Heat Transfer in Fixed-bed Reactors

The rate of reaction becomes zero at a conversion of about x = 0.69 and a temperature of 1445°R, as determined from Eqs. (B) and (D). From Fig. 13-6 it is found that a bed depth of 3.8 ft is required for a conversion of 45%. The production of styrene from each reactor tube would be 

Hence two 4-ft-diameter reactor tubes packed with catalyst to a depth of at least 3.8 ft would be required to produce 15 tons/day of crude styrene. 

Because radial temperature profiles have a large effect on reactor performance, a summary of the heat-transfer characteristics of fixed beds is given in this section. We shall return to the design problem in Sec. 13-6. 

Wall Heat-transfer Coefficients If 7], is the bulk mean temperature of the reaction fluid and 7 is the wall temperature, is defined by 

Leva and others have correlated fixed-bed transfer coefficients for air over a wide range of variables. These ban be used to predict the heat-transfer rate by Eq. (13-17) when the variation in temperature within the bed itself is neglected. Leva s expressions are 

---

Figure 17.18. Heat transfer in fixed-bed reactors (a) adequate preheat (b) internal heat exchanger (c) annular cooling spaces (d) packed tubes (e) packed shell (f) tube and thimble (g) external heat exchanger (h) multiple shell, with external heat transfer (Walas, 1959).

Figure 7.32 Some correlations for wall heat-transfer in fixed-bed reactors -f, Beck, cylinders A, Beek, spheres O, Yagi and Wakao. Np, = 0.7 in all cases.

The book is divided into three parts. Part I surveys concepts for heat-integrated chemical reactors, with special focus on coupling reactions and heat transfer in fixed beds and in fuel cells. Part II is dedicated to the conceptual design, control and analysis of chemical processes with integrated separation steps, whilst Part III focuses on how mechanical unit operations can be integrated into chemical reactors. 

FIG. 19-20 Thermal conductivity and wall heat transfer in fixed beds, (a) Effective thermal conductivity, (h) Nusselt number for wall heat transfer. (Figs. 11.7.1-2 and 11.7.1-3 in Froment and Bischoff. Chemical Reactor Analysis and Design, Wiley. 1990.)... 

This reaction is responsible for the deposition of carbon in the reactor tubes in fixed-bed reactors and reducing heat transfer efficiency. 

Experimental data on heat transfer in fixed and fluidized bed reactors are correlated in terms of a j factor for heat transfer. 

There are reactions where the heat of reaction can be employed to preheat the feed when an exothermic reaction is operated at a high temperature (e.g., ammonia N2 + 3H2 <-> 2NH3 or methanol CO + 2H2 CH3OH synthesis, water-gas shift reaction CO + H20 <-> H2 + C02). These processes may be performed in fixed-bed reactors with an external heat exchanger. The exchanger is primarily used to transfer the heat of reaction from the effluent to the feed stream. The combination of the heat transfer-reaction system is classified as autothermal. These reactors are self-sufficient in energy however, a high temperature is required for the reaction to proceed at a reasonable rate. 

The second of the above requirements presupposes an assortment of heat transfer media that covers the whole temperature range of interest for gas-phase reactions in fixed-bed reactors. It is convenient to distinguish between gaseous, liquid, and vaporizing heat transfer media. Gaseous heat transfer media in the form of hot flue gases arc used in the temperature range above 500 °C exclusively to supply heat for endothermic reactions. 

The linear velocity (LV) or superficial velocity is an important engineering term because it relates to pressure drop and turbulence. This parameter is often increased in fixed bed reactors to enhance bulk mass transfer and heat transfer. 

---