Characteristic reaction time Determination

The reaction system, the experiment procedure, and the analytical method used for the determination of micromixing in the TIJ mixer are the same as those described in the last section of this book but Mahajan et al. correlated their experimental data not with impinging velocity w() but with the jet Reynolds number Re. Also, the researchers employed the measure of increasing both the initial concentration CBo and the reaction temperature to raise the sensitivity of the procedure. The characteristic reaction time constant tK = 200 ms at 25 °C and CBo = 2.5 mM, while rR = 65 ms at 35 °C and CB0 = 4.7 mM, which can be used to bound the micromixing times, rM, no greater than them, respectively. 

The four examples below illustrate how the characteristic reaction time is determined. 

An aqueous solution with a concentration of 2 mol/L of A is charged into a batch reactor. For the enzyme type and concentration used, = 0.1 mol/L min and Km = mol/L. Determine the characteristic reaction time. 

To determine the characteristic reaction time, first the reference concentration, Co, is calculated. Selecting the inlet stream as the reference stream, for gas-phase reactions, assuming ideal-gas behavior... 

Definition and determination of the characteristic reaction time as a measure... 

To solve the energy balance equation (Eq. 6.1.17), we have to specify flie value of HTN. However, its value depends on the selection of die reference state, Q, and the selection of the characteristic reaction time, ter- Also note that HTN is proportional to the heat-transfer coefficient, U, which depends on the flow conditions, the properties of the fluid, and the heat-transfer area per unit volume (S/V). These quantities are not known a priori. Therefore, we develop a procedure to estimate the range of HTN. For isothermal operation (dQ/di = 0), and we can determine the HTN at any instance from Eq. 6.1.17 (taking the operating temperature as the reference temperature, 9 = 1) ... 

The main difficulty in determining the reaction rate r is that the extent is not a measurable quantity. Therefore, we have to derive a relationship between the reaction rate and the appropriate measurable quantity. We do so by using the design equation and stoichiometric relations. Also, since the characteristic reaction time is not known a priori, we write the design equation in terms of operating time rather than dimensionless time. Assume that we measure the concentration of species j, Cj(t), as a function of time in an isothermal, constant-volume batch reactor. To derive a relation between the reaction rate, r, and Cj(t), we divide both sides of Eq. 6.2.4, by obtain... 

Select a leading (or desirable) reaction and determine the expression of the characteristic reaction time, and its numerical value. 

In the present chapter we focus our discussion on external mass transfer in MSR. Thus, we assume that the reactions occur on the outer surface of the catalyst particle or of the wall catalytic layer. The observable effective reaction rate is determined by the ratio of the characteristic mass transfer time, t, and the characteristic reaction time, commonly known as second Damkohler number Dull (see Section 2.6.1). 

The effective reaction rate is determined by the ratio of the characteristic mass transfer time, to, and the characteristic reaction time, the second Damkohler number ... 

How fast do chemical reactions occur The speed of a reaction is described by its rate. Rate is the number of events per unit time, such as the number of molecules reacting per second. Every elementary reaction has a characteristic rate. Some reactions are so fast that they are complete in the smallest measurable traction of a second, whereas others are so slow that they require almost an eternity to reach completion. The observed rate of an overall chemical reaction is determined by the rates of the elementary reactions that make up the mechanism. 

The value of this ratio is characteristic of the reaction order. Table 3.1 contains a tabulation of partial reaction times for various rate expressions of the form r = kCAn as well as a tabulation of some useful ratios of reaction times. By using ratios of the partial reaction times based on experimental data, one is able to obtain a quick estimate of the reaction order with minimum effort. Once this estimate is in hand one may proceed to use a more exact method of determining the reaction rate parameters. 

Dimerization is the characteristic reaction of radical-anions from activated alkenes. The rate constants for dimerization are high and the conjugate acids from such alkene radical-anions in many cases have low pKa values and. The data in Table 3.4 were obtained by following the changes in uv-absorbance after pule-radiolysis of the substrate in an aqueous buffer. Attachment of a solvated electron leads to the radical-anion. Changes in the initial absorbance with pH lead to determination of the pKg value, while the dimerization rate can be determined from changes in absorbance over a longer time scale. Radical-anions from esters and amides are pro-... 

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