Plant-wide Control- Part3 CH EN 5253 – Process Design II Plant-wide Control- Part3

Book Product and Process Design Principles: Synthesis, Analysis and Evaluation by J. D Seader, Warren D. Seider and Daniel R. Lewin Chapter 20 (4th Edition) Individual Control Loops These are all simple 1 input, 1 output controllers with alarms for safety Control for Unit Operation Reactor, Heat Exchanger, Distillation Column Have Not Talked About Flash Unit Furnace Plant-wide Control

Distillation Control Schemes Use when Feed is Fixed L, V control (b) D, V control (a) Use ratio when Feed is variable L/F, V control D/F, V control DV control LV control

Flash Control Scheme This design works great for a cold liquid feed Pressure Control Temperature Control Level Control Hot Gas Fed to Flash Spray of cold liquid to Flash Tank Heat Exchanger Placed at Vapor Exit Always use a De-mister at Flash Exit

Plant Wide Control Alan Foss (“Critique of chemical process control theory”, AIChE Journal,1973): “The central issue to be resolved ... is the determination of control system structure. Which variables should be measured, which inputs should be manipulated and which links should be made between the two sets? “

Plantwide Control Individual Control Loops Control for Unit Operation These are all simple 1 input, 1 output controllers with alarms for safety Control for Unit Operation Reactor, Heat Exchanger, Distillation Column, Flash Unit, Furnace Plant-wide Control

Plant-wide Control Plant Wide Control is Different Profit Optimizer 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

Qualitative Design Procedure for Plantwide Control Step 1: 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 Step 2: Determine the Process Degrees of Freedom Position Control Valves Step 3: Establish Energy Management System Temperature control for exothermic/endothermic reaction Step 4: Set the production rate

Qualitative Design Procedure for Plantwide Control Step 5: Control the product quality and handle safety, environmental, and operational constraints Step 6: Fix a flow rate in every recycle loop and control vapor and liquid Inventories ( vessel pressure and level) Step 7: Check component balances ( if build up in the process) Step 8: Control the individual process units Step 9: Optimize economics and improve dynamic controllability

SELECTION OF CONTROLLED VARIABLES Rule 1: Select variables that are not self-regulating. Self Regulating dx/dt=f(u) Non Self Regulating dx/dt=f(u,x) Rule 2: Select output variables that would exceed the equipment and operating constraints without control. Rule 3: Select output variables that are a direct measure of the product quality or that strongly affect it. Also those that are a direct measure of process emissions Rule 4: Choose output variables that seriously interact with other controlled variables. Rule 5: Choose output variables that have favorable static and dynamic responses to the available control variables. 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

SELECTION OF MANIPULATED VARIABLES Rule 6: Select inputs that significantly affect the controlled variables. Rule 7: Select inputs that rapidly affect the controlled variables. Rule 8: The manipulated variables should affect the controlled variables directly rather than indirectly. Rule 9: Avoid recycling disturbances. 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

SELECTION OF MEASURED VARIABLES Rule 10: Reliable, accurate measurements are essential for good control. Rule 11: Select measurement points that are sufficiently sensitive. Rule 12: Select measurement points that minimize time delays and time constants. 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

Example 1 Plant-wide Control System Configuration for an exothermic reaction Process

A  B 1 Jacketed CSTR, R-100 1 Flash vessel, V-100 3 Heat Exchanger Exothermic reaction

Nine-step design procedure for plantwide control A  B Step 1: Establish Control Objectives Production rate of B at specified level Conversion in reactor highest possible Constant composition for B from the flash vessel

Where to set the production rate? Very important! Should it be at the inlet or outlet? Determines structure of remaining inventory (level) control system

On-demand Product or Feed Flow Control System Controlling Outlet (product) flow Controlling Inlet (feed) flow On-demand product flow FC on Product B Feed of A controlled as needed by reactor Level Control dictates feed flow rates FC on Feed A Flow Level Controls Product B Flow Rate Level Controls Reactor Product Flow Note: Reactor Temp is controlled with Cascade Control To meet Composition Requirements!

A  B Step 2: Determine Degrees of Freedom for Control System 7 DOF for this system, 7 Manipulated Variables (7 control valvues) 3 Utility streams ( controlled by V-2, V-3, V-6) 1 Feed flow rate (controlled by V-1) 1 Reactor effluent flow rate (controlled by V-4) 2 Product flow rates (controlled by V-5 and V-7) If on-demand flow control is decided B Product flow rate is controlled independently by V-7

A  B Step 3: Establish Energy Management System 2 Manipulated Variables – critical for safety Reactor feed temperature (controlled by V-2) Reactor effluent temperature(controlled by V-3) Flash Temperature (V-6)

A  B Step 4: Set the production rate If on-demand product flow is selected Desired flow rate by V-7

A  B Step 5: Control the product quality and handle safety, environmental, and operational constraints Which parameters determine product quality? Pressure and temperature of the flash vessel, V-100 Temperature regulated by coolant flow rate through V-6 Pressure is controlled by adjusting flow rate of unreacted A through overhead valve V-5 Rapid and direct effects Satisfy the Objective 3: constant composition

A  B Step 6: Fix recycle flow rates and vapor and liquid Inventories (vessel pressure and level) Flash vessel level Outlet control valve already assigned for product rate Inlet flow i.e., reactor effluent flow is controlled (controlled by V-4) Reactor level Inlet flow i.e., feed flow rate (controlled by V-1) Both direct and rapid control Vapor inventory Control valve V-5, already assigned for pressure

A  B Step 7: Check component balances Not needed- no build up of A and B

A  B Step 8: Control the individual process units No need for individual unit control All control valves have been assigned No additional control loops can be designed

Single loop vs. cascade Single loop control Cascade control Master controller Slave controller Single loop control Cascade control

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

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

A  B Step 9: Optimize economics and improve dynamic controllability 2nd Objective: Conversion in reactor highest possible Temperature control system already established What is the temperature set point? Cascade control: Reactor temperature controller (controlled by V-2) Temperature Set point adjusted to control the concentration of B Irreversible reaction Highest possible temperature ( safe limit) Controller unnecessary Cascade control

EXAMPLE 3: CYCLIC PROCESS (Cont’d) Steps 1 & 2: Establish the control objectives and DOFs: Maintain the production rate at a specified level. Keep the conversion of the plant at its highest permissible value. 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

EXAMPLE 3: CYCLIC PROCESS (Cont’d) Step 3: Establish energy management system. Need to control reactor temperature: Use V-2. 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

EXAMPLE 3: CYCLIC PROCESS (Cont’d) Step 4: Set the production rate. For on-demand product: Use V-7. 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

EXAMPLE 3: CYCLIC PROCESS (Cont’d) Step 5: Control product quality, and meet safety, environmental, and operational constraints. To regulate V-100 pressure: Use V-4 To regulate V-100 temperature: Use V-5 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

EXAMPLE 3: CYCLIC PROCESS (Cont’d) Step 6: Fix recycle flow rates and vapor and liquid inventories Need to control recycle flow rate: Use V-6 Need to control vapor inventory in V-100: Use V-4 (already installed) Need to control liquid inventory in V-100: Use V-3 Need to control liquid inventory in R-100: Cascade to FC on V-1. 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

EXAMPLE 3: CYCLIC PROCESS (Cont’d) Steps 7, 8 and 9: Improvements Install composition controller, cascaded with TC of reactor. 10 - Design and Control DESIGN AND ANALYSIS II - (c) Daniel R. Lewin

Example2 Plantwide Control System Configuration for a Vinyl Chloride Process

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

Vinyl Chloride Process C2H4 + Cl2  C2H2Cl2 C2H2Cl2  C2H3Cl + HCl

Nine-step design procedure for plantwide control - Vinyl Chloride Process Step 1: Establish Control Objectives >90% conversion in dichloroethane reactor, R-100 Fixed conversion (60%) in the pyrolysis furnace (F-100) Feed flow rate of Ethylene ( V-1 control valve) C2H4 + Cl2  C2H2Cl2 C2H2Cl2  C2H3Cl + HCl

Vinyl Chloride Process Step 2: Determine Degrees of Freedom for Control System 20 control valves placed C2H4 + Cl2  C2H2Cl2 C2H2Cl2  C2H3Cl + HCl

Vinyl Chloride Process Step 3: Establish Energy Management System Reactors temperature control Temperature of R-100 : V-3 Temperature of F-100 : V-6 Reduction the effects of temperature disturbances Effluent temperature in the Evaporator, E-101: V-5 Effluent temperature in the Quench tank, V-100: V-7 Effluent temperatures in the partial condenser, E-103: V-8 Effluent temperatures in the recycle cooler, E-108: V-20 Rapid and direct No recycle of temperature disturbances

Vinyl Chloride Process Step 4: Set the production rate V1 assigned to control feed Set point regulates production rate C2H4 + Cl2  C2H2Cl2 C2H2Cl2  C2H3Cl + HCl

Vinyl Chloride Process Step 5: Control the product quality and handle safety, environmental, and operational constraints Top products of 2 distillation columns by adjusting reflux Distillation column T-100 : V-11 Distillation column T-101 : V-16 Bottom products of 2 distillation columns by adjusting vapor rate (via stream rate) Distillation column T-100 : V-13 Distillation column T-101 : V-18 Inlet pressure of distillation column T-100: V-9 Sump level of distillation column T-100 : V-14

Vinyl Chloride Process Step 6: Fix recycle flow rates and vapor and liquid Inventories (vessel pressure and level) Liquid inventory control Reactor R-100 : Cascaded V-1 Distillation column T-101 : V-19 Reflux drum V-102 : V-17 Distillation column T-100 : V-14 Reflux drum V-101 : V-10 Vapor inventory control Distillation column T-101 : V-15 Distillation column T-100 : V-12

Vinyl Chloride Process Step 7 and 8: Check component balances and Control the individual process units Stoichiometric ratio of feed Chlorine feed adjusted for >90% conversion : V-2

Vinyl Chloride Process Step 9: Optimize economics and improve dynamic controllability Improve the toleration for a range of production levels 120% over design, 50% turndown Setpoint of recycle flow controller is proportional to feed flow rate (cascade loops)

DESIGN AND ANALYSIS II - (c) Daniel R. Lewin SUMMARY Provided motivation for handling flowsheet controllability and resiliency as an integral part of the design process Outlined qualitative approach for unit-by- unit control structure selection Outlined qualitative approach for plantwide control structure selection DESIGN AND ANALYSIS II - (c) Daniel R. Lewin 10 - Design and Control

Alternatives structures for optimizing control Step 3: What should we control? Setpoint optimization RTO controller Real Time Optimizing

Step 3. What should we control? (primary controlled variables) Intuition: “Dominant variables” Systematic: Define cost J and minimize w.r.t. DOFs

HW 8 1) rewrite Variables L,D  D, L/D then optimize 2) use V=D*(R+1) optimize column to minimize V w.r.t. R and NT This minimizes capital and operating costs of column.