RAPPORT. Abstract. Jeg erklærer at arbeidet er utført selvstendig og i samsvar med NTNUs eksamensreglement. Dato og underskrifter:

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1 NTNU Norwegian University of Science and Technology Faculty of Natural Sciences and Technology Department of Chemical Engineering RAPPORT Felleslab, TKP 4105 og TKP 4110 Tittel: Ultrafiltration Sted: Trondheim Versjon: - Forfattere: Anders Leirpoll & Kasper Linnestad Veileder: Georg Voss Utført i tiden: Kl. 10:00-13:00 Antall sider: Hovedrapp: Bilag: Abstract The throughput, permeability and rejection rates of two membranes were studied in a stirred ultrafiltration cell. Both the pure water permeability as well as permeability of a filtrate sample of leaves was tested. The experiment was carried out using a transmembrane pressure of 2 bars. The second membrane was found to have a higher throughput and permeability than the first membrane. Using UV-Vis spectroscopy, the rejection rate was found to be higher for the second membrane. Jeg erklærer at arbeidet er utført selvstendig og i samsvar med NTNUs eksamensreglement. Dato og underskrifter: Address Location Tel Sem Sælands vei 4 NO-7491 Trondheim Fax Org. no. NO

2 i Table of Contents 1 Introduction Theory Ultrafiltration UV/Vis-spectroscopy Experimental Setup Pure water permeability Filtration Analysis Results and discussion Conclusion... 9 References List of symbols and abbreviations... 11

3 1 1 Introduction This experiment was carried out as a part of the felleslab for the courses TKP4105 and TKP 4110 at NTNU during fall The understanding of filtration and separation techniques is important to a chemical engineer, and testing of these in the lab, using different parameters is essential in fine-tuning of membranes and other separating equipment. The purpose of this exercise is to study the permeability and throughput of two different membranes in an ultrafiltration cell, and compare the two permeates with a UV/Vis spectrum to find a connection between throughput and the ability to separate. 2 Theory 2.1 Ultrafiltration Ultrafiltration is a pressure driven membrane process where the solvent, and possibly small solute molecules, pass through the membrane. Larger molecules such as proteins and polymers do not pass through the membrane, thereby separating the macromolecules from the solute and smaller molecules possibly present. The instantaneous pure water flux is given by: d (2.1) d where V is the filtration volume; A is the surface area of the membrane and t is the filtration time. Two membranes with the same surface can be compared using the throughput. The permeability with respect to the solvent is given as: where is the volumetric filtration flux and is the pressure drop over the membrane. The permeability is often normalized by viscosity, making the equation for, permeability at reference temperature: where is the viscosity of the filtrate, and is the viscosity at the reference temperature. In Table 1, viscosity and density of water at relevant temperatures are shown. Using Table 1 we can calculate the permeability at reference temperature,. (2.2) (2.3) Table 1:Viscosity and density of water at relevant temperatures [1]. Temperature Viscosity Density [ ] In an ultrafiltration experiment the sieving and rejection coefficients are also of interest. The sieving coefficient is defined as the ratio of the solute concentration in the filtrate to that in the bulk solution. The sum of the rejection coefficient and the sieving coefficient is equal to one which gives:

4 2 (2.4) where is the sieving coefficient; the rejection coefficient; the permeate concentration of component and the feed (bulk) concentration of component. Among the biggest problems of ultrafiltration are concentration polarization and fouling, both phenomena give a flux decline [2]. Membrane fouling is defined as the reduction of the flux of pure water filtration, caused by substances accumulating in or on the membrane [3]. This impacts the water s ability to permeate through the membrane, hence the flux decline. Concentration polarization is defined as the concentration gradient of permeating solute between the bulk region and the film near the membrane surface, and it becomes a limiting factor in membrane separations due to the reduced diffusion between the bulk and the film along the membrane surface [4]. It can also be defined as the ratio of the solute concentration at the membrane film to the concentration of the bulk, and this causes the solvent flux to decrease because it increases the osmotic pressure and the solute flux to increase due to increased concentration at the membrane film [1]. To avoid polarization of concentration and fouling high fluid velocity can be applied along the membrane surface [3]. In this exercise a rotational stirrer is used. 2.2 UV/Vis-spectroscopy In UV/Vis-spectroscopy we use this to determine different samples by measuring their absorbance at different wavelengths [5]. In this experiment spectrums of diluted samples and filtrated samples were compared [6]. The absorbance as a function of concentration can be found using Beer-Lambert law [7]: where is the absorbance; is the molar absorption coefficient; is the absorption path length and is the solute concentration. The sieving and rejection coefficients can be calculated using Beer-Lambert law by assuming that the molar absorption coefficient is the same for the original solution and the permeates. per eate solutio where per eate is the absorbance of the permeate, and solutio is the absorbance of the original solution. (2.5) (2.6) 3 Experimental 3.1 Setup The Stirred Ultrafiltration Cell Model 8400 from Millipore was set up according to the script [2], using circular sheet-formed membranes (area, c ) and rinsed with deionized water.

5 3 Figure 1: Stirred Ultrafiltration Cell Model 8400 from Millipore. [8] 3.2 Pure water permeability First feed of deionized water ( l was added during assembly of the cell. A beaker was placed on a laboratory scale next to the cell, monitored with the Labview-software. Transmembrane pressure was set ( bar), stirrer was turned on ( rp ), as the timer was started. This was performed twice for each of the two membranes. Temperature of filtrate was noted for every filtration. 3.3 Filtration A third filtration was performed for each of the two membranes, using a given solution. A sample was taken of the permeate ( i utes i ), using a glass beaker. Temperature of filtrate was noted for every filtration. 3.4 Analysis UV/Vis-spectroscopy was used to compare the absorbance of the sample with serial dilutions. The serial dilutions were made with the original sample and pure deionized water. The dilutions contained 100 %, 75 %, 50% and 25 % of the original sample, respectively. The UV/Vis measures were taken with a photometer placed in the solutions. Aluminum foil was used to cover up the samples to block out background light. Each sample was measured with the photometer, and the data were saved for later analysis. 4 Results and discussion The throughput was calculated and plotted against time in Figure 2 and Figure 3.

6 Throughput [L/h] Throughput [L/h] t [s] Test 1 Test 2 Filtration Figure 2: Throughput plotted against time,, for each sample used with the first membrane. Note that the first test and the second are so similar their lines overlap. Figure 2 shows the constant membrane performance for deionized water filtration in test 1 and in test 2, while membrane performance decreases for the filtration of the solution. Around 200 s for the filtration a sample was taken, leading to a measured throughput of 0. The decline at the end of the first filtration with deionized water occurs because the filtration chamber got empty. The first membrane did not experience any performance decline when deionized water was filtrated, but the throughput and permeability decreased throughout the filtration sample. This is due to fouling and concentration polarization. The solution sample contained large molecules which were held back in or on the membrane, resulting in fouling of the membrane. Furthermore, as the sample were filtrated the concentration of solute increased resulting in co ce tratio polarizatio This explai s why the e bra e s throughput a d per eability both decreased during the filtration of the solution sample Test 1 Test 2 Filtration t [s] Figure 3 Throughput plotted against time,, for each sample used with the second membrane.

7 Permeability [L/h m 2 bar] 5 Figure 3 shows membrane performance decreasing for each deionized water filtration, while performance is reduced over time by filtration of the solution. At approximately 100 s, a sample was taken from the permeate giving a throughput of 0. The second membrane experienced extensive performance decline for the duration of the experiment when deionized water was filtrated. This could be due to compaction of the membrane due to the exerted pressure [9]. This also explains why the throughput of the second deionized water filtration does not have the same starting value as the throughput of the first deio ized water filtratio s e di g value a ely because the e bra e had ti e to expa d between the two filtrations. The throughput of the solution sample filtration decreased for the whole duration of the experiment, this is due to fouling and concentration polarization and possibly also to compaction to some extent. Fouling is most likely the principal contributor to the flux declination because concentration polarization can be minimized by the use of a stirrer with a relatively high rotational velocity [1]. Such a stirring was applied in this experiment, consequently reducing the concentration polarization. However it cannot be said anything quantitative about how much each of these factors contributed to the flux decay, with the data procured in this experiment. From the throughput, permeability was calculated from (2.2), and was plotted in Figure 4 and Figure 5. Values are temperature corrected to using (2.3) t [s] Test 1 Test 2 Filtration Figure 4: Permeability, plotted against time,. Values are temperature-corrected for. Note that the first and the second test are so similar their lines overlap. Figure 4 shows the permeability of membrane 1 staying constant for deionized water tests, while decreasing with time for filtration sample. This is expected from the throughput in Figure 2, because the relationship between them is constant, as can been seen in (2.3).

8 Permeability [L/h m 2 bar] Test 1 Test 2 Filtration t [s] Figure 5: Permeability, plotted against time,. Values are temperature-corrected for. The graphs are cut where test finished. Figure 5 shows the permeability of membrane 2 decreasing for each deionized water test, and decreasing with time for filtration sample. This is expected from the throughput in Figure 3, because the relationship between them is constant as seen in (2.2). In Table 2 the time for each experiment to reach a mass of 40 g is shown. Table 2: The time each experiment used to reach a permeate mass of 40 grams. Experiment Time to reach 40 g permeate The first membrane, first water test 237 The first membrane, second water test 237 The first membrane, solution sample 487 The second membrane, first water test 13 The second membrane, second water test 21 The second membrane, solution sample 83 Table 2 displays how long each filtration took to reach a permeate mass of 40 g. Here it is ascertainable that the second membrane is significantly faster than the first membrane, but it is also undergoing a greater amount of deterioration. This is most likely due to increased throughput, hence the increased plugging and concentration polarization, and possibly also compaction as discussed earlier. The samples taken from filtration of the sample solution were analyzed using UV/Visspectroscopy, and compared to a dilution-series of the sample, shown in Figure 6.

9 Absorbance % 50 % 75 % 100 % First Membrane Second Membrane λ [nm] Figure 6: The UV/Vis spectrum of the dilution serial and the two permeates. Here is the wavelength. In Figure 6 it is clear that there are excessive amounts of distortions in the absorbance for wavelengths in the range of 200 nm to 400 nm. Furthermore in the wavelengths ranging from 400 nm to 470 nm some of the absorbance lines cross each other, this makes it inadequate for analysis. Around 570 nm to 650 nm there are unappreciable amounts of distortion, and for wavelengths in the range of 700 to 800 nm the absorbance of the filtration through the second membrane is negative. By these arguments further analysis of the absorbance is done with the wavelength range of 470 nm to 570 nm. Figure 7 displays the UV/Vis spectrum of the dilution serial and the filtrated solutions are plotted with wavelengths ranging from 470 nm to 570 nm. This is done because this range is the range with reasonably small amounts of distortion and large enough difference between the absorbance of each test. Here a moving average of 12 nm is used to account for the distortion.

10 Absorbance λ [nm] Figure 7: UV/Vis-spectroscopy of the dilution serial and the filtered solutions between 470 nm and 550 nm. The plot is made of a 12 nm moving average to remove distortions. Here λ is the wavelength. Figure 7 displays the UV/Vis spectrum of the dilution serial of the original sample together with the two permeates. The range of wavelengths between 470 nm and 570 nm is the range with least fluctuations and distortions in this spectrum, and hence this range was chosen for further comparison amid the different filtrations and dilutions. Figure 7 presents this wavelength range, and the plot is made of a 12 nm moving average of the absorbance to minimize the effects of aberrations. This figure shows that the absorbance of the dilutions of the original sample decrease with decreasing concentration, in accordance with (2.5), conversely one would expect the permeate of the first membrane to have a lower absorbance than that of the second membrane due to its lower permeability. Membranes with lower permeability usually yield a better separation than those with a higher permeability [1]. In this experiment the first membrane obtained a lower permeability than the second membrane, this would predict the first membrane to yield a better separation than the second membrane, thus yielding a permeate with a lower absorbance than that of the second membrane. Figure 7 shows the exact opposite, the absorbance of the permeate from the second membrane is significantly lower than the absorbance of the permeate from the first membrane. This could occur because the one of the membranes were put in upside down, or the data could have gotten interchanged. The last explanation is exceedingly improbable because the data were kept and edited meticulously. The rejection coefficient is calculated using (2.6) for every measured absorbance in the wavelengths ranging from 470 nm to 570 nm. The average rejection coefficient is given in Table 3 with two standard deviations. The sieving coefficient is then calculated via (2.4) Table 3: The rejection coefficient for each membrane, and the sieving coefficient, calculated by (2.4) and (2.6) for every measured absorbance in the wavelengths ranging from 470 nm to 570 nm. The values are averaged and given with two standard deviations. The first membrane The second membrane The sieving and rejection coefficients are displayed in Table 3, and they concur with what has already been discussed. Namely that the permeate of the second membrane has a lower absorbance than that of the first membrane, thus giving the second membrane a higher rejection coefficient than the first membrane.

11 5 Conclusion The throughput and permeability were calculated to be higher for second membrane than the first. Using UV-Vis spectroscopy and Beer-Lambert law, the rejection rate was found to be lower for the first membrane than for second membrane. 9

12 10 References [1] Christie John Geankoplis, Transport Processes and Separation Process Principles, 4th ed. Westford, Massachusets: Prentice Hall, [2] Georg Voss, "Membrane Ultrafiltration Script," Department of Chemical Engineering, Norwegian University of Technology and Science, Trondheim,. [3] David Paul and Abdul Rahman M. Abanmy, "Reverse Osmosis Membrane Fouling - The Final Frontier," Ultra Pure Water, vol. VII, no. 3, pp , [4] S. X. Liu, L. M. Vane, and M. Peng, "Theoretical Analysis of Concentration Polarization Effect on VOC Removal by Pervaporation," Hazardous Substance Research, vol. IV, no. 5, pp. 1-21, [5] ThermoScientific. (2007) ThermoScientific.com. [Online]. [6] Peter Atkins and Julio de Paula, Physical Chemistry, 9th ed. Oxford, New York: W.H. Freeman and Company, [7] IUPAC, Compendium of Chemical Terminology, 2nd ed., A. D. McNaught and A. Wilkinson, Eds. Oxford: Blackwell Scientific Publications, [Online]. [8] Millipore Corporation. Stirred Ultrafiltration Cells. [Online]. 88faba2f dc /$FILE/99228.pdf [9] D. A. Musale and S. S. Kulkarni, "Effect of Membrane-Solute Interactions on Ultrafiltration Performance," Journal of Macromolecular Science, Part C: Polymer Reviews, vol. 38, no. 4, pp , 1998.

13 11 List of symbols and abbreviations Symbol Unit Description Area Absorbance ol Feed concentration of component ol Permeate concentration of component ol Concentration h Flux h bar Permeability h bar Permeability at reference temperature c Length g ol Molecular weight of component g Mass g h Mass flow bar s a s a s g c Pressure drop Rejection coefficient Sieving coefficient Temperature Reference temperature Time Volume ol c Molar absorption coefficient Wavelength Viscosity Viscosity at reference temperature Density