Factors influencing Venturi SrubberPerformanceMark Omara, Punith Dev Nallathamby

Overview

Introduction Approach/Literature Review Objectives Results/Discussion Conclusion

Introduction

Venturi scrubbers are one of the most important devices used to control the emission of particulate and gaseous pollutants.

Venturis fall under the category of wet scrubbers. Other wet scrubbers include: spray tower, cyclonic spray, orifice, fan (mechanically aided) and moving bed scrubbers.

Venturis are considered a standard for effluent gas stream cleaning because:

-They can handle flammable and explosive dusts with little risk;

-Can handle mists;-Relatively low maintenance;

Intro.contd.

-Simple in design and easy to install;-Collection efficiency can be varied;-Provides cooling for hot gases;-Corrosive gases and dusts can be neutralized;-They also work well when sticky or high viscosity

particles must be collected. The removal phenomenon is based on the inertial

impaction. Other collision mechanisms such as interception and

Brownian diffusion also exist, but they are very weak in comparison to the inertial compaction.[Boll(1973)].

Intro.contd.

Contaminated gas enters at the venturi inlet. Pressure differential is used to accelerate the gas to increasingly higher velocities until it enters the venturi throat.

Liquids are fed to the plug-bob through a central nozzle and to the converging walls through tangential inlets that create a uniform film spinning down the throat section.

Gases impact the liquid curtain in the throat section and atomize it. The swirling motion of the resulting mixture forces the entrained liquid to the separator wall and the gas is drawn out. Liquid cascades down the walls and is drained at the bottom for discharge or recirculation.

Two fundamental operating parameters considered in the design of a venturi are [Valdberg and Dubinskaya(2002)];

-Gas throat velocity,-L/G ratio.

The performance of a venturi scrubber is measured by consideration of its collection efficiency and pressure drop.

The literature contains a number of models used to predict the venturi scrubber efficiency. The models are useful in optimizing and designing new scrubbers or predicting the effect of changes in existing ones.

Models by Goel and Hollands(1977), Leith and Cooper(1980), Haller et al(1989) have limitations that include: the often very complex mathematical relationships involved; and the need to estimate important data inputs.

Approach

Approach contd

In this work, we used a more refined model proposed by Yung et al(1977)-the infinite throat model-for estimating the scrubber collection efficiency.

According to Yung, the penetration for one particle size is given by:

lnPt(dp)= -B [4Kpo+4.2-5.02po0.5(1+0.7/Kpo)tan-1(Kpo/0.7)0.5]Kpo +0.7

Where Kpo is the inertial parameter at the throat entrance, B is a parameter characterizing the liquid-to-gas ratio, and Pt is the penetration for one particle size.

Through documented procedures one can calculate the overall penetration for a given particle size, and thus make an estimate of the collection efficiency [Calvert et al(1972)].

Approach contd

In determining the pressure drop, we employed the more widely accepted equations for estimating pressure drop as given by Calvert(1972) where

p = 5.4x10-4 vt2 (L/G)

We also established a relationship between throat gas velocity and the throat radius by making simplifying assumptions:-for a rectangular throat, the width can be calculated from (EPA, 1998)

dt = (At)0.5

-and the throat length can be estimated as

lt = 3dt

Approach contd

The throat area can be determined from the following relationship:

At = Qm/Vt If Qmand Vt are known, then the throat area can be calculated. With the knowledge of the industrial operating conditions such as the pressure drop, the L/G ratio, we can then calculate the useful throat length beyond which no much collection can be obtained.

Throat width

The useful throat length is obtained from the relationship (Ananthanarayanan and Viswanathan(1998)):

Lt = [369.561(L/G)0.293/vt1.127]

Approach contd

Ananthanarayanan and Viswanathan(1998) obtained a parameter that could define collection efficiencies for varying L/G ratio, aspect ratio, nozzle diameter and number of nozzles: the Venturi number. They defined the Venturi number as

VN=[L/G*Ro/do*Z/nj]

Here, Ro = 0.5dt

They were able to show that an optimal venturi number occurs for which there is an optimal collection efficiency. This was confirmed in their subsequent work where they studied the effect of nozzle arrangement on Venturi performance [Ananthanarayanan and Viswanathan,(1999)]. However, they did not evaluate the effect of the venturi number on the pressure drop.

Objectives

To theoretically estimate venturi scrubber performance based on the collection efficiency and pressure drop by examining important factors that affect these variables.

To establish the effect of the venturi number on the pressure drop within a given venturi scrubber, and thus determine the useful throat length for a given venturi scrubber.

Parameters/DataMass median particle size(m)

0-5

Geometric Standard deviation

2.5

L/G ratio(m3/1000m3) 0.5-2.5Throat gas velocity(m/s)

30-120

Venturi throat radius(mm)

10-35

Venturi number 0.5-2.0x10-3

Table 1: Variables considered in this work

Effect of throat gas velocity.It can be seen from fig.1 that the

collection efficiency increases with increase in particle size, and that the venturi is nearly 100% efficient for particle sizes greater than 5m.

Although the Infinite throat model is basically an estimate, we see that the theoretical calculations do agree excellently with experimental data such as that given by Fathikaljahi et al.(1995).

The effect of throat gas velocity on the collection efficiency is shown below in fig.2

Fig.1 Collection efficiency versus particle size.

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Presentwork

Experimental data

vt=75m/sL/G=0.9m3/m3

Effect of throat gas velocity

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vt=40m/s

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L/G=0.9m3/m3

Fig. 2 Effect of throat gas velocity on collection efficiency for a given range of particle sizes.

Here, the L/G ratio is 0.9m3/1000m3,and it can be seen that increasing the velocity increases the collection efficiency such that even for smaller particles the collection is beyond 98% efficient. This is due to the increased momentum of the particulate thus allowing smaller particles to be collected by momentum.

This velocity can be increased by narrowing the throat, injecting the scrubbing liquid counter-current tothe flow, or spraying the liquid into the throat.

Effect of throat gas velocity

Fig. 3 Effect of throat gas velocity on collection efficiency.

In fig.3, it is evident that there is a maximum throat velocity beyond which the collection efficiency is not significantly affected. Higher throat gas velocities give higher collection efficiencies when all conditions such as L/G ratio, particle size and scrubber geometry remain constant.

Furthermore, Fathikalajahi et al.(1996) observes that most optimal gas velocities are between 70-90 m/s. In this study, we see that increasing the throat gas velocity from 40m/s to 60 m/s(50%) result into an increase in efficiency from about 93 to 98(6%). The optimal velocity here is 75m/s. This agrees well with literature.

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L/G=1.3m3/1000m3dp=5micron

Effect of throat gas velocity

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Fig.4 Effect of throat gas velocity on pressure drop (G=0.13m3/1000m3)

In fig.4 the increase in pressure drop occurs almost linearly with increase in throat gas velocity. Similar observations were made by Nasseh et al.(2007) who used artficial neural networks to predict pressure drops in a venturi.

They offer an explanation based on the fact that the friction of the gas against the wall increases with axial distance as velocity increases. They also observed that the maximum pressure drop occurred at the end of the venturi throat. Too high a pressure drop is associated with high energy demand and operating costs.

Effect of L/G ratio

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Fig.5 The effect of L/G ratio on collection efficiency; Vt = 75m/s

In fig.5 it is evident that an increase in L/G ratio results in an increase in collection efficiency up to an optimum value beyond which no significant increase in collection efficiency is noted. Many researchers have noted that liquid flow rates between 7 and 10 gal/1000ft3(0.936 to 1.337 m3/1000m3) give optimum results.

Here, we obtain an optimal value of 1.3m3/1000m3 which compares well with results obtained by Ananthanarayanan and Viswanathan(1998) value of 1.2m3/1000m3. However, while increasing the L/G ratio increases collection efficiency, operating costs are increased as well due to greater scrubbing liquid and pump usage.

Effect of L/G ratio

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L/G=1.3m3/1000m3L/G=10m3/1000m3L/G=0.5m3/1000

Fig.6 The effect of L/G ratio on the pressure drop given a wide range of varying throat gas velocity.

Fig.6 shows the effect of throat gas velocities on pressure drop at selected values of L/G. It is clear that an increase in throat gas velocity results into an increase in pressure drop. Too high an L/G ratio corresponds to an abnormally high pressure drop.

Its thus important to note that according to Schifftner and Hesketh(1996), increasing the pressure drop above 45 in H2O does not significantly increase the removal efficiency for conventional venturi designs.

Fig. 7 shows explicitly the linear relationship between L/G and pressure drop.

Effect of L/G ratio

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Fig.7 The relationship between pressure drop and L/G

Effect of Venturi Number

According to Ananthanarayanan and Viswanathan(1998), venturi numbers between 1.0-1.5x10-3 offers conditions for maximum efficiency. The table below shows their optimum venturi scrubber parameters:

The Venturi number is

VN=(L/G*Ro/do*Z/nj)

It can be seen that VN is proportional to L/G ratio, and the throat radius, Ro. This is particularly true if we consider a variable throat venturi scrubber. For our analyses, we only looked at the effect of the Venturi number on the pressure drop of a rectangular venturi scrubber.

Optimum venturi number, VN

1.0-1.5x10-3

Nozzle diameter, do

2.1mm

Number of nozzles, nj

34

Aspect ratio, Z 2-3

L/G 1.2m3/1000m3

Table 2: Optimum parameters for estimating maximum removal efficiency.

Effect of Venturi Number

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Vt=75m/sDp=5micronZ=2.5L/G=1.3m3/1000m3

Effect of Venturi Number on Collection Efficiency.

Effect of Venturi Number

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Fig.8 The effect of Venturi number on the pressure drop

By carefully choosing optimum values of L/G, do, Z and nj we have calculated values of VN as a function of Pressure drop. The results are shown in fig.8.

It is evident that, based on the assumption of variable throat radius, the venturi number increases with decreasing pressure drop. It appears that increasing the venturi number beyond values of 2.0x10-3 does not yield any significant change in pressure drop. This is because the VN is directly proportional to the throat gas velocity which has an optimal value for optimum collection efficiency.

Also, a decrease in Vt corresponds to an increase in throat radius, Ro and thus an increase in VN

Effect of Venturi Number

From the Venturi number concept and by looking at the results obtained for maximum venturi performance we can determine a useful throat length beyond which not much collection is expected.

This is desired because too long a throat will cause extra pressure drop with minimal gain in collection efficiency, and a very short throat would not allow liquid drops to approach the throat gas velocity and would result in achieving less than desired collection efficiencies. Based on the model provided by Ananthanarayanan and Viswanathan(1998), we estimate the useful throat length as

Lt = [369.561(L/G)0.293/Vt1.127]

Whence, Lt = 0.2m

Conclusion

By using the infinite throat model, venturi collection efficiency was estimated for varying particle size, throat gas velocity, and L/G ratio. Results were found to agree well with experimental data and theory.

An analyses of the factors influencing pressure drop in a venturi scrubber was made and optimum values of L/G and throat gas velocities identified. The effect of the Venturi number on the pressure drop was also studied. From these results, an estimate of the useful throat length was made.

References

Boll, R.H, Particle collection and pressure drop in venturi scrubber, Ind. Eng. Chem. Fund.12(1973) 40-50.

Valdberg, A. Yu and F.E. Dubinskaya,Design and operational aspects of Venturi scrubbers, Chemical And Petroleum Eng. 38(2002) 7-8

Goel, K.C, and K.G.T Hollands,Optimum design of Venturi scrubbers, Atmos. Environ., 11(1977)837

Cooper, D.W, and D.Leith,Venturi scrubber optimization revisited, Aerosol Sci. and Technol, 3(1984)63

Haller H., E. Muschelknautz, and T. Schultz,Venturi Scrubber calculation and Optimization, Chem. Eng. Technol.,12(1989)188

Yung, S., S. Calvert, and J. F. Barbarika. 1977. Venturi Scrubber Performance Model. EPA 600/2-77-172. U.S. Environmental Protection Agency. Cincinnati, OH.

References

Calvert, S., J. Goldschmid, D. Leith, and D. Mehta. 1972, August. Wet Scrubber System Study. Vol.1, Scrubber Handbook. EPA-R2-72-118a. U.S. Environmental Protection Agency.

EPA, 1998. U.S. EPA, Office of Air Quality Planning and Standards,Design Evaluation of Particulate Wet Scrubbing Systems, EPA-452/R-97-001, Research Triangle Park, NC, October.

Ananthanarayanan, N.V.; Viswanathan, S. Estimating Maximum Removal Efficiency in Venturi Scrubbers. AIChe J. 1998, 44, 2549.

Ananthanarayanan, N.V.; Viswanathan, S.Effect of Nozzle arrangement on Venturi Scrubber Performance. Ind. Eng. Chem. Res. 1999, 38, 4889-4900.

Fathikalajahi, J., M.Taheri, and M.R.Talaie,Theoretical Study of non-uniform droplets concentration distribution on Venturi Scrubber Performance, Particulate Sci. and Technol.,14,153(1996)

Nasseh, S., A. Mohebbi, Z.Jeirani, A.Sarrafi,Predicting Pressure drop in Venturi Scrubbers with artificial neural networks,J.of Hazardous Materials 143(2007) 144-149.

Schifftner, K.C. and H.E. Hesketh, Wet Scrubbers (2nd Edition), Technomic Publishing, Lancaster, PA, 1996.

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Factors influencing Venturi Srubber PerformanceOverviewIntroductionIntro.contd.Intro.contd.ApproachApproach contdApproach contdApproach contdApproach contdObjectivesParameters/DataEffect of throat gas velocity.Effect of throat gas velocityEffect of throat gas velocityEffect of throat gas velocityEffect of L/G ratioEffect of L/G ratioEffect of L/G ratioEffect of Venturi NumberEffect of Venturi NumberEffect of Venturi NumberEffect of Venturi NumberConclusionReferencesReferencesThank you