Water vapor residence time

Fast pyrolysis has a short hot vapor residence time typically less than 1 s (Bridgwater et al., 1999). The residence time has to be kept as short as possible to prevent secondary reactions taking place which will convert the condensable fast pyrolysis vapors into permanent gases, water vapor, and char (Liden et al., 1988 Scott et al., 1999). Typical liquid yields are around 75% from clean wood (Bridgwater et al., 1999). 

Liquid condensate recovered by thermal treatment of lignocellulosic biomass at short hot vapor residence time (typically less than about 5 s) typically at between 450 and 600°C, at near atmospheric pressure or below, in the absence of oxygen, and using small (typically less than 5 mm) dry (typically less than 10% water) biomass particles. 

Both vapor-phase and Hquid-phase processes are employed to nitrate paraffins, using either HNO or NO2. The nitrations occur by means of free-radical steps, and sufftciendy high temperatures are required to produce free radicals to initiate the reaction steps. For Hquid-phase nitrations, temperatures of about 150—200°C are usually required, whereas gas-phase nitrations fall in the 200—440°C range. Sufficient pressures are needed for the Hquid-phase processes to maintain the reactants and products as Hquids. Residence times of several minutes are commonly required to obtain acceptable conversions. Gas-phase nitrations occur at atmospheric pressure, but pressures of 0.8—1.2 MPa (8—12 atm) are frequentiy employed in industrial units. The higher pressures expedite the condensation and recovery of the nitroparaffin products when cooling water is employed to cool the product gas stream leaving the reactor (see Nitroparaffins). 

Upward diffusion of water vapor through the cold temperatures of the tropopause is very inefficient in fact, the upper limit of cloud formation often occurs at the tropopause. Thus the stratosphere is so dry as to prevent rain formation, and particles and gases have very much longer residence times there than in the troposphere. Stratospheric removal requires diffusion back through the tropopause, which then may be followed by precipitation scavenging. 

The turnover time of water vapor in the atmosphere obviously is a function of latitude and altitude. In the equatorial regions, its turnover time in the atmosphere is a few days, while water in the stratosphere has a turnover time of one year or more. Table 7-1 Qunge, 1963) provides an estimate of the average residence time for water vapor for various latitude ranges in the troposphere. Given this simple picture of vertical structure, motion, transport, and diffusion, we can proceed to examine the behavior of... 

Table 7-1 Average residence time of water vapor in the atmosphere as function of latitude...

Although thermodynamically it is relatively simple to determine the amount of water vapor that enters the atmosphere using the Clausius-Clapeyron equation (see, e.g.. Chapter 6, Equation (1)), its resultant atmospheric residence time and effect on clouds are both highly uncertain. Therefore this seemingly easily describable feedback is very difficult to quantify. 

A fixed-bed reactor system was employed (Figure 32.2). Each of the two reactors was charged with 38 cc of Amberlyst BD20 catalyst. Sample ports located at the exit of each reactor enabled increased acquisition of residence time data. Pressure was maintained by a back pressure control valve to maintain methanol in the liquid phase. After charging, the 1st and then 2nd reactors were connected to the pumps and filled with the reaction mixture while vapor was released from each through the top vent valve. Once each reactor was filled with liquid and emptied of vapor, the pressure regulator was connected to the output and both reactors were immersed into the water bath. 

The above behavior of narrow-pore supported cobalt catalysts toward co-fed water can also be explained in terms of relative size of cobalt clusters, pore network of support, expected location of cobalt clusters within the pore network, and relative differences in the residence time of water vapor within and outside the... 

Due to its chemical inertness, vaporizable nature (enthalpy of vaporization = 59.15 kJ/mol), and low water solubility (at 20°C, 2 x 10 6 g/g), elemental mercury vapor has over one year of residence time, long-range transport, and global distribution in the atmosphere [3-8]. 

In the atmosphere, the vapor pressure of the isomeric cresols, 0.11+0.30 mmHg at 25.5 °C (Chao et al. 1983 Daubert and Danner 1985), suggests that these compounds will exist predominantly in the vapor phase (Eisenreich et al. 1981). This is consistent with experimental studies that found all three isomers in the gas phase of urban air samples, but they were not present in the particulate samples collected at the same time (Cautreels and Vancauwenbergh 1978). The relatively high water solubility of the cresol isomers, 21,520- 25,950 ppm (Yalkowsky et al. 1987), indicates that wet deposition may remove them from the atmosphere. This is confirmed by the detection of cresols in rainwater (Section 5.4.2). The short atmospheric residence time expected for the cresols (Section 5.3.2.1) suggests that cresols will not be transported long distances from their initial point of release. 

This situation describes an emulsion reactor in which reacting drops (such as oil drops in water or water drops in oil) flow through the CSTR with stirring to make the residence time of each drop obey the CSTR equation. A spray tower (liquid drops in vapor) or bubble column or sparger (vapor bubbles in a continuous liquid phase) are also segregated-flow situations, but these are not always mixed. We wiU consider these and other multiphase reactors in Chapter 12. 

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