Introduction to Process Control ChE 220 Lecture PDF

Introduction to Process Control ChE 220 Objective To define and determine importance of Process Control to Chemical Engineering To determine the necessity of Control To determine where Controls are needed Feedback and Feed forward Control Systems Importance of Process Control...

Introduction to Process Control ChE 220 Objective To define and determine importance of Process Control to Chemical Engineering To determine the necessity of Control To determine where Controls are needed Feedback and Feed forward Control Systems Importance of Process Control Describe your method in driving a car. Could you drive a car without looking out from the windshield? What must be provided by the car designers? Can a “good design” eliminate the need to steer? Importance of Process Control Importance of Process Control controlled variables - these are the variables which quantify the performance or quality of the final product, which are also called output variables. manipulated variables - these input variables are adjusted dynamically to keep the controlled variables at their set-points. disturbance variables - these are also called "load" variables and represent input variables that can cause the controlled variables to deviate from their respective set points. Importance of Process Control set-point change - implementing a change in the operating conditions. The set-point signal is changed and the manipulated variable is adjusted appropriately to achieve the new operating conditions. Also called servomechanism (or "servo") control. disturbance change - the process transient behavior when a disturbance enters, also called regulatory control or load change. A control system should be able to return each controlled variable back to its set-point. Feedback System Necessity of Process Control We want to achieve the following: Safety Environmental Protection Equipment Protection Smooth Operation Product Quality Profit Monitoring and diagnosis Where is Control Done? Where is Control Done? Feedback Control System Distinguishing feature: measure the controlled variable Feedback Control System Advantages:  Corrective action is taken regardless of the source of the disturbance.  Reduces sensitivity of the controlled variable to disturbances and changes in the process (shown later). Disadvantages:  No corrective action occurs until after the disturbance has upset the process, that is, until after x differs from xsp.  Very oscillatory responses, or even instability… Feed Forward Control System Feed Forward Control System Distinguishing feature: measure a disturbance variable Advantage:  Correct for disturbance before it upsets the process. Disadvantage:  Must be able to measure the disturbance.  No corrective action for unmeasured disturbances.