WORKING PRINCIPAL

Drying is an essential unit operation in a variety of chemical process industries (CPI) sectors. Food, pharmaceuticals, chemicals, plastics, timber, paper and other industries use drying equipment to eliminate moisture during product processing. Most dryers are classified as direct dryers, where hot air (at near Atmospheric pressure) is used to supply the heat to evaporate water or other solvents from the product. Another important dryer category, vacuum dryers, involves the use of a reduced-pressure atmosphere to surround the product. Drying is among the most energy-intensive unit operations, due to the high latent heat of vaporization of water and the inherent inefficiency of using hot air as the (most common) drying medium. Depending on the specific product attributes required, different industry sectors require different types of drying technology. Drying high-value products that are likely to be heat-sensitive, such as food, pharmaceuticals and biological products, demands special attention. When dried by convection at higher temperatures, these heat-sensitive products degrade, change color and appearance and have lower vitamin or nutrient content. Vacuum dryers offer an alternate path. This article discusses the operation and selection of vacuum dryers, and provides examples of applications in which vacuum drying is used.

DRYING PRINCIPALS

Drying involves two distinct drying periods, known as the constant drying period and the falling drying period. Drying occurs when liquid is vaporized by supplying heat to the wet feedstock. The liquid removed by the drying process could be either free moisture (unbound) or bound within the structure of the solid. The unbound moisture, normally present as a liquid film on the surface of a solid particle, is easily evaporated, while the bound moisture could be found within the solid material, trapped in the microstruc-ture of the solid. In this case, the moisture must travel to the surface to be evaporated. When a solid product is subjected to drying, removal of unbound and bound moisture depend on the rates at which these two pro- cesses proceed. Removal of unbound moisture depends on external conditions of air or gas temperature, flow, humidity, area of exposed surface and pressure. The movement of bound moisture depends on the nature of the product being dried and the extent of moisture within the product. Unbound moisture normally is removed by evaporation and vaporization. Raising the temperature facilitates the evaporation and air draws the moisture away. If the product being dried is heat-sensitive, then the temperature at which evaporation occurs (at the boiling point of water or other solvent) can be reduced by lowering the pressure with a vacuum.

VACUUM-DRYING OPERATION

The majority of dryers are of the direct (or convective) type, where hot air is used both to supply the heat for evaporation and to carry away the evaporated moisture from the product. Notable exceptions are freeze and vacuum dryers, which are used almost exclusively for drying heat sensitive products because vacuum dryers tend to be significantly more expensive than dryers that operate near atmospheric pressure. Vacuum drying is a process in which materials are dried in a reduced pressure environment, which lowers the heat needed for rapid drying. Vacuum dryers offer low-temperature drying of thermolabile materials and are suitable for solvent recovery from solid products containing solvents. Heat is usually supplied by passing steam or hot water through hollow shelves Drying temperatures can be carefully controlled and, for the major part of the drying cycle, the material remains at the boiling point of the wetting agent. Drying times are long, usually about 12 to 48 h. Unlike a direct-heat dryer in which the material is immersed directly into the heating media (usually a hot gas stream) and is dried by convection a vacuum dryer is an indirect-heat dryer That is, the heat is transferred to the material as it contacts the dryer’s heated surface, drying the material by conduction. Understanding this distinction is essential for grasping the advantages and limitations of vacuum drying, as well as for selecting a vacuum dryer that efficiently and economi-cally achieves process goals.To understand how vacuum operation can aid drying, consider the following equation, which represents a simplified drying theory.A major advantage to vacuum drying is its energy conservation less energy is needed for drying, cutting down on the economic and environmental costs associated with drying a product for storage, sale or other purposes. Vacuum-drying processes also tend to work faster than other drying methods, cutting down on processing times, which can be important in some facilities where products are being moved through quickly. Another advantage of drying materials in this way is a less damaging drying process. Some materials can experience problems at high temperatures, such as developing hard, leathery crusts from heat exposure during the drying process. Vacuum drying tends to retain the integrity of the original item without damaging it with heat. For foods and pharmaceuticals, this can be valuable, as other drying processes can degrade quality and make the foodless appealing or affect potency of heat-sensitive drug product.Using vacuum-drying equipment also reduces risks to workers. With other types of drying equipment, there are vented fumes and particles that can make people sick or that.heat to the dryer, facilitating faster drying than at normal atmospheric pressure. Hence, heat-sensitive materials such as foods, pharmaceuticals and antibiotics can be dried with vacuum drying with shorter drying times and at lower temperatures. The closed system also offers the advantage of handling reactive compounds or haz-ardous solvents in the product being dried. The vacuum dryer safely con-tains and condenses the hazardous vapors from such substances without any threat to the workplace environ-ment or to the outside atmosphere. Vacuum drying is predominately operated as a batch unit operation. However, a vacuum-drying unit can also be integrated as part of a continuous process. In those cases, proper control of the infeed and dis-charge materials is critical, along with proper process-control parameters. Limitations of vacuum dryers are generally related to the heat-transfer mode of the equipment. A vacuum dryer’s upper temperature limit (typi-cally about 600°F) is lower than that of a direct-heat dryer. The rate at which material temperature can be raised in a vacuum dryer is also limited. This is because the indirect-heat vacuum dryer is limited by the surface area available for heat transfer, unlike a direct-heat dryer, which is limited only by the hot-gas volume in the drying chamber. The vacuum pump is primarily responsible for the vacuum level inside the dryer.

CONSTRUCTION

The vacuum oven consists of a jacketed vessel sufficiently stout in construction to withstand vacuum within the oven and steam pressure in the jacket. In addition, the supports for the shelves form part of the jacket, giving a larger area of conduction heat transfer.

The oven can be closed by the door that can be locked tightly to give an airtight seal. The oven is connected through condenser and receiver to a vacuum pump, although if the liquid to be removed is water and the pump is of the ejector type that can handle water vapour, the pump can be connected directly to the oven.

CONDENSER

Shell and tube heat exchangers consist of series of tubes. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and tube heat exchangers are typically used for high-pressure applications (with pressures greater than 30 bar and temperatures greater than 260 °C). This is because the shell and tube heat exchangers are robust due to their shape.
Several thermal design features must be considered when designing the tubes in the shell and tube heat exchangers: There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes.

RECEIVER

The main purpose of air receiver tanks, are to give you the air storage capacity to meet high demand events that last for short periods of time (up to 30 seconds). This could be anything from a production worker sandblasting to someone using a blowgun to quickly dust themselves off. Air receiver tanks work in a similar manner to a battery. They allow you to use a smaller horsepower compressor to complete a larger task by utilizing stored energy. Receiver tanks also help steady the compressor controls in order to eliminate short cycling and over-pressurization. If receivers are too small or not present, the compressor will rapid cycle which will lead to a variety of issues. The air tank acts as a second heat exchanger because as the air passes through it, its temperature is lowered approximately another 10 degrees below what the first heat exchanger has already cooled it to.

VACUMM PUMP

Vacuum pumps are categorized by their operating pressure range and as such are classified as: primary pumps, booster pumps or secondary pumps. Within each pressure range are several different pump types, each employing a different technology, and each with some unique advantages in regard to pressure capacity, flow rate, cost and maintenance requirements.
Regardless of their design, the basic principle of operation is the same. The vacuum pump functions by removing the molecules of air and other gases from the vacuum chamber (or from the outlet side of a higher vacuum pump if connected in series). While the pressure in the chamber is reduced, removing additional molecules becomes exponentially harder to remove. As a result, an industrial vacuum system must be able to operate over a portion of an extraordinarily large pressure range, typically varying from 1 to 10-6 Torr of pressure. In research and scientific applications this is extended to 10-9 Torr or lower. In order to accomplish this, several different styles of pumps are used in a typical system, each covering a portion of the pressure range, and operating in series at times.

SALIENT FEATURES

• Main Body/Vacuum chamber SS316 with heavy duty SS316 flange and Stiffeners.
• Door SS 316 (hemisphere shape) with heavy duty SS316 flange.
• Shelves Hollow Type pads SS316 with flow Baffles.
• Condenser (Shell & tube) and Receiver in SS 304 with Isolation valve and Flow Glass in between to monitor condensation.
• Trays (without back folding) in SS316 rounded corners and edges.
• Explosion Vent/Rupture Disc on vacuum chamber.
• Digital Temperature Controller (NFLP) with solenoid controlled Pneumatic operated valve at Hot Water inlet (Optional).
• Provision for Validation Port on Body.
• Digital Temp., Indicator (NFLP) at H.W. inlet & outlet (Optional).
• Dial type Vacuum gauge and Digital Temp., Indicator (NFLP) in Vapor line (Optional).
• Pressure release valve in Steam line.
• View Glass/Light glass provided on vacuum chamber Door, Body and Receiver.
• Silicon transparent gasket for door.
• Nitrogen purging valve provided on Vacuum chamber.
• Vacuum break valve provided on vacuum chamber.
• Drain valve for Vacuum chamber and Receiver.
• Temperature Accuracy : _+ 2 to 30C
• FINISH : Internal 320 Grit Mirror Polish and Outer 180 Grit Mat Finish.

KEY ADVANTAGE

Faster Drying Times : The vacuum lowers the boiling point of water, allowing for efficient moisture removal at lower temperatures.

Energy Efficiency : Reduced temperatures and pressures lead to lower energy consumption and operating costs.

Preservation of Heat-Sensitive Materials : Drying at lower temperatures minimizes the risk of damage to heat-sensitive products.

Flexibility and Scalability : Vacuum tray dryers can be adapted to various materials and industries, including pharmaceuticals, chemicals, and food.

Easy to Maintain and Clean : The design of vacuum tray dryers, with their tray-based system, makes them relatively simple to maintain and clean. Cost-Effectiveness : Efficient drying and lower energy consumption can lead to long-term cost savings. 

Environmental Friendliness : Reduced energy consumption and solvent recovery contribute to a more sustainable process.

Safety : Inert gas purging and containment options enhance safety, especially when handling hazardous or potent materials.