Tower-bottom temperature decrease

As the amount of liquid overflowing the draw pan increases, and the amount of liquid flowing to the reboiler decreases, the tower bottoms temperature will start to approach the tray temperature. For example, with valve A wide open, I had observed the following temperatures ... 

After these moves, pressure decreased and the heater temperature was increased another 5°F. Finally, the vacuum tower bottoms stripping steam rate, heater outlet temperature, and heater pass flow were optimized to obtain maximum gas oil production. These last items usually take some trial and error. 

Due to the entrainment of the colder gas from the bottom of the spray tower, the temperature level is strongly decreased in the axial and radial direction. That means that in the hot gas atomization process, the high temperatures zones are located only close to the atomizer. The consequence is that the thermal energy is concentrated on the spray jet, which is desired to improve the atomizer performance. The temperature profiles on the spray axis and at the wall, measured at different operation times (until 40 min), are illustrated in Fig. 19.6. 

Classical Adiabatic Design Method The classical adiabatic method assumes that the heat of solution serves only to heat up the liquid stream and that there is no vaporization of solvent. This assumption makes it feasible to relate increases in the hquid-phase temperature to the solute concentration x by a simple eutnalpy balance. The equihbrium curve can then be adjusted to account For the corresponding temperature rise on an xy diagram. The adjusted equilibrium curve will become more concave upward as the concentration increases, tending to decrease the driving forces near the bottom of the tower, as illustrated in Fig. 14-8 in Example 6. 

The temperature at the base of the de-butanizer determines the vapor pressure of the gasoline product. If its vapor pressure is too high, the temperature must be increased or the tower pressure decreased to drive more butanes-minus out of the bottoms liquids. 

The tray temperatures in our preflash tower, shown in Fig. 4.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays, in the absorption section of such a tower, are drying out. The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios, and a multicomponent overhead product composition. 

Stable column operation is guaranteed by keeping the internal reflux of the distillation tower constant. Consequently, internal reflux controls are designed to compensate for changes in the temperature of the external reflux caused by ambient conditions. Figure 2.90a is controlled by a typical internal reflux control system (top) and the equations that need to be solved in calculating the required external reflux rate are shown at the bottom. This control system corrects for either an increase in overhead vapor temperature or a decrease in external reflux liquid temperature. 

Mechanical Draft Towers Two types of mechanical draft towers are in use today the forced-draft and the induced-draft. In the forced-draft tower the fan is mounted at the base, and air is forced in at the bottom and discharged at low velocity through the top. This arrangement has the advantage of locating the fan and drive outside the tower, where it is convenient for inspection, maintenance, and repairs. Since the equipment is out of the hot, humid top area of the tower, the fan is not subjected to corrosive conditions. However, because of the low exit-air velocity, the forced-draft tower is subjected to excessive recirculation of the humid exhaust vapors back into the air intakes. Since the wet-bulb temperature of the exhaust air is considerably above the wet-bulb temperature of the ambient air, there is a decrease in performance evidenced by an increase in cold (leaving) water temperature. 

The temperature inside the fractionating tower is controlled so that it remains near 400°C at the bottom, where the petroleum is boiling, and gradually decreases toward the top. The condensation temperatures (boiling points) generally decrease as molecular mass decreases. Therefore, as the vapors travel up through the column, the hydrocarbons condense and are drawn off, as shown in Figure 21.6. 

The most satisfactory temperature datum is the vaporizer temperature because this temperature can be accurately estimated and is the temperature about which the entire design of tower and pipestill binges. By using this datum plane, the heat balance consists simply of the sensible heat that is required to (1) cool each product from the vaporizer temperature to its withdrawal temperature and (2) condense the products that are withdrawn as hquids. The reflux that is computed by such a heat balance is about the minimum amount by which the process can function. Upon casual inspection it appears to provide no flow of liquid into the feed plate from higher plates in the column, but rince the decrease in temperature of the bottoms product is not caused primarily by reflux, the heat balance mentioned above actually provides a small flow of hquid into the feed plate. At each ride-draw plate the internal reflux is depleted by the amount of ride-draw product that is withdrawn, as shown in Fig. 

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