Temperature-liquid residence time

An interesting feature of the conversion profile is the existence of a maximum located between the wall and the axis of the mold. This effect is explained by the superposition of the influence of temperature and residence time on the kinetics of the reaction a liquid moves faster in the central zone than at the wall, therefore the residence time is longer near the walls, but temperature is higher in the center therefore the reaction rate of the material near the walls is lower than in the center. As a result, there is a point between the center and a wall where the degree of conversion is maximum. The results in Fig. 4.55, also answer the question about the role of the fountain effect ... 

On this basis, conversion is limited by coal structure. And in terms of the conventional homolytic sclsslon/H-capping view of conversion, increased yields of coal liquids are therefore obtainable only through increases in conversion temperature or residence time. Unfortunately, increases in the thermal severity of the process result in products reflecting the rise of dealkylation and aromatization reactions at higher temperatures. Thus increased product yields are brought about at a considerable cost to product quality (5). 

Use of rotary kilns for hazardous waste incineration is becoming more common for disposal of chlorinated hydrocarbons such as polychlorinated biphenyls (PCBs). Flow in these kilns is cocurrent. Major advantages include high temperature, long residence time, and flexibility to process gas, liquid, solid, or drummed wastes. 

To dampen flow-rate fluctuations, requires liquid holdup at the bottom of the column. Ludwig [54] recommends 5 to 20 min for the surge time, i.e., the liquid residence time at the bottom of a colunm. On the other hand, it is desirable to minimize the solvent inventory in a process to minimize cost and to minimize the amount of flammable hquids. Also, if the liquid contains heat-sensitive organic conpoxmds, it is necessary to reduce the residence time, particularly in strippers, where the temperature is high. For this problem, select a surge time of 5.0 min to keep the residence time low for the stated reasons. Therefore, the liquid height in the absorber. 

Liquid holdup Liquid holdup, mean residence time, and liquid residence time distribution are important in determining conversion and selectivity. Catalyst deactivation Catalyst deactivation is often accounted for during design by use of excess catalyst, and increase in reaction severity by increasing reflux (for increased residence time) or by increasing reaction temperature. 

Short liquid residence time (may allow the use of higher processing temperatures). 

Temperatures measured in the reactor bed during the entire experiment are plotted in Fig.3. We can see that steady state is reached after 1.5-2 hours from the start of the process, which is equivalent to 4-5 times the liquid residence time. At a hydrogen supply ratio - defined as molar hydrogen feed rate over the stoichiometric molar rate needed to convert all DNT to DAT - of ctDAT < 1 equilibration of the reactor took usually as much as 2 to 3 hours. 

Operating pressure and temperature constrain practical size of vessel. Design codes for pressure vessels vary slightly with the country. In general, for operating pressures >10 MPa, vessel volume usually <1 m Pressure decreases as temperatures exceed 250°C. For temperatures above 350°C, consider carbon/molybdenum, and for temperatures >500°C, consider austenitic steels. Corrosion allowance 1.5 mm for corrosion rates 0.08 mm/a 3 mm for rates 0.09 to 0.3 nun/a 4.5 nun for 0.31 to 0.4 mm/a 6 mm for >0.4 mm/a. If pressure <400 kPa, use L/D of 2 to 3 1. For pressures >400 kPa, use L/D of 4 to 5 1. For surge, allow 2-min liquid residence time for draw-off, use 15 min for reflux, use 5 min, provided this allows sufficient time for controllers to function. Total volume = 1.3 X holdup if the holdup volume is >3 m ... 

Let the thermocouple beads be considered as the control volumes. Then in mist flow, if the drops of liquid are small relative to the control volume, the liquid temperature would be observed momentarily. If the liquid droplets or plugs are large relative to the control volume, the saturation temperature would be observed for finite periods (liquid residence times). The plug flow model implies that temperatures measured at the center of the stream are representative of the entire cross section and that vapor film thicknesses accompanying liquid plugs are negligible. 

Due to the continuous transfer of heat to the fluid along the length of the test section, liquid was vaporized as it flowed down the test section, and the slug flow phenomenon at the exit of the test section was not observed until a later period. By this time, the line had cooled down considerably so that the temperature oscillations (between vapor and liquid) were less pronounced and liquid residence times could not be determined as readily. However, from the vapor fractions calculated for the inlet to the test section, those at the outlet can be computed from energy balances. 

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