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Acid gas removal (acid gas treating) is the removal of acidic gases such as hydrogen sulfide and carbon dioxide from gas streams. In addition to removal water and gas stream liquids removal, one of the most important parts of gas processing involves the removal of hydrogen sulfide and carbon dioxide. Gas from some sources contains significant amounts of hydrogen sulfide and carbon dioxide and is usually referred to as sour gas. Sour gas is undesirable because the sulfur compounds it contains can be extremely harmful, even lethal, to breathe and the gas can also be extremely corrosive. The process for removing hydrogen sulfide from sour gas is commonly referred to as sweetening the gas.

Acid gas removal (i.e., removal of carbon dioxide and hydrogen sulfide from gas streams) is achieved by application of one or both of the following process types: (i) absorption and (ii) adsorption.

Acid gas removal processes involve the chemical reaction of the acid gases with a solid oxide (such as iron oxide) or selective absorption of the contaminants into a liquid (such as ethanolamine) that is passed countercurrent to the gas. Then the absorbent is stripped of the gas components (regeneration) and recycled to the absorber. The process design will vary and, in practice, may employ multiple absorption columns and multiple regeneration columns.

Liquid absorption processes (which usually employ temperatures below 50°C (120°F) are classified either as physical solvent processes or chemical solvent processes. The former processes employ an organic solvent, and absorption is enhanced by low temperatures, or high pressure, or both. Regeneration of the solvent is often accomplished readily. In chemical solvent processes, absorption of the acid gases is achieved mainly by use of alkaline solutions such as amines or carbonates. Regeneration (desorption) can be achieved by the use of reduced pressure and/or high temperature, whereby the acid gases are stripped from the solvent.

Adsorbers are widely used to increase a low gas concentration prior to incineration unless the gas concentration is high in the inlet air stream. Adsorption also is employed to reduce problem odors from gases. There are several limitations to the use of adsorption systems, but it is generally felt that the major one is the requirement for minimization of particulate matter and/or condensation of liquids (e.g., water vapor) that could mask the adsorption surface and drastically reduce its efficiency.

The precise area of application of a given process is difficult to define, and several factors must be considered: (i) the types and concentrations of contaminants in the gas, (ii) the degree of contaminant removal desired, (iii) the selectivity of acid gas removal required, (iv) the temperature, pressure, volume, and composition of the gas to be processed, (v) the carbon dioxide-hydrogen sulfide ratio in the gas, and (vi) the desirability of sulfur recovery due to process economics or environmental issues.

A number of processes are available for the removal of hydrogen sulfide from gas streams. These processes can be categorized as those based on physical absorption, adsorption by a solid, or chemical reaction. Physical absorption processes suffer from the fact that they frequently encounter difficulty in reaching the low concentrations of hydrogen sulfide required in the sweetened gas stream.

Solid bed adsorption processes suffer from the fact that they are generally restricted to low concentrations of hydrogen sulfide in the entering sour gas stream. The development of a short-cycle adsorption unit for hydrogen sulfide removal might help remove part of this low-concentration restriction for the solid bed absorption processes. In general, chemical processes are able to meet the regulated hydrogen sulfide levels.

The most well-known hydrogen sulfide removal process is based on the reaction of hydrogen sulfide with iron oxide (iron sponge process or dry box method) in which the gas is passed through a bed of wood chips impregnated with iron oxide:

The bed is then regenerated by passage of air through the bed:

The bed is maintained in a moist state by circulation of water or a solution of soda ash.

The method is suitable only for small-to-moderate quantities of hydrogen sulfide. Approximately 90% of the hydrogen sulfide can be removed per bed but bed clogging by elemental sulfur occurs and the bed must be discarded and the use of several beds in series is not usually economical. Removal of larger amounts of hydrogen sulfide from gas streams requires continuous processes, such as the Ferrox process or the Stretford process.

The Ferrox process is based on the same chemistry as the iron oxide process except that it is fluid and continuous. The Stretford process employs a solution containing vanadium salts and anthraquinone disulfonic acid.

Most hydrogen sulfide removal processes involve fairly simple chemistry with the potential for regeneration with return of the hydrogen sulfide. However, if the quantity involved does not justify installation of a sulfur recovery plant, usually a Claus plant, it will be necessary to select a process which produces elemental sulfur directly:

The conversion can be achieved by reacting the hydrogen sulfide gas directly with air in a burner reactor if the gas can be burnt with a stable flame.

Other equilibria which should be taken into account are the formation of sulfur dimer, hexamer, and octamer as well as the dissociation of hydrogen sulfide:

Carbonyl sulfide and carbon disulfide may be formed, especially when the gas is burned with less than the stoichiometric amount of air in the presence of hydrocarbon impurities or large amounts of carbon dioxide.

Equilibrium conversion is almost complete (approximately 99 to 100%) at relatively low temperatures and diminishes at first at higher temperatures, in accordance with the exothermic nature of the reaction. A further rise in temperature causes the equilibrium conversion to increase again. This is a consequence of the dissociation of the polymeric sulfur into monatomic sulfur.

Catalysis by alumina is necessary to obtain good equilibrium conversions: the thermal Claus reaction is fast only above 500°C (930°F). There is also a lower temperature limit which is not caused by low rates but by sulfur condensation in the catalyst pores and consequent deactivation of the catalyst. The lower limit at which satisfactory operation is still possible depends on the pore size and size distribution of the catalyst; with alumina-based catalysts having wide pores, the conversion proceeds satisfactorily at approximately 200°C (390°F).

In all Claus process configurations several conversion steps in adiabatic reactors are used, with intermittent and final condensation of the sulfur produced. There are three main process forms, depending on the concentration of hydrogen sulfide and other sulfur compounds in the gas to be converted, i.e., the straight-through, the split-flow oxidation process.

The straight-through process is applicable when the gas stream contains more than 50% v/v hydrogen sulfide. Feed gases of this type can be burnt with the stoichiometric amount of air to give sulfur. The combustion reactor is followed by a combined waste heat boiler and sulfur condenser from which liquid sulfur and steam are obtained. The gases are then reheated by in-line fuel combustion to the temperature of the first catalytic convertor, which is usually kept at approximately 350°C (660°F) to decompose any carbonyl sulfide and any carbon disulfide formed in the combustion step. A second catalytic convertor, operating at as low a temperature as possible, is also employed to obtain high final conversions.

Molecular sieves and membranes have been undergoing development for the removal of hydrogen sulfide and carbon dioxide from gas streams, especially when the amount of the acid gas(es) is low. The most appropriate use of the sieves and the membranes would be use of the sieve to selectively remove hydrogen sulfide and/or use of membranes permeable to hydrogen sulfide but not to carbon dioxide.

See also: Gas Cleaning, Gas Processing, Gas Treating.