Heat effects reactors

Empirical grey models based on non-isothermal experiments and tendency modelling will be discussed in more detail below. Identification of gross kinetics from non-isothermal data started in the 1940-ties and was mainly applied to fast gas-phase catalytic reactions with large heat effects. Reactor models for such reactions are mathematically isomorphical with those for batch reactors commonly used in fine chemicals manufacture. Hopefully, this technique can be successfully applied for fine chemistry processes. Tendency modelling is a modern technique developed at the end of 1980-ties. It has been designed for processing the data from (semi)batch reactors, also those run under non-isothermal conditions. 

The highly exothermic nature of the butane-to-maleic anhydride reaction and the principal by-product reactions require substantial heat removal from the reactor. Thus the reaction is carried out in what is effectively a large multitubular heat exchanger which circulates a mixture of 53% potassium nitrate [7757-79-1/, KNO 40% sodium nitrite [7632-00-0], NaN02 and 7% sodium nitrate [7631-99-4], NaNO. Reaction tube diameters are kept at a minimum 25—30 mm in outside diameter to faciUtate heat removal. Reactor tube lengths are between 3 and 6 meters. The exothermic heat of reaction is removed from the salt mixture by the production of steam in an external salt cooler. Reactor temperatures are in the range of 390 to 430°C. Despite the rapid circulation of salt on the shell side of the reactor, catalyst temperatures can be 40 to 60°C higher than the salt temperature. The butane to maleic anhydride reaction typically reaches its maximum efficiency (maximum yield) at about 85% butane conversion. Reported molar yields are typically 50 to 60%. 

Turbulence. Turbulence is important to achieve efficient mixing of the waste, oxygen, and heat. Effective turbulence is achieved by Hquid atomization (in Hquid injection incinerators), soHds agitation, gas velocity, physical configuration of the reactor interior (baffles, mixing chambers), and cyclonic flow (by design and location of waste and fuel burners). 

A useful classification of lands of reaclors is in terms of their concentration distributions. The concentration profiles of certain limiting cases are illustrated in Fig. 7-3 namely, of batch reactors, continuously stirred tanks, and tubular flow reactors. Basic types of flow reactors are illustrated in Fig. 7-4. Many others, employing granular catalysts and for multiphase reactions, are illustratea throughout Sec. 23. The present material deals with the sizes, performances and heat effects of these ideal types. They afford standards of comparison. 

Micro reactors are seen to have smaller inhomogeneities of the electrical field and less temperature rise in the reaction medium due to the Joule heating effect between the electrodes [70]. Submillimeter interelectrode gaps are expected to reduce the ohmic loss. 

Models may be used in training and education. Many important aspects of reactor operation can be simulated by the use of simple models. These include process start-up and shut down, feeding strategies, measurement dynamics, heat effects and control. Such effects are easily demonstrated by computer, as shown in the accompanying simulation examples, but are often difficult and expensive to demonstrate in practice. 

Energy balances are needed whenever temperature changes are important, as caused by reaction heating effects or by cooling and heating for temperature control. For example, such a balance is needed when the heat of reaction causes a change in reactor temperature. This is seen in the information flow diagram for a non-isothermal continuous reactor as shown in Fig. 1.19. 

For reactions involving heat effects, the total and component mass balance equations must be coupled with a reactor energy balance equation. Neglecting work done by the system on the surroundings, the energy balance is expressed by... 

Chapter 3 concerns the dynamic characteristics of stagewise types of equipment, based on the concept of the well-stirred tank. In this, the various types of stirred-tank chemical reactor operation are considered, together with allowance for heat effects, non-ideal flow, control and safety. Also included is the modelling of stagewise mass transfer applications, based on liquid-liquid extraction, gas absorption and distillation. 

For reactors 2 and 3, there will not be any sensible heat effects. 

Consider the reaction used as the basis for Illustrations 10.1 to 10.3. Determine the volume required to produce 2 million lb of B annually in a plug flow reactor operating under the conditions described below. The reactor is to be operated 7000 hr annually with 97% conversion of the A fed to the reactor. The feed enters at 163 C. The internal pipe diameter is 4 in. and the piping is arranged so that the effective reactor volume can be immersed in a heat sink maintained at a constant temperature of 160 °C. The overall heat transfer coefficient based on the... 

No rate enhancement was observed when the reaction was performed under microwave irradiation at the same temperature as in conventional heating [47]. Similar reaction kinetics were found in both experiments, presumably because mass and heat effects were eliminated by intense stirring [47]. The model developed enabled accurate description of microwave heating in the continuous-flow reactor equipped with specific regulation of microwave power [47, 48]. Calculated conversions and yields of sucrose based on predicted temperature profiles agreed with experimental data. 

The adiabatic temperature rise can be calculated from the total heat effect and the specific heat of the reactor contents. Both parameters can, for example, be determined by using appropriate procedures for the Mettler-Toledo RC1 or Contalab. The adiabatic temperature is calculated by ... 

The complete design of any reactor must also take into account questions concerning the stability of operation, particularly in relation to heating effects. 

We follow a three-step procedure First, we must find how equilibrium composition, rate of reaction, and product distribution are affected by changes in operating temperatures and pressures. This will allow us to determine the optimum temperature progression, and it is this that we strive to approximate with a real design. Second, chemical reactions are usually accompanied by heat effects, and we must know how these will change the temperature of the reacting mixture. With this information we are able to propose a number of favorable reactor and heat exchange systems—those which closely approach the optimum. Finally, economic considerations will select one of these favorable systems as the best. 

The recycle reactor with large recycle acts as a mixed flow reactor and shares its advantages. Actually, to minimize heat effects the catalyst need not be all at one location, but can be distributed throughout the recycle loop. 

It is frequently difficult to maintain reactors strictly isothermal because aU reactions liberate or absorb considerable heat. These effects can be rninintized by diluting reactants and using low temperatures, thus making reaction rates sufficiently slow that the system can be thermostatted accurately. However, kinetics under these conditions are not those desired in a reactor, and one must be careful of the necessary extrapolation to operating conditions. We will discuss heat effects in detail in Chapters 5 and 6. 

These reactors operate near equilibrium, and therefore the first reactor must be heated to high temperatures because this reaction is endothermic, and the second must be cooled to fairly low temperatures. The kinetics of these reactions are very important if one is designing a reactor in detail, but the major features of the process are governed by equilibrium limitations and heat effects. 

For the pseudohomogeneous analysis criterion, a predictive expression for the temperature difference between the gas- and solid-phase temperatures for a fixed bed reactor is given by Wei et al. (1984). In this expression, p is the dimensionless heat transfer coefficient, AA is the total heat generation in the reactor, and y is the fraction of the heat effect going to the solid phase. For the example packed bed reactor used throughout this chapter, the temperature difference between the gas and solid phases is in many cases up to 10 K. 

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