Exp 3 Packed Absorption Column Raschig Ring

1.0

INTRODUCTION Packed columns are used for continuous counter current gas absorption applications and to an extent in distillation plant as well. The tower consists of a cylindrical column containing a gas inlet and distributing space at the bottom, a liquid inlet and distributing device at the top, a gas outlet at the top, a liquid outlet at the bottom, and a packing or filling in the column. The gas enters the distribution space below the packed section and rises upward through the openings and contact the descending liquid flowing through the same opening. A large area of intimate contact between liquid and gas is provided by the packing. Many different types of tower packings have been developed and some are being commonly used. Such packings and other commercial packings are available in various sizes such as from 3mm to about 75mm. Most of the tower packings are made of materials such as clay, porcelain, metal or plastic. High void spaces of 65-96% are characteristic of good packings. The packings allow relatively huge volumes of liquid to flow countercurrent to the gas flow through the openings with relatively low pressure drops for the gas. These same types of packings are also used in vapor-liquid separation processes of distillation. Ceramic Raschig ring and Berl saddles are older types of random packing and are seldom used these days. Pall rings (second generation packing) are made of plastic or metal; they are much more efficient and are still widely used now. They have porosities or void spaces of 0.90-0.96 and areas of about 100-200m 2/m3. The latest or third generation packing is the Intalox metal type, which is a combination of the Berl saddle and the Pall ring. Porosities range from 0.95 to 0.98. Stacked packings having sizes of 75mm or so and larger are also used. The packing is stacked vertically, with open channels running uninterruptedly through the bed. The advantage of the lower pressure drop of the gas is offset in part by the poorer gas-liquid contact in stacked packings. Typical stacked packings are wood grids, drip point grids and spiral partition rings.

1.1

Objective 1

The objective of this experiment is to determine the flooding point and the velocity of flooding through observation and experimental data. Apart from that, it is also used to study the effects of the manipulation of water and air flow rate towards the flooding point in the packed column. From the study itself, relationship between the changes in water and air flow rate and the pressure drop can be drawn out. 1.2

Scope In this experiment, Raschig ring is used for packing in a packed tower in order to find its flooding point. Surrounding air is pumped into the packed column at a specific location just below the packing by using a compressor while the water is pumped to enter from the above of the packed column. The flow rate of each of its constituent is controlled in order to study the effects of the flow rate manipulation. Each different sets of flow rate will generate various pressure drops. Flooding point will occur when certain pressure drop is achieved which will caused the water to accumulate and gas flow through a packed tower will become turbulent

2.0

THEORY 2

In absorption process, the phases that involved in this separation process are gas and liquid. A certain liquid is flowing through the packed column to separate the counter current flowing of one or more components in gas mixture. The liquid would be absorbed the component as the separation of the components in the gas mixture by the mass transfer occurred. Packing materials that filled in the column were used to increase the contact surface area for the absorption process to ensure an efficient and faster process. Packed columns are the units most often used in absorption operations (Fig. 1). Usually, they are cylindrical columns up to several metres in diameter and over 10 metres high. The packing is placed on a support whose free cross section should be at least equal to the packing porosity. One of the various supports is shown schematically in Fig.2 .Liquid is fed in at the top of the column and distributed over the packing through which it flows downwards (Roman Zarzycki and Andrzej Chacuk, 1993).

Figure 1 Packed column

Figure 2 Packing supports

In this experiment, Raschig rings are used as the one of the typical random or dumped tower packings.This types of random packing have much more packing efficient than Intalox metal and are still used now. Geankoplis mentioned that the Raschig rings have porosities or void spaces of 0.90-0.96 and areas of 100-200 m2/m3 (30-60 ft2/ft3). Other than that, Raschig rings in Figure 3 are cylinders made from ceramics, metal or plastic, and are used to increase efficiency in gas, petroleum and chemical refinery 3

towers. The rings are packed randomly into the tower to increase the surface area available for chemical reactions. They are typically shaped like hollow cylinders, with a diameter equal to the length of the cylinder. In an actual, operating tower, the gas velocity is well below flooding. The optimum economic gas velocity is about one-half or more of the flooding velocity. It depends upon a balance of economic factors including equipment cost, pressure drop, and processing variables. Pressure drop in the packing is an important consideration in design of a tower.

Figure 3 Raschig Rings For pressure drop in random packings, empirical correlations for various random packings based on experimental data are used to predict the pressure drop in the gas flow. The original correlation by Eckert (K1) correlated the gas and liquid flow rates and properties with pressure drop. It is important for proper design to be able to predict the flooding pressure drop in towers and hence, the limiting flowrates at flooding. Kister and Gill (K2) have developed an empirical equation to predict the limiting pressure drop at flooding. The equation is ∆ P flood = 0.115 Fp0.7 ……………………………………(1) where ∆Pfloodis in in. H20/ft height of packing and Fp is the packing factor in ft-1.

At a

packing factor of 60 or higher, Equation 1 should not be used. In addition, packed absorption column (Raschig Rings) can be used in the petrochemical distillation and extraction applications. Besides that, it also can be used in manufacture of absorption in gas processing, combustion plants and desorption in water treatment as written by Jiangxi Jintai Special Material LLC (2005). 3.0

METHODOLOGY 4

3.1

Equipment/Materials:     

3.2

Packed absorption column –Raschig rings Air compressor Stop watch Water Air

Procedure First before the experiment start, all valves had been checked and components a in place and in good conditions. Then, the water flow rate was set at 2.0 L/min and air flow rate also had been adjusted to 30 L/min. At the same time, the stop watch is started and continues for 5 minutes. After 5 minutes, the pressure difference, ∆P was recorded. These steps are repeated for 10L/min increment of air flow rate for every 5 minutes until flooding occurs.When flooding occurs, the ∆P was recorded and the air flow rate was reduce back to 30L/min. These steps also are repeated for water flow rates of 2.5, 3.0 and 3.5 L/min.Next, the air flow rate was set at 50L/min and for every 2 minutes, the pressure difference was recorded at water flow rate 2.0 to 3.5 L/min with each increment of 0.5L/min.

4.0

RESULTS AND DISCUSSION 4.1

Experimental Data

PART A Table 1: Manipulated air flow rate. 5

Water flow

Air flow rate,

Pressure drop,

Log

Log

Time

rate, W

G

(cm H20)

Air flow

Pressure

(every 3

(L/min) 2.0

(L/min) 30 40 50 60 70 80 90 100 30 40 50 60 70 80 90 30 40 50 60 70 30 40 50 60

1.1 1.8 2.5 3.8 4.6 6.5 8.1 10.5 1.7 2.5 5.0 6.1 9.0 11.7 19.5 2.2 4.0 5.9 11.0 15.0 3.4 4.9 10.0 18.0

rate 1.4771 1.6021 1.6990 1.7782 1.8451 1.9031 1.9542 2.0000 1.4771 1.6021 1.6990 1.7782 1.8451 1.9031 1.9542 1.4771 1.6021 1.6990 1.7782 1.8451 1.4771 1.6021 1.6990 1.7782

drop 0.04139 0.25527 0.39794 0.57978 0.66276 0.81291 0.90849 1.02119 0.23045 0.39794 0.69897 0.78533 0.95424 1.06819 1.29003 0.34242 0.60206 0.77085 1.04139 1.17609 0.53148 0.69020 1.00000 1.25527

minutes) 3 6 9 12 15 18 21 24 3 6 9 12 15 18 21 3 6 9 12 15 3 6 9 12

2.5

3.0

3.5

6

PART B Table 2: Manipulated water flow rate. Air flow rate, G

Water flow rate,

Pressure drop,

Log Pressure

Time

(L/min)

W

(cm H20)

Drop

(every 2 minutes)

(L/min) 2.0 2.5 3.0 3.5

5.5 6.1 7.0 10.3

0.74036 0.78533 0.84510 1.01284

2 4 6 8

50

4.2

Data analysis and Discussion

PART A 1.4 1.2

f(x) = 2.16x 2.43x - 3.01 3.11 f(x) = 2.29x - 3.06

1 f(x) = 1.86x - 2.73

W = 2.0 L/min

0.8

log ∆P

Linear (W = 2.0 L/min) W= 2.5 L/min Linear (W= 2.5 L/min)

0.6

W= 3.0 L/min Linear (W= 3.0 L/min)

0.4

W= 3.5 L/min Linear (W= 3.5 L/min)

0.2 0 1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

log G

Figure 4 : Graph of Log Pressure Drop vs. Log Air Flow Rate

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From figure 1, the air flow rate, G is proportional to the pressure drop, ∆P when the water flow rate is constant. Besides that, we also can see that the gradient of the graph increase as the value of water flowrate, W is increased except for water flow rate at 2.5 L/min. It proves that the time taken for the column to reach flooding point becomes shorter when the water flowrate, W is increased. From figure 1, W = 2 L/min,

gradient = 2.0481

W = 2.5 L/min,

gradient = 1.6202

W = 3.0 L/min,

gradient = 2.1926

W = 3.5 L/min,

gradient = 2.4307

By the comparison with the reference, the pressure drop is proportional to the flow rate to the power of 1.8 when the gas flow rate increased at low velocity. P = KQ1.8 For this experiment, based on the graph, P = KGn Then when taking logarithm both sides, Log ∆P = Log K + n Log G Therefore n is equal to the gradient of the graph, m. The correlation between ∆P and the flow rate might be a bit different from the reference’s equation due to several errors that occur during the experiment.

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PART B 1.05 1 f(x) = 0.18x + 0.36

0.95 0.9 0.85 log ∆P

0.8 0.75 0.7 0.65 0.6 1.5

2

2.5

3

3.5

4

water flow rate, W (L/min)

Figure 5 : Graph of log Pressure Drop vs. Water Flow Rate By referring to figure 2, the graph illustrated the linear line with positive slope at constant air flow rate, G (50 L/min). It showed that the pressure drop is proportional to the water flow rate, W. this correlation can be indicated by equation below, P = K’ W m Then taking logarithm on both sides, Log P = Log K’ + m Log W From the graph plotted the slope of the graph, m is 0.1754 and the intersection at the y axis, Log K’ is 0.3634. Thus the value of K’ is 2.3089. By referring to the graph, the relation between P and water flow rate is, P = 2.3089W 0.1754 Hence, by the comparison with the reference, the pressure drop is proportional to the flow rate to the power of 1.8 when the gas flow rate increased at low velocity. P = K’ W 1.8

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4.3

Definitions of flooding, flooding velocity, visual flooding and load point Loading Point is defined as the gas flow rates which the gas starts to hinder the

liquid downflow, and local accumulations or pools of liquids start to appear in the packing. The pressure drop of the gas starts to rise at a faster rate. As the gas flow rate is increased, the liquids accumulation increases. At the flooding point (the point where the capacity of the column to carry both liquid and vapour stream is exceeded) the liquid can no longer flow down through the packing and is blown out with the gas. The flooding velocity of the column is the velocity of the vapour rising through the column at which the liquid on each stage is suspended. The flow of vapour up through the column will not allow the liquid to fall down through the column causing the stages to "flood". This means that the tower cannot operate. Visual Flooding is the gas stream that flow counter current with water stream which flow through packed column in form of bubble. Operation of a gas absorption column is not practical above the loading point. For optimum

design, the

recommended

gas

velocity

is

half

of

the

flooding

velocity. Alternatively, some design can be based on a specified pressure drop condition, usually well below the pressure drop at which flooding would occur. 4.4

Comparison the operation and the efficiency of the absorption column using packing materials of Intalox saddle and Raschig ring It was observed that the dry packing factor of the latter in Intalox saddle was

lower than the former. This showed that the pressure drop for Intalox saddle would be lower than that contributed by Raschig ring. Meanwhile, Intalox saddle has higher efficiency compared to the Raschig ring. This was due to the geometrical design of Intallox saddle that would significantly enhance the porosity, surface area per unit volume of the packing, and packing efficiency.

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5.0

CONCLUSION From experiment, pressure drop, P is proportional to the flow rate. The higher the flow

rate, the higher the pressure drop, P and less time needed for the column to reach flooding point. This relationship is verified by the theoretical equation below: P = KQ1.8 6.0

RECOMMENDATION Based on this experiment, some recommendation are made for the improvement of the results. There are: a) The eyes of the absorber must be perpendicular to the scale to avoid parallax error. b) Ensure that the knobs for adjusting the water and gas flow rate is tightly close and no splitting occurs. c) The zero error can be overcome by substitute the final value with the zero error value therefore the actual value of the reading can be obtain. d) Ensuring that the flow rate of gas and water is controlled and constant

REFERENCES 11

1. Holman, J. P., “Heat Transfer”, 8th edition, Mc Graw-Hill Inc., U.S.A, 1997. 2. Geankoplis, C. J., “Transport Processes and Separation Process Principles”, 4th Edition, Pearson Education,Inc.,2003 3. Geankoplis, C. J., “Transport Processes & Unit Operations”, 3rd Edition, Prentice-Hall, 1995. 4. Jiangxi Jintai Special Material LLC., “JINTAI Raschig Ring Packing”, 2005. Retrieved from http://www.ceramichoneycombs.com/tower_packing/Raschig_Ring_Tower_Random_Col umn_Packing.htm 5. Mc Cabe, W. L., Smith, J. C. &Harriot, P., “Unit Operation of Chemical Engineering”, 4th Edition, Mc Graw-Hill, 1985. 6. Roman Zarzycki and Andrzej Chacuk., “Fundamental &Applications:Absorption”.pg 410-502, 1993

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APPENDICES Sample calculation: PART A For W=2.0 L/min The value of n is 2.0481,

|experimental value−theoretical value|

error =

theoretical value

x 100 =

|2.0481−1.8| 1.8

x 100

= 13.78 % The percentage error for other flow rate is listed below W =2.5 L/min

% error = 9.99 %

W =3.0 L/min

% error = 21.81%

W =3.5 L/min

% error = 35.04 %

This large percentage error is due to some error occurring during the experiment PART B The value of m is 0.1754,

|experimental value−theoretical value|

error =

theoretical value ¿

x 100

|0.1754−1.8| 1.8

x 100

= 90.26% Based on the large of percentage of error, there are some errors occur during the experiment.

.

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