COS and CS.SUB.2 .abatement method

Abstract

Disclosed is method for removing carbonyl sulphide and/or carbon disulphide from a sour gas stream. The method comprises subjecting the gas stream to simultaneous contact with an absorption liquid, such as an aqueous amine solution, and with a catalyst suitable for hydrolyzing carbonyl sulphide and/or carbon disulphide. To this end, the invention also provides a reactor system wherein both an absorption liquid and a catalyst are present. In a preferred embodiment, the catalyst is a heterogeneous catalyst present on or in an absorption column, either coated on the trays of a column with trays, or contained in the packing of a packed column.

Claims

1. A sulfur recovery facility comprising: a) an acid gas removal unit comprising an absorber and a low pressure (LP) regenerator, wherein the absorber is configured for operating at high pressure (HP), medium pressure (MP), or low pressure (LP); b) an acid gas enrichment and/or tail gas treatment unit comprising a low pressure/very low pressure (LP/LLP) absorber and an LP regenerator; c) a sulfur recovery unit; and d) a carbonyl sulfide (COS) and/or carbonyl disulfide (CS.sub.2) hydrolysis reactor; wherein the hydrolysis reactor comprises a catalyst suitable for hydrolyzing carbonyl sulphide and carbonyl disulphide; wherein said catalyst is a transition metal or a salt of a transition metal; wherein the COS and/or CS.sub.2 hydrolysis reactor is: (i) in the acid gas removal unit between the bottom of the absorber of the acid gas removal unit and the LP regenerator of the acid gas removal unit; or (ii) in the acid gas enrichment and/or tail gas treatment unit between the bottom of the absorber of the acid gas enrichment and/or tail gas treatment unit and the LP regenerator of the acid gas enrichment and/or tail gas treatment unit.

2. The sulfur recovery facility of claim 1, wherein the sulfur recovery facility comprises the acid gas enrichment unit, the regenerator of the acid gas removal unit has an outlet for gas connected to an inlet of the acid gas enrichment unit, and the acid gas enrichment unit has an outlet for gas connected with an inlet of the sulphur recovery unit.

3. The sulfur recovery facility of claim 2, wherein the hydrolysis reactor is filled with absorption liquid.

4. The sulfur recovery facility of claim 3, wherein said acid gas removal unit comprises a HP absorber having an inlet for sour gas, an outlet for treated sweet gas, and an outlet for absorption liquid connected to an inlet of said hydrolysis reactor, wherein the hydrolysis reactor comprises the catalyst and has an outlet for liquid connected to an inlet of said regenerator, wherein the regenerator has an outlet for liquid connected with an inlet for liquid of said HP absorber, and wherein said regenerator further has an outlet for gas connected to an inlet of said acid gas enrichment unit.

5. The sulfur recovery facility of claim 3, wherein said absorber of said acid gas enrichment unit has an outlet for liquid connected with an inlet for liquid of said hydrolysis reactor, wherein said hydrolysis reactor comprises the catalyst and has an outlet for liquid connected with an inlet of said regenerator of said acid gas enrichment unit, wherein said regenerator of said acid gas enrichment unit has an outlet for gas connected with an inlet of said sulfur recovery unit, and has an outlet for liquid connected with an inlet of said absorber of said acid gas enrichment unit.

6. The sulfur recovery facility of claim 1, wherein the COS and/or CS.sub.2 hydrolysis reactor is in the acid gas removal unit between the bottom of the absorber of the acid gas removal unit and the LP regeneration of the acid gas removal unit.

7. The sulfur recovery facility of claim 1, wherein the COS and/or CS.sub.2 hydrolysis reactor is in the acid gas enrichment and/or tail gas treatment unit between the bottom of the absorber of the acid gas enrichment and/or tail gas treatment unit and the LP regenerator of the acid gas enrichment and/or tail gas treatment unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic drawing of a conventional sulphur recovery facility.

(2) FIG. 2 is a schematic drawing of an acid gas removal unit of a conventional sulphur recovery facility.

(3) FIG. 3 is a schematic drawing of an acid gas removal unit of a sulphur recovery facility as modified in accordance with an embodiment of the invention, having a hydrolysis reactor on top of an HP absorber.

(4) FIG. 4 is a schematic drawing of an acid gas removal unit of a sulphur recovery facility as modified in accordance with an embodiment of the invention, having a hydrolysis reactor between the bottom of an absorber and an LP regenerator.

(5) FIG. 5 is a schematic drawing of an acid gas enrichment section of a sulphur recovery facility as modified in accordance with an embodiment of the invention, having a hydrolysis reactor between the bottom of an LP absorber and an LP regenerator.

DETAILED DESCRIPTION OF THE INVENTION

(6) In a broad sense, the invention is based on the judicious insight that the removal of COS and/or CS.sub.2 from a sour gas stream can be conducted more efficiently, and at a relatively low temperature, if one combines the actions of an absorption liquid and catalytic hydrolysis into one and the same medium.

(7) To this end, the gas stream is subjected to contact with an absorption liquid so as to provide an absorbed gas-containing liquid, and the absorbed gas-containing liquid is contacted with a catalyst suitable for hydrolyzing carbonyl sulphide and/or carbonyl disulphide. It will be understood that the absorbed gas can refer to gas bubbles introduced in the absorption liquid (i.e. a two-phase system having the gas still in the gas phase), to molecularly absorbed compounds (i.e. the gas, components from the gas, or compounds converted from the gas such as H.sub.2S being in the liquid phase), or to a combination thereof.

(8) In one embodiment, the foregoing is realized by subjecting the gas stream to simultaneous contact with the absorption liquid and with the catalyst. Particularly, the method of the invention is thereby conducted in a single reaction vessel. Without wishing to be bound by theory, the present inventors believe that the hydrolysis of COS and/or CS.sub.2 advantageously affects also the action of the absorption liquid, as it converts COS and/or CS.sub.2 into compounds that are more easily absorbed. It will be understood that this advantage will be particularly exhibited in the event that the absorbed gas-containing liquid contains the absorbed gas as bubbles (i.e. in the gas phase). For, in that case, the conversion will lead to compounds that are more easily molecularly retained in the absorption liquid, or even absorbed therein molecularly.

(9) In this description, the impurities addressed (e.g. in respect of the removal, absorption, hydrolysis, and abatement) are described as carbonyl sulphide and/or carbon disulphide (COS and/or CS.sub.2). It will be understood that this refers to either or both of the compounds mentioned. I.e., it can be foreseen to address COS only, CS.sub.2 only, or COS and CS.sub.2. In other words, the impurities addressed are selected from the group consisting of carbonyl sulphide, carbon disulphide, and mixtures thereof.

(10) The absorption liquid serves to absorb said impurities COS and/or CS.sub.2 from the gas stream treated. Liquids suitable for this purpose are known to skilled person. These can be based, e.g., on alcohol (methanol) or water. Absorption liquids can be physical solvents or solutions. A preferred absorption liquid is a basic aqueous solution, more preferably an amine solution. Suitable amines include MEA (mono ethanol amine), DEA (diethanol amine), MDEA (methyl diethanol amine), DIPA (diisopropyl amine), DGA (diglycol amine) or special formulated amine solution. Commercial physical solvents, or their mixtures, with or without additives can also be used. The absorption liquid can be a mixture of liquids.

(11) The catalyst comprises a catalytically active metal, preferably a transition metal or a salt of a transition metal, more preferably selected from the group consisting of vanadium, iron, molybdenum, ruthenium, manganese, chromium, zinc, nickel, combination of them and salts thereof.

(12) Thereby, typically if in the salt form, the catalyst can be a homogenous catalyst dissolved in the absorption liquid. In this embodiment, the invention is particularly useful in a method for removing carbonyl sulphide and/or carbonyl disulphide from a sour gas stream, i.e. a gas comprising H.sub.2S and COS which is not necessarily an untreated natural gas.

(13) More preferably, however, the catalyst is a heterogeneous catalyst, deposited on a suitable support. The latter is of advantage with a view to recovering and regenerating the catalyst particles. Suitable catalysts, and optionally supports, adjuvants and/or promoters, are generally known to the skilled person.

(14) The present inventors do not particularly aim at providing a novel catalyst for COS and/or CS.sub.2 hydrolysis. Rather, based on the state of the art in COS and/or CS.sub.2 catalytic conversion, the invention particularly resides in the novel combination of such a catalytic step into a step of scrubbing with an absorption liquid. The invention also allows to optimize the process scheme configuration, as well as to profit from optimized operating parameters, mainly temperature and residence time.

(15) In a particularly preferred embodiment, the catalyst is a heterogeneous catalyst presented on an absorption column as used in the absorption step. Thus, the catalyst can, e.g., be deposited on the trays of a column or on the packing of the same. It can be also deposited, in addition to the column or in lieu thereof, in special devices like filtering systems, special packing system outside the column, particularly downstream from where the gas stream is contacted with the absorption liquid.

(16) Advantageously, in accordance with the invention the carbonyl sulphide and/or carbon disulphide are preferably hydrolyzed at a temperature below 150° C., preferably below 130° C.

(17) In preferred embodiments, the reactor for COS and/or CS.sub.2 hydrolysis operates at low pressure (generally 0.01 to 0.1 barg, typically about 0.05 barg). This represents an advantage as compared to known COS and/or CS.sub.2 removal, which typically occurs in a high-pressure absorption step. In fact, this adds to the residence time of the gas in the reactor, which contributes to further lowering the amount of COS in the gas stream. The simultaneous hydrolytic conversion of COS and/or CS.sub.2 also helps to overcome the drawbacks associated with the fact that the basicity of the scrubbing liquid (i.e. the amine solution) inevitably becomes reduced during COS and/or CS.sub.2 absorption.

(18) The invention further pertains to a reactor system for removing carbonyl sulphide and/or carbon disulphide from a sour gas stream. The reactor system is filled with an absorption liquid (i.e., in particular an absorption liquid is made to pass through the system, preferably involving regeneration and recirculation of the regenerated liquid), and comprises a catalyst suitable for hydrolyzing carbonyl sulphide and carbonyl disulphide. In one embodiment, this reactor system comprises a single reaction vessel filled with the absorption liquid, said liquid also comprising the catalyst. In one preferred embodiment, the reactor is a column with trays, wherein the catalyst is a heterogeneous catalyst applied as a coating on the trays. In another preferred embodiment, the column is a packed column, wherein the catalyst is coated on the packing. Suitable packing materials are known to the skilled person and include, e.g., stainless steel or ceramic material, such as alumina.

(19) It is also possible, in embodiments of the reactor system that the catalyst is deposited on one or more available contacting devices other than a column, such as a filtering media, plates designed to separate liquid from gas such as vane packs, corrugated plates, coalescing media, and flashing devices.

(20) A particular advantage of the invention is represented by the flexibility in the location of the additional step for COS and/or CS.sub.2 abatement. The first embodiment foresees the use of this reactor on the treated gas stream of an AGR unit (see FIG. 3), i.e. on the top of the absorber (which can be a HP, an MP, or an LP absorber, with HP being preferred. In this embodiment, after the COS and/or CS.sub.2 hydrolysis reactor, a subsequent stage (not shown) is dedicated to the removal of the produced H.sub.2S and thereby reactivating the used absorption liquid, e.g. amine. Thus, in this embodiment, a two stage type of vessel can be employed, if desired a plurality thereof.

(21) The abbreviations used for the various pressure sections referred to in this description have the following meanings:

(22) LLP stands for very low pressure, typically 0.01-1 barg;

(23) LP stands for low pressure, typically 1-20 barg;

(24) MP stands for medium pressure, typically 20-40 barg;

(25) HP stands for high pressure, typically greater than 40 barg.

(26) In further preferred embodiments, the COS and/or CS.sub.2 hydrolysis reactor is positioned either between the bottom of the HP/MP/LP absorber and the LP regenerator, or between the bottom of the LP/LLP absorber and the LP regenerator.

(27) Thus, with reference to the foregoing embodiments, the invention pertains to a sulfur recovery facility comprising a. an acid gas removal unit comprising an absorber, operating at high pressure (HP), medium pressure (MP), or low pressure (LP), and an LP regenerator; b. an acid gas enrichment/tail gas treatment unit comprising an LP/LLP absorber and an LP regenerator; c. a sulfur recovery unit; d. a COS and/or CS.sub.2 hydrolysis reactor;

(28) wherein the COS and/or CS.sub.2 hydrolysis reactor position is selected from (i) the top of the HP/MP/LP absorber; (ii) between the bottom of the HP/MP/LP absorber and the LP regenerator; and (iii) between the bottom of the LP/LLP absorber and the LP regenerator. The reactor mentioned under d. can be a customary-type hydrogenator/hydrolysis reactor, operating with the reagents in the gaseous phase, and wherein the catalyst preferably is a CoMox based catalyst (cobalt molybedenum oxide catalyst).

DETAILED DESCRIPTION OF THE FIGURES

(29) FIG. 1 is a schematic drawing of a conventional sulphur recovery facility. Depicted is (a) a feed stream of sour gas entering into an Acid Gas Removal Unit (AGRU); (b) a stream of treated gas exiting the AGRU; (c) a stream of acid, COS-containing gas fed to a sulphur recovery unit (SRU) via an Acid Gas Enrichment unit (AGE); (d) a resulting stream of enriched acid gas fed to the SRU and rejected CO.sub.2 fed to an incinerator; (e) a stream of tail gas from the SRU fed to the incinerator; (f) a stream of recovered sulphur from the SRU.

(30) FIG. 2 is a schematic drawing of the acid gas removal unit (AGRU) of the above conventional sulphur recovery facility. Shown is how an HP absorber and a regenerator are connected.

(31) FIG. 3 is a schematic drawing of an acid gas removal unit of a sulphur recovery facility as modified in accordance with an embodiment of the invention; shown is the presence of a hydrolysis reactor on top of an HP absorber.

(32) FIG. 4 is a schematic drawing of an acid gas removal unit of a sulphur recovery facility as modified in accordance with an embodiment of the invention; shown is the presence of a hydrolysis reactor between the bottom of an absorber and an LP regenerator.

(33) FIG. 5 is a schematic drawing of an acid gas enrichment section of a sulphur recovery facility as modified in accordance with an embodiment of the invention, having a hydrolysis reactor between the bottom of an LP absorber and an LP regenerator.

(34) The main operating conditions preferably employed are:

(35) Feedstock: Sour gas (C1, C2, C3, C4, C6+)

(36) Feedstock temperature: 20° C.-80° C.

(37) Feedstock pressure: >0.01 barg

(38) Type of absorption liquid: MEA, DEA, MDEA, DIPA, DGA, special formulated amine solution, commercial physical solvents, any mixture of them, with or without additive addition.

(39) Absorption liquid temperature: 20° C.-150° C.

(40) Absorption liquid concentration of amines: 5 wt. %-95 wt %

(41) COS and/or CS.sub.2 conversion in liquid phase via hydrolysis with catalyst: >>50% depending on the type of amine (prior art typically achieves <50%)

(42) Percentage of deposited catalyst on structured support or any contacting device: <30 wt. %

(43) Catalyst installation: upgrading of conventional equipment (columns, 3 phase separators, heat exchangers, filters, flash vessel, tank) or in a dedicated equipment (reactor).

(44) The invention is hereinafter illustrated with reference to the following non-limiting Example.

EXAMPLE

(45) In this example, a homogeneous catalyst is used, i.e. a metal salt in aqueous solution. Examples of salts employed are: NH.sub.4VO.sub.3 (ammonium metavanadate); C.sub.10H.sub.14FeO.sub.4 (ferrous acetylacetonate); (C.sub.5H.sub.8O.sub.2).sub.3Ru (ruthenium(III)acetylacetonate); (NH.sub.4)Mo.sub.7O.sub.24*4H.sub.2O (diammonium heptamolybdate tetrahydrate).

(46) P=0.1 barg

(47) Absorption liquid temperature=25° C.

(48) Absorption liquid concentration, the liquid being an aqueous solution of diethanol amine (DEA) of 15 wt. %

(49) Feedstock COS concentration: 125 ppm-1000 ppm

(50) Vanadium concentration in the solution: 0.13 M

(51) In the presence of the catalyst, the removal of COS through a hydrolysis reaction is one order of magnitude higher than that obtained in the absence of catalyst.