For years now, specialized sands known as frac sands have been utilized to augment the production of natural gas and oil from wells. The mining and processing of sand for hydraulic fracturing, or “fracking,” has grown, creating an increased demand for both natural and manmade proppants such as ceramic beads and silica sand. Better known as frac sand because it is most commonly used for hydraulic fracturing, silica sand often is the most cost-effective proppant choice and has become a growing industry in its own right.
The hydraulic fracturing process begins with the drilling of a well into a rock formation. A high pressure fracking fluid is injected into the well. This fluid, made up of water mixed with frac sand and a blend of chemicals, acts as a propping agent or proppant. The proppant prevents the fracture from closing and permits gas to flow through the well.
While silica sand can be used for numerous applications, fracking is the largest use in the United States. About 57 percent of all silica sand produced goes to hydraulic fracturing applications. Fracking currently is used in 90 percent of the natural gas and oil wells in North America.
Frac sand is mostly made up of silica quartz — found in sandstone deposits around the country — that have been extensively eroded. The erosion produces rounded and spherical grains that are well suited for use as proppants. Frac sand also is dense and hard, enabling it to withstand the intense pressures within the rock that would pulverize ordinary sand.
In order to be used in hydraulic fracturing operations, frac sand must meet specific standards set by the American Petroleum Institute (API). Usable frac sand has high silica content, is spherical and rounded and is capable of withstanding extremely high pressures without breaking apart. The API recognizes several frac sand size gradations, but 20/40 and 30/50 generally are in the highest demand.
Because frac sand must have less than one percent moisture, drying is an essential step in processing the material. A lower moisture content also is economical because it reduces the amount of heat required in the drying process.
Rotary dryers are a proven and preferred method to dry frac sand. A varying feed rate (tph) often can have a substantial effect on the efficiency of the drying process. A rotary dryer allows the user to vary the mass airflow for different moisture-content levels or operation at lower throughputs. Flights within the shell are used to cascade the material through the hot gas stream as it moves through the dryer.
Dryers are available in countercurrent- and parallel-flow models. In parallel flow, the burner and feed system are located on the same end of the dryer. The material and exhaust gas exit the dryer on the opposite end. The primary advantage of using a parallel-flow dryer is the direct relationship between the dryness of the product and the temperature of the exhaust gas. This allows the dryer to adjust to changes in feed conditions in seconds rather than minutes.
When designing a dryer for frac sand or any other material, several factors must be taken into consideration, including the moisture content. The moisture content of the feed as well as the desired discharge moisture content level are important considerations. They dictate energy consumption and residence time in the drum. The flights must be arranged for optimum heat transfer. The system is designed to make sure that the material temperature never exceeds 185°F (85°C) because higher temperatures can weaken or damage the frac sand.
Rotary drums are easy to operate and economical to run in comparison to some of the alternatives such as vibrating fluidized bed dryers. Rotary dryers also generally have smaller environmental footprints and are quick and simple to set up.
With proper maintenance and care, a rotary drying system can endure the rigors of frac sand processing. The dryer must be inspected regularly for preventive maintenance, thereby avoiding unnecessary downtime cost and production losses. Some of the most common maintenance requirements on a frac sand dryer are dryer alignment, flighting wear and replacement, and bag wear in baghouses. Burners and fuel trains also need regular inspection to ensure system safety and integrity.
Most drying systems consist of a correctly sized drum and a burner mounted to a combustion chamber. After passing through the dryer, the dried material is discharged to a transfer conveyor for further sorting and separation. The vapor from the process is pulled through a cyclone that is specifically designed to deal with the fine frac sand as well as a high temperature baghouse that removes all fine particulates from the vapor stream.
All oversized material is removed from the feed stream by a trommel screen, which is placed in front of the feed conveyor. Conveyors transfer the material to and from the dryer system.
During the process, the frac sand can be dried in a parallel-flow rotary dryer with specially designed flights and an oversized knock-out box. Because of the fine particulate in the sand, a multi-clone and baghouse combination is utilized to comply with EPA PM2.5 and PM10 regulations. The drying unit is fed by a belt conveyor at a rate of 100 tons per hour. Once inside the drum, the material is transported through the drum by the flights, ensuring that the material comes into contact with the hot gas stream from the burner and combustion gases.
Dryers can be utilized at both the beginning and the end of the fracking process. Prior to its transportation to and use at a job site, frac sand must be dried and treated. After the process is completed, the waste fluids that remain form a sludge. Dryers can extract the liquid from this sludge and turn it back into a solid for proper disposal.
Drying is an essential step in the processing of frac sand for use in oil and gas drilling operations. A rotary dryer provides the sturdiness and reliability required to dry frac sand, resulting in a quality and economically valuable product.