Continuous plug-flow reaction

At the point where the MIP is at a minimum, injection of ammonia gas results in a steep increase in the amount of high-molecular-weight materials with a concurrent rise in pH value. Both these processes are highly desirable. Ammonia is added until the desired pH value is obtained ( 3.5 to 4.0). At this point, the resin preparation is a viscous emulsion of dark brown color. The advantage of this method of resin preparation is its adaptability to continuous plug-flow reaction, unlike the Phase I resin, which is more suited to batch reaction. 

Example 2-7 The reaction A B with k =0.01 sec takes place in a continuous-plug-flow tubular reactor What residence time in the tube is required for 90% conversion ... 

Biochemical reactors can be operated either batchwise or continuously, as noted in Section 1.5. Figure 7.1 shows, in schematic form, four modes of operation with two types of reactors for chemical and/or biochemical reactions in Uquid phases, with or without suspended solid particles, such as catalyst particles or microbial cells. The modes of operation include stirred batch stirred semi-batch continuous stirred and continuous plug flow reactors (PFRs). In the first three types, the contents of the tanks arc completely stirred and uniform in composition. 

The application of the equations to chemical reactions requires the proper definition of the above quantities as well as correctly defining the transition probabilities pjj and pjk this is established in the following. It should also be noted that the models derived below for numerous chemical reactions, are applicable to chemical reaction occurring in a perfectly-mixed batch reactor or in a single continuous plug-flow reactor. Other flow systems accompanied with a chemical reaction will be considered in next chapters. 

Deflnitions. The basic elements of Markov chains associated with Eq.(2-24) are the system, the state space, the initial state vector and the one-step transition probability matrix. Considering refs.[26-30], each of the elements will be defined in the following with special emphasize to chemical reactions occurring in a batch perfectly-mixed reactor or in a single continuous plug-flow reactor. In the latter case, which may simulated by perfectly-mixed reactors in series, all species reside in the reactor the same time. 

One of the ways of influencing the chemical reaction behaviour in the diffusion region (chemical reaction rate, process selectivity, product characteristics, temperature mode, and so on) is the intensification of turbulent mixing in the chemical vessel. The most effective solution to this problem is the use of continuous plug flow tubular... 

Continuous plug flow reactors are also unsuitable for these purposes because it is usually impossible to obtain an isothermic mode in such reactors, even for reactions with a relatively low rate of reaction. Plug flow reactors usually operate in adiabatic or intermediate modes, which are far from isothermic even with an external heat removal modification. In can be stated that almost all industrial reactors employed for fast processes are not optimally designed and are therefore ineffective. The quality of products is also far from optimal and the processes are generally not perfect from an engineering, economical, or social point of view (decrease of final product yield and quality, increase of nonrecyclable wastes, excessively high consumption of raw materials and low energy efficiency). 

Now let us analyze the same reaction, A B, in a continuous plug-flow reactor at optimum conditions, which are certainly not equilibrium conditions. The overall reaction rate is given by... 

In a continuous plug flow reactor we can use the same equation, if we use the residence time instead of the reaction time. 

The modeling methodology is shown in Figure 6.17.7 for the example of a discontinuous slurry reactor. First, the concentration profiles within the catalyst particles are calculated. This information is then coupled (for each time step) with the change of concentrations in the bulk phase (Cj t)- The link between both procedures, that is, between the bulk phase and the porous catalyst particles, is the concentration gradient of each reactant at the external particle surface. Note that this calculation is also applicable for a continuous plug flow reactor simply by using the residence time t (= x/u) instead of the reaction time, whereby x represents the axial coordinate x in a tubular reactor and u the fluid velocity. 

In a continuous plug-flow reactor, the reaction mixture flows through a pipe without hack mixing. This type of reactor is dominated by dead time. The residence time of the reactants traveling through a volume V at a flow F is y/F. Thus the concentration at the exit of the vessel is... 

Solution, l ree possibilities are sketched in Fig. 12.4. With a semibatch reactor, the more reactive monomer is replenished as the reaction proceeds to maintain/i (and therefore FJ constant. A method for calculating the appropriate rate of addition has been described. In a continuous stirred tank (backmix) reactor, both fi and Fx are constant with time. In a continuous plug-flow reactor, the variation in Fx can be kept small by limiting the conversion per pass in the reactor. Note that the last two techniques require facilities for separating unreacted monomer from the polymer, and in most cases, recycling it. 

Multiple reactions in parallel producing byproducts. Consider again the system of parallel reactions from Eqs. (2.16) and (2.17). A batch or plug-flow reactor maintains higher average concentrations of feed (Cfeed) than a continuous well-mixed reactor, in which the incoming feed is instantly diluted by the PRODUCT and... 

In general terms, if the reaction to the desired product has a higher order than the byproduct reaction, use a batch or plug-flow reactor. If the reaction to the desired product has a lower order than the byproduct reaction, use a continuous well-mixed reactor. 

But what is the correct choice a byproduct reaction calls for a continuous well-mixed reactor. On the other hand, the byproduct series reaction calls for a plug-flow reactor. It would seem that, given this situation, some level of mixing between a plug-flow and a continuous well-mixed reactor will give the best... 

Polymerization reactions. Polymers are characterized by the distribution of molecular w eight about the mean as well as by the mean itself. The breadth of this distribution depends on whether a batch or plug-flow reactor is used on the one hand or a continuous well-mixed reactor on the other. The breadth has an important influence on the mechanical and other properties of the polymer, and this is an important factor in the choice of reactor. 

Solution We wish to avoid as much as possible the production of di- and triethanolamine, which are formed by series reactions with respect to monoethanolamine. In a continuous well-mixed reactor, part of the monoethanolamine formed in the primary reaction could stay for extended periods, thus increasing its chances of being converted to di- and triethanolamine. The ideal batch or plug-flow arrangement is preferred, to carefully control the residence time in the reactor. 

Reactor types modeled A, stoichiometric conversion B, equiUbrium/free-energy minimization, continuous stirred tank, and plug flow C, reactive distillation. Some vendors have special models for special reactions also, private company simulators usually have reactors of specific interest to their company. 

Much of the basic theory of reaction kinetics presented in Sec. 7 of this Handbook deals with homogeneous reaclions in batch and continuous equipment, and that material will not be repeated here. Material and energy balances and sizing procedures are developed for batch operations in ideal stirred tanks—during startup, continuation, and shutdown—and for continuous operation in ideal stirred tank batteries and plug flow tubulars and towers. 

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