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182 Applied Process Design for Chemical and Petrochemical Plants

5. Add area requirements of sensible heat to the area Vaporization Inside Vertical Tubes;
required for vaporization to obtain the total area. Natural Thermosiphon Action
6. Follow steps 7 (Gilmour method), etc., of the proce-
dure for vaporization only. If baffles are added for sensi- The vertical thermosiphon reboiler is a popular unit for
ble heat (not assumed in free convection), then heating distillation column bottoms. However, it is indeed
pressure drop will be affected accordingly. Gr a is the surprising how so many units have been installed with so lit-
Grashof number using properties at average fluid tem- tle data available. This indicates that a lot of guessing, usu-
3
2
perature, D i g t/ . ally on the very conservative side, has created many
uneconomical units. No well-defined understanding of the
70
Procedure for Horizontal Natural Circulation performance of these units exists. Kern’s recommended
Thermosiphon Reboiler procedure has been found to be quite conservative on plant
scale units; yet it has undoubtedly been the basis for more
These units normally do not have a disengaging space but designs than any other single approach. For some systems at
allow the vapor-liquid mixture to enter the distillation unit and below atmospheric pressure operation, Kern’s proce-
or other similar item of equipment. Feed is from the bottom dure gives inconsistent results. The problem is in the evalu-
with a split flow on the shell side by means of a shell-side baf- ation of the two-phase gas-liquid pressure drop under these
fle in the center being open at each end. conditions.
This unit is usually used as the reboiler for the distillation For units that are vertical one-pass in tubes with liquid in
column and, in this service, operates by the thermosiphon the bottom entrance and a top exit of the liquid-vapor mix-
action of the difference in static head in the column and in ture, the separation is accomplished in the equipment to
the vapor-liquid phase leaving the reboiler. When tied into which it is attached, usually a distillation column (Figure 10-
the bottom chamber, the liquid is usually recirculated many 96D).
times, vaporizing only 10—25% of the reboiler feed per pass; For services in which fouling is high or in which down-
however, when used as a draw-off from the bottom tray seal time cannot be tolerated, two reboilers may be installed on
pan, the feed to the reboiler is not recirculated flow. The the same distillation column. These reboilers may each be
basic operation is the same, however. half sized so that downtime will be limited to a half-capacity
operation; each may be two-thirds sized; or each may be a
full 100% spare. The latter is, of course, the most expensive
Kern Method 70 from an equipment investment standpoint but may pay for
itself in uptime.
1
1. Follow the procedure for steps 1, 2, 3, 4, and 5 of the ear- The tubes are usually 1 / 4 -in. O.D. but never smaller
lier section, “Nucleate or Alternate Designs Procedure.” than 1-in. O.D. because the flow contains vapor as well as liq-
2. If sensible heat exchange exists, follow steps 2—6 of the uid. The recirculation ratio; i.e., liquid-to-vapor ratio in the
“Suggested Procedure for Vaporization with Sensible outlet, is seldom less than 5 and more often is 10—15, some-
Heat Transfer,” previously in this chapter. times reaching 50.
3. Pressure drops must be kept low through the piping to
the reboiler and through the reboiler to avoid Fair’s Method 45
expensive elevation of the distillation equipment. Kern
suggests 0.25 psi as preferable to 0.50 psi; however, the This method for vertical thermosiphon reboilers is based
final economic balance of the system will determine the on semi-empirical correlations of experimental data and is
allowable pressure drop, because for the same system, stated to predict heat transfer coefficients 30 percent,
either the piping or the exchanger must become larger which is about the same range of accuracy for most boiling
if the pressure drop is to be reduced. coefficient data. The advantage of this method is that it has
had significant design experience in the industry to support
The length of the tubes should not be selected more than it. It is also adaptable to other types of reboilers used in the
4.5—5.5 times the shell diameter. This performance may be industry. See Figures 10-110 and 10-111.
increased by placing two inlets in the bottom and two vapor Fair’s 45 presentation provides the development of the
outlets in the top, and at the same time adding shell-side design technique. The method recognizes two-phase flow in
longitudinal baffling to split the flow into four paths upon a vertical reboiler and points out that slug-type flow is most
entrance. The paths recombine before leaving. predominate and that mist-flow should be avoided.
The recirculation ratio for a unit is the lb rate of liquid This procedure develops stepwise calculations along the
leaving the outlet compared to the lb rate of vapor leaving. tube length, using increments of length or vaporization.
The liquid recirculation flow rate entering the unit is set by The increments are chosen small enough so that average
the differential pressure driving the system. values of R L , R g ,
, X tt , and h t may be used in the difference