Reactor flow models

This involves knowledge of chemistry, by the factors distinguishing the micro-kinetics of chemical reactions and macro-kinetics used to describe the physical transport phenomena. The complexity of the chemical system and insufficient knowledge of the details requires that reactions are lumped, and kinetics expressed with the aid of empirical rate constants. Physical effects in chemical reactors are difficult to eliminate from the chemical rate processes. Non-uniformities in the velocity, and temperature profiles, with interphase, intraparticle heat, and mass transfer tend to distort the kinetic data. These make the analyses and scale-up of a reactor more difficult. Reaction rate data obtained from laboratory studies without a proper account of the physical effects can produce erroneous rate expressions. Here, chemical reactor flow models using matliematical expressions show how physical... 

The physical model of the reactor is a 350 mm high cylindrical vessel, with a diameter of 200 mm and an elliptical bottom. The operation volume is V = 12 10 m. The entrance of the reactants is placed near the middle of the reactor, more exactly at 130 mm from the bottom. The reactor s exit is positioned on the top of the vessel but below the liquid level. At the vessel centre is placed a mixer with three helicoidal paddles with d/D = 0.33. It operates with a rotation speed of 150 mirnf In order to establish the reactor flow model, this is filled with pure water which continuously flushes through the reactor at a flow rate of 6.6 10 5 m /s (similar to the reactants flow rate). At time t = 0, a unitary impulse of an NaCl solution with a Cq = 3.6 kg/m is introduced into the reactor input. The time evolution of the NaCl concentration at the exit flow of the reactor is measured by the conductivity. Table 3.5 gives the data that show the evolution of this concentration at the reactor exit. 

However, the molecular reaction did not consider the temperature variation, as well as the internal and external diffusion during the reaction. Therefore, due to the integral accumulation effect along the catalyst bed, the modified ASF distribution has appeared, even though the reactor is strict isothermal and the internal and external diffusion of catalyst can be ignored. Therefore, if we want to control the product distribution of FTS, we must clarify the detailed dynamic information on the particle-reactor level, which needs to couple the catalyst particle model and the reactor flow model. 

To go from volume element models to reactor models the macro flow patterns in the reactor need to be considered. For stirred tank reactors this can be quite simple, in those cases where volume elements in the stirred tank can be described in terms of average conditions. This is not so when macro mixing or residence time distribution are scale dependent, see Chapter 7. When the reactor is tubular, with two countercurrent or parallel flows, the volume element models have to be combined with reactor flow models, including axial mixing. Also this is treated in Chapter 7, for various cases. 

The use of the macroscopic mechanical energy balance for radial reactor flow modelling has been criticized by Li [4], partly due to some assumptions made in deriving the equations, but mainly because of the assumptions regarding the friction factor. Li suggested that the use of a macroscopic momentum balance was more appropriate, involving a wall force friction factor rather than the energy... 

It was suspected that fluid bypassing was occurring in the plant reactors which would necessitate operation at higher temperatures to effect oxygen siass transfer and reaction. Residence time distribution studies using idealized reactor flow models strongly confirmed this suspicion. 

E+ square average error of prediction reactor flow model... 

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