Plant-wide Control- Part2 CH EN 5253 – Process Design II Plant-wide Control- Part2

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)

Symbols in P&ID International Society of Automation (ISA)

Outlines of the topic: Plant-wide Control Basics Objectives Controlled /Manipulated variables Sensors Instrumentation Few guidelines Fail safe strategy 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

Control of individual variables

Flow Control Flow Transducer/Transmitter Orifice, venturi, many others Flow Indicator Controller – FIC PID Centrifugal /reciprocating Pump Fixed speed/constant volume output If Valve fails Dead-heading Check valve to prevent back flow By-pass loop to prevent dead-heading pump

Flow Control : Energy efficient way Variable Frequency Drive (VFD) can be costly ( higher capital cost) Energy saving ( lower operating cost)

Temperature Control with Utility The Simplest arrangement in process plant Control the flow rate of cooling or heating medium When is bypass line of utility stream used?

Temperature Control Arrangement for cooler Cooling medium : Air Temperature of air varies seasonal basis ( even hourly basis)

Temperature Control Arrangement for cooler Cooling medium : Air Temperature of air varies seasonal basis ( even hourly basis) For fixed fan speed

Ratio Control Ratio Control Valve – FFV – fail Closed May have two control valves – one for each stream 2 Flow Transducers/transmitters Orifice, venturi, many others Example Reactor Feeds ( stoichiometric ratio) Distillation column reflux 2H2+O2  2H2O

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

DOF Analysis : Implications on control system design Complex Systems Heat Exchanger Networks Control Reactor Control Distillation Column Control

Heat Exchanger Network Control

Heat Exchanger Network Control Total variables: Nv = 15 3 Flows: F1, F2, F3 9 Temperatures: T0, T1, T2, T3, 0, 1, 2, 3, 4 3 Heat duties of 3 Hx : Q1, Q2, Q3 Externally defined variables: Ndef =4 1 Flow: F1 3 Temperatures: T0, 0, 1 Total equations Neq=3x3=9 for each heat exchanger Q1=F1Cp1(To-T1) Q1=F3Cp3(4 - 3) Q1= U1A1ΔTLMTD DoF = Nv- Ndef – Neq = 15-4-9 = 2 2 Control valves for 2 manipulated variables 2 manipulated variables : F2 and F3

Heat Exchanger Network Control Controlled variables: 3 Target temperatures T3, 2, 4 Manipulated variables: 2 flow rates of two cold streams F2, F3 Conclusion: One temperature can not be controlled Controlling any two temperatures 3 possible configurations F2- 2 , F3- 4 ( shown here) F2-4, F3- 2 ( reverse, unstable) F2- 2 , F3- T3 Solutions to control all three target temperatures?

Solution: HEN Control with bypass stream Total variables: Nv = 17 3 Flows: F1, F2, F3 1 bypass flow fraction:  10 Temperatures: T0, T1, T2, T3, 0, 1, 2, 3, 3’, 4 3 Heat duties of 3 Hx : Q1, Q2, Q3 Total equations Neq=3x3+1=10 𝜃 3 = 1− 𝜃 3 ′ + 𝜃 0 With By-pass, DoF = Nv-NDef-Neq=17-4-10=3

Solution: HEN Control with bypass stream Controlled variables: 3 Target temperatures T3, 2, 4 Manipulated variables: 3 2 flow rates of two cold streams F2, F3 1 flow fraction of bypass stream  F2  HE2 directly F3  HE1 and HE3   T3 directly

Reactor Control

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) DoF = 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 FA : direct on reactor temperature, T Fo : same reasons, h

Distillation Column Control

Distillation Column Control ( Binary and total condenser) 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) DoF = Nv- Ndef – Neq = (4NT+13) -2 –(4NT+6) = 5 5 manipulated variables (# control valves) 5 controlled variables

Distillation Column Control 5 Controlled variables -PD, LR, LD, xB, xD 5 manipulated variables -L, D, B, QC, QR PD is controlled with CV QC LR is controlled by CV B xB is controlled by CV QR LD is controlled by 4/5: CV L if so what does CV D control?? 4/5 : CV D if so what does CV L control??

With Composition Analysis LV control DV control

Distillation Control Schemes L, V control D, V control L/F, V control D/F, V control

Control of entire plant

Next class ( 5-Mar-2018)