Principle. 

The Agitated Thin Film Dryer (ATFD) operates on the principle of continuous heat transfer and rapid evaporation of solvent or moisture from a thin film of material under agitation and vacuum. It is a highly efficient drying system designed to handle heat-sensitive, viscous, or paste-like materials that are difficult to dry using conventional dryers. In this system, a feed slurry or solution is continuously distributed as a thin film on a heated surface by a high-speed rotating agitator. The agitation spreads the material uniformly, constantly renewing the surface for evaporation and preventing the formation of crust or buildup. As heat is transferred from the wall to the product film, the solvent or moisture vaporizes rapidly, and the vapors are removed by a vacuum or vapor outlet system. The dried product, which may be in the form of powder, flakes, or granules, is continuously discharged from the bottom of the dryer.

The basic principle of the ATFD is to maximize the rate of heat and mass transfer by creating an agitated thin film of material on a heated surface. Because the film thickness is typically only a few millimeters, the heat transfer coefficient is very high, resulting in short residence times and efficient drying even at low temperatures. Operation under vacuum lowers the boiling point of the solvent, allowing drying at moderate wall temperatures (60°C to 150°C, depending on material), which minimizes thermal degradation. This makes the ATFD particularly suitable for drying heat-sensitive and viscous materials such as pharmaceutical residues, chemical slurries, food extracts, polymers, and effluents from evaporators or crystallizers.

Construction 

An Agitated Thin Film Dryer is a vertical or horizontal cylindrical vessel designed to provide a large heat transfer area with mechanical agitation. The main components of its construction are as follows:

• Drying Cylinder / Shell:
  The core of the ATFD is a jacketed cylindrical shell fabricated from stainless steel (SS304 or SS316L) or other corrosion-resistant material, depending on the application. The inner surface is highly polished to reduce friction and prevent product adhesion. The outer jacket serves as the heating surface, through which steam, hot oil, or other heat transfer fluids circulate to supply the required thermal energy. The heating medium typically flows through the jacket in countercurrent to the material flow for maximum efficiency. The cylinder is designed for operation under vacuum or slight pressure and is fitted with vapor outlets and inspection ports.
• Agitator Assembly:
  A central rotating shaft runs through the length of the drying cylinder and carries a series of agitator blades, wipers, or paddles. These blades are specially designed to maintain a uniform, turbulent thin film of the feed material on the inner wall. The blades are usually spring-loaded or hinged to adjust automatically to the wall contour, ensuring continuous wiping action and preventing material buildup. The agitator typically operates at high speed (100–500 rpm depending on size and product viscosity) and is driven by an electric motor with gearbox and mechanical seal. In some designs, the agitator is top-driven; in others, it may be bottom-driven with a stuffing box or mechanical seal to maintain vacuum integrity.
• Feed Inlet System:
  The feed slurry, paste, or concentrate enters the dryer through a feed nozzle or distributor located at the top (in vertical ATFDs) or one end (in horizontal ATFDs). The feed is spread immediately by the rotating blades into a thin, uniform film on the heated wall, ensuring efficient contact and rapid evaporation.
• Vapor Outlet System:
  As the solvent or moisture evaporates from the thin film, the generated vapors rise or flow toward a vapor outlet located near the top of the shell. The vapors are directed to a condenser and vacuum pump system, which maintains the required vacuum and removes non-condensable gases. The vapor duct is designed to minimize entrainment of product particles.
• Product Discharge System:
  The dried material, typically in the form of powder, flakes, or granules, is discharged continuously from the bottom outlet through a scraper or discharge screw. Depending on the material characteristics, the discharge may be gravity-assisted or screw-conveyed to a collection vessel or cooling zone.
• Heating and Vacuum Systems:
  The heating medium (usually steam or hot oil) circulates through the jacket to provide the required thermal energy for drying. A vacuum system consisting of a vacuum pump, condenser, and receiver maintains the operating pressure, which may range from 50 mbar to 200 mbar, depending on the product and solvent boiling point. The combination of heat and vacuum ensures gentle drying and high product quality.
• Instrumentation and Controls:
  Modern ATFDs are equipped with temperature and pressure sensors, feed rate controls, and rotational speed adjustments. These parameters are monitored and controlled through a PLC–HMI control system to ensure consistent operation and safe performance. Safety interlocks prevent overheating, overpressure, or mechanical failure.

 Construction of a Freeze Dryer

A freeze dryer (lyophilizer) consists of several key components integrated to perform the three stages above under controlled conditions.

Main Components and Their Functions