Agitated reactors interfacial area

In addition to agitation, the interfacial areas are significantly affected by the following acid/hydrocarbon ratio, acid composition (and especially the amounts of dissolved conjunct polymers), and temperature. Conjunct polymers are surfactants that collect in appreciable concentrations at the interface. Here, they act as a reservoir of H s in the transfer steps from isobutane or other isoparaffins to the i-Cs to i-Cie cations. Conjunct polymers also have a major effect on the viscosity and other physical properties of the acid phase. In the alkylation reactor, the preferred sulfuric acid and HF phases contain appreciable amounts of conjunct polymers optimum amounts result in higher RON values, higher yields, less byproducts, etc. 

Stirred suspensions of droplets have proven to be a popular approach for studying the kinetics of liquid-liquid reactions [54-57]. The basic principle is that one liquid phase takes the form of droplets in the other phase when two immiscible liquids are dispersed. The droplet size can be controlled by changing the agitator speed. For droplets with a diameter < 0.15 cm the inside of the drop is essentially stagnant [54], so that mass transfer to the inside surface of the droplet occurs only by diffusion. In many cases, this technique can lack the necessary control over both the interfacial area and the transport step for determination of fundamental interfacial processes [3], but is still of some value as it reproduces conditions in industrial reactors. 

A reaction occurring in a bulk phase will show an increase in the rate with the area as shown in Fig. 5.3 for a reaction occurring in the film or at the interface, the rate will be linearly dependent on the interfacial area. The interfacial area in a dispersed two-phase liquid-liquid system can be estimated by measuring the rate of a suitable test reaction in a reactor with the known interfacial area (a flat interface, Section 5.3.2.1), and comparing it with the reaction rate in a dispersed system [6, 15]. A convenient reactive system for this purpose is a formate ester and 1-2 M aqueous NaOH. Formate esters are very reactive to hydroxide ion (fo typically around 25 M 1 s 1), so the reaction is complete inside the diffusion film, and the reaction rate is proportional to the interfacial area. A plot of the interfacial area per unit volume against the agitator speed obtained in this way in the author s laboratory for the equipment shown in Fig. 5.12 is shown in Fig. 5.14 [8]. 

Fig. 5.14 Plot of the interfacial area per unit volume against the agitator speed for the reactor shown in Fig. 5.12 for the reactive pair, n-hexyl formate and 1 M aqueous NaOH.

Power or energy dissipated in the aerated suspension has to be large enough (a) to suspend all solid particles and (b) to disperse the gas phase into small enough bubbles. It is essential to determine the power consumption of the stirrer in agitated slurry reactors, as this quantity is required in the prediction of parameters such as gas holdup, gas-liquid interfacial area, and mass- and heat-transfer coefficients. In the absence of gas bubbling, the power number Po, is defined as... 

Gas holdup is an important hydrodynamic parameter in stirred reactors, because it determines the gas-liquid interfacial area and hence the mass transfer rate. Several studies on gas holdup in agitated gas-liquid systems have been reported, and a number of correlations have been proposed. These are summarized in Table VIII. For a slurry system, only a few studies have been reported (Kurten and Zehner, 1979 Wiedmann et al, 1980). In general, the gas holdup depends on superficial gas velocity, power consumption, surface tension and viscosity of liquids, and the solid concentration. The dependence of gas holdup on gas velocity, power consumption, and surface tension of the liquid can be described as... 

As described later, liquid-liquid reactors are mechanically agitated in order to achieve a good dispersion and large interfacial area between two immiscible liquids. The increase in interfacial area due to stirring enhances the reaction rate (e.g., saponification, bead polymerization, etc.). It should be noted that the interfacial area is also increased by the addition of surfactants. This process is called emulsification and is governed by completely different principles than the ones described here. 

Mechanical agitation is very prevalent in liquid-liquid reactors because it provides the required high interfacial area, interphase mass transfer, and good dispersion in the case of reactions between two immiscible liquids or... 

In this section we consider the rate of absorption of gases into liquids that are agitated so that dissolved gas is transported from the interfacial surface to the interior by convective motion. The next section, based on this one, treats chemical methods for determining interfacial areas and mass-transfer coefficients in agitated gas-liquid reactors. 

Fig. 26. Influence of gas velocity on specific interfacial area at low agitator speed in a mechanically agitated reactor (MI2).

Fig. 32. Interfacial area in ejector reactor comparison with mechanically agitated reactor (N2, N5).

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