Chemical Engineering April 2012 - 51

Heat transfer performance
Flow within kettle reboilers
Top of
weir
3
5
104
2
2
3
5
Single tube
104
Liquid
circulation
pattern
Tube bundle
Shell
FIGURE 4. This schematic shows the operation
of a kettle reboiler, where liquid enters
the tube bundle from the base and sides
The cold composite curve
In pinch analysis, the concept of " composite
curves " is based upon the assumption
of counter-current flow. As
will be shown below, co-current flow
should be used in a thermosiphon reboiler.
So, how can such heat demand
be represented in a composite curve?
The option recommended here is that
it is represented as an " isothermal "
operation conducted at the vapor exit
temperature (Figure 2). (For more on
composite curves, see Energy Optimization
Using Pinch Analysis, Chem.
Eng., November 2011, pp. 36-41.)
Matching of reboiler streams
If the stream being used to provide
heat to the reboiler is a liquid, then its
flow should be co-current with the flow
of the stream being vaporized. This is
particularly important with vertical
thermosiphon reboilers (Figure 3).
The mixture being vaporized flows
through the tubes of a vertical thermosiphon.
It enters the tubes as a subcooled
liquid (the liquid in the reservoir
will be at saturation temperature,
but the surface of the reservoir can
be 3-4 m above the lower tube-sheet
of the reboiler). The point at which
nucleation (the formation of bubbles of
vapor) first occurs within the tube is
dependant upon both the temperature
of the liquid and the temperature of
the hot wall. Maximizing the temperature
driving force reduces the length of
5
1
Change in saturation temperature, °C
100
10
1,000
FIGURE 5. The thermal behavior of kettle reboilers has led the authors to
conclude that this type of unit is best avoided when hot liquid is used as the
heat source
tube prior to the onset of nucleate boiling.
This driving force is maximized if
the hot liquid flows co-currently.
The heat transfer coefficient on the
cold-side of the unit increases rapidly
once two-phase flow has been
established. Consequently, there is a
large variation of heat transfer coefficient
along the length of the tube.
This variation is " stabilized " if cocurrent
flow is used. If the hot liquid
flows counter-current to the vaporizing
mixture, then its temperature
profile is affected by changes in the
heat transfer distribution on the coldside.
These changes affect the point
at which nucleation commences and
the heat transfer distribution on the
cold-side. So, the use of counter-current
flow can result in instability and
control difficulties.
Startup and control
Startup and control are important
considerations that must made when
evaluating reboiler integration. While
a plant may be designed to operate at
a given condition (such as a specific
throughput), it will be operated under
a range of conditions. This means
that reboiler load will be subject to
variation, and therefore control will
be required.
In many situations it will be found
that startup and control cannot be
undertaken using an integrated unit,
and that the overall duty must be undertaken
using two separate reboilers:
an integrated unit having a fixed duty
and an non-integrated unit used for
both startup and for control functions.
The engineer needs to determine
the following two items:
1. The load required for startup purposes
(this is set by the minimum
loading on the column not the ultimate
operating condition);
2. The variation in load required for
the control function
The design of the non-integrated reboiler
will involve identifying a geometry
that will provide either the startup
performance or the load associated
with maximum control load (whichever
is the larger) at a point that is at
least 20% below the maximum operating
condition for the design (set by
critical heat-flux conditions).
This reboiler needs to be onstream
all of the time that the plant is operational.
Consequently, its load will never
be zero. The minimum value should
be set at a value that is around 20%
above the lowest stable condition (the
minimal sustainable reboiler load).
The load for the integrated unit will
be the total load minus that required
at the minimum throughput minus
that needed by the non-integrated
unit operating at its minimum load.
Minimum temperature
While the reboiler is represented by
an isothermal line set at the temperaCHEMICAL
ENGINEERING WWW.CHE.COM APRIL 2012 51
Calculated single-tube
maximum heat flux
Bundle data
Common commercial
design method
(Kern 1950)
Heat flux, W/m2
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Chemical Engineering April 2012

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