Vertical thermosyphon reboilers. Maximum heat flux and separation efficiency

Abstract

A simplified model of a vertical thermosyphon reboiler near the maximum heat flux shows the pressure balance to be dominated by two-phase pressure drop, acceleration, and two-phase static pressure drop. These terms can be developed by using the Martinelli parameter to provide the circulation rate of fluid in the tubes and the heat flux. The maximum heat flux is shown to be a function of the reduced pressure and gives a global maxima when PR = 0.25 for a wide variety of pure components. The other variables that affect (Q/A)MAX are the vapor fraction a t the tube exit and the tube geometry. A list of 11 important characteristics has been obtained to assist in reboiler design. A new reboiler separation efficiency is derived for binary systems, which provides a sound basis for reboiler testing.The boiling of liquids from a metal surface when natural convection dominates has been called pool boiling. A large number of experiments show an increase in the heat flux with temperature difference until a maximum heat flux is obtained. Thereafter, the thermal characteristics show a decline in the heat flux to a minimum and then a further rise when vapor blankets the surface. A photographic study to determine the heat-transfer mechanisms in the three regimes of nucleate, transition, and film boiling is given by Westwater (1956). The maximum heat flux was found to be dependent on the roughness of the surface, the number of nucleation sites, and the degree of agitation of the surrounding liquid. Cichelli and Bonilla (1945) conducted experiments with ethanol, propane, pentane, and benzene over wide pressure ranges and found a global maximum heat flux when the reduced pressure was near 0.35, although there is considerable scatter in the results. Global maximum heat fluxes of over 1 MW/m2 were obtained. An important feature of their results is the low value of the maximum heat flux under vacuum conditions and also under conditions approaching the critical temperature and pressure. Thermosyphon reboilers (see Figure 1) with a vertical or horizontal configuration are widely used in the chemical processing industries. The maximum heat flux through the tube surface is an important design and control consideration. Lee et al. (19561, Johnson (1956), Shellene et al. (1968), andVolejnik (1979) obtained extensive thermal data on water, benzene, acetone, cyclohexane, propylene glycol, ethylbenzene, and butane. Maximum heat fluxes of 325 kW/m2 for water were obtained, but the maximum values for the organics was 170 kW/m2. Lee et al. (1956) attempted to correlate the results using tube length and physical properties including the interfacial tension (a) and the absolute temperature ( T I . Lee et al. (1956) note that the maximum heat flux occurred when there was a departure from smooth operation to surging operation and vapor locking was the cause of the problem. The very high heat fluxes obtained in pool boiling could not be observed in the thermosyphon reboiler, and different controlling mechanisms exist in these two different geometries....

Reboiler Separation Efficiencies for Binary Systems

“…Reboiler efficiency can be defined as the Murphree efficiency for the liquid phase E normalM normalL = ( m x N + n ) − x N ( L̇ / Ġ ) normals normals EF [ false( m x N + n false) − x N ] + ( m x N + n ) − x N = 1 1 + ( L̇ / Ġ ) normals normals EF In this case the reboiler efficiency is E ML < 1, as given in ref and explained, but not clearly quantified in ref . It is obvious that the efficiency will be E ML → 1 for EF → 0, that is, when L̇ rec → ∞.…”