Rates, chemical reactions plug flow reactor

Figure 6-3.2 Yield of acetaldehyde as a function of ethanol conversion. Data were obtained at 518 K. Data points (in order of increasing ethano conversion) were obtained at space velocities of 26,000,52,000,104,000, and 208,000 h"h The curves were calculated for a first-order series reaction in a plug-flow reactor and show yield of the intermediate species B as a function of the conversion of reactant for various ratios of rate constants and. [Reprinted with permission fiom Ind. Eng. Chan. Prod. Res. Dev., 22, 212 (1983). Copyright 1983 American Chemical Society.]...

Consider a plug flow reactor - a cylindrical tube with reactants flowing in one end and reacting as they flow to the outlet. A mole balance is made here for the case when the operation is steady. Let F be the molar flow rate of a chemical, which changes dF in a small part of the tube take the volume of that small part to be d V. The rate of reaction is expressed as the moles produced per unit of time per unit of volume. If a chemical specie reacts, then the r for that specie is negative. Thus, the overall equation for a smaU section of the tube is... 

To express the rates of the chemical reactions in terms of the reaction extents, we have to relate the species concentrations to Z s and t. For plug-flow reactors, flie concentration of species j at a given point in the reactor is... 

Below, we analyze the operation of isothermal plug-flow reactors with single reactions for different types of chemical reactions. For convenience, we divide the analysis into two sections (i) design and (ii) determination of the rate expression. In the former, we determine the size of the reactor for a known reaction rate, specified feed rate, and specified extent (or conversion). In the second section, we determine the rate expression and its parameters from reactor operating data. 

Coal Residence Time. Because the oxidation rate is fast, there is essentially no residence time required for chemical reaction. In a plug flow reactor, coal feed rate would depend only on the oxygen input rate and the unit oxygen consumption. Moreover, if gas distribution is uniform, the oxidation would be uniform. In a continuous fluidized-bed reactor, however, residence time must be long enough to minimize the effect of the short-circuiting of untreated feed into the product. The... 

The solution strategy described above is based on writing a differential plug-flow reactor mass balance for each component in the mixture, and five coupled ODEs are solved directly for the five molar flow rates. The solution strategy described below is based on the extent of reaction for independent chemical reactions, and three coupled ODEs are solved for the three extents of reaction. Molar flow rates are calculated from the extents of reaction. The starting point is the same as before. The mass balance is written for component i based on molar flow rate and differential reactor volume in the presence of multiple chemical reactions ... 

This assumes that all chemical species have the same residence time distribution, and is very convenient to compute the reaction paths for different contacting patterns. Matsuyama and Miyauchi [16] have also considered some aspects of this. An important conclusion of Wei [15] is that for a reactor with distribution of residence times, all reactions are slowed down in comparison with those in a plug flow reactor, but the faster reactions are slowed down a great deal more than the slower ones. Consequently, the occurrence of distribution of residence times makes all reaction rates of the characteristic species nearly equal. That is, the differences between the various reaction rates are decreased, thereby decreasing the selectivity. This is similar to the diffusion effects considered in Chapter 3. 

To illustrate the Euler and trapezoidal methods, we apply them to the following problem of chemical decomposition of nitrogen dioxide in a plug flow reactor. The chemical reaction rate is second order with respect to the nitrogen dioxide concentration that is. 

The ideal reactors mostly studied in chemical engineering are the batch or semi-batch reactor, the plug flow reactor (PFR) and the continuous flow stirred tank reactor (CSTR). Abatch reactor has no input or output of mass, and the stirring of the reactor avoids temperature or concentration gradient in the reaction medium. The reaaion rate is uniform and can be considered everywhere equal to the average value. 

The parameter p measures an activating influence of intermediate A, on the rate consttuit for the first step. This law has been followed by diverse chemical reactions [27]. Some biological reactions catalyzed by allosteric enzymes also give such reaction scheme [12]. The relevant differential equations are written for the isothermal plug flow reactor for carrying out the reaction given by Equation 1. 

In all three models the rate of mixing of the reactants is assumed not to influence the rate of the chemical reactions. In the ideal batch reactor, the reactants are mixed before they react in the plug flow reactor the reactants are mixed immediately with each other, and in the perfectly mixed CSTR the entering reactants are mixed immediately with the reactor contents. In addition we present two other reactor models that approach reactor types that are frequently used in practice ... 

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