Industrial reactors, performance

Thermal stability of chemical reactors is a classic yet active area within chemical engineering science. Considerable research has focused on determining safe operating criteria for batch, CSTR, and tubular reactors. Current work has been directed towards understanding thermal stability in the presence of multiple phases (fluid/solid and gas/liquid) and multiple reactions with realistic, complex reaction rates expressions. The advent of computational methods has allowed for this field to continue to thrive. A sound understanding of these principles may help improve industrial reactor performance by reducing waste and costly separation operations and help maintain a clean environment. 

Figure 13.33 shows a comparison of semi-industrial and bench-scale performance at LHSV = 0.2 h and 380° C. It is evident that the semi-industrial scale outperforms the bench scale as a result of the effect of liquid flow rate on catalyst utilization. The use of wetting efficiency as the scale-up parameter allows accounting for such an effect on reactor performance. From the previous analysis, it was determined that a wetting efficiency of 0.7 correlates sufficiently well bench-scale and semi-industrial reactor performances. 

The application of residence time distribution theory is an established method for identifying process system fluid flow and mixing characteristics. Tracer data analysis via this theory can give valuable insight into a system s hydrodynamic characteristics. It has been applied to understand and improve industrial reactor performance. Two applications are discussed. 

Scale-Up Principles. Key factors affecting scale-up of reactor performance are nature of reaction zones, specific reaction rates, and mass- and heat-transport rates to and from reaction sites. Where considerable uncertainties exist or large quantities of products are needed for market evaluations, intermediate-sized demonstration units between pilot and industrial plants are usehil. Matching overall fluid flow characteristics within the reactor might determine the operative criteria. Ideally, the smaller reactor acts as a volume segment of the larger one. Elow distributions are not markedly influenced by... 

The effect of physical processes on reactor performance is more complex than for two-phase systems because both gas-liquid and liquid-solid interphase transport effects may be coupled with the intrinsic rate. The most common types of three-phase reactors are the slurry and trickle-bed reactors. These have found wide applications in the petroleum industry. A slurry reactor is a multi-phase flow reactor in which the reactant gas is bubbled through a solution containing solid catalyst particles. The reactor may operate continuously as a steady flow system with respect to both gas and liquid phases. Alternatively, a fixed charge of liquid is initially added to the stirred vessel, and the gas is continuously added such that the reactor is batch with respect to the liquid phase. This method is used in some hydrogenation reactions such as hydrogenation of oils in a slurry of nickel catalyst particles. Figure 4-15 shows a slurry-type reactor used for polymerization of ethylene in a sluiTy of solid catalyst particles in a solvent of cyclohexane. 

The space velocity SV is often used with conversion to descrihe die overall reactor performance. It is common in die petroleum and petrochemical industries to plot conversion against space velocity to descrihe die effect of feed rate on die performance of a flow system. 

Use Scalable Heat Transfer. The feed flow rate scales as S and a cold feed stream removes heat from the reaction in direct proportion to the flow rate. If the energy needed to heat the feed from to Tout can absorb the reaction exotherm, the heat balance for the reactor can be scaled indefinitely. Cooling costs may be an issue, but there are large-volume industrial processes that have Tin —40°C and Tout 200°C. Obviously, cold feed to a PFR will not work since the reaction will not start at low temperatures. Injection of cold reactants at intermediate points along the reactor is a possibility. In the limiting case of many injections, this will degrade reactor performance toward that of a CSTR. See Section 3.3 on transpired-wall reactors. 

Firstly, there are technical reasons concerning catalyst and reactor requirements. In the chemical industry, catalyst performance is critical. Compared to conventional catalysts, they are relatively expensive and catalyst production and standardization lag behind. In practice, a robust, proven catalyst is needed. For a specific application, an extended catalyst and washcoat development program is unavoidable, and in particular, for the fine chemistry in-house development is a burden. For coated systems, catalyst loading is low, making them unsuited for reactions occurring in the kinetic regime, which is particularly important for bulk chemistry and refineries. In that case, incorporated monolithic catalysts are the logical choice. Catalyst stability is crucial. It determines the amount of catalyst required for a batch process, the number of times the catalyst can be reused, and for a continuous process, the run time. 

In this context, Benson and Ponton declare that while the chemical industry has made considerable achievements in reactor performance, safety and control, comparable to those in the microelectronics business, this success is by no means evident to the public, in deep contrast to the latter [139], It is said that this is mainly and in a way simply due to the visual recognition of chemical production plants. From a distance and for somebody outside the field, the chemical plants of the late 1940s and the early 1990s look virtually similar, whereas one is able immediately to see the big differences in, e.g., television sets and automobiles. Hence it is not evident that notable improvements were made over the decades. 

As illustrated by the examples above, the possibility of removing the generated heat from the reaction zone decreases with an increase in reactor size. As proven above, it can happen that the temperature of the reaction mixture in a full-scale reactor becomes higher than in the laboratory flask reactor. If multiple chemical reactions of distinctly different temperature sensitivities take place, differences in yields and selectivities between small and large reactors will be observed. This has a large influence on safety also. The laboratory reactor might still show satisfactory performance, while the industrial reactor might even explode. 

In any real reactor, the flow will not follow the plug-flow pattern precisely. Non-ideal flow in chemical reactors is the subject of Chap. 6 where the various models used to predict the performance of industrial reactors are discussed at some length. 

Chapter 1 reviews the concepts necessary for treating the problems associated with the design of industrial reactions. These include the essentials of kinetics, thermodynamics, and basic mass, heat and momentum transfer. Ideal reactor types are treated in Chapter 2 and the most important of these are the batch reactor, the tubular reactor and the continuous stirred tank. Reactor stability is considered. Chapter 3 describes the effect of complex homogeneous kinetics on reactor performance. The special case of gas—solid reactions is discussed in Chapter 4 and Chapter 5 deals with other heterogeneous systems namely those involving gas—liquid, liquid—solid and liquid—liquid interfaces. Finally, Chapter 6 considers how real reactors may differ from the ideal reactors considered in earlier chapters. 

We have presented a general reaction-diffusion model for porous catalyst particles in stirred semibatch reactors applied to three-phase processes. The model was solved numerically for small and large catalyst particles to elucidate the role of internal and external mass transfer limitations. The case studies (citral and sugar hydrogenation) revealed that both internal and external resistances can considerably affect the rate and selectivity of the process. In order to obtain the best possible performance of industrial reactors, it is necessary to use this kind of simulation approach, which helps to optimize the process parameters, such as temperature, hydrogen pressure, catalyst particle size and the stirring conditions. 

Most industrial reactors and high pressure laboratory equipment are built using metal alloys. Some of these same metals have been shown to be effective catalysts for a variety of organic reactions. In an effort to establish the influence of metal surfaces on the transesterification reactions of TGs, Suppes et collected data on the catalytic activity of two metals (nickel, palladium) and two alloys (cast iron and stainless steel) for the transesterification of soybean oil with methanol. These authors found that the nature of the reactor s surface does play a role in reaction performance. Even though all metallic materials were tested without pretreatment, they showed substantial activity at conditions normally used to study transesterification reactions with solid catalysts. Nickel and palladium were particularly reactive, with nickel showing the highest activity. The authors concluded that academic studies on transesterification reactions must be conducted with reactor vessels where there is no metallic surface exposed. Otherwise, results about catalyst reactivity could be misleading. 

Industrial gas-liquid hydrogenation reactions are carried out in slurry and trickle-bed reactors (Ref. 3). Modeling of the latter has been advanced significantly in the last two decades (Refs. 4-6). Predictions of trickle-bed reactors performance were in good agreement with experimental data (Ref.7). 

Organizational measures are based on human action for their performance. In the fine chemicals and pharmaceutical industries, reactor-charging operations are often manual operations and the product identification relies on the operator. In this context, quality systems act as support to safety, since they require a high degree of traceability and reliability. Examples of such measures are labeling, double visual checks, response to acoustic or optical alarms, in process control, and so on. The efficiency of theses measures is entirely based on the discipline and instruction of the operators. Therefore, they must be accompanied by programs of instructions, where the adequate procedures are learned in training. 

The equipment heat transfer coefficient determined from a cooling (or heating) experiment performed in the industrial reactor containing a known amount of a compound with known physical properties. 

Industrial reactors generally operate adiabatically. Cholette and Blanchet [8] compared adiabatic plug flow reactor to the CSTRmm. For exothermic reactions, they inferred that the performance of a CSTRmm is better than that of a plug flow reactor at low values of conversion, and vice-versa at high values of conversion. They further showed that the design considerations for endothermic reactions are similar to those for isothermal reactions. 

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