CHE 185 – PROCESS
CONTROL AND DYNAMICS
CONTROL OBJECTIVES
CATEGORIES OF OBJECTIVES
• PROCESS OBJECTIVES
– QUANTITY
• MEET PRODUCTION TARGETS
• OPERATE AT CONSTANT LEVELS
– QUALITY
• ALL PRODUCT TO MEET MINIMUM CRITERIA
• MINIMIZE PRODUCTION OF OFF-SPEC OR
BYPRODUCT COMPONENTS
CATEGORIES OF OBJECTIVES
• PROFITABILITY
– MAXIMIZE YIELDS
– MINIMIZE UTILITY CONSUMPTION
• PRODUCTS WITH REDUCED
VARIABILITY
– REDUCED VARIABILITY PRODUCTS ARE
IN HIGH DEMAND AND HAVE HIGH VALUE
ADDED
– PRODUCT CERTIFICATION (E.G., ISO
9000) ARE USED TO GUARANTEE
PRODUCT QUALITY
EXAMPLE OF IMPROVED
CONTROL
PLANT OPERATIONAL
OBJECTIVES
• RELIABILITY
– ON-STREAM TIME
– MINIMIZE UNSCHEDULED OUTAGES
• SAFETY - FAIL SAFE OPERATION
– OUT-OF-RANGE ALARMS
– EMERGENCY SHUTDOWN – PANIC
BUTTON
– EMERGENCY INTERLOCKS – AUTOMATIC
OPERATION
SAFETY RELIEF SYSTEMS
• STANDARDS AND CODES
– ASME (AMERICAN SOCIETY OF
MECHANICAL ENGINEERS) BOILER &
PRESSURE VESSEL CODE, SECTION VIII
DIVISION 1 AND SECTION I
– API (AMERICAN PETROLEUM INSTITUTE)
RECOMMENDED PRACTICE 520/521, API
STANDARD 2000 ET API STANDARD 526
– ISO 4126 (INTERNATIONAL
ORGANISATION FOR STANDARDISATION)
MODEL DERIVATION
• INVENTORY TANK
• DESIGN BASES
– STEADY STATE FLOWS
– DISCHARGE FLOW IS
A FUNCTION OF h
– CONSTANT AREA A
– CONSTANT DENSITY ρ
DERIVE EQUATIONS
• MASS BALANCE
d (  Ah )
dt



w  q
 wi  w0
 
in
out

dh
dt

qi  qo
A
accumulati on
• ASSUMPTION OF STEADY STATE
h ( 0 )  h0
DERIVE EQUATIONS
• VALVE CHARACTERISTICS
LINEAR
qo  Cv h
NONLINEAR
• LEVEL CHANGES
– LINEAR ODE
– NONLINEAR ODE
qo  Cv
h
MODEL DERIVATION
• HEATING TANK
• DESIGN BASES
– CONSTANT VOLUME
– PERFECT MIXING IN
VOLUME
– PERFECT INSULATION
– CONSTANT FLUID PROPERTIES, DENSITY
ρ AND HEAT CAPACITY cP
DERIVE EQUATIONS
• MASS BALANCE
• ENERGY BALANCE
d
dt
 VC
p

(T  T ref )  w i C p (T i  T ref )  wC p (T  T ref )  Q
 VC
dT
dt
dT
p

dt
w
V
 wC p (T i  T )  Q
(T i  T ) 
1
 VC
Q
p
DERIVE EQUATIONS
• AS INITIAL VALUE PROBLEM
• GIVEN
– PHYSICAL PROPERTIES (, Cp)
– OPERATING CONDITIONS (V, w, Ti, Q)
– INITIAL CONDITION T(0)
• INTEGRATE MODEL EQUATION TO FIND T(t)
MODEL DERIVATION
• CSTR
– REACTION A → B
• DESIGN BASES
–
–
–
–
–
–
CONSTANT VOLUME
FEED IS PURE A
PERFECT MIXING
INSULATED
CONSTANT FLUID PROPERTIES (, Cp, DH, U)
CONSTANT COOLING JACKET TEMPERATURE
OTHER RELATIONSHIPS
• CONSTITUTIVE RELATIONS
– REACTION RATE/VOLUME
– r = kcA = k0exp(-E/RT)cA
– HEAT TRANSFER RATE:
– Q = UA(Tc-T)
DERIVE EQUATIONS
• MASS BALANCE
d ( V )
dt
 0  wi  w   q i   q

qi  q
• COMPONENT BALANCE ON A
d ( M AVc A )
dt
V
dc A
dt
 M A q i c Ai  M A qc A  M AVr
 q ( c Ai  c A )  Vk 0 exp(  E / RT ) c A
DERIVE EQUATIONS
• ENERGY BALANCE
d
dt
 VC
p
 VC p
(T  T ref )   w i C p (T i  T ref )  wC p (T  T ref )  (  D H ) rV  Q
dT
dt
  qC p (T i  T )  (  D H )Vk 0 e
(  E / RT ) c A
 UA (T c  T )
SOLUTION CONSTRAINTS
• EQUATION PROPERTIES
– 2 ODES
– FOR DYNAMIC MODEL TIME IS THE
INDEPENDENT VARIABLE
– NONLINEAR AND COUPLED
– INITIAL VALUE PROBLEM REQUIRES
NUMERICAL SOLUTION
• DEGREES OF FREEDOM
– 6 UNKNOWNS
– 2 EQUATIONS
– MUST SPECIFY 4 VARIABLE VALUES
MODEL DERIVATION
• BIOCHEMICAL REACTOR (GENERAL)
• DESIGN BASES
–
–
–
–
–
CONTINUOUS OPERATION
STERILE FEED
CONSTANT VOLUME
PERFECT MIXING
CONSTANT REACTION
TEMPERATURE & pH
– SINGLE RATE LIMITING NUTRIENT
– CONSTANT YIELDS
– NEGLIGIBLE CELL DEATH
DERIVE EQUATIONS
• CELL MASS
VR
dX
dt
  FX  V R  X

dX
  DX   X
dt
– DEFINITION OF TERMS
– VR = REACTOR VOLUME
– F = VOLUMETRIC FLOW RATE
– D = F/VR = DILUTION RATE
– NON-TRIVIAL STEADY STATE: 
– WASHOUT: X  0
D
DERIVE EQUATIONS
• PRODUCT RATE
VR
dP
dt
  FP  V R qX

dP
  DP  qX
dt
• SUBSTRATE CONCENTRATION
VR
dS
dt
 FS 0  FS 
1
YX / S
VR X

dS
dt
 D (S0  S ) 
– S0 = FEED CONCENTRATION OF RATE
LIMITING SUBSTRATE
– STEADY-STATE: X  Y X / S ( S 0  S )
1
YX / S
X
SOLUTION CONSTRAINTS
• EQUATION STRUCTURE
– STATE VARIABLES: x = [X S P]T
– THIRD-ORDER SYSTEM
– INPUT VARIABLES: u = [D S0]T
– VECTOR FORM:
YEAST METABOLISM
• BIOCHEMICAL REACTOR (ETHANOL)
acetaldehyde/
pyruvate (S4ex)
glucose
extracellular
J0
intracellular
NAD+ NADH
(N1)
(N2)
glycerol
r6
r7
J
glucose (S1)
ATP (A3)
r1
degraded
products
NADH NAD+
acetaldehyde/
ethanol
pyruvate (S4)
ADP (A2)
NAD+ NADH
G3P/DHP (S2)
r2
r4
r3
ATP
AD
P
1,3-BPG (S3)
r5
MODEL COMPONENTS
• INTRACELLULAR CONCENTRATIONS
– INTERMEDIATES: S1, S2, S3, S4
– REDUCING CAPACITY (NADH): N2
– ENERGY CAPACITY (ATP): A3
• MASS ACTION KINETICS FOR r2-r6
r2  k 2 S 2 N 1
r3  k 3 S 3 A2
r5  k 5 A3
r6  k 6 S 2 N 2
r4  k 4 S 4 N 2
• MASS ACTION KINETICS AND ATP
INHIBITION FOR r1

 A3  
 
r1  k 1 S 1 A3 1  

K

 I  
4
1
DYNAMIC MODEL EQUATIONS
• MASS BALANCES
dS 1
dt
dS 4
dt
dS 2
 J 0  r1
dt
dN
 r3  r4  J
dt
dS 3
 2 r1  r2  r6
2
 r2  r4  r6
dt
dA 3
dt
• CONSERVED METABOLITES
A2  A3  At
• MATRIX
dx
dt
N1  N 2  N t
 f (x, u )
 r2  r3
  2 r1  2 r3  r5
REVIEW OF OBJECTIVES FOR
CONTROL SYSTEMS
• PLANT OBJECTIVES - OVERALL
PRODUCTION FROM THE FACILITY
• COMPONENT OBJECTIVES INDIVIDUAL STEPS IN THE PROCESS
• PROVISION FOR OPERATOR
CONTROL
• OPTIMIZATION OF OPERATIONS
PLANT OPERATIONAL
OBJECTIVES
• ENVIRONMENTAL PROTECTION
– MINIMIZE EMISSIONS FROM PROCESS
UPSETS
– RELIABLE OPERATION OF ALL
POLLUTION CONTROL EQUIPMENT
• VENTS
– FLARES
– SCRUBBERS
• PRESSURE RELIEF
http://www.corrocare.com/air_pollution_control_equipment.htm
PLANT OPERATIONAL
OBJECTIVES
• FLEXIBILITY - DYNAMIC RESPONSE
– SYSTEM TO ADJUST AUTOMATICALLY TO
ANTICIPATED CHANGES IN:
•
•
•
•
PRODUCTION RATES
QUALITY SPECIFICATIONS
COMPOSITIONS OF FEED
INTERMEDIATE STREAMS
PLANT OPERATIONAL
OBJECTIVES
• USER FRIENDLY OPERATOR
INTERFACE
– MINIMIZE NUMBER OF VARIABLES
NECESSARY TO CONFIRM THE PROCESS
STATUS
– DESIGN THE SYSTEM SO THE “NATURAL”
OPERATOR REACTION TO PROCESS
VARIATIONS IS ANTICIPATED
– PROVIDE AN INFORMATION INTERFACE
FOR OPERATION/ENGINEERING
PLANT OPERATIONAL
OBJECTIVES
• MONITORING AND OPTIMIZATION
– DETERMINE THE CONTROL LIMITS FOR
THE PROCESS
– DETERMINE THE OPTIONS FOR COST
REDUCTION
PLANT OPERATIONAL
OBJECTIVES
• STARTUP/SHUTDOWN
– ROUTINE START-UP CONTROL
– MINIMIZE START-UP TIMES
– ROUTINE SHUTDOWN CONTROL
– RESPOND TO SHORT TERM SHUTDOWNS
WITH MINIMUM RESTART TIME
– SAFE EMERGENCY SHUTDOWN
PLANT OPERATIONAL
OBJECTIVES
• EQUIPMENT PROTECTION
– INTEGRATE DESIGN SO FAILURE OF ONE
PART OF THE FACILITY DOES NOT
TRANSFER TO FAILURE IN ANOTHER
PART
– INTERLOCK SYSTEMS TO PREVENT
EQUIPMENT DAMAGE IN THE EVENT OF A
PROCESS INTERRUPTION
COMPONENT OPERATIONAL
OBJECTIVES.
• SIMILAR TO PLANT OBJECTIVES
• COMPONENT RELIABILITY
– MINIMIZE COMPONENT DEGRADATION
OR FAILURE.
– REDUNDANCY WHEN PRACTICAL.
– MINIMAL LOCAL ADJUSTMENT FOR
NORMAL PROCESS VARIATIONS
COMPONENT OPERATIONAL
OBJECTIVES.
• SAFE OPERATION – COMPONENT DESIGNS FOR SAFE
OPERATION WITHIN THE ANTICIPATED
OPERATING RANGES FOR THE PROCESS
– RELIEF SYSTEMS TO AVOID
CATASTROPHIC FAILURE IF THE
PROCESS EXCEEDS THE SAFE
OPERATING RANGES.
COMPONENT OPERATIONAL
OBJECTIVES.
• ENVIRONMENTAL PROTECTION
– DESIGNS TO AVOID LEAKS OF PROCESS
MEDIA
– DESIGNS TO INDICATE LEAKS OF
PROCESS MEDIA
– DESIGNS TO AVOID SUPERSONIC FLUID
CONDITIONS OR OTHER FORMS OF
SOUND POLLUTION
COMPONENT OPERATIONAL
OBJECTIVES.
• EASE OF OPERATION
– LOCAL OPERATION
– REMOTE OPERATION
• MONITORS
– TO DETERMINE CURRENT STATUS OF
COMPONENT
– TO DETERMINE THE NEED FOR
MAINTENANCE OR REPLACEMENT
COMPONENT OPERATIONAL
OBJECTIVES.
• PROVIDE THE OPERATOR WITH
ADEQUATE INFORMATION
– FOR ROUTINE START-UP AND
SHUTDOWN FROM A REMOTE LOCATION.
– FOR LOCAL OPERATION DURING
STARTUP OR SHUTDOWN
COMPONENT OPERATIONAL
OBJECTIVES.
• EQUIPMENT PROTECTION
– DESIGNS TO INDICATE OUT-OF-RANGE
CONDITIONS SO OPERATORS CAN TAKE
PROPER ACTION
• DESIGNS TO INITIATE AUTOMATIC
SHUTDOWN SEQUENCES FOR OUTOFCONTROL CONDITIONS.
TYPES OF CONTROL
•
•
•
•
CONTINUOUS
BATCH
SEMI-CONTINUOUS
COMBINATIONS OF THE ABOVE
http://www.controlloopfoundation.com/continuous-chemical-reactorprocess.aspx
http://www.controlloopfoundation.com/batch-chemical-reactorworkspace.aspx