Laboratory reactors evaluation

The ROTOBERTY internal recycle laboratory reactor was designed to produce experimental results that can be used for developing reaction kinetics and to test catalysts. These results are valid at the conditions of large-scale plant operations. Since internal flow rates contacting the catalyst are known, heat and mass transfer rates can be calculated between the catalyst and the recycling fluid. With these known, their influence on catalyst performance can be evaluated in the experiments as well as in production units. Operating conditions, some construction features, and performance characteristics are given next. 

Schlatter, J. C. Sinkevitch, R. M. Mitchell, P. J. "A Laboratory Reactor System for Three-Way Catalyst Evaluation" GM Research Publication GMR-2911 presented at 6th N. Amer. Mtg. Catal. Soc., Chicago, IL, March 1979. 

The other major issue in reactor design concerns catalyst deactivation and membrane fouling. Both contribute to loss of reactor productivity. Development of commercially viable processes using inorganic membrane reactors will only be possible if such barriers are overcome. These subjects will receive greater attention as current R D efforts expand beyond laboratory scale evaluations into field demonstrations. 

Evaluation of literature data on the correlation between the particle Peclct number (also referred to as Bodenstcin number) and particle Reynolds number for single phase and trickle flow yielded Fig. 6. At low Rep (<20) a value between 0.3 and 0.7 holds for single phase flow, whereas for trickle flow this is one order of magnitude lower. These conditions are usually encountered in small scale laboratory reactors. 

Evaluation techniques and equipment are as varied as the individual catalytic processes themselves. The long term goal of catalyst evaluation is to reduce the size of the testing equipment consistent with reliable and accurate data as it relates to the commercial process. Invariably, the farther removed in physical size the process simulation attains, the more likely that errors will be introduced which can affect data accuracy, accuracy being defined as commercial observations. In addition, smaller equipment size also places less demand on the physical integrity of a catalyst particle therefore, additional test methods have been developed to simulate these performance characteristics. Despite these very important limitations, laboratory reactors fully eight orders of magnitude (100 million times) smaller are routinely used in research laboratories by both catalyst manufacturers and petroleum refiners. 

The PFR is efficient for screening solid catalyst in a single fluid phase. It can also be used in later research stages to assess commercial criteria. Consider the evaluation of the ultimate commercial performance of a newly developed fixed-bed catalyst. The theory of similarity teaches that for the laboratory and the industrial reactor, the Damkohler number (NDa), the Sherwood number (Nsh), and the Thiele modulus (<)>) need to be kept constant (Figure 2). As a result, the laboratory reactor must have the same length as the envisioned commercial reactor (7). In this case, scale up is done by increasing the diameter of the reactor. This example further illustrates that laboratory reactors are not necessarily small in size. 

A variety of laboratory reactors have been developed for the determination of the kinetics of heterogeneous reactions, all with specific advantages and disadvantages. Several reviews of laboratory reactors are available [28-33]. The evaluations of the available methods in these reviews are different because of the variation of chemical reactions and catalysts investigated and the different viewpoints of the authors. It is impossible to choose a best kinetic reactor because too many conflicting requirements need to be satisfied simultaneously. Berty [34] discussing an ideal kinetic reactor, collected 20 requirements as set forward by different authors. From these requirements it is easy to conclude, that the ideal reactor, that can handie all reactions under all conditions, does not exist For individual reactions, or for a group of similar reactions, not all requirements are equally important. In such cases it should be possible to select a reactor that exhibits most of the important attributes. 

To analyze reaction mechanisms in complex catalytic systems, the application of micropulse techniques in small catalytic packed beds has been used. Christoffel [33] has given an introduction to these techniques in a comprehensive review of laboratory reactors for heterogeneous catalytic processes. Mtlller and Hofmann [59,61] have tested the dynamic method in the packed bed reactor to investigate complex heterogeneous reactions. Kinetic parameters have been evaluated by a method, which employs concentration step changes and the time derivatives of concentration transients at the reactor outlet as caused by a concentration step change at the reactor inlet. 

Various laboratory reactors listed in Table 5-1 are evaluated below, based on the above factors. The pertinent references for these reactors are listed at the end of the chapter. 

Chapters 2, 3, and 4 review the tools for modeling the performance of three-phase reactors. Chapter 2 evaluates the use of film and penetration theory for the calculation of absorption rate in three-phase reactors. Chapter 3 describes various techniques for characterizing residence time distribution and the models which take into account the macromixing in a variety of three-phase reactors. The concepts described in these two chapters are vital to the simulation of an entire reactor. Chapter 4 illustrates the development of the mathematical models for some important pilot scale and commercial reactors. In Chapter 5 some advantages and disadvantages of three-phase laboratory reactors are outlined. 

The successful design of industrial reactors lies primarily with the reliability of the experimentally determined parameters used in the scale-up. Consequently, it is imperative to design equipment and experiments that will generate accurate and meaningful data. Unfortunately, there is usually no single comprehensive laboratory reactor that could be used for aU types of reactions and catalysts. In this section we discuss the various types of reactors that can be chosen to obtain the kinetic parameters for a specific reaction system. We closely follow the excellent strategy presented in the article by V. W. Weekman of Mobd Oil. The criteria used to evaluate various types of laboratory reactors are listed in Table 5-3. 

Table 5-3. CftrrERiA Used To Evaluate Laboratory Reactors...

From a bench-scale laboratory reactor designed to operate at nearly constant temperature and composition. Usually operating conditions are chosen to facilitate separating the effects of diffusion and heat transfer (the physical processes) from the observed measurements, so that the rate of the chemical step can be accurately evaluated. This is the most successful of the three methods. 

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... 

Experiments on a laboratory scale will serve to test different catalytic systems for the synthesis of alcohols, namely Cu/Zn-type and Cu/Co-type catalysts. Therefore, recycle reactors will be used in the laboratory to evaluate catalyst performance in terms of activity and selectivity. 

The criteria used to evaluate various types of laboratory reactors are listed in Table 5-2. 

Commercially-prepared Pt-Rh and Pd monolith catalysts were thermally aged then characterized for catalytic performance using a laboratory reactor to evaluate the magnitude and reversibility of the impact of sulfur on three-way activity. The SO2 concentration in the feedstream was varied from 0 ppm to 30 ppm, which was comparable to sulfur levels in gasoline ranging from 0 to 450 ppm. Tests were first conducted using propylene and repeated using propane to represent the hydrocarbon mixture in exhaust. 

When performing such DoEs, the use of automated laboratory reactors is highly recommended. In addition to having recipe capabilities and agitation and temperature controls, they also have data acquisition and analysis modules. Eor example, the Mettler Toledo RCI (reactor calorimeter) is an automated laboratory reactor capable of measuring heats of reaction. Such measuranents are important in mechanistic investigations and safety evaluations. We also believe that after the development chemist, reaction calorimetry is the development engineer s best Mend. 

The concentration-controlled, gradientless differential circulating reactor is best suited for kinetic measurements. Such modem laboratory reactors are now of major importance. They allow kinetic data to be measured and evaluated practically free of distortion by heat- and mass-transport effects [17]. Depending on the material flow, a distiction is made between reactors with outer and inner circulation. Evaluation of the kinetic measurements is straightforward because the simple algebraic balance equation for a stirred tank reactor (Eq. 13-8) can be applied (prerequisite high recycle ratio R). In practice it is foimd that recycle ratios of R = 10-25 are sufficient to achieve practically ideal stirred tank behavior [8]. 

A laboratory and on-reactor evaluation of three commercial Intermediate Range Monitors has been completed. Although none of the Instruments evaluated have all the features desired for a reactor process instrument, at least one of the instruments will satisfy the requirements with minor modifications. 

Central to the evaluation presented here is the assessment of the catalyst s reaction performance, carried out using one or more of the many forms of laboratory reactor available. 

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