Environmental Protection

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Drying Frac Sand

Why frac sand use will become more popular over the next few years and the best methods for drying frac sand

With the frequent rises in oil prices and the increase in oil and gas exploration in the United States, experts anticipate that the demand for frac sand will increase by at least 4.8 percent every year until 2016.

In 2009, the average price of industrial sand was $28.82 per ton, and the average price of frac sand was $44.33 per ton. With the addition of transportation costs from the Midwest to oil and gas sites in places like Colorado, Texas and Canada, the total cost of frac sand can be over $300 per ton. Many wells use more than 2,000 tons of frac sand, leading oil and gas producers to look for more cost-effective alternatives.

For years now, frac sands have been utilized to augment the production of natural gas and oil from wells, and frac sand use will most likely become more common and frequent in the years to come.

The hydraulic fracturing, or “fracking,” 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.

Frac sand is a natural silica-based proppant. In 2009, 6.5 million metric tons of sand were mined to meet the demands of the fracking industry—an amount worth a total of $319 million. The U.S. Geological Survey estimated that frac sand mining doubled in 2010.

Frac sand requires pure quartz with as few contaminating materials as possible. The American Petroleum Institute (API) has specific criteria that must be met for frac sands, including its weight percentage, sphericity and roundness, crush resistance, acid solubility and turbidity. To ensure that the quality of sand is acceptable, mining companies wash and dry the frac sand to rid it of all possible impurities.

To meet API standards, frac sand particles must be well rounded and relatively clean of other mineral and impurities. High-purity quartz sands are common in the U.S., but most silica sand deposits are already being exploited because of the numerous uses for the material in a variety of industrial applications, including glass making and filtration media.

The way in which frac sand is mined and processed depends on the location and quality of the sand deposit. In general, though, a typical frac sand flow sheet is made up of wet flow and dry flow. The goal of wet flow is to eliminate clay slimes and disintegrate any agglomerates, or clumps of material.

This portion of the flow sheet can also act as a preliminary sizing by rejecting excess fines—a process that is much more cost-effective if performed wet instead of dry. The first step in wet processing is to remove any clays. The sand is washed with water and then pumped to a cyclone for “desliming.” Slimes are materials that are usually in the form of clays or very fine silica. Since these materials are often harmful to frac sand processing, they must be removed through the use of classifiers.

In the dry flow portion, the sand is sized into a variety of products and any magnetic contaminants are removed because they would impede crush and acid solubility. Sizes vary, but the most commonly used for frac sand are 12/20, 20/40, 40/70 and 70/140 mesh.

Shape and size have a significant effect on the sand’s final permeability through the fracture. A wide variety of particle shapes and sizes will result in a tight packing arrangement, reducing conductivity and permeability. A controlled range of sizes and a spherical shape, however, will lead to greater conductivity.

Rotary Dryers: The Frac Sand Drying Solution

Rotary dryers are the most proven and preferred method to dry frac sand. A varying TPH rate can often have a substantial effect on the efficiency of the drying process. A rotary dryer allows the user to vary the mass air flow for different moisture content levels or operation at lower throughputs.

In contrast, a fluid bed dryer features a drying gas medium that must produce heat for the drying process while also conveying the material. Because the mass air flow must stay constant, a fluid bed dryer can only be turned down by decreasing the inlet gas temperature, which has a detrimental effect on efficiency. Fluid bed dryers also require an increase in fluidizing velocities for larger particles, resulting in high power usage and lower evaporation and efficiency.

Rotary dryers, however, are far more efficient and capable of enduring the varying production requirements that accompany different drying projects. A rotary dryer is a sturdy piece of equipment that, when well-built and maintained, can have maintenance costs that are comparable to a fluid bed dryer.

Dryers are available in countercurrent and concurrent flow models. In concurrent flow, the burner and feed system are located on the same end of the dryer, and the material and exhaust gas exit the dryer on the opposite end. The primary advantage of using a concurrent 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, instead of minutes.

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.

Worldwide Recycling Equipment Sales, LLC has a line of custom-built Vulcan rotary dryers that can be manufactured to suit any job site or drying application, including frac sand. These units are available in direct or indirect heated models and feature burners ranging from one to 30 MMBtu and rotary drums ranging from 18 to 70 feet with a variety of diameters. Vulcan rotary kilns are ideal for quick and efficient drying of numerous materials in a wide range of industries, from soil remediation to food processing to mining.

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