Modelling the distribution of micro-nutrient metals in the anaerobic digestion for fischer-tropsch reaction water.
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Date
2017
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Abstract
Sasol, one of South Africa’s largest petrochemical producers; derives water from the Fischer
Tropsch process and is referred to as Fischer-Tropsch Reaction Water (FTRW). FTRW has high
hydrocarbon content and a very low pH, hence, it must be treated prior reuse in the system. In
the purification process, the organic content in the FTRW is mainly broken down to CO2 and
CH4 by microorganisms which use the organic content as nourishment, bringing about its
decomposition. Macro and micronutrients are necessary for survival of microorganisms, and this
study is focused on the optimisation of micronutrients (micro-metals) to supplement the
treatment of FTRW.
This study was aimed at developing a model to predict the distribution of micronutrients in the
various phases present in anaerobic digester sludge: soluble ions, precipitates and an adsorbed
phase. The aim was to introduce an adsorbed phase to the precipitates-soluble phase model to
reduce the deviation between model and experimental sludge concentrations. The model would
thereafter be used for optimization of micronutrients dosing techniques employed by Sasol to
reduce operating costs of anaerobic treatment of FTRW.
The model was sufficiently extended to include the adsorbed phase by ionic representation of the
biomass, however, the accuracy to which the model represents reality could not be tested by
model validation due to these crucial limitations: incomplete set of experimental soluble phase
concentrations, imprecise experimental data for metals entrapped in precipitates only and in the
adsorbed phase only, to allow for regression of governing equations, as well lack of
experimental representation of the relationship between sulphide and sulphate by concentrations.
The integrated Ionic speciation model was used to point out the importance of the sulphatesulphide
system, as the phase control varies between the precipitates and the adsorbed phase,
depending on the behaviour of the anions, specifically the sulphides.
A series of further experimental work needs to be completed to ensure a robust model outcome,
such that the model best represents the speciation of metals in an anaerobic digester. The
hypothesis therefore could not be proven to be true based on the data at hand. The next steps will
be to carry out detailed experimental work; showing initial conditions, sulphate and sulphide
concentration as well as experimental partitioning of metals in the various phases. In the
meantime, Sasol can look into incorporation of a donor cation with a greater affinity to sulphide
ions and adsorption on biomass while not impacting the decomposition reaction, such that the
metal cations essential for anaerobic digestion are kept in solution to enhance the microbial
activity for the treatment of FTRW.
Description
Master of Science in Chemical Engineering. University of KwaZulu-Natal, Durban 2017.
Keywords
Theses - Chemical Engineering.