SECADOR FLASH

INSTITUTO POLITÉCNICO NACIONAL

Escuela Superior de Ingeniería Química e Industrias Extractivas

[pic 1][pic 2]

Procesos de separación por membrana y los que involucran una fase sólida.

Trabajo de investigación: “Secador Flash”

PROCESS OF A FLASH OR PNEUMATIC DRYER

This operation consists of introducing a wet solid in a vertical column through which circulates a current of hot air at high speed.[pic 3]

The product enters the equipment through a kicker mounted on the base of the drying column that rotates at high speed and drives the product vertically and upward, facilitating its drag by the drying air and preventing its accumulation in the horizontal parts of the dryer flash. Another way is through a cascade feeder.

The materials to be processed must be disintegrable for a greater contact "air-particle" that facilitates the drying in the short time of residence.

A hot gas stream controlled by temperature from the air heater is sucked by the extraction fan through the main body and fluidizes the fine particles in the grinding chamber. This guarantees an optimal heat exchange and an instantaneous evaporation of moisture.

The combination of these elements allows an excellent dispersion of the product with a relatively low loss of load that minimizes energy consumption, optimizing the speed of heat and mass exchange between solid and gases.

The dry particles are transported with the gas stream to the top of the dryer where a separator classifies the particles by weight. Particles that are too large or too wet return to the lower parts.

This type of drying is very efficient because each particle is surrounded by an air column producing uniformity of temperature, distribution composition and particle size.

. The particles that pass the classifier at the determined cutoff point are transported with the extracted air to a dust-air separation system such as a cyclone or a cyclone filter.

The high-efficiency, bag-type, low-pressure, high-retention, extended-surface filters are manufactured to improve indoor air quality, also used as a pre-filter to absolute or HEPA

They are composed of individual bags whose number varies according to specifications. Each of these bags is subdivided, in turn, through sealed intermediate seams that run along the bag, in tubular sections. These seams create a uniform channel through which air flows with minimal resistance and ensure a high retention capacity; The internal reinforcement keeps the bags in a controlled way under different conditions during the operation, maximizing the use of the filter media and lengthening its useful life.

[pic 4]

Features

• Short residence time
• A buffer hopper or back-mixing system if required
• Can be designed for open circuit, partial gas recycle and SSD™
• Can be designed for operation in an inert atmosphere
• Simple system with few moving parts

[pic 5]

Applications

Basic operation principle and applications of flash dryers

The simple flash drying system includes six basic components: the gas heater, the wet material feeder, the drying duct, the separator, exhaust fan, and a dried product collector. The wet particles are fed into the hot gas stream sometimes with special mixing devices. The stream flows up the drying tube. The gas velocity must be greater than the free fall velocity of the largest particle to be dried. The gas velocity in relation to the particle velocity is high. Thermal contact between the conveying air and the solids as mentioned above is usually very short and therefore flash dryers are most suitable for removal of external moisture (surface moisture) and are less suit-able for removal of internal moisture. At the end of the drying process a dust separation arrangement is installed. It must comply with the regulations for pollution control. For this purpose cyclone dust separators, fabric filters, electrostatic precipitators, wet scrubbers, and fabric filters are used.

 [pic 6]

Figure 1. Simple Flash Drying system

High rates of evaporation in flash dryers are leading to low temperatures of the dried material and indicate that flash dryers are particularly useful for drying granular, crystalline, pasty, and powdery products, etc. Flash dryers are used in various branches of the chemical, pharmaceutical, ceramic, gypsum, wood, and mining industries. Flash dryers are simple in construction and have low capital cost and they are almost trouble free. Vertical type of construction, which facilitates installation in existing buildings, is advantageous for the flash drying systems. The tube of most flash dryers is of circular and uniform cross section. In some cases, the tube may diverge and converge and may have sudden expansions and contractions. The tube may be heated through the wall to keep up the temperature of the gas. In order to shorten the drying time, recirculation of the material is used. In this case the number of cycles of different particles may be different and as a result the residence time of the particles will not be the same.

In order to achieve efficient pneumatic drying process, the air velocity should be as low as possible to achieve materials transport, the mass flow rate of the gas should be the minimum necessary to achieve the specified drying rate, the temperature of the hot gas should be as high as possible without exceeding limits imposed by the thermal sensitivity of the solids or safety considerations, and the construction of the dryer should allow to achieve thermal equilibrium between the gas and the solid.

The advantages of flash dryers are the following:

• Short contact time and parallel flow make possible to dry thermolabile materials.
• The dryer needs only a very small area and can be installed outside a building. However, the gas cleaning system should be located inside the building in order to avoid moisture and dust deposition.
• The dryer is easy to control. The low material content in the dryer enables equilibrium conditions to be reached very quickly.
• Due to small number of moving parts the maintenance cost is low. The capital costs are low in comparison with other types of dryers.
• Simultaneous drying and transportation is useful for materials handling process.

The disadvantages of flash dryers are as follows:

• High efficiency of gas cleaning system is required.
• Because of powder emission, the dryer cannot be used for toxic materials. In some cases this disadvantage can be avoided using superheated steam as a drying agent.
• For lumped materials difficult to disperse, drying is impossible to carry out in this apparatus.
• There is a risk of fire and explosion, so care must be taken to avoid flammability limits in the dryer.
• In general, especially when recirculation is applied, not all material particles have the same residence time in the dryer.

Components of spin flash dryer

[pic 7]

(A) HOT AIR GENERATION SYSTEM

This will consist of a hot air generator - direct/indirect/finned tube heat exchanger type. The unit would have to be connected to the dryer by means of a Hot Air Duct which will have to be insulated.

(B) LUMP BREAKER CUM SCREW CONVEYOR

This unit would be connected to the bottom of suitable wet product storage and consists of two shafts mounted with special lump breakers meshing with each other, below a dual screw conveyor will feed material in to the dryer. The lump breakers shafts and the screw conveyors will run by a single motor with gear box through a system of meshing gears, bearings, stuffing box etc. This unit is of a special design for the type of cake and is very robust and sturdy.

...