Page 202 - Environmental Control in Petroleum Engineering

188 Environmental Control in Petroleum Engineering

6.2.2 Removal of Hydrocarbons

A variety of methods are available to remove hydrocarbons from
solids, such as drill cuttings, contaminated soil, and produced sand.
These methods include washing, adsorption, filtration, heating, solvent
extraction, incineration, and biological degradation (U.S. Environ-
mental Protection Agency, 1990). These methods are described below.
The effectiveness of these methods varies significantly. Pyrolysis
can reduce most hydrocarbon concentrations on solids to nondetectable
levels, while solvent extraction and distillation can reduce concen-
tration to a few tens of mg/kg. Hydrocarbon concentrations following
simple filtration can be in the hundreds of mg/kg range (American
Petroleum Institute, 1987).

Washing

One of the least expensive ways to remove most of the hydro-
carbons from solids is to wash them. The solids can be entrained in a
fluidized bed of upward-flowing, high-velocity water. This stream
agitates the solids and opens the pore system to release the oil. The
efficiency of this process can be enhanced by adding a surfactant
(soap) to the water to lower the interfacial tension holding the oil to
the solids. Washing is more effective in sandy soils containing low
amounts of clay.
A related process is to slurry the solids in a low-toxicity base oil.
Although this process does not necessarily result in a lower hydrocar-
bon concentration in the solids, it can replace the original hydrocarbon,
e.g., diesel, with a less toxic one.
If the volume of solids is small, they can be placed in an ultra-
sound bath for cleaning. The high-frequency acoustic pulses in the bath
help release the hydrocarbons from the solids. Ultrasound baths work
well for laboratory scale operations, but are not appropriate for
oilfield-scale applications.

Adsorption

Another relatively low-cost method of removing some of the hydro-
carbons contaminating solids is to mix the soil with a material that is
strongly oil-wet, like coal or activated carbon (Ignasiak et al., 1990).