Introduction to Process Control and Theoretical Process Models

Flashcards covering fundamental process control concepts, variable definitions, hierarchy levels, design methodologies, and first-principles dynamic process modeling equations.

Primary Objective of Process Control

To maintain a process at the desired operating conditions safely and economically while satisfying environmental and product quality requirements.

Process

The conversion of feed materials to products using chemical and physical operations; in practical application, it refers to both the processing operation and the processing equipment.

Continuous Processes

Processes where feed streams enter and product streams exit the processing equipment continuously without interruption, such as heat exchangers, jacketed chemical reactors, cracking furnaces, and kidney dialysis units.

Controlled Variable (CV)

A process variable ($CV$) that is monitored and maintained at a specific target state or value.

Setpoint (SP)

The desired operating value ($SP$) for a controlled variable.

Manipulated Variable (MV)

A process variable ($MV$) that is adjusted by a control action to bring a controlled variable to its desired setpoint.

Disturbance Variable (D)

A process variable ($D$) that affects controlled variables but cannot be manipulated directly, typically arising from ambient conditions or changing feed properties.

Feedback Control

A control strategy in which control systems compare measurements of controlled variables with their setpoints and adjust manipulated variables accordingly after a disturbance has occurred.

Feedforward Control

A control strategy that continuously measures the source of a disturbance and adjusts manipulated variables based on those measurements before the controlled variable deviates from its setpoint.

Proportional Control

A control method implemented in feedback systems where the corrective action taken is directly proportional to the magnitude of deviation between the controlled variable and its setpoint.

Feedback Controller

A control element that receives a measured signal ($x_m$) from a transmitter, converts the setpoint into a matching signal ($x_{sp}$), compares both signals via subtraction to generate an error signal $e(t)$, and computes an output control signal $p(t)$.

Control Valve

A final control element that receives an electrical controller signal $p(t)$ to move its valve stem, thereby regulating a fluid flow rate ($w_2$).

Feedback Control Block Diagram

A functional block diagram illustrating the dynamic flow of mathematical signals and operational components in a feedback control system.

Process Control Schematics vs. Block Diagrams

Process control schematics show physical connections between hardware components, whereas block diagrams illustrate the directional flow of information and signal operations between components.

Multivariable Control

A control strategy in complex processes where a single manipulated variable or integrated controller is utilized to simultaneously control multiple controlled variables.

Hierarchy of Process Control Activities

A structured 5-level operational hierarchy that organizes process control activities by execution priority and time scale.

Measurement and Actuation (Hierarchy Level 1)

The foundational layer of the process control hierarchy operating at time scales of $< 1\,\text{second}$, consisting of primary sensors (e.g., temperature, level, flow) and actuators interfaced with control devices.

Safety and Environmental/Equipment Protection (Hierarchy Level 2)

A mandatory process control layer operating at time scales of $< 1\,\text{second}$, comprising dedicated sensing and actuating systems designed to protect personnel, equipment, and the environment.

Regulatory Control (Hierarchy Level 3a)

The process control hierarchy layer operating at time scales of $\text{seconds-minutes}$ that utilizes feedback and feedforward strategies to maintain key process variables near setpoint targets.

Multivariable and Constraint Control (Hierarchy Level 3b)

The control layer operating at time scales of $\text{minutes-hours}$ that manages variable cross-coupling and enforces operational and economic constraints.

Real-Time Optimization (RTO) (Hierarchy Level 4)

A control hierarchy layer operating at time scales of $\text{hours-days}$ that uses plant economic data and steady-state models to recalculate optimal operating conditions that minimize cost or maximize profit.

Planning and Scheduling (Hierarchy Level 5)

The highest layer in the process control hierarchy operating at time scales of $\text{days-months}$, responsible for scheduling production targets, managing plant logistics, and tracking inventory limits.

Traditional Design Approach

A method of control system design where control strategies and hardware are chosen based on operator intuition, past experience, and qualitative process knowledge, followed by physical controller tuning.

Model-Based Approach Flowchart

A systematic workflow for control system design that begins with creating a dynamic mathematical model of the process to evaluate control strategies in computer simulation prior to physical installation.

Dynamic Model

A mathematical model formulated from fundamental conservation principles that describes unsteady-state process behavior where variable values change with time.

Theoretical Model

A dynamic model generated directly using first principles of chemistry, physics, or biology (such as fundamental mass and energy conservation laws).

Empirical Model

A process model developed by fitting experimental data to algebraic or differential formulas, which is easy to construct but cannot extrapolate reliably beyond tested conditions.

Semi-Empirical Model

A hybrid modeling formulation that combines core theoretical first-principles equations with empirical data fitting for unmeasured parameters.

Degree of Freedom Analysis

A method used to determine model solvability calculated as $DOF = N_V - N_E$, where $N_V$ is the number of process variables and $N_E$ is the number of independent equations; a model is solvable if $DOF = 0$.

Unsteady-State Mass Balance Equation

The general balance law governing changing systems: $\text{Accumulation} = \text{In} - \text{Out} + \text{Generation} - \text{Consumption}$.

Isothermal Stirred Tank Mass Balance

The differential dynamic mass conservation equation for a liquid blending vessel: $\frac{d(\rho V)}{dt} = w_1 + w_2 - w$.

Blending Process Component Mass Balance

The dynamic component concentration model $V \rho \frac{dx}{dt} = w_1(x_1 - x) + w_2(x_2 - x)$, derived assuming constant volume ($V$) and constant fluid density ($\rho$).

General Energy Balance Equation

The dynamic energy balance relationship $\frac{dU}{dt} = -\Delta(H) + Q$, where internal energy change rate $\frac{dU}{dt}$ depends on enthalpy transport $\Delta(H)$ and heat addition rate $Q$.