Calculating Alkalinity

Transcription

1 Cacuating Akainity Overview How to cacuate acid:anion ratios at varying ionic strengths Use the Mixing Water cacuation as a ph titration apparatus Understand how weak acid chemistry affects akainity Compare true bicarbonate and carbonate concentrations to carbonate akainity Investigating the Components that Create Akainity The akainity measurement is used to compute carbonate scae tendencies and ph buffer capacity. Bicarbonate, and to a esser degree, borates, organic acids and bisufides, provide akainity to produced water. Therefore, this chapter is an introduction to quantifying akainity beyond the standard contributions from HCO 3, CO 3 2 and OH. Carbonate Akainity Contributions The Standard Methods for the Examination of Water and Wastewater codified the measurements to determine akainity. In its second edition (1912), the book inked akainity and tota hardness to CaCO 3 scae (an interesting side note, in the year of pubication, oive oi-derived castie soap was used to measure tota water hardness instead of inductivey couped pasma spectrometry). Measurements reated to Lacmoid, Erythrosine, Methy orange, siver nitrate and phenophthaein, were a designed to measure carbonate and hydroxide akainity. The contribution of non- carbonate and non- hydroxide buffers, ike organic acids, do not have supporting text. Oified water anayses continue to use the origina experimenta design with the same endpoint, despite the measurement does not capture the produced water species compexity. Akainity equations are a series of chemica reactions with measureabe endpoints, described beow: HCO H + = CO 2 + H 2 O considerd compete by ph < 4.5 (25 C) CO H + = HCO 3 considerd compete by ph < 8.5 (25 C) OH + H + = H 2 O considerd compete by ph < 10 (25 C) Thus, a simpe definition for carbonate akainity is: Carbonate Akainity = Tota H + required to convert a CO 3 2, HCO 3, and OH to CO 2 = H + (from CO 3 2 ) + H + (from HCO 3 and neutraized CO 3 2 ) + H + (from OH ) = tota acid added to bring the ph to < 4.5 Lasty, a mass baance equation for carbonate: = 2CO HCO 3 + OH SSC: Cacuating Akainity 10-1

2 Carbonate Tota = HCO 3 + CO 3 2 A Bjerrum pot, in which carbonate concentration is fixed, shows the reative concentration of each species as functions of ph (aso shown is the reative OH - concentration): The cacuations provide measurabe hydroxide, carbonate and bicarbonate concentrations at a reative ph: free hydroxide (OH-) exists above 10 ph; carbonate exists above 8.5 ph; and bicarbonate exists between 4.5 and 8.5 ph. An akainity titration procedure is as foows: 1. Start with a known sampe voume 2. Add two indicators - the first activates at 8.5 ph or higher and the second is active beow 4.5 ph 3. Titrate the sampe with a strong acid of known composition When the first indicator deactivates (the ph drops beow 8.5), the amount of titrant added is the carbonate akainity and hydroxide akainity point. The foowing equation converts the acid voume to the carbonate akainity. CO 3 Ak ( mg ) as HCO 3 = Acid voume used to 8.5 ph () Acid Conc. ( moes ) ( mg HCO 3 1 ) moe Sampe Vo () 4. The sampe is titrated further unti it unti the second indicator appears (ph<4.5). This is the bicarbonate akainity endpoint. This information is used as foows: HCO 3 Ak ( mg ) as HCO 3 = Vo Acid for Bicarbonate Ak Titration () Acid Conc. ( moes ) ( mg HCO 3 ) moe 1 Sampe Vo () The two titrations combine to cacuate the tota akainity SSC: Cacuating Akainity 10-2

3 Tota Ak ( mg ) as HCO 3 = Vo Acid for Carbonate Ak Titration () + Vo Acid for Biarbonate Ak Titration () Acid Conc. ( moes ) ( mg HCO 3 1 ) moe Sampe Vo () OH Ak ( mg ) as HCO 3 = CO 3 Ak ( mg ) HCO 3 Ak ( mg ) 5. In the absence of carbonates or in addition to carbonate, the OH akainity can aso be determined from the difference between the carbonate and bicarbonate akainity and is ony reevant above 9.5 or 10pH (OH - > M or ~0.5ppm). Ak OH = 2 Ak 8.5pH EP Ak 4.5pH EP If the vaue is negative, then there is no hydroxide akainity. Hydroxide akainity is not common in production waters because the ph is never that high. It is present in driing and akaine injection fuids. Probem Statement Oified waters do not conform chemicay, to the fresh water used in the origina akainity method. Organic acids, borates, phosphates, sufides and other proton-accepting species produce akainity. Consequenty, the standard akainity titration cannot be used to measure the bicarbonate/carbonate concentration. We wi evauate this in the foowing exampes. Task 1: Quantifying Akainity of Certain Anions You wi compute the akainity of 100 mg/ of the foowing anions: HCO 3-, CO 3-2, OH -, CH 3CO 2-, B(OH) 4-, SO 4-2, C - and HS. The counter-ion wi be sodium making the system basic. There wi be no additiona sat (sainity) added the first set of cacuations. This wi aow you to quantify akainity at near-idea activities. You wi cacuate a second time in the presence of 1M NaC. You wi concude this section by producing a quantitative tabe showing the contribution to akainity as a function of species and sainity. We wi be using C and atm units for Temperature and Pressure so pease adjust accordingy. Create a new fie caed Akainity Add a new brine anaysis Rename it Akainity Bicarbonate Seect the Design tab Find HCO3-1 in the Anions grid and give it a vaue of 100 mg/ Seect the Reconcie tab Cacuate at defaut reconciiation conditions (Equiibrium Cacuation) Document the ph and akainity SSC: Cacuating Akainity 10-3

4 The cacuated ph is The cacuated akainity is mg/ and is entirey the bicarbonate akainity. The 0.4 mg/ addition is due to the ph endpoint and minor activity coefficient effects. The next three steps wi make it easier for us to perform titrations ater. Aternativey, we can continue working in the same Akainity fie and repace the anions as we go aong. Subsequent instructions wi be appicabe for both approaches. In the Navigator pane, right-mouse cick Akainity and press copy Right-mouse cick the Streams icon, then seect paste, repeating this 7 times The software designates the new streams as Akainity -1, Akainity-2, and so on. Rename the objects so that the anion they contain wi be easiy identifiabe: Akainity- Bicarbonate, Akainity-Carbonate, Akainity-Hydroxide, Akainity-Acetate, Akainity- Borate, Akainity-Sufate, Akainity-Choride, and Akainity-Bisufide Carbonate Seect the Akainity-Carbonate brine anaysis stream SSC: Cacuating Akainity 10-4

5 Remove the 100 mg/ HCO3-1 and add 100 mg/ CO3-2 We can either type in CO 3-2 in an empty ce, or seect the Expanded Tempate to ocate it in the Anion grid. Remember to switch back to the Standard tempate for a quicker cacuation. Seect the Reconcie tab and recacuate The akainity is mg/ as HCO 3 twice the CO 3 2 concentration added, and is consistent with the above equation. The HCO 3 formua weight is g/moe and CO 3 2 is g/moe a moe ratio of 1.7%. Two factors contribute to the akainity increase. The first is that CO 3 2 accepts two H + ions one above 8.5 ph and one above 4.5pH. The second is that 1.7% additiona moes of carbonate are added because of the weight difference. Thus, the theoretica akainity 100 mg/ akainity as HCO 3 *2 * 1.7% = mg/ akainity as HCO 3. The computed vaue is Note the sight bias forming in the resuts. Instead of exacty mg/, the vaue is 0.50 mg/ higher. This difference is due to non-idea effects and the precise ph choice. The equations provided above are for idea conditions. However, we are we aware that there are non-idea effects that must be considered. Hydroxide Seect the origina Akainity or the Hydroxide brine anaysis stream Remove the HCO 3 - concentration and add 100 mg/ OH - SSC: Cacuating Akainity 10-5

6 Seect the Reconcie tab and recacuate The computed akainity is 360 mg/ as HCO 3-1. The formua weight of OH is g/moe compared with g/moe for HCO 3 1, so times more moes of OH is added compared with HCO 3. Therefore, mg as HCO 3 = mg as HCO Compete OH neutraization actuay happens at about 10 ph, not 4.5 ph, which is the experimenta endpoint. Observing the neutraization effect requires a titration curve. Task 2: Non-Carbonate Akainity Contributions We wi see compications arising when other species contribute to akainity because they affect the direct use of the akainity vaue in the carbonate/scae tendency equation. Contributing species incude borate, phosphate, formate, acetate, propanate, bisufide, and any anion that wi accept H + ion above 4.5 ph. Acetate Seect the origina Akainity or the Acetate brine anaysis stream Zero out any existing concentrations and add 100 mg/ acetate C2H3O2-1 Seect the Reconcie tab and recacuate The computed akainity is mg as HCO 3. The acetate formua weight is g/moe, cose to HCO 3 (61.02 g/moe). So, if a the acetate contributes to akainity, then 100 mg/ acetate shoud be cose to that for 100 mg/ HCO 3 as shown in the foowing equation: mg HCO 3 mo mg Acetate 100 = mg as HCO mg Acetate mo The theoretica akainity contributed by acetate is mg as HCO 3. This means that = 67% of the acetate ion accepted an H + at 4.5 ph (33% remained as the C 2 H 3 O 2 ion). Acetate concentrations therefore have significant impact on tota akainity. SSC: Cacuating Akainity 10-6

7 Borate Seect the origina Akainity or the Borate brine anaysis stream Remove any existing concentrations and add 100 mg/ borate, B(OH)4-1 Seect the Reconcie tab and recacuate The borate contribution is mg as HCO 3 for 100 mg/ added. If 100% borate contributed to akainity the maximum vaue woud be: mg HCO 3 mo mg B(OH) 4 mo 100 mg B(OH) 4 = mg as HCO 3 The theoretica akainity is sighty ower than the computed amount (we wi investigate this next). Thus, borate appears to be significant contributor to akainity. However, the ph is high, Borate is generay neutraized to boric acid by ~7 ph. Consequenty, ony a fraction of the tota boron in produced water is in anion form. Most is found in the acidic form H 3 BO 3. Sufate Seect the origina Akainity or the Sufate brine anaysis stream Remove any existing concentrations and add 100 mg/ sufate (SO 4 2 ) SSC: Cacuating Akainity 10-7

8 Seect the Reconcie tab and recacuate The sufate contribution is 2.2 mg/ as HCO 3 - for 100 mg/ SO 4-2 added. If 100% sufate contributed to akainity the maximum vaue woud be: mg HCO 3 mo mg SO mg SO 4 2 = mg as HCO 3 mo 2 Sufate has negigibe effects on akainity. This is not surprising, since SO 4 is a sat of an acidic anion HSO 4. To reach the theoretica contribution of 63.5 mg as HCO 3, the endpoint ph woud be near 0.3. Choride Seect the origina Akainity or the Choride brine anaysis stream Remove any existing concentration and add 100 mg/ choride (C-) then Seect the Reconcie tab and recacuate The choride contribution is 2.04 mg/ as bicarbonate for 100 mg/ C - added. If 100% choride contributed to akainity the maximum vaue woud be: SSC: Cacuating Akainity 10-8

9 61020 mg HCO 3 mo mg C 100 = mg as HCO mg C mo Choride has negigibe effects on akainity for the same reason as sufate. Both are conjugate bases of strong acids. To reach the theoretica contribution of mg as HCO 3, the endpoint ph woud be about 5. Bisufide Seect the origina Akainity or the Bisufide brine anaysis stream Remove any existing concentration and add 100 mg/ bisufide (HS ) Seect the Reconcie tab and recacuate The cacuated bisufide contribution is mg as HCO 3 for 100 mg/ added. If 100% sufate contributed to akainity the maximum vaue woud be: mg HCO 3 mo mg HS 100 = mg as HCO mg HS mo The theoretica bisufide ion akainity is mg as HCO 3. It has a significant impact on akainity. Bisufide concentrations however are generay ow in produced water. Therefore, the absoute effects are sma. SSC: Cacuating Akainity 10-9

10 A summary of a the cacuation resuts are shown in the tabe beow. 100 mg/ of the foowing ion Initia ph Akainity as mg/ HCO3 HCO CO OH Acetate B(OH) SO C HS Task 3: Akainity Titration Curves The akainity endpoint provides a snapshot of fuid chemistry. A compete titration curve shows the system behavior across the ph profie and therefore the buffering region of different bases. In this section, we wi create akainity titration curves for 100 mg/ of HCO 3-, CO 3-2, OH -, Acetate -, and B(OH) 4-. The first two sections beow are aternatives. Cients can either use OLI Studio: ScaeChem objects to create the 1N HCL reagent or use the Stream Anayzer modue. In some cases, due to a bug in the software, the first aternative may not work. Task 3-1 Create the 1N HC Reagent with Studio ScaeChem Add a new brine anaysis Name it 1 N HC Type HCL in the neutras section with 36,453 mg/ Seect the show non-zero box Seect the Reconcie tab then cacuate It shoud not be surprising that the reagent ph is amost zero. HC is a strong acid, and thus the H-C bond is weak, resuting in near compete dissociation. Thus, there is 1 moe H + in soution or zero ph. SSC: Cacuating Akainity 10-10

11 Save the fie Task 3-2 Create the 1N HC Reagent with Stream Anayzer Seect the Goba Streams icon in the Navigator pane Doube-cick the Add Stream icon in the Actions pane Press the <F2> key and change the name to 1N HCL Seect the Units Manager then use the Quickist button to pick Metric, concetration Press OK then enter HC as an infow with a concentration 36,453 mg/l Task 3-3 Create an Akainity Titration Once we compete either of the above methods for creating the HC reagent stream, we can proceed with the foowing tasks. Add a Mixing Water object Rename it Akainity Titrator Change the Brine Voumes to m (per day) by cicking a bue-coored unit and opening the Units Manager Seect the bottom right corner of the Name ce and pick 1 N HCL as the first brine Remember that first brine is the stream that the Mixer wi adjust. Set the tota voume to 1000 ml/day The 1L voume is the tota iquid voume (sum of 1 st and 2 nd brine). In this case, the 1L wi be primariy the akainity iquid that we wi enter next. The 1 N-HC reagent wi be added to the Akainity iquid, but its voume wi be reativey sma. It is an imperfect exampe, but it serves to show another appication of the SSC: Cacuating Akainity 10-11

12 Mixing too. The actua fow rate is unimportant, but for reaism, we wi use a voume simiar to aboratory conditions. Task 3-4 Bicarbonate Titration Curve Seect the bottom right corner of the Name ce in the Second Brine row and pick the Akainity Bicarbonate (the origina Akainity brine) as the second brine Seect the bue, vertica Conditions tab Type 2 in the first coumn and first row then press tab and type 0 in the second row Change the Auto Steps amount to 20 then seect the Auto Step button The software wi add the HC at 0.1m increments up to 2 m. It wi aso remove 0.1 m of Akainity-HCO3 voume at each step to maintain the 1000 m voume. Consequenty, the titration we are doing is not 100% correct. In an actua titration, the sampe fuid voume is fixed at 100 m. Fortunatey, the HC reagent is concentrated, and the voume added is sma. This imits the dispacement error to 0.2% SSC: Cacuating Akainity 10-12

13 Press the Cacuate button Seect the Pot tab and seect the Variabes button Remove a existing pot variabes in the Y1 Axis by highight the Y1 axis header then seect the << arrow button Expand the Aqueous category and add HCO3-1 to the Y1 axis Expand the Additiona Stream Parameters category and add ph-aqueous to the Y2 axis Press OK and view the pot SSC: Cacuating Akainity 10-13

14 The ph curve is the typica bicarbonate-buffering curve. The sope of the curve is shaow between ph 7 and 5.5, where bicarbonate buffering is strong. Beow 5.5 ph, the curve drops sharpy and has its steepest sope at about 4.5 ph. Cick the View Data button Copy the data to a spreadsheet program by hoding down the eft arrow, highighting a the points and pressing <Ctr+C> For the foowing titrations, we can either add a new Mixing Water object or work from the existing object. The instructions beow assume the atter. Task 3-5 Carbonate Titration Curve Return to the Design >> Inets screen Repace the second brine with the Akainity Carbonate stream Seect the Conditions tab and change the 1N HC fow from 2 m to 4 m (top ce) We wi need twice as much acid to titrate the CO 3-2 Make sure Auto steps are at 20 and seect the Auto Step button then cacuate SSC: Cacuating Akainity 10-14

15 Seect the Pot tab then seect the curves button Add CO3-2 from the Aqueous categories to the Y1 Axis Press OK and view the Pot Cick on the View Data button then copy data to a spreadsheet program. Save the fie Task 3-6 Hydroxide Titration Curve Return to the Design>Inets tabs and repace the Carbonate stream with Hydroxide Seect the Conditions, tab then overwrite the first ce with 7 m Press the auto step button, press cacuate then pot ph and OH - (mg/) SSC: Cacuating Akainity 10-15

16 Cacuate Remove the carbonates Add OH- from the Aqueous section to the Y1 axis Seect the view data button and copy the data to a spreadsheet and Save the fie Task 3-7 Acetate Titration Curve Repeat the same procedure for Acetate Determine what the maximum HC voume needs to be (argey through tria and error) Aways reset the auto steps to 20 Press cacuate then pot ph on the Y2 Axis and acetate ion (C2H3O2-1)to the Y1 axis SSC: Cacuating Akainity 10-16

17 Save the fie Cick on the View Data button then copy the data to a spreadsheet program Task 3-8 Borate Titration Curve Redo the cacuation for Borates Decide upon which species are important to pot Cick on the View Data button and copy data to a spreadsheet program Save the fie Task 3-9 Sufate Titration Curve Repeat for Sufate SSC: Cacuating Akainity 10-17

18 The sufate titration cacuation was taken we beyond the akainity endpoint in order to show the equiibrium between the SO 4-2 and HSO 4 - species. At ~2 ph, SO 4-2 and HSO 4- have roughy the same concentration: this is a sufate buffering region. Cick on the View Data button and copy the data to a spreadsheet program Save the fie Task 3-10 Bisufide Titration Curve Change the mixer to accept the bisufide akainity as the 2 nd brine either by adding the Akainity Bisufide brine or by redoing the akainity brine for bisufide Set the maximum fow in the conditions tab to 4 e - 6 Set the steps to 20 then press Auto Step button and cacuate Pot ph on the Y2 Axis and bisufide (H2S) ion on the Y1 axis Cick on the View Data button copy data to a spreadsheet program Task 3-11 Interpreting the Titration Data SSC: Cacuating Akainity 10-18

19 Concentration (mg/) ph Move to the spreadsheet pot the ph vs. Ratio of brines ph (HCO3-1) ph (CO3-2) ph (OH-) ph (acetate) ph (BOH4-) This pot shows the buffering (ph) regions and the tota buffering capacity of each weak acid. The highest buffering capacity, though not reevant to produced waters, is OH -. It s reativey ow moecuar weight (17 g/mo) resuts in more buffering per concentration (a are fixed at 100 mg/). Its buffering region ends at about 10.5 ph (extent of shaow sope). At this ph virtuay a the OH is consumed by the HC titrant. Consequenty, at typica produced water ph (between 5 and 8) no OH - buffering occurs. Pot the species concentrations vs. ph SO4-2 Acetate-1 HCO3-1 HC ratio BOH4-1 CO3-2 OH ph HCO3-1 CO3-2 OH- The pot above shows the concentration impact in greater detai. It represents the concentration of free ion vs. ph for the different buffers. The red vertica ine to the far eft is the ph titration end point. The shaded vertica section to the right is the typica produced water ph as measured in the aboratory. From this diagram we can see that very itte OH - exists in produced water between 7 and 8 ph. The carbonate ion is dibasic (accepts two H + ) and therefore has two buffering regions and endpoints. The first buffering zone exists from 10.7 to 9 ph, and the sharp ph drop refects CO 3-2 depetion from soution. Few produced waters have ph above 8, and therefore CO 3-2 akainity is rarey observed. The second buffering region of CO 3-2 is associated with protonating HCO 3-, and this second zone paraes exacty the SSC: Cacuating Akainity 10-19

20 100 m/ HCO 3-1 titration curve. This second curve is the main akainity component in produced waters. Its existence is due to the continuous production of CaCO 3 (s) in seawater. The acetate buffering zone is between about 5.7 and 3.5 (end of curve). If the titration were extended, then the infection point ~2.7 ph woud have been observed. This buffering zone straddes the akainity endpoint, which is why it creates compications when measuring tota akainity without an organic acid content. Organic acids are products of organics/kerogen catagenesis, which creates the fossi fue. Therefore, it is commony observed in produced waters. Lasty, the borates buffering region is between 10 and 8 ph with an end point at about 6 ph. Borates exist at ph s higher than the typica produced water, but when borate concentrations are high and the aboratory measured ph approaches or exceeds 8, then this base wi affect tota akainity. Borates exist in seawater and deposit in the interstices of sediments. They are aso fairy soube (ike C and Na) and are generay found in produced waters. The tabe beow contains the ph and concentration data from the cacuations we ran and potted above. The shaded ces represent the regions in the titration curves that woud contribute to a produced water akainity. At the bottom of the tabe is a % range vaue. This vaue represents the fraction of the species concentration that woud contribute to the akainity. HCO3-1 CO3-2 OH- Acetate-1 BOH4-1 SO4-2 HSpH mg/ ph mg/ ph mg/ ph mg/ ph mg/ ph mg/ ph mg/ % in range Consider the foowing with this tabe: Anions are now being measured using ion chromatography. How does the akainity vaue measured reate to these ion chromatography measured ions. SSC: Cacuating Akainity 10-20

21 Task 4 Effects of Temperature and Sainity on Carbonate Akainity Return to the origina Akainity (or Akainity-Bicarbonate) brine Add mg/ Na + and mg/ C - Return to the Akainity Titrator object and seect the Inets tab Change the second brine to the new Akainity-Bicarbonate brine Set the Max HC to 2m and press the Auto Steps Cacuate, Seect the Pot tab, then the Variabes button Remove a the variabes from the Y1 and Y2 axes Add ph-aqueous from the Additiona Stream Parameters section the Y1 Axis Copy the data to the Spreadsheet and pot origina HCO 3 - titration with this one Cick on the Akainity Titration s Inets tab and change the temperature to 75C Cacuate Pot the ph vs. HC ratio for the three 100 mg/ HCO 3 - cacuations (25C, 25C & 1N NaC, and 75C & 1N NaC). SSC: Cacuating Akainity 10-21

22 ph ph (HCO3-1) ph (HCO3-1), 1 M NaC ph (HCO3-1), 1 M NaC, 75C HC ratio Consider the foowing when viewing the pot: Under idea conditions (25C & 0 M NaC) the titration endpoint occurs at ~ HC ratio (1.6 m HC per 1000 m sampe). At more reaistic conditions (1m NaC) the endpoint occurs at ~ HC ratio. This is a difference of ~3%. Task 5 Effects of Temperature and Sainity on Acetate Akainity Return to the Akainity brine Keep the mg/ Na + and mg/ C - erase the HCO3-1 vaue and add 100 mg/ acetate -1 Baance and cacuate Return to the Akainity Titration stream and set the maximum HC ratio to Recacuate to create the foowing ph vs. HC ratio pot Commented [JK1]: Baance? SSC: Cacuating Akainity 10-22

23 ph Consider the foowing when viewing the pot: Under idea conditions (25C & 0 M NaC) the titration endpoint occurs at ~ HC ratio (1.1 m HC per 1000 m sampe). At more reaistic conditions (1m NaC) the endpoint occurs at ~ HC ratio (0.75 m HC per 1000 m sampe). A difference of ~32%! Summary The purpose of this chapter was to present the quantitative chemistry of akainity and to expose some additiona cacuation features within ScaeChem. From this, we can see that additiona measurements for organic acids, bisufides, and borates can be important when considering carbonate scae risks. Further Reading ph (acetate) ph (acetate-1), 1 M NaC ph (acetate-1), 1 m NaC, 75C HC ratio SPE Akainity in Oified Waters. What akainity is and how it is measured B. Kaasa and T Ostvod SPE internationa symposium on Oified Chemistry. Houston TX. James Drever The Geochemistry of Natura Waters, Chapter 4: The Carbonate System and ph contro. Prentice Ha. SSC: Cacuating Akainity 10-23