Cooled Reactor with Hot Reaction

The impact of reaction kinetics on the steady-state design and dynamic controllability of the cooled reactor system is studied in this section. All the results presented in previous sections used a moderate activation energy (69,710 kJ/kmol) and a moderate specific reaction rate. Now the activation energy is doubled (E = 139,420 kJ/kmol). In addition, the reaction rate at 500 K is increased by a factor of 4 (the preexponential factor a = 1.46 x 107kmols 1 bar 2 kg-cat 1). These changes in reaction kinetics make the system highly nonlinear and very sensitive to changes in temperature. 

A comparison of this design with the moderate reaction design (Table 6.8) shows that the larger specific reaction rate produces a smaller reactor with less recycle, less heat transfer area, and lower TAC. 

TABLE 6.8 Steady-State Design Results for the Cooled Reactor System 

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Figure 6.26 Two designs for cooled reactor with hot reaction.

Table 6.10 gives the tuning parameters for the suboptimum design. The ZN peak temperature controller gain is 0.36 compared to 2.67 found for the cooled reactor with moderate kinetics studied in previous sections. The low controller gain of the peak temperature controller is required because of the high sensitivity of the reactor with the hot reaction. The lower gain of the pressure controller is due to the smaller gas volume of the hot reactor system. 

The reaction is exothermic, and multitubular reactors are employed with indirect cooling of the reactor via a heat transfer medium. A number of heat transfer media have been proposed to carry out the reactor cooling, such as hot oil circuits, water, sulfur, mercury, etc. However, the favored heat transfer medium is usually a molten heat transfer salt which is a eutectic mixture of sodium-potassium nitrate-nitrite. 

The reaction produces additional hydrogen for ammonia synthesis. The shift reactor effluent is cooled and tlie condensed water is separated. The gas is purified by removing carbon dioxide from the synthesis gas by absorption with hot carbonate, Selexol, or methyl ethyl amine (MEA). After purification, the remaining traces of carbon monoxide and carbon dioxide are removed in the methanation reactions. 

Transition metal oxides or their combinations with metal oxides from the lower row 5 a elements were found to be effective catalysts for the oxidation of propene to acrolein. Examples of commercially used catalysts are supported CuO (used in the Shell process) and Bi203/Mo03 (used in the Sohio process). In both processes, the reaction is carried out at temperature and pressure ranges of 300-360°C and 1-2 atmospheres. In the Sohio process, a mixture of propylene, air, and steam is introduced to the reactor. The hot effluent is quenched to cool the product mixture and to remove the gases. Acrylic acid, a by-product from the oxidation reaction, is separated in a stripping tower where the acrolein-acetaldehyde mixture enters as an overhead stream. Acrolein is then separated from acetaldehyde in a solvent extraction tower. Finally, acrolein is distilled and the solvent recycled. 

A temperature of 15°C is maintained in the nitrator by cooling and the reaction mixture enters the reactor below, which is equipped with a number of vertical pipes warmed externally with hot water. Here the reaction mixture is heated to 80°C and this temperature is maintained for 30 min while the reaction (42) involving ammonium nitrate takes place. The mixture is then introduced into the container below where the whole is cooled to 20°C. 

The high heat transfer rates achievable in micro heat exchangers and reactors avoid unfavorable reaction conditions resulting from hot spots or thermal runaway effects. An optimum temperature or temperature profile for the reaction can be chosen with respect to spatial distribution and time. Thus, a fast-flowing fluid element can be cooled down or heated up very rapidly, in fractions of a millisecond. Because of the small thermal mass of microdevices, a periodic change of temperature of the reactor can be realized, with a typical time constant of some seconds. All these examples offer possibilities to improve yield and selectivity. 

The MTG process utilised by Synfuel is based on a fixed-bed adiabatic reaction system. This reaction is highly exothermic and heat generated is removed by recycle gas which limits the temperature rise in the MTG reactors to 420°C at the reactor outlet. Hot reactor effluent is cooled with waste heat being used to preheat recycle gas and to vaporise methanol feed to the DME reactor. 

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