Heat Transfer to and from Reactors

Heat transfer is usually effected by coils or jackets, but can also be achieved by the use of external loop heat exchangers and, in certain cases, by the vaporisation of volatile material from the reactor. The treatment, here mainly concerns Jackets and coils. Other instances of heat transfer are illustrated in the simulation examples of Chapter 5. 

The rate of heat transfer is most conveniently expressed in terms of an overall heat transfer coefficient, the effective area for heat transfer and an overall temperature difference, or driving force, where 

In simple cases the Jacket or cooling temperature, Tj, may be assumed to be constant. In more complex dynamic problems, however, it may be necessary to allow for the dynamics of the cooling jacket, in which case Tj becomes a system variable. The model representation of this is shown in Fig. 3.3. 

Under conditions, where the reactor and the jacket are well insulated and heat loss to the surroundings and mechanical work effects may be neglected 

Energy required to heat coolant from Tjin to T 

Assuming the liquid in the jacket is well-mixed, the heat balance equation for the jacket becomes 

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Heat transfer to and from moving streams in the reactor... 

Mass transfer of reactants and products to and from the catalytically active surface in a reactor equipped with an OCFS is a function of the geometry of the structure and of the flow regime within it alone. Heat transfer to and from the reactor, on the other hand, is additionally a function of the flow regime between the structure and the reactor wall. For this reason mass transfer and heat transfer are treated separately in the following. 

In Chapter 8 the important problem of heat transfer to and from chemical reactors is discussed briefly. It is shown that the rate of heat removal will often necessitate the choice of a certain reactor type, so that aspects of physical contacting of reactants have to be reconsidered. Important though heat transfer is, we examine it here in the second place, after studying the contacting of reactants, the interactions of reactions and transport phenomena, and the possible formation of another phase. The reason is, that for reactor development it is often necessary to select optimum reaction conditions first, and try to adjust reactor temperatures as required. It is true that this is not alway possible, so that an iterative procedure is then unavoidable. 

In any catalytic system, not only should chemical reactions be considered, but also mass and heat transfer effects. For example, mass and heat transfer effects are present inside the porous catalyst particles, as well as in the surrounding fluid films. In addition, heat transfer to and from the catalytic reactor gives an essential contribution to the energy balance. The core of modehng for a two-phase catalytic reactor is related to processes in the catalyst particle namely, simultaneous reaction and diffusion in the pores of the particle should be accounted for. These effects are completely analogous to reaction-diffusion effects in liquid films appearing in gas-hquid systems. Thus, the formulae presented in the next section are vahd for both catalytic reactions and gas-hquid processes. 

The thermal profile through the reactor will, in most circumstances, be carefully optimized to maximize selectivity, extend catalyst life, and so on. Because of this, direct heat integration with other process streams is almost never carried out. The heat transfer to or from the reactor is instead usually carried out by a heat transfer intermediate. For example, in exothermic reactions, cooling might occur by boiling water to generate steam, which, in turn, can be used to heat cold streams elsewhere in the process or across the site. 

For a continuous-flow reactor, such as a CSTR, the energy balance is an enthalpy (H) balance, if we neglect any differences in kinetic and potential energy of the flowing stream, and any shaft work between inlet and outlet. However, in comparison with a BR, the balance must include the input and output of H by the flowing stream, in addition to any heat transfer to or from the control volume, and generation or loss of enthalpy by reaction within the control volume. Then the energy (enthalpy) equation in words is... 

In the Inflow and Outflow terms (1) and (2), the heat flow may be of two kinds the first is transfer of sensible heat or enthalpy by the fluid entering and leaving the element and the second is heat transferred to or from the fluid across heat transfer surfaces, such as cooling coils situated in the reactor. The Heat absorbed in the chemical reaction, term (3), depends on the rate of reaction, which in turn depends on the concentration levels in the reactor as determined by the general material balance equation. Since the rate of reaction depends also on the temperature levels... 

Moving from batch experiments to continuous reactors, one major problem is to ensure a good flow of the material inside the reactor (mechanical aspect). The efficiency of the global process depends not only on the material transport in the reactor, but also on the heat transfer to and inside the material. The feed preparation is then essential as well as the characterization of the eventual side material (water, metals, minerals, pollutants, etc.). 

Although the assumption of perfect mixing in the CSTR implies that the reactor contents will be at uniform temperature (and thus the exit stream will be at this temperature), the reactor inlet may not be at the same temperature as the reactor. If this is the case and/or it is necessary to determine the heat transferred to or from the reactor, then an energy balance is required. 

It should generate an inlet-outlet enthalpy table based on elemental species at 25 C as references and then calculate the required heat transfer to or from the reactor, 2(kJ). The spreadsheet should be tested using the species and reactions of Problem 9.21 and should appear as shown below. (Some of the input data and calculated results are shown.)... 

For noncatalytic homogeneous reactions, a tubular reactor is widely used because it cai handle liquid or vapor feeds, with or without phase change in the reactor. The PFR model i usually adequate for the tubular reactor if the flow is turbulent and if it can be assumed tha when a phase change occurs in the reactor, the reaction takes place predominantly in one o the two phases. The simplest thermal modes are isothermal and adiabatic. The nonadiabatic nonisothermal mode is generally handled by a specified temperature profile or by heat transfer to or from some specified heat source or sink and a corresponding heat-transfer area and overall heat transfer coefficient. Either a fractional conversion of a limiting reactant or a reactoi volume is specified. The calculations require the solution of ordinary differential equations. 

For fixed-bed catalytic reactors, a PFR model with a pseudo-homogeneous kinetic equation is usually adequate and is referred to as a 1-D (one-dimensional) model. However, if the reactor is nonadiabatic with heat transfer to or from the wall, the PFR model is not usually adequate and a 2-D model, involving the solution of partial differential equations for variations in temperature and composition in both the axial and radial directions, is necessary. Simulators do not include 2-D models, but they can be generated by the user and inserted into the simulator. 

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