Permeation, membrane extraction

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. 

In an attempt to overcome the significant difficulties that the presence of water vapor poses to the analysis of very volatile compounds, purge-and-membrane extraction techniques have been developed that largely prevent the introduction of water into the analytical system. Typical implementations of this form of sample introduction have been called by its developers membrane extraction with a sorbent interface (MESI),97 or membrane introduction mass spectrometry (MIMS).98 " They are based on a silicone hollow-fiber membrane that is inserted into the sample to be monitored, and the passing of a certain volume of inert gas through the membrane. Volatile compounds permeate the membrane and are swept to the adsorbent trap from which they are desorbed into the GC. This method of sample introduction is particularly suited for field and process monitoring and for dirty samples, since it prevents any nonvolatile compounds from entering the analytical system.100... 

The greatest challenge in membrane extraction with a GC interface has been the slow permeation through the polymeric membrane and the aqueous boundary layer. The problem is much less in MIMS, where the vacuum in the mass spectrometer provides a high partial pressure gradient for mass transfer. The time required to complete permeation is referred to as lag time. In membrane extraction, the lag time can be significantly longer than the sample residence time in the membrane. An important reason is the bound-... 

In all types of membrane extraction, the membrane separates the sample phase (often called donor or feed solution) from the acceptor or strip phase and the analyte molecules pass through the membrane from the donor to the acceptor. This process is often called pertraction (permeation-extraction). The membrane extraction techniques can be divided into porous and nonporous membrane techniques. Another distinction is between one-, two-, or three-phase membrane extraction techniques. 

Membrane extraction (ME) is a solventless extraction method that has gained popularity for the analysis of VOCs in water. In ME the sample is contacted with a membrane surface. Analytes will migrate according to the polymer affinity from the aqueous phase to the surface of the membrane and selectively permeate through it. 

Immersed MBRs have been developed out of a need to simplify the use of these systems and to operate more cost-effectively than the external loops with respect to both energy consumption and cleaning requirements. In these configurations, the membranes are directly immersed in the tanks containing the biological sludge, and the treated permeate is extracted. 

The main thermodynamic properties of the methanol and ethanol steam reforming reactions are plotted in Fig. 18.3 for comparison. Both are endothermic. However, they are spontaneous, due to large entropy changes associated with the dissociation of the alcohols. Spontaneous methanol dissociation is obtained at a lower temperature (AG < 0 at - 325 K) compared to ethanol (AG < 0 at 475 K), which requires practical dissociation temperatures of 600 K. It is therefore possible to incorporate a palladium permeation membrane into a methanol steam reformer (in such cases, both processes are operating at the same temperature) whereas for ethanol, reforming and extraction of hydrogen are usually performed in two separate... 

Similar to the pervaporation process, in membrane extraction, concentration differences between feed and permeate act as the driving force for mass transport across the membrane two immiscible hquids are in contact with each other inside of the membrane or at either surface of the membrane, facilitating the transfer of low concentrated solutes across the hquid—hquid interface (Abels et al., 2013). Hydrophobic (PVDF or polypropylene) membranes, typically in a hollow-fiber or even spiral-wound format, can be apphed (Bayer et al., 2012). Mainly organic solvents such as n-heptane or other solvents such as ionic liquids can be utilized in the process... 

Membrane extraction (ME) techniques are a set of solvent-free extractions, which have gained popularity for VOC analysis in water (for applications see Table 23.8). The sample is in contact with one side of the membrane surface called feed or donor side. Analytes permeate selectively (according to their membrane affinity) through the membrane to the other side, called permeate or acceptor side, where they are retained by an acceptor phase. This process is called pertraction (permeation-extraction). 

HFCLM-based Perstraction Processes. Perstraction, the combination of permeation and extraction, is selective permeation of a species from a liquid feed through a membrane into a strip or sweep liquid which selectively extracts the permeating species (Sirkar (30) has a brief review). Cahn and Li (31) report separation of toluene and n-heptane through an aqueous ELM and kerosene as the strip liquid. Papadopoulos and Sirkar (32) explored the separation of a 2 vol% isopropanol-n-heptane mixture through a highly polar liquid membrane in a HFCLM... 

Minimum energy required for membrane gas permeation, distillation, extraction and other separation... 

In this section, we wiU illustrate the calculation of the minimum energy required for a variety of separation processes, e.g. membrane gas permeation, distillation, extraction and adsorption. 

Membrane-retained components are collectively called concentrate or retentate. Materials permeating the membrane are called filtrate, ultrafiltrate, or permeate. It is the objective of ultrafiltration to recover or concentrate particular species in the retentate (eg, latex concentration, pigment recovery, protein recovery from cheese and casein wheys, and concentration of proteins for biopharmaceuticals) or to produce a purified permeate (eg, sewage treatment, production of sterile water or antibiotics, etc). Diafiltration is a specific ultrafiltration process in which the retentate is further purified or the permeable sohds are extracted further by the addition of water or, in the case of proteins, buffer to the retentate. 

Various types of detector tubes have been devised. The NIOSH standard number S-311 employs a tube filled with 420—840 p.m (20/40 mesh) activated charcoal. A known volume of air is passed through the tube by either a handheld or vacuum pump. Carbon disulfide is used as the desorbing solvent and the solution is then analyzed by gc using a flame-ionization detector (88). Other adsorbents such as siUca gel and desorbents such as acetone have been employed. Passive (diffuse samplers) have also been developed. Passive samplers are useful for determining the time-weighted average (TWA) concentration of benzene vapor (89). Passive dosimeters allow permeation or diffusion-controlled mass transport across a membrane or adsorbent bed, ie, activated charcoal. The activated charcoal is removed, extracted with solvent, and analyzed by gc. Passive dosimeters with instant readout capabiUty have also been devised (85). 

The concentration of cations at discrete pH values which resulted in the highest observed PE permeability was obtained at pH 8.0 and 0.15M NaCl, pH 3.8 and 0.4M NaCl, or pH 3.8 and O.IM CaCl2. An increase of pH and/or cation concentration promoted an increase in PE permeability. Conversely, flux decreased with an increase in cation concentration or pH. At similar pH and cation concentrations that stimulate PE activity by competitive displacement, permeation of PE through a 30 kD MWCO UF membrane was observed. At low pH and no salt, the low level of pectin present in the PE extracts bound PE and prevented permeation. 

If the photoequilibrium concentrations of the cis and trans isomers of the photoswitchable ionophore in the membrane bulk and their complexation stability constants for primary cations are known, the photoinduced change in the concentration of the complex cation in the membrane bulk can be estimated. If the same amount of change is assumed to occur for the concentration of the complex cation at the very surface of the membrane, the photoinduced change in the phase boundary potential may be correlated quantitatively to the amount of the primary cation permeated to or released from the membrane side of the interface under otherwise identical conditions. In such a manner, this type of photoswitchable ionophore may serve as a molecular probe to quantitatively correlate between the photoinduced changes in the phase boundary potential and the number of the primary cations permselectively extracted into the membrane side of the interface. Highly lipophilic derivatives of azobis(benzo-15-crown-5), 1 and 2, as well as reference compound 3 were used for this purpose (see Fig. 9 for the structures) [43]. Compared to azobenzene-modified crown ethers reported earlier [39 2], more distinct structural difference between the cis... 

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