A. Sharma, Contributing
Author, Jaipur, India
Of the condensers in process industries such as
refineries and petrochemical facilities, many are flooded-type condensers.
Flooded condensers are fundamental heat exchangers that condense hot vapor or
gas into a liquid. During typical operation, 10%–40% of the condenser's area is
covered or filled with liquid, while the balance is used for vapor
These flooded condensers are typically employed
in total condensers, where hot vapor fully condenses into liquid and the
outflow is completely liquid. Usually, pure liquid is produced by fully condensing
hot vapor. This means that ethylene and propylene fractionators, debutanizers,
depropanizers, depentanizers, iso-butane and propylene compressors and other
systems with mostly pure hot vapor can be developed with total and flooding
Flooded condensers are generally favored in
pure component condensers since their dew point and boiling point (bubble
point) are the same for pure (single component) components. A small pressure
drop can drastically vaporize the condensed liquid in the reflux drum and lower
the liquid level. Subcooling of the liquid is essential to avoid such a level
fluctuation; therefore, flooded condensers are chosen to provide subcooling.
Typical flooded condenser designs fall into two
categories. The first contains a control valve installed at the condenser inlet
or outlet and the condenser outlet is submerged in the reflux drum. Due to
backpressure, the condensed liquid is resisted and flows back into the
condenser (FIG. 1).
In the second design, the reflux drum is
elevated above the condenser and liquid is lifted to the reflux drum due to
liquid subcooling (FIG.
article will delve into following challenges and providing recommendations for
condenser design to avoid process disturbances.
Challenges. Non-condensable gas buildup primarily
affects this flooded condenser. Small bubbles can clump together and generate
vapor pockets. Non-condensable gases such as methane and hydrogen (H2),
and inert gases such as nitrogen, air and carbon dioxide (CO2) build
in the flooded condenser because its bottom section or condenser outlet is back
flooded with liquid, which acts as a liquid seal for this vapor and prevents it
from escaping through the liquid. Vapor tends to lift rather than fall due to
This inert or non-condensable gas insulates by
covering the tubes and minimizes heat transfer (the vapor heat transfer
coefficient is very low), known as vapor binding. In 1983, Bell, et al.,
stated that more than half of all condenser problems are due to poor venting.1
One of the fluid catalytic cracker (FCC)
facilities' debutanizer towers had a flooded condenser (Type 1), and the
reboiler was swapped over to stand by after nitrogen pressurization-depressurization
(PDP). This residual nitrogen within the column accumulated in the top
condenser as overhead vapor. The nitrogen concentration was high enough to
destabilize the column. Column pressure varies, and the column tripped due to
high pressure. The issue was that there was no vent nozzle on the flooded
condenser's shell side to vent out non-condensable gases.
Another challenge comes from an ethylene plant,
where the pressure in the ethylene fractionator (C2 splitter) varies
at high pressure despite a high propylene flow (as a coolant) in the condenser.
The non-condensable vent nozzle was present to vent out; however, it was
blinded without an isolation valve.
The iso-butane refrigeration compressor in the
refinery tripped on high pressure due to an accumulation of nitrogen in the
condenser is the only place where the nitrogen can vent out inert or
non-condensable (lighter) gas to minimize hydrocarbon loss. The vent was
present, but it took a long time to vent non-condensable gas because the
vertical baffle in the heat exchanger trapped most of the non-condensable gas and
impeded the vapor flow, limiting the heat transfer area (FIG. 3).
A similar issue occurred in a steam cracker
where the propylene refrigeration compressor faced a high-pressure scenario:
the vent location was incorrect, resulting in the high likelihood of inert
accumulation. Additionally, numerous case studies have also been reported.2,3
The preceding situation demonstrates why a
permanent vent with an isolation valve is essential in a flooded condenser and
why the vent should be located where there is a high risk of inert
Solutions. These flooded
condensers necessitate the installation of a permanent vent with a valve to
exhaust non-condensable gases. It is always preferable to redirect into a flare
so that non-condensable gases can exit the system. Some plant vents are, in
some manner, linked to reflux drums. The gases have been vented but are still
in the system. When hot vapor condenses in the shell side of the
condenser, the vent must be located far enough away from the vapor inlet to
avoid bypassing hydrocarbon vapor rather than inerts.
A vertical baffle is often installed on the
shell side of the condenser to support the tube and produce turbulence in the
fluid, improving heat transmission. These baffles also trap most
non-condensable gas, preventing it from reaching the vent, always resisting
heat transmission and bottlenecking the condenser.4 Drilling a 6-mm
hole at both the top and bottom is always preferable to vent non-condensable
gas and drain-trapped liquid (FIG. 4).
If the vapor condenses
in the tube side of the flooded condenser, most non-condensable gases are prone
to become trapped in the condenser's bottom pass partition of tube bundle;
however, vent nozzles are often provided in the condenser's top compartment.
Venting should always be done through the bottom pass partition of the tube compartment.5
When the bottom
section of the partition vent is unavailable, an alternative method is to drill
a 6-mm hole in the pass partition plate to let non-condensable gas enter the
top compartment and exit through the vent to flare (FIG. 5).
flood-back varies in flooded condensers; therefore, it must be considered. Due
to backpressure in the reflux drum caused by friction drop, piping or other
drum restrictions can flood the condenser. The safety factor in the surface
area of the condenser must be greater than the normal necessary for heat
transfer for condensation.6,7
A steam reboiler can have similar flood-back
and variable area issues. Where the condensate header is well elevated, and the
condensate cannot lift, the steam reboiler is back-flooded
with water, reducing the heat transfer area and bottlenecking the reboiler.
Thermal scanning is the best way to diagnose
the problem of inert accumulation and liquid level in the condenser. A thermal
assessment of the shell can provide enough information regarding its current
state. The age of the condenser is also affected by inert buildup. Heat
transfer is reduced because the flooded area of the tube is colder than the top
or inert accumulated tubes, and this temperature difference affects the
To vent out non-condensable gas from flooded
condensers incessantly, direct control of the flow controller is utilized in
some plants where non-condensable gas slippage from upstream units (such as
methane or H2) is expected in normal operation—this flow controller
should be a pressure controller. This flow controller always maintains a
constant amount of non-condensable flow, but some regular hydrocarbon or
product is also vented out and some lighter hydrocarbons remain in the system,
causing pressure to fluctuate. To avoid such a problem, always use a pressure
controller instead of a flow controller.
Safe practices for handling a condenser include:
This work is
based on the author’s experience and learning. It is not affiliated with any
SHARMA works at an ethylene plant as a process engineer. He
has more than 5 yr of experience working in a steam cracker unit. Sharma earned
a BS degree in chemical engineering with honors from the National Institute of
Technology Raipur in India and finished a process equipment design course at
the Indian Institute of Technology Roorkee. He is an associate member of the
IChemE, an active professional member of the American Institute of Chemical
Engineers, and the author of eight technical articles in industry publications.