Design methods fixed beds

Reactor design is a key element in each process listed in Table XX. The method of feed introduction, the arrangement of the catalyst bed, and the mode of operation have an impact on the ability to process residua. For this reason, classification by reactor type provides a convenient and appropriate distinction for discussing hydroprocessing technology. The most common reactor designs include fixed beds, ebullated or expanded beds and slurry beds, and moving-bed reactors. These classifications are discussed in more detail next. 

T. Baron, Generalized Graphical Method for the Design of Fixed Bed... 

T. Baron, Generalized Graphic Method for the Design of Fixed Bed Catalytic Reactors, Chem. Eng. Prog., (48) 118-124,1952. 

In the analysis and design of fixed-bed catalytic reactors, catalyst effectiveness factors must be evaluated several times in a single calculation, and it is therefore necessary that the numerical method for solution of the mathematical model is reliable and fast. 

The slow water removal is obvious within the synthesis of, for example, myristyl myristate determining the total reaction time. In a stirred-tank reactor it takes 24 h to reach a conversion of 99.6% and in a fixed-bed reactor 14 h. Therefore, a new synthesis platform (Figure 4.11) which also enables conversion of highly viscous polyols and fatty acids from renewable resources to ester-based surfactants was designed. It is used by Evonik on a pilot scale, outperforming conventional methods, such as stirred-tank or fixed-bed reactors. In contrast to the setups introduced before, conversion of >99.6% is already obtained after 5.5 h in the bubble column reactor [44-47]. 

This chapter discusses the steps involved in the development and design of a new S02 oxidation catalyst VK69, which was introduced to the market in 1996 by Haldor Topsoe. The strategy and many of the methods are generally applicable to heterogeneous fixed bed catalysts, partly to fluid and slurry bed catalysts, and less relevant for homogeneous catalysts as found in organic synthesis and enzymatic reactions. 

The conditions in countercurrent fixed beds have been investigated for many years in order to improve the understanding of two-phase flow and to develop reliable design methods. However the proposed correlations available for fluid dynamics and mass transfer are practically all based on experimental data obtained at atmospheric pressure. Extrapolation of the results to high pressure is questionable and not recommended. Moreover the results of systematic investigations in the high-pressure range are scarce in the open literature. 

Further advancements in the theory of fixed bed reactor design have been made(56,57) but it is unusual for experimental data to be of sufficient precision and extent to justify the application of sophisticated methods of calculation. Uncertainties in the knowledge of effective thermal conductivities and heat transfer between gas and solid make the calculation of temperature distribution in the bed susceptible to inaccuracies, particularly in view of the pronounced effect of temperature on the reaction rate. 

Then, a survey of micro reactors for heterogeneous catalyst screening introduces the technological methods used for screening. The description of microstructured reactors will be supplemented by other, conventional small-scale equipment such as mini-batch and fixed-bed reactors and small monoliths. For each of these reactors, exemplary applications will be given in order to demonstrate the properties of small-scale operation. Among a number of examples, methane oxidation as a sample reaction will be considered in detail. In a detailed case study, some intrinsic theoretical aspects of micro devices are discussed with respect to reactor design and experimental evaluation under the transient mode of reactor operation. It will be shown that, as soon as fluid dynamic information is added to the pure experimental data, more complex aspects of catalysis are derivable from overall conversion data, such as the intrinsic reaction kinetics. 

With regard to application and construction, it is convenient to differentiate between fixed-bed reactors for adiabatic operation and those for nonadiabatic operation. Since temperature control is one of the most important methods to influence a chemical reaction, adiabatic reactors are used only where the heat of reaction is small, or where there is only one major reaction pathway in these cases no adverse effects on selectivity or yield due to the adiabatic temperature development are expected. The characteristic feature of an adiabatic reactor is that the catalyst is present in the form of a uniform fixed bed that is surrounded by an outer insulating jacket (Fig. 1A). Adiabatic reactor designs are discussed in Section 10.1.3.1. 

In the inertial microbalance, the mass located at the tip of an oscillating tapered quartz element is detected as a change in its vibrational frequency. The design of this equipment provides a packed bed of catalyst through which all the gas is forced to flow, and the classical methods of testing for differential operation in an ideal plug-flow fixed-bed reactor can therefore be applied. 

The use of two-phase homogeneous continuum models in packed bed modelling has often been avoided due to the computational difficulties. Recently, Paspek and Varma (15) have found a two-phase model to be necessary to describe an adiabatic fixed-bed reactor, while Dixon and Cresswell (16) have shown that the effective parameters of the one-phase model may be interpreted in terms of the more fundamental parameters of a two-phase model, thus demonstrating more clearly their qualitative dependencies on the operating and design characteristics of the bed. When two phases and several species are involved, the computational advantages of the cubic Hermite method may be anticipated to be high. 

An analysis of radial flow, fixed bed reactor (RFBR) is carried out to determine the effects of radial flow maldistribution and flow direction. Analytical criteria for optimum operation is established via a singular perturbation approach. It is shown that at high conversion an ideal flow profile always results in a higher yield irrespective of the reaction mechanism while dependence of conversion on flow direction is second order. The analysis then concentrates on the improvement of radial profile. Asymptotic solutions are obtained for the flow equations. They offer an optimum design method well suited for industrial application. Finally, all asymptotic results are verified by a numerical experience in a more sophisticated heterogeneous, two-dimensional cell model. 

A new light naphtha aromatization process has been developed using a conventional fixed bed reactor. Fundamental study revealed the importance of preparation method, morphology, and acid property to increase the catalyst stability. Based on fundamental and scale-up studies, a demonstration plant was designed and operated. This operation confirmed the good stability of the catalyst. 

One of the most common catalytic reactors is the fixed-bed type, in which the reaction mixture flows continuously through a tube filled with a stationary bed of catalyst pellets. Because of its importance, and because considerable information is available on its performance, most attention will be given to this reactor type. Fluidized-bed and slurry reactors are also considered later in the chapter. Some of the design methods given are applicable also to fluid-solid noncatalytic reactions. The global rate and integrated conversion-time relationships for noncatalytic gas-solid reactions will be considered in Chap. 14. 

Quantitative design methods of increasing complexity are considered in Secs. 13-3,13-4,13-6, and 13-7. First, however, let us summarize construction and operating characteristics of fixed-bed reactors. 

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