Small-scale reactors

The ratio Zt/Zp is in technical reactors much higher than 1. It becomes, e.g. also for a small scale reactor of V-IOOL (H/D = 2 D = 0.4 m) equipped with three turbines (d/D = 0.3) and working at a average impeller power per mass of only = lmVs in media with water like viscosity to Zt/Zp>36...72. The maximal energy dissipation in the impeller zones, required for the calculation of length scale of turbulence here taken from Eq. (20). 

Quality control tests or improvement of existing processes. Raw materials from various sources can be used in the manufacture of fine chemicals and pharmaceuticals. The raw materials can contain different impurities at various concentrations. Therefore, before the raw material is purchased and used in a full-scale batch its quality should be tested in a small-scale reactor. Existing full-scale procedures are subject to continuous modifications for troubleshooting and for improving process performance. Laboratory reactors used for tests of these two kinds are usually down-scaled reactors or reactors being a part of the full scale-reactor. 

The fine chemicals business is characterized by a small volume of products manufactured. Therefore, batch production predominates and small-scale reactors are used. The need to implement fine chemistry processes into existing multiproduct plants often forces the choice of batch reactors. However, safety considerations may lead to the choice of continuous processing in spite of the small scale of operation. The inventory of hazardous materials must be kept low and this is achieved only in smaller continuous reactors. Thermal mnaways are less probable in continuous equipment as proven by statistics of accidents in the chemical industries. For short reaction times, continuous or semicontinuous operation is preferred. 

In fact none of these models of the mass-transfer coefficient are of much use for the calculation of Kia values in small scale reactor conditions and we have to obtain them by experiments. However, these models can be used as a guide to estimate the influence of the physical properties of the medium. They also make it possible to consider relative values of Ka for compounds for which in experiments the value of Ka is not measurable as easily as for gases such as oxygen. 

In some cases, direct scale-up may be impracticable, for example because of blockage of the small-scale relief line. The requirement for complete emptying of the small-scale reactor by two-phase relief may also not be met in practice. If this occurs for a tempered system, the problem could be overcome by using a small-scale relief system from the bottom of the test reactor to simulate one at the top of the large-scale reactor. This procedure would not be safe for untempered reactions. 

The first example for small-scale reactors is a stirred vessel for a maximum pressure of 32.5 MPa and 350°C (Fig. 4.3-25). It has a volume of 0.4 1 and can be used batchwise or in continuous operation, preferably for gas-liquid reactions, without- or with soluble or suspended catalysts. 

Platinum on alumina reforming catalysts are commonly used commercially in the form of cylindrical pellets about X i in. in size, since this is about the smallest size acceptable from the standpoint of avoiding excessive pressure drop. For fundamental studies in small-scale reactors, where pressure drop limitations are less severe, it may be preferable to use the catalyst in the form of small granules to minimize diffusional limitations. 

If a continuous process is to he used for commercial production, a similar small-scale reactor system should he utilized in this second stage of product development. There are a number of reasons for this recommendation. The earher discussion of the difference between batch reactors and CSTRs lists some of these reasons, if, for example, engineering data are to he obtained for design of a commercial unit, the variable relation ps might be quite different for the different reactors. The Smith-Ewart CSTR model predicts a linear relationship between or N and the surfactant concentration [5]. The same mechanistic model for a batch reactor predicts a 0.6 power relationship between Rp or N and... 

The most commonly used high pressure reactor is the stirred autoclave, also referred to as a stirred tank reactor (STR). These reactors are commercially available in sizes ranging from 50 ml to 500 gallons. A typical small scale reactor... 

Carlsmith and Johnson (C2) have reviewed the research and development of the FCC process. Drawing on their data, Ikeda (14) has recalculated scale-up ratio and operating results, as shown in Table II. The feed rate of oil is roughly proportional to the cross-sectional area of the reactor. Thus a large superficial velocity was chosen for the small-scale reactors, and scale-up was conducted under a constant superficial velocity. 

Operation of a fluid bed depends on both height and diameter. Slugging properties have been studied extensively by Davidson and co-workers (H17, K8, K9, S21). Generally the ratio Lf/Dx is much larger than unity for small-scale reactors (C2). For solids particles in a teeter bed, this ratio will lie well into the slugging region. However, the fluidity in fluid bed is quite different from that in a teeter bed, as explained in Section II,A. 

The theories of dilution techniques have been explained in detail by various researchers. The selection of the proper size of diluent is very important. Equal volumes of diluent and catalyst were used for this comparison. For the case of an undiluted bed, only commercial size catalyst is packed in a small-scale reactor. The wall effect is very significant and in this case causes channeling of liquid. Because of the high void space inside the catalyst bed, the liquid holdup in the catalyst bed is also very low. As a whole, there is incomplete wetting of catalyst and only partial utilization is achieved in this case. Besides this, an appreciable amount of axial backmixing is present in the undiluted catalyst bed. When a larger size of diluent is used, it cannot enter the void space between the catalyst particles. Thus, it does not increase liquid holdup and hence only partial utilization of catalyst is also obtained. However, the addition of diluent increases the bed height, which in turn reduces liquid axial dispersion to some extent. When the diluent size is smaller, it can enter the narrow void space between the catalyst particles and can increase the liquid holdup... 

In essence, diluting the catalyst bed with appropriate size of inert particles increases liquid holdup, improves catalyst wetting, and reduces liquid backmix-ing. The selection of diluent size depends on several factors such as the length and inner diameter of the reactor, the size, shape and amount of catalyst, and the fiow rate of reactants. Therefore, there is a lot of research interest to determine the appropriate size of the diluent for various sizes of small-scale reactors to overcome their limitations. 

When reactions are fast relative to the mixing rate, not only are the apparent reaction rates affected but the whole time and temperature history of the reaction mechanism is also affected, yielding different selec-tivities and yields, depending on the intensity of the mixing. This often leads to a scale-up/scale-down problem, where yields of the desirable products in a plant-scale reactor are not as good as those in a small-scale reactor in the laboratory or the pilot plant. If the yield drops from the pilot-scale to the plant-scale reactor when all other important variables (temperature, pressure, and composition) have been held constant, then there is a mixing problem. Fast... 

Fixed-bed catalytic reactors are widely applied to reaction systems in which the reactants are present in a single vapor phase. The scale-up and performance of commercial reactors can be predicted from experiments in small-scale reactors. On the other hand, the mixed-phase trickle bed reactor is considerably more complex to analyze and scale up. The performance of trickle bed reactors is influenced by many factors associated with mixed-phase (gas-liquid-solid) processing. Some of... 

From a small-scale reactor (pilot plant) in which the composition, temperature, and pressure may change. Here calculations similar to. 

In flow systems such small-scale reactors are commonly called differential reactors, since the changes in temperature, pressure, and composition in the reactor are small. 

The conversion of methanol to olefins over ZSM-5 was discovered by Mobil scientists in the 1970 s, together with the similar process of conversion of methanol to gasoline. Initial process development was in small-scale reactors (ref. 15). 

The effects of the variation of temperature on enzymatic reactions usually do not require precautions, as in typical small scale reactors the capacity of heat exchangers is sufficient to maintain a constant temperature. 

Small-Scale Reactors for Catalyst Evaluation and Process Optimization... 

In a continuous process the substrate and hydrogen move continuously through the catalyst with the product removed at the same rate. There are two general types of continuous reactors those in which the reactants pass through a sliury of the catalyst particles and those in which the catalyst particles are packed into a fixed bed through which the reactant fluid (substrate and hydrogen) is passed (14). Here, too, small-scale reactors can provide a considerable amount of information about a continuous catalytic reaction in a relatively short time. This is particularly trae for catalyst activity and longevity studies as well as the effect which reaction parameters can have on product selectivity. 

Batch, recirculating batch, extractive semibatch, semicontinuous flow, continuously stirred tank (CSTR) and continuous packed bed reactors have alt been succesfully tested as enzyme reactors for SCFs (Figure 4.9-1). References to helpful descriptions for designing small-scale reactors for enzymatic studies are collected in Table 4.9-1. 

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