Distillation Column and Reactor Control Strategies

Overhead Pressure Control in Distillation Columns

Level in the Overhead Drum

Column Bottom Level Control

Concentration Measurement

Concentration measurement is usually replaced by temperature measurement due to its simplicity, speed, and reliability. To choose the best tray for temperature control, various criteria are applied. One common method is the slope criterion, which selects the tray exhibiting the largest temperature changes relative to the adjacent tray (excluding the first tray).

Distillation Column Flow Control

For a distillation column with a single liquid phase and a simple partial condenser where only inerts remain in the vapor phase, several flow control possibilities exist. These must adhere to the following rules:

• Rule 1: Only one stream flow of a partition can be directly manipulated through a fixed setpoint (SP). The other is left free or manipulated via a level controller.
• Rule 2: The directly manipulated stream should be the smallest one.

Note: Rule 1 takes precedence over Rule 2.

Partitions

• Partition 1: Feed into overhead product (D) and bottoms product (B).
• Partition 2: Condensate into reflux (L) and overhead product (D).
• Partition 3: Bottoms into boilup (G) and bottoms product (B).

Equations: 1. F = D + B (balance on one of the products); 2. V = L + D; 3. B = V + W.

Reactor Control Strategies

Standard pressure and level control configurations are shown in the figure. In cases where mechanical stirring is not feasible, product recycling and gas reactant bubbling are employed to ensure a homogeneous mixture within the reactor.

Thermal jackets are typically used to control reactor temperature as they simplify utility management. When a jacket is not applicable, recycling is used for temperature regulation. In specific scenarios, an inert air flow may be introduced to control temperature.

Endothermic Plug Flow Fixed Bed

In these systems, temperature decreases along the bed. The feed temperature is regulated via an exchanger or furnace for higher temperatures. Pressure is typically controlled downstream of the reactor, following the separation stages.

Exothermic Plug Flow Fixed Bed

Temperature increases along the bed, necessitating cooling after each stage. If the temperature rise is significant, direct injection of a cold stream (quench) may be required, as seen in hydrogenation processes.