Mean temperature difference boiling

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. ... 

AT = mean temperature difference between the bulk of the boiling liquid and the bulk of the heating medium, °F... 

AT = overall temperature difference, F or, mean temperature difference between bulk of boiling liquid and bulk of heating medium, F or, temperature difference of fluid, (end points), F or, (T — TJ, R, in Levy Boiling equation. 

Ak = overall At between average tube-side bulk temperature, F, between average tube-side bulk temperature and boiling film, F or, critical temperature differential in boiling, F. At,jj = mean temperature difference, (f, — k)/2, F or, corrected mean temperature difference, F. 

If the feed is sub-cooled, the mean temperature difference should still be based on the boiling point of the liquid, as the feed will rapidly mix with the boiling pool of liquid the quantity of heat required to bring the feed to its boiling point must be included in the total duty. 

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. 

JFor boiling of mixtures, the saturation temperature (bubble point) of the final liquid phase (after the desired vaporization has taken place) is to be used to calculate the mean temperature difference. A narrow-boiling-range mixture is defined as one for which the difference between the bubble point of the incoming liquid and the bubble point of the exit liquid is less than the temperature difference between the exit hot stream and the bubble point of the exit boiling liquid. Wide-boiling-range mixtures require a case-by-case analysis and cannot be reliably estimated by these simple procedures. 

If one or both of the streams undergoes isothermal condensation or boiling, the specific heats are constant, and the above assumptions 1, 3, and 5 apply, the log-mean temperature difference applies to all heat exchanger configurations, including multiple tube- or shell-pass arrangements. 

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. 

Apart from the question of scale, the nature of the clean surface has a pronounced influence on the rate of boiling. Thus Bonilla and PERRY(79) boiled ethanol at atmospheric pressure and a temperature difference of 23 deg K, and found that the heat flux at atmospheric pressure was 850 kW/m2 for polished copper, 450 for gold plate, and 370 for fresh chromium plate, and only 140 for old chromium plate. This wide fluctuation means that caTe must be taken in anticipating the heat flux, since the high values that may be obtained initially may not persist in practice because of tarnishing of the surface. 

For heat pumping to be economic on a stand-alone basis, it must operate across a small temperature difference, which for distillation means close boiling mixtures. In addition, the use of the scheme is only going to make sense if the column is constrained to operate either on a stand-alone basis or at a pressure that would mean it would be across the pinch. Otherwise, heat integration with the process might be a much better option. Vapor recompression schemes for distillation therefore only make sense for the distillation of close boiling mixtures in constrained situations3. 

The vapor pressure against temperature data obtained with a Knudsen cell set-up are handled as already described for a low boiling temperature liquid. The main difference stems from the very low pressure of the vapor in equilibrium with the solid, which justifies the adoption of the ideal gas model in this case. ASub ° at the mean temperature can then be derived from equation 2.40 (with Z = 1) and the correction to 298.15 K can be made with an equation similar to 2.41. 

The advent in 1952 of PTGC ° and the subsequent introduction of commercial equipment for temperature programming provided the necessary means to analyze complex mixtures that contained components of widely differing boiling points and solved some of the problems previously described in the section dealing with isothermal separations. 

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