seborg - chapter2

1234567898

1234567898 5

2.1

a)

Overall mass balance: d (ρV ) = w1 + w2 − w3 dt

(1)

Energy balance:

C

d 1V 2T3 − Tref 3  dt

= w1C (T1 − Tref ) + w2C (T2 − Tref )

− w3C (T3 − Tref )

(2)

Because ρ = constant and V = V = constant, Eq. 1 becomes: w3 = w1 + w2 b)

(3)

From Eq. 2, substituting Eq. 3

1CV

d (T3 − Tref ) dt

= 1CV

dT3 = w1C 2T1 − Tref 3 + w2C 2T2 − Tref 3 dt − ( w1 + w2 ) C (T3 − Tref )

(4)

Constants C and Tref can be cancelled: ρV

dT3 = w1T1 + w2T2 − ( w1 + w2 )T3 dt

The simplified model now consists only of Eq. 5. Degrees of freedom for the simplified model: Parameters : ρ, V Solution Manual for Process Dynamics and Control, 2nd edition, Copyright © 2004 by Dale E. Seborg, Thomas F. Edgar and Duncan A. Mellichamp.

2-1

(5)

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: Cp

d 1V 2T − Tref 3  dt

= wC p (Ti − Tref ) − wC p (T − Tref ) − UAs (T − Ta ) + Q

Simplifying 1VC p

dT = wC p Ti − wC p T − UAs 2T − Ta 3 + Q dt dT 1VC p = wC p 2Ti − T 3 − UAs 2T − Ta 3 + 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

a)

Mass Balances:

2-2

ρA1

dh1 = w1 − w2 − w3 dt

(1)

dh2 = w2 dt

(2)

ρA2

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. w2 =

Then

P1 − P2 ρg = (h1 − h2 ) R2 g c R2

(3)

P1 − Pa ρg = h1 R3 g c R3

(4)

w3 = 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 Assume constant liquid density, ρ . The mass balance for the tank is d (ρAh + m g ) dt

= ρ( q i − q )

Because ρ, A, and mg are constant, this equation becomes

2-3

A

dh = qi − q dt

(1)

The square-root relationship for flow through the control valve is   ρgh q = C v  Pg + − Pa  gc  

1/ 2

(2)

From the ideal gas law,

Pg =

(m g / M ) RT

(3)

A( H − h)

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

 (mg / M ) RT ρ gh  dh + − Pa  A = qi − Cv  dt gc  A( H − h) 

(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

a)

For linear valve flow characteristics,

Pd − P1 P − P2 , wb = 1 , Ra Rb Mass balances for the surge tanks wa =

dm1 = wa − wb , dt

wc =

P2 − Pf Rc

dm2 = wb − wc dt

where m1 and m2 are the masses of gas in surge tanks 1 and 2, respectively. If the ideal gas law holds, then

2-4

(1)

(2)

P1V1 =

m1 RT1 , M

P2V2 =

m2 RT2 M

(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, V1M dP1 V2 M dP2 = wa − wb and = wb − wc RT1 dt RT2 dt

(4)

The dynamic model consists of Eqs. 1 and 4. b)

For adiabatic operation, Eq. 3 is replaced by γ

V  V P1  1  = P2  2  m1   m2

or

 PV γ m1 =  1 1  C

   

γ

  = C , a constant 

1/ γ

and

 PV γ m2 =  2 2  C

   

(5)

1/ γ

(6)

Substituting Eq. 6 into Eq. 2 gives, 1 γ

 V1 γ   C 

   

1/ γ

1 γ

 V2 γ   C 

   

1/ γ

P1

(1− γ ) / γ

dP1 = wa − wb dt

(1− γ ) / γ

dP2 = wb − wc dt

P2

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: 2-5

Overall balance (No accumulation of mass): 0 = ρq − ρq1

thus

q1 = q

(1)

Energy balance (No change in volume): V11C

dT1 = 1qC 2Ti − T1 3 − UA2T1 − T2 3 dt

(2)

Compartment 2: Overall balance: 0 = ρq1 − ρq2

thus

q2 = q1= q

(3)

Energy balance: V21C

b)

dT2 = 1qC 2T1 − T2 3 + UA2T1 − T2 3 − U c Ac 2T2 − Tc 3 dt

(4)

Eight parameters: ρ, V1, V2, C, U, A, Uc, Ac Five variables: Ti, T1, T2, q, Tc Two equations: (2) and (4) 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 Let the volume of the top tank be γV, and assume that γ is constant. Then, an overall mass balance for either of the two tanks indicates that the flow rate of the stream from the top tank to the bottom tank is equal to q +qR. Because the two tanks are perfectly stirred, cT2 = cT.

2-6

Component balance for chemical tracer over top tank: 4V

dcT 1 = qcTi + qR cT − 2 q + qR 3cT 1 dt

(1)

Component balance on bottom tank: (1 − 43V

dcT 2 = 2q + qR 3cT 1 − qR cT − qcT dt

or (1 − 43V

dcT = 2 q + qR 32cT 1 − cT 3 dt

(2)

Eqs. 1 and 2 constitute the model relating the outflow concentration, cT, to inflow concentration, cTi. Describing the full-scale reactor in the form of two separate tanks has introduced two new parameters into the analysis, qR and γ. Hence, these parameters will have to be obtained from physical experiments.

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: ρ

dV = q F − q = 0 ; thus q = qF dt

Energy balance for tank: ρVC

dT 0.8 = q F ρC (TF − T ) − Kq J A(T − TJ ) dt

(1)

Energy balance for the jacket: ρ J VJ C J

dTJ dt

= q J ρ J C J (Ti − TJ ) + Kq J

2-7

0.8

A(T − TJ )

(2)

where A is the heat transfer area (in ft2) between the process liquid and the coolant. Eqs.1 and 2 comprise the dynamic model for the system.

2.9

Additional assumptions: i. The density ρ and the specific heat C of the process liquid are constant. ii. The temperature of steam Ts is uniform over the entire heat transfer area iii. Ts is a function of Ps , Ts = f(Ps) Mass balance for the tank: dV = qF − q dt Energy balance for the tank:

1C

d V (T − Tref )  dt

= qF 1C 2TF − Tref 3 − q1C 2T − Tref 3

(1)

(2)

+UA(Ts − T ) where: Tref is a constant reference temperature A is the heat transfer area Eq. 2 is simplified by substituting for (dV/dt) from Eq. 1, and replacing Ts by f(Ps), to give ρVC

dT = q F ρC (TF − T ) + UA[ f ( Ps ) − T ] dt

Then, Eqs. 1 and 3 constitute the dynamic model for the system.

2-8

(3)

2.10

Assume that the feed contains only A and B, and no C. Component balances for A, B, C over the reactor give. dc A = qi c Ai − qc A − Vk1e− E1 / RT c A dt

(1)

dcB = qi cBi − qcB + V (k1e − E1 / RT c A − k2e− E2 / RT cB ) dt

(2)

dcC = − qcC + Vk2e − E2 / RT cB dt

(3)

V

V

V

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:

d (Vc A M A S A + VcB M B S B + VcC M C SC ) T  dt

= ( qi c Ai M A S A + qi cBi M B S B ) (Ti − T )

−UA(T − Tc ) + (−∆H1 )Vk1e − E1 / RT c A + (−∆H 2 )Vk2 e− E2 / RT cB

(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: 1 j S jV j

dTc = 1 j S j qci 2Tci − Tc 3 + UA2T − Tc 3 dt

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.

2-9

(5)

2.11

Model (i) : Overall mass balance (w=constant= w ): d ( ρV ) dh = Aρ = w1 + w2 − w dt dt

(1)

A component balance: d (ρVx) = w1 − wx dt

or

Aρ

d (hx) = w1 − wx dt

(2)

Note that for Stream 2, x = 0 (pure B).

Model (ii) : Mass balance: d (1V 3 dh = Aρ = w1 + w2 − w dt dt

(3)

Component balance on component A: d (ρVx) = w1 − wx dt

or

Aρ

d (hx) = w1 − wx dt

(4)

2-10

2.12

a)

Note that the only conservation equation required to find h is an overall mass balance:

dm d (ρAh) dh = = ρA = w1 + w2 − w dt dt dt

Valve equation: w = C v′

ρg h = Cv h gc

where C v = C v′

ρg gc

(1)

(2) (3)

Substituting the valve equation into the mass balance, dh 1 = ( w1 + w2 − C v h ) dt ρA

(4)

Steady-state model : 0 = w1 + w2 − C v h b)

c)

Cv =

w1 + w2 h

=

2.0 + 1.2 3.2 kg/s = = 2.13 1/2 1.5 2.25 m

Feedforward control

2-11

(5)

Rearrange Eq. 5 to get the feedforward (FF) controller relation, w2 = C v hR − w1

where hR = 2.25 m

w2 = (2.13)(1.5) − w1 = 3.2 − w1

(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, w2 = 3.2 − 1.1w1 (flow transmitter 10% higher), w2 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 C v h = 2.13 h = 2.0 + 1.0 or

h=

3 = 1.408 and h = 1.983 m (instead of 2.25 m) 2.13

Error in desired level =

2.25 − 1.983 ×100% = 11.9% 2.25

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

2-12

conclusion, however, one should check how well the operating FF controller works for a change in w1 (e.g., ∆w1 = 0.4 kg/s).

2.13

a)

Model of tank (normal operation): dh = w1 + w2 − w3 dt π (2) 2 A= = π = 3.14 m 2 4

ρA

(800)(3.14)

(Below the leak point)

dh = 120 + 100 − 200 = 20 dt

20 dh = = 0.007962 m/min dt (800)(3.14) Time to reach leak point (h = 1 m) = 125.6 min.

b)

Model of tank with leak and w1 , w2 , w3 constant: 1A

dh = 56 − δ q4 = 56 − 12676583 h − 9 = 20 − 20 h − 1 , h ≥ 1 dt

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

hm − 1 or

hm = 2 m

Thus, overflow does not occur for a leak occurring because hm < 2.25 m. 2.14 Model of process Overall material balance:

2-13

ρAT

dh = w1 + w2 − w3 = w1 + w2 − C v h dt

(1)

Component: ρAT

d (hx3 ) = w1 x1 + w2 x2 − w3 x3 dt

ρAT h

dx3 dh + ρAT x3 = w1 x1 + w2 x2 − w3 x3 dt dt

Substituting for dh/dt (Eq. 1) ρAT h

dx3 + x3 ( w1 + w2 − w3 ) = w1 x1 + w2 x2 − w3 x3 dt

ρAT h

dx3 = w1 ( x1 − x3 ) + w2 ( x 2 − x3 ) dt

dx3 1 = [w1 ( x1 − x3 ) + w2 ( x2 − x3 )] dt ρAT h

or a)

(2)

(3)

At initial steady state ,

w3 = w1 + w2 = 120 + 100 = 220 Kg/min 220 Cv = = 166.3 1.75 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, dx3 1 = [120(0.6 − x3 ) + 100(0.5 − x3 )] dt (800)(1.75)π =

1 [ (72 + 50) − 220 x3 )] (800)(1.75)π

= 0.027738 − 0.050020x3 Integrating,

2-14

x3 f

∫

x3 o

t

dx3 = dt 0.027738 − 0.050020 x3 ∫0

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

dh = 100 + 100 − Cv h dt dh 1  = 200 − 166.3 h dt 800π 

 

= 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. S2.14 1.8

1.7 h(m) 1.6

1.5

1.4 0

50

100

150 200 time (min)

250

300

Figure S2.14. Numerical solution of the ode for part c) 2-15

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.

a)

The dynamic model for the chemostat is given by:

2.15

dX = Vrg − FX dt

or

dX F = rg −   X dt V 

(1)

Product: V

dP = Vrp − FP dt

or

dP F = rp −   P dt V 

(2)

Substrate:

V

Cells:

V

dS 1 1 = F (S f − S ) − Vrg − VrP dt YX / S YP / S

or 1 1 dS  F  rg − rP =  ( S f − S ) − YX / S YP / S dt  V 

b)

At steady state,

then,

dX =0 dt

∴

rg = DX

µX = DX

∴

µ=D

A simple feedback strategy can be implemented where the growth rate is controlled by manipulating the mass flow rate, F. c)

Washout occurs if dX/dt = 0 is negative for an extended period of time; that is, rg − DX < 0

or

µ