Laboratory reactors limitations

Measurements of the true reaction times are sometimes difficult to determine due to the two-phase nature of the fluid reactants in contact with the solid phase. Adsorption of reactants on the catalyst surface can result in catalyst-reactant contact times that are different from the fluid dynamic residence times. Additionally, different velocities between the vapor, liquid, and solid phases must be considered when measuring reaction times. Various laboratory reactors and their limitations for industrial use are reviewed below. 

Figure 4-17. Differential reaotor. (Source V. W. Weekman, Laboratory Reactors and Their Limitations/ A ChEJ, Vol. 20, p. 833, 1974. Used with permission of the AlChEJ.)...

The value for is conservatively interpreted as the particle diameter. This is a perfectly feasible size for use in a laboratory reactor. Due to pressure-drop limitations, it is too small for a full-scale packed bed. However, even smaller catalyst particles, dp 50 yum, are used in fluidized-bed reactors. For such small particles we can assume rj=l, even for the 3-nm pore diameters found in some cracking catalysts. 

It is important to be able to identify mass transfer limitations that occur when the reaction rate is high compared with the rate of mass transfer. For a heavily mass transfer limited reaction, preliminary experiments in a non optimised laboratory reactor... 

The reaction is carried out over a silver gauze or low surface supported catalyst at 600—700°C, indicating a very fast chemical reaction. This implies that determination of the intrinsic reaction rate in laboratory reactors is complicated by the interference of heat and mass transfer limitations. To avoid this problem, studies have been made at much lower temperatures, which in turn run the risk of being non-representative. 

Three ideal reactors—the batch reactor, the plug-flow reactor and the perfectly stirred reactor—are mathematical approximations to corresponding laboratory reactors that are used regularly to study chemical kinetics (Section 13.3.2). The batch reactor (or static reactor) is particularly useful to characterize explosion limits [241] and kinetic behavior at temperatures below 1000 K (e.g., [304,351]), while stirred reactors (e.g., [151,249,296, 367,397]) and flow reactors (e.g., [233,442]) have proved highly valuable in the study of chemical kinetics at higher temperatures. 

The initial dehydration reaction is sufficiently fast to form an equilibrium mixture of methanol, dimethyl ether, and water. These oxygenates dehydrate further to give light olefins. They in turn polymerize and cyclize to form a variety of paraffins, aromatics, and cycloparaffins. The above reaction path is illustrated further by Figure 3 in terms of product selectivity measured in an isothermal laboratory reactor over a wide range of space velocities. ( 3) The rate limiting step is the conversion of oxygenates to olefins, a reaction step that appears to be autocatalytic. In the absence of olefins, this rate is slow but it is accelerated as the concentration of olefins increases. 

Rates of catalytic reactions are obtained by measurement of the conversion of a key component, often the rate limiting reactant, in laboratory reactors and relating this to the amount of catalyst used and the amount or flow rate of reactants used, to obtain an intrinsic quantity, mols-1 amount-1. For practical application the mass or volume of a catalyst is most relevant as the amount but, for comparitive studies the amount of active phase on a supported catalyst, its specific surface area or the number of active sites may be preferred. In the latter case this yields the turnover frequency (TOF) [3], which is quite relevant for fundamental studies. The number of active sites is, however, usually hard to determine and the mass of the catalyst W will be used, resulting in a rate dimensions of mol s 1 kg-1. Other quantities are easily derived from this. 

The limits of the Biot heat number Bi are 0.01 and 50 [8], so it will depend on the particular conditions which criterion is the most severe. In the laboratory reactors it is often <1. It is obvious that decreasing the particle size will shift the largest gradient to the film layer around the particle. 

Temperature control for laboratory reactors is typically easy because of high heat transfer area-reactor volume ratios, which do not require large driving forces (temperature differences) for heat transfer from the reactor to the jacket. Pilot- and full-scale reactors, however, often have a limited heat transfer capability. A process development engineer will usually have a choice of reactors when moving from the laboratory to the pilot plant. Kinetic and heat of reaction parameters obtained from the laboratory reactor, in conjunction with information on the heat transfer characteristics of each pilot plant vessel, can be used to select the proper pilot plant reactor. 

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 most important methods for the determination of kinetics of catalyzed reactions are described here. We emphasize the problems and pitfalls in obtaining reliable reaction rates. The many diagnostic tests are briefly discussed and some warnings are given to limitations of commonly used laboratory reactors. Finally, it is worth noting that reaction rates can be expressed per unit mass of catalyst, per unit catalytic surface, per unit external particle area or per unit volume of the reactor, fluid or catalyst. For chemical reactor design it is best to express reaction rates in terms of unit catalyst volume. 

Employing a high recirculating flow rate in this small laboratory reactor, the following assumptions can be used (i) there is a differential conversion per pass in the reactor, (ii) the system is perfectly stirred, (iii) there are no mass transport limitations. Also, it can be assumed that (iv) the chemical reaction occurs only at the solid-liquid interface (Minero et al., 1992) and (v) direct photolysis is neglected (Satuf et al., 2007a). As a result, the mass balance for the species i in the system takes the following form (Cassano and Alfano, 2000) ... 

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