Transcript
Ryerson University
Department of Chemical Engineering
CHE 415 Unit Operations II
Lab Report
Experiment #6 – Reverse Osmosis
Experiment Performed on September 21, 2004
Report Submitted to
Dr.
By Group # 3 Section # 012 1. / Group Leader and Partial Inspector
2. / Data Reporter and Partial Inspector
Date Report Submitted: September 28, 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 CONTENT
1.0 Background/Introduction………...……………………………………………...…………… 3
2.0 Experimental Procedure and Design……………………….................................................... 6
2.1 Equipment and Process Design...………………………………..………………… 6
2.2 Pre-experimental Procedures ………………………………… ………………….. 6
2.2.1 Safety Concerns………………..….……... …………………..…………... 7
2.2.2 Set-Up ………………………………….………………………………….. 7
Experimental Method…….…………………..…………………………... 8
3.0 Results / Discussion …………….…………………………………………………………….. 8
4.0 Conclusions …………………………………………………………………………………… 10
5.0 References ………………………………………………………………………………..….... 11
6.0 Appendix ...……………………………………………………………………………………. 12
Background / Introduction
Municipalities and industrial facilities use reverse osmosis permeate as a pure drinking water supply and to convert drinking water to high cleanliness water for industrial use at microelectronics, food and beverage, power, and pharmaceutical facilities. The technology is also very useful at removing bacteria, pyrogens, and organic contaminants. (Bird and Stewart, 2002)
When two solutions with diverse concentrations of a solute are mixed, the full amount of solutes in both solutions will be equally circulated in the whole quantity of solvent from the two solutions (Filmtech Membranes, 2006). This is accomplished by diffusion, where solutes will shift from areas of superior concentration to areas of subordinate concentrations until and unless the concentration in all the different areas of the resulting mixture is at equilibrium.
Instead of mixing the both solutions collectively, they can be put in two different compartments separated by a semi permeable membrane. The semi permeable membrane regulates the solutes from moving one section to the other, but allows the solvent to move. Since equilibrium won’t be attained by the movement of solutes from the compartment with a large solute concentration to the one with small solute concentration, it is instead achieved by the movement of the solvents from areas of little solute concentration to areas of excessive solute concentration. When the solvent shifts from areas of less concentration, it causes this area to become highly concentrated. On the other hand, when the solvent moves to places of high concentration, the solute concentration will reduce; hence this process is known as osmosis.
Whenever a solute movement is blocked by the membrane it will transfer momentum to it and generate pressure. The pressure will be the same as the pressure of an ideal gas of the same molecular concentration. Therefore, the osmotic pressure p, is given by Van’t Hoff formula:
p = cRT
Where c is the molar solute concentration, R is the gas constant, and T is the absolute temperature. (Bird and Stewart, 2002)
For reverse osmosis, pressure is given to the high concentration compartment. There are two forces causing the movement of water: the osmotic pressure and the outwardly applied pressure. The solute will not be able to move from areas of great pressure to areas of little pressure since the membrane is not permeable, only the solvent can go through the membrane. When the effect of the outwardly applied pressure is larger than that of the concentration variation, the net solvent movement will be from areas of great solute concentration to little solute concentration, thus reverse osmosis occurs.
Figure 1.0 – A diagram of how reverse osmosis functions. (Ryerson University, 2006)
The design of the spiral wound membrane is used in modular systems. Systems are built with multiple vessels connected in series, to increase permeate production, or in parallel, to produce higher quality water. The reverse osmosis membrane systems is competent in taking away substances from the feed stream with a molecular weight of superior than 150-250 Daltons ((Bird and Stewart, 2002)).
Fig 1.1 - RO Spiral Wound Membrane (Filmtec Membranes, 2006)
The rate at which permeate flows through the membrane is called the mass flux rate. The rate at which the fluid flows from the retentate side to the permeate side of the membrane is the mass average (and also known as cross flow velocity) (Bird and Stewart, 2002). The proportional relationship between mass flux rate and cross velocity is:
? = V ? A
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) and A = membrane cross sectional area (m2) (Bird and Stewart, 2002).
The relationship between the solute mass flux and mass average (cross flow) velocity is shown in the below equation. To decrease the occurrence of fouling, cross flow filtration systems have been designed such that any feed that does not pass through the membrane is carried away in the reject stream (Ryerson University, 2006).
NA = ?A ? V - dAb ? ? ?A
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).
Figure 1.2: Cross-Flow Reverse Osmosis System, (Ryerson University, 2006)
Experimental Procedure and Design
The main objective of this experiment was to evaluate the performance of the Filtration Concepts Inc. Commercial Tap-Water Reverse Osmosis System Model #TW 1-1/2-120-60 by examining the relationship between increasing reject recycles and product/reject flow rates (at a fixed retentate pressure).
2.1 Equipment and Process Design
The Filtration Concepts Inc. Commercial Tap-Water Cross-Filtration RO System Model # TW 1–1/2–120–60 used a 5 micron filter cartridge and two reverse osmosis membrane units joined in series. This system is setup with a high pressure valve which changes the retentate exit stream to increase reject membrane pressure, permeate/retentate flow meters, a retentate recycle valve which adjusts the amount of water being re-circulated through the system, pressure gauges at the retentate stream exit and high pressure suction and also the retentate stream is connected to the reject stream while the permeate stream was gathered into the product storage tank.
Figure 2.0: PFD for Reverse Osmosis Cross Filtration System (Ryerson University, 2006)
2.2 Pre-experimental Procedures
A number of pre-experiment procedures were dealt with to ensure safety and proper set-up of the lab.
2.2.1 Safety Concerns
Ensure the reject pressure (high pressure gauge) does not exceed 150 psi.
The low pressure gauge is at a minimum of 20 psi in the feed tap water is presented to the suction area of the high pressure pump.
An utmost suction pump pressure of 75 psi is surpassed.
2.2.2 Set-Up (Ryerson University, 2006)
Close Inlet Shut-off Valve.
Verify 5 micron filter cartridge is installed.
Close Recycle Valve completely.
Open the Inlet Feed Water Shut-off Valve Completely.
Verify that Product Storage Tank Float Level Controls are connected properly.
Verify that Reject and Product tubes are connected to drain and storage tank respectively.
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.
Experimental Method:
This part of the experiment applied a fixed pressure condition for the system to observe the effects of escalating the retentate recycle on the retentate/permeate flow. The retentate membrane pressure was held constant by nearly fully opening the recycle stream to keep a constant low pressure gauge reading of 20psi and also keeping a constant high pressure of 108 psi. Two readings for the product and reject flow rates (gal/min) were recorded down, respectively.
At the completion of the lab, to lower the pressure within the system, the high pressure valve was fully opened, then the switch of the reverse osmosis system control was turned off, and finally the inlet feed water shut off valve was closed.
3.0 Results/Discussion
The 5 micron cartridge filtered the entering tap water, which was then pumped in by the high pressure pump into the reverse osmosis membrane units. The RO membrane separated the feed flow into permeate and retentate streams. A minimum feed pressure of 20 psi was produced on the suction of the pump thus the high pressure pump was able to function throughout the duration of the experiment.
A level of osmotic pressure was created in the retentate membrane solution (Van’t Hoff Eqn), although it was known about the solute contaminants within the feed. The increase in hydrostatic pressure was able to move the water out of the reject solution and able make an increase in the product stream (flow rate). This was possible since the hydrostatic pressure was greater than the total amount of the osmotic pressure of the solution and the pressure on the product side of the membrane, by steadily fixing the reject exit stream. This finding is dependable with the mass flux rate of permeate across membrane. Likewise, there is a higher boundary to the quantity of solute is be rejected by the membrane at an increased pressure.
It was understood that by integrating the retentate recycle stream with the incoming feed stream by slowly opening the recycle valve, the final concentration of the reject would increase. Osmotic pressure is related to the concentration of solute in the reject membrane solution, thus an increase in the solute concentration should raise the osmotic pressure of the waste solution. Since the pressure was kept stable in the lab, it was likely that the high reject concentrate would increase the permeate flux. Therefore, a low amount of salt would join with the product water and pass through the membrane to produce a relatively descent to high purity product.
The findings in this experiment did support the above theory. The permeate flow rates changed with the increase in retentate recycle at constant retentate membrane pressures of 108 psi and low pressure gauge of 20 psi. The similarity between the experimental results and the expected theoretical trends largely relay that the selected retentate membrane pressures of 108 psi was not sufficiently high 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 was maintained at a flow rate of 70% recovery (Appendix A1 and Figure 3.0). Since both results had a flow meter reading of product and reject flow rates at 1.1 GPM and 0.4-0.5 GPM, respectively, it can be directly seen that at a constant retentate membrane pressures of 120psi and low pressure gauge of 20 psi the flow rate functions at 70% recovery.
Figure 3.0- The expected theoretical and observed values for flow rate at 70% recovery. (Ryerson University, 2006)
CONCLUSION
This experiment was appropriate for determining the relationship between increasing reject membrane pressure on the flow rates for reject/product. The failure to calculate the retentate and product stream and feed concentrations restricted the chance to use the calculations mentioned previously. However, the assumptions for the lab and the experimental results coincided with one another. Thus, the relationships between increasing reject concentration and product and reject flow rates was somewhat determined. But with the addition of the ability to calculate the feed and product pressure can help in finding the osmotic pressure differences within the reverse osmosis semi-permeable membrane
5.0 REFERENCE
1. Bird, R., Stewart, W. and Lightfoot, E. (2002). Transport Phenomena (2nd Ed.). John Wiley and Sons, Inc.: New York. p. 84.
2. Filmtec Membranes. Factors Affecting RO Membrane Performance. Retrieved on September 19, 2006 from the World Wide Web: _http://www.ionics.com/technologies/ro/#_
3. 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 t_https://www.my.ryerson.ca/webapps/portal/frameset.jsp_.
4. 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: _http://www.twdb.state.tx.us/Desalination_.
6.0 APPENDIX
Table 1.0: Product/Reject Flow Rates with Increasing Recycle.
Low Pressure Gauge (PSI) Permeate Flow Rate
(Gal/min) Retentate Flow Rate (Gal/min)
20 0.4 1.1
20 0.5 1.1
Retentate Membrane Pressure (constant) = 108 psi
Sample Calculation(s):
For Calculating the GPD for the 1st set of results with a constant low pressure gauge of 20PSI and constant high pressure gauge of 108 PSIL
Permeate flow rate + retentate flow rate = overall flow rate
0.4 gal/min + 1.1 gal/min = 1.5 gal/min * 60 min/Hour * 24 Hour / day = 2160 GPD (This value is located on the previous graph of the flow rate at 70% recovery.
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