MEASURING H 2 S IN CRUDE OIL FOR QUALITY CONTROL & TRANSPORTATION SAFETY

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Muriel Marshall
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1 MEASURING IN CRUDE OIL FOR QUALITY CONTROL & TRANSPORTATION SAFETY COQA Meeting 2015 Marriott West Loop, Houston Wesley Kimbell Analytical Systems Keco

2 Why measure in crude oil? Safety Product Quality

3 Why measure in crude oil? Safety Personnel protection Transportation Product Quality

4 Why measure in crude oil? Safety Product Quality Personnel protection Transportation Off-spec products Poisoned catalysis Environmental concern

5 Why measure in crude oil? Safety Personnel protection 1. gas is toxic and deadly 2. Even dissolved in crude is deadly

6 CHARACTERISTICS Gas with rotten egg smell Specific gravity: vs air LEL 4.3%, UEL 46% Highly corrosive Toxic

7 Why measure in crude oil? Safety Transportation 1. Prevent car derailments 2. Corrosion in vessels Pipelines Railcars Holding tanks Truck tanks

8 Why measure in crude oil? Safety Product Quality Transportation Personnel protection Off-spec products Poisoned catalysts Environmental concern

9 Why measure in crude oil? Product Quality Control 1. Off-spec products

10 Why measure in crude oil? Product Quality Control 1. Off-spec products 2. Poisoned catalysts

11 Why measure in crude oil? Product Quality Control 1. Off-spec products 2. Poisoned catalysts 3. Environmental concern

12 How do we measure H2S in crude oil? Direct optical measurement difficult: Crude oil is opaque in nature Complex compositional characteristics High viscosity crude oils difficult to transport in flow lines/tubing

13 How do we measure H2S in crude oil? Question: What is the best method to measure H2S in liquid samples like crude oil?

14 How do we measure H2S in crude oil? Answer: Transfer from liquid sample to headspace Upon saturation, the headspace concentration and the liquid sample concentration are proportional (Henry s Law)

15 How do we measure H2S in crude oil? Henry s Law: "At a constant temperature, the amount of a given gas that dissolves in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid."

16 Method of sampling Headspace Headspace concentration function of: 1. Saturation time/volume 2. Temperature 3. Vapor pressure of analyte 4. Concentration of analyte

17 Method of sampling Headspace

18 Method of sampling Headspace Glass Container Headspace Proportional concentration Crude Oil Zoomed to molecular level

19 Method of sampling Headspace Goal: Extract the headspace vapor of the liquid sample for analysis in the gas phase Solution: Headspace sampling Sample Transfer Stripper with ASI Membrane Technology (proprietary)

20 Method of sampling Headspace

21 Method of sampling Headspace

22 Method of sampling Headspace (dynamic) Purge Air In Headspace vapor to STS / Detector Dip tube REMEMBER: Headspace concentration is a function of: 1. Saturation time/volume 2. Temperature 3. Vapor pressure of analyte 4. Concentration of analyte

23 Method of sampling Headspace Headspace vapor can be dirty Mists will coat the detector and detector flow lines Results in in-gassing and out-gassing Leads to inaccurate readings over a short period of time

24 Method of sampling Headspace Clean the headspace vapor before analysis Significantly increase accuracy Significantly decrease maintenance

25 Method of sampling Membrane technology (proprietary) Sample Transfer Stripper with ASI Membrane Technology 5

26 Method of sampling Membrane technology (proprietary)

27 Method of sampling Membrane technology (proprietary) Membrane physically blocks headspace vapor mists Prevents fouling of detector Prevents liquid carry-over 5 Radically reduced maintenance Maintains accuracy of analyzer Maintains composition of headspace vapor

28 Method of sampling Membrane technology (proprietary) Headspace ASI Membrane Technology Dirty Crude oil vapor Zoomed to molecular level

29 Method of Analysis Difficulty of quantifying H2S in crude oil

30 Method of Analysis Difficulty of quantifying H2S in crude oil Crude oil is compositionally complex Aromatics make quantification difficult Common problematic components: SO2, ethyl-mercaptan, methylmercaptan and other sulfur compounds Crude compositional changes not uncommon

31 Method of Analysis Comparison Measurement Technology Colorimetric Ratiometric Tape Method - ASTM approved ASTM methods YES D , D , & D related Specific to H2S only YES Directly measures H2S without interference. No false positives. UV Absorbance Spectrophotometry X NO X NO Interference with mercaptans (naturally occurring), SO2 and other sulfurs. May give false positives. Requires complicated chemometric modeling but not full-proof Accuracy ± 2% of full scale typical ±10% at 0-10ppm FS ± 3-5% of FS Electrochemical Cell X NO X NO Interference with mercaptans, hydrogen, CO, ammonia, etc. giving false positives. Sensor Zero Drift Calibration Stability NO Does not suffer from zero drift YES Maintains accuracy despite severe process changes X YES 5% in 24 hours for low ppm ranges X NO Even slight process changes requires re-calibration. X YES X NO Requires monthly or bi-monthly calibrations Range PPB, PPM and up to 100% ranges with high accuracy X Low ppm ranges not reliable X Low ppm ranges not reliable Calibration Stability Long-term even in demanding environments and process changes X Unstable if process changes even slightly. Requires calibrations Maintenance Consumables LOW Tape change only 4 times per year LOW tape & solution change every 3 or 4 months only LOW Optics require frequent cleaning once a month or more X Light source and scrubber materials require replacement X HIGH Minimal NOTE: Data based on typical manufacturers published data

32 Method of Analysis Colorimetric Ratiometric Tape Detector

33 Method of Analysis Colorimetric Ratiometric Tape Detector The chemical formulation for this reaction is as follows: H 2 O Pb(OAc) 2 + PbS + 2HOAc White colored This reaction has three characteristics which allow it to be applied with unique analytical precision: 1. The reactants are colorless (white). 2. The product (PbS) is colored. 3. In more than a century of use, hydrogen sulfide is the only practical reactant found that produces a colored product on lead acetate (Pb(OAc) 2 tape.

34 Method of Analysis Colorimetric Ratiometric Tape Detector The rate (r) of this reaction (forming PbS) is as follows: r = k Pb(OAc) 2 x Where k, the proportionality factor, is called the rate constant. NOTE: The very large (more than 1000 times) concentration of Pb(OAc) 2 on the sensing tape simplifies the reaction as follows: [Pb(OAc) 2 ] 0 = [Pb(OAc) 2 ] 1 PbS 1 = K 2 (<.1%) Therefore, r n = kk 2 ( - PbS n ) For simplicity: kk 2 = K r n = K( = PbS n )

35 Method of Analysis Colorimetric Ratiometric Tape Detector By maintaining a constant concentration, with continuous flow of the sample into the sample chamber, the rate of reaction equation is further reduced to: r = K Therefore, by maintaining the control conditions of: 1. A large quantity of Pb(OAc) 2 2. Constant concentration. 3. Sufficient moisture. The rate of reaction (i.e. the rate of darkening) is linearly proportional concentration. Measuring the rate of darkening is therefore directly equivalent to measuring the concentration.

36 Method of Analysis Colorimetric Ratiometric Tape Detector The rate of reaction (i.e. the rate of darkening) is linearly proportional concentration.

37 Method of Analysis Colorimetric Ratiometric Tape Detector In actual practice, the rate of darkening is determined by measuring the change in darkness from second to second.

38 Method of Analysis Colorimetric Ratiometric Tape Detector 80 PPM tape darkness Y 40 PPM 0 b 0 X time The general equation for the lines of the graph above is: y = ax + b b is the zero drift or offset term in conventional equipment that is often adjusted to zero (zero drift)

39 Method of Analysis Colorimetric Ratiometric Tape Detector 80 PPM tape darkness Y 40 PPM 0 b 0 X time

40 Colorimetric Ratiometric Tape Detector Advantages Significant advantages of Tape Method Detector Only method inherently specific to No vapor processor or scrubbers required to block interfering components No complicated chemometric calibration modeling needed (spectrophotometry) Measures only Hydrogen Sulfide no matter the compositional background Sensor cannot be damaged by samples

41 Colorimetric Ratiometric Tape Detector Improvements to design Significant technological advancements to Tape Method: Improved tape drive system Reliable and rugged One tape reel > 150 analyses Fewer moving parts Simplest detection method available

S is referred to as sour. The sulfide loading lends an unpleasant odor and makes the water corrosive. Additionally, the presence of H 2

S is referred to as sour. The sulfide loading lends an unpleasant odor and makes the water corrosive. Additionally, the presence of H 2 Application Summary Analytes: S (hydrogen sulfide), (ammonia) Detection Technology: OMA-300 Process Analyzer Process Stream: stripped sour water Introduction In the context of petrochemical refining and