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Chemical Process Control. Chapter 1. Introduction. A Career in Process Control. Requires that engineers use all of their chemical engineering training (i.e., provides an excellent technical profession that can last an entire career) Can become a technical “Top Gun”
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Chapter 1 Introduction
A Career in Process Control • Requires that engineers use all of their chemical engineering training (i.e., provides an excellent technical profession that can last an entire career) • Can become a technical “Top Gun” • Allows engineers to work on projects that can result in significant savings for their companies (i.e., provides good visibility within a company)
A Career in Process Control • Provides professional mobility. There is a shortage of experienced process control engineers. • Is a well paid technical profession for chemical engineers.
Importance of Process Control • PC directly affects the safety and reliability of a process. • PC determines the quality of the products produced by a process. • PC can affect how efficient a process is operated. • Bottom Line: PC has a major impact on the profitability of a company.
Safety and Reliability • The control system must provide safe operation • Alarms, safety constraint control, start-up and shutdown. • A control system must be able to “absorb” a variety of disturbances and keep the process in a good operating region: • Thunderstorms, feed composition upsets, temporary loss of utilities (e.g., steam supply), day to night variation in the ambient conditions
Benefits of Improved Control Old Controller
Benefits of Improved Control Old Controller New Controller
Better Control Means Products with Reduced Variability • For many cases, reduced variability products are in high demand and have high value added (e.g., feedstocks for polymers). • Product certification procedures (e.g., ISO9000) are used to guarantee product quality and place a large emphasis on process control.
Benefits of Improved Control Old Controller New Controller Improved Performance
Maximizing the Profit of a Plant • Many times involves controlling against constraints. • The closer that you are able to operate to these constraints, the more profit you can make. For example, maximizing the product production rate usually involving controlling the process against one or more process constraints.
Constraint Control Example • Consider a reactor temperature control example for which at excessively high temperatures the reactor will experience a temperature runaway and explode. • But the higher the temperature the greater the product yield. • Therefore, better reactor temperature control allows safe operation at a higher reactor temperature and thus more profit.
Driving a Car: An Everyday Example of Process Control • Control Objective (Setpoint): Maintain car in proper lane. • Controlled variable- Location on the road • Manipulated variable- Orientation of the front wheels • Actuator- Driver’s arms/steering wheel • Sensor- Driver’s eyes • Controller- Driver • Disturbance- Curve in road
Heat Exchanger Control • Controlled variable- Outlet temperature of product stream • Manipulated variable- Steam flow • Actuator- Control valve on steam line • Sensor- Thermocouple on product stream • Disturbance- Changes in the inlet feed temperature
The key feature of all feedback control loops is that the measured value of the controlled variable is compared with the setpoint and this difference is used to determine the control action taken.
Characteristics of Effective Process Control Engineers • Use their knowledge of the process to guide their process control applications. They are “process” control engineers. • Have a fundamentally sound picture of process dynamics and feedback control. • Work effectively with the operators.
Operator Acceptance • A good relationship with the operators is a NECESSARY condition for the success of a control engineer. • Build a relationship with the operators based on mutual respect. • Operators are a valuable source of plant experience. • A successful control project should make the operators job easier, not harder.
Process Control and Optimization • Control and optimization are terms that are many times erroneously interchanged. • Control has to do with adjusting flow rates to maintain the controlled variables of the process at specified setpoints. • Optimization chooses the values for key setpoints such that the process operates at the “best” economic conditions.
Economic Objective Function • VB > VC, VA, or VAF • At low T, little formation of B • At high T, too much of B reacts to form C • Therefore, the exits an optimum reactor temperature, T*
Optimization Algorithm • 1. Select initial guess for reactor temperature • 2. Evaluate CA, CB, and CC • 3. Evaluate F • 4. Choose new reactor temperature and return to 2 until T* identified.
Process Optimization • Typical optimization objective function, F: F = Product values-Feed costs-Utility costs • The steady-state solution of process models is usually used to determine process operating conditions which yields flow rates of products, feed, and utilities. • Unit costs of feed and sale price of products are combined with flows to yield F • Optimization variables are adjusted until F is maximized (optimization solution).
Process Control Terminology • Important to be able to communicate with operators, peers, and boss. • New terminology appears in bold in the text • New terminology is summarized at the end of each chapter. • Review the terminology regularly in order to keep up with it.
Overview • All feedback control loops have a controller, an actuator, a process, and a sensor where the controller chooses control action based upon the error from setpoint. • Control has to do with adjusting flow rates to maintain controlled variables at their setpoints while for optimization the setpoints for certain controllers are adjusted to optimize the economic performance of the plant.
