Membrane-based separation studies

FIGURE 2.6 Variation of U, Th, and nitric acid transport with feed Th concentration. Carrier 1 M D2EHIBA/ -dodecane. Strip pH 2 solution. Feed acidity 4 M HN03. [U]feed = -200 mg/L. Duration 8 h. (From Shailesh, S. Pathak, P.N. Mohapatra, P.K. Manchanda, V.K., J. Membr. Sci. 272, 143-151, 2006. With permission.) 

---

According to lUPAC a membrane reactor is a device for simultaneously carrying out a reaction and membrane-based separation in the same physical enclosure . This type of reactor has been studied intensively by many groups, resulting in more than 1400 publications since 1994 [5]. The membrane may act in several ways. A rough classification is illustrated in Figure 1. Examples of each type are listed in Table 1. 

Membrane-based separation processes to capture either H2 or CO2 from the gasifier are new and less studied methods of CO2 separation and capture. Membranes separate the desired gas component without requiring phase changes or chemical or physical sorption. The cost of membrane separation is generally dictated by the overall pressure drop. Membranes made of various types of materials such as polymers, metals, and rubber composites have been investigated. Palladium and molecular sieves are currently under study. ° ... 

Extensive studies on hollow fiber membrane based separation technology (HFMST) carried out for efficient removal of toxic heavy metals like Cr(VI), Cd, Zn, Ni, separation and concentration of gold from alkaline hydrometallurgical solution [13-26] were described in [2]. Further, recovery of valuable solutes from aqueous phases—for example, citric acid, carboxylic acid, amino acids, and L-phenylalanine [27,28]—such as removal of phenol from industrial waste water [29] are well demonstrated by using this technology and are described in detail in a later section of this chapter. 

The same time-invariant mode of feed stream introduction and feed-phase flow perpendicular to the force direction is also utilized in separations where any one stream enters the separator, provided at least two product streams are continuously withdrawn from the separator. Crossflow membrane separators and external force based separators studied in Sections 7.2 and 7.3 are particular examples. Of the at least two product streams, one is the depleted feed stream. The other product stream continuously removes selected species from the feed stream (Figure 6.1.9(b)). 

In the recent past separation of isomers has been attempted using aqueous liquid membranes based on p-cyclodextrin. Thus, separation of a mixture of o- and p-nitroaniline (in 80% i-octanol, 20% -heptane) has been studied, with the p-isomer showing a selectivity of 5 at 0.7 molar p-cyclodextrin. Even stereoisomers of stilbene cis and trans) were separated using a 0.02 to 0.2 M cyclodextrin solution, but the selectivity was less than 2 (Mandal et al, 1998). 

Although rum ammonia levels are not routinely measured, it is a useful indicator of Reye s syndrome and should be monitored in newborns at risk of developing hyperammonemia Ammonia is produced in many analytically useful enzyme reactions and the ammonium ISE has been used as the base sensor in several enzyme electrodes (see next section). In addition to valinomycin, other antibiotics such as the nonactin homalogs and gramicidins also behave as ionophores. The nonactin homolo were originally studied for their ability to selectively bind potassiiun ions It was then discovered that ammonium ions were preferred over potassium ions, and the selectivity coefficient Knh+ = 0.12 was reported. Since ammonia is present at fairly low levels in serum, this selectivity is not sufficient to to accurately measure NH4 in the presence of K. An extra measure of selectivity can be gained by using a gas permeable membrane to separate the ammonia gas from the sample matrix... 

Schlosser, S., Kertesz, R. and Martak, J. (2005) Recovery and separation of organic acids by membrane-based solvent extraction and pertraction - An overview with a case study on recovery of MPCA. Separation and Purification Technology, 41, 237. 

Much effort has been expended in attempting to use membranes for separations. Reverse osmosis membranes are used worldwide for water purification. These membranes are based on size selectivity depending on the pores used. They do not have the ability to selectively separate target species other than by size. Incorporation of carrier molecules into liquid membrane systems of various types has resulted in achievement of highly selective separations on a laboratory scale. Reviews of the extensive literature on the use of liquid membrane systems for carrier-mediated ion separations have been published [15-20]. A variety of liquid membranes has been studied including bulk (BLM), emulsion (ELM), thin sheet supported (TSSLM), hollow fiber supported (HFSLM), and two module hollow fiber supported (TMHFSLM) types. Of these liquid membranes, only the ELM and TMHFSLM types are likely to be commercialized. Inadequacies of the remaining... 

Electrodialysis is a membrane-based process which can be used for separation, removal, or concentration of ionic species present in aqueous solutions. These operations are accomplished by the selective transport of ions through an ion exchange membrane under the influence of a direct current. One of the earliest applications of electrodialysis was the desalting of brackish water. However, since the 1970s, extensive studies have been performed on the application of electrodialysis for waste-water treatment, especially in the electroplating and metal-finishing industries. 

---