Analyzing Stripped Sour Water

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Application Summary

Analytes H2S (hydrogen sulfide), NH3 (ammonia)
Detector OMA-300 Process Analyzer
Process Stream stripped sour water

Introduction

In the context of petrochemical refining and wastewater treatment, water containing H2S is referred to as “sour.” The sulfide loading lends an unpleasant odor and makes the water corrosive. Additionally, the presence of H2S in certain water pipes can serve as an indicator of equipment problems, e.g. heat exchanger leaks.

In a refinery, sour water must be cleaned of its sulfide content before being recycled or released to the environment. This cleaning process is known as “stripping” because it flows gas (air or steam) through the sour water to strip H2S and NH3 out of the water. To verify that the sour water is being effectively stripped, online analysis is required in the stripped water stream.

The OMA system applies the power of dispersive UV-Vis absorbance spectroscopy to this analysis. In applications where the water is too dark or dirty to transmit a light signal, Applied Analytics utilizes our expertise in headspace sampling design, thus providing a proven analytical solution for water streams where other analyzers have failed.

OMA Benefits: Stripped Sour Water

  • Continuously measures H2S and NH3 concentrations in water using dispersive UV-Vis absorbance spectrophotometer
  • Totally solid state build with no moving parts — modern design for low maintenance
  • Capable of analyzing opaque/dirty water using headspace sampling option
  • Fully automated using Auto Zero to normalize spectrophotometer on zero-analyte process water
  • Decades of field-proven performance in diverse water compositions

The Headspace Solution for Opaque Sour Water

Henry’s Law states that the amount of a certain gas dissolved in a solution at a given temperature is directly proportional to the partial pressure of that gas above the solution. The major implication of this for water analysis is that, given certain constant conditions, the chemical composition of the headspace gas correlates directly to the composition of the opaque liquid sample. The conditions which must remain constant (temperature, pressure, carrier gas flow rate, and liquid sample flow rate) are all held constant by the headspace system.

To create a representative vapor-phase sample from the water, the headspace system uses a temperature-controlled column (24” high by 2” diameter). The opaque liquid sample flows in from the top while carrier gas (typically nitrogen) flows in from the bottom. The carrier gas picks up the molecules that evaporate from the liquid sample and carries them out of the column and into the flow cell for optical analysis.

Analyzing Stripped Sour Water--headpace-schematic-column

The held temperature determines the partial pressure mix of compounds in the headspace gas. This allows us to hold the temperature of the column at a point where we get an extremely useful vapor-phase sample that contains the analyte but very low levels of interfering compounds like aromatics and phenols (due to the difference in Henry’s law constants between the analyte and the interfering compounds).

The OMA Double Headspace System for H2S and NH3

Measuring H2S and NH3 simultaneously in the effluent water from a stripping operation requires twofold application of the headspace principle, since the two analytes have different Henry’s Law constants.

The double headspace system uses two separate headspace columns, each with a corresponding optical flow cell for the emerging headspace gas. The H2S column is held at a temperature optimized for H2S stripping, while the NH3 column is held at a different temperature optimized for NH3 stripping. Each column feeds the headspace gas into a dedicated flow cell with a specialized optical path for the analyte.

Analyzing Stripped Sour Water--double-headspace-sample conditioning system

Absorbance Curves and the Power of Collateral Data

Any single photodiode measurement is vulnerable to noise, signal saturation, or unexpected interference. This susceptibility to error makes a lone photodiode data point an unreliable indicator of one chemical’s absorbance.

As accepted in the lab community for decades, the best way to neutralize this type of error is to use collateral data in the form of ‘confirmation wavelengths,’ i.e. many data points at many wavelengths instead of a single wavelength:

H2S Absorbance Curves

Analyzing Stripped Sour Water--spectra

In the spectra above, each diamond represents a single photodiode and data point. The spectrophotometer measures absorbance at each integer wavelength within the 200-800 nm UV-Vis range and produces an H2S absorbance curve.

After being calibrated on a full spectrum of pure H2S, the OMA knows the absorbance-concentration correlation for each measurement wavelength. The system averages the modeled concentration value from each wavelength to completely eradicate the effect of noise at any single photodiode.

The OMA visualizes the H2S absorbance curve in this manner and knows the expected relation of each data point to the others in terms of the curve’s structure. This curve analysis enables the OMA to automatically detect erroneous results at specific wavelengths, such as when a single photodiode is saturated with light. The normal photometer, with a single data point, is completely incapable of internally verifying its measurement.

The same principle is used in the NH3 measurement:

NH3 Absorbance Curves

Analyzing Stripped Sour Water--spectra

As with the H2S calibration explained above, the NH3 analysis is calibrated on a full spectrum of standard NH3 solution. In the double headspace configuration, each analyte has its own flow cell with path length specialized for the concentration range.

The specifications below represent performance of the OMA-300 Process Analyzer in a typical stripped sour water application.

For technical details about the OMA-300 Process Analyzer, see the data sheet:
DS-001A: OMA-300 Process Analyzer

All performance specifications are subject to the assumption that the sample conditioning system and unit installation are approved by Applied Analytics. For any other arrangement, please inquire directly with Sales.

Application Data
Performance Specifications
Accuracy Custom measurement ranges available; example ranges below. Accuracy specifications represent headspace gas sample analysis validated with span gas.
H2S 0-10 ppm: ±1 ppm
0-100 ppm: ±1% full scale or 1 ppm*
0-10,000 ppm: ±1% full scale
NH3 0-10 ppm: ±1 ppm
0-100 ppm: ±1% full scale or 1 ppm*
0-10,000 ppm: ±1% full scale
*Whichever is larger

Note: Subject to modifications. Specified product characteristics and technical data do not serve as guarantee declarations.

Further Reading

SubjectType
OMA-300 H2S AnalyzerBrochure
OMA-300 Process AnalyzerData sheet
Advantage of Collateral DataTechnical Note
Multi-Component AnalysisTechnical Note
How do we Measure H2S in Crude Oil? Video