Conventional Contactors

The mass transfer coefficients obtained in microstructured devices and in conventional gas - hquid contactors are hsted in Table 7.11. The liquid-side k a and interfacial area in microstructured devices are at least 1 order of magnitude higher than those in conventional contactors such as bubble columns and packed columns, being up to21s .  

Packed bed column (Pall/Raschig ring, Intalox saddles) [58] 80-450 0.0034-0.005 

The volumetric mass transfer coefficients found in the liquid-liquid microstructured devices at various flow rates were compared with those for conventional equipment in Table 7.12. Identical to gas-hquid devices, the mass transfer coefficients found in liquid-hquid microstructured devices are well above those of conventional contactors. 

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Solvent extraction carried out in conventional contactors like mixer-settlers and columns has certain limitations, including (a) controlling optimum dispersion and coalescence, (b) purifying both phases to ensure that stable emulsions are avoided (c) temperature control within a narrow band (d) high entrained solvent losses and related environmental and process economic effects and (e) large equipment dimensions and energy requirements when the density differential or selectivity is low. 

As Cu11 is substitution-labile,154 the rates of mass transfer are dependent on interfacial processes, which have been shown155 to be fast for both loading and stripping in conventional contactors, but possibly too slow in stripping for the industrial application of columns. 

Table 7. Comparison of typical dealkalization results obtained with the fluidized bed of whisker-type magnetic resin and a conventional contactor of Asahi type...

Mass transfer coefficients in micro-reactors are much higher than those obtained in conventional macro-contactors as it can be seen in Table 2.4. One common drawback of conventional contactors is the inability to predict precisely flow characteristics and interfacial area, because of the complexities of the governing hydrodynamics. This often leads to uncertainties in the design and causes limitations on the performance than can be achieved. Mass transfer rates in micro-reactors can be up to 2 orders of magnitude higher. In addition, consecutive reactions can be efficiently suppressed by strict control of residence time and its distribution (Kashid et al. 2011). 

In all the above cases the mass transfer coefficients varied between 0.049 and 0.23 s. These are comparable to the values found in other plug flow microchannel mass transfer units (Dessimoz et al. 2008 Ghaini et al. 2010 Kashid et al. 2011), whilst they are higher than those found in conventional contactors (see Table 2.4) (Fernandes and Sharma 1967 Verma and Sharma 1975). 

It can be seen that relative to capacity the size of the manifold capillaries is still much smaller than that of conventional contactors. In addition, the residence time within each extractor is lower in the case of the capillary than the other conventional contactors, which will prevent solvent degradation. 

Table 7.11 Comparison of gas-liquid microstructured devices with conventional contactors.

Therefore, high transformation rates can be obtained in fluid-fluid devices with high interfacial area. Microstructured multiphase reactors are characterized by interfacial areas, which are at least 1 order of magnitude higher compared to conventional contactors, and, therefore, suited particularly for very fast reactions. 

The aforementioned discussion was general in nature and also included conventional contactors such as tray and packed columns. In the case of three-phase (G-L-S) reactions, such conventional contactors are not used. The stirred reactor is the workhorse of the fine chemicals industry. The gas-inducing reactor can be considered as an alternative to stirred reactors when a pure gas is used. However, this reactor has several drawbacks (Chapter 9). In view of this, the venturi loop reactor has been widely used as a safe and energy-efficient alternative to the conventional stirred reactor. Table 3.3 summarizes the preceding discussion in the form of a multiphase reactor selection guide. 

Despite the wide availability of flat-sheet membranes and the impressive permeability they offer, the hollow fibre configuration is usually preferred due to its high packing density. The membrane surface area of commercial hollow fibre membrane modules varies in the contactor volume range of 1500-3000 m /m (Kumar et al, 2002), whereas in conventional contactors (bubble column, packed and plate columns) it is in the range of 100-800 mVm. Table 2.4 clearly shows that MC offers a much larger contact area per unit volume than other conventional absorbers (Yan et al, 2007). 

MCs present an alternative to more conventional contactor systems. The main advantages of MC systems over conventional column contactors are the reduction in size, the operational flexibility, elevated mass transfer rate and Unear scale-up, and the possibiUty of integration. Independent of their... 

Power input, a decisive parameter for benchmarking technical reactors, has been investigated using the experimental pressure drop and compared with conventional contactor as shown in Table 15.5. The comparison reveals that the liquid-liquid slug flow microreactor requires much less power than the alternatives to provide large interfacial area - as high as a = 5000 m m in a 0.5 mm capillary microreactor, which is way above the values in a mechanically agitated reactor (a 500 m m ). 

Table 15.7 Comparison of mass transfer data for microsystems and conventional contactors [76]. 

C02 buffer solution of NaHCOs, Na2C03, NaOH Conventional contactors for liquid-liquid systems [62] Ul = 0.09-1 in the order of 0.3 — 0.5 s (uL 0.09ms )... 

Conventional contactors may be fabricated from a wide range of materials, including metals, polymers, ceramics, and glass, in which the chemical resistance is suitable for many operating environments. The chemical compatibility of the materials of construction must be carefully assessed in advance for each application. 

The mass transfer coefficients obtained in micxochannels as well as in conventional gas-liquid contactors are listed in Table 2.3. From this list it can be concluded that liquid side volumetric mass transfer coefficient kja and interfacial area in microchannels are at least one order of magnitude higher than those in conventional contactors. 

Table 23 Comparison of MSR performance with conventional contactors for gas—liquid mass transfer.

Table 2.4. It can be seen that the mass transfer coefficients found in microchannels are well above those of conventional contactors.

Mass Transfer in Gas-Liquid Systems As in conventional contactors, mass transfer rates in membrane contactors for gas-liquid systems are generally described by means of an overall mass transfer coefficient, K, and the gas-liquid interfacial area per unit device volume, a. The overall mass transfer coefficient based on the liquid phase for any species i, Ku, is usually described via the principle of the following resistances in series liquid film resistance (1 /fe,/), membrane resistance (//,/, > ), and the gas film resistance (//,/ kig) for the gas-filled membrane pore case in series leading to the overall resistance (1 /Ku) ... 

Mass Ti ansfer The rate of mass transfer in a liquid-liquid extraction system implemented in a nondispersive membrane contactor is analyzed in the manner followed in conventional contactor analysis. The overall organic phase-based mass transfer coefficient Kio of a species i being extracted (or back extracted) from an aqueous solution into a solvent wetting the pores of a hydrophobic membrane is described via the resistances-in-series model ... 

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