Reactors for Measuring Reaction Rates

We also discussed how to model differential reactors, which are convenient reactors for measuring reaction rates as a function of species concentrations. 

The main aim of the methods described in this chapter is to obtain data for the design of chemical reactors, for the simulation of their operation behaviour, and, last but not least, to evaluate the influence of temperature and pressure on reaction rate. For this purpose, the techniques for measuring reaction rates at high pressures are presented. The details of the apparatus are mentioned in Chapter 4.3.4. 

The goal is to determine a functional form for (a, b,. .., T) that can be used to design reactors. The simplest case is to suppose that the reaction rate has been measured at various values a,b,..., T. A CSTR can be used for these measurements as discussed in Section 7.1.2. Suppose J data points have been measured. The jXh point in the data is denoted as S/t-data aj,bj,..., Tj) where Uj, bj,..., 7 are experimentally observed values. Corresponding to this measured reaction rate will be a predicted rate, modeii p bj,7 ). The predicted rate depends on the parameters of the model e.g., on k,m,n,r,s,... in Equation (7.4) and these parameters are chosen to obtain the best fit of the experimental... 

Figure 1 shows two reactor configurations we have used to measure reaction rates on clean surfaces. In Figure 1(a) is shown a high pressure cell inside the UHV system ( ) with the sample mounted on a bellows so it can be moved between the reaction cell and the position used for AES analysis. In Figure 1(h) is shown a reaction cell outside the analysis system with the sample translated between heating leads in the reactor and in the UHV analysis system ( ). 

In dispersed-metal catalysts, the metal is dispersed into small particles, on the order of 5 to 500 A in diameter, which are generally located in the micropores (20-1000 A) of a high surface area support. This provides a large metal surface area per gram for high, easily measurable reaction rates, but hides much of the structural surface chemistry of the catalytic reaction. The surface structure of the small particles is unknown only their mean diameter can be measured and the pore structure could hide reactive intermediates from characterization. Some of the same difficulties also hold for thin films. However, we can accurately characterize and vary the surface structure of our single-crystal catalysts, and in our reactor the surface composition can also be readily measured both are prerequisites for the mechanistic study of the catalysis on the atomic scale. 

The rate of steam consumption is equal to the steam flow rate times the steam conversion, and the rate of HBr formation is twice the rate of steam consumption. The formation of HBr at a given reaction time tR depends upon the melt composition. A second-order reaction of CaBr2 was found to match the experimentally measured reaction rates far better than a first-order reaction. The reaction constant is then derived from the rate of HBr formation, which is experimentally measured. The observed kinetic constant was 2.17 10-12 kmol s-1 m-2 MPa-1 (1.30 1CH g-mol min-1 cm-2 bar-1) for the hydrolysis reaction, which is 24 times greater than the constant reported for solid CaBr2 reaction. This higher rate promises to significantly reduce the size and design complexity of the hydrolysis reactor. 

A differential characteristic which demands a lower degree of standardization is the reaction rate. The rate of a chemical reaction with respect to compound B at a given point is defined as the rate of formation of B in moles per unit time per unit volume. It cannot be measured directly and is determined from the rates of change of some observable quantities such as the amount of substance, concentration, partial pressure, which are subject to measurements. Reaction rates are obtained from observable quantities by use of the conservation equations resulting from the mass balance for the given reactor type. 

What are the types of problems that need to be addressed by measuring reaction rates The answers to this question are very diverse. For example, in the testing of catalysts, a new catalyst may be evaluated for replacement of another catalyst in an existing process or for the development of a new process. Accurate, reliable laboratory reaction rate data are necessary for the design of an industrial reactor... 

When choosing a laboratory reactor for the measurement of reaction rate data, numerous issues must be resolved. The choice of the reactor is based on the characteristics of the reaction and for all practical matters by the availability of resources (i.e., reactors, analytical equipment, money, etc.). A good example of the issues involved in selecting a laboratory reactor and how they influence the ultimate choice is presented by Weekman [AIChE J., 20 (1974) 833]. Methods for obtaining reaction rate data from laboratory reactors that approximate the ideal reactors listed in Table 3.5.1 are now discussed. 

The reactor shell was about 8 feet tall and 8 inches in outer diameter (O.D.). Provision was made for simultaneous reaction rate measurements on five small catalyst charges mounted in series inside a 3/8 inch I.D. thin-wall stainless steel tube centered in the shell the data from the upstream bed are considered here. 

The general idea of the three previous cases (i.e., Figures 10-31 through 10-33)is to arrange the data in such a fashion as to arrive at functional groupings of measured variables that will be linear with time. The particular functional groups will vary with (1) type of reactor used to collect the data, (2) reaction order of the main reaction, and (3) the decay reaction order. For the three main types of reactors, three main reaction rate laws, and three decay rates, 27 different types of plots could result. We leave derivation of the equation for each of these plots to the reader and point out that only one additional step is needed in our solution algorithm. That step is the decay rate law ... 

The use of these criteria requires an experimentally measured point value for the reaction rate, the solubility of gas phase reactant and an estimation of gas to liquid mass transfer coefficient k,a. Some correlations for calculating k,a values in different multiphase reactor systems are presented in Table 3. 

We emphasize that equations 5.9 - 5.11 do not have to be solved. Indeed, they are only suggestive of what a suitable set of equations might be. Furthermore, any suitably complete set of equations would surely be intractable. Our purpose in examining these equations is to see that rate-values can be measured by measuring dX/dt, directly or indirectly. This, of course, is quite obvious in the case of the batch reactor. We will see subsequently that it turns out to be the case for other reactors, where it is not nearly so obvious how to measure dX/dt. Measuring reaction rates using a TS-BR may therefore be simple in principle, but might initially confuse those accustomed to conventional methods. 

For the three tested catalysts, the addition of tripropylamine or benzoic acid in the reactor decreases the reaction rate. The decrease of the heptanal conversion measured after 60 min is quite significant. It decreases, for instance, from 66% to 18% over AIPO in the presence of tripropylamine, and from 66% to 32% over the same catalyst in the presence of benzoic acid. This indicates the participation of basic as well as acid sites to the reaction, over both phosphate precursors and oxynitrides. 

For platinum catalysts no interference with the measured reaction rate has been observed with acidic catalyst present in the reactor. For example, mixing equal parts of 46 A.I. (CAT-A evaluation) silica-alumina cracking catalyst with platinum catalyst did not alter the rate measurements obtained from the platinum catalyst alone. 

Now we turn to the single most important parameter estimation problem in chemical reactor modeling determining reaction-rate constants given dynamic concentration measurements. We devote the rest of the chapter to developing methods for this problem. 

The gas-phase production of methanol from carbon monoxide and hydrogen is carried out in a small constant-volume batch reactor under isothermal conditions and the pilot-plant operator measures the total pressure within the reactor vs. time for subsequent reaction-rate data analysis. A stoichiometric feed of carbon monoxide and hydrogen is introduced to the reactor at time t = 0, and the total pressure is 3 atm. Sketch (he raw data as total pressme vs. time. Be sure to indicate the appropriate equation that describes the shape of the curve. 

There are two fundamental types of experimental reactors for measuring solid-catalyzed reaction rates, integral and differential. The integral reactor consists essentially of a tube of diameter less than 3 cm filled with, say, IF g of catalyst. Each run comprises steady-state operation at a given feed rate, and based on several such runs, a plot of the conversion X/ versus IF/F o is prepared. Differentiation of this curve gives the rate at any given (i.e., concentration) as... 

Stirring gases is inconvenient and inellicient at any scale. F >r the study of gas phase reactions catalyzed at the surface of a solid, the mechanical problem of setting up a gas-solid stirred-flow reactor is a difficult one. If, however, a recirculation pump for gases is available, a stirred-flow reactor can be built readily for measuring the rate of a reaction catalyzed by a solid. Show how. 

---