Mean temperature difference reboilers

Figure 10-97A compares horizontal and vertical units in the same hydrocarbon boiling service at low pressures and shows that the horizontal units are more favorable in the same service than vertical units and even more so when the mean temperature difference is low. Figure 10-97B compares horizontal and vertical thermosiphon units with kettle reboilers when boiling the same hydrocarbon mixture also see Fair, Jacobs, and Rubin. ... 

For vertical tubes, the superficial vapor velocity (based on the total heat-transfer surface) can be obtained by multiplying the value calculated from the preceding equation by 0.22. This assumes that there is adequate liquid circulating past the surface to satisfy the mass balance. For thermosiphon reboilers, a detailed analysis must be made to establish circulation rate, boiling pressure, sensible heat-transfer zone, boiling heat-transfer zone, and mean temperature difference. If hquid circulation rates are not adequate, ah hquid will be vaporized and superheating of the vapor wih occur with a resultant decrease in heat-transfer rates. 

SubcooUng heat load is transferred at the same coefficient as latent heat load in kettle reboilers, using the saturation temperature in the mean temperature difference. For horizontal and vertical thermosiphons, a separate calculation is required for the sensible heat transfer area, using appropriate sensible heat transfer coefficients and the liquid temperature- profile for the mean temperature difference. 

When desired, the default utility applied by Aspen IPE can be altered interactively for a particular equipment item, such as a condenser or reboiler, after it has been mapped. For example, when the resulting surface area of a reboiler is too large due to a small log-mean-temperature-difference, the steam utility can be replaced with steam at a higher pressure to reduce the area, being careful to stay in the nucleate boiling region. 

Because of high column pressure, the column bottom temperature increases, which will have some impact on the reboiler heat input. In case steam or hot oil is used as the heat source, the LMTD (log mean temperature difference) of the reboiler is reestimated. Because of the increase in bottom temperature, the LMTD and reboiler heat duty would reduce. In case of a fired heater, reboiler heat duty is assumed to be the same. 

A first possibility consists of using an external heat pump (Fig. 11.15). The heat taken from the condenser is raised at a sufficient high temperature by means of a compressor. Note that the working fluid may be chosen independently of separation mixture. It is easy to imagine another solution where the heat pump would connect not condenser and reboiler, but an intermediate cooler or reboiler (side integration). This combination would enable to manage better the temperature difference, as well as the variation of internal heat streams. 

If the condenser uses the flow rate of a cooling medium (typically cooling water) or if the reboiler uses the flow rate of a heating medium (hot oil), a model, using a log-mean temperature differential driving force (temperature differentials at outlet and inlet ends), can be used. The inlet medium temperature and the minimum approach temperature difference between the process and the medium are specified. Then Aspen calculates the required UA product (overall heat-transfer coefficient U and heat-transfer area A) and the required flow rate of the medium from the known heat-transfer rate. 

Sustained oscillation could not be found by means of continuation analysis for the lab-scale methyl formate column. However, oscillations have been observed during experimental investigations by Reder [87] as displayed in Fig. 10.6. Various temperature readings in the reboiler and on different trays are shown in the lower diagram. A step change in heat duty is applied at time 0. Large amplitude temperature oscillations with a period of about 0.3 h can be observed before and after the step change. The oscillations have been reproducible in various experiments. No source of external excitation could be located. An explanation of the phenomenon has not yet been achieved, however. 

---