Residence time steam effect

The production of synthesis gas (CO, H2) from methane by partial oxidation is investigated over commercial steam reforming catalyst at several flow rates, temperatures, and at different methane/oxygen ratios (R). Optimum synthesis gas selectivity and yield achieved are 70% and 60%, respectively at methane/oxygen ratio close to 2 and at flow rates of500 cm /min. An initial temperature (665 °C) is necessary to initiate the reaction and then the reaction is stabilized at 883 °C. Tbe effect of methane/oxygen ratios and residence time are effective in determining the synthesis gas selectivity and yield. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

An increase in droplet size with axial position is observed for all three gases. However, the relative trend of smallest droplet mean size with steam and largest with normal (unheated) air remains unchanged. As an example, at 50 mm downstream from the nozzle exit at r = 0, droplet mean size for steam, preheated air, and normal air were found to be 69, 86, and 107 pm, respectively see Fig 16.3. The droplet size with steam is also significantly smaller than air at all radial positions see Fig. 16.3. The droplet size with preheated air is somewhat smaller than normal air due to the decreased effect of preheated air at this location and increased effect of combustion. Early ignition of the mixture with preheated air (see Fig. 16.1) must provide a longer droplet residence time which results in a smaller droplet size. In addition, the increased flame radiation with preheated air increased droplet vaporization at greater distances downstream from the nozzle exit. Indeed, the results indicate that the measured droplet sizes with preheated atomization air are smaller than normal air in the center... 

The aim of the present study was to apply steam pretreatment to corn stover and investigate the influence of different pretreatment conditions— temperature, residence time, and concentration of H2S04—on sugar yield and the effect of inhibitors on subsequent ethanol yield. 

The increased residence time provides time for excess steam to react with carbon. These effects contribute to lowering the maximum temperature in a dry ash gasifier like the Lurgi, and the combustion zone moves upwards. (1)... 

Antal, M. J. The Effects of Residence Time, Temperature and Pressure on the Steam Gasification of Biomass , American Chemical Society Division of Petroleum Chemistry, Honolulu, 1979. 

The Effects of Residence Time, Temperature, and Pressure on the Steam Gasification of Biomass... 

Research described in this paper focuses on the second step of the gasification process, and details the effects of temperature and residence time on product gas formation. Cellulose is used as a feedstock for pyrolytic volatiles formation. Earlier papers (JS.M) have discussed the effect of steam on cellulose pyrolysis kinetics. Two recent papers (1, 1 6) presented early results on pelletized red alder wood pyrolysis/gasification in steam. Future papers will discuss results using other woody materials, crop residues, and manures (17,1 ). Research to date indicates that all biomass materials produce qualitatively similar results in the gasification reactor described in the following section of this paper. Effects of pressure on the heat of pyrolysis of cellulose are also discussed as a prelude to future papers detailing the more general effects of pressure on reaction rates and product slates. 

Some efforts have also been made to experimentally measure the temperature rise of the steam entering the gas-phase reactor. Qualitative agreement with the results of the heat transfer calculations was found however, a brief calculation of the effect of radiation on the thermocouple s measurement of the gas temperature pointed to a significant error in the measurement. For example, with a steam flow of 0.12 g/min and a gas-phase temperature of 600°C, the thermocouple temperature was calculated to exceed that of the gas by IS C. Carefully constructed radiation shields are needed to eliminate this effect. Because our research effort in this area was drawing to a close, it was decided to use the methodology described earlier (Equation 15) to estimate the residence time of the volatiles at temperature. 

Figure III shows the effect of solids residence time on net Btu yield. There is a question as to whether the high Btu yield shown for the pellets containing 20 percent recycled wood ash is due to the catalytic effect of the wood ash or the relatively high steam/wood ratio used for that run. Further experiments are needed to isolate the catalytic effects from the effects of other process parameters.

Figure 3. Effect of residence time on net BTU yield (gas) in steam gasification of...

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