PROBLEMS IN ENVIRONMENTAL SCIENCES AND ENGINEERING
PROBLEMS IN ENVIRONMENTAL SCIENCES AND ENGINEERING ENVR 890
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NOx amp Ozone Gas Phase Titration GPCalibration August 2008 rk The Gas phase titration method to calibrate NO NOx N02 and 03 is based on the one to one reaction of NO 03 to form N02 NOO39 N02 Things to turn on A l 9 Check that NOx and 03 meters are on usually they are on all the time lfthye are not tune on the computer remote switch 8 outlet mixing fanquot Check that the GPC air tank is on in the utility area If it is empty you will have to use the back up pump on the wall Figure 1 Remote switch 8 inlet mixing fanquot shold be on This turns on the solenoid onoff valve on the GPT air tank in the utility area and the Tylan flow digital meters It also switches the NOx and 03 meters to the GPT cal flask from the Mahogany sampling board When finished turn off the remote switch on data system set to South inlet mixing fan to off Doing this will return the NOx and O3 sampling lines to the chamber gas sampling manifold behind the particle instruments Make sure the NO cal tank is off The general Idea The NO instrument is 1St calibrated by flowing and diluting known amounts of NO from a calibrated tank usually at 50 ppm 1 2 with NOx free clean air into a flask This produces a known concentration of NO at the outlet of the flask A NOx instrument and an 03 instrument look at these known concentrations NOx free air usually flows into the flask that will be called a GPT flask at a flow of 25 to 4 Lmin See Figures 1 and 2 below C8urrently we are using a Tylan flow of 35 lmin 03 generator Tylan flow meter flask nnr 50 ppm NO drynte Cal tank rotameter 3Wa valvej39 onloff GPT tank air y L V3 south inlet mixing fan relay 10 lein 55 UV 03 calibrator or metabelows pump nebulizer particle size calibrator Figure1 Schematic of GPT cal system Figure 2 Picture of GPT Cal system In a complete calibration four different NO concentrations are added to the ask starting with 01 ppm and going to 06 to 08 ppm To do this you will start with an NOx ow of 001 Ilmin and an air ow of400min012 ppm NO A er a stable 39 mI e ask Typically NO is in excess over 03 so that not all ofthe NO reacts This change in NO gives the amount of No that forms and the amount of03 that had to react to give the change in NO Not a l ofthe 03 will react in the as and the amount of ozone that makes in through the ask is recorded The NO flow to the ask is then removed and the total 03 unreacted and reacted is observed The unreacted 03 is subtracted from the total 03 and this equals the calibrated change in NO 1To begin The NOx and 03 instruments in the new aerosol lab should be on for at least 2 hours Make suret e NOx meter ump is on T e old NOx metervacuum should be 26 inches ong There is a lter in back ofthe NOx meter and replacement ofthe lter is necessa a er eve 34 ex eriments lfyou are USlng tne HeWMOHllOrLaDS NOx meter currentlyltls hrokenyou need to checktne dryerontnetop oftne NOxlnstmrnent lfltdoes not navel lnch ofhlue zone ltneedsto be replaced plnk lndlcatestnattne dryer l5 Spent We are currently using an old Bendix Nox meter Figure 3 The vacuum pump should read at least 25quotHg Figure 4 Note tnere l5 a back up Monltor Lab Nox meter ln tne utlllty morn There are two components to this NOx meter They are in separate blue instrument boxes that are on the other side of the wall from the GPT cal area The one on top is the pneumatics and pump box and the other is the electronics box The directions on the meter say to power up the pneumatic box 5 minutes after the main power is turned on I don t knowwhy but this message is left over from the 1980s at the site The power cord usually is plugged in If you have to use this NOx meter there is a sampling line that run into the For the old Bendix Nox meter that we are currently using make sure the put the NO and NOx settings on the range scale of 1 ppm Do not move the span or zero settings If using the back up Monitor labs Nox meter on the pneumatic box on top see that the 03 rotameter is 3040 of full scale and the other two are 80 of full scale If you add more than 1ppm of NOx change the NOx amp N02 to 2ppm Do this for both instruments Figure 3 Old Bendix Chemilumenescent Nox meter 2 Bring up the NOx computer usually it is on but the monitor may be off it is next to the black data logger The Tylan flow controller boxes see Figure 2 should be on because they are turned on by remote switch 8 inlet mixing fan If you do not see the digital numbers on the Tylan flow meters turn on remote switch South Inlet Mixing Fan This also switches the sampling of the NOx and O3 instruments to the GPT cal flask form the Mahogany board chamber sampling manifold 3 The general procedure to calibrate the NOx meter will be to first take zero NOx and 03 readings with just NOx free air flowing through the r flask then you will create dilute mixtures of NO in lililjluiiiliii nitrogen in the GPT dilution flask NOx free air to the GPT flask can come from an airtank in the utility area called GPT air or if the tank is empty from a pump on the wall Note the air goes Figure 4 Pressure Gauge on Old through a dryer column should has at least 5 cm Bendix Chemilumenescent NOx of blue indicator and a purifil column to remove meter NOx Adjust the airflow to 35 litersmin on the Tylan airflow controller read out by adjusting the airflow rotameter in Figure 2 The Tylans read in litersmin at zero uC Beginning the calibration 5 It is very important to purge the NO regulator before conducting a cal This procedure is performed so that any air in the regulator that results during no operation does not back diffuse in the NO tank If it does 02 will slowly oxidize NO and ruin the tank Typically you may find the NO flow controller needle valve connected to the regulator and this supplies NO to the Tylan flow meter You will have to disconnect this apparatus see red male quick disconnect and the NO needle valve since this is the normal position you will typically find the system in see picture above with labeled No ow needle valve After removal replace this with another male quick disconnect that is attached to an 18quot tube that vents directly into the vertical waste pipe behind the NOx instrument To purge the regulator close the low pressure regulator outlet valve near the quick disconnect Then quickly open the main valve of the tank and close it quickly You should see the pressure on the main tank gauge of the regulator to 1000 psi Now vent the regulator by opening the regulator outlet valve When all of the pressure on the main regulator gauge goes to zero close the outlet valve and quickly open the main tank valve Repeat this 57 times Again this is very important because it purges oxygen from the regulator which can backdiffuse into the tank and slowly oxidize NO to N02 Now adjust the tank outlet pressure to 10 to 15 psi 7 The Tylan flow controller Flows It would be great if we could read the Tylans directly apply a calibration factor and then correct them for the temperature in the GPT flask to get the real flow For now we are checking our previous Tylan airflow calibration with a spot check flowquot from a calibrated flow meter If this is ok there is no need for a more rigorous bubble meter flow calibration The Tylan airflow when corrected to room airtemperatures reads about 25 low A check on the Tylan flow meter is performed by adjusting the airflow with the airflow rotameter valve on wall to 25 to 4 lmin When the flow is stable on the Tylan read out check the flow the large 496 ml soap bubble flow meter or the dry cal test meterquot Get 3 stable stopwatch times say 1236 sec 008 sec record these and the air flow from the Tylan flow readout in the spread sheet cells under the graph Tylan air flow cal on the left side of the spread sheet Also enter the temperature from the temperature sensor near the GPT flask The spread sheet will now estimate based on the last cal the predicted flow to the flask This should agree with in 5 of the soap bubble meter Now remove the air flow from the bubble meter and reattach it to the GPT flask adjust the air flow to 35min so we can get GPT zero readings with the NOx and 03 meters The NO inlet to the GPT flask should be capped and the NO flow should not be attached to the GPT flask because we will spot check its flow Check the NO Tylan flow the same way you did the airflow but use the 100 ml burette bubble flow meter This however must be done for each NO flow that is used the Tylan flow meter is not stable from day to day A note on the bubble meters make sure you get good clean bubbles that are circular Your should be level with the bubble as it crosses the line on the bubble meter at the bottom and at the top ldeally only one bubble should be in the bubble tower at a time Readings should agree to i008 sec 8 Adding NO to the GPT flask Bring up the NOxcal template excel spread sheet and give it today s date Chart readings NOx cal worksheet change grey shaded cells only and save under a different name blue cell values are calculated Tank ID National specialty gases 0051730 2 2098 Tank NO conc 505 ppm N02 082 ppm Our new tank With NOx free clean air often called zero air flowing to the GPT flask and the NOx and 03 meters sampling form the rear of the flask record the the NOx and O3 GPT zero values Note there are two places to record zero values There is a bank set of cells the other says GPT air concentrations onlyquot These should be the same but if say were to humidify the air and look at how the NOX zeros change with different humidities we could record this in the top boxes Typically we just record the GPT air concentrations only NO 0000 NOX 03 0000 0 GPT air concentrations only NO 0005 NOX 03 0002 0 The calibration can now proceed with either a one point or a multipoint calibration One point calibrations are done before each experiment To do a 4 point calibration 4 different NO flows have to be introduced to the to the flask say 001 002003 and 004min waiting 7 10 minutes between flows The airflow can be left constant Note in the spread sheet below the is a column to enter the airflow Over the years we have found the Tylan air flow to be within 5 of the soap bubble meter so typically we just take the Tylan airflow reading and correct it from STP 0 C to the temperate at the GPT flask The current version of the spread sheet reflects this To increase the NO flows open the needle valve slightly till you get the desired flow Record the raw Tylan air and NO flows and enter these into the spread sheet 3rd 1st Calibration 2nd Calibration Calibration air flow NO flow airflow NO flow NO flow air flow min seconds seconds seconds seconds seconds 15Jan 636 3783 00 64 3805 1576 646 3796 1563 643 378 1569 636 157 Soap bubble meter volumecc 49600 10 496 10 avgsec seconds s flowLmin LMin Again enter the temperature at the GPT flask during each flow change The spread sheet will compute the correct flows for air and Tank NO based on the temperature of the flask In the cells below the spread sheet calculates the NO and NOx concentrations in the GPT flask Record the the observed measured values in the appropriate cells Based on these flows the following NO and NOX conc result from N0 ppmtank X NO flowtotal flow NOppmfask Cal predicted meter Number concentration readings Calibration factors NO NOX 03 NO NOX 03 measured 1 0004 0718 0720 bkg cal factor measured 2 bkg 3 I Calfactor 3 4 l Calfactor 4 The way the NOx spread sheet computes the NO concentration in the GPT flask is to start with a mass balance on the flask That is the massunit time of NO flowing into the flask the masstime fo NO flowing out of the flask or Notank X tank ow NOGPT X tOtal owllask Say that the Tylan corrected air flow at 27 C 400 litersmin to the GPT flask and the NO corrected flow 002 Lmin at 27 C and the NO tank concentration is 5008 ppm NOGPT NOtank X NO tank ow tota oW ask 5008X 002400002 0249 ppm After you are finished with the cal enter the calibration slopes into a file called NOx cal history This way we can keep track of how the NOx meter is performing Make sure your NO predicted values do not exceed 09 ppm The NOx meter overloads beyond 1 ppm The multipoint cal spread sheet computes a regression plot in the spread sheet and will give you NO and NOx calibration lines in y mx b format 9 Calibrating 03 to the GPT flask Starting with a stable NO and NOx concentration in the range of 090 to 06 ppm record the 03 reading at the back of the flask On a one point cal the NO and NOx before the titration is keyed to the observed NO and NOx Ozone Calibration Titration meter NO NOX 03 before 0718 0720 00022 OBback A NO after of flask adding 03 0323 0707 0075 Corrected ANO Correcte ANO xcal factor d ANO ANOX The reading for 03 should be equivalent to an O3 zero Then turn on the O3 generator This is the switch on the black box above the GPT flask on the wall see picture NO should decrease and NOX should more or less stay the same Wait until NO and NOx are stable This may take 10 minutes because the UV lamp needs to warm up Remember NOx should stay stable even though NO and N02 are changing lf NOx changes by more than 5 there may be a problem with the NOx catalytic converter If we got a good NO and NOx cal first even if NOx changes slightly due to flow changes we can get backto the numbers The assumption here is that the catalyst is working correctly It is important to be taking 03 data at this point because we need to assess the amount of 03 which does not react in the flask and makes it to the outlet of the flask before reaction 03 flask outlet When NO and N02 look reasonably stable after adding 03 to the GPT flask record these values A delta NO is calculated next by subtracting uncorrected NO both before and after 03 is added The correct ANO is estimated by multiplying by the NO cal factor slope 10 After N02 and NO are stable remove the NO flow from the flask and shut off the NO tank this is very important Also quickly cap the GPT flask and take 03 data You will need to take at least 68 readings Record the raw 03 value remove NO flow to flask predicted 03 corrected delta NO NO Correcte corrected d flow 03 cal factor delta raw meter 03 factor correct NOOS03flask NO 03 meter OSOstk NO ii 1 flask factor 0409 Recall that the amount of 03 which made it through the flask is entered in or near cell D59 This will be subtracted from the total 03 measured when the NO tank flow is removed from the flask and this difference is equal to the corrected NO The corrected A NO conc is repeated in 03 calibration calculation above Lastly when we removed NO from the flask the total flow to the flask flow decreased by a small amount equal to the NO tank flow This must is taken into account since the NO tank flow is no longer diluting the flask It is computed by taking the original total flow and dividing itjust by the airflow This number is entered in cell for NO flow factorquot and then factored in computing the corrected meter 03 Finally this corrected 03 is compared with the meter 03 and a calibration correction factor is estimated to be 124 in the above calculations Record the 03 cal factor in the NOx cal history file Turn off the O3 generator Check that the NO tank is off 101696 rk rev 33197 kl rev 32304 rk rev aug 14 2008 rk Theory Distribution of analytes between phases The distribution of analytes between phases can often be described quite simply An analyte is in equilibrium between the two phases A mobile A stationary The equilibrium constant K is termed the partition coef cient defined as the molar concentration of analyte in the stationary phase divided by the molar concentration of the analyte in the mobile phase The time between sample injection and an analyte peak reaching a detector at the end of the column is termed the retention time tR Each analyte in a sample will have a different retention time The time taken for the mobile phase to pass through the column is called tM Detector signal E A term called the retention factor k is often used to describe the migration rate of an analyte on a column You may also find it called the capacity factor The retention factor for analyte A is defined as 39AZfRfMfM tR and tM are easily obtained from a chromatogram When an analytes retention factor is less than one elution is so fast that accurate determination of the retention time is very difficult High retention factors greater than 20 mean that elution takes a very long time Ideally the retention factor for an analyte is between one and five We define a quantity called the selectivity factor oc which describes the separation of two species A and B on the column OCZkIBk39A When calculating the selectivity factor species A elutes faster than species B The selectivity factor is always greater than one Band broadening and column ef ciency To obtain optimal separations sharp symmetrical chromatographic peaks must be obtained This means that band broadening must be limited It is also beneficial to measure the efficiency of the column The Theoretical Plate Model of Chromatography The plate model supposes that the chromatographic column is contains a large number of separate layers called theoretical plates Separate equilibrations of the sample between the stationary and mobile phase occur in these quotplatesquot The analyte moves down the column by transfer of equilibrated mobile phase from one plate to the next lt The commn r Theoretica mate It is important to remember that the plates do not really exist they are a figment of the imagination that helps us understand the processes at work in the columnThey also serve as a way of measuring column efficiency either by stating the number of theoretical plates in a column N the more plates the better or by stating the plate height the Height Equivalent to a Theoretical Plate the smaller the better If the length of the column is L then the HETP is HETPLN The number of theoretical plates that a real column possesses can be found by examining a chromatographic peak after elution 39339 1 in J quotJ I h quotH U U 2 1 E where W12 is the peak Width at halfheight As can be seen from this equation columns behave as if they have different numbers of plates for different solutes in a mixture The Rate Theory of Chromatography A more realistic description of the processes at work inside a column takes account of the time taken for the solute to equilibrate between the stationary and mobile phase unlike the plate model which assumes that equilibration is in nitely fast The resulting band shape of a chromatographic peak is therefore affected by the rate of elution It is also affected by the different paths available to solute molecules as they travel between particles of stationary phase If we consider the various mechanisms which contribute to band broadening we arrive at the Van Deemter equation for plate height HETPABuCu where u is the average velocity of the mobile phase A B and C are factors which contribute to band broadening A Eddy diffusion The mobile phase moves through the column which is packed with stationary phase Solute molecules will take different paths through the stationary phase at random This will cause broadening of the solute band because different paths are of different lengths B Longitudinal diffusion The concentration of analyte is less at the edges of the band than at the center Analyte diffuses out om the center to the edges This causes band broadening If the velocity of the mobile phase is high then the analyte spends less time on the column which decreases the effects of longitudinal diffusion C Resistance to mass transfer The analyte takes a certain amount of time to equilibrate between the stationary and mobile phase If the velocity of the mobile phase is high and the analyte has a strong af nity for the stationary phase then the analyte in the mobile phase will move ahead of the analyte in the stationary phase The band of analyte is broadened The higher the velocity of mobile phase the worse the broadening becomes Van Deemter plots A plot of plate height vs average linear velocity of mobile phase Atypical Van Deemter plot Optimum velumly Plate helght Mlmmum plate mam Mums phase velumty Such plow are of considerable use in determining the optimum mobile phase ow rate Resolution Although the selectivity factor on describes the separation of band centers it does not take into account peak widths Another measure of how well species have been separated is provided by measurement of the resolution The resolution of two species A and B is defined as 7 ay542911 WNWs R Baseline resolution is achieved whenR 15 It is useful to relate the resolution to the number of plates in the column the selectivity factor and the retention factors of the two solutes R 7 4 7 kg To obtain high resolution the three terms must be maximised An increase in N the number of theoretical plates by lengthening the colurnn leads to an increase in retention time and increased band broadening which may not be desirable Instead to increase the number of plates the height equivalent to a theoretical plate can be reduced by reducing the size of the stationary phase particles It is often found that by controlling the capacity factor k separations can be greatly improved This can be achieved by the temperature in Gas Chromatography or the composition of the mobile phase in Liquid Chromatography The selectivity factor on can also be manipulated to improve separations When on is close to unity optimising k and increasing N is not sufficient to give good separation in a reasonable time In these cases k is optimised first and then on is increased by one of the following procedures