Process Dynamics and Control: Solutions for Problems (4th Edition) Dale Seborg, Thomas Edgar

2-1
Chapter 2
2.1
a) Overall mass balance:
3
2
1
)
(
w
w
w
dt
V
d
−
+
=
ρ
(1)
Energy balance:
( )
3
1 1 2 2
3 3
ρ ( )
( ) ( )
ref
ref ref
ref
d V T T
C w C T T w C T T
dt
w C T T
 
−
 = − + −
− −
(2)
Because ρ = constant and V
V = = constant, Eq. 1 becomes:
2
1
3 w
w
w +
= (3)
b) From Eq. 2, substituting Eq. 3
( ) ( )
3 3
1 1 2 2
1 2 3
( )
ρ ρ ( ) ( )
ref
ref ref
ref
d T T dT
CV CV w C T T w C T T
dt dt
w w C T T
−
= = − + −
− + −
(4)
Constants C and Tref can be cancelled:
3
2
1
2
2
1
1
3
)
( T
w
w
T
w
T
w
dt
dT
V +
−
+
=
ρ (5)
The simplified model now consists only of Eq. 5.
Solution Manual for Process Dynamics and Control, 4th edition
Copyright © 2016 by Dale E. Seborg, Thomas F. Edgar, Duncan A. Mellichamp,
and Francis J. Doyle III
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2-2
Degrees of freedom for the simplified model:
Parameters : ρ, V
Variables : w1, w2, T1, T2, T3
NE = 1
NV = 5
Thus, NF = 5 – 1 = 4
Because w1, w2, T1 and T2 are determined by upstream units, we assume
they are known functions of time:
w1 = w1(t)
w2 = w2 (t)
T1 = T1(t)
T2 = T2(t)
Thus, NF is reduced to 0.
2.2
Energy balance:
ρ ( )
( ) ( ) ( )
ref
p p i ref p ref s a
d V T T
C wC T T wC T T UA T T Q
dt
 
−
 
= − − − − − +
Simplifying
ρ ( )
p p i p s a
dT
VC wC T wC T UA T T Q
dt
= − − − +
ρ ( ) ( )
p p i s a
dT
VC wC T T UA T T Q
dt
= − − − +
b) T increases if Ti increases and vice versa.
T decreases if w increases and vice versa if (Ti – T) < 0. In other words, if
Q > UAs(T-Ta), the contents are heated, and T >Ti.

2-3
2.3
a) Mass Balances:
3
2
1
1
1 w
w
w
dt
dh
A −
−
=
ρ (1)
2
2
2 w
dt
dh
A =
ρ (2)
Flow relations:
Let P1 be the pressure at the bottom of tank 1.
Let P2 be the pressure at the bottom of tank 2.
Let Pa be the ambient pressure.
Then )
( 2
1
2
2
2
1
2 h
h
R
g
g
R
P
P
w
c
−
ρ
=
−
= (3)
1
3
3
1
3 h
R
g
g
R
P
P
w
c
a ρ
=
−
= (4)
b) Seven parameters: ρ, A1, A2, g, gc, R2, R3
Five variables : h1, h2, w1, w2, w3
Four equations
Thus NF = 5 – 4 = 1
1 input = w1 (specified function of time)
4 outputs = h1, h2, w2, w3

2-4
2.4
Assume constant liquid density, ρ . The mass balance for the tank is
)
(
)
(
q
q
dt
m
Ah
d
i
g
−
ρ
=
+
ρ
Because ρ, A, and mg are constant, this equation becomes
q
q
dt
dh
A i −
= (1)
The square-root relationship for flow through the control valve is
2
/
1








−
ρ
+
= a
c
g
v P
g
gh
P
C
q (2)
From the ideal gas law,
)
(
)
/
(
h
H
A
RT
M
m
P
g
g
−
= (3)
where T is the absolute temperature of the gas.
Equation 1 gives the unsteady-state model upon substitution of q from Eq. 2 and
of Pg from Eq. 3:
1/ 2
( / )
( )
g
i v a
c
m M RT
dh gh
A q C P
dt A H h g
ρ
 
= − + −
 
−
 
(4)
Because the model contains Pa, operation of the system is not independent of Pa.
For an open system Pg = Pa and Eq. 2 shows that the system is independent of Pa.

2-5
2.5
a) For linear valve flow characteristics,
a
d
a
R
P
P
w 1
−
= ,
b
b
R
P
P
w 2
1 −
= ,
c
f
c
R
P
P
w
−
=
2
(1)
Mass balances for the surge tanks
b
a w
w
dt
dm
−
=
1
, c
b w
w
dt
dm
−
=
2
(2)
where m1 and m2 are the masses of gas in surge tanks 1 and 2,
respectively.
If the ideal gas law holds, then
1
1
1
1 RT
M
m
V
P = , 2
2
2
2 RT
M
m
V
P = (3)
where M is the molecular weight of the gas
T1 and T2 are the temperatures in the surge tanks.
Substituting for m1 and m2 from Eq. 3 into Eq. 2, and noticing that V1, T1,
V2, and T2 are constant,
b
a w
w
dt
dP
RT
M
V
−
=
1
1
1
and c
b w
w
dt
dP
RT
M
V
−
=
2
2
2
(4)
The dynamic model consists of Eqs. 1 and 4.
b) For adiabatic operation, Eq. 3 is replaced by
C
m
V
P
m
V
P =








=








γ
γ
2
2
2
1
1
1 , a constant (5)
or
γ
γ








=
/
1
1
1
1
C
V
P
m and
γ
γ








=
/
1
2
2
2
C
V
P
m (6)
Substituting Eq. 6 into Eq. 2 gives,
b
a w
w
dt
dP
P
C
V
−
=








γ
γ
γ
−
γ
γ
1
/
)
1
(
1
/
1
1
1

2-6
c
b w
w
dt
dP
P
C
V
−
=








γ
γ
γ
−
γ
γ
2
/
)
1
(
2
/
1
2
1
as the new dynamic model. If the ideal gas law were not valid, one would
use an appropriate equation of state instead of Eq. 3.
2.6
a) Assumptions:
1. Each compartment is perfectly mixed.
2. ρ and C are constant.
3. No heat losses to ambient.
Compartment 1:
Overall balance (No accumulation of mass):
0 = ρq − ρq1 thus q1 = q (1)
Energy balance (No change in volume):
1
1 1 1 2
ρ ρ ( ) ( )
i
dT
V C qC T T UA T T
dt
= − − − (2)
Compartment 2:
Overall balance:
0 = ρq1 − ρq2 thus q2 = q1= q (3)
Energy balance:
2
2 1 2 1 2 2
ρ ρ ( ) ( ) ( )
c c c
dT
V C qC T T UA T T U A T T
dt
= − + − − − (4)
b) Eight parameters: ρ, V1, V2, C, U, A, Uc, Ac
Five variables: Ti, T1, T2, q, Tc
Two equations: (2) and (4)

2-7
Thus NF = 5 – 2 = 3
2 outputs = T1, T2
3 inputs = Ti, Tc, q (specify as functions of t)
c) Three new variables: ci, c1, c2 (concentration of species A).
Two new equations: Component material balances on each compartment.
c1 and c2 are new outputs. ci must be a known function of time.
2.7
As in Section 2.4.2, there are two equations for this system:
( )
1
( )
i
i
i
dV
w w
dt
w
dT Q
T T
dt V VC
ρ
ρ ρ
= −
= − +
Results:
(a) Since w is determined by hydrostatic forces, we can substitute for this
variable in terms of the tank volume as in Section 2.4.5 case 3.
( )
1
i v
i
i
dV V
w C
dt A
w
dT Q
T T
dt V VC
ρ
ρ ρ
 
= −
 
 
 
= − +
This leaves us with the following:
5 variables: , , , ,
i i
V T w T Q
4 parameters: , , ,
v
C C A
ρ
2 equations
The degrees of freedom are 5 2 3
− =. To make sure the system is specified, we
have:
2 output variables: ,
T V

2-8
2 manipulated variables: , i
Q w
1 disturbance variable: i
T
(b) In this part, two controllers have been added to the system. Each controller
provides an additional equation. Also, the flow out of the tank is now a
manipulated variable being adjusted by the controller. So, we have
4 parameters: , , ,
sp sp
C T V
ρ
6 variables: , , , , ,
i i
V T w T Q w
4 equations
The degrees of freedom are 6 4 2
− =. To specify the two degrees of freedom, we
set the variables as follows:
2 output variables: ,
T V
2 manipulated variables (determined by controller equations): ,
Q w
2 disturbance variables: ,
i i
T w
2.8
Additional assumptions:
(i) Density of the liquid, ρ, and density of the coolant, ρJ, are constant.
(ii) Specific heat of the liquid, C, and of the coolant, CJ, are constant.
Because V is constant, the mass balance for the tank is:
0
=
−
=
ρ q
q
dt
dV
F ; thus q = qF
Energy balance for tank:
)
(
)
(
8
.
0
J
J
F
F T
T
A
Kq
T
T
C
q
dt
dT
VC −
−
−
ρ
=
ρ (1)
Energy balance for the jacket:
)
(
)
(
8
.
0
J
J
J
i
J
J
J
J
J
J
J T
T
A
Kq
T
T
C
q
dt
dT
C
V −
+
−
ρ
=
ρ (2)
where A is the heat transfer area (in ft2
) between the process liquid and the
coolant.

2-9
Eqs.1 and 2 comprise the dynamic model for the system.
2.9
Assume that the feed contains only A and B, and no C. Component balances for
A, B, C over the reactor give.
1 /
1
E RT
A
i Ai A A
dc
V q c qc Vk e c
dt
−
= − − (1)
1 2
/ /
1 2
( )
E RT E RT
B
i Bi B A B
dc
V q c qc V k e c k e c
dt
− −
= − + − (2)
2 /
2
E RT
C
C B
dc
V qc Vk e c
dt
−
=
− + (3)
An overall mass balance over the jacket indicates that qc = qci because the volume
of coolant in jacket and the density of coolant are constant.
Energy balance for the reactor:
( )
( )( )
A A A B B B C C C
i Ai A A i Bi B B i
d Vc M S Vc M S Vc M S T
q c M S q c M S T T
dt
 
+ +
  = + −
1 2
/ /
1 1 2 2
( ) ( ) ( )
E RT E RT
c A B
UA T T H Vk e c H Vk e c
− −
− − + −∆ + −∆ (4)
where MA, MB, MC are molecular weights of A, B, and C, respectively
SA, SB, SC are specific heats of A, B, and C.
U is the overall heat transfer coefficient
A is the surface area of heat transfer
Energy balance for the jacket:
ρ ρ ( ) ( )
c
j j j j j ci ci c c
dT
S V S q T T UA T T
dt
= − + − (5)
where:
ρj, Sj are density and specific heat of the coolant.
Vj is the volume of coolant in the jacket.
Eqs. 1 - 5 represent the dynamic model for the system.

2-10
2.10
The plots should look as shown below:
Notice that the functions are only good for t = 0 to t = 18, at which point the tank
is completely drained. The concentration function blows up because the volume
function is negative.

2-11
2.11
a)
Note that the only conservation equation required to find h is an overall
mass balance:
1 2
( )
dm d Ah dh
A w w w
dt dt dt
ρ
= =
ρ =+ − (1)
Valve equation: w = h
C
h
g
g
C v
c
v =
ρ
′ (2)
where
c
v
v
g
g
C
C
ρ
′
= (3)
Substituting the valve equation into the mass balance,
)
(
1
2
1 h
C
w
w
A
dt
dh
v
−
+
ρ
= (4)
Steady-state model:
0 = h
C
w
w v
−
+ 2
1 (5)
b) 1 2
1/2
2.0 1.2 3.2 kg/s
2.13
1.5
2.25 m
v
w w
C
h
+ +
= = = =
c) Feedforward control

2-12
Rearrange Eq. 5 to get the feedforward (FF) controller relation,
1
2 w
h
C
w R
v −
= where 2.25 m
R
h =
1
1
2 2
.
3
)
5
.
1
)(
13
.
2
( w
w
w −
=
−
= (6)
Note that Eq. 6, for a value of w1 = 2.0, gives
w2 = 3.2 –1.2 = 2.0 kg/s which is the desired value.
If the actual FF controller follows the relation, 1
2 1
.
1
2
.
3 w
w −
= (flow
transmitter 10% higher), 2
w will change as soon as the FF controller is
turned on,
w2 = 3.2 –1.1 (2.0) = 3.2 – 2.2 = 1.0 kg/s
(instead of the correct value, 1.2 kg/s)
Then 0
.
1
0
.
2
13
.
2 +
=
= h
h
Cv
or 408
.
1
13
.
2
3
=
=
h and h = 1.983 m (instead of 2.25 m)
Error in desired level =
2.25 1.983
100% 11.9%
2.25
−
× =

2-13
The sensitivity does not look too bad in the sense that a 10% error in flow
measurement gives ~12% error in desired level. Before making this
conclusion, however, one should check how well the operating FF
controller works for a change in w1 (e.g., ∆w1 = 0.4 kg/s).
2.12
a) Model of tank (normal operation):
1 2 3
dh
A w w w
dt
ρ = + − (Below the leak point)
2
2
(2)
3.14 m
4
A
π
π
= = =
(800)(3.14) 20
200
100
120 =
−
+
=
dt
dh
20
0.007962 m/min
(800)(3.14)
dh
dt
= =
Time to reach leak point (h = 1 m) = 125.6 min.
b) Model of tank with leak and 3
2
1 ,
, w
w
w constant:
4
ρ 20 20 ρ(0.025) 1
dh
A q h
dt
d
= − = − − = 20 − 20 1
−
h , h ≥ 1
To check for overflow, one can simply find the level hm at which dh/dt =
0. That is the maximum value of level when no overflow occurs.
0 = 20 − 20 1
−
m
h or hm = 2 m
Thus, overflow does not occur for a leak occurring because hm < 2.25 m.

2-14
2.13
Model of process
Overall material balance:
3
2
1 w
w
w
dt
dh
AT −
+
=
ρ = h
C
w
w v
−
+ 2
1 (1)
Component:
3
3
2
2
1
1
3 )
(
x
w
x
w
x
w
dt
hx
d
AT −
+
=
ρ
3
3
2
2
1
1
3
3
x
w
x
w
x
w
dt
dh
x
A
dt
dx
h
A T
T −
+
=
ρ
+
ρ
Substituting for dh/dt (Eq. 1)
=
−
+
+
ρ )
( 3
2
1
3
3
w
w
w
x
dt
dx
h
AT 3
3
2
2
1
1 x
w
x
w
x
w −
+
)
(
)
( 3
2
2
3
1
1
3
x
x
w
x
x
w
dt
dx
h
AT −
+
−
=
ρ (2)
or [ ]
)
(
)
(
1
3
2
2
3
1
1
3
x
x
w
x
x
w
h
A
dt
dx
T
−
+
−
ρ
= (3)
a) At initial steady state ,
Kg/min
220
100
120
2
1
3 =
+
=
+
= w
w
w
Cv = 3
.
166
75
.
1
220
=
b) If x1 is suddenly changed from 0.5 to 0.6 without changing flowrates, then
level remains constant and Eq.3 can be solved analytically or numerically
to find the time to achieve 99% of the x3 response. From the material
balance, the final value of x3 = 0.555. Then,
[ ]
3
3 3
1
120(0.6 ) 100(0.5 )
(800)(1.75)
dx
x x
dt
= − + −
π

2-15
[ ]
3
1
(72 50) 220 )
(800)(1.75)
x
= + −
π
3
0.027738 0.050020x
= −
Integrating,
3
3
3
3 0
0.027738 0.050020
f
o
x t
x
dx
dt
x
=
−
∫ ∫
where x3o=0.5 and x3f =0.555 – (0.555)(0.01) = 0.549
Solving,
t = 47.42 min
c) If w1 is changed to 100 kg/min without changing any other input variables,
then x3 will not change and Eq. 1 can be solved to find the time to achieve
99% of the h response. From the material balance, the final value of the
tank level is h =1.446 m.
800π 100 100 v
dh
C h
dt
= + −
1
200 166 3
800
dh
. h
dt
 
= −
 
π
0 079577 0 066169
. . h
= −
where ho=1.75 and hf =1.446 + (1.446)(0.01) = 1.460
By using the MATLAB command ode45 ,
t = 122.79 min
Numerical solution of the ode is shown in Fig. S2.13

2-16
Figure S2.13. Numerical solution of the ode for part c)
d) In this case, both h and x3 will be changing functions of time. Therefore,
both Eqs. 1 and 3 will have to be solved simultaneously. Since
concentration does not appear in Eq. 1, we would anticipate no effect on
the h response.
2.14
a) The dynamic model for the chemostat is given by:
Cells: FX
Vr
dt
dX
V g −
= or (1)
Product: FP
Vr
dt
dP
V p −
= or P
V
F
r
dt
dP
p 





−
= (2)
Substrate: )
( S
S
F
dt
dS
V f −
=
/
1
g
X S
Vr
Y
−
or
P
S
P
g
S
X
r
Y
r
Y /
/
1
1
−
− (3)
b) At steady state,
0 50 100 150 200 250 300
1.4
1.5
1.6
1.7
1.8
time (min)
h(m)
X
V
F
r
dt
dX
g 





−
=
)
( S
S
V
F
dt
dS
f −






=

2-17
0
=
dt
dX
∴ DX
rg =
then,
X DX
µ = ∴ D = µ (4)
A simple feedback strategy can be implemented where the growth rate
is controlled by manipulating the mass flow rate, F, so that F/V stays
constant.
c) Washout occurs if dX/dt is negative for an extended period of time;
that is,
0
<
− DX
rg or D µ
>
Thus, if D µ
> the cells will be washed out.
d) At steady state, the dynamic model given by Eqs. 1, 2 and 3 becomes:
0 = rg - DX DX = rg (5)
0 = rp - DP DP = rp (6)
0 = 𝐷𝐷�𝑆𝑆𝑓𝑓 − 𝑆𝑆� −
1
𝑌𝑌𝑋𝑋/𝑆𝑆
𝑟𝑟
𝑔𝑔 (7)
From Eq. 5,
g
r
DX = (8)
From Eq. 7
Substituting Eq. 9 into Eq. 8,
From Eq. 4
max
S
DK
S
D
m
=
−
f
S
X D (10)
S
S
Y
DX / )
( −
=
f
S
X
g D
S
S
Y
r (9)
/ )
( −
=

2-18
Substituting these two equations into Eq. 10,
/
max
S
X S f
DK
DX Y S D
D
m
 
= −
 
−
 
(11)
For Yx/s = 0.5, Sf = 10, Ks = 1, X = 2.75, μmax = 0.2, the following plot can be
generated based on Eq. 11.
Figure S2.14. Steady-state cell production rate DX as a function of dilution rate D.
From Figure S2.14, washout occurs at D = 0.18 h-1
while the maximum
production occurs at D = 0.14 h-1
. Notice that maximum and washout points
are dangerously close to each other, so special care must be taken when
increasing cell productivity by increasing the dilution rate.

2-19
2.15
a) We can assume that ρ and h are approximately constant. The dynamic
model is given by:
(1)
Notice that:
∴
dt
dV
dt
dM
ρ
= (2)
h
r
V 2
π
= ∴
dt
dr
A
dt
dr
rh
dt
dV
=
π
= )
2
( (3)
Substituting (3) into (2) and then into (1),
s
kAc
dt
dr
A =
r
− ∴ s
kc
dt
dr
=
r
−
Integrating,
0
ρ
o
r t
s
r
kc
dr dt
= −
∫ ∫ ∴ t
kc
r
t
r s
o
r
−
=
)
( (4)
Finally,
2
hr
V
M rπ
=
r
=
then
b) The time required for the pill radius r to be reduced by 90% is given by
Eq. 4:
t
kc
r
r s
o
o
r
−
=
1
.
0 ∴ 54
)
5
.
0
)(
016
.
0
(
)
2
.
1
)(
4
.
0
)(
9
.
0
(
9
.
0
=
=
ρ
=
s
o
kc
ρ
t min
Therefore, 54min.
t =
s
d kAc
dt
dM
r =
−
=
V
M ρ
=
2
)
( 







ρ
−
ρπ
= t
kc
r
h
t
M s
o

2-20
2.16
For V = constant and F = 0, the simplified dynamic model is:
X
S
K
S
r
dt
dX
s
g
+
µ
=
= µax
X
S
K
S
Y
r
dt
dP
s
X
P
p
+
µ
=
= µax
/
P
X
P
g
S
X
r
Y
r
Y
dt
dS
/
/
1
1
−
−
=
Substituting numerical values:
S
SX
dt
dX
+
=
1
2
.
0
S
SX
dt
dP
+
=
1
)
2
.
0
)(
2
.
0
(






−
−
+
=
1
.
0
2
.
0
5
.
0
1
1
2
.
0
S
SX
dt
dS
By using MATLAB, this system of differential equations can be solved. The time
to achieve a 90% conversion of S is t = 22.15 h.
Figure S2.16. Fed-batch bioreactor dynamic behavior.

2-21
2.17
(a) Using a simple volume balance, for the system when the drain is closed
(q = 0)
𝐴𝐴
𝑑𝑑ℎ
𝑑𝑑𝑑𝑑
= 𝑞𝑞1 (1)
Solving this ODE with the given initial condition gives a height that is increasing
at a rate of 0.25 ft/min.
So the height in this time range will look like:
(b) the drain is opened for 15 mins; assume a time constant in a linear
transfer function of 3 mins, so a steady state is essentially reached. (3 < t < 18).
Assume that the process will return to its previous steady state in an exponential
manner, reaching 63.2% of the response in three minutes.

2-22
(c) the inflow rate is doubled for 6 minutes (18 < t < 24)
The height should rise exponentially towards a new steady state value double that
of the steady state value in part b), but it should be apparent that the height does
not reach this new steady state value at t = 24 min.. The new steady state would
be 1 ft.
(d) the inflow rate is returned to its original value for 16 minutes (24 < t < 40)

2-23
The graph should show an exponential decrease to the previous steady state of 0.5
ft. The initial value should coincide with the final value from part (c).
Putting all the graphs together would look like this:

2-24
2.18
Parameters (fixed by design process): m, C, me, Ce, he, Ae.
CVs: T and Te.
Input variables (disturbance): w, Ti. Input variables (manipulated): Q.
Degrees of freedom = (11-6) (number of variables) – 2 (number of equations) = 3
The three input variables (w, Ti, Q) are assigned and the resulting system has zero
degrees of freedom.

2-25
2.19
(a) First we simulate a step change in the vapor flow rate from 0.033 to 0.045
m3
/s.
The resulting plots of xD and xB are shown below.
Figure: Plot of xD, xB, and V versus time for a step change in V from 0.033 to 0.045
m3
/s.
By examining the resulting data, we can find the steady-state values of xD and xB
before and after the step change in V.
(b) Next we simulate a step change in the feed composition (zF) from 0.5 to 0.55.
Note that the vapor flow rate, V, is still set at 0.045 m3
/s.
Start End Change
xD 0.85 0.73 -0.12
xB 0.15 0.0050 -0.145

2-26
Figure: Plot of xD, xB, and zF versus time for a step change in zF from 0.5 to 0.55
By examining the resulting data, we can find the steady-state values of xD and xB
before and after the step change in zF.
(c) Increasing V causes xD and xB to decrease, while increasing zF causes both
xD and xB to increase. The magnitude of the effect is greater for changing V
than for changing zF. When changing V, xB changes more quickly than xD.
Start End Change
xD 0.73 0.80 +0.066
xB 0.0050 0.0068 +0.0018

2-27
2.20
(a) First we simulate a step change in the Fuel Gas Purity (FG_pur) from 1 to
0.95.
The resulting plots of Oxygen Exit Concentration (C_O2) and Hydrocarbon
Outlet Temperature (T_HC) are shown below.
Figure: Plot of C_O2, T_HC, and FG_pur versus time for a step change in FG_pur from 1
to 0.95.
By examining the resulting data, we can find the steady-state values of C_O2 and
T_HC before and after the step change in FG_pur.
(b) Next we simulate a step change in the Hydrocarbon Flow Rate (F_HC_sp)
from 0.035 to 0.0385. Note that the Fuel Gas Purity, FG_pur, is still set at
0.95.
Start End Change
C_O2 0.92 1.06 0.14
T_HC 609 595 -14

2-28
Figure: Plot of C_O2, T_HC, and F_HC_sp versus time for a step change in F_HC_sp
from 0.035 to 0.0385.
By examining the resulting data, we can find the steady-state values of C_O2 and
T_HC before and after the step change in F_HC_sp.
(c) Decreasing FG_pur causes C_O2 to increase, while T_HC decreases.
Increasing F_HC_sp causes T_HC to decrease while C_O2 stays the same.
The change in T_HC occurs more quickly when changing F_HC_sp versus
changing FG_pur.
Start End Change
C_O2 1.06 1.06 0
T_HC 595 572 -23

2-29
2.21
The key to this problem is solving the mass balance of the tank in each part.
Mass balance:
( ) i o
d
Ah q q
dt
ρ ρ ρ
= −
- ρ (density) and A (tank cross-sectional area) are constants, therefore:
i o
dh
A q q
dt
= −
- The problem specifies o
q is linearly related to the tank height
1
o
q h
R
=
1
i
dh
A q h
dt R
= −
- Next, we can obtain R (valve constant) from the steady state information in the
problem
0 at steady state
dh
dt
=
1
0 i
q h
R
= −
1
0 2 (1)
R
= −
2
1 ft
2 0.5
min
R
R
∴ = =
- In addition, we can find that

2-30
( )
1
4 2
2
AR
τ
 
= = =
 
 
min
Part a
i o
dh
A q q
dt
= − (Mass Balance)
4 2
dh
dt
= (Separable ODE)
1
2
dh dt
=
∫ ∫
1
( ) (0) 1
2
h t t C h
=
+ =
1
( ) 1 0 3
2
h t t t
= + ≤ <

2-31
Part b
1
i
dh
A q h
dt R
= − (Mass Balance)
4 2 2
dh
h
dt
= −
1 1
2 2
dh
h
dt
+ = (Solution by integrating factor = / 2
t
e )
t/ 2 / 2
1
( )
2
t
d e h e dt
=
∫ ∫
/2 /2
1 (3) 2.5
t t
he e c h
=
+ =
/ 2
1 t
h ce−
= +
3/ 2
2.5 1 ce−
= +
3/ 2
1.5
c e
=
( 3)/ 2
( ) 1 (1.5) 3 18
t
h t e t
− −
= + ≤ <
Part c
4 4 2
dh
h
dt
= − (Mass balance)
1
1
2
dh
h
dt
+ = (Solution by integrating factor)
/ 2 / 2
( ) 1 (18) 1
t t
d e h e dt h
= =
∫ ∫
- Method is same as part b.
( 18)/ 2
( ) 2 18 33
t
h t e t
− −
= − ≤ <

2-32
Part d
Same as part b with h (33) = 2
( 33)/2
( ) 1 33 50
t
h t e t
− −
= + ≤ ≤

2-33
2.22
To solve the problem, we start by writing the mass balance for each tank 1-4.
To write the mass balance for each tank, we start with the most general form,
where the change in mass in the tank over time is equal to the mass flowing into
the tank minus the mass flowing out of the tank. The general form of the
equations are shown below, where i represents the tank number (1, 2, 3, 4). The
mass can be written as the density multiplied by the tank volume, and the mass
flow rates can be written as the density multiplied by the volumetric flow rate.
, ,
( )
i
in i out i
d V
q q
dt
ρ
ρ ρ
= −
With density assumed constant over time, it can be pulled out of the derivative.
Also, we write the volume of the tank as the height of liquid in the tank, hi,
multiplied by the cross-sectional tank area, Ai.
, ,
, ,
( )
( )
i i
in i out i
i i
in i out i
Ad h
q q
dt
Ad h
q q
dt
ρ
ρ ρ
= −
= −
The flow exiting each tank through the bottom can be written as:
,
exit i i i
q C h
=
Where Ci is the proportionality constant for each tank.
Results:
a) The final equations for the height of liquid in each tank are shown below.

2-34
3
1 1 1
1 3 1
1 1 1
2 2 4 2
2 4 2
2 2 2
3 3 2
3 2
3 3
(1)
(2)
1
C
dh C
h h F
dt A A A
dh C C
h h F
dt A A A
dh C ( )
h F
dt A A
γ
γ
γ
=
− + +
=
− + +
−
=
− +
4 4 1
4 1
4 4
(3)
1
(4)
dh C ( )
h F
dt A A
γ
−
=
− +
b) Now we can substitute 1 2 0.5
γ γ
= =
3
1 1
1 3 1
1 1 1
2 2 4
2 4 2
2 2 2
3 3
3 2
3 3
0 5
0 5
0 5
C
dh C .
h h F
dt A A A
dh C C .
h h F
dt A A A
dh C .
h F
dt A A
=
− + +
=
− + +
=
− +
4 4
4 1
4 4
0 5
dh C .
h F
dt A A
=
− +
The differential equations for the tank heights are coupled, so the heights
cannot be solved for or controlled independently. F1 and F2 can be used to
control h3 and h4 independently, but h1 and h2 will be affected in an
uncontrolled manner.
c) In the extreme case where 1 2 0
γ γ
= = , we get:
3
1 1
1 3
1 1
2 2 4
2 4
2 2
3 3 2
3
3 3
4 4 1
4
4 4
C
dh C
h h
dt A A
dh C C
h h
dt A A
dh C F
h
dt A A
dh C F
h
dt A A
=
− +
=
− +
=
− +
=
− +
These equations make sense with the process diagram because now F1 and
F2 only affect tanks h3 and h4 directly (they no longer flow into tanks 1
and 2 at all). However, F1 and F2 indirectly affect tanks 1 and 2 through h3
and h4.

Process Dynamics and Control: Solutions for Problems (4th Edition) Dale Seborg, Thomas Edgar

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Process Dynamics and Control: Solutions for Problems (4th Edition) Dale Seborg, Thomas Edgar