WO2001056683A1

WO2001056683A1 - Treatment of gaseous fuels, acid gases and off gases

Info

Publication number: WO2001056683A1
Application number: PCT/PL2000/000009
Authority: WO
Inventors: Stefan Ermich; Elżbieta PRUSZYŃSKA-ZAJDA
Original assignee: Stefan Ermich; Pruszynska-Zajda Elzbieta
Priority date: 2000-02-07
Filing date: 2000-02-07
Publication date: 2001-08-09
Also published as: AU2333100A

Abstract

A process for removing hydrogen sulphide or hydrogen sulphide and organic sulphur compounds from a gas mixture, where the process comprises: contacting the gas mixture with a chelate solution in a vessel (11); regenerating the spent first chelate solution, thereby producing oxidised polyvalent metal chelate solution and a separate stream (22) containing elemental sulphur or elemental sulphur and organic disulphides; containing the said separate stream (22) in a vasel (23) with a solution of a chelating agent, thereby reducing the content of insoluble degradation products of the metal chelates in the system, and producing a spent chelating-agent solution; adding fresh chelate solution to a vessel (30); recirculating the chelate solution from vessel (30) to vessel (11).

Description

TREATMENT OF GASEOUS FUELS, ACID GASES AND OFF GASES

FIELD OF THE INVENTION

The present invention relates to a method of environmental friendly complex desulfurization of gases, it is: removing hydrogen sulfide and organic sulfur compounds from the gas stream, organic sulfur compounds and other impurities from the spent regeneration air and the solution degradation products from the byproduct elemental sulfur. Two different solutions are being used: one to convert hydrogen sulfide to elemental sulfur and the mercaptanes to sulfides, and the second solution to clean the sulfur pulp out of the solid impurities and solution degradation products and recycle it to the first solution.

BACKGROUND OF THE INVENTION

Many gaseous fuels, tail gases, acid gases, off gases, and vent air streams contain components which are undesirable and which need to be removed from the gas stream prior to burn, further processing, or discharge to the atmosphere. Most common such a component is hydrogen sulfide (H₂S), next are mercaptanes (RSH) like: methyl mercaptane, ethyl mercaptane, propyl mercaptane, buthyl mercaptane and heavier, then organic sulphides, disulphides etc. Sometimes one has also heavy hydrocarbons as well as nitrogen compounds.

There exist several commercial processes for effecting hydrogen sulfide removal using modern liquid phase oxidation processes, such as a STRETFORD, LO- CAT, IGNIG-CHELATE, SULFEROX, BIOSULFEX, UNISULF, HBPERION, PROTHERM, etc.

It is well known in the prior art that transition metal chelates solutions are useful to convert hydrogen sulfide to elemental sulfur. This is illustrated by the following patents U.S. Pat. Nos. 3,097,925; 3,363,989; 4,009,251 ; 4,014,983; 4,076,621; 4,189,462; 4,218,342; 4,238,462; 4,526,773; 4,880606; BP Nos. 948,270; 999 800; 1,136,901; 2 088 839; PL Pat. Nos. 171214, 173430, 173792; Czech Rep. Pat. Nos. l 17274; 117277; DE Pat. No. 26 06277 C2; EU Pat. Nos. 0 141 872 Al; 0 186 235 Bl; 0 215 505 Bl; 0 244 249 A2.

It also has been proposed that various organic compounds have to be added as stabilizers to be added as stabilizers for the transition metal chelates to slow down the degradation of the chelate solution for example U.S. Pat. Nos. 4,382,918; 4,388,293; 4,421,733; 4,461,754.

It is also known, U.S. Pat. No. 4,036,942; that in the oxidative removal of mercaptanes from gas streams in contact with a suitable aqueous catalyst system alkyl mercaptanes are oxidized to dialkyl disulfides:

4RSH + 0₂ -» 2RSSR + 2H₂0 An immiscible liquid disulfides are formed in the regenerator, and then removed out of the desulfurization unit. One has to notice however that the removal of hydrogen sulfide from the gas is faster than for organic sulfur compounds and to the contrary of sulfur , the disulfides as well as other products may recirculate back to the gaseous phase, or to the air. Considerable amount of organic sulfur compounds left in treated gas stream after hydrogen sulfide removal make the stream odoriferous, and still highly toxic. Moreover the existence of organic sulfur compounds in the gas treated in the sulfur removal unit gives the spent air used for chelate solution regeneration and sulfur a persistent odor. So the air can not be vented and the storage of elemental sulfur may cause problems. Existence of considerable amount of solid impurities and solution degradation products in the sulfur produced from the hydrogen sulfide increase the specific cost of desulfurization and make the produced sulfur even more hard to sell.

SUMMARY OF THE INVENTION

The present invention is a process for environmental friendly, complex desulfurization, ie: removing H₂S and organic sulfur compounds from the gas stream, organic sulfur compounds from the regeneration air and the solution degradation products from the sulfur pulp produced during the desulfurization process, by the use of two different solutions.

The first chelate solution is used to remove the sulfur compounds from the treated gas stream and then is contacted with the air circulating in a closed loop to: regenerate the chelate solution, flotate the sulfur and to remove organic sulfides out of the solution. The removal of the organic sulfur compounds is done by oxidizing most of them to sulfides in the first solution, and then, by stripping the resulted sulfides by the use of flotation air. The air is kept in circulation and the solution in the regenerator is kept a little warmer to speed up the oxidation of the mercaptanes and to concentrate the organic sulfur compounds in the air. A smal portion of the circulating air is directed to the catalytic burner . The second solution of a different composition to the first one is used for the process of washing out the solid impurities and the chelate solution degradation products from the sulfur pulp separated from the first chelate solution. A further aspect of the invention is to use the second solution to activate the chelate solution separated from the sulfur after sulfur melting - if molten sulfur is to be produced. An other aspect of the invention is the use of a second solution combined with heating up the washed sulfur pulp, below the sulfur melting temperature, to deodorize the elemental sulfur - if sulfur cake is to be produced for sale. The advantage of the present invention over the conventional washing is that the composition of the second solution makes possible the conversion of the spent second solution to the first chelate solution - what keeps down the total chelate and water consumption and avoids any sewage to be produced.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for removing hydrogen sulfide or hydrogen sulfide and the organic sulfur compounds in the absence or in the presence of other impurities like heavy hydrocarbons, carbon dioxide, ammonia, and other nitrogen compounds from the treated natural gas, associated gas, tail gas , acid gas or off gas stream by the use of two different solutions.

The first chelate solution of the present process, circulating in a closed loop between a contactor and the regenerator, is used to remove the sulfur compounds from the treated gas stream. The removal is done by way of oxidizing hydrogen sulfide to elemental sulfur, mercaptanes to sulfides, next during the regeneration process, the products of the removal process are separated from the first chelate solution : the solid one by way of flotation, and the gaseous products by carrying it out by the regeneration air . The solid product of the process in a form of a wet pulp consists of water, elemental sulfur and 0.5 to 7 per cent of solid impurities including metal chelates in form of insoluble degradation products as well as soluble components of the chelate solution. The amount of degradation products is a function of proper mixture of adopted chelating agents, to less extent of the adopted transition metal , but may be even larger than 7 per cent if considerable amount of dust , carbon dioxide, alcohols, and other impurities in the treated gas are present . An other source of the solids in sulfur pulp may be water used for the preparation of the chelate solution. The gaseous products of the desulfurization process consist of oxidation products of mercaptanes and traces of unoxidized mercaptanes and hydrogen sulfide mixed with flashed inert gases like carbon dioxide or nitrogen , sometimes other impurities of the processed gas, like heavy hydrocarbons, ammonia, nitrogen compounds, etc. The gaseous products are carried out of the chelate solution by the regeneration air, what is probably a normal procedure , but in the present invention the regeneration air is circulating in a closed loop to complete the oxidation, and concentrate the disulfides in the air. Additional possibility of this invention is to add flash gases to the gaseous products. For some acid gases it is reasonable to add also the outgoing stream of acid gas, and direct it all to the catalytic burner. So in the catalytic burner usually no additional fuel is needed, because some amount of hydrocarbons, present in the off gas or flash gas , is sufficient to be the fuel for the catalytic burning. A further aspect of the invention is the use of water vapor to carry over the gaseous products of the desulfurization process out of the chelate solution , and to concentrate the disulfides by way of condense the water vapor out of the gaseous products stream. This procedure is recommended if a considerable amount of heat is to be removed out of the chelate solution .

An aqueous chelate solution for removing hydrogen sulfide from gas stream useful in the process of this invention comprise the following metal ions: iron, nickel, chromium, molybdenum, platinum, vanadium, palladium, titanium or copper with acids having the formula: 2X-N-R-N-2X, wherein from two to four of the groups X are selected from the following: acetic and propionic acid groups; and from zero to two of the X are selected from 2-hydroxyethyl, 2- hydroxypropyl and CH₂CH₂N-2X where X is a acetic or propionic acid group, and R is ethylene, propylene or isopropylene or alternatively cyclohexane or benzene where the two hydrogen atoms replaced by nitrogen are in 1,2 position, and mixture thereof. The solubilized metal chelate may be formed in aqueous solution by reaction of an appropriate salt, oxide or hydroxide of metal and the chelating agent in the acid form, in the presence of alkali metal or ammonium ions. In the first chelate solution of the present process at least two of the metal ions like iron, nickel, copper, are most advantageously employed. Also at least two chelating agents of the following: nitrilo triacetic acid, 2-hydroxyethyl ethylene diamine triacetic acid, ethylene diamine tetracetic acid, and diethylene triamine pentaacetic acid, are most advantageously employed. It is well known in the art that to slow down the degradation of the solution as a result of degradation of the chelating agent, the compounds having higher affinity for hydroxyl radicals than the chelating agents are used to stabilize the solution. In the first solution the following stabilizers are most advantageously employed: aromatic compounds, compounds having unsaturated carbon-carbon bond, bromide ions, cyanides, nitrites, amino acids, sugars, ascorbates, alcohols, polyols, aliphatic aldehydes, dimethy sulfoxide, organic disulfides. The pH range of the first solution is between 6 and 9 , preferably between 7 and 8.

The second solution of the present invention is used for washing the sulfur pulp removed out of the regeneration tank by the way of flotation . The washed sulfur is mixed with second solution and heated . The second solution dissolves the solid impurities and degradation products present in the pulp, and deodorize the pulp. The spent second solution is then mixed with fresh chelate solution and pumped back to the desulfurization loop. The present invention is that the second solution is an aqueous solution of at least two of the following acids: nitrilo triacetic acid, ethylene diamine tetraacetic acid, 2-hydroxyethyl ethylene diamine tetraacetic acid, diethlene triamine pentaacetic acid, and other components which selection is possible when the particular composition of the existing impurities, other than hydrogen sulfide in the gas is known- what gives, for those skilled in the art., the possibility to calculate mutatis mutandis the proper composition of the second solution , and then the composition of the first chelate solution. Additional possibility of this invention is periodic or continuos admixing a part or all of the second solution in to the fresh first chelate solution ,when high sulfur purity is not essential. The present invention is also the discovery that an industrial water may be used for the fresh chelate solution preparation if the two mentioned chelate solutions are adopted. Heating of the sulfur mixed with the second chelate solution produces additionally some vapors to be recycled and added to the regeneration air . The regeneration air in the organic sulfur treatment loop circulate many times through the regenerator and a heat exchanger to complete the oxidation of mercaptanes, concentrate the organic vapors, and to simplify cooling of the regenerator if needed. Sulfur pulp after washing and deodorization is to be filtered to recover the chelate solution, mixed with the fresh chelate solution, and recycle the solution back to the desulfurization. The present process leaves so little of insoluble components in the pulp that additional water washing make possible sulfur of high purity to be produced. Additional possibility of this invention is the melting of the sulfur pulp after or instead of filtering and using the second chelate solution as activator for the solution coming out of the melter to reduce the production of the sewage caused by degradation of chelate solution when heating .

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to one embodiment of the process of the invention illustrated in FIG. 1, natural gas containing hydrogen sulfide, carbon dioxide, mercaptanes and havy hydrocarbons Cg₊ is introduced through line 10 into a turbulent absorber 11 to contact cocurrently with the first chelate solution which enters the turbulent absorber through line 12 which is fed by pump 13 from regenerator 14. The gas free of hydrogen sulfide and most of the organic sulfur compounds leaves the turbulent absorber through line 15 and the spent first chelate solution with the absorbed sulfur compounds and sulfur passes through line 16 back to the regenerator 14. The regeneration and flotation air provided by the blower 17 is kept in a closed circle between the regenerator and the cooler 18. A small part of the air in circulation is purged through line 19 and directed to the catalytic burner 20 to burn the organic disulfides, hydrocarbons and impurities stripped by the air from the first chelate solution. A part of the clean gas is directed via the line 21 to provide the fuel for the catalytic burner 20. Referring to FIG 2, there is shown another embodiment of the invention in which the sulfur foam separated from the first aqueous solution in the regenerator 14 is supplied via the line 22 into the tank 23 to be mixed with the second solution stored in the tank 24 and fed to the tank 23 via the line 25. The sulfur pulp mixed with the two of the solutions is fed to the heat exchanger 26, to be heated below the sulfur melting point, and then sulfur is separated in filter 27 and send out via the line 28, and the solution is directed by line 29 to the tank 30, where is activated by admixing the fresh solution fed by line 31 from the tank 32 producing the first aqueous chelate solution, which is supplied by line 33 to the regenerator 14. In another embodiment of the process on FIG 2, in the heat exchanger 26 the sulfur pulp, mixed with the two of the solutions, is heated above the sulfur melting point and the melted sulfur is separated in the separator 27 and send out of the installation. The solution separated from the melted sulfur is directed by line 29 to the tank 30 to be activated by admixing the fresh solution fed by line 31 from the tank 32. Mixing those two solutions in the tank 30 produces also the first chelate solution which is supplied by line 33 back to the regenerator 14.

Claims

What is claimed is:

1. A process for removing hydrogen sulfide or hydrogen sulfide and organic sulfur compounds from a gas mixture which process comprises contacting the gas mixture in a contactor with a first chelate solution to produce a purified gas stream having a reduced hydrogen sulfide or hydrogen sulfide and organic sulfur compounds content, and a spent first chelate solution containing solid sulfur or solid sulfur and organic disulfides and having an increased content of the reduced polyvalent metal chelates, and regenerating the spent first chelate solution to obtain oxidized polyvalent metal chelate solution, and products elemental sulfur pulp or sulfur pulp as well as organic disulfides to be removed from the first solution with the regeneration air, and a second solution to contact the elemental sulfur pulp in a separate vessel to produce a sulfur pulp with lower insoluble degradation products content and a spent second solution to be separated from the sulfur pulp and by adding the fresh solution to form the first chelate solution to be recirculated back to the contactor.

2. The process of claim 1 wherein the first chelate solution is the aqueous chelate solution of a mixture of at least two of the metal ions like iron, nickel, copper.

3. The process of claim 1 wherein the second solution is a aqueous solution of a mixture of at least two of the following : nitrilo triacetic acid, ethylene diamine tetraacetic acid, ethylene diamine tetraacetic acid, diethylene triamine pentaacetic acid.

4. The process of claim 1 wherein the fresh solution is a aqueous buffered solution of a mixture of at least two of the metal ions like: iron, nickel, copper, vanadium, chromium and a mixture of at least two of the following: nitrilo triacetic acid, ethylene diamine tetraacetic acid, ethylene diamine tetraacetic acid, diethylene triamine pentaacetic acid.

5. The process of claim 1 wherein the organic disulfides are removed from the first chelate solution by water vapor or water vapor mixed with the air .

6. The process of claim 1 wherein the spent regeneration air containing organic disulfides is recirculated to the regeneration.

7. The process of claim 1 wherein the second solution mixed with the sulfur pulp is heated below the sulfur melting temperature.

8. The process of claim 1 wherein the second solution mixed with the sulfur pulp is heated above the sulfur melting temperature in the sulfur melter, and the second solution separated from the molten sulfur and mixed with the fresh solution is recirculated back to the contactor.

9. The process of claim 1 wherein the vapors produced during the contact of second solution with the sulfur pulp are recirculated back to the regeneration loop.