Plug-flow reactors parallel reactions

Figure 10. Evolution with time of the effective rate profile of the main reaction in a plug flow reactor. Parallel coking. Diffusion-limited process on a ZSM-5 type catalyst.

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... 

The series byproduct reaction requires a plug-flow reactor. Thus, for the mixed parallel and series system above, if... 

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... 

Adiabatic plug flow reactors operate under the condition that there is no heat input to the reactor (i.e., Q = 0). The heat released in the reaction is retained in the reaction mixture so that the temperature rise along the reactor parallels the extent of the conversion. Adiabatic operation is important in heterogeneous tubular reactors. 

Figure 8.13 Distribution of materials in a batch or plug flow reactor for the elementary series-parallel reactions...

In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of... 

Fig. 12.18. Comparison of the optimized reduced amounts that should be dosed and the corresponding internal compositions for a fixed-bed reactor (discrete dosing, top) and a membrane reactor (continuous dosing, bottom). A triangular network of parallel and series reactions was analyzed using an adapted plug-flow reactor model, Eq. 48. One stage (left) and 10 stages connected in series (right) were considered. All reaction orders were assumed to be 1, except for those with respect to the dosed component in the consecutive and parallel reactions (which were assumed to be 2) [66].

TABLE 7-2 Consecutive and Parallel First-Order Reactions in an Isothermal Constant-Volume Ideal Batch or Plug Flow Reactor. 

Solution This is an example of series-parallel reversible reactions. The reactor design formulation of diese chemical reactions was discussed in Example 4.1. Here, we complete die design for an isothermal plug-flow reactor and obtain the reaction and species curves. Recall that we select Reactions 1, 3, and 5 as a set of independent reactions. Hence, the indices of the independent reactions are m = 1, 3, and 5, the indices of the dependent reactions are = 2, 4, and 6, and we express the design equations in terms of Zi, Z3, and Z5. The stoichiometric coefficients of the three independent reactions are... 

The kinetic measurements were performed by monitoring the gas phase composition along the length of a fixed bed of catalyst. The reactor was treated as an isothermal plug flow system. The reaction kinetics can be described with a simple triangle network consisting of the main reaction (aldehyde to carboxylic acid), a consecutive reaction (carboxylic acid to byproducts) and a parallel reaction (aldehyde to by-products). 

In a plug flow reactor all fluid elements move along parallel streamlines with equal velocity. The plug flow is the only mechanism for mass transport and there is no mixing between fluid elements. The reaction therefore only leads to a concentration gradient in the axial flow direction. For steady-state conditions, for which the term IV is zero the continuity equation is a first-order, ordinary differential equation with the axial coordinate as variable. For non-steady-state conditions the continuity equation is a partial differential equation with axial coordinate and time as variables. Narrow and long tubular reactors closely satisfy the conditions for plug flow when the viscosity of the fluid is-low. 

Plug flow is a simplified and idealized picture of the motion of a fluid, whereby all the fluid elements move with a uniform velocity along parallel streamlines. This perfectly ordered flow is the only transport mechanism accounted for in the plug flow reactor model. Because of the uniformity of o>nditions in a cross section the steady-state continuity equation is a very simple ordinary differential equation. Indeed, the mass balance over a differential volume element for a reactant A involved in a single reaction may be written ... 

Now for kjCjio kj, or o, > 02, it seems reasonable to expect that the parallel reaction is more critical than the consecutive step in decreasing the yield of Q, and based on the above paragraphs the optimum choice would be a perfectly mixed reactor rather than a plug flow reactor—this will be verified by calculations. Also, for kjCjio < k2< or o, <02, the consecutive reaction should dominate, and the plug flow reactor should be best However, for a, 02, it is not so clear which is (he optimum reactor type. 

The plug-flow reactor is generally accepted as the most favorable for intermediate selectivity in series-parallel reactions. The results of this example clearly show that the membrane reactor can significantly outperform the PFR. 

In the case of known formal kinetics, the reactor performance can be determined directly from the RTD. We can imagine, for example, that the RTD in the reactor under consideration can be represented by a series of ideal plug flow reactors of different lengths arranged in parallel through which the reaction mass flows at equal rates (see Figure 3.17). 

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