Chemical Engineering
https://researchspace.ukzn.ac.za/handle/10413/6527
2020-02-23T05:51:56ZHydrocracking of short residue over unsupported and supported magnetite nanocatalysts.
https://researchspace.ukzn.ac.za/handle/10413/16887
Hydrocracking of short residue over unsupported and supported magnetite nanocatalysts.
With the rapid depletion of crude oil and current cracking methods of heavy petroleum residue all resulting in the production of undesirable coke formation, a better solution must be found. This project investigated the use of an unsupported molybdenum-doped magnetite nano-catalyst, as well as a magnetite nanocatalyst on a mesoporous silica support, to determine if the use of these catalysts can be successful in cracking petroleum residue. Short residue from the vacuum distillation column supplied by SAPREF, was used throughout the experimental work.
A lot of effort went into the preparation of the feedstock due to the high viscosity of short residue. The solvent used during experimental work was toluene, which was used to dilute the short residue. A temperature range between 350˚C and 400˚C was used in order to determine temperature effects on product distribution from the cracking reaction. The feedstock to catalyst ratio was also varied, using the unsupported catalyst, in order to determine the effects of the amount of catalyst on the reaction. Kerosene and gas oil are the desired products due to their higher heating value and use as liquid fuels compared to the heavier residue. There is a strong interaction between temperature and catalyst to feedstock ratio. The high temperature-high catalyst combination gave improved gas oil yields over the low temperature-high catalyst combination. Results carried out at 400˚C with a high catalyst amount showed the most favourable results with a yield of 49.3% and 6% of gas oil and kerosene respectively. Aquaprocessing (catalytic splitting of water that occurs on the surface complexes of the iron-based catalyst, at a relatively low pressure) was simulated at the experimental conditions using kinetics from literature for a nickel-based catalyst. The simulated composition profiles proved that the unsupported magnetite nanocatalyst was much more efficient in upgrading residue than the nickel based catalyst, due to the presence of greater amounts of lighter components. Analysis of the catalyst after the cracking reaction shows that no major phase changes had taken place and that the catalyst could be regenerated to be used again.
The supported magnetite nanocatlyst was compared to conventional nickel-molybdenum and cobalt-molybdenum catalyst, in a fixed bed reactor set up. The supported catalyst proved to be the most consistent, and was able to shift the residue into the lighter fractions more effectively than the conventional catalysts. The supported catalyst was the most effective in cracking the vacuum residue, mostly into vacuum gas oil. The yields using the catalyst compared quite favourably with the unsupported catalyst, with the unsupported catalyst yielding more lighter components. The most favourable results implementing a supported catalyst were also at 400˚C, due to the extensive decrease in vacuum residue and a corresponding increase in lighter components.
Ultimately this investigation proved that hydrocracking can take place with the use of a supported and unsupported magnetite nanocatalyst, at lower temperatures than that of conventional methods and aquaprocessing. It was also proven that the process can be upscaled to industry level, as shown with the performance of the supported catalyst. A larger temperature range could give better clarity in the performance of the catalyst for future petroleum residue cracking.
Masters Degree. University of KwaZulu-Natal, Durban.
2017-01-01T00:00:00ZEconomic recovery of biobutanol-a platform chemical for the sugarcane biorefinery.
https://researchspace.ukzn.ac.za/handle/10413/16768
Economic recovery of biobutanol-a platform chemical for the sugarcane biorefinery.
In recent years, the South African sugar industry has faced challenges, such as drought, low
prices and labour issues that have impacted negatively on the perceived sustainability. The
adoption of the sugarcane biorefinery concept by the sugar industry is a possible solution to
improving the sustainability of the industry amid these challenges. In this envisioned
biorefinery, multiple products are created within an integrated system that maximises
sustainability, as opposed to relying on producing one or very few products. In this study,
the potential economic viability of the recovery of biobutanol was explored with the
ultimate intention of using this biobutanol as a platform chemical for the production of
higher value products to include in the biorefinery’s product portfolio. Biobutanol is
produced from biomass via the ABE (acetone, butanol, and ethanol) fermentation process.
Biobutanol production is characterised by very low butanol concentrations in the
fermentation broth (around 2 wt. %) due to high inhibition, resulting in a very high cost of
recovery (distillation) and the need for several downstream purification steps. Following a
literature search on technologies that have been proposed and previously implemented for
biobutanol production, processes integrating gas stripping and extraction were simulated on
Aspen Plus® and techno economic analyses performed to determine the profitability based
on cash flows over a 25 year period.
Gas stripping and liquid-liquid extraction experiments were first carried out in order to have
a way of validating simulation results. Gas stripping experiments created scenario-based
results of the expected butanol concentration in the gas phase once a steady state butanol
concentration can be maintained in the fermenter. The extraction experiments were
conducted to establish a quick way of evaluating the extractive properties of a solvent based
on the distribution coefficients and selectivities with respect to butanol. Five solvents were
evaluated including hexyl acetate and diethyl carbonate, which have not been reported on
but have been previously applied in biomass processing. Distribution coefficients of 3.57
and 6.15 and selectivities of 367.09 and 396.00, with respect to butanol, were obtained for
hexyl acetate and diethyl carbonate, respectively.
Four processes were then simulated on Aspen Plus® and they all assumed a fermentation
process that make use of 281.67 t/h clear juice from a South African generic sugar mill
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model. A study estimate type economic evaluation, accurate within ±30% error, was
performed with profitability being assessed in terms of the Net Present Value (NPV) and the
Internal Rate of Return (IRR) over the 25 year period. Process Scheme 1 was the
benchmarking case and consists of the conventional series of five distillation columns. For
this process a Total Capital Investment (TCI) of US$124.85 million was obtained and based
on the sales and production costs a negative NPV of US$3.80 million was obtained. This
indicates a non-viable process under the current economic conditions. Process Scheme 2
included in situ recovery by gas stripping and final purification using distillation. Five
distillation columns were still required to purify the condensate from the stripper due to a
large amount of water that is carried in. The increased productivity in the fermenter and the
reduction the downstream column sizes in this process, compared to the benchmarking
case, resulted in a reduced capital cost of US$67.43 million. This recovery process also
yielded a potential to be profitable with a positive NPV of US$505.88 million and an IRR of
31%. This was attributed to the reduced TCI as well as the ability of the process to yield all
the three ABE solvents to sellable purities.
Process Scheme 3 that included gas stripping and liquid-liquid extraction had almost the
same TCI as Process Scheme 2 (US$68.94 million) but could only yield butanol to sellable
quality due to the selective property of the solvent used (2-ethyl-hexanol). This reduction in
sales led to an IRR of 6% which is below the discounted rate used (10%) although a positive
NPV of US$82.38 million resulted. Process Scheme 4, making use of a two-stage gas
stripping and distillation, was the most profitable process and it was concluded it would be
the process to attach to the sugar mill model and also to be considered for the higher value
chemical production. An NPV of US$524.09 and an IRR of 32% were realised for this process.
Sensitivity analyses on these four processes showed that the cost of the substrate (clear
juice) and the butanol selling price have the major effects on the profitability. It was,
therefore, recommended that other streams from the sugar mill be considered as substrates
for higher value chemical products which can attract higher prices than butanol which is
regulated by the petro based butanol. Finally, a structure of a functionalised ionic liquid was
suggested based on group contribution methods to be a potential reactive extraction
reactant for converting butanol to a higher value ester product.
Master of Science in Chemical Engineering. University of KwaZulu-Natal. Durban, 2017.
2018-01-01T00:00:00ZPhase equilibrium studies of NFM and toluene with heavy hydrocarbons and the conceptual process design of an aromatics recovery unit.
https://researchspace.ukzn.ac.za/handle/10413/16764
Phase equilibrium studies of NFM and toluene with heavy hydrocarbons and the conceptual process design of an aromatics recovery unit.
Distillation and extraction are commonly employed phase separation techniques, and improved efficiency and cost reduction in these large-scale processes are motivating factors behind thermodynamic equilibrium investigations. This first objective of the research undertaken was phase equilibrium studies of two ternary systems comprising of a heavy hydrocarbon and toluene, with the suitability of NFM as an extraction solvent investigated, due to its good selectivity and heat stability (Xia et al., 2008). The other objective was the development and simulation of a conceptual process design using Aspen Plus V8.4 to demonstrate the separation and recovery of aromatics using NFM, and to make a comparison to an existing process in terms of energy and cost efficiency.
Ternary liquid-liquid equilibrium (LLE) phase compositions were generated for the systems n-nonane (1) + toluene (2) + NFM (3), as well as n-decane (1) + toluene (2) + NFM (3). The measurements were conducted at 303.15 K, 323.15 K, and 343.15 K for each system. The modified apparatus of Raal and Brouckaert (1992) was used, with the latest modifications to the cell incorporating an adjustable temperature sleeve and magnetic stirrer (Narasigadu et al., 2014). The uncertainty in temperature of each cell was 0.02 and 0.01 respectively. Composition uncertainty was minimized by ensuring that phase composition samples were within 1% of the repeatability error for the average absolute deviation of at least 3 samples taken. Samples were analysed using gas chromatography.
The ternary systems measured in this work were modelled in terms of the NRTL model (Renon and Prausnitz, 1968) and the UNIQUAC model (Abrams and Prausnitz, 1975). Calculated RMSD values were between 0.002 and 0.02 for both models, indicating that the models represented the data satisfactorily, with the NRTL model displaying superior representation due to lower RMSD values compared to UNIQUAC. The effectiveness of using NFM an alternative solvent to extract toluene from a mixture containing n-nonane and n-decane was evaluated by determining the distribution coefficient, selectivity, and separation factor.
A process design simulation was developed using Aspen Plus V8.4 for the separation of benzene, toluene, ethylbenzene and xylene (BTEX) isomers from a hydrocarbon mixture using NFM as the
ABSTRACT
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solvent. Process conditions and column specifications were optimized by investigating numerous unit configurations and running sensitivity analyses on these parameters. The aim was to target a recovery of at least 99% aromatics, which was achieved. A sequence of columns was used to effect the aromatics recovery, consisting of a counter-current liquid-liquid extraction column, followed by four distillation columns in series. The simulation results indicated that the process would consume at least 11 kcal/kg extract less energy than the sulfolane process. This manifests as lower heating and steam requirements, resulting in reduced costs of at least R19 million per annum.
Masters of Science in Chemical Engineering. University of KwaZulu-Natal. Durban, 2017.
2017-01-01T00:00:00ZThe development of a weighted directed graph model for dynamic systems and application of Dijkstra’s algorithm to solve optimal control problems.
https://researchspace.ukzn.ac.za/handle/10413/16759
The development of a weighted directed graph model for dynamic systems and application of Dijkstra’s algorithm to solve optimal control problems.
Optimal control problems are frequently encountered in chemical engineering process control applications as a result of the drive for more regulatory compliant, efficient and economical operation of chemical processes. Despite the significant advancements that have been made in Optimal Control Theory and the development of methods to solve this class of optimization problems, limitations in their applicability to non-linear systems inherent in chemical process unit operations still remains a challenge, particularly in determining a globally optimal solution and solutions to systems that contain state constraints.
The objective of this thesis was to develop a method for modelling a chemical process based dynamic system as a graph so that an optimal control problem based on the system can be solved as a shortest path graph search problem by applying Dijkstra’s Algorithm. Dijkstra’s algorithm was selected as it is proven to be a robust and global optimal solution based algorithm for solving the shortest path graph search problem in various applications. In the developed approach, the chemical process dynamic system was modelled as a weighted directed graph and the continuous optimal control problem was reformulated as graph search problem by applying appropriate finite discretization and graph theoretic modelling techniques. The objective functional and constraints of an optimal control problem were successfully incorporated into the developed weighted directed graph model and the graph was optimized to represent the optimal transitions between the states of the dynamic system, resulting in an Optimal State Transition Graph (OST Graph). The optimal control solution for shifting the system from an initial state to every other achievable state for the dynamic system was determined by applying Dijkstra’s Algorithm to the OST Graph.
The developed OST Graph-Dijkstra’s Algorithm optimal control solution approach successfully solved optimal control problems for a linear nuclear reactor system, a non-linear jacketed continuous stirred tank reactor system and a non-linear non-adiabatic batch reactor system. The optimal control solutions obtained by the developed approach were compared with solutions obtained by the variational calculus, Iterative Dynamic Programming and the globally optimal value-iteration based Dynamic Programming optimal control solution approaches. Results revealed that the developed OST Graph-Dijkstra’s Algorithm approach provided a 14.74% improvement in the optimality of the optimal control solution compared to the variational calculus solution approach, a 0.39% improvement compared to the Iterative Dynamic Programming approach and the exact same solution as the value–iteration Dynamic Programming approach. The computational runtimes for optimal control solutions determined by the OST Graph-Dijkstra’s Algorithm approach were 1 hr 58 min 33.19 s for the nuclear reactor system, 2 min 25.81s for the jacketed reactor system and 8.91s for the batch reactor system. It was concluded from this work that the proposed method is a promising approach for solving optimal control problems for chemical process-based dynamic systems.
Master of Science (Chemical Engineering). University of KwaZulu-Natal. Durban, 2017.
2017-01-01T00:00:00Z