Hydrocarbon Processing - May 2022 - 47

Digital Technology
* PSA and membrane operation
* Hydrocracker (or another
selected unit) load to balance
hydrogen consumption.
Pre-requirements are usually
minimal. Unlike the applications
on production circuits, a hydrogen
network optimizer does not
require an APC layer on most of
the units in the envelope. If the
load (throughput) of a process
unit is to be optimized, then an
APC layer is required on the unit.
FIG. 6 illustrates four consecutive years
, with a
minimum 70% reduction in losses.
Steam network optimization. Steam
production systems must satisfy the demand
coming from process units at the
lowest possible cost. These networks
can be very complex with multiple
headers, boilers, gas turbines and heat
recovery steam generators (HRSGs),
as well as different types of turbines
and turbogenerators.
FIG. 7 shows a complex network in
an ethylene plant with seven headers,
two boilers, six turbines, numerous
turbomachines, hundreds of users and
more than 10 steam pressure controllers.
Of course, optimizing this network
is quite challenging. Header pressures
for high-, medium- and low-pressure
steam must be controlled tightly, and the
overall efficiency must be maximized by
properly addressing nonlinearity, which
is dependent on load distribution and
equipment availability.
Another issue to consider is that steam
networks are continuously affected by
disturbances from process units. Within
the network, header pressures interact
between themselves. Moreover,
there
are constraints on power import/export
and a need to balance steam/power production.
Cost variability may impact the
optimal operating point dynamically, as
often as daily or even more frequently.
Steam letdowns and venting must be
minimized or avoided, as they can result
in excessive fuels consumption and higher
CO2
emissions/t of steam produced.
Consumptions and related emissions
strongly depend on the steam production
infrastructure, efficiencies and fuels
used. A reasonable estimate is that 12-t
of high-pressure steam consumes 1 t of
fuel gas, and each ton of fuel gas emits
2.78 t of CO2
. Therefore, 0.23 t of CO2
gets in mind. The following are several
examples of units to consider for APC
applications:
Recent advancements in APC and optimization
technologies provide strong options for oil and gas and
chemical companies faced with the challenge of achieving
energy efficiency and reducing carbon emissions.
of hydrogen losses to flare, two years
before and after implementation of dynamic
optimization technologyc
are emitted for each ton of high-pressure
steam produced.
Any amount of steam lost or downgraded
to a lower pressure level without
producing energy impacts the site's energy
bill and CO2
balance. It is necessary to
dynamically coordinate energy producers
and consumers, often under different
operators' responsibility or even operated
in separate control rooms.
A steam network can be effectively
optimized by leveraging the nonlinear
adaptive process control solutiona
, typically
in combination with an optimizer
that sets targets for the APC layer considering
the overall utilities system. This
must be considered, given that the steam
and fuel networks interact, and a single
overall optimum exists. Such an application
typically enables users to:
* Stabilize high-, medium- and lowpressure
steam header pressures
by properly managing boiler/
turbine loads and turbine spills
* Maximize boiler efficiency,
while guaranteeing proper
high-pressure control
* Optimize boiler(s) combustion
* Maximize overall efficiency
by optimally distributing loads
across equipment.
* Continuously minimize direct
letdowns (and vents if any) if this
is the most economical solution
* Significantly reduce CO2
emissions.
In addition, every steam user must be evaluated
to ensure that energy targets are actively
pushed by local APC applications.
For example, columns must be operated
at their minimum pressure against process
constraints and not at pressure limits set
by an operator. Also, live steam constraints
of columns or fractionators must be active
(e.g., flash point or hydrogen limit instead
of with steam/flow ratios).
Every APC application objective
should be revised with the energy tarFIG.
7. Ethylene cracker steam network.
* Distillation columns
* Amine regenerators
* Sour water strippers
* Fractionators live steam
* Side strippers
* Compressors/steam turbine drives.
Multiple opportunities exist in using the
steam network. For example, FIG. 8 shows
that more than 10 t/hr of high-pressure
steam is downgraded to medium-pressure
header during normal operations.
Some typical available control handles
include the following:
* Boiler/turbine/HRSG loads
* Turbine spills
FIG. 8. Letdown flowrate and valve position.
Hydrocarbon Processing | MAY 2022 47
Hydrocarbon Processing - May 2022

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