Adiabatic continuous reactor

The isothermal and nonisothermal reactors are the most used when the reactions are exothermic or endothermic and one can provide or withdraw heat to keep the reactor isothermal (or not) and also determine how the temperature varies with the progress of the reaction. The adiabatic reactor is a particular case in which there is no heat exchange and the reactor is thermally insulated. The temperature and conversion vary differently within the reactor. The term Qextemal is zero into Equation 14.69 or 14.73. 

The temperatnre varies linearly with conversion. However, the term  

Depends on each component as seen in Eqnation 14.71. One may consider the enthalpy of reaction —AH and specific heat Cpj to be constant at the temperature range. 

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Fig ure 6-22. Temperature versus conversion for a first order irreversible reaction in an adiabatic continuous flow stirred tank reactor. 

Let us assume an adiabatic, semi-continuous reactor (see Sec. 3.2.4) with negligible input of mechanical energy (Fig. 1.22). 

The gas leaving an ammonia oxidation unit in a continuous process is cooled rapidly to 20°C and contains 9 mol % NO, 1% N02, 8% O2, and 82% N2 (all the water formed by reaction is assumed to be condensed). It is desirable to allow oxidation of NO to N02 in a continuous reactor to achieve a molar ratio of N02 to NO of 5 before absorption of the N02 to make HNO3. Determine the outlet temperature of the reactor, if it operates adiabatically (at essentially 6.9... 

In this case history, the control of the TMRaa (adiabatic Time-to-Maximum-Rate) is to be achieved in a semi-continuous reactor process by the dynamic optimization of the feed rate. Here it is desired to have the highest possible space-time-yield STY and it is necessary to achieve a thermally safe process (Keller, 1998). The reaction involves the addition of a sulfur trioxide on a nitro-aromatic compound... 

One of the simplest practical examples is the homogeneous nonisothermal and adiabatic continuous stirred tank reactor (CSTR), whose steady state is described by nonlinear transcendental equations and whose unsteady state is described by nonlinear ordinary differential equations. 

Consider an exothermic irreversible reaction with first order kinetics in an adiabatic continuous flow stirred tank reactor. It is possible to determine the stable operating temperatures and conversions by combining both the mass and energy balance equations. For the mass balance equation at constant density and steady state condition,... 

FINDING REQUIRED VOLUME FOR AN ADIABATIC CONTINUOUS-FLOW STIRRED-TANK REACTOR 5.6... 

While the adiabatic batch reactor is important and presents many control issues in its own right, we are concerned here primarily with continuous systems. We consider in detail two distinct reactor types the continuous stirred tank reactor (CSTRj and the plug-flow reactor. They differ fundamentally in the way the reactants and the products... 

It is the purpose of this chapter to discuss presently known methods for predicting the performance of nonisothermal continuous catalytic reactors, and to point out some of the problems that remain to be solved before a complete description of such reactors can be worked out. Most attention will be given to packed catalytic reactors of the heat-exchanger type, in which a major requirement is that enough heat be transferred to control the temperature within permissible limits. This choice is justified by the observation that adiabatic catalytic reactors can be treated almost as special cases of packed tubular reactors. There will be no discussion of reactors in which velocities are high enough to make kinetic energy important, or in which the flow pattern is determined critically by acceleration effects. 

The search continues for better and more economical processes for the production of ethylene. Those processes include catalytic thermal cracking, methanol to ethylene, oxidative coupling of methane, advanced cracking technology, adiabatic cracking reactor, fluidized bed cracking, membrane reactor, oxydehy-drogenation, ethanol to ethylene, propylene disproportionation, and coal to ethylene. Much work is still needed before any such process can compete with current processes. 

Vejtasa, S.A. Schmitz, R.A. An experimental study of steady state multiplicity and stability in an adiabatic stirred reactor. AIChE J. 1970,16, 410 19. Schmitz, R.A. Multiplicity, stability, and sensitivity of states in chemically reacting systems - a review. Adv. Chem. Ser. 1975, 148, 156-211. Razon, L.F. Schmitz, R.-A. Multiplicities and instabilities in chemically reacting systems - a review. Chem. Eng. Sci. 1987, 42, 1005-1047. Uppal, A. Ray, W.H. Poore, A.B. On the dynamic behavior of continuous stirred tank reactors. Chem. Eng. Sci. 1974, 29, 967-985. 

A major breakthrough with regard to the understanding of this phenomenon in the field of chemical reaction engineering was achieved by Ray and co-workers (Uppal et ai, 1974, 1976) when in one stroke they uncovered a large variety of possible bifurcation behaviours in non-adiabatic continuous stirred tank reactors. In addition to the usual hysteresis type bifurcation, Uppal et al. (1976) uncovered different types of bifurcation diagrams, the most important of which is the isola which is a closed curve disconnected from the rest of the continuum of steady states. 

The analysis that is illustrated in Figure 8-2 can be performed for any type of reactor. The only requirements are that the reactor be adiabatic and that the reaction be at chemical equilibrium at the exit condition of a continuous reactor, or at the final condition of a batch reactor. 

Nitrobenzene is produced by the nitration of benzene in a mixed nitrating (nitric/sulfuric) acid solution, using either the direct nitration process or adiabatic nitration. The first process involves the direct nitration of benzene in a batch or a continuous reactor. In the adiabatic process, an excess of benzene and a mixture of nitric and sulfuric acid are added to a series of reactors. Product nitrobenzene and dilute sulfuric acid are separated, water is flashed off under vacuum, and the sulfuric acid is recycled. 

Many important gas-liquid reactions such as oxidation, hydrogenation, nitration, sulfonation and chlorination are carried out in adiabatic continuous stirred-tank reactor over a wide range of temperature in vdiich a continuous shift from chenical to mass transfer control can happen. The interactions between the solubility, the diffusional resistances and the chenical reaction may cause the occurence of sustained periodic oscillations and steady... 

The preceding equation assumes the reaction is completely quenched immediately after the relief point is reached. This behavior is closely approximated if the reaction stops in the quench pool and the reactor empties quickly and thoroughly. If the reaction continues in the quench pool, the temperature Tr should be increased to the maximum adiabatic exotherm temperature. An equation is presented by CCPS (AIChE-CCPS, 1997) that includes the heat of reaction. In some cases, an experiment is necessary to confirm that the reaction indeed stops in the quench pool. 

There are a variety of limiting forms of equation 8.0.3 that are appropriate for use with different types of reactors and different modes of operation. For stirred tanks the reactor contents are uniform in temperature and composition throughout, and it is possible to write the energy balance over the entire reactor. In the case of a batch reactor, only the first two terms need be retained. For continuous flow systems operating at steady state, the accumulation term disappears. For adiabatic operation in the absence of shaft work effects the energy transfer term is omitted. For the case of semibatch operation it may be necessary to retain all four terms. For tubular flow reactors neither the composition nor the temperature need be independent of position, and the energy balance must be written on a differential element of reactor volume. The resultant differential equation must then be solved in conjunction with the differential equation describing the material balance on the differential element. 

ILLUSTRATION 10.3 ADIABATIC OPERATION OF CONTINUOUS STIRRED TANK REACTORS OPERATING IN SERIES... 

In a plant for the continuous nitration of chlorobenzene, maloperation during startup caused the addition of substantial amounts of reactants into the reactor before effective agitation and mixing had been established. The normal reaction temperature of 60°C was rapidly exceeded by at least 60° and an explosion occurred. Subsequent investigation showed that at 80° C an explosive atmosphere was formed above the reaction mixture, and that the adiabatic vapour-phase nitration would attain a temperature of 700° C and ignite the explosive atmosphere in the reactor. See l,3-Bis(trifluoromethyl)benzene, above... 

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