Hydrocarbon Engineering - August 2019

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August 2019 HYDROCARBON
ENGINEERING 42
ethylene purity and recovery suffered. It was clear that both
the capacity and efficiency of the trays were limiting the
operating rates of this plant. The engineering company/licensor
considered several alternatives, including trays with a large
number of downcomers at a lower tray spacing. Due to the
lower efficiency of such trays, the number of trays would have
had to be increased by 23%.4 The trays would have had to
exhibit an increased capacity that would overcome a
23% reduction in tray spacing and give the customer the extra
24% capacity it was targeting. The amount of work needed to
reduce the trays spacing would also have raised hot work issues
and would have extended the shutdown.
The owner of the plant decided to use optimised
SUPERFRAC trays for this revamp. The number of trays and tray
spacing in the different zones of the tower were adjusted to
achieve the maximum capacity and recovery given the purity
constraints. This study showed that a one-for-one tray
replacement with optimised trays would meet the objectives.
Changes in downcomer arrangements could be
accommodated using the OMNI-FIT technology. This approach
removed the need for welding and significantly reduced the
duration of the shutdown. In addition to the tray features
discussed in case study 2, this revamp configuration also
optimised the feed arrangement, reboiler return, side reboiler
draw, and return and reflux distribution. Based on Koch-Glitsch
calculations, the new tower arrangement would increase the
ethylene production to 25% above the target set by the
operating company. At that point the capacity would be
limited by the reboilers, condenser and the pumps, and not the
trays.
The tray installation and piping modifications were
successfully completed within the allotted turnaround time.
The tower was then started up without any issues and the
product purity and recovery was reached quickly. Test runs
were conducted to assess the separation performance and
capacity of the revamped tower. At 96% of the desired rate,
the upstream units reached their maximum capacity and it was
not possible to push the tower to its limit. Even with the
limitation of the upstream units the plant was able to increase
its ethylene production capacity by 26%. Over the whole
operating range of the tests, the overall tray efficiency was
higher than 96%.
Case study 4 – a new C3 splitter in a
grassroots PDH plant
An even larger propane/propylene splitter has been in service
for over two years with the first ever 8-pass SUPERFRAC XT
trays.5 The 33.5 ft (10.2 m) dia. column has exceeded design
capacity and product purity requirements from the initial
start-up.
This grassroots PDH plant has a capacity that called for one
of the largest C3 splitters in the world at just over 10 m in dia.
and over 100 m tall. The plant owner selected SUPERFRAC XT
trays based on criteria including proven performance, minimum
tower size and ease of accessibility. Minimising the pressure
drop was also an important factor since the splitter has a heat
pumped reboiler driven by the compressed overhead vapours.
The reliably high tray efficiency allowed fewer trays to be
specified, resulting in a shorter vessel height, and lower
pressure drop, requiring less compressor horsepower.
Koch-Glitsch worked closely with the owner’s subject
matter experts and their chosen engineering contractor to
define the scope of the splitter tower internals. An 8-pass
SUPERFRAC XT tray design with several of the features discussed
above was selected for this application. With this being a new
tower, the tray design, tower diameter and tray spacing could be
optimised. Sectionalised beams were used to support the tray
decks, with tray levelness being an important factor to ensure
that the maximum performance would be achieved.
To minimise fit up errors, OMNI-FIT technology was used
to significantly simplify the design of the supporting tower
attachments that had to be welded inside the tower by the
vessel fabricator. This provided greater flexibility to the tray
installer to ensure an accurate installation was achieved. Even
with the much-simplified tower attachment scope, several
deficiencies in the fabrication of the tower had to be
overcome during the installation.
On all towers the support rings must be levelled within a
manufacturer-specified tolerance. The support ring level should
be checked at the vessel shop before customer acceptance. A
final check of the support rings should be made prior to
beginning installation of the trays. If any of the rings exceed the
specified tolerance for level, the tray manufacturer should be
contacted immediately to agree upon a plan for rectification. For
the PDH splitter, Koch-Glitsch sent a Certified Tower Specialist
to the site to verify the tray installation, and he and the project
teamworked with the tray installers to ensure a quality
outcome.
The PDH unit started up in November 2015 and by
February 2016, it was running at full capacity, delivering
polymer-grade quality propylene from the initial startup
without any issues. The tower is meeting all capacity, purity
and recovery expectations. The overall tray efficiency is above
90%, which is higher than what the engineering company
assumed during the design phase. This allows the owners to
adjust the reflux ratio to optimise the production and minimise
the energy consumption.
Conclusions
The case studies presented in this article show that high tray
efficiencies and capacities can be obtained with optimised
SUPERFRAC XT trays. In C2 and C3 splitters, efficiencies of 92%
to 100% have been obtained. This allows debottlenecking of
existing splitters or a reduction in diameter and height for
grassroots splitters. It also allows the energy consumption of
these mega towers to be minimised. In the case of heat-pump
splitters, the energy consumption and size of the heat-pump
system may also be minimised.
References
1. NIEUWOUDT, I., and SANDFORD, N., ‘Overcoming challenges: part one’,
Hydrocarbon Engineering, Vol. 24, No. 4, (April 2019), pp. 39 – 42.
2. NIEUWOUDT, I., KARPILOVSKIY O., and LOCKETT, M.J., ‘Revamp
and Retune’, Hydrocarbon Engineering, Vol. 14, No. 7, (July 2009),
pp. 56 – 60.
3. NIEUWOUDT, I., ‘Tray efficiency of SUPERFRAC high-performance trays
in C2 splitter service’, Hydrocarbon Asia , April-June, (2011),
pp. 64 – 68.
4. SUMMERS, D. R., MCGUIRE, P. J., GRAVES, C. E., HARPER, S. E., and
ANGELINO, S. J., ‘High-Capacity Trays Debottleneck Texas C3 Splitter’,
Oil & Gas Journal, Vol. 93, No. 45, (November 1995).
5. ‘Dow PDH unit completes performance test’, Dow, https://www.
performance-test