Note: Descriptions are shown in the official language in which they were submitted.
001 -1-
002 METHOD FOR DESULFURIZING GASES WITH IRON OXIDE
004 BACKGROUND OF THE_INVENTION
005 The present invention relates to an improved method
006 for removing hydrogen sulfide from gases. More specifically,
007 the present invention relates to an improvement in a method for
008 removing hydrogen sulfide from produced natural gas with iron
009 oxide.
010 Produced fuel gases such as natural gas and petroleum
011 light gas fractions often contain substantial concentrations of
012 hydrogen sulfide. Normally, the hydrogen sulfide must be
013 removed before the produced gas is suitable for sale or distri-
014 bution through pipelines, in order to meet the product specifi-
015 cations required by commercial gas suppliers. It is also desir-
016 able to remove hydrogen sulfide from fuel gases before they are
017 burned to reduce sulfur oxides emissions. In addition to pro-
018 duced fuel gases, other gases, such as petroleum refinery off-
019 gas streams, are often contaminated with hydrogen sulfide. The
020 presence of hydrogen sulfide can be detrimental when such gases
021 are used in hydrocarbon or petrochemical processing. The
022 presence of hydrogen sulfide also complicates the disposal of
023 refinery gases by, for example, flaring, since the sulfur
024 dioxide produced during burning po es an emissions problem.
025 Various techniques have been suggested for removing hydro-
026 gen sulfide from fuel and other gases. The term "acid gases"
Q27 includes carbon dioxide, as well as hydrogen sulfide, since
028 both the~e compounds can dissolve in water to form corrosive,
02g acidic solutions. In many cases, conventional purification
030 treatment of gas streamst such as fuel gas streams, has as its
031 purpose the removal of the acid gas components, not just hydro-
032 gen sulfide. In such cases, removal of substantial amounts of
033 carbon dioxide in addition to removal of hydrogen sulfide is
034 desirable~ On t~e other hand, produced hydrocarbon gases such
035 as natural gas may contain a substantial amount of carbon
036 dioxide, which is not detrimental to the commercial value of
037 the natural gas. Gas purification processes which remove
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001 -2-
002 carbon dioxide from a gas, as well as hydrogen sulfide, may
003 therefore be needlessly complicated and expensive for use in
004 purifying natural gas, and can reduce the volume of natural gas
005 available for sale or distribution. Accordingly, it is some-
006 times advantageous to remove hydrogen sulfide from natural gas
007 and other produced fuel gases without substantially changing
008 the concentration of carbon dioxide in ~he gas.
009 Amon~ the methods proposed for the removal of hydro-
010 gen sulfide from gases has been the use of solid, dry materials
011 such as zeolites or iron sponge. The use and regeneration of
012 zeolites is often~impractical for processing large quantities
013 of relatively inexpensive natural gas, and zeolites usually
014 remove carbon dioxide as well as hydrogen sulfide. Another
015 technique used for removing hydrogen sulfide from gases has
016 been scrubbing the gases with an aqueous or other liquid solu-
017 tion. Generally, the liquid materials such as aqueous
018 solutions of amines, alkanolamines, potassium carbonate and the
019 like, which have been used to remove hydrogen sulfide from
020 gases by liquid phase scrubbing also remove substantial amounts
021 of carbon dioxide. These aqueous and other liquid aqueous
022 scrubbing processes usually operate by forming a chemical addi-
023 tion product from hydrogen sulfide and, for example, an amine
024 in liquid solution and then stripping the hydrogen sulfide out
025 of the liquid solution in concentrated form. Disposal of this
026 concentrated hydrogen sulfide usually requires a Claus unit, or
027 the like, for converting the hydrogen sulfide to sulfur. Some
028 types of liquid scrubbing-type purification processes actually
029 require a substantial concentration of carbon dioxide in the
030 gas to be treated for operability in desulfurization. Various
031 other solvents, which may be termed physical solvents for hydro-
032 gen sulfide, have also been employed or suygested. Generally,
033 these physical solvents simultaneously remove carbon dioxide
034 from the gas, in addition to hydrogen sulfide. Direct conver-
035 sion of hydrogen sulfide to sulfur in aqueous or other liquid
036 solutions has also been proposed. For example, it has been
037 suggested to dissolve hydrogen sulfide in an alkaline solution
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001 -3~
002 and to oxidize the hydrogen sulfide in the liquid solution with
003 one of a variety of oxidation catalysts to provide elemental
004 sulfur in the liquid.
005 Removal of hydrogen sulfide from gases by contacting
006 .hem with iron oxide has long been known, and is com~ercially
007 practiced. Iron oxide has been used in the for~ of pellets,
008 powder, etc., and has been used on essentially inert solid
009 supports such as wood shavings, clays, etc. Iron oxide on wood
010 shavings, known as iron sponge, is used commercially for
011 removing hydrogen sulfide from produced fuel gases such as
012 natural gas. Iron sponge desulfurization has the advantages of
013 simplicity, econom~ and ability to remove hydrogen sulfide
014 selectively without affecting carbon dioxide in the gas. Iron
015 sponge desulfurization is sometimes called the "dry-box"
016 process. In natural gas purification, the gas is passed
017 through a bed of iron sponge. Hydrogen sulfide reacts with
018 iron oxide (e.g., ferric oxide) to form iron sulfide (e.g.,
019 ferric sulfide). Enough air is added to the natural gas to pro-
020 vide molecular ox~gen to react with the iron sulfide to regene-
021 rate the iron o~ide and form elemental sulfur. The bed of iron
022 sponge can be used until it becomes plugged or deactivated with
023 elemental sulfur. The deactivated bed is then removed from the
024 purification vessel and discarded. Alkali metal salt solutions
025 may be added to the iron sponge bed periodically to increase
0~6 its efficiency.
027 It is often difficult to remove a spent bed of iron
028 sponge from the purifi~ation vessel, since the bed tends to
029 harden into a cohesive mass resistant to convenient removal
030 means such as water spraying. Disposal of the spent sponge is
031 often complicated by a sulfurous odor present in the spent
032 sponge. The improvement of the present invention is directed,
033 in part, to overcoming these and other drawbacks in conven-
034 tional iron-sponge gas-purification methods.
035 SUMMARY OF THE INVENTION
036 In an embodiment, the present invention relates to an
037 improvement in a method for removing hydrogen sulfide from a
038 gas, wherein the gas is contacted with a bed of subdivided
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002 _4_
003 solids containing iron oxide, hydrogen sulfide is reacted with
00~ iron oxide to form iron sulfide; iron sulfide is reacted with
005 molecular oxygen in the gas to form iron oxide and elemental
OOQ sulfur; elemental sulfur is deposited in said bed; and the gas
007 is recovered from the bed, the improvement comprising: intro-
008 ducing into the bed a liquid comprising an amine having the
009 formula
010 R
011
012 N-H
013 l2
014 R
015 where Rl and R2 are selected from hydrocarbon radicals having
016 from 1 to 10 carbon atoms, and where Rl maY be hydrogen ion.
017 I have found that addition of a primary or preferably a
018 secondary amine to a bed of iron sponge used for removing hydro-
019 gen sulfide from natural gas has several surprising and benefi-
020 cial effects. The life of the iron sponge appears to be
021 increased. The spent iron sponge does not form a hard, cohe-
022 sive mass and can be removed from the purification vessel
023 easily. The pressure drop through the iron sponge bed is
024 reduced. The spent sponge has essentiall~ no detectable sul-
025 furous odor, facilitating disposal of the used material.
026 DETAILED D~:SCRIPTION OF THE INVENTION
027 The improved method of the present invention is
028 directed to the removal of hydroqen sulfide from a gas. Gases
029 conventionally treated with iron oxide to remove hydroqen sul-
030 fide can suitably be purified. Gases which can suitabl~ be
031 treated for the removal of hydrogen sulfide include, ~or
032 example, methane, ethane, propane, isGpropane, n-butane, iso-
033 butane, and like para~fins, ethene, propylene, and like
034 olefins, carbon dioxide, molecular nitrogen, molecular oxyqen,
035 lecular hydroqen and water vaPor. The method of the inven-
03~ tion is Particularly adaptable for removinq hydroqen sulfide
037 from natural qas and light qas fractions of Petroleum~ which
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typically contain a major proportion of methane, along with a mixture of
paraffins and olefins. Produced hydrocarbon fuel gas usually also contains
at least a substantial amount of carbon dioxide. For various reasons, air
often becomes mixed with produced hydrocarbon gases, such as natural gas,
during their gathering and transmission. A mixture of natural gas and air
can suitably be treated according to the present invention. Preferably,
mixtures of gaseous hydrocarbons with carbon dioxide, nitrogen, oxygen,
water vapor, hydrogen, and the like are subjected to hydrogen sulfide re-
moval.
According to the invention, a liquid containing a primary or pref-
erably a secondary amine is added to a bed of iron oxide. Amines suitable
for use in the present invention have the general formula
R
~-H
~2
where Rl and R can each be a hydrocarbon radical selected from Cl-C10
hydrocarbons, and where Rl may be hydrogen ion. Suitable hydrocarbon rad-
icals include methyl, ethyl, propyl, benzyl, cycloalkyl, and the like. Pre-
ferred alkyl radicals include 2-propyl, 2-n-butyl~ 3-n-pentyl, 3-methyl, 2-
butyl and cyclohexyl. Preferably, a carbon atom in at least one of R and
R2 is bonded to the nitrogen atom of the amine, and to two other carbon
atoms, so that the carbon atom is bonded to a single hydrogen atom. Aro-
matic substituent radicals should be separated from the nitrogen component
of the amine by at least one carbon atom. Amines which are suitable for use
in the present process are preferably those which have a substantial vapor
pressure at the temperature and pressure employed in the operation. Second-
ary amines are particularly preferred. It will be within the ability of
those skilled in the art to select an amine having a substantial vapor pres-
sure in relation to the particular purification conditions used.
8~
001 -~ -
002 If molecular oxygen is not already present in the gas
003 to be purified by hydrogen sulfide removal with the iron oxide,
004 molecular oxygen is conventionally added to the gas prior to
005 the purification treatment. Preferably, the concentration of
006 molecular oxygen in the gas is adjusted to be at least suffi-
007 cient to permit conversion to elemental sulfur of all the
008 sulfur present in the hydrogen sulfide in the gas according to
009 the stoichiometric reaction of oxygen and hydrogen sulfide to
010 form water and sulfur. In removing hydrogen sulfide from a
011 fuel gas by the improved procedure of the invention, enough air
012 should be mixed with the fuel gas to provide the desired amount
013 of molecular oxygen for reaction with the iron sulfide, e.g.,
014 one mol of molecular oxygen for each two mols of hydrogen sul-
015 fide present in the gas. It will be apparent to those skilled
016 in the art that the concentration of molecular oxygen required
017 will depend, in part, on the degree of hydrogen sulfide removal
018 which is desired. When complete removal of hydrogen sulfide is
019 desired, it is preferred to include at least the stoichiometric
020 amount of molecular oxygen, and particularly pre~erably at
021 least 1.5 times the stoichiometric amount of molecular oxygen
022 re~uired for complete conversion of all the hydrogen sulfide.
023 In cases when a relatively large concentration of molecular
024 oxygen is already present in the gas to be treated, with
025 respect to the hydrogen sulfide concentration, e.g., when air
026 or pure oxygen is to be purified, it is obviously not necessary
027 to add molecular o~ygen to the gas to be treated. When addi-
028 tion of molecular oxygen is necessary, a molecular oxygen-
029 containing gas such as air can be metered into the gas to be
030 purified by any well-known means at the desired rate.
031 Conventional purification conditions may be employed
032 in the improved cperation, including a tempera~ure in the range
033 from 0C to 200C Preferably a temperature between 25C and
034 100C is employed. Conventional purification conditions also
03S include pressures between atmospheric and 50 atmospheres. Pre-
036 ferred pressures are those between atmospheric and 20 atmo-
037 spheres. Conventional purification contact times of from 0.001
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001 ~7~
002 minute to 10 minutes are suitable for carrying out the purifica-
003 tion treatment. Preferably a purification time between 0.01
004 minute and 5 minutes is employed.
005 The iron oxide can be used in any convenient solid,
006 subdivided form, e.g., as pellets, extended on solid supports,
007 etc. Preferably, the iron oxide is disposed on a cellulosic
008 support such as wood shavings. The iron oxide is preferably in
009 the form of ferric oxide, but less-oxidized forms of iron can
010 be used, since they can be oxidized to ferric oxide simply by
011 contact with oxygen at moderate temperature. A variety of suit-
012 able conventional forms of iron oxide will be apparent to those
013 skilled in the art.
014 The iron oxide-containing solid is preferably formed
015 into a bed through which a stream of the gas to be purified is
016 passed. Batch-type contact b~tween the gas and solid bed can
017 be used, but is not practical for most purposes. Conventional
018 purification apparatus can suitably be used for holding the bed
019 of solids and effecting contact between the gas and iron oxide-
020 containing solids.
021 According to the invention, a liquid including an
022 amine, preferably a secondary amine, is introduced into the
023 bed. The amine can be introduced continuously or intermit-
024 tently in either a single treatment or a plurality of periodic
025 treatments. The amine-containing liquid can be any convenient
026 amine, or solution or suspension of an amine such as a water
027 solution, but is preferably a non-aqueous liquid having the
028 amine in solution. ~ preferred non-aqueous solvent is
029 dimethylsulfoxide. The concentration of the amine in a solvent
030 liquid is not critical. A pure liquid amine can be used, or a
031 dilute amine solution can be used. In aqueous solutions, a
032 saturated solution of the amine is preferably used. It is pre-
033 ferred to introduce relatively small quantities of the amine
q34 into the bed of iron oxide periodically, e.g., weekly or daily.
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001 -8-
002 The amount of amine added to the iron oxide at any
003 given time is preferably from about 0.01 part per million, by
004 weight, to about 1 weight percent of the iron oxide present in
005 the bed being treated, The quantity of amine-containing liquid
006 used should be sufficient, during a given treatment, to wet a
007 substantial portion of the bed, and preferably enough liquid is
008 used in each treatment so that more than one-half of the bed is
009 wetted.
010 EXAMPLE
011 The improvement of the invention was employed in a
012 conventional iron sponge unit in commercial nat~ral gas purifi-
013 cation service. Prior to introduction of the amine, removal of
014 spent sponge was found to be extremely difficult and time-
015 consuming, and the spent iron sponge had a distinctly
016 unpleasant sulfurous odor. The life of a bed of iron sponge
l? was felt to be undesirably short. According to the invention,OlB a saturated aqueous solution of equal volumes of
019 ethylcyclohexylamine and N-isopropylcyclohexyamine was added to
020 the soda ash liquid normally used to maintain an alkaline
021 condition in the bed. The amine solution was added to the iron
022 sponge bed every 7 days. Each treatment included about 40 cc
023 of total amine. The amine treatment was continued for about 90
024 days. The useful lie of the iron sponge bed in use was sub-
025 stantially increased. When the iron sponge became spent, it
026 was found to be easily removable with a low-velocity water
027 spray, and the spent iron sponge had essentially no odor. The
028 pressure drop was reduced from 10-15 psi to 4-5 psi~
029 A preferred embodiment of the invention having been
030 described in the foregoing example, a variety of modifications,
031 variations and equivalents of the invention will be apparent to
032 those skilled in the art. These modifications and equivalents
033 are intended to be included within the scope of the invention
034 as defined in the appended claims.
