Microchannels contactors

Owing to the complex and often dedicated equipment required to perform gas-liquid phase reactions within research laboratories, this area of synthetic chemistry is somewhat underutilized. Over the past decade, however, numerous research groups have developed an array of continuous flow reactors capable of conducting such reactions in a safe and efficient manner, including microchannel contactors, falling film micro reactors, and packed-bed reactors [68, 69]. 

A microchannel contactor has been developed and tested with water and cyclohexane streams extracting cyclohexanol [199]. Using this device, the relative importance of mass transfer resistance in the flow channels versus the contactor plate was explored. Both micromachined contactor plates and commercial polymeric membranes were configured with various channel heights both on the feed and solvent sides. Data indicate that contactor plate mass transfer becomes... 

Figure 8.4 Packed microchannel contactor with microfabricated columns. From [56].

Figure 8.13 Disassembled falling film microchannel contactor and schematic showing the gas-liquid contacting. From [92].

Figure 8.27 MicroChannel contactor with guiding ridges, (a) Schematic cross-sectional view and (b) 3D image of the guide structures. From [160],...

TeGrotenhuis, W. E., Cameron, R., Viswanathan, V. V., and Wegeng, R. S., Solvent extraction and gas absorption using microchannel contactors, Proc. 3rd Inti. Conf. Microreaction Tech., 1999. Also available online, at http //www.pnl.gov/microcats. 

Microseparation units make use of the enhancement of mass transport between different phases. PNNL workers reported a microchannel gas-liquid contactor... 

Continuous Contactor with Partly Overlapping Channels Solute transfer can occur between immiscible phases each flowing in separate adjacent but displaced microchannels, having only a small conduit in which the fluid interface is stable (partial overlap) [267,268]. 

Most work in microchannel extraction focuses on improving the extraction efficiency or the phase separation or system development through adding multiple imit operations on a single chip or by scaling up. Because laminar flow exists in the microchannel devices, the intimate mixing of turbulent flow in traditional contactors is not present. Most studies have shown that the dissimilar phases flow parallel to each other with movement of solute molecules caused by molecular diffusion only. Thus, extraction is governed by contact time between phases [202]. 

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). 

Microchannels with Mesh or Sieve-Like Interfadal Support Contactors... 

Similarly to partially overlapping channels, microchannels with mesh contactors (Figure 7.2h) are used to create the partial contact of fluids. The advantage of these contactors is that both modes of operation, cocurrent and countercurrent, can be apphed. Besides, the flow is stabilized because of the solid support between two fluids. The solid contactors are porous membrane [9, 10] and metal sheets with sieve-like structure [11]. Similarly to parallel flow, the mass transfer in both cases is only by diffusion and the flow is under laminar flow regime dominated by capillary forces. The membrane contactor has the advantage of being flexible with respect to the ratio of two fluids. In addition to flow velocities, the mass transfer is a function of membrane porosity and thickness. In another type of microextractor, two microchaimels are separated by a sieve-like wall architecture to achieve the separation of two continuous phases. However, the hydrodynamics in both types of contactors is more complex because of interfadal support and bursting of fluid... 

The fading film MSR is one of the most commonly used devices for gas-liquid reactions (examples are given in gas-liquid reactions section). The liquid flows downward because of gravity in the form of film and gas flows through the open space that lies in the top cover of the housing. The falling film contactor consists in general of a stainless steel plate with open channels, typically 300 pm deep, separated by about 100 pm thick walls. The role of open microchannels is to prevent the breakup of the liquid film. 

The parallel flow can also be formed at low flow velocity, especially in rectangular cross section microchannel, if the interfacial tension between two fluids is very low, for example, wedge-shaped parallel flow contactor [38], or modifying the wettability of the channel walls (see Figure 7.28). The flow could be stabilized by placing membranes or sieve plates inside the channels [9, 39]. 

Figure8.il SEM image of cross-section of silicon lass bonded structure with overlapping microchannels to form contactor.

Figure 8.15 Disassembled and partially assembled helicoidal microchannel falling film contactor with path length 540 mm. From [97],...

A high-throughput tube-in-tube microchannel reactor was recently designed and developed as a novel gas-liquid contactor [102,103]. 

The mass transfer efficiency of different gas-liquid contactors as a function of residence time was compared, including an interdigital micromixer, a caterpillar minimixer, a mixing tee, and three microbubble column with microchannels of varying diameter (Figure 9.35) [141]. The two microbubble columns comprising the... 

Figure 2.11 Schematic representation of fluid-fluid microstructured reactors (a micromixer settler b cyclone mixer c interdigital mixer d microchannel with partial overlap e microchannel with membrane or metal contactor f microchannels...

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