Distillation Tower Operations and Furnace Fundamentals

Distillation Tower Dynamics and Heat Transfer

After changing a process variable, the tower will reflect this change from top to bottom slowly. Each tray of a tower has a different temperature and vapor pressure than any other tray in the tower.

Flooding occurs when the liquid flow in the downcomers and onto the trays is too high, or when the vapor flow is too high. Heat is removed from the tower when hot or warm overhead and bottom products exit the tower, and through leaks in insulation.

Approximately 85% of the distillation tower heat is supplied by the reboiler. Heat sources for a distillation tower include:

• Radiant heat
• Sparging
• Reboilers

Tray Design and Liquid Flow

A downcomer is a device that allows liquid to flow from one tray to another between each level. Three common tray types include:

• Valve
• Sieve
• Bubble cap

As fluid flows down a tower from tray to tray, its density increases. Conversely, as fluid flows down a tower from tray to tray, its temperature decreases.

Distillation Classifications and Processes

Distillation systems are classified as either continuous or batch. Three special types of distillation processes are:

• Azeotropic
• Vacuum
• Extractive

An azeotrope is a mixture that acts like a pure substance and boils at a single temperature. A decrease in pressure inside a column causes boiling points to decrease, while an increase in column pressure causes boiling points to increase.

Safety Hazards and Operational Stability

Reflux is chilled and condensed overhead product returned to the column to improve rectification (purification) and column stability. High temperatures, high pressures, flammable chemicals, toxic chemicals, and leaks are some safety and health hazards associated with the distillation process.

Furnace Operations and Combustion Principles

The purpose of a furnace is to heat a process fluid through combustion. Heat released by the combustion of fuel is measured in BTUs (British Thermal Units).

Primary air is the air mixed with fuel at the burners to provide the proper fuel-air mixture for lighting. Flame impingement occurs when burner flames touch the tubes.

Oxygen and combustibles are two chemical measurements made on flue gas in the furnace stack. Draft is defined as the difference between atmospheric pressure and the pressure inside the furnace.