Refluxing Condensation Systems (Dephlegmators)

Refluxing Condensation Systems (Dephlegmators)

0 INTRODUCTION/PURPOSE
1 SCOPE
2 FIELD OF APPLICATION
3 DEFINITIONS
4 BACKGROUND
5 DESIGN CALCULATIONS
5.1 Flooding
5.2 Estimation of Thermal Performance
5.3 Estimation of Pressure Drop

6 NON-CONDENSING GASES

7 RECOMMENDED AREAS OF APPLICATION
7.1 Distillation Columns
7.2 Knock-back Condensers
7.3 Boiling Coolants
7.4 Economic Considerations

8 ADVICE ON INSTALLATION
8.1 Vertical ’U’-Tube Exchangers

9 NOMENCLATURE

10 BIBLIOGRAPHY

1. 1 CATALYST, PROCESS TECHNOLOGY

CONSULTANCY

Process Engineering Guide:

GBHE-PEGHEASIS

Refluxing Condensation Systems
(Dephlegmator)

Information contained in 1

2. FIELD OF APPLICATION

DEFINITIONS

BACKGROUND

DESIGN CALCULATIONS

Flooding
Estimation of Thermal Performance
Estimation of Pressure Drop

NON-CONDENSING GASES

RECOMMENDED AREAS OF APPLICATION

3. FIGURES

DIFFERENTIAL AND INTEGRAL FLASHES

VERTICAL TUBESIDE REFLUX CONDENSER

VERTICAL 'U'-TUBE SHELLSIDE REFLUX CONDENSER

'STAB-IN' HORIZONTAL BUNDLE REFLUX CONDENSER,

ROSS FLOW SPIRAL HEAT EXCHANGER CONDENSER 8

ALFA-LAVAL TYPE G SPIRAL CONDENSER,

4. estimation
whether the condenser is liable to flood. Duties
appropriate are discussed,

2 FIELDOF 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
f refluxing condens

DEFINITIONS
ecificdefinitions apply to this Guide.

BACKGROUND

5. temperature gral bubble point,
and the liquid is at point C. This process is

from contact with the

ondensing until the differential bubble
point, Tes is reached. The liquid follows the line DE, where E, the point of total
condensation, has the s: he 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 fm
and interactions between the phases complicate the process.

FIGURE 1 DIFFERENTIAL AND INTEGRAL FLASHES

6. 4

Four types of reflux condenser geometry are common:

Vertical tubeside, (see Figure 2). This design is att
materials are necessary for the process fluid, as the
fabricated from carbon steel. Design methods are mc
geometry

Vertical 'U'-tube shellsido, (see Figure 3). Units can be unbaffled as
files to enhance the performance or
d battles

12. jooding

If 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 fm. As the

flow is increased, further thickening of the film takes place, and was

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.

13. sise

loc:

14. For battled shellside flow, no recommendations can be made.

An alternative approach to the flooding problem for ‘Utube 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 dis

15. -ondenser, because of the fractional
the condenser isthe total ofthat which has be
not possible to determine what the liquid and vapor
be at any point. HTFS (Ref. [3)) recommend the use of a
differential condensation curve, in which i 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 cun
BG and DE in Figure 1

At present, there is no easy way of calculating differential condensation curves; it
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
deg paration achieved is likely to be less than indicated by this method,
due to a combination of gas pha a e mixing,

16. but the overall effect cannot be readily determined. It is
ore the effects of vapor shear. The effects of
than half the floodin

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
y from the flooding point, the pressure drop is likely to be low. The pressure

drop will ise 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

fooding inthe lower part ofthe tube wil resultin high gas phase ve
ign here could be the
pointed out above,

17. inating 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.

73 Bolling Coolants

If the heat isto be rem sing 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. Ifa reflux
condenser with tubeside condensation is required, the boling 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 ketle shells. Points to
consider are the avoidance of carry-over of liquid with the
battling is required for tube support. Nevertheless, this arranc

th care,

7.4 Economic Considerations

18. a

I wiper ring, fixed betv
be fitted to divert condensate flowing down the wall nto the collector pan.
If this is not fitted there isa risk that insufficient condensate will enter the
ollector pan to m product draw-off requirements, particularly at
reflux ratios below 1:1

The product draw-off nozzle should be sized correctly to avoid sucking
/apor into the liquid off-ake (see GBHE-PEG-FLO-301 - Overflows and
Drainage System:

If the inerts concentration is higher than 10% viv, it may be advantageou
to make the bundle diameter less than the column diameter and reduc
the column diameter approximately one third of the way up from the base
of the "U-tubs rease 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 inerts concentrations at the inlet, the

19. number, defined in equation 4
Number of tubes in the exchanger

Tube pitch

Liquid volumetric flow

Bubble point for differential condensation

Bubble point or integral condensation

Dew point

Maximum gas velocity for drops to be collected

Terminal velocity of droplet

Relative velocity between gas and drop

Flooding superficial gas velocity (m/s)

Weber number, defined in equation 9

Liquid viscosity

Ge (kgim3)

Liquid density (Kama)
Surface tension (Nim)

20. 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 lems (referred to

21. GBH ENTERPRISES, LTD

VULCAN Catalyst Process

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