Process for removal of H2S from gas processing streams

A process for removing H2S and mercaptans from gas streams. The process comprises contacting such gas streams with an aqueous solution of copper complex of a water soluble amine to form water insoluble copper sulfide and regenerate free water soluble amine. The copper sulfide is removed from the system and recovered. Lastly, additional copper complex of the water soluble amine is formed by reacting the regenerated water soluble amine with a copper compound.

FIELD OF THE INVENTION 
This invention relates to the removal of H2S and mercaptans gas streams. 
BACKGROUND OF THE INVENTION 
In the article, Hydrocarbon Gas Processing, Chemical Engineering, 
September, 1991; pp 41-47, the author G. Samdani makes the following 
remarks which relate to problems associated with hydrogen sulfide and 
mercaptan contaminated gas processing streams: 
"Hydrogen sulfide is the major nuisance facing gas processors. Present in 
refinery fuel and natural gas--along with modicums of carbonyl sulfide, 
carbon disulfide and thiols or mercaptans--H.sub.2 S is highly toxic and 
corrodes pipelines. It is also a severe odor nuisance, even in minute 
concentrations, and is categorized as a hazardous air pollutant--with COS 
and CS.sub.2 --in Title III of amendments to the new U.S. Clean Air Act 
(CAA) (CE, December 1990, p. 24)." 
"In the past, many sour natural-gas streams have been treated by removing 
the H.sub.2 S by an amine process, which generates an offgas stream 
containing the H.sub.2 S along with other acid gases, such as CO.sub.2. 
Sulfur has usually been recovered from this tailgas only when marketable 
quantities are present. The sulfur recovery process most often used has 
been the Claus process (sidebar, p. 45). If the amine tailgas contains 
less than economically recoverable amounts of sulfur, the practice has 
typically been to vent or flare the stream. `New environment pressures are 
making the venting or flaring of sulfur less and less acceptable." 
This article as well as the paper, Research Needs for Acid Gas Kinetics and 
Equilibria in Alkanolamine Systems, G. T. Rochelle, 70th Annual 
Association Convention , Mar. 11-12, 1991, San Antonio, Tex. are 
incorporated herein by reference for purposes of illustrating the state of 
the art with respect to H2S and mercaptan removal. 
Several of the presently used commercial processes for H2S removal are 
summarized below: 
Summary of Current Technology for H2S Removal 
Hot Potassium Carbonate System 
EQU K2CO3+H2S .fwdarw. KHS+KHCO3 
H2O 
EQU K2CO3+H2CO3 .fwdarw. 2 KHCO3 
Amine Based Removal Systems 
EQU RNH2+H2S .fwdarw. RNH3.sup.+ +HS.sup.- 
Solvent 
EQU RNH2+H2CO3 .fwdarw. RNH3.sup.+ +HCO3.sup.- 
Amine Based Systems Include 
Sulfinol--Uses sulfolane as the solvent and diisopropanolamine (DIPA) as 
the collector. 
Rectisol--Uses cold methanol as the solvent and DIPA as the collector. 
Selexol--Uses the dimethylether of polyethylene glycol as the solvent and 
DIPA as the collector. 
Flexsorb PS--Uses sulfolane as the solvent and a proprietary hindered amine 
as the collector. 
Flexsorb SE--Uses water as the solvent and a proprietary hindered amine as 
the collector. 
DEA/MEA/H2O--Numerous systems have been developed that utilize water as the 
solvent and a mixture of 
MEA and/or DEA as the collector(s). 
Designer Amines--Various designer amines, similiar to Flexsorb SE, have 
been developed as collectors. These amines are sterically hindered and use 
water or a polar organic as the solvent. 
Iron Spronge Removal System 
EQU Fe2O3+3 H2S .fwdarw. Fe2S3+3 H2O 
Claus Removal System 
Catalyst 
EQU H2S+O2 .fwdarw. 2 H2O+2 S 
This process usually is used in conjunction with amine collectors, since 
the gas stream can not be treated directly in the presence of O2. 
In many instances the streams treated contain amounts of CO2 in addition to 
the H2S and mercaptans. CO2 being acidic is also removed along with the 
H2S or mercaptans using current processes. This severely affects the 
effectiveness and efficiency of the prior art systems in removing H2S and 
mercaptans. Many of the prior art systems are incapable of reducing the 
amount of H2S removed to a level where it meets federal and local 
pollution laws. 
OBJECTS OF THE INVENTION 
It would be a valuable contribution to the technology of H2S and mercaptan 
removal if a process were available which could selectively remove H2S to 
a level below 0.1 ppm even though CO2 were present in high concentrations. 
Also of benefit would be a gas scrubbing process that could remove large 
amounts of H2S and mercaptans economically using catalytic amounts of a 
regenerable active carrier in combination with inexpensive readily 
available chemicals. Also advantageous would be a system that would 
operate efficiently under broad ranges of temperature and pressure. A 
further desirable feature of such a process would be that it could operate 
in either acidic or basic environments. These valuable and desirable 
properties and characteristics are afforded by practicing the invention 
hereafter described and, therefore, become the objects of this invention.

THE INVENTION 
The present invention comprises a process for removing H2S and mercaptans 
from gas streams. It consists of the following sequential steps: 
The first step comprises contacting such gas streams with an aqueous 
solution of copper complex of a water soluble amine. This contact should 
be done under conditions to allow for the formation of a water insoluble 
copper sulfide and the regeneration of free water soluble amine. In the 
next step, the copper sulfide and regenerated water soluble amine are 
separated and the copper sulfide recovered. Finally, as a last step, 
additional copper complex of the water soluble amine is formed by 
contacting the regenerated water soluble amine with a copper compound. In 
this step the copper compound desirably is present in a large molar 
excess. This contact of the regenerated free water soluble amine with the 
copper compound is repeated until the copper compound is exhausted. 
In preferred embodiments of the invention the water soluble amine is a 
primary, sterically hindered alkanol amine. The alkanol amines desirably 
contain a terminal hydroxy alkyl group. A most preferred practice of the 
invention utilizes as the water soluble amine, 
2-amino-2-hydroxymethyl-1,3-propanediol, sometimes hereafter referred to 
as "tris". 
The source of copper used to prepare the amine complex may be selected from 
any cuprous or cupric compound, either water soluble or insoluble. Simple 
water insoluble copper compounds are preferred, particularly the copper 
oxides and most preferably cupric oxide. When water soluble salts are 
used, the selection should be governed by the operating pH of the process 
and the anion of the salt since certain anions may add undesirable 
characteristics to the process stream being treated. 
The Water Soluble Amines 
To be operative in the practice of the invention, the water soluble amines 
should have the ability to form stable copper complexes yet at the same 
time be incapable of forming complexes with copper sulfides. These amines 
benefically contain at least one amino group and more preferably hindered 
primary amines with one or more hydroxy alkyl groups. An amine meeting 
these criteria is 2-amino-2-hydroxymethyl-1,3-propanediol which is a 
relatively non-toxic material and is water soluble. Tris in its free amine 
form, or complexed with copper, produces a buffered pH ranging between 
5-11 which represents a preferred pH range for practicing the process of 
the invention. 
Examples of Water Soluble Amines 
Amines 
Methyl amine, ethyl amine, n-propylamine, n-butylamine, dimethylamine, 
diethylamine, di-n-propylamine, di-n-butylamine, trimethylamine, 
triethylamine, tri-n-proylamine, tri-n-butylamine, 2-methoxyethylamine, 
2-methoxypropylamine, di-2-methoxyethylamine, ethylene diamine. 
Hindered Amines 
Tertiary butylamine. 
Alkanol Amines 
Monoethanol amine, monopropanol amine, monoisopropanol amine, diethanol 
amine, dipropanol amine, diisopropanol amine, triethanol amine, 
tripropanol amine, triiopropanol amine, methyldiethanol amine, 
dimethylmonoethanol amine, diethylmonoethanol amine, 2-amino-1-propanol, 
2-amino-1-butanol, 2-2'-aminoethoxyethanol amine. 
Hindered Alkanol Amines 
2-amino-2-methyl-1,3-propanediol, 2-amino-2-ethyl-1,3-propanediol, 
2-amino-2-hydroxymethyl-1, 3-propanediol. 
The invention utilizes these amines in the form of aqueous solutions. An 
important feature of the invention is that the amount of amine in relation 
to the source of copper used to make the amine complex is such that there 
is present in such solutions between 0.05 to 5 moles of amine. In the case 
of 2-amino-2-hydroxymethyl-1,3-propanediol, amounts ranging between 0.1-4 
moles will produce excellent H2S and mercaptan removal. 
The Copper Compounds 
When copper oxides, the preferred copper source for preparing the amine 
copper complexes are used, the preferred pH of the tris solutions is 
within the alkaline range, desirably between 9-11. Cupric oxide represents 
an excellent source of copper to prepare the amine copper complexes. 
Copper salts such as the sulfates, nitrates and chlorides may be used but 
are not preferred as a source of copper for the amine complexes. Finely 
divided copper may be used, e.g. copper metal or mixed copper-copper 
oxides having particle size preferably less than a few angstroms in size. 
Such elemental forms of copper are considered as being within the term, 
"copper compound". 
Examples of Copper Salts 
Cuprous Salts 
Cuprous bromide, cuprous carbonate, cuprous chloride, cuprous fluoride, 
cuprous oxide, cuprous sulfate, cuprous thiocyanate. 
Cupric Salts 
Cupric acetate, cupric diammine dichloride, cupric hexaammine dichloride, 
cupric tetrammine sulfate, cupric bromide, cupric carbonate, cupric 
chloride, cupric citrate, cupric fluoride, cupric hydroxide, cupric 
nitrate, cupric oxide, cupric sulfate, cupric tartrate, cupric 
thiocyanate. 
When water insoluble copper compounds are used in the practice of the 
invention they are employed as a aqueous slurry. They are present in such 
slurries in relatively large amounts in relation to the amine dissolved in 
the aqueous phase. Generally the copper compound is present in the slurry 
to provide at least one mole of copper metal. It is desirable that the 
number of moles of copper present in the slurry represent 1-300 or more 
moles of copper. 
The Chemistry of the Removal System 
x tris 
EQU CuO(s)+H2O .fwdarw. Cu(tris)x!.sup.2 +2 OH.sup.- +CuO(s) 
A catalytic amount of 2-amino-2-hydroxymethyl-1, 3-propanediol (tris) is 
added to a large excess of cupric oxide in water to form a slurry. The 
tris complexes with the cupric oxide where x preferably is 1 to 4. This 
cupric-tris complex is soluble even at the basic pH of the system, which 
is approximately 10.0 to 10.5. Since the cupric oxide is insoluble under 
basic conditions, the pH of the system is determined totally by the 
concentration/buffering capacity of the tris. 
Other cupric salts may be utilized resulting in the system being operated 
at a lower pH. 
Absorption of H2S: 
H2O 
EQU H2S .fwdarw. H.sup.+ +HS.sup.- 
H2O 
EQU HS.sup.- H.sup.+ +S.sup.-2 
H2O 
EQU S.sup.-2 +Cu(tris)x!.sup.+2 .fwdarw. CuS(s)+x tris 
The cupric-tris complex reacts with all available sulfide anion in 
solution. The tris that is liberated solubilizes more CuO thereby creating 
a catalytic system for sulfide removal. 
CO2 Absorption 
Tris does not absorb any CO2 because of a combination of steric hindrance 
and the strong affinity for the tris to complex with the divalent copper. 
Tris forms a very stable complex with copper. Since the tris is utilized 
in catalytic amounts, all of the tris is coordinated to the divalent 
copper until the CuO has been totally consumed to form insoluble CuS. 
Low Level H2S Removal 
The solubility product for CuS is 8.5.times.10.sup.-45. If the 
concentration of tris that is used is 0.1 molar, and 4 molecules of this 
tris coordinate to 1 molecule of divalent copper, then the theoretical 
level of H2S that is left in solution is 3.4.times.10.sup.-43 molar. 
H2S and Mercaptan Removal is Independent of pH 
Cupric sulfide is insoluble even under acidic conditions because of its 
very small solubility product. An illustration of this pH independence is 
given below. 
Assume a pH of 2, a tris concentration of 0.1 molar which yields a divalent 
copper concentration of 0.025 molar, and a H2S concentration of 0.1 ppm. 
This yields, 
##EQU1## 
Solving for S.sup.-2 ! gives 
EQU S.sup.-2 !=1.1.times.10.sup.-24 molar 
Substituting this value and the copper concentration, the solubility 
product gives: 
EQU Cu.sup.+2 !S.sup.-2 !=0.025!1.1.times.10.sup.-24 !=2.8.times.10.sup.-26 
which is &gt;Ksp=8.5.times.10..sup.-45 
Therefore, even at a pH of 2, the system will remove H2S to well below 0.1 
ppm. 
After all of the cupric oxide has been exhausted, the spent slurry is 
discharged to regeneration and the absorption unit is charged with a fresh 
CuO/tris/H2O mixture. 
The tris is recovered by simple filtration of the slurry to isolate the 
aqueous tris from the solid CuS. The aqueous tris is adjusted to the 
proper molarity and added to a fresh charge of CuO. This slurry is then 
transferred back to the absorption unit. 
The solid cupric sulfide that has been isolated from the tris can be 
oxidized to either elemental sulfur or sulfur dioxide: 
EQU 2 CuS(s)+Air (O2) .fwdarw. 2 S(s)+2 CuO(s) 
EQU 2 CuS(s)+Air (3 O2) .fwdarw. 2 SO2(g)+2 CuO(s) 
Commercial Practice of the Invention 
One of the advantages of the invention is that it is capable of utilizing 
conventional gas scrubbing equipment. Generally upflow scrubbing systems 
give the best results. The process thus described allows for a closed loop 
type operation wherein fresh or regenerated copper is added to the system. 
In many systems this can be done incrementally so that as copper used to 
make the tris complex is converted to insoluble copper sulfide which is 
removed from the system, fresh copper compound is added to maintain the 
amount of copper compound substantially constant. Similarly, a side stream 
of tris solution can be steadily or incrementally withdrawn, adjusted for 
molar concentration and be returned to the scrubber. 
Time and Temperature Relationships 
The temperature at which the process may be varied with increasing 
temperatures causing the conversion of tris to copper complex and its 
subsequent conversion to copper sulfide being more rapid. The reactions 
can be done at atmospheric or superatmospheric pressures. Vacuum 
environments may be used also. The reactions proceed readily at room 
temperature (Approximately 24 degrees C.) Elevated temperatures causing 
loss of water due to vaporization or loss of reactants should be avoided. 
Operating temperatures will in many instances be governed by the 
temperatures utilized in existing recovery systems. In nearly all cases 
the conversion of the tris and the other amines to their copper complexs 
occurs within a matter of a few minutes upon contact with the copper 
compound. 
EXAMPLES 
Example of A Typical Pilot Unit For H2S Removal 
Conditions 
H2S concentration--100 ppm H2S 
Flow rate--100,000 scf/day 
Temperature--25 degrees Centigrade 
Pressure--atmospheric 
Size of absorption unit--10'.times.10" I.D. 
Capacity 
Volume of Absorption Unit 
EQU V=(pi)(r).sup.2 (l) 
##EQU2## 
Loading CuO=10% by volume=942.0 cu. in. 
##EQU3## 
CuO=97.6 kilograms Loading 
H2O=65% by volume=6123 cu. in. 
##EQU4## 
H2O=100 kilograms=100 liters Tris=1.0 molar 
##EQU5## 
W=12.1 kilograms H2S Absorbed Under Test Conditions 
Overall Reaction 
##EQU6## 
Moles H2S=1227 moles Lifetime of Absorption Solution Under Test Conditions 
##EQU7## 
Time=106 days 
Applications 
The invention may be used to replace or modify existing amine-based H2S 
collector systems. It can be used in a variety of refinery sweetening 
operations. It also can be set up to treat small remote gas and oil wells. 
It is suitable for treating H2S generating processes.