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CHAPTER 7

INVESTIGATION ON APPLYING SAND FILTRATION

TECHNIQUE TO REDUCE SECONDARY POLLUTION IN

WET SCRUBBER USING COMPUTATIONAL FLUID

DYNAMICS MODELING

7.1 INTRODUCTION

In many foundries, the presence of fine particles is a source of

concern for both the industrial process itself and the human health, as well.

Health hazards include heart diseases (strokes, high blood pressure,

arteriosclerosis, heart attack) and altered lung functions (asthma, difficult or

painful breathing, chronic bronchitis), especially in children and elder people.

In order to capture the dust,three mechanisms, namely impaction, interception

and diffusion are considered. It appears that one could efficiently consider the

cleaning process with temperature and humidity control into a single unit.

Very fine inhalable particles can remain suspended in the atmosphere for a

long time, travel long distances from the emitting sources and, once inhaled,

they can reach the deepest regions of the lungs and even enter in the

circulatory system (Renwick et al 2004).

Adverse consequences of particulate matters released from

foundries on the environment are related to reduction of visibility in cities

and scenic areas, as well as to large scale effects on climate due to its

influence on atmospheric radioactive phenomena (Menon et al 2002). Hence

there is increasingly restricting legislation for the emission of pollutants from

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foundry industries. Due to this situation many researchers have concentrated

on end of pipe technologies. Wetscrubber is one of the devices that is

incorporated with end of pipe technologies, As shown in Table 3.2, in

majority of the iron foundries wet scrubber is used as a device control

pollution. Hence iron foundries may prefer to continue to use wet scrubber to

control pollution. However, the drawback of utilizing wet scrubber is that, its

usage results in secondary pollution. In order to overcome, this secondary

pollution a improved version of wet scrubber system that is incorporated with

sand filtering technique was designed during this doctoral work being

reported here. The performance of this modified wet scrubber system was

carried by analysis using solid works software package. The details of this

work are presented in this chapter.

7.2 RESEARCHES ON WETSCRUBBER SYSTEM

The trend of researches on wetscrubber system reported in literature

arena is appraised in this section.

Lehner et al (2001) developed a compact wet scrubber that consists

of several venturi scrubbers working in self-priming mode. The scrubber has

been employed at a hazardous waste incineration plant in order to test its

applicability under real conditions. Carl-Johan Lothgren and Bert van Bavel

(2008) presented a model that estimates the effect of pollution control using

wet scrubbers. Bhave et al (2008) evaluated a compact, wet packed bed

scrubber-based gas cooling and cleaning system, suited for small-volume

applications. Mercedes et al (2007) evaluated the influence of different

scrubber parameters on mercury removal efficiency. When the operational

parameters were slightly modified, a reduction in oxidized mercury removal

reduction with a corresponding increase in mercury removal efficiency was

observed.

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Chandrasekara et al (2009) studied the effect by operating

conditions on the removal of NOx and SO2 from simulated NO–SO2–air flue-

gas mixtures in a scrubber column. The gaseous components were absorbed

into HNO3 electrolyte in the scrubber in a counter-current mode. The flow

rate effects were analyzed in terms of gas/liquid residence time and superficial

liquid velocity/superficial gas velocity ratio SO2 removal was total under all

conditions. Pourmohammad et al (2011) designed a novel swirl wet scrubber

system. The effect of some operating conditions on simultaneous removal of

NO, NO2, SO2, and CO2 is reported. The gaseous pollutants were absorbed

into NaOH solution. As the absorbent media was circulated continuously for

removal purpose, the production of chemical wastes were minimized. Carl

and Bert 2008 measured dioxin levels after wet scrubbing systems was

installed.

Strock et al (1994) discussed the experimental techniques that are

used for measuring wet scrubber flow distribution, pressure drop, spray

nozzle droplet size characteristics and wet scrubber liquid-to-gas ratio along

with the characteristics and capabilities of a new hydraulic test facility.

Perevezentsev et al (2010) confirmed high efficiency of scrubber column for

deterioration of air contaminated with deteriorated water vapor and developed

simulation program that satisfactorily describes operation of columns. Pierre

et al (2009) investigated the removal of Dimethyl disulphide of columns in an

original process compact scrubber which combined an advanced oxidation

process. Dimethyl disulphide gas–liquid equilibrium was reached at the

outlet.

The reporting of the above researches in the literature arena reveals

that a significant researchers on wet scrubber system have been carried by the

researchers.

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7.3 REDUCTION OF SECONDARY POLLUTION THROUGH THE USE OF WET SCRUBBER

Although quite a many researchers have concentrated on addressing

air pollution problems, only very few of them have addressed pollution

control in foundries using end of pipe technology. It is discernable that,

there are no sufficient contribution in addressing secondary pollution

reduction through the use of wet-scrubber in iron foundries. In order to

overcome this deficiency an improved version of the wetscrubber

incorporated with sand filteration technique was designed during the last stage

of pursing the doctrol work being reported here.

Figure 7.1 Existing method of wetscrubber

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Figure 7.2 Proposed method of wet scrubber along with the processing tank

The existing design of wet scrubber is shown in Figure 7.1. As

shown in this existing design, while using wet scrubber, the disposal of water

and sledges are carried out by a solar evaporation tank. The water gets

vaporized by natural sunlight. After vaporization, the sledges deposited on the

bottom of the solar evaporation tank are collected and disposed safely. Since

the evaporation process is possible only during day time and hot summer

periods, it is not possible to vaporize the water and collect the sludge during

rainy seasons and night time. Because of this condition, the sand is not

disposed properly. This cause pollution and affect the ecosystem. In order to

overcome this deficiency, a new design of wet scrubber is evolved. This new

design of wet scrubber is shown in Figure 7.2. As shown, in this new design

of wetscrubber system, the solar evaporation tank is replaced by sand filter

and a polluted water processing tank is introduced. The flue gases coming out

from the furnace are passed through a duct line. The blower sucks the gases

and delivers it into wet scrubber unit. The harmful gases coming out from the

furnace is most dangerous to exhaust it into the atmosphere. So it needs to be

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treated for exhaust in the atmosphere. There is a barrel of water placed under

wet scrubber unit. The water is pumped into the wet scrubber at two stages.

The water is sprayed into the wet scrubber unit. The flue gases are washed by

the water on the scrubber unit. The heavy particles and ashes are absorbed by

the water and stored in the same barrel. and the same barrel. The same water

is re circulated continuously to the wet scrubber unit. Thus the water gets

polluted. A secondary pollution is effected in the water. Thus secondary

pollution affects the performance of the wet scrubber unit,. In the new design

with sand filteration, the solar evaporation tank is replaced by sand filter.

Additionally a polluted water filtration tank is introduced. This unit is placed

under wet scrubber unit. Here the polluted water is purified. A sand filters is

placed inside the barrel in three different heights. First of all, the sands used in

this method are washed thoroughly. The dirty particle in the sand is removed

by this washing .The sand gets purified. Crusher sand is filled in the sand

filter at first stage.

Natural river sand is filled in the sand filter at second stage. White

sand is filled in the sand filters at third stage. The water containing heavy

particles and dust that comes out from the wet scrubber unit is passed into the

barrel tank. The sludge is sieved on the sand particles in first stage. The

minute particles escaping from the first stage is captured in the second and

third stages. Thus the water collected at the bottom of the tank is purified.

Thus the secondary pollution through water is eliminated and the pure water

passes into the wet scrubber unit. This prevention of secondary pollution

improves the performance of wet scrubber unit.

7.4 MODEL BUILDING AND TESTING

Since it was not possible to actually construct the sand filtration

technology incorporated wet scrubber in iron foundries, the model of the

existing wet scrubber system and proposed wetscrubber system were

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developed using Solid works software. After building the model in Solid

works, it was subjected to testing in CFD. The boundary conditions that were

input during this testing are given below.

Inlet boundary conditions: 1.2 m / sec

Outlet boundary conditions: Open to ambient that is, static

pressure of 1.01325 X 105 N/m2

Mass flow rate 0.1 kg

The mesh built during this process is shown in Table 7.1.

Table 7.1 Building a mesh

Mesh Count Count Quality

Existing Proposed Existing Proposed

Surface mesh 15,623 65,325 16,546 85,745

Volume mesh 21,834 99,030 22,874 1,80,811

As shown, as the geometrical construction mimics a closed loop

system, the mesh count is quite higher in the case of the proposed design as

than that is observed in the existing wet scrubber system.

7.5 COMPARISON OF THE PERFORMANCE OF EXISTING

AND PROPOSED WET SCRUBBER SYSTEM

After testing the existing and proposed models, the performance of

the existing and proposed wet scrubber system using Ansys fluent software

package was compared by developing and studying the pressure contours,

turbulence curves and velocity vector graphs. These comparisons are

presented in the following subsections

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The pressure contour obtained by making use of Ansys fluent in the

case of existing wet scrubber system is shown in Figure 7.3. As shown, water

enters the inlet pipe at high pressure. This is indicated using red color. This

high pressure decreases when the water enters the wet scrubber. Due to the

gravity, polluted water flows down through the outlet. As shown in

Figure 7.3, the pressure of the water decreases when it is moved through the

outlet. This is a drawback, as the release of polluted water cannot be effective

under low pressure. Due to small difference of variation of pressure drop, the

toxic and carcinogenic odors are still present in the polluted water. This

deficiency indicates the need for improving the design of the existing wet

scrubber system to control the toxic emissions.

Figure 7.3 Total pressure contours in existing method

The pressure contour obtained by making use of Ansys fluent in

the case of proposed sand filtration technique incorporated wet scrubber

system is shown in figure7.4.In this Figure, variation of pressure of fluid

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particles at the inlet pipe is shown using dark yellow color and the changes

occurring due to pressure drop variation are visibly seen in the form of

changing of color inside the wet scrubber of proposed model. Due to gravity,

partially wetted fine particles fall down through the outlet pipes in the water

processing tank kept below the scrubber.As shown in Figure 7.4 when the

water falls inside the water processing tank, pressure is reduced and is

indicted by Ansys fluent using light green color.

Figure 7.4 Pressure contours in the proposed model

A view on Figures 7.3 and 7.4 would indicate that, the final

pressure of polluted water is higher in the case of proposed model shown that

is observed in the existing system. This higher pressure will enable the

flushing away of pollutants effectively. Thus, the comparison of these

pressure countours (shown in Figures 7.3 and 7.4) indicates that, the

efficiency of the sand filtration is incorporated proposed now wet scrubber

system in controlling pollution is higher.

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7.6 COMPARISON BY STUDYING TURBULENCE EFFECT

The turbulence effect of the existing wet scrubber was derived by

making use of Ansys fluent. This turbulence effect is shown in Figure 7.5.as

shown, the polluted water stays stagnant and is not swirled. Absence of swirling

action prevents the mixing of the pollutants uniformly in the water and thus the

release of water does not result in the evacuation of the pollutants.

The turbulence effect of sand filtration techniques incorporated

model was obtained using Ansys fluent. This turbulence effect is shown in

Figure 7.6. As shown in this proposed wet scrubber system, the water

containing pollutants is subjected to severe swirling. This swirling action

causes the pollutants to get mixed uniformly to the water. Because of this

action, when the polluted water is released , pollutants too get flushed out.

thus the study of turbulence of the existing and proposed wet scrubber system,

which are shown in Figures 7.5 and 7.6, indicates that the latter is superior in

controlling pollution than the former.

Figure 7.5 Turbulence effect of the existing wetscrubber system

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Figure 7.6 Turbulence effect of wetscrubber system

7.7 COMPARISON ON THE STUDYING VELOCITY VECTOR

GRAPH

The Velocity vector graph of existing wet scrubber system was

obtained using Ansys fluent. This velocity vector as shown in Figure 7.7.As

shown, the pollutants move in different direction with less velocity. This

situation causes the poor controlling on pollution in the existing wet scrubber

system.

The velocity vector graphs of the proposed sand filtration

techniques in corporate wet scrubber system and its water processing tank

were obtained. These velocity vector graphs are shown in Figures 7.8 and 7.9.

As shown in both proposed sand filtration incorporated wet scrubber system

and its water processing tank, the velocity of the pollutants is higher and their

movement occur in definite direction.

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This aspects results in higher efficiency towards controlling

pollution by employing the proposed sand filtration techniques incorporated

wet scrubber system.

Figure 7.7 Velocity vector of the existing wet scrubber system

Figure 7.8 Velocity vector of the proposed wet scrubber system

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Figure 7.9 Velocity vector of the water processing tank of the proposed wet scrubber system

7.8 COMPARISON BY STUDYING THE QUANTUM OF

POLLUTANTS RELEASED

The overall performance of the existing wet scrubber system and

proposed sand filtration techniques in corporate wet scrubber system were

studied by drawing the performance details of this system by using Ansys

fluent. The screen output thus drawn are shown in Figures 7.10 and 7.11.

A remarkable observation on viewing in this figures is the number of

pollutants particles released by existing wet scrubber system and sand

filtration technique incorporated proposed system. As shown in these figures,

the number of pollutant particles released by the existing wet scrubber system

is 880, where as it is 435 in the case of the proposed sand filtration

incorporated wet scrubber system. Thus the overall performance of the

proposed sand filtration techniques incorporated wet scrubber is going to be

superior by 45% in comparison to the performance of the existing wet

scrubber system.

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Figure 7.10 Overall performance measuring the existing wetscrubber system

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Figure 7.11 Overall performance of the proposed wetscrubber system

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7.9 CONCLUSION

The overall finding of this doctoral work is that, the cassette filter is

the PCD that is most efficient in controlling pollution in iron foundries.

However, cassette filter is rarely employed in modern iron foundries. Rather

wet scrubber is employed in many modern iron foundries to control pollution.

Despite the benefits of using cassette filter, the modern iron foundries which

have employed wet scrubber would not prefer to replace the wet scrubber

with the cassette filter due to the need of heavy investment. Hence it is

required to improve the design of wet scrubber system so that its efficiency in

controlling pollution is improved. In order to meet this requirement, before

ending this doctoral work, an improved model of scrubber incorporated with

the sand filtration technique was designed.

The construction and working principles of proposed sand filtration

incorporated wet scrubber are described in this chapter. The efficiency of

controlling pollution using these two systems in iron foundries was

investigated by modeling these systems in Solid works and analysing their

performance using Ansys fluent. These results indicated that the efficiency of

the proposed sand filtration incorporated wet scrubber in controlling pollution

in iron foundries will be higher by 45% than that is achieved in the existing

wet scrubber system.