| dc.description.abstract | Water in South Africa is becoming limiting due to economic growth, social
development and the country's water demand that exceed its water availability. Water
conservation in the industry can be accomplished by the reuse of process water instead
of direct treatment and discharge. By reusing a process effluent as cooling water in
cooling towers, the water requirements of an industry, such as Sasol, will be lower and a
zero effluent discharge scenario could be achieved.
At Sasol, during the gas-to-liquid (GTL) conversion process, natural gas is converted to
diesel and other products. During this process an aqueous effluent stream is produced
in the Fischer-Tropsch (F-T) reactors known as Primary Column Bottoms. Primary
Column Bottoms can be re-used as cooling water within cooling towers. Although this
approach is technically feasible, the re-use of process water in cooling systems is
characterised by major problems (fouling, scaling and corrosion) due to the complicated
chemistry of the process water and the increased nutrient loads within the system.
The aim of this study was to evaluate the suitability of Fischer-Tropsch gas-to-liquid
Primary Column Bottoms as process cooling water by analysing the microbial
community dynamics, fouling, scaling and corrosion. Due to the corrosive nature of
this process effluent, stabilisation of the water was essential. To determine whether
efficient stabilisation was attained, an accelerated corrosion test was performed.
Influence of the external operating parameters within the cooling tower on the rate of
fouling, scaling and corrosion were also determined. Structural and functional diversity
of planktonic and sessile communities were studied by making use of conventional
microbiological techniques (plate counts, MPN technique) and molecular methods
(PLFA, DGGE).
The accelerated corrosion test of 28 days conducted on mild steel and stainless steel
(316L) corrosion coupons accelerated corrosion by immediately establishing the mature
natural environment that causes corrosion. The test solution was stabilised as well as
non-stabilised synthetic Primary Column Bottoms, in order to compare the effect of
stabilisation. Scaling and corrosion indices were also calculated on stabilised and non-stabilised
water to determine the scaling and corrosive tendencies of the water and how this correlates with the actual corrosion results obtained. According to the Langelier
Saturation Index (LSI), Rymar Stability Index (RSI) and the Puckorius Scaling Index
(PSI) the stabilised water was slightly scale forming with little corrosion and the non-stabilised
water being more corrosive than scale forming. Average corrosion rate of the
stabilised water was 0.032 m d y and 0.049 m d y for non-stabilised water. Average
scaling rate was calculated as 7.269 mg/dm2/d for stabilised water and 5.853 mg/dm2/d
for non-stabilised water. It can therefore be concluded that effective stabilisation was
achieved since stabilised water was less corrosive than non-stabilised water which was
also confirmed through experimental data (corrosion rates from accelerated corrosion
test) and corresponded with the corrosive tendencies obtained from the scaling and
corrosion indices.
A lab-scale cooling tower was operated with stabilised synthetic Primary Column
Bottoms as cooling water. Five experiments were conducted under varying flow rates
and cycles of concentration. Influence of the external operating parameters (linear flow
velocity and cycles of concentration) on fouling, scaling and corrosion rates of mild
steel and stainless steel (316L) corrosion coupons and heat exchanger tubes were
determined through weight loss measurements. Routine physico-chemical analyses,
EDS (energy dispersive spectrometry) microanalysis as well as scaling and corrosion
indices of each experiment were also compared, in order to evaluate the influence of
cycles of concentration and linear flow velocity. Based on the results obtained, it was
evident that the variation in cycles of concentration and linear flow velocity had a
significant effect (p0.05) on the fouling, scaling and corrosion rates on the mild steel
corrosion coupons and heat exchanger tubes. Experimental runs operated at low flow
rates of 0.6mIs and 0.9 d s resulted in relative high fouling, scaling and corrosion rates.
Operation at 3 and 4 cycles of concentration had the highest scaling and corrosion rates.
The COD within the cooling tower was not removed by the microorganisms within the
planktonic and sessile communities and resulted in a build-up of COD in the sump.
Thus, the cooling tower cannot be used as a bioreactor to biologically degrade volatile
organic acids and hydrocarbons.
To evaluate the structural and functional diversity of the bacterial and fungal
communities. plate counts, most probable number technique, phospholipid fatty acid
(PLFA) analysis as well as denaturing gradient gel electrophoresis (DGGE) was used. According to PLFA profiles the community structure within the planktonic and biofilm
samples of the experiments operated at low linear flow velocities were similar. The
same percentages of Gram-positive, Gram-negative bacteria and fungi occurred,
The community structure composition of the planktonic and sessile phases in the
experiments operated at higher linear flow velocities was also similar. PLFA analysis
concluded that the highest estimated viable biomass was in experiment 1 which had a
low linear flow velocity of 0.6 d s . Shannon-Weaver index analysis of DGGE profiles
(general structural diversity) indicated that the planktonic bacterial diversity of
experiment I and 2 were the highest. Experiment I and 2 were operated at a linear flow
velocity of 0.6 and 0.9 m/s respectively. The biofilm samples that had the highest
Shannon-Weaver diversity index were experiment 1 and 5. Both experiment 1 and 5
were operated at a linear flow velocity of 0.6 m/s. Morphological changes between
planktonic and sessile communities were monitored through scanning electron
microscopy (SEM). SEM results illustrated that the planktonic and sessile microbial
populations throughout the five experiments were similar, based on morphology.
According to the results obtained from the MPN technique, the experiment operated at
the lowest linear flow velocity had the highest numbers of sulphate reducing bacteria
and also resulted in the highest corrosion rate. Both experiments that were operated at a
low linear flow velocity of 0.6 d s had the highest bacterial numbers and also resulted
in high fouling rates. However, no relationship exists between the percentage increase
in the numbers of aerobic bacteria and the cycles of concentration at which the cooling
tower was operated. These observations are supported by results from PLFA profiles
that showed that the community structure within the planktonic and sessile samples of
the experiments operated at low linear flow velocities were similar. The planktonic and
sessile phases of these two experiments had similar levels of Gram-positive-, Gram-negative-
bacteria and fungi. The community structure composition of the planktonic
and sessile phases in the experiments operated at high linear flow velocities was also
similar. PLFA analysis further demonstrated that the highest estimated viable biomass
was in the experiment operated at a low linear flow velocity of 0.6 m/s. Shannon-
Weaver index analysis of DGGE profiles (general structural diversity) also indicated
that the planktonic bacterial diversity during operation at low linear flow velocities were
the highest. Although scanning electron microscopy results illustrated that the
planktonic and sessile microbial populations throughout the five experiments were generally similar. these results supported the observations of the other techniques.
These techniques all supported the notion that corrosion rates may not be directly
related to the total microbial biomass or the number of species on mild steel or stainless
steel. Corrosion rates seem to be more profoundly affected by biofilm composition
within the sessile phase.
Based on the results obtained when using Primary Column Bottoms as cooling water, it
were evident that variation in cycles of concentration and linear flow velocity had a
significant effect (p>0.05) on the fouling, scaling and corrosion rates on mild steel
corrosion coupons and heat exchanger tubes. Low linear flow velocities resulted in
high fouling rates, increased bacterial numbers as well as high bacterial and fungal
diversities. High cycles of concentration resulted in high scaling and corrosion rates
and also had the result of similar community structure profiles. This research study
could facilitate the selection of optimised operational parameters for the re-use of
industrial process water (such as Primary Column Bottoms) as cooling water to
minimise fouling, scaling and corrosion. | |
