Analytical Solutions by the Sea The 60th Annual Symposium of the Analysis Division Galveston Island...

Analytical Solutions by the SeaThe 60th Annual Symposium of the Analysis DivisionGalveston Island Convention Center, Galveston, Texas, USA; 26th – 30th April 2015

MEASURING H2S IN CRUDE OIL FOR QUALITY CONTROL & TRANSPORTATION SAFETY

Wesley Kimbell Analytical Systems Keco


Slide 2

Why measure H2S in crude oil?

Safety Industrial Process Control


Slide 3


Safety

• Personnel protection

• Transportation (rail, roads, pipelines)

Industrial Process Control


Slide 4


Safety

• Personnel protection

• Transportation

Industrial Process Control

• Meet product specifications

• Protect process equipment

• Environmental concern


Slide 5


•SafetyPersonnel protection

1. H2S gas is toxic and deadly2. Even H2S dissolved in crude is deadly


Slide 6

H2S CHARACTERISTICS

•Gas with rotten egg smell•Specific gravity: 1.1895 vs air•LEL 4.3%, UEL 46%•Highly corrosive•Toxic


Slide 7


•SafetyPersonnel protection

1. H2S gas is toxic and deadly2. Even H2S dissolved in crude is deadly

Transportation3. Prevent car derailments4. Corrosion


Slide 8


•Industrial Process Control1.Meet product specifications


Slide 9


•Industrial Process Control1.Off-spec products2. Protect process equipment


Slide 10


•Industrial Process Control1.Off-spec products2.Industrial process control3.Environmental concern


Slide 11

How do we measure H2S in crude oil?

Direct optical measurement of crude oil is difficult:

• Crude oil is opaque in nature• Complex compositional characteristics


Slide 12


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


Slide 13


Answer: Measure the headspace

• Upon saturation, the headspace concentration and the liquid sample concentration are proportional


Slide 14


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."


Slide 15

Methods of sampling

Headspace concentration function of:

1. Saturation time/volume2. Temperature3. Vapor pressure of analyte4. Concentration of analyte


Slide 16

Methods of sampling


Slide 17

Methods of sampling

Headspace

Crude Oil H2S

H2S

H2S

H2S

H2SH2S

H2S

H2SH2S

H2S

H2S

H2S

H2S

Proportionalconcentration

H2S


Slide 18

Methods of sampling

Goal: Extract the ‘headspace’ vapor of the liquid sample for analysis in the gas phase

Two methods:• ‘Headspace’ stripping column• Sample Transfer Stripper™ with ASI Membrane Technologies (proprietary)


Slide 19

Methods of sampling

Carrier / Stripping Gas In

Crude Oil In

Crude Oil to Drain

Stripped H2S to Detector

~ 3 to 4 feet

Str

ipp

ing C

olu

mn

Headspace stripping column


Slide 20

Methods of sampling

Carrier / Stripping Gas In

Crude Oil In

Crude Oil to Drain

Stripped H2S to Detector

~ 3-4 feet

Str

ipp

ing C

olu

mn


Drawbacks• Suffers from liquid

“carry-over”

• Cleaning requirements

• Many moving parts = more maintenance


Slide 21

Methods of sampling

Membrane technology (proprietary)

Sample Transfer Stripper™with ASI Membrane Technologiesby


Slide 22

Methods of sampling



Slide 23

Methods of sampling


ASI MembraneTechnology

Headspace

Crude Oil H2S

H2S

H2S

H2S

H2SH2S

H2S

H2SH2S

H2S

H2S

H2S

H2S


Slide 24

Methods of sampling


• Membrane physically blocks mists from headspace vapor

• Prevents fouling of detector

• Prevents liquid “carry-over”

• Radically reduced maintenance

• Prevents in-gassing / out-gassing of H2S

• Maintains compositional representation of the headspace vapor


Slide 25

Methods of sampling | Comparison

Sampling Technology: Sample Transfer Stripper™ (STS)with exclusive ASI Membrane Technologies


Principles incorporated Henry’s Law & proprietary methods Henry’s Law

Maintenance requirements LOW

STS Membrane creates ultra-clean sample for detector & physically blocks liquids from passing through to detector. Clean only one to two times per year typical

X HIGH Plagued by frequent liquid “carry-overs”. No physical block to prevent liquids from contaminating detector and gas sample lines. User reported constant cleaning & upkeep

Moving parts NONE No moving parts

X HIGHComplicated system using 85% more parts than STS Membrane

Cost of ownership Cost-Effective Minimal maintenance means less cost over time

X HIGHLong term expense due to cleaning & maintenance requirements

Accuracy HIGHSTS creates ultra-clean sample for analysis

X LOWSystem fouls as liquids frequently pass through to detector and gas sample lines requiring frequent cleaning

Range PPB, PPM and up to 100% ranges with high precision

X Low ppm ranges not reliable

Safety SAFE

Closed loop system for analysis and cleaning. No H2S exposure to user

X CAUTIONUser potentially exposed to residual H2S when cleaning is required


Slide 26

Method of Analysis

Difficulty of quantifying H2S in crude oil

• Crude oil is compositionally complex• Many components interfere with H2S:

Aromatics, SO2, ethyl-mercaptan, methyl-mercaptan and other sulfur compounds

• Crude composition changes not uncommon• Low ppm especially a challenge


Slide 27

Method of Analysis

Colorimetric-Rateometric Tape Detector


Slide 28

Colorimetric Rateometric Tape DetectorAdvantagesSignificant advantages with tape method• Only method inherently specific to

H2S• No complicated chemometric

modeling needed• No need to “trick” system to

minimize interferences• Measures only H2S, no matter the

compositional background• Inherently accurate—measure low

ppb, ppm to 100% H2S without false positives even in crude oil headspace


Slide 29

Colorimetric Rateometric Tape DetectorImprovements to design

Significant technological advancements to Tape Method:

• Improved tape drive system• 75% less parts• 3-6 month tape life• Reliable and rugged• Remote diagnostics &

troubleshooting• Simplest detection & most

straight forward method available


Slide 30

Method of analysis | Comparison

NOTE: Data based on typical manufacturers published data

Measurement Technology

Colorimetric-Rateometric Tape Method

UV-Absorbance Spectrophotometry

TDL Absorbance Spectroscopy

Specific to H2S only

YES Innately and practically free of cross-interference. No false positives.

X NOProblems with mercaptans (including naturally occurring mercaptans), hydrogen, ammonia, etc. giving false positives

X NO Problems with moisture and other sulfur components giving false positives

Accuracy ± 2% of full scale typical ±10% at 0-10ppm FS X ± 4ppmv 0-10ppm FS

Sensor Zero Drift NO zero drift (automatic self zero, no zero air required)

X YES 5% in 24 hours for low ppm ranges. Requires zero gas.

X YES. Normally requires zero gas.

Sample pressure requirements

Pressure between 5 to 2,000 PSI acceptable

Normally requires minimum of ~150 PSI

Normally requires minimum of ~150 PSI

Range

PPB, PPM and up to 100% ranges with high accuracy

X Low ppm ranges not reliable. PPB range not possible

X Low ppm ranges not reliable. PPB range not possible

Calibration Stability

Long-term even in demanding environments and process changes

X Unstable if process changes even slightly. Requires calibrations

X Unstable if process changes even slightly. Requires calibrations

Maintenance

LOWTape change only 3 to 4 times per year

X HIGH Optics / Flow cell require frequent cleaning

Mirrors tend to fog or produce haze. Requires cleaning

Consumables

Tape change every 3 to 6 months. ~$25 per tape reel

Zero gas required. Light source and scrubber materials require replacement. Costly.

Zero gas required. Scrubber (copper nanoparticle) replaced annually or more. Costly.


Slide 31

Method of Analysis

Colorimetric-Rateometric Tape Analyzer

Detection of H2S concentrations by the use of H2S sensing tape is achieved by exposing the film to an H2S sample through an aperture in the sample flow system, called the sample chamber. The reaction of photographic film to the light is analogous to the way that chemically saturated H2S sensing tape reacts to hydrogen sulfide


Slide 32

Method of Analysis


The chemical formulation for reaction is as follows: H2OPb(OAc)2 + H2S →→→ PbS + 2HOAcWhite 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.


Slide 33

Method of Analysis

Colorimetric-Rateometric Tape DetectorThe rate (r) of this reaction (forming PbS) is as follows: r = k Pb(OAc)2x H2S 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 H2S sensing tape simplifies the reaction as follows: [Pb(OAc)2]0 = [Pb(OAc)2]1 – PbS1 = K2 (<.1%) Therefore, rn = kk2(H2S - PbSn) For simplicity: kk2 = K rn = K(H2S = PbSn)


Slide 34

Method of Analysis


By maintaining a constant H2S concentration, with continuous flow of the sample into the sample chamber, the rate of reaction equation is further reduced to: r = KH2S

Therefore, by maintaining the control conditions of:1. A large quantity of Pb(OAc)2

2. Constant H2S concentration.3. Sufficient moisture.

The rate of reaction (i.e. the rate of darkening) is linearly proportional H2S concentration. Measuring the rate of darkening is therefore directly equivalent to measuring the H2S concentration.


Slide 35

Method of Analysis


The rate of reaction (i.e. the rate of darkening) is linearly proportional H2S concentration.


Slide 36

Method of Analysis


In actual practice, the rate of darkening is determined by measuring the change in darkness from second to second.


Slide 37

Method of Analysis


tape darkness

Y

80 PPM

40 PPM

Xtime

b

00

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)


Slide 38

Method of Analysis


tape darkness

Y

80 PPM

40 PPM

Xtime

b

00


Slide 39

Case Study

Measuring H2S for quality control

Location: In-plant study in Middle East

Participants: ASI Personnel, Customer Plant Manager, Customer Lab Manager

Activity: Startup/commissioning and accuracy test


Slide 40

Case Study

Measuring H2S for quality control

Equipment:

Model 205 H2S in Crude Oil Analyzer

• Sample Transfer Stripper™ with ASI Membrane Technologies (proprietary)

• Colorimetric-Rateometric Tape Detector

• Measuring Range: 0 to 80 ppm by wt.


Slide 41

Case Study

Verification of Calibration MethodVolumetric Blending Process

The calibration procedure calls for a known concentration of H2S, which is injected into the crude sample in a container. This will cause the concentration of H2S in the sample to read proportionally more based on the concentration of H2S injected.


Slide 42

Case Study

Results

The model 205 H2S in Crude Oil Analyzer measured continuously while two field samples were taken by lab personnel which were tested in the laboratory. The cross reference of the readings of the analyzer and the lab results can be seen below:

SAMPLE #/ TIME MODEL 205 LABSample 1: 12:00 5.201 ppm/w 4 ppm/wSample 2: 15:00 15.02 ppm/w 14 ppm/w

ASI Prepared Standard 10 ppmw

10.07 ppm/w 14 ppm/w


Slide 43

Case Study

ConclusionThe Model 205 H2S in Crude Oil Analyzer utilizing the membrane Sample Transfer Stripper™ and colorimetric-rateometric tape method of detection was tested by the plant laboratory specialists and found to be accurate.

The Volumetric Blending method of calibration was also verified for accuracy. Both the plant engineers and Process Analyzer Engineer specialists along with ASI engineers were very pleased with the testing method and the accuracy of the model 205 H2S in Crude Oil Analyzer.

The Sample Transfer Stripper combined with Tape detector results in an accurate and stable analyzer to measure H2S in crude oil:• The analyzer maintains calibration long term (no chemometric modeling

required)• Field calibrations not required • Zero gas not required• Membrane stripper eliminates liquid carry over and cleanup• Advancements in tape technology results in low maintenance• Tape change required only 2-4 times annually

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Analytical Solutions by the Sea The 60th Annual Symposium of the Analysis Division Galveston Island...

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