- Michigan State University
Contents
1. DO represents the actual dissolved oxygen concentration in the liquid phase DO the dissolved oxygen concentration in equilibrium with the gas phase calculated according to Henry s law and k a the liquid side volumetric mass transfer coefficient After integration and linearization this equation can be adopted to obtain kra as follows DO DO vf 22120 k a t ty The experiment was repeated three times re oxygenation profiles from each experiment analyzed separately and kra reported as the aremathic mean and standard deviation of the independent determinations 18 6 Results and discussion The dynamic re oxygenation method was adopted to assess the gas liquid volumetric mass transfer coefficient kia using polarographic sensors to monitor dissolved oxygen concentrations in the water column The sensor lag dynamics was determined to verify the assumption that sensor lag dynamics was not significant in comparison with re oxygenation dynamics The kya was then estimated for distilled water under ambient pressure and temperature and the assumption of perfect mixing in the reactor justified 6 1 Sensor lag constant and effect on ka measurements Sensor lag was evaluated by subjecting the sensors to a near instantaneous alteration in dissolved oxygen concentration and modeling the sensor lag according to a first order dynamics A rapid sensor response was observed and a steady state measurement was usually observed in less t2. about the operating conditions is vital to accommodate the interpretation of kra The liquid phase was in all experiments distilled water and experiments were performed at ambient temperatue and pressure Table 2 lists a number of reactor operating parameters used during this study as measured by visual inspection Gas and liquid hold up were estimated according to the volume expansion method based on measurement of the height of the water column in the presence H and absence Ho of air bubbles using the following equations H H ad H and E liquid T E eas 15 Superficial gas velocity was estimated by interrupting the gas flow to the reactor and monitoring the time required for the gas bubbles to reach the air water interface around 3 5 s Notwithstanding potential inaccuracies in the measured reactor operating conditions the provided estimates should be useful in the design of further experiments However more advanced methods such as a high speed camera with dedicated software tools may be required to measure the superficial gas velocity and bubble size distribution more accurately Table 2 Estimated reactor operating conditions Parameter Condition Superficial gas velocity 0 3 m s Gas hold up 0 009 Bubble diameter 7 5 mm 5 2 Determination of the sensor lag constant Sensor lag was determined by subjecting the sensors to a near instantaneous alteration in dissolved oxygen concentration and colle
3. from each experiment analyzed separately and s and related parameters reported as the aremathic mean and standard deviation of the independent determinations 5 3 Determination of the volumetric mass transfer coefficient The volumetric mass transfer coefficient kra was determined using the dynamic oxygen absorption method Letzel et al 1999 Dissolved oxygen in the water column was first removed by N sparging until the concentration fell below 1 of the dissolved oxygen concentration at saturation estimated at 8 66 mg L at 23 C and a oxygen partial pressure of 0 21 atm This step was generally achieved in about 30 min Oxygen was then introduced in the reactor as compressed air using a diffuser and the dissolved oxygen concentration monitored during re oxygenation of the water phase every second until saturation was reached Switching between nitrogen and air sparging was achieved by opening and closing the main valve of the gas cylinders The gas cylinder outlet pressure was furthermore adjusted prior to the experiment to yield a constant gas flow as observed using a non calibrated rotameter level 11 at the rotameter used during our 17 experiments However this approach did not yield information on the actual air flow and it is suggested to calibrate the rotameter in order to accurately measure the gas flow rate Re oxygenation profiles were modeled according to the CSTR model without sensor lag a o k a DO DO where
4. in the order of 4 This in excellent agreement with the value of 0 57 s observed in this study 24 7 Outlook An experimental unit for the determination of volumetric mass transfer coefficient kra was designed and fabricated The developed method adopts dissolved oxygen sensors for monitoring of dissolved oxygen concentration during re oxygenation of the water column according to the dynamic method Results obtained from an initial set of experiments demonstrated that sensor lag was sufficiently small and could be ignored during estimation of kra In addition the bubble column reactor behaved similar to a CSTR and no difference in kya estimates at various axial positions was observed The experimental unit can be adopted to study the effect of various operating conditions such as superficial gas velocity and bubble size on the volumetric mass transfer coefficient A number of relevant studies pertaining to the effect of various reactor operating conditions are provided in the references These and similar studies may serve as the basis for the design of future experiments 25 8 References Vandu et al 2004 Volumetric mass transfer coefficient in a slurry bubble column operating in the heterogeneous flow regime Chem Eng Sci 59 22 23 5417 5423 Han et al 2007 Gas liquid mass transfer in a high pressure bubble column reactor with different sparger designs Chem Eng Sci 62 1 2 131 139 Gourich et al 2006 Improve
5. plastic tube and emptied rapidly less than 5 min by removal of the sensor closed to the bottom of the reactor Table 1 Overview and cost of the components of the experimental unit Items Acrylic tower aquarium 50 12 5 Dissolved oxygen sensor Vernier LabPro Monitor Central processing unit Reducing coupling 1 2 Reducer bushing 3 4 x1 2 O ring washer 3 4 Gas cylinder Air amp Nitrogen 230 scf Gas regulator nitrogen single stage Gas regulator air single stage Rotameter Air diffuser top fin round airstone 3 Air pump Stellar S 30 Chemical resistant tubing Tygon 2 and 5 Glass beaker 600 mL Glass flask 500 mL Glass flask 1 L Magnetic stirrer Stirrer bar Catalog Number HT 2 DO BTA LABPRO CGA 580 CGA 590 OE1042 R 3603 FB 2610 27060 500 27060 1000 M 2200 SBMS51080TH Company The Billiard Warehouse Inc Vernier Software and Technology Vernier Software and Technology Panasonic Colfax Home Depot Home Depot Home Depot Linde gas LLC AIRCO Scott PetSmart PetSmart Saint Gobain KIMAX USA KIMAX USA KIMAX USA VMR Scientific STIR BARS Total Cost 699 3x199 220 200 400 3x1 3x1 3x0 5 2x5 per month 100 100 2 99 20 99 30 5 22 06 34 82 3x98 33 3 50 2 752 Air diffuser The air diffuser 7 6 cm diameter Figure 2 used during our initial set of experiments was purchased from a local pet store PetSmart E
6. Design of an Experimental Unit for the Determination of Oxygen Gas Liquid Volumetric Mass Transfer Coefficients using the Dynamic Re oxygenation Method ENE806 Laboratory Feasibility Studies in Environmental Engineering Spring Semester 2007 Submitted to Dr Syed A Hashsham Department of Civil and Environmental Enginneering A126 Research Complex Engineering Michigan State University East Lansing MI 48824 Dieter Tourlousse amp Farhan Ahmad Table of Contents 1 Summary 2 Introduction 3 Experimental Unit 3 1 Reactor configuration 3 2 Real time data acquisition 4 Experimental procedures 4 1 Preparation dissolved oxygen sensors 4 1 1 Sensor preparation 4 1 2 Sensor warm up 4 1 3 Sensor calibration 4 1 3 1 Zero dissolved oxygen calibration point 4 1 3 2 Saturated dissolved oxygen calibration point 4 2 Communication with LabVIEW and data collection 5 Experiments 5 1 Reactor operating conditions 5 2 Determination of the sensor lag constant k 5 3 Determination of the volumetric mass transfer coefficient kya 6 Results and discussion 6 1 Sensor lag constant and effect on kra measurements 6 2 Volumetric mass transfer coefficient kya 7 Outlook 8 References 9 Appendix nH HR gt Q 12 12 12 12 13 13 13 13 15 15 16 17 19 19 21 25 26 27 1 Summary We report on the design and fabrication of an experimental laboratory unit for the determination of the oxygen gas liquid volumetric mass transfer c
7. actors and aeration systems During the last decades various techniques for the determination of kya have been developed Poughon et al 2003 and references therein These methods can be separated into chemical and physical methods Deront et al 1998 Chemical absorption methods can be classified depending on the reaction rates with the sulfite oxidation method being the most widely adopted approach Physical oxygen absorption methods based on gassing in gassing out or pressure step can be dynamic or steady state In the dynamic gassing in or re oxygenation method dissolved oxygen concentration is monitored in the liquid phase during oxygen absorption Though inherent limitations of the method are well recognized Gourich et al 2006 the dynamic re oxygenation method using dissolved oxygen sensors has been increasingly adopted in current studies on oxygen gas liquid mass transfer A number of factors can confound the measurement of kya First sensor lag can lead to inaccurate dissolved oxygen concentration measurements and inferred kya estimates when the characteristic time of the sensor response and re oxygenation dynamics are of comparable magnitude Vandu et al 2004 Gourich et al 2006 Increased sensor lag leads to reduced estimates of kia when this is not considered during modeling of the re oxygenation dynamics Second the hydrodynamic conditions in the reactor and position of the dissolved oxygen sensors are critical for kya
8. and constant oxygen concentration in the gas phase For bubble column reactors with high water column height to diameter ratios however the axial dispersion AD model might be more appropriate to account for non perfect mixing by incorporation of a dependence of the gas and liquid phase oxygen concentration on the axial position in the column Han et al 2007 The latter authors observed that the CSTR and AD model produced different kya values in particular at 21 positions with increasing distance from the bottom of the reactor expressed as the axial position divided by the diameter of the water column z d In this study no large difference was observed for kya estimates at different axial distances Table 4 This was attributed to the fact that sensor furthest from the bottom of the water column has an axial position of 3 8 and Han et al 2007 observed that for sensor position to column diameter ratios below 3 to 4 the AD and CSTR model yield nearly identical kya estimates This demonstrated that our assumption of ideal mixing was justified and the BC reactor essentially behaved as a CSTR Table 4 Volumetric mass transfer coefficients kya n 3 Sensor number ka axial position z d min 1 0 6 0 30 0 01 2 2 0 0 30 0 00 3 3 4 0 32 0 00 Mean 0 31 0 01 The volumetric mass transfer coefficient kta in bubble column reactors is dependent on various interrelated parameters including the gas hold up gas th
9. ast Lansing MI However it is suggested that for future experiments air diffusers specifically designed for bioreactor aeration should be adopted to yield a better control of the bubble size distribution The plastic tubing providing air flow to the diffuser was placed in a plastic pipe taped to the reactor wall The air diffuser was placed under the dissolved oxygen sensors away from the center of he reactor to ensure adequate water circulation in the vicinity of the sensors Figure 2 Figure 2 Left panel Air diffuser Right panel Positioning of the air diffuser with respect to the dissolved oxygen sensor Dissolved oxygen sensors Three dissolved oxygen sensors were purchased from Vernier Software amp Technology Vernier Beaverton OR The sensors were relatively inexpensive 199 per sensor but it should be recognized that it is recommended by the manufacturer to adopt the sensor for teaching purposes only It was however observed that the sensors performed very well in terms of measurement of changes in dissolved oxygen concentrations even without calibration Since estimation of oxygen gas liquid mass transfer coefficients merely relies upon the rate of changes in dissolved oxygen concentration the sensors were deemed appropriate for our purposes Adopting the sensors to accurately measure absolute dissolved oxygen concentrations might however require additional careful calibration and testing of the sensors The Vern
10. cting sensor readings until a constant measurement was observed Vandu et al 2004 Philichi and Stenstrom 1989 The sensor was first equilibrated in a conically shaped beaker 500 mL continuously sparged with N provided using a gas cylinder The sensor was then rapidly transferred to a beaker 1000 mL continuously stirred using a stir bar and magnetic stirrer The dissolved oxygen concentration was maintained at saturation by continuous sparging of air using an air pump The sensor dynamics was obtained by monitoring the dissolved oxygen concentration every 0 2 s for a period of 1 min Important It should be ensured that no gas build up exists under the membrane of the sensor This was achieved by placing the stirrer bar away from the center touching the glass of the flask Sensor lag was assumed to follow a first order dynamics Philichi and Stenstrom 1989 16 Letzel et al 1999 Vandu et al 2004 Gourich et al 2006 according to the following equation dDO a k DO DO where DO represents the measured dissolved oxygen concentration DO the actual dissolved oxygen concentration and sp the sensor lag constant After integration and linearization this equation yields DO DO In I k t t paaa pl with DO the initial sensor dissolved oxygen measurement This equation allows to obtain Sp based on the measured sensor dynamics The experiment was repeated four times re oxygenation profiles
11. e superficial gas velocity Ugas and the specific interfacial gas liquid surface area a The specific interfacial gas liquid surface area is related to gas hold up and the mean bubble diameter dp Wongsuchoto et al 2003 Assuming that the bubbles in reactor posses an uniform diameter of 7 5 mm and a gas hold up of 0 009 Table 2 the specific gas liquid surface area was estimated at 7 2 m This in turn yields an estimated liquid side mass transfer coefficient k of 7x10 m s This estimation is in good agreement with the value of 4x10 m s reported by Painmanakul et al 2005 22 100 oo gt a ie kh DO DO il S DO DO DO saturation Pu Time min Figure 8 Typical re oxygenation dynamics The arrow indicates start of the re oxygenation process by air diffusion 23 The estimated liquid side mass transfer coefficient in bubble column reactors may vary with superficial gas velocity Usas depending on the range of Ugas Vandu and Krishna 2004 and references therein The experiments in this initial study were performed in the so called churn turbulent flow regimes Ugas 0 3 m s gt 0 08 m s Vandu and Krishna 2004 previously observed that the volumetric mass transfer coefficient per unit volume kia Egas Was independent of Ugas for velocities higher than 0 08 m s The authors reported a kya gqs of 0 48 s for a bubble column reactor with a height to diameter ratio
12. ed re oxygenation profiles 2 Introduction Efficient oxygen supply is a principal requirement for all aerobic chemical and biological processes Aeration refers to the process of addition of oxygen to water by utilization of the principles of mass transfer In most aerated biological processes the oxygen transfer rate OTR is modeled to be directly proportional to the driving force generated by the difference between the saturation DO and actual dissolved oxygen concentration DO in the liquid phase The proportionality constant is defined as the volumetric mass transfer coefficient Kra expressed in reciprocal time units yielding the following well known relationship describing oxygen gas liquid mass transfer d DO dt OTR K a DO DO with DO obtained by Henry s law The Kra is a lumped parameter incorporating the overall resistance to mass transfer and the total specific surface area available for mass transfer 1 1 T 1 K a k a Hk a where k and kg represent the liquid and gas phase mass transfer coefficient respectively and H the dimensionless Henry constant In most cases the liquid phase resistance to mass transfer is dominating and the volumetric mass transfer coefficient is approximated by kra The Kra depends on numerous parameters including liquid phase properties reactor geometry and operating conditions The volumetric mass transfer coefficient kya is a critical parameter in the design of re
13. er filled with 100 mL of distilled water 4 1 2 Sensor warm up 1 Plug the dissolved oxygen probe into the computer interface and open the LoggerPro or LabVIEW software The program will automatically identify the dissolved oxygen probe 2 It is necessary to warm up the dissolved oxygen probe for 10 minutes before taking readings To warm up leave it in water connected with the software for 10 minutes The probe must stay connected at all times to keep it warmed up If disconnected for a few minutes it will be necessary to warm up the probe again 12 4 1 3 Sensor calibration 4 1 3 1 Zero dissolved oxygen calibration point 1 Choose calibrate from the experimental menu and click on calibrate now button 2 Remove the probe from the water and place the tip of the probe into the sodium sulfite calibration solution Important No air bubbles can be trapped below the tip of the probe or probe will sense an inaccurate dissolved oxygen level If the voltage does not rapidly decrease tap the side of the bottle with the probe to dislodge any bubbles The readings should be in the 0 2 to 0 5 V range 3 Type 0 the known value in mg L in the edit box 4 When the displayed voltage reading for reading 1 stabilizes 1 min click keep 4 1 3 2 Saturated dissolved oxygen calibration point 1 Rinse the probe with distilled water and gently blot dry 2 Unscrew the lid of the calibration bottle provided with the probe S
14. estimations when perfect mixing is not present Gourich et al 2006 Models to extract kta from re oxygenation profiles need to incorporate the hydrodynamics conditions in the reactor if perfect mixing is not present and spatial gradients exist This is of particular importance in bubble column reactors where complete mixing is often not obtained The goal of this project was to design and build an experimental unit for the measurement of the oxygen gas liquid volumetric mass transfer coefficient kta A bubble column type reactor was selected and kya estimated using the dynamic re oxygenation method Polarographic dissolved oxygen sensors were implemented to measure dissolved oxygen concentrations at various axial positions in the water column during oxygen absorption A series of experiments was performed to test the performance of the experimental unit with focus on i determination of the sensor lag constant and ii determination of ka in distilled water as a demonstration of the developed experimental set up In conjunction with this set of experiments a number of assumptions inherent to the estimation of kya based on the continuous stirred tank reactor CSTR model without sensor lag were evaluated 3 Experimental Unit 3 1 Reactor configuration The following section provides an overview of the various components of the experimental unit All components were commercially available Table 1 and required little or no modification A
15. han 30 s Fig 7 Sensor lag constants were estimated at 0 18 0 16 and 0 24 s for the different sensors equivalent to a 95 sensor response time of 16 4 18 2 and 12 3 s Table 3 These estimates are in good agreement with Gourich et al 2006 who determined a sensor lag constant of 0 14 s Vandu and Krishna 2004 observed a sensor lag constant in the order of 0 47 s The determined 95 sensor response time was however roughly two times smaller than the response time reported by the manufacturer of the probes Table 3 Parameters of sensor lag dynamics n 4 Sensor number ks s5 t s tos S axial position z d 1 0 6 0 18 0 01 5 6 0 2 16 44 0 5 2 2 0 0 16 0 00 6 1 0 1 18 2 0 4 3 3 4 0 24 0 00 4 10 0 POSi 19 100 80 60 f DO DO 40 5E Do po Time s 20 Normalized response 0 10 20 30 40 50 Time s Figure 7 Typical sensor dynamics obtained after a near instantaneous alteration in dissolved oxygen concentration 20 Sensor lag can drastically influence the accuracy of dissolved oxygen measurements and inferred kra if the characteristics mass transfer time tm 1 k a is similar to the probe response time ts 1 k Philichi and Stenstrom 1989 estimated that the product of the sensor lag constant kp should be 50 times smaller than k a to limit errors in kya estimates to less than 1 Considering that the slowest responsing sensor displayed a k of 0 16 it
16. ier dissolved oxygen sensor is a Clark type polarographic electrode sensing the dissolved oxygen concentrations in liquid samples A platinum cathode and Ag AgCl reference anode in KCl electrolyte are separated from the surrounding sample solution by a oxygen permeable membrane A fixed voltage is applied to the platinum electrode and oxygen reaching the cathode undergoes the following reduction reaction O2 H20 2e 20H Simultaneously the following reaction occurs at the anode Ag Cl AgCl e As a result an electric current flows is generated proportional to the dissolved oxygen concentration in the sampled solution This current is converted to a proportional voltage amplified and recorded membrane platinum cathode Ag AgCl anode KCI aq Figure 3 Schematic depictionof the polarographic dissolved oxygen sensor Important Polarographic dissolved oxygen sensors continuously consume oxygen and sufficient water flow around the sensor membrane should be ensured to eliminate an apparent drop in dissolved oxygen concentration The sensors were placed across the water column at various axial positions It was important to reduce potential end effects that might occur if the sensors were placed too close to the bottom of the column where aeration occurs or too close to the top of the water column The lower sensor was placed 19 cm above the bottom of the reactor and the two other probes were p
17. laced 45 cm apart yielding a distance of 14 cm between the upper sensor and the level of the water column Figure 1 Sensor seal Design of a seal allowing to readily insert and remove the dissolved oxygen sensors was a critical part of the reactor We were able to construct a simple sensor seal using items readily available in a hardware store A detailed view of the seal is presented below along with the dimensions of the various components Importants The threads of the various components reducing coupling and reducer bushing purchased from the local hardware store were tapered and needed to be modified to allow for a tight seal Figure 4 Sensor seal Upper panel Individual components of the sensor seal From left to right reducer bushing 3 4 1 2 O ring washer 3 4 reducing coupling 1 2 rubber gronmmet provided with dissolved oxygen sensors Lower panel Sensor inserted in seal 3 2 Real time data acquisition Data acquisition constitutes sampling of the real world to generate data that can be manipulated by a computer The components of data acquisition systems DAS include sensors to convert the measurement parameter to an electrical signal acquired by the data acquisition hardware Acquired data is displayed analyzed and stored on a personal computer either using vendor supplied software or custom displays Data acquisition begins with the physical phenomenon or physical property of an object under i
18. lide the lid and the rubber grommet about 1 2 into the probe body 3 Add water to the bottle to a depth of 1 4 and screw the bottle into the cap 4 Type the correct saturated dissolved oxygen value in mg L a table with dissolved oxygen saturation values at different temperatures and barometric pressures is provided in the user guide 5 When the displayed voltage reading for reading 2 stabilizes reading should be above 2 V click keep and then done 4 2 Communication with LabVIEW and data collection Instrument drivers simplify instrument control and reduce test program development time by eliminating the need to learn the programming protocol for each instrument An instrument driver is a set of software routines that control a programmable instrument Each routine corresponds to a programmatic operation such as configuring reading from writing to and triggering the instrument Use an instrument driver for instrument control when possible National Instruments provides thousands of instrument drivers for a wide variety of instruments Use the NI Instrument Driver Finder to search 13 for and install instrument drivers without leaving the LabVIEW development environment Select Help to find instrument drivers to launch the Instrument Driver Finder One can also visit the NI Instrument Driver Network at www ni com idnet to find a driver for an instrument If a driver is not available for an instrument
19. lthough fabrication of a watertight and user friendly sensor seal was deemed challenging we were able to manufacture a straightforward seal using components available in a local hardware store Home Depot East Lansing MI 14 cm E 45cm E AN t 45 cm 19 cm 32 cm as Figure 1 Experimental apparatus Left panel Major components of the experimental unit 1 compressed air and nitrogen cylinders connected in parallel 2 non calibrated rotameter 3 bubble column reactor 4 air diffuser 5 dissolved oxygen sensors Vernier Beaverton OR 6 interface LabPro Vernier 7 personal computer equipped with LabView and LoggerPro 30 day demonstration version Right panel Dimensions of the bubble column water column height to diameter ratio 3 8 and axial positioning of the dissolved oxygen sensors Bubble column The bubble column reactor used in this study was an acrylic hexagonally shaped tower aquarium with a height of 1 27 m and a diameter of 31 8 cm The tower aquarium was purchased from www plasticsonline com at a cost of 699 excluding shipping costs Considering the cost of individual acrylic sheets the option of purchasing an aquarium to serve as bubble column was deemed preferable in comparison with in house fabrication Three holes were carefully drilled in the reactor to accommodate the sensor seal discussed below The reactor was filled with water from the top of the reactor using a
20. ment of oxygen mass transfer estimation from oxygen concentration measurements in bubble column reactors Chem Eng Sci 61 18 6218 6222 Vandu and Krishna 2004 Volumetric mass transfer coefficients in slurry bubble columns operating in the churn turbulent flow regime Chem Eng Process 43 8 987 995 Philichi and Stenstrom 1989 Effect of dissolved oxygen probe lag on oxygen transfer parameter estimation J Water Pollut Control 61 S3 Letzel et al 1999 Gas holdup and mass transfer in bubble column reactors operated at elevated pressure Chem Eng Sci 54 2237 2246 Painmanakul et al 2005 Effect of surfactants on liquid side mass transfer coefficients Chem Eng Sci 60 22 6480 6491 Wongsuchoto et al 2003 Bubble size distribution and gas liquid mass transfer in airlift contactors Chem Eng Sci 92 81 90 Data Acquisition Basics Manual National Instruments January 1998 Edition Function and VI Reference Manual National Instruments January 1998 Edition 26 Appendix The appendix contains the following documents Material safety data sheet for sodium sulfite solution Material safety data sheet for potassium chloride solution Vernier dissolved oxygen sensor user s guide Vernier LabPro user s manual Dissolved oxygen saturation levels as function of temperature and pressure 27
21. nvestigation to be measured This physical property or phenomenon could be the temperature or temperature change of a room the intensity or intensity change of a light 10 source the pressure inside a chamber the force applied to an object etc A transducer converts this physical property into a corresponding electrical signal such as voltage or current The ability of a data acquisition system to measure different phenomena depends on the transducers to convert the physical phenomena into signals measurable by the data acquisition hardware DAQ hardware is what usually interfaces between the sensor and a personal computer It can be in the form of modules that can be connected to the computer s ports parallel serial USB etc or cards connected to slots PCI ISA in a mother board Driver Software that usually comes with the DAQ hardware or from other vendors allows the operating system to recognize the DAQ hardware and programs to access the signals being read by the DAQ hardware In this experiment a data acquisition interface named LabPro Vernier Software and Technology was used A schematic depiction of the module is presented below Figure 5 LabPro can be connected to the USB port or serial port of a personal computer The interface contains two digital and four analog channels for connection of the sensors LabPro is compatible with LabVIEW a highly user friendly environment for acquiring analyzing displaying and storing da
22. oefficient kya in a bubble column reactor The total cost of the unit was around 2 000 and required little or no in house modification of commercially available items The reactor was hexagonal in shape and had a column height to diameter ratio of 4 Three polarographic sensors were implemented to measure dissolved oxygen concentrations at various axial positions in the water column during oxygen absorption To illustrate the performance of the experimental unit the kya was determined in distilled water at ambient pressure and temperature Alongside factors important in the determination of kia in bubble column reactors are discussed and evaluated with focus on the impact of sensor lag and hydrodynamic conditions in the reactor It was verified that the estimated kra 0 31 min was much larger than the sensor lag constant k 0 16 s thereby justifying the elimimation of sensor dynamics in modeling of the re oxygenation profiles In addition it was observed that re oxygenation profiles were independent of the axial position of the sensors in the reactor thus allowing implementation of the continuous stirred tank reactor CSTR model to estimate kra In conclusion we constructed an experimental laboratory unit for the estimation of kya using the dynamic re oxygenation method using polarographic dissolved oxygen sensors We provided experimental evidence that the CSTR model without sensor lag can be adopted to extract kya values from the measur
23. ta 1 lt AC adaptor port USB connection analog channels Figure 5 Set up of LabPro interface for collection of sensor readings LabVIEW provides a graphical programming environment highly suitable for data acquisition LabVIEW programs are called virtual instruments or VIs because their 11 appearance and operation imitate physical instruments LabVIEW VIs contain the following main components the front panel block diagram and icon connector The front panel provides the user an interface for data inputs and outputs The user operates the front panel by using the computer s keyboard and mouse Behind the front panel is the block diagram that is responsible for the actual data flow between the inputs and the outputs 4 Experimental procedures 4 1 Preparation of the dissolved oxygen sensors The protocol for the preparation warm up and calibration of the sensors is discussed below and the protocol describing both steps can also be found in user s guide provided by the manufacturer a reprint is provided in the appendix 4 1 1 Sensor preparation 1 Remove the blue protective cap from the tip of the probe This protective cap can be discarded once the probe is unpacked 2 Unscrew the membrane cap from the tip of the probe 3 Using a pipet fill the membrane cap with 1 mL of DO electrode filling solution 4 Carefully thread the membrane cap back on to the electrode 5 Place the probe into a beak
24. was estimated that kya values higher than 0 19 min could be accurately measured with the sensors used in this project As demonstrated below the ka observed under the specified reactor operating conditions was higher than this limit and justified the assumption that sensor lag did not significantly influence kya estimates in our experimental set up 6 2 Volumetric mass transfer coefficient ka The volumetric mass transfer coefficient kya was determined using the dynamic re oxygenation method Re oxygenation was initiated by sparging of compressed air using an air diffuser and complete re oxygenation was usually observed in less than 20 min Fig 8 The linearized plot demonstrated that a mono exponential process governed the re oxygenation dynamics further substantiating the assumption that sensor lag did not noticeably influence the monitoring of the reoxygenation dynamics Interestingly overlapping re oxygenation profiles were obtained for all three sensors at different axial positions with coefficients of variations among the always less than 5 not shown The kya was estimated at 0 31 0 01 min expressed as the arhematic mean and standard deviation of the three independent determinations for all probes Table 4 translating into a time of 9 79 min to achieve 95 re oxygenation of the water column The CSTR model is the most widely applied method to estimate kya This model assumes perfect mixing in both the gas and liquid phase
25. you can use the Instrument I O Assistant Express VI to communicate with the instrument Before starting to work with LabVIEW download the VIs from www ni com and install it on the computer The following section provides a step by step guide describing the LabVIEW start up real time data collection and storage Additional information can be found in the user s manuals present in the laboratory Step 1 Open LabView and Click to open the user friendly VI program specially designed for measuring real time data Step 2 The below presented window Figure 6 will be displayed which shows the overall block diagram for acquiring data We have modified the real time measurement VI to acquire the data for all the three probes and saving the data to the specified folder Step 3 Wire the resulting signal to either the graph for waveforms or the numeric for scalar values Step 4 Configure the Write LabVIEW Measurement File Express VI by double clicking it and make sure to provide a correct path for the file name Step 5 Save the file 14 BH a 4 2516 _ mp ie gt _ Figure 6 Programmed LabVIEW VI for collection of dissolved oxygen sensor readings 5 Experiments 5 1 Reactor operating conditions Reactor operating variables including type of air diffuser gas flow rate and superficial gas velocity influence the re oxygenation dynamics and inferred kra Hence knowledge
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