Tutorial 1
Feedback Concepts & Benefits
1.1.
Drawing symbols: Determine all letters that would be used to designate each of
the following instruments on process and instrumentation (P&I) drawings.
For example, for a level controller, the designation would be LC.
i)
ii)
iii)
iv)
Liquid level alarm high,
Pressure indicator,
Temperature indicator in a packed bed,
Volume flow rate of butane in a pipe,
v)
vi)
vii)
viii)
Mass flow rate of hydrogen,
Weight of a solid in a vessel,
Speed of rotation of a shaft, and
Mole % of propane in a gas stream
1.2.
Common examples of automation: Discuss whether each of the common
systems below uses automatic feedback to achieve its desired performance.
a.
b.
c.
Boiling water on a burner in a home stove.
Maintaining a temperature in an oven in a home stove.
An alarm clock used to wake you for class.
1.3.
A Chemical Engineering Example: A chemical reactor with recycle is depicted
in textbook Figure 1.8 and repeated below.
a.
Can the following variable be controlled by feedback? Hint: determine which
valves have a causal effect on each sensor.
b.
Select the best valve to control each, if more than one valve can effect the sensor.
c.
Select a sensor principle for each of the sensors. (Hint: Check the WEB site!)
i.
ii.
T4, reactor feed temperature
T1, feed temperature
iii.
iv.
F3, reactor effluent flow
L1, reactor liquid level
v8
F2
F1
T1
T3
v3
F5
T5
P1
T4
F3
T6
F4
L1
v5
v1
v7
v2
T2
Hot Oil
Figure 1.1
v6
T8
T9
Hot Oil
F6
L2
T7
1.4
Economic benefits: Discuss the economic benefits achieved by reducing the
variability (and, in some cases changing the average value) of the key controlled variable
for the situations in the following.
a.
Crude oil is distilled, and one segment of the oil is converted in a chemical reactor
to make gasoline. The reactor can be operated over a range of temperatures; as
the temperature is increased, the octane of the gasoline increases, but the yield of
gasoline decreases because of increased by-products of lower value. (It’s not
really this simple, but the description captures the essence of the challenge.) The
customer cannot determine small changes in octane. You are responsible for the
reactor operation. Is there a benefit for tight temperature control of the packed
bed reactor? How would you determine the correct temperature value?
Gasoline
yield, %
Octane
By-products
T
Crude
oil
Gasoline
Octane
Minimum octane, don’t go
below this!
Time
Figure 1.2
b.
You are working at a company that produces large roles of paper sold to
newspaper printers. Your client has many potential suppliers for this paper. Your
customer can calibrate the printing machines, but after they have been calibrated,
changes to paper thickness can course costly paper breaks in the printing
machines. Discuss the importance of variance to your customer, what your
product quality goal would be. Is this concept different from the situation in part
(a) of this question?
Tutorial 1. Feedback Concepts and Benefits
Learning Inventory
1
(low)
1.
I can recognize common examples
of feedback control and distinguish these
from examples in which actions are 1
implemented in a “pre-programmed”
manner, without feedback.
2.
Using my chemical engineering
skills, I can identify and explain casual
relationships between adjusted variables, 1
usually valves, and measured variables.
3.
I am beginning to build my
understanding
of
instrumentation
principles, because a) I will deal with data 1
based on sensor measurements (for my
entire career) and b) sensor accuracy will
influence my analysis.
4.
I can recognize the value of reduced
variability in chemical processes.
1
Confidence
5
(O.K.)
10
(high)
5
10
5
10
5
10
5
10