Comparison of Different Collection Efficiency Models for Venturi Scrubbers

7/28/2019 Comparison of Different Collection Efficiency Models for Venturi Scrubbers

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to the liquid droplets) promotes particle collection. For gaseous pollutant collection, the pollutant

must be soluble in the chosen scrubbing liquid. In addition, the system must be designed to provide

good mixing between the gas and liquid phases, and enough time (residence time) for the gaseous

pollutants to dissolve.

Other important considerations for both particulate and gaseous pollutant collection are theamount of liquid injected into the scrubber per given volume of gas flow (referred to as the liquid-

to-gas ratio) and the removal of any entrained liquid droplets. The liquid-to-gas ratio is important to

provide sufficient liquid for effective pollutant removal. Also, the system must be designed to

remove entrained mists, or droplets, from the cleaned exhaust gas stream before it leaves the stack.

If not removed, the "captured" pollutants could be emitted from the stack.

TABLE 1: Particle collection mechanisms in venturi scrubbers

Mechanism Explanation

Impaction Particles too large to follow gas streamlines around a

droplet collide with it.Diffusion Very tiny particles move randomly, colliding with

droplets because they are confined in a limited space.

Direct interception An extension of the impaction mechanism. The center

of a particle follows the streamlines around the

droplet, but a collision occurs if the distance between

the particle and droplet is less than the radius of the

particle.

Electrostatic attraction Particles and droplets become oppositely charged and

attract each other.

Condensation When hot gas cools rapidly, particles in the gas

stream can act as condensation nuclei and, as a result,become larger.

Centrifugal force The shape or curvature of a collector causes the gas

stream to rotate in a spiral motion, throwing larger

particles toward the wall.

Gravity Large particles moving slowly enough will fall from

the gas stream and be collected.

The literature contains a number of empirical models to predict venturi scrubber efficiency

and/ or pressure drop, that are useful in optimizing and designing new venturi scrubbers as well as

predicting the effects of changing parameters in existing ones. For the calculation of collectionefficiency, some of the most popular models are those developed by Johnstone et al (1954), Calvert

(1972), Boll (1973), Young et al (1977) and Concalves et al (2004). For the calculation of pressure

drop the models developed by Calvert (1970), Boll (1973), Azzopardi (1991) and Pulley (1997) are

amongst the most widely used.

Some of the models mentioned above are explicit, analytical expressions that have

straightforward solutions. Others are more complex and require numerical solution with a computer.

A common feature in these models is that although most start from firm scientific concepts, they

give only qualitative results when predicting collection efficiencies or pressure drops. The

interaction of particulate matter having a given particle size distribution with water droplets having

another size distribution is not easy to express in quantitative terms. As a result of this complexity,experimentally determined parameters are usually required in order to perform engineering


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Alley, 2004). The Reynolds number can be calculated using Equation 13 (where G

the gas density,

in g/cm

3

).

3 116

t D G

d L

l CXd

= +

(11)

,

( )1/3

24 4Re Re

DC = + (12)

,

Red G g

G

d V

= (13

)

3. RESULTS AND DISCUSSION

As has been mentioned above, some of the best educational textbooks for air pollution

abatement technologies (e.g., Cooper and Alley, 2004; Wang et al 2004; Theodore, 2008), consider

the models chosen for the development of the software presented herein, useful in the training of

future engineers in the design of venturi scrubbers. Naturally, these models are not without flaws.Further, of the numerous models developed in the past 30 years, some were bound to give more

accurate predictions. Thus, a short discussion, comparing the models used in this paper, with other

models reported in the literature is warranted.

Calvert et al (1970) presented the first model for pressure drop in venturi scrubbers,

however, they neglected wall friction and momentum recovery in the divergent section, so other

researchers tried to improve this model. Boll (1973) solved simultaneous equations of drop motion

and momentum exchange for variable cross section ducts with acceptable results, except for very

high and low liquid to gas ratios, where it did not show agreement with the experimental data.

Azzopardi and Govan (1984) considered momentum losses due to accelerating droplets entrained

from the film and the interfacial drag between the fast moving core and the slower moving liquid

film. However, they had little successes with this procedure (Nasseh et al, 2006). Pulley (1997)

carried out various experiments and suggested more effective variables such as drop size,

entrainment at liquid injection and entrainment and deposition along the venturi length. He also

compared pressure drop predictions from various models (amongst these were the models

developed by Young, Boll and Azzopardi) and concluded that the corrected proposed model of

Azzopardi et al. (1991) gave a better prediction of pressure drop for a wider variety of data.

Goncalves et al (2001) studied a large number of models for the prediction of pressure drop in

venturi scrubbers and concluded that all of them must be used with caution.

In regards to collection efficiency, Rudnick et al (1986), vigorously compared the modelsdeveloped by Calvert et al (1972), Boll (1973) Young et al (1978) and concluded that the model of

Yung et al (1978) is probably best for most applications because it is an explicit algebraic

expression and gave the best results of the models tested. The model of Calvert et al. (1972), while

also an explicit algebraic expression (and thus easy to use), is very dependent on the choice of the

correlative parameter, f, and thus should be used with caution. One of the latest attempts at

modeling the collection efficiency was undertaken by Concalves et al (2004) who studied the

atomization of the liquid jets injected transversally to a gas stream in a venturi scrubber. A

mathematical model was developed to predict the trajectory, breakup and penetration of the liquid

jets. With this model for liquid jet dynamics, Concalves et al (2004) calculated the spatial

distribution of droplets for the case where liquid was injected through a single orifice in a

rectangular venturi scrubber.


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Comparison of Different Collection Efficiency Models for Venturi Scrubbers