Hydraulic fracturing, or fracking, is a mining technique that greatly enhances oil- and gas-well productivity and is being aggressively deployed all over the world and particularly in North America. Frac sand is used in hydraulic fracturing for the recovery of gas and oil from previously inaccessible deposits and dormant wells.
The basic process was first used by oil producers in the 1860s as a method to stimulate shallow hard-rock wells. It was next applied to water and gas wells with the water-and-sand based technique in the 1940s. With the advent of specialized frac sand in the late 1990s, the process was perfected for use in oil and gas wells.
The fundamental fracking methodology consists of pumping a fracturing fluid — usually water — into the wellbore at a pressure that exceeds the pressure of the base rock formation. The speed of the decline into the well and the width of the fracture can be controlled by using frac sand as a proppant to prevent fractures from closing prematurely when the injection is stopped and the pressure is removed.
A single fracking job at one well can require a few thousand tons of sand. As the demand for frac sand exploded, the need to process the sand more efficiently also has increased in a dramatic fashion. Customers in the oil and gas industry require a specific type of sand, hence the need for frac sand processing. The sand must be extremely pure in a size that matches job requirements, with a spherical shape that is more easily carried in the fluid.
Large amounts of the raw sand are deposited in glacial landforms called drumlins throughout the midsection of the United States. Most of the currently active mines are located in Wisconsin, Minnesota and Ohio.
Frac sand is a high-purity quartz sand with durable and round grains, and it has crush-resistant properties. In order to be used, frac sand must be dried and refined after it is mined. Specifically, in order for the desirable sand to be separated from unusable impurities, it must be washed and dried using specialized thermal processing equipment before it is transported to the sifting process. Two types of equipment are used for drying the sand — rotary and fluid bed. Rotary dryers are more forgiving regarding feed material properties such as moisture content and particle size; fluid bed dryers are more thermally efficient.
Rotary Dryers for Frac Sand Processing
A rotary dryer is a rotating cylinder, or shell, that is slightly inclined to the horizontal. Hot air enters the rotating cylinder at one end, passes through it and exits at the other end. This action directly heats the frac sand as it is fed into one end of the cylinder and, by virtue of the cylinder’s rotation and slope, exits continuously from the other end.
Flights welded to the interior of the cylinder lift and shower the wet sand, bringing it into direct contact with hot air. The configuration of the flights depends on the characteristics of the material being processed. The length of the cylinder is usually about four to ten times the diameter in order to achieve efficient heat transfer.
The rotating cylinder operates at neutral or slightly negative pressure and is sealed at both ends to stationary breechings or end boxes. Various seal configurations are used to connect the breechings to the cylinder, ranging from simple fabric segments to more robust metallic seals, depending on the application. Both wet feed material and heated air enter the cylinder through one of the breechings and exit through the opposite one. These breechings are sealed to any stationary downstream equipment such as the air heater, conveyors, airlocks and off-gas handling equipment.
Airflow in rotary dryers is either counter or parallel to the conveyance of the sand inside. In counter-flow dryers, air flows against the flow of material, exposing the driest material to the highest air temperature. This is usually best for processes that require a specific elevated discharge product temperature or very low moisture content. In parallel-flow dryers, air flows with the flow of material, exposing the wettest material to the highest air temperature. The rapid heat exchange creates significant evaporative cooling of the product without thermally degrading the material.
Forged steel tires or riding rings support the cylinder as it rotates and rests on trunnion rolls. In addition to the trunnions, thrust rolls help maintain the cylinder in its correct longitudinal position. A cylinder generally rotates at two to six rotations per minute using either a gear or chain drive. Gear drives are reversible for longer life.
Fluid Bed Dryers for Frac Sand Processing
Fluid bed dryers are used to process granular, free-flowing materials such as sand, minerals, clays, organic salts, coal and specialty chemicals. In a fluid bed, the material being dried is suspended and completely surrounded by the drying gas or air, causing the material to act like a liquid. This intimate fluidizing action provides a high rate of heat transfer while gently handling the solids. Because there are no moving parts inside the fluid bed, maintenance costs are greatly reduced compared to other systems.
Almost any material, ranging from sand to chemicals to spent grains, can be dried with a fluidized bed as long as the material has a suitable particle-size distribution and other characteristics. Generally, only submicron powders or extremely large, dense solids are not candidates for this type of drying.
As with the rotary dryer, a fluid bed dryer’s heat source can be a standard fuel such as natural gas, propane, oil or steam. Electric heat typically is not used because of an industrial-scale fluid bed dryer’s large electrical demands. When a standard fuel is used, the chamber’s combustion air can be supplied by a fluidizing air blower or by a dedicated combustion-air fan. For high-temperature applications (greater than 1000°F [538°C]), the chamber typically is lined with refractory materials such as fire brick.
In conclusion, both rotary and fluid bed dryers are used in the frac sand market. Rotary dryers may be more expensive to purchase, but they are more forgiving and allow greater process turndown capability. Fluid bed dryers cost less, but they are more thermally efficient. Also, they have minimal maintenance because there are few moving parts. If your frac sand drying requirements are consistent — that is, your throughput, particle size and moisture do not change much — then fluid bed drying is the way to go. Otherwise, the rotary dryer will provide the versatility you need for now and in the future.