Definition of spray dryer
Spray dryer is one of the common dryers is used by different industries. In this dryer, materials are first dispersed into fine particles and then are dried by hot air. Generally, the materials are sprayed by an atomizer into fine particles. A spray dryer consists of a pump for feed transfer, an atomizer, air generating, air distributor, drying chamber and systems for removing particles from the exhaust air. This type of dryer is widely used in industries such as food and pharmaceutical industries, since materials in these industries are temperature sensitive. Properties of final product depends on the design of dryer, feed characteristics and operational parameters.
Spray Dryer Applications
Spray dryers are used in the pharmaceutical industry for drying pharmaceuticals such as vaccines, blood products, penicillin, etc. Moreover, spray dryer is used in the food industries (e.g. milk, soya protein, whey, etc.), chemical industries (e.g. catalysts, amino acids and so on), fertilizer industries (e.g. nitrates, ammonium salts, phosphate, etc.), detergents industry, biochemical industries, and ceramic industries and so on.
How spray dryer works
Figure 1 shows a spray dryer. The feed enters the atomizer using a suitable pump (usually mono pump). The atomizer converts the feed into fine particles at a speed of 15 to 20,000 rpm to maximize heat transfer. The particle size can vary from 20 to 180 microns depending on the feed. On the other hand, hot air is generated using a heater or burner and enters the system through an air distributor. The sprayed particles come in contact with the hot air flow in the drying chamber. Heat and mass transfer occurs quickly and powder or dry fine particles are obtained in a short time. Larger particles are exited from the dryer bottom, while smaller particles enter the cyclone with hot air flow. The air tangentially enters into the cyclone. Solids often collide the wall because of higher density and move tangentially to the bottom of the cyclone and the air moves upward from the central part. If the fine particles were still in the air, a bag filter should also be installed after the cyclone. However, it is recommended that installing bag filter be postponed after the operational test so that unnecessary cost is not imposed on the process. If the bag filter is needed, number of bags, diameter and the length are designed and simply added to the process. At the bottom of cyclone (or bag filter), a rotary valve is installed to materials are get out continuously and simultaneously is prevented from entering or exiting of air. Moreover, it should be mentioned that there are some pneumatic hammers in drying chamber; they help to detach the possible attached particles at the wall.
Figure 1) schematic of spray dryer
The main equipment of the spray dryer
Feed tank: a suitable feed tank should be considered. The size and material of tank are chosen based on the weight and type of feed. The feed of tank is pumped into the dryer.
Pump: the feed (in the form of slurry, paste, solution, etc.) should be pumped into the system. The type of the pump is selected based on the type of feed. For example, if the feed is slurry, high energy is needed to pump it, so high-power pump is used. Mono pump is one of the common pumps used in these dryers.
Atomizer: atomizer plays a major role in the spray drying. The atomizer convert feed into fine particles to reduce surface tension and increase contact surface, so heat and mas transfer increase at the next stage of operation. The final shape, size and structure of product are influenced by the performance of atomizer.
Inlet fan: there is a blower fan in the system. The fan directs the air into the heater with the designed amount and pressure. After heating the air transferred into the main body of dryer and causes materials drying. As mentioned, choosing the type of fan is very important.
Hot air generation system: Electric elements or burner can be used to generate hot air. Hot air system with electric element is relatively simple, but imposes a huge monthly cost on the process due to the electricity price. The gas burner consists of two categories, 1) use exhaust gas of burner and 2) generation of hot air conditioning (without combustion products). In the first case, gases from combustion at specific temperature (between 70 to 140 ºC) collide to the materials and cause drying. In the second case, clean air passes through a heat exchanger and receives heat from the combustion gases and then enters the system. The cost of hot air conditioning (clean) making is higher, but it improves product quality. Moreover, it is necessary to use hot air conditioning (clean) at food and pharmaceutical industry. Capacity of generated hot air is determined by system type.
Hot air distributor: the distributor is used to hot air enters uniformly into the system. Thus, all particles are received the heat uniformly and are dried under the same condition. Moreover, it provides the best condition for heat and mass transfer between feed and hot air.
Drying chamber: it includes main body (cylindrical) and a conical bottom. The main operation is performed in this chamber. Particles from the atomizer collide with the hot air flow in this chamber. As a result, moisture is removed and powder or dried particles (the desired product) are obtained. Some of these particles (larger particles) are removed from the bottom of dryer. It should be mentioned that there are pneumatic hammers in this chamber to detach any attached particles in to the wall of the chamber. Moreover, drying chamber is insulated using materials such as ceramic wool to minimize heat transfer to the environment.
Cyclone: cyclone is used to separate solids that may be present in the exhaust air. The separation mechanism of the cyclone separator is based on centrifugal force. Design of diameter, length of cylindrical part, length of conical part, making inlet air section tangential and etc. are very important.
Bag filter: if there were particles in the outlet air of cyclone, it is necessary to install a bag filter after the cyclone to remove 100 % of material from the air. The dry particles are retained on the bag surface while the clean air passes through it, being expelled from the device. Number of bags, diameter, length of cube section, length of funnel and etc. are important points in the design of bag filter.
Rotary valve: rotary valve is used to remove materials from the cyclone and bag filter continuously. In fact, role of rotary valve is transfer solids from one region to another region without changing the pressure or connecting the air of two regions. Suitable design of rotary valve is important especially in the cyclone. If the rotary valve passes much air, the performance of the cyclone will not be acceptable.
Exhaust fan: getting inlet air from the system into the surrounding is performed by this fan. Choosing the right fan (in terms of pressure and air flow) is directly related to the performance of the overall system (from the main body to cyclone and bag filter).
Advantages of spray dryer
- The quality and properties of products are controllable and maintained through the drying process. It can be claimed that final product quality is higher in comparison to other dryers.
- Pharmaceutical and temperature-sensitive materials can be dried at ambient pressure and low temperature.
- Operation is relatively simple in these dryers and high-tonnage production in continuous operation can be obtained.
- Any feed that can be pumped, including emulsion, paste, slurry, etc. can be dried in this dryer.
- Corrosion is minimum in this dryer; since the anhydrous materials collide the surface, the choice of material for construction of this dryer is simple.
- This dryer has low energy consumption and high efficiency.
Figure 3: Spray dryer
Design, manufacture and after sales services
Vandad Chemical Industry Company has designed and manufactured these dryers with the highest quality using the engineering department and Iranian elite workforces. Many points are important for design and manufacturing of this type of dryer, all of which have been obtained through various laboratory, pilot, bench and industrial scale tests and are observed in the final product. The wide range of Vibrating Fluid Bed Dryer are designed and manufactured by the engineering department. Table 1 illustrate some of the design parameters for mentioned dryer. It should be mentioned that all parameters are checked again for requested materials and the mentioned numbers may vary for each project. Manufactured machines have up to 1 year repair warranty. It is taught in person by company engineers that how to work. Moreover, different points such as how to start, operation setting, how to shut down the system, maintenance , etc. will be given in a separate booklet (final book).
Table 1) design parameters for Vandad spray dryer.
|Outlet Temperature (||Inlet Temperature (||Evaporation rate (kg/hr)||Model|
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