Calcium-Chloride Wellhead Gas Dehydrators
Authors: Fowler O.W. 
Published: December 2007

Introduction

The calcium-chloride dehydrator is a combination water-vapor absorption tower and solid chemical bed desiccant unit. The passing of the gas through the upper bed of anhydrous calcium chloride forms a concentrated brine of approximately 1.40 specific gravity. The bed settles as the calcium chloride is consumed from the bottom and will give efficient dehydration until only 2 ft of bed remains. The brine formed drips down into an absorption section that has three to five self-circulating trays. In the absorber section, the brine absorbs water from the gas as it flows downward by gravity from tray to tray, countercurrent to the wet gas rising from the bottom. The brine is continuously recirculated on each tray as a fine mist, when it is picked up by the gas at the tray nozzle. The specific gravity of the brine, at the bottom of a five-tray column, is normally between 1.15 and 1.20. Because of the low cost of the calcium chloride, the brine is considered expendable and dumped into a pit along with any free water produced by the well.

It is the efficient utilization of the calcium-chloride brine in the absorption-tray section that makes the use of calcium chloride as a drying agent economically feasible. Inasmuch as the unit consumes calcium chloride in direct relation to the water vapor contained in the gas at operating pressures and temperatures, the needed time of recharging the bed can be forecast accurately. The calcium-chloride pellets that are used for charging the dehydration unit are especially made for this use.

DISADVANTAGES OF THE UNIT

We have found the following faults with the calcium-chloride wellhead dehydration unit.The field men do not like to recharge the units because the salt dust is irritating to the skin and the old method of recharging where the drums were lifted to the top of the 20-ft unit was unwieldy. Recently we have been using a vacuum blower for loading the beds, and the men prefer it, although it needs some minor mechanical improvements.The field men have no way to check the effective dehydration, except by visual inspection of the bed or running a dew-point test.The fire tube has burned through on most of the old units, and trouble has been experienced in keeping the heaters lighted. These are mechanical problems that either have been corrected or can be. We have this trouble with all heater and regenerator fires because of the strong winds and rough terrain.Solidification of the brine dump line occurs on approximately 2 percent of the units per year. On gas flow conditions below 70F, this phenomenon vary rarely occurs. This is minimized on those units where we know it occurs by steam cleaning in the fall. We occasionally experience plugging of the separator water dump line on the glycol dehydration units due to deposition of salts.The calcium-chloride bed channels and bridges on approximately 5per cent of the San Juan basin units and 10 per cent of the Big Piney field, Wyo., units, thus reducing the effective dehydration. The use of large pellets of calcium chloride reduces channeling and bridging. Steam cleaning the bed section reduces channeling and bridging. No conclusion has been reached as to the effectiveness of plastic lining the bed section to reduce this problem.

High daily temperature variation to as much as 50F increases channeling and bridging. The use of the vacuum-blower bed loader reduces channeling and bridging by eliminating most of the powdered calcium chloride. When recharging the bed by dumping the calcium chloride from the barrels it is shipped in, the powdered material is sometimes dumped into the unit. This fine material on absorption of water vapors forms a semi-impervious mass that causes the gas flow to concentrate, thus resulting in a channel. If this mass reaches across the bed and bonds to the side wall, a bridge will be formed that will support the pellets above. The field men, on visual checking of a bridged bed, may assume the unit does not need recharging.6. On unsaturated gas streams, calcium-chloride units are not as effective as other types of dehydration.