Micro counter-current

Aota, A., Hibara, A. and Kitamori, T., Dependence of the number of theoretical plates of micro counter-current extraction of flow rate, Proc. pTAS 2005 Symposium, Boston, USA, 118, 2005. 

Figure 2.30 Schematic drawing of a counter-current micro heat exchanger.

Figure 2.31 Characteristic temperature profiles in a counter-current micro heat exchanger for a very low (left), intermediate (middle) and very high (right) thermal conductivity of the wall material and equal volume flows inside the two channels, reproduced from [125],...

Figure 1.17 Schematic of a micro channel equipped with many electrodes at the upper (U ) and lower (Lf) walls for control of the C, potential. The arrows in the channel denote the directions of the electroosmotic velocities creating one type of flow pattern, here a counter-current arrangement of top and bottom flows (top) alternating-flow arrangement, demonstrating another type of control over the potential (bottom) [28] (by courtesy of ACS).

Figure 4.112 Monolithic counter-current heat exchanger manufactured from a stack of micro structured plates and sealed by laser welding (source IMM).

If our main interest is in the total volumetric mass transfer between the liquids, the role of shear rate and blend time is relatively minor. However, if we are interested in the bubble-size distribution, and we often are because that affects the settling time of an emulsion in a multi-stage co-current or counter-current extraction process, then the change in macro- and micro-rates on scale up is a major factor. Blend time and circulation time are usually not a major factor on scale up. 

H2S can be separated frran the gas by the use of semi-permeable membranes. H2S (and CQ2) can pass the membrane whereas CH4 cannot [18, 37]. In addition, gas-liquid absorption membranes can be used. The membranes are micro pwous and have hydro-phobic pK ierties the molecules in the gas stream, flowing in one direction, difiiise through the membrane and are absorbed on the other side by the liquid, flowing in counter current At a temperature of 25—35 °C the H2S concentration of the raw gas of 2 % could be reduced to less than 250 cm% thus yielding an efficiency of more than 98 %. NaOH is used as the absobent in liquid [ 18,46]. 

The counter electrode is the current carrying electrode and it must be inert and larger in dimension. Platinum wire or foil is the most common counter electrode. For work with micro- or ultramicroelectrode where the maximum current demand is of the order of few microamperes, the counter electrode is not necessary. At very low current, a two-electrode system with the reference electrode can function as the current-carrying electrode with very little change in the composition of the reference electrode. Many commercial glucose sensors and on-chip microcells have such electrode configuration. 

Integrated reactors One type of integrated reactor is micro structured heat exchanger/reactor concepts, which may work as cross- or counter-flow reactors. Another type couples endothermic and exothermic reactions in two separate flow paths normally operated in the co-current mode. Both reactor types are designed as prototype components of future fuel processors for mobile applications. 

The gas-phase microreactor can be used on the laboratory scale under maximum conditions of 3 bar and 500 °C. It is made up of a stack of stainless-steel micro-structured plates that are arranged for counter-flow or co-current flow practice. Already tested applications of this reactor include the dehydration of 2-propanol [109]. 

A nitrate sensor has been also reported by Kim et al.." This sensor uses a simple electrochemical system composed of a silver sensing electrode, a silver oxide reference electrode, and a platinum counter electrode in 0.01 M NaOH concentrically distributed electrolyte. These micro-electrodes are microfabricated on a silicon substrate, providing an electrochemical microcell ion which a single microfluidic channel is used to deliver the reagents. Nitrate concentration is detected using double-potential step chronocoulometry in which the current accrued from nitrate reduction to nitrite is integrated. 

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