Thermal desorption describes a treatment technique in which the oily cuttings are heated and the volatile liquids are driven off, resulting in two phases. The liquid phase containing water and oil is separated. The solids component is generally de-oiled to less than 0.5% oil by weight. This is considered a treatment technique, because something must still be done with both phases. It is important to understand that this technique does not address anything except oil. The salinity remains with the solids, as does the heavy metal content (barium, zinc, lead, etc.).
In thermal desorption, oily cuttings are fed to a heating unit. Many types of heating units exist, but the purpose is to efficiently transfer heat to the drilled cuttings to where oil and water are driven off. The water and oil are separated. Depending on quality, the recovered oil is used to further fuel the thermal desorption process, as makeup fluid for the fluid system; or it can be sold to industries needing boiler fuel (cement kilns and power plants).
In order to apply thermal desorption to the treatment of oily cuttings from offshore, the cuttings are transported to shore. Large-scale thermal desorption has not yet been applied at an offshore location. As of this writing, the maximum throughput for any thermal plant on an offshore location has been about 5 to 6 tons per hour. At the dock, the cuttings are transported to the treatment site by truck, box, or other transport method. Cuttings arriving at the desorpton facility are weighed in and kept segregated by operator (generator). The cuttings are screened for foreign materials before being fed into the feed hopper of the desorption unit. All of this is simplified if the thermal desorption unit is located near the receiving dock.
At this point, the individual processes vary considerably. There are single and twin screws, hollow-flight augers, hollow-paddle augers, rotary kilns, and hammer mills. They may be direct fired, as in a rotary kiln type of incinerator; indirect fired as in calciners; indirectly heated by circulating a high-temperature silicone oil through the hollow augers; or, in the case of the hammer mills, use the heat generated by friction to vaporize the liquid components. There is a perceived advantage to being able to control the temperature and the amount of oxygen in the process. This is partially to reduce fire hazard potential and partially to prevent cracking of the hydrocarbons.
The use of nitrogen to purge the units (called a nitrogen blanket) effectively excludes oxygen from the process. For cost reasons, the ‘‘steam quench’’ method of preventing fire is frequently used in lieu of a nitrogen blanket. The hazard from having volatile vapors above the flashpoint in the presence of oxygen is obvious. The propensity of hydrocarbons to crack and form polycyclic aromatic hydrocarbons and other known carcinogens in the presence of oxygen is a serious concern. Further, cracked hydrocarbons do not retain their original fluid properties and become unsuitable for reuse as base oil in a drilling fluid In addition to process type, it is important to understand how the presence of water in the cuttings reduces the process rate while increasing fuel requirements.
Two problems must be addressed as the hot, de-oiled cuttings leave the desorption unit. The cuttings will be around 500C and must be cooled rapidly before exposure to air. This is usually addressed by having the cuttings pass through an enclosed cooling auger. A second problem is that fine solids are stripped from the unit with the vapor phase. There will usually be a centrifuge to remove solids from the liquid phase before oil/water separation. Bag houses to control dust have proven to be susceptible to catching fire and their use has generally been discontinued.
The processing rate for a typical unit is about 100 metric tons per day. This is equivalent to about 350 bbl of cuttings per day. Feed consistency is critical to the overall processing rate. High water content will require the feed to be diluted, so to speak, with processed cuttings containing no water or set aside under cover to allow the material to drain before processing. A water content below 10% is desired, while most cuttings will contain more than that. Oil content is a concern for those units that allow oxygen in the process. Many of the rotary kiln desorption units restrict the feed rate to control the process or attempt to maintain the concentration of oil in the feed at about 10%.
The main advantage of thermal desorption is in removing almost all of the oil from the waste solids. If disposal criteria demand extremely low hydrocarbon content, the only way to obtain it is by thermal desorption. A second advantage is in reclaiming oil that can theoretically be reused. However, this is dependent on the process, the temperature control, and the ability to control oxygen.
The primary disadvantages are cost and safety. Most reputable companies have addressed the safety concerns, but some fly-by-night companies still exist, especially in developing countries. Other disadvantages include:
- Processed solids must still be handled and disposed of (see the
preceding sections on commercial options, land farming, and burial).
- The quality of the recovered oil varies, depending on the process.
- There are high capital costs.
- Maintenance is relatively high.
- The technique is highly dependent on operator experience and
- Air emissions must be controlled and monitored.