Instantaneous reaction liquid-film control

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

Note that the enhancement factor E is relevant only for reaction occurring in the liquid film. For an instantaneous reaction, the expressions may or may not involve E, except that for liquid-film control, it is convenient, and for gas-film control, its use is not practicable (see problem 9-12(a)). The Hatta number Ha, on the other hand, is not relevant for the extremes of slow reaction (occurring in bulk liquid only) and instantaneous reaction. The two quantities are both involved in rate expressions for fast reactions (occurring in the liquid film only). 

Consider a packed tower for the absorption of A from a gas containing inert material (in addition to A) by a liquid containing B (nonvolatile) under continuous, steady-state conditions. Absorption is accompanied by the instantaneous reaction A(g) + bB(() - products. Assume the overall process is liquid-film controlled. 

Figure 2-4 Typical concentration profiles of instantaneous reaction between the gas A and the reactant C, based on film theory, ids Diffusion controlled - slow reaction, (fcl kinetically controlled-slow reaction, (c) gas-film-controlled desorption - fast reaction, 0 liquid-film-controlled desorption-fast reaction, (e) liquid-film-controlled absorption -instantaneous reaction between A and C, (/) gas-film-controlled absorption-instantaneous reaction between A and C, (g) concentration profiles for A, B, and C for instantaneous reaction between A and C-both gas- and liquid-phase resistances are comparable.1 2...

We now discuss some of the main features of LLPTC models developed for reaction under neutral conditions. Evans and Palmer (1981) were among the first to consider the effect of diffusion and mass transfer inPTC. They considered PTC in liquid-liquid systems by considering two well-mixed bulk phases of uniform composition separated by a uniform stagnant mass-transfer layer at the interface, and set up equations for bulk phase species balance and mass conservation equations for simultaneous diffusion and reaction in the film. Dynamics of the interaction between reaction and diffusion were studied under these assumptions for two special cases (a) reaction which is pseudo-first-order in the quaternary ion-pair (b) mass-transfer controlled instantaneous reaction. 

Example 7.3 showed that a very fast reaction in the liquid can make the gas-film resistance important even for a slightly soluble gas such as CO2. For an instantaneous reaction in the liquid and a high value of SC o, diffusion in the gas film may be the controlling step. The gradients for this case are shown in Figure 7.8, which is based on the film theory for simplicity. 

A special case of instantaneous reaction is when it occurs entirely at the gas-liquid interface. Clearly, when reaction occurs so fast as to prevent any penetration of A into the film, the controlling step is the rate at which gas A is supplied to the interface. In other words, the reaction is gas-film controlled, and the rate is given by... 

Middleton indicates that for his regime 1 (very slow reaction), where Rl is little affected by the chemical reaction, the interface surface area per unit volume, a, is of little importance since the reaction takes place in the bulk liquid phase, so a bubble colunm is the typical reactor of choice. For Middleton s regimes 11, IV, and V—diflfusional control, very fast reaction, and instantaneous reaction, respectively—both high a and k [ are needed, so a stirred tank is the typical reactor recommended. In regime III—reaction in the mass transfer film—the most important variable is the interface area, so a packed column yielding much liquid surface area may be appropriate. 

When A and B react instantaneously, the reaction occurs at a plane parallel to the interface, or at the gas-liquid interface if the gas-film resistance controls. For the case where is not zero, the gradients for reaction in a stagnant film are shown in Figure 7.6. The steepness of the gradients reflects... 

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