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Student_Projects_for_SO2_Absorption/Student Projects for SO2 Absorption/t2_f99.PDFSO2 Absorption

Unit Operations LabCarnegie Mellon University

Dr. Gary PowersMr. Matt Cline

Friday December 9, 1999

Group 2:Jason Liu

Vinnie PaganoMelissa Proch

Wes RandallGina Venezia

Table of Contents:

Section Page

Abstract 1

Introduction 2

Background and Theory 2

Procedure 3

Results and Discussion 4

Conclusion 6

Appendix A Sample Calculations

Appendix B Tables

Appendix C Figures

Appendix D Works Cited

Abstract

The objective of the following experiment was to become familiar with the operation of agas absorber by determining various physical characteristics. The characteristics in questionwere: the flowrates required to flood the column, the effects of column height on stripping ability,and the stripping ability of the column at different flowrates. The primary result observed wasthat the maximal interfacial surface area leads to maximal mass transfer. The optimalflowrate for mass transfer occurred at 78% of the flooding velocity. Depending upon theheight of the column, the pressure drops at flooding were both higher and lower than theexpected pressure drop (20-30%).

Introduction:

To determine the optimal operating conditions of a gas absorber, various physical

parameters were measured. The parameters in question were: to measure the flowrates

required for flooding the column, to measure the effects of column height on stripping ability, and

to determine the stripping ability of the column at different flowrates. From these measurements,

it is possible to develop a model for the maximum efficiency for a binary separation. This proves

to be extremely important in the chemical industry. When column efficiency is maximized,

larger profits can be attained.

Background and Theory:

Absorption is commonly utilized to perform binary separations in the chemical industry.

A gaseous stream containing the component to be removed flows upward through a packed

tower, while a fluid stream flows downward through the tower. The concentration gradient of the

transfer component between the two streams causes mass transfer to occur. Packing increases the

internal surface area of the tower, thus creating a large surface area for mass transfer. In the

following experiment inch ceramic Raschig rings were utilized to accomplish this.

The theoretical height required to produce a product with a specified concentration is

dictated by the following equation:

ZT = HTU * NTU (1)

ZT is the tower height, HTU is the height of a transfer unit, and NTU is the number of transfer

units. HTU is calculated from the following equation:

HTU = V/SKya (2)

V is the vapor flowrate, S is the cross sectional area of the tower, and Kya is the overall mass

transfer coefficient. NTU is obtained from the following equation:

=Yb

Ya yydy

NTU*

(3)

Ya is the concentration of the entering gas, Yb is the concentration of the exiting gas, and y* is

determined from the operating line and the equilibrium curve. For dilute solutions, the integral

for NTU can be evaluated as (yb ya)/(y-y*)L, where (y-y*)L is the log mean of (y-y*). Thus,

equation 3 simplifies to the following:

**

ln

*)(*)()(

yyyy

yyyyyy

NTU

a

b

ab

ab

= (4)

The operating line used to find y* is calculated from Henrys Law:

y* = Hx (5)

H is a constant (1.2 for SO2 in water), and x is the mole fraction of SO2 dissolved in the stripping

liquid.

As the flowrate of gas increases, the overall mass transfer coefficient also

increases. Unfortunately, there is a physical constraint to this relationship. When the gas

flowrate is large enough to impede the flow of liquid, flooding occurs. The liquid

completely retards the column. Thus, the vapor has to flow around the packing. This

greatly decreases the surface area available for mass transport. The efficiency of the

column plummets. Thus, flooding is to be avoided at all costs.

Procedure:

The equipment utilized throughout this experiment was a gas absorber. This particular

column consists of removable one foot sections. Thus, adding or removing these sections could

alter the height of the column. To increase the surface area of the column, the absorber was filled

with inch Raschig rings. The SO2 exit flow tube was placed beneath a fume hood to ensure that

no SO2 entered the lab environment. At the columns console, the main power button was then

turned. Once this was accomplished, the water flow was turned on. The flowrate of water was

adjusted on the console. Water was allowed to flow through the column alone for several

minutes to soak the Raschig rings. Next, the airflow was turned on followed by the SO2 flow and

air mixer. After allowing several minutes for the column to stabilize, the air and SO2 flowrates

were adjusted. This was done to ensure that SO2 output was being detected at a measurable level

by the SO2 meter. When the SO2 output readings had stabilized, water, air, and SO2 flowrates

were measured. The pressure drop across the height of the column was also measured. If the SO2

reading failed to stabilize, measurements were recorded at regular time intervals. The measured

values were then averaged to determine a reasonable time-averaged estimate by which the

columns behavior could be modeled.

Results and Discussion:

The flooding profile of the column and the stripping ability of the SO2 absorber were

measured throughout the course of three weeks. During the first week, the effects the height of

the column had upon the stripping ability of the column were measured. Increased height clearly

improved the stripping ability of the column. With a 0.61 m (2 ft) column, 1.4 gpm (gallons per

minute) of water were required to leave 180 ppm SO2 in the effluent stream. When the height of

the column was increased by 1 ft to a total column height of 3 ft, a lower water flowrate (0.85

gpm) significantly improved the columns stripping ability (effluent stream of 23.5 ppm SO2).

By increasing the height of the column, the number of transfer units contained within the column

increased by 50%.

The following week, the flowrates required to flood the column were measured. When

the gas flowrate is large enough to impede the flow of liquid, flooding occurs. A packed column

usually floods when the pressure drop exceeds 12.5 cm H2O/m of packing (1.5 in H2O/ft of

packing). The height of the column had a great effect on the pressure drop required for flooding.

Height (m) Air flow*10-4 Water flow*10-5 P P/m of packingm m3/s m3/s cm H2O cm H2O / m

1.22 14 8.70642 11.2 9.1803278691.22 7 12.1764 8.75 7.1721311481.22 5.6 13.5644 9 7.377049181.22 11.66667 10.0313 11.75 9.631147541

0.915 11.66667 10.5991 15 16.393442620.915 9.33333 11.9871 14.5 15.846994540.915 7 13.1227 15 16.39344262

With a height of 4 ft, the column flooded at a pressure drop beneath 12.5 cm/m packing. At, 4 ft,

the column would flood at approximately 8 cm/m packing. When the height was reduced to 3 ft,

the opposite effects were observed. A pressure drop of 16 cm/m caused flooding to occur

The final experiment conducted was the determination of the stripping ability of the

column at different flowrates. The overall mass transfer coefficient Kya increased as the water

flowrate through the column increased. This was simply because there was a larger surface area

available for mass transfer to occur. Unfortunately, there was a physical limit upon this quantity.

When the column began to flood, the vapor flow bypassed the Raschig packing. This significantly

reduced the interfacial surface area, which ultimately caused Kya to decrease.

Kya vs. Relative Flowrate

0.000E+00

5.000E-05

1.000E-04

1.500E-04

2.000E-04

2.500E-04

0.00E+00 2.00E-03 4.00E-03 6.00E-03 8.00E-03 1.00E-02 1.20E-02 1.40E-02 1.60E-02 1.80E-02

Relative Flowrate

Kya

Figure 1.a Plot of overall mass transfer coefficient vs. relative flowrate

Through countless years of experience, chemical engineers have determined that the

optimal flowrate for a gas absorber is at approximately 80% of the flooding rate. The maximum

on figure 1.a occurs at 78% of the flooding rate. The overall mass transfer coefficient declined

dramatically when the flooding occurred.

In order to study the efficiency of the column, stripping power was defined. Stripping

power was simply the concentration difference between the inlet and outlet streams. Stripping

power was the amount of SO2 that the column was physically capable of stripping from the vapor

phase. When the system flooded, there were two counteracting forces at work. Kya dropped

dramatically, which decreased the stripping power. However, the increased water flowrate

elevated the stripping power of the column. To determine which force dominated the system, the

stripping power of the column was plotted against the relative flowrate.

Kya vs. Relative Flowrate

0.000E+00

5.000E-05

1.000E-04

1.500E-04

2.000E-04

2.500E-04

0.00E+00 2.00E-03 4.00E-03 6.00E-03 8.00E-03 1.00E-02 1.20E-02 1.40E-02 1.60E-02 1.80E-02

Relative Flowrate

Kya

Figure 2.a Stripping Profile

Figure 2.a clearly demonstrates that stripping power greatly decreased whenflooding occurred. Kya dominated the efficiency of the column, while the increase of liquidflow had insignificant effects upon the efficiency of the column. The maximum stripping andefficiency of this column occurred at approximately 78% of flooding.

Conclusion

Several results came from this study of a stripping column. The primary resultobserved was that the maximal interfacial surface area leads to maximal mass transfer.The optimal flowrate for mass transfer occurred at 78% of the flooding velocity. Thisresult is consistent with that found in literature. However, the results regarding thepressure drop at flooding did not correspond with those that were anticipated. Dependingupon the height of the column, the pressure drops at flooding were both higher and lowerthan the expected drop by 20-30%. As seen above, both expected and unexpected resultswere products of this series of experiments. Some ideas for future experiments are listedbelow:

Change the liquid in the column to see which provides optimal masstransfer.

Catalytic scrubbing of the SO2 gas. Liquid-liquid extraction utilizing liquid of differing densities.

Appendix D:Works Cited

Armitage, M., et al. Modeling of an SO2 Absorption Column for an Industrial Steam Plant.Carnegie Mellon University Unit Operations Lab (06-311), Group 12 Report, 15 October1999.

Bennion, N., et al. Sulfur Dioxide Scrubber. Carnegie Mellon University Unit Operations Lab(06-311), Group 1 Report, 15 October 1999.

Biegler, L. T., Grossmann, I. E., Westerberg, A. W. Systematic Methods of Chemical ProcessDesign. Prentice Hall PTR, Upper Saddle River, 1997.

Flowmeter Calibration Data Document No. 392. Cole-Parmer Instrument Company, VernonHills.

Flowmeter Calibration Data Document No. 396. Cole-Parmer Instrument Company, VernonHills.

Geankoplis, C. J. Transport Processes and Unit Operations. 3rd Ed. Prentice Hall PTR,Englewood Cliffs, 1993.

McCabe, W. L., Smith, J. C., Harriott, P. Unit Operations of Chemical Engineering. 5th Ed.McGraw-Hill, New York, 1993.

Prieve, D. C. Notes on Unit Operations of Chemical Engineering. 06-202 Class Notes, CarnegieMellon University, Pittsburgh, 1998.

This is a solid effort -- quality work. A-/B+.