Water temperature conversion profiles

Figure 17.34. Temperature and conversion profiles in a water-cooled shell-and-tube phosgene reactor, 2-in. tubes loaded with carbon catalyst, equimolal CO and Cl2.

After completion of phase inversion, the polymer solution is dispersed in a 2-4-fold amount of water. Suspension aids used are water-soluble organic polymers, such as poly(vinyl alcohol) or polyvinylpyrrolidone, or inorganic compounds, such as Pickering systems. In order to achieve a final conversion of 99.5 %, initiator combinations with different decomposition times are used, and the polymerization follows a defined temperature-time profile. The suspension is then centrifuged, dried and compounded. 

Two regions can be seen on the reaction profiles the low-temperature domain corresponding to CO oxidation (limited at a 40% conversion) and the high temperature one where the CO having not been oxidized react with water (WGS domain). For every reaction, Pt is the best metal and ceria acts as a promoter. However, an exceptional increase of conversion can be observed in WGS when the metals are deposited on CeA. 

In the case of 44 % moisture there is liquid water present in the structure and also the permeability of liquid influences the conversion time of the sample. Figure 6 shows the measured temperature profiles of Figure 1 and the two simulated Cases 6 and 7 in Table 1, simulated for liquid axial permeabilities of 10 and 10 respectively. In both cases the radial permeability is assumed to be 10 lower. It is seen from Figure 6 that the intrinsic permeability of liquid has a large influence on the pyrolysis time. A higher permeability leads to a larger transport of water through the wood and less water evaporates inside the sample, which reduces the time of conversion. 

An example of how the model works is shown in Figure 252, in which a typical commercial activation is represented. 273 kg of Cr/silica was charged to an activator. The catalyst was heated to 800 °C at the standard linear ramp rate of 1.4 °C min 1 and an air velocity of 6.4 cm s Then the temperature was held at 800 °C for 12 h. To obtain a predicted conversion, the activation profile (temperature vs. time) was first plotted (Figure 252) and the concentration of water vapor in the gas stream was calculated from the temperatures shown in the plot and a library of laboratory TGA curves that indicate how much water is evolved at each temperature and heat-up rate. The conversion of the chromium to Cr(VI) was calculated at each temperature from the calculated concentration of water vapor by use of the stability curves shown in Figure 251. The Cr(VI) content was found to be high when the temperature reached 500 °C, but it dropped quickly as the temperature was raised, reaching only 0.37% Cr(VI) at 800 °C (Figure 252). 

Each of the three ramp profiles took the same time to reach 800 °C, but there was a major difference in the conversion to Cr(VI). The "convex bent ramp" led to the release of water vapor slowly as the highest temperature was reached, which produced higher Cr(VI) levels. The improvement in conversion resulting from the "convex bent ramp" has been confirmed in commercial operations in many manufacturing plants where it has now been adopted. 

Figure 6.34 shows the strong effect of steam to methane ratios on the process gas, inner and outer temperature profiles. Increase of the steam to methane ratio decreases the process gas temperature and accordingly the inner and outer wall temperatures (Figure 6.34a). A high pressure drop results from the increase of the steam to methane ratios (Figure 6.34b). Figure 6.34c shows the conversion and yield variation corresponding to the change in the steam to methane ratio while the yield of carbon dioxide is affected more strongly by the change of steam to methane ratio. The conversion of carbon dioxide increases with the increase of steam to methane ratios and this may be due to the enhancement of the water-gas shift reaction.

The superior heat transfer achievable in engineered mieroehannels can be used to optimize temperature profiles in a reactor, which can lead to smaller reactor sizes and improved eonversion. For reversible exothermie reactions such as water gas shift, the mieroehannels make it possible not only to remove the heat of reaction, but also to reduce the reaction temperature. By maintaining heat transfer length scales on the order of 100 mierons, minimal temperature gradients across the eatalyst were maintained while achieving precise eontrol of temperature down the reactor. This approach provides an optimum balance between rapid kineties at high temperature and favorable thermod5mamies for conversion as the temperature is redueed. 

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