WO2005053820A1

WO2005053820A1 - Desulfurization of gases for gaseous fuels treatment or odour control

Info

Publication number: WO2005053820A1
Application number: PCT/PL2003/000133
Authority: WO
Inventors: Stefan Ermich; Elzbieta Pruszynska-Zajda
Original assignee: Stefan Ermich; Elzbieta Pruszynska-Zajda
Priority date: 2003-12-03
Filing date: 2003-12-03
Publication date: 2005-06-16
Also published as: AU2003295292A1

Abstract

A method for better sulfur compounds removal from gas streams by using modem liquid phase oxidation process is disclosed. The present invention relates to treating: large gaseous fuels streams, small biogas streams, as well as to control odour problems for streams of biogas or vent air etc; it is for: removing hydrogen sulfide and organic sulfur compounds from the gas or air stream, organic sulfur compounds from the spent regeneration air and the solution impurities & and degradation products from the by-product elemental sulfur. Three solutions are to be used: first one to desulfurize gas stream, and the second solution to treat elemental sulfur. The spent second solution is recycled to the first solution for gas stream treatment. The third solution is also used to lower the siloxanes content of the desulfurized biogas. Byproduct: elemental sulfur is dry -so no centrifuge or press is needed .

Description

DESULFURIZATION OF GASES FOR GASEOUS FUELS TREATMENT OR ODOUR CONTROL

FIELD OF THE INVENTION

The present invention relates to a method of environmental friendly complex desulfurization of large gaseous fuels streams, as well as for treatment of small streams of biogases or streams with low sulfur compounds content, i.e. the method of more complex treating of natural gas, acid gas off gases from chemical plants and, preferably in simplified version, for control of odour derived from biogas or other vent air streams. Complex desulfurization means: removing hydrogen sulfide and organic sulfur compounds from the gas stream, organic disulfides and other impurities from the spent regeneration air and the solution degradation products from the by-product elemental sulfur, and dehydrate sulfur pulp, whereby the stream is treated with two different solutions: one to convert hydrogen sulfide to elemental sulfur and the mercaptanes to disulfides, 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. Moreover, in a further aspect of the invention a third solution is used for removing silicone compounds from biogases after desulfurization.

A simplified version of the present invention for treating small streams of ventilation air or biogas is possible, because the resulting by-product powder like dry sulfur pulp is easy to be separated from the solution - so no large installation for treating such a small air or biogas stream is needed.

BACKGROUND OF THE INVENTION

Many gaseous fuels, tail gases, acid gases, off gases as well as biogases, off air from sewage plants and vent air streams contain components which are undesirable and which need to be removed from the gas stream prior to bum, 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 sulfides, disulfides etc. Sometimes one has also silicone compounds in biogases: siloxanes. 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, 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. 173792; 173430; 171214; Czech Rep. Pat. Nos. 117274; 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 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 sulfur produced from the hydrogen sulfide increase the specific cost of desulfurization and make the produced sulfur even more hard to sell. Moreover, the sulfur pulp takes away substantial amount of solution, so centrifuges and filter press is needed to control solution loss. Industrial facilities have focused on treatment of gases to meet gaseous fuels standards or environmental emissions limits. For odour issues however the regulations are less clear; the only factor seems to be the complains the management receives from the neighborhood of their factory. There exist many well known processes for odour control like dry scrubbing, i.e.: solid bed towers with activated carbon, iron sponge or biofilters, as well as wet scrubbing. When large amounts of sulfur compounds are to be treated, the towers are going to be large, and many operational and waste disposal problems arise. Recently, with increasing applications of small biogas streams for local electric power generation, there arise a problem of build up of hard deposits of siloxides on pistons, spark plugs etc. , hazardous to motor and motor oil : so biogas has to be cleaned not only out of sulfur but also out of silicone compounds.

SUMMARY OF THE INVENTION

The present invention relates to the desulfurization method useful for treating both: large gaseous fuels streams, as well as small streams of biogases or streams with low sulfur compounds content - as it is for deodorization, or removing both: sulfur and silicone compounds.

The present invention is a complex desulfurization method, i.e.: removing H₂S and organic sulfur compounds from the gas stream, organic sulfur compounds from the regeneration air and the solution as well as the solution degradation products from the sulfur pulp produced during the desulfurization process, by the use of three 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 small 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 third solution is used to allow the second solution to be recirculated to the first solution for the gas treatment. A further aspect of the invention is to use the third 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 existing methods for large gaseous fuels streams, is that the composition of the third solution makes possible the admixture 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. Sulfur more easy to dehydrate may be taken also as an advantage. For small units however: the dry pulver like sulfur pulp, obtained right after flotation , instead of wet sulfur pulp for existing methods, is essential - because no centrifuge or filter press is needed to control solution loss within the sulfur pulp. A further aspect of the invention is complex removal of sulfur and silicone compounds from biogas.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for removing hydrogen sulfide or hydrogen sulfide and the organic sulfur compounds, or sulfur and silicone 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 off gas, biogas, or air stream by the use of three 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, diethylene triamine pentaacetic acid and citric acid are most advantegeously 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, nitrites, amino acids, sugars, ascorbates, alcohols, polyols, aliphatic aldehydes, dimethy sulfoxide, organic disulfides. The pH range of the first solution is between 6 and 10 , preferably between 8 and 9,5. 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, citric 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 fresh chelate solution. In the third solution of the present invention at least two of metal ions as have been selected for the first chelate solution are most advantageously employed. The third solution of the present invention is also used for removing siloxanes from gas stream after desulfurization. The siloxane removal is done in a separate contactor providing recirculation of the second solution as well as fresh water addition Additional possibility of this invention is periodic or continuous admixing a part or all of the second solution and the third solution into the first solution ,when high sulfur purity is not essential. The present invention is also the discovery that an industrial water may be used for the chelate solution preparation . Heating of the sulfur mixed with the second 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 deodorizing is to be filtered to recover the chelate solution. 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 third 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. A further aspect of the invention is treatment of small streams of ventilation off air or biogas . In this particular case to reduce the investment costs the process is to be simplified: i.e. the removal of sulfur compounds and the regeneration is carried out in one stage using the first solution only when gases free of siloxanes are to be treated, or using in one stage the first solution mixed with the second and the third solution. Here the most promising advantage: the production of a dry sulfur pulp right after flotation means small compact desulfurization installation with no need of big filter press etc to control consumption of the solution. For treating the biogas with siloxanes in the amount of more than 10 mg/m³ the biogas is contacted with the first solution to remove hydrogen sulfide, and then in a separate column with the third solution, preferably diluted with cold water to remove siloxanes. The existing redox desulfurization methods give elemental sulfur partially to flotate and to sedimentate, so a typical procedure was to add some defoaming agents and to remove sulfur by the way of sedimentation/centrifuge and filtration. The present invention is to flotate all elemental sulfur: eliminating fouling of the equipment on one hand, and avoiding excessive foaming on the other; so the critical process here is the way to carry out the regeneration together with flotation to remove the elemental sulfur: the present invention proposed way is to supply the regeneration and flotation air with the excess to the stechiometric one in form of large bubbles, by using submerged one or more reverse "T" shape pipes each of them with open ends and having cut diagonal slits located on both arms of the "T" shape pipe while the total area of the slits being equal or exceeding the cross section of the "T" pipe itself.

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 heavy hydrocarbons Cg₊ is introduced through line 10 into a turbulent absorber 11 to contact concurrently 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 bum the organic disulfides, hydrocarbons and impurities stripped by the air from the first aqueous alkaline 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 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 third 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 third solution fed by line 31 from the tank 32. Mixing those two solutions in the tank 30 produces also the first 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 or sulfur and silicone compounds from a gas mixture which process comprises contacting the gas mixture in a contactor with a first solution to produce a purified gas stream having a reduced hydrogen sulfide or hydrogen sulfide and organic sulfur compounds content, and a spent first 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 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 a third solution to be recirculated back to the first solution . 2. The process of claim 1 wherein the first solution is an 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, citric acid. 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, citric acid. 4.The process of claim 1 and 2 wherein the third solution is an aqueous solution of a mixture of at least two of the metal ions selected for the first solution. 5.The process of claim 1,2 and 4 wherein the third solution is used to treat the desulfurized biogas or biogas containing air to lower the silicone compound content. 6. The process of claim 1,2,3 and 4 wherein the gas mixture is spent air or biogas and the second and the third solution is admixed to the first solution . 7.The process of claim 1,2,3,4 and 6 wherein the gas mixture is spent or vent air and the regeneration air is provided by the purified gas stream. 8. The process of claim 1,2,3 and 4 wherein the organic disulfides are removed from the first solution by water vapor or water vapor mixed with the air 9. The process of claim 1,2,3 and 4 wherein the spent regeneration air containing organic disulfides is recirculated to the regeneration. lO.The process of claim 1,2,3, and 4 wherein the second solution mixed with the sulfur pulp is heated below the sulfur melting temperature. 11. The process of claim 1,2,3, and 4 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 third solution ,is recirculated back to the contactor.

12. The process of claim 1,2,3 and 4 wherein the vapors produced during the contact of second solution with the sulfur pulp are recirculated back to the regeneration loop.

13. The process of claim 1 wherein the regeneration and flotation air is supplied with many times excess to the stechiometric one in form of large bubbles and is supplied to the regenerating chelate solution by using submerged one or more reverse"T" shape pipes each of them with open ends and having cut diagonal slits located on both arms of the "T" shape pipe while the total area of the slits being equal or exceeding the cross section of the "T" pipe itself.