CLAUS PROCESSPrepared by 6520s
Reference: Petroleum Refining-Technology and Economics. James Hary and Glenn HandwerkThe most practical method for converting hydrogen sulfide to elemental sulfur
is the modified Claus process. The original process was reported by Chance and
Claus in 1885. Today, various modifications of the process are used depending
upon the hydrogen sulfide concentrations in the gas. The modified Claus
process best suited for acid gas containing 50% or more hydrogen sulfide is the
partial combustion (‘‘once-through’’) process.
Present refinery practice generally provides for removal of hydrogen sulfide
from refinery gas streams by solvent absorption as discussed in the previous section.
The acid gas stream recovered from these treating processes contains some
carbon dioxide and minor amounts of hydrocarbons, but in most cases the hydrogen
sulfide content is over 50%. Therefore, the once-through Claus process is
used in most sour crude refineries to convert the hydrogen sulfide to elemental
sulfur.
In the partial combustion (once-through) process, the hydrogen sulfide–
rich gas stream is burned with one-third the stoichiometric quantity of air and
the hot gases are passed over an alumina catalyst to react sulfur dioxide with
unburned hydrogen sulfide to produce free sulfur. The reactions are:
Burner: 2H2S + 3O2 → 2H2O + 2SO2
Reactor: 2H2S + SO2 → 2H2O + 3S
The burner is located in a reaction chamber which may be either a separate vessel
or a part of the waste heat boiler. The purpose of the reaction chamber is to
allow sufficient time for the combustion reaction to be completed before the gas
temperature is reduced in the waste heat boiler.
Ammonia frequently is present in the Claus unit feed streams and must be
completely destroyed in the reaction furnace to avoid plugging of equipment with
ammonium salts. Specially designed burners and combustion zones have been
developed for this purpose.
The waste heat boiler removes most of the exothermic reaction heat from
gases by steam generation. Many types of waste heat boilers are in use. Usually
they are arranged so that the gas flows through several tube passes in series with
chamber, or ‘‘channels,’’ where a portion of the gases may be withdrawn at
elevated temperatures to use for reheating the main gas flow stream prior to the
catalytic converters. Some elemental sulfur is often condensed and removed from
the gas in the waste heat boiler. In some plants a separate condenser is used after
the waste heat boiler. The gas temperature entering the first catalytic converter
is controlled at about 425 to 475°F (218 to 246°C) which is necessary to maintain
the catalyst bed above sulfur dewpoint in order to avoid saturating the catalyst
with sulfur and thereby deactivating the catalyst. The reaction between hydrogen
sulfide and sulfur dioxide in the converter is also exothermic. Gases from the
converter are cooled in the following condenser for removal of most of the elemental
sulfur as liquid.
The condenser outlet temperatures must be maintained above about 275°F
(135°C) to avoid solidifying the sulfur.
When refineries process crude oils with sulfur contents higher than design,
it is necessary to recover more sulfur in the Claus unit. An economical method
for increasing the capacity of Claus units is the substitution of oxygen for a
portion of the combustion air needed in the reaction furnace. This modification
can increase the capacity for sulfur production by 50% or more at a relatively
small capital cost
