Process Unit Control System Design

Transcription

1 Process Unit Control System Design 1. Introduction 2. Influence of process design 3. Control degrees of freedom 4. Selection of control system variables 5. Process safety

2 Introduction Control system requirements» Safe operation» Satisfy environmental regulations» Stable plant operation» Satisfy specifications on product quality and production rate» Economic plant operation Control system design» Select controlled, manipulated and measured variables» Specify control structure and controller type» Select controller tuning parameters

3 Multivariable Control Strategies Multivariable processes» Multiple input and output variables» Requires multivariable control strategy Multiloop (decentralized) control» Manipulated inputs and controlled outputs are paired together» A controller is designed for each input/output pair» Simple to design and often provides satisfactory performance Multivariable control» Manipulated inputs and controlled outputs are not explicitly paired» All the inputs are used simultaneously to control all the outputs» Often necessary for processes with slow dynamics, strong variable interactions, and/or variable constraints

4 Influence of Process Design Process design» Invariably performed prior to control system design» Based on steady-state analysis» May produce processes that are very difficult to control dynamically Integrated process design and control» Dynamic operability considered during the process design stage» Currently a popular research topic» Starting to be practiced in industry

5 Heat Integration Increase interactions between columns Lose second column reboiler duty as a manipulated variable Increase interactions between units Introduces positive feedback Lose hot oil flow as a manipulated variable

6 Control Degrees of Freedom Process degrees of freedom (N F )» N F = N V - N E» N V = number of process variables» N E = number of independent model equations Control degrees of freedom (N FC )» N FC = number of process variables that can be independently controlled» N FC = N F - N D» N D = number of disturbance variables» N FC usually can be calculated as the number of independent material and energy flows that can be manipulated Feedback control usually does not change N FC

7 Distillation Column Example Manipulated variables: B, D, R, q c, q h N FC = 5 Controlled variables: x B, x D, h D, h B, P

8 Selection of Control System Variables Output variables» Measured variables variables that can be measured» Controlled variables usually a subset of the measured variables Input variables» Manipulated variables inputs that can be manipulated» Disturbance variables inputs that cannot be manipulated and are determined outside the control system Basic requirement» Number of manipulated variables must be greater than or equal to the number of controlled variables

9 Selection of Controlled Outputs All outputs that are not self-regulating must be controlled» Liquid level in a tank with outlet pump Select outputs that must be maintained within equipment and operating constraints» Polymer reactor temperature Select outputs that directly represent product quality or strongly affect product quality» Distillation column overhead product composition Select outputs that strongly interact with other output variables» Steam header pressure in steam generation unit Select outputs that have favorably steady-state and dynamic characteristics» Large steady-state gains, small time constants, and small time delays

10 Selection of Manipulated Inputs Select inputs that have large effects on controlled variables» Large steady-state gains and large input ranges Select inputs that rapidly affect the controlled outputs» Small time constants and small time delays Select inputs that directly (rather than indirectly) affect the controlled outputs» Condenser duty rather than reboiler duty for column overhead composition control Avoid selecting inputs that recycle disturbances» Flow rate of column product stream recycled to chemical reactor

11 Selection of Measured Variables Select measurements that are reliable and accurate» Stream temperature rather than composition Select measurements that exhibit sensitivity to the manipulated inputs» High purity distillation column tray temperature Select measurements that minimize time constants and time delays» Distillation column product composition measurement with gas chromatograph

12 Evaporator Example N FC = 3 Controlled outputs: h, x B, P Manipulated inputs: B (for h), D (for P), P s (for x B ) Measured variables: h, x B, P, B, D, P s

13 Evaporator Control System

14 Process Safety

15 Alarms and Interlocks