Millions of tonnes of oil contaminated drill cuttings are produced every year. By nature the cuttings are contaminated by drilling mud. The level of contamination depends on the efficiency of the mud return system carried out by means of shell shakers. The cuttings may origin from drilling with water based mud (WBM) or oil based mud (OBM). Many different composition of mud and many different properties in the well result in a vast variety of cuttings properties. Defining characteristics of the cuttings is difficult and it is often not made easier by the drillers and mud suppliers who tend to protect details about their mud for competitive reasons.

Drill cuttings have different destinies in different areas. Some is dumped, some is reinjected, sometimes the material is cleaned and parts of the waste are reused in new mud. It appears as if the destiny of drilling wastes often is decided upon limited input and without overall considerations. In some cases the decision seems based on un-proportional focus in only a few areas like ease of operation, emissions to sea/air, cost, storage, etc. Often the value of oil in the cuttings is not given appropriate attention.

High surface temperatures and long retention time can degrade the oil. For applications where the OBM oil is of high quality and high value, it could make a significant impact on the business case if all or parts of the output oil from the process can be re-used in new OBM or used as fuel for diesel engines.

Heat generated by friction is a well known phenomenon and evaporation by friction heat in the Thermomechanical Cuttings Cleaner (TCC^®) has been available in the drilling waste market for approximately 10 years. This method offers the best quality of recovered oil and is also the safest method to use.

Base oils used for drilling mud are well defined, low sulphur, low aromatics oils within the diesel range of distillation. Among the quality specifications for the base oil are density and flash point. In addition HSE requirements like low aromatics, BTEX and low sulphur applies.
 
Fingerprint
This Example with oil from a TCC^® site shows a GC MS profile of virgin base oil (top) and base oil recovered in the hammermill (bottom).
 
The recovered oil has the same fingerprint as virgin base oil. The recovered oil is directly re-usable as base oil in new mud.
 
The recovered oil is cleaned in several stages before it is discharged in two different fractions; the main fraction comes directly from the oil condenser and a small fraction is separated from the water which is evaporated in parallel with the oil.
 
BTEX
Despite the virgin base oil being low on BTEX GC analysis sometimes demonstrates small BTEX concentrations in the feed. In cases where the total BTEX is higher than spec one can separate it out by keeping the smallest fraction of recovered oil separate from the larger. BTEX will, if present, accumulate in the oil fractions separated from recovered water.
 
Flash point
Often a maximum flash point reduction is used as a quality assurance in thermal desorption of OBM drill cuttings, but if the flash point specified for virgin base oil in the MSDS is used as reference this may lead to misleading conclusions. It is important to compare flash point of the actual oil in cuttings and recovered oil. The TCC^® unit does not change flash point of oil.
 
Conclusions
The hammermill is used for recovery of high quality oil from OBM drill cuttings without degrading oil quality. In fact the thermal separation process can be used to raise the quality of oil compared to the actual feed to the unit by removing oil fractions outside of the virgin base oil specification. Compared to other drying methodologies TCC^® friction driers offer a gentle evaporation, low residence time in the reactor and low required process temperature. These, in addition to the homogenous mixing, are all factors that ensure the best possible oil recovery.

Click here to watch our animation of the TCC^® process

In the same way as any other thermal technology, the capacity of a TCC^® is depending on the energy input and the content of the waste. Therefore, any capacity indication for thermal desorption processes has to be based on the composition of the waste.

The energy required to heat and evaporate the various components in the waste is defined by thermodynamics.

In particular, capacity is depending on the amount of water in the waste. When Thermtech gives an indication of the processing capacity this is based on assumptions regarding the content. A "3 ton per hour" unit will have that capacity with 70/15/15 solids/water/oil ratio (by weight) and will have a main motor/engine of approx. 700kW. If the water content is lower, the capacity of the same unit will be higher than three tons per hour. Up-scaling the equipment, also above 4-5 tons per hour is probably doable, but that may leave less flexibility than what multiple units will give.

The TCC^® is based on the basic principle of thermal separation. The OBM cuttings is heated to a temperature sufficiently high to evaporate the oil (and water) from the mineral solids by heating the waste to a temperature higher than the evaporation point of the base oil in the OBM. The oil and water will be condensed back to liquids in later process steps. A common name for such technologies is “thermal desorption” technologies.

The TCC^® is based on a completely different principle than the indirect thermal technologies. The TCC^® converts kinetic energy to thermal energy by creating friction in the waste. A drive unit sets a series of shaft mounted hammer arms in motion inside a barrel shaped process chamber (also referred to as the hammermill or just the mill). The solid particles are forced towards the inner wall of the process chamber where the kinetic energy from the rotating arms will be transformed to heat by friction. The unit can run continuously, automatically controlled by an advanced Plc system. Frictional heat is constantly created by the hammering and motions.

Whereas the TCC^® is based on transforming kinetic energy to thermal energy,the conventional thermal desorption technologies are heating the waste indirectly. The waste is placed inside a box, container or other storage facility and is heated by a medium outside the storage facility through the surfaces of the storage facility. In all such technologies the waste is heated gradually to the temperature required for the oil to evaporate, and the oil in the waste is under influence of high temperature for a long period of time, normally 30 minutes or more. In addition, the temperature of the heating medium in an indirect technology needs to be higher than the evaporation temperature of the oil, meaning that the oil is also influenced by a higher maximum temperature. For these reasons the output oil from the conventional technologies has a reduced quality and is normally not used as a component in new OBM.