Permeate flux maintenance, membrane

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

FIGURE 15.6 Schematic of constant flux maintenance manifold, which can be used with SBCR utilization of active back-flushing of membrane surface with permeate solution. 

An active flux maintenance procedure was initiated at this point (about 330 h TOS), beginning with a 2 s back-flush of clean permeate through the filter membrane. This active flux maintenance cycle was continued every 30 min for just over 24 h. The flux initially recovered to 0.90 lpm/m2 (32.0 GPD/ft2), but declined again within 24 h to a baseline value of 0.76 lpm/m2 (26.7 GPD/ft2) without clean permeate back-flush. The flux maintenance method was then returned to passive (no back-flush with clean permeate) mode, only increasing the flux off-time to 60 s every 30 min. Thereafter, the flux steadily declined over the next 120 h TOS from 0.77 to 0.58 lpm/m2 (27.3 to 20.4 GPD/ft2). At 480 h TOS, a 1 h flux off-cycle was attempted, resulting in an increase of the flux back to 0.82 lpm/m2 (29.1 GPD/ ft2), a 42.6% increase. When the flux off-cycle was returned to the 60 s off-cycle for the next 48 h, it was found that the permeate flux decreased to 0.62 lpm/m2 (21.9 GPD/ft2). Applying another 1 h flux off-cycle returned the flux to 0.721pm/... 

The variation of iron content in both the slurry and permeate samples against time on stream is represented in Figure 15.14. The permeate purity (in terms of iron concentration) was consistently below 35 ppm (as Fe) for the entire experiment, with over 85% below the 16 ppm level. The variation over iron content could be due to sampling during or after flux maintenance cycles, which can disturb the boundary layer of submicron particles on the membrane surface. 

The concentration of iron present in the permeate wax was found to be consistently less than 35 ppm, with over 85% below the 16 ppm level. Following an active flux maintenance procedure results in short-term recovery of flux, which declines to base value within 24 h. The passive flux maintenance procedure of interrupting the permeate flow for 30 or 60 s per 30 min was effective in recovering the initial membrane fouling temporarily. Better flux stability was attained only after increasing the permeate off-cycle to 1 h per day in addition to 30 s off per half-hour cycle. Variation of flux magnitude with TMP was found to follow a linear relationship within the range studied. 

Four parameters related to the membrane, feed stream and operating conditions determine the technical as well as economic performance of an inorganic membrane system. They are the transmembrane flux, permselectivity, maintenance of the permeating flux and permselectivity over time and stability toward the applications environment These parameters are the primary considerations for all aspects of the membrane system design, ai lication, and operation. 

How well economically a gas separation membrane system performs is largely determined by three parameters. The first parameter is its permselectivity or selectivity toward the gases to be separated. Permselectivity affects the percentage recovery of the valuable gas in the feed. For the most part, it is a process economics issue. The second is the permeate flux or permeability which is related to productivity and determines the membrane area required. The third parameter is related to the membrane stability or service life which has a strong impact on the replacement and maintenance costs of the system. 

In a pressure regime high enough, the permeate flux becomes independent of the applied pressure, which is the critical flow of the process. The presence of a layer of particles trapped and compressed on the surface of the membrane leads to the maintenance of a constant pressure drop in the gel layer polarization, and this pressure is the critical pressure of the system. Considering that the thickness of the layer retained on the membrane surface is very small, relative to the diameter of the pore channel, we can neglect its effect in relation to the hydraulic conditions of flow, and thus, the flow on the surface can be given as zero, thus characterizing the critical flow. 

An increased electrolyte concentration within the membrane may enhance accessibility and improve rates of proton sorption and permeation though the membrane. Maintenance of constant electrolyte concentration may be desirable for obtaining stable proton conductivity, and replenishment of vaporized or leached electrolyte may be continuously performed during operation. On a morphological level, dynamic fluctuations between electrolyte domains may provide conductive pathways through the polymeric continuous phase. Thus, the ability of the polymeric phase to mechanically comply with the anodic proton flux may enable proton percolation though the membrane and enhance conductivity. 

More recently, a new class of high-performance-thin-fllm nanocomposite (TFNC) ultraflltration membrane was introduced by You et al. (2013). In their research, electrospun PAN nanoflbrous substrates, coupled with a thin hydrophilic PVA barrier layer incorporating surface-oxidized multiwalled nanotubes (MWNTs) cross-linked by GA, were used to fabricate a composite membrane. As a result of incorporating MWNTs into the PVA barrier layer, there was a twofold improvement in the permeate flux, with maintenance of a high rejection rate (99.5%) at low pressure (0.1 MPa) during oil/water emulsion separation (You et al. 2013). 

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