Isothermal reactors scale

Reactor design usually begins in the laboratory with a kinetic study. Data are taken in small-scale, specially designed equipment that hopefully (but not inevitably) approximates an ideal, isothermal reactor batch, perfectly mixed stirred tank, or piston flow. The laboratory data are fit to a kinetic model using the methods of Chapter 7. The kinetic model is then combined with a transport model to give the overall design. 

In the above equation, C s are assumed to be in grams per cubic centimeter. Any other convenient units for C can also be chosen. The above equations can be used to correlate the data obtained in large-scale isothermal reactors such as hydrodesulfurization, hydrodenitrogenation reactors, etc. 

Design Stmcture for Isothermal Reactors 125 Scale-up of Liquid-Phase Batch Reactor Data to the Design of a CSTR 129... 

Feed properties and operation conditions determine catalyst life in the residue hydrodesulfurization. In a high conversion operation of vacuum residue, catalyst deactivation due to coke is as important as the one due to metals. Though many researchers have worked on understanding and modelling deactivation of residue hydrodesulfurization catalysts, there has still been a controversy in a coke deactivation mechanism [2, 3]. Very few publications are available discussing an effect of a bed temperature profile on catalyst deactivation in large scale adiabatic commercial reactors. Most of the studies on deactivation of residue hydrodesulfiirization catalysts have been done with small-scale isothermal reactors [2,3,4,5]. The activity tests of the used catalysts were conducted to study the catalyst deactivation in the commercial reactors. This paper also describes an effect of a bed temperature profile on coke deactivation, which was tested in the commercial reactors. 

In a commercial adiabatic reactor there is some concern about temperature distributions inside the reactor, in comparison with the isothermal small-scale pilot plant. We investigated, therefore, temperature profile in the catalyst bed to scale-up the process. The following study was conducted to investigate the temperature distributions during regeneration of large packed column. 

The results found in this study are less promising then those reported in literature [45-49]. There are several reasons for this difference. In some publications experiments have been reported in which process conditions and/or feed compositions have been used that are not realistic or feasible on an industrial scale but do have a large impact on the performance of the membrane reactor. Also, when results are reported from modelling this process, incorrect assumptions were sometimes made, e.g. side-reactions which have a large influence on the performance of this process have been neglected [47]. In other publications a very large heat input is taken, which leads to a more or less isothermal reactor, and as a consequence to higher conversions [45,46,48]. 

Over the years, several processes for the catalytic dehydrogenation of propane to propylene have been developed, which can be divided into processes based on an adiabatic or an isothermal reactor concept, respectively. The processes currently apphed on an industrial scale are based on adiabatic systems, such as the Catofin (Lummus/Air Products) and the Oleflex (UOP) process. As the dehydrogenation of propane to propylene comprises an equihbrium reaction (11), selective removal of hydrogen from the reaction mixture can shift the reaction towards the product side. At high temperatures, thermal cracking may occur. 

Large-scale hydroprocessing trickle-bed reactors normally operate under adiabatic conditions therefore, heat effects caused by the reaction must also be included. Shah [61] showed that in this case the critical Bodenstein number for elimination of axial dispersion effects is a function of a heat parameter as well as a modified Damkohler number. For low Damkohler numbers smaller critical Bodenstein numbers than in isothermal reactors are sufficient to eliminate axial dispersion in adiabatic reactors, whereas the inverse is true for large Damkohler numbers. 

In Chapter 7 the effects of transport phenomena on the scale of the reactor are considered. We call these macro flow effects. These can be described in terms of macro-mixing. For continuous reactors macro>mixing causes residence time distribution. Combined with micro-mixing this will lead to backmixing. When two or more phases are present in the reactor, the way these are each introduced into and removed from the reactor are quite essential for the performance of the reactor. These various effects are considered in this chapter in order to arrive at an integral reactor model. As in Chapter 3, only isothermal reactor models are considered so far. 

It is desired to prinluce 20() million pounds per year of EG. The reactor is to be operated isothermally. A 16.1 mol/dm solution of ethylene oxide (EO) in water is mixed (see Figure ES-2.1) with an equal volumetric solution of water containing 0.9 wi % of the catalyst H SOj and fed to the reactor. The specific reaction rate con- stant is 0..1)) min", as determined in Example 5-1, Practical guidelines for reactor scale-up are given by Mukesb. ... 

Since we are concerned here with the development of a chemical process for use on a commercial scale, it is evident that the kinetics equation should describe the process not only under laboratory conditions, but also on a large-scale industrial production scale. In many cases the processes are carried out under identical conditions, both in the laboratory and industrially. Thus, for example, the hydrochlorination process mentioned above can be carried out in an isothermal reactor consisting of a set of tubes filled with catalyst. The size of the tubes and the catalyst particles, and the conditions under which the process is carried out, arc the same as in the laboratory equipment. As an example of a process that is already carried out commercially, we can mention the reaction of the hydrogenation of iso-octene in the U.O.P. (Universal Oil Product) process. However, in many other cases the dimensions of the individual items and of the apparatus as a whole, and the conditions under which the process is carried out, change radically on transition from the laboratory to the industrial process as a result, the kinetics equations derived from laboratory tests are not applicable in industry. 

The three-phase isothermal reactor model was applied to analyze and simulate the performance of a bench-scale reactor. The model solution for the experimental reactor is an initial-value problem as the concentrations of reactants and products are known at the reactor inlet. The model was solved with the kinetic parameters estimated from experiments as reported previously. 

Figure 7.17c and d show the predicted dynamic liquid molar concentration profiles of sulfur along the commercial catalytic bed at different times ranging from 60 to 1700 s for an inlet reactor temperature of 340°C. The dynamic simulation was carried out at the same reaction conditions than those employed for the simulation of the bench-scale reactor. The value of sulfur concentration reported at the exit of the isothermal bench-scale reactor is represented by symbol o . The profiles with a pronounced reduction of sulfur concentration in the first section of the reactor have already been reported by Jimenez et al. (2007). They attributed those sulfur concentration shapes in the catalytic bed to the kinetic model considered and to the operating conditions simulated. Also, the bench-scale experimental sulfur concentration value was higher than that predicted for the commercial reactor because of the increasing catalytic bed temperature observed in the liquid phase of the adiabatic reactor. 

In predicting the time required to cool or heat a process fluid in a full-scale batch reactor for unsteady state heat transfer, consider a batch reactor (Figure 13-2) with an external half-pipe coil jacket and non-isothermal cooling medium (see Chapter 7). From the derivation, the time 6 to heat the batch system is ... 

The research programme into n-butyl lithium initiated, anionic polymerization started at Leeds in 1972 and involved the construction of a pilot scale, continuous stirred tank reactor. This was operated isothermally, to obtain data under a typical range of industrial operating conditions. 

Section 1.5 described one basic problem of scaling batch reactors namely, it is impossible to maintain a constant mixing time if the scaleup ratio is large. However, this is a problem for fed-batch reactors and does not pose a limitation if the reactants are premixed. A single-phase, isothermal (or adiabatic) reaction in batch can be scaled indefinitely if the reactants are premixed and preheated before being charged. The restriction to single-phase systems avoids mass... 

If the pilot reactor is turbulent and closely approximates piston flow, the larger unit will as well. In isothermal piston flow, reactor performance is determined by the feed composition, feed temperature, and the mean residence time in the reactor. Even when piston flow is a poor approximation, these parameters are rarely, if ever, varied in the scaleup of a tubular reactor. The scaleup factor for throughput is S. To keep t constant, the inventory of mass in the system must also scale as S. When the fluid is incompressible, the volume scales with S. The general case allows the number of tubes, the tube radius, and the tube length to be changed upon scaleup ... 

The same result is obtained when the fluid is compressible, as may be seen by substituting Sr = Si = S into Equations (3.40) and (3.41). Thus, using geometric similarity to scale isothermal, laminar flows gives constant pressure drop provided the flow remains laminar upon scaleup. The large and small reactors will have the same inlet pressure if they are operated at the same outlet pressure. The inventory and volume both scale as S. 

As a general rule, scaled-down reactors will more closely approach isothermal operation but will less closely approach ideal piston flow when the large reactor is turbulent. Large scaledowns will lead to laminar flow. If the large system is laminar, the scaled-down version will be laminar as well and will more closely approach piston flow due to greater radial diffusion. 

Show that the Equation (3.34) is valid if the large and small reactors have the same value for /2 and that this will be true for an isothermal or adiabatic PER being scaled up in series. 

Most kinetic experiments are run in batch reactors for the simple reason that they are the easiest reactor to operate on a small, laboratory scale. Piston flow reactors are essentially equivalent and are implicitly included in the present treatment. This treatment is confined to constant-density, isothermal reactions, with nonisothermal and other more complicated cases being treated in Section 7.1.4. The batch equation for component A is... 

Previous chapters have discussed how isothermal or adiabatic reactors can be scaled up. Nonisothermal reactors are more difficult. They can be scaled by maintaining the same tube diameter or by the modeling approach. The challenge is to increase tube diameter upon scaleup. This is rarely possible and when it is possible, scaleup must be based on the modeling approach. If the predictions are satisfactory, and if you have confidence in the model, proceed with scaleup. 

Figure 5. Effect of reactor shapes on Isotherms, flow streamlines and relative deposition rates of GaAs. The absolute growth rates scale as 0.88 (a) 0.91 (b) 0.94 (c) 1.0 (d) 1.08 (e).

---