COPYRIGHT AND USE OF THIS THESIS This thesis must be used in accordance with the provisions of the Copyright Act 1968. Reproduction of material protected by copyright may be an infringement of copyright and copyright owners may be entitled to take legal action against persons who infringe their copyright. Section 51 (2) of the Copyright Act permits an authorized officer of a university library or archives to provide a copy (by communication or otherwise) of an unpublished thesis kept in the library or archives, to a person who satisfies the authorized officer that he or she requires the reproduction for the purposes of research or study. The Copyright Act grants the creator of a work a number of moral rights, specifically the right of attribution, the right against false attribution and the right of integrity. You may infringe the author’s moral rights if you: - fail to acknowledge the author of this thesis if you quote sections from the work - attribute this thesis to another author - subject this thesis to derogatory treatment which may prejudice the author’s reputation For further information contact the University’s Director of Copyright Services sydney.edu.au/copyright THE KINETICS OF INDUSTRIAL AMMONIA COMBUSTION A thesis submitted in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY By Maximilian Warner School of Chemical and Biomolecular Engineering The University of Sydney May 2013 Declaration I hereby declare that the work presented in this thesis is solely my own work. To the best of my knowledge, the work presented is original except where otherwise indicated by reference to other authors. No part of this work has been submitted for any other degree or diploma. __________________________ Maximilian Warner May, 2013 i Abstract This thesis describes experiments and modelling to determine the kinetics of industrial ammonia combustion over platinum gauzes. The study is motivated a by lack of understanding of the kinetics under industrial conditions. To investigate the industrial combustion, a burner was built whose design parameters were based on individual plant data found in the literature. The collection of this literature data is presented in chapter three of this thesis, and is the first time it has been collected together in this way. Two models of the system are presented, one for the kinetics, and another for the temperatures in the catalyst bed. The literature review gives a historical study of industrial ammonia combustion, without the limitation of examination in the context of nitric acid manufacture only. This broader scope has been crucial in trying to understand the field, and the rediscovery of much of the early work has been instrumental in not only directing the research carried out in this thesis, but challenging the way researchers currently look at the problem. For example, despite the process having been known to be mass-transfer-limited since the 1920’s, only four academic papers have specifically focused on the mass transfer kinetics1. In comparison there are hundreds that only look to the surface kinetics in order optimize combustion. In essence one must look at both the mass transfer and surface kinetics to gain any understanding of the combustion system, and this thesis is the first to do so. The lack of historical insight has also left many researchers to build a heuristic view of the field. In the literature review, seven “old saws” are presented, which are found in the all the highly cited reviews of ammonia combustion2. These “old saws” claim to optimize the combustion of ammonia. Alarmingly, many of the “old saws” continue to be peddled, despite published research showing them to be wrong, With an understanding of the importance of this mass transfer limit, and a gap in the literature linking the mass transfer and surface reactions, a simple model has been developed to look at how the overall kinetics respond to the interactions between mass transfer and surface reaction. The surface kinetics have been taken from a validated micro kinetic model, and mass transfer data from a review of literature of heat and mass transfer in gauzes. 1 The mass transfer limitation was discovered by Bodenstein. Only Appl’baum and Temkin, Oele, Roberts and Gillespie, and Andrews seem to realize its relevance in the study of NH3 combustion under industrial conditions. 2 There are more than 7, but these are the ones which one could consider to be core to the understanding of the combustion system, and optimized burner operation. ii The model resulted in two expressions for the product ratio of N2 to NO, and N2O to NO. At selectivities of N2 and N2O much lower than that of NO, these expressions approximate the N2 and 3 N2O selectivity . The expression for N2,at low N2 and N2O selectivity is: ( ) 1 1 1 + = ( ) = 2 2 , > 0 [ ] , T 2 푆푒푙 푁2 푘푁 � �퐾푂Θ� 푎 푂 푏 2 3 푓 퐾 Θ 푓 � � Θ 푁퐻3 ∞ 푘푔 푁퐻 푘푁푂 퐾푂Θ and an analogous expression for N2O: ( ) , 1 1 = ( ) = 2 , > 0 [ ] 2 푔, 푁푂 T 푁2푂 푁푂 푆푒푙 푁 푂 푘 푎 푏 푘 퐾 3 ∞ 푔 푁퐻3 푔 Θ 푔 � � 푁푂 Θ 푁퐻 푘 푘 �퐾푂 √Θ where is the oxygen excess above the surface, T is the gauze temperature, [ ] the bulk ammoniaΘ inlet concentration, , the mass transfer coefficient of species , the푁 퐻reaction3 ∞ rate constant for the formation of species푘푔 푖 on the surface, and and the equilibrium푖 푘푗 constants for the chemisorption of O2 and NO on the푗 surface respectively.퐾 푂 퐾푁푂 For both N2 and N2O, these expressions can be plotted either against at constant temperature, or as an Arrhenius plot at constant . Plotted in this way, the experimentalΘ data over wide range of conditions - from both this thesis Θand from other studies - collapse on to a single line. These results suggest that the model well describes the effects occurring in ammonia combustion to form N2 and N2O. The model and experiments elucidate four aspects of N2 formation which were hitherto not understood: 1. The N2 selectivity function is constant for varying > 0. 2. The variables that control the N2 selectivity overΘ a particular metal are limited to [ ] , , , and the gauze temperature. 푁퐻3 ∞ 3 3. 푘Increasing푔 푁퐻 the pressure does not increase the N2 selectivity. 4. At a constant temperature the selectivity to N2 is only increased by increasing the product , [ ] . The mass transfer coefficient can be increased by increasing the burner 푘loading,푔 푁퐻3 푁 or퐻 3alternatively∞ reducing the gauze wire diameter and/or fractional open area of the gauze. 3 In the case where the N2 and N2O selectivities are comparable to NO, a correction was applied to the expression. For industrial conditions the product ratio is within a few percent of the selectivity. iii Similarly, the model and experiments reveal three conditions that lead to an increased selectivity to N2O, which were also not previously known: 1. Decreasing 2. Increasing pressureΘ 3. Increased ammonia concentration Experiments have been carried out to examine the degree of decomposition of NO and N2O on the gauze. Neither increasing the layers of gauze, nor (in the case of N2O) doping of the feed led to any change in selectivity, suggesting that the rates of NO and N2O decomposition are negligibly small. This result, supported by the work of Apel’baum and Temkin (Apel'baum & Temkin, 1948) is contrary to the prevailing belief that NO is decomposed at appreciable rates on the gauze pack, despite no studies which show it to be the case. The significance of this conclusion is that the target of a catalyst “contact time” for burner design is not well founded. A model of the temperature in the catalyst gauze pack is presented. Previous models have assumed the Lewis number of ammonia in air ( ) to be 1, which would lead to the maintenance of a constant surface temperature throughout퐿푒푎푖푟 the gauze pack. In fact, the assumption of unity Lewis number for ammonia in air is erroneous: with a more accurate value of = 0.9, it is shown that the first gauze temperature is likely to be higher than the subsequent layers퐿푒푎푖푟, perhaps by as much as 50 °C. This result, which has been seen experimentally before (Apel'baum & Temkin, 1948) but not previously explained, has significant impacts on the performance of ammonia burners in practice: for example, the significant restructuring and loss of platinum from the leading gauze may just be the result of the higher temperatures experienced there. The models for the reaction kinetics showed the burner parameters which can be used to optimise NO selectivity and reduce N2O formation in industrial ammonia oxidation. Where previously this optimization has been discussed in terms of “rules of thumb”, the master plots presented in this thesis give the universal design data that can be used over a very wide range of conditions. Below is a proposed correction of the “old saws”, which also act to summarize the findings of this thesis: 1. The NO selectivity is a function of the mass transfer coefficient (which incorporates the burner loading and gauze type), the inlet ammonia and oxygen concentrations ([ ] and [ ] ), and the gauze temperature. 푁퐻3 푖푛 2. Pressure푂2 푖푛 does not affect the selectivity to N2, but does affect the N2O selectivity. 3. NO and N2O are not decomposed on the gauze at an appreciable rate. 4. The maximum NH3 mole fraction in the feed is set by the lower of either: iv a. The flammable limit b. The lowest value of that still gives good NO selectivity, as determined by the relative diffusivities ofΘ O2 and NH3 in the bulk gas environment 5. The gauze type can improve the selectivity to NO.