Reaction, exothermic tubular reactor

This equation yields the optimum temperature at each conversion for a reversible, exothermic reaction. Note that we generally conduct reversible, exothermic reactions in tubular reactors since they respond faster to a desired temperature change than do batch reactors. However, such reactions can be performed in batch reactors equipped with external heat exchangers for removing heat. The circulation flow rate must be high to achieve the desired operating temperature profde. 

The influence of the inlet temperature on the axial temperature profile in the microchannel reactor is illustrated in Figure 5.18. All other reaction conditions are kept constant. The initial temperature difference between the cooling fluid and the reaction mixture at the inlet is in all cases identical To-T = 10K). At the reactor outlet the temperature reaches nearly the cooling temperature for all inlet conditions. But, the maximum temperature increases drastically with an increase of Tq. The important increase of the maximum temperature because of a small change of the inlet conditions is called parametric sensitivity and can be observed for fast and exothermic reactions in tubular reactor. In the domain of high parametric sensitivity the reactor is difficult to control and important temperature excursions carmot be avoided. High local temperatures may lead to important... 

The alternate possibility of building a laboratory tubular reactor that is shorter and smaller in diameter is also permissible, but only for slow and only mildly exothermic reactions where smaller catalyst particles also can be used. This would not give a scaleable result for the crotonaldehyde example at the high reaction and heat release rates, where flow and pore-ditfusion influence can also be expected. 

In the petrochemical industry close to 80% of reactions are oxidations and hydrogenations, and consequently very exothermic. In addition, profitability requires fast and selective reactions. Fortunately these can be studied nowadays in gradientless reactors. The slightly exothermic reactions and many endothermic processes of the petroleum industry still can use various tubular reactors, as will be shown later. 

As the name implies, these reactors are mostly used for the study of exothermic reactions, although they can be applied to endothermic reactions, too. Figure 2.2.6 shows a liquid-jacketed tubular reactor (Berty 1989). 

Although fluidized sand or alumina can also be used in the jacket of these somewhat larger reactors, the size makes the jacket design a problem in itself, hence these reactors are seldom used. An advantage of the jacketed reactor is that several—usually four—parallel tubes can be placed in the same jacket. These must be operated at the same temperature, but otherwise all four tubes can have different conditions if needed. This type of arrangement saves time and space in long-lasting catalyst life studies. Jacketed tubular reactors come close, but still cannot reproduce industrial conditions as needed for reliable scale-up. Thermosiphon reactors can be used on all but the most exothermic and fast reactions. 

Several patents exist on carrying out exothermic reactions for manufacture of reactive intermediates where high selectivity is essential. Even this author has a patent to make ethylene oxide in a transport line reactor (Berty 1959). Yet no fluidized bed technology is in use today. Mostly fixed bed, cooled tubular reactors are used for that purpose. 

Hydrochloric acid may conveniently be prepared by combustion of hydrogen with chlorine. In a typical process dry hydrogen chloride is passed into a vapour blender to be mixed with an equimolar proportion of dry acetylene. The presence of chlorine may cause an explosion and thus a device is used to detect any sudden rise in temperature. In such circumstances the hydrogen chloride is automatically diverted to the atmosphere. The mixture of gases is then led to a multi-tubular reactor, each tube of which is packed with a mercuric chloride catalyst on an activated carbon support. The reaction is initiated by heat but once it has started cooling has to be applied to control the highly exothermic reaction at about 90-100°C. In addition to the main reaction the side reactions shown in Figure 12.6 may occur. 

In a tubular reactor system, the temperature rises along the reactor length for an exothermic reaction unless effective cooling is maintained. For multiple steady states to appear, it is necessary that a... 

It is generally desirable to minimize the diameter of a tubular reactor, because the leak rate in case of a tube failure is proportional to its cross-sectional area. For exothermic reactions, heat transfer will also be more efficient with a smaller tubular reactor. However, these advantages must be balanced against the higher pressure drop due to flow through smaller reactor tubes. 

The curves in Figure 5.2 are typical of exothermic reactions in batch or tubular reactors. The temperature overshoots the wall temperature. This phenomenon is called an exotherm. The exotherm is moderate in Example 5.2 but becomes larger and perhaps uncontrollable upon scaleup. Ways of managing an exotherm during scaleup are discussed in Section 5.3. 

ILLUSTRATION 10.4 DETERMINATION OF THE VOLUME REQUIREMENTS FOR ADIABATIC OPERATION OF A TUBULAR REACTOR WITH EXOTHERMIC REACTION... 

A packed tubular reactor is used to produce a substance D at a total pressure of 100 kN/m2 (1 bar) utilising the exothermic equilibrium reaction ... 

Schutz, J. Chem. Eng. Sci. 43 (8) (1988) 1975. Agitated thin film reactors and tubular reactors with static mixers for rapid exothermic multiple reactions. 

It is well known that self-oscillation theory concerns the branching of periodic solutions of a system of differential equations at an equilibrium point. From Poincare, Andronov [4] up to the classical paper by Hopf [12], [18], non-linear oscillators have been considered in many contexts. An example of the classical electrical non-oscillator of van der Pol can be found in the paper of Cartwright [7]. Poore and later Uppal [32] were the first researchers who applied the theory of nonlinear oscillators to an irreversible exothermic reaction A B in a CSTR. Afterwards, several examples of self-oscillation (Andronov-PoincarA Hopf bifurcation) have been studied in CSTR and tubular reactors. Another... 

Truly isothermal operation of a tubular reactor may not be feasible in practice because of large enthalpies of reaction or poor heat transfer characteristics. Nor is it always desirable, as, for example, in the case of a reversible exothermic reaction (see Sect. 3.2.4). In an exothermic catalytic reaction, it may be necessary to provide adequate means for heat transfer to prevent the development of local hot-spots on which coking may occur and reduce the catalyst activity. An excessive temperature rise may also cause the catalyst particles to sinter, thereby reducing their surface area and causing an irreversible decrease in catalytic activity. 

In designing a wall-cooled tubular reactor, we want to operate such that the trajectory stays near the maximum rate for all temperatures. Thus for an exothermic reversible reaction the temperature should increase initially while the conversion is low and decrease as the conversion increases to stay away from the equilibrium constraint. One can easily program a computer to compute conversion and T versus t to attain a desired conversion for rninimum T in a PFTR. These curves are shown in Figure 5-17 for the three situations. 

An exothermal reaction with an adiabatic temperature rise of 100 K is to be performed in a tubular reactor with internal diameter of 30 mm, wall thickness of 2 mm, and surrounding jacket of thickness 30 mm containing water. Calculate the effective temperature rise that would occur if the reactor suddenly lost the utilities. In this situation, the reactant flow is stopped and there is no water flow in the jacket. 

Tubular reactors often have high-temperature limitations because of the occurrence of undesirable reactions, catalyst degradation, or materials of construction. This means that the maximum temperature anywhere in the reactor cannot exceed this limit. An exothermic reaction in an adiabatic reactor produces a maximum temperature at the exit under steady-state conditions. An exothermic reaction in a cooled reactor can... 

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