Page 357 - Fluid mechanics, heat transfer, and mass transfer

REBOILERS 339

& Suitable for high viscosity and fouling liquids or pump and power and possibly severe maintenance.
when fraction vaporized must be kept low. This mode of operation is a last resort with viscous or
& Force recirculation reboilers may be designed so fouling materials, or when the fraction vaporized
that boiling occurs inside vertical tubes, inside must be kept low.
horizontal tubes, or on the shell side. . How is a kettle reboiler different from a U-bundle heat
& Normally used in fired heaters as reduction or exchanger?
loss of circulation results in tube failures. Forced & Shell diameter for a kettle reboiler is much larger
circulation ensures higher flow velocities. than that for a U-bundle heat exchanger on account of
& Since the feed liquid is at its bubble point, provision of vapor disengaging space in a kettle
adequate NPSH must be assured for the pump reboiler.
if it is a centrifugal type. & In a kettle reboiler, shell side liquid submerges the
& Linear velocities in the tubes of 4.6–6 m/s tube bundle with provision of an overflow baffle/weir
(15–20 ft/s) usually are adequate. at the end of the bundle to ensure submergence of the
tube bundle. Liquid product overflows into a reser-
& Very high equipment, maintenance, and operat-
voir section from which it is withdrawn.
ing costs with additional piping and pumps.
. “In a kettle reboiler, liquid vaporizes inside tubes.”
& Larger space requirements.
True/False?
. What are the recommended maximum heat flux for a (i)
& False. Heating medium is admitted into the tubes.
natural circulation reboiler and (ii) forced circulation
reboiler? . What are the advantages of kettle reboilers?
2
(i) Natural circulation reboiler: 37.85 kW/m (12,000 & Vapor disengaging space with reduced entrainment.
2
Btu/(h ft )). & Can operate at low DP.
2
(ii) Forced circulation reboiler: 94.64 kW/m (30,000 & High heat fluxes are possible.
2
Btu/(h ft )). & Can handle high vaporization rates (up to 80%).
. What is the normally recommended ratio of liquid & Low skirt height.
circulated to vapor generated in a circulation reboiler? & Ease of maintenance.
& 4:1. & Ease of control.
. What are the consequences of high vaporization rates in & Handles higher viscosity liquids.
a reboiler? . What are the disadvantages of kettle reboilers?
& Large temperature differences in vaporizers/reboi-
& Difficult to determine degree of mixing.
lers are not conducive to good heat transfer rates on
& High residence time and hence not suitable for heat-
account of film boiling, which prevent liquid reach-
sensitive liquids.
ing heat transfer surfaces as a result of vapor blanket-
& Liquid distribution along the entire length of the shell
ing. In this condition, the heat transfer rates decrease
because of the lower thermal conductivity of the is not uniform within the shell, resulting in excessive
vapor. local vaporization and vapor binding.
& Fouling problems. Reboiler shell side fouling may
& If a design analysis shows that the temperature
difference is close to causing film boiling, the va- lead to tray flooding in the column, as fouling can
porizer should be started with the boiling side full of cause DP buildup on the shell side, leading to liquid
relatively cooler liquid. This way, flashing of the backup in the column.
liquid is avoided. If the vaporizer is steam heated, the & Shell side is difficult to clean.
steam pressure should be reduced which will reduce & Excessive pressure drop in kettle reboiler circuits
the temperature difference. With steam heating, the is the prominent kettle malfunction, causing liquid
design should consider keeping the mean tempera- to backup in the column base beyond the reboiler

ture be kept below 50 C, to avoid film boiling. return elevation. This high liquid level leads to
. What are the issues involved in the selection of forced premature flooding and capacity loss.
circulation reboilers? & Requires extra space and piping.
& Since the feed liquid is at its bubble point, adequate & High cost, mainly due to large shell size.
NPSH must be assured for the pump if it is a . What is a column internal reboiler and what are its
centrifugal type. characteristic features? Illustrate.
& Linear velocities in the tubes of 3–5 m/s usually are & Column internal reboiler is a U-bundle inserted into
adequate. The main disadvantages are the costs of the side of the column (Figure 11.8). It works as a