cbe495 lecture vicbe495 lecture vi batch process control … · · 2007-05-22cbe495 lecture...

CBE495 LECTURE VICBE495 LECTURE VIBATCH PROCESS CONTROL

Professor Dae Ryook YangProfessor Dae Ryook Yang

Spring 2007Spring 2007Dept. of Chemical and Biological Engineering

Korea University

Korea University VI-1CBE495 Process Control Application

y

Batch Processes (1)Batch Processes (1)

• Alternative to continuous processingS f t i d fi d d– Sequence of one or more steps in a defined order

– In a single vessel or multiple vessels– Yielding a specifying quantity of finished product– Volume of product is normally small: repeated runs– Suitable for small-amount, multiple-product production

• Challenges• Challenges– Produce each product in accordance with its specification– Maximizing the utilization of available equipment

• Benefits– Reduced inventory– Shortened response time to make a specialty product


Shortened response time to make a specialty product

Batch Processes (2)Batch Processes (2)

• ApplicationsS i lt h i l– Specialty chemicals

– Metals– Electronic materials– Ceramics– Polymers– Food and agricultural materialsFood and agricultural materials– Biochemicals– Pharmaceuticals

M lti h t i l /bl d– Multiphase materials/blends– Coatings– Composites


• Example: Batch Distillationp– Charge the feedstock– Heating the reboiler– Cooling the condenserg– Maintain total reflux– Produce product

• Top comp. spec.p p p– Manipulate distillate flow rate– Manipulate reboiler heat duty

– Switching product tanks• At selected times• Multi-product case

– Shut down the columnDi h– Discharge

• Bottom residue• Receiver holdup

Cl d t f t b t h


– Clean-up and setup for next batch

Batch Control SystemsBatch Control Systems

• Batch Control Hierarchy– Batch sequencing

• Follow a recipe• Charging/Processing/Transferring

– Logic control• Discrete logic: valves, pumps, …

Batch productionmanagement

g p p• Safety interlock: personnel, equipment, …• Process interlock: time sequence

– Control during the batch• Feedback control: F, T, L, P, x, … Control during

batch

Run-to-run control

, , , , ,• Advanced control: override, selective, …• Ramping the CV’s up/down• Detecting end point: product quality

– Run-to-run controlBatch sequencingand logic control

batch

Run to run control• Learn from previous batch operations

– Batch production management• Information database for whole processing• Scheduling of production/process units

Batch process


Scheduling of production/process units

Sequential and Logic ControlSequential and Logic Control

• Sequential logic– It is used to ensure that the batch process attains the proper sequencep p p q

of states because the time order of steps is important.– Example: the reactor discharge valve must be closed or the vent must

be open in order for the feed valve to be opened.Representation of batch steps and sequential logic• Representation of batch steps and sequential logic– Information flow diagram– Sequential function chart

Ladder logic diagram– Ladder logic diagram– Binary logic diagram

• PLC (Programmable Logic Controller)– Execute the desired binary logic operation and to implement theExecute the desired binary logic operation and to implement the

desired sequencing– Set of relays are operated based on the defined logic.– Process interlock


– Safety interlock

• Typical example– A typical batch sequence

1 T f ifi d t f t i l f t k A t t k R1. Transfer specified amount of material from tank A to tank R.2. Transfer specified amount of material from tank B to tank R.3. Agitate for a specified period of time after the feeds are added.4. Discharge the product to storage tank C.

– For step 1 to proceedp p• Tank R inlet valve: open• Tank R agitator: off• Tank R cooling valve: closed

– Then, proceed step 1 by opening Tank A discharge valve and turning on theTank A transfer pumpTank A transfer pump.

– For step 2 to proceed• The totalizer of A is within the allowable range of specified amount of A• Then, close the Tank A discharge valve and turning off the Tank A transfer pump.

Also, check if– Conditions for step 1– Tank A discharge valve: closed– Tank A transfer pump: off

– Then, process to step 2. And so on.– If any of the conditions are not met, indicate ‘abnormal’.


If any of the conditions are not met, indicate abnormal .

Programmable Logic Controller (PLC)Programmable Logic Controller (PLC)

• Components of PLC– Central Processing Unit (CPU)Central Processing Unit (CPU)

• Update input and output at a given interval, perform programmedcalculation, accessing memory units

– Memory• Storing the program code and variables for calculations

– Input modules• Read the states of input signal from equipment• Analog/Digital inputs• TTL level signal

– Output modulesSend the calc lated signal to o tp t de ices

ProgrammingTerminal

• Send the calculated signal to output devices• Analog/Digital outputs• TTL level signal

– Power supplyCPU

OutputModule

Memory

InputModule


Power supply• Supply power for PLC operation Power Supply

• PLC operationPLC i t f ti l il f– PLC program consists of vertical rails of‘ladder’ and ‘rungs’ in between and it iscalled Ladder Logic Diagram.I ‘RUN’ d PLC ill d t ll i t– In ‘RUN’ mode, PLC will update all inputsand outputs before calculation, and performthe calculation of each rung from the top.Repeat this procedure indefinitelyRepeat this procedure indefinitely.

– During the calculation, the CPU of PLC willscan the rungs of the program and decide thevalues for memories and perform the actionvalues for memories and perform the actionspecified depending on the input states.

– Total scan time for calculation depends onthe length of the program


the length of the program.

• Ladder logic and digital (binary) logic functionBi t ti 0(F l ) d 1(T )– Binary representation: 0(False) and 1(True)

– Binary variable operators• AND, OR, NOT, Flip-Flop, and etc.

– AND operator• A*B A

A

B D

• ‘On’ when A and B are 1 at the same time.

– OR operator

C

• A+B• ‘On’ when one or more of A and B are 1.

ORA

B

C

D


– NOT operator• The result is the opposite of AThe result is the opposite of A.

– Flip-Flop (S-R latch)• Use output as input

A B

• Use output as input.• Avoid S=R=1.• =1 when Q=0.• When S=R=0 Q is unchanged (Latch)

Q• When S=R=0, Q is unchanged. (Latch)• When S becomes 1, Q=1. (Set)• Unless R becomes 1, Q is unchanged even S changes.• When R becomes 1 Q 0 (Reset)• When R becomes 1, Q=0. (Reset)• Unless S becomes 1, Q is unchanged even R changes.• Used as Memory

A CS


B DR

– Timer• Delay Initiation (DI): start after a period

A BDItDelay Initiation (DI): start after a period

• Delay Termination (DT): stop after a period• Positive Pulse Output: Repeat On and Off with a period• Negative Pulse Output: Repeat Off and On with a period

A BDT

t

A Bt

• Negative Pulse Output: Repeat Off and On with a period

– Counter• Counter Up (CU): count when input changes from 0 to1

A Bt

• Counter Up (CU): count when input changes from 0 to1• Counter Down (CD): count when input changes from 1 to0• If counter number is met, then turn on.• Resetting the counter by setting the counter memory to 0• Resetting the counter by setting the counter memory to 0

– Others


• XOR, NAND (Not AND), NOR (Not OR)BABA:XOR +

InterlockInterlock

• InterlockA system that checks the conditions for safety and performs required– A system that checks the conditions for safety and performs requiredsequence of actions for safety.

– If there are required states of the process condition in order tooperate a valve, for example, it is difficult and not reliable to check allope ate a va ve, o e a p e, t s d cu t a d ot e ab e to c ec athe conditions by an operator.

– Thus, the interlock ensures that all the required conditions for safetyare met before performing actions on behalf of operator.

– Interlock system is programmed in PLC.– Example

• When more furnace fuel needs to be supplied for higher temperature,• The combustion air needs to be increased together with fuel.• If fuel is increased but air is not, incomplete combustion may cause highly

dangerous condition in the furnace.• Thus, the interlock system will increase air automatically ahead of the


Thus, the interlock system will increase air automatically ahead of thefuel increase, then increase fuel automatically.

• Interlock Example– Incineration of hazardous waste in furnace– Incineration of hazardous waste in furnace– The waste can be fed only when the flame in the

furnace long enough.– Install several flame sensors (3 for example)Install several flame sensors (3 for example)– Case 1: Maximizing waste incineration

• Feed the waste if at least one sensor detects flame.• There is a risk of waste effluence due to sensor

malfunction.– Case 2: Minimizing the risk of waste effluence

• Feed the waste if all the sensors detect flame.• Reduced waste treatment due to sensor malfunction.

– Case 3: Optimum operation• Feed the waste if at least few sensors detect flame

considering the rates of sensor malf nction


considering the rates of sensor malfunction.

Control during the BatchControl during the Batch

• Within the batch control– Follow the operating trajectoryp g j y

• CV changes as a function of time• Tracking of set points

• Operational challenges– Time-varying characteristics– Nonlinear behavior– Model inaccuracy

Sensors– Sensors– Constrained operation– Unmeasured disturbance– Irreversible behaviorIrreversible behavior

• Advantages over a continuous process– The batch duration can be adjusted to meet quality spec.– The repetitive nature helps the performance run-to-run.


p p p– The slow response allows sufficient computation time.

Example: Batch Reactor ControlExample: Batch Reactor Control

• Feed charging control– Feedstock ratio

Feeding system

– Feedstock ratio– Total amount of feed– May require sequential feeding

• Reactor pressure control

Time-varyingset points

• Reactor pressure control– Prescheduled set point change

• Reactor temperature control– By both heating and cooling

• Split-range control– Exothermic reaction phase: cooling– Circulating pump is essential

• Minimize time delay• Maintain the jacket temperature


• Cascade control

• Batch startup– From low T to high reaction TFrom low T to high reaction T– Reset windup occurs.– Use preload (or bias term): u0

1 t⎡ ⎤

– Other control methods

* *0 0

1( ) ( ) ( )t

cI

u t u K e t e t dtτ

⎡ ⎤= + +⎢ ⎥

⎣ ⎦∫

Other control methods• Dual mode approach• On/off control for initial period• Then, switched to PID control


Run-to-run ControlRun to run Control

• ProblemsP d t lit ill t b il bl til th d i t th– Product quality will not be available until the end point, thusthe control during the batch is based on the recipe.

– Direct feedback control for quality is impossible.– Process model may be quite inaccurate.– Initial recipe is not satisfactory.

• RemedyRemedy– Learn from the past batch runs.– Modify the recipe to meet the quality specification.

F db k t l i t b t h t b t h ( t d i th b t h)– Feedback control against batch to batch (not during the batch)– Adjust the time profile of the preload of the controller.– The performance will get better as the batch repeats.


Batch Production ManagementBatch Production Management

• Standard for batch process operationISA SP 88 t i l– ISA SP-88: terminology

• procedure-operation-phase-control steps/instruction– ISO 9000 (ISO 9001-9004)

• Every manufactured product should have an established,documented procedure.

• The manufacture should be able to document that the procedurewas followedwas followed.

• Batch scheduling and planning– Multiple products are manufactured within a series of batch

equipments at the same time.– Batch processing steps should be carried out in a

predetermined order (wait until the equipment is ready)


– Minimizing the equipment idle time relates to productivity

• Gantt chart4 d t– 4 products

– Each product requires 3 steps– Each step for each product has different processing time.– Cycle time: total time required for the production requirement– Cycle time should be minimized for maximum production.– The nature of the problem is mixed integer nonlinear problem.The nature of the problem is mixed integer nonlinear problem.


SupplementSupplement

• Snowball EffectReactor/Distillation Col mn Plant– Reactor/Distillation Column Plant

• Feed: mainly A with some B fed to the reactor• Isothermal reactor: A B

20 t bi di till ti l• 20 stage binary distillation column:– B is the main product as bottom product– A-rich distillate is recycled to the reactor

• Model equations (steady state material balance)• Model equations (steady state material balance)– Reactor:

– Distillation:

0

0 0 D r r

F F DFz F z Dx k H z= += + +

F D B= +

0

0 0 B r r

B FF z Bx k H z

=⇒

= +

• If the products are relatively pure, xD≈1 and xB≈ 0.• Compositions and levels for column are controlled conventional

D B

F D BFz Dx Bx

+= +

Korea University VI-21

manner.

CBE495 Process Control Application

• Alternative 1Alternative 1


• The reactor holdup (HR) is controlled by manipulating F.• If the level is controlled perfectly, F0z0 ≈ krHrz and D ≈ Fz.If the level is controlled perfectly, F0z0 krHrz and D Fz.• The variables can be obtained in terms of F0 and z0.

0 0 0 02

0 0 0 0 0

/

( ) ( ) /( )r r r r

r r

F z k H z z F z k H

D D F z D F z k H F z

= ⇒ =

= + ⇒ = −

• Analysis

0 0 0 0 0

0 0 0 0

( ) ( ) /( )/( )

r r

r r r r

kF D F F F k H k H F z

⇒= + ⇒ = −

Analysis– If F0 or z0 are changed so that the denominator becomes near zero,

D will be amplified considerably and so does F.– Thus, D and F is very sensitive to changes in F0 and z0.– The high sensitivity to a disturbance is termed the snowball effect.– The high sensitivity is caused by the specific control structure.


• Alternative 2Alternative 2


– F is controlled constant by flow controller. However, z is stillaffected by the feed to the reactor.affected by the feed to the reactor.

– The reactor level is not controlled. (floating level)– The variable are function of F, F0 and z0 not HR.

F D F D F F0 0

0

0 0 0 0 0 0 0

( ) // ( ) /[ (1/ 1/ )]r r r r r

F D F D F FD Fz z F F FF z k H z H FF z k F F z k F F

= + ⇒ = −= ⇒ = −

= ⇒ = − = −

– Analysis• There is no snowball effect.• The HR changes proportional to z0 and is strongly related to F0.• However, is self-regulating. (Larsson et al., 2003)• For safety, for example, tank overflow, level controller can be

added with the manipulation of F0.

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• The snowball effect can appear in recycle system generally.

CBE495 Process Control Application

• Other alternatives (Wu and Yu, 1996)Alt ti 2 li i t b ll ff t D b t i il ff t– Alternative 2 eliminates snowball effect on D but similar effectis introduced in HR.

– Alternative 3• HR is controlled by manipulating D.• The set point of HR controller is manipulated to control z as

cascade.F/F h ld b i t i d b ti t l• F/F0 should be maintained by ratio control.

– Alternative 4• HR is controlled by manipulating D.• The set point of HR controller is manipulated to control xD as

cascade.• F/F0 should be maintained by ratio control.


Case StructureRelativegain (λ)gain (λ)

Alternative 1XD-RXB-V

2.8

Alternative 2 XD-RX V

12.2XB-V

Alternative 3 z-HR,spXB-V

0.78

Alternative 4XD-HR,sp

XB-V0.59

XB V

A lti it l t ith • Any multi-unit plant with arecycle stream from a separationunit is likely to exhibit slowerdynamics.

•The process response becomesslower as either the degree ofseparation or the recycle flow ratei c ea e


increases.

cbe495 lecture vicbe495 lecture vi batch process control … · · 2007-05-22cbe495 lecture...

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