Supply phase

At first, a drop of surfactant aqueous solution was formed in a cell filled with pure hexane, such fliat a ratio Q + 10 existed between the volume of the drop (supplying phase) and that of the hexane (reeipient phase). The dynamic interfacial tension y(r) was monitored by a eomputer-enhanced pendant-drop teehnique. The evolution of y for some initial eoneentrations of aqueous solution is shown in Fig. 15A—C for C13DMPO, C12DMPO, and CiqDMPO, respeetively. 

Speed, HP, frequency, supply voltage, supply phases, and frame size of all the motors which are fitted confirm that they are as per specified on the machinery name plate. 

Further relevant parameters are the specific interfacial area a, now defined as the interfacial area divided by the volume of phase II, and the distribution (or partitioning) coefficient y, that may be defined as follows (the dash refers to the "supply" phase I). 

Again, the symbols with a dash refer to the supply phase (I) the other symbols refer to the reaction phase (II). It is assumed that reactant B is present in sufficient excess. 

Lx)wer the concentration of A in the supply phase I, and increase the interfacial area and the volume of the reaction phase (II). T is would raise the ratio... 

In case of chemically enhanced absorption or extraction, the mass transfer resistance of the supply phase usually cannot be neglected. One can then use the following equation that is more generally applicable tfian eqs. (5.20) and (5.39) ... 

When reactant A can be converted in the reaction phase to an unwanted byproduct via a competitive reaction, an excess of B throughout the liquid phase is desirable, so the situations depicted in figures 5.12a and b, should be avoided. If one should find that one of these situation prevails, one may attempt to increase the selectivity by lowering the A concentration in the supply phase (e.g., by applying a lower gas pressure), and increase the surface area proportionally. 

Another interesting selectivity problem arises when there are two different reactants in the supply phase, say A and C, that both react with the liquid phase reactant B, forming P and Q respectively, where the formation of Q is undesired. An example of practical importance is the selective absorption of hydrogen sulfide from an inert gas containing also carbon dioxide, in an alkaline solution (containing, e.g., alkanol amines). Conditions can be such that carbon dioxide (C) reacts rapidly with the alkanol amine, whereas hydrogen sulfide (A) reacts instantaneously. The consequence is that the absorption rate of hydrogen sulfide is practically determined by the gas phase mass transfer rate, and Ae rate of carbon... 

An interesting situation arises in processes where the reaction product P evaporates and is taken out of the reactor with the gas phase (the supply phase). Let us assume that there are no chemical reactions in the gas phase, e.g., l ause the liquid phase reaction is catalysed. We consider the case of rapid reactions, so that all the desired product P is formed in the diffusion layer in the liquid phase, close to the interface. When P can undergo undesired reactions in the liquid phase it is essential to remove P as effectively as we can, e.g., by creating a large surface area and very high gas-phase mass transfer coefficients. At the same time it is essential that the volume of the liquid phase is minimized, since decomposition of P will occur just there. The obvious choice would then be a configuration where the liquid is the dispersed phase, such as in a spray tower or a spray cyclone, provided the heat removal rate is sufficient. Another suitable arrangement could be a gas/liquid packed bed or a wetted wall column. The latter reactor type is very suitable for heat removal (section 4.6.3.1)... 

When two phases, each containing one reactant, are brought together in a reactor, the nature of the physical contact between the phases should be such, that a certain amount of at least one reactant can be transported to the other phase, per unit time. In most cases, there is chemical reaction in one phase only, the other one is the "supply" phase. This situation has been describe in some detail in Chapters 4 and 5. These descriptions were very clearly limited to volume elements of reactors. The ways in which the two phases flow through the reactor were not considered. 

Consider a stirred gas/liquid contactor, where die dispersed phase is the supply" phase and contains reactant A, that dissolves in the continuous phase and reacts there. The conversion in each gas bubble can be expressed, in analogy to a first order batch process, as a function of its individual residence time r, see eq, (3.15). Of course, the degree of conversion is defined (for constant density) as follows... 

This is a situation that is quite common in such processes as oxidations, hydrogenations, etc. The generd conclusion is that segregation of the dispersed "supply" phase is of no consequence for chemical reactions as long as their rates are first order with respect to the reactant in that phase (A). This is often realistic, particularly when the mass transfer rate of i4 is the rate determining factor in the effective rate constant K ... 

Reaction phase well mixed, supply phase in unidirectional flow. 

Let us first consider the case where the reaction phase moves in plug flow, while the supply phase is well mixed, such as in not too tall spray columns. 

Figure 7.9. Concentration profiles of reactant A in the supply phase, of reactant B and product P in the reaction phase, in cocurrent (a) and countercurrent (b) flow reactors (schematically).

The solution (see Appendix) gives the exit concentration of By so that the entrance concentration of A can be calculated. By iterative calculations both exit concentrations may be found. It turns out that the assumption of a finite exit concentration of A is realistic. The process is first order with respect to A, so the A-concentration in the supply phase approaches zero, but never becomes zero. However, the process in the reaction phase is of the order 0.5 with respect to reactant B, so the B>concentration will become zero after a finite length (see figure 7.9b). 

---