Module II - Modeling and Analysis of Process Dynamics · 2016. 1. 20. · Dynamic Models of Control...
Transcript of Module II - Modeling and Analysis of Process Dynamics · 2016. 1. 20. · Dynamic Models of Control...
Chapter 2
Model Development
Mathematical Modelingof Chemical Processes
• Process Model
A mathematical abstraction of a real process.
– The equation or set of equations that comprise the model
are at best an approximation to the true process.
• Rationale for Mathematical Modeling
– To improve understanding of the process
– To train plant operating personnel
– To design controller strategy for a new process
– To select controller setting
– To design the control law
– To optimize process operating conditions
Introduction to Process Dynamics
• Chemical Engineering courses are generally taught
from a steady-state point-of-view.
• Dynamics is the time varying behavior of
processes.
• Chemical processes are dynamically changing
continuously.
• Steady-state change indicates where the process is
going and how far it is going.
• Dynamic characteristics of a system indicates what
dynamic path it will take and how certain variables
evolve with time I.e. dynamic characteristics shows
the evolution of certain variables with time.
Uses of Dynamic Process Models
• Evaluation of process control configurations
– For analysis of difficult control systems for both
existing facilities and new projects.
• Process design of batch processes.
• Operator Training (Simulation).
• Start-up/shut-down strategy development
Classification of Models
• Lumped parameter models- assume that the
dependent variable does not change with spatial
location within the process, e.g., a perfectly well
mixed vessel.
• Distributed parameter models- consider that the
dependent variable changes with spatial location
within the process.
Example of a Lumped Parameter Process
F1
T1
T
TT
F2
T2
FT
FC(F
1)spec
Example of a Distributed Parameter Process
TTPT
PC
Condensate
Steam
Feed
Toutlet
Modeling Approaches
• Lumped parameter processes- Macroscopic balances are typically applied for conservation of mass, moles, or energy and result in ODE’s.
• Distributed parameter processes- Microscopic balances are typically applied yielding differential equations for conservation of mass, moles, or energy for a single point in the process which result in PDE’s.
Significances of Dynamic Modeling are:
A. To know how fast an input change influences output.
B. To know how far output will change for a given input change.
C. To know impact of parameters change on the output behaviours.
you may need to specify more i.e. what effects?
Conservation Equations:
Mass, Moles, or Energy Balances
SystemthewithinReaction
byGenerationofRate
Systemthe
LeavingRate
Systemthe
EnteringRate
onAccumulati
ofRate
Mass Balance Equation
system
theleaving
massofRate
_
system
theentering
massofRate
systemtheinmassof
onaccumulatiofRate
Mole Balance Equation
reactionbymolesof
nconsumptioofRate
reactionbymolesof
generationofRate
system
theleaving
molesofRate
_
system
theentering
molesofRate
systemtheinmolesof
onaccumulatiofRate
Thermal Energy Balance Equation
systemtheof
boundariesthethrough
transferheatofrateNet
reactionby
generationheat
ofrateNet
systemtheleaving
transferheat
convectiveofRate
systemtheentering
transferheat
convectiveofRate
energythermalof
onaccumulatiofRate
Constitutive Relationships
• Usually in the form of algebraic equations.
• Used with the balance equations to model chemical engineering processes.
• Examples include:
– Reaction kinetic expressions
– Equations of state
– Heat transfer correlation functions
– Vapor/liquid equilibrium relationships
Degree of Freedom Analysis
evf NNN
• The number of degrees of freedom (DOF) is equal to the number of unknowns minus the number of equations.
• When DOF is zero, the equations are exactly specified.
• When DOF is negative, the system is overspecified.
• When DOF is positive, it is underspecified.
Different Types of Modeling Terms
• Dependent variables are calculated from the
solution of the model equations.
• Independent variables require specification by the
user or by an optimization algorithm and represent
extra degrees of freedom.
• Parameters, such as densities or rate constants, are
constants used in the model equations.
Dynamic Models of Control Systems
• Control systems affect the process through the actuator system which has its own dynamics.
• The process responds dynamically to the change in the manipulated variable.
• The response of the process is measured by sensor system which has its own dynamics.
• There are many control systems for which the dynamics of the actuator and sensor systems are important.
A Systematic Approach for Developing
Dynamic Models
1. State the modeling objectives and the end use of the model.
They determine the required levels of model detail and model
accuracy.
2. Draw a schematic diagram of the process and label all process
variables.
3. List all of the assumptions that are involved in developing the
model. Try for parsimony; the model should be no more
complicated than necessary to meet the modeling objectives.
4. Determine whether spatial variations of process variables are
important. If so, a partial differential equation model will be
required.
5. Write appropriate conservation equations (mass, component,
energy, and so forth).
6. Introduce equilibrium relations and other algebraic
equations (from thermodynamics, transport phenomena,
chemical kinetics, equipment geometry, etc.).
7. Perform a degrees of freedom analysis to ensure that the
model equations can be solved.
8. Simplify the model. It is often possible to arrange the
equations so that the dependent variables (outputs) appear
on the left side and the independent variables (inputs)
appear on the right side. This model form is convenient
for computer simulation and subsequent analysis.
9. Classify inputs as disturbance variables or as manipulated
variables.
Rationale for Dynamic Process Models
• Improve understanding of the process
• Train plant operating personnel
• Develop a control strategy for a new process
• Optimize process operating conditions
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Variable Selection
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Example 3.1 - Mixing Tank
• goal - determine dynamic behaviour in response to step changes in feed concentration, find time to 90% change
• information - size of vessel, standard operating concentrations and flow rates
• assumptions – well-mixed
– similar densities of components, no volume change on mixing
– constant flow in
• note - concentrations in moles/volume
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Example 3.1 - Mixing Tank
F0, cA0
F1, cA
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Example 3.1 - Total Material Balance
• Total mass balance: Mass accumulating = Mass in – Mass out
» Assuming inflow and outflow have constant and identical densities.
• constant level in overflow tank --> dV/dt=0 (flow in = flow out)
0 1
( )d VF F
dt
0 1
dVF F
dt
0 10 F F
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Example 3.1 - Component Balance
• Material balance on component A
Mass A accumulating = Mass A in – Mass A out
• solvent balance - not independent from total and component A
balances - unnecessary
d MW Vc
dtMW F c MW F c
d c
dt
F
Vc
F
Vc
A A
A A A A
A
A A
( )
( )
0 0 1
0
0
1
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Example 3.1 - Final Model and Degrees of
Freedom
• model equations
• Variables to solve for: F1, cA
• external variables: F0, cA0
• degrees of freedom = (2 variables - 2 equations) = 0
0 10
0 10
AA A
Fd c Fc c
dt V V
F F
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Example 3.1 - Solution
• model consists of an algebraic equation and a 1st order, linear ordinary differential equation
• solve algebraic equation for exit flow and substitute,
leaving o.d.e.
• solve o.d.e using integrating factor
0 10
0 10
AA A
Fd c Fc c
dt V V
F F
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Integrating Factor
• for first-order linear differential eqns.
• “complete the derivative” to form separable differential eqn. Separate and solve to get
• How does this work?
)(tqpydt
dy
Cedtetqey tptptp .)(
1 10
AA A
d c F Fc c
dt V V
Summary of Integrating Factor
)()( xfyxdx
dy
dxxexpxI
xI
CdxxfxI
xIy
1
Write the general equation of 1st order linear ODE
Determine integrating factors I (x)
Determine dependent variable or unknown function y
Arbitrary constant C is determined by the boundary condition.
(See the in-class example)
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Example 3.1 - Analytical Solution
• o.d.e. is of form
• solution
• C - integration constant - determined from initial
conditions of o.d.e.
• (which has units of time) is called the “time constant”. determines how quickly the system gets to the new steady state after a change in cA0.
dc
dtc c V FA
A Ao 1 1
, /
ceecec tt
A
t
A
//
0
/ ][ 0
t
//
00
tt
AAA ceeccc
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Example 3.1 - Solution ...
• at t=0, cA=cAinit , so CAinit =CAo-CAo + C, and thus;
C = cAinit giving:
• if we are initially at steady-state,
and solution can be written in deviation form as
• change in cA due to change in cA0
/
00 )( t
AAinitAA ecccc
c cAinit A init 0
c c eA A
t
0 1( )/
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Example 3.1 - Solution ...
• If at S.S., CA=0.9 mol/l and CAo has been change from 0.9 to 1.8 mol/l. volume (V) =20 l and volumetric flow (F) is 20 l/min gives time constant=1 min, the solution will be
• if we are initially at steady-state, and solution can be written in deviation form as
• change in cA due to change in cA0
t
A ec )8.19.0(8.1
9.00 initAAinit cc
)1(9.0 t
A ec
at t=0 CA=0.9
t ∞ CA=1.8
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Example 3.1 - Response
0 20 40 60 80 100 0.5
1
1.5
2
time
ou
tle
t co
nce
ntr
atio
n
outlet concentration
0 20 40 60 80 100 0.5
1
1.5
2
time
inle
t co
nce
ntr
ation
inlet concentration
ΔCAo
ΔCA
Solve Example 2.1
if you have any question
ask me !
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40
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Steady-State Gain
• Steady-state gain is the final ratio as process
settles (t)
• for our example, the steady-state gain is 1:
c
c
A
A0
1)1(
limlim0
/0
0
A
tA
tA
A
t c
ec
c
c
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Time Constant
• focus on (1-e-t/)
• dictates fraction of change through process
• for t= , (1-e-t/) is 0.632 -- 63.2% change
• for t= 2 , (1-e-t/) is 0.865 -- 86.5 % change
c c eA A
t
0 1( )/
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Gain-Time Constant Form of a Model
• 1st order linear o.d.e. model:
where u(t) is the input variable of interest.
• Kp - process steady-state gain
• - process time constant
• if model is in this form, these quantities can be
identified directly.
)(tuKydt
dyp
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Step Response Solution for
Gain-Time Constant Model Form
The solution to:
where the input, u(t), is a step change at time zero in
the input of size u, starting from steady-state
operation is:
y K e up
t ( )/1
)(tuKydt
dyp
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Example 3.2 - Stirred Tank Reactor Model
• same overflow tank - constant level
• reaction - first order: A B
• reaction rate for consumption of A is kCA (mol/L/min)
• we now have a generation term (consumption)
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Example 3.2 - Model
• component mass balance on A
0 0 1
0 10
( )A AA A A A
A A
AA A A
d MW VcMW F c MW F c
dt
MW Vkc
Fd c Fc c kc
dt V V
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Example 3.2 - Gain-Time Constant Form
• group terms:
• time constant is V/(F+kV)
» time constant is now smaller than when there was no reaction.
• gain is F/(F+kV)
» gain is now smaller than when there was no reaction.
AOAA
FCCkVFdt
dCV )(
AOpAA
CKCdt
dC
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Example 3.2 - Initial Conditions
• Assume that we start at steady state with feed concentration
cA0init
– reactor initial concentration is
cA = [F/(F+kV)] cA0init
How do we know?
• We can place model solution in deviation form (perturbation) if
we are interested in changes from the old steady-state condition
where =V/(F+kV) and Kp= F/(F+kV)
0/ )1( A
tA ce
kVF
Fc
3.3 Stirred-Tank Heating Process
Figure 2.3 Stirred-tank heating process with constant holdup, V.
3.3 Stirred-Tank Heating Process
Assumptions:
1. Perfect mixing; thus, the exit temperature T is also the
temperature of the tank contents.
2. The liquid holdup V is constant because the inlet and outlet
flow rates are equal.
3. The density and heat capacity C of the liquid are assumed to
be constant. Thus, their temperature dependence is neglected.
4. Heat losses are negligible.
5. Changes in potential energy and kinetic energy can be neglected because they are small in comparison with changes in internal energy.
6. The net rate of work can be neglected because it is small compared to the rates of heat transfer and convection.
For the processes and examples considered in this book, it
is appropriate to make two assumptions:
1. Changes in potential energy and kinetic energy can be
neglected because they are small in comparison with changes
in internal energy.
2. The net rate of work can be neglected because it is small
compared to the rates of heat transfer and convection.
For these reasonable assumptions, the energy balance in
can be written as
int (2-10)dU
wH Qdt
int the internal energy of
the system
enthalpy per unit mass
mass flow rate
rate of heat transfer to the system
U
H
w
Q
denotes the difference
between outlet and inlet
conditions of the flowing
streams; therefore
-Δ wH = rate of enthalpy of the inlet
stream(s) - the enthalpy
of the outlet stream(s)
For a pure liquid at low or moderate pressures, the internal energy
is approximately equal to the enthalpy, Uint , and H depends
only on temperature. Consequently, in the subsequent
development, we assume that Uint = H and where the
caret (^) means per unit mass.
H
ˆ ˆintU H
intˆ ˆ (2-29)dU dH CdT
where C is the constant pressure heat capacity (assumed to be
constant). The total internal energy of the liquid in the tank is:
int intˆ (2-30)U VU
Model Development - I
An expression for the rate of internal energy accumulation can be
derived from previous equations:
int (2-31)dU dT
VCdt dt
Note that this term appears in the general energy balance
Suppose that the liquid in the tank is at a temperature T and has an
enthalpy, . Integrating from a reference temperature Tref to T
gives,
H
ˆ ˆ (2-32)ref refH H C T T
where is the value of at Tref. Without loss of generality, we
assume that , equation above can be written as:
ˆrefH H
ˆ 0refH
ˆ (2-33)refH C T T
Model Development - II
ˆ (2-34)i i refH C T T
Substituting into the convection term gives:
ˆ (2-35)i ref refwH w C T T w C T T
Finally, substitution into the original differential equation gives
(2-36)i
dTV C wC T T Q
dt
Model Development - III
For the inlet stream
Solve Example 2.4
if you have any question
ask me !
3.4 Liquid Level Systems
Assumption ; constant density, uniform cross-sectional area
– Mass Balance; [Accumulation]=[In]-[Out]
oi qqdt
Ahd
)()( oi qq
dt
dhA
hCqqqdt
dhA Vioi
Figure: A liquid-level storage system.
qo=Cv√h
Flow resistance
qo
qi
The Continuous Stirred-Tank Reactor(CSTR) – Reaction; A B,
– Assumption; the mass densities of feed and products stream are equal and constant.
Figure: A nonisothermal continuous stirred-tank reactor.
andkCr A )/exp(0 RTEkk
• Mass balance for component A.
• Energy balance.
AiAAiA kCVqCqC
dt
CVd
)(
OutInAccumulation nceDisappeara
)()()}((
RPRiPRP TTqCTTqC
dt
TTCVd
)()( TTUAVkCH CCA
Accumulation OutIn
additionheatofrateNet
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Why do we write solutions in deviation (perturbation) form?
• We focus on changes from old steady-state
• We will use this approach when we determine linear
approximations to nonlinear ODE models
– linearise about initial steady-state point
• Laplace transform analysis - done in deviation form to
avoid initial condition problems
– if we use deviation variables, and start at steady-state, initial
condition for the model output variable is 0.