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

• ASSUMPTION OF STEADY STATE

0

out

0

inonaccumulati

)0()(

hhA

qq

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.

TYPES OF CONTROL

• CONTINUOUS• BATCH • SEMI-CONTINUOUS• COMBINATIONS OF THE ABOVE

http://www.controlloopfoundation.com/continuous-chemical-reactor-process.aspx

http://www.controlloopfoundation.com/batch-chemical-reactor-workspace.aspx