vious research, which indicates that water-rich
SUMMARY AND CONCLUSIONS
paraffin hydrocarbon phases reach equilibrium
with the gas phase at atmospheric pressure and
Air stripping experiments performed as part
25C. A model to predict liquid heating and air
of this work demonstrated that diesel range pe-
stripping power requirements per volume of wa-
troleum hydrocarbons (TPH) can be effectively
ter treated was determined (eq 11), but is based
stripped from water in conjunction with VOCs at
only on the batch mode process used in this study.
modest liquid flow rates and temperatures (using
A significant advantage of the ShallowTray strip-
a tray-type air stripper). The batch process em-
per appears to be the large air/water ratio it gen-
ployed was capable of treating the contaminated
erates (100900), in comparison to air/water ra-
source water to within discharge permit standards
tios typical of counter-current packed towers
at a lower cost per volume treated than high tem-
(50150) treating similar contaminant mass load-
perature airstripping (HTAS) systems or high
ings. This high air/water ratio allows very effec-
performance oil/water separation combined with
tive mass transfer of VOCs/SVOCs over a rela-
air stripping.
tively small surface area (just 2.8 m2 for the unit
VOC stripping efficiency did not appear to be
affected by TPH stripping. High concentrations
used in this research).
(4050 ppm) of VOCs were reduced by ≥ 99.99%
during all experiments, regardless of TPH con-
centration, liquid flow rate or temperature. VOC
RECOMMENDATIONS
removals were so consistently high that no statis-
tically discernible trends were observed as a func-
The ShallowTray air stripper was proven to be
tion of liquid flow rate and temperature. Mass
just as efficient at removing VOCs as packed tower
transfer of VOC from liquid to gas phase was
systems. It also proved capable of treating heated
driven by the contaminant's physical and ther-
water to strip significant fractions of petroleum
modynamic properties, not the concentration gra-
hydrocarbons without fouling, based on achiev-
ing 100% ( sampling and analytical error) mass
dient. Mass transfer coefficients (KLa) were deter-
balance of hydrocarbons. The medium tempera-
mined for TCE and PCE using removal data, and
ture air stripping process appears to not only be
these showed good agreement with published val-
effective but economically competitive with other
ues of VOC mass transfer coefficients in packed-
processes. Remediation projects which require or
tower air strippers.
already employ air stripping/solid phase adsorp-
Significant amounts of TPH were stripped dur-
tion for VOC removal from groundwater can be
ing all experiments, but variations in liquid tem-
cost effectively retrofitted with a modest liquid
perature affected TPH stripping more than varia-
heating capacity so that petroleum hydrocarbons
tions in flow rate. The low residual error associated
can be removed concurrently with the VOCs. Ad-
with the TPH removal data indicates that liquid
ditional research should be conducted to evaluate
temperature and flow rate were the only signifi-
the effects of increased detention time, higher liq-
cant factors controlling TPH stripping efficiency.
uid temperatures and continuous mode treatment
Mass transfer of TPH from liquid to gas phase
on TPH stripping efficiency. Detention time is
was driven by concentration gradient; however,
easily increased in ShallowTray strippers by add-
liquid heating affected the contaminants' thermo-
ing the modular trays, so that for essentially the
dynamic properties sufficiently to improve remov-
same power requirements (i.e., operating costs)
als to the 8090% range. For similar liquid heat-
higher removals are achieved. The continuous
ing and ShallowTray air stripping applications,
mode, medium temperature process should also
TPH removal can be predicted using the multi-
linear regression model (eq 8), or the TPH re-
be used to treat other contaminated source waters
moval predicted response surface (Fig. 7). Equa-
in order to more fully explore the applications of
tion 8 is probably only valid when predicting
the process.
removals below 90%, and removals above this
level are expected to approach some asymptotic
value as a function of liquid temperature and
LITERATURE CITED
flow rate. The liquid temperatures predicted by
eq 8 as necessary to achieve maximum ("100%")
APHA (1992) Standard Methods for the Examination
TPH (diesel-range or paraffin hydrocarbons)
of Water and Wastewater, 18th Edition. American
removal, 27C to 31.8C, are corroborated by pre-
Public Health Association, Washington, D.C.
14
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