Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2002-03-14
2003-02-18
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S106000, C095S123000, C095S143000, C095S235000, C095S236000, C423S228000, C423S567100
Reexamination Certificate
active
06521020
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a novel integrated process for removing hydrocarbon and other organic contamination from feed gas streams for Claus reactors. This invention also relates to the use of unique inorganic molecular sieves of the type containing octahedral coordinated metal sites, such as coordinated octahedral titanium, in processes for removing hydrocarbon and other organic contamination from hydrogen sulfide-containing streams.
BACKGROUND OF THE INVENTION
Natural gas as well as refinery gas streams are commonly contaminated with sulfur compounds, especially hydrogen sulfide (H
2
S). If substantial amounts of hydrogen sulfide are present, regulatory restrictions dictate special precautions must be taken to purify the gas streams. In non-polluted areas, generally a maximum of two tons per day of sulfur are allowed to be vented as sulfur oxide (SO
2
) flare-off gas per processing plant. In populated areas even more stringent restrictions are applied.
The first step in H
2
S removal from natural gas and/or refinery streams is accomplished by an acid gas removal unit. This unit removes substantial amounts of H
2
S and CO
2
from the processing stream. The off-gas of this stream contains predominantly CO
2
and H
2
S. The sulfur from this off-gas stream is removed by the Claus reaction which produces salable elemental sulfur. The remaining CO
2
may be safely vented to the atmosphere.
The Claus process was discovered over 115 years ago and has been employed by the natural gas and refinery industries to recover elemental sulfur from hydrogen sulfide-containing gas streams for the past 50 years. Briefly, the Claus process for producing elemental sulfur comprises two major sections. The first section is a thermal section where H
2
S is converted to elemental sulfur at approximately 1,800-2,200° F. No catalyst is present in the thermal section. The second section is a catalytic section where elemental sulfur is produced at temperatures between 400-650° F. over an alumina catalyst. The reaction to produce elemental sulfur is an equilibrium reaction, hence, there are several stages in the Claus process where separations are made in an effort to enhance the overall conversion of H
2
S to elemental sulfur. Each stage involves heating, reacting, cooling and separation. A flow diagram of the Claus process is shown in
FIG. 1
which will be explained in more detail below.
In the thermal section of the conventional Claus plant, a stoichiometric amount of air is added to the furnace to oxidize approximately one-third of the H
2
S to SO
2
and also burn all the hydrocarbons and any ammonia (NH
3
) present in the feed stream. The primary oxidation reaction is shown as follows:
2H
2
S+3O
2
→2SO
2
+2H
2
O (1)
This reaction is highly exothermic and not limited by equilibrium. In the reaction furnace, the unconverted H
2
S reacts with the SO
2
to form elemental sulfur. This reaction is shown as follows:
2H
2
S+SO
2
⇄3S°+2H
2
O (2)
Reaction (2) is endothermic and is limited by equilibrium.
In the catalytic section of the Claus process, the unconverted hydrogen sulfide and sulfur dioxide from the thermal stage are converted to sulfur by the Claus reaction (2) over an alumina catalyst. Typically, there are three stages of catalytic conversions. Important features of the Claus reaction in the catalytic stage are that the reaction is equilibrium limited and that the equilibrium to elemental sulfur is favored at lower temperatures.
The Claus process was modified in 1938 by I.G. Fabenindustrie and various schemes of the modified process are utilized today. For feed gas streams containing approximately 40% H
2
S, the balance carbon dioxide (CO
2
) and water (H
2
O), the once through Claus process is generally employed in which all of the acid gas is fed directly to the burner. Three catalytic stages are typically utilized after the initial thermal stage. This scheme will generally produce an overall recovery of 95-97% sulfur. If this recovery efficiency is acceptable, no further processing is required. However, if the recovery efficiency is not high enough (for a variety of reasons and, in particular, environmental constraints) an advanced Claus process such as Comprimo's Super Claus process which has a sulfur efficiency of 99.0% can be utilized. This process consists of the replacement of the final Claus reaction stage by, or the addition of, a reaction stage featuring a proprietary catalyst to promote the direct oxidation of hydrogen sulfide to sulfur selectively in the Claus tail-gas. Air is injected upstream of the reactor. The hydrogen sulfide and oxygen react over the catalyst via the following reaction:
2H
2
S+O
2
→2S°+2H
2
O (3)
If a sulfur recovery efficiency of greater than 99% is required, a tail-gas cleanup unit (TGCU) needs to be employed. This type of unit allows for an overall sulfur recovery efficiency of 99.8%. In the United States, a sulfur recovery efficiency of 99.8+% is required for Claus production units generating greater than or equal to 50 STSD of elemental sulfur, hence, a TGCU such as the Shell Scot process is often required. Such processes coupled with a sulfur recovery unit (SRU) can meet and exceed a sulfur recovery efficiency of 99.8+%.
There are other modifications to the basic Claus process. One particular modification to the Claus process that is widely used today is the “Split-Flow” process for feed gas streams containing 30-35% H
2
S or less concentrations. In this scheme, 40-60% of the feed gas is passed directly to the catalytic section, bypassing the noncatalytic reaction furnace. This process is utilized to achieve a hotter temperature and a more stable flame in the furnace. The bypassed feed joins the furnace effluent after the condenser and the combined flow enters the first catalytic converter. The sulfur recovery efficiency for this scheme is normally 1-3% lower than the conventional once-through or straight-through process. Basic descriptions of Claus process schemes and additional tail-gas cleanup units are given in the
Kirk Othmer Encyclopedia of Chemical Technology
, Vol. 23, pp. 440-446, the contents of which are incorporated herein by reference.
In the Claus reaction scheme, it can be seen that combustion air is a critical variable in maintaining a high efficiency operation in the thermal section. Hydrocarbon impurities and other feed gas contaminants not only cause a high temperature operation (up to 2,500° F.) such contaminants cause problems in maintaining the correct amount of combustion air. Additionally, it should be noted that in the first catalytic stage, any carbonyl sulfide (COS) and carbon disulfide (CS
2
) that are formed in the reaction furnace and/or any such materials entering the catalytic section with the feed gas such as in the split flow process must be hydrolyzed to hydrogen sulfide and CO
2
if they are to be removed. Any sulfur in the form of COS or CS
2
leaving the first catalytic stage cannot be recovered by the Claus process because of the lower temperatures used in the second and subsequent catalytic stages. A bottom bed temperature of 600-640° F. is required in the first catalytic stage for good hydrolysis which in turn requires an inlet bed temperature greater than 500° C. Normal operation for the inlet bed temperature is generally 450-460° F., hence the higher temperature for the former does not favor the equilibrium to elemental sulfur formation.
In the Claus process design and operation to date, it is the design and operation of the reaction furnace, reaction furnace burner and the first catalytic converter or stage which are critical in an effort to achieve a successful operation. The burner is a critical piece of equipment in that it must be able to burn one-third of the incoming H
2
S while also burning all the impurities in the feed gas stream, namely, paraffin and aromatic hydrocarbons, ammonia and low molecular weight organics at substoichiometric air conditions. This is critical not only to
Butwell Kenneth F.
Dolan William B.
Kuznicki Steven M.
Engelhard Corporation
Lindenfeldar Russell G.
Spitzer Robert H.
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