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Supercritical fluid cleaning - McHardy J.

McHardy J., Sawan P.S. Supercritical fluid cleaning - Noyes publications, 1998. - 304 p.
Download (direct link): spercrificalfluidcleaning1998.pdf
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Table 3 (constructed from Refs. 21-24) lists various physical properties for a variety of candidate SCF solvents. A cursory inspection of the entries in this table shows that many hydrocarbons have a critical pressure close to 45 bar and the critical temperature of SCF solvents increases as the molecular weight of the solvent increases or as the polarity or intramolecular hydrogen bonding of the solvent increases. This means that the solvents vapor pressure curve extends to very high temperature.
Water is also included in the table to make one pointthe solvent that we are all most familiar with is a poor candidate from both engineering and safety standpoint. The critical temperature and pressure are among the highest for common solvents. Ammonia is very unpleasant to work with since a fume hood or other venting precautions are needed to keep it out of the laboratory atmosphere. One of the alternative fluids of potential interest is nitrous oxide. It is attractive since it has molecular weight and critical parameters similar to carbon dioxide, yet has a permanent dipole moment and is a better solvent than carbon dioxide for many solutes. There are evidences of violent explosive reactions of nitrous oxide in contact with oils and fats. For this reason, nitrous oxide should be used with great care and is not suitable as a general purpose extraction fluid J251
As Table 3 shows, the critical temperatures of gases and liquids can differ by hundreds of degrees; this difference suggests the use of specific supercritical fluids for specific applications. For example, because the critical temperatures of carbon dioxide, ethane, and ethylene are near ambient, they are attractive solvents for processing heat-sensitive flavors, pharmaceuticals, labile lipids, and reactive monomers. Substances that are less temperature-sensitive, such as most industrial chemicals and polymers, are readily treated with C3 and C4 hydrocarbons with critical temperatures in the range 100-150C; C3 and C4 hydrocarbons are generally better solvents for polymers than C2 hydrocarbons. Still higher molecular weight hydrocarbons, such as cyclohexane and benzene, with high critical temperatures of 250-300C, are used to process nonvolatile substances such as coal and high molecular weight petroleum fractions.^
Table 3. Candidates for supercritical fluids and their various physical properties. The critical pressure is represented in three different units. (Constructed from Refs. 21-24.)
Molecular Density Temp. Dipole Critical Temp. Critical Critical Critical
weight, Mw of Liquid of liquid moment Factor, (T/t,.) Pressure (Pc) Volume, Vc Compress. Fluid (gm/mol) (gm/mol) (K) (debyes) (K/C) (bar) (atm) (psi) (cm3/mol) (Z)c
Methane, (CH4) 16.04 0.425 112 0.0 190.6/-82.4 46.0 46.6 667.2 98.7 0.288
Ethane, (C^Hs) 30.07 0.548 183 0.0 305.4/32.4 48.8 49.5 707.8 148.3 0.285
Propane (C3H8) 44.09 0.582 231 0.0 369.8/96.8 42.5 43.1 616.4 203.0 0.281
n-Butane, (4) 58.12 0.579 293 0.0 425.2/155.2 38.0 38.5 551.1 255.0 0.274
n-Pentane, (C3H12) 72.15 0.626 293 0.0 469.6/196.6 33.7 34.1 488.8 304.0 0.263
n-Hexane, (CeH^) 86.18 0.659 293 0.0 507.4/234.4 30.1 30.5 436.6 370.0 0.264
n-Heptane, (C7H]6) 100.21 0.648 293 0.0 540.3/267.3 27.4 27.8 397.4 432.0 ---
Acetonitrile, (CH3CN) 41.05 0.782 293 3.5 548.0/275.0 48.3 48.9 700.6 173.0 0.184
Acetone, (CH3-CO-CH3) 58.08 0.790 293 2.9 508.1/235.1 47.0 47.6 681.7 209.0 0.232
Methanol, (CH3-OH) 32.04 0.791 293 1.7 513.1/240.1 80.9 82.0 1173.4 118.0 0.224
Ethanol, (CH3-CH2-OH) 46.07 0.784 293 1.69 516.2/243.2 61.4 62.2 890.5 167.1 0.240
Isopropanol, (CH3-CHOH-CH3) 60.10 0.786 293 1.7 508.8/235.8 47.6 48.2 690.4 220.0 0.248
Carbon dioxide, (C02) 44.01 --- --- 0.0 304.2/31.1 73.8 74.8 1070.4 94.0 0.274
Carbon disulfide (CS2) 76.13 1.293 273 0.0 552.0/279.0 79.0 80.1 1145.8 160.0 0.276
Ammonia, (NH3) 17.03 0.639 273 1.47 405.4/132.4 113.5 115.0 1646.2 72.5 0.242
Nitrous oxide, (N20) 44.01 1.226 184 0.2 309.6/36.6 72.4 73.4 1050.1 97.4 0.274
The Supercritical State
Table 3. (Contd)
Fluid Molecular Density Temp, Dipole Critical Temp. Critical Critical Critical
weight, M of Liquid of liquid moment Factor, (TJtc) Pressure (Pc) Volume, Vc Compress.
(gm/mol) (gm/mol) (K) (debyes) (K/C) (bar) (atm) (psi) (cm3/mol) (Z)
Xenon, (Xe) 131.3 3.06 165 0.0 289.7/16.7 58.4 59.2 847.0 118.4 0.287
Water, (H20) 18.02 0.998 293.0 1.85 647.4/374.4 221.2 224.1 3208.2 55.3 0.227
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