Modified Claus Sulfur Recovery Process

The modified Claus (also known as conventional Claus) process is the workhorse in sulfur recovery, and is used worldwide in more plants than any other process. In the typical modified Claus process, the acid gas feed is partially oxidized to generate sulfur dioxide (SO2), which then reacts with the remaining hydrogen sulfide (H2S) over a catalyst to produce sulfur. Most Claus sulfur plants contain two or three catalytic stages and can achieve 94-97% sulfur recovery efficiency.

The Claus process utilizes the following chemical reactions to convert hydrogen sulfide to elemental sulfur:

(1) H2S + 3/2 O2 ⇌ SO2+H2O

(2) 2 H2S + SO23/nSn + H2O

The overall reaction for the process is:

(3) 3 H2S + 3/2 O23/nSn + 3H2O

Most sulfur plants contain one non-catalytic conversion stage (the reactor furnace) and two or more catalytic conversion stages in series. The Claus reaction is highly exothermic, releasing a great deal of heat energy that can be recovered by generating steam in heat exchangers following the conversion stages.

Modified Claus Process Flow Diagram

The process is described below:

1 – For refinery sulfur plants, the ammonia-bearing sour water stripper off-gas is routed to the acid gas burner mounted on the reactor furnace, along with most of the amine acid gas. The remainder of the amine acid gas is usually routed to side injection ports on the furnace so that the front zone of the furnace operates at a higher temperature to ensure good ammonia destruction. For gas plant sulfur plants, there is normally only amine acid gas, which is usually all routed to the burner. Combustion air is routed to the burner, with the amount of air controlled to combust 1/3 of the H2S to SO2 via reaction (1). Sufficient air is also provided to combust the ammonia and hydrocarbons entering with the acid gas streams. The combustion products from the burner pass into the reactor furnace which provides sufficient residence time for all of the reactions to reach equilibrium. The furnace usually functions as a non-catalytic conversion stage, as the high temperature in the furnace will cause H2S and SO2 to combine via reaction (2) and form sulfur.

2 – The hot combustion products from the furnace enter the waste heat boiler and are partially cooled, usually to 600-800°F, by generating steam. Steam can be produced at up to 600 PSIG pressure from this boiler, depending on the needs at a particular location.

3 – The combustion products are further cooled (to 340-375°F typically) in the first sulfur condenser, usually by generating medium pressure steam (40-65 PSIG typically). This will normally cool the gas enough to condense the sulfur formed in the furnace, which is then separated from the gas and drained to a collection pit.

4 – In order to avoid forming liquid sulfur in the downstream catalyst bed, the gas leaving the sulfur condenser must be heated before entering the reactor. The reheat exchanger heats the gas stream (to 420-450°F typically), often by using some of the high pressure steam generated in the waste heat boiler. Heating oil or an electric heater may also be used.

5 – The heated stream enters the first reactor, containing a 36″-48″ deep bed of sulfur conversion catalyst. Most sulfur conversion catalyst is alumina-based, and often contains special promoting agents to give longer service life and to enhance hydrolysis of the trace organic sulfur compounds that are often formed by side reactions in the furnace. Typically, about 70% of the H2S and SO2 in the gas will react via reaction (2) to form sulfur, which leaves the reactor with the gas as sulfur vapor.

6 – The hot gas leaving the first reactor (550-650°F typically) is cooled in
the second sulfur condenser, usually by generating medium pressure
steam. As the gas is cooled (to 320-350°F typically), the sulfur
formed in the reactor is condensed, separated, and drained to the collection
pit. This is usually followed by one or two more heating, reaction,
and condensing stages to react most of the remaining H2S
and SO2, with the sulfur plant tailgas leaving the final sulfur
condenser at a temperature between 250°F and 300°F. This temperature
depends on the steam pressure in the last condenser, which can be operated
as low as 15 PSIG to maximize sulfur recovery.

7 – The sulfur plant tailgas is routed to either a Tailgas Cleanup Unit (see
Modified Claus Process with Tailgas Cleanup)
for further processing, or to a Tailgas Thermal
to incinerate all of the sulfur compounds in the tailgas to
SO2 before dispersing the effluent to the atmosphere.

When the H2S concentration is 50% or higher, the straight-through version of the modified Claus process (where all of the acid gas is routed to the acid gas burner) is generally used. Below this concentration, it is usually necessary to use the split-flow version of the process (where only a portion of the acid gas is combusted in the burner) in order to maintain a stable flame in the burner. Below an H2S concentration of about 15%, a stable flame usually cannot be maintained in the burner, but special design techniques (such as supplemental fuel gas firing) can be employed to extend the range of the process to very lean acid gas streams.

The modified Claus process is generally limited to a sulfur recovery efficiency of 94-97%, depending on the H2S concentration in the acid gas and the number of catalytic stages used. Although it is possible in some cases to achieve higher recoveries using additional catalytic stages, reaction equilibrium considerations generally limit the recoveries to these levels. For sulfur recoveries in the intermediate range of 97.5-99.5%, the Cold Bed Adsorption Process is usually a better choice. For recoveries in excess of 99.5%, the Modified Claus Process with Tailgas Cleanup is normally used.