O/C ratio

Coke on the catalyst is often referred to as delta coke (AC), the coke content of the spent catalyst minus the coke content of the regenerated catalyst. Delta coke directly influences the regenerator temperature and controls the catalyst circulation rate in the FCCU, thereby controlling the ratio of catalyst hydrocarbon feed (cat-to-od ratio, or C/O). The coke yield as a fraction of feed Cpis related to delta coke through the C/O ratio as ... 

High resolution and high S/N spectra of 32 metal poor stars observed with various 4-m class telescopes (Israelian et al. 1998, 2001, Bihain et al. 2004) have been used to analyse N/O and C/O ratios from near-UV molecular lines of CH, NH and OH. We have carried out an independent study of the (N/O) ratio using the NH band at 3360 A and the OH lines employed by Israelian et al. (1998, 2001). Details of the analysis and stellar parameters are provided in Israelian et al. (2004). 

We stress that any possible abundance errors produced by uncertainties in the near-UV continuum, 3D and/or non-LTE effects are not critical for the present analysis. These errors will cancel out when forming abundance ratios from lines formed in the same atmospheric layer. This philosophy led Tomkin and Lambert (1984) to derive C/O ratio from the bands NH 3360 and CH 4300 A. 

We have selected several unblended CH lines located in the near-UV between 3145-3190 A modified their oscillator strengths (gf-values) by fitting the solar high-resolution spectrum and assuming solar abundance of carbon 8.56 from Anders and Grevesse (1989). These lines are measurable in dwarfs down to the metallicities —3. Our results are shown in the Fig. 1. We confirm the metallicity dependence of the C/O ratio (Tomkin et al. 1992, Akerman et al. 2004). On the other hand, our plot for C/O shows a steep rise at [0/H]< —1. It is not clear if this effect is real. The work is in progress to address this issue using other abundance indicators such as CH 4300 A and better quality spectra. 

Fig. 1. Comparison of C/O ratios for programme stars (filled diamonds) with observations of Akerman et al. (2004) (crosses). Empty boxes are metal-rich stars from Israelian et al. (2004).

AGB stars constitute excellent laboratories to test the theory of stellar evolution and nucleosynthesis. Their particular internal structure allows two important processes to occur in them. First is the so-called 3(,ldredge-up (3DUP), a mixing mechanism in which the convective envelope penetrates the interior of the star after each thermal instability in the He-shell (thermal pulse, TP). The other is the activation of the s-process synthesis from alpha captures on 13C or/and 22Ne nuclei that generate the necessary neutrons which are subsequently captured by iron-peak nuclei. The repeated operation of TPs and the 3DUP episodes enriches the stellar envelope in newly synthesized elements and transforms the star into a carbon star, if the quantity of carbon added into the envelope is sufficient to increase the C/O ratio above unity. In that way, the atmosphere becomes enriched with the ashes of the above nucleosynthesis processes which can then be detected spectroscopically. 

The chemical analysis has revealed that rather low C/O ratios are found in metal-poor extragalactic carbon stars, as found for galactic carbon stars of the solar vicinity. Furthermore, the three analyzed stars show similar s-elements enhancements [ls/Fe]=0.8-1.3 and [hs/Fe]=l.l-1.7. This leads to new constraints for evolutionary models. For instance, the derived C/O and 13C/12C ratios are lower than model predictions at low metallicity. On the contrary, theoretical predictions of neutrons exposures for the production of the s-elements are compatible with observations (see Fig. 1). Finally, from their known distances, we have estimated the luminosities and masses of the three stars. It results that SMC-B30 and Sgr-C3 are most probably intrinsic carbon stars while Sgr-Cl could be extrinsic. 

Table 6.2 gives an overview of some of the stages of stellar evolution where carbon and/or s-process anomalies occur (see Fig. 3.37). The C/O ratio increases down the series. In addition to the types listed there, there are infrared carbon stars such as IRC +10216,1 proto-planetary nebulae and a whole zoo of peculiar carbon stars, including J stars (strong 13C as in the case of HD 52432 shown in Fig. 1.7) and hydrogen-deficient carbon stars which can be cool, e.g. R Cor Bor, RY Sag and HD 137613 shown in Fig. 1.7, or hot (when they look like extreme helium stars) such stars may have lost their envelopes by binary mass transfer, or they may be born-again AGB stars. 

Fig. 8.3. C/O ratio vs. O/H in Galactic and extragalactic H n regions, based on International Ultraviolet Explorer (IUE) and Hubble Space Telescope (HST) observations. After Garnett (2004).

Russell explains spectra of carbon stars (types R, N, S) as consequence of reversal of the usual C/O ratio. 

Hoyle successfully predicts existence of a 7.6 MeV resonance state of the carbon-12 nucleus on grounds that otherwise little carbon would survive further processing into oxygen during stellar nucleosynthesis by helium burning, whereas in fact the C/O ratio is about 0.5. Discovery of strange particles. 

Para-t-butylphenyl glycidyl ether BPGE had a similar viscosity and C/O ratio as those of NA and had the best properties of the photocurable epoxies that were surveyed, but this monomer dewetted from Si substrates immediately after spin coating and formed a puddle at the substrate center. Other monofunctional epoxies exhibited the same behavior. Mixtures of BPGE with multifunctional aromatic epoxies wetted Si substrates and could be used as planarizing layers. 

For the cracking of fraction No.6, a higher c/o-ratio was required for H2 compared with H6 in order to obtain a given conversion level. This reflects the additional activity supplied by the alumina part of the matrix. The contribution of the alumina in the matrix is also seen when fraction No. 6 is cracked over spray dried samples of the matrices only. At a c/o-ratio of 3.0, conversions of 33% and were obtained using the kaolin and the kaolin-alumina matrix, respectively. The tendency for higher coke and gas production at the expense of gasoline over catalyst H6 compared with H2 is also seen for oil No. 6. 

Figure 6.15 Variation of c/o ratio in rutile-type oxides with d electron configuration. (After Rogers et al, 1969.)...

Third, carbon, nitrogen, and oxygen, the elements that catalyze hydrogen burning in the Sun, are relatively abundant in the solar system. Oxygen is somewhat more abundant than carbon the C/O ratio is 0.54. Because CO is a very stable molecule, most of the carbon and oxygen in the early solar system was tied up in CO. The excess oxygen that remained controlled the chemical environment of the early solar system. 

At luminosities above the MS and S stars in the Magellanic Cloud clusters, we find cool (N type) carbon stars in which the third dredge-up has increased the C/O ratio to >1. There have been many studies of the carbon stars in the Magellanic Clouds a good review and comparison of this work with theory is given by Iben and Renzini (1983), with some more recent work being found in Lattanzio (1986). 

According to Johnson (1974) the LOI values of many common materials can be reasonably well predicted by the expression, provided the atomic C/O ratio is larger than 6 ... 

---