Interface reaction regime

Initially, when the ApBq layer is very thin, the reactivity of the A surface is realised to the full extent because the supply of the B atoms is almost instantaneous due to the negligibly short diffusion path. In such a case, the condition kom kW]/x is satisfied. Therefore, if the surface area of contact of reacting phases A and ApBq remains constant, chemical reaction (1.1) takes place at an almost constant rate. In practice, this regime of layer growth is usually referred to as reaction controlled. The terms interface controlled regime and kinetic regime are also used, though less suited. 

Figure 2. Concentration profile across a gas-liquid interface with very rapid reaction (Regime I). Reactants A and B diffuse into a narrow reaction zone in the liquid film. [J + D CBt/CnD c jj].

Figure 3. Concentration profile across a gas-liquid interface with moderately fast reaction (Regime II).

Figure 4. Concentration profile across a gas-liquid interface with slow chemical reaction (Regime III), Reactant A diffuses across the liquid film without appreciable chemical reaction. Virtually all chemical reaction takes place in bulk of liquid.

Pore-water profiles of Fe and from three stations in Long Island Sound have general depth-dependent concentration distributions similar to those reported from other sedimentary basins concentrations rise above seawater values to a maximum below the interface and then decrease again or remain constant deeper in the deposit. Beyond these general features, specific features of the profiles reflect the internal transport-reaction regime effective at each station. 

When the chemical reaction is very slow, the absorbed component A will diffuse Into (he bulk of the liquid before reection occurs. As a result of the reaction, the concentration or A in the balk of the liquid is kept low, and the driving force for transfer from the interface remains higher lhan ii would be in the absence of chemical reaction. In this case, called "the slow reaction regime, kL = kl and E = 1. There is no enhancement effect, and the only effect of the chemical reaction is its effect on the driving force. 

The slow reaction regime is characterized by the fact that the amount of the aromatic reactant that reacts in the film at the interface between phases is negligible compared to the amount that diffuses into the acid phase. Either the intrinsic kinetics or the rate of bulk-diffusion of the aromatic reactant may be the rate controlling step. The second type of reaction system is designated as a fast reaction system, and benzene would react in... 

Eq.9 indicates that, in the fast reaction regime, the mass transfer rate increases with increasing rate of the chemical reactions, though less than linearly. However, this is true only as long as the condition in Eq.23 is fulfilled. As the rate of reaction becomes very large, the concentrations of the non-vol tile components near the interface become appreciably different from their bulk-liquid values, and the Eq.24 ceases to be valid. 

A reaction taking place on the liquid-solid interface could ther fore result in appreciable rate enhancement only if a significant number of solid particles are present at distances from the gas-liquid interface less than 10 cm. This in turn would require particle diameters no more than 10 cm, an unrealistically low value. It therefore appears that, whenever the reaction takes place at the liquid-solid interface, no significant rate enhance ment will be observed for the gas-liquid mass transfer process the latter will essentially proceed in the slow-reaction regime. 

The statements above require some clarification. In the instantaneous reaction regime, Eq.28 must hold both in the bulk of the liquid and at the interface. Furthermore, if all diffusivities are equal, Eq.45 holds true. Straightforward algebra then leads to the following equation ... 

Reaction 82 is in general slow enough as to result in the mass transfer of CO2 taking place at most in the fast reaction regime (the kinetics of Reaction 82 will be discussed in some more detail below). This means that the occurrence of Reaction 82 does not appreciably influence the concentration distribution of the nonvolatile components B and BH near the interface. Consequently, that distribution in governed only by the occurrence of Reaction 81. 

By suitably choosing the solubility, the concentration of the reactant and the rate of reaction, either the mass transfer coefficients, or the interfacial area or both groups of parameters can be deduced from the overall rate of absorption (lA). Generally but not always, a steady flow of each phase through the reactor is assumed. Indeed the competition between the phsyical and chemical kinetics at the level of mass transfer between gas and liquid (the mass transfer reaction regime where the reaction belongs) may allow for the choice of the type of gas-liquid contactor (I). This is clearly shown in Fig. I that represents schematically the concentration profiles for A and B on each side of the interface. 

Alper and co-workers(5,44) have recently studied the absorption of Op or COp into solutions containing finely powdered activated carbon in stirred cells with unbroken interfaces.They employed both non-stationary physical absorption or pseudo-stationary chemical absorption experiments.In the latter,the reaction regime was in the transition between diffusional to fast pseudo mth order regime so that the information for k, could be deducted. For instance,for a first order reaction,the absorption rate is given by... 

BCR are particularly well suited to carry out reactions in the slow reaction regime of absorption. Due to the high liquid holdup BCR provide for a large liquid volume where the reaction can take place. Also, in slurry reactors where the reaction takes place at the surface of the solid catalyst particles belong to the slow reaction regime. Only a few exceptions are known where absorption enhancement due to the slurry phase reaction has been observed [6, 20 - 22]. Strictly speaking, enhancement and hence transition to the fast reaction regime can only be expected if the diameters of the particle fines are considerably less than the liquid film thickness at the gas/liquid interface. 

This regime is characterized by the presence of two continuous fluid phases and an interface which can easily be described. The term separated flows is frequently employed to describe these situations in both horizontal and vertical systems. Some flow patterns in Regime I are advantageous for transferring heat between the tube wall and the fluid mixture or for carrying out two-phase reactions. The special case of laminar-laminar flow is included in this regime, and two studies seem to be of interest, Byers and King (B7) and Bentwich and Sideman (B3). 

---