Mixing pressure-driven

In chemical micro process technology there is a clear dominance of pressure-driven flows over alternative mechanisms for fluid transport However, any kind of supplementary mechanism allowing promotion of mixing is a useful addition to the toolbox of chemical engineering. Also in conventional process technology, actuation of the fluids by external sources has proven successful for process intensification. An example is mass transfer enhancement by ultrasonic fields which is utilized in sonochemical reactors [143], There exist a number of microfluidic principles to promote mixing which rely on input of various forms of energy into the fluid. 

P 21] Solutions of 10 M 4-(l-pyrenyl)butyric acid in ethanol and 10 M sulfuric acid in ethanol were contacted in a micro-mixing tee/micro channel flow configuration at room temperature and at 50 °C [91]. Pressure-driven feed was used. The glass surface of the micro channels was either tuned hydrophobic (by exposure to octadecyltrichlorosilane) or hydrophilic (by wetting with a sulfuric acid/hydrogen peroxide mixture). 

Elangovan, S., B. Nair, J. Hartvigsen, and T. Small, Mixed Conducting Membranes for Pressure Driven Hydrogen Separation from Syngas, 225th American Chemical Society National Meeting, Fuels Division, New Orleans, LA, March 2003. 

Static mixers. Static mixers or motionless mixers are pressure-driven continuous mixing devices through which the melt is pumped, rotated, and divided, leading to effective mixing without the need for movable parts and mixing heads. One of the most commonly used static mixers is the twisted tape static mixer schematically shown in Fig. 3.25. 

A passive method for mixing streams of steady pressure-driven flows in microchannels at low flow rates has been developed. It is known to be difficult to mix solutions in microchannels at low flow rates because of the laminar nature of the flow. In such cases, molecular diffusion across the channels is slow. To solve this problem, a protocol for mixing based on transverse flows has been developed. To generate transverse flow. 

Figure 1. Schematics of the continuous chaotic stirrer developed by Kim and Beskok [25], The stirrer consists of periodically repeating mixing bocks with zeta potential patterned surfaces (a) and an electric field parallel to the x-axis is externally applied resulting in an electroosmotic flow (b). Combining a unidirectional (x-direction) pressure-driven flow (c) with electroosmotic flow under time-periodic external electric field (in the form of a Cosine wave with a frequency ra), a 2-D time-periodic flow is induced to achieve chaotic stirring in the mixer. Two fluid streams colored with red and blue are pumped into the mixer from the left and are almost well mixed after eight repeating mixing blocks for Re = 0.01, St = I2%, Pe= 1,000, and T = 0.8(d).

The basic unit operation on the pressure driven laminar flow platform is the contacting of at least two liquid streams at a microfluidic channel junction (see Fig. 7). This leads to controlled difflisional mixing at the phase interface, e g. for initiation of a (bio-) chemical reaction [105]. It can also be applied for the lateral focusing of micro-objects like particles or cells in the channel [95]. The required flow focusing channel network consists of one central and two S5munetric side channels, connected at a junction to form a common outlet channel. By varying the ratio of the flow rates, the lateral width of the central streamline within the common outlet channel can be adjusted very accurately. Consequently, micro-objects suspended in the liquid flowing... 

For this type of system, diffusion alone is not sufficient to achieve suitable mixing. Therefore, it was proposed to inject the droplets into a pressure driven flow in a serpentine channel, as shown in Fig. 7.8 [103, 104]. Flows internal to the drop were caused by changing the geometry of the channels, and resulted in intra-drop mixing. 

When equation (10) is applied to the base model (Fig. 1) with 1-10° API gravity fluid density differences between the segments, the resulting mixing times are dependent on the fluid type and the initial density difference (Fig. 8). Gas would mix in about 20000 to 200 000 years, while oil would take 400 000 to 4 million years. This time scale is over 5 orders of magnitude longer than pressure-driven mixing (Fig. 8),... 

The static mixers (SM) operate on the principle of repetitive dividing of a flow channel into at least two new channels, reorienting them by 90°, and dividing again. The flow is a pressure driven, laminar shear. Mixing by SM is related to the numbers of striations (N ) generated by a number of SM elements (n ) and the number of divisions (new channels) engendered by each element (n ) ... 

Convective or pressure-driven fluid flow is remarkably efficient for carrying agents over large distances (i.e. > 1 cm) and for mixing agents within the human body. 

---