Refluxing Condensation Systems (Dephlegmators)

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-HEA-516

Refluxing Condensation Systems
(Dephlegmator)

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Process Engineering Guide:

Refluxing Condensation
Systems (Dephlegmator)

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

BACKGROUND

2

5

DESIGN CALCULATIONS

10

5.1
5.2
5.3

Flooding
Estimation of Thermal Performance
Estimation of Pressure Drop

10
12
13

6

NON-CONDENSING GASES

13

7

RECOMMENDED AREAS OF APPLICATION

13

7.1
7.2
7.3
7.4

Distillation Columns
Knock-back Condensers
Boiling Coolants
Economic Considerations

13
14
14
14


8

ADVICE ON INSTALLATION

14

8.1

Vertical ’U’-Tube Exchangers

14

9

NOMENCLATURE

15

10

BIBLIOGRAPHY

16

FIGURES

1

DIFFERENTIAL AND INTEGRAL FLASHES

3

2

VERTICAL TUBESIDE REFLUX CONDENSER

5

3

VERTICAL ‘U’-TUBE SHELLSIDE REFLUX CONDENSER

18

4

'STAB-IN' HORIZONTAL BUNDLE REFLUX CONDENSER

7

5

CROSS FLOW SPIRAL HEAT EXCHANGER CONDENSER 8

6

ALFA-LAVAL TYPE G SPIRAL CONDENSER

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

9

16


0

INTRODUCTION/PURPOSE

Most condensers are designed such that the vapor and condensate flow in a cocurrent fashion. There is a class of condenser, known variously as ’Reflux
condenser’, ’Dephlegmator’ or ’Knock-back condenser’ in which the condensate
flows in a counter-current fashion to the vapor. This design offers certain
advantages, but poses some design problems.

1

SCOPE

This Process Engineering Guide discusses the design of condensers in which
the condensate flows by gravity in the opposite direction to the vapor. It covers
estimation of the heat transfer and pressure drop as well as how to estimate
whether the condenser is liable to flood. Duties for which the design is
appropriate are discussed.

2

FIELD OF APPLICATION

This Guide is intended for process engineers and plant operating personnel in
the GBH Enterprises worldwide, who may be involved in the design or operation
of refluxing condensers.

3

DEFINITIONS

No specific definitions apply to this Guide.
4

BACKGROUND

Reflux condensers are usually proposed either for distillation columns, or to be
mounted on reactors in which there is a boiling solvent which it is required to
return to the reactor. Their principal advantage is that they eliminate the need for
vapor pipework, reflux drums and pumps, as they are mounted directly onto the
vapor generating item. They also act as fractionating devices, the vapor
becoming progressively richer in the more volatile components as it passes up
the condenser. If used as the partial condenser of a distillation column, this can
be an advantage; one or more separation stages can be achieved.


However, if total condensation of a multi-component mixture is required, the
fractionating tendency can be a real disadvantage. In the extreme case, it may
prove impossible to condense totally a mixture which would condense in a
normal co-current flow system.
This can be understood with reference to Figure 1. If a vapor is cooled from point
A, it initially starts to condense at the dew point B, at a temperature of Td. The
composition of the initial condensate corresponds to point D. If the vapor and
condensate flow in a co-current fashion in intimate contact, they will remain
approximately in equilibrium. As the stream is cooled further, the vapor
composition follows the line BF, while the condensate follows DC. When the
temperature reaches the integral bubble point, Tbi, all the vapor has condensed
and the liquid is at point C. This process is known as ’integral condensation’.
If the condensate is continuously removed from contact with the vapor, the vapor
then follows the line BFG, not completely condensing until the differential bubble
point, Tbd , is reached. The liquid follows the line DE, where E, the point of total
condensation, has the same composition as the starting vapor at A. This process
is known as ’differential condensation’. (This is an idealized model; in practice it
is not possible to remove all the condensate as it forms, and the degree of
fractionation indicated will not be achieved.)
The processes in a refluxing condenser are in many ways similar to the
differential condensation model. However, partial mixing in the condensate film
and interactions between the phases complicate the process.

FIGURE 1

DIFFERENTIAL AND INTEGRAL FLASHES


4
Four types of reflux condenser geometry are common:
(a)

Vertical tubeside, (see Figure 2). This design is attractive if exotic
materials are necessary for the process fluid, as the shell can be
fabricated from carbon steel. Design methods are most established for this
geometry.

(b)

Vertical ’U’-tube shellside, (see Figure 3). Units can be unbaffled as
shown; it may be desirable to add baffles to enhance the performance or
to prevent tube vibration. Either conventional segmental or rod baffles can
be used. Design methods for predicting flooding are uncertain. It is not
possible to drain the fluid from the tubeside.

(c)

’Stab-in’ horizontal bundle, (see Figure 4). This design is only suitable for
columns of a reasonable diameter. Tubeside drainage is not a problem.
The use of low fin tubing to enhance the heat transfer is possible. Design
methods for avoiding flooding are uncertain.


(d)

Cross flow spiral heat exchanger, (see Figure 5). (Note that
Alfa-Laval also offer their type G spiral condenser, which although
mounted directly onto a distillation column, has a central pipe up which the
vapor flows before passing back down the spirals in co-current flow with
the condensate, (see Figure 6)). This latter type is not a reflux condenser;
it can be treated as a conventional down-flow condenser.

FIGURE 2

VERTICAL TUBESIDE REFLUX CONDENSER


FIGURE 3

VERTICAL ‘U’-TUBE SHELLSIDE REFLUX CONDENSER


FIGURE 4

’STAB-IN’ HORIZONTAL BUNDLE REFLUX CONDENSER


FIGURE 5

CROSS FLOW SPIRAL HEAT EXCHANGER REFLUX
CONDENSER

Note:
The exchanger may be mounted directly onto a distillation column, whereupon
the lower end plate and nozzle may be omitted.

FIGURE 6

ALFA-LAVAL TYPE G SPIRAL CONDENSER


5

DESIGN CALCULATIONS

The presence of counter-current condensate and vapor flow presents certain
problems when undertaking the thermal design of a reflux condenser. These can
be divided into:
(a)

The prevention of flooding.

(b)

Estimation of the heat transfer performance.

5.1

Flooding

If a liquid film is flowing down the walls of a vertical tube and a vapor is flowing
upwards, the interfacial shear will result in a thickening of the film. As the vapor
flow is increased, further thickening of the film takes place, and waves are
induced on the surface. Finally, a point will be reached where some of the liquid
will be carried back up the tube by the vapor. This phenomenon is known as
flooding, and represents the limits of operation of a vertical tubeside reflux
condenser.
Flooding is a complex phenomenon, and there are considerable uncertainties in
data and disparities between the various correlations.


5.1.1 Tubeside Flooding
HTFS, in the Handbook sheet TM11 (Ref. [1]), recommend the correlation due to
Alekseev et al.
Calculate a Froude number:

. Calculate the flooding gas superficial velocity:

In order to allow for uncertainties, HTFS recommend a maximum design gas
superficial velocity of 0.625 vgf .
.The flooding velocity can be increased, and hence the tendency to flooding
reduced, either by inclining the condenser by 10-20° to the vertical, which may
not be practical, or by cutting the tube ends at an oblique angle (see Figure 2).
However, the exact magnitude of the increases due to these changes cannot be
predicted at present. It is recommended that obliquely cut tubes be used to
provide an additional safety factor above the flooding velocity as calculated
above.

5.1.2 Flooding In Other Geometries
There are no published methods known to the author for predicting flooding in
shellside condensers or spiral condensers. Tentatively, the Alekseev correlation
could be used in conjunction with a hydraulic mean diameter. For a spiral
exchanger, the hydraulic diameter is twice the separation between the plates, 2a.
For longitudinal (unbaffled) flow on the shellside of an exchanger with triangular
pitch it is:

For baffled shellside flow, no recommendations can be made.
An alternative approach to the flooding problem for 'U'-tube condensers, which
has been successfully used in the NE, is to check that the vapor velocities at all
points in the bundle never exceed the terminal velocity of the condensate drops
falling onto the collector pan. The terminal velocity of a droplet of diameter d is
given by:

Droplets will be unstable and shatter if the Weber number (We) exceeds 10.

Combining these equations:

This gives the maximum velocity for drops to be collected. As for the tubeside
flooding, a margin of safety should be placed on this figure; 75% is suggested.

5.2

Estimation Of Thermal Performance

5.2.1 Vapor-Liquid Equilibrium Calculations
For normal condensers, with co-current flow of condensate and vapor, the usual
design method for calculating the condensing coefficient is that of Silver which is
described in Ref. [2]. This uses an integral condensation curve, where the heat
load as a function of temperature is calculated assuming that the vapor and
condensate are in thermal and compositional equilibrium along the exchanger.
Such a curve can be readily calculated, for example by using the
programs/Tasks option in the “VAULT”, as at any point along the exchanger the
total composition is known.
This is not the case for a reflux condenser, because of the fractionating effects;
the condensate at the bottom of the condenser is the total of that which has been
condensed higher up. It is not possible to determine what the liquid and vapor
compositions will be at any point. HTFS (Ref. [3]) recommend the use of a
differential condensation curve, in which it is assumed that the vapor is in
equilibrium with the local newly forming condensate. This is equivalent to
assuming there is no mixing of the condensate, and corresponds to the curves
BG and DE in Figure 1.
At present, there is no easy way of calculating differential condensation curves; it
is possible that an option will be added to the “VAULT” for the automatic
generation of the curves at some future date, but there are no immediate plans.
The data can be generated with the assistance of the “VAULT”, but only with
considerable user intervention. The calculation of the differential condensation
curve gives some indication of the fractionation effects. However, in practice the
degree of separation achieved is likely to be less than indicated by this method,
due to a combination of gas phase resistance effects and liquid phase mixing.
For a single condensable component with inerts (non-condensables), the vapor
composition depends only on the temperature. For this case, the integral
condensation curve could be used. However, this will over-estimate the heat load
for a reflux condenser, as it assumes that the condensate is cooled to the vapor
temperature; this is a safe assumption for design.
For a pure component, ignoring the effects of pressure drop, the condensation is
isothermal, and the differential and integral condensation curves are equivalent.


5.2.2 Heat Transfer Coefficients
Having calculated the differential condensation curve, HTFS recommend the use
of the Silver method (Ref. [2]) for the gas phase thermal resistance. . However,
beware of the effects of shear on the liquid film coefficient; see below.
5.2.3 Estimation Of Condensate Film Coefficient
For vertical down-flow condensation, the effects of gas shear on the liquid film
are to thin the film and increase its turbulence, both of which will increase the film
coefficient. For a reflux condenser, on the other hand, vapor shear will thicken
the liquid film, reducing the coefficient. This will to some extent be offset by the
increased turbulence, but the overall effect cannot be readily determined. It is
likely to be non-conservative to ignore the effects of vapor shear. The effects of
shear are likely to be minor for vapor velocities less than half the flooding
velocity, but above this could be significant.
5.3

Estimation of Pressure Drop

There are no reliable methods for estimating pressure drop in the counter-current
flow of liquids and vapors in tubes or other geometries. However, for designs well
away from the flooding point, the pressure drop is likely to be low. The pressure
drop will rise sharply as the flooding point is approached.

6

NON-CONDENSING GASES

The in-tube reflux condenser may not be well suited to duties where there is a
significant quantity of inert gas present, as the low velocities necessary to avoid
flooding in the lower part of the tube will result in high gas phase resistances in
the upper parts of the tube. A more appropriate design here could be the
shellside condenser with a variable baffle pitch, but, as was pointed out above,
there are no reliable flooding correlations for this geometry.
However, even though the high resistance in the upper part of the exchanger
results in a larger unit than is desired, it could still be financially attractive
because of the large savings in pipework, reflux drum and pumps.


7

RECOMMENDED AREAS OF APPLICATION

7.1

Distillation Columns

The cost saving and separation effects of a reflux condenser have been exploited
to our advantage in, for example, the Amines and Alkyl Phenols areas, using
internal ’U’-tube condensers with shellside condensation. Experience suggests
that as a general rule, 2m of tube gives approximately one separation stage.
Spiral exchangers are increasingly being used in the NE for design pressures
below 10 bar. Both these designs lead to a compact plant without the vapor and
liquid return pipework.
7.2

Knock-back Condensers

Because of the fractionating effects, the use of a reflux condenser configuration
to return condensed vapors to a reactor is not recommended for wide boiling
range multi-component mixtures where it is desired to return all of the vapors as
condensate to the reactor.
7.3

Boiling Coolants

If the heat is to be removed using a boiling coolant, such as a refrigerant, the
normal design of condenser in the process industries would be a kettle boiler,
with the refrigerant in the horizontal shell, and condensing in the tubes. If a reflux
condenser with tubeside condensation is required, the boiling refrigerant will be
on the vertical shellside of the exchanger. Design methods for boiling on the
outside of vertical tube bundles are less sure than for kettle shells. Points to
consider are the avoidance of carry-over of liquid with the vapor, and whether
baffling is required for tube support. Nevertheless, this arrangement can be used
with care.
7.4

Economic Considerations

The reflux configuration can save significant quantities of large diameter
pipework, as well as saving on reflux pumps and drums. However, in order to
avoid the flooding problem, units have to be designed with relatively low inlet
velocities. This can lead to larger exchangers than would be necessary for a cocurrent design, especially where the vapor contains significant inerts (see
above), or in vacuum duties where there is a large volume of vapor. If the inerts
concentration is low, the exchanger size is almost independent of condensing
side velocity.


8

ADVICE ON INSTALLATION

8.1

Vertical ’U’-Tube Exchangers (see Figure 3)

(a)

To avoid excessive pressure drop, the collector pan diameter should be
such that the annular area is not less than 40% of the column crosssectional area.

(b)

A distance of 0.5 - 1.0 m, depending on column diameter, should be left
between the ’nose’ of the condenser bundle and the collector pan to
reduce the turbulence in the vapor as it decelerates from the annulus to
the space above the collector pan.

(c)

A wall wiper ring, fixed between the collector pan and the bundle should
be fitted to divert condensate flowing down the wall into the collector pan.
If this is not fitted there is a risk that insufficient condensate will enter the
collector pan to meet the product draw-off requirements, particularly at
reflux ratios below 1:1.

(d)

The product draw-off nozzle should be sized correctly to avoid sucking
vapor into the liquid off-take (see GBHE-PEG-FLO-301 - Overflows and
Gravity Drainage Systems).

(e)

If the inerts concentration is higher than 10% v/v, it may be advantageous
to make the bundle diameter less than the column diameter and reduce
the column diameter approximately one third of the way up from the base
of the ’U’-tube, to increase the vapor velocity and enhance the film
coefficient. It may also be advantageous to fit baffles at the top of the
exchanger to improve heat transfer further. Some baffles may be required
for tube support in any case. Vessels Section should be consulted for
advice. Baffles should be sloped 1.5 - 2.0° to the horizontal to assist
drainage For vapors with low inerts concentrations at the inlet, the
coefficient will not be significantly enhanced by the above, so the
additional complications can be avoided.


9

NOMENCLATURE

a
Bo
Di
Do
d
F
Fr
g
Kg
n
p
Ql
Tbd
Tbi
Td
U crit
Ut
u
v gf
We
ml
rg
rl
s

Gap between plates in a spiral exchanger
Bond number, defined in equation 2
Tube inside diameter
Tube outside diameter
Droplet diameter
Viscosity correction factor, defined in equation 3
Froude number, defined in equation 1
Gravitational acceleration
Kutateladze number, defined in equation 4
Number of tubes in the exchanger
Tube pitch
Liquid volumetric flow
Bubble point for differential condensation
Bubble point for integral condensation
Dew point
Maximum gas velocity for drops to be collected
Terminal velocity of droplet
Relative velocity between gas and drops
Flooding superficial gas velocity (m/s)
Weber number, defined in equation 9
Liquid viscosity
Gas density
Liquid density
Surface tension

(m)
(m)
(m)
(m)

(m/s2)

(m)
(m3/s)
°C
°C
°C
(m/s)
(m/s)
(m/s)

(Ns/m2)
(kg/m3)
(kg/m3)
(N/m)


10

BIBLIOGRAPHY

Ref

Source

[1]

HTFS Handbook sheet TM11. ’Correlation for flooding in vertical tubes’.
P B Whalley, April 1984.

[2]

HTFS Handbook sheet CM 15. ’Silver method for multi-component
condensation’. D Butterworth, August 1979.

[3]

HTFS Handbook sheet CP15. ’Guidelines for the design of reflux
condensers’. J M McNaught, February 1986.

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
PROCESS ENGINEERING GUIDES

GBHE-PEG-FLO-301

Overflows and Gravity Drainage Systems (referred to
in Clause 8.1).


Refluxing Condensation Systems (Dephlegmators)

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