Hydrocarbon Processing - December 2021 - GP-31

PLANT OPTIMIZATION
sition of the fluid being processed changes,
the control strategy can be adapted to
that change.
FIG. 1 shows the phase envelope and
hydrate curves for a refrigeration process.
Both graphs show the same facility. In the
left graph, the facility is operating in an inappropriate
state, with hydrates forming
in one refrigeration cycle and the fluid becoming
all liquid in another cycle. In the
right graph, intelligent throughput optimization
software is operating, providing
closed-loop control to the control strategy
to avoid hydrates and achieve the correct
amount of cooling without causing
liquids to form when they are not wanted.
With throughput optimization software,
the shape and location of the hydrate
curve and phase envelopes are
known in real time. The process operation
can be moved to the correct side of
both physical entities. The adjustments
can take place either through direct
closed-loop control with the control system,
or the software advises the operator
through an human-machine interface
(HMI) and allows the operator to adjust
the control system.
The software helps the facility on the
right increase reliability through the prevention
of shutdowns, reducing energy
by optimizing the refrigeration cycle and
not exceeding what is needed. Additionally,
by being able to operate efficiently
close to the phase envelope, the facility's
productivity increases.
How throughput optimization works.
In throughput optimization, the targeted
software application communicates with
the controller running the plant or facility
to provide a physics-based way to optimize
the processing of fluids to avoid
process upsets, and solves issues caused
by failing to operate in the proper phase
and/or the formation of hydrate or ice
particles. As a result, the automated tuning
control solution allows operations engineers
to easily and reliably control the
process to optimize operations based on
real-time conditions and composition.
This can measurably reduce energy costs
in an array of applications, including:
* Avoiding hydrate formation
in piping as the oil and gas
comes out of the ground
* Ensuring liquids do not form
in the piping as product flows
to its consumption destination
FIG. 2. A cutaway of a centrifugal compressor on a natural gas pipeline or processing plant that
uses dry gas seals.
* Monitoring the temperature in
sulfur recovery units (SRUs).
For example, FIG. 2 shows a cutaway
of a centrifugal compressor on a natural
gas pipeline or processing plant that uses
dry gas seals. A microscopically thin
film of gas provides lubrication and separation
for the rotating elements. During
startup and shutdown-particularly
quick-stop and sudden shutdowns-the
pressure and temperature of the gas film
may move into the liquid phase envelope.
A single, microscopic drop of fluid
in the dry gas seal can destroy the seal.
Throughput optimization can identify
if the potential for this exists, and, if
necessary, operate a heater on the dry gas
seal feed to avoid the issue.
For another use-case scenario, consider
an SRU in which hydrogen sulfide
(H2
S) is converted to elemental sulfur
using the Claus process. The temperature
of the furnace in the SRU is critical
to proper and continuous operation
but is difficult to measure due to the
high temperatures, which consequently
also rapidly destroy thermocouples.
Throughput
optimization can determine
temperature based on composition
and flowrate of inputs to the furnace.
The software can adjust automatically
or alert an operator via a userinterface
showing phase envelope and
hydrate curve to make adjustments.
FIGS. 3-5 show how the software demonstrates
the value of understanding
the phase envelope and hydrate curve
in operating a thermal natural gas separation
system:
* FIG. 3: Without intelligent
throughput optimization control,
the process conditions are
crossing the hydrate curve.
Without action, hydrates will
form, eventually causing vessels
and piping to plug and the
process to be shutdown.
* FIG. 4: Without intelligent
throughput optimization control,
the process conditions are so
close to the vapor saturation
curve that little if any liquid
is forming, minimizing the
effectiveness of the separator.
* FIG. 5: With closed-loop intelligent
throughput process control, the
Gas Processing & LNG | NOVEMBER/DECEMBER 2021 31

Hydrocarbon Processing - December 2021

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