4.1 Scope and Importance of Natural Gas Sweetening

The desulfurization of natural gas, commonly referred to as “gas sweetening,” is a widespread and essential part of natural gas processing. Sour gas, which contains hydrogen sulfide (H₂S), must be treated before it can be used commercially. The scale of this operation is immense: approximately 33% of the natural gas produced in the United States and over 90% of that processed in Canada undergoes sweetening. The sulfur recovered from these operations is a major industrial commodity, accounting for about 25% of the free world’s total sulfur production.

4.2 Classifying Natural Gas Desulfurization Processes

Current commercial processes for natural gas sweetening can be classified into four major categories:

1. Dry Bed—Catalytic Conversion
2. Dry Bed—Absorption—Catalytic Conversion
3. Liquid Media Absorption—Air Oxidation
4. Liquid Media Absorption—Air Conversion

4.3 Detailed Analysis of Key Processes

The Modified Claus Process (Dry Bed Catalytic Conversion)

The Modified Claus Process is the workhorse of the industry for large-scale sulfur recovery. It is not used on the main sour gas stream directly but rather on the concentrated acid gas stream (containing 15-100 mole % H₂S) that has already been extracted by another process, such as amine treating.

In the “once-through” process (Figure 8), a portion of the H₂S is combusted with air to form SO₂, which then reacts with the remaining H₂S over a catalyst to produce elemental sulfur and water. This scheme generally provides the highest overall sulfur recovery and allows for maximum heat recovery. For feeds with lower H₂S concentrations or high hydrocarbon content, a “split stream” process is used. Seven pertinent design criteria for Claus plants include feed composition, combustion conditions, retention time, converter feed gas temperature, reheat schemes, space velocity, and sulfur condensing temperatures.

The Haines Process (Dry Bed Absorption)

The Haines Process (Figure 9) uses zeolites, also known as molecular sieves, to remove sulfur from sour natural gas. The process operates in a two-step cycle:

1. Absorption: Sour gas is passed through a bed of zeolites, which selectively absorb the H₂S until the bed becomes saturated.
2. Regeneration: The bed is taken offline and regenerated using hot, SO₂-bearing gases. These regeneration gases are produced by burning a portion of the liquid sulfur produced by the plant. The zeolite catalyzes the reaction between the absorbed H₂S and the SO₂ to form sulfur vapor, which is then condensed and collected.

Liquid Media Absorption – Air Oxidation

This category of processes is common in Europe, particularly for treating manufactured gases like coal gas or coke oven gas. The typical scheme (Figure 10) involves absorbing H₂S into a slightly alkaline solution that contains oxygen carriers. This solution is then regenerated by bubbling air through it. The air oxidizes the absorbed H₂S to elemental sulfur, which forms a froth on the surface of the regenerator. This sulfur froth is skimmed off and then filtered or centrifuged to produce a sulfur cake.

The Townsend Process (Liquid Media Direct Conversion)

The Townsend Process is one of the newest technologies in this field and was still in development at the time of this review. Its primary potential advantage is its ability to combine three separate operations—sweetening, dehydration, and conversion to elemental sulfur—into a single step. The process uses an aqueous solution of an organic solvent, such as triethylene glycol, to contact the sour gas. This offers the potential for a much more efficient and compact system compared to a conventional setup requiring three separate plants (amine, dehydration, and Claus).