Forced-Recirculation Reboilers

Forced-Recirculation Reboilers In forced-recirculation reboilers, a pump is used to ensure circiilation of the liquid past the heattransfer surface. Force-recirculation reboilers may be designed so that boiling occurs inside vertical tubes, inside horizontal tubes, or on the shell side. For forced boihng inside vertical tubes. Fair s method (loc. cit.) may be employed, making only the minor modification that the recirculation rate is fixed and does not need to be balanced against the pressure available in the downcomer. Excess pressure required to circiilate the two-phase fluid through the tubes and back into the column is supphed by the pump, which must develop a positive pressure increase in the hquid. 

Fair s method may also be modified to design forced-recirculation reboilers with horizontal tubes. In this case the hydrostatic-head-pressure effect through the tubes is zero but must be considered in the two-phase return Tines to the column. 

The same procedure may be applied in principle to design of forced-recirculation reboilers with shell-side vapor generation. Little is known about two-phase flow on the shell side, out a reasonable estimate of the fric tion pressure drop can be made from the data of Diehl and Unruh [Pet Refiner, 36(10), 147 (1957) 37(10), 124 (1958)]. No void-fraction data are available to permit accurate estimation of the hydrostatic or acceleration terms. Tnese may be roughly estimated by assuming homogeneous flow. 

The still for this system is a simple flash still with a forced circulation reboiler. The saponified, crude glycerine stream is fed into the recirculation pump on the suction side. The recirculation rate is in the range of 40-50 times of the crude glycerine feed rate. The top of the stiU vessel is equipped with a pad style entrainment separator to eliminate carryover. 

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