THE GLOBAL RATE AND LABORATORY REACTORS

As intrinsic rate equations cannot yet be predicted, they must be evaluated from laboratory data. Such data are measurements of the global rate of reaction. The first part of the problem is to extract the equation for the intrinsic rate from the global rate data. Since laboratory reactors are small and relatively low in cost, there is great flexibility in designing them. In particular, construction and operating conditions can be chosen to reduce or eliminate the differences between the global and intrinsic rates, so that more accurate equations for the intrinsic rate can be extracted from the 

---

As mentioned earlier, in laboratory reactors the global rate is measured directly, and the question is whether these rates are influenced by extenlal physical processes. In this section we shall consider the method of solution for an isothermal case. Combined mass- and energy-transfer limitations are discussed in Sec. 10-4. 

In the laboratory either integral or differential (see Sec. 4-3) tubular units or stirred-tank reactors may be used. There are advantages in using stirred-tank reactors for kinetic studies. Steady-state operation with well-defined residence-time conditions and uniform concentrations in the fluid and on the solid catalyst are achieved. Isothermal behavior in the fluid phase is attainable. Stirred tanks have long been used for homogeneous liquid-phase reactors and slurry reactors, and recently reactors of this type have been developed for large catalyst pellets. Some of these are described in Sec. 12-3. When either a stirred-tank or a differential reactor is employed, the global rate is obtained directly, and the analysis procedure described above can be initiated immediately. 

In homogeneous reactors that are not isothermal, scale-up is dangerous because it is difficult to allow for the differences in heat-transfer conditions in laboratory and large-scale units. When precautions are taken to obtain the same global rate and the same heat-transfer conditions (for example,... 

Laboratory reactors and industrial-scale equipment are seldom operated under similar flow and heat transfer conditions. To obtain a global rate that is useful for design purposes, one must combine the intrinsic chemical reaction rate expression with expressions for heat and mass transfer rates corresponding to industrial operating conditions. As a general rule, the global rate reduces to the intrinsic... 

In this sequence, steps 3-5 are the chemical rate processes laboratory analysis of these steps in the absence of physical effects yields the intrinsic reaction rate. Steps 1 and 7 are external physical rate processes separated from and in series with the chemical rate processes, while steps 2 and 6 are internal physical rate processes occurring simultaneously with chemical rate processes. The external and internal physical transport effects existing in a particular system are superimposed on the intrinsic reaction rate to obtain the global reaction rate, which is used in the macroscopic mass and energy transport equations required for reactor design. 

Example 11-7 The rate of isomerization of o-butane with a silica-alumina catalyst is measured at 5 atm and 50°C in a laboratory reactor with high turbulence in the gas phase surrounding the catalyst pellets. Turbulence ensures that external-diffusion resistances are negligible, and so Q = Q. Kinetic studies indicate that the rate is first order and reversible. At 50°C the equilibrium conversion is 85%. The effective diffusivity is 0.08 cm /sec at reaction conditions, and the density of the catalyst pellets is 1.0 g/cm, regardless of size. The measured, global rates when pure n-butane surrounds the pellets are as follows ... 

The book has been written from the viewpoint that the design of a chemical reactor requires, first, a laboratory study to establish the intrinsic rate of reaction, and subsequently a combination of the rate expression with a model of the commercial-scale reactor to predict performance. In Chap. 12 types of laboratory reactors are analyzed, with special attention given to how data can be reduced so as to obtain global and intrinsic rate equations. Then the modeling problem is examined. Here it is assumed that a global rate equation is available, and the objective is to use it, and a model, to predict the performance of a large-scale unit. Several reactors are considered, but major attention is devoted to the fixed-bed type. Finally, in the... 

---