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part. These fl uid mechanics were coupled with the chemical transport of mobile phase compounds to help understand the mixing of two different concentrated liquids. The model was studied under time dependent specifi cations of 10 seconds for ease of computing while still fi nding a comparable solution. Theoretical data was generated in the time dependent study using the point probe projection tool where a point in the middle of the outlet was selected to gather data.
The CFD model and experimental testing utilised two different solvents through a proportional sampling valve and pumping system, thereby resulting in alternative plugs of each solvent in the sample line. These solvents were then subsequently mixed in the static mixer.
Modelling simulations for fl ow through a standard tubing (to simulate no mixer) and the Mott static mixer are shown in Figures 2 and 3, respectively. Modelling was performed on a 5 cm long by 0.25 mm ID straight tube to demonstrate the concept of alternating plugs of water and pure acetonitrile entering the tube, shown in Figure 2, without the presence of a static mixer. The exact tube and mixer design dimensions and a fl ow rate of 0.3 ml/min were used in the simulations. Figure 3 shows the CFD mixing simulation for the 30 mL mixer.
Figure 4. Graph of mixing effi ciency versus mixing volume of the static mixer series. The theoretical mixing follows the same trend of experimental mixing data validating the CFD modelling.
Figure 5. Schematic diagram of the low pressure gradient experimental test system.
Figure 2. Shows CFD modelling on fl ow in a 5 cm long by 0.25 mm ID tube to represent what is happening in the HPLC tubing, i.e., if no mixer is in place. The full red represents water as a mass fraction. The blue represents the lack of water, which is pure acetonitrile. A diffusion region can be seen between the alternating plugs of the two distinct liquids.
The HPLC system utilised for this testing was an Agilent 1100 Series HPLC with a UV detector controlled using a PC with Chemstation Software. Presented in Table 1 are the typical setup conditions for measuring mixer effi ciency by monitoring baseline sinusoid in two case studies.
Experimental tests were conducted for two different solvent case studies. The two solvents mixed in Case 1 were Solvent A (20 Millimolar solution of Ammonium Acetate in DI water) and Solvent B (80% Acetonitrile (ACN) / 20% DI water). In Case 2 study, Solvent A was a solution of 0.05% acetone (tracer) in DI water. Solvent B was an 80/20% mixture of methanol and water. The pump was set to ramp from 0.25 ml/min to 1.0 ml/min in Case 1 and to a constant fl owrate of 1 mL/min for Case 2. In both cases the mixing ratio of Solvents A and B was 20% A / 80% B. The detector was set at 220 nm in Case 1 and the maximum absorbance of acetone, 265 nm wavelength for Case 2.
Table 1. HPLC Confi gurations for Case 1 & 2 Case 1
20 Millimolar Ammonium Acetate in DI water
Case 2
Pump Speed 0.25 ml/min through 1.0 ml/min 1.0 ml/min Solvent A
Solvent B
80% Acetonitrile (ACN) / 20% DI water
Solvent Ratio 20% A / 80% B Detector
220 nanometers 0.05% Acetone in DI Water 80% Methanol / 20% DI Water
20% A / 80% B 265 nanometers
Figure 3. The 30 mL static mixer modelled in COMSOL CFD software package. The legend represents the mass fraction of water within the mixer. Pure water is represented by red while pure acetonitrile is represented by blue. As the two fl uids mix the colour changes, to represent a simulate the changing mass fraction of water.
Figure 4 is a validation study of the model relating mixing effi ciency to mixing volume. As the mixing volume increases the mixing effi ciency will increase. It is understand by the authors that there are other complex physical forces acting within the mixer that were unable to captured in this CFD model, thereby resulting in greater mixing effi ciency when the experimental testing was performed. The experimental mixing effi ciency was measured as a percentage reduction in baseline sine wave. Furthermore, increased back pressure generally results in a higher level of mixing, something the modeling also does not consider.
Experimental Procedure
The following HPLC conditions and test setup were used to measure baseline sine wave to compare the relative performance for various static mixers. Presented in Figure 5 is a schematic diagram showing a typical layout of a HPLC/UHPLC system. Testing of static mixers was performed by locating the mixer immediately downstream of the pump and upstream of the sample injector and separation column. Most background sinusoid measurements were performed by bypassing the sample injector and column using a capillary tube between the static mixer and the UV detector. When analysis of signal to noise ratios and/or peak shape were evaluated, the system was confi gured as shown in Figure 5.
Figure 6. Plots of measured mixing sinusoid before and after a low-pass fi lter was applied to remove the baseline drift component of the signal.
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