Gas-phase catalytic reactions

Catalytic gas-phase reactions play an important role in many bulk chemical processes, such as in the production of methanol, ammonia, sulfuric acid, and nitric acid. In most processes, the effective area of the catalyst is critically important. Since these reactions take place at surfaces through processes of adsorption and desorption, any alteration of surface area naturally causes a change in the rate of reaction. Industrial catalysts are usually supported on porous materials, since this results in a much larger active area per unit of reactor volume. 

The rate of an electrochemical reaction depends, not only on given system parameters (composition of the catalyst and electrolyte, temperature, state of the catalytic electrode surface) but also on electrode potential. The latter parameter has no analog in heterogeneous catalytic gas-phase reactions. Thus, in a given system, the potential can be varied by a few tenths of a volt, while as a result, the reaction rate will change by several orders of magnitude. 

Heterogeneous catalytic gas-phase reactions are most important in industrial processes, especially in petrochemistry and related fields, in which most petrochemical and chemical products are manufactured by this method. These reactions are currently being studied in many laboratories, and the results of this research can be also used for synthetic purposes. The reactions are usually performed [61] in a continuous system on a fixed catalyst bed (exceptionally a fluidized bed). 

The most studied catalytic gas-phase reaction has been the transformation of methane to the higher hydrocarbons or oxygenated products. This reflects the large effort being made by catalytic chemists to find a simple process by which world s large resources of natural gas can be utilized. 

Microwave-induced, catalytic gas-phase reactions have primary been pursued by Wan [63, 64], Wan et al. [65] have used pulsed-microwave radiation (millisecond high-energy pulses) to study the reaction of methane in the absence of oxygen. The reaction was performed by use of a series of nickel catalysts. The structure of the products seemed to be function of both the catalyst and the power and frequency of microwave pulses. A Ni/Si02 catalyst has been reported to produce 93% ethyne, whereas under the same irradiation conditions a Ni powder catalyst produced 83% ethene and 8.5 % ethane, but no ethyne. 

An example of a catalytic gas-phase reaction is the decomposition of diethyl ether catalyzed by iodine (I,) ... 

Sloot, H. 1991. A non-permselective membrane reactor for catalytic gas phase reactions. Thesis, University of Twcntc, Enschede. 

Sample integrations similar to pharmaceutical approaches were already examined in 1997 [39]. Here, a chip-like microsystem was integrated into a laboratory automaton that was equipped with a miniaturized micro-titer plate. Microstructures were introduced later [40] for catalytic gas-phase reactions. The authors also demonstrated [41] the rapid screening of reaction conditions on a chip-like reactor for two immiscible liquids on a silicon wafer (Fig. 4.8). Process conditions, like residence time and temperature profile, were adjustable. A third reactant could be added to enable a two-step reaction as well as a heat transfer fluid which was used as a mean to quench the products. 

Here, a chip-like microsystem was integrated into a laboratory robot which was equipped with a miniaturized micro titer-plate. Micro structures were introduced later [52] for catalytic gas-phase reactions. The authors also demonstrated... 

Veser G, Friedrich G, Freygang M, Zengerle R, A Simple and flexible microreactor for investigation of heterogeneous catalytic gas phase reactions, in Reaction kinetics and development of catalytic processes, Froment GF, Waugh KC, Eds., Studies in Surface Science and Catalysis, 122 237-246, Elsevier, Amsterdam, 1999. 

The attrition of solid particles is an unavoidable consequence of the intensive solids motion resulting from the presence of bubbles in the fluidized bed. The attrition problem is especially critical in processes where the bed material needs to remain unaltered for the longest possible time, as in fluidized-bed reactors for heterogeneous catalytic gas-phase reactions. Catalyst attrition is important in the economics of such processes and may even become the critical factor. 

In this section the industrial uses of fluidized-bed reactors for heterogeneous catalytic gas-phase reactions and the polymerization of alkenes are presented. The most important applications are listed and a few typical examples are analyzed in more detail. Complete descriptions of industrial uses of the fluidized-bed reactor can also be found in Refs 2, 6, 12 and 13. 

The fluidized-bed reactor offers the following principal advantages over the fixed-bed reactor for heterogeneous catalytic gas-phase reactions ... 

Zwahlen A. G., Agnew J. Modification of an Internal Recycle Reactor of the Berty Type for Low-Pressure High Temperature Catalytic Gas-Phase Reaction CHEMECA 1987, I, 50.1-50.7, Melbourne, Australia. 

As with every catalytic gas-phase reaction, the course of ammonia synthesis by the Haber-Bosch process can be divided into the following steps ... 

Slool, H J., 1991, A Nonpcrmsclcciivc Membrane Reactor for Catalytic Gas-Phase Reactions, PhD. dissertation, Twenic Univ. of Technol., Netherlands. 

Schlunder, On the Mechanism of the Constant Drying Rate Period and Its Relevance to Diffusion Controlled Catalytic Gas Phase Reactions, Chem. Eng. Sci. 43 2685-2688 (1988). 

Many of the modern combustion processes can be characterized by relatively low reaction rates compared to the modern catalytic processes operated in chemical reactors [67]. Therefore, these combustion processes do require lower gas velocities and higher solids circulation rates. On the other hand, many catalytic gas-phase reactions, including FCC, Fischer-Tropsch synthesis and oxidation of butane, utilize a relatively high gas velocity in the riser to promote plug flow operating conditions and short contact times between the gas and solids. 

The production of high hydrogen selectivities requires further comment. Hydrogen has been observed previously in the non-catalytic gas phase reaction... 

A honeycomb shape has been considered as the most desirable structure for the combustion catalyst due to a small pressure drop across the channel and a large surface-to-volume ratio. Stable combustion can be attained with laminar flow of gas mixture along the channel of the honeycomb, whereas turbulent flow and back-mixing are operative for the conventional flame combustion. The temperature at the honeycomb wall rises rapidly with fuel contact. The rate of homogeneous reaction depends on the fuel concentration and temperature therefore, the non-catalytic gas-phase reaction initiates from this hot wall where the temperature is raised by catalytic combustion. Once this catalytically initiated gas-phase reaction started, the reaction propagates rapidly toward the center of the channel. Then the high combustion efficiency can be attained. ... 

It is necessary here to draw attention on the way in wliich tliese experiments were conducted in what concerns to the oxygen feed. As already stated in the Experimental section, oxygen was incorporated to the gas phase very close to the catalyst bed. Experiences perfonned mixing oxygen at the reactor inlet, showed lower nitric oxide conversions. Tliis is due to the fact tliat methanol oxidation takes place in the reactor void voliune before reacliing tlie catalyst bed, tlius obtaining NO conversions similar to those of tlie non-catalytic gas phase reaction. 

Kolaczkowski, S.T., Chao, R., Awdry, S., and Smith, A. (2007) Application of a CFD code (FLUENT) to formulate models of catalytic gas phase reactions in porous catalyst pellets. Chem. Eng. Res. Des., 85, 1539-1552. 

In order to enhance space-time yields of catalytic gas-phase reactions, two strategies are in principle possible the improvement of catalyst activity or the implementation of more intense process conditions. This usually leads to an increase of heat production that can only be partially released, if at all, using conventional reactor technology. 

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