Performance membranes

A key factor determining the performance of ultrafiltration membranes is concentration polarization due to macromolecules retained at the membrane surface. In ultrafiltration, both solvent and macromolecules are carried to the membrane surface by the solution permeating the membrane. Because only the solvent and small solutes permeate the membrane, macromolecular solutes accumulate at the membrane surface. The rate at which the rejected macromolecules can diffuse away from the membrane surface into the bulk solution is relatively low. This means that the concentration of macromolecules at the surface can increase to the point that a gel layer of rejected macromolecules forms on the membrane surface, becoming a secondary barrier to flow through the membrane. In most ultrafiltration appHcations this secondary barrier is the principal resistance to flow through the membrane and dominates the membrane performance. 

Flux response to concentration, cross flow or shear rate, pressure, and temperature should be determined for the allowable plant excursions. Fouling must be quantified and cleaning procedures proven. The final design flux should reflect long-range variables such as feed-composition changes, reduction of membrane performance, long-term compaction, new foulants, and viscosity shifts. 

RO membrane performance in the utility industry is a function of two major factors the membrane material and the configuration of the membrane module. Most utility applications use either spiral-wound or hollow-fiber elements. Hollow-fiber elements are particularly prone to fouling and, once fouled, are hard to clean. Thus, applications that employ these fibers require a great deal of pretreatment to remove all suspended and colloidal material in the feed stream. Spiral-wound modules (refer to Figure 50), due to their relative resistance to fouling, have a broader range of applications. A major advantage of the hollow-fiber modules, however, is the fact that they can pack 5000 ft of surface area in a 1 ft volume, while a spiral wound module can only contain 300 ftVff. 

Two common types of membrane materials used are cellulose acetate and aromatic polyamide membranes. Cellulose acetate membrane performance is particularly susceptible to annealing temperature, with lower flux and higher rejection rates at higher temperatures. Such membranes are prone to hydrolysis at extreme pH, are subject to compaction at operating pressures, and are sensitive to free chlorine above 1.0 ppm. These membranes generally have a useful life of 2 to 3 years. Aromatic polyamide membranes are prone to compaction. These fibers are more resistant to hydrolysis than are cellulose acetate membranes. 

The foregoing equations assume that membrane performance is time independent. In some cases, a noticeable reduction in permeability occurs over time primarily due to membrane fouling. In such cases, design and operational provisions are used to maintain a steady performance of the system (Zhu et a ., 1997). 

Zeolite A is a very successful membrane for separation of water from alcohols, but it suffers from stability issues under acid conditions [23]. Usually, a Hquid phase should be avoided and, for this reason, vapor permeation is preferred. Recent developments show that the hydrophilic MOR [23] and PHI [50] membranes are more stable under acidic conditions in combination with a good membrane performance. 

The dead-end setup is by far the easiest apparatus both in construction and use. Reactor and separation unit can be combined and only one pump is needed to pump in the feed. A cross-flow setup, on the other hand, needs a separation unit next to the actual reactor and an additional pump to provide a rapid circulation across the membrane. The major disadvantage of the dead-end filtration is the possibility of concentration polarization, which is defined as an accumulation of retained material on the feed side of the membrane. This effect causes non-optimal membrane performance since losses through membrane defects, which are of course always present, will be amplified by a high surface concentration. In extreme cases concentration polarization can also lead to precipitation of material and membrane fouling. A membrane installed in a cross-flow setup, preferably applied with a turbulent flow, will suffer much less from this... 

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

Therefore, a team, led by the University of Alaska-Fairbanks, was formed to study these practical issues (75), including the composition of the ceramic membrane, seals that would join the ceramic and metal materials, membrane performance, and development of a ceramic that would resist warping and fracturing at the high temperatures of the conversion process. 

Research and development in electrolysers and membranes is in progress to meet the requirement for large-scale electrolysers and higher current densities, and to resolve the questions relating to the effect of operation at 6 kA m-2 and higher on-membrane performance and service life, in contrast to the widely known performance and reliability as proven through many years of operation at 4 kA m-2. 

Asahi Chemical has investigated the effects of this low-concentration region on the membrane performance using the three-compartment cell shown in Fig. 17.6, which... 

Needless to say, uniform concentration distribution of an electrolyte over a membrane surface is the most important factor to maintain good membrane performance. Sufficient internal circulation results in good electrolyte mixing inside the chamber. As shown in Fig. 19.2,12 ribs installed in both anode and cathode chambers work as downcomers in the same manner as in B-l. The horizontal cross-sectional area of the downcomer in the Improved B-l has been approximately doubled. All electrolyte concentrations measured at various points over the whole electrolysis area are maintained at specification, even at 6 kA m-2 through to 8 kA m-2, as is shown in Fig. 19.3 (with the relevant data at the downcomer positions 1-4 given in Table 19.1). 

Hybrid membranes composed of poly(vinyl alcohol) (PVA) and tetraethylorthosilicate (TEOS), synthetised via hydrolysis and a co-condensation reaction for the pervaporation separation of water-isopropanol mixtures has also been reported [32], These hybrid membranes show a significant improvement in the membrane performance for water-isopropanol mixture separation. The separation factor increased drastically upon increasing the crosslinking (TEOS) density due to a reduction of free volume and increased chain stiffness. However, the separation factor decreased drastically when PVA was crosslinked with the highest amount of TEOS (mass ratio of TEOS to PVA is 2 1). The highest separation selectivity is found to be 900 for PVA TEOS (1.5 1 w/w) at 30°C. For all membranes, the selectivity decreased drastically up to 20 mass % of water in the feed and then remained almost constant beyond 20 mass %, signifying that the separation selectivity is much influenced at lower composition of water in the feed. 

The laboratory-scale fluoride transport tests revealed severe attack on the Pt plating of the electrodes. AEA is now seeking resolution of this problem with electrode manufacturers. Pt and Ti released in these attacks may plug electrode cavities or impact membrane performance. 

Below /ps, the membrane performs under uniform saturation conditions, like a linear ohmic resistance. According to Equation (6.53), two modes of water management can be applied to compensate for electro-osmotic drag and keep the membrane in a well-hydrated state. Sufficient replenishment of water in the membrane can be accomplished by (1) providing a steady external water supply j > at the anode side, or (2) applying an external gas... 

Recently, it was shown that the hydraulic permeation model could explain the response of the membrane performance to variations in external gas pressures in operating fuel cells. i Figure 6.15 shows data for the PEM resistance in an operational PEFC,... 

When the membrane performs only a separation function and has no catalytic activity, two membrane properties arc of importance, the permeability and the selectivity which is given by the separation factor. In combination with a given reaction, two process parameters are of importance, the ratio of the permeation rate to the reaction rate for the faster permeating component (c.g. a reaction product such as hydrogen in a dehydrogenation reaction) and the separation factors (permselectivities) of all the other components (in particular those of the reactants) relative to the faster permeating gas. These permselectivities can be expressed as the ratios of the permeation rates of... 

Despite improvement to synthesis methods used it is known that all membranes possess unavoidable defects due to intercrystalline porosity, which are formed during membrane growth [7, 8]. The goal of many characterization techniques is to determine the size and concentration of such defects and evaluate how their presence affects membrane performance. 

The membrane performance for separations is characterized by the flux of a feed component across the membrane. This flux can be expressed as a quantity called the permeability (P), which is a pressure- and thickness-normalized flux of a given component. The separation of a feed mixture is achieved by a membrane material that permits a faster permeation rate for one component (i.e., higher permeability) over that of another component. The efficiency of the membrane in enriching a component over another component in the permeate stream can be expressed as a quantity called selectivity or separation factor. Selectivity (0 can be defined as the ratio of the permeabilities of the feed components across the membrane (i.e., a/b = Ta/Tb, where A and B are the two components). The permeability and selectivity of a membrane are material properties of the membrane material itself, and thus these properties are ideally constant with feed pressure, flow rate and other process conditions. However, permeability and selectivity are both temperature-dependent... 

The many details of this theory are omitted here. Nothing dealing with chemical groups and the forces that drive the morphology of ionomers is factored into this model, which limits its use in predicting fuel cell membrane performance. Moreover, it seems impossible to relate the quasi-percolation threshold to the real structure. Nonetheless, the view of conductance from the perspective of percolation is very appropriate. 

For a membrane specified in terms of A and D /K6, eq 14 and 15, together with eq 11, enable one to predict membrane performance (Xy 3 and N3, and hence f and (PR)) for any feed concentration X i and any chosen feed flow condition as specified in terms of k. Several theoretical and experimental methods of specifying k for different solutes under different conditions are available in the literature (6c,6d,18b,90,100). The quantities f and (PR) are related to Xy 3 and Ng through the following equations ... 

---