Catalytic reactors, descriptions

Process Description. Reactors used in the vapor-phase synthesis of thiophene and aLkylthiophenes are all multitubular, fixed-bed catalytic reactors operating at atmospheric pressure, or up to 10 kPa and with hot-air circulation on the shell, or salt bath heating, maintaining reaction temperatures in the range of 400—500°C. The feedstocks, in the appropriate molar ratio, are vaporized and passed through the catalyst bed. Condensation gives the cmde product mixture noncondensable vapors are vented to the incinerator. 

Scanning electron microscopy and other experimental methods indicate that the void spaces in a typical catalyst particle are not uniform in size, shape, or length. Moreover, they are often highly interconnected. Because of the complexities of most common pore structures, detailed mathematical descriptions of the void structure are not available. Moreover, because of other uncertainties involved in the design of catalytic reactors, the use of elaborate quantitative models of catalyst pore structures is not warranted. What is required, however, is a model that allows one to take into account the rates of diffusion of reactant and product species through the void spaces. Many of the models in common use simulate the void regions as cylindrical pores for such models a knowledge of the distribution of pore radii and the volumes associated therewith is required. 

Another reason for describing surface reaction kinetics in more detail is that we need to examine the processes on a microscopic scale. While we are interested primarily in the macroscopic description of catalytic reactor behavior, we cannot do this intelligently until we understand these processes at a molecular level. 

The link between the microscopic description of the reaction dynamics and the macroscopic kinetics that can be measured in a catalytic reactor is a micro-kinetic model. Such a model will start from binding energies and reaction rate constants deduced from surface science experiments on well defined single crystal surfaces and relate this to the macroscopic kinetics of the reaction. 

B. Flow Reactors. Laboratory-scale catalytic reactors and reactors for the reaction of solids with gases arc often constructed from metal. One of the principal objectives in the use of laboratory-scale catalytic reactors is the determination of rate data which can be associated with specific physical and chemical processes in a catalytic reaction. Descriptions are available for these kinetic analyses as they relate to reactor designs and reaction conditions. ... 

Description Three RAM processes are available to remove arsenic (RAM I) arsenic, mercury and lead (RAM II) and arsenic, mercury and sulfur from liquid hydrocarbons (RAM III). Described above is the RAM II process. Feed is heated by exchange with reactor effluent and steam (1). It is then hydrolyzed in the first catalytic reactor (2) in which organometallic mercury compounds are converted to elemental mercury, and organic arsenic compounds are converted to arsenic-metal complexes and trapped in the bed. Lead, if any, is also trapped on the bed. The second reactor (3) contains a specific mercury-trapping mass. There is no release of the contaminants to the environment, and spent catalyst and trapping material can be disposed of in an environmentally acceptable manner. 

Description N-butane and air are fed to a fluid-bed catalytic reactor (1) to produce maleic anhydride. The fluid-bed reactor eliminates hot spots and permits operation at close to the stoichiometric reaction mixture. This results in a greatly reduced air rate relative to fixed-bed processes and translates into savings in investment and compressor power, and large increases in steam generation. The fluid-bed system permits online catalyst addition/removal to adjust catalyst activity and reduces downtime for catalyst change out. 

Description The flowsheet for an oxygen-based unit is one of several possible process schemes. Compressed oxygen, ethylene and recycle gas are mixed and fed to a multitubular catalytic reactor (1). The temperature of oxidation is controlled by boiling water in the shell side of the reactor. 

Description In the direct oxidation process, ethylene and oxygen are mixed with recycle gas and passed through a multi-tubular catalytic reactor (1) to selectively produce EO. A special silver catalyst (high-selectivity catalyst) is used it has been improved significantly over the years. Methane is used as ballast gas. Heat generated by... 

Description N-butane and air are fed to a fluid-bed catalytic reactor (1) to produce maleic anhydride. The fluid-bed reactor elimi-... 

Description The SUPERFLEX process is a proprietary technology patented by ARCO Chemical Tech nology, Inc. (now Lyondell Chemical Co.), and is exclusively offered for license by Kellogg Brown Root. It uses a fluidized catalytic reactor system with a proprietary catalyst to convert low-value feedstocks to desirable propylene and ethylene products. The catalyst is very robust thus, no feed pretreatment is required for typical contaminants such as sulfur, water, oxygenates or nitrogen. Attractive feedstocks include C4 and CB... 

A quantitative description of heterogeneous catalytic reactors for design, scaling-up, control or optimization purposes requires several parameters. Some of them, including the effective diffusivity and some parameters for the transport models and also the intrinsic chemical rate, should be determined in special experiments. 

It is the purpose of this chapter to discuss presently known methods for predicting the performance of nonisothermal continuous catalytic reactors, and to point out some of the problems that remain to be solved before a complete description of such reactors can be worked out. Most attention will be given to packed catalytic reactors of the heat-exchanger type, in which a major requirement is that enough heat be transferred to control the temperature within permissible limits. This choice is justified by the observation that adiabatic catalytic reactors can be treated almost as special cases of packed tubular reactors. There will be no discussion of reactors in which velocities are high enough to make kinetic energy important, or in which the flow pattern is determined critically by acceleration effects. 

With this methodology, the problem is analyzed from the smallest to the largest scales, as appearing in the process description. As an example, in the case of a catalytic reactor, we consider the process on the following scales ... 

Modeling of catalytic combustors has been the subject of a number of studies. The models used varied in degree of complexity and could therefore answer various types of questions. General issues of modeling monolith catalytic reactors are discussed in Chapter 8 of this book and in the reviews of Irandoust and Andersson [57] and Cybulski and Moulijn [58]. Hence, only topics that are specific to the modeling of catalytic combustion in monolith catalysts are considered here. A description of some important aspects of different types of models are as follows. 

An example of a mapping from the equipment representation to the thermodynamic state representation is shown in Fig. 5. It represents an isothermal vertical packed-bed catalytic reactor equipped with temperature and pressure sensors, an explosion vent, and a distributor plate. Notice that the equipment and sensors are not associated with the state representation. They are contained in the base representation and reside in the process description at the equipment level. As discussed earlier, flow, work, heat, and mass interactions are all modeled independently. This allows us to evaluate independently the effect of these processes. Independent evaluation assists in the identification, evaluation, and assessment of event pathways leading to hazardous states. 

The H-Oil reactor (Fig. 21) is rather unique and is called an ebullated bed catalytic reactor. A recycle pump, located either internally or externally, circulates the reactor fluids down through a central downcomer and then upward through a distributor plate and into the ebullated catalyst bed. The reactor is usually well insulated and operated adiabatically. Frequently, the reactor-mixing pattern is defined as backmixed, but this is not strictly true. A better description of the flow pattern is dispersed plug flow with recycle. Thus, the reactor equations for the axial dispersion model are modified appropriately to account for recycle conditions. 

In such systems modeling meets very serious difficulties, since the problem of formulation of a kinetic description of reactions in the adsorbed layer on the active metal is added and interferes with other problems stated above. One of the first attempts to suggest such a description was done by Hickman and Schmidt (1992, 1993). Analyzing a nearly 10-year period of development in the area, Schmidt (2001) concluded that ... these apparently simple processes are in fact far more complicated than the usual packed bed catalytic reactor assumptions used for typical modeling. First, the temperatures are sufficiently high that some homogeneous reaction may be expected to occur, even at very... 

Description The INEOS acrylonitrile technology uses its proven fluidized-bed reactor system. The feeds containing propylene, ammonia and air are introduced into the fluid-bed catalytic reactor, which operates at 5 psig-30 psig with a temperature range of 750°F-950°F (400°C-510°C). This exothermic reaction yields acrylonitrile, byproducts and valuable steam. 

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