Nonsteady state methods

The various methods for the measurement of t/, the actual lifetime, can be classified as steady state and nonsteady state methods. In the steady state methods are included lifetime determinations from fluorescence quenching data and depolarization studies, the theoretical aspect of which have already been discussed in Sections 6.4 and 4.10, respectively. 

Nonsteady State Methods for Determination of Fluorescent Lifetimes... 

The nonsteady state methods may be conveniently divided into two categories (A) pulse methods and (B) phase-shift methods. 

Which experimental method should be used depends on the type of reactor and how it will be operated, and if clean or process water is to be used for the measurement. Nonsteady state methods are generally simpler and faster to perform if kLa is to be determined in clean water without reaction. For processes that are operated at steady state with a reaction, determination of kLa using steady state methods are preferred, since continuous-flow processes need not be interrupted and operating conditions similar to the normal process conditions can be used. This is especially important for systems with reactions because the reaction rate is usually dependent on the concentration of the reactants present. They are thus often applied for investigations of the mass transfer coefficient under real process conditions with chemical reactions kLa(02) or biological activity kLa(02), e. g. in waste water treatment systems. 

Nonsteady State Methods without Mass Transfer Enhancement... 

The nonsteady state methods described in this section are all based on no or negligible reactions taking place in the system. If reactions are present, the treatment of the mass balances becomes more complicated since... 

Common Problems Inherent to the Determination of Mass Transfer Coefficients with the Nonsteady State Method... 

A variety of problems encountered with the measurement of oxygen mass transfer coefficients by using the nonsteady state method are well known and well understood. Libra (1993) gives a comprehensive discussion based on oxygen mass transfer measurements. The most important problems are found in the following table. 

Table 3-4 Problems inherent to the determination of mass transfer coefficients with the nonsteady state method.

The determination of kLa from an instantaneous reaction is rather complex and the experimental procedure complicated, requiring an extensive knowledge of the theoretical background. Since it is not within the scope of this book to go into the necessary details, the basic experimental procedure is summarized in Figure 3-6, and only a few remarks shall be made here. For complete information the reader is referred to the original literature in which the nonsteady state method has been applied in ozonation experiments (e. g. Beltran and Gonzalez, 1991 Beltran et al., 1992 a Beltran and Alvarez, 1996) and the basics (e. g. Levenspiel and Godfrey, 1974 Charpentier, 1981),... 

Table 5.7 gives an overview of the most commonly used electrochemical nonsteady-state methods (in parentheses alternative names). Typically, one imposes a variation in either the current or the potential at the working electrode and then records its response as a function of time or frequency. 

The temperature modulation technique is advantageous for observing the complex physical properties in the relaxation region. The temperature wave analysis (TWA) method is a nonsteady-state method for measuring the thermal diffusivity of materials. 

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