Temperature Control of Industrial Reactors

Different technical solutions are used in the temperature control of industrial reactors. The heat carriers mentioned in Section 9.2.f may be used by different technical means the direct way whereby the heat carrier is directly mixed with the reaction mass, internal or external coils, jacket, simple circuits, and indirect systems with a double circulating system. These techniques with their advantages and drawbacks, in terms or process safety, are reviewed in the following sections. 

B.J. Cott and S. Macchietto. Temperature control of exothermic batch reactors using generic model control (GMC). Industrial Engineering Chemistry Research, 28 1177-1184, 1989. 

Consider the following problem. In the petrochemical industry, many reactions are oxidations and hydrogenations that are very exothermic. Thus, to control the temperature in an industrial reactor the configuration is typically a bundle of tubes (between 1 and 2 inches in diameter and thousands in number) that are bathed in a heat exchange fluid. The high heat exchange surface area per reactor volume allows the large heat release to be effectively removed. Suppose that a new catalyst is to be prepared for ultimate use in a reactor of this type to conduct a gas-phase reaction. How are appropriate reaction rate data obtained for this situation ... 

Conventional microwave ovens are used less often for microwave chemistry today. Microwave reactors for chemical synthesis are commercially available and widely used in academia and in industry. These instruments have built-in magnetic stirring, direct temperature control of the reaction mixture, shielded thermocouples or IR sensors, and the ability to control temperature and pressure by regulating microwave output power. 

Continuous processes have recently been described in two patents for the preparation of cyclopropylamine, 10, (Figure 13.11), an intermediate for both the pharmaceutical and agricultural industries. In the earlier patent a two-reactor train was used in a CSTR approach [20]. To the first flask were charged simultaneously a solution of 8 in H20 and an aqueous solution of NaOCl.The overflow from this reactor was collected in a second reactor which was charged with 45% aqueous NaOH at temperatures below 30 °C. The extended addition allowed for temperature control of the very exothermic hydrolysis and decarboxylation. Subsequently the basic mixture was transferred to a distillation apparatus, and 10 was isolated by distillation in yields as high as 96%. 

An important issue for packed bed reactors is temperature control. Insufficient heat removal may lead to local overheating of the catalyst pellets with the consequence of rapid deactivation. Therefore, multitubular reactors with up to 35 000 parallel tubes are used in chemical industry for the temperature control of highly exothermic reactions. 

The feedstock used in this study was a hydrodesulfurized straight-run naphtha recovered from an industrial naphtha HDS unit. The catalyst used in this investigation was a conunercial available Pt-Re reforming sample (Pt 0.29 wt%. Re 0.29 wt%). The tests were performed in a fixed-bed pilot plant with H2 recycle. The unit consists of a stainless-steel reactor (internal diameter of 2.5 cm and length of 25 cm), which was operated in isothermal mode by independent temperature control of a three-zone electric furnace. 

The feedback effect of heat evolution on the rate of exothermic reactions may cause thermal runaway. This is a major issue in the operation of industrial reactors, as the loss of control of a chemical reactor constitutes a serious hazard everybody has in mind the SEVESO accident or those which occured recently in the Swiss industry. Thermal instability is due to the irreducible coupling between heat accumulation and the quasi-exponential increase of reaction rate as a function of temperature accounted for by Arrhenius equation. This problem can be studied by the methods of non linear dynamics. Here again, characteristic times make it possible to establish simple criteria which give at least an order of magnitude for dangerous and safe ranges of operation. 

Catalyst Development. Traditional slurry polypropylene homopolymer processes suffered from formation of excessive amounts of low grade amorphous polymer and catalyst residues. Introduction of catalysts with up to 30-fold higher activity together with better temperature control have almost eliminated these problems (7). Although low reactor volume and available heat-transfer surfaces ultimately limit further productivity increases, these limitations are less restrictive with the introduction of more finely suspended metallocene catalysts and the emergence of industrial gas-phase fluid-bed polymerization processes. 

Figure 4-8 shows a continuous reactor used for bubbling gaseous reactants through a liquid catalyst. This reactor allows for close temperature control. The fixed-bed (packed-bed) reactor is a tubular reactor that is packed with solid catalyst particles. The catalyst of the reactor may be placed in one or more fixed beds (i.e., layers across the reactor) or may be distributed in a series of parallel long tubes. The latter type of fixed-bed reactor is widely used in industry (e.g., ammonia synthesis) and offers several advantages over other forms of fixed beds. 

The maximum rate of polymerization has been confirmed to occur at the laminar-turbulent flow transition. The rate of polymerization was observed to be maximum at the transition for both straight reactors as well as for the helically-coiled reactor for which the transition is at a Reynolds number higher than that of the straight tube. The helically coiled tubular reactor is of industrial interest since it is much more compact and, consequently, the cost and the temperature control problems are more tractable. 

Another industrially important reaction of propylene, related to the one above, is its partial oxidation in the presence of ammonia, resulting in acrylonitrile, H2C=CHCN. This ammoxidation reaction is also catalyzed by mixed metal oxide catalysts, such as bismuth-molybdate or iron antimonate, to which a large number of promoters is added (Fig. 9.19). Being strongly exothermic, ammoxidation is carried out in a fluidized-bed reactor to enable sufficient heat transfer and temperature control (400-500 °C). 

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