of Department Chemical Engineering Ch.E. 333.2 Laboratory Manual 2008
Course Outline
I.
PURPOSE OF THIS COURSE This course is intended to develop skills, which will be of use to you as a practicing chemical engineer. You will be expected to use typical items of equipment and to conduct simple measurements and tests. You will be expected to communicate the results in a clear and effective manner.
II.
ORGANIZATION OF THE COURSE 1. Data Recording* Each student in the course must have a hard-cover laboratory notebook. Experimental investigations will be conducted in groups of two or three students. One student from each group will be designated to be responsible for: a) Planning the investigation. This task is described more fully below. b) Producing a notebook record of what was done in the laboratory. This will be signed by the Demonstrator, who will certify that all students were present. c) Preparation of a suitable apparatus diagram. d) Sample calculations, showing how the data were used. e) Preparation of graphs and/or tables showing the salient conclusions from the experiment as clearly as possible. f) Preparation of formal or brief report. Items b) to f) are to be included in the notebook of the designated student. Only one notebook record of this type is required from each group. *refer to section VI for rules for laboratory notebooks 2. Planning The designated student leader is responsible for planning the experiment and for ensuring that sufficient data of an appropriate quality are obtained. This will require background reading, visits to the laboratory to view the apparatus and discussion with the instructors. Both partners will be assessed for their contribution to the experiment by the Demonstrator. 3. Reporting Each student is required to hand in one full formal report, one brief formal report and two technical memos. When one partner submits a formal report, the other partner must 2
submit a technical memo. Reports and memos are due two weeks from the completion of the experiment, unless a time extension has been granted. Reports and tech memos are to be handed in to D. Claude in Room Eng 1D43 and marks will be deducted for tardiness
III. EXPERIMENTS 1. Viscosity 2. Fluid Friction in Pipes, Valves and Fittings 3. Fluid Meters
4. Heat Exchanger Shell and Tube Exchanger - Water/Water 5. Heat Transfer a) Boiling and Condensing b) Radiation and Convection 6. Ion Exchange in water softening
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IV.
GUIDES FOR PREPARING REPORTS Full Formal Reports: All formal reports must be done on a word processor. A formal report should include the following: 1. Title: Name of the experiment (use title page template given in the ChE 333 class website) Date of Experiment Due Date Students participating (group members present) 2. Abstract: This is written after the Discussion has been completed. It may be prepared with a word processor and pasted into the notebook. It is intended to be read by persons who will not read the rest of the report. Both qualitative observations and quantitative results must be included. 3. Table of Contents: Give titles of sections with their page numbers. This should include the Appendix with titles of each section of the Appendix. 4. Nomenclature: List all symbols used in alphabetical order. Greek symbols should be kept in a separate list. 5. Purpose. This is a brief statement of the purpose of the experiment. It serves as an introduction to the rest of the report. 6. Theory. A brief summary, giving the equations to be used, is required. 7. Apparatus and Procedure. The apparatus diagram can be prepared with a drawing program or drawn by hand. Chemical Engineering symbols for the unit operations should be used wherever possible so that a proper Process Flow diagram (PFD) is prepared. The procedure should record what was done. It must not be written as instructions. 8. Presentation and Discussion of Results: In this section, indicate where the data and results are presented. Any significant observations and what effect they had on the outcome of the experiment should be reported . Data and result are probably presented most effectively in tabular form in an Appendix. Graphs can be presented either in this section or in an Appendix. Your results should then be fully discussed in this section. All conclusions and recommendations must be supported by your results. Error analyses may or may not be useful in this regard. Figures or graphs can be either included in the body (insert as the page following the first mention of the graph in the body of the report) of the report or presented in an Appendix. If there are several graphs that are similar, a representative graph can be included in the body of the report.
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9. Conclusions: Give your conclusions in numbered statements, each one concise and precise. No discussion is given here. All conclusions must be fully discussed and supported in the Discussion Section. Add introductory and closing paragraph before and after the numbered statements to make your report easy to follow. 10. 11. Recommendations: A similar format to that of the Conclusions should be followed here. Reference: A list of references must be presented in this section. Use a standard format (list in alphabetical order and in the body of the report refer to the last name of the first author followed by the year of publication in brackets). Appendix: Identify them by A, B, C and in the following order: data, results, sample calculations. All tables and figures should have headings (e.g., Table B1, Figure A1, etc.) and full titles. In your sample calculation, indicate the run number used and which table(s) the information can be found.
12.
Brief Technical Reports A brief technical report should include the following: title page, summary, results and discussion, conclusions, data, results and sample calculation. It is equivalent to the formal report but with the abstract replaced by a summary and the omission of the introduction, theory/literature review, materials and methods sections. The summary should include a brief introduction stating the nature and purpose of the investigation, a brief explanation of the procedures used, and a descriptions of the important results. The data, (calculated) results, samples calculations and any derived theoretical equations, etc., should be put in an Appendix (A, B, C, etc.) Technical Memos A technical memo is a brief memorandum to the supervisor. It should state concisely the experimental conditions, results, discussion, conclusions and recommendations. A brief table of results or a graph should be included to support the conclusions.
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V.
MARK DISTRIBUTION:
Careful measurements, correct calculations, logical deductions and clear conclusions are necessary to a good report. However, even if all these are present but the report is not well written, some of the positive effects of the investigation will be lost. Although proper spelling, grammar and general use of the English language are somewhat less important than clarity, conciseness and technical contents, they will also have an effect on the marking. Both formal reports, brief technical reports and technical memos will be marked out of 10 grade points. However, for the final mark, each formal report will be worth 35 marks, the brief technical report, 25, and the technical letter, 10 marks. The lab demonstrator will be reviewing your performance and your lab notebook while you are in the lab and will assign a mark (out of 5) at the end of each lab period. A summary of the marking scheme is given in Table 1 on the next page. In general, we recommend that reports and technical memos be completed and submitted within two weeks, i.e. while the experiment is still fresh in the student's mind. Therefore, the deadline for receiving reports and technical memos without any penalty will be two weeks after the experiment was performed. A penalty of 10% per week will be deducted from late reports or memos. Submissions will not be accepted after the last day of classes and will be given a mark of zero.
Table 1. Distribution of Marks
Item Individual Final Late Penalty/Report Mark Mark 10% _________________________________________________________________________ Full Report 1 35 35 3.5 Brief Technical Report 1 25 25 2.5 Technical Memo 2 10 20 1.0 Lab Performance & Notebook 4 5 20 _________________________________________________________________________ Total Mark 100 Number
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VI. RULES FOR LABORATORY NOTEBOOKS: 1. 2. 3. 4. 5. Use a hard-covered and numbered record book (purchase from University Bookstore) Label research ideas/proposal to differentiate them from experiments that are performed Explain all abbreviations or terms that you use that are not universally known Make all entries in ink Do not erase any entry. Instead draw a line neatly through the error and then initial and date the correction in the margin 6. Record data and observations when they are made. Date each entry 7. Stick to the facts (positive and negative). Your notebook is not the place for your opinion 8. Leave no blank space between entries. Cancel all blank spaces (including blank pages) with diagonal lines drawn across the space. Initial and date the cancellation in the margin 9. Have each page of your notebook witnesses by someone who is not an inventor but who understands the experiment and its objectives (ask your Lab Demonstrator as the witness) 10. Make no changes or insertions on a page after it has been signed and witnessed 11. Attach support records to the notebook where practical. If not practical, then, crossreference the notebook with the material and witness as above 12. Maintain safe custody of your notebook
After completing all the experiments, use MS Word to create a Table of Contents (TOC) and pastes it to the beginning of your laboratory notebook. To create a TOC using MS Word, at the menu bar, click on Insert/Index and Tables, and choose Table of Contents.
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1.
Introduction
VISCOMETRY
This experiment involves the use of a cone and plate viscometer. You will be asked to characterize a fluid which may or may not be Newtonian. Newtonian fluids should be tested at different shear rates for a range of temperatures. Non-Newtonian fluids should be tested at a range of shear rates. Discuss the choice of a fluid with the instructor before planning the experiment.
Procedures The viscometers are operated empty at first to find deflections at zero load. The viscometers must be operated according to the procedures in the literature provided in the laboratory. Because they are sensitive and expensive instruments, please read the procedures carefully before operating them. If you are unaware about any procedure, ask the demonstrator before proceeding. When you are placing the fluid in the viscometer, try to avoid entrapping air bubbles as these may cause significant errors. Also, before changing fluids in a viscometer, wash it thoroughly since small amounts of contamination may distort the results. Be careful not to scratch the surfaces of the measuring elements. Before testing an unknown fluid, a Newtonian standard fluid should be used to verify instrument performance.
Data 1. Brookfield Viscometers (springs are linear) (i) LV, full scale deflection = 673.7 dyne-cm
(ii) RV, full scale deflection = 7187 dyne-cm (iii) Cone Angle, = 0.8 degrees (iv) Cone Radius, r = 2.4 cm (v) Cone and Plate, sample = 0.5 ml 2. Working equations: i) Cone and Plate ( dyne / cm 2 ) = 3T 2 r 3 8
(sec 1 ) = where
sin
= shear stress = shear rate T = torque (dyne-cm2)
Characterizing a Fluid: For Newtonian fluids, comment on the effect of temperature upon viscosity by comparing your results with those predicted by the Eyring theory(4) . For nonNewtonian fluids, select a suitable model and evaluate the coefficients in its equation of state relating shear stress to shear rate. For non-Newtonian fluids calculate the pressure drop per meter of pipe in horizontal flow if the velocity is 1.0 m/s and the pipe diameter is just small enough to ensure laminar flow (i.e., the flow is not turbulent).
References 1.1 Streeter, V.L., Handbook of Fluid Dynamics, Chapter 7. McGraw-Hill Book Company Inc., 1961. 1.2 Middleman, S., The Flow of High Polymers, Interscience Publishers, 1968. 1.3 Cheremisinoff, N.P. and Gupta, R., Handbook of Fluids in Motion, Ann Arbor Science, 1983. 1.4 Tabor, D., Gases, Liquids and Solids, 2nd Ed., Cambridge Univ. Press, 1979.
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2. FLUID FRICTION IN PIPES, VALVES AND FITTINGS
A variety of studies will be conducted with this equipment, including the determination of: a) the friction factor - Reynolds number relationship for pipe flow, b) valve coefficients (Cv values) and characteristics (f(x) vs x) for gate valves, and globe valves, c) friction losses for various fittings. Since the piping systems used for a, b, and c contain substantial lengths of pipe, it will be necessary to correct the measured pressure drops for the lengths of straight piping between the fittings. These corrections should be calculated assuming the pipe is smooth. The main apparatus is a flow loop with ten (10) separate lines, each incorporating different types of fittings. The pressure differences across the piping and from the venturi meter are measured with variable reluctance differential pressure transducers. The transducer output is a voltage signal that is collected by Voltage Meters and Demodulators, which are connected to a SCB-68 National Instruments Shielded I/O Connector Block for DAQ Devices with 68-pin connectors. The Connector Block is then used to send voltage signals to a NI PCI-MIO-16E-4 I/O Terminal installed in an IBM Intel computer. The resulting signals are then picked up by LabVIEW 8.2 and displayed in Fluid Friction.vi in a series of graphs and indicators. A cold junction-compensated (CJC) thermocouple is used to measure the temperature of the fluid. The CJC occupies channel ai0 and the thermocouple is attached to channel ai1 and ai9 of the connector block. Each transducer is calibrated individually. The Demodulators are set to zero by allowing the individual transducers to be open to atmospheric pressure (with both bleed screws open). Once stabilized, the spans of the Demodulators are set at 10V with the bleed screws closed and air applied at the appropriate pressure. The exact procedure will be discussed by the TA. To measure the flow rate for comparison with the Venturi meter, a bucket, stopwatch, and scale are required. A 3/32 Allen wrench is also necessary for adjusting the pressure transducer bleed screws. The inside diameter of the tubing is 0.527 inches. The equipment is fairly complex and you will have to spend some time tracing the flow lines and thinking about what you are going to do. Dont turn anything on until the demonstrator has given his/her approval. From your experimental measurements on the Venturi meter, calculate the coefficient of discharge, C, in the relation:
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w = CA 2 where W is the mass rate of flow,
2 ( p )1 1 4
A2 is the cross-sectional area of the throat of the Venturi, 1 is the density of the fluid just upstream of the throat, (-) is the pressure difference across the Venturi, is the ratio of throat diameter to inside pipe diameter (14.3 mm and 25.3 mm respectively) The valve coefficient and valve stem function are defined as: Q = Cv f ( x) where Q Cv f(x) S Pv x Pv S
= flowrate (US gallon/minute) = valve coefficient (usgpm/psi0.5) = dimensionless stem function (0, closed; 1, fully open) = fluid density/water density = pressure drop over valve (psi) = stem position (fraction open)
A possible f(x) vs x relationship is of the form f(x) = xm. In examining the valve performances the data should be used to find the best-fit values of C v and m.
References 2.1 Anon, Flow of Fluid Through Valves, Fittings and Pipe, Technical Paper No. M-409, Crane Ltd., 1950. 2.2 Perrys Chemical Engineers Handbook, 6th Edition, 1984.
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3. FLUID METERING
Introduction In this experiment you will be pumping water through a system which contains a number of different kinds of flowmeters. They are: 1) Nutating disc meter (domestic water meter) 2) Gear meter 3) Vortex flowmeter 4) Venturi meter 5) Ultrasonic (Doppler) flowmeter 6) Orifice meter 7) Rotameter 8) Turbine meter 9) Magnetic flowmeter 10) V-notch weir (see Perrys Handbook (6)) You may use the gear meter values as the correct flow rates. Using these values for flow rates, you can then determine the meter coefficients for the Orifice and Venturi meters as functions of Reynolds number. These can be compared to the expected values found in the literature. The Magnetic flowmeter readings should be linear with flow rate. Evaluate the magnetic flowmeter coefficient for converting EMF to flow rate. The Ultrasonic meter should also give a linear signal with respect to flow rate. For the other meters, flow rate comparisons can be made. Some background information is given in References 2.2, 3.1 and 3.2 Procedure Determine the direction the water flows and decide how you will adjust the flow rate. Be sure to start your measurements at a low flow rate and then increase between readings. Determine where each meter is located and how to make a measurement for it (discuss this with the demonstrator). Note that for the Orifice and Venturi meters you have to make pressure measurements and this is accomplished with either the manometer or pressure transducer (compare results from these two). The connections from the two meters to the pressure measuring devices are quite complex and you will have to spend several minutes studying them before you can determine which to open and close. Make sure your procedure is correct before starting as opening the wrong valve may blow the manometer. Make at least eight measurements by first increasing the flow rate and then reducing it to zero.
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Calculations (1) Orifice or Venturi: The flowrate Q is related to the pressure drop, minimum (throat) area and density by the equation: Q = CA 2P (1 4 )
where C is the coefficient of discharge. (2) V-Notch Weir: Q= (0.31h02.5 2 g ) / tan Where ho is the height of the liquid above the bottom of the weir and is the angle between the side of the notch and the horizontal.
Data Diameter of pipe Diameter of orifice Diameter of venturi Angle of weir Assessment Each of these meters is useful in particular circumstances. The factors to be considered are capital cost (including data processing), operating cost (primarily energy losses, reliability (whether calibration is required or not). Several possible situations are shown below: a) b) c) d) e) f) g) measuring the flow of water into households measuring the flow of water in a 5 foot diameter pipe measuring the flow of heavy crude oil in a 3 inch pipe measuring the flow of coal-water in a 12 inch pipe measuring the flow of water into a laboratory reactor measuring the flow of water in a small creek measuring the flows of petroleum derivatives in an automated refinery = = = = 1.049 in 0.45 in 0.33 in 53o
Choose one to use in each case and explain your choice.
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References 3.1 Donald Ginesi, Choices Abound in Flow Measurement, Chemical Engineering, Vol 98, April 1991, pp 88-100. 3.2 Donald Ginesi, A Raft of Flowmeters on Tap, Chemical Engineering, Vol 98, May 1991, pp 146-155
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4.
HEAT EXCHANGE
Reading Fairly extensive (but not very difficult) reading will be necessary before you do the experiment so that you will be able to do a good job and understand what is involved. Read (if you have not already done so) the following sections in the book by Incropera et al.: 1.2.2, 6.1, 7.1, 8.5, 11.1, 11.2, 11.3. Further Background Shell-and-tube and cross-flow heat exchangers involve fairly complex flow patterns. Of course the flow pattern affects the rate of heat transfer. A common approach is to calculate transfer coefficients from empirical correlations, combine resistances in series at steady state, calculate a logarithmic mean T for counter-current flow, find a correction factor for the complex flow pattern, and to combine factors to give the heat transfer rate. This is the LMTD-correction factor method. The equations for the counter-flow heat exchanger are 11.14, 11.15 and 11.17 with Fig. 11.8. The mean T for the complex flow pattern is given by: Tm = F(T)lm,CF where F is given by Figures 11.10 to 11.13. The subscript lm indicates logarithmic mean and CF denotes a hypothetical counterflow exchanger. Purpose To determine the heat transfer rates and coefficients for one of the following sets of heat exchangers: Shell and Tube (water-water) Double Pipe - (water-water) Double Pipe - (air-water) Each evaluation should consist of a check on the enthalpy balance and a comparison of experimental and literature values of heat transfer coefficients. Some discussion of pressure drop may also be appropriate.
Procedures 15
1. The flow rates of the fluids can be controlled by adjustment of appropriate valves. 2. Sufficient time must be allowed for the system to come to steady state before measurements are made. In your report, indicate how you knew that steady state has been achieved. How much time was required? 3. Measurements are made of temperature using thermometers and/or thermocouples and flow rates using bucket and stopwatch and/or calibrated meters. If your experiment is for the double-pipe heat exchanger, check with the demonstrator as to which meter to use. 4. Several different operating conditions (flow rates, etc.) should be studied in order to obtain as much information as possible to characterize the system. Discuss choice of operating conditions with your demonstrator. 5. Heat transfer rates, heat transfer coefficients, pressure losses and energy losses should all be evaluated if possible and compared to values and/or trends reported in the literature. 6. When heat balances are of interest, be sure that the temperature differences for the streams can be determined reasonably accurately. For the air-water exchanger this will require low water flowrates to be used. Equipment Data Use Equation 24-11 in B & M for h for the shell and tube heat exchangers. hD DG max = b k u
n
See Appendix for information on other heat exhangers. Report: In your write-up, report the heat transfer rates and coefficients for the exchanger that you studied and discuss the problem of scaling up your heat exchanger to allow for a one hundred fold increase in flowrate of the cold fluid but still maintain the same temperature rise.
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5. HEAT TRANSFER Boiling and Condensing Heat Transfer
A schematic diagram of this equipment is given in the Appendix. This experiment is concerned with measurement of heat transfer coefficients for boiling. It is also possible to make an evaluation of heat transfer processes which occur in a condenser. Visual observations of flow regimes play an important role in this experiment since there are substantial effects on heat transfer coefficient when the flow regime changes. For the condenser, it is important to remember that this is a heat exchanger for which the wall temperature is unknown. Overall coefficients can be measured and they can also be predicted from the values of the individual (inside and outside) heat transfer coefficients. The purpose of the experiment is to extract as much information as possible from the measurements, and to report this information. Reading: Fairly extensive (but not very difficult) reading will be necessary before you do the experiment. Read (if you have not already done so) the following sections in the book by Incropera et al.: 1.2.2, 6.1, 8.5, Introduction to Chapter 10, 10.2, 10.3, 10.6, 11.3.3, and equations 11.14, 11.15 and 11.17. Procedures: 1. In this experiment, a copper element is heated in Forane R141b. You will be given a pH diagram for this fluid. The heating occurs because of the passage of an electric current and is measured with a voltmeter and ammeter. 2. The power supply is controlled by an on/off switch and a Variac. The water flow rate is controlled by a valve and pre-calibrated rotameter. There are built-in safety mechanisms which shut the apparatus down if either the pressure or temperature gets too high (220 kN/m2 or 160 oC). State in your note book whether this occurred. 3. Visual indications of convection heat transfer, nucleate boiling and film boiling will be observed and should be recorded along with the corresponding operating conditions. 4. A set of experiments is done at constant pressure. To do this the heater is turned on to some low value (5 to 50 watts) with no cooling water flowing. The pressure is allowed to come to steady state and conditions are recorded (V, I, P Cooling Water Temperatures, Cooling Water Flow rate, Temperature of Heater, Temperature of Refrigerant). The heat supply is then increased and the water supply is adjusted in order to maintain the same operating pressure. Conditions are recorded when steady state is reached. State how you know the steady state
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was attained. How much time was required? This procedure is repeated for six or seven conditions spanning nucleate to film boiling. Go back to two earlier conditions as a check. 5. Finally, the pressure is adjusted by varying the heating rate and cooling water flow rate and observing critical heat flux necessary to cause transition from nucleate to partial film boiling at various pressures. 6. Calculations should include heat fluxes and heat transfer coefficients as a function of temperature difference for both the heating element and condenser, critical heat flux vs pressure, and energy lost (or gained) from the system vs fluid temperature. The types of heat transfer observed should be described in your report. 7. The Software provided by P.A. Hilton contains six experiments. These experiments cannot be changed although when a data point is taken all the available data is recorded. The experiments to be performed are: Experiment 2, determination of heat flux and heat transfer coefficient at constant pressure; Experiment 3, investigation of the effect of pressure on the critical heat flux; Experiment 6, pressuretemperature relationship for a pure fluid. Experiment 1, visualization of the 3 modes of boiling is incorporated into the other experiments. 8. Note: The effect of air in a condenser is not to be done
References: P.A. Hilton Limited. Experimental Operating and Maintenance Manual: Boiling Heat Transfer Unit H655. Hampshire, England: P.A. Hilton LTD. Dec. 1997.
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Free Convection and Radiation
Introduction In this experiment heat is transmitted in parallel by two different mechanisms, free (or natural) convection and thermal radiation. We are able to distinguish between them because the rate of thermal radiation is not affected by the pressure of air between emitter and absorber, whereas a convection process is affected by fluid density and so by pressure. The total rate of transfer at steady state is determined by the rate of input of electrical energy to a heater. You will certainly need to do some reading as preparation for the experiment. In the book by Incroperaet al., read section 1.2, the introduction to Chapter 9, and sections 9.1 and 9.6, with definitions of emissivity, Nusselt number, Grashof number, Prandtl number, and Rayleigh number, all before you do the experiment. Note: This experiment is long. Examine the equipment carefully beforehand and plan your operation. Check with the demonstrator you whether need to start the experiment early. Apparatus: A small horizontal cylindrical element is suspended in a steel pressure vessel. The element is heated electrically. Its surface consists of circular plates and a cylinder, made of copper with a matt black surface. Temperatures are measured by two thermocouples, one at the middle of the surface of the element and the other in the wall of the vessel. The temperature of the air in the vessel is assumed to be equal to the temperature of the vessel wall under the conditions of this experiment. The element is 6.34 mm in diameter by 159.68 mm long. Its surface area is given by A=
d 2 + dl 2
The power input, corrected for resistance of leads and transfer of heat from the leads, is given by Q = 0.96VI - 0.0017 (TE TV) With a correction factor to the surface area of 1.02. The correction factor should be applied to the calculated surface area. TE is the element temperature and TV is the vessel wall temperature.
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Important Notes for Operation Whenever the pressure in the vessel is above atmospheric, be sure that the valve between the vessel and the vacuum gauge transducer is closed. This is because exposure of the transducer to pressure above atmospheric will harm it seriously. The vacuum-pipe-isolator valve must always be closed before the vacuum pump is shut off. If the isolator valve is open when the pressure in the pump is more than the pressure in the vessel, oil from the pump can be sucked into the vessel. Procedure: Turn on the heater and adjust the thermostat. The heat input to the element should be about 5 watts. Close the air-release valve and turn on the vacuum pump. Open the valves that isolate the pump and the vacuum gauge transducer. Let the vacuum pump run until the maximum vacuum of the apparatus is closely approached (this is about 0.03 torr). Keep the pressure constant by adjusting the vacuum-pump isolator valve. When temperature of the heating element changes by no more than 0.1 oC in 60 seconds, steady state can be assumed. Record the temperature of the element, temperature of the vessel, voltage, current and pressure. By adjusting the vacuum-pump isolator valve, let the pressure in the vessel rise. Take two more readings at steady state when the pressure is less than 0.1 tarr. Firmly close the valve that isolates the vacuum-gauge transducer. Close the air-release valve, then turn on the pressure supply valve. With the pressure line isolation valve open, adjust the pressure regulator to increase the pressure in the vessel. Take two or three readings at pressures above atmospheric. Close the air supply valve. Open the air-release valve and allow the pressure to return to barometric. Turn off the valve. Theoretical Considerations: Various empirical correlations for heat transfer by free convection at usual pressures are given in the literature, e.g. Equations 9.26, 9.27, 9.30, 9.33, 934 given by Incropera et al. However, at low pressures the mechanism of convective heat transfer changes because of the increase in mean free path of the gaseous molecules with falling pressure. Once the length of the mean free path becomes comparable with the dimensions of the body and thickness of the boundary layer, the simple empirical equations no longer apply. This is analogous to Knudsen diffusion.
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An empirical procedure for extrapolating to heat transfer at zero pressure has been devised by the makers of the equipment used in this experiment. They suggest plotting (T E TV) vs the fourth root of absolute pressure. This gives approximately a straight line, which should be calculated using the three lowest pressures and extrapolated to zero pressure. Note that for this extrapolation to be reasonable, measurements at the three lowest pressures must be done at the same conditions otherwise. Calculations: Determine the emissivity of the surface using Equation 1.7. Compare it with values given in Table A.11 of the textbook and with the value for an ideal radiator. Plot the Nusselt number for free convection against the Rayleigh number on log-log coordinates. This similar to Figure 9.6 but for a different geometry. Plot Equation 9.34 on this graph for comparison. At approximately what absolute pressure does the Knudsen effect begin to be significant? Find the fraction of the total heat transfer which occurs by free convection at each set of conditions. Show this by a suitable plot.
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6. ION EXCHANGE IN WATER SOFTENING Objective: Determine the exchange capacity of a cationic resin in water softening. Introduction: Water softening is a process to reduce hardness in water and prevent the build-up of lime scale and calcium deposits in pipes and equipment. Hardness is normally measured by the amount of calcium and magnesium that is present in water and is reported as the concentration of CaCO3. To get an idea of scale, the Saskatoon Water Treatment and Meters Branch reports the potable water has an average hardness of 126 mg/L as CaCO 3. The river water has an average hardness of 176 mg/L. 120 mg/L as CaCO3 or greater is considered hard. Ion exchange is an important technique to reduce hardness in water. It is the reversible interchange of ions between a solid (ion exchange material) and a liquid. The ions in solution become attached to the solid and the displaced ions will be forced into solution. The process of exchange continues until both ions reach equilibrium on the surface and in solution. This process is dynamic and can be reversible depending on the relative concentrations of the ions in solution. Ion exchange has been used on an industrial basis since 1910 with the introduction of water softening. Cation exchange is widely used to soften water. The most usual ion exchange material employed in water softening is a sulphonated styrene-based resin, supplied by the makers in the sodium form. In the process, calcium and magnesium ions in water are exchanged for sodium ions on the resins. Ferrous iron and other metals such as manganese and aluminum, sometimes present in small quantities, are also exchanged
Figure 1. Ion Exchange Columns (Picture courtesy to www.stockinterview.com/News/) after calcium and magnesium are removed, but are unimportant in the softening process. Removal of the hardness, or scale-forming calcium and magnesium ions, produces soft water. Softening can be carried out as a batch process by stirring a suspension of the ion exchange resin in the water for a period until equilibrium, or an acceptable level of hardness, is reached. 22
However, it is more convenient to operate a continuous flow process by passing the water downwards through a column of resin beads. Theory: The exchange reaction for water softening with a sulphonated styrene-based resin in the form of sodium can be described below. 2Na+R- + Ca2+(aq) Ca2+R-2+ 2Na+(aq) where R represents the resin chain and the exchange point on the beads. The reaction takes place rapidly enough for the upper layers of the bed to approach exhaustion before the lower layers being able to exchange ions. There is thus, a zone of active exchange which moves down the column until the resin at all depths becomes exhausted. The position at an intermediate stage can be illustrated as shown in Figure 2a. Plotting the hardness readings as CaCO3 (mg/L) in the effluent against the volume of water treated (L) generates the breakthrough curve as shown in Fig.2b. The breakthrough point can be determined at which the concentration of CaCO3 in the effluent reaches an acceptable level of hardness or the hardness of the feed. It is usually the limit of the exhaustion cycle. (1)
Figure 2a. Ion Exchange Zone
Figure 2b. Idealized Breakthrough Point
When the resin is exhausted, it can be regenerated with a copious amount of sodium salt such as sodium chloride. The excess salt will shift the equilibrium (Eq.1) to the left and sodium ions will replace calcium and be present on the solid. If the hardness is measured by CaCO3, the ion exchange capacity of the resin can be determined as follows:
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Exchange Capacity (meq/mL) = where Volume of Wet Bed (mL) =
Removed Mass of CaCO 3 (mg) X0.02 Volume of Wet Bed (mL)
(2)
D 2 XFinal Depth of Resin Bed (cm) 4
(3)
where D is the diameter of the column (cm), equal to 1.5 cm in this investigation. The mass of CaCO3 removed from the tap water up to the breakthrough point can be calculated. Graphically, this is given by the area in the graph of breakthrough curve between the curve plotted and the horizontal line, representing the original hardness of the water. The mass of CaCO3 (mg) can be converted to milliequivalent (meq) by multiplying a factor of 0.02. (DOWEX, Ion Exchange Resins, Water Conditioning Manual, p.74.) Knowing the wet volume of the resin bed, the exchange capacity of the resin can be calculated as meq/mL. If other minerals such as magnesium carbonate, calcium oxide and so on are removed, they can be converted to the equivalent concentration as CaCO3 by certain conversion factors. The conversion factors of common substances are given in literature (DOWEX, Ion Exchange Resins, Water Conditioning Manual, p.75). Once the exchange capacity of the resin bed is determined, it can be used to design a column packed with the resins for water softening at large scale. The required resin volume can be determined in the following equation: Resin Volume (L) = Feed Hardness (meq/L)X Throughput (L) X10 3 (L/mL) Exchange Capacity (meq/mL) (4) where the throughput refers to the volume treated per exhausted cycle of resin.
Apparatus: Water softening in this investigation is carried by the Armfield Ion Exchange Apparatus W9. The sketch is shown below in Figure 3. The system consists of a column packed with a sulphonated styrene-based resin, a pump to supply liquids to the column, four tanks to store solutions of HCl, NaCl, test and deionized water and a sump tank. A rotameter is used to measure the flowrate of the feed. A conductivity meter is used to monitor the concentration of sodium in the effluent. An anion exchange column was set up for the experiment of demineralization but not used in this investigation. A Schott Instruments Titroline easy auto-titrator is used for determination of CaCO3 concentration in the effluent. For this purpose, 250 mL Erlenmeyer flasks and 1L beakers are used.
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Figure 3. Diagram of Ion Exchange Apparatus
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Materials and Methods: The hardness of the test water passing through the ion exchange column is determined by titrating the effluent of the W9 Ion Exchange apparatus with a complexometric reaction. (Appendix A). The concentration of sodium ions [Na+] in solution is measured by the conductivity of the collected effluent. The required chemicals are as follows: Calcium chloride anhydrous Sodium chloride pH 4, 7, & 10 buffer solutions Disodium ethylenediamine-tetraacetic acid dihydrate (EDTA) solution 0.015 M Calcium carbonate ACS grade HCl conc. 12.1 M Ammonium hydroxide conc. (30% w/w) Calmagite 1% solution - indicator Procedure: A. Adding Cation Resin to the Column 1. 2. 3. 4. B. Drain the column by first placing a waste vessel at valve 10, then opening valves 6 and 10 Remove the cation column by undoing the plastic holders on each end of the column. The column pulls out towards you and has no catches. Fill the cation column to ~300 mm of cationic resin (golden-brown colored granules). Replace the column and close valves 6 and 10
Regenerate the Cationic Resin Regeneration is required at the beginning of the experiment to ensure that the cation column has the requisite amount of Na+ ions. You are assuming that the column is depleted. For apparatus configuration see Data Sheet I, Figure 1c. 1. 2. Select Tank B, open valves 2 and 12 turn on the main switch. Add 30 g of salt to a beaker. Add RO water.
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3.
Set the flow meter to 20 -50 mL/min and add the salt to the column. Continue until the conductivity reaches 1.1 x 10-2 Siemens for three minutes or all the salt has been placed in the column. This means that the column has reached the saturation point and has an excess of Na+ ions. Select Tank D and flush the column for 5 minutes at 70 ml/min. Close all valves afterwards.
3. 4. C.
Backwashing the Cation Column Backwashing ensures that the remaining regeneration solution and debris from the last experimental run are washed out of the column. Plus it expands the resin beads so that no air pockets remain in the resin bed. For apparatus configuration see Data Sheet I (Page 9 of this manual), Figure 1b. 1. 2. 3. Make sure all valves are closed. Open valves 3 and 6. Select Tank D and turn on the pump, then backwash the cation column at a flow rate of 50-70 ml/min for 5 minutes.
Large air bubbles can be gotten rid of by closing valve 6 and opening 12 until the water has reached the top of the column. Then close 12 and re-open 6. Repeat if necessary.
4. 5. D.
When the air bubbles have been eliminated and the resin has settled, turn off valves 3 and 6. Measure the final depth of the resin.
Softening of Water Sample For apparatus configuration see Data Sheet I, Figure 1d. 1. 2. 3. 4. 5. Select Tank C containing the test water. Open valves 2 and 10. Ask the TA which flow rate is used for the week. Collect samples at ~800 mL intervals. Determine the hardness of each sample as per Appendix A and continue testing each sample until hardness rises above 100 mg/L as CaCO3.
E.
Shutdown
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1. 2. 3.
Select Tank D and flush the column at max flow for 5 minutes Turn off the pump, open valve 10 and completely drain the resin column. Rinse the beakers and equipment with RO water.
Data Analysis: a) Plot the hardness as CaCO3 concentration (mq/L) against effluent volume (L). Identify the breakthrough point. Determine the total amount of CaCO3 (mg) removed by the column up to the breakthrough point. b) Calculate the exchange capacity (meq/mL). c) Design a column (area and height) to reduce the hardness of 10,000 L of water to 100 mq/L of CaCO3. The initial hardness of water is 700 mq/L CaCO3. Keep the same ratio of the height to diameter of the wet resin bed as that in this lab. Provide brief discussion on your design results including the feasibility of using one column and one exhausted cycle to complete the task. Hint: you need to first determine the required resin volume for this project. A safety factor should be applied to the exchange capacity figure to compensate for non-ideal operating conditions and resin aging on a working plant. Typical safety margin is 5% for cation resins. Column sizing should be adjusted to allow for resin expansion if backwashing is performed (80 100% of the settled resin bed height) and resin swelling during service, approximately 5-8% for strong acid cation resin.
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Data Sheet I Configuration of Ion Exchange Apparatus
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Appendix A
Complexometric Titration for the Determination of Water Hardness Titration Procedure Water Hardness Titration Disodium Ethylenediamine-tetraacetic acid dihydrate (EDTA or Na2H2Y2H2O) forms a chelated soluble complex when added to a solution of certain metal cations. If a small amount of dye (Calmagite) is added to a solution containing calcium and magnesium ions at a pH of 10 1, the solution becomes wine red due to the MgIn- formation. If EDTA is added as a titrant, the calcium and magnesium will become complexed. The calcium will be complexed out first as it has a larger formation constant with EDTA than magnesium. When all of the calcium ions have been complexed with EDTA, the trace amount of magnesium ions in the buffer will react. Once the trace amount of magnesium is complexed, the solution will change to a blue. The addition of the trace amount of magnesium is required for the complexometric titration and eliminates the need for a blank correction titration. pH is very important in this experiment as having a higher value than 10.5 will precipitate out CaCO3 or Mg(OH)2 immediately. However, even at a pH of 10 CaCO3 will precipitate out eventually. Thus a 5 minute deadline from the addition of the buffer solution is needed to prevent interference from CaCO3 precipitation. Schott Instruments Titroline easy auto-titrator The Schott Instruments Titroline easy auto-titrator is used for the titrations. The main switch is in the back. This experiment is a colored endpoint thus a manual titration is required. From the main menu on the auto-titrator, press F1 three times. Operation is simple. Press the left hand button to refill the ...