Vacuum dryer - Pharmaceutical Engineering B. Pharm Third Semester PDF Notes

Vacuum dryer

Principle

In vacuum dryer, material is dried by the application of vacuum.

When vacuum is created, the pressure is lowered so that water boils at a lower temperature. Hence, water evaporates faster. The heat transfer becomes efficient i.e. rate of drying enhances substantially.

Construction

• It is made up of a cast iron heavy jacketed vessel. It is so strong that it can withstand high vacuum within the oven & steam pressure in the jacket.

• The enclosed space is divided into a number of portions by means of 20 hollow shelves, which are part of jacket. These shelves provide large area for conduction of heat.

• Over the shelves, metal trays are placed for keeping the material

• The oven door can be locked tightly to give an air tight seal. Oven is connected to a vacuum pump by placing condenser in between

Working

• Material to be dried is spread on trays. Trays are placed on the shelves

• Pressure is decreased up to 30 to 60 kilopascals by means of vacuum pump. Door is closed firmly

• Steam or hot air is supplied into the hollow space of jacket & shelves. Heat transfer by conduction takes place

• At this vacuum, evaporation of water from the material takes place at 25-30°C, on account of lowering of boiling point.

• Water vapour passes into the condenser where condensation takes place

Advantages

1. Large surface area for heat transfer

2. Handling of material, trays & equipment is easy

3. Easy to switching over to next material

4. Hot water of desired temperature can be supplied

5. Electrically heated hollow shelves can be used

Disadvantages

1. Heat transfer coefficient are low

2. Limited capacity & used for batch process

3. More expensive than tray dryer, Labour & running cost is also high

4. There is danger of overheating as the material is in contact with steam heated surface for longer period

Uses

1. Heat sensitive materials, which undergo decomposition

2. Dusty & hygroscopic material

3. Drugs containing toxic solvents can be separated into closed containers.

4. Feed containing valuable solvents. These are recovered by condensation

5. Drugs which required as porous end products