UOP 209-2000 ALKALINITY SULFIDE AND MERCAPTIDE ANALYSES OF USED REFINERY CAUSTIC SOLUTIONS.pdf

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1、 COPYRIGHT 1969, 1976, 1983, 1999, 2000 UOP LLCALL RIGHTS RESERVEDUOP Methods are available through ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken PA 19428-2959,United States. The Methods may be obtained through the ASTM website, www.astm.org, or by contacting Customer Ser

2、vice atserviceastm.org, 610.832.9555 FAX, or 610.832.9585 PHONE.ALKALINITY, SULFIDE AND MERCAPTIDEANALYSES OF USED REFINERY CAUSTIC SOLUTIONSUOP Method 209-00SCOPEThis method is for the analysis of used refinery caustic solutions. Part A is for determining the strongbase, weak base (spent caustic) a

3、nd total alkalinity. Part B is for determining the concentration of sulfurpresent as sodium sulfide and sodium mercaptide. The data generated using this method are consideredquantitative over the concentration ranges generally encountered in these types of refinery caustic solutions.Refinery caustic

4、 samples are inherently unstable, and must be analyzed as soon as possible after they aredrawn.Part A, a potentiometric titration, is an accurate procedure for use in distinguishing between strong bases,i.e., sodium phenolate and sodium hydroxide. The APPENDIX contains a procedure, Caustic SolutionA

5、nalysis - Double Indicator, which can be used as a convenient, although not rigorous, test for the plantcontrol of refinery caustic solutions used to remove hydrogen sulfide, mercaptans and certain other weakacids from petroleum products.OUTLINE OF METHODPart A - Determination of Alkalinity of Used

6、Refinery Caustic SolutionsThe strong bases, weak bases, and total alkalinity are determined by titrating with standard acid using apotentiometric procedure. The titration of the strong bases to an endpoint in a pH range from 9 to 6 includessodium hydroxide, the first endpoint of sodium carbonate, th

7、e first endpoint of sodium sulfide, sodium alkylmercaptides and sodium phenolates (see Note 1). The titration of the weak bases, from a pH of about 6 to anendpoint in a pH range from 4 to 3, includes sodium bicarbonate, sodium bisulfide, sodium naphthenatesand sodium thiophenolates (sodium aryl merc

8、aptides), see Note 2. The salts of strong acids, such as sodiumchloride, sodium sulfate, sodium thiosulfate and sodium alkane sulfonates, are not basic and do not titrate(see Note 7).IT IS THE USERS RESPONSIBILITY TO ESTABLISH APPROPRIATE PRECAUTIONARY PRACTICES AND TODETERMINE THE APPLICABILITY OF

9、REGULATORY LIMITATIONS PRIOR TO USE. EFFECTIVE HEALTH ANDSAFETY PRACTICES ARE TO BE FOLLOWED WHEN UTILIZING THIS PROCEDURE. FAILURE TO UTILIZE THISPROCEDURE IN THE MANNER PRESCRIBED HEREIN CAN BE HAZARDOUS. MATERIAL SAFETY DATA SHEETS(MSDS) OR EXPERIMENTAL MATERIAL SAFETY DATA SHEETS (EMSDS) FOR ALL

10、 OF THE MATERIALS USED INTHIS PROCEDURE SHOULD BE REVIEWED FOR SELECTION OF THE APPROPRIATE PERSONAL PROTECTIONEQUIPMENT (PPE).2 of 16209-00Part B - Determination of Mercaptide and Sulfide in Caustic SolutionsThe sample is dissolved in a titration solvent containing equal volumes of 2-propanol and a

11、queoussodium hydroxide, with 1% by volume of concentrated ammonium hydroxide. The solution is titratedpotentiometrically with standard alcoholic silver nitrate using a combination glass/silver electrode. Theconcentration of sulfur present as sodium sulfide and sodium mercaptide is calculated as mass

12、-% sulfur.Caustic solutions containing high levels of mercaptide and low levels of sulfide are titrated separately forthe two components. A sample is titrated to the mercaptide endpoint, and a larger sample is titrated only tothe sulfide endpoint. The mercaptide concentration is determined by differ

13、ence. If the ratio of sulfide sulfurto mercaptide sulfur is greater than 10:1, then the mercaptide result is considered to be qualitative.DEFINITIONSStrong Base1. When only one break in the titration curve is present in the pH range from 9 to 6, this break will beconsidered the endpoint of all stron

14、g bases present (see Fig. 2, Curve A).2. When two breaks are present in the titration curve, the first break will be considered the endpoint ofall the strong bases present (see Fig. 2, Curve C and Fig. 3, Curves A and B).3. When three breaks are present in the titration curve, the second break will

15、be considered the endpointof all the strong bases present (see Fig. 1, Break B; Fig. 2, Curves B and D; and Fig. 3, Curve C).Weak Bases1. When only two breaks are present, the second break in the titration curve will be considered theendpoint of all the weak bases present (see Fig. 2, Curve C and Fi

16、g. 3, Curves A and B).2. When three breaks are present, the third break in the titration curve will be considered the endpoint ofall the weak bases present (see Fig. 1, Break C; Fig. 2, Curves B and D; and Fig. 3, Curve C).Total Alkalinity1. When only two breaks are present, the second break in the

17、titration curve will be considered theendpoint of all the bases present (see Fig. 2, Curve C and Fig. 3, Curves A and B).2. When three breaks are present, the third break in the titration curve will be considered the endpoint ofall bases present (see Fig. 1, Break C; Fig. 2, Curves B and D; and Fig.

18、 3, Curve C).Percent SpentThe expression “% Spent” is an approximate but useful measure of the condition of a refinery causticused to remove weak acids.()Weak Base% Spent 100Total Base= (1)or3 of 16209-00()()()Total Base - Strong Base% Spent 100Total Base= (2)APPARATUSReferences to catalog numbers a

19、nd suppliers are included as a convenience to the method user. Othersuppliers may be used.Balance, capacity 200 g, readability 0.1-mgBeaker, electrolytic, tall form, 300-mL, Fisher Scientific, Cat. No. 10-310-8Cylinder, graduated, Class B, 100-mL, Fisher Scientific, Cat. No. 08-549-5EDesiccator, Fis

20、her Scientific, Cat. No. 08-631AElectrode, combination glass, pH 0-14, Brinkmann Instruments, Cat. No. 20-91-050-0 (see Note 8)Electrode, combination glass/silver Titrode, Brinkmann Instruments, Cat. No. 20-94-850-7. The electrodeshould be dedicated to sulfur analysis.Flask, Erlenmeyer, 500-mL, Fish

21、er Scientific, Cat. No. 10-040HFlasks, volumetric, Class A, 250- and 500-mL, Fisher Scientific, Cat. Nos. 10-211E and -211F,respectivelyOven, drying, suitable for operation at 110oCPipets, volumetric, Class A, 5-, 10-, 25- and 50-mL, Fisher Scientific, Cat. Nos. 13-650-2F, -2L, -2P and-2S, respectiv

22、elyRegulator, nitrogen, two-stage, high purity, Matheson Gas Products, Cat. No. 3122-580Scotchbrite abrasive cleaning pads, or equivalent, local supplyStirring bars, magnetic, Teflon-covered, Fisher Scientific, Cat. No. 14-511-94Titrator, potentiometric, recording, 2000-mV range, 1-mV resolution wit

23、h a dispenser having a volumereadout of 0.00 to 20.00 mL and 0.01% resolution, Brinkmann Instruments, Cat. No. 20-87-254-3REAGENTS AND MATERIALSAll reagents shall conform to the specifications established by the Committee on Analytical Reagents ofthe American Chemical Society, when such specificatio

24、ns exist, unless otherwise specified. References towater mean deionized or distilled water. Unqualified references to solutions mean aqueous solutions.References to catalog numbers and suppliers are included as a convenience to the method user. Othersuppliers may be used.4 of 16209-00Ammonium hydrox

25、ide, concentrated, Fisher Scientific, Cat. No. A669-212Buffer solutions, pH 4, 7 and 10, color-coded, Fisher Scientific, Cat. Nos. SB101-500, SB107-500 andSB115-500, respectivelyDrierite desiccant, indicating, Fisher Scientific, Cat. No. 07-578-3AHydrochloric acid, 0.1-M, standardized to 0.0001, Fis

26、her Scientific, Cat. No. SA54-1Nitrogen, cylinder, 99.9% minimum purityPaper, test, lead acetate, vial, Fisher Scientific, Cat. No. 14-8622-Propanol, oxygen free, Fisher Scientific, Cat. No. A416-4. Purge the contents of a freshly-openedbottle with nitrogen for one hour and store under nitrogen.Silv

27、er nitrate, alcoholic, 0.01-M, standardized to 0.0001, Fisher Scientific, Cat. No. LC22745-2Sodium hydroxide, approximately 1-M and 5-M, stripped with nitrogen, Fisher Scientific, Cat. Nos.SS266-1 and SS256-500, respectivelySodium sulfide, 9-hydrate, Fisher Scientific, Cat. No. S425-500Sodium sulfid

28、e, 1% solution. Dissolve 1.0 g + 1.0 mg of sodium sulfide in 100 mL of water.THAM, tris(hydroxymethyl)aminomethane, alkalimetric standard, Fisher Scientific, Cat. No. T395. Dryin an oven at 110oC for 2 hours before using. Cool in a desiccator.Titration solvent, oxygen free, equal volumes of nitrogen

29、-purged 2-propanol and 1-M sodium hydroxide.Add 1% by volume of concentrated ammonium hydroxide. Store under nitrogen.PROCEDURESilverSilver Sulfide Electrode Preparation and ReconditioningProper electrode preparation is essential to obtain reproducible and noise-free titration curves havinggood endp

30、oints. The electrode should be reconditioned on a weekly basis when in use, and must bereconditioned if it is used for any titrations other than sulfur compounds. An electrode should be dedicatedto sulfur analysis. Prepare and recondition the silver-silver sulfide electrode as follows:1. Clean the s

31、ilver surface with a Scotchbrite pad. Rinse with water and dry.2. Immerse the electrodes in a solution containing 90 mL of 2-propanol, 2 mL of ammonium hydroxideand 8 mL of 1% sodium sulfide solution.3. Slowly add approximately 10 mL of 0.01-N silver nitrate solution over a period of 10 minutes whil

32、estirring at a moderate speed. A film of silver sulfide will be deposited on the silver electrode.5 of 16209-004. Wipe the excess silver sulfide from the electrode with a soft paper towel. The electrode should be cleaned after each titration by rinsing with water.Standardization1. Weigh approximatel

33、y 0.1 g of previously dried THAM, to the nearest 0.1 mg, into each of two clean,dry titration beakers.2. Add a magnetic stirring bar and approximately 100 mL of deionized water to each beaker.3. Titrate the contents of both beakers with 0.1-M hydrochloric acid using the automated titrator, withcombi

34、nation glass electrode, according to the manufacturers directions.4. Record the volume of titrant required to reach the endpoint of each titration.5. Calculate the molarity of the hydrochloric acid to the fourth decimal place for both titrations asfollows:11000 WM121.14 V= (3)where:M1= molarity of h

35、ydrochloric acid, mol/LV = volume of hydrochloric acid required to reach the endpoint, mLW = mass of THAM weighed into beaker, g121.14 = molecular weight of THAM, g/mol1000 = factor to convert L to mL The duplicates should agree within 0.001 mol/L. If not, redry the THAM or check the equipment and r

36、epairas necessary. Then rerun the standardization until the guideline is met.6. Record the average molarity of the hydrochloric acid for use in CALCULATIONS.Part A - Determination of Alkalinity of Used Refinery Caustic Solutions1. Prepare the automatic titrator using the combination glass electrode

37、according to the manufacturersdirections (see Note 8). Calibrate with pH standard buffers.2. Pipet 10 mL of the sample of used caustic solution into a tared 500-mL volumetric flask. Weigh theflask containing the sample, calculate and record the sample mass. Some caustic samples have a hydrocarbon la

38、yer floating on the surface. The caustic sample taken foranalysis must not contain any of this hydrocarbon layer.3. Dilute to the mark with water and thoroughly mix the contents of the flask.4. Pipet an aliquot of the diluted sample that will yield a titration of 5 to 15 mL into a 250-mLelectrolytic

39、 titration beaker. Add approximately 100 mL of water and a magnetic stirring bar.5. Titrate with 0.1-M hydrochloric acid using the combination glass electrode. Samples should be titrated immediately after dilution to prevent loss of basic sulfur compounds byoxidation.6 of 16209-00 More than one infl

40、ection may be observed in the titration curve. The endpoint for any one inflection is takenwhere pH / mL is a maximum, i.e., where the change in pH per unit of titrant added is the greatest. Thefirst break at a pH in the range of 9 to 6 represents the strong bases. The break at a pH in the range of

41、4 to3 represents the weak bases (see Figs. 1, 2 and 3). Calculate the strong base using the break at thehigher pH value as the endpoint. Calculate the total alkalinity using the break at the lower pH value as theendpoint. The weak base (spent caustic), as previously defined, is the difference betwee

42、n the totalalkalinity and the strong bases present.Part B - Determination of Mercaptide and Sulfide in Caustic Solutions1. Prepare the automatic titrator using the combination glass/silver Titrode according to manufacturersdirections. The electrode should be conditioned according to the procedure in

43、 Electrode Preparation.2. Weigh a portion of the original caustic sample, to the nearest 0.1 mg, into a 250-mL electrolyticbeaker. Use Table 1 as a guide in selecting the proper sample size. The sample size should be selected according to Table 1 to give a titration of at least 2 mL. Some caustic sa

44、mples have a hydrocarbon layer floating on the surface. The caustic sample weighed foranalysis must not contain any of this hydrocarbon layer. 3. Add approximately 100 mL of the titration solvent to the sample and titrate with 0.01-M alcoholicsilver nitrate. Caustic containing sulfide and mercaptide

45、 should be titrated so that a minimum amount of oxygen comesin contact with the caustic. Provide a nitrogen blanket over the titration beaker while titrating. Do not bubblenitrogen through the caustic solution because this would result in stripping out hydrogen sulfide, mercaptanand ammonia. The tit

46、rator should be programmed to add the titrant at a rate not to exceed 0.1 mL/min as the endpoint isapproached.If both sulfide and mercaptide ions are present, two inflections will be noted in the titration curve (seeFig. 4). If only one inflection is present, confirm the identity of the species pres

47、ent with lead acetate testpaper. Moisten a piece of lead acetate test paper with water and add one drop of the caustic. If the paperturns silver-black, the test is positive and sulfide ion is present in the sample. Some mercaptides will causelead acetate paper to darken with a tan to yellow colorati

48、on; this is not to be confused with a positive sulfidetest.Table 1Suggested Sample SizeExpected NaSR andNa2S Content, mass-%SuggestedSample Mass, g0.50 0.17 of 16209-00Titration of Sulfide in the Presence of High Concentrations of MercaptideCaustic used to extract sulfur compounds can be quite high

49、in mercaptide. If sodium sulfide is present insmall concentrations, the high mercaptide content will have the effect of masking the sulfide break. It is,therefore, necessary to titrate separately for the sulfide and mercaptide.1. Weigh a sample sufficiently large so that the sulfide break can be discerned, if sulfide is present.2. Titrate the sample according to Steps 1 through 3 in Part B - Determination of Mercaptide and Sulfidein Caustic Solutions.3. Discontinue the titration after the sulfide break is complete.4. Titrate a smaller sample portion for mercaptide ana