Design of Tubular Reactors

Tubular reactors are normally used in the chemical industry for extremely large-scale processes. When filled with solid catalyst particles, such reactors are referred to as fixed-bed or packed-bed reactors. In this section we treat general design relationships for tubular reactors in which isothermal homogeneous reactions take place. Nonisothermal tubular reactors are treated in Section 10.4 and packed-bed reactors in Section 12.7. 

1 The Plug Flow Reactor Model Basic Assumptions and Design Equations 

There will be velocity gradients in the radial direction, so all fluid elements will not have the same residence time in the reactor. Under turbulent flow conditions in reactors with large length/diameter ratios, any disparities between values observed and model predictions arising from this factor should be small. For short reactors and/or laminar flow conditions, the disparities can be appreciable. Some of the techniques used in the analysis of isothermal tubular reactors that deviate from plug flow are treated in Chapter 11. 

There may be radial temperature gradients in the reactor that arise from the interaction between the energy released by reaction, heat transfer through the walls of the tube, and convective transport of energy. This factor is the greatest potential source of disparities between the predictions of the model and the behavior observed for real systems. The deviations are most significant in nonisothermal packed-bed reactors. 

The tubular reactor is a convenient means of approaching the performance characteristics of a batch reactor on a continuous basis, since the distance-pressure-temperature history of the various plugs as they flow through the reactor corresponds to the time-pressure-temperature protocol 

A reactor design can be carried out if we know the following information  

External restrictions imposed by the reactor setup on fluid flow 

In reactors used for carrying out radical polymerization, there is a continual change in the physical properties such as heat capacity, density, and viscosity. The following examples examine these effects on reactor design. 

Example 6.1 Polymerization of styrene is carried out in an isothermal tubular reactor at 60°C up to 30% conversion. Assiune average rate constants at 60°C  

At this temperature, styrene has a density of 0.869 g/mL and that of polystyrene is 1.047 g/mL. Determine the residence time, 

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The SA concept will prove to be particularly useful in the design of tubular reactors for gas phase reactions. 

STEADY-STATE DESIGN OF TUBULAR REACTOR SYSTEMS... 

The discussion above points out one of the most important tradeoffs in chemical reactor design. The smaller the reactor size, the larger the recycle flowrate. This reactor/recycle tradeoff dominates the steady-state economics of the design of tubular reactor systems, as we will illustrate in several examples in this chapter. It also has a major impact on dynamic control, as we will see in the next chapter. 

The design of tubular reactor systems is dominated by the classical tradeoff between reactor size and recycle flowrate. Gas phase systems are particularly affected because of the high cost of compression. 

P. Iedema, Design of tubular reactors in recycle systems, Comput. Chem. Eng., 28, 63-72 (2004)... 

The main problem in the design of tubular reactors for quantitative studies of homogeneous reactions is to confine the reaction sharply to the reactor itself. This requires rapid mixing of the reactants at the entry, and equally excellent quenching of the exiting fluid. Both are easier to achieve for liquid-phase than for gas-phase reactions. Tubular reactors are not suited for gas-liquid reactions because gas sparging would disrupt the flow pattern. 

Bildea, C. S., S. Cruz, Dimian, A. C., ledema, P., 2002, Design of tubular reactors in recycle systems. Proceedings ESCAPE-12, Elsevier, 439-444 Dimian, A. C., A. J. Groenendijk, P. ledema, 2001, Recycle interaction effects on the ontrol of impurities in a complex plant, 2001, Ind. Eng. Chem. Res., 40,5784-5794 Downs, J., 1992, Distillation control in a plantwide control environment, in W. Luyben (ed), Practical Distillation Control, van Nostrand Reinhold, New York Fisher, W. R., M. F. Doherty, J. M. Douglas, 1988, The Interface between design and control, Ind. Eng. Chem. Res., 27, 597-611... 

Design of Tubular Reactors with Plug Flow... 

The necessity to implement new conditions for carrying out heat and mass exchange processes directly in the reaction zone of fast chemical reactions, resulted in unprecedented designs of tubular reactors with the hydrodynamic flow mode of a reaction mixture as a key parameter [33-35]. These devices are characterised by high specific productivity and make it possible to achieve a quasi-plug flow mode in a reaction zone, providing heat and mass exchange processes with duration comparable to that of a chemical reaction. 

For the design of tubular reactors an a priori estimation of the axial dispersion is indispensable. The dispersion in tubular reactors depends on the flow regime, characterized by the Reynolds number. Re, and the physical properties of the fluid, characterized by the Schmidt number. Sc. In addition, the presence of internal packings influences the flow behavior and, in consequence, the axial dispersion of the fluid. 

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