Hydrocarbon production rate

It was found that an iron carbide catalyst produced by laser pyrolysis and a commercially available ultrafine iron oxide catalyst are not as active for FTS as a precipitated iron catalyst. Operating under industrial conditions, it was found that the unpromoted precipitated catalyst had a hydrocarbon productivity 93% of that reported by Kolbel while the novel catalysts were far below Kolbel s benchmark. It was found, however, that at similar CO conversion, the iron carbide catalyst had a higher hydrocarbon production rate and had a better selectivity for C5+ hydrocarbons. 

The HTFT process operates with an iron based catalyst at about 350°C with the syngas passing through a fluidized bed of finely divided catalyst. Low temperatures cannot be used as the two phase system (gas and catalyst) would become defluidized by the formation of liquid waxes. At the higher temperature of the HTFT process, the catalyst is much more active than it is in the LTFT process and the hydrocarbon production rate is much higher. 

For the Fischer-Tropsch hydrocarbon synthesis (FTS) it has been shown that alkali modifiers are effective in improving a catalyst s behavior (refs. 11-16). For FTS higher-hydrocarbon production rates drop upon alkali addition but that of methane drops more dramatically, resulting in improved selectivity for ihe desired higher-hydrocarbon... 

These routes have some very attractive features, with hydrogen, acetylene and a little coke being almost the sole products. The systems can be made self-quenching which is a major advantage however, the conversion per pass is at most 50% with unacceptable power consumption. Indeed, we estimate that the power generating facilities for such a route alone would cost about as much as a Fischer-Tropsch plant with the same net hydrocarbon production rate. To the best of our knowledge this route is not presently practised on a commercial scale. 

The average H2/CO feed ratio, 0.19, is far away from the stoichiometric ratio of slightly above two. Consequently, the conversion of carbon monoxide is low, 5.1%. The hydrogen conversion is at a higher level, 53%. The response of the product fractions to the applied feed cycle is shown in Figure 2a. The Diesel fraction never exceeds 36 wt%, but this selectivity maximum coincides with a maximum in the hydrocarbon production rate as shown in Figure 2b. 

The hydrocarbon feed rate to the reactor also affects the burning kinetics in the regenerator. Increasing the reactor feed rate increases the coke production rate, which in turn requires that the air rate to the regenerator increase. Because the regenerator bed level is generally held constant, the air residence time in the dense phase decreases. This decrease increases the O2 content in the dilute phase and increases afterbum (Fig. 5). 

The principal advance ia technology for SASOL I relative to the German Fischer-Tropsch plants was the development of a fluidized-bed reactor/regenerator system designed by M. W. Kellogg for the synthesis reaction. The reactor consists of an entrained-flow reactor ia series with a fluidized-bed regenerator (Fig. 14). Each fluidized-bed reactor processes 80,000 m /h of feed at a temperature of 320 to 330°C and 2.2 MPa (22 atm), and produces approximately 300 m (2000 barrels) per day of Hquid hydrocarbon product with a catalyst circulation rate of over 6000 t/h (49). 

With respect to the formation of unwanted polyaromatic hydrocarbons in the pyrolytic process, it has been shown that conditions can be maintained where such fonuation is negligible according to EPA and OSHA standards. As production rates are increased, it will be incumbent on any manufacturer to maintain a set of operating parameters which produce an environmentally-benign product however, current information regarding the process for fiber formation reveals no barriers to accomplishing this. 

It is obvious that one can use the basic ideas concerning the effect of alkali promoters on hydrogen and CO chemisorption (section 2.5.1) to explain their effect on the catalytic activity and selectivity of the CO hydrogenation reaction. For typical methanation catalysts, such as Ni, where the selectivity to CH4 can be as high as 95% or higher (at 500 to 550 K), the modification of the catalyst by alkali metals increases the rate of heavier hydrocarbon production and decreases the rate of methane formation.128 Promotion in this way makes the alkali promoted nickel surface to behave like an unpromoted iron surface for this catalytic action. The same behavior has been observed in model studies of the methanation reaction on Ni single crystals.129... 

The gases used were purchased premixed in aluminum cylinders to avoid carbonyl formation. The high purity gas mixture was further purified by a zeolite water trap and a copper carbonyl trap. The gas pressure in the reactor was measured with a capci-tance manometer and the fTow monitored with a mass fTow controT-ler. The typical gas flow rates were 15 cc/min (STP) and the maximum conversion was 1% based on integration of hydrocarbon products. The hydrocarbon products were analyzed by gas chromatography (temperature programmed chromosorb 102, FID). 

Experimental temperature determination reveals little difference between the gases at 355 kHz, as is evident in Figs. 14.15a and b, similar to the results of Okitsu et al. The likely explanation for this seemingly contradictory result is that the MRR method, which requires the presence of a significant amount of hydrocarbon within the bubble core, yields a temperature value that is dictated by the hydrocarbon itself. In other words, the probe itself determines the temperature measured. This becomes apparent at higher frequency (1,056 kHz Fig. 14.15c) where it is clear that the temperature that is obtained when one extrapolates to zero hydrocarbon concentration varies depending on the probe molecule. The lower temperature for /(77-butanol is attributed to a greater rate of hydrocarbon product formation. 

CO can be converted into either hydrocarbon products and water (via FTS) or C02 and Fl2 via the water-gas shift (WGS) reaction. The reversible WGS reaction accompanies FTS over the iron-based catalyst only at high temperature conditions. The individual rates of FTS (rFTS) and the WGS reaction (rWGS) can be calculated from experimental results as rWGS = r(,and rFTS = rco-rc02, where rCo2 is the rate of C02 formation and rco is the rate of CO conversion. 

The data available for heterogeneous Fischer-Tropsch catalysts indicate that with cobalt-based catalysts the rate of the water gas-shift reaction is very slow under the synthesis conditions (5). Thus, water is formed together with the hydrocarbon products [Eq. (14)]. The iron-based catalysts show some shift activity, but even with these catalysts, considerable quantities of water are produced. 

No evidence of ruthenium metal formation was found in catalytic reactions until temperatures above about 265°C (at 340 atm) were reached. The presence of Ru metal in such runs could be easily characterized by its visual appearance on glass liners and by the formation of hydrocarbon products (J/1J) The actual catalyst involved in methyl and glycol acetate formation is therefore almost certainly a soluble ruthenium species. In addition, the observation of predominantly a mononuclear complex under reaction conditions in combination with a first-order reaction rate dependence on ruthenium concentration (e.g., see reactions 1 and 3 in Table I) strongly suggests that the catalytically active species is mononuclear. 

After the initial volume estimate has been determined, testing of a pilot recovery system should be initiated to evaluate recovery rates. However, factors that significantly affect recovery rates include the areal distribution and geometry of the free-hydrocarbon product plume, type(s) and design of recovery system selected, and the performance and efficiency of the system with time. 

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