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DEPARTMENT OF CHEMICAL ENGINEERING FACULTY OF ENGINEERING, ARCHITECTURE AND SCIENCE Course Number CHE 315 Section No. 02 Course Title Unit Operation Laboratory I Semester/Year Fall 2011 Instructor Dr. Ginette Turcotte Teaching Assistant Samira Ghafoori Lab Report for Experiment NO. 7 Report Title Pressure Drops Across Packed Columns Total Pages 21 Experiment Date October 21st 2011 Submission Date October 28th 2011 Due Date October 28th 2011 Group No. M Students’ Name Student ID Signature Leader: Talal Ali 70764 Avinash Krishnan 03168 Harison Perinparayagam 72818 Tawsif Zaman 68324 Table of Contents Introduction 3 Theoretical Background 4 Experimental Procedure 6 Results and Discussion 8 Error Analysis 14 Conclusion and Recommendations 15 References 16 Sample Calculations 17 Appendix 18 Introduction The purpose of this experiment was to observe the effects of the pressure drop due to various fittings and valves in a packed column, as well as to determine the flooding velocities of the column. Throughout this experiment the relative humidity, water flow rate, air flow rate, differential pressure, water pressure, and the water and air in and out temperatures were recorded under different water and air flow rates varying from fully closed to fully open. The water flow rate going to the absorption tower from the feed tank was controlled using a recycle valve, which when fully closed would allow the maximum water flow rate achievable. For every water flow rate used, individual trials were recorded under different air flow rates. The flooding velocity was found to not change with respect to water flow rate, but it would change with respect to air flow rate. The flooding velocity would be at its highest at lower blower settings, when more air is released into the column. Theoretical Background Packed columns are pressure columns used in a variety of chemical processes that require the constant interaction of gas and liquid. These processes include absorption and stripping. Absorption purifies gas mixtures by using a liquid to remove a gas phase component. Stripping is where the liquid phase is purified, rather than the gas. Pressure drop is an important aspect of packed columns. This pressure drop occurs through the packing of the column, where the largest contact area between the liquid and the gas occur.[3] FIGURE 1 – Flow diagram of a Packed Column(1) The flooding velocity of a packed column occurs when the column is filled with liquid, usually caused by high gas velocity, and the operation is difficult to carry out. This flooding may damage the packing inside and is detrimental to the operation. The flooding velocity can be determined with a Generalized Pressure Drop Correlation chart. For the chart, the first step is to calculate the value X[3]: X=LMLGMG(?G?L)12 EQUATION 1 where: L = Molar flow rate of liquid (lbmol/min) ML = Molar weight of liquid (lbm/lbmol) G = Molar flow rate of gas (lbmol/min) MG = Molar weight of gas (lbm/lbmol) ?G = Density of gas (lb/ft3) ?L = Density of liquid (lb/ft3) Once the X value is calculated, the Y value can be read from the chart.[3] Y=u02gFp?G?Lf?Lf?L EQUATION 2 where: u02 = Superficial velocity of gas (ft/s) g = Gravity (ft/s2) Fp = Packing factor (ft2/ft3) ?G = Density of gas (lb/ft3) ?L = Density of liquid (lb/ft3) f?Lf?L = Correction factor for liquid density and viscosity (other than water) EQUATION 2 can then be rearranged to calculate the flooding velocity. Experimental Procedure In this experiment an absorption tower of 4 ft diameter and 10.925 ft height was used. The size and schedule of each type of pipe, as well as the number of each type of fitting were recorded. The LabVIEW based program “New Packed Bed Experiment” was accessed from a computer. The water pump was then turned on, and the recycle valve was fully closed. The air blower was then turned on, and was set to 0. After about a minute, the “New Packed Bed Experiment” program was used to record the relative humidity, water flow rate, air flow rate, differential pressure, water pressure, and the water and air in and out temperatures for approximately 4 minutes. The data was then saved in an excel file. This process was repeated for the air blower settings 15, 45, and 75. The air blower was then turned off. The recycle valve was turned one and third complete rotation and the air blower was turned on again, where 4 more trials were recorded at the previously mentioned air blower settings. This process was repeated until the recycle valve was fully closed, yielding a total of 4 different recycle valve positions. FIGURE 2 – Schematic of Pump and Piping Equipment Results and Discussion During the experimental procedure, LABVIEW was used to collect data such as time, relative humidity in, relative humidity out, water flow high through the 4’’ pipe, water flow low through the 1’’ pipe, air flow, differential pressure, water pressure at the top of Tower (Point 2), water pressure at the pump (Point 1), water in temperature, air in temperature, water out temperature and air out temperature. This raw data can be seen in the appendix. From the raw data, TABLE 1 was created. This table shows the values of the mass flow rate of air and the pressure drop per unit length of packing. TABLE 1 – Pressure Drop for Blower Setting Recycle Valve Blower Setting Mass Flowrate of Air - ? (lb/s) log(?) Pressure Drop per unit Height (?P/z) log(?P/z) Closed Open 122.638 2.088625 0.768897 -0.11413 15 122.61 2.088526 0.753881 -0.1227 45 103.4576 2.014762 0.413411 -0.38362 75 103.5301 2.015066 0.414582 -0.38239 First Opening Open 122.6484 2.088662 0.859404 -0.0658 15 122.6265 2.088584 0.81785 -0.08733 45 122.6449 2.088649 0.850142 -0.07051 75 122.6436 2.088645 0.849787 -0.07069 Second Opening Open 122.6512 2.088672 0.848628 -0.07128 15 122.6519 2.088674 0.80534 -0.09402 45 100.9468 2.004092 0.432803 -0.36371 75 16.89904 1.227862 0.02941 -1.5315 Fully Open Open 122.6914 2.088814 0.858814 -0.0661 15 122.6991 2.088841 0.813948 -0.0894 45 90.85697 1.958358 0.380176 -0.42002 75 17.58726 1.245198 0.028387 -1.54688 These values are displayed in FIGURE 3. FIGURE 3 – Correlation between Mass Flowrate and Pressure Drop per unit Height By investigating the correlation shown in FIGURE 1, there exists a linear proportional relationship between the mass flowrate of air and the pressure drop per unit height. As observed, a higher mass flowrate of air (ie lower water flowrate) results in a higher pressure drop. In other words, when the blower is set to a lower setting, the pressure drop is maximized. This is because the air resists the flow of water through the packed column causing a water build up in the column. This excess accumulation of water blocks the openings in the packed bed resulting is less space for the air flow which in turn causes a higher pressure drop. Using the data obtained from LABVIEW, the flooding velocities can be found for all three trials as seen in TABLE 2. TABLE 2 – Blower Setting Flood Velocity Recycle Valve Blower Setting Liquid Flow Rate (kg/min) Gas Flowrate (kg/s) X Y Flood Velocity U (m/s) Closed Open 29.81393 122.638 0.008426 0.27 3.202501 15 30.37856 122.61 0.008587 0.26 3.142636 45 30.35201 103.4576 0.010168 0.25 3.081608 75 30.89507 103.5301 0.010343 0.24 3.019347 First Opening Open 39.22168 122.6484 0.011083 0.24 3.019347 15 38.68084 122.6265 0.010933 0.24 3.019347 45 38.89739 122.6449 0.010992 0.24 3.019347 75 38.57237 122.6436 0.0109 0.24 3.019347 Second Opening Open 40.27583 122.6512 0.011381 0.24 3.019347 15 39.77303 122.6519 0.011239 0.24 3.019347 45 40.05147 100.9468 0.013751 0.22 2.890805 75 38.40488 16.89904 0.078765 0.03 1.0675 Fully Open Open 39.52378 122.6914 0.011165 0.24 3.019347 15 40.0511 122.6991 0.011313 0.24 3.019347 45 40.68855 90.85697 0.015521 0.2 2.756274 75 40.56742 17.58726 0.079944 0.03 1.0675 Throughout the experiment, the flooding velocity remains more or less equal for constant flowrate of water, the discrepancies are addressed in the error analysis. This means that the flood velocity does not depend on the flowrate of water. On the other hand, the flooding velocity does change with blower setting and this change is not due to an error. It can be seen that the flooding velocity is highest at lower blowing settings, when more air is released into the column. This trend coincides with the correlation seen in FIGURE 3; the air flows opposite to the flow of water, creating a resistance. In a packed column, it is important to increase the area of contact between the liquid and gas phase by using the appropriate packing material. Also, this should be done while maintaining a minimal pressure drop. The packing arrangement can either be random or structured. Random packings are affordable and reusable type of packing. They are generally used for smaller separation columns or for batch systems. Structured packings are the expensive type of packing and stacked in a fixed manner for specific purposes, such as increasing the surface area and efficiency of the packing. This packing has better performance than random packing. Structured packings are used in low pressure operations and liquid flow rates. The materials that are used for packing can be different. The packing with different material is selected based on the expected performance, components in the packed column, the pressure, flow rates, and temperature of the process taking place. There are three types of materials for packing: ceramic, metal, and plastic. Ceramic packing is generally used for high temperatures and suited for organic and non-organic acids since this material does not reach with them. Ceramic packing also generates little pressure drop. This packing is widely used in drying towers, absorption towers, and cooling towers. Metal packing is generally used in processes where multiple stages are required, such as absorption and stripping. Since metal packing has high heat transfer capacity, it is also used for heat transfer, gas mixing, and extraction where packing’s heat conductivity can improve the process. Plastic packing are used in corrosive applications with low to moderate operating temperatures. Plastic packings are typically good because they are much cheaper than the metal and ceramic packings. Also, plastic is not as bulky as ceramic, so there is less pressure drop than the latter. Plastic packings are frequently used for processes, such as absorption, stripping, and scrubbing. In the FIGURE 4 below, different types of random and structured packings and characteristics are shown. Additional information regarding the packing mentioned in the following tables and other packing can be found in Table 6.6 of Separation Process Principles by Seader and through the reference list of this lab report[3]. FIGURE 4A - Characteristics of Packings[3] FIGURE 4B - Characteristics of Packings[3] As seen in the FIGURE 2 above, each type of packings has a different packing factor, FP. So, this has an effect on the pressure drop across a packed column. Random packing will have a moderate drop because random packing has random water flow path and as a result, this increases the chance of pressure loss in many areas. Structured packing will have a low pressure drop because of the organized water flow path and thus, it creates lower pressure loss. As mentioned earlier, each type of packing will have a different effect on pressure drop due to different packing factors. There is an empirical expression: ?Pflood=0.115Fp0.7 by which the pressure drop at flooding is strongly dependent upon[3]. As the packing factor increases, the pressure drop increases by a power of 0.7. Also, when the pressure drop increases, the flooding velocity (uo) increases due to their proportionality as shown in the expression: . Error Analysis The error in this experiment can be attributed to several things such as the pipes used during this experiment, which were quite old, showing signs of use and age. The use of the pipes over the years, may have morphed the pipes and or the lining of the pipes. This would cause the flow rate to decrease and would lead to inaccurate values. Another error involves the assumption that the system reached steady state during each of the trials. If the system was not at steady state, then the recorded flow rates could have been affected leading to inaccurate results. Another error is that the packing may have been affected due to the flooding velocities. The flooding velocities may have damaged the packing, which in turn could change the void fraction of the packing, and thus the surface area would change as well. Lastly, the Generalized Pressure Drop Correlation chart was difficult to read, and thus the values used were estimated, which caused the calculated values to change and may be inaccurate. Conclusion and Recommendations The purpose of this experiment was to observe the pressure drop observed due to various fittings and valves in a packed column and to determine the flooding velocities of the column. The flooding velocity was determined to not change with respect to water flow rate, but it would change with respect to air flow rate. The flooding velocity would be at its highest at lower blower settings, observed to be 3.02 m/s when the blower was set to 0 and 1.07 m/s when the blower was set to 75 (under the third water flow rate setting). Some recommendations on how to improve the experiment for better results is to replace or upgrade much of the equipment used in this experiment. Equipment to be upgraded includes the pipes, the pump, and possibly the sensors. The packing material should be cleaned periodically and or checked for damage or alteration. Eventually the packing should be replaced. References http://en.citizendium.org/images/f/f1/Packed_Bed_Absorption_Column.png (Last Viewed 28/10/2011) Merhvar, M, CHE 315: Unit Operations Laboratory Manual, September 2008 Seader, J., Henley, E.J., Separation Process Principles, Jon Wiley & Sons, Inc., 1998 AceChemPack Tower Packing Co., Ltd. http://www.towerpacking.com/Dir_column_packing.htm. November 6, 2010 Perry, R., Green, D. Perry’s Chemical Engineering Handbook. McGraw Hill, 1999. Sample Calculations Calculations for Pressure Drop per unit Height: Air mass flowrate (?) : V x ? = (3607.6ft3min)m335.3ft3×(1.2kgm3) = 122.63 kg/m3 log (?) = log (122.63) = 2.088 Pressure Drop per unit Height (?P/z) = 8.4 mmHg / 10.925 ft = 0.769 log (?P/z) = log (0.769) = -0.114 Calculations for Flood Velocities: X=L'G'?G?L0.5 L'=7.88galminft37.48 galm335.3 ft3999 kgm3=29.81 kg/min G’ = ? = 122.63 kg/m3 X=29.81 kg/min122.63 kg/min1.2 kg/m3999 kg/m30.5= 0.008 From Generalized Pressure Drop Chart (Figure 6.35)[3] @ the flood line: Y = 0.27 Y=u02Fpg?G?Lf?Lf(?L) u0= Y gFP (?L?G) u0= 0.279.81ms(3600)215 (999 kg/m31.2kgm3) u0= 3.2 m/s Appendix Raw Data: (an average with of all values with time was taken)
