Frac Sand Drying Methods: Rotary vs. Fluid Bed

Jun 11, 2015

To reach gas that was previously thought to be unreachable, pressure is used to fracture rock formations deep underground to allow a better flow of gas through a well. When the pressure is removed, the fracture collapses, which dramatically diminishes its effectiveness. A special type of refined sand is used as a "proppant" to keep the well linings propped open after water-based slurry has been injected into the casing. Hydraulic fracturing, or "fracking," is a mining technique that greatly enhances oil and gas well productivity and is being aggressively deployed around the world, and North America in particular. The sand remains in the fracture when the pressure is removed, keeping the fracture propped open and allowing the gas to continue to flow through the well. Thanks to the Marcellus Shale Formation, the Northeast region has experienced a recent economic boom. This turnabout came after technological advancements were made in horizontal drilling and fracking. Although fracking was used by the oil industry in the 1860s with liquid nitroglycerin in shallow wells, it wasn't until the modern version of this old technique was developed, in which the extraction of deeper pockets of natural gas became a reality.Frac sand is a high-purity quartz sand with very durable, round grains, and is a crush-resistant material produced for use by the petroleum industry. The U.S. is the largest consumer and producer of frac sand, and according to Wall Street Daily, about 25-30 million metric tons of it are used to supply the 1.1 million active gas and oil wells in the U.S. The American Petroleum Institute (API) determines frac sand specifications that include grain size, sphericity, crush resistance, and solubility. Mining companies are therefore eager to sell only frac sand that meets these API specifications to their well operator customers. To ensure an acceptable quality of sand, mining companies wash and dry it to remove all possible impurities. Two types of industrial dryers are ideally engineered to meet the stringent requirements that sand mining companies must adhere to, in order to consistently provide the highest quality sand to their energy producing customers. These dryers are fluid bed and rotary. On the surface, a fluid bed dryer is similar to many other types of drying equipment. It processes granular, free-flowing material at product temperatures ranging from 140 to 300°F, and suitable materials include sand, minerals, clays, organic salts, coal, and specialty chemicals. But the similarities end there. In a fluid bed, the material being dried is suspended and completely surrounded by the drying air or gas, causing that material to behave like a fluid. Besides keeping the material in a seemingly liquid state, the gas stream is the media for heat and mass exchange. As a result of the intimate gas-to-solids contact, very high rates of heat transfer are accomplished while the sand grains are gently handled. The bedplate allows the uniform distribution of the gas stream during operation, and supports the bed of material during shutdown. A fluid bed dryer provides lower capital equipment costs at installation and lower operating costs over the life of the machine, including reduced baghouse and scrubber costs, while also increasing productivity. Because there are no moving parts inside the fluid bed, maintenance costs are greatly reduced compared to other systems.In contrast, rotary dryers handle a much broader range of materials, regardless of their conveying and handling characteristics. Whether the process requires the drying of fine and dusty powders, lumpy solids, sticky semi-plastics, sludges, pellets, agglomerates, or even a mixture of all of the above, a rotary dryer will successfully accomplish its task. A rotary dryer is a rotating cylinder, or shell, that is slightly inclined to the horizontal, and its length can be four to ten times its diameter. Material is fed into one end of the cylinder and, by virtue of its rotation and slope as well as the head effect of the material, and it exits continuously from the opposite end. The interior of the shell is equipped with specially designed flights that advance the feed material into the shell's active zone and efficiently expose the material to the heat source. An important determination is the usage of direct or indirect heat in the drying process. Rotary dryers use either direct convection heat transfer or indirect heat transfer, which depends upon hot surfaces within the dryer to provide heat by conduction and radiation. The determination of direct versus indirect heat is contingent upon the properties of the material to be dried, the process conditions and the desired end product. The optimal method of drying frac sand is with direct heat, with hot air entering the rotating shell at one end, passing through it and exiting at the other end. Flights welded to the internal shell walls lift and shower the wet sand, bringing it into direct contact with the hot air. The configuration of the flights depends on the characteristics of the material being processed, and the length and diameter of the shell depends on the drying capacity. Indirect heat drying is most frequently used for products with small particle sizes that could result in excessive dust generation. The frac sand industry utilizes both types of dryers. Rotary dryers are the more expensive of the two, but they are more forgiving and allow greater process turndown capability. Fluid bed dryers cost less, are more thermally efficient and have minimal maintenance due to fewer moving parts. When taking into account certain frac sand drying requirements that must remain consistent throughout the process, such as throughput, particle size and moisture content, then fluid bed drying is the optimal method. However, rotary dryers provide greater versatility, which allows the frac sand market to better adjust to the needs of its industry. David Phillips is marketing & communications manager, Heyl & Patterson Inc., Carnegie, PA. Founded in 1887 in Pittsburgh, Heyl & Patterson is an industry leader in the design and construction of bulk transfer and thermal processing equipment for customers in a wide range of industries, including chemical, steel, biomass, energy, ports, and mining & minerals. For more information, visit www.heylpatterson.com.

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