Evaluation of manufacturing processes for the production of atrazine
- Authors: Schaefer, Melissa Claire
- Date: 2002
- Subjects: Atrazine , Pesticides
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10960 , http://hdl.handle.net/10948/96 , Atrazine , Pesticides
- Description: This report describes the results of investigations carried out with the view to find an alternative for MIBK as solvent for the production of atrazine as currently practised by Dow AgroSciences in South Africa. The main motivating factors for the said investigation was: · to increase the yield of atrazine produced, · to reduce the amount of organics, consisting essentially of reaction solvent containing dissolved product, in the aqueous process effluent, and · to improve the properties of the solid (crystalline) product to enable easier product formulation. Synthetic reactions carried out in the absence of organic solvent, i.e. in essentially a 10% NaCl solution containing a surfactant, proved rather disappointing. Low yields of atrazine were obtained together with relatively large amounts of by-products such as propazine and simazine, irrespective of the nature of the surfactant. The reason for the low yield of atrazine and high yields of by-products were established in competing substitution reactions. In these reactions, IPA was reacted with an equimolar mixture of cyanuric chloride and mono-i (first reaction intermediate) in both aqueous medium and in toluene as reaction solvent. The results of these experiments indicated that in aqueous medium IPA reacts faster with mono-i than cyanuric chloride to give propazine as by-product. In toluene, however, the preferred reaction is with cyanuric chloride to give more mono-i as product. Toluene was investigated as an alternative organic solvent to MIBK in view of its desirable properties such as low solubility in water and ease of recovery and recycling. The synthesis of atrazine was optimised in terms of addition sequence and rates of amine reagents and base (HCl acceptor), both by means of benchscale reactions and reaction calorimetry. Reaction energy profiles indicated that both the reaction of secondary amine/NaOH and primary amine/NaOH were virtually instantaneous. This implies that the reaction can be performed under feed control conditions. Of particular importance in ensuring high yields of high purity product was accurate temperature control (since both reaction steps are highly exothermic) and mixing. The latter was important in view of the rapid reaction of amine/NaOH with cyanuric chloride, as well as the possible reaction of mono-i, the first reaction intermediate, with IPA in cases of local excesses of reagent. Under optimum conditions, a yield of atrazine > 97% could be achieved where the resultant product was well within stipulated product specifications. In view of the results obtained, the following recommendations regarding the synthesis of atrazine in toluene as reaction solvent can be made: · Use a reagent addition sequence that staggers the addition of amine and NaOH in such a manner that amine is added first for a short while, followed by the simultaneous addition of amine and NaOH, and ending with NaOH. Use two reaction vessels in series, one for the IPA addition reaction and one for the MEA addition reaction. In this manner the reaction can be run on a continuous basis since no lag time between amine additions is required. Also, smaller reactors may be used whilst maintaining high production rates. Smaller reactors will improve both temperature control and mixing of reagents.
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- Date Issued: 2002
Process for the preparation of vanillin from a mixed m-cresol/p-cresol stream
- Authors: Buddoo, Subash
- Date: 2002
- Subjects: Vanillin
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10968 , http://hdl.handle.net/10948/138 , Vanillin
- Description: The Vanillin project was undertaken by AECI as it was believed that with the raw materials available from SASOL i.e. cresols, it would be possible to develop a process that would be globally competitive. A process for preparing both vanillin and ethyl vanillin was developed by AECI’s Research and Development Department using a mixed m- and p-cresol stream as feed. The displacement of bromide by methoxide is very successful when using a dimethyl formamide (DMF)/alcohol solvent system and excellent conversions (>95%) and selectivities (>95%) may be obtained. However, the use of DMF, which poses a serious chronic health risk, is unacceptable in this process since the products are intended for use as food and flavouring chemicals. In view of the above the main objectives of this study were: · To find a suitable alternative solvent system, which could produce comparable results while still being economically viable; · To develop an appropriate experimental protocol in the laboratory based on the alternative solvent system; · To determine the important reaction variables by conducting statistically designed experiments; · To optimise the reaction to produce a reproducible and robust experimental protocol; and · To test the reaction thoroughly at bench-scale level and to obtain experimental data for scale-up to pilot plant The most promising alternative solvent system was a methanol/methyl acetate mixture, which produced satisfactory results in the preliminary assessment (conversion of 98.3% and selectivity of 92.0%). DMA and acetonitrile also produced promising results but were not considered for further investigation because of toxicity and cost issues. A set of statistically designed experiments was carried out on the methanol/methyl acetate solvent system where four variables were tested i.e., substrate concentration, temperature, catalyst loading, and methanol to methyl acetate volume ratio. The experimentally determined response surface model showed that the most important variable was catalyst loading (63.2%) for conversion. With respect to selectivity, the most important variables were catalyst loading (31.9%) and methanol to methyl acetate ratio (33.1%). The optimum reaction conditions were as follows: · Temperature: 120°C · Methanol:methyl acetate: 15:1 vol/vol · Catalyst loading: 8 mol % to substrate · Substrate concentration: 22 %m/m on solvent · Catalyst: Copper(I) bromide · Sodium Equivalents: 2.7 wrt substrate · Time: 3 hours The optimum conditions were tested for reproducibility in a 1 Labmax pressure reactor. Replicated reactions, two at a 10% and two at a 20% substrate concentration gave conversions and selectivities all greater than 90%. Although the reaction mixture was a slurry at these concentrations, the reactions were very fast and virtually complete within the first hour (~95% conversion). Initial scale-up studies were conducted in an 8 Parr reactor where five reactions were carried out using the optimum conditions described above. The conversion of substrate and vanillin selectivity was consistently high and compared favourably to the Labmax reactions. The average conversion was 97.3% (96.3 to 98.5%) at an average selectivity of 98.2% (97.4 to 99.1%). A study of the reaction kinetics confirmed that the reaction was first order with respect to the substrate as a plot of substrate concentration versus reaction rate gave a straight line. The rate constant was calculated as 1.1096 k(h-1). The reaction mechanism proposed for the copper assisted nucleophilic aromatic substitution involves the formation of an adduct between sodium methoxide, methyl acetate and copper(I) bromide. The formation of a transient intermediate with the substrate allows intramolecular delivery of the methoxide ion to the aryl moiety through a CuI – CuIII type cycle.
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- Date Issued: 2002
The electrochemical hydroxylation of aromatic substrates
- Authors: Rautenbach, Daniel
- Date: 2002
- Subjects: Aromatic compounds , Hydroxylation
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10959 , http://hdl.handle.net/10948/94 , Aromatic compounds , Hydroxylation
- Description: The electrochemical hydroxylation of aromatic substrates was investigated in some detail, with the view to develop a method, which could produce dihydroxybenzenes in acceptable yields. Of particular interest was the selectivity and yield of the 1,4-dihydroxybenzenes. Two distinctly different methods were investigated in order to achieve this goal, acyloxylation and direct electrochemical hydroxylation. Acyloxylation is the process where radical cations generated at the anode undergoes nucleophilic attack by acetate anions. The resulting aromatic acetates so produced can then be hydrolysed to the phenolic compounds. Two nucleophile systems were considered in the investigation, acetates (acetoxylation) and trifluoro-acetates (trifluoro-acetoxylation). These investigations were conducted under a variety of conditions using phenol and phenyl acetate as starting materials. From the results it was, however, concluded that the acetoxylation of these aromatic compounds occurs in unacceptable product and current yields. Trifluoro-acetoxylation on the other hand showed promise, but due to the nature and cost of the reagents it was deemed to be an impractical process. Direct electrochemical hydroxylation: in which the radical cations produced at the anode undergoes nucleophilic attack by water producing the corresponding dihydroxybenzenes. These dihydroxybenzenes are then further oxidised to the benzoquinones, which then undergo reduction at the cathode in order to produce the corresponding dihydroxybenzene. In this process phenol, 2-tert-butylphenol and 2,6-di-tert-butylphenol were investigated as substrates. The results indicated that the yield towards the 1,4-dihdroxybenzenes increased as the degree of substitution on the ring increased.
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- Date Issued: 2002
The evaluation of waste minimization/waste treatment strategies for a commercial production process of 4-methyl-3-thiosemicarbazide
- Authors: Bennen, Wilroy
- Date: 2002
- Subjects: Hazardous wastes , Hazardous substances -- Analysis
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10951 , http://hdl.handle.net/10948/97 , Hazardous wastes , Hazardous substances -- Analysis
- Description: Chemical synthesis is closely related to waste minimization. There is no chemical process that does not produce waste. The methods used by industry to deal with this waste is a major environmental concern. This thesis describes the laboratory scale waste minimization and waste treatment strategies for the commercial production process of 4-methyl-3-thiosemicarbazide (MTSC). The production process of 4-methyl-3-thiosemicarbazide was investigated with the aim of increasing the isolated yield of MTSC and at the same time decrease the amount and toxicity of effluent obtained. During this study, parameters were investigated such as the use of excess DIPEA and the temperature of the reaction. Preliminary studies clearly showed that both factors have a significant influence on the final yield of the product. The next part of the investigation was to optimize the two parameters influencing the isolated yield of the MTSC. For this investigation, a multi factorial design was used to determine the optimum conditions in the MTSC yield response. From the results obtained, it was clear that the excess of DIPEA and the temperature of the reaction both need to be high to obtain high yields. These theoretical results were confirmed by results obtained practically, where yields of up 82 % were obtained, but it became clear that even higher yields could be obtained since chromatographic results showed yeilds as high as 90 %. The mass balance of the MTSC synthesis showed a loss of approximately 30 grams per reaction. This loss may have an influence on the final yield. The effluent obtained during the synthesis of MTSC was investigated and a waste treatment protocol was established to reduce the high COD value of the MTSC effluent. The protocol consists of two steps used for the clean up of the effluent. The first being a cooling step; the effluent was cooled at 0oC to induce precipitation of a solid, consisting mostly of MTSC. The second step is a high pressure wet oxidation of the effluent with oxygen in a high pressure reactor. The remaining compounds in the effluent were oxidized, resulting in another precipitate, consisting mostly of sulphur. After the oxidation the COD value of the effluent was decreased by 98 % to a value of 0.4 %. The MTSC present in the precipitate obtained after cooling could be isolated and purified, to add to the yield of the synthesis. The sulphur obtained during the oxidation could also be isolated and reused, or sold to prevent it from contaminating the environment.
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- Date Issued: 2002
The investigation of the hydrolysis reaction of m-phenylene to resorcinol
- Authors: Khaile, Thebeeapelo John
- Date: 2002
- Subjects: Resorcinol
- Language: English
- Type: Thesis , Masters , MTech (Chemistry)
- Identifier: vital:10961 , http://hdl.handle.net/10948/107 , Resorcinol
- Description: The purpose of this study was to characterise the resinous material formed during the acid catalysed hydrolysis of m-phenylenediamine (MPDA) to resorcinol, and to establish a reaction mechanism that could explain the formation of both resorcinol and the resinous materials in the reaction. A further objective was to determine reaction conditions that would lead to reduced formation of the resins during the hydrolysis reaction. The number of compounds present in a sample taken during the course of the hydrolysis reaction was determined by HPLC fitted with a photodiode array detector. Five main components were detected. These compounds were identified as mphenylenediamine, m-aminophenol, resorcinol, 3,3’-aminodiphenylamine and 3- amino-3’-hydroxydiphenylamine by means of GC-MS. 3,3’-Diaminodiphenylamine is formed by self-condensation of MPDA and 3-amino-3’-hydroxydiphenylamine is formed by the reaction of MPDA with resorcinol. The degree of formation of these intermediates is substantially reduced when MPDA is added dose-wise to phosphoric acid at a reaction temperature of 220oC. The reaction mechanism of the hydrolysis reaction was investigated by isotopic lable incorporation, followed by analysis by NMR. This was done by hydrolysing MPDA using deuterated phosphoric acid (D3PO4). D3PO4 was generated by dissolving phosphorus pentoxide in deuterium oxide. The resorcinol product obtained from the hydrolysis using D3PO4 was found to be deuterated on the C-2, C-4 and C-6 positions. This suggests that the hydrolysis reaction involves protonation of MPDA on these positions, and this results in the formation of an iminium ion as one of the resonance forms. Hydrolysis then occurs on the positively charged carbon of this species. Fertiliser grade phosphoric acid can be used in the hydrolysis reaction, provided sulphates are removed before the acid is used in the hydrolysis reaction. The hydrolysis reaction using either ammonium sulphate or sulphuric acid produces resorcinol in yields lower than 60% if the hydrolysis is conducted in one pass. If these catalysts are used in the hydrolysis reaction, the reaction mixture needs to be re-heated after removal of resorcinol in order to increase the yield. Hydrolysis of MPDA using zirconium phosphate as catalyst gave resorcinol yields in excess of 90% over 24 hours. These reactions were carried out at very low concentrations of MPDA (0.3%). The mode of catalysis in these reactions is unclear. There is a possibility that the reaction is catalysed by phosphoric acid that leaches out of zirconium phosphate at high temperatures. The reaction might therefore not differ mechanistically from the phosphoric acid catalysed reaction. Further studies are required to clarify this point.
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- Date Issued: 2002