Transcript
Ryerson University
Department of Chemical Engineering
CHE 415 Unit Operations II
Lab Report
Experiment # 6: Reverse Osmosis
Experiment Performed on September 28,2006
Report Submitted to
Dr.
By Group # 4 Section #02 1. / Leader
2. Data Recorder
3. Inspector
Date Report Submitted: October 05, 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
33
33
66
77
99
1010
1111
11
LIST OF FIGURES
Figure 1: Osmotic Flow vs. Reverse Osmotic Flow with a Semi-Permeable Membrane 4
Figure 2: Typical Spiral Membrane Element
5
Figure 3: Process Flow Diagram of Reverse Osmosis Cross Filtration System
6
Figure 4: Flow Rate vs. Retentate Membrane Pressure
12
Figure 5: Flow Rate vs. % of Retentate Recycle at 120 psi
12
Figure 6: Flow Rate vs. % of Retentate Recycle at 130 psi
13
Figure 7: Flow Rate vs. % of Retentate Recycle at 140 psi
13
LIST OF TABLES
Table 1: Increasing Reject Pressure, Constant Inflow, Without Recycle
11
Table 2: Constant Reject Pressure of 120 psi with increased % Recycle
11
Table 3: Constant Reject Pressure of 130 psi with increased % Recycle
11
Table 4: Constant Reject Pressure of 140 psi with increased % Recycle
11
INTRODUCTION
Reverse osmosis is a water purification process based upon the movement of solvent molecules through a semi-permeable membrane based on a concentration difference. The reverse osmosis system operates by feeding municipal water through a carbon filter. The water then passes through the reverse osmosis system where the pressure and flow rate differences are encountered.
The objective of the reverse osmosis experiment is to verify the relationship between the retentate and permeate flow rates based on the graphs provided in the operations manual. Also, the relationship between the pressure and the flow rate of each respective stream will be verified by adjusting the valves to generate different values.
THEORETICAL BACKGROUND
Reverse osmosis is the process of water purification, based upon the movement of solvent molecules through a semi-permeable membrane from an area of low concentration to an area of high concentration. Reverse osmosis applies pressure to reverse the natural flow of water. This forces the water to move from the more concentrated solution to the weaker. Figure 1 illustrates the difference between osmotic flow and reverse osmotic flow:
Figure 1: Osmotic Flow vs. Reverse Osmotic Flow with a Semi-Permeable Membrane
The Van’t Hoff Equation may be used to calculate the theoretical osmotic pressure:
(1) ? = c R T
Where:
? = osmotic pressure (atm)
c = concentration (mol/L)
R = Universal Gas Constant (L atm/ K mol)
T = temperature (K)
Reverse osmosis membranes are set in hollow fibres or flat sheets, and are typically made of cellulose acetate. Packaging the membranes into this form allows one to package a large amount of membrane area in a small volume. Figure 2 is an illustration of a typical spiral membrane element.
Figure 2: Typical Spiral Membrane Element
In order to determine the mass flux rate of the permeate flow through the membrane, a relationship between the mass average velocity and the cross sectional area of the membrane may be related by Equation 2:
(2) ? = v * A
Where:
? = Mass Flux Rate (kg/s m2)
v = Mass Average Flow Velocity of fluid through membrane (m/s)
A = Cross Sectional Area of the membrane (m2)
Another relationship pertinent to reverse osmosis is that between the solute mass flux and the mass density is Equation 3:
(3) NA = ?A * V - dAb * ? ?A
Where:
NA = Mass flux of the 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)
TECHNICAL PROCEDURE
The feed water source was turned on and the equipment was plugged in.
The Reject and Product tubes were ensured that they were connected to the drain and Storage tank.
That Level Controls were ensured to be properly connected.
The High Pressure Valve was opened all the way counter-clockwise.
The control switch was turned on.
The High Pressure gauge was monitored to insure it did not exceed 150 psi.
With the Recycle valve closed, the product and reject flow rates were recorded at different High Pressure Gauge readings.
Keeping the pressure constant, the product and reject flow rates were recorded at different recovery percentages.
Step 8 was repeated two more times at a different pressure.
The high pressure valve was opened to relieve the system pressure.
The system was turned off.
The inlet feed and shut off valves were closed.
Figure 3 : Process Flow Diagram of Reverse Osmosis Cross Filtration System
RESULTS AND DISCUSSION
For the first part of the experiment, the permeate and retentate flow rates were found when retentate pressure was varied from 117 psi to 150 psi. This occurred under a zero recycle condition. As seen in Figure 4, there is a linear relationship between increasing retentate membrane side pressure and permeate flow rates. This is verified by the R2 values of 0.9961 and 0.925 for the retentate and permeate respectively. It is also seen that when the pressure is increased, the permeate flow rate increases as the retentate flow rate decreases. According to the Van’t Hoff equation (1) there is an osmotic pressure. However once the hydrostatic pressure becomes greater than the osmotic pressure, the tap water will be driven out of the retentate solution and forced across the membrane resulting in a higher product flow rate. This is consistent with equation (2) which says that when the mass flux rate of the permeate flow through the membrane increases, the mass average velocity will also increase.
For the second part of the experiment, the retentate side pressure was held constant at 120 psi, 130 psi and 140 psi as the retentate recycle was increased. The recycle valve was opened at values of 0%, 20%, 40%, 60%, 70% and 80%. The effect on the permeate and retentate flow rates was observed. According to equation (1), it states that a higher retentate concentrate would cause the osmotic pressure to increase. Since the retentate recycle stream was mixed with the feed stream, it was believed that the concentration of the retentate would be increased. Therefore it would require more retentate side pressure to overcome the osmotic pressure. Since this pressure was held constant, it was expected that the permeate flux would decrease. However, this did not happen as seen in Figure 5, 6, and 7. The permeate flow rates stayed relatively constant as the recycle was increased. The reasoning behind these unexpected results could have been that the retentate was not as concentrated as was expected. There was no way to determine the actual concentration of the retentate since the equipment did not allow us to do so. Another reason could be that the pressure was held at a high enough value to overcome the osmotic pressure and therefore not cause a decrease in the permeate flow rate.
The total dissolved solids were also observed in this experiment. This value ranged from 0.06 to 0.07 ppm. This value lies within the limits of drinking water, however since the TDS of the incoming feed could not be determined, it is impossible to compare these results and determine if the reverse osmosis system was effective.
There were many factors that could have caused a deviation in actual results. The gauges tended to fluctuate, causing inaccurate readings for the flowrates and pressures. The recycle valve had no indicator on it to determine the actual recycle percentage. We were also unable to determine the actual total dissolved solids for the incoming tap water and the retentate concentrations. This made it impossible to compare the results obtained during the lab to the theoretical results which were expected.
CONCLUSION AND RECOMMENDATIONS
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 and 2) increasing retentate recycle and permeate/retentate flow rates at constant retentate membrane side pressure.
In this experiment, the linear relationships between increasing retentate membrane side pressure on permeate and retentate flow rates were examined by using a reverse osmosis membrane. It was determined that as the retentate side pressure is increased, the permeate flow increases and the retentate decreases. This occurred in a linear fashion as determined by the R2 values displayed on Figure 4.
When the recycle was increased at constant pressure the permeate flow rate stayed relatively the same. This did not agree with our expected results. It was assumed that the concentration of the retentate would increase when the recycle is opened but the data obtained did not reflect this. Not being able to measure the feed, retentate and product stream concentrations caused this discrepancy and also limited the chance to use equations 1 and 3 to perform any calculations. Concentration could have been measured by using a conductivity meter, because the conductivity of the stream increases as the concentration of electrolytes increases. Measuring the feed and the product pressures could have also helped in determining the hydraulic and osmotic pressure differences across the semi permeable RO membrane.
REFERENCES
Baker, R.W.; et al. (1991). Membrane Separation Systems - Recent Developments and Future Directions. William Andrew Publishing/Noyes.
Online version available at:
_http://www.knovel.com/knovel2/Toc.jsp?BookID=312&VerticalID=0_
Bird, R., Stewart, W. and Lightfoot, E. (2002). Transport Phenomena (2nd Ed.). John Wiley and Sons, Inc.: New York. p. 84.
Ryerson University. (2006). CHE 415 Unit Operations II Laboratory Manual. Department of Chemical Engineering.
_http://www.historyofwaterfilters.com/reverse-osmosis-pc.html_
_http://www.trisep.com/SpiraSep/SpiraSep%20Technology.pdf#search=%22spiral%20membrane%22_
APPENDIX
Table 1: Increasing Reject Pressure, Constant Inflow, Without Recycle
Trial # Municipal Water Reject Product Permeate Retentate TDS
Pressure (psi) Pressure (psi) Pressure (psi) Flow Rate(GPM) Flow Rate(GPM) (ppm)
1 11 117 7 1.2 4.6 0.06
2 11 120 7 1.2 4.5 0.06
3 10.5 130 7 1.25 3.9 0.06
4 10.5 140 7.5 1.3 3.5 0.06
5 11 150 7.5 1.3 3 0.06
Table 2: Constant Reject Pressure of 120 psi with increased % Recycle
Trial # Municipal Water % Recycle Product (Low) Permeate Retentate TDS
Pressure (psi) Pressure (psi) Flow Rate(GPM) Flow Rate(GPM) (ppm)
1 12 0 7 1.2 4.5 0.06
2 12 20 9 1.2 3.25 0.06
3 12 40 11 1.2 2.65 0.06
4 12 60 14 1.2 2 0.06
5 12 70 14 1.2 1.6 0.06
6 12 80 17 1.2 0.8 0.06
Table 3: Constant Reject Pressure of 130 psi with increased % Recycle
Trial # Municipal Water % Recycle Product (Low) Permeate Retentate TDS
Pressure (psi) Pressure (psi) Flow Rate(GPM) Flow Rate(GPM) (ppm)
1 12 0 7 1.25 3.9 0.06
2 12 20 9.5 1.25 2.7 0.06
3 12 40 11.5 1.25 2.1 0.06
4 12 60 14 1.25 1.25 0.06
5 12 70 15 1.25 0.9 0.06
6 12 80 19 1.25 0 0.07
Table 4: Constant Reject Pressure of 140 psi with increased % Recycle
Trial # Municipal Water % Recycle Product (Low) Permeate Retentate TDS
Pressure (psi) Pressure (psi) Flow Rate(GPM) Flow Rate(GPM) (ppm)
1 8 0 2.4 1.3 3.25 0.06
2 8 20 5 1.3 1.8 0.06
3 8 40 7 1.3 1.05 0.06
4 8 60 10.5 1.3 ---------- 0.07
5 8 70 -------- -------- ---------- ------
6 8 80 -------- -------- ---------- ------
Figure 4
Figure 5
Figure 6
Figure 7
13
