Gas-phase separations

Gas-phase separations may be classified as enrichment, sharp, or purification separation, depending on the purity, recovery, and magnitude of the pertinent separation. The classification system allows for a certain amount of synergy, as several separation methods may be combined in order to achieve the desired result. Certain separation methods ate favored for each category (26). 

Gas chromatography, depending on the stationary phase, can be either gas—Hquid chromatography (glc) or gas—soHd chromatography (gsc). The former is the most commonly used. Separation in a gas—Hquid chromatograph arises from differential partitioning of the sample s components between the stationary Hquid phase adsorbed on a porous soHd, and the gas phase. Separation in a gas—soHd chromatograph is the result of preferential adsorption on the soHd or exclusion of materials by size. 

The ability to selectively excite a particular ion (or group of ions) by irradiating the cell with the appropriate radiofrequencies provides a level of flexibility unparalleled in any other mass spectrometer. The amplitude and duration of the applied RF pulse determine the ultimate radius of the ion trajectories. Thus, by simply turning on the appropriate radiofrequency, ions of a single m/z may be ejected from the cyclotron. In this way, a gas-phase separation of analyte from matrix is achieved. At a fixed radius of the ion trajectories the signal is proportional to the number of orbiting ions. Quantitation therefore requires precise RF control. 

Jess, A., Kern, C., Schrogel, K., Jung, A., and Schiitz, W. 2006. Manufacturing of carbon nanotubes and -fibers through gas phase separation. Chemie Ingenieur Technik 78 94-100. 

Whilst improvements have been made in the efficiency of some individual items, the components of the standard block flow-diagram of Fig. 7.1 have changed little over the years. In this chapter, the potential improvements offered by a new concept in operation are considered. The subject here is a different kind of membrane that allows the gas-phase separation of chlorine from some of its accompanying impurities. 

For separation of colloidal particles and for breaking down emulsions, the ultra-centrifuge is used. This operates at speeds up to 30 rpm (1600 Hz) and produces a force of 100,000 times the force of gravity for a continuous liquid flow machine, and as high as 500,000 times for gas phase separation, although these machines are very small. The bowl is usually driven by means of a small air turbine. The ultra-centrifuge is often run either at low pressures or in an atmosphere of hydrogen in order to reduce frictional losses, and a fivefold increase in the maximum speed can be attained by this means. 

Adsorption as a gas phase separation process fills a space in the spectrum of separations processes that encompasses both purification and bulk separations. The market for gas phase adsorptive separations is of the order of several billion US dollars armuaUy when aU sorbent, equipment and related products are included. 

While the Union Carbide organization, which is now in UOP, was the leader in gas phase separations, liquid bulk separations were brought to a high degree of maturity by UOP s Don Broughton and his successors in UOP s liquid phase separations research and development groups. 

In discussing gas phase separations, a few definitions will help in understanding the subject matter. Adsorbents, sometimes referred to here as sorbents, are solid chemical substances that possess micro-porous surfaces that can admit molecules to the interior surface of the structure. Zeolites in particular are solid, micro-porous, alumino-silicates with adsorption and or ion exchange capability. They affect separations by adsorbing molecules into their micro-structures. 

While inert and displacement purge regeneration is widely used in liquid phase separations, there are few industrially relevant inert purge systems employed in gas phase separations. It is sufficient to note that an inert purge regeneration can be done and it will generally be most effective at relatively high adsorption temperatures. 

The transport of an adsorbable species from the bulk fluid flowing around an individual bead is a problem of molecular diffusion. With the fluid in motion the rate of transport to the surface of a bead or pellet of adsorbent material is generally treated as a linear driving force. Eor gas phase separations there are a variety of correlations available to describe the mass transport to the surface in terms of the particle Reynolds number, the Schmidt number, the size of the adsorbent particle and of course the binary diffusivity of the species of interest. 

Industrial gas phase separations span a wide variety of apphcations, from ultrapurifications to bulk separations. In the preceding sections I touched upon some of the more widely practiced separations technologies. 

Gas-phase liquid particle. The gas-phase liquid particle size of choice is generally 150 pm or less, which results in excellent gas-phase separation of liquids. For more critical liquid particle removal, such as with turbine blade gas suction, a 100-pm particle size input is recommended with a supporting demister pad in the separator. 

Volatilization processes, combined with gas adsorption chromatographic investigations, are well established methods in nuclear chemistry. Fast reactions and high transport and separation velocities are crucial advantages of these methods. In addition, the fast sample preparation for a-spectroscopy and spontaneous fission measurements directly after the gas-phase separation is a very advantageous feature. Formation probabilities of defined chemical compounds and their volatility can be investigated on the basis of experimentally determined and of predicted thermochemical data, the latter are discussed in Part II of this chapter. 

In transactinide chemistry research, gas phase separation procedures play an important role. Already, the very first investigation of rutherfordium has been conducted in form of frontal isothermal gas chromatography in a chlorinating atmosphere [1]. The success of gas chemical separations in transactinide research is quite remarkable since gas chromatography is, in general, of minor importance in inorganic analytical chemistry. 

There are several reasons for this exceptional situation. First, production of transactinides at accelerators implies a thermalization of the primary products in a gas, usually helium. It is rather straightforward to connect such a recoil chamber to a gas chromatographic system. Second, gas phase separation procedures are fast and may be performed in a continuous mode. Third, at the exit of the chromatographic column separated volatile species can be easily condensed as nearly weightless samples on thin foils. This enables detection of a decay and spontaneous fission (SF) of the separated products with supreme energy resolution. 

Magnetic heterogeneity of FC-4 was studied by magnetic separation both in gas and in liquid. Gas phase separation was done in a small cyclone-type separator, where the agglomerates were destroyed by the turbulent flow. More than 97 % of the powder was... 

The generic permselectivity of a membrane can be described by the retention coefficient for liquid phase or the separation factor for gas phase. Separation factor will be defined and discussed in Chapter 7. In the case of liquid-phase membrane separation, the retention coefficient of the membrane can be characterized by some commonly used model molecules such as polyethylene glycol (PEG) polymers which have linear chains and arc more flexible or dextians which arc slightly branched. The choice of these model molecules is due to their relatively low cost. They are quite deviated from the generally... 

In contrast to liquid-phase tqiplications reviewed in Chapter 6. the current sales volume and application varieties of inorganic membranes in the gas-phase separation market are still quite limited. Their commercial usage in the gas- and vapor-phase environments is far from being a significant presence. Even the first largest gas-phase separation application, gas diffusion for uranium enrichment, discussed in Chapter 2 no longer requires any major production efforts. 

The methods and literature are briefly reviewed for solid-suspension separations, solution-phase separations, liquid-phase separations, and gas-phase separations. In the terminology used, the objective is to separate a feed stream (or streams) into a permeate phase and a reject phase, either of which may contain the compon-ent(s) of more interest. For a single membrane, say, the permeate phase remains on the feed side or high-pressure side of the membrane, and is subsequently discharged, whereas the reject or raffinate phase builds up on the opposite or low-pressure side of the membrane, and is then discharged. 

The previous discussion illustrates the difficulties in attaining high gas-phase separation factors with zeolite membranes, even when good quality membranes are employed. In spite of these difficulties, van de Graaf et al. [185,186] managed to obtain a considerable increase in conversion (13% higher than the thermodynamic value under optimal operation conditions) and a very significant shift in product selectivity (a 34% increase in the ratio rran -2-butene/cw-2-butene) in the metathesis of... 

J. H. Wang and M. B. Ray, Application of ultraviolet photooxidation to remove organic pollutants in the gas phase, Separation and Purification Technol. 19, 11-20 (2000). 

CA Srebalus, JW Li, WS Marshall, DE Clemmer. Determining synthetic failures in combinatorial libraries by hybrid gas-phase separation methods. J Am Soc Mass Spectrom 11 352-355,2000. 

Fig. 22.10(c) shows the following surprising results (i) The predicted energy of adsorption (70 kj mol i at 0 K) is of the same order of magnitude as estimates based on experiments for related molecules (50-63 kJ moh ). (ii) With respect to isobutene in the gas phase separated from the zeolite, the tert-butyl cation is much less stable (-17 kJ moTi) than the isobutoxide (-48 kJ moh ). The reason is that dispersion contributes substantially less to the stabilization of the tert-butyl cation than to the stabilization of the adsorption complex or the isobutoxide. As result, the proton transfer energy increases from 24 kJ moH (DFT) to 59 kJ moh (MP2/DFT) and it seems very unlikely that the fert-butyl cation will be detected in zeolites, even as a short-lived species. 

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