Dust-laden streams

Dust-laden streams can also cause operational problems. A turboexpander that can efficiently process condensing streams (gas with fog droplets suspended) can usually handle a stream with suspended solid particles, as long as the particle size does not exceed 2-3 p. The newer designs reduce erosion of expander back rotor seals by disposing of... 

Baghouses are high-maintenance items due to internal movement in a dust-laden stream. They operate at low air-to-cloth ratios and the collectors are large and more costly than some other devices described in this section. 

Wet Dynamic Scrubbers. These scrubbers are also known as mechanical scrubbers, as seen in Figure 20.7. They have a power-driven rotor to produce a spray that is centered in the inlet of the unit such that the blades of the rotor are coated with water. As the dust-laden stream enters, it contacts the water surfaces and the dust-water mixture is thrown outward against the walls. 

Adhesion of Particles in Relation to Velocity of a Dust-Laden Stream... 

An important factor in determining adhesion is the velocity during the process of particle contact with the surface, i.e., the particle velocity at which adhesion takes place. For a stationary object, this velocity is absolute. For a moving object, the velocity of the object must be compared with the velocity of the dust-laden stream, i.e., we must be concerned with the relative contact velocity. 

Thus we see that the adhesion of particles to the bottom of an air duct depends on the ratio between the particle-settling velocity and the flow velocity of the dust-laden stream and also on the properties of the contiguous bodies. 

The coefficient Kq, in contrast to the coefficient does not take into account the conditions of flow of the dust-laden stream across the walls of the air duct [250]. The coefficient Kq, for the movement of particles with a diameter of 1 fim in a rising vertical flow in a tube with a diameter of 2.5 cm, changes from 0.84 to 0.89 as the flow velocity changes from 1.5 to 4.5 m/sec [251]. 

Adhesion in Bends in Air Ducts. When the flow direction of a dust-laden stream is changed, the probability of particle contact with the surface and the probability of particle adhesion are increased as a result of inertia. 

An empirical approach to the determination of adhesion under these conditions gives only a qualitative picture. In [255] an attempt was made at quantitative evaluation of adhesion under conditions of rising flow in a duct. To this end, a study was performed on particles of milled potassium dichromate with diameters from 9 to 189 /xm, in the movement of a dust-laden stream in a vertical... 

In the present case, r o is the ratio of the number of adherent particles to the total number of particles passing across the midsection of the obstacle. The amount of adherent dust and the value of the capture coefficient will depend on the conditions of flow around the obstacle by the dust-laden stream, on the possibility of particle rebound from the surface, and on the adhesive forces capable of holding these particles. The capture coefficient will have a value less than unity. 

The capture coefficient is a quantity characterizing not only the conditions of flow around the surfaces by the dust-laden stream, but also the adhesion of particles to these surfaces. 

The adhesion of particles from a stream can be evaluated on a probability basis [257]. The probability is related to the velocity of the flow around the surface by the dust-laden stream. Minimum adhesion is achieved at a certain velocity Uad, which for spherical latex particles with a diameter of 1.27 /rm has been found to be 83 and 91 cm/sec for surfaces of steel and quartz, respectively. For the conditions of the experiment (horizontal flow, vacuum), the probability of adhesion is given by... 

This equation is valid for vertical flow. The plus sign is used ahead of Vff when the direction of particle deposition is the same as the flow direction of the dust-laden stream the minus-sign is used if the directions of flow and deposition do not coincide. 

Cylindrical surfaces may be placed not only in the center of the stream, but also adjacent to the wall of the duct. For example, in reinforcing the walls of certain air ducts such as in mine drifts, timber sets are used, usually with cylindrical surfaces. The air flow passes around these surfaces. The particular features of flow around cylindrical surfaces in contact with flat surfaces determine the specific features of deposition and adhesion of aerosol particles. The reduction in dust concentration because of adhesion as the dust-laden stream passes through a series timber set can be determined from the formula [248]... 

If the metal surface is precoated with a layer of particles and this layer is fixed on the surface, the adhesion of particles from the dust-laden stream to this surface is characterized by the following results ... 

Influence of Flow Velocity on Adhesion of Particles to Plates. The adhesion of particles to plates will depend on the velocity of the dust-laden stream (see Fig. IX.7). As the air-flow velocity is increased from 5 to 25 m/sec, we also see an increase in the relative amount of fine particles attached to the surface (lines a and b), apparently because of the specific features of the flow around the obstacle. The influence of air-flow velocity is taken into account only indirectly in Eq. (IX.42), through the number n. Only at the initial moment, when there are no adherent particles on the surface, will the number of deposited particles be proportional to the number of particles striking the surface. Thereafter, there is an increased probability that the incident particles will strike particles previously stuck to the surface, so that the probability of particle rebound is increased. 

The relationship between the velocity of the dust-laden stream and the velocity 5 0 for quartz particles with a diameter of 5-15 /xm when these particles were deposited on a polyamide surface, was found in [155] to be as follows As the particle diameter was increased from 5 to 15 /xm, the velocity of the undisturbed flow dropped from 19 to 8.5 m/sec, whereupon the velocity 50 changed form 0.28 to 0.84 m/sec. 

With the electric field, the rate constant of the process was found to be practically independent of particle concentration over a range from 30 to 400 particles per cm, but the rate constant fell off with increasing time of residence of the dust-laden stream in the closed space. For example, when this time was increased to 18 min, the constant Xgp dropped from 0.150 to 0.040 min. On the whole, when the electric field was acting, approximately 75% of the particles were deposited on the walls and adhered to these walls. 

In conclusion, let us once more examine the two processes taking place in flow of a dust-laden stream around an obstacle that has adherent particles, i.e., the detachment of previously adherent particles and the precipitation of particles present in the stream. 

It has been found that the particle-size distribution, number, and shape of the contaminations falling out of the flow depend on the direction of gas flow. Thus, small particles stick in the rear zone. The shape of edge contaminations depends on the manner in which the dust-laden stream flows around the obstacles. The frontal part of the pipes is subjected to the impact of the large particles hence, sticking to this part only occurs at low velocities. 

One of the oldest, simplest, and most efficient methods for removing solid particulate contaminants from gas streams is by filtration through fabric media. The fabric filter is capable of providing high collection efficiencies for particles as small as 0.5 pm and will remove a substantial quantity of particles as small as 0.01 pm. In its simplest form, the industrial fabric filter consists of a woven or felted fabric through which dust-laden gases are forced. A combination of factors results in the collection of particles on the fabric filters. When woven fabrics arc used, a dust cake eventually forms. This, in turn, acts predominantly as a sieving mechanism. When felted fabrics are used, the dust cake is minimal or nonc.xistent. 

Cyclone Separators. The most commonly used equipment for the separation of dust particles from an air/gas stream is the cyclone separator. The literature on design and operation of cyclones has been extensively reviewed by Rietemer and Vetver (1961), Maas (1979), Zenz (1982), and Pell and Dunson (1999). A sketch of a cyclone separator and typical dimensional ratios are found in Figure 20.1(b). The dust-laden gas stream enters near the top of the collection chamber tangentially. The force on the larger particles is greater than the force on the smaller ones because the latter... 

Shaker filters have the drawback that they must be taken off stream to clean the bags. Since continuous on stream operation is required, several baghouse chambers are installed in parallel with dampers, permitting one section to be in the cleaning cycle while the rest of the baghouse is filtering the dust-laden gas stream. 

Spray Scrubbers. Spray scrubbers consist of an empty cylindrical chamber in which dust-laden air is contacted with water from spray nozzles, as shown in Figure 20.6(b). The dust-laden air enters the tower near the bottom and passes upward through the water spray. This type of equipment is similar to the spray towers used in mass transfer operations. Proper water distribution can be a problem, so multiple banks of spray nozzles mounted on a manifold produce better air-water contact. The spray knocks down the dust that leaves the bottom of the unit as a dust-water mixture. An entrainment separator is mounted in the upper part of the chamber to reduce the potential spray carryover into the exiting gas stream. 

Fig. 10.50 Detail of a dust laden fiber from Fig. 10.49 showing how particles extend into the gas stream.

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