Today, process engineers are responsible for many project
activities, including conceptual design, revamp studies and
operational troubleshooting. Increasingly, the process
simulator is an essential tool central to these activities.
Process simulators are very powerful tools for modeling all or
parts of a process. While they are excellent for
generalpurpose process modeling, it is the process
engineer’s responsibility to understand to what extent
these tools can be applied, and how combining their application
with more specialized tools might be appropriate. This choice
is ultimately based on the business and technical objectives to
be achieved.
This article examines three different applications where
rigorous heat exchanger models can enhance value derived from
process simulation and provide more accurate results. These
applications include conceptual designs of new plants, revamps
of existing facilities, and operations
support.
Conceptual design
One of the key responsibilities of the process engineer is
related to the conceptual design of processes. With conceptual
design, the use of process simulation is central to project activities. The initial
stages of conceptual design consider the main process synthesis
and separation operations required to convert feedstocks to products. At this
early stage, the process flowsheet typically involves
simplified models of reactors, distillation columns, and the
heating and cooling services required to facilitate the
essential parts of the process. At this stage, the type and
design of equipment required, for example, to preheat reactants
before they enter a reactor, are less important. The
traditional functionality of process simulation in providing
heat and mass balance over the conceptual process is
paramount.
As the conceptual design evolves, it becomes important to
take account of the actual equipment involved. The reactors,
separators and heat exchangers need to be evaluated to further
develop their designs to ensure desired performance; to size
them adequately; and to obtain estimates of the capital cost of
the process, the heating and cooling utility requirements and
the energy cost to operate the process. Heat transfer equipment
can typically be up to 30% of the capital cost of process
equipment. Therefore, as the process design progresses, it is
important to take account of the real design requirements for
the major heat transfer equipment items. For any heat
exchanger, two main aspects must be considered:
 How much duty does the heat exchanger need to
provide?
 How much pressure drop can be consumed?
The first aspect can be modeled by a simple equation:
Q = m (h_{o} –
h_{i}) (1)
where Q is the rate of heat transfer, m is
the mass flow, h is specific enthalpy, and the
subscripts o and i refer, respectively, to
outlet and inlet. Where there is no phase change, this can be
expressed as:
Q = m Cp (T_{o}
– T_{i}) (2)
where Cp is the specific heat of the fluid, and
T is the temperature.
Consider the following simple example, where a water/water
exchanger has been modeled with one side heating up from
20°C to 90°C, while the other side is cooling down from
90°C to 20°C (Fig. 1).

Fig.
1. Water/water exchanger model
showing one side heating up and one side
cooling down. 
However, if the exchanger must be designed to determine how
much surface it will require, the basic heat transfer equation
(for pure countercurrent flow) must be considered:
Q = UA 3
LMTD (3)
where U is the overall heat transfer coefficient,
A is the effective area in the heat exchanger, and
LMTD is the logarithmic mean temperature
difference.
If a generic heat exchanger is assumed to have two ends
(here referred to as “A” and
“B”) at which the hot and cold streams enter
or exit on either side, then LMTD is defined by the
logarithmic mean, as follows:
LMTD = (ΔT_{A} −
ΔT_{B}) ÷ [ln
(ΔT_{A} ÷
ΔT_{B})]
(4)
where ΔT_{A} is the temperature
difference between the two streams at end A, and
ΔT_{B} is the temperature difference
between the two streams at end B. In this case, the
LMTD will have a limit of 0, so it will need a
UA with an infinite limit.
The second aspect to consider for any real equipment is the
pressure drop that will be consumed on the hot and cold sides
as the respective streams flow through the heat exchanger. It
is normal for the process engineer to designate how much
pressure drop will be allocated to a particular exchanger. For
example, in turbulent ﬂow inside tubes, the local
heat transfer coefficient varies approximately with the mass
velocity raised to the power 0.8. The pressure drop varies
approximately with the mass velocity squared. This means that,
if pressure drop is kept low, the heat transfer coefficient
will be very low, and a large surface area will be needed for
the heat exchanger. A realistic pressure drop must be estimated
at this stage to enable the design of the heat exchanger later
without having to rework the process design.
A more realistic way to model the exchanger is to assume
that one side of the exchanger is between 90°C and
25°C, with the other side heating up from 20°C to
85°C. Both sides have a pressure drop of 0.5 bar
(Fig. 2).

Fig.
2. Heat exchanger model showing one
side heating up and one side cooling down. 
This type of idealized approach is often used to model an
exchanger where a process stream is heated by utility steam in
a heat exchanger. Pure fluids, like steam, condense
isothermally at constant pressure. If isothermal condensation
is present, then EQ. 3 can be applied to good effect; however,
in reality, any pressure drop on the steam side will result in
a lower saturation temperature, and then the exit temperature
will be lower than the inlet temperature.
The main issue with this approach is that it is easy for the
process engineer to specify conditions that later make it
difficult to achieve a practical exchanger design. This hampers
effective collaboration between process engineers and thermal
design specialists, resulting in additional cycles of
engineering to refine the overall process and equipment
designs.
One way to promote better collaboration between disciplines
and achieve better designs quickly is to use a rigorous
exchanger modeling tool within the process simulation to
achieve a preliminary design. This approach enables the process
engineer to get a better first approximation for evaluating the
feasibility of the process, and to give the thermal specialist
a useful starting point for full design optimization. Where
this technique is employed, it has been shown to reduce project
schedules and eliminate costly rework.
Revamp studies
The second type of project where rigorous heat exchanger
modeling can improve the engineering workflow is a revamp.
Typically, revamp projects have two main aspects.
First, there is a check that the actual proposed equipment in
the process is accurately simulating the plant performance
data. Secondly, “what if” options can be explored for
process and capital improvements, with different equipment
geometries and stream sequencings validated against the
revamp’s performance objective.
Modeling an existing exchanger can be easy if plant data is
available. The process simulator allows the specification of
process conditions for the exchanger. This, in turn, allows
simple modeling of an exchanger based on EQ. 3, and it enables
the simulator to estimate the exchanger duty. The inherent
assumption is that UA will remain constant. The
pressure drop will not be recalculated by the simulator, so any
variation will need to be estimated with a manual calculation.
As mentioned earlier, for singlephase turbulent flow inside
tubes, the local heat transfer coefficient will vary according
to:
a = f(m^{0.8})
(5)
where a is the local tubeside heat transfer coefficient,
and m is the mass velocity in the tubes. This
indicates that, as the flow of either stream in an exchanger is
varied, the simple modeling of the simulator will result in an
error in the estimated duty of an exchanger. Change in steam
properties will also be unaccounted for in this simple modeling
approach.
In the following example, the first exchanger downstream of
the desalter in a crude preheat train is subject to examination
in a revamp study where the overall aim is to recover more
pumparound energy and increase the throughput of the refinery (Fig. 3).
The first step is to model the existing exchanger. The crude on
the tube side of this exchanger is focused on in Table
1.

Fig.
3. Revamp study of an exchanger
in a crude preheat train. 
The first two columns are the values of the pressure drop and
the temperature changes on the tube side of the exchanger. The
last two columns represent the difference between the simple
UA modeling and the rigorous modeling
approaches.
In the first set of process conditions, the rigorous model
and UA model values are close. This is expected, since
the UA model is based on the result of the rigorous
calculation performed during the design stage. However, when
the process conditions change, the UA model and
rigorous model diverge, with the relative difference increasing
from less than 1% to more than 3% for the temperature drop, and
from less than 2% to more than 20% for the pressure drop. The
rigorous modeling shows that the pressure drop increased to a
value higher than the limit of 0.6 bar defined in the process.
After the revamp and a redesign of the heat exchanger, it is
possible to calculate the pressure drop for the rigorous model
below the limit of 0.6 bar.
The rigorous modeling of the heat exchanger is needed to
check the performance with new process conditions and to
properly design a revamped heat exchanger. The integration of rigorous modeling
inside the simulator allows the engineer to check the
anticipated heat exchanger performance and take any corrective
design actions without leaving the simulator environment.
Operation support
In this case study, an existing exchanger on a gas
compression system is watercooled. The process is modeled with
a control operation that simulates the adjustment of the water
flow to achieve a specified outlet temperature for the gas
being cooled on the tube side of the heat exchanger
(Fig. 4).

Fig.
4. Watercooled exchanger on a
gas compression system. 
The operator is seeking to reduce the outlet temperature of the
heat exchanger to reduce the power consumed by a large
compressor. In the process simulator, it is simple to set a
lower gas outlet temperature target in the control block, and
the coolant flow rate will be increased until the new, higher
duty is achieved.
In the rigorous exchanger simulation shown in Fig.
5, it is clear that the pressure drop on the water
side is below the maximum allowable for the existing operating
conditions.

Fig.
5. Rigorous simulation for a water
cooled exchanger. 
If a lower gas outlet temperature is prescribed to affect the
desired reduced compressor power, the rigorous model in the
simulation responds to the increased coolant flow that the
adjust mechanism imposes. The exchanger can now achieve the new
duty. However, because a rigorous tool is being used, other
beneficial calculations can be performed. The results
highlighted in Fig. 6 show three issues to
consider:
Pressure drop. The increase in water flow
has resulted in a pressure drop on the shell side, which
exceeds the design allowable. This may mean that sufficient
pumping capacity will not be available to achieve the required
flow.
Dynamic pressure. The Tubular Exchanger
Manufacturers Association (TEMA) defines maximum dynamic
pressure as:
q = rho v^{2} (6)
where rho is the fluid density, and v is
the fluid velocity.^{1}
The maximum dynamic pressure will be different based on the
exchanger geometry. Exceeding these values brings the risk of
excessive erosion and the potential for premature failure of
tubes or other pressure parts of the exchanger.
Vibration. The rigorous exchanger model
performs a vibration analysis for the exchanger bundle. It can
be seen that the increase in the coolingwater flowrate has
resulted in a possible risk of flowinduced vibration for this
exchanger bundle. This can lead to tube failure, which, in some
cases, can be rapid.

Fig.
6. Results of rigorous simulation
for a watercooled exchanger. 
The process simulation, coupled with the rigorous heat
exchanger analysis, can reveal potential operational problems
that go far beyond the simple considerations of heat and mass
balance. In this case, the operator can choose to work within
limits that avoid the risks of erosion, flowinduced vibration
and other operational problems. The simulator and the rigorous
exchanger tools can be used to evaluate an alternative control
scheme, such as controlling the coolingwater temperature
instead of the flowrate.
Best practice in exchanger/process modeling
Today, leading engineering and operating companies in the
chemical and energy sectors are exploiting the integration of rigorous exchanger
models within process simulation to reduce project schedules,
minimize rework, and provide better overall optimization of
their processes.
However, traditional organizations often separate process
engineering, thermal design and mechanical design functions,
which can be a barrier to the adoption of integrated
technologies. As companies recognize the benefits provided by
closer cooperation between the disciplines involved, many are
seeing that they can make much more effective use of specialist
skills when process engineers undertake preliminary designs
using rigorous models in their simulations. Such simulations
can then be fully optimized by the thermal specialist as
process activities proceed.
In many smaller engineering organizations, a broader skill
base for process engineers allows them to directly exploit the
benefits of the integration discussed here. In a case
study^{2} presented at the OPTIMIZE 2011 conference,
one chemical company discussed a feasibility study wherein a
reduction in capital equipment costs of 15% and an annual
energy savings of $200,000 were discovered through the
integration of rigorous equipment modeling with process
simulation.
Another company obtained an estimated $5.5 million in
additional revenue from increased liquefied petroleum gas (LPG)
production, while reducing equipment costs by $0.5
million.^{3} This was achieved through the integration
of a platefin rigorous modeling tool inside a process
simulator. The integration allowed the evaluation of various
alternative process solutions and their direct impact on
temperature approach in the heat exchanger type selected.
The integration of rigorous modeling
tools for heat exchanger modeling inside process simulators
allows a faster delivery of projects by shortening the
discussion time between different disciplines. Process
engineers can be confident with the results of the process
modeling by using the real geometry and the most rigorous tool
for the heat exchanger calculation. Finally, plant operations
are made safer by modeling all aspects of the heat exchanger
operation, such as vibration.
LITERATURE
CITED
^{1} Standards of the Tubular Exchanger
Manufacturers Association, 9th Ed., New York, New York,
2007.
^{2} Roy, E., presentation at the AspenTech OPTIMIZE
2011 Conference, Washington, DC, May 2011.
^{3} Venkatesh, L., Petrofac Engineering India Ltd., presentation at
Aspentech OPTIMIZE 2011 Conference, Washington, DC, May
2011.
The author 
Julien Cazenave is an Aspen
exchanger design and rating (EDR) business consultant
for AspenTech, based at the company’s European
headquarters in Reading, UK. He has more than 10
years of experience in working with customers of
AspenTech’s EDR and simulation products across
Europe, the Middle East and Africa. Mr. Cazenave
ensures that customers derive maximum value from
their investment and are regularly updated on new
developments in the software and the underlying technology.
