Drying is used in most different processes and solid line productions. This process is often used as a final production step before selling or packaging products. Sometimes, the drying process is used at the beginning of the production process. In these cases, the aim is drying and preparation of feed to enter the production process. Generally, drying is one of the most important parts of any production process. Understanding the methods of drying and choosing the suitable equipment will not only produce a quality and market-friendly product, but also can save on production cost. Moreover, selection the kind of drying process depends on the purpose of the drying and kind of materials. Thus, scientists and researchers have suggested different methods for drying. Some of these methods are suitable for laboratory scale; others are industrialized and widely used in various industries. The most common industrial dryers cab be spray dryer, vibrating fluid bed dryer, rotary dryer, tray dryer, sublimation dryer and so on. The following points should be checked by the line product designers before selecting the kind of dryer:
- State of intended matter for drying (liquid such as dry milk or solid such as coffee)
- Physical properties of feed (viscosity, density, temperature sensitivity, etc.)
- Examination of corrosion in feed (This can be checked by examining the feed and its composition percentage; feed corrosion has a significant effect on dryer selection and final cost of system)
- The final percentage of moisture in the product (sometimes, the purpose of drying is simple separation of solid from liquid and in some cases, the purpose of drying is a solid with close to zero moisture)
- Production capacity or feed input capacity (design of dryer is directly related to the production capacity, so production capacity should be determined prior to design and making)
- Safety issues (incendiary, toxic or any safety related issues should be considered)
In this article, we are going to introduce one of the best kind of dryers which is used in various industries and its technical know-how developed by Vandad Chemical Industry company.
Definition of Vibrating Fluid Bed Dryer
Vibrating Fluid Bed Dryer is one the common types of dryers at various industries. In this type of dryer, material is poured onto a perforated plate and they move forward by system vibration. A hot air stream from under the plate collides with the material and causes drying wet material along the way. This type of dryer is one of the best type of dryers in terms of heat transfer efficiency, product quality and power consumption. The applications and performance of this dryer are discussed below.
Vibration Fluid Bed dryer is used in several industries such as food, pharmaceutical, petrochemical, chemical fertilizer and so on. Some example materials which are dried by this dryer include: various type of food grains (sugar, salt, rice, coffee, tea, etc.) and various types of chemicals (ammonium sulfate, mono ammonium phosphate, di- ammonium phosphate, urea phosphate, calcium ammonium sulfate, oxalate, etc.).
How Vibration Fluid Bed Dryer works
As shown in Figure 1, the materials are first poured into the dryer. The input materials amount should be proportional to the drying capacity. Materials entry control is simply performed by a screw pipe or a rotary valve. A suitable grid is installed on the dryer depending on the size and type of materials. Diameter of the grid holes is such that the materials are not spilled down. This grid can be made of steel 304 or steel 316. Moreover, type of steel can be changed according to the customer’s need. Diameter of holes can also be changed.
Diameter of holes can be changed from 3 cm to 0.03 mm. The materials move on the grid by two vibrating motors. Vibration motor power, vibration percentage and rotation speed value can be adjusted according to the requested capacity. Hot air should be generated to dry the materials. 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 industry. Capacity of generated hot air is determined by system type. After colliding hot air and materials (vertically), moisture of materials is removed from the top of the Vibrating Fluid Bed Dryer along with hot air at temperature between 40 to 70 ºC and much of the materials spills out of the main body of dryer. It is necessary to decrease the temperature after drying in order to properly collect the materials from the main part. For this purpose, dry air at ambient temperature is injected into the system in the main body and the bottom part of the Vibration Fluid Bed Dryer to reduce the temperature of the dried particles.
Some lightweight materials may be left along with air from the top of the system. Thus, a suitable cyclone is designed for the system according to the amount of inlet air. 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.
The system is assembled so that vibration is applied only to the main body. Moreover, designing a suitable power switch enables operator to test different materials and develop different residence times for different materials so that drying process can be
completely adapted to the desired quality.
Figure 1. Vibrating Fluid Bed dryer
Here is a summary of the main equipment available in the dryer and their roles:
Inlet fans: there are two blower fans in the system. The first fan directs the air into the heater with the designed amount and pressure; the second fan directs the cool and dry air into the dryer to lower the materials’ temperature before leaving the system. As mentioned, choosing the type of fan is very important.
Heater or hot air generation system: different types of heater can be designed to increase the output temperature of the first fan. For this type of dryers, maximum temperature of 140 ºC is sufficient. The precise and engineering design of heater (of any type) reduces the power consumption of gas, electricity and used materials in the heater.
Vibration motor: two vibration motor are handled moving the materials. Vibration motor power, motor speed, crank rate are the important factors for selecting vibration motor. Vibration motor is shown in figure 2.
Figure 2. Vibration motor
Main body of Vibrating Fluid Bed dryer: in this section, hot air (and cool air at the bottom) collides the materials vertically and eliminate moisture from the material. The design of diameter, length and overall configuration of system for proper vibration transmission is important. Main body of Vibrating Fluid Bed dryer is shown in Figure3.
Figure 3. Main body of the Vibrating Fluid Bed dryer
Cyclone: cyclone is used to separate solids that may be present in the exhaust air. Design of diameter, length of cylindrical part, length of conical part, making inlet air section tangential and etc. are very important. Figure 4 shows the cyclone.
Figure 4. Cyclone
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. 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. Figure 6 shows the rotary valve.
Figure 5. Rotary valve
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).
Vibrating Fluid Bed Dryer has several advantages. Some of its important advantages include:
- Flexibility: it is quite flexible respect to material type, material size, and retention time and so on. It can be designed and constructed according to the needs of any company. If you buy a dryer, you can use it for wide range of materials with slight modifications.
- Appropriate temperature distribution: as described in the previous section, hot air uniformly collides all materials on the grid and eliminates moisture.
- Low maintenance cost: this system has medium power consumption; its maintenance cost and other operating costs are also economical.
- Cooling: after heating process, the materials are cooled in the same system so that the operator can simply collect the materials. This is a unique feature of the Vibrating Fluid Bed Dryer.
- Invariant quality of materials: this type of dryer makes the slightest change in the state of matter and causes no clogging.
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. Characteristics of some Vibration Fluid Bed Dryer models of Vandad Chemical Industry Company
|Kw of vibrating motor||Rate of vaporization (kg/hr)||Temperature of outlet (||Temperature of inlet (||Area of fluidized bed (m2)||Model|