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Lesson 15. THEORY OF SOLID GAS SEPEARATION, CYCLONE SEPARATORS, BAG FILTERS ETC.
Module 2. Drying
Lesson 15
THEORY OF SOLID GAS SEPARATION, CYCLONE SEPARATORS, BAG FILTERS etc
Introduction
15.1 Powder Separation System
Fig. 15.1 Classification of method of powder recovery
The most used separators in the milk powder industry are
2. Bag filter
3. Wet scrubber
15.2 Cyclone
The cyclone has some advantages, such as high efficiency, it is easy to maintain and easy to clean.
Fig. 15.2 Cyclone separator
Fig. 15.3
Powder and air pass tangentially into the cyclone at equal velocities. Powder and air swirl in a spiral form down to the base of the cyclone separating the powder out to the cyclone wall. Powder leaves the bottom of the cyclone via a locking device. The clean air spirals upwards along the center axis of the cyclone and passes out at the top. (Fig. 15.2.1).
The centrifugal force each particle is exposed to can be seen in this equation:
Where
C= centrifugal force
Vt= tangential air velocity
r= radial distance to the wall from any given point
However, the big capacity dryers are require many cyclones (Fig. 15.2.2). As each cyclone has to have an outlet for powder in form of a rotary valve, pneumatic valve or flap valve, this means that there is a big risk of air leaks which will reduce the cyclone efficiency. The small cyclones can also be connected to one central hopper, and only one valve is then necessary. This means however, that unless there is exactly the same pressure drop over each cyclone, air and powder will pass from one cyclone to another via the bottom outlet. This will result in decreased efficiency and increased powder loss. Cleaning the many small cyclones is a problem, as it is a time consuming job, and with the many corners there is a risk of a bacterial infection. For above reasons the cyclones have become bigger and bigger and are now constructed with diameters of 2.5-3 m, each handling 25,000-30,000 kg of air/h.
When designing a cyclone various key figures should be taken into account in order to obtain the highest efficiency. This is achieved if
Cyclone diameter : Exit duct diameter = 3:1
Cyclone diameter : exit duct diameter = 10:1
In order to know a cyclone’s efficiency the following terms have to be defined:
b) The cut size
c) The overall cyclone efficiency
b) The cut size is defined as the size for which 50% collection is obtained and is a much better value for starting the efficiency of cyclones.
c) The overall cyclone efficiency is obtained when handling a product of definite size distribution.
Another method of learning the cyclone efficiency is by a simple powder loss measurement after the cyclone. A very small fraction of the out-going air is passed through a high-efficient mini cyclone or through micro dust filters. The amount of powder collected is directly proportional to the powder loss, which will mainly due to
· High outlet air temperature
· Low particle density (as a result of the above, for example)
· Leaking product outlet from old non-adjusted rotary valves
· Blocked cyclone
· Change in drying parameter resulting in decrease of mean particle size
15.3 Bag Filters
Average powder loss from a normal high-efficient cyclone should not exceed 0.5% when spray drying skimmilk. However, local authorities may conclude that 0.5% is too high due to pollution, thus requiring a final cleaning of the air. This is usually done in bag filters consisting of numerous bags or filters arranged so that each bag receives almost equal quantities of air. The direction of the air is from outside in through the filter material to the inner part of the bag from where the cleaned air enters an exhaust manifold. With a correct selection of filter material high efficiencies can be obtained and collection of 1 micron particles is reported from the manufacturers. The collected powder is automatically shaken off by blowing compressed air through the filter bags from the inner side. The powder is collected at the bottom via a rotary valve. (Fig. 15.3).
The bag filter may also replace the cyclones, a solution often chosen in dryers for whey protein powder or egg white. To prevent condensation, especially on the conical part of the filter housing, war air, water circulation or heat tracing is established. Powder loss of 15-20 mg/Nm3 is reported.
Fig. 15.4 Bag filters
a. High collection efficiency for smallest size particles. For particles up to 1 micron size it provides 99% collection efficiency. However , it can remove a substantial quantity of particles as small as 0.01 micron size
b. Moderate pressure drop of the order of 7-15 cm of water gauge.
c. Dry recover , and
d. Absences of corrosion and rusting problem.
a. Large size of the unit
b. Difficult maintenance and need for frequent replacement
c. Plugging of fabric pores by hygroscopic materials
15.4 Wet Scrubbers
Fig. 15.4.1 Sanitary wet scrubber
Fig. 15.4.2 Wet scrubber recycled with water
Table 15.1 Some of the general advantages and disadvantages of wet scrubbers are
Advantage |
Disadvantages |
1.Ability to recover solids as well as liquids. |
1.May create liquid disposal problem if the same is hot reused. |
2.Ability to handle high temperature, high humidity gas streams. |
2.Product is collected wet which subsequent drying. |
3.Initial cost is usually low. |
3.Pressure drop is higher. Thus operating cost is more. |
4.Small space requirement. |
4. Sever corrosion and maintenance problem. |
|
|
15.5 Electrostatic (ES) Precipitators
The device works on the principle that if one or more of the materials in a mixture can be imparted surface charge, on or before entering the electrostatic field, those charged grains will be repelled from one electrode and attracted towards other depending upon the polarity of the electrode. Thus in ES precipitator, strong electric field is created with the help of rectifier and transformer unit and dust laden air flows, that chamber. Powder particles develop negative polarity by combination with air ions. These particles move towards positively charged collecting plates where from they fall in separate chute after getting neutralized. High D.C voltage of the order of 50,000 volt is used.
The advantages of ES precipitator are
- High collection efficiency above 99% for particles upto 0.01 micron size,
- Low pressure drop,
- Low maintenance cost.
The disadvantages of the unit are
ii. Higher energy consumption and difficult operation,
iii. Ionization of air and production of ozone.
Because of these disadvantages ES precipitators are not frequently used in dairy plants. They are preferred in chemical industries, especially those dealing with hazardous chemicals where near complete trapping of even the finest particles is essential.