Membrane processes pulsed flow

Recent developments in LM module design, including rotational, vibrational membrane devices, pulsed-flow fluid management for polarization control, use of low-cost refractory monoliths as membrane supports, and use of electric potentials to minimize macrosolute polarization and fouling, may permit practical and economic application of membrane processes to liquid and gaseous streams which today are untreatable by such methods. 

The performance of a membrane process is a function of the intrinsic properties of the membrane, the imposed operating and hydrodynamic conditions, and the namre of the feed. This chapter describes methods available to enhance performance by various techniques, mainly hydrodynamic but also chemical and physical. The focus is on the liquid-based membrane processes where performance is characterized by attainable flux, fouling control, and separation capabilities. The techniques discussed include secondary flows, flow channel spacers, pulsed flow, two-phase flow, high shear devices, electromagnetic effects, and ultrasound. 

In a record obtained by the patch clamp technique, the channel is closed for much of the time (i.e. no current flows across the patch of membrane that contains it), but at irregular intervals the channel opens for a short time, producing a pulse of current. Successive current pulses are always of much the same size in any one experiment, suggesting that the channel is either open or closed, and not half open (there are exceptions to this rule). The durations of the pulses, however, and the intervals between them, vary in an apparently random fashion from one pulse to the next. Hence the openings and closings of channels are stochastic events. This means that, as with many other molecular processes, we can predict when they will occur only in terms of statistical probabilities. But one of the most useful features of the patch clamp method is that it allows observation of these stochastic changes in single ion channels as they actually happen individual protein molecules can be observed in action. 

MESI operation requires processing of the whole sample to be extracted and has to reach steady-state permeation, which usually takes a long time. Thus, a new technical modification of MESI, called pulse introduction (flow injection-type) membrane extraction (PIME), has been developed, in which the sample is introduced to the membrane as a pulse pushed by a stream of eluent (usually water).55 This means that attaining a steady state is no longer crucial. PIME therefore provides not only a faster response and higher sensitivity, but also allows extraction of individual samples via discrete injections in addition to continuous on-line monitoring by sequential injection of a series of samples. Guo et al.56 described a mathematical model for the PIME permeation process, which showed that (a) there was a trade-off between the sensitivity and the time lag (the time taken to complete the permeation process) and (b) a large sample volume and a low flow rate enhance the sensitivity but also increase the time lag. 

The second solution to increasing the throughput of pulsed ultrafiltration mass spectrometry has been to miniaturize the ultrafiltration chamber volume while maintaining the flow rate and chamber pressure. Because the ultrafiltration membrane cannot withstand high pressure without rupturing, the ultrafiltration process cannot be accelerated simply by increasing the flow rate through the chamber. The approach of Beverly et al. (72) was to fabricate a 35-/u,L ultra-... 

A radioactive solution containing Co of the total radioactivity of ca. 4000 counts/100 sec was anployed as a radioactive feed. The respective radioactivity counting rate (pulse/sec), treated as the intensity of the y-radiation of a 5 cm liquid sample containing Co, was measured by a y-analyzer, LG-IB (INCT). The obtained values were compared with the radiation intensity of the Co standard sample. Therefore, the specific activity of the feed (Ap), the retentate (Ar), and the permeate (Ap) were calculated. The parameters (/ ), (DF), and if) were determined when the radioactive solutions were treated. In all the experiments, the transman-brane hydrostatic pressure was maintained at 0.22 MPa and the feed flow rate at 401/h. Substantially high DFs for radioactive Co were obtained for the SMM-modified membranes used in the UF/complexation process 223 for SMM3/PES and 163 for SMM41/PES (Table 1.2). For the unmodified membranes, the DFs were lower 44 for the commercial PES membrane and 75 for the prepared unmodified PES membrane. Additionally, the SMM-modified membranes showed smaller adsorption of the radioactive cobalt than the modified membranes, which was beneficial, taking into account the considered applications. After a 2 h operation, the adsorption of Co by the SMM-modified membranes was four to five times smaller than that of the unmodified PES membrane. 

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