## 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

As the drying air will contain a small proportion of powder (10-30%) when it leaves the chamber, it is necessary for economic reasons, but also because of pollution problems, to clean the drying air by separating the powder particles. This powder fraction is usually referred to as the fines, as they normally represent the smallest particles.

Fig. 15.1 Classification of method of powder recovery

The most used separators in the milk powder industry are

1. Cyclone
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 Battery of cyclone separator with central hopper

The operation theory is based on a vortex motion where the centrifugal force is acting on each particle and therefore causes the particles to move away from the cyclone axis towards the inner cyclone wall. However, the movement in the radial direction is the result of two opposing forces where the centrifugal force acts to move the particle to the wall, while the drag force of the air acts to carry the particles into the axis. As the centrifugal force is predominant, separation takes place.

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:

C = mVt2/r

Where

C= centrifugal force

m= mass of particle
Vt= tangential air velocity
r= radial distance to the wall from any given point

From this equation it can be concluded that the higher particle mass, the better efficiency. The shorter way the particle has to travel the better efficiency, and the closer the particle is to the wall the better efficiency, because the velocity is highest and the radial distance is short. However, time is required for the particles to travel to the cyclone wall, so a sufficient air residence time should be taken into consideration when designing a cyclone. From above equation it seems evident that small cyclones (diameter less than 1m ) will have the highest efficiency, a fact generally accepted.

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

Air through-put (velocity V0 and increased pressure drops will also increase the efficiency, but the energy requirement will increase simultaneously, so in general the upper limit is 175-200 mm WG for skimmilk powder. 140-160 mm WG is the maximum for whole milk in order to avoid deposits and final blocking. In most cases rotary valves are used as air lock and product discharge at the bottom of the cyclone.

In order to know a cyclone’s efficiency the following terms have to be defined:

a) The critical particle diameter
b) The cut size
c) The overall cyclone efficiency

a) The critical particle diameter is defined as the particle size that will be completely removed from the air flow (100% collection 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

· Feed with low solids or feed containing air
· 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

Fabrics filters consist of woven fabrics of tubular shape which are held over certain frames. Among the various designs available the commonest is bag filters. It consists of freely suspended tubes which are open at both the ends and mounted inclined or vertical. Dust laden air enters from the top and escapes from the pores. Where as solid particles leaves from the bottom. The sleeve type needs internal wire frame for supports .It is mounted inverted .Air enters from the bottom opening and is filtered on the inner side of the sleeve .Pocket type is mounted with opening on the top . Air, enters from the sides and gets filtered on outer surface and leaves through the inside of the tube while the dust is collected on outer surface. For large capacity plants large numbers of sleeves/pockets might be mounted in a chamber. Continuous build up dust layer on the fabric surface results in increase in pressure drop and thus in drop of capacity. To avoid this, the dust is continuously dislodged by mechanical sacking or by reverse air pulsations.
The advantages of fabric filters are

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.

However the disadvantages of the system are

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

The wet scrubber has now been known for many years in the chemical industry. It is based on the venturi scrubber principle. When introduced in the dairy industry, it proved especially efficient, i.e. an efficiency very close to 100%. The outlet separator is designed according to the well-known cyclone principles, however with a modified outlet, resulting in a minimum liquid level, thereby minimizing bacteria growth, and a design ensuring de-aerating thus avoiding from building.

Fig. 15.4.1 Sanitary wet scrubber

Fig. 15.4.2 Wet scrubber recycled with water

The outlet air from the spray dryer containing powder particles is accelerated to a high velocity in the venturi inlet, where also the liquid is injected through full-cone nozzles. Due to different velocities between the air/particles and the liquid droplets, they will collide, and the powder will dissolve in the liquid droplets. Passing through the subsequent diffuser this process will continue simultaneously with a certain pressure recovery of the air/droplet mix. (Fig. 15.4.1 and 15.4.2). Passing through the separator, air and liquid are separated again. The air leaves through the centre duct and the liquid through the bottom outlet for further processing or recycling depending on what system is selected.

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

i. High initial cost.

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.