The need for a means of efficient recovery of otherwise hazardous hydrocarbons and remediation of soils and waters — without venting volatile organic

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compounds (VOCs) into the atmosphere — continues to grow. One emerging technology provides a way to treat oil sludge ponds, oilfield drill cuttings, tank bottoms dregs and soils contaminated with hydrocarbons.

The technology — indirect thermal desorption — provides remediation and recycling of soil, sludge and other materials. Designed to separate the organic constituents from a waste stream, the process allows useful materials such as hydrocarbons to be recovered in such a manner that they are preserved for collection and recycling.

The concept is much different than the traditional approach, which typically included transporting the waste material to a hazardous waste landfill or an incineration facility, where the hydrocarbons were invariably burned and released to the atmosphere in the form of greenhouse gases.

By contrast, with thermal desorption technology, materials can be treated on site. The feed material is indirectly heating in an anaerobic (oxygen-deficient) environment during treatment. The separation process allows the operator to recover the hydrocarbons in the waste, and it prevents the formation of dioxins, furans or other toxic gases that otherwise would escape to the atmosphere. The remaining solid fraction (e.g., soil) is rendered non-hazardous and may be de-listed, allowing for disposal in a non-hazardous landfill.

One manufacturer of indirect thermal desorption systems has seen more than 30 of its anaerobic thermal desorption unit (ATDU) systems installed in refineries, oilfields and waste processing facilities. RLC Technologies, founded by Dr. Robert L. Carroll, the company’s president and CEO, credits its anaerobic thermal desorption unit’s success to its effective hydrocarbon removal and recycling capabilities. The company also notes that the system’s ease of regulatory emission permitting is a key factor in its acceptance among the refineries.

Handling the Heat

Heat is an essential ingredient in the anaerobic thermal desorption unit’s process. Rather than burn the contaminated materials being treated, the materials are placed inside a drum and heated from the outside. The system elevates the temperature of the waste stream beyond the boiling point of the contaminants, yet the oxygen deficiency prevents the hydrocarbons from burning. So, instead of combusting, they phase change into a gas and are extracted via vacuum from the rotary chamber. Vaporized hydrocarbons are delivered to a vapor-recovery unit, where they are cooled and condensed back into liquid form. In this process, the hydrocarbons are separated from the waste material rather than oxidized or otherwise destroyed.

A water-cooled heat exchanger draws off the heat from the process driving the hydrocarbon gas condenser unit, facilitating recovery so that recovered hydrocarbons can be bottled for future use. Because it is a continuous system, the water must be cooled before it loops back through the system.

The vapor stream generated in the desorption unit is typically 4,000 to 5,000 cfm. By the time they come through the condensation system, the vapors are down to 200 to 400 cfm, and the temperature drops from approximately 1,000°F (538°C) to approximately 55°F (13°C). Ultimately, all heat must be removed by the cooling towers.

This makes cooling towers an essential component of the anaerobic thermal desorption unit plant. Because the systems are designed to operate continuously for 15 to 20 years, towers that are reliable and have a long service life must be included.

In applications such as the thermal desorption technology, high-density polyethylene (HDPE) plastic cooling towers often are used. Plastic cooling towers are designed with a seamless shell that is not affected by water treatment chemicals and harsh ambient environments — either natural or industrial.

Uptime Critical

The impervious cooling tower shell is especially beneficial for systems located in hot, humid environments, or areas in close proximity to salt water, which can be

The impervious cooling tower shell is especially beneficial for systems located in hot, humid environments, or areas in close proximity to salt water, which can be damaging to metal cooling towers.

damaging to metal cooling towers.

Cooling efficiency also is important to the volume of hydrocarbon recovery processing. With the RLC Technologies’ system, the HDPE cooling tower designs are able to maintain that efficiency. The cooler the water, the more efficient the condensation process is going to be, and the more hydrocarbons are going to be able to be condensed and preserved for recycling.

Other benefits of plastic cooling towers include ease of maintenance. Rust and corrosion problems do not occur, and their direct-drive fan motors require less maintenance than gear- or belt-driven conventional towers. In some applications, the direct-drive fan motors consume less energy. The factory-assembled plastic towers are available to provide up to 2,000 cooling tons in a single, modularized unit. The modularity of these systems also makes the cooling tower scalable — a desirable feature for those with smaller or intermediate cooling requirements but who want the convenience of easily expanding cooling capacity as the business grows.