Heterogeneous-homogeneous reactions residence time

Vocabulary of Terms Used in Reactor Design. There are several terms that will be used extensively throughout the remainder of this text that deserve definition or comment. The concepts involved include steady-state and transient operation, heterogeneous and homogeneous reaction systems, adiabatic and isothermal operation, mean residence time, contacting and holding time, and space time and space velocity. Each of these concepts will be discussed in turn. 

Heterogeneous catalysis is preferred over homogeneous catalysis. A critical issue is the catalyst design, which should ensure compatibility between the reaction and the separation. The temperature profile dominated by the vapor-liquid equilibrium, as well as the residence-time distribution controlled by the hydrodynamics of internals must comply with the achievable reaction rate and with the desired selectivity pattern. 

Residence-time distributions for the gas and liquid phases in this type of reactor can be evaluated easily. The reactor is operated under transient conditions if the catalyst decays rapidly. Otherwise, steady-state operation is obtained. Baffles can be installed to obtain better contact. When both homogeneous and heterogeneous reactions occur simultaneously, their rates can be separated by obtaining the results at various stirrer speeds. This type of reactor has several dis-... 

Up to now it was assumed that reaction and distillation can favourably be combined in a column - in a normal distillation coluitui in the case of homogeneous catalysis, and in a column with special internals or an additional exterior volume in the case of heterogeneous catalysis. This was discussed in the previous chapter under the aspect of scale-up in connection with separation and reaction performance. However, columns are an appropriate solution only for reactions that are so fast as to achieve considerable conversions in the residence time range of such columns. The question is whether the full potential for comhining reaction and distillation can be found and industrially implemented using columns only. 

Let us now assume that the residence time of a system is equivalent to the period of time that the system behaves as a closed system thermodynamically. With this assumption it is useful to qualitatively compare the residence times of different aqueous systems in the hydrosphere to the halftimes of some example reactions and reaction types. This has been done schematically in Fig. 2.2. In essence, as we examine the diagram, we can assume reactions are at equilibrium in waters whose residence times significantly exceed the half-times of reactions of interest. Note that the half-times of some solute-solute and solute-water reactions (these include some complexation and acid-base reactions [see Chaps. 3 and 5]) are shorter than the residence times of raindrops and so can be assumed to be at equilibrium in rain. These are homogeneous reactions. However, the other types of reactions shown, including atmospheric gas exchange, which is heterogeneous, are too slow to have... 

In contrast to detection systems that are based on physicochemical properties e.g. UV-visible absorption, fluorescence) and typically involve homogeneous solutions, it is clear that ED must be a heterogeneous process since interfaces are involved. Furthermore, electrochemical detectors (EDs) are reaction detectors and therefore the responses may be influenced not only by the amount(s) of electroactive analyte(s) present, but also by factors such as temperature and residence time. 

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