S. Mcbul, OQ Specialty Chemicals, Masqaţ, Oman
When addressing process controls, the immediate interest is
to invest in advanced process control (APC) technology without knowing the
performance and stability of complex wide primary (PID) control. An APC typically
sits on top of the PID control—if the PID control is unstable, the APC cannot do much to
resolve the instability because there are no known shortcuts in process
control. There is a tendency in APC projects to tune controllers that are
identified for APC control while ignoring the rest; however, the process
interactions creep and affect those controls that APC needs directly or
indirectly. The lesson learned is that there are no shortcuts.
Organizations tend to jump into APC mainly due to advertised
benefits by vendors. However, the value of well-performing PID receives only
partial attention mainly due to the difficulty in identifying the value of the
PID tuning stabilization.
It has always been challenging to address the business value
of improving PID controls because they only reduce the variability or standard
deviation around the mean value. With improved PID control, the operation
continues to operate at its previous average value (FIG. 1).
For example, if the average production rate were 100
throughputs per labor hour (tph) with a standard deviation of 3, a PID would be
performed, producing 100 tph with a standard deviation of 0.5. The finance team
will not confirm any improvement in the bottom line as the production rate remains
In certain cases, PID stabilization directly improves the
plant's bottom line or improves reliability; in various other cases, it enables
moving closer to optimum production (FIG. 2). The three phases of process control are:
This case is an example from
a propylene fractionator in a residue fluid catalytic cracking (RFCC) unit. The
operator kept a low reboiling duty due to high reboiler steam variability,
mainly because of an unstable valve coupled with poor tuning, leading to
increased bottom impurity, propylene and propane loss. When the controller
tuning was optimized, the variability was reduced, allowing the operator to
increase the reboiling, reducing propylene loss. The reboiling duty increased,
leading to increased bottom purity and improved recovery of higher value olefin
from the column top product. FIG. 3 shows that the standard deviation was reduced, and
the mean was moved closer to the column flooding limits.
FIG. 4 shows the
amine top pressure control in oscillation, leading to higher than normal amine
losses. Once stabilized, it leads to a significant reduction in amine losses.
This is an example of how the tuning could sometimes lead to reduced losses.
On another occasion, instability in the hydrogen (H2)
header led to higher-than-normal flaring, and the tuning optimization of
pressure control led to operating conditions below flaring limits and increased
plant profits by conserving H2 produced from the steam methane
reformer (FIG. 5).
FIG. 6 is an
example of loss to the plant’s bottom line due to lower PID control
utilization. In this case, the air-to-fuel ratio control of the furnace
remained in manual mode. When taken in auto, it adjusted the air-to-fuel ratio so
the same energy was delivered to the process with lower fuel consumption.
Takeaway. Dozens of such examples occur daily; however, FIG. 6 illustrates
the concept. It is recommended to continuously monitor the utilization and
stability of all controllers, identify if the issues remain in the controller,
valve or instrument, and resolve them as soon as possible to improve the
An average refinery can have
more than 1,000 PID loops, while an integrated petrochemical complex can exceed
3,000 loops. It is hard to monitor each of them, so it is common practice to
use a PID monitoring tool to automatically identify and rank the best- to
worst-performing PID loops and tune the poor-performing loops, valves or
transmitters daily. Once the process is stabilized, there will be loops
affected by multivariable interactions (causing oscillatory behavior) that will
be the target of the APC. Tuning alone may not completely resolve multivariable
interactions without detuning, which may again not be preferable.
The well-tuned plant will
ensure greater stability and reliability of process equipment and valves,
allowing direct and indirect benefits by shifting the mean closer to
constraining limits. Once this is done, the next step will be to use APC to
stabilize multivariable interfacing loops and allow it to automatically shift
to the optimum levels across the plant. HP
SAQIB MCBUL is the Head of
process control and digitization at OQ Specialty Chemicals’ Oman refinery and
petrochemicals complex. He has served in various plants helping owner-operators
in the hydrocarbon processing industry increase their bottom lines using
regulatory and advanced process control, manufacturing execution and digital
information systems. Mcbul is the founder of Apex Digitization.