Plant-wide Control- Part2 CH EN 5253 – Process Design II Plant-wide Control- Part2 6th March 2019

Books Product and Process Design Principles: Synthesis, Analysis and Evaluation by J. D Seader, Warren D. Seider and Daniel R. Lewin Chapter 20 (4th Edition) Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design by Gavin Towler, and Ray Sinnott Chapter 5 (2nd Edition)

Outlines : Plant-wide Control Control of individual variables Control of individual units Control of entire plant Pressure Level Flow Ratio Temperature Vaporizer Heat Exchanger Network Control Reactor Control (e.g., CSTR) Distillation Column Control previous class Example 1 Example 2

Control of individual units

Degrees of Freedom (DoF) Analysis Determine number of manipulated variables permissible Before selecting controlled variables and manipulated variables DoF = Nvariables – Nequations (Universal definition) DoF = NManipulated + Nexternally defined NManipulated = Nvariables – Nequations- Nexternally defined

Reactor Control AB, exothermic Reactant Inlet (A) Coolant Inlet Product Outlet (A+B) outlet

CSTR (AB, exothermic, jacketed cooling) Total variables: Nv = 10 3 Flows: Fi, Fo, Fc 4 Temperatures: Ti, T, Tc, Tco 2 Concentrations : CA, CAi 1 Liquid level: h Externally defined variables: Ndef =3 1 Concentrations : CAi 2 Temperatures: Ti, Tco

CSTR (AB, exothermic, jacketed cooling) Total equations Neq=4 Overall mass balance Mass balance on component Energy balance on reacting mixture Energy balance on jacket coolant

CSTR (AB, exothermic, jacketed cooling) Nmanipulated = Nv- Ndef – Neq = 10-3-4 = 3 3 manipulated variables (# control valves) 3 controlled variables 3 Controlled variables CA : product quality T : safe operation h : non-self-regulating 3 Manipulated variables Fi : direct and rapid on conversion, CA FC : direct on reactor temperature, T Fo : same reasons, h

Distillation Column Control ( Binary and total condenser)

Primary objectives of distillation column control Separating two components Primary objectives of distillation column control Maintain specified composition of the top and bottom products ( and any side streams) correcting for the effects of disturbances Feed flow rate, composition and temperature Steam or other hot utility supply Cooling water or air cooler conditions Ambient conditions

Distillation Column Control Total variables: Nv = 4NT+13 Externally defined variables: Ndef =2 Feed Flow: F Feed composition ( overall) : xF yn , xn Ln , hn LD, xD L, D ysump , xsump LR B, V F, xF PD QC Sump

Distillation Column Control Total equations Neq= 4NT+6 yn = kn xn Correlations Jeronimo & Sawistowski (1973) Bennett (1983) Nmanipulated = Nv- Ndef – Neq = (4NT+13) -2 –(4NT+6) = 5 5 manipulated variables (# control valves) 5 controlled variables

Distillation Column Control Selection of 5 Controlled variables (out of 4NT+13 total variables) Variable 1: Condenser pressure, PD (Guidelines 3 & 4) Direct measure or strongly affect the product quality Variable 2: Sump liquid level, LR (Guideline 1) Not self regulating Variable 3: Reflux drum liquid level, LD (Guideline 1) Variable 4: Compositions of bottom stream, xB (Guidelines 3) Direct measure of the product quality Variable 5: Compositions of distillate, xD (Guidelines 3)

Distillation Column Control Selection of 5 Manipulated variables Variable 1: Condenser pressure, PD is controlled by QC Variable 2: Sump liquid level, LR is controlled by B Variable 4: Compositions of bottom stream, xB is controlled by QR ( equivalent to V (vapor) control in sump) Variable 3: Variable 5: Reflux drum liquid level, LD Compositions of distillate, xD If controlled by L (R>4) -Controlled by D If controlled by D -Controlled by L

Distillation Column Control Desired Separation by purity xB and xD or recovery D/F or B/F LV control L controls xD V ( or QR) controls xB DV control D controls xD V ( or QR) controls xB TC Alternative control TC TC Near the top of the column

Distillate Composition Distillate Composition Other Control Schemes Direct Control of Bottom Composition Indirect Control of Bottom Composition Direct Control of Distillate Composition Indirect Control of Distillate Composition

Control of entire plant Guidelines Example 1 : Exothermic Reaction Example 2: Vinyl Chloride Process

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 control 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

Example 1 Plantwide 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 flow (product) Controlling Inlet flow (feed) On-demand product flow FC on feed A flow Level controls product B flow rate FC on product B Feed of A is controlled as needed by reactor 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 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)

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 Before moving to last point, Step 9: Optimize Cascade control should always be used a process with relatively slow dynamics like level, temperature, composition a liquid or gas flow relatively-fast process, has to be manipulated to control the slow process.  Master controller Slave controller Single loop control Cascade control

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 2 Plantwide Control System Configuration for a Vinyl Chloride Process

Vinyl Chloride Process Reactor 1 FeCl3 solid Catalyst Conversion is >90% C2H4 + Cl2  C2H4Cl2 Fired Heater for Pyrolysis Reaction Conversion is 60% C2H4Cl2  C2H3Cl + HCl

Vinyl Chloride Process

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

Vinyl Chloride Process Step 2: Determine Degrees of Freedom for Control System 20 control valves placed

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

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 range of production levels Setpoint of recycle flow controller is proportional to feed flow rate