Page 361 - Fluid mechanics, heat transfer, and mass transfer

REBOILERS 343

pump energy. Pumping in reboiler systems involve & True.
handling hot liquids with pump NPSH, leakage and . “Maintenance for a vertical thermosiphon reboiler is
other pump maintenance problems. easy.” True/False?
& Savings in energy which is required if pump is used. & False.
& No leakage problems as there is no pump. . Briefly explain the impact of relative volatility on the
& Not easily fouled because of the relatively high two- operation of thermosiphon reboilers.
phase velocities obtained in the tubes. For a fluid that & High relativevolatility gives rise to large temperature
is considered to have average fouling tendencies, gradients.
using a vertical thermosiphon reboiler is quite & Large variations in reboiler feed temperatures result
common.
from compositional differences in the reboiler feed
& Heavy components are not likely to accumulate.
arising from different configurations. Tendency to
& Less cost than kettle type, the later requiring large create foaming will be more with high relative vol-
diameter shell and more space for installation. atility fluids. Foaming creates control, operability,
product quality, and stability problems.
Disadvantages:
. What are the symptoms for loss of circulation in a once-
& Requires more headroom and column skirt height.
through thermosiphon reboiler?
& Its main disadvantage is the problem of hydrody-
& Inability to achieve normal reboiler duty.
namic instability, as the upward flow of the vapor
should be capable to support the flow of liquid slugs. & Low reflux drum level, accompanied by low column
Experience has shown that the instability at high heat pressure even at low reflux ratio.
fluxes is very sensitive to the size and hence to the DP & Too light bottoms, that is, bottoms composition with
in the exit piping. respect to low boiling component(s) increases.
& More difficult to design satisfactorily than kettle & Reboiler outlet temperature is higher than column
reboilers, especially in vacuum operation. Lack of bottoms temperature.
understanding of design issues which include inte- & On opening heat source (steam/hot oil) inlet valve
gration of complex columns, reboiler systems and will not increase heat input.
control schemes, temperature gradient analysis, and
. What are the causes for loss of circulation? (cf. above
imbalances in multiexchanger thermosiphon reboiler
question)
circuits.
& Leaking column bottom tray due to low dry DP.
& Control over circulation rates is not easy with upsets
& Damaged bottoms tray seal pan/drawoff pan.
in operation due to energy input variations.
& Partially plugged reboiler.
& Poor thermosiphoning for high viscosity liquids.
& Restricted flow to reboiler feed line.
& Steam being on the shell side, which involves low
& Excessive design DP for the reboiler.
velocities,foulingtendenciesarehighontheshellside.
& Bottoms liquid level covering reboiler vapor return
& Suspended solids in the process fluid are recirculated
nozzle.
between the column and the reboiler.
. How would one detect malfunctioning of thermosi-
& Large surface area requirements.
phon circulation in a once-through thermosiphon
. Give general guidelines for thermosiphon reboilers.
reboiler?
& Inclined piping should never be used for two-phase
& By observing temperatures of column bottoms and
flow in a process plant. This is particularly true for
reboiler outlet. If column bottoms are cooler than the
reboiler return piping. Only horizontal or vertical
reboiler outlet liquid, it means that some thing has
runs should be used.
gonewrong with circulation and remedial steps are to
& If the reboiler heating medium is condensing steam, a
be taken.
desuperheater should be provided, if the superheat is
. What are the consequences of submerged reboiler

more than 4–15 C.
return inlet below the column base liquid?
. “For a horizontal thermosiphon reboiler, process fluid is
& The high velocity vapor entering the column from the
taken on tube side.” True/False?
reboiler outlet, when below the liquid level in the
& False. Process fluid is on shell side, unless forced
column, gives rise to entrainment and carry-over
circulation is employed.
large amounts of liquid on to the trays or packing
. “Land area required for a horizontal thermosiphon above. This liquid gives rise to flooding the column to
reboiler is more than that for a vertical one.” True/False? considerable heights.