wet srub design

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>> Discussions of Acid/Alkaline Wet Scrubber Design Methodology

Acid/Alkaline wet scrubbers applied in earlier chemical industries have been developed and considered as ordinary

strategies to control acid/alkaline contamination in exhaust air stream. Since they were already mature

technologies, why we have to emphasize its importance here again, particularly in the aspect of design for this

specific application in semiconductor/optoelectronic industries?

The answer to this question was originated from dramatic requirements of contract technical specification due to

perpetually environmental concerns; 95% Removal Efficiency for all kinds of acid/alkaline contaminants under

specified flows and concentrations. In order to demonstrate and prove their design to be efficient and effective,

whoever was granted the contract, functional calculation is demanded, mandatory by submission process in the

contract. However, regardless of who acquires the contract, their calculations and designs all seem to be so

“consistent” and constructed in a same methodology that constant doubts arise from this skeptical “routine” among

customers, consultants, and main contractors.

This fact gives us a cue, as a main contractor, how are we supposed to pull through this?

On the other hand, as so called professional vendors of scrubbers, what’s the true story behind the scene?

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In this edition of R&D News, we’ll be focused on discussions of the specific topic related to scrubber design. It might give us another point of view to look at this, and come up with another design approach to cope with the growingly demanding reality of high removal efficiency imposed on the acid/alkaline scrubbers.

>> Preface

Preface

According to ITRI’s early studies, results show that the most significant difference between the natures of process

exhausts from semiconductor/optoelectronic and those from traditional chemical industries are of their

discrepancies in concentration levels of contaminants.

Compared to chemical industries, contaminant concentrations in exhaust air stream from semiconductor/

optoelectronic industries are much lower.

The subject of this edition will be discussing this point to begin with and be exploring the wonder of that “consistent

and routine” calculation on theoretical basis.

>> Discussion of Traditional Methodology

Discussion of Traditional Methodology

At the beginning, please look into the ordinary methodology and concept we’ve been used to design a packed bed

reactor in the past, which determines packing height Lt, by the product of HOG and NOG.


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These equations are actually derived from gas/liquid concept and two-film theory


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According to two-film theory, and by the Control Volume illustrated below, we can derive a basic equation for LT, in

which, HOG is obtained from experimental data, NOG can be calculated by another equation.


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NOG can be further developed into other forms :


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Eventually, when K=0, the Scrubber Basic Equation can be further reconstructed into the following form, more

simplified and easier to use in calculation


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From a practical point of view, either in the microscopic view of fluid field and mass transfer, or in the macroscopic

view of a reactor system, the major flaw in this simplified equation is that K could not possibly always be zero

throughout the entire packed bed by assuming an infinite solubility of a contaminant. This is only an idealistic but

inconvincible assumption.

The only part in this simplification we can concur with is that at top of scrubber, with sufficient amount of

neutralizing agent, an assumption of infinite solubility with the K value being zero is merely appropriate.

>> Discussion of Modified Methodology

Discussion of Modified Methodology

Due to the imperfections of the traditional design methodology, we proposed following corrected models to deal

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At top of scrubber, with sufficient amount of neutralizing agents, a reasonable assumption of infinite solubility with

the K value being zero is presumed.

Along the path of mass transfer and chemical reaction between contaminants and neutralizing agents, standard

Henry’s Law dominates, which means, K varies.

In the next section, we’ll be showing and modeling how K value would vary along the reaction path of a scrubber.


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We proposed four different kinds of curves to describe how K value varies with “stages” along the path.

Characteristics of the curves would be further manipulated by changing powers of the curves.

Before we proceed to come up with our “Modified Design Method”, some premises have to be made to clarify some

points:

1. The contaminant inlet concentration in water phase at top of a scrubber is approximately zero (xin•0)

because the soluble contaminants from gas phase have been “neutralized” with neutralizing agents . Ex. a

neutralization reaction of HF and NaOH results in a salt of NaF, thus, NaF is no longer considered as part of

the contaminant concentration in water.

2. As long as an excessive and sustainable amount of neutralizing agent exists during a scrubber reaction

process, xin•0 is indisputable.file:///C|/Documents%20and%20Settings/Mobile%2002/Desktop/list.php.htm (9 of 12) [1/26/2010 12:18:32]

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3. With regard to K value, we have following assumptions and corrections:

(1) K=0 is acceptable at the top of the scrubber, considering its infinite solubility with an excessive

amount of neutralizing agent.

(2) At the bottom of the scrubber, where the exhaust stream just begins to come into contact with the

scrubbing water, the infinite solubility is not an acceptable assumption due to the lack of a significant

amount of neutralizing agents. Therefore, the general Henry’s Law is applied in this situation.

Based on the assumptions above, Scrubber Basic Equation can be re-written as the following form:

>> Results and Analysis

Results and Analysis

We can obtain following results in response to the change of K value by using the equations derived above and

some simplifications with some manipulations of K value.


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In fact, we can see that, by applying different values to the power of the curves (n• when n=1, K value

varied as Linear Approach; when n>1 K value varied as Power Law–Infinite Solubility

Approach; when n<1 K value varied as Power Law–Finite Solubility Approach), results in different

characteristics of varied K values, which subsequently change the profiles of the removal efficiency.

>> Conclusion

Conclusion

We brought up some points in this edition with regard to the traditional scrubber design methodology, particularly in

terms of its imperfections in practical use. At the same time, we came up with another idea that, by approximating


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practical reality with different “pseudo” curves to illustrate how the total efficiency profiles of the scrubber change in

response to the variations of K value, one could use it as reference for future research.

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