Hydrocarbon Processing - June 2021 - GP-29

HEAT EXCHANGE
Improve gas-gas exchanger footprint
and economics with a PSHE
R. BROAD, API Heat Transfer, Bretten, Germany; and A. BAYATI, McDermott, The Hague, the Netherlands
Plate-type heat exchangers are known
for high heat transfer coefficients. As
a result, they are compact and lightweight
compared to conventional shelland-tube
(S&T) exchangers. Another
advantage of plate-type exchangers is
a lower fouling tendency due to higher
wall shear stress; however, the construction
of widely used exchangers, such as
gasketed and bloc-type exchangers, limits
their application to low and medium
design pressures (typically less than 50
barg). The plate-and-shell heat exchanger
(PSHE) combines the advantages of
plate-type exchangers with the robustness
of the S&T exchanger into one exchanger.
Design pressure can rise to 400
barg for the PSHE.
This article focuses on a specific application
in upstream facilities where
weight reduction and smooth operation
in turndown is always a concern. The
design comparison, advantages and limitations
between S&T exchangers and
PSHEs are presented and discussed using
an actual example.
Exchanger background. Gas-gas exchangers
in gas treatment processes
(FIG. 1) are usually the largest exchanger
in an offshore plant, with special design
criteria. In this article, the possibility of
replacing conventional S&T exchangers
with PSHEs is reviewed, and the advantages
in this application are evaluated.
Due to the risk of hydrate formation
in gas-gas exchangers, the design of S&T
exchangers are required to meet certain
criteria, which goes back to work done
by P. W. Knight in 1975.2
The satisfactory
feedback from practice encourages
designers to follow Knight's recommendations.
Further studies by HTRI have
demonstrated similar criteria.3
As per Knight's recommendation,
three parameters play a crucial role in
sizing a gas-gas exchanger:
* Maximum tube-side velocity
(to prevent foaming and formation
of unstable emulsion)
* Minimum tube-side velocity
(to prevent hydrate formation)
* Maximum tube length per shell
based on selected tube size
These three parameters determine the
size and number of S&T exchangers. It is
clear that the maximum gas velocity influences
achievable heat transfer coefficient,
while minimum velocity and maximum
tube length determine the number of
shells in series. To minimize the risk of separation,
it is normal to select one tube pass.
The minimum velocity limit is recommended
to prevent stratification inside
tubes. This limit is usually easy to observe
for design flow, but very difficult to adhere
to during turndown conditions. To overcome
the low velocity issue in turndown, a
recycle line from the compressor discharge
to the exchanger inlet has been considered
by some designers to boost the velocity in
turndown conditions and in late-life scenarios.
This modification requires additional
piping and creates energy losses.
Due to the smaller channel size, the
corrugated plate and the flow direction
Pipeline gas
constantly breaking up the boundary
layer, the chance of stratification is unlikely
with PSHE. Turndown can be handled
without the additional recycle line,
which reduces piping requirements and
power consumption.
As per common practice, minimum
approach temperature for an S&T exchanger
should not be less than 5°C
(preferably 8°C), while it can be as low
as 2°C for the PSHE. This means lower
compressor power consumption, which
results in considerable savings of longterm
operation costs.
Lighter equipment with a smaller
footprint is always a challenging activity
in offshore platform design. The following
example demonstrates significant
savings achieved by replacing an S&T exchanger
with a PSHE.
Case study: Exchanger design comparison.
The design of the S&T calculation
is shown in FIG. 2; the design
includes the highest velocity possible to
maintain the minimum required velocity
in turndown condition. This design also
boosts the tube-side heat transfer coefficient,
which is the main resistance.
Compact exchangers. In recent years,
increased attention on environmental
PC
Recompressor
Overhead compressor
Stabilizer
High-pressure feed gas
TEG
TC
dehydration
Gas-gas exchanger
FIG. 1. Process flow for a gas-gas exchanger in a gas treatment facility.1
Gas Processing & LNG | MAY/JUNE 2021 29
LC
JT valve

Hydrocarbon Processing - June 2021

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