Industrial synthesis of terephthalic acid in near/supercritical water

Housley, Samuel Duncan (2018) Industrial synthesis of terephthalic acid in near/supercritical water. PhD thesis, University of Nottingham.

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Terephthalic acid is an industrial commodity chemical produced as an intermediate in the manufacture of polyethylene terephthalate polymer used in the manufacture of polyester textiles, plastic bottles and packaging material. This thesis undertakes an industrial assessment of an alternative process technology for the manufacture of this intermediate. It is divided into nine Chapters which can be summarised as follows.

Chapter 1 discusses current industrial TA processes. TA is produced by the autoxidation of p-xylene (pX) with air oxidant in acetic acid solvent, using a homogeneous catalyst at a temperature of around 160 - 220 ˚C. The TA precipitates as a crude material containing contaminants and must be further purified to a purer grade material suitable for making polymer. The chapter considers the problems with the current processes and introduces pX oxidation in near critical or supercritical water (N/SCWO). It considers the difficulties in introducing new processes and displacing established, incumbent technology. The chapter finally considers the aims of this PhD project.

Chapter 2 reviews the state of the art of N/SCWO at the commencement of this project. This includes total oxidation and synthetic oxidations including pX N/SCWO.

Chapter 3 describes the experimental rigs, methods and analysis used in this project. It covers assessments, validations and improvements that were made.

Chapter 4 describes mixing aspects. The reaction requires the mixing of SCW and NCW streams and this was challenging due to their significant density difference. It was best achieved by the downward injection of the SCW stream into a rising NCW stream, an arrangement that exploits the buoyancy of the less dense stream to aid efficient mixing. A methodology was developed to assess the likely performance of a mixer under actual process conditions based on the turbulence, density difference and momentum ratio of the streams to be mixed.

Chapter 5 describes the influence of residence time distribution on the reaction outcomes. The reaction was undertaken in tubular and continuous stirred tank reactors and the outcomes were compared and related to their industrial significance.

Chapter 6 considers chemistry aspects. TA is produced by pX N/SCWO at a temperature around 300 – 400 ˚C and a pressure around 200 bar. The TA remains in solution during the oxidation. The N/SCWO can be catalysed by several transition metal bromides and Cu/Co/Br with a Co/Cu ratio of 85/15 was found to be the most effective. The process produces similar by-products to the acetic acid based process of carbon oxides and other aromatics but in higher amounts. Optimum TA yields were obtained under turbulent plug flow conditions using low temperature (~300 ˚C), high catalyst concentration and minimising oxygen concentration.

Chapter 7 considers the evolution of aromatic mass accountability and TA yield during the project and relates this to key reactor and chemistry developments. It was important to weld reactor joints to avoid yield losses through leaking compression joints and to accurately pump the pX. Reactors were constructed of Hastelloy C-276 though this material corrodes slowly under N/SCWO conditions and would not be suitable for commercial deployment. A high aromatic mass balance of up to 100 % has been obtained with a maximum yield of ~90 % of TA.

Chapter 8 considers the economics and value of the pX N/SCWO process relative to the acetic based process. The N/SCWO eliminates losses of acetic acid and generates higher grade steam from the reaction exotherm at the higher temperature. However, these gains trade off against the lower yield of TA obtained. The high make of by-products prevents significant process simplification and capital reduction compared to the acetic acid based process.

Chapter 9 reviews the extent to which the aims of the project were achieved and draws conclusions. The yield and quality of the TA obtained in the N/SCWO process is currently short of that needed for commercial deployment of the technology. Any future work will need to address these limitations and to further address materials of construction issues.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Poliakoff, Martyn
Licence, Peter
Keywords: p-xylene paraxylene supercritical water near critical water oxidation
Subjects: Q Science > QD Chemistry > QD241 Organic chemistry
Faculties/Schools: UK Campuses > Faculty of Science > School of Chemistry
Item ID: 49040
Depositing User: Housley, Samuel
Date Deposited: 27 Sep 2021 13:53
Last Modified: 28 Sep 2021 04:30

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