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1 Design of an experimental setup and procedure for the measurement of the moisture exchange between a sheet of paper and airflow through it Anagnostidis, F.; Technische Universiteit Eindhoven (TUE). Stan Ackermans Instituut. Design and Technology of Instrumentation (DTI) Published: 01/01/2013 Document Version Publisher s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: A submitted manuscript is the author's version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website. The final author version and the galley proof are versions of the publication after peer review. The final published version features the final layout of the paper including the volume, issue and page numbers. Link to publication Citation for published version (APA): Anagnostidis, F., & Technische Universiteit Eindhoven (TUE). Stan Ackermans Instituut. Design and Technology of Instrumentation (DTI) (2013). Design of an experimental setup and procedure for the measurement of the moisture exchange between a sheet of paper and airflow through it Eindhoven: Technische Universiteit Eindhoven General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal? Take down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Download date: 25. Nov. 2018

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3 Design of an experimental setup and procedure for the measurement of the moisture exchange between a sheet of paper and airflow through it by Filippos Anagnostidis 17/09/2012 ISBN: One year project presented to Eindhoven University of Technology towards the degree of Professional Doctorate in Engineering in Design and Technology of Instrumentation

4 A catalogue record is available from the Eindhoven University of Technology Library ISBN: (Eindverslagen Stan Ackermans Instituut ; 2012/073)

5 Contents 1 Introduction Experimental setup Design parameters Initial experimental setup Initial setup challenges Decision point New experimental setup Idea of the design Determination of the design parameters Design and manufacture of the parts for the setup Initial moisture content experiments Optimization of the setup Instruments and accuracy of the methods Climate chamber Near Infrared Spectroscopy (NIR) Accuracy of the moisture content measurement Attention points and recommendations Air flow measurement Corresponding air velocities to the speed of the fan Accuracy of the flow measurement Modeling Modeling using Comsol Introduction of the convection term Extraction of the mass transfer coefficient (K me ) Results and Discussion Results Future work References 32

6 Chapter 1 Introduction Océ is one of the leading providers of document management and printing systems for professionals. It develops and produces office printing and copying systems, high speed digital production printers and wide format printing systems. Some examples of the Océ products are presented in fig One of the main goals of Océ is to improve the print quality of its printers and in order to achieve this an understanding is necessary of how paper exchanges moisture with the environment in different relative humidity conditions and also according to the design of the printer and the parameters that the printer is set to work in. Figure 1.1: Products of Océ Technologies The project which is presented in this report is an extension of a previous project performed by Maurice Fransen and Paula Marin. Their model describes the moisture transport in paper and experimental data is presented. The research performed in their project neglects the effect of convection in the transport of moisture in the paper and incorporates the diffusion as a mechanism. The goal of this project is to design an experimental setup to measure the transfer of moisture between a sheet of paper and airflow through it and also a procedure to extract the mass transfer coefficient. In fig. 1.2 the principle of the inkjet process in a roll-to-roll type printer is shown. The paper is stored in a cylindrical roll inside the printer. At the moment that the printing starts the external end of the cylindrical roll is unrolled and is positioned onto the print surface. There the paper is pressed onto the print surface by an airflow that is passing through. Another surface that is positioned next to the print surface provides additional time for the drying of the toner. 2

7 Figure 1.2: Schematic of the process of a roll-to-roll inkjet printer This project is focused on the effect of the airflow in the transport of moisture in the paper. The presence of the ink is neglected as we are in an initial stage of this research. It is focused on the print surface where airflow is passing through the paper. The project consists of three main parts: The design of the experimental setup, which includes the idea generation for the optimum solution for the experimental setup, the design and manufacturing of any parts needed and all the measurement devices that must be used in order to obtain the desired data from the experiments. The second part is a modeling part, where the transport of moisture into paper is modeled including all the parameters of the experiment. The third part is assessment of the accuracy of the experimental setup and the methods used. 3

8 Chapter 2 Experimental Setup In this chapter the experimental setup is presented. All the parameters for the design are analyzed and an initial experimental setup with its challenges is discussed. Moreover a decision point will lead to a new design of the setup including all the problems, solution and optimizations that took place Design Parameters As mentioned above the project focuses on the behavior of the paper on the print surface. First we need first to understand how the airflow affects the paper only, before introducing more parameters which would complicate the underlying physics of the phenomenon. Figure 2.1: Focused area of the printing process where the project is focused in The print surface is heated and it also allows airflow to pass through using fans positioned under it. So when the paper is positioned on top of it, it will reach equilibrium with the temperature of the plate and also the airflow passes through the paper. Therefore the design parameters for the experimental setup are: Airflow to pass through the paper This airflow depends on the type of paper. Pressure drop over the paper : ΔP = 600 Pa This is the pressure drop used in the printer Heating of the paper. Control of the environmental conditions: Relative humidity ranges from 15% to 95%. 4

9 2.2 Initial experimental setup The idea for a first approach of the experimental setup is to use a smaller version of the print surface that is generally used by Océ in the printers. This way the temperature of the paper can be controlled and also the airflow to pass through the paper is achieved. A climate chamber is used in order to control the environmental conditions by means of temperature and relative humidity. In Figure 2.2 the setup is shown. Figure 2.2: Initial experimental setup The setup consists of the print surface, a box positioned underneath with a fan which performs the suction and a water heating system. The print surface is designed as presented in Figure 2.3. On the top side of the surface is an array of holes which are connected with channels running through the length of the surface and connect with the bottom side of the surface through the slits in the back view. Under the slits, the fan is positioned to perform the suction. From the section B- B of the print surface channels are shown to run along the length of the print surface. These connect the holes to the bottom surface and there are also water channels in order to let warm water flow through the surface and heat it to the desired temperature. Figure 2.3: Geometry and dimensions of the print surface Initial setup challenges The proposed setup is entered inside the climate chamber in order to control the temperature and the humidity of the environment. The moisture content of the paper can be measured with a Near InfRared spectrometer (NIR), of which details are given in the next chapter. Experiments have been performed with the setup to measure the moisture content of a paper sheet for a specific change of relative humidity and several problems with the setup have been discovered. A major problem was that the sealing of the paper could not be achieved as the paper is lying on top of the surface and air is passing through and also from the side of the paper, which is undesired. Also the system cannot be sealed in a way that we are positive that the flow we measure is the one that passes through the paper as there are many leaks in the system of the surface and the box. 5

10 Another problem is that the measurement spot of the NIR measurement has a larger diameter than the holes of the surface (see fig. 2.4). If a NIR measurement is performed on the part of the paper which is placed on the aluminum then we will measure the transport of moisture due to diffusion because there is no airflow in the places where the aluminum is. For the part of the paper sheet that lays over the hole the NIR will measure the moisture transport due to diffusion but also due to convection. This is undesirable as we need to understand how the airflow affects the transport of moisture so we need to measure only the convection. In order to measure only the convection the measurement spot of the NIR should be smaller than the hole. The difficulty is that the NIR spot cannot reach such small diameters and the holes cannot be larger. The water channels of the print surface prevents us from making a suitable redesign in the plate with larger holes. An additional problem which occurs due to the configuration of the measurement, which is presented in fig. 2.4, is the background in the NIR measurement (the material behind the paper). The signal of the NIR is affected by the background as it penetrates the paper when it measures. This affects the measurement and in the case of aluminum and void space as a background, it is hard to identify each contribution and interpret the measurements correctly. Figure 2.4: Measurement spot of the NIR Finally another obstruction is with the temperature of the paper. The paper is heated by the surface and it will have a different temperature over the holes and over the aluminum. Also the air flowing through the paper has a different temperature than the paper. It is a choice to keep the modeling rather easy in this initial stage of the research Decision Point The complications that have been described for the initial experimental setup led us to a decision point where we needed to decide whether to continue with the print surface and solve the problems or to redesign the experimental setup. The print surface is used in the real printer and is designed with requirements and parameters that are not useful in this project. So a redesign of the print surface, by solving all the problems, would be a time consuming and difficult work and also would not be the ideal solution that would meet our requirements. The other option is to design a new simpler experimental setup which would focus on our requirements and would neglect the print surface and the problems that occurred. Also the setup should successfully resemble the real printing situation. In order to design the setup some requirements are considered. We need to find a way to have airflow passing through the paper and to heat the paper, the NIR measurement should be undisturbed and the background should be homogeneous and finally a way to control of the environmental conditions should be provided. We choose to follow the idea of the new experimental setup as it is a more dedicated approach to the problem and will provide us with better results. 6

11 2.3 New experimental Setup Idea of the design The idea behind the new experimental setup is to focus on the requirements of the project and find the most suitable solution. The experimental setup needs to be designed in a way that there is airflow passing through the paper and the paper can be heated. Also the airflow should pass only through the paper and a system to measure the airflow should be designed. In fig. 2.5 the idea for the experimental setup is presented. The setup consists of a bottom part where the fan is entered and a top part which fits to the bottom part and closes the system leaving only an area open to the environment from where the air is able to pass through. At the top face of the top part and on the periphery of the hole there is a rubber ring placed. The paper is placed on top of the rubber ring and sealing part is screwed on top of the paper. This cap will press the paper with the rubber ring and will seal the system so air can only pass through the paper. There is also an intermediate sealing part which is positioned between the fan and the top part of the setup. This part uses two rubber rings on its top and bottom faces and is used to seal the system inside the setup so that the fan will suck air only through the paper and not from the environment inside the setup. This way the airflow measured with the flowmeter is only passing through the paper. Figure 2.5: New experimental setup Determination of the design parameters There are a number of parameters that need to be determined in order to design the parts of the setup. First the determination of the area of the hole in order to have a sufficient airflow through the paper is necessary. Furthermore we need to find whether the displacement in the z direction of the paper due to the pressure drop across it and also due to the change in moisture content of the paper is significant and whether it affects the NIR measurement. The NIR measurement is calibrated, before each set of measurements, in a specific height from the sample. For the specific height the signal of the NIR is measured, for paper samples with known relative humidity, and therefore the translation of the signal of the NIR to relative humidity is feasible. If this height is changed then the calibration is no longer valid, as the signal of the NIR will change. The displacement of the paper will have the same effect on the NIR measurement so it needs to be checked whether the displacement is large enough to affect it. Diameter of the hole In order to determine the hole diameter use the previous setup with the plate is used and the airflow for a different number of holes that are open for air flow (in that setup) is measured. Then a total area from a specific number of holes is calculated and for this area the corresponding radius for one hole is found. The calculations and the results from the calculations can be found in Appendix B. From the calculations it is derived that a minimum radius for the hole of the new setup should be: r = 12.5 mm. 7

12 Displacement of the paper in Z-direction The displacement in z direction of the paper, when this is positioned over the hole, is due to the pressure difference but also due to the moisture content difference of the paper. The displacement due to pressure difference can be approximated using the equation for the displacement of a membrane [3]: 3 p K Et (1) 4 Where: Δp is the pressure difference K is a coefficient which varies from 2.93 to 3.22 δ is the displacement α is the radius of the hole E is the Young modulus t is the thickness of the paper Table 2.1: Input data for the calculation Pressure difference Δp 600 Pa Young modulus E 2.00E+10 Pa Paper thickness t 1.10E-04 m Coefficient K 3 To calculate the displacement due to the change in moisture content the following equation is used: l / l (2) MC Where: β is the hygro expansion coefficient with β cd =0,16 (%/%) in the cross direction and β md =0.03 (%/%) in the machine direction ` ] Δl/l is the relative change in the length of the paper ΔMC is the change in the moisture content If a maximum change in the moisture content of 10% is considered then we are able to calculate the displacement of the paper from the above equation for different diameters of the paper. The calculation for the machine direction (MD) and the cross direction (CD) is performed. The machine direction of the paper has a greater orientation of the fibers than the cross direction. All the calculations for the displacements of the paper can be found in Appendix B. In Diagram 2.1 the displacement due to the moisture content difference in the MD and CD directions, the displacement due to the pressure drop and the total displacement for different diameters can be found. Note here that the displacements due to change in moisture content have been transformed in the vertical dimension and also the total displacement is the sum of all the displacements which is calculated. From the Diagram 2.1 it is derived that for a diameter of 30 mm the total displacement is calculated at 4mm. 8

13 Displacement (mm) Displ. CD Displ. MD Displ. due to flow Total Displ. Total Displacement Diameter (mm) Diagram 2.1: Displacement of the paper Effect of the displacement of the paper to the NIR measurement After calculating the displacement of the paper for different diameters of the hole we need to investigate whether this displacement affects the NIR measurement. To check an experiment is performed to measure the moisture content of the paper for different heights of the paper. Figure 2.6 illustrates the experiment. The paper is positioned on a platform which can change its height. A calibration is performed in one position which is the initial position of zero height. The moisture content of the paper is measured, for constant relative humidity, for this position and then the height of the platform is changed by 0.5mm without calibrating again. In this new position the moisture content of the paper is measured again. Then the experiment continuous by changing the height of the platform 0.5mm from its previous position until the final point of 4mm. The results from the experiment are included in Diagram 2.2. Figure 2.6: Experimental setup for the measurement of the moisture content of the paper versus the displacement 9

14 Effect of the displacement on the NIR measurement Moisture Content (%) Distance (mm) Diagram 2.2: Effect of the displacement of the paper on the NIR measurement In Diagram 2.2 every point is the average of one measurement of 20 minutes in 25% relative humidity conditions and 25 C temperature. A regression analysis was performed for the experimental data and the regression line was calculated y = x The regression coefficient with its confidence intervals is calculated: < a < The regression coefficient is not different from zero and this leads us to the conclusion that there is no significant correlation between the moisture content of the paper and the change in the height. Therefore the displacement of the paper due to the pressure difference and also to the moisture content difference will not affect the NIR measurement as long as it is smaller than 4mm. The choice for the diameter of the hole is now feasible. A diameter of 30mm is chosen as the airflow through the hole is sufficient for this diameter and also the displacement of the paper does not significantly affect the measurement. The accuracy of the NIR measurement will be presented in Chapter Design and manufacture of the parts for the setup After all the design parameters are determined the parts of the setup must be designed and manufactured. The parts were designed using the 3D CAD software Co create. The 3D parts are the ones presented in fig.2.5 and the dimensioned 2D drawings can be found in Appendix B. All the parts have been manufactured with aluminum except the connector to the flow meter which was manufactured using a 3D printer because of its complicated geometry. In fig. 2.7 manufactured parts and also the assembly of the setup are presented. (a) (b) Figure 2.7: Photos of: a) parts of the setup and b) the assembled setup 10

15 2.3.4 Initial moisture content experiments After the setup was manufactured and assembled, experiments can be performed to measure the moisture content of the paper for different airflows. Initially we need to check whether the moisture content of the paper is the same for different airflows. So the first experiments that are performed are equilibrium moisture content experiments at a relative humidity of 90% and 25 C temperature. The papers are left inside the climate chamber for 20 minutes in 90% relative humidity in order to reach equilibrium with the environment and then the moisture content of the paper is measured for 5 minutes. The experiment is performed for four different airflows (four different speeds of the fan: four different values of the fan voltage). In Diagram 2.3 the results of the experiment are shown. 16 Average Moisture Content comparison - Input 90% 14 Moisture Content (%) V 15V 20V 25V 30V Fan volts (V) Diagram 2.3: Moisture content of the paper in equilibrium conditions of 90% relative humidity for different airflows (different fan voltage) In diagram 2.3 each point is the average moisture content of the paper for the specific fan speed. The fan speed can be translated into air speed through the paper with 0V being no airflow and 30V being the maximum airflow that the fan can achieve. It would be expected that the moisture content of the paper would be the same for different airflows. This is derived from the fact that the setup is positioned inside the climate chamber were the environment is constant at 90% relative humidity. So the air that flows through the paper is also of 90% relative humidity and should not be affected by the flow rate. But from the results of Diagram 2.3 a decrease in the moisture content of the paper as the airflow increases is observed. Another experiment is performed to confirm these unexpected results. In this experiment the paper is left inside the climate chamber at constant 90% relative humidity for 20 minutes in order to reach equilibrium. Then the moisture content of the paper is measured for 10 minutes. Initially the fan is switched off and at 3 minutes time the fan is switched on at its maximum speed (30V). In diagram 2.4 the result of the experiment are presented. 11

16 Fan inside the setup - Fan from 0V to 3min Moisture Content (%) Time (sec) Diagram 2.4: Moisture content of the paper in equilibrium conditions of 90% relative humidity, fan turned on at 3 minutes at 30V From the result that is presented in Diagram 2.4 it is clear that the moisture content decreases when the fan is turned on. The mean value for the time 0 sec 180 sec is calculated at 14.3% with a standard deviation of 0.75% of moisture content and for time 300 sec 600 sec is calculated at 12.3% with a standard deviation of 0.60% of moisture content. There is a difference of 2% of moisture content which is significant. As it is expected to remain constant we need to investigate whether there is a problem in the setup or a physical phenomenon that we have not considered yet. An increase in the temperature of the paper could explain the decrease in the moisture content. So the temperature of the paper inside the climate chamber is measured, which has a constant temperature of 25 C. Initially the fan is turned off and at 30 seconds time after the measurement starts we switch on the fan. After 12 minutes the fan is switched off. The temperature of the paper is measured using an Infrared thermometer (Optris CT hot). Diagram 2.5 includes the results from the experiment. Diagram 2.5: Temperature of the paper for different airflows 12

17 The temperature of the paper increases when we turn on the fan. As the fan is the only heating source in the system and it is positioned under the paper inside the setup, it can be easily derived that it radiates heat to the paper. So the setup needs to be optimized by positioning the fan outside the setup and also outside the climate chamber. It is necessary to position it outside the climate chamber in order not to have a heating source inside the environment that the experiment is performed as it will change the environmental conditions Optimization of the setup The idea behind the optimization is to take the fan out of the setup. This way various goals are achieved. The paper will not be heated and therefore we are able to check if the moisture content change is due to the temperature difference or due to the difference in the airflow. The fan will be also outside the climate chamber, so there will be no heat source inside the measurement system. Finally it will be very convenient to use another source to perform the suction, as the fan can be easily changed with any other instrument. Finally the flow meter is positioned between the setup and the fan so the airflow can be easily measured with no need to design any extra parts or to seal the fan. In Figure 2.7 a schematic explanation of the optimization of the setup is shown. Figure 2.8: Optimization of the setup After the optimization of the setup several parts were modified and the optimized setup needs to be validated in order to check if it can be used to measure the transport of moisture in the paper. The temperature of the paper and the moisture content of the paper for different airflows are measured. In Diagram 2.6 and 2.7 we can see the temperature of the paper and the moisture content of the paper for different airflows respectively. In the experiment that is performed the temperature of the paper is measured for 15 minutes. The paper is placed on the optimized setup and the setup is placed inside the climate chamber in a constant temperature of 25 C. Initially there is no airflow and at three minutes time the fan is turned on at its maximum speed. Then the fan is again switched off at 12 minutes time. 13

18 Diagram 2.6: Temperature of the paper for different airflows using the optimized setup From Diagram 2.6 it is shown that the temperature of the paper remains constant for the different airflows which means that the change of the airflow does not affect the temperature of the paper and the reasoning that the fan radiates heat to the paper was correct. The next experiment that needs to be performed is to check whether the moisture content of the paper changes for different airflows. The experiment is the same as the one whose results were presented in Diagram 2.4. The paper is left inside the climate chamber, in the optimized setup, at constant 90% relative humidity for 20 minutes in order to reach equilibrium. Then the moisture content of the paper is measured for 10 minutes. Initially the fan is switched off and at 3 minutes time the fan is switched on at its maximum speed. In diagram 2.7 the result of the experiment is presented. Opimized setup - Fan from 0V to 3min Moisture Content (%) Paper Linear (Paper) Time (sec) Diagram 2.7: Moisture content of the paper in equilibrium conditions of 90% relative humidity, fan turned on at 3 minutes at 30V Optimized setup As it is shown in Diagram 2.7 the moisture content of the paper remains constant for the whole measurement and is independent from the change in the airflow. The mean value for the time 0 sec 180 sec is calculated at 15.2% with a standard deviation of 0.46% of moisture content and for time 300 sec 600 sec is calculated 14

19 at 14.88% with a standard deviation of 0.51% of moisture content. There is a small decrease of 0.3% of moisture content between the initial and the final point of the measurement but it is an acceptable error. Comparing Diagram 2.8 with Diagram 2.4 it is clear that the effect of the fan in the moisture content has been decreased. The setup is now ready to measure the moisture content of the paper for different airflows. Fig. 2.8 shows the optimized setup as it was designed using CoCreate and also the manufactured and assembled setup. (a) (b) Figure 2.9: Optimized setup: a) CAD design, b) Manufactured setup 15

20 Chapter 3 Instruments and accuracy of the methods In this chapter the instruments that are used to control and to measure various parameters are presented. 3.1 Climate chamber A climate chamber is used in the experiments to control the relative humidity and also the temperature of the paper. In fig. 3.1 we can see the climate chamber. Appendix C includes the technical drawings of the climate chamber and the instruments that are included. The climate chamber works in a temperature range from 10 to 90 C and a relative humidity range from 10 to 90% ± 5%. The input of the climate chamber can be controlled using a computer and can be a ramp or a step for the temperature or the relative humidity inside the range of its operation. For the requirements of this project we need to measure the moisture content of the paper when the relative humidity of the environment is increasing from a low level to a high level. The climate chamber can be used to perform this change but it need to be checked whether its response is faster than the one of the paper. If this is not true then the NIR will measure the response of the climate chamber and not the one of the paper. In Appendix C the initial experiments that were performed are presented. The conclusions from these experiments show that it is probably able to measure the response of the paper in a step input to the climate chamber. In any other case the paper is faster and we finally measure the response of the climate chamber. Nevertheless, the sensor that is used to measure the relative humidity of the climate chamber is positioned away from the setup so the conclusion needs to be validated with another experiment that is presented below. Another conclusion from these experiments is that the final level of the step function must be up to 90% in order to minimize the overshoot of the climate chamber. Figure 3.1: Climate chamber used in the experiments In the experiment that will follow the response of the climate chamber is measured using the humidity sensor of the climate chamber and also another humidity sensor which is positioned close to the paper. The use of two sensors is necessary as the sensor of the climate chamber is positioned far away from the paper and we need to check if this has an impact on its response. Finally the response of the paper, using the NIR sensor, is measured. Figure 3.2 shows the experimental setup and the response of the climate chamber. Figure 3.2: Setup to measure the climate chamber response

21 Diagram 3.1 illustrates the response of the paper measured with the NIR sensor (blue line), of the climate chamber measured with the humidity sensor of the climate chamber (green line) and of the climate chamber measured with the Vaisala humidity sensor just above the paper (red line). In Appendix C the Vaisala sensor time response is determined. Response in Relative Humidity step input 20% - 90% Relative Humidity (%) Time (sec) NIR sensor Climate chamber sensor Vaisala sensor Diagram 3.1: Comparison of the response of the climate chamber with the response of the paper. In Diagram 3.1 there is a clear difference in the response of the climate chamber measured with different sensors in different positions inside the climate chamber. The response measured above the paper (red line) confirms that there is a lag between the two sensors and that its result is more reliable than the one measured with the climate chambers sensor (green line). This response can be used to compare it with the response of the paper (blue line). Comparing the two responses the conclusion that the climate chamber is faster than the paper is reached so the climate chamber can be used in the experiments. 3.2 Near InfRared spectroscopy (NIR) Near Infrared spectroscopy is used to measure the moisture content of the paper. It is a spectroscopic method that uses the near-infrared region of the electromagnetic spectrum to measure the absorbance of water in a material. The near infrared light refers to light within the wavelengths of 800nm 2500nm. In the near-infrared range strong absorbance by liquid water occurs at wavelengths around 2100, 1950 and 1450 (nm), with weaker absorption around 1200 and 970 nm. During the transmission of the electromagnetic radiation through a paper, that contains water, portions of the electromagnetic spectrum are absorbed by water molecules. This way different moisture content of the paper can be measured. Figure 3.2: NIR measurement in the experimental setup 17

22 Figure 3.4: Water absorbance in the NIR signal Accuracy of the moisture content measurement Equilibrium moisture content experiments are performed to check for repeatability and reproducibility of the results derived from the NIR measurement. Three sets of five experiments each are performed. Before each set of experiments a new calibration is performed. Each experiment is performed at 25% relative humidity and 25 C temperature. The paper is inside the climate chamber for 20 minutes, in order to reach equilibrium, and then the moisture content of the paper is measured for 5 minutes. For the next experiment the paper is changed and the measurement is performed again. When the experiments are finished the average moisture content of every experiment is calculated. In Diagram 3.2 the results of the experiments are presented. Each set of experiments is performed in a different day so the papers that are used in the calibration were inside the humidity boxes for at least 16 hours. 5 Average moisture content comparison Moisture Content (%) Set 1 Set 2 Set 3 Linear (Set 1) Linear (Set 2) Linear (Set 3) Number of experiment Diagram 3.2: NIR measurement repeatability and reproducibility Average moisture content comparison 18

23 The average moisture contents are calculated at 4.65% with a standard deviation of 0.078% of moisture content, for the experiments of set 1, 4.75% with a standard deviation of 0.162% of moisture content, for the experiments of set 2 and 4.84% with a standard deviation of 0.097% of moisture content, for the experiments of set 3. The maximum difference is 0.19% of moisture content which is acceptable according to the requirements of the project. An analysis of variance (ANOVA) is performed, with a confidence interval of a = 0.05, to check whether there is a significant difference among the mean values of the three sets of measurements. Table 3.1: ANOVA table NIR accuracy Source D.F. Sum of Mean square F-value P-value squares Between Samples Within Samples Total The F-value is calculated at 3.23 and the F-critical, as derived from a F-distribution table, is calculated at F.05,2,12 = Since F<F critical the null hypothesis is not rejected and the conclusion that the mean values of the three sets of experiments are not significantly different is reached Attention points and recommendations From the experimental procedure described above we conclude that the calibration is a very important step in the measurement of the moisture content of a paper. The papers that are used in the calibration must stay in the humidity boxes for at least 16 hours and must be undisturbed during this time (the boxes should not be opened). This conclusion is derived from an experimental procedure that was performed before the one presented here. The results of this experimental procedure are presented in Appendix C. The operator must be fast and careful when measuring a paper for the calibration. From the time that he opens the humidity box, to take out the paper, till the time he is measuring its moisture content, the paper is exchanging moisture with the environment. So the faster the operator will take the measurement, the closer to the real value the measurement will be. This rule also applies for the gravimetric method which relates the moisture content of the paper to the relative humidity. The faster the operator will measure the weight of the paper the closer to the real value the measurement will be. Finally it is advised to measure the relative humidity inside the humidity boxes after every calibration so that the conversion from the signal of the NIR to relative humidity will be accurate for every calibration. 19

24 3.3 Air flow measurement The flow of air through the paper is measured using the Bios DryCal DC-Lite flowmeter. The DryCal DC-Lite measures in a range of airflow between ml/min. In Figure 3.4 the installation of the setup in the experimental setup is shown. (a) (b) Figure 3.4: a) Flowmeter connected to the setup, b) Flowmeter Drycal DC-Lite Corresponding air velocities to the speed of the fan The corresponding air velocities and pressure drop for the different speeds of the fan (different voltage of the fan) are calculated for the LFM 090 paper. The flow rate is measured for the different voltages of the fan and the speed of the air is calculated for the specific area of the hole. The area of the hole is calculated at A = 7.07*10-4 m for a diameter of 30mm. The air velocity is calculated if we divide the airflow (in m 3 /sec) with the calculated area. It should be mentioned here that the reported flow rates are the average values of 20 measurements with each measurement being the average of 10 measurements. Table 3.2: Corresponding velocities and pressure drop for the different voltage of the fan. Fan voltage (Volt) Pressure drop (Pa) Flow rate (mil/min) Flow rate (m 3 /sec) Air Velocity (m/sec) * * * * * * * * * * Accuracy of the flow measurement Experiments are performed to access the accuracy of the air flow measurement. Two experimental procedures of three sets of measurements each are performed. Each measurement is the average measurement of 10 measurements and for each set 20 average measurements are performed. For all of the measurements the LFM 090 paper is used and the fan is set to each maximum power (30V). For the first experimental run the paper is changed before each set of measurements, while in the second 20

25 experimental run the measurements are performed with the same paper for every set. The results of the experiments are included in Diagram 3.3 and 3.4. In Appendix C the measurements are presented. 300 Accurasy of the airflow measurement - Different paper for each set Airflow (ml/min) Set 1 Set 2 Set 3 Linear (Set 1) Linear (Set 2) Linear (Set 3) No of measurement Diagram 3.3: Accuracy of the airflow measurement Different paper for each set of measurements 300 Accurasy of the airflow measurements - Same paper for all sets of measurement Airflow (ml/min) Set 1 Set Set 3 Linear (Set 1) 210 Linear (Set 2) Linear (Set 3) No of measurement Diagram 3.4: Accuracy of the airflow measurement Same paper for each set of measurements In both Diagrams 3.3 and 3.4 it is clear that each set of measurements is decreasing in time. This could be due to the fact that the fan is heating in time, frictions increase so the rpm of the fan is decreasing, and therefore the fan does not produce a constant airflow to pass through the paper. As it is shown Diagram 3.3 the airflow is different for each set of experiments. The means for the first experimental run are calculated ml/min with a standard deviation of 5.02 for Set 1, ml/min with a standard deviation of 3.09 for Set 2 and ml/min with a standard deviation of 4.60 for Set 3. The means of the three sets of experiments are significantly different as the p-value of the analysis of variances is calculated at zero. The maximum difference is calculated at 8.5 %. In Diagram 3.4 where each set of measurements is performed with the same paper we observe that the difference 21

26 between the measurements has decreased in comparison with the first experimental run. The means for the second experimental run are calculated at ml/min with a standard deviation of 4.31 for Set 1, ml/min with a standard deviation of 4.76 and 266 ml/min with a standard deviation of 4.11 for Set 3. The maximum difference now is calculated at 1.6 %. An analysis of variance is performed and the p-value is calculated at which suggests that the means of the three sets are statistically different but the difference is not important. From the two results the conclusion that the paper has a high effect on the measurement of the airflow is derived. The accuracy of the airflow measurement as derived from the second experimental run is 1%-2%. This is the accuracy of the measurement device as the same paper is used for these experiments. In the experiments that the setup will perform different papers will be used and therefore the accuracy is 8%-9% as derived from the first experimental run. We need to check whether this 9% difference in the airflow will have an effect in the transfer of moisture in the paper and whether this effect is significant. In order to do that a simulation is performed for three different airflows. The air velocity used is m/sec and two more velocities with a ± 9% difference. The model used for the simulation is presented in Chapter 4. In Diagram 3.5 the result of the simulation is presented. 12 Effect of airflow on the transport of moisture in the paper 11 Moisture Content (%) V = m/sec V = m/sec V = m/sec Time (sec) Diagram 3.5: Effect of airflow measurement on the transfer of moisture into the paper It is clear from Diagram 3.5 that the 9% difference in the airflow does not affect the moisture content of the paper but a statistical test is also performed to validate the conclusion. An analysis of variance (ANOVA) is performed, with a confidence interval of a = Table 3.3: ANOVA table Airflow accuracy Source D.F. Sum of Mean square F-value P-value squares Between Samples Within Samples Total The F-value is calculated at and the F-critical, as derived from a F-distribution table, is calculated at F.05,2,600 = Since F<F critical the null hypothesis is not rejected leading to the conclusion that the mean values of the three sets are not significantly different. This means that the 9% accuracy of the airflow measurement does not affect the transfer of moisture into the paper. 22

27 Chapter 4 Modeling The modeling of the transport of moisture into paper is essential for this project. Using the simulation the behavior of the paper in different relative humidity environments can be predicted and also the effect of different parameters of the paper on the transport of moisture can be evaluated. Finally the model is essential because it is used to extract the mass transfer coefficient as it fits the simulation results to the experiments. 4.1 Modeling using Comsol 4.2 The general model that describes the moisture uptake by a single sheet of paper in a homogeneous relative humidity environment is described in Modeling and computation of heat and moisture Transport in Paper by Paula Marin [1]. This model describes the transport of moisture into paper due to diffusion and neglects the effect of convection. Two mass balance equations, one for the water in pores and one for the water in the fibers, accompanied with the boundary and initials conditions are used to describe the moisture transport in the paper. The exchange of moisture between the paper and the environment, in this project, is simulated using Comsol 4.2. The general model by Paula Marin has been coded in Matlab, but there are some practical issues in the implementation for the needs of this project. The final goal is to include in the model the convection term so that we are able to simulate the exchange of moisture when airflow is passing through the paper. This prevents us from using the Matlab code as it has to be rewritten with our requirements and we choose to use Comsol as it is more convenient and user friendly than Matlab. As a first approach of the simulation the convection term is neglected and a number of results as they are presented in the thesis of Paula Marin [1] is reproduced. This way we are positive that the simulation is correct and we can continue including the convection term 1. In this model the same parameters as in the thesis of Marin Zapata [1] are used. These parameters are presented in Table 4.1: Table 4.1: Parameters used in the model 0.47 porosity 2 p 8.2*10 f 1550 L 4 Kg / m Kg / m 3 3 paper density Density of the dry fibers 1.1*10 m Paper thickness D 6 eff 1.7*10 m 2 / sec Effective diffusivity of water in the pores D 3 Wair 2.53*10 m 2 / sec Diffusivity of water in the air K 3 mi 7.4*10 1/ sec Internal mass transfer coefficient K 3 me 8.9*10 m / sec External mass transfer coefficient T 20 C Temperature of the paper and the environment K GAB GAB parameter C GAB GAB parameter X m GAB parameter 1 The reason that Paula Marin has coded her model in Matlab is that her model was to be used also in cases that the boundary conditions for the temperature of the paper show discontinuities as function of time. Such boundary conditions are not easy to implement in Comsol. A finite difference method is more suited to solve such problems than a finite element method and this has been implemented in Matlab.

28 In the simulation that is performed the paper is initially in equilibrium with the environment at 30% relative humidity and after 6 minutes the relative humidity is raised to 70% with a linear profile in a time interval of 0.5 minute. The concentration in the pores, the fibers and also the moisture content of the paper can be calculated. Diagram 4.1 presents the evolution in time of the moisture content of the paper. Moisture content of the paper Moisture Content Moisture content of the paper Time (sec) Diagram 4.1: Evolution in time of the moisture content of the paper The moisture content increases instantaneous after the relative humidity step starts. This suggests that the resistance to moisture exchange with the environment is not high. Also the moisture content reaches its equilibrium value in 1080sec (18min) after the relative humidity ramp starts. The equilibrium moisture content is calculated at MCeq=8.4%. This value is the same as the one calculated in the thesis of Paula Marin [1]. The equilibrium values for the concentrations in the pores and in the fibers are calculated at: Cpeq = kg/m^3 and Cfeq = kg/m^3. These values are also the same as the ones calculated in the thesis of Paula Marin [1]. In Appendix D the analysis and the diagrams for the concentrations in the pores and the fibers are included. Diagram 4.1 is also produced in a logarithmic scale in order to check whether there is a linear relation and if there are multiple time constants. 24

29 Moisture content of the paper 0.1 log(moisture Content) Moisture content of the paper Time (sec) Diagram 4.2: Evolution in time of the moisture content of the paper From the result presented in Diagram 4.2 we reach the conclusion that there is no linear relation between the moisture content of the paper and no time constants are derived. 4.2 Introduction of the convection term The calculations that were performed and compared with the literature showed that the model is correct and works for a condition where there is no airflow. The next step in the simulation is to introduce the convection term in the model and see how this affects the transfer of moisture in the paper. The convection term will include the averaged velocity of the air within the pores of the paper multiplied by the evolution in the thickness direction of the concentration of water in the pores. This term has been introduced in the mass balance for the pores equation as the pores are void space and the airflow will affect the transfer of moisture in this part of the paper. The transfer of moisture in the fibers will also be affected but not directly by the airflow as the fibers are solid material and no airflow can pass through. Introducing the convection term, the equation for the mass balance in the pores becomes: 2 C p C p Cp V Deff K (1 )( 2 mi C f, eq C f ) t z z (3) Where: V is the averaged air velocity in the pores of the paper (m/sec) C p and C f the concentrations in the pores and the fibers respectively. η the porosity of the paper. D the effective diffusivity of water in the pores. eff K mi the internal mass transfer coefficient C is the equilibrium water concentration in the fibers and is given by: f, eq 25

30 The mass balance for the water concentration in the fibers remains: C f t K ( C C ) mi f, eq f (4) The simulation is performed for a number of different velocities. This way we can find a range of velocities that have a significant influence in the transfer of moisture into the paper. The velocities used in the simulation are 0 m/s, 0.1 m/s, 0.01 m/s and m/s. Diagram 4.3 and 4.4 show the difference in the moisture content of the paper for the different airflows. 9 Moisture content of the paper for different airflows Moisture Content (%) V = 0 m/s V = 0.1 m/s 4.5 V = 0.01 m/s V = m/s Time (sec) Diagram 4.3: Evolution in time of the moisture content of the paper for different airflows Moisture content of the paper for different airflows - focused area Moisture Content (%) V = 0 m/s V = 0.1 m/s 4.5 V = 0.01 m/s V = m/s Time (sec) Diagram 4.4: Focused area of the evolution in time of the moisture content of the paper for different airflows 26

31 Diagrams 4.3 and 4.4 show how moisture is transferred into the paper for different airflows and also a focused area to have a better view of the results. For small air velocities, of the order of 10-3 m/s, there are no changes in the transfer of moisture in the paper, compared to the situation where the velocity is zero. Increasing the air velocity to the order of 10-2 there is a higher rate in the transfer of moisture in the paper and the paper reaches equilibrium with environment faster than the situations for zero or very small velocities. A higher increase in the velocity (order of 10-1 m/s) will result in a faster response of the paper. The results of the simulation suggest that an airflow of the order of 10-2 m/s or higher is needed to have a significant effect on the transfer of moisture into the paper. Concluding, the simulation of the transfer of moisture between the paper and the environment is successfully performed, starting by simulating the phenomenon for diffusion and then introducing the convection term in the model. The results for the diffusion model are in full agreement with the literature [1], which indicates that the model works correctly, and the results for the diffusionconvection model are as expected. This simulation tool can be used in order to fit the results from the experiments and find the mass transfer coefficient. 4.3 Extraction of the mass transfer coefficient (K me ) The final part of the project includes the extraction of the mass transfer coefficient K me : the coefficient that describes the exchange of moisture between the pores and the environment. After obtaining the results from the experiments a procedure to extract the mass transfer coefficient is necessary. Using the model we can obtain results for different papers, different inputs of relative humidity and also different K me. In the case that we need to extract the K me we can simulate the transport of moisture into the paper using the same parameters for the paper and the relative humidity input as in the experiment changing every time the K me. This way we are able to fit the results from the simulation to the ones from the experiment for a specific K me. Below an example of the procedure is presented. In principle it would be better to perform the fit automatically with a numerical scheme like the Marquardt algorithm. In this case it was decided to not use such a method because of the elaborate implementation that would be needed for such a procedure (coupling between Matlab and Comsol would have to be realized). The timeframe of the project does not leave room for this. Also, as will be shown, a step-by-step approach can be used without much loss of accuracy and robustness. The following experiment is performed for a step input of 35% to 90% of relative humidity using LFM 090 paper. The characteristics of the paper and the input used in the experiment are used in order to simulate the exchange of moisture. These values are presented in Table 4.1. Initially the simulation is performed for a wide range of values for the K me. The values 0, , 0.001, 0.01 and 0.1 (m/sec) are used. In Diagram 4.5 the result of the simulation and the fitting of the results to the experiment are presented. 27

32 Extraction of the mass transfer coefficient (Kme) Moisture content (%) Experiment 6.5 Kme = m/sec 6 Kme = 0.01 m/sec 5.5 Kme = 0 m/sec 5 Kme = m/sec 4.5 Kme = 0.1 m/sec Time (sec) Diagram 4.5: Extraction of K me Fitting of the simulation to the experiment for values of the K me in a range of [0, 0.1]. From Diagram 4.5 it is clear that the experiment is best fitted by the values of K me in the range [0.001, 0.01]. So we are going to focus in this area and perform the simulation again. Extraction of the mass transfer coefficient (Kme) Moisture content (%) Experiment 6.5 Kme = m/sec 6 Kme = m/sec 5.5 Kme = m/sec 5 Kme = m/sec 4.5 Kme = 0.01 m/sec Time (sec) Diagram 4.6: Extraction of K me Fitting of the simulation to the experiment for values of the K me in a range of [0.001, 0.01]. For the results of Diagram 4.6 we need to calculate the sum of squares for the difference between the experimental values and the results from the simulation for each value of K me. In this way we are able to find which values of the simulation are closer to the experimental values. By performing this calculation we find that the experimental values are best fitted by values of K me in the range of [0.008, 0.01]. Performing one more time the simulation for this range of values we can find the value of K me that best fits the experimental data. 28

33 Extraction of the mass transfer coefficient (Kme) Moisture content (%) Experiment 6.5 Kme = m/sec 6 Kme = m/sec 5.5 Kme = m/sec 5 Kme = m/sec 4.5 Kme = 0.01 m/sec Time (sec) Diagram 4.7: Extraction of K me Fitting of the simulation to the experiment for values of the K me in a range of [0.008, 0.01]. If we compute the sum of squares for the difference between the experimental values and the results from the simulation for each value of K me we find that the minimum value is corresponding to K me = m/sec. Another calculation could be performed to find a better estimate of K me but this would not bring a better accuracy as the difference would be very small. 29

34 Chapter 5 Results and discussion In this chapter the results, comments and suggestions for future work of this project are presented. 5.1 Results The main result of this project is the successful design of the experimental setup in order to measure the moisture content of the paper for different airflows and for different inputs of the relative humidity. Also the design of the experimental procedure in order to extract the mass transfer coefficient was successfully finished. The experimental setup is presented in Chapter 2 and the procedure to extract the mass transfer coefficient in Chapter 4. The experimental setup is able to measure the moisture content of the paper for a wide range of airflows from 0 ml/min up to 5000 ml/min (air velocity of 0.12 m/sec for the specific setup). An experiment is performed that validates the above conclusion. The moisture content of the paper is measured for different air flows using the fan that is used in the real printer and also a vacuum cleaner in order to achieve a high flow. The moisture content of the paper is measured for 10 minutes with a step input of relative humidity from 30% to 90%. Moisture Content experiments for different airflows Moisture Content (%) Time (sec) No airflow V = m/sec V = 0.12 m/sec Diagram 5.1: Moisture content experiments for a step input from 30% to 90% of relative humidity for conditions without airflow and air velocities of m/sec and 0.12m/sec From Diagram 5.1 we can see that for high airflow (v=0.12 m/sec) the rate of the moisture content in the paper is really high in comparison with the condition where the airflow is small (v= m/sec) or zero. It is possible that the response of the paper in first case is faster than the climate chamber so finally the climate chamber is measured but this does not cancel the conclusion that there is a high influence of this high airflow on the transfer of moisture into the paper. Comparing the condition where the airflow is low (6.5*10-3 m/sec) with the condition of no airflow we can see that there is a difference in the response of the paper. More experiments should be performed to reach a solid conclusion on the influence of the airflow in this range. Finally we observe that the equilibrium moisture levels for the different curves are not the same but we can see that for the case of the high airflow the equilibrium moisture content is decreasing and for

35 the case of no airflow it is increasing. So a measurement for a longer time is necessary in order to reach the equilibrium moisture content levels. In the real printing conditions the pressure drop over the paper is around 400 to 600Pa. In the experimental setup designed here this can be translated to a flow rate of 135 ml/min which can be achieved if we set the fan to operate at 20V. The corresponding air velocity in this case is 3.25*10-3 m/sec. It is interesting to see if there is an influence of the airflow in the transport of moisture in the paper for this range of low airflows produced by the fan. Experiments are performed for three different conditions, one without airflow, one with the fan operating at 20V and one at 30V (corresponding air velocity 6.5*10-3 m/sec). The moisture content of the paper is measured for 20 minutes and the input of the climate chamber is a step input from 30% to 90% of relative humidity. Two experiments for each condition are performed and the results are averaged to one curve. In Diagram 5.2 the results of the experiments are presented. Moisture content experiments for different airflows Moisture content (%) Time (sec) No airflow m/sec m/sec Diagram 5.2: Moisture content experiments for a step input from 30% to 90% of relative humidity for conditions without airflow and air velocities of m/sec and m/sec From the results of the experiments we can see that, for the different airflows, there is a difference in the rate that the moisture is transferred into the paper. Also there is a difference of maximum 1% in the final equilibrium moisture content levels of each set of experiments. It is important to mention here that the equilibrium level is not obvious in these experiments and the experiment needs to be performed for longer period in order to be positive that the equilibrium has been reached. Nevertheless first conclusions can be derived from this set of experiments. The results are unexpected, as from the simulations performed in Chapter 4 we would expect that the difference in the rate of the moisture transfer for these low flows would not differ and also the equilibrium moisture content would be the same. The mass transfer coefficients for the three experimental sets are determined, in order to check if there is a significant difference. The Diagrams which include the fitting of the simulation to the experiments are presented in Appendix E. The mass transfer coefficients (K me ) are calculated m/sec, m/sec and m/sec for the conditions without airflow, air velocities of m/sec and m/sec respectively. There is a 20% difference between the K me for the conditions of m/sec velocity and no airflow and an 8% difference for the conditions of the m/sec and m/sec velocities. This would lead to the conclusion that the airflow, although low, has an effect on the transport of moisture into the paper. The point of attention here is that the three experiments seem to have different equilibrium moisture content levels. If this is true then the GAB parameters would change and should be determined separately for the conditions with airflow. Also the model is based on the GAB parameters determined 31

36 for conditions without airflow, so the extraction of the mass transfer coefficient for conditions with airflow using this model would not be valid. As mentioned above the equilibrium moisture content of the experiments presented in Diagram 5.2 is not clear so we need to perform another set of experiments which will record the moisture content of the paper for a longer time. In this set of experiments the moisture content of the paper (LFM 090) is measured for 40 minutes for two different settings of the power of the fan (0V and 30V). Two experiments of each setting are performed and the results are averaged to one curve. The input of the climate chamber is a step input from 35% to 90% of relative humidity. 2 In Diagram 5.3 the results of the experiments are presented. Moisture content experiments for different airflows Moisture content (%) Time (sec) m/sec No Airflow Diagram 5.3: Moisture content experiments for a step input from 35% to 90% of relative humidity for conditions without airflow and air velocity of m/sec In the results of Diagram 5.6 the moisture content of the paper is measured for 40 minutes and the equilibrium moisture content is reached. The difference now is approximately 0.5% of moisture content while at the time 1200 sec the difference is again 1% as also observed in the results of Diagram 5.2. The difference in the rate of the transfer of moisture has the same behavior as in the previous experiments which suggest that there is a difference between the two conditions. The mass transfer coefficients are calculated for this set of experiments. For the conditions without airflow the mass transfer coefficient is calculated at K me = m/sec while for the conditions with a m/sec air speed the mass transfer coefficient is K me = m/sec. The diagrams that include the fitting of the simulation and the experiment are presented in Appendix E. 5.2 Future work In this project the experimental setup and also the procedure to extract the mass transfer coefficient when airflow is passing through a paper were successfully designed. The results presented above concluded to interesting phenomena that need to be investigated further. The airflow seems to have an effect in the transfer of moisture in the paper but the equilibrium moisture content for the different airflows is different. The timeframe of the project did not allow the performance of extended 2 The input of the climate chamber is changed in this set of experiments due to a problem occurred in the function of the climate chamber and due to the time constrains of the project it is decided to perform the experiments with this input as it does not affect the conclusions. 32

37 experimental procedures in order to validate these results and to explain in detail the reason for the above phenomena to occur. A dedicated experimental procedure should be designed and performed. One optimization of the specific setup would be to include additional equipment in order to control the temperature of the paper. As presented in Chapter 2 the temperature of the paper has a significant effect in the moisture content. With this addition the paper would be able to have different temperature from the environment and will enable the user to perform experiments with different airflows but also with different temperatures of the paper. In this case the model will need to be optimized as the different temperature of the paper should be included. With this optimization the setup will reflect much better the real printing conditions. 33

38 References [1] Marin Zapata, P. Modeling and computation of heat and moisture transport in paper. Master's thesis, Eindhoven University of Technology, [2] M.A.L.J. Fransen, Transient Moisture exchange between paper and environment. Master thesis, Eindhoven University of Technology, 2010 [3] S. P. Koenig, N. G. Boddeti, M. L. Dunn, J. S. Bunch, Ultra-strong adhesion of graphene membranes. University of Colorado, 2011

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