Transcript
Inspector: Leader:
Data Reporter: Experiment #6, Reverse Osmosis
Section #011, Group #3
September 24, 2006
Ryerson University
Department of Chemical Engineering
CHE 415 Unit Operations II
Lab Report
Experiment #6: Reverse Osmosis
Experiment Performed on September 19, 2006
Report Submitted to
Dr.
By Group # 3 Section # 011 1. Leader:
2. Inspector:
3. Data Reporter
Date Report Submitted on September 24, 2006
Marking Scheme
Formatting Answer to all 6 questions in each Report Section / 10
General Appearance; Grammar and Spelling / 5
Complete and Informative Tables and Graphs / 15
Contents Accuracy and Precision of Results / 20
Comparison with Literature Data / 10
Influence of Procedural Design on Results / 10
Logic of Arguments / 20
Sample Calculations / 10
_____
Total: / 100
TABLE OF CONTENTS
1.0 Introduction and Background………...……………………………………………...……….. 2
2.0 Experimental Section…………………………………………………...................................... 6
2.1 Process Schematic and Equipment ...……………………………………………… 6
2.2 Technical Design Procedure…………………………………… ………………….. 8
2.2.1 Review of Safety and Environmental Concerns ………………………... 8
2.2.2 Pre-Experiment Equipment Set-Up …………………………………….. 8
Experimental Method…….………………………………………………. 8
3.0 Results and Discussion …………….………………………………………………………… 9
4.0 Conclusions …………………………………………………………………………………… 12
5.0 References ………………………………………………………………………………..….... 13
6.0 Appendix A ……………………………………………………………………………………. 14
Tables and Figures:
Figure 1.1: Selective Movement of Solvent (Water) During Osmotic
and Reverse Osmotic Processes. …………………………………………………….. 4
Figure 1.2: Spiral wound and hollow fiber RO membrane modules …………………………… 5
Figure 1.3: Cross-Flow Reverse Osmosis Filtration System …………………………………… 6
Figure 2.1: Process Flow Diagram of Reverse Osmosis Cross Filtration System …………….. 7
Figure 3.1: Effect of Increasing Retentate Pressure on Permeate/Retentate Flow Rates…….. 10
Figure 3.2: Effects of Increasing Retentate Recycle on
Permeate/Retentate Flow Rates at 120 psi…………………………………………. 11
Figure 3.3: Effects of Increasing Retentate Recycle on
Permeate/Retentate Flow Rates at 130 psi …………………………………………. 12
Table A1: Permeate/Retentate Flow Rates at Increasing Retentate Pressure (No Recycle)…. 14
Table A2: Permeate/Retentate Flow Rates at Increasing Recycle (Constant Pressure)……… 14
Table A3: Permeate/Retentate Flow Rates at Increasing Recycle (Constant Pressure)…….. 14
EXPERIMENT #6: REVERSE OSMOSIS
INTRODUCTION AND BACKGROUND
The ability to purify water and waste streams is critically necessary given the increasing depletion of natural water resources due to contamination and climate changes (Rautenbach and Albrecht, 1989). In addition, the rising costs of fresh water production and wastewater disposal have magnified the above need for efficient and cost-effective methods of water and wastewater purification (Dababneh and Al-Nimr, 2002). Specific applications of reverse osmosis include: 1) the desalinization and purification of seawater, 2) treatment of industrial wastewater to remove heavy metal ions, non-biodegradable substances, and other components of commercial value, 3) treatment of rinse water from electroplating processes to obtain a metal ion concentrate and a permeate that can be reused as a rinse, 4) separation of sulfites and bisulfites from pulp and paper process effluent, 5) treatment of wastewater from dyeing processes, 6) recovery of constituents having food value from food processing plant wastewater, 7) dewatering of certain food products such as coffee, soups, tea, milk etc., 8) concentration of amino acids and alkaloids, and 9) treatment of municipal water to remove inorganic salts, certain organic compounds, viruses, and bacteria such that it is suitable for human consumption (Seader and Henley, 1998).
Reverse osmosis (RO) is based on the process of “osmosis” (Dababneh and Al-Nimr, 2002). Osmosis involves the movement of solvent molecules through a semipermeable membrane from a low concentration region to a region of high concentration (Kotz and Treichel, 1999). The transfer direction of solvent through the membrane can be reversed by applying sufficient pressure to the concentrated side (reject or retentate side) of the membrane. If the applied pressure is higher than the sum of osmotic pressure and the pressure on the other side of the membrane (product or permeate side), water is forced out of the concentrated solution through to the product side of the membrane in a process known as “reverse osmosis” (Seader and Henley, 1998). A comparison of osmotic and reverse osmotic processes is shown in Figure 1.
(Ryerson University, 2006)
Figure 1.1: Selective Movement of Solvent (Water) During Osmotic and Reverse Osmotic Processes.
The theoretical osmotic pressure of any single compartment can be calculated using the Van’t Hoff Equation as shown in Equation 1.1. Van’t Hoff derived a precise linear relationship between osmotic pressure, ?, and solute molar concentration, c, proportionality gas constant, R, and absolute temperature of solution, T, in an ideal solution (Kotz and Treichel, 1999).
? = c R T where: ? expressed by atm (Equation 1.1)
c expressed by mol/L
R expressed by 0.0821 L? atm / K ? mol
T expressed by º K
Therefore, as the more concentrated the feed solution, the hydrostatic force necessary to overcome this osmotic pressure in order to drive water out of solution and through the membrane to the product side.
The semipermeable membrane of the RO system is comprised of spiral wound and hollow-fiber modules of cellulose acetate or aromatic polyamide (Seader and Henley, 1998) as shown in Figure 2. Based on the pore size diameter of the membrane, RO membrane systems are typically capable of removing constituents with a molecular weight of greater than 150-250 Daltons (the larger the size and charge of the solute “contaminants”, the more likely RO membranes will reject these particles) from the feed stream (Rautenbach and Albrecht, 1989). Since most of the contaminants do not generally move with the solvent as it passes across the membrane, a relatively pure permeate is produced.
Spiral Wound Hollow Fiber
(Sagle and Freeman, 2006)
Figure 1.2: Spiral wound and hollow fiber RO membrane modules
The rate at which permeate flows through the membrane is termed the “mass flux rate” while the rate at which the fluid flows from the retentate side to the permeate side of the membrane is known as “mass average or cross flow velocity” (Bird and Stewart, 2002). The proportional relationship between mass flux rate and cross velocity is illustrated by Equation 1.2.
? = V ? A (equation 1.2)
where: ?= mass flux rate of permeate across membrane (kg/s ? m2)
V = mass average or cross flow velocity of fluid through the membrane (m/s)
A = membrane cross sectional area (m2)
(Bird and Stewart, 2002)
A technique which throttles the retentate stream outlet to increase pressure on the retentate side of the membrane has been shown to increase cross flow velocity of the water through the membrane (Filmtec, 2001). However, if pressure is increased too much, the membrane will not be able to prevent solute from crossing into the permeate and potentially cause membrane fouling (Sheiknoleslami and Bright, 2002). Such phenomena can be explained by the relationship between solute mass flux and mass average (cross flow) velocity shown in Equation 1.3. In an effort to minimize the occurrence of fouling, cross flow filtration systems have been designed as shown in Figure 1.3 such that any feed that does not pass through the membrane is carried away in the reject (concentrate) stream (Ryerson University, 2006).
NA = ?A ? V - dAb ? ? ?A (equation 1.3)
where: NA = mass flux of solute component (kg/s ? m2)
?A = mass density of component A (kg/m3)
V = mass average velocity of fluid through the membrane (m/s)
dAb = diffusion coefficient of component A in membrane (cm2/s)
(Bird and Stewart, 2002)
(Ryerson University, 2006)
Figure 1.3: Cross-Flow Reverse Osmosis Filtration System
EXPERIMENTAL SECTION
The objectives of this experiment were to examine the relationship between: 1) increasing retentate membrane side pressure and permeate/retentate flow rates (with zero recycle of retentate stream) with reference to Equation 1.1 and 1.2, and 2) increasing retentate recycle and permeate/retentate flow rates (at constant retentate membrane side pressure) with reference to Equation 1.1 and 1.3. The experiment was subdivided into two parts in order to investigate each of the objectives outlined above.
2.1 Process Schematic and Equipment
City of Toronto Tap Water was fed to a Filtration Concepts Inc. Commercial Tap-Water Cross-Filtration RO System Model # TW 1–1/2–120–60 which utilized a 5 micron filter cartridge and two RO membrane modules connected in series as shown in Figure 2.1.The system was equipped with a High Pressure Valve which could throttle the retentate exit stream to increase reject membrane side pressure, Retentate Recycle Valve which recycle retentate back into the feed stream, permeate/retentate flow meters, and safety pressure gauges at the High Pressure Pump suction and retentate stream Exit. The retentate stream was sent to waste while the permeate stream was collected in a storage tank.
(Ryerson University, 2006)
Figure 2.1: Process Flow Diagram of Reverse Osmosis Cross Filtration System
2.2 Technical Design Procedure
Prior to performing the above two experiments, several pre-experiment procedures were implemented as outlined in sections 2.2.1 and 2.2.2.
2.2.1 Review of Safety and Environmental Concerns:
Ensure the reject pressure stays within normal operating parameters (High Pressure Gauge < 150 psi).
Monitor low pressure gauge to ensure that a minimum of 20 psi in the feed tap water is presented to the suction side of the High Pressure Pump and maximum suction pump pressure of 75 psi is not exceeded in order to meet normal operating conditions.
2.2.2 Pre-Experiment Equipment Set-Up:
Close Inlet Shut-off Valve.
Verify 5 micron filter cartridge is installed in blue prefilter housing.
Verify that Reject and Product tubes are connected to drain and storage tank respectively.
Verify that Product Storage Tank Float Level Controls are connected properly.
Close Recycle Valve completely.
Open the Inlet Feed Water Shut-off Valve Completely.
Turn RO System control switch to ON.
Press RO System Start switch if storage tank water level is between High and Low float levels, otherwise, system will automatically start if storage tank water level is below the Low float level.
Monitor Low Pressure and High Pressure gauges throughout the experiment as per safety and environmental concerns outlined above.
Experimental Method:
The first experiment implemented a zero recycle condition (recycle valve closed) in order to analyze the effects of increasing retentate membrane side pressure on permeate/retentate flow rates. With the pre-experiment set-up completed as per section 2.2.2 above, the High Pressure Valve was partially closed until the High Pressure Gauge read approximately 110 psi. At this pressure, the corresponding the permeate and retentate flow rates (L/min) were recorded. The above procedure was repeated to achieve retentate membrane side pressures of 120, 130, 140 and 150 psi respectively and the corresponding flow rates were documented.
The second part of the experiment implemented a constant system pressure condition in order to analyze the effects of increasing retentate recycle on permeate/retentate flow rates. The retentate membrane side pressure was held constant by partially closing the High Pressure Valve until a High Pressure gauge reading of 120 psi was achieved. At zero recycle, the corresponding the permeate and retentate flow rates (L/min) were recorded. The recycle valve was then opened by 20%, 40%, 60%, 80%, and 100% and the respective flow rates were recorded.
At the completion of the second part of the experiment, the High Pressure Valve was fully opened in order to reduce system pressure, the RO System control switch was turned off, and the Inlet Feed Water Shut-off Valve was closed.
3.0 RESULTS AND DISCUSSION
The tap water was pre-filtered in the 5 micron cartridge filter prior and was then pumped into the RO membrane modules by the High Pressure Pump. The RO membrane separated the feed flow into permeate and retentate streams. Unfortunately, a feed pressure of only 9 psi could be produced on the suction of the pump which was below recommended pump operating parameters. This was due to a limitation in the local tap water service line pressure. Nevertheless, the High Pressure Pump was able to function in some capacity over the duration of the experiment.
Several findings were noted regarding the effects of increasing retentate membrane side pressure on permeate/retentate flow rates. Firstly, system retentate pressure could only be reduced to 110 psi under the zero recycle condition. As such, the relationship between the above variables could only be investigated between the range of 110 and 150 psi. Over this pressure range, the permeate and retentate flow rates are shown in Appendix A – Table A1. A linear relationship (R2 =1) was found between increasing retentate membrane side pressure and permeate flow rate as seen in Figure 3.1. A concomitant linear reduction (R2 =0.9466) in permeate flow rate was also observed.
Given the solute contaminants contained in the feed, a level of osmotic pressure was produced in the retentate membrane side solution according to the Van’t Hoff Equation 1.1. By gradually throttling the retentate exit stream, the hydrostatic pressure was greater than the sum of the osmotic pressure of the solution and the pressure on the product side of the membrane. Thus, the gradual increase in hydrostatic pressure was sufficient to drive water out of the retentate solution and across the membrane to generate an increase in the product stream flow rate. This finding is consistent with Equation 1.2 and previous experimental findings by Jonsson and Tragardh (1990) that shown increases in cross flow velocity resulting in an direct increase in permeate flux. Furthermore, there is an upper limit to which the amount of solute than can be rejected by the membrane at increasing pressure. At that limit, salt flow remains coupled with the water flux through the membrane and reduces the purity of the permeate (Jonsson and Tragardh (1990).
Figure 3.1: Effect of Increasing Retentate Pressure on Permeate/Retentate Flow Rates
It was believed that by mixing the retentate recycle stream with the incoming feed stream by gradually opening of the recycle valve, the resulting concentration of the retentate would increase. Since osmotic pressure is a function of the concentration of solute in the retentate membrane side solution as per Equation 1.1, an increase in the solute concentration should increase the osmotic pressure of the retentate solution. Therefore, the amount of pressure need to overcome the osmotic pressure must also increase in order to maintain permeate flux. Since the pressure was held constant in this experiment, it was expected that the higher retentate concentrate would decrease permeate flux. In addition, it should become increasing difficult to force the water out of the retentate solution as the intermolecular force between the salt and the water become difficult to overcome (Jonsson and Tragardh (1990). As such, more salt would couple with the water and pass through the membrane to the other side to produce low purity product.
The findings in this experiment did not support the above theory. The permeate flow rates remained largely unchanged with the increase in retentate recycle at constant retentate membrane side pressures of 120 and 130 psi respectively as seen in Appendix A – Tables A2 and A3 and Figures 3.2 and 3.3. The lack of similarity between the experimental results and the expected theoretical trends largely stems from two possible reasons. Firstly, concentration of the retentate stream could not be measured based on the existing experimental equipment. As such, the retentate stream may not have been as concentrated as was anticipated. Secondly, it is possible that the selected retentate membrane side pressures of 120 and 130 psi may be high enough to offset possible increases in retentate osmotic pressure associated with increased retentate concentration. It should also be noted that condition of constant pressure on the retentate side of the membrane could only be maintained for percentages of up to only 60% of Full Recycle.
Figure 3.2: Effects of Increasing Retentate Recycle on Permeate/Retentate Flow Rates at 120 psi
Figure 3.3: Effects of Increasing Retentate Recycle on Permeate/Retentate Flow Rates at 130 psi
CONCLUSIONS
This experiment was suitable for examining the linear relationship between increasing retentate membrane side pressure on permeate and retentate flow rates. The strength of the experimental findings could be greatly enhanced by performing engineering calculations using Equation 1.1 and 1.3. However, the inability to measure feed, retentate and product stream concentrations limited the opportunity to apply such calculations. In order to measure concentration, conductivity meter could be used to measure a stream’s conductance which increases as the concentration of electrolytes, such as salts, increase. As well, the ability to measure feed and product pressure would assist in determining the hydraulic and osmotic pressure differences across the semipermeable RO membrane. In doing so, the relationship between increasing retentate concentration and permeate and retentate flow rates could be much more accurately determined.
5.0 REFERENCES
Baker, R. (2004). Membrane Technology and Applications (2nd Ed.). John Wiley and Sons, Inc.: Chichester. p. 34.
Bird, R., Stewart, W. and Lightfoot, E. (2002). Transport Phenomena (2nd Ed.). John Wiley and Sons, Inc.: New York. p. 84.
Dababneh, A. and Al-Nimr, M. (2002). A Reverse Osmosis Desalination Unit. Desalination, Vol. 153: p 265.
Filmtec Membranes. Factors Affecting RO Membrane Performance. [Publication posted on the World Wide Web]. Retrieved on September 19, 2006 from the World Wide Web: _http://www.pacificro.com/DeFilmFal.pdf_ .
Jonsson, A. and Tragardh, G. (1990). Fundamental Principles of Ultrafiltration. Chemical Engineering Process, Vol. 27: p. 67.
Kotz, J. and Treichel, P. (1999). Chemistry and Chemical Reactivity (4th Ed.). Saunders College Publishing: Philadelphia, p. 668.
Rautenbach, R. and Albrecht, R. (1989). Membrane Processes. John Wiley and Sons, Inc.: New York. p 12.
Ryerson University. (2006). CHE 415 Unit Operations II Laboratory Manual. Department of Chemical Engineering. [Manual posted on the World Wide Web]. Retrieved on September 19, 2006 from the World Wide Web: _https://www.my.ryerson.ca/webapps/portal/frameset.jsp_.
Sagle, A. and Freeman, B. (2006). Fundamentals of Membranes for Water Treatment. [Publication posted on the World Wide Web]. Retrieved on September 19, 2006 from the World Wide Web: _http://www.twdb.state.tx.us/Desalination_.
Seader, J. and Henley, E. (1998). Separation Process Principles. John Wiley and Sons, Inc.: New York. p. 757.
Sheiknoleslami, R. and Bright, J. (2002). Silica and Metals Removal by Pretreatment to Prevent Fouling of Reverse Osmosis Membranes. Desalination, Vol. 143: p 255.
APPENDIX A
Table A1: Permeate/Retentate Flow Rates at Increasing Retentate Pressure (No Recycle)
Retentate Pressure (psi) Permeate Flow Rate (L/min) Retentate Flow Rate (L/min
110 4.5 17
120 5 16.6
130 5.5 15
140 6 13.5
150 6.5 11
Table A2: Permeate/Retentate Flow Rates at Increasing Recycle (Constant Pressure)
Percent of Full Retentate Membrane Side Recycle Permeate Flow Rate
(L/min) Retentate Flow Rate (L/min)
0 5 16.5
20 5 12
40 5.5 8
60 5.5 6
Note: Retentate Side Membrane Pressure = Constant = 120 psi
Table A3: Permeate/Retentate Flow Rates at Increasing Recycle (Constant Pressure)
Percent of Full Retentate Membrane Side Recycle Permeate Flow Rate
(L/min) Retentate Flow Rate (L/min)
0 5.5 15
20 5.5 10
40 5.5 7
60 6.0 3
Note: Retentate Side Membrane Pressure = Constant = 130 psi
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