1. T&S Chapter 5, SS&L Chapter 20 Terry A. Ring University of Utah
Plant-wide Control
T&S Chapter 5, SS&L Chapter 20
Terry A. Ring
University of Utah

2. PROCESS DESIGN STAGES AND TOOLS
10 - Design and Control
DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

3. Instrumentation and Control Objectives

4. Control Valves Safety Aspects Types of Control Valves
Fail Open, Fail Closed
Types of Control Valves
Linear
Non-linear
Restricted open range (gain scheduled controller)

5. Pressure Control Loop Pressure Control Valve – fail closed
Is fail closed best?
Pressure Transducer – PT
Pieziolelectric, diaphragm displacement
Pressure Indicator Controller – PIC
PID, maybe gain scheduling if valve is non-linear
Gain scheduling for pressure relief above a given pressure.
Alarm High
Alarm Low

6. Level Control Loop pump Level Control Valve – fail open
Level Transducer – LT
DP cell, guided radar, conductivity
Level Indicator Controller – LIC
PID, maybe gain scheduling if valve is non-linear
Alarm High
Alarm Low
Pump may dead head
Interlock or gain schedule valve to never go to zero

7. Flow Control Centrifugal Pump Flow Control Valve – Fail ??
Ratio
Centrifugal Pump
Flow Control Valve – Fail ??
Flow Transducer
Orifice, venture, v-cone, many others
Flow Indicator Controller – FIC
PID, maybe gain scheduling if valve is non-linear
Note, check valve to prevent back flow and by-pass loop to prevent dead- heading pump
VFD can be costly
Missing Alarms on Pressure Indicator (PI)

8. Ratio Control Ratio Control Valve – FFV – fail Closed
May have two control valves – one for each stream
2 Flow Transducers
Orifice, venture, v-cone, many others
Flow Ratio Controller
PID with calculator for total flow(FT=FT1+FT2) to be held constant
CV1 SP=Ratio*SPLC
CV2 SP=(1-Ratio)*SPLC
Add
Pressure Relief

9. Temperature Control w Utility
Utility Control Valve – TV – Fail ??
Temperature Sensing Element (TE) and Temperature Transducer (TT) or Temperature Indicator (TI)
Temperature Indicator Controller
PID, maybe gain scheduling if valve is non-linear

10. Vaporizer Control Flow Control on Liquid and Level Control in HX
Pressure relief valve???

11. CSTR Contol System Utility Temperature Control Pressure Control
TT and FT feed signals to Flow control – Cascade Control
Valve Fail???
Pressure Control
PID, gain scheduling = on/off
Valve Fail ???
Level Control
PID, gain scheduling if non-linear Valve
Valve Fail??
Feed Flow Rate Control
This needs ratio control!!
Valves Fail ??

12. PID Tuning – Lamda Tuning
P only P=1
Step Test – CV +/- 10%
Observe increase/decrease in PV
Τd and τ (63.2%) and Kp=%PV/%CV(output)
Kc=(Kp)-1*(Tr)/(λ+Td)
Tr=τ, λ=0.5 to 4 (typically 3) times Max(Td, Tr)
Cascade Control inner loop 5x to 10x faster than outer loop

13. Lamda Tuning Results Max(Td, Tr) = 4 s

14. These are all simple 1 input, 1 output controllers with alarms for safety
Plant Wide Control is Different
Active Control System Objectives
Meet production rate and product quality targets
Keep system in Safe Operating Range for equipment, catalysts and materials of construction
Minimize Energy Utilized
Minimize Process Variability
Meet environmental regulations
Air quality
Water discharge quality
Profit Optimizer
Provide inputs for Active Control System
Separate Safety Control System
separate sensors, valves, controllers, power system
Identify Unsafe conditions
Take Action to Safely Bring the Plant/System Down to a Safe Point of Operation or Shut Down

15. Procedure for Plant Wide Control
I. Top-down analysis to identify degrees of freedom and primary controlled variables (look for self- regulating variables) II. Bottom-up analysis to determine secondary controlled variables and structure of control system e.g. pairings (CVs with Sensors).

16. Degrees of Freedom (DoF) Analysis
DoF (= Control Valves)
DoF=Nvariables-Nexternally_defined-Nequations
Practice DoF on Smaller Problems

17. Heat Exchanger Network
Flow Rate Set
Floating Flow Rates for C1 & C2

18. Heat Exchanger Network Control
Hot Stream
DoF = Nv-NDef-Neq=15-4-9=2 (# Control Valves)
Nv=15 (9Ts,3 flows, 3 Hx Qs)
NDef=4 (F1,To, Θo, Θ1)
Equations for Each Exchanger Neq=3x3=9
Q1=F1Cp1(To-T1)
Q1=F3Cp3(Θ4- Θ3)
Q1= U1A1ΔTLM
With By-pass, DoF = Nv-NDef-Neq= =3
Θ3 =(1-φ) Θ0+ Θ’3 Neq=10, Nv=17(& φ,Θ’3)
Streams 2 and 3 Cold

19. Distillation Control with Total Condenser
Control Valves ????
Degrees of Freedom Analysis DoF= Nv-NDef-Neq
Variables, Nv= 4 NT+13, NDef=2 (Feed Flow and Composition)
Equations = 4 NT+6, DoF=5 (# Control Valves)
Additional Control Valve on Feed is Possible which is used for Production Rate Control

20. Distillation Control What measurements?
What should be the pairings of measurements to control valves?
PD controlled with CV QC
LR controlled by CV B
LD controlled by
CV L if so what does CV D control??
CV D if so what does CV L control??
What does CV QR and CV B control?

21. Distillation Control Material Balance Control 4 Control Schemes
Inferred Composition Analysis = Temp. of Stage
Top CA or Bottom CA

22. With Composition Analysis
LV control
DV control
L/F V control
D/F V control

23. Key Concerns for Plant-wide Control System
Establish Control Objectives
Safe Operation – Process Within its Constraints
Meets Environmental Constraints
Control Production Rate
Feed Flow Controls
Product (on-demand) Flow Controls
Control Product Quality
Determine Degrees of Freedom for Control System
Position Control Valves
Establish Energy Management System
Recycle Loop Flows Fixed – watch out for snowball effects
Vapor and Liquid Inventories Fixed
Improve Dynamic Controllability

24. Acrylic Process A  B Objectives Production Rate of B high & constant
Conversion in reactor highest possible
Constant Composition for B

25. Where to set the production rate? Should it be at the inlet or outlet?
Very important!
Determines structure of remaining inventory (level) control system
Set production rate at (dynamic) bottleneck
Link between Top-down and Bottom-up parts of Plant-wide Control System

26. On-demand Product or Feed Flow Control System
Note: Flash Vessel will have a spray head for cold B to condense all possible Vapors
Vapor Typically is fed above Liquid level
Demister on outlet of Vessel
20.13
FC on Product B
Feed of A controlled as needed by reactor
20.14
FC on Feed A Flow
Level Controls Product B Flow Rate
Reactor Temp is controlled with Cascade Controller
To meet Composition Requirements
Note:
Trim steam heater for Feed A for accurate temperature control at reactor

27. Plantwide Control Design
Determine all manipulated variable. Number of manipulated variables MAY be equal to the number of control valves.
Determine how production rate is set: (a) upstream process; (b) downstream process; (c) free to vary. Determine the best way to control the production rate: (a) valve selection; (b) setpoint on temperature, recycle, etc. can sometimes be used to control production rate.
Decide how to control product quality.
The closed loop system for product quality control should have adequately small time constant, time delays, and sufficiently large gains.
Consider interaction between different loops and decide if multivariable control is needed.
Make sure that flows are not too small to achieve the objective (i.e. it is difficult to control condenser level using small flow rate of distillate in the column with high internal flows).
Determine and stabilize unstable units.
Design inventory control loops.
Develop component balance control loops (control of makeup streams of reactants, makeup gaseous streams to maintain pressure, liquid makeup to control level, etc.).
Be careful with recycle loops: They introduce feedback (positive or negative), which makes it difficult to analyze the consequences of the particular control design on the overall performance. For instance, it was found that unless one flow somewhere in the recycle loop is fixed, the recycle flow might grow to very high rate when disturbance occurs or when throughput is increased (snowball effect).
Heat integration saves cost but can make plantwide control more difficult because integrating loops also “spread” disturbances.
Use cascade control to reduce the effect of disturbances.
Using steady state simulations to study controllability of the designed system in face of disturbances. For example, perturb the feed flow rate or composition and study how flows and other process variables change to compensate for the disturbances. If small disturbance requires large changes in flow rate or other manipulated variables to compensate for its effect, the control system must be redesigned.
Using steady state simulations, determine the ranges of acceptable disturbances, which can be taken care of while maintaining the production goals.
Use dynamic simulation to determine dynamic response of the designed system. Redesign as needed. For instance, redesign to eliminate inverse response, if at all possible.
Use dynamic simulation to find the range of disturbances, which can be compensated for using the designed control system.
Use dynamic model to determine the range of stability of the closed-loop system when different model parameters change.
Use the remaining degrees of freedom (manipulated variables) for steady state optimization (maximize the profit, etc.) and/or to improve controllability of the plant (disturbance rejection, flexibility in changing operation point or product mix, etc).
Ensure safe and environmentally sound operation.
Startup shutdown, safety and emergency handling control.

28. Vinyl Chloride Process
Reactor 1 FeCl3 solid Catalyst
Conversion is >90%
C2H4 + Cl2  C2H2Cl2
Fired Heater for Pyrolysis Reaction
Conversion is 60%
C2H2Cl2  C2H3Cl + HCl
Product

29. Vinyl Chloride Process

30. Vinyl Chloride Process w Control System

31. Approach to Optimizer DoF Analysis for Optimizer Constraint Analysis
Levels are not important in Optimization
Constraint Analysis
Optimization Itself

32. Example of self-optimizing structure Recycle process: J = V (min
Example of self-optimizing structure Recycle process: J = V (min. boil-up => min. energy) 5 4 1 2
Given feed rate F0 and
column pressure: 3
Constraints:
Maximize reactor volume (residence
time and thus conversion)
Product spec: xB > 0.98
Nm = 5
N0y = 2 Levels
DoF=Nss = = 3

33. Recycle process: Selection of controlled variables
Step 3.1 J=V (minimize energy with given feed)
Step 3.1 DOFs for optimization: Nss = 3
Step 3.3 Most important disturbance: Feedrate F0
Step 3.4 Optimization: Constraints on max. Mr and xB always active
Step DOF left, candidate controlled variables: F, D, L, xD, ...
Step 3.6 Loss with constant setpoint:

34. Recycle process: Loss with constant setpoint, cs
Large loss with c = F
Negligible loss with c = L/F

35. Proposed control structure for case with J = V (minimize energy)
Active constraint
Mr = Mrmax
Active constraint
xB = xBmin= 98%
2 Active Constraints for Optimization
Reactor Full
Min Composition Required

36. Recycle process: Selection of controlled variables
Step 3.1 J=V (minimize energy with given feed)
Step 3.1 DOFs for optimization: Nss = 3
Step 3.3 Most important disturbance: Feedrate F0
Step 3.4 Optimization: Constraints on max. Mr and xB always active
Step DOF left, candidate controlled variables: F, D, L, xD, ...
Step 3.6 Loss with constant setpoint: Good: xD, L/F. Poor: F, D, L

37. Given feedrate, the production rate set at inlet of the column

38. Reconfiguration is required when the bottleneck (max
Reconfiguration is required when the bottleneck (max. vapor rate in column) is reached (rate set at the outlet of the column)
MAX

39. HW 5 Heat Integration Optimization of minimum temperature approach in a heat exchanger:
Set up a ΔT for E1 =(TS1-TS4)
VP = (1-t)(S-C)-i(Cequipment)
S=0,
C= Utilities for E2 and E3
Qi = mi Cpi ΔTi
Ci =Σ Qi * Price of Utilityi
Cequipment
Cost of HX
E1+E2+E3
Cost α f(HX Area)
Area = Q/(U ΔTLM)

40. Capital and Operating Cost Optimization
Capital cost goes down when A is less. This is caused by delta T being larger for Q to remain the same.
ΔTthres

41. Problem 2 - Pinch Analysis
To exchange heat between four streams with ΔTmin= 20°C, the HEN in the figure below is proposed. Determine if the network has the minimum utility requirements. If not, design a network with the minimum utility requirements.
As an alternative design a network with the minimum number of heat exchangers.