Carbon deposition rate

Chemical vapor deposition (CVD) of carbon from propane is the main reaction in the fabrication of the C/C composites [1,2] and the C-SiC functionally graded material [3,4,5]. The carbon deposition rate from propane is high compared with those from other aliphatic hydrocarbons [4]. Propane is rapidly decomposed in the gas phase and various hydrocarbons are formed independently of the film growth in the CVD reactor. The propane concentration distribution is determined by the gas-phase kinetics. The gas-phase reaction model, in addition to the film growth reaction model, is required for the numerical simulation of the CVD reactor for designing and controlling purposes. Therefore, a compact gas-phase reaction model is preferred. The authors proposed the procedure to reduce an elementary reaction model consisting of hundreds of reactions to a compact model objectively [6]. In this study, the procedure is applied to propane pyrolysis for carbon CVD and a compact gas-phase reaction model is built by the proposed procedure and the kinetic parameters are determined from the experimental results. 

Experiments of propane pyrolysis were carried out using a thin tubular CVD reactor as shown in Fig. 1 [4]. The inner diameter and heating length of the tube were 4.8 mm and 30 cm, respectively. Temperature was around 1000°C. Propane pressure was 0.1-6.7 kPa. Total pressure was 6.7 kPa. Helium was used as carrier gas. The product gas was analyzed by gas chromatography and the carbon deposition rate was calculated from the film thickness measured by electron microscopy. The effects of the residence time and the temperature... 

Measurements of S cycling in Little Rock Lake, Wisconsin, and Lake Sempach, Switzerland, are used together with literature data to show the major factors regulating S retention and speciation in sediments. Retention of S in sediments is controlled by rates of seston (planktonic S) deposition, sulfate diffusion, and S recycling. Data from 80 lakes suggest that seston deposition is the major source of sedimentary S for approximately 50% of the lakes sulfate diffusion and subsequent reduction dominate in the remainder. Concentrations of sulfate in lake water and carbon deposition rates are important controls on diffusive fluxes. Diffusive fluxes are much lower than rates of sulfate reduction, however. Rates of sulfate reduction in many lakes appear to be limited by rates of sulfide oxidation. Much sulfide oxidation occurs anaerobically, but the pathways and electron acceptors remain unknown. The intrasediment cycle of sulfate reduction and sulfide oxidation is rapid relative to rates of S accumulation in sediments. Concentrations and speciation of sulfur in sediments are shown to be sensitive indicators of paleolimnological conditions of salinity, aeration, and eutrophication. 

Curry W.B. and Lohmann G.P. (1985) Carbon deposition rates and deep water residence time in the equatorial Atlantic Ocean throughout the last 160,000 years. In The Carbon Cycle and Atmospheric CO2 Natural Variations Archean to Present (eds. E.T. Sundquist and W.S. Broecker), pp. 285-301. Amer. Geophys. Union, Washington, D.C. 

Earlier studies in nickel catalysts resistance to coking in steam reforming showed that the carbon deposition rate depends not only on such direct factors as nickel dispersion [10] or the support composition [11], but also on indirect factors, connected with the preparation and pretreatment conditions of the systems, the latter influence the coking rate by causing changes of the direct factors [12]. 

Carbon deposition rates were measured in a microreactor connected to a Sartorius 4436 high pressure microbalance [11]. The catalyst (17-70 mg) was placed on quartz wool in a perforated quartz basket in a stainless steel reactor lined with an alumina tube (i.d. = 15 mm) and hung from one arm of the microbalance by a quartz fiber. Water was fed using a Lewa M3 pump. A flow of inert gas was always maintained through the microbalance. The composition of the product gas could be determined by on-line gas chromatography. 

Small quantities of HjO increased the observed rate of carbon formation. This behavior was reproduced at all three levels of temperature. The maximum carbon deposition rate was observed at about 0.1 - 0.2 bar H O. 

Systematic assays for organic carbon carried out on Phanerozoic rocks as well as observed oscillations of the carbon isotope age function (Figure 31) indicate that organic carbon deposition rates have just moderately oscillated around a mean of perhaps 0.5% in the average sediment over this time span. 

In the case of high-temperature, fluidized-bed reactors operation at higher pressures has an additional benefit in that the rate of carbon deposition on iron catalysts is proportional to Pgg/P H2 (ref. 2). As the partial pressures increase, the carbon deposition rate decreases. Because the deposited carbon has a high area and acts like a sponge for retaining wax, the rate of wax accumulation on the catalyst also decreases with increasing pressure (ref. 2). The benefit of this is that the catalyst particles are less likely to become "sticky" and result in defluidization of the bed. The CFB reactors at Sasol Two and Three operate at higher pressures than the older units at Sasol One, and the lower deposition rates of carbon and wax on... 

Figure 10. Apparent reaction order with respect to hydrogen for carbon deposition rate at constant temperature, constant pressure (1 atm), constant benzene partial pressure on nickel foils (T = 650°C, P = 0.132 atm) and iron foils (T = 625°C, PB = 0.132 atm). Key , Ni foils and , Fe foils.

Anderson, O. R. Faber, W. W. 1984. An estimation of calcium carbonate deposition rate in a planktonic foraminifer Globigerinoides sacculifer using Ca as a tracer a recommended procedure for improved accuracy. Journal of Foraminiferal Research, 14, 303-308. 

The late Cenozoic was therefore a time of unusually high organic carbon deposition rates, leading to an increase in the size of the sedimentary organic carbon reservoir. The organic subcycle thus acted as a carbon sink over the course of the Himalayan uplift. There are two possible causes of this evolution. 

Catalysts deactivation was determined by measuring catalytic activity after circulating a coking feed (Cff4 /H2 /N2 45/5/50) at 923 K for 3 hours. Under the same conditions, carbon deposition rate was measured in a thermogravimetric reactor (17), which allowed to monitore variations in sample weight versus the time. 

A value of = 0.5 in Eq. (2.61) would result if the carbon deposition rate, dCddt, varied inversely with the coke content. Although some tests do show n = 0.5, there is no basic reason for this dependence. As active sites are covered by carbon, the rate of carbon deposition should decrease, unless the coke is a catalyst for further deposition. 

Table 1. Reactor Entry Gas Composition and Carbon Deposition Rate ...

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