Page 205 - Environmental Control in Petroleum Engineering

Waste Treatment Methods 191

with the oily solids to wash the oil from the solids and dilute what
remains trapped. The solvent is then separated from the hydrocarbons
and solids by low-temperature distillation and reused. Solvent extrac-
tion is routinely used in the petroleum industry for extracting fluids
from cores during core analysis. Like distillation, solvent extraction
is expensive. Solvent extraction is more effective in sandy soils
containing little clay. Several commercial solvent extraction processes
are available (Ruddy et al., 1990).
A more exotic solvent extraction process uses critical or super–
critical fluids. In this process, the cuttings are placed in a pressure
chamber with a fluid near its critical point. Commonly used fluids
include carbon dioxide, propane, ethane, and butane. The pressure
is increased until the fluid passes above its critical point and becomes
a liquid. The liquid is then used as a solvent to wash the oil
from the solids. After the liquid mixture is separated from the
solids, the pressure is lowered. With the lower pressure, the super-
critical fluid reverts to a gaseous state, leaving the extracted hydro–
carbons behind. The gas is then recycled. The process is expensive,
but eliminates many of the problems associated with high-temperature
thermal processes.

Biological Processes

Most hydrocarbons encountered in the upstream petroleum industry
can be biologically converted to carbon dioxide and water by microbes
like bacteria and fungi. During biological degradation, the hydro-
carbons are eaten as food by the bacteria. This biological degrada-
tion can be enhanced by providing the optimum conditions for mic-
robe growth. The deliberate enhancement of biological degradation is
called bioremediation.
The effectiveness and speed of bioremediation in degrading hydro-
carbons depends on a variety of environmental conditions, includ-
ing temperature, salinity, pH, hydrocarbon type, heavy metal concentra-
tion, soil texture, moisture content, and hydrocarbon concentration.
Because of this, the chemical composition of the hydrocarbon, the type
and level of background microorganisms, and the nutrient level at
the site must be determined and the environmental conditions con-
trolled for optimum degradation (American Petroleum Institute, 1986b;
Hildebrandt and Wilson, 1991).