CHE 185 – PROCESS CONTROL AND DYNAMICS CONTROL OBJECTIVES
Dec 18, 2015
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
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
• ASSUMPTION OF STEADY STATE
0
out
0
inonaccumulati
)0()(
hhA
dt
dhww
dt
Ahd oiqw
i
DERIVE EQUATIONS
• VALVE CHARACTERISTICS
• LEVEL CHANGES– LINEAR ODE
– NONLINEAR ODE
hCqhCq vovo NONLINEARLINEAR
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
QVC
TTV
w
dt
dT
QTTwCdt
dTVC
QTTwCTTCwTTVCdt
d
pi
ipp
refprefipirefp
1)(
)(
)()()(
DERIVE EQUATIONS
• AS INITIAL VALUE PROBLEM• GIVEN
– PHYSICAL PROPERTIES (r, 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 (r, 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
• COMPONENT BALANCE ON A
qqqqwwdt
Vdiii
0)(
AAAiA
AAAAiiAAA
cRTEVkccqdt
dcV
VrMqcMcqMdt
VcMd
)/exp()(
)(
0
DERIVE EQUATIONS
• ENERGY BALANCE
)()()(
)()()()(
)/( TTUAeVkHTTqCdt
dTVC
QrVHTTwCTTCwTTVCdt
d
ccRTE
ipp
refprefipirefp
A
0
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
– DEFINITION OF TERMS– VR = REACTOR VOLUME
– F = VOLUMETRIC FLOW RATE– D = F/VR = DILUTION RATE
– NON-TRIVIAL STEADY STATE: – WASHOUT:
XDXdt
dXXVFX
dt
dXV RR
D
0X
DERIVE EQUATIONS
• PRODUCT RATE
• SUBSTRATE CONCENTRATION
– S0 = FEED CONCENTRATION OF RATE LIMITING SUBSTRATE
– STEADY-STATE:
qXDPdt
dPqXVFP
dt
dPV RR
XY
SSDdt
dSXV
YFSFS
dt
dSV
SXR
SXR
/0
/0
1)(
1
)( 0/ SSYX SX
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)
extracellular
intracellular
glycerol
NAD+ NADH
G3P/DHP (S2)
ATP (A3)
NADH(N2)
NAD+
(N1)
ADP (A2)
ethanol
acetaldehyde/pyruvate (S4
ex)
1,3-BPG (S3)
ADP
NAD+NADH
ATP
acetaldehyde/pyruvate (S4)
degradedproducts
glucose
glucose (S1)
r2r6
r1 r5r3
r4
J0 J
r7
MODEL COMPONENTS• INTRACELLULAR CONCENTRATIONS
– INTERMEDIATES: S1, S2, S3, S4
– REDUCING CAPACITY (NADH): N2
– ENERGY CAPACITY (ATP): A3
• MASS ACTION KINETICS FOR r2-r6
• MASS ACTION KINETICS AND ATP INHIBITION FOR r1
2444
2266
2333
355
1222 NSkr
NSkr
ASkr
Akr
NSkr
14
33111 1
IK
AASkr
DYNAMIC MODEL EQUATIONS
• MASS BALANCES
• CONSERVED METABOLITES
• MATRIX
5313
6422
434
323
6212
101
22
2
rrrdt
dArrr
dt
dNJrr
dt
dS
rrdt
dSrrr
dt
dSrJ
dt
dS
tt NNNAAA 2132
),( uxfx
dt
d
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.html
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 OUT-OFCONTROL CONDITIONS.