Size-selective fractionation

An apparatus to fractionate size-selectively small quantities (sub-milligram quantities of nanoparticle material) is presented in Figure 2.4b [19]. This apparatus consists... 

Zeohte 5A is used because its pores can size-selectively adsorb straight-chain molecules while excluding branched and cycHc species. The normal hydrocarbon fraction has better than 95% purity, and the higher octane isomer fraction contains less than 2% normal hydrocarbons (64). 

Molecular sieves have had increasing use in the dehydration of cracked gases in ethylene plants before low temperature fractionation for olefin production. The Type 3A molecular sieve is size-selective for water molecules and does not co-adsorb the olefin molecules. 

Because a filter sample includes particles both larger and smaller than those retained in the human respiratory system (see Chapter 7, Section III), other types of samplers are used which allow measurement of the size ranges of particles retained in the respiratory system. Some of these are called dichotomous samplers because they allow separate measurement of the respirable and nonrespirable fractions of the total. Size-selective samplers rely on impactors, miniature cyclones, and other means. The United States has selected the size fraction below an aerodynamic diameter of 10 /xm (PMiq) for compliance with the air quality standard for airborne particulate matter. 

Figure 2.5 Schematic diagram of an apparatus capable of size-selectively fractionating large quantities of nanoparticles. Reproduced from [18] 2010 IOP Publishing.

Anand, M., McLeod, M.C., Bell, P.W. and Roberts, C.B. (2005) Tunable solvation effects on the size-selective fractionation of metal nanoparticles in C02 gas-expanded solvents. Journal of Physical Chemistry B, 109 (48), 22852-22859. 

Saunders, S.R. and Roberts, C.B. (2009) Size-selective fractionation of nanoparticles at an application scale using C02 gas-expanded liquids. 

Anand, M., You, S.S., Hurst, KM., Saunders, S.R., Kitchens, C.L., Ashurst, W.R. and Roberts, C.B. (2008) Thermodynamic analysis of nanoparticle size selective fractionation using gas-expanded liquids. Industrial and Engineering Chemistry Research, 47 (3), 553-559. 

Purification of Air Prior to Liquefaction. Separation of air by cryogenic fractionation processes requires removal of water vapor and carbon dioxide to avoid heat exchanger freeze-up. Many plants today are using a 13X (Na-X) molecular sieve adsorbent to remove both water vapor and carbon dioxide from air in one adsorption step. Since there is no necessity for size selective adsorption, 13X molecular sieves are generally preferred over type A molecular sieves. The 13X molecular sieves have not only higher adsorptive capacities but also faster rates of C02 adsorption than type A molecular sieves. The rate of C02 adsorption in a commercial 13X molecular sieve seems to be controlled by macropore diffusion 37). The optimum operating temperature for C02 removal by 13X molecular sieve is reported as 160-190°K 38). 

Figure 3a is a schematic of the functionalized zeolite beta. Figure 3b plotted the catalytic conversion of HEX and PYC over 6 A zeolites as a function of time. For sulfonated zeolite (Z-S03H), more than 60 % HEX was converted in 4 hours, and nearly complete conversion was observed over 12 hours. On the other hand, PYC, which has a large molecular size and cannot enter the microporosity, showed less than 8 % conversion over extended reaction time with same Z-S03H as catalyst. Both HEX and PYC were also reacted over pure zeolite beta (Z), and the TMMPS functionalized zeolite (Z-SH) before it was treated with H202. Pure zeolite and Z-SH showed low catalytic activity, and only a small fraction of either HEX or PYC was converted. Further evidence of the size selectivity is provided when amines of different sizes are used to poison (neutralize) the acid sites (19). As shown in Figure 3c, the... 

All of the tests conducted in the pilot plant were with a single batch of Colorado oil shale from the U.S. Bureau of Mines mine at Rifle, Colo. Mine run material was crushed to 1/4-in. size at the mine and sieved elsewhere into various fractions. We selected a —6+10 U.S. Standard sieve size material for most of these tests because it was the largest size which we could successfully feed with our existing equipment. This material had a Fischer assay oil yield in the 20-25 gal/ton range. 

Impurity ions in wurtzite lattices are described by the same expressions for P2, and P3c, with a numerically insignificant difference in P3o. These expressions are only quantitatively accurate in the dilute limit, but many of the doped nanocrystals discussed in this chapter fall in this limit. The reader is referred to Ref. 42 for a generalized treatment of the problem. Figure 2(b) plots the probabilities calculated from Eq. 4a-d as a function of impurity concentration. The fraction of dopants having at least one nearest-neighbor dopant is quite high even at moderate impurity concentrations (<5%). Needless to say, whereas purification to ensure size uniformity is possible (size-selective precipitation), no purification method has yet been developed for ensuring uniform dopant concentrations in an ensemble of nanocrystals. 

Excluded from this list is sieving, to which the concept of selectivity is not applicable. For completeness, we have subdivided the FFF family into sedimentation FFF, thermal FFF, flow FFF, and steric FFF to show how the selectivity of each of these subtechniques compares to that of the other fractionation methods. The values reported here differ from S values reported elsewhere (12), which refer to mass rather than size selectivity. 

FIGURE 8 Displacement histogram and UV detector trace for a selective displacement process. (A) Displacement separation of a three-component protein mixture using streptomycin sulfate A as a displacer. Column 100 X 5 mm i.d. strong cation exchange (8 m) carrier 30 mM sodium phosphate buffer, pH 6.0 feed 1.6 mL of 0.392 mAI ribonudease A, 0.42 mM horse cytochrome c and 0.34 mM lysozyme in the carrier. Total column loading 12.7 mg/mL column displacer 25 mM streptomycin sulfate A flow rate 0.2 mL/min fraction size 200 /iL. (Kundu et al.43) (B) UV detector trace monitored at 280 nm for the displacement separation shown below. 

In the past 20 years size-selective sampling has been applied to ambient sampling as well as personal sampling [John (1984), EPA (1987)]. A method known as the PM-10 method samples particles into two size segments, one greater than 10 pm and the other less than 10 pm (the thoracic fraction). If a conventional mass respirable sampler operates at a flow rate of 1.7 L/min to collect 3.5-pm particles with a 50 percent efficiency, what flow rate would be necessary to collect 10-pm particles with a 50 percent efficiency Assume unit-density spheres. 

Determine the breakage functions that pertain to the size fraction of the feed material Cumulative breakage functions can be calculated [1] from the relation Bi J = Fj/Sj. Therefore, in the case of the feed-size selection function 5), the equation is B ,i = Fi/S, and the breakage functions are as follows ... 

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