Thermodynamic environmental fate modelling.
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Date
2002
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Abstract
The labelling of methyl tertiary butyl ether (MTBE), an oxygenate additive used extensively in
gasoline blending, as an environmentally harmful chemical has led to the banning and
subsequent phasing-out of this additive in California (USA). In response, the global petroleum
industry is currently considering replacement strategies, which include the use of tertiary amyl
methyl ether (TAME) or ethanol. Subsequently, SASOL (South African Coal and Oil Limited),
a local petrochemical company, in its capacity as an environmentally responsible player in the
global petroleum and aligned chemical markets, has commissioned this investigation into the
environmental fate of the fuel oxygenates: TAME, ethanol and MTBE.
In order to evaluate the environmental fate of the oxygenates, this dissertation has formed a
three-tiered approach, using MTBE as a benchmark. The first tier assessed the general fate
behaviour of the oxygenates using an evaluative model. A generic evaluative model, developed
by Mackay et al. (l996a), called the Equilibrium Criterion (EQc) model was used for this
purpose. This fugacity based multimedia model showed MTBE and TAME to have similar
affinities for the water compartment. Ethanol was demonstrated to have a pre-disposition for the
air compartment. Parameterisation of the EQC model to South African conditions resulted in
the development of ChemSA, which reiterated the EQC findings.
The second tier quantified the persistence (P), bioaccumulation (B) and long-range
transport (LRT) potential of the additives. This tier also included a brief toxicity (T) review.
MTBE and ethanol were demonstrated to be persistent and non-persistent, respectively,
according to three threshold limit protocols (Convention on the Long Range Trans-boundary
Air Pollution Persistent Organic Chemical Protocol; the United Nations Environment
Programme Global Initiative; and the Track 1 criteria as defined by the Canadian Toxic
Substances Management Policy, as referred to by the Canadian Environmental Protection
Act 1999). These protocols were not unanimous in the persistence classification of TAME.
Further investigation of persistence was conducted using a persistence and long-range transport
multimedia model, called TaPL3, developed by Webster et al. (1998) and extended by
Beyer et al. (2000). TaPL3 reiterated the conclusions drawn from the threshold limit protocols,
indicating that TAME's classification worsened from non-persistent to persistent on moving
from an air emission to a water emission scenario. This served to emphasise the negative water compartment affinity associated with TAME. Using classification intervals defined by
Beyer et al. (2000), TaPL3 demonstrated that the long-range transport potential of the
oxygenates increased in the order of TAME, ethanol and MTBE; however, it was concluded
that none of the oxygenates were expected to pose a serious long-range transport threat.
Bioaccumulation was not expected to be a pertinent environmental hazard. As expected, the
oxygenates were dismissed as potential bioaccumulators by the first level of a screening method
developed by Mackay and Fraser (2000); as well as by the threshold limit protocols listed
above. Simulation of biomagnification, using an equilibrium food chain model developed by
Thomann (1989), demonstrated that none of the oxygenates posed a biomagnification threat. A
review of toxicity data confirmed that none of the three oxygenates are considered particularly
toxic. LDso values indicated the following order of increasing toxicity: ethanol, MTBE and
TAME.
The third tier focussed on oxygenate aqueous behaviour. A simple equilibrium groundwater
model was used to analyse the mobility of the oxygenates in groundwater. TAME was found to
be 21 % less mobile than MTBE. Ethanol was shown to be very mobile; however, the
applicability of the equilibrium model to this biodegradable alcohol was limited. An analysis of
liquid-liquid equilibria comprised of oxygenate, water and a fuel substitution chemical was
performed to investigate fuel-aqueous phase partitioning and the co-solvency effects of the
oxygenates. Ethanol was shown to partition appreciably into an associated water phase from a
fuel-phase. Moreover, this alcohol was shown to act as a co-solvent drawing fuel chemicals into
the water phase. MTBE was found to partition sparingly into the water phase from a fuel-phase,
with TAME partitioning less than MTBE. Neither ether was shown to act as a co-solvent.
It was concluded that TAME and ethanol pose less of a burden to the environment than MTBE.
Ethanol was assessed to be environmentally benign; however, it was concluded that ethanol's
air compartment affinity and the extent of its co-influence on secondary solutes justified the
need for further investigation before its adoption as a fuel additive. This project showed
sufficient variation in the environmental behaviour of TAME and MTBE to justify the
abandonment of the axiom that MTBE and TAME behave similarly in the environment.
However, as MTBE is a significant water pollutant, and TAME has been shown to share a
similar water affinity, it is cautiously recommended that the assumption of environmental
similarity be discarded, except for the water compartment.
Description
Thesis (M.Sc.Eng.)-University of Natal, Durban, 2002.
Keywords
Motor fuels--Additives--Environmental aspects., Reformulated gasoline--Environmental aspects., Gasoline--Environmental aspects., Butyl methyl Ether--Environmental aspects., Alcohol--Environmental aspects., Oxygenated gasoline--Environmental aspects., Theses--Chemical engineering.