1、CR 135255 SIGMA RESEARCH. INC. 2950 GEORGE WASHINGTON WA Y RICHLAND. WASHINGTON 99352 ,.*s“ . ; (:3 U, N78-16329 Unclas 59489 “;“7. 0.66 atm) a linear relationship was indicated (moles/cm3 sec) (2.l-9) whee AI, A, and B are curve-fitting rate constants, and P is pressure. At both high and low pressu
2、re, the rate constants were xponential in temperature in accordance with models discussed in Section 2.1.1, but differed consider ably in value, as shown in Table 2.1-2. TIBLE 2.1-2. EFFECT OF PRESSURE ON THE ACTIVATION ENERGY E+ (Kcal/g-mole) FOR DECOMPOSITION OF NORMAL HY DROCARBONS 1 ) Method of
3、Calculation n-propane -butane n-pentane n-heptane Extrapolation to zero pres sure Extrapol adon to infinite pressure 71 68 (1) From Ingold et al. (2) 75 93 88 59 63 59 + Energy E is Kcal/g-mole. Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2-8 As
4、an example of decomposition being rate-limited by free radical production, in the pyrolysis of toluene at a constant pressure of 0.008 atm and tempera tures between 738 and 964oC Szwarc obtained a reaction product gas which was essentially 40% methane and 60% hydrogen. The gases were also accompanie
5、d by dibenzyl in the ratio of 1 mole per mole of gases. The rate constant was exponential in temperature and corresponded to an activation energy of 77.5 Kcal/mole. (3) Later measurements by Price indicate a more accurate value of 85 Kcal/mole. (4) Because of the products formed, the decomposition m
6、echanism was attributed to be rate-limited by production of benzyl and hydrogen free radicals (2.1-10) and the 77.5 Kcal/mole is equivalent to activation energy E+ in Figure 2.1-1. For free radicals, E2 z 0, so that equations (2.1-6) and (2.l-7) are appropriate for description of the reaction rate.
7、The energy E+ is the bond dissociation energy for free radical production. This important quantity is discussed in more deta i1 in Secti on 2.2 . In summary, the thermal decomposition of a gaseous species is generally characterized by 1. an Arrhenius-type of rate equation with a pre-exponential fact
8、or IOI3 sec-I; 2. a concentration-dependent rate constant at low pressure; 3. a relatively concentration-independent rate constant at high pressure. These concl us;ons hold for many simple syste1ls, but theye are some exce;:tions to these generalizations and care must be exercised in their use. 2.2
9、Definition of Stable Organic Specie A stable molecule is obviously one in which the various intramolecular bonds have high stability, and bond stability is the basic factor determined in pyrolysis experiments. A large body of data exists on both the pyrolysis of organic compounds to form stable mole
10、cules and the pyrolysis of organic Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-1 1 j 1 1 I 2-9 compounds to form free radicals. These data can be of significant help in defining high temperature two-phase working fluids, particularly if the data
11、are interpreted on the basis of bond strength. In the previous section, the mechanisms associated with pyrolysis were described. A critical bond dissociation energy E+ was invoked, but no attempt was made to characterize and define bond strength in terms of organic structure. That is, the number of
12、possible organic compounds is infinite for all practical purposes and pyrolysis data are available for only a very small number of these compounds. However, the physical structures of many organic molecules are closely related, and examination of bond strengths for groups of homologous compounds is
13、possible and practical. In addition, covalent electron pair bonds reta.in much of their identity rega-rdless of what is occurring in the remainder of the molecule, and hence bond disso ciation energies can often be used even with considerable change elsewhere in the molecule. 2.2.1 Bond Theory of St
14、able Molecules Chemical bonding in organic molecules is chiefly covalent and dominated by the formation of hybrid electron orbitals. Although accurate quantum mechanical representations for compounds of carbon cannot be calculated, it is possible to show that certain linear combinations of the stand
15、ard s, p, and d electronic orbitals possess minimum energy and, hence, high stability. One type of hybrid orbital is designated liSp“, and is formed by combining an s orbital with a p orbital to form two equivalent sp orbitals. The orbitals are oriented 1800 from each other. In the sp2 hybridization
16、, a single s orbital and two p orbitals form three equivalent hybrid sp2 orbitals, on a 3-cornered arrangement. The sp3 hybrid is formed from one s orbital and three p orbitals and is tetrahedrai. Because of their greater s character, sp orbitals are generally smaller than sp2 orbitals and sp2 orbit
17、als are smaller than sp3 orbitals. A small orbital generally indicates a short, strong bond, and bond strength increases as sp3 Rcr) an d that no paraffinic hydroca rbons have bot a low decompo si tion rate an d a reasonabl e vapor pressure at 3500C. Iso-bu:ne, for example, has a rate con s ant of 1
18、.6E-5 at 3500C, and a boil i ng point of -120C. Other higher mol ecular weight an d lower pressure paraffins have hi gn er decompositi on :ates . 80th toluene an d methyl - napthalene have dissociation rates on the order of 1.E-9 or l ess , and il l ustrate the sperior high temperature properties of
19、 he aromat cs. One of the most serious results of the pyrolysis of a two-phase heat transfer f l uid is the production of low boiling point secondary compounds, or non cond ensable gases (NCG) . Th ese gases, typi cally hy droge n in he cas e of organics, in hi bit condensation heat transfer. By its
20、elf , the py rolysis data above are useful but not comp lete in that there is still unce r tainty in the rate of NCG roduction since t he py rolysis may not terminate with a gaseous product. Figure 2.2-1 shows t he formation rate of NCG by pyrolysis for ei ght fairl y stabl e organic wor king f l ui
21、ds s a function of temperature in t he Arrhenius fa shion (liT) . Since Table 2. 2- 3 showed that t he most stable fluids had activation energies (E*) in the region around 70 Kcal/mol e, the ab ov e i ure shows a performance un cel ainty band around this value. Fluids behav ing within the band can b
22、e considered as suitab le for heat pipe use be low about 4000C, depending on lon gevity requirements . 1. Some perti nen work on hig temperature stabili y of heat transfe r flui ds wa s carried out by Seifert , Jackson , and Se ch . (l7) Their interest was in liquid phase stability of 4 compounds or
23、 mix tures in the temperature range of Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-2-18 343-4130C (650-7750F). Samples were heated in stainless stee
24、l tubes contain ing 02 free nitrogen. After testing, percent weight loss, presence of residual pressure (at -700C), and physical state were noted before the fluid was analyzed by gas chromatography. Analyses were reported as low boiling products, original fluid and high boiling products. Results fro
25、m 16 organic fluids (8 other halogen organic fluids are discussed later) are sum marized in Table 2.2-4. Quantitative pyrolytic damage is only indicated by percent oririnal material lost, or percent higher and lower boiling fractions. Qualitativf: damage is indicated by physical state, color and pre
26、sence of NCG. The lower boiling fractions would probably appear as noncondensable gas in a heat pipe or other two-phase heat transfer device. The most significant observations to be made are the high stability of the arcatic ethers and the variable stability of alkylated aromatic ethers. For the lat
27、ter, dealkylation seems to be the primary form of pyrolysis, with ortho substituted alkyl groups being least stable. Mi 11 er et a 1 Another search for high temperature working fluids, this time for Automotive Rankine engines, provides a somewhat qualitative set of results for apparent vapor phase p
28、yrolysis of organic working fluids. One hundred ten fluids were studied by heating small quantities to 3820C (7200F) in carbon steel con tainers without removing atmospheric air. Minimum test times were 24 hours and decomposition was ascertained by visual inspection (color, solids content) and/or pr
29、esence of strong acid. Of thp 110, eleven organlc fluids not containing halogens passed the minimum 24-hour test and nine (nonhalogen) organic-water mixtures were tested at the same temperature. The results are summarized in Tables 2.2-5 and 2.2-6. At the elevated temperature (382C) all the specimen
30、s were in the gas phase since the critical temperature was exceeded. Even the water-fluid mixtures were vaporized (To = 3740C for water). With the small amount of air present, some oxidative degradation was likely. Most organic compounds readily oxidize in contact with liquid water and air under pre
31、ssure at about 2500C and higher. (18) This could have been a mode Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-1- TABLE 2.2-4 STATIC THERMAL STABILITY COMPARISONS OF SELECTED ORGANIC FLUIDS Gas Chromatographic Analysis Compound Length Physicala* R
32、esidualb* Weeks d* e* (freezing point and of Test Temp. Sample % Lights % Higher % normal boiling point, C) (weeks) (oC) Lost-% State Color Pressure at c (L) (H) L+H Diphenyl ether - diphenyl 0 L Lt. Yellow 2,413 9.5 eutectic 3 413 1.9 L Yellow No 4,413 5.5 (12.2, 257) 2 413 4 413 Biphenylyl phenyl
33、ether 0 S White (isomer mixture) 4 343 2.1 VL Brown No (37.2, 360) 6 399 5.7 VL Black No 2 413 3.1 VL Brown No 2,413 4.9 4 413 2.6 VL Brown No 2,413 11.9 1 o-biphenylyl phenyl ether 0 S White N l (50, 354) 3 399 1.0 VL Tan No 2,413 4.5 I n - , 4 413 1.3 VL Brown No 4,413 10.4 ;1 i m-biphenylyl pheny
34、l ether 0 L Yellow ;1 II (not available) 2 413 2.5 L Brown No 4 413 2.3 L Brown No il di- and triaryl ethers2* :; 0 - L Yellow i 2_4c “ -18, 300) 8 343 L Brown No 4,343 0.3 iT :1 4 371 2-4c L Brown No 4,385 4.4 i) 3 399 2-4c L Brown No 1,413 0.7 5.1 5.8 Ii 3 413 2-4c L Brown No- 4,413 2.4 7.0 9.4 Ii
35、 II Dimethyl diphenyl ether3* II 0 L Colorless II (isomer mixture) 4 343 3.2 L Brown No I! i (-40 pour point, 290) 3 371 17 L Black Yes I 3 399 24 VL Black Yes Tetramethyl diphenyl ether 0 L Yellow (isomer mixture) 2 343 1.5 L Yellow No (N.A., 310) 4 343 1.7 L Brown No 2 371 4.5 L Black No 4 371 8.5 L Black No * See last page of table for footnotes. _ . _ - - _ _ - _.a. _ _ - - - - -, - - - - - .- -Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-
