Chemical Engineering 3P04

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• Chemical Engineering 3P04
• Process Control
• Tutorial 6
• Learning goals
• Learn basic principles of equipment in a control
  loop
• Build understanding of feedback loop

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Loop Elements Sensor ? Computer ? Valve Why must
we transmit these signals?
What is wrong with this picture?
controller
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Loop Elements Sensor ? Computer ? Valve Why must
we transmit these signals?

• Transmitted to/from
• Central control room

• Displayed locally
• Manual valves

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Loop Elements Sensor ? Computer ? Valve Why must
we transmit these signals?

• Transmitted to/from
• Central control room
• Safety related or time critical
• Used for control
• Important for quality, reliability, performance
• Trouble shoot and monitor longer-term behavior

• Displayed locally
• Manual valves

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Loop Elements Sensor ? Computer ? Valve Why must
we transmit these signals?

• Transmitted to/from
• Central control room
• Safety related or time critical
• Used for control
• Important for quality, reliability, performance
• Trouble shoot and monitor longer-term behavior

• Displayed locally
• Used for local maintenance/ operation
• Not safety or time critical
• Manual valves
• Infrequently adjusted
• Not safety or time critical

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Loop Transmission Why learn about it?

• We need to understand the closed-loop
• We select equipment to achieve required
  performance
• We trouble-shoot problems
• These are our senses and our handles

?
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Loop Transmission Why learn about it?
Class workshop What are general features that we
seek for the transmission of signals from the
sensor ? computer and from the computer ? valve?
Hint We have lists of features for sensors and
for valves already
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Loop Transmission What features do we seek?

• Accuracy and reproducibility
• Noise sensitivity
• Reliability
• Dynamics
• Distance
• Interoperability
• Safety
• Diagnostics
• Cost

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Loop Transmission What features do we seek?
Dynamics Transmission delays are in the
feedback loop. Delays in transmission are as
bad as delays in the process. Good news
Electronic transmission is very fast compared
with other elements in the loop. Caution Old
transmission systems using air pressure
(pneumatic signals) can be slow for distance over
50 meters.
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Loop Transmission What features do we seek?
Distance Process plants can extend over 1000s
of meters. The transmission must be capable of
these distances. Good news Electronic
transmission via hard wire has a large enough
range. Caution Pneumatic signals have limited
range. Note Telemetry is not now used for
process control. It is used for monitoring
remote equipment (wells)
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Loop Transmission What features do we seek?
Interoperability When you purchase one loop
element from a company, do you want to buy all
other elements from the same company for the life
of the plant? NO! Standards are recognized so
that equipment from various manufacturers can be
used interchangeably. This was easy for older,
analog technology. Standards are available for
digital technology.
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Loop Transmission Two typical designs. Life is
exciting during a revolution!
Analog transmission Continuous electronic signal
Digital transmission Digital numeric
representation
Older technology, but widely employed and will be
in use for decades
Newer technology, generally used in new
facilities and when replacing analog technology
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Loop Elements A Typical Analog Loop
Thermocouple temperature sensor, mV signal
Analog signal transmission (4-20 mA)
Analog to digital conversion
Digital number
A/D
Digital controller
transmitter
Pneumatic signal transmission (3-15 psig)
D/A
i/p
Digital number
fc
Valve stem position 0-100)
Analog signal transmission (4-20 mA)
Digital to analog conversion
Heating medium
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Loop Elements A Typical Analog Loop
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Loop Elements All digital transmission
?-Processor at every sensor and valve
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Loop Elements Life is exciting during a
revolution!
Why have a micro-processor at every sensor and
valve?
Valve
Flow Sensor
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Loop Elements Life is exciting during a
revolution!
Why have a micro-processor at every sensor and
valve?

• Flow Sensor
• Improve accuracy
• Correct for density changes
• Diagnose performance and warn when degradation
  begins
• Calibrate quickly
• Power supply error

• Valve

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Loop Elements Life is exciting during a
revolution!
Why have a micro-processor at every sensor and
valve?

• Flow Sensor
• Improve accuracy
• Correct for density changes
• Diagnose performance and warn when degradation
  begins
• Calibrate quickly
• Power supply error

• Valve
• Diagnose performance and warn when degradation
  begins
• Valve sticking
• Air pressure low
• Signal not received

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Loop Elements Life is exciting during a
revolution!
Note that both have two-way communication
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Loop Transmission Two typical designs. Life is
exciting during a revolution!
Analog transmission Continuous electronic signal
Digital transmission Digital numeric
representation
Older technology, but widely employed and will be
in use for decades
Newer technology, generally used in new
facilities and when replacing analog technology
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• Chemical Engineering 3P04
• Process Control
• Tutorial 6
• Learning goals
• Learn basic principles of equipment in a control
  loop
• Build understanding of feedback loop

Lets look at some examples from Tutorial 7
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FC

• Flow Control
• Centrifugal pump with constant speed (rpm)
• Orifice plate sensor
• Globe valve

(a)
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FC

• Flow Control
• Positive displacement pump
• Orifice plate sensor
• Butterfly valve

(b)
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FC

• Flow Control
• Centrifugal pump with variable speed driver
• Orifice plate sensor

(c)
c) The pressure increase from a centrifugal pump
depends on the rotor speed the fast the
rotation, the higher the pressure. A variable
speed motor can be adjusted to achieve the
desired flow rate, which is more energy efficient
than adjusting a variable pressure drop (valve)
in the pipe. Increasing the speed increases the
flow rate.   Yes, feedback control is possible.
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• Pressure Control
• Manipulate one exiting flow
• Flexible diaphragm
• Globe valve

(h)
h) The pressure in a pipe can be controlled by
adjusting one of the flows. We can prove this by
formulating a dynamic material balance.
Naturally, successful control can only be
achieved over a range of flows when the valve is
either fully opened or closed, control is no
longer possible.   Yes, feedback control is
possible. A pressure sensor that deflected
because of pressure and converted the deflection
to an electronic signal is used in such
circumstances. A globe valve is acceptable here.
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• Pressure Control
• Manipulate exiting flow from vessel
• Piezoelectric
• Globe valve

(i)
PC
i) The pressure in a vessel can be controlled
using the exit (or inlet) flow. The principles
are identical to the previous design.   Yes,
feedback control is possible.   A piezoelectric
sensor generates a small electronic signal when a
pressure is applied it can be used in this
application.
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LC
(k)
AC
CB
Reaction A ? B ? C

• Composition Control in isothermal CSTR
• Manipulate the inlet flow
• Control CB
• Ball valve
• Level maintained constant by LC

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CB cannot be controlled by adjusting F
CB can be controlled increase the flow rate to
increase CB
CB can be controlled decrease the flow rate to
increase CB
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• The conversion
• (or extent of reaction) depends
• on the space time in the reactor.
• Clearly, the flow rate affects
• the space time.
•  
• However, this process is more complex, some might
  say, Tricky. For control to be successful, we
  need to have a controller gain that has a
  non-zero gain. The gain can be either positive
  or negative, but it should not change sign! What
  happens in this example? The figure below shows
  that the gain changes sign, because of the two
  reactions. In two regions, control is possible,
  but would only function within the region. At
  the maximum CB point, control is not possible by
  adjusting the feed flow rate.
•  
• While control is possible, great care would have
  to be employed when implementing. A different
  manipulated variable, such as feed concentration
  should be investigated.
•  
• A ball valve would be an acceptable choice.