disenho de um control de processo

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1All rights reserved by Adrie Huesman

Process Systems Design CH3801

Process control

Adrie Huesman

Version 28 November 2007

Delft Center for Systems and Control


Introduction I

• About me:

• 1984 – 1990 Student Chemical Technology (Delft University of Technology).

• 1990 – 1996 Control technologist for Shell Nederland atPernis.

• 1996 – 1999 Senior control technologist for Shell Singapore atPulau Bukom.

• 1999 – now Assistant professor with DCSC, current researcharea is “Economic dynamic process optimization”.

• Personal webpage: http://www.dcsc.tudelft.nl/~ahuesman/




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Introduction II

• Content

1. Design and operation.

2. Safety.

3. Process and instrumentation diagrams.

4. Plantwide control

a. Degrees of freedom.

b. Objectives.

c. Decomposition.



Introduction III

• Content (continued)

4. Plantwide control

d. Production rate control.

e. Standard quality schemes.

f. Recycles.g. Procedure.

h. Example.

• Course scope limited to mainly continuous processes.




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Design and operation I

• Plant design means defining:

1. the process.

2. the operation (control).

• In practice this is done in three sequential stages:


1. Operation mode

2. Process design

3. Control design


Design and operation II

• Note that the process design is done before the control design.This facilitates the control design; the process dynamics can beused as a starting point for the control design. However theprocess dynamics can severely limit the controlled behavior.

• The process and control design deal with different DOF’s:




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Design and operation III

• Often a lot of attention is directed to process design (because itdetermines capital costs).

• However for a continuous plant two to three years is spent onthe design while it is operated for 20 years!

• During design money is spent, only by operating the plantmoney is earned.



Design and operation IV

• There are two operation modes:

1. Batch; the product flow is discontinuous.

2. Continuous; the product flow is continuous.

• Note that in a batch process the product flow frequently drops tozero while in a continuous process it never drops to zero.




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Design and operation V

• This implies that a batch process at least accumulates product.So it can never reach a steady state (a batch process is alwaysin transition). A continuous process can reach one or multiplesteady states.

• The control design for a batch process boils down to theautomation of a recipe (discrete control) while for a continuousprocess it means maintaining one or more operating points.

• The functionality of a batch control system is often described by

Sequential Function Charts (SFC’s).



Design and operation VI


0

1

2

state

Filling, set V1 to100%. Set V2 and V3

to 0%.

timer1 > 90 s.

LT > 80%.

TT > 60 oC.

Heating, set V1 andV2 to 0%. Set V3 to100%.

Initialize, set V1 andV3 to 0%, set V2 to100%. Start timer1.

transition

3



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Design and operation VII


Not flexible

High capital cost

High efficiency

High safety

Continuous

Lower safety

Lower efficiency

Flexible

Low capital cost

Batch

DisadvantagesAdvantages


Safety I

• Operational definition of safety:


No uncontrolled loss of process inventory

•How safety is achieved:1. Inherent safety. Thickness of vessels, columns etc. ensures

containment.

2. Safety Relief Valves (SRV). At high pressures controlled blowdown to safe location.

3. Instrument Protective functions (IPF). At high pressures,temperatures etc. perform a (partial) shutdown (trip).



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Safety II

• Note that control does not guarantee safety* (but rathersupports it).


SRV

coolingwater

PC

steam

PZ

safelocation

SRV

IPF

Pinherent > PSRV > PIPF > Pcontrol

* Not always true for batch operation.


Process and instrumentation diagrams I

• A PI&D makes use of “standard” symbols. The standard variesper location…

• It should be noted that a P&IDis a “marriage” between a flow

sheet and a block diagram; so itcontains flows and signals.Flows are indicated by solidlines, signals by broken lines.


column

vessel

process line

heat exchanger

compressor or turbine

pump

AB

nn

control valve

control valvefail close

control valve

fail open

instrument or control function

with process

connection

signal line

ABnn

instrument or control function



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Process and instrumentation diagrams II

• The letter-number combination “ABnn” is called a tag. The firstletter specifies the process variable type, the second the function.

• So LA stands for level alarm and FC for flow controller. Thenumber serves to differentiate between similar lettercombinations; for example FC1 from FC2.


IPFZ (or E)OtherX

CalculationY QualityQ

TransmittingTTemperatureT

SwitchingSPressureP

ControlCLevelL

AlarmA FlowF

FunctionSecond letterVariableFirst letter


Process and instrumentation diagrams III

• Standard feedback control is normally done by ProportionalIntegral Derivative (PID) controllers…

• The Process Value (PV, measured value) is driven to theSetPoint (SP, reference) by manipulating the OutPut (OP,typically a valve position).


FC

PV

OPSP

( ) ( )( )

dt

PV SPd K dt PV SP

K PV SPK OP DC

t

I

C C

−+−+−= ∫ τ

τ 0



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Process and instrumentation diagrams IV



Degrees of freedom

• The Degrees Of Freedom (DOF) are the (remaining) number of equations that are needed to calculate the dependent variables:

DOF = Number of Variables (NV) – Number of Equations* (NE).

1. DOF = 0; exactly specified → simulation.2. DOF > 0; under specified → control/optimization.

3. DOF < 0; over specified → something is wrong.

• The DOF also equal the number of inputs (typically flows thatcan be manipulated). This is a more robust way to determine theDOF since NV and NE are large while the DOF are small.


* Independent equations.



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Objectives

• The objectives of a plantwide control system are:

1. Satisfy operational constraints.

2. Maintain production rate and product quality.

3. Improve economics (e.g. minimize operational costs).

• The objectives are mentioned in the order of economicimportance (1. being most important).

• The second objective leads to a decomposition of the overalldesign problem into:

1. one production rate control system and2. various quality control systems.



Decomposition

• The decomposition clarifies the most common interaction in anyprocess system; between production rate and quality.

• For example in the plant below a change in the production ratewill affect the quality of the flows F1 and F2.

• A standard way to deal with this interaction is to make productionrate changes slow; so the various quality control systems getmore time to cope with a production rate change. In practice SPchanges of the production rate controller are filtered or ramped.




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Production rate control I

• For production rate control there are at least two possibilities…

• The upper one is called a push scheme, the lower one a pullscheme. Note that the level controllers automatically propagatethe production rate through the plant.



Production rate control II

• The pull scheme has several inherent disadvantages:

1. Larger variability in the product quality loops.

2. Larger surge capacity requirements.

3. Control loops are more interacting and difficult to tune.

4. Stronger non-linear behavior.

5. More switching of loop pairing when flowsheet changesoccur.

• Therefore normally a push scheme is preferred. However ifproduct storage is difficult or even impossible (utilities!) a pullscheme is used.




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Standard quality schemes

• The quality of F1 and F2 still needs to be controlled. These twoqualities are not interactive since unit operation 1 and 2 aredifferent (say a reactor respectively a separator).

• This is done by standard quality control schemes; such ascheme shows how to control the quality of a particular unitoperation. The production rate is treated as an unmeasured ormeasured disturbance…



Recycles I

• An aspect that has been neglected until now is the presence of one or more recycles (material and/or heat).

• Depending on its size a recycle can slow down plantwidedynamics considerably. So be conservative with large recycles.

• From a system point of view this slow down can be explained bythe fact that a recycle introduces positive feedback . Anexplanation from a process of view would be that the same flowis passed through the same dynamics several times.




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Recycles II

• A tank plus recycle example;from the comp. balances:

V=constantF0=1,C

AOF1=10,C

A1F2=10,C

A2

F3=9


V V

Kuydt

dy

C C dt

dC V

C C dt

dC V

C C dt

dC V

C C C

totalvessel

AAA

AAA

AAA

AAA

==⇒

=+

=+⇒

=+⇒

−=

+=

τ τ

τ

and 10/

:hatRemember t

:total

10:vessel

1010

910

02

2

12

2

21

2

021


Procedure

• The various insights have led to plantwide control designprocedures like the one below:

• The first 4 steps will be explained by means of an example.


8. Evaluate

7. Simulate

6. Do simple checks

5. Minimize the operating costs

4. Develop quality control schemes

3. Develop a production rate control system

2. Determine DOF

1. Determine control objectives



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Example I

• The flowsheet shows a simplified formaldehyde process. Thisprocess is based on the dehydrogenation and partial oxidation of methanol:

• The first reaction partly consumes the heat of the secondreaction. Besides these two main reactions also side reactionstake place.


kJ158∆HOHOCH0.5OOHCH

kJ85∆HHOCHOHCH

0

2982223

0

298223

−=+→+

=+↔


Example II

• Fresh methanol is mixed with recycle methanol and evaporated inA1 by means of steam. In A1 also air is added, care should betaken that the mixture is outside the explosive limits. Steam isadded to the mixture and the total is heated to reaction

temperature in E1.

• The reaction is catalyzed by a shallow bed (1 - 5 cm) of silvercatalyst (upper part of R1). The hot (around 600 °C) productsare cooled to 145 °C in a waste heat boiler generating steam(lower part of R1). The operating pressure of R1 should bearound 1.5 bar.




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Example III

• The gases are cooled further to 100 °C against cooling water inE2 and led to absorber C1. In the absorber formaldehyde,methanol and water go to the bottom while the light components(like nitrogen and hydrogen) are removed over the top.

• The bottoms of C1 are fed to distillation column C2 thatseparates methanol from the formaldehyde solution. Themethanol is recycled back to A1.



Example IV

• It should be noted that:

1. The maximum temperature is limited by the excess of methanol and the presence of steam in the feed. The ratiorecycled methanol/fresh methanol is in the range 0.25 - 0.50.The conversion of oxygen in R1 is complete.

2. The distillation column operates at a reflux ratio of 2.0.

3. The process produces between 75 and 150 tons of formaldehyde solution per day. The formaldehyde solutionproduced should contain >50.0% formaldehyde, <1.0%methanol and <49.0% water.





Example V


16 Objectives RemarksDescriptionEquipmentNumber

16

15

14

13

12

11

10

9

8

7

6

5

4

3

2

1

C2

C2

C2

C2

C2

C1

C1

C1

E2

R1

R1

E1

A1

A1

A1

Plantwide

Related to quality bottomRatio steam/feed

Related to quality topRatio reflux/feed

Level bottom

Level top

Pressure

Related to quality bottomRatio water/feed

Pressure

Level bottom

100 °COutlet temperature

145 °COutlet temperature

1.5 barPressure

Reaction temperatureOutlet temperature

Level

Related to maximum temperatureRatio steam/methanol

Related to maximum temperatureRatio air/methanol

Directly proportional to airProduction rate

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