Reaction rate from commercial-scale reactors

The discussion in the previous sections has evidenced that the use of biphasic systems has solved, at least in various cases, the problem of homogeneous catalyst recovery and recycle, but there still exists the problem of the cost of recycle and especially of reaction rate per volume of reactor, which derives in large part from mass- and heat-transfer limitations, but also from the low amount of catalytic centers per volume of reactor necessary to avoid side reactions and maintain a high selectivity, and/or limit catalyst deactivation or loss. These aspects often emerge only during the scaling-up and industrialization of the reaction and this is one of the reasons why many interesting reactions at the laboratory scale fail in commercialization. 

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... 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

Flow Reactors Fast reactions and those in the gas phase are generally done in tubular flow reaclors, just as they are often done on the commercial scale. Some heterogeneous reactors are shown in Fig. 23-29 the item in Fig. 23-29g is suited to liquid/liquid as well as gas/liquid. Stirred tanks, bubble and packed towers, and other commercial types are also used. The operadon of such units can sometimes be predicted from independent data of chemical and mass transfer rates, correlations of interfacial areas, droplet sizes, and other data. 

Bench-Scale Reactor. The bench-scale reactor is 0.81 in. i.d. and 48 in. long. The nominal feed gas rate for this unit is 30 standard cubic feet per hour (scfh) the feed gas is supplied from premixed, high-pressure gas cylinders. Except for reaction temperature, the bench-scale unit is substantially manually operated and controlled. The catalysts used in these studies were standard commercial methanation catalysts ground to a 16-20 mesh size which is compatible with the small reactor diameter. 

On a commercial scale, hexamine is manufactured from anhydrous NHi and a 45% solution of methanol-free formaldehyde. These raw materials, plus recycle mother liquor, arc charged continuously ai carefully controlled rates 10 a high-velocity reactor. Tile reaction is exothermic. "Die reactor effluent is discharged into a vacuum evaporator which also serves as a... 

A patented process has been developed for the production of electrode binder pitch from petroleum-based materials. Carbon anodes produced from the petroleum-based pitch and coke have been used successfully on a commercial scale by the aluminum industry. One stage of the process involves the pyrolysis of a highly aromatic petroleum feedstock. To study the pyrolysis stage of the process a small, sealed tube reactor was used to pyrolyze samples of feedstock. The progress of the reaction is discussed in terms of the formation of condensed aromatic structures, defined by selective solvent extraction of the reaction product. The pyrolysis of the feedstock exhibits a temperature-dependent induction period followed by reaction sequences that can be described by first-order kinetics. Rate constants and activation energies are derived for the formation of condensed aromatic structures and coke. 

A batch reactor by its nature is a transient closed system. While a laboratory batch reactor can be a simple well-stirred flask in a constant temperature bath or a commercial laboratory-scale batch reactor, the direct measurement of reaction rates is not possible from these reactors. The observables are the concentrations of species from which the rate can be inferred. For example, in a typical batch experiment, the concentrations of reactants and products are measured as a function of time. From these data, initial reaction rates (rates at the zero conversion limit) can be obtained by calculating the initial slope (Figure 3.5.1b). Also, the complete data set can be numerically fit to a curve and the tangent to the curve calculated for any time (Figure 3.5. la). The set of tangents can then be plotted versus the concentration at which the tangent was obtained (Figure 3.5.1c). 

Liquid distribution in trickle bed reactors has been mainly discussed from the aspect of flow channels between particles [6, 7]. However, since most of the commercial catalysts are extrudates, an effect of the particle orientation on liquid distribution is much more important than flow channel, which relates to mass flow rate and a particle size. Shaped catalysts have a higher volume activity than cylindrical catalysts when an effect of diffusion on the reaction rate is large [8]. Therefore, the shaped catalysts have been commonly used for hydrodemetallation of residue. However, since an effect of liquid distribution on the catalyst performance is important in large-scale commercial reactors, catalyst shape should be carefully selected to maximize the effectiveness of the catalyst usage in a commercial application. 

Isoparaffin alkylation reactions are very fast and they suffer from severe pore diffusion limitations. As a result, when catalyst particle size is increased from 100 pm (for slurry reactors) to 1.6 mm for fixed-bed reactors, the catalyst activity reduces by 10-fold according to basic mass transfer models using experimental values of the intrinsic rate constant, as shown in figure 4. To match the catalyst productivity of a slurry reactor, one would need to build a fixed-bed reactor with ten times the volume - not practical for a commercial scale system. In addition to using a fixed-bed reactor, we wanted to ensure that the solid-acid catalyst was both robust as well as benign (i.e. environmentally fiiendly). 

Pilot plant fixed-bed reactors are traditionally designed at space velocities equivalent to commercial scale fixed-bed reactors. Thus the film diffusion resistance of the process at the two scales is different. In general, pilot plant fixed-bed reactors are film diffusion rate limited, whereas commercial-size fixed-bed reactors are either pore diffusion rate or, more rarely, reaction rate limited. This shift from film diffusion rate limited to, more generally, pore diffusion rate limited occurs due to the high volumetric fluid flow through the catalyst mass in a commercial-size fixed-bed reactor. Thus reactant consumption or product formation is faster in the commercial-size fixed-bed reactor than in the pilot plant fixed-bed reactor. 

The proper design of commercial pyrolysis reactors requires a suitable expression for the intrinsic rate of the reactions. As intrinsic rate equations cannot yet be predicted, especially for the ultrapyrolysis regime, experimental data is required. This data is best obtained from bench-scale laboratory reactors, rather than from pilot plants or commercial-scale units. In laboratory scale pyrolysis reactors, the design and operating conditions can be chosen to reduce or eliminate the effects of mass and heat transfer, contaminants and catalytic surfaces from the observed measurements, thus allowing for the development of accurate expressions. It is most advantageous if the laboratory reactor is operated isothermally (in space and time), so that the temperature can be considered as an independent variable. Also, the pressure should be ideally kept constant. 

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