I am trying to use GEKKO on PYTHON to control a cstr. The CVS are the temperature and the level of the tank - gekko

Question

Attached is the code I wrote: When it runs, the level controlled variable is not tracking its setpoint.
On the other hand, the Temperature controlled variable is tracking its setpoint very well. I am using manipulating the cooling temperature and inlet flow rate. I am trying to control the level of the tank, temperature and concentration.
import numpy as np
import matplotlib.pyplot as plt
from scipy.integrate import odeint
from gekko import GEKKO
# Steady State Initial Condition
u1_ss = 300.0
u2_ss=100.0
Ca_ss = 0.87725294
T_ss = 324.47544313
h_ss=75.82018806
# Feed Temperature (K)
Tf = 350
# Feed Concentration (mol/m^3)
Caf = 1
# Steady State Initial Conditions for the States
x0 = np.empty(2)
x0[0] = Ca_ss
x0[1] = T_ss
p0=np.empty(1)
p0[0]=h_ss
#%% GEKKO nonlinear MPC
m = GEKKO(remote=False)
m.time = [0,0.02,0.04,0.06,0.08,0.1,0.12,0.15,0.2]
c1=10
Ac=400.0
# Volume of CSTR (m^3)
V = 100
# Density of A-B Mixture (kg/m^3)
rho = 1000
# Heat capacity of A-B Mixture (J/kg-K)
Cp = 0.239
# Heat of reaction for A->B (J/mol)
mdelH = 5e4
# E - Activation energy in the Arrhenius Equation (J/mol)
# R - Universal Gas Constant = 8.31451 J/mol-K
EoverR = 8750
# Pre-exponential factor (1/sec)
k0 = 7.2e10
# U - Overall Heat Transfer Coefficient (W/m^2-K)
# A - Area - this value is specific for the U calculation (m^2)
UA = 5e4
# initial conditions
Tc0 = 300
T0 = 324.47544313
Ca0 = 0.87725294
h0=75.82018806
q0=100.0
tau = m.Const(value=0.5)
Kp = m.Const(value=1)
m.Tc = m.MV(value=Tc0,lb=250,ub=350)
m.T = m.CV(value=T_ss)
m.rA = m.Var(value=0)
m.Ca = m.CV(value=Ca_ss,lb=0,ub=1)
m.h=m.CV(value=h_ss)
m.q=m.MV(value=q0,lb=0,ub=1000)
m.Equation(m.rA == k0*m.exp(-EoverR/m.T)*m.Ca)
m.Equation(m.T.dt() == m.q/V*(Tf - m.T) \
+ mdelH/(rho*Cp)*m.rA \
+ UA/V/rho/Cp*(m.Tc-m.T))
m.Equation(m.Ca.dt() == (m.q)/V*(Caf - m.Ca) - m.rA)
m.Equation(m.h.dt()==(m.q-c1*pow(m.h,0.5))/Ac)
#MV tuning
m.Tc.STATUS = 1
m.Tc.FSTATUS = 0
m.Tc.DMAX = 100
m.Tc.DMAXHI = 20
m.Tc.DMAXLO = -100
m.q.STATUS = 1
m.q.FSTATUS = 0
m.q.DMAX = 10
#CV tuning
m.T.STATUS = 1
m.T.FSTATUS = 1
m.T.TR_INIT = 1
m.T.TAU = 1.0
DT = 0.5 # deadband
m.h.STATUS = 1
m.h.FSTATUS = 1
m.h.TR_INIT = 1
m.h.TAU = 1.0
m.Ca.STATUS = 1
m.Ca.FSTATUS = 0 # no measurement
m.Ca.TR_INIT = 0
m.options.CV_TYPE = 1
m.options.IMODE = 6
m.options.SOLVER = 3
# define CSTR model
def cstr(x,t,u1,u2,Tf,Caf):
# Inputs (3):
# Temperature of cooling jacket (K)
Tc = u1
q=u2
# Tf = Feed Temperature (K)
# Caf = Feed Concentration (mol/m^3)
# States (2):
# Concentration of A in CSTR (mol/m^3)
Ca = x[0]
# Temperature in CSTR (K)
T = x[1]
# Parameters:
# Volume of CSTR (m^3)
V = 100
# Density of A-B Mixture (kg/m^3)
rho = 1000
# Heat capacity of A-B Mixture (J/kg-K)
Cp = 0.239
# Heat of reaction for A->B (J/mol)
mdelH = 5e4
# E - Activation energy in the Arrhenius Equation (J/mol)
# R - Universal Gas Constant = 8.31451 J/mol-K
EoverR = 8750
# Pre-exponential factor (1/sec)
k0 = 7.2e10
# U - Overall Heat Transfer Coefficient (W/m^2-K)
# A - Area - this value is specific for the U calculation (m^2)
UA = 5e4
# reaction rate
rA = k0*np.exp(-EoverR/T)*Ca
# Calculate concentration derivative
dCadt = q/V*(Caf - Ca) - rA
# Calculate temperature derivative
dTdt = q/V*(Tf - T) \
+ mdelH/(rho*Cp)*rA \
+ UA/V/rho/Cp*(Tc-T)
# Return xdot:
xdot = np.zeros(2)
xdot[0] = dCadt
xdot[1] = dTdt
return xdot
def tank(p,t,u2,Ac):
q=u2
h=p[0]
dhdt=(q-c1*pow(h,0.5))/Ac
if p[0]>=300 and dhdt>0:
dhdt = 0
return dhdt
# Time Interval (min)
t = np.linspace(0,10,410)
# Store results for plotting
Ca = np.ones(len(t)) * Ca_ss
T = np.ones(len(t)) * T_ss
Tsp=np.ones(len(t))*T_ss
hsp=np.ones(len(t))*h_ss
h=np.ones(len(t))*h_ss
u1 = np.ones(len(t)) * u1_ss
u2 = np.ones(len(t)) * u2_ss
# Set point steps
Tsp[0:100] = 330.0
Tsp[100:200] = 350.0
hsp[200:300] = 150.0
hsp[300:] = 190.0
# Create plot
plt.figure(figsize=(10,7))
plt.ion()
plt.show()
# Simulate CSTR
for i in range(len(t)-1):
ts = [t[i],t[i+1]]
y = odeint(cstr,x0,ts,args=(u1[i+1],u2[i+1],Tf,Caf))
y1=odeint(tank,p0,ts,args=(u2[i+1],Ac))
Ca[i+1] = y[-1][0]
T[i+1] = y[-1][1]
h[i+1]=y1[-1][0]
# insert measurement
m.T.MEAS = T[i+1]
m.h.MEAS= h[i+1]
# solve MPC
m.solve(disp=True)
m.T.SPHI = Tsp[i+1] + DT
m.T.SPLO = Tsp[i+1] - DT
m.h.SPHI = hsp[i+1] + DT
m.h.SPLO = hsp[i+1] - DT
# retrieve new Tc value
u1[i+1] = m.Tc.NEWVAL
u2[i+1]= m.q.NEWVAL
# update initial conditions
x0[0] = Ca[i+1]
x0[1] = T[i+1]
p0[0]=h[i+1]
plt.clf()
# Plot the results
plt.subplot(5,1,1)
plt.plot(t[0:i],u1[0:i],'b--',linewidth=3)
plt.ylabel('Cooling T (K)')
plt.legend(['Jacket Temperature'],loc='best')
plt.subplot(5,1,2)
plt.plot(t[0:i],u2[0:i],'g--')
plt.xlabel('time')
plt.ylabel('flow in')
plt.subplot(5,1,3)
plt.plot(t[0:i],Ca[0:i],'r-',linewidth=3)
plt.ylabel('Ca (mol/L)')
plt.legend(['Reactor Concentration'],loc='best')
plt.subplot(5,1,4)
plt.plot(t[0:i],Tsp[0:i],'r-',linewidth=3,label=r'$T_{sp}$')
plt.plot(t[0:i],T[0:i],'k.-',linewidth=3,label=r'$T_{meas}$')
plt.ylabel('T (K)')
plt.xlabel('Time (min)')
plt.legend(loc='best')
plt.subplot(5,1,5)
plt.plot(t[0:i],hsp[0:i],'g--',linewidth=3,label=r'$h_{sp}$')
plt.plot(t[0:i],h[0:i],'k.-',linewidth=3,label=r'$h_{meas}$')
plt.xlabel('time')
plt.ylabel('tank level')
plt.legend(loc='best')
plt.draw()
plt.pause(0.01)

Answer 1

One problem is that the function pow is not supported by Gekko and is evaluating that part to a constant. Here is a modified version of your equation that should work better:
m.Equation(m.h.dt()==(m.q-c1*m.h**0.5)/Ac)
One other issue is that your similar is broken into two parts and should be one model:
def tank(p,t,u2,Ac):
q=u2
h=p[0]
dhdt=(q-c1*pow(h,0.5))/Ac
if p[0]>=300 and dhdt>0:
dhdt = 0
return dhdt
You should add a third state to your simulator
# Return xdot:
xdot = np.zeros(3)
xdot[0] = dCadt
xdot[1] = dTdt
xdot[2] = dhdt
return xdot
When you have a variable height, the volume is changing so you can't assume that it is constant in the other equations. You'll need to modify your energy balance and species balance as shown in the material on balance equations.