Plant-wide Control : part1 CH EN 5253 – Process Design II Plant-wide Control : part1

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)

Overview 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

Instrumentation and Control Objectives

Manipulated and controlled variables Those streams or conditions to control or to maintain at some desired level (known as the set point). Flow rates Levels Pressures Temperatures Compositions etc. Manipulated variables An associated variable which can be changed to get desired value of controlled variable as a result. Flowing streams Disturbances Variables tend to drive the controlled variables away from their set points, Change in inlet/outlet Change in set point In process control, flow rate of the stream is manipulated through the use of a control valve.

Manipulated and controlled variables Heat generation Heat loss Fuel Supply Temperature

Selection of Manipulated Variables Guideline 1: Select manipulated variables that significantly affect the controlled variables If R>4, Controlled Variable: level in reflux drum Manipulated variable: reflux flow rate ( not distillate) Guideline 2: Select manipulated variables that rapidly affect the controlled variables Avoid variables with large delayed impact on controlled variable  X

Selection of Manipulated Variables Guideline 3: Select manipulated variables that effect the controlled variables directly rather than indirectly If Exothermic reaction, Controlled Variable: temperature Inject coolant directly (do not use cooling jacket) Guideline 4: Avoid recycling disturbances Impurities in process Better to purge than recycle

Single most important component to maintain parameters at desired values

Feedback Control System

Control Valves Instrument Connection I to P converter (Electro-pneumatic converter) Output air from positioner 4-20 mA to 3-15 psi Electric control signal Pneumatic control signal Instrument Air Supply Instrument Air Supply Positioner Positioning valve stem following signal Feedback to the controller the actual valve 

Control Valve Schematic

Control Valve Maintenance Bypass valve Drain valves Isolation valves

Control Valves: Fail Safe Strategy Fail safe does not mean “ failure is impossible or improbable” “Prevents or mitigates unsafe consequences of the system's failure” Traffic lights are designed to blink red in all directions if their controller goes down ( often during rain and snow) An elevator is designed with special brakes that are held back by the tension of the elevator cable. If the cable snaps, loss of tension causes brakes to be applied. Dead man's switch : Lawnmowers and snow blowers. Hand closed lever/held down all times If it is released, it stops rotating.

Control Valves: Fail Safe Strategy Reasons of failure in process plant Process abnormality Instrument air failure Power shutdown Emergency situations fire, storm etc.

Control Valves Example: Polymerization: reactor should be emptied Gas Processing: Gas should be flared

Sensors

Type of sensors in Process Control Transmitter Current output directly proportional to a variation in measured variable  Typical output of 4 to 20 mA  While transmitters consume more power during operation Less susceptible to electrical noise as cable lengths increase in the applications Transducer Voltage output directly proportional to a variation in measured variable.  lower operating power and consumption requirements Switch Active in two conditions like switch Either “On” or “Off” signal Used for alarms for high or low values Float Type

Why is “Switch” important? Why is the value from transmitter/transducer not always used for alarm ( like switch)? Why is transmitter output 4 to 20 mA ,not 0-16mA ?

Location of Sensor/Switch Ultra-sound pulses Float type Level Switch Glass Liquid Level Gages Level Transmitter (differential pressure)

Controlled variables Pressure Level Flow Ratio Temperature Vaporizer

Pressure Control Loop Pressure Control Valve – fail closed Venting of noncondensables after a condenser Pressure Control Valve – fail closed Is fail closed best? Pressure Transducer/Transmitter – PT Piezoelectric, diaphragm displacement Pressure Indicator Controller – PIC PID Alarm Low- PAL Alarm high -PAH Direct venting Condenser Condenser Condenser Varying HT area ( dependent on liquid level) Controlling coolant flow

Level Control Loop Level Control Valve – fail open Level Transducer /transmitter– LT DP cell, Ultra-sound pulses Level Indicator Controller – LIC PID Alarm High-LAH Alarm Low-LAL Pump may dead head Interlock valve to never goes to zero