FLUIDIZED AND FIXED BED REACTORS

Catalytic or heterogeneous reactors are an alternative to homogenous reactors. If a solid catalyst is added to the reactor, the reaction is said to be heterogeneous. For simple reactions, the effect of the presence of a catalyst is to 

permit the reaction to occur at a more favorable pressure, 

The basic problem in the design of a heterogeneous reactor is to determine the quantity of catalyst and/or reactor size required for a given conversion and flow rate. In order to obtain this, information on the rate equaiion(s) and their parameter(s) must be made available. A rigorous approach to the evaluation of reaction velocity constants has yet to be accomplished for catalytic reactions at this time, industry still relies on the procedures set forth in the previous chapter. For example, in catalytic combustion leac-tioas, the rate equation is extremely complex and cannot be obtained either analytically or numerically. A number of equations may result and some simplification is often warranted. As mentioned earlier, in many cases it is safe to assume that the expression may be satisfactorily expressed by the rate equation of a single step. 

It is common (xactice to write the describing equations for mass and energy transf for homogeneous and heterogeneous flow reactors in the same way. However, the (units of the) rate of reaction may be expressed as either 

Chetmeal Jteaaor Analysis and Applications for the Practicing Engineer. By Louis Theodore 2012 John WUey Sons, Inc. Published 2012 by Wiley Sons, Inc. 

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Extensive research on hydrogen sulfide removal from natural gas was carried out by Tollefson and coworkers [65, 67—72]. The oxidation of hydrogen sulfide was studied in both fluidized and fixed bed reactors at a wide temperature range (398-473 K) and a pressure range between 230-320 kPa with H2S concentration between 0.(K)3 to 6 % and oxygen content from one to four times the stoichiometric ratio with respect to hydrogen sulfide concentration present in the mixture. The typical experimental set up for fixed bed reactor is presented in Fig.25. 

The flow pattern of solids in fluidized bed can, as a first approximation, be assumed to be perfectly mixed—Chapter 13 deals with this in detail. Weekman [17] used this concept together with a time variable rate constant to compute its mean value. This result was then used in a comparison of fluidized and fixed bed reactors for catalytic cracking. 

Can be used in fluidized and fixed-bed reactors and in stirred suspension... 

The hrst part serves as an introduction to the subject title and contains chapters dealing with history, process variables, basic operations, chemical kinetic principles, and stoichometry and conversion variables. The second part of the book addresses traditional reactor analysis chapter topics include batch, CSTRs, and tubular flow reactors, plus a comparison of these classes of reactors. Part IH keys on reactor applications that include thermal elfects, interpretation of kinetic data, non-ideal reactors, and reactor design. The book concludes with other reactor topics chapter titles include catalysis, catalytic reactions, fluidized and fixed bed reactors, biochemical reactors, open-ended questions, and ABET-related topics. An Appendix is also included. 

Ethyl Chloride. Hydrochlorination of ethylene with HC1 is carried out in either the vapor or the liquid phase, in the presence of a catalyst.182-184 Ethyl chloride or 1,2-dichloroethane containing less than 1% A1C13 is the reaction medium in the liquid-phase process operating under mild conditions (30-90°C, 3-5 atm). In new plants supported AlClj or ZnCl2 is used in the vapor phase. Equimolar amounts of the dry reagents are reacted in a fluidized- or fixed-bed reactor at elevated temperature and pressure (250-400°C, 5-15 atm). Both processes provide ethyl chloride with high (98-99%) selectivity. 

Until recently only a few papers were available on moving beds in cross flow [11-18]. This type of reactor is sometimes a favorable process solution for a selective catalytic process with a moderate catalyst rcsidence time and with a short gas residence time, especially when the process is accompanied by a continuous catalyst regeneration. The use of conventional short-contact-time reactors like fluidized-bed reactors, risers, and fixed-bed reactors does not always yield satisfactory results. This may be explained by problems connected with gas back-mixing, channeling of gas, low catalyst holdup, attrition of the solid catalyst, or difficulties in temperature control. 

Tubular fluidized and fixed bed fermenters are deviations from the simple bubble column fermenter. Often utilized in producing beer and ciders, these fermenters contain immobilized microorganisms or microbial films on support surfaces. Microbes lost with the product are continuously replenished by adding fresh microorganisms into the packed bed fermenters. In the fixed bed case, slow downward flow of the medium significantly reduces the shear removal (mobilization) of the microbes from the support materials and increases the residence time in the packed column. This is a typical characteristic of the trickle bed fermenter for continuous operation. Readers are referred to the packed bed reactor entry in this volume for a more... 

The types of reactors used for catalytic and noncatalytic gas-solid reactions are also often similar. Moving-bed reactors are used in blast furnaces and cement kilns. Fluidized-bed reactors are used for the roasting of sulflde ores and regeneration of catalytic cracking catalyst, and fixed-bed reactors are used to remove sulfur compounds from ammonia synthesis feed gas. When regeneration of the solid reactant is desired, two or more reactors operating in parallel are required if continuous, steady-state operation is to be achieved. 

Comparing the fluidized bed and fixed bed reactor investment costs, physical characteristics and operation performance the important advantages and disadvantages of fluidized beds relative to fixed beds can be summarized as follows. 

Brief discussion of a total of over 25 reactors of all categories, such as fixed-, fluidized- and moving-bed reactors, bubble columns, sectionalized bubble columns, loop reactors, stirred-tank reactors, film reactors, rotating disk reactors, jet reactors, plunging jet reactors, spray columns, surface aerators... 

Newton s law is applied in order to derive the linear momentum balance equation. Newton s law states that the sum of all forces equals the rate of change of linear momentum. The derivation/application of this law ultimately provides information critical to any meaningful analysis of most chemical reactors. Included in this information are such widely divergent topics as flow mechanisms, the Reynolds number, velocity profiles, two-phase flow, prime movers such as fans, pumps and compressors, pressure drop, flow measurement, valves and fittings, particle dynamics, flow through porous media and packed beds, fluidization— particularly as it applies to fluid-bed and fixed bed reactors, etc. Although much of this subject matter is beyond the scope of this text, all of these topics are treated in extensive detail by Abulencia and Theodore. ... 

In addition to this introduction, the chapter primarily addresses two types of catalytic reactors. Although there are numerous types of catalytic units, this chapter solely reviews fluidized bed reactors and fixed bed reactors. Details on other reactors are available in the literature The remaining sections of this chapter include ... 

The process of a species undergoing a chemical reaction in the presence of a solid or catalyst can occur in a variety of ways. For reasons that should now be obvious, this chapter addresses fluidized bed reactors and fixed bed reactors. The material presented in the three previous sections attempted to provide some simple design procedures. 

Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

Recent advances in Eischer-Tropsch technology at Sasol include the demonstration of the slurry-bed Eischer-Tropsch process and the new generation Sasol Advanced Synthol (SAS) Reactor, which is a classical fluidized-bed reactor design. The slurry-bed reactor is considered a superior alternative to the Arge tubular fixed-bed reactor. Commercial implementation of a slurry-bed design requires development of efficient catalyst separation techniques. Sasol has developed proprietary technology that provides satisfactory separation of wax and soHd catalyst, and a commercial-scale reactor is being commissioned in the first half of 1993. 

More recently, Sasol commercialized a new type of fluidized-bed reactor and was also operating a higher pressure commercial fixed-bed reactor (38). In 1989, a commercial scale fixed fluid-bed reactor was commissioned having a capacity similar to existing commercial reactors at Sasol One (39). This effort is aimed at expanded production of higher value chemicals, in particular waxes (qv) and linear olefins. 

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