Gas sweetening is the industrial process of removing toxic and corrosive contaminants, primarily hydrogen sulfide (\(\text{H}_2\text{S}\)) and carbon dioxide (\(\text{CO}_2\)), from raw natural gas. Gas containing these acidic compounds is referred to as “sour gas.” The sweetening procedure transforms sour gas into “sweet gas,” which is safe and suitable for commercial transportation and use. This purification step is mandatory before the gas enters the pipeline network.
Why Gas Sweetening is Essential
The contaminants in raw natural gas pose significant risks to human safety and industrial infrastructure. Hydrogen sulfide (\(\text{H}_2\text{S}\)) is extremely poisonous and lethal even at very low concentrations. Its removal is a fundamental safety requirement for any facility handling natural gas.
Both \(\text{H}_2\text{S}\) and \(\text{CO}_2\) become highly corrosive when they encounter water vapor present in raw gas streams. The resulting acidic solutions rapidly degrade carbon steel pipelines and processing equipment. This damage leads to metal loss corrosion and can cause stress cracking in the steel, potentially resulting in equipment failures.
Untreated sour gas cannot meet the quality standards necessary for commercial sale or transport. Pipeline specifications severely limit the allowable concentration of \(\text{H}_2\text{S}\), often to 4 parts per million (ppm) or less, to protect the network’s integrity. Sweetening is necessary to comply with environmental regulations and commercial contracts, ensuring the gas is safe to handle and has an acceptable heating value.
The Primary Method: Amine Treatment
The most widespread method for gas purification is chemical absorption using aqueous amine solutions, often called amine scrubbing. Amines are weak bases that chemically react with the acidic \(\text{H}_2\text{S}\) and \(\text{CO}_2\) molecules. Common solvents used include Monoethanolamine (MEA) and Diethanolamine (DEA).
The sweetening operation takes place in an absorber tower. Raw sour gas flows upward, countercurrent to the downward-flowing, cool “lean” amine solution. The amine selectively captures the acid gases, forming a temporary chemical bond, while the purified “sweet” gas exits the top of the tower. This liquid, now enriched with contaminants, is called “rich” amine.
The rich amine is sent to a regenerator unit to release the captured acid gases and recycle the solvent. In the regenerator, the solution is heated, typically around 225°F, which reverses the chemical reaction. The heat breaks the bonds, causing the \(\text{H}_2\text{S}\) and \(\text{CO}_2\) to strip out as a concentrated stream. The regenerated, lean amine is then cooled and sent back to the absorber to repeat the cycle.
Alternative Sweetening Technologies
While amine treating is generally the standard for large-volume streams, other technologies are used for specialized applications or when specific contaminant removal is required. Physical solvents, such as Selexol or Rectisol, rely on physical solubility rather than chemical reaction to capture acid gases. These methods are particularly effective when the gas stream is at a high pressure, which increases the solubility of the contaminants in the solvent.
Physical solvents are also favored when a high concentration of \(\text{CO}_2\) is present, as they require less energy for regeneration compared to chemical amines. Instead of relying on heat, physical solvents can often be regenerated simply by reducing the pressure, flashing the absorbed gases out of the liquid.
Another approach involves solid bed scavengers, like the iron sponge process, which use a non-regenerative medium such as hydrated iron oxide. These solid scavengers are used for smaller gas volumes or as a final “polishing” step to remove trace amounts of \(\text{H}_2\text{S}\) to meet stringent specifications. Membrane separation technology is a viable option for bulk removal of \(\text{CO}_2\), especially in high-pressure streams. This method uses a pressure differential to separate gases based on their differing permeation rates through a semipermeable membrane.
Handling the Removed Contaminants
The final step in the sweetening process involves dealing with the concentrated stream of \(\text{H}_2\text{S}\) and \(\text{CO}_2\), known as acid gas, which is stripped from the solvent. This stream is hazardous and cannot be released directly into the atmosphere, necessitating its safe disposal or conversion.
The most common method for handling streams with high \(\text{H}_2\text{S}\) concentration is the Claus process. This sulfur recovery technique chemically converts the toxic hydrogen sulfide into elemental sulfur, a stable and marketable yellow solid. The conversion involves a thermal stage, where one-third of the \(\text{H}_2\text{S}\) is burned to form sulfur dioxide (\(\text{SO}_2\)). This is followed by a catalytic stage where the remaining \(\text{H}_2\text{S}\) reacts with the \(\text{SO}_2\) to produce elemental sulfur and water.
For acid gas streams that are not suitable for sulfur recovery, often due to low volume or low \(\text{H}_2\text{S}\) content, a method called acid gas injection is used. This involves compressing the acid gas and injecting it deep underground into porous geological formations for permanent containment.