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Oxidative dehydrogenation of ethane in short contact time reactors

Oxidative dehydrogenation of ethane at short contact times in the last years has been proven to be effective in achieving high yields and selectivities to ethylene. Nevertheless, a number of issues remains unresolved that we discuss in Chapter 1.
In Chapter 2 we try to provide insights into the role of the catalyst. To do this, we propose a novel non-noble metal catalyst alternative to Pt, which until now is the only reliable catalyst for performance and lifetime. LaMnO3-based catalysts are cheaper than Pt and more thermally stable, and above all perform better than the noble metal. From the experimental results in short contact time reactors we have seen higher ethane conversion and ethylene selectivity on LaMnO3 under a wide range of conditions.
The role of the catalyst was further investigated by comparison with a blank reactor. Both experimental results and numerical simulations showed that the catalyst is important not only as ignitor to sustain gas phase reaction and to limit their extent to a well defined length, but also to produce the heat to drive the homogeneous dehydrogenation reactions by combustion to CO2 rather than to CO, thus sacrificing less ethane.
In Chapter 3 we addressed the investigation of different catalysts, alternative to noble metals. We studied mixed catalysts, where Pt is substituted into the perovskite structure, and Pt/CeO2. Experimental results showed good performance of substituted perovskite for the ODH process, as high as on Pt/Sn, but without the highly volatile Sn. Experimental results also suggested that a catalyst suitable for this process is required to oxidize C2H6 under fuel-rich conditions preferentially to CO2 and H2O rather than to syngas. Thus we interpreted the experimental results in terms of C and H oxidation and built a surface mechanism based on these considerations.
In Chapter 4 the catalyst supports were object of our investigation. We explained the experimentally observed differences among ceramic supports commonly used in short contact time reactors, with the estimation of gas dispersion and in terms of specific geometric surface and pore size. Also the effect of washcoat was critically evaluated.
In Chapter 5 we investigated the effect of the main operating parameters of the process, such as C2H6/O2, dilution, preheat temperature and flow rate. Particular attention was devoted to fuel addition. As alternative to H2, we proposed CO addition, which on a suitable catalyst active in CO oxidation, such as LaMnO3, yields to significantly improvements in ethane conversion (~10%) and ethylene selectivity (~10%). Also the transient behavior of the system was followed and coke formation investigated.
Finally, in Chapter 6 we used a mathematical model to provide deeper understanding of the process. With a purely homogeneous model we studied ethylene formation at short contact times, together with the formation of undesired PAH, and the effect of O2. Oxygen presence is important to boost the kinetics of ethylene formation and to provide the heat to drive an autothermal process.
With a hetero-homogeneous 2-D model, we tried to assess the concurrent phenomena of gas-phase reactions, mass transfer and surface reactions occurring in the system. We proposed a three-zone model, where the first zone is dominated by heterogeneous reactions producing heat to increase gas temperature, in the second zone, still rich in oxygen, hetero-homogeneous reactions occur, and most of ethylene is formed, and in the third one, when homogeneous reactions are faster than mass transfer rate because of temperature, purely homogeneous reactions occur. We also studied the hetero-homogeneous model parametrically with the surface reaction rate and the inlet temperature.
In chapter 7 we conclude with a summary of the results.

Mostra/Nascondi contenuto.
Chapter 1 Introduction 1 Chapter 1 Introduction 1.1 Ethylene production Ethylene (C 2 H 4 ) is an organic molecule where the 2 carbon atoms are connected by a double bond, which causes the molecule to be highly reactive towards the reactions of halogen addition, hydration and polymerization (Solomons, 1976). Ethylene is the largest-volume petrochemical produced worldwide, but has no direct end uses. Half of the ethylene produced is polymerized to polyethylene, through high or low pressure processes. The remaining half is transformed into chemicals, such as ethylene glycol, oxirane, styrene, ethanol, 1,2-dichloro ethane, acetaldehyde and vinyl acetate. The extremely high versatility of ethylene, due to the chemical properties determined by its double bond, puts such molecule as building block in a large range of products, above all in the field of the fine chemicals. Currently, ethylene is produced by steam cracking in furnace reactors, accordingly to a well established technology, which was constantly improved since the 1940s, when U.S. oil and chemical company began to produce it from ethane obtained from refinery by-product streams and from natural gas. Today, various feedstocks are employed for ethylene production, from ethane to naphta, to LPG, to fuel oil, according to the convenience of the locations and the cost of the raw materials. For instance, in the U.S. ethane is the main feedstock, since it is largely available, while in Europe and Japan, where natural gas is more expensive, the fuel feeds are mainly constituted by light naphtas (Kirk-Othmer, 1978). Ethylene is produced by steam cracking, as sketched in Fig. 1.1. As reported in the Kirk-Othmer Encyclopedia, a hydrocarbon stream is heated and mixed with steam to incipient cracking temperature (500-650°C) before entering a fired tubular reactor (radiant tube or coil), where under controlled residence time, temperature profile and partial pressure is heated to 750-875°C. Saturated hydrocarbons crack into smaller molecules, amongst which ethylene, other olefins and diolefins are the major products. The steam cracking of saturated hydrocarbons to olefins is highly endothermic and requires high energy input rates. On leaving the fired tubular reactor, the products are cooled immediately to 550-650°C to prevent degradation by secondary reactions. These gases are then separated into the desired products. Large ethylene yields require high temperature, short residence times and low hydrocarbon pressure in the reactor. Fast and efficient heat transfer is achieved in the furnace,

Tesi di Dottorato

Dipartimento: Ingegneria Chimica

Autore: Francesco Donsì Contatta »

Composta da 194 pagine.

 

Questa tesi ha raggiunto 1060 click dal 04/11/2004.

 

Consultata integralmente 5 volte.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.