Differential continuous contacting

The second type of mass-exchange units is the differential (or continuous) contactor. In this category, the two phases flow through the exchanger in continuous contact throughout without intermediate phase separation and recontacting. Examples of differential contactors include packed columns (Fig. 2.6), spray towers (Fig. 2.7), and mechanically agitated units (Fig. 2.8). 

For continuous contact equipment the material and energy balances are written around a section of column of differential height as shown in Figure 12.8. For the vapor phase the component material balance reads... 

The continuous contact between phases provided by packed columns requires a differential treatment for a horizontal slice of the colutiM. followed by iniegnaion over the column height. 

Figure 4 is a schematic representation of a Perkin-Elmer DSC 2 differential scanning calorimeter, whose principles are given by Gray [17]. The calorimeter head is constituted of two identical plates in continuous contact with a cold source and heaters that supply the required energy to impose the programmed temperature Tp to the plates. The heaters run independently, and an electronic system compensates for temperatures between the plates. 

For packed towers, the continuous differential nature of the contact between gas and liqiiid leads to a design procedure involving the solution of differential equations, as described in the next subsection. 

Liquid-liquid contacting equipment may be generally classified into two categories stagewise and continuous (differential) contact. 

Various experimental methods to evaluate the kinetics of flow processes existed even in the last centuty. They developed gradually with the expansion of the petrochemical industry. In the 1940s, conversion versus residence time measurement in tubular reactors was the basic tool for rate evaluations. In the 1950s, differential reactor experiments became popular. Only in the 1960s did the use of Continuous-flow Stirred Tank Reactors (CSTRs) start to spread for kinetic studies. A large variety of CSTRs was used to study heterogeneous (contact) catalytic reactions. These included spinning basket CSTRs as well as many kinds of fixed bed reactors with external or internal recycle pumps (Jankowski 1978, Berty 1984.)... 

Mass transfer processes involving two fluid streams are frequently carried out in a column countercurrent flow is usually employed although co-current flow may be advantageous in some circumstances. There are two principal ways in which the two streams may be brought into contact in a continuous process so as to permit mass transfer to take place between them, and these are termed stagewise processes and continuous differential contact processes. 

The main process variables in differential contacting devices vary continuously with respect to distance. Dynamic simulations therefore involve variations with respect to both time and position. Thus two independent variables, time and position, are now involved. Although the basic principles remain the same, the mathematical formulation, for the dynamic system, now results in the form of partial differential equations. As most digital simulation languages permit the use of only one independent variable, the second independent variable, either time or distance is normally eliminated by the use of a finite-differencing procedure. In this chapter, the approach is based very largely on that of Franks (1967), and the distance coordinate is treated by finite differencing. 

Differential extractors, in which the phases are continuously in contact in the extractor and are only separated at the exits for example, in packed column extractors. 

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