vessel. If the bed has dropped 1ʼ to 0” over a three day period and the bed height is 4ʼ to 0”, this probably means that the vessel will need to be filled every nine days, since 1ʼ to 0” of desiccant should remain in the vessel at all times. Over time the person responsible for filling the vessel will get a feel for when each vessel needs to be refilled with desiccant. Changes in the inlet gas temperature, pressure and the duration of down time at the facility will affect the time interval at which the vessel needs refilling. Each vessel is designed with a 1-1/4” 3000# sight port on the side of the vessel, which is located 1ʼ to 0” above the bottom of the desiccant bed. It is important to never let the desiccant height in the vessel drop below 1ʼ to 0”, as this will start to affect the outlet gas stream.

Fill each vessel with the correct desiccant until the desiccant height is approximately 1” below the gas outlet.. The dryer vessel has a stainless steel screen welded inside to cover the nozzle opening. This screen will prevent tablets from falling into the gas outlet piping should the dryer be overfilled. Overfilling the dryer will not cause any operational problems in a properly designed desiccant dehydration system.

Almost everyone asks about the disposal of the brine (salt water) at the bottom of the dryer vessel. The brine is piped to the customer’s water storage vessel and disposed of with all other production fluids. It is important to note that dryers produce a very small amount of brine when compared to other production fluids. A dryer flowing 20MMscfd at 1250 psig and 70ºF, drying 5# water/MMscf outlet gas spec produces only one barrel per day, assuming the inlet gas stream is completely saturated with water vapor.

Deliquesant desiccants are primarily made from various blends of alkali earth metal halide salts and are naturally hygroscopic. Water vapor is removed from gas as it flows through a bed of desiccant tablets in a pressure vessel. Moisture is attracted to salts in the deliquescent tablets and coats them with hygroscopic (water attracting) brine. This brine continues to attract water and forms a droplet and then flows down the desiccant bed into a sump. An automatic level controller and control valve then discharge the effluent waterinto a storage tank. Common brine water is the only byproduct. Since the desiccants dissolve upon attracting and absorbing water vapor (deliquesce), new desiccant is simply added to the vessel when needed.

For corrosion to occur, free oxygen must be present. The brine inside the vessel will not cause corrosion in the dehydration of any gas streams in which oxygen is not present. Natural gas, CO2 and H2S contain no free oxygen. In any dry desiccant systems in which free oxygen is present, corrosion is a major design concern. The vessels can be coated internally with an appropriate system. Carboline and other major coating manufactures have systems specifically designed for salts. Additionally, the drain piping and valves must be appropriately protected through the use of stainless steel trim in the valve and internal coating in the piping of the use of stainless steel pipe.