Carbon monoxide residence time

Weinstock, B Carbon Monoxide Residence Time in the Atmosphere, Science, 166, 224-225 (1969). 

Weinstock, B. (1969). Carbon monoxide residence time in the atmosphere. Science 166, 224-225. 

The second example of an air pollutant that affects the total body burden is carbon monoxide (CO). In addihon to CO in ambient air, there are other sources for inhalation. People who smoke have an elevated CO body burden compared to nonsmokers. Individuals indoors may be exposed to elevated levels of CO from incomplete combustion in heating or cooking stoves. CO gas enters the human body by inhalation and is absorbed directly into the bloodstream the total body burden resides in the circulatory system. The human body also produces CO by breakdown of hemoglobin. Hemoglobin breakdown gives every individual a baseline level of CO in the circulatory system. As the result of these factors, the body burden can fluctuate over a time scale of hours. 

Enclosed flares are composed of multiple gas burner heads placed at ground level in a staeklike enclosure that is usually refractory or ceramic lined. Many flares are equipped with automatic damper controls that regulate the supply of combustion air depending on temperature which is monitored upstream of the mixing, but inside the staek. This class of flare is becoming the standard in the industry due to its ability to more effectively eontrol emissions. Requirements on emissions includes carbon monoxide limits and minimal residence time and temperature. Exhaust gas temperatures may vary from 1,000 to 2,000 F. 

GP 9] [R 16] When using metal catalysts (Rh, Pt or Pd) supported on alumina particles (-3 mg 53-71 pm) in a wide mini fixed bed, residence times in the range 0.6-8.0 ms were applied (1% carbon monoxide, 1% oxygen, balance helium 20-60 seem up to 260 °C) [78]. 

Figure 4.2 presents a simplified flow diagram of the ENCOAL Liquid from Coal (LFC) process. The process upgrades low-rank coals to two fuels, Process-Derived Coal (PDF ) and Coal-Derived Liquid (CDL ). Coal is first crushed and screened to about 50 mm by 3 mm and conveyed to a rotary grate dryer, where it is heated and dried by a hot gas stream under controlled conditions. The gas temperature and solids residence time are controlled so that the moisture content of the coal is reduced but pyrolysis reactions are not initiated. Under the drier operating conditions most of the coal moisture content is released however, releases of methane, carbon dioxide, and monoxide are minimal. The dried coal is then transferred to a pyrolysis reactor, where hot recycled gas heats the coal to about 540°C. The solids residence time... 

Jacoby et al. (1994) studied the photocatalytic reaction of gaseous trichloroethylene in air in contact with UV-irradiated titanium dioxide catalyst. The UV radiation was kept less than the maximum wavelength so that the catalyst could be excited by photons, i.e., X <356 nm. Two reaction pathways were proposed. The first pathway includes the formation of the intermediate dichloroacetyl chloride. This compound has a very short residence time and is quickly converted to the following compounds phosgene, carbon dioxide, carbon monoxide, carbon dioxide, and hydrogen chloride. The second pathway involves the formation of the final products without the formation of the intermediate. 

Carbon monoxide is an important trace gas, which has a mean residence time of about two months and a mean concentration of the order of 0.1 ppm. The principal sources of CO are (1) oxidation of methane and other higher hydrocarbons, (2) biomass burning, (3) traffic, industry and domestic heating, (4) oceans, and (5)... 

Increase regenerator dense bed level to increase residence time and minimizing channeling of oxygen and/or carbon monoxide throngh the dense bed... 

Catalytic tests of n-pentane oxidation were carried out in a laboratory glass flow-reactor, operating at atmospheric pressure, and loading 3 g of catalyst diluted with inert material. Feed composition was 1 mol% n-pentane in air residence time was 2 g s/ml. The temperature of reaction was varied from 340 to 420°C. The products were collected and analyzed by means of gas chromatography. A FlP-l column (FID) was used for the separation of C5 hydrocarbons, MA and PA. A Carbosieve Sll column (TCD) was used for the separation of oxygen, carbon monoxide and carbon dioxide. 

The optimum residence times are to be 11-12 sec. for 400°C., 8-10 sec. for 430°C., 6-7 sec. for 450°C. and 4-6 sec. for 480°C. Carbon monoxide, methanol, formaldehyde, and acetone are decreased with the increase in reaction temperature. At 480°C. the yield of carbon monoxide is a little less and that of propylene oxide is more than those at the other temperatures because of the smaller content of oxygen in the feed gas. 

This was ascribed to the short residence times applied (50-100 ms). Under these conditions, assuming the reaction mechanism proposed by Takahashi et al. shown above, carbon monoxide could only be formed by the reverse water-gas shift reaction, which is known to be slower than the reforming reaction. This is the case especially for catalyst systems with low activity towards water-gas shift. Holladay et al. [19] compared the performance of the same proprietary catalyst with that of a Cu/Zn catalyst which produced a higher carbon monoxide concentration of 3.1% in the reformate. 

The experiments were carried out at residence times between 200 and 100 ms and total flow rates between 500 and 900 Ncm3 min-1 using five coated plates. A 15% decrease in conversion was found when the flow rate was increased from 500 to 800 Ncm3 min-1, leading to a 36% relative decrease in the hydrogen content of the product. This was attributed to the slow kinetics of the reverse water-gas shift reaction. Increasing the temperature from 200 to 275 °C increased the conversion from 37 to 65%. At temperatures exceeding 250 °C, carbon monoxide formation started [26]. 

Experimental results supported the assumption that this temperature was necessary to gain the PdZn alloy on the catalyst surface. No catalyst deactivation was detectable during the experiments. At 300 °C full conversion was achieved at a 100 ms residence time [32] and 5% and lower carbon monoxide selectivity. First order kinetics were determined, revealing 7.04 1013 h 1 for the pre-exponential factor and 92.8 kj mol 1 for the activation energy. 

A feed of 100 Ncm3 min-1 methane and 50 Ncm3 min-1 oxygen was introduced into the reactor at a pressure loss of < 2.5 mbar. The residence time of the reaction was 50 ms. 60% conversion was achieved along with a high carbon monoxide selectivity of 70% at 700 °C reaction temperature. Owing to the short residence times applied, no coke formation was observed and carbon monoxide selectivity was higher than expected from the thermodynamic equilibrium [46],... 

The nickel-containing sample showed no detectable activity up to a reaction temperature of 650 °C within the short residence times applied for the tests. Increasing the reaction temperature to 750 °C led to a conversion of 6% for this sample. However, the catalyst was mostly active for propane dehydrogenation (44% selectivity) and had a selectivity of 28% towards both carbon dioxide and carbon monoxide [52]. 

The feed was composed of 10% steam and 2-10% carbon monoxide, balance argon (Figure 2.49). Up to 60% conversion was achieved at a 280 °C reaction temperature and a modified residence time of 280 gcat (s mmol)4. 

Figure 2.61 Carbon monoxide conversion over various catalysts with respect to reaction temperature at an 02/C0 ratio of 4 and 200 ms residence time [89] (source IMM).

Figure 2.88 CO conversion found for low-temperature water-gas shift at various reaction temperature vs. modified residence time (catalyst weight/carbon monoxide flow). Results from a micro channel stack reactor (closed symbols) are compared with conventional cordierite monoliths (open symbols) [82].

Germani et al. [82] compared the performance of their catalyst coating developed for water-gas shift in a micro structured reactor with that of the same catalyst coated on a cordierite monolith under identical reaction conditions. Higher conversion was achieved in the micro channels at same modified residence time under all experimental conditions applied. Figure 2.88 shows the CO conversion vs. a modified residence time (catalyst weight/flow of carbon monoxide) measured at various reaction temperatures. 

The partial combustion (partial oxidation) of natural gas (Fig. 1) is probably the most widely used method of producing acetylene. The overall reaction of the methane (combustion and splitting) is 90 to 95 percent whereas the oxygen is 100 percent converted. The residence time is 0.001 to 0.01 seconds. The acetylene and gases are cooled rapidly by quench oil or water sprays to 38°C and have the following typical composition (percent by volume acetylene, 8 to 10 hydrogen, 50 to 60 methane, 5 carbon monoxide, 20 to 25 and carbon dioxide, <5. The soot is removed in a carbon filter and the clean gases are compressed to 165 psi (1.14 MPa). 

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