Jawaharlal Nehru Technological University Hyderabad B.Tech III PROCESS DYNAMICS AND CONTROL Previous Question Paper (November/December - 2020 - R16)

A batch process is a manufacturing method where a finite quantity of raw materials is processed through a series of steps in a single vessel or a sequence of vessels to produce a specific amount of product. The process is characterized by a distinct start and end point, and the system is typically in an unsteady state as conditions like concentration and temperature change over time.

In a continuous process, materials are fed into the system and products are removed at a constant rate. The system operates in a steady state where the process variables at any given point remain constant over time, allowing for high-volume, consistent production.

Diagrammatic Representation:

A batch process is represented by a single reactor vessel equipped with an agitator, inlet valves for raw materials, and an outlet valve for the finished product. The system operates in cycles: Charge → React → Discharge → Clean. A continuous process is represented by a flow system where raw materials enter through a feed stream into a reactor, and the product is continuously withdrawn through an exit stream, often involving heat exchangers and control loops to maintain steady-state parameters.

Applications:

Batch processes are primarily used in the pharmaceutical industry for drug manufacturing, specialty chemicals, food processing (such as brewing or baking), and situations where product variety is high but production volume is low. Continuous processes are essential in large-scale industries such as oil refineries, petrochemical plants, power generation, and mass-produced consumer goods where high efficiency, consistent quality, and lower operating costs per unit are required.

In process control, control problems are generally categorized into two types based on the objective of the controller: the servo problem and the regulator problem. These classifications depend on whether the controller is tasked with tracking a changing setpoint or maintaining a constant setpoint despite disturbances.

The servo problem (or setpoint tracking problem) occurs when the primary objective of the control system is to force the process output to follow a desired change in the setpoint. In this scenario, the disturbance is assumed to be constant, and the controller must adjust the manipulated variable to minimize the error between the setpoint and the measured output as the setpoint varies over time. An example is a temperature control system in a chemical reactor where the operator frequently changes the desired setpoint temperature to follow a specific reaction profile or recipe.

The regulator problem occurs when the primary objective is to keep the process output at a constant setpoint despite the influence of external disturbances. Here, the setpoint is typically fixed, and the controller acts to reject the effects of unpredictable load changes or environmental fluctuations. An example is a pressure control system in a distillation column where the setpoint is kept constant, but the controller must continuously adjust the valve position to compensate for variations in feed flow rate or feed composition that act as disturbances to the system.

The fundamental difference lies in the source of the error: in the servo problem, the error is driven by the intentional change of the setpoint, whereas in the regulator problem, the error is driven by unintended changes in the process environment (disturbances). Both problems require the controller to calculate the error signal and apply a control action, but the tuning requirements and performance criteria for tracking versus rejection can differ significantly in practical applications.