| Summary: | Heightened concerns for cleaner air and increasingly more stringent regulations on
sulphur content in transportation fuels will make desulphurization more and more
important. The sulphur problem is becoming more serious in general, particularly for
diesel fuels as the regulated sulphur content is getting an order of magnitude lower,
while the sulphur contents of crude oils are becoming higher. This thesis aimed to
develop a desulphurisation process (based on oxidation followed by extraction) with
high efficiency, selectivity and minimum energy consumption leading to minimum
environmental impact via laboratory batch experiments, mathematical modelling and
optimisation.
Deep desulphurization of model sulphur compounds (di-n-butyl sulphide, dimethyl
sulfoxide and dibenzothiophene) and heavy gas oils (HGO) derived from Libyan crude
oil were conducted. A series of batch experiments were carried out using a small reactor
operating at various temperatures (40 ¿ 100 0C) with hydrogen peroxide (H2O2) as
oxidant and formic acid (HCOOH) as catalyst. Kinetic models for the oxidation process
are then developed based on `total sulphur approach¿. Extraction of unoxidised and
oxidised gas oils was also investigated using methanol, dimethylformamide (DMF) and
N-methyl pyrolidone (NMP) as solvents. For each solvent, the `measures¿ such as: the
partition coefficient (KP), effectiveness factor (Kf) and extractor factor (Ef) are used to
select the best/effective solvent and to find the effective heavy gas oil/solvent ratios.
A CSTR model is then developed for the process for evaluating viability of the large
scale operation. It is noted that while the energy consumption and recovery issues could
be ignored for batch experiments these could not be ignored for large scale operation.
Large amount of heating is necessary even to carry out the reaction at 30-40 0C, the
recovery of which is very important for maximising the profitability of operation and
also to minimise environmental impact by reducing net CO2 release. Here the heat
integration of the oxidation process is considered to recover most of the external energy
input. However, this leads to putting a number of heat exchangers in the oxidation
process requiring capital investment. Optimisation problem is formulated using
gPROMS modelling tool to optimise some of the design and operating parameters (such
as reaction temperature, residence time and splitter ratio) of integrated process while
minimising an objective function which is a coupled function of capital and operating
costs involving design and operating parameters. Two cases are studied: where (i) HGO
and catalyst are fed as one feed stream and (ii) HGO and catalyst are treated as two feed
streams.
A liquid-liquid extraction model is then developed for the extraction of sulphur
compounds from the oxidised heavy gas oil. With the experimentally determined KP
multi stage liquid-liquid extraction process is modelled using gPROMS software and the
process is simulated for three different solvents at different oil/solvent ratios to select the best solvent, and to obtain the best heavy gas oil to solvent ratio and number of
extraction stages to reduce the sulphur content to less than 10 ppm.
Finally, an integrated oxidation and extraction steps of ODS process is developed based
on the batch experiments and modelling. The recovery of oxidant, catalyst and solvent
are considered and preliminary economic analysis for the integrated ODS process is
presented.
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