Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture
Utility Patent
1998-01-23
2001-01-02
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
C423S220000, C423S228000, C423S229000, C423S230000
Utility Patent
active
06168768
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for producing low sulfur synthesis gas (syngas) from natural gas with C
4+
C
5+
hydrocarbon recovery. More particularly the invention relates to producing very low sulfur syngas from sour natural gas with C
4+
C
5+
hydrocarbon recovery and to the use of the syngas for hydrocarbon synthesis. The sulfur content of the syngas is less than 10 vppb and preferably less than 3 vppb. The process includes treating the natural gas by amine scrubbing, low temperature hydrocarbon and sulfur separation, followed by contact with zinc oxide and then nickel.
2. Background of the Invention
Hydrocarbon synthesis (HCS) processes are well known and include fixed bed, fluid bed and slurry type processes in which a synthesis gas (syngas) comprising a mixture of H
2
and CO is reacted in the presence of a suitable Fischer-Tropsch type of hydrocarbon synthesis catalyst at conditions effective to form hydrocarbons, and preferably paraffinic hydrocarbons which are solid at standard room temperature conditions of temperature and pressure. The syngas is produced by reacting a low molecular weight hydrocarbon gas with oxygen and steam via well known processes which include partial oxidation, catalytic steam reforming and combination thereof, using a fixed or fluid catalyst bed. In a fluid bed syngas generation (FBSG) process, partial oxidation and steam reforming both occur in the presence of the steam reforming catalyst. This process has the advantage of superior heat and mass transfer. In autothermal reforming the hydrocarbon is first partially oxidized and then separately catalytically steam reformed. These and other syngas processes and their relative merits are discussed, for example, in U.S. Pat. Nos. 4,877,550; 4,888,131 and 5,160,456. A preferred source of the low molecular weight hydrocarbon is natural gas in which the hydrocarbon comprises primarily methane with minor amounts (e.g., ~1-10%) of C
2+
hydrocarbons, including C
4+
hydrocarbons. Other natural gas components include nitrogen, carbon dioxide, water vapor and sulfur in the form of sulfur bearing compounds including H
2
S, mercaptans (RSH), other organic sulfides generally, carbonyl sulfide (COS) and sometimes minor amounts of carbon disulfide. Sulfur in the feed to a syngas generator will poison the steam reforming catalyst and result in a loss of syngas productivity. Certain HCS catalysts are easily poisoned and permanently deactivated by these sulfur bearing compounds. Those comprising a cobalt catalytic component are particularly sensitive and as little as 0.1 vppm (volume parts per million) of sulfur compounds present in the syngas feed to the HCS reactor will permanently deactivate the catalyst in less than 10 days. Even levels as low as, for example, 10 vppb (volume parts per billion) are unacceptably high for a commercial HCS plant. As the catalyst deactivates, hydrocarbon production decreases and the reactor has to be taken off line for catalyst replacement. Consequently, the ability to achieve highly productive hydrocarbon synthesis with such catalysts, on a sustainable basis, has not yet been achieved. It would be an improvement to the art to be able to produce syngas having less than 10 vppb of sulfur compounds from sour natural gas by a method which also recovers the valuable C
4+
and/or C
5+
hydrocarbons.
SUMMARY OF THE INVENTION
A process for producing low sulfur synthesis gas (syngas) from natural gas which contains C
4+
C
5+
hydrocarbons, with recovery of these hydrocarbons from the gas, comprises scrubbing or contacting the gas with a liquid sulfur absorbent to remove most of the sulfur, followed by low temperature cooling to remove more sulfur compounds and the C
4+
C
5+
hydrocarbons, and then contacting the sulfur and hydrocarbon reduced gas first with zinc oxide and then nickel to reduce the sulfur content to less than 0.1 vppm (volume parts per million) and preferably less than 80 vppb (parts per billion), before it is passed into the syngas generator. The syngas exiting the syngas generator is then contacted with zinc oxide to remove remaining sulfur from the gas. This process produces a syngas feed having less than 10 vppb of sulfur in the form of sulfur bearing compounds and recovers the valuable C
4+
C
5+
hydrocarbons from the feed, so that they are not wasted by being passed into the syngas generator. The C
4+
C
5+
hydrocarbons recovered from the natural gas are upgraded by hydrorefining and fractionation. The zinc oxide and nickel react with the sulfur compounds remaining in the gas after the scrubbing and cooling, to form zinc sulfide and nickel sulfide. The nickel is preferably nickel metal and in a particularly preferred embodiment the nickel is supported on a support material. Reducing the sulfur content of natural gas fed into an FBSG unit down to less than 0.1 vppm and preferably less than 80 vppb substantially reduces catalyst deactivation in a fluid bed syngas generator and increases the syngas productivity. When the sulfur content in natural gas fed into a FBSG containing a nickel reforming catalyst was reduced to less than 80 vppb, it resulted in less than a 1% per day activity loss. The low sulfur feed comprising primarily methane, is then fed into a syngas generating unit, along with steam and oxygen or air, and preferably oxygen, to produce a syngas comprising a mixture of H
2
and CO. The syngas is then contacted with zinc oxide to reduce the sulfur level in the syngas to the less than 10 vppb and preferably less than 3 vppb levels desired for feeding the syngas to a hydrocarbon synthesis (HCS) reactor. It also serves as a guard bed in the event of a sulfur breakthrough upstream of the syngas generator and from sulfur contaminants present in the syngas generating unit and from the other feed components. The very low sulfur syngas is then fed into an (HCS) reactor in which the H
2
and CO react in the presence of a suitable Fischer-Tropsch type of hydrocarbon synthesis catalyst at conditions effective to form hydrocarbons. In a slurry HCS process, at least a portion of the synthesized hydrocarbons comprise the slurry liquid and are solid at standard room temperature conditions of temperature and pressure (e.g., 75° F. and atmospheric pressure).
In a broad sense, the invention comprises removing sulfur compounds and C
4+
C
5+
hydrocarbons from natural gas to form a low sulfur gas comprising mostly methane, which contains less than 0.1 vppm and preferably less than 80 vppb of sulfur, by liquid absorption, low temperature separation and contact with zinc oxide followed by nickel. The low sulfur methane gas is then passed into a syngas generating unit to produce a syngas comprising a mixture of H
2
and CO having a low sulfur content or used for any other purpose. In a further embodiment, the syngas is contacted with zinc oxide to insure that the level of sulfur in the gas remains at less than 10 vppb and preferably less than 3 vppb of sulfur. By sulfur is meant sulfur compounds which include predominantly H
2
S and one or more of mercaptans (RSH), other organic compounds generally, carbonyl sulfide (COS) and CS
2
. Except for the H
2
S, all of the other sulfur bearing compounds may be considered as organic sulfur compounds.
The liquid absorption or scrubbing may selectively remove only the sulfur compounds or it may remove both the sulfur compounds and CO
2
, if desired. The choice depends on the amount of CO
2
present in the natural gas and the extent to which various CO
2
and sulfur compound removal methods, such as amine scrubbing, are feasible. As a practical matter, if the amount of CO
2
exceeds about 2 mole % of the gas, removal is necessary to prevent plugging of the low temperature hydrocarbon separation unit downstream of the scrubbing. The use of amine scrubbing for either sulfur or both sulfur and CO
2
removal from gas streams, including natural gas, is well known and is disclosed, for example, in
Alexion Dennis G.
Fedich Robert B.
Glass, Jr. James P.
O'Connor Richard P.
Say Geoffrey R.
Exxon Research and Engineering Company
Griffin Steven P.
Nave Eileen E.
Provoost Jonathan N.
Simon Jay
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