Page 360 - Fluid mechanics, heat transfer, and mass transfer

HEAT TRANSFER EQUIPMENT INVOLVING PHASE TRANSFER
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& Inability to achieve normal reboiler duty. & These benefits result in a reduction in the size of the
& Bottoms product is very light. exchanger, the space requirements, and the initial
investment.
& Steam inlet valve does not function well, resulting
inadequate heat supply to the column. & However, use of inserts increases pressure drop in the
exchangers and increase cleaning and maintenance
& Low reflux drum level, accompanied by low column
pressure, even at low reflux rate. costs, especially for viscous and dirty fluids.
. “For thermosiphon reboilers, hydraulic aspects are as
& Causes include leaking bottom tray, bottom tray seal
important as heat transfer aspects.” True/False?
pan, or drawoff pan damaged, partially plugged
reboiler, restricted reboiler feed line, excessive re- & True. A single pass TEMA E-type shell is generally
boiler design DP, and submerged reboiler return line used as shown in Figure 11.10.
in column bottoms. & General characteristics of this type of reboiler are
. “In a circulating thermosiphon reboiler, the reboiler a large diameter vapor exit pipe with a cross-
outlet temperature is lower than the column bottoms sectional area equal to the total cross-sectional area
temperature.” True/False? of the tubes arranged to minimize the vertical dis-
tance between the top tube sheet and the column
& False. The reboiler outlet temperature is always
nozzle.
higher than the column bottoms temperature. Part of
the bottom product is recirculated back into the & The liquid level in the column is usually kept near the
column through the reboiler, thus further heating the top tube sheet level in order to provide for maximum
circulating part of the liquid and hence the higher circulation.
temperature. & The driving force for circulation is the density dif-
. “Chances of flooding are more in a circulating reboiler ference between the liquid in the column and the two-
than in a once-through reboiler.” Comment. phase mixture in the tubes.
& In a once-through reboiler, liquid flow is limited to & The exit vapor weight fraction for hydrocarbons is
amount of liquid overflowing bottom tray. in the range of 0.1–0.35 and for water it is about
0.02–0.1.
& In a circulating reboiler, velocities are higher as
liquid flow can be very high making it more suscep- & The E-type shell and associated piping is relatively
tible for flooding of the column. Due to this reason, inexpensive, easy to support and compact.
fouling will be low making it better suited for dirty . What is the effect of choking in a vertical thermosiphon
bottom product. reboiler on heat transfer rates?
. Illustrate by means of a diagram, flow arrangements & Choking on the outlet nozzle and piping reduces the
with a vertical thermosiphon reboiler with respect to a circulation rate through a thermosiphon reboiler.
distillation column. Since the tube side heat transfer rate depends on
& Figure 11.10 illustrates flow arrangements with a velocity, the heat transfer is lower at reduced recir-
vertical thermosiphon reboiler. culation rates. A rule of thumb is that the inside flow
area of the channel outlet nozzle and piping should be
. What are the bottlenecks in the operation of a vertical
the same as the flow area inside the tubing. For
thermosiphon reboiler? Suggest measures in perfor-
example, a ratio of 0.7 in nozzle flow area/tube flow
mance improvements.
area reduces the heat flux by about 10%. A ratio of 0.4
& Due to size and fluid flow restrictions, its heat transfer
reduces the heat flux by almost 50%.
rate is often quite low. The subcooled region formed
. What are the advantages and disadvantages of thermo-
at the tube bundlebase usually causes reduction in the
siphon reboilers?
average heat transfer coefficient on the tube side.
Advantages:
& Improvements in performance can be achieved with
the application of heat transfer enhancement devices & High circulation rates.
such as twisted tape, wire matrix, or other types of & Compact with simple piping.
inserts in the subcooled zone of the reboiler. These
& Reliable operation.
enhancement devices reduce the required heat trans-
& No/low maintenance requirements during normal
fer surface area, subcooled zone length, and maxi-
operation.
mum temperature of the reboiler tube wall, while
& Low investment relativeto a pumped reboiler system.
increasing the overall heat transfer coefficient, sub-
cooled zone overall heat transfer coefficient and & No auxiliary equipment such as a pump for recircu-
average heat flux rate. lation and flow control system are required, saving