Typical process units control Prof. Cesar de Prada Dpt. Systems Engineering and Automatic Control University of Valladolid, Spain
[email protected]
Outline Chemical
Reactors
Distillation
columns
Boilers Compressors Control
structures design methodology
Examples
Outline Chemical
Reactors
Distillation
columns
Boilers Compressors Control
structures design methodology
Examples
Reactor control cA
A→B 0 = Fc Ai
− Fc A − Vke
0 = xF − Vke x 1− x
=
V F
ke
−E
−E
RT
−E
RT
=
(1 − x )c Ai
c A using conversion x :
(1 − x )
RT
A
u
Reactant
TT
Tr
T Reactor
Coolant LT
AT
Product B, A
Ti Coolant Jacket
Reactor Control Temp TC
Reactant TC
TT
TT
Ti
FC FT
Tr
T Reactor
FC
FT
q
Coolant AT LT
LC
Product
AC
Comp.
Reactor Control Temp TC
Reactant TC
TT
TT
FC FT
Tr
FC
T Reactor
Coolant
Ti q
AT
Comp.
AC
LT
LC
Product
FT
Coolant
Reactor Control Reactant B
Temp
A+B→C
TC FC
FT FF
TC
TT
TT
Ti
FC FT
Tr
T Reactor
FC
q
FT
Reactant A
Coolant AT LT
LC
Product
AC
Comp.
Reactor control React B
Temp TC FC
TC
TT
FT
React A
TT
FC
FT
Ti q
Tr
FC
T
FT
FF
Reactor
Coolant AT LT
LC
Product C
AC
Comp.
Reactor Control Temp FT
AT
Reactant B
TC FC
TC
TT
React A
TT
FC FT
FY
Tr
T Reactor
Ti q
FC
FT
Coolant AT
Strong changes in the composition of B
AC
LT
Comp. LC
Product
FF
Reactor Control Reactant B
Temp TC FC
TC
TT
React A
TT
Ti
FC FT
FT
Tr
T Reactor
FC
FT
FF
q
Coolant
HL AT
Non measurable strong changes in the composition of A or B
AC
LL
LT
Comp. LC
Product
Reactor: constraining production Temp
React B TC FC
TC
95%
React A
TT
FC VPC FT
Ti q
TT
FT
Tr
FT
FC
T Reactor
FF
LS
Coolant
% AT LT
LC
Product C
AC
Comp.
Loop interaction Temp TC
Reactant TC
TT
TT
Ti
FC FT
Tr
T Reactor
FC
FT
q
Coolant AT LT
LC
Product
AC
Comp.
Multivariable control, MPC SPtemp
SPConc.
u
u
2
1
MPC
FC FT TT
AT
Feed
FC
FT
Reactor Coolant Product
Bed reactor temperature control
TT
TC
TC
TT
Inert product
FC
TT
Feed
TC
FT
Destillation Column Coolant PT
PC
LT
R
LC
Feed F
FT
FC
FC
V
Steam
LT
LC
B
FT
Inventory and pressure basic control
D
Alternatives Coolant PT
PC
R
LC
LT
D
Feed F
FT
LT
LC
V Steam FT
B
FC
FC
With high R/D or V/B inventory control should be implemented using R and V
Destillation Column Coolant PT
PC
TT LT
R F
Feed
FC
TC TC
FT
Steam
LC
FC
TT
V
LT FT FC
B
LC
FT
D
Destillation column PT
PC
Refrigerant TT LT
R
LC
F
Feed
FC
TC
FT
TC
FT
Steam
FC
Pressure control with a partial condenser
TT
V
LT FT
B
FC
LC
D
Feedforward F Coolant PT
PC
TT LT
R F
Feed FT
FY
TC TC
FT
Steam
LC
FC
TT
V
LT
LC
B
FC
FT
D
Feedforward V/F Coolant PT
PC
TT LT
R F
Feed FT
LC
FY
TC
V/F
FC
FT
D
TC
FT
Steam
FC
V
TT
V
LT
LC
B
Output of the TC is asumed to be proportional toV/F, so that, if F changes, V is adjusted automaticaly
Feedforward + cascade Coolant PT
F
Feed FT
PC
TT LT
R
FY
LC
TC
Q
TC
QC
FC
FT
FaceΔT FC
FT
ce
TT
V
LT
TDT
Heating liquid
LC
B
Energy changes in the heating fluid are compensated
D
Superfractionator column Coolant Components with low relative volatility Small temperature differences between head and bottom
PT
TT LT
R F
Feed FT
FT
Steam
LC
FC
FC
TT
V
LT
LC
B
FT
D
Many plates High purity in D High R/D High R y V Slow response Conventional schemes do not work well
Superfractionator column Coolant
V Aim: Keep R/V or B/F constant
PT PC TT LT
R
FT
V=R+D
F
Feed
R
+
-
FFY
FFY
D/V
FY
D FC
LC
FY
LC
FT
R/D AC
FC
FT
AT FT
FC
V
LT
TT
D R FT
V
B
B
FC
B/F
FFY
F
=
R D
D
1+ R D
V
=
1 1+ R D
Steam Boiler Steam LC
PT
LT
Stack
FT
Air
FC
Burner
Gas FF
FT
FC
Furnace
Smoke
PC
Steam Boiler
Steam PT
LC
PC
LT
Smoke FT
Air
FC
AT
O2
AC
Gas
LL FF
FT
FC
Furnace HL
> <
Steam Boiler
Steam FT PT
Opacity of smoke
PC
LC
FT
FC
LT
Smoke FT
FC
Air Gas FF
O2
AT
AT
AC
HS
LL FT
FC
HL
> <
AT
CO
Security Fuel/Air
Burner Gas
Air
Centrifugal compressors
Control system
PC
wP
HP LP
Gas
PT
High pressure steam Compressor Turbine
PT
Low pressure steam
The turbine starts with the automátic valve then the regulation is made with the HP valve
Anti-surge Control PDT
Δp
Gas
Compressor
Turbine
Δp ω1
ω2
q
q
On the left hand side of the surge line, operation is unstable: if the flow q decreases, then Δp decreases too, which, in turn, decreases q
Anti-surge Control FC
FT PC
PT
Compute q ~ k Δp2 SC
ST PDT
Δp
Turbine
Gas
Compressor
q
Anti-surge Control A certain amount of gas is recirculated in order to maintain the flow through the compressor below the surge line
Unit / Department control Control
loops do not work in isolation. They should not disturb the operation of other control loops and must cooperate in fulfilling the overall aims PT PC LT LC FC FT FT FC LT LC
Methodology The number of automatic valves, other actuators, ... Can be considered as the number of degrees of freedom that can be used in order to maintain a given production level, product quality, security, etc. Order: 1 Choose first those loops that fix production level 2 Then, design the security and quality loops 3 Inventory control loops 4 Check that the balances (mass, energy) can be satisfied 5 The remaining degrees of freedom can be used to optimize the plant behaviour 6 Validate the design using dynamic simulation
Interactions Plant Wide Control XC
YC
Only one automatic valve is allowed in a pipe
Interactions XC XC
XT
XT
One variable should be controlled using only one controller
Interactions
FC
FC
Process 1
Process 2 LC
Level control loops must operate in the same direction (backward, forward)
LC
Interactions
LC FC
FC
If a product is recirculated, a flow control loop must be placed somewhere in the loop
Reactors in series C A
B
Reactor 1
Reactor 2 Refrigerant
Steam
D Product A reacts in the endothermic reactor 1 providing a product B to the exothermic reactor 2. Here, B reacts with C in order to obtain the desired product D. A and C are obtained from two storage tanks. The level of both reactors depends on the inputs and outputs flows while the speed of reaction is quite sensitive to their temperature. Also it is known that steam flow is affected by big changes in the supply pressure. A control structure should be drawn that it is able of maintaining with precision D product concentration as well as other possible requirements.
LC
C
A
LC
LT FF
FC
FT LT
TC
TT
PC
PT
FT
B TT
TC
Refrigerant
Reactor 1
Reactor 2
Steam AT
LT
LC
LT
AT
LC
AC
FT
AC
D
FC
LC
C
A
LC
LT FF
FC
FT LT
TC
FT
TT
FC
B PC
TT
PT
TC
Refrigerant
Reactor 1
Reactor 2
Steam AT AC
LT
LC
AT
LT
AC
LC
D
Double effect evaporator
Condenser
Steam
Fresh juice
Syrup
The schematic of the figure shows a double effect evaporator that processes a fresh juice in order to convert it into a syrup. The evaporator is heated using steam that comes from a previous process that experience changes in pressure and cannot be manipulated. The steam flowing out of the second effect goes to a condenser that can experiment some changes too. A control system must operate the process being the main aims maintaining a desired production of syrup of a given density in spite of the disturbances acting on the process and taking into account other possible control aims that must be implemented.
Double effect evaporator
PC
Condenser
PT
PY
PT
Steam DC
LC
LT
LC
LT FT DT
Fresh juice
Syrup
FC
Pulp Dryer pulp
Smoke gases
air
Furnace
Gas Dried pulp
The schematic represents a pulp dryer in which a certain flow of wet pulp must be dried up to a specified value that must be maintained in spite of possible disturbances. The amount of pulp entering the dryer is proportional to the speed of the belt conveyor, which is fed from an storage tank and must be fixed according to the production needs. The dryer has a combustion chamber (where a mixed of natural gas and air is burned in order to produce a flow of hot gases) and a main body, which is a cylinder rotating at constant speed. The pulp, push by the hot gases, moves along the cylinder and, at the same time, loses water by evaporation. Smoke goes out by a chimney while the dried pulp leaves the dryer at the end of the cylinder. For security reasons, it is desired that the temperature at the furnace output is below a given upper limit. It is also known that the feeding pressure of the natural gas changes frequently. Design a control structure that is able to cope with the above mentioned requirements.
