|Title:||Concentration of small-volume samples by centrifugal and tangential flow nanofiltration|
This thesis aims at studying adequate techniques to concentrate low molecular weight (MW) solutes (MW < 1000 g/mol) in small-volume liquid samples (less than 100 mL). This is a theme of interest for biological and chemical applications like sample concentration for the detection of drugs and toxins in body fluids and in environmental specimens. The main objective of the present work is to develop techniques and devices capable of performing the sample concentration in a cheap, replicable and easy way, based on pressure-driven membrane processes, such that concentration factors (CF) above 10 are obtained.
Three main subjects were focused in this work: 1) the use of microfluidic tangential flow filtration (micro-TFF); 2) the development of a hybrid computational / semi-empirical method to predict nanofiltration (NF) performance; 3) the assessment of centrifugal nanofiltration (CNF) performance. To prove the feasibility of these techniques, and discard fouling effects, only binary aqueous solutions of small inorganic salts and neutral organic molecules were used. Concentration of samples with two osmotic pressures were tested (0.6 bar and 2.6 bar), while the applied transmembrane pressure varied between 7 and 42 bar.
Both micro-TFF and CNF result in appreciable concentration factors (CF between 15 and 22) when applied to the samples with the lowest osmotic pressure. For the samples with highest osmotic pressure, lower concentration factors are obtained (CF between 5 and 10). In these scenarios, CNF provides better concentration performance due to its self-cleaning mechanism that disrupts the concentration polarization (CP) layer, while in micro-TFF the CP layer is stable, which decreases the permeate flux and the permeate quality.
The CP phenomenon can be minimized in micro-TFF devices using improved channel or spacer designs. To speed up the selection of the best designs, the hybrid method developed herein can be used, since its estimations for the NF performance deviates less than 10 % from the results obtained from the full CFD simulation of the permeation process. The validation of the momentum and mass transport models, used for the CFD simulations, was performed by comparing the numerical results against the corresponding experimental ones obtained with micro-PIV and holographic interferometry.
Similarly, the CNF performance can also be improved by optimizing the filtration chamber geometry of the centrifugal device. It was verified that the obtained CF value depends on both the height of the filtration chamber, h, and the angle between the centrifugal force and the membrane surface, β. For the range of tested values of these two parameters (h from 0.1 mm to 2.4 mm; and β from -10° to +10°) the optimal values were h = 0.2 mm and β = -10°. These results suggest that h should be high enough to permit efficient mixing of the liquid inside the filtration chamber, but also small enough to minimize the volume of the filtration chamber. At the same time, the β angle should be as negative as possible provided that the normal to the membrane surface points radially outwards.
Overall, the aim to develop methods for the concentration of small samples of low MW solutes by pressure-driven membrane processes was accomplished and CNF proved to be the most efficient one.
Concentration by nanofiltration, Centrifugal nanofiltration, Microscale tangential flow filtration, Hybrid computational / semi-empirical method, Micro-PIV, Holographic interferometry