Environmental Engineering Reference
In-Depth Information
cost of additional time and energy. Mass transfer has been shown to limit rates of DBT
degradation in Pseudomonas Dsz systems [12, 17] but presents less of a problem during use
of the comparatively hydrophobic Rhodococcus cells that tend to adhere to oil-water
interfaces [1]. Not only is the transfer of substrate into the cells improved, but the mixture is
then also easily manipulated by means of patented devices known as hydrocyclones that
readily and inexpensively separate oil-water emulsions [18]. These devices are ~1 meter long
conical tubes that cause fluid to spin as it is pumped from the wide end to the narrow end,
driving the denser fraction to the outside where it can be drawn off continuously. In a system
where bacteria partition to the oil-water interface, the cells stay with the discontinuous phase.
In a water-in-oil emulsion, cells associate with water droplets and allow separation of the
clean oil phase; in an oil-in-water emulsion, cells can be concentrated with the oil for return
to the reactor [1]. Additional reactor design research has reduced the influence of mass
transport limitations, and current BDS reactors use staging, air sparging, and media
optimization with lower water-to-oil ratios to reduce reactor size, although these conditions
increase the difficulty of downstream separations [7].
As with any biological technology, the maintenance of an active catalytic population is a
challenge in the context of a conventional industrial system. An important advance in this
area incorporated the production and regeneration of microbial cells within the BDS process,
lengthening biocatalyst activity to 200-400 hours [14] and setting an example for other
bioprocesses.
3. Research Priorities
While a number of issues described above have been fully addressed, additional effort is
still needed to expand the substrate specificity of the desulfurization process to include
smaller compounds, allowing efficient desulfurization of gasoline, as well as to include
compounds with sterically obstructed S atoms, allowing more complete desulfurization of all
fossil fuels. In addition, metabolic engineering to increase the availability of FMNH
2
to the
mono-oxygenases has the potential to improve the rate of desulfurization further, while
additional increases in thermotolerance and solvent tolerance of the catalytic microbes would
improve the robustness of commercial-scale systems.
4. Commercialization
Commercialization efforts in the United States were initially spearheaded by Energy
BioSystems Corporation of The Woodlands, Texas, which obtained a broad patent in 1999
covering recombinant microbes based upon the Rhodococcus erythropolis IGTS
3
genome and
worked extensively to isolate, characterize, and manipulate desulfurization genes and develop
bioprocess concepts [19]. This company, later incorporated under the name Enchira
Biotechnology Corporation, constructed and operated several small pilot plants, later entering
an agreement with Petro Star, Inc. to design and build a 5,000 barrel-per-day BDS facility at
their Valdez, Alaska refinery. The company also entered a number of technology
development alliances with oil refiners, including TOTAL Raffinage Distribution S. A. of
France, Koch Refining Company, Kellogg, Brown & Root, and the Exploration & Production
