A dissertation submitted in fulfilment of the requirement for the degree of Master of Science in Engineering, to the Faculty of Engineering and the Built Environment, School of Chemical and Metallurgical Engineering, University of the Witwatersrand, Johannesburg, 2018 / The two hydrofluoric acid (HF) storage tanks used for holding 70% technical grade HF product at the HF plant at Necsa started leaking in March 2012. An evaluation of the failure was conducted in the form of a corrosion failure analysis. It was confirmed that a higher than usual nitric acid (HNO3) content in the technical grade HF stream changed the corrosion mechanisms typically experienced within the HF storage vessels, which then caused the tanks to fail. Immersion type corrosion experiments were done to safely simulate the corrosive environment experienced by the mild steel, stainless steel and nickel alloys used on site, and to predict the change in corrosion rates and characteristics associated with the HNO3 contamination in the HF production plant circuit. Since the corrosion resistance of mild steel in HF is heavily dependent on the thickness of the protective scale on the steel, a series of planned interval corrosion tests (PICTs) was done to reproduce and then examine the oxidefluoride barrier on mild steel coupons in pure 70% HF prior to corrosion tests. These shorter PICTs were also done on the stainless steel and nickel alloys and showed that the prepassivation step had a surface cleaning effect when exposed for only 24 h. Eleven day corrosion tests were conducted to establish the effect of HNO3 concentration and temperature on mild steel corrosion in 70% HF, and to determine the change in corrosion rates and mechanisms associated with HNO3 contamination (0.1-1% HNO3) of the downstream HF products. The corrosion was characterized by analysing the corroded coupons for mass loss, apparent corrosion rates, acid consumptions, visual observations of scale formation and pits, as well as depth profiles from scanning electron microscopy and energy dispersive spectroscopy analyses. Linear relationships were frequently observed when analysing mass losses for the coupons over time, making it possible to define corrosion rates in terms of first order reaction kinetics. The harshest corrosive condition for mild steel in HF was determined to be 1% HNO3 in 70% HF at a constant temperature of 25ºC. The corrosion characteristics of alloys used in the HF plant, as affected by HNO3 impurities (in the range 50–10000 ppm) in the final HF acid product (70% Technical grade) were successfully established. Normalized SA516 Grade 70 mild steel and Monel 400 were found not adequate for use as construction materials in a plant where HNO3 contamination was >100 ppm. However, the corrosion resistance of SS 904 L was suitable under these conditions and was recommended for applications in HF solutions where the presence of an oxygen-containing acid (e.g. HNO3) is consistent. It was recommended that Alloy 31, Alloy 33 or Nirosta 4565S, with higher chromium content (>20 wt% Cr), should be considered for construction material of the HF plant when HNO3 contamination becomes unavoidable. However, if the continued use of mild steel at the plant cannot be avoided, other inhibition strategies tailored to the selective consumption of HNO3 in the HF product stream need to be investigated. / XL2019

M.Phil. / In modern society, professional engineers, technologists and technical managers are responsible for the planning, design, manufacture, maintenance and operation of the processes and systems ranging from simple processes to complex systems. The failure of these can often cause effects that range from inconvenience and irritation to severe impact on the society and its environment. Users, customers and society in general expect that products be reliable and safe at all times (Allan & Ballinton 1992). The biggest investment in any plant is, arguably, on individual plant equipment. It is therefore reasonable to give the greatest attention possible to the health and integrity of equipment that form part of the chemical process plant.Most of plant failures occur without warning and this result in equipment breakdowns, huge production losses and expensive maintenance. The reaction to plant failures has, in most cases, been a reactive maintenance which means that the plant equipment must fail before the cause of fault is investigated and the equipment is repaired. Reactive maintenance has shortcomings in that it is successful in solving problems temporarily but does not guarantee prevention of fault recurrence. Equipment and process failures waste money on unreliability problems. The question that arises is. ‘How reliable and safe is the plant during its operating life?’ This question can be answered, in part, by the use of quantitative reliability evaluation. The growing need to achieve high availability for large integrated chemical process systems demands higher levels of reliability at the operational stage. Reliability is the probability of equipment or processes to function without failure when operated correctly for a given interval of time under stated conditions. This research dissertation is aimed at developing equipment optimisation program for the chemical process plant by introducing a logical approach to managing the maintenance of plant equipment. Some relevant reliability theory is discussed and applied to the Short – Path Distillation (SPD) plant of SASOL WAX. An analysis of the failure modes and criticality helps to identify plant equipment that needs special focus during inspection.