Observations Through the Membrane

A major advance vas made by Tony Fane and co-workers at the University of New South Wales when they developed a method to perform visual observation from the permeate side of the membrane. This technique, called direct observation through membrane (DOTM), was first described by Hodgson, PUlay and Fane [9] and then discussed more fully by Li et al. [10]. The key idea is to use a membrane which becomes transparent when wetted, and then observe partide deposition by placing a microscope on the permeate or back side of the membrane. Because of the relatively short focal length, compared to looking into the center of a membrane molecule from the side, high magnification may be used to observe individual particles as small as 1-10 pm in diameter. 

When the permeate flux was controlled at a value near the critical flux, Li et al. 

Additional applications of DOTM include observing deposition and critical fluxes for yeast, algae and bacterial cells [10, 15-17], and particle deposition patterns that occur when spacers are used in the membrane channels [18]. Spacers were shown to increase the critical flux by up to two times [19]. Observations of the deposition and removal of submicron bacteria required the use of fluorescence. Removal of the bacterial cake when the flux was reduced below the critical value was observed to occur in floes [16]. In a recent application of DOTM, Zhang, Fane and Law [20] showed that the presence of larger particles increased the critical flux of smaller particles in a mixture and that only the smaller particles selectively deposited from a mixture when the imposed permeate flux was above this critical flux. This finding is consistent with the earlier observation of Li et al. ]10] that the smaller particles are preferentially deposited from a feed distribution of particle sizes. 

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Li H, Fane AG, Coster HGL, and Vigneswaran S, An assessment of depolarisation models of crossflow microfiltration by direct observation through the membrane, J. Memb. Sci. 2000 172 135-147. 

The plastic supports exposed approximately 3 mm2 of the membrane surface. Protein-containing solution (350 pi) was placed under the membrane in contact with it. This was the source solution. Air was excluded from the membrane s pores by soaking them in ethanol, and then water (double distilled in glass) prior to use. Bubbles were excluded from the chambers by carefully placing the membrane onto the surface of the liquid in the test chamber. Since the membranes are thin and translucent, the presence of even very tiny bubbles could be observed through the membranes with a dissecting microscope. The mix was stirred continuously with a magnetic stir bar. Buffer (30 yd of phosphate buffered saline,... 

Critical Coagulation Concentration Chromatographable Organic Carbon Chloral Hydrate Forming Potential Disinfection By-Product Diethylaminoethyl Diffusion Limited Aggregation Dissolved O anic Carbon Dissolved Organic Matter Direct Observation through the Membrane Technique Diffusive Reflectance Fourier Transform Infrared Spectroscopy Electronic Conductive Carbon Black Electron Dispersive Spectra Ethylene Diamine Tetra Acetic Acid Fulvic Acid... 

DOTM Direct observation through the membrane n Viscosity... 

Figure 2.3 Schematic of a crossflow microfiltration device employing direct observation through the membrane. (From Li et al. [10]).

A significant advance in optical characterization was made with the introduction of an approach now known as direct observation through the membrane (DOTM) [5, 6]. The microscope objective is positioned on the permeate side and focused through the permeate channel and the membrane onto the membrane surface on the feed side. Thus the membranes used must be transparent. Although this is a major limitation, significant findings have been made, as discussed elsewhere. 

Each of the membranes acts like a hard wall for dimer molecules. Consequently, in parts I and III we observe accumulation of dimer particles at the membrane. The presence of this layer can prohibit translation of particles through the membrane. Moreover, in parts II and IV of the box, at the membranes, we observe a depletion of the local density. This phenomenon can artificially enhance diffusion in the system. In order to avoid the problem, a double translation step has been applied. In one step the maximum displacement allows a particle to jump through the surface layer in the second step the maximum translation is small, to keep the total acceptance ratio as desired. 

In this section we describe a cellular automata model of a semipermeable membrane separating two compartments [5]. A solute in one compartment has varied parameters to reflect its relative polarity or lipophilicity. The passage of this solute into and through the membrane is observed, as this property is varied. 

In the following studies, the water temperature is increased to observe the effect on water passage through the membrane. Change the Pb(WW) and J(WW) values to accomplish these changes. See Chapter 3, Table 3.2 for these values. 

Cavitations generate several effects. On one hand, both stable and transient cavitations generate turbulence and liquid circulation - acoustic streaming - in the proximity of the microbubble. This phenomenon enhances mass and heat transfer and improves (micro)mixing as well. In membrane systems, increase of fiux through the membrane and reduction of fouling has been observed [56]. 

The membrane-bound preparation from kidney is easily solubilized in non-ionic detergent and analytical ultracentrifugation shows that the preparation consists predominantly (80 85%) of soluble af units with 143000 [28]. The soluble a)S unit maintains full Na,K-ATPase activity, and can undergo the cation or nucleotide induced conformational transitions that are observed in the membrane-bound preparation. A cavity for occlusion of 2K or 3Na ions can be demonstrated within the structure of the soluble a)S unit [29], as an indication that the cation pathway is organized in a pore through the aji unit rather than in the interphase between subunits in an oligomer. 

Combining these equations and integrating yield Cf = CioX for a volume reduction factor X = Q/Qo and the observed component passage Si. This allows one to determine either final concentrations from crossflow rates or the reverse. For a fully retained product (Sj= 0), a 10-fold volume reduction (X = 10) produces a 10-fold more concentrated product. However, if the product is only partially retained, the volume reduction does not proportionately increase the final concentration due to losses through the membrane. 

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

Lassgaard-Jorgensen et al. [19] calculated that the rate of reactions involved in SMR is much higher than the rate of penetration of methane through the membrane. Lin et al. [20] have observed that the methane conversion strongly depends on the space velocity and the amount of methane per membrane surface area... 

Based on GebeTs calculations for Nafion (where lEC = 0.91 meq/g),i isolated spheres of ionic clusters in the dry state have diameters of 15 A and an intercluster spacing of 27 A. Because the spheres are isolated, proton transport through the membrane is severely impeded and thus low levels of conductivity are observed for a dry membrane. As water content increases, the isolated ionic clusters begin to swell until, at X, > 0.2, the percolation threshold is reached. This significant point represents the point at which connections or channels are now formed between the previously isolated ionic clusters and leads to a concomitant sharp increase in the observed level of proton conductivity. 

MeOH is transported through the membrane by two modes diffusion and electro-osmotic drag. ° When MeOH comes into contact with the membrane, it diffuses through the membrane from anode to cathode and is also dragged along with the hydrated protons under the influence of current flowing across the cell. Therefore, a correlation between the MeOH diffusion coefficient and proton conductivity is observed. The diffusive mode of MeOH transport dominates when the cell is idle, whereas the electro-osmotic drag... 

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