Determination of Hydrogen Sulfide, Mercaptan Sulfur, and Carbonyl Sulfide in Hydrocarbon Gases by Potentiometric Titr...

Understanding Determination of Hydrogen Sulfide, Mercaptan Sulfur, and Carbonyl Sulfide in Hydrocarbon Gases by Potentiometric Titration

Accurate quantification of sulfur compounds in hydrocarbon streams is a cornerstone of process efficiency, product quality, and environmental compliance within the oil and gas, petrochemical, and natural gas industries. This UOP method provides a robust analytical framework for determining specific sulfur species – hydrogen sulfide (H₂S), mercaptan sulfur (RSH), and carbonyl sulfide (COS) – in gaseous hydrocarbons and liquefied petroleum gas (LPG). By employing potentiometric titration, this practice offers a precise and reliable alternative to methods that may struggle with the complex matrices or low concentrations encountered in these critical applications.

Purpose & Problem Solved

This practice addresses the challenge of accurately identifying and quantifying individual sulfur compounds, which can significantly impact downstream processes, catalyst performance, and emissions. Traditional methods might offer a total sulfur analysis, but often lack the specificity required for targeted process adjustments or regulatory reporting. Potentiometric titration, as outlined in this method, overcomes these limitations by providing distinct endpoints for each sulfur species through a carefully controlled sequential scrubbing and titration process. This allows for precise measurement, enabling engineers to optimize operations, ensure product specifications are met, and maintain environmental stewardship.

When to Apply This Practice

• • Required Situations:
• • Quality control of hydrocarbon gas streams and LPG destined for further processing or sale.
• • Monitoring sulfur levels in feedstock to prevent catalyst poisoning.
• • Verifying compliance with environmental regulations concerning sulfur emissions.
• • Recommended Situations:
• • Process stream analysis to diagnose operational issues related to sulfur contamination.
• • Troubleshooting off-spec product related to sulfur content.
• • Evaluating the effectiveness of sulfur removal units.
• • Not Applicable When:
• • Only total sulfur analysis is required, and individual species differentiation is not necessary.
• • The concentration of sulfur compounds is significantly outside the method's specified range (less than 1 to several thousand mass-ppm sulfur).
• • Analysis of liquid hydrocarbons where specific methods for liquid matrices are more appropriate (e.g., UOP Method 163).

Implementation Framework

Phase 1: Sample Preparation and Apparatus Setup

This phase involves rigorous preparation, including ensuring all sampling equipment (as per UOP Method 516 for C3-C4 fractions) and analytical apparatus are clean, calibrated, and correctly assembled. Crucially, the gas washing bottles for carbonyl sulfide must be shielded from light to prevent degradation.

Phase 2: Sample Introduction and Scrubbing

The hydrocarbon gas or LPG sample is introduced and sequentially passed through a potassium hydroxide solution (containing a chelating agent to prevent oxidation) to capture H₂S and RSH, followed by a monoethanolamine solution to capture COS. Careful control of the gas flow rate is essential for efficient scrubbing.

Phase 3: Potentiometric Titration

The scrubber solutions are then titrated potentiometrically using a silver nitrate titrant. A specialized silver/glass electrode system is employed. The H₂S and RSH in the first scrubber are titrated, followed by the COS in the second. The distinct inflection points on the titration curve identify the endpoint for each component.

Phase 4: Data Analysis and Reporting

Titration data, obtained from an automatic