Page 359 - Fluid mechanics, heat transfer, and mass transfer

REBOILERS 341

60% of the shell diameter. Other criteria include & True.
keeping a minimum height of 0.3 m or 1.3 to 1.6 . “In horizontal thermosiphon reboilers, fluid is not taken
times the bundle diameter (whichever is greater) on the tube side.” Comment.
above the liquid level.
& Due to their inability to establish a vertical density
& Normally the vapor outlet is in the center above the gradient inside horizontally aligned tubes.
bundle. Then the vapor comes from two different . What is the maximum percent vaporization that may be
directions as it approaches the outlet nozzle. Only in
acceptable in a vertical thermosiphon vaporizer illus-
rare cases are these two vapor streams equal in
trated in Figure 11.10. Give reasons for this choice.
quantity. A simplification that has been extensively
& About 30% of the feed. For higher fouling fluids, a
used is to assume the highest vapor flow is 60% of
lower percent vaporization should be used to avoid
the total. One case where this would cause an un-
fouling deposits.
dersized vapor space is when there is a much larger
& Experience dictates that a 30% vapor fraction helps
temperature difference at one end of the kettle than
keep a 1.8–2.4 m (6–8 ft.) long tube wet along its
the other. The minimum height of the vapor space is
entire length (or nearly the entire length) as when the
typically 18 cm (8 in.). It is higher for high heat flux
tube begins to dry out, fouling occurs.
kettles.
. What is the maximum recommended DT between the
& Using internal baffles or demisters to reduce entrain-
fluids in a thermosiphon reboiler?
ment will entail maintenance problems and frequent
& About 50 C.

blockages.
. Is mist flow a desirable phenomenon in a reboiler?
. What are the effects of an undersized kettle reboiler?
& No. In this type offlow, the tubewall is almost dry and
& The effect will be a decrease in the boiling coeffi-
the liquid droplets are carried along in a vapor core.
cient. A boiling coefficient depends on a nucleate
Therefore, the heat transfer is much lower because
boiling component and a two-phase component that
the much higher thermal conductivity of the liquid is
depends on the recirculation rate. An undersized
invery little contact with the tubewall. The higher the
kettle will not have enough space at the sides of the
percent vaporization, the lower should be thevelocity
bundle for good recirculation. Another effect is high
to avoid mist flow.
entrainment or even a two-phase mixture going back
to the column. & If the mist flow region cannot be avoided, then use of
twisted tape turbulence enhancers can be used to
. If reboiler heating medium is steam, a desuperheater is
increase heat transfer. They will throw the liquid in
provided if superheat is more than 4–15 C. Why?

the vapor core toward the tube wall.
& Heat transfer coefficient is very low for superheated
. What are the reasons for loss of circulation through a
steam;desuperheatingincreaseshby100to200times.
once-through thermosiphon reboiler?
. What is a thermosiphon reboiler?
& Reboiler outlet temperature is higher than column
& Thermosiphon: Convective movement of liquid
bottoms temperature.
starts when liquid in the loop is heated, causing it
to expand and become less dense and thus more
buoyant than the cooler liquid in the bottom of the
loop. Convection moves heated liquid upwards in the
system as it is simultaneously replaced by cooler
liquid returning by gravity. The liquid may be heated
beyond its boiling temperature, causing vaporization
of part of the liquid. Since the vapor is much more
buoyant than the hot liquid, the convective process is
increased considerably.
& Thermosiphon reboiler operates making use of the
buoyancy effect. Typically 25–30% of the liquid is
vaporized in the reboiler. The two-phase flow is sent
back to the distillation column where the vapor is
released to move up through the trays and the liquid
drops back to the bottom of the column.
. “Thermosiphon reboilers can be horizontal or vertical.”
True/False? FIGURE 11.10 Vertical thermosiphon reboiler.