Chemical Engineering

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Development of a pilot scale black liquor gasifier.
(2011-05-04) Avidi, Vernon Alastair.; Arnold, David R.; Hunt, John.; Leske, Anthony.
The use of black liquor gasification as an alternative to conventional chemical and energy recovery systems for spent liquors is an area of particular interest to the pulp and paper industry. The motivation to explore this technology is to improve the thermal efficiency of the recovery process by utilizing the energy content of the spent black liquor more effectively and provide chemical recovery for sodium and sulphur containing liquors for a local pulp and paper mill. A study of the available gasification technologies showed that the steam reforming process marketed by ThermoChem Recovery International is particularly suited to the mill in that it can handle a change to a sulphite pulping chemistry and also handle silica removal which is an inherent problem with the bagasse raw material that the mill uses. However the technology required further development and confirmation of process suitability before implementation at the mill. This aim of this project was to build and operate a gasifier based on the TRI concept to determine if this process is suitable for recovery of SASAQ black liquor from bagasse pulping. This included gaining an understanding of the process variables like the black liquor solids composition and the non-process element levels and required carrying out a mass balance on inorganic components across the reactor as well. The focus of this investigation was primarily on the front end of the project and entailed basic and detailed design of a pilot gasification unit. The pilot unit was subsequently constructed, commissioned and operated to prove the unit met the design intent. Preliminary results showing the conceptual proof of the technology are presented as well as performance tests showing the unit capability of gasifying a 3.1 1Ihr 60% solid content black liquor feed. Problematic areas that could influence the design of a scale-up unit were identified and highlighted for further development, with proposed solutions.
Fluidised bed gasification of spent soda and sulphite liquors from the paper industry.
(2004) Sewnath, Pravesh.; Arnold, David R.
The pulp and paper industry uses pulping chemicals for the treatment of bagasse, straw and wood chips. Spent liquor or effluent liquor, with high carbon content is produced and sent to chemical recovery to recover pulping chemicals. In addition, energy from the spent liquor is recovered and utilised to generate steam for electricity supply, thereby reducing fossil fuel power consumption. Spent liquor is destroyed using conventional incineration technology, in a recovery furnace or recovery boiler, which is the heart of chemical recovery. These units have over the past few decades been prone to numerous problems and are a major concern to the pulp and paper industry. They pose a threat to the environment, are expensive to maintain and constitute a safety hazard. Thus the pulp and paper industry is now looking at a replacement technology; an alternative that will effectively regenerate pulping chemicals and recover energy for generating electricity, ultimately to make the plant energy self-sufficient. Gasification technology may be the chosen technology but is yet to be applied to the pulp and paper sector. However, this technology is not new. It has been integrated and used successfully in the petroleum industry for decades, with applications in coal mining and the mineral industry. The overall objective of tills study is to develop a better understanding of gasification using a pilot-scale fluidised bed reactor which was designed and developed at the University of Natal. The reactor, "the Gasifier", is operated at temperatures below the smelt limits of inorganic salts (<750°C) in the spent liquor. In this investigation, spent liquor is injected directly into an inert bed of alwninium oxide grit, which is fluidised by superheated steam. The atomized liquor immediately dries when it contacts the grit in the bed, pyrolyses and the organic carbon is gasified by steam. Pyrolysis and steam gasification reactions are endothennic and require heat. Oxidised sulphur species are partially reduced by reaction with gasifier products, which principally consist of carbon monoxide, carbon dioxide and hydrogen. The reduced sulphur is said to be unstable in the gasifier environment, and reacts with steam and carbon dioxide to form solid sodium carbonate and gaseous hydrogen sulphide. (Rockvam, 2001). The focus of this study will be to determine the Gasifier's ability to gasify spent liquor, from soda and sulphite pulping of bagasse, at different operating conditions. In addition, the fate of process and non-process elements will be investigated. The product gas generated in the gasification of spent soda and sulphite liquors consisted of hydrogen, carbon dioxide, carbon monoxide and methane. In the gasification of spent sulphjte liquor, hydrogen sulphide was also produced. The water-gas shift reaction, which was the main reaction, was found to be temperature dependent. In adilition, organic carbon conversion increased with temperature. Furthermore, most of the sulphur in the bed predominated in the form of hydrogen sulphide with very little sulphur in the form of sulphate. This indicated that gasification would reduce sulphate levels, which are responsible for dead load in a chemical recovery cycle. Finally, an important result was that the aluminium oxide grit was successfully coated. It was previously speculated that this would not be possible.
An investigation into a lower temperature and low cost direct reduction process for iron-making.
(2003) Chellan, Reubendran.; Pocock, Jonathan.; Arnold, David R.
The blast furnace process for the reduction of iron ore to pig iron faces problems such as emission of air pollutants, high investment cost and the current major problem of decreasing supplies of coke. Coke is used in large quantities to promote a combination of direct and indirect reduction within the furnace. Due to the lack of good coking coal within South Africa, and dwindling supplies worldwide, new iron-making processes, are being developed using coal and/or natural gas to replace coke as the reductant. The new processes allow efficient use of carbon, fed in the form of coal pellets (coalbased processes) or natural gas (gas-based processes), as the reducing agent. Presently, most coal-based processes Use an excess of coal, up to 500% stoichoimetric addition, and are run at temperatures up to ±1200°C, although reduction tends to proceed at ±850°C. This project developed a low temperature process using mixed pellets of fine waste iron oxide and fine domestic coal with a natural carbonaceous binder (a by-product from local pulping industry). Reduction tests performed on composite pellets in a tube furnace and thermobalance indicated, upon analysis by X-Ray Diffraction and Scanning Electron Microscope, that reduction occurred gradually at 900°C. Implementing induction heating of bulk pellets reduced heating times substantially. Induction heating also resulted in direct reduced iron [DRI] containing 75 - 80% metallic iron. Energy consumption based on coal usage amounted to 23.71 GJ/ton DRI, which compares with the calorific consumption of most coal-based processes, i.e. coal consumption range between 15 and 25 GJ/ton DRI. Energy consumed during induction heating amounted to 9.94 GJ/ton DRI, as electricity. This energy consumption value does not take into account the efficiency of the primary energy required to generate electricity.
Investigation of the effects of polychlorinated biphenyl (PCB) dechlorination on the natural inhibitors and oxidation stability of uninhibited naphthenic based mineral oils.
(2010) Nassiep, Sumaya.; Arnold, David R.
PCBs are persistent organic pollutants that have intentionally and unintentionally (through contamination) been added to mineral insulating oil to improve its insulating and cooling properties within electrical equipment. The Stockholm Convention on Persistent Organic Pollutants (2001) orders the eradication of PCBs from use by 2025. Sodium based dechlorination is a PCB destruction process that is non-thermal, relatively cost effective and allows for the recovery of a reusable end product. A comparative benefits study, contained in chapter 2 of this dissertation, describes the increased financial and environmental impact associated with incinerating large volumes of PCB contaminated mineral oil. The results of the comparative analysis indicate a cost ratio of 1: 2.5, in favour of sodium dechlorination. In addition to the financial benefit, the sodium based PCB dechlorination process is versatile and can be either batched or skid mounted and is typically combined with an oil regeneration step, allowing for transformers to be treated onsite and whilst energised. Eskom is currently considering obtaining the mobile dechlorination unit for the purpose of conducting dechlorination and regeneration on its PCB contaminated transformers while energised. Mineral insulating oil is considered a strategic asset within most industries. Eskom uses mainly uninhibited mineral oil in its older transformers and the effects of PCB dechlorination on the natural inhibitor content of the oil is uncertain. The objective of this study was to investigate the effects of sodium dechlorination on the oxidation stability and thereby indirectly the natural inhibitor content of uninhibited naphthenic based mineral oil. The study involved the dechlorination, regeneration and subsequent chemical analyses of PCB contaminated oils in the PCB ranges <50ppm, 50 to 500 ppm and >500 ppm as stipulated by the Stockholm Convention on Persistent Organic Pollutants (2001). The study confirmed the reduction in oxidation stability and thereby the natural inhibitor content of the mineral oil after sodium dechlorination. Based on the results obtain a preliminary algorithm was established to predict the reduction in oxidation stability after sodium dechlorination, as a function of the PCB concentration prior to dechlorination. This will provide an indirect indication of the rate of natural inhibitor depletion of the oil, based on its exposure to the sodium dechlorination reagents and process conditions.
Production of activated carbon from South African sugar-cane bagasse.
(2003) Devnarain, Prathisha Baruth.; Arnold, David R.; Loveday, Brian Kelsey.
The South African sugar industry generates excessive amounts of sugar cane bagasse (~ 25 wt% of feed) as a byproduct during the extraction of sugar juice from cane. Although bagasse is extensively consumed in various processes, a substantial amount remains unexploited. The industry's core business is the production of refined sugar which involves among others, a step of decolourising raw sugar liquor. Activated carbons are well known adsorbents and their excellent decolourisation capabilities have been established since 1800 in the sugar industry. The possibility of making suitable in-house activated carbons from sugar cane bagasse to aid the decolourisation process of raw sugar liquor is of interest to the growing South African sugar industry. The purposes of this research study were to develop an understanding on the manufacture of activated carbons from sugar cane bagasse, produce suitable activated carbons on a laboratory scale, characterize them and subsequently determine their sugar decolourisation capabilities under simulated conditions. The application of the two-step physical method of processing was found to be the most effective and feasible route to produce activated carbons from sugar cane bagasse for the purposes of decolorizing unrefined sugar. A semi-batch process was developed whereby compressed sugar cane bagasse was pyrolysed under a nitrogen atmosphere at a heating rate of 10 °C/min to the final pyrolysis temperature for a desired hold time resulting in bagasse chars with a rudimentary pore structure. These bagasse chars were subsequently subjected to partial and controlled gasification with a steam/nitrogen mixture at higher temperatures to produce the final activated carbon product. Both pyrolysis and activation were carried out in a pyrolysis furnace that was modified to represent a fixed bed reactor system. The process was designed such that it included a steam supply and a gas cleaning system. Feasible processing conditions were established by varymg the temperature, hold time and partial pressure of steam in the pyrolysis furnace. The bagasse chars and final activated carbons were characterized with respect to surface area, pore volume, pore size distribution, methylene blue number, iodine number and molasses number. The optimum pyrolysis conditions were found to be at heating rate of 10°C/min to the final pyrolysis temperature of 680 °C for a hold time of 1 hour, which gave rise to microporous carbons. Increasing the steam partial pressure and activation temperature during activation of bagasse chars resulted in the gasification reaction proceeding at a much faster rate leading to well developed mesoporous activated carbons having high adsorption capacity for large colour bodies present in molasses and sugar liquor. This was achieved by activating bagasse chars at a temperature of 900°C for 2 hours with a steam / nitrogen mixture of 1:0.6 which resulted in 50% bum-off being reached. Excellent powder and granular activated carbons were produced from sugar cane bagasse fibres by the established process with the latter being mixed with refined sugar prior to pyrolysis and activating for half an hour extra. A typical final activated carbon produced in this research possessed a BET surface area of 995 m2/g, pore volume of 0.82 crrr'zg, iodine number of 994 mg/g, molasses number of 700 and methylene blue number of 256 mg/g. High ash content in the bagasse raw material tends to decrease the surface area and pore volume for adsorption of the final activated carbon. Both granular and low ash bagasse activated carbons possess high adsorption capacity to remove large colour bodies from molasses and brown liquor solutions and compare well with commercial Norit N2 carbon . Approximately 80% colour removal was achieved using 0.5 g carboni 100g brown liquor. The bagasse activated carbons were stable in acidic and basic brown liquor solution and maintained their high decolourisation potential. The ability of bagasse activated to replace commercial activated carbons has been proven in this study. The option of producing both granular and powder activated carbons provide flexibility of the sugar industry to choose between batch and continuous adsorption systems during sugar decolourisation. This research has established that the fact that excellent sugar decolourising activated carbons can be produced from South African sugar cane bagasse fibres. However, more research needs to be carried out in order for the sugar industry to take this project to the commercial stage and it is suggested that a pilot study and an economic study be carried out.
Production of activated carbon from South African sugarcane bagasse.
(2005) Mwasiswebe, Denny.; Arnold, David R.
South Africa has an annual sugarcane milling capacity of about 22 million tonnes on average producing about 3.3 million tonnes of dry bagasse, of which one third is surplus to factory requirements. Currently surplus bagasse is used for furfural, pulp and paper and cogeneration but significant amounts still remain . This prompted the need to find viable alternative and appropriate technology to utilize the surplus. A laboratory pilot plant was used to investigate the production of activated carbon from bagasse. Experiments were carried out to investigate conditions for making the best activated carbon in a rotary batch kiln, and also to examine potential ene rgy recovery from process gases using Gas Chromatography. Derived results from the laboratory experiments were used to develop a conceptual design for a demonstration plant sited within a sugar mill. The conceptual design was evaluated for economic and environmental impacts using a robust Excel spreadsheet and GABI-3 modelling software respectively. Excellent activated carbon was produced from sugarcane bagasse by a two-stage physical process involving pyrolysis and gasification with steam. The best operating conditions were pyrolysis at 700°C for 1 hr and activation at 850°C for 1hr, a heating rate of 10°C/min and a steam flow of 15g/g of char per hour. The active carbon yield was 7% on dry bagasse basis with a Methylene Blue Number of 257mglg of carbon. The active carbon had a sugar decolourisation capacity of 20% at a carbon dosage rate of 0.7 wt% on Brix using clear juice (l2°Brix) and 70% at 0.5 wt% on Brix using brown liquor (65°Brix) . The Freundlich isotherm showed that the bagasse-based activated carbon was a suitable adsorbent for sugar colour bodies. Gas analysis results revealed that the off gases from the pyrolysis and activation stages had calorific values of about 63MJ and 31MJ per kg of activated carbon respectively . The total combustion energy of 94 MJ/kg of active carbon was enough to satisfy the process energy requirements for drying, pyrolysis and activation. By burning combustibles like tar, methane, carbon monoxide, ethylene and hydrogen for process thermal energy needs, the environmental impact of the manufacturing process was reduced to a Global Warming Potential of llkg CO2 Equiv per kg of carbon produced. The demonstration plant requires a capital investment of US$lOA million to give a competitive bagasse-based activated carbon (BPAC) selling price of US$1.80 per kg and IRR, ROI and Investment payback time of 17.93%, 23.93% and 3.80 years respectively. A sensitivity analysis was also carried out to investigate the effect of possible variation in the main project forecasts which are BPAe selling price , bagasse buying price, capital investment and production costs on IRR, ROI and payback time . The benefits of process integration within a sugar mill would be expected to improve the business feasibility ; If bagasse was free the IRR would increase to 28.59% and even better to 32.12% if extra boiler and electricity capacity was also available at the mill.
The production of furfural from sunflower husks using the s-suprayield process.
(2010) Schay, Samantha Rachel.; Starzak, Mathew.; Arnold, David R.
Since the early 1920s, when furfural was first produced, several other processing routes have been developed but none have been able to produce yields comparable to those obtained in the standard TAPPI procedure for xylan which almost completely converts xylan to furfural. Karl Zeitsch, a German chemist, believed that the key feature of a process which could achieve high yields was rapid removal of the furfural on formation. Zeitsch suggested using gas phase HCl catalysis to produce gaseous furfural from xylan containing material, the process was titled s-Suprayield. The experimental apparatus heated a water and HCl solution to a superheated vapour phase and then allowed for contact of the vapour and a bed of pentosan-containing material (in this case sunflower husks). The raw material was analysed by the TAPPI procedure for xylose while the product solutions were analysed for HCl, acetic acid and furfural by titration and refractive index. Tests were performed at four acid concentrations of 0.5, 1.1, 2.2, 4.3% wt and three different temperatures viz. 163ºC, 152 ºC and 144 ºC. The best yields of over 80% were achieved when an acid concentration of 4.3% was used. Temperature did not appear to be as significant a factor as acid concentration in affecting the furfural yield. At an acid concentration of 0.5% the yield was low ranging from 33% to 42%. The reactor modelling was used to verify the results. The s-Suprayield process has been demonstrated to be successful at mini-pilot plant scale indicating that a process using gaseous catalysis to produce furfural at moderate temperatures and low acid concentrations can work and that further exploration of this process should be undertaken for potential industrial use. Acid concentration was observed to have a significant effect on the reaction yield while the effect of temperature was not clear from the experimental results. Further work should focus on understanding the reaction kinetic and the development of a laboratory scale test method for which parameters such as gas flow rate and temperature can be properly controlled. Product analysis should be more rigorous with the use of an HPLC.
Pyrolysis of chlorinated hydrocarbons using induction heating.
(2004) Pillay, Kruben.; Arnold, David R.; Rawatlal, Randhir.
Chemical and allied industries produce significant quantities of chlorinated wastes each year. Thermal treatnent of these chlorinated wastes has a long and controversial history. The most common and contentious method of waste destruction is incineration. Although waste incinerators are designed to provide greater control over the combustion process, toxic products are inevitably formed from incomplete combustion and released in stack gases and other residues. The most notable group belonging to the products of incomplete combustion (PICs) are dioxins and furans. The fact that oxygen is an integral part of the molecular structure of dioxins and furans suggests that the formation of these particular PICs may be reduced or avoided by minimizing or completely excluding oxygen from thermal waste treatment. Pyrolysis using induction heating is a relatively new technology that has shown much promise from the initial work performed by Pillay (2001). This research was an extension of that study, and investigated equipment and process optimization as well as macroscopic modeling of different systems. The objective of this study was to establish the technology of pyrolysis using induction heating as a competitive alternative to existing waste destruction systems. The novel approach of pyrolysing compounds using induction heating was demonstrated by destroying chlorinated aliphatic, aromatic and a mixture of these compounds. These experiments were conducted at atmospheric pressure in a tubular laminar flow reactor (5.2cm I.D) under a thermally transparent argon atmosphere. In this system heat was generated in an embedded graphite tube using induction heating. Thermal degradation occurred through the bombardment of the compounds by the photons emitted from the heated graphite tube. The compounds were pyrolysed at temperatures ranging from 330°C to 1000°C and at mean residence times from 0.47s to 2.47s. In addition to these process variables the effects of reactant concentration and additives were investigated The major species formed from this thermal treatment were solid carbon black and gaseous hydrogen chloride. Destruction efficiencies (DE) of the order of 99.9999% (six nines) and greater were obtained for the different feed mixtures at their respective operating conditions. A minimum DE of six nines adequately satisfies the regulation set by the Environmental Protection Agency (EPA) for successful waste destruction.
Pyrolysis of chlorinated organic chemicals.
(2001) Pillay, Kleantha.; Arnold, David R.; Ramjugernath, Deresh.
At present, South Africa has inadequate technology to destroy its hazardous waste, with approximately 18000 litres of chlorinated hazardous waste stored in this country. Approximately 800 tons of banned or obsolete chemicals are to be sent to Pontypool. Wales, for incineration, at a considerable cost. Because of the toxic nature of chlorinated waste and their long-term effects on the environment , a sustainable method of dealing with this type of waste is essential. Gas phase destruction of methylene chloride, trichlorobenzene and lindane by pyrolysis (i.e. heating in the absence of oxygen) was attempted. Destruction was effected by high temperature thermal degradation of molecules into free radicals. These radicals then combine to form hydrogen chloride and carbon as major products. This method was chosen so as to eliminate the possible formation of highly toxic oxygenated derivatives such as polychlorinated dibenzofurans and dibenzodioxins that can be formed during incineration if strict control is not excercised. The reactor assembly was built in the Department of Chemical Engineering at the University of Natal. 11 incorporates aspects of many different previously designed reactors, as discussed in the text. Heat for the reactions was supplied by induction. A high frequency induction unit supplied current to a copper coil. The resulting magnetic field induced current to flow in a susceptor housed within the copper coil. The susceptor in this case was a graphite tube, which served as both the heating element and the thermal radiation source, in addition to forming the walls of the reaction zone. Up and down stream processes were designed and experiments were carried out in which reaction temperatures (348-1400°C) and residence times (1.3-5.6 seconds) were varied. Destruction efficiencies of 100% and 99.99% were obtained for methylene chloride and trichlorobenzene respectively, with inert argon used as the carrier gas. These destruction efficiencies comply with the 99.99% stipulated by the United States Conservation and Recovery Act. A cause for concern was the formation of chlorinated benzenes and naphthalenes. Destruction of lindane proved unsuccessful due to limitations in the vapourisation and feed system and will have to be investigated further. The method of induction heating was evaluated to be 98.9% thermally efficient. Raw material and utility consumption per ton of waste destroyed by the pyrolysis process was compared to values for incineration as well as the plasma arc and catalytic extraction processes. Consumption for pyrolysis compares favourably with all three processes and suggests that the process could be competitive. Claims to the success of the technology on a wide scale are limited by the small number of compounds that were successfully pyrolysed. Results do however indicate much promise for this technology to be used as a fi nal chlorinated waste destruction unit on an existing process. Modifications to the existing reactor to improve product recovery and analys is will allow for temperature and residence time optimisation for a variety of wastes. Additional in strumentation and process control will allow for kinetic studies to be undertaken in future. This project should be considered as the first step in an ongoing series of research and subsequent improvements to the technology presented here.

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