Reactors full-scale

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

Scale- Up of Electrochemical Reactors. The intermediate scale of the pilot plant is frequendy used in the scale-up of an electrochemical reactor or process to full scale. Dimensional analysis (qv) has been used in chemical engineering scale-up to simplify and generalize a multivariant system, and may be appHed to electrochemical systems, but has shown limitations. It is best used in conjunction with mathematical models. Scale-up often involves seeking a few critical parameters. Eor electrochemical cells, these parameters are generally current distribution and cell resistance. The characteristics of electrolytic process scale-up have been described (63—65). 

SASOL has pursued the development of alternative reactors to overcome specific operational difficulties encountered with the fixed-bed and entrained-bed reactors. After several years of attempts to overcome the high catalyst circulation rates and consequent abrasion in the Synthol reactors, a bubbling fluidized-bed reactor 1 m (3.3 ft) in diameter was constructed in 1983. Following successflil testing, SASOL designed and construc ted a full-scale commercial reac tor 5 m (16.4 ft) in diameter. The reactor was successfully commissioned in 1989 and remains in operation. 

In many cases, two identical reaction systems (e.g., a pilot plant scale and a full-scale commercial plant) exhibit different performances. This difference in performance may result from different flow patterns in the reactors, kinetics of the process, catalyst performance, and other extraneous factors. 

Because there are advantages in maintaining geometric similarity between the pilot plant and the full-scale plant reactors, the larger unit often has the same aspect ratio as the small unit. That is, Rj = Rj. Equation 13-27 then becomes... 

Determine the aspect ratio of a full-scale batch reactor for SUF = SUFy and at various values of Rj and SUFy as shown in Table 13-5. 

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 ... 

Consider the scale-up of a batch reactor from a pilot plant reactor to a full-scale reactor. Rewriting Equation 13-82 to the full-scale reactor yields ... 

If the pilot plant and the full-scale reactors have the same aspect ratio, then (3=1 and... 

In specifying the number of jacket zones and the aspect ratio for a full-scale reactor, there is a limitation on the temperature adjustment time. This implies that it must be of the same duration as experienced in the pilot plant reactor. Combining Equations 13-89 and 13-97 yields... 

Most reactors are equipped with safety rupture disks to protect the operator and equipment from destructive pressures. The operating pressure in a vessel should never exceed 70% of the range covered by the rupture disk. Similarly, gauges should not be stressed beyond about 70% of full-scale readings for safety and to ensure reliable readings. 

The combination of highly exothermic reactions with a sharp increase in viscosity as conversion proceeds controls reactor design and operational conditions in full-scale operations. The art of sulfonation is to maintain the optimal reaction temperature and reaction time, resulting in products with small amounts of byproducts and good color. 

Table 12.4. Conversion rates for near full scale reactor measured on 1.8 1 test engine. Conversion rates in % of raw material...

Figure 12.12. Near full scale monolithic Dinex reactor for electrochemically promoted soot combustion.18 20 Reprinted with permission from the Society of Automotive Engineers.

Fig.2. HBr conversion during catalyst life testing in single full-scale reactor tube showing high conversion throughout the test. The brief time at lower conversion was due to a unit upset.

Also assume that the pilot- and full-scale vessels will operate at the same temperature. This means that A(o-out,bout, . )and/i/2 will be the same for the two vessels and that Equation (1.49) will have the same solution for provided that 7 is held constant during scaleup. Scaling with a constant value for the mean residence time is standard practice for reactors. If the scaleup succeeds in maintaining the CSTR-like environment, the large and small reactors will behave identically with respect to the reaction. Constant residence time means that the system inventory, pV, should also scale as S. The inventory scaleup factor is defined as... 

The value for is conservatively interpreted as the particle diameter. This is a perfectly feasible size for use in a laboratory reactor. Due to pressure-drop limitations, it is too small for a full-scale packed bed. However, even smaller catalyst particles, dp 50 yum, are used in fluidized-bed reactors. For such small particles we can assume rj=l, even for the 3-nm pore diameters found in some cracking catalysts. 

If kiAi is known with good accuracy, it may be possible to back out the intrinsic kinetics using the methods of Section 7.1. Knowing the intrinsic kinetics may enable a scaleup where kiAj(af — ai) is dilferent in the large and small units. However, it is better to adjust conditions in the pilot reactor so that they are identical to those expected in the larger reactor. Good pilot plants have this versatility. The new conditions may give suboptimal performance in the pilot unit but achievable performance in the full-scale reactor. 

Suppose now that a pilot-plant or full-scale reactor has been built and operated. How can its performance be used to confirm the kinetic and transport models and to improve future designs Reactor analysis begins with an operating reactor and seeks to understand several interrelated aspects of actual performance kinetics, flow patterns, mixing, mass transfer, and heat transfer. This chapter is concerned with the analysis of flow and mixing processes and their interactions with kinetics. It uses residence time theory as the major tool for the analysis. 

We have described two highly Instrumented, data-logged and versatile pilot reactors which Integrate Into a rigorous scale-up regime. This Is necessitated by our highly complex products and the precise definition the computer-controlled full-scale plant can exploit. 

Microreactor scale-up is built upon the premise of numbering up channels. Figure 11.1. A single channel is demonstrated with the same geometry and fluid hydrodynamics as a full-scale reactor. Numbering up rehes on creating a massively... 

Scahng up will probably continue to be a problem since large reactors carmot be as efficient as small laboratory reactors. However, it may be possible to make laboratory or pilot-plant reactors that are more similar to large-scale reactors, allowing more rebable validation of the simulations and process optimization. The time from laboratory-scale to full-scale production should be shortened from years to months. 

Resident Time Distribution (RTD) is widely employed in the chemical engineering industry, as an analytical tool for characterizing flow dynamics within reactor vessels. RTD provides a quantitative measure of the back-mixing with in a reactor system [2]. However the cost and time involved in building and operating a pilot- or full scale reactor for RTD analysis can be economically prohibitive. As such we have implemented a numerical RTD technique through the FLUENT (ver. 6.1) commercial CFD package. 

Reactors Often yes Direct scale-up from the laboratory to the full scale often possible for homogeneous systems. 

Factors re.sponsible for the occurrence of scale-up effects can be either material factors or size/shape factors. In addition, differences in the mode of operation (batch or semibatch reactor in the laboratory and continuous reactor on the full scale), or the type of equipment (e.g. stirred-tank reactor in the laboratory and packed- or plate- column reactor in commercial unit) can be causes of unexpected scale-up effects. A simple misuse of available tools and information also can lead to wrong effects. 

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