Control reactor-inlet stream fixing

Control Structures Fixing Reactor-Inlet Stream... 

Fig. 13.18 depicts reaction systems involving material recycles that are common in the industrial practice. The plant receives the feed Fq of concentration cq. The reactor effluent (F, C2) is firstly processed by a separation section, and only afterwards recycled. Fixed composition of product (4) and recycle (3) streams is achieved by local control of the separation units. Hence, the composition at reactor inlet is not directly dependent on the reactor effluent. The temperature at the reactor inlet has a constant value by an upstream heat exchanger. This excludes the energy feedback. Such plant will be designated here by Reactor-Separator-Recycle system. 

The oil feed is mixed with hydrogen-rich gas and then preheated to the proper reactor inlet temperature. The combined fe (oil+hydrogen) enters the top of the fixed bed reactor, called a trickle-bed reactor. Generally the catalyst bed is divided into several beds, which are separated by quenching zones where cold hydrogen-rich streams are injected in order to control the temperature inside the reactor. ITie catalytic beds are adiabatic and therefore temperature increases along each bed since the reactions are exothermic. 

The CFD model described above has been used by Liu and Fox (2006) to simulate the experiments of Johnson and Prud homme (2003a) in a confined impinging-jets reactor. In these experiments, two coaxial impinging jets with equal flow rates are used to introduce the two reactant-streams. The jet Reynolds number Re, determines the fluid dynamics in the reactor. Typical CFD results are shown in Fig. 6 9 for a jet Reynolds number of Re, = 400 and a reaction time of tr — 4.8 msec. The latter is controlled by fixing the inlet concentrations of the reactants. Further, details on the reactor geometry and the CFD model can be found in Liu and Fox (2006). 

Table 10.7 presents the stream table around the chemical reactor. As mentioned above, the inlet mixture is fixed because of control requirements. In this way, the outlet reflects the transformation of composition due to the chemical reactor. The molar conversions for ethylene and acetic acid are in agreement with industrial data. The amount of C02 formed by reaction is quite limited, denoting good catalyst selectivity only about 11% from ethylene is consumed by combustion, the rest, 89%, going into vinyl acetate. Finally, the computation gives a catalyst productivity of 326.6kgVAM/m3-catalyst h. 

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