R.J. Bordeos, R.J. Candare, K. Dimalanta, M. Lerin, R. Rasco, P.B. Rose, Gas Absorption, 2011
Determination of the Effects of Varying Fluid Flow Rates on the Pressure Drop across a Packed Column R.J. Bordeos, R.J. Candare, K. Dimalanta, M. Lerin, R. Rasco, P.B. Rose Department of Chemical Engineering, College of Engineering, University of the Philippines Diliman February 23, 2011
ABSTRACT In this experiment, gas absorption and pressure drop across a packed column were observed and studied. This experiment helps the students determine and comprehend the effects of varying the rate of flow of fluids on the pressure drop they produce across a packed column. The water pump was switched on and C1 was set to provide given flow rates which were from 0-4L/min as given in the table of data. The air flow was also set from 40-160L/min at each of the used water flow rates and the pressure drop for each setting was recorded. The experiment showed that air flow rising below the column as well as the packing affects the pressure drop produced by the water flowing down. A comparison was made using a dry column by passing different air flow rates. Results tell us that as we increased the air flow rate for a specified water flow rate, the pressure drop also increased which means the pressure drop is a function of both the water and the air flow.
------------------------------------------------------------------------------------------------------------------------------1. Introduction Gas absorption is one of the basic mass transfer units in the industry. It can be used to wash, purify and obtain needed gaseous substances from mixtures. An example is the washing of ammonia by liquid water. As the name implies, the process involves bringing a gaseous mixture into contact with a liquid in order to obtain a product from the gas mixture. The commonly used equipment for this process is the gas absorption tower. It consists of a cylindrical column and inside contains solid objects called packing material. A liquid inlet and distributor is placed at the top of the tower and a gas inlet is located at the bottom. The liquid and gas comes into contact inside the tower and is efficiently mixed by the packing material. The washed gas comes out at the top and
the desired solute-containing liquid or commonly known as liquor is obtained at the bottom. The desired solute may be recovered from the liquor through distillation. 2. Methodology In determining the effect of air flow rate on the pressure differential across a dry column , the column must first be completely dried by passing sufficient airflow until all evidence of moisture in the packing has disappeared. Connect the column top and bottom to the water manometer with stopcocks S1 and S2. Take manometer readings of pressure differential across the column for a range of airflow rates. Airflow rates are to be increased by 20 L / min for every trial starting at 60 L / min of airflow until 140 L /min. Record the pressure readings for all trials. P a g e | 1
R.J. Bordeos, R.J. Candare, K. Dimalanta, M. Lerin, R. Rasco, P.B. Rose, Gas Absorption, 2011
involved phases during the stability of each period.
F i g u r e 1. G a s A b s o r p t i o n C o l u m n S e t- u p
In determining the effect of air flow rate with different water flow rates on the pressure differential across the column, fill the water reservoir tank up to half with water. Switch on the water pump and set C1 to give a flow rate at 3 liters/minutes. After a minute, close C1, switch off the pump and allow the column to drain for five minutes. Measure the air pressure differential across the wet column as a function of the air flowrate. Measure the air pressure differential across the column as a function of the airflow rate for different water flow rates, noting the appearance of the column at each setting. Flow rate of water increases by 1 liter / minute per trial up to 6 liters / minute. Record the pressure readings for all trials.
When the packing is dry, the pressure drop increases with the air flow rate across the column. If the packing is diluted with a constant flow of liquid (wet column), the relationship between the pressure drop and air flow rate initially follows a curve almost parallel to that for dry packing. Theoretically, as the air flow rate increases, the pressure drop for wet packing becomes greater than that for the dry packing because the liquid in the column lessens the space available for gas to flow. Also, as the liquid flow rate increases, pressure drop also increases. The pressure drop due to varying water and air flow rates for both wet and dry column are tabulated below. These were graphed against logarithmic scales. As seen from the graph, the pressure drops across the wet tower do not significantly deviate from that of the dry tower. A dry tower has a graph with almost a straight line representing a nearly constant pressure drop. The wet tower also has the same curve. As we introduce a constant flow of water to the tower, and as the air flow is increased, the graph doesn’t change significantly. Table 1. Data for Dry Co lum n
3. Discussion A transfer unit is concerned with the change in composition for the gas phase (i.e. vapor concentration) divided by the average driving force and based on the overall driving force for the gas phase. It is a measure of the performance of the equipment or the difficulty of separation in the system. Equilibrium stage, on the other hand, is guided by the mechanism of the
Airflow Rate (L/min)
ΔL (cm H2O)
ΔP (mmH2O)
140
6.8
666852.2
120
5.2
509945.8
100
3.6
353039.4
80
2.4
235359.6
60
1.5
147099.75
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R.J. Bordeos, R.J. Candare, K. Dimalanta, M. Lerin, R. Rasco, P.B. Rose, Gas Absorption, 2011
Table 2. Data for Wet Colu mn
10
ΔP at a Specific Water Flowrate
3 Air Flowrate
80
980665 843371. 9 666852. 2 490332. 5 353039. 4 235359. 6 117679. 8
60
980665
180 160 140 120 100
980665
980665
980665 P
843371.9
823759
823759
666852.2
686466
686466
509945.8
509946
509946
372652.7
333426
372653
215746.3
235360
235360
137293.1
117680
137293
980665
980665
980665
4
Δ
5 6 1
dry
55 Air Flowrate
Figure 2. Pressure drop (mmH 2 O) against logarithm ic scales.
If we take a closer look on the graphs, we can see that the slope of the graph in the wet column is nearly equal the slope of the dry column all throughout. This is not the case for an ideal set-up. The group suspected that the data gathered behaved in the said way because the equipment’s maximum air flowrate possible to observe is too small that the loading point and flooding point will be visible. Theoretically, the slope of the graph in the wet column will deviate from the slope of the dry column as it approaches its loading point where liquid is starting to holdup. At this point, the slope changes further which signals that it has already reached its flooding point which means that the water rapidly stacks up and fills the column. This is supposedly witnessed when the gas bubbles through the liquid.
The
group
experienced
a
lot
of
problems in doing the experiment which results to a poor set of data gathered. Some of the common problems involved in gas absorber column that were encountered during the conduct of the experiment may be that the equipment is not well calibrated and the tubes must is not connected to the right manometer, the Inconsistency of air and water flow rates during the experiment proper due to faulty control valve and when flooding occurs during the process, the pressure can’t be measured accurately as it fluctuates while measuring. 4. Conclusion and Recommendations Variation of air flow rate in gas absorption shows an increasing or decreasing effect on pressure drop across the packed column. From the plot, air flow rate is in direct proportion with pressure drop and has slope of the line of about 0.067. As the air flow rate increases, the pressure differential increases.
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R.J. Bordeos, R.J. Candare, K. Dimalanta, M. Lerin, R. Rasco, P.B. Rose, Gas Absorption, 2011
When liquid flow rate is introduced in the column, the plot is relatively parallel to the plot of the dry packing. This means that the pressure drop also increase with increasing air flow rate. However, this does not give the over-all effect of introducing water and varying its flow rate to the packed column. Increasing water flow rate increases pressure drop relative to that of the dry packed column and when the loading point is reached, a steeper slope of the plot is observed. With still further increase in gas velocity, the line becomes almost vertical. From the problems have undergone by the group during experiment, gas absorber column with a smaller diameter is recommended in replaced of the one. With such, more data points will be gathered because the loading point of the packed column is easily reached. In terms of the equipment faulty, proper maintenance of the control valve is recommended as well as the
checking the tube connection and probable leaks in it. Errors from fluctuating measurement can be minimized by having readings be taken by one person. 5. References [1] Foust, Alan S., et. al. Principles of Unit Operations 2nd Edition. John Wiley and Sons: Singapore. 1980. [2] Geankoplis, Christi J. Transport Processes and Unit Operations, 3rd Edition. Prentice-Hall International, Inc.: Singapore. 1993. [3] McCabe, Warren L.; Smith, Julian C.; Harriott, Peter. Unit Operations of Chemical Engineering, 4th Edition , Singapore. McGraw-Hill International Book Co., c. 1985
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