Nanofiltration pore size distribution

W. Bowen and J. Welfoot, Modelling of membrane nanofiltration pore size distribution effects, Chem. Eng. Sci., 57 (2002) 1393-1407. 

Separation processes as a whole have grown in importance because of increasingly stringent requirements for product purity [1]. Among the different membrane techniques, pressure-driven processes such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO) were the first to undergo rapid commercialization [2-A], These processes basically differ in pore size distribution of membranes used and the types of compounds recovered. A typical schematic of the exclusion of various compounds through different membrane processes is illustrated in Figure 42.1. 

Transport through nanofiltration membranes is controlled primarily by electrostatic and steric interactions. The extended Nemst-Plank equation commonly is used with Donnan and steric partitioning to predict transport rates based on effective membrane charge density, pore radius, and thickness to porosity ratio [131-132]. Inclusion of solute-pore hydrodynamic interactions and a pore size distribution improves the predictive and correlative capabilities of the models [133]. 

For many years, polymeric membranes have been widely utilized in practical appHca-tions without having precise information on their pore size and pore size distribution, despite the fact that most commercial membranes are prepared by the phase inversion technique, and the performance of those membranes is known to be governed by their pore characteristics in a complicated manner [1]. These pore characteristics are influenced both by the molecular characteristics of the polymer and by the preparative method [2]. Crudely, membranes applied for pressure-driven separation processes can be distinguished on the basis of pore diameter as reverse osmosis (RO, < 1 nm), dialysis (2-5 nm), ultrafiltration (UF, 2-100 nm), and microfiltration (MF, 100 nm to 2 J,m). Nanofiltration (NF) membranes are a relatively new class and have applications in a wide range of fields [3]. The pore sizes of NF lie between those of RO and UF membranes. 

What all of these have in common is their ability to remove or separate species on the basis of size. UF can remove fine particles and colloids RO can remove all ionic species from water nanofiltration, being an intermediate process, actually distinguishes between ions of different size. It is the ability to manufacture materials not only with very fine pores but also with closely controlled pore size and narrow pore size distribution that has made all these processes commercially feasible. Table 7.24 shows the spectrum of applications. Nanofiltration is the newest application in the chlor-alkali field. We can expect the number of membrane-based installations to continue to grow. 

Ionic membranes are characterised by the presence of charged groups. Charge is, in addition to solubility, diffusivity, pore size and pore size distribution, another principle to achieve a separation. Charged membranes or ion-exchange membranes are not only employed in electrically driven processes such as electrodialysis and membrane electrolysis. There are a number of other processes that make use of the electrical aspects at the interface membrane-solution without the employment of an external electrical potential difference. Examples of these include reverse osmosis and nanofiltration (retention of ions), microfiltration and ultrafdtration (reduction of fouling phenomena), diffusion dialysis and Donnan dialysis (combination of Dorman exclusion and diffusion) and even in gas separation and pervaporation charged membranes can be applied... 

Various pressure-driven membrane processes can be used to concentrate or purify a dilute (aqueous or non-aqueous) solution. The characteristic of these processes is that the solvent is the cominueous phase and that the concentration of the solute is relatively low. The particle or molecular size and chemical properties of the solute determine the structure, i.e. pore size and pore size distribution, necessary for the membrane employed. Various processes can be distinguished related to the panicle size of the solute and consequently to membrane structure. These processes are microfiJtration, ultrafiltration, nanofiltration and reverse osmosis. The principle of the four processes is illustrated in figure VI - 2. 

Composite membranes constitute the second type of structure frequently used in reverse osmosis while most of the nanofiltration membranes are in fact composite membranes. In such membranes the toplayer and sublayer are composed of different polymeric materials so that each layer can be optimised separately. The first stage in manufacturing a composite membrane is the preparation of the porous sublayer. Important criteria for this sublayer are surface porosity and pore size distribution and asymmetric ultrafiltration membranes are often used. Different methods have been employed for placing a thin dense layer on top of this sublayer ... 

Otero, JA Mazarrasa, 0 Villasante, J Silva, V Pradanos, P Calvo, JI Hernandez, A. Three independent ways to obtain information on pore size distribution of nanofiltration membranes. Journal of Membrane Science, 2008, 309, 17-27. 

The principle of pore size distribution measurement is shown in the Fig. 5. At first, the porous matrix, in occurrence nanofiltration membrane is filled with a specific wetting fluid . The latter is then pushed out, by successively increasing pressure, by another immiscible fluid (gas or liquid). 

One of the new techniques to scan membranes and determine pore size distributions is positron, annihilation spectroscopy (PAS). With this method also the free spaces in nanofiltration membranes can be determined. For instance in a study by Boussu et al. [9] it was found that some much-studied NF membranes (like Desal-5 DL, NTR7450) have two sizes of spaces, one size about 0.12-0.15 nm and the other between 3nm and 4nm. It could be speculated that diffusive transport would happen through the smaller spaces (the size of a water molecule), depending on the hydrophilicity properties of the membrane, and convective transport through the larger spaces, depending on size and charge conditions. 

Figure 6.5 Single pore and pore size distribution of XP117 nanofiltration membrane.

An important challenge in science is to boldly go where no man (or woman) has gone before. Thus, Figure 6.5 shows an image and the corresponding pore size distribution of a single pore in a nanofiltration membrane, XP117 from PCI Membranes. 

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