Reactor-size increase

Notice that the temperature difference between the cooling jacket and the reactor must be increased as the size of the reactor increases. The flow rate of cooling water also increases rapidly as reactor size increases. 

Figure 6 shows the coolant flow rate and the feed rate curves vs. the reactor volume. Note that the flow rate of cooling increases rapidly as the reactor size increases, whereas the inlet flow rate increases according to Eq.(39). 

These problems are easy Note that the reactor residence time (proportional to reactor size) increases markedly as the required conversion increases. Note also that for this example (first-order kinetics) we did not need even to specify Cao because the equation involves only the ratio Cao/Ca-... 

This proportionality means that the relative importance of diffusion in mass transfer decreases as the reactor size increases. 

As the design conversion x increases, reactor size increases, which provides more heat transfer area. Although the heat transfer rate increases, the jacket temperature is higher because the larger area requires a smaller AT driving force. 

Figure 2.23a shows how conversion is affected by reactor size and the temperature of the second reactor. The temperature in the first reactor is set at 350 K (the temperature that maximizes conversion in a single 100-m3 CSTR). There is a second-reactor temperature that maximizes conversion for a given reactor size. The maximum conversion increases as reactor size increases, and the temperature at which it occurs shifts to lower values. For example, two 200-m3 reactors can achieve a 72.4% conversion if the second reactor is operated at 328 K. Two 100-m3 reactors can achieve a 68.7% conversion if the... 

The heat transfer coefficient UA for ambient temperature loss is a function of how well insulated the reactor is. Jacket heat transfer coefficient, UAj, is a function of the reactor geometry, agitator blade design and RPMs, the viscosity of the medium, and relative amount of fouling in the reactor. The viscosity of the reaction medium as discussed before is dependent on solids level, M, and of the polymer as well as reactor temperature. In general, removing heat from the CSTR becomes more difficult as the reactor size increases due to the surface to volume ratios. 

Our interest in chemical reaction kinetics arises from our need to use large-sized batch reactors to produce enough product to satisfy market demand. There is another reason for using large batch reactors as reactor size increases, manufacturing cost decreases to a minimum. Management enjoys the sound of that proposition therefore, they demand that product be produced in as large a batch reactor as possible. 

Reduction of channel diameter it increases the specific interfacial area a, which is inversely proportional to the reactor diameter d/,. However, further reduction in reactor size increases the risk of clogging and results in higher specific energy consumption because of higher pressure drop to overcome during pumping. 

Figure 4.3.4 shows the influence of Da on the conversion for a zero-, first-, and second-order reaction. Note that for a reaction order n 7 1 [Eq. (4.3.19)], Da depends on the initial concentration. Only for a first-order reaction are Da and thus the conversion independent of concentration [Eq. (4.3.13)]. The reason for this is that for n = 1 the rate increases not only proportionally to the concentration but also the amount of reactant A that has to be converted. These effects nullify each other. Figure 4.3.4 also indicates that the curvature flattens for an increasing order, that is, the expenditure (reaction time or reactor size) increases if a high conversion is needed. 

Heat removal is also an important issue to consider during process scale-up. As reactor size increases, ffie system dynamics become increasingly slow, and therefore it takes longer for desired changes (for example decreasing the reactor temperature) to occur. This may in itself not be a serious problem, provided the reactor... 

If k-2 increases faster than kx, operate at low temperature (but beware of capital cost, since low temperature, although increasing selectivity, also increases reactor size). Here there is an economic tradeoff between decreasing byproduct formation and increasing capital cost. 

Reaction and Transport Interactions. The importance of the various design and operating variables largely depends on relative rates of reaction and transport of reactants to the reaction sites. If transport rates to and from reaction sites are substantially greater than the specific reaction rate at meso-scale reactant concentrations, the overall reaction rate is uncoupled from the transport rates and increasing reactor size has no effect on the apparent reaction rate, the macro-scale reaction rate. When these rates are comparable, they are coupled, that is they affect each other. In these situations, increasing reactor size alters mass- and heat-transport rates and changes the apparent reaction rate. Conversions are underestimated in small reactors and selectivity is affected. Selectivity does not exhibit such consistent impacts and any effects of size on selectivity must be deterrnined experimentally. 

Approach to Equilibrium As equilibrium is approached the rate of reaction falls off, and the reactor size requireci to achieve a specified conversion goes up. At some point, the cost of increased reactor size will outweigh the cost of discarded or recycled unconverted material. No simple rule for an economic appraisal is really possible, but sometimes a basis of 95 percent of equilibrium conver-... 

A salient feature of the fluidized bed reactor is that it operates at nearly constant temperature and is, therefore, easy to control. Also, there is no opportunity for hot spots (a condition where a small increase in the wall temperature causes the temperature in a certain region of the reactor to increase rapidly, resulting in uncontrollable reactions) to develop as in the case of the fixed bed reactor. However, the fluidized bed is not as flexible as the fixed bed in adding or removing heat. The loss of catalyst due to carryover with the gas stream from the reactor and regenerator may cause problems. In this case, particle attrition reduces their size to such an extent where they are no longer fluidized, but instead flow with the gas stream. If this occurs, cyclone separators placed in the effluent lines from the reactor and the regenerator can recover the fine particles. These cyclones remove the majority of the entrained equilibrium size catalyst particles and smaller fines. The catalyst fines are attrition products caused by... 

Tubular reactors often offer the greatest potential for inventory reduction. They are usually simple, have no moving parts, and a minimum number of joints and connections that can leak. Mass transfer is often the rate-limiting step in gas-liquid reactions. Novel reactor designs that increase mass transfer can reduce reactor size and may also improve process yields. 

Heat transfer problems become more severe as reaction rates are increased and water-to-monomer ratios are reduced. In addition, as reactor sizes are increased for improved process economics, the amount of wall heat transfer surface area per unit volume will drop and result in a lower reactor space-time yield. 

Section 5.3 discusses a variety of techniques for avoiding scaleup problems. The above paragraphs describe the simplest of these techniques. Mixing, mass transfer, and heat transfer aU become more difficult as size increases. To avoid limitations, avoid these steps. Use premixed feed with enough inerts so that the reaction stays single phase and the reactor can be operated adiabatically. This simplistic approach is occasionally possible and even economical. 

With increasing reactor size, the withdrawal of the heat evolved during the reaction becomes more difficult. This gives rise to a nonuniform temperature distribution within the reactor and thus a differentiation of conditions for the efectrochemical reaction in different reactor parts. In a number of cases the thermaf conditions can be improved through coofed efectrodes. 

As illustrated by the examples above, the possibility of removing the generated heat from the reaction zone decreases with an increase in reactor size. As proven above, it can happen that the temperature of the reaction mixture in a full-scale reactor becomes higher than in the laboratory flask reactor. If multiple chemical reactions of distinctly different temperature sensitivities take place, differences in yields and selectivities between small and large reactors will be observed. This has a large influence on safety also. The laboratory reactor might still show satisfactory performance, while the industrial reactor might even explode. 

The blend time increases significantly with reactor size. To keep the average energy dissipation constant one has to decrease the speed of rotation. However, as the stirrer diameter increases, the tip speed becomes greater despite the lower rotation speed. In this situation energy dissipated in the vicinity of the stirrer increases with reactor size. 

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