Removal of Sulphur Together With Other Contaminants From Fluids

This invention relates to a purification process and in particular to the removal of sulphur compounds together with other contaminants from fluid streams by absorption using particulate absorbent materials.

As a fluid stream containing a contaminant is passed through a bed of an absorbent for that contaminant, the contaminant is absorbed, initially at the inlet region of the bed, and the effluent from that bed contains little or none of the contaminant. Gradually the inlet region of the absorbent becomes saturated with the contaminant and the region where the absorption occurs moves gradually towards the outlet of the bed. Often the absorption front is relatively sharp: i.e. there is a clear distinction between the region of the bed where absorption has occurred (where the bed is partially or fully saturated with the contaminant) and downstream regions where the bed is essentially free of contaminant. When the adsorption front reaches the outlet of the bed, break-through is said to have occurred since the contaminant can then be detected in significant quantities in the effluent from the bed. Continued passage of the contaminated fluid through the bed will result in little or no further absorption of the contaminant.

Fluid streams, such as hydrocarbon liquids and gases, for example natural gas, are often contaminated with sulphur compounds and other contaminants such as elemental mercury, phosphine, stibine, arsine and/or organo-arsenic compounds such as mono-, di- or tri-alkyl arsines. Various references, for example GB 1 533 059 and EP 0 465 854, disclose that mercury and such arsenic compounds can be removed by passing the fluid through a bed of a copper sulphide absorbent. U.S. Pat. No. 4,593,148 discloses that arsines and hydrogen sulphide can be removed together by the use of a bed of copper oxide and zinc oxide. EP 0 480 603 discloses that sulphur compounds and mercury may be removed together by passing the fluid stream through a bed of an absorbent containing copper compounds: the sulphur compounds are absorbed, forming copper sulphide which then serves to remove the mercury.

The fluid stream generally contains a far greater amount of sulphur compounds, particularly hydrogen sulphide than other contaminants. It is generally necessary to remove essentially all the mercury and arsenic compounds, but often it is permissible for the product to contain a small amount of hydrogen sulphide. For example a typical natural gas may contain about 50 .mu.g/ml of mercury and about 10 ppm by volume of hydrogen sulphide and it is desired that this gas is purified to a mercury content of less than 0.01 .mu.g/m.sup.3 and to a hydrogen sulphide content of 1-3 ppm by volume.

We have devised a simple process whereby essentially all of the mercury and for arsenic compounds can be removed and the sulphur compounds content decreased to a specified level.

Accordingly the present invention provides a process for the purification of a fluid stream containing at least one sulphur contaminant selected from hydrogen sulphide, carbonyl sulphide, mercaptans and hydrocarbon sulphides and at least one second contaminant selected from mercury, phosphine, stibine, and arsenic compounds comprising passing said fluid stream through a bed of a particulate absorbent containing a sulphide of a variable valency metal that is more electropositive than mercury whereby said second contaminant is removed from said fluid stream but little or none of said sulphur contaminant is absorbed and then passing at least part of the effluent from said bed containing the variable valency metal sulphide through a bed of a particulate sulphur absorbent comprising at least one compound selected from oxides, hydroxides, carbonates and basic carbonates of said variable valency metal, whereby said sulphur contaminant is absorbed from that part of the effluent passing through said sulphur absorbent and converting said sulphur absorbent to a sulphide of said variable valency metal, characterised in that said bed of the variable valency metal sulphide has been produced by absorbing sulphur contaminants from a previous portion of said fluid stream from which said second contaminant has been removed.

In a preferred form, the present invention provides a process for the purification of a fluid stream containing at least one sulphur contaminant selected from hydrogen sulphide, carbonyl sulphide, mercaptans and hydrocarbon sulphides and at least one second contaminant selected from mercury, phosphine, stibine, and arsenic compounds comprising passing said fluid stream through a primary bed of a particulate absorbent containing a sulphide of a variable valency metal that is more electropositive than mercury, and having essentially no capacity for absorption of said sulphur contaminant under the prevailing conditions, whereby essentially all of said at least one second contaminant is removed from said fluid stream, passing part of the effluent from said primary bed through at least one secondary bed of a particulate sulphur absorbent comprising at least one compound selected from oxides, hydroxides, carbonates and basic carbonates of said variable valency metal, whereby at least part of said sulphur contaminant is absorbed from said part of the effluent from the primary bed by said variable valency metal compound by conversion thereof to a sulphide of said variable valency metal giving a first product stream that has a decreased sulphur contaminant content, mixing said first product stream with the remainder of the effluent from said primary bed to give a final product stream, the proportion of said effluent stream that is passed through said at least one secondary bed being such that the final product stream has the desired sulphur contaminant content, and, after at least one secondary bed is saturated so that it can no longer absorb said sulphur contaminant under the prevailing conditions, switching the flow of said fluid stream so that a saturated secondary bed is used as the primary bed of absorbent, replacing the absorbent in the previous primary bed with a fresh charge of particulate absorbent comprising said variable valency metal compound and then using said previous primary bed as a secondary bed.

It is seen that the absorption of the sulphur contaminant, e.g. hydrogen sulphide, by the secondary bed converts the aforesaid sulphur absorbent, i.e. oxide, hydroxide, carbonate or basic carbonate of the variable-valency metal, in that bed to a sulphide of said variable valency metal which is then used as the bed, i.e. primary bed, of a sulphide of the variable valency metal required for removal of the second contaminant. When the process is first started up it is necessary that the absorbent in the primary bed comprises a sulphide of the variable valency metal. A pre-sulphided variable valency metal absorbent may be charged to the vessel as the primary bed. Alternatively the absorbent may be the product of sulphiding an absorbent comprising an oxide, hydroxide, carbonate or basic carbonate of the variable valency metal in situ, for example as described in aforesaid EP 0 480 603. Thus an unsulphided absorbent may be charged to the vessel and then a fluid containing a sulphur compound that reacts with the variable valency metal compounds to give the variable valency metal sulphide may be passed through the bed until the variable valency metal compounds have been converted to the sulphide. At that stage, flow of the fluid containing the second contaminant may be commenced.

As will be described hereinafter, it is preferred to employ a series of three secondary beds, and the fluid stream flow is switched after the second of the secondary beds has become saturated with sulphur, with the second of the secondary beds being used as the new primary bed and the replenished previous primary bed being used as the second of the secondary beds. In this case, the first of the secondary beds will also be saturated with sulphur when the second secondary bed is saturated and this saturated first secondary bed is also replenished and is then used as the third secondary bed. Thus at each switchover operation, the primary bed and the first secondary bed are replenished. While these beds are being replenished, only two beds are on absorption duty, namely the previous second secondary bed (which is now the new primary bed), and the previous third secondary bed (which is now the first secondary bed, and until the previous primary and first secondary beds have been replenished, is the only secondary bed). When the previous primary and first secondary beds have been replenished, they are brought into line as the second and third secondary beds respectively.

In the aforementioned arrangement utilising four beds, i.e. a primary bed and three secondary beds in series, it is preferred that the beds are located in two vessels. Thus the primary bed and the first secondary bed are located in one vessel and the second and third secondary beds are located in a second vessel. In a preferred arrangement, the two beds in each vessel form a single continuous bed but fluid off-take means is provided within the bed to withdraw part of the fluid from within the bed after the fluid has passed through the first part of the bed. The first part of the bed thus forms the primary bed. The fluid off-take means conveniently takes the form of a plurality of perforate pipes disposed within the bed with a mesh or cage round each pipe to prevent the particulate absorbent from entering the pipe perforations.

The variable valency metal may be any variable valency metal that is more electropositive than mercury. Examples of such metals include copper, manganese, chromium, tin, iron, cobalt, nickel and lead. Copper is the preferred variable valency metal. The sulphur absorbent charged to the secondary beds comprises an oxide, hydroxide, carbonate or basic carbonate of the variable valency metal. It may also contain other components such as oxides, hydroxides, carbonates and/or basic carbonates of zinc and for aluminium. The presence of such other components is desirable as they appear to stabilise the variable valency compounds enabling the high absorption capacity of the latter to be maintained. The presence of alumina in the absorbent is desirable where the fluid stream being treated contains carbonyl sulphide as the alumina catalyses the reaction of carbonyl sulphide with water (formed by the reaction of hydrogen sulphide and the variable valency metal compound) to give carbon dioxide and hydrogen sulphide. The absorbent is preferably in the form of porous high surface area agglomerates, typically of size in the range 2 to 10 mm average dimension. The agglomerates preferably have a BET surface area of at least 10 m.sup.2 /g. Such agglomerates may be obtained by forming a finely divided high surface area variable valency metal, e.g. copper, compound, or a precursor thereto, for example by a precipitation method, adding a binder such as a calcium aluminate cement, and a little water, insufficient to form a paste, and granulating the mixture. Alternatively the absorbent may be formed by extruding a paste of the aforesaid finely divided high surface area variable valency metal compound, or precursor thereto, binder and water into short extrudates. The agglomerates or extrudates may then be dried and, if desired, calcined to convert the component compounds to oxides. It is however preferred to employ hydroxides, carbonates, or, more preferably, basic carbonates, as the variable valency metal compound in the sulphur absorbent and so it is preferred not to calcine the agglomerates or extrudates. Where other components, such as zinc .and for aluminium compounds, are required in the sulphur absorbent, an intimate mixture of the variable valency metal compound and such other components may be formed, for example by co-precipitation, or by precipitation of the variable valency metal compound, or a precursor thereto, in the presence of the other components in a finely divided particulate form, and then the agglomerates or extrudates formed from this intimate mixture by addition of the binder etc. Examples of suitable agglomerates are described in EP 0 243 052 and PCT publication WO 95 24962.

Where the agglomerates also contain zinc compounds, the latter may also exhibit some capacity for the absorption of sulphur. However the present invention Is of particular utility at relatively low temperatures, particularly below 50.degree. C. At such temperatures zinc compounds exhibit little capacity for the absorption of sulphur. Under such conditions it is believed that essentially all the absorbed sulphur is absorbed by the variable valency metal compound and any zinc compounds merely act as stabilisers. It is therefore preferred that the variable valency metal compounds form at least 75% by weight of the agglomerates.

The fluid being treated may be a hydrocarbon stream, e.g. natural gas, substitute natural gas, natural gas liquids, naphtha, reforming gases, for example hydrocarbon streams such as propylene separated from the product of cracking naphtha; synthesis gas produced, for example, by the partial oxidation of a carbonaceous feedstock; organic compounds such as alcohols, esters, or chlorinated hydrocarbons; or other gases such as carbon dioxide, hydrogen, nitrogen, or helium.

The process is conveniently carried out at a temperature in the range -10.degree. C. to 50.degree. C. The absorption process may be effected at any suitable pressure; typical pressures range from atmospheric up to about 200 bar abs. Under these conditions the fluid may be gaseous, or liquid, or in the case of fluids which are mixtures of components such as hydrocarbons, for example natural gas, in the so-called dense phase, i.e. at a temperature between the critical temperature and the temperature of the maxcondentherm point but at a pressure above that of the upper dew point at that temperature.

The invention is illustrated by reference to the accompanying drawings wherein

FIG. 1 is a diagrammatic flowsheet of the process of the invention,

FIGS. 2 to 5 are diagrammatic flowsheets showing the progressive absorption of the impurities in the flowsheet of FIG. 1,

FIGS 6 to 8 are flowsheets similar to FIG. 1 showing successive stages of the process.

FIG. 9 is a diagrammatic cross section of a reactor containing two beds with a fluid take-off means, and

FIG. 10 is a section along the line IX--IX of FIG. 9.

In FIGS. 1 to 8 control valves are omitted for clarity. Broken lines indicate flow paths not in use at the stage indicated. In FIGS. 1, 6, 7 and 8 the beds are shown as separate entities whereas in FIGS. 2 to 5 two vessels are used each containing two beds.

FIGS. 1 and 2 show the process at the start of operation. The fluid feed, e.g. natural gas at a temperature of 20.degree. C. and a pressure of 120 bar abs. containing 8 ppm by volume of hydrogen sulphide and 50 .mu.g/m.sup.3 of elemental mercury, is fed via lines 1 and 2a to a primary bed 3a of absorbent. At the start of operation, as shown in FIG. 2, primary bed 3a contains agglomerates comprising a sulphide of a variable valency metal, e.g. copper sulphide, while secondary bed 4a (in the same vessel as bed 3a) and secondary beds 3b and 4b (both in a second vessel) each contain fresh absorbent comprising agglomerates comprising at least one compound selected from oxides, hydroxides, carbonates, or, preferably, basic carbonates, of the variable valency metal.

As shown in FIGS. 1 and 3, during passage through bed 3a, the mercury is absorbed by the variable valency metal sulphide, forming mercury sulphide, e.g. via the reaction

while little or none of the hydrogen sulphide in the feed is absorbed. The effluent from bed 3a thus contains hydrogen sulphide in essentially the same concentration as in the feed to bed 3a. Part of the effluent from bed 3a is passed through the first secondary bed 4a and then via lines 5a and 6a through the second and third secondary beds 3b and 4b. After passage through beds 3b and 4b, the fluid leaves bed 4b via lines 5b and 7b to give a product stream 8.

As shown in FIG. 3, during passage of the fluid through bed 4a, hydrogen sulphide is absorbed from the fluid, converting the oxides, hydroxides, carbonates, or, preferably, basic carbonates, of the variable valency metal, to the variable valency sulphide.

Eventually bed 4a becomes saturated with hydrogen sulphide so that break-through occurs and hydrogen sulphide is detectable in line 5a. Thereafter, as shown in FIG. 4, hydrogen sulphide is absorbed by bed 3b. Eventually, as shown in FIG. 5, bed 5b becomes saturated with hydrogen sulphide so that bed 4b starts to absorb hydrogen sulphide. The beds are sized such that the beds 4a and 3b become saturated with sulphur before the mercury absorption front reaches the exit of bed 3a.

While part of the effluent from bed 3a is passing through beds 4a, 3b and 4b, the remainder is taken via line 9a and mixed with the fluid from fine 7b to give the final product stream 8. The hydrogen sulphide content of stream 8 is controlled by controlling the proportion of hydrogen sulphide containing fluid taken via line 9a. Since the fluid that has passed through beds 4a, 3b, and 4b is essentially free from hydrogen sulphide, it is seen that the proportion of the fluid that is taken via line 9a depends directly on the ratio of the desired hydrogen sulphide content of the product to the hydrogen sulphide content of the feed. Control may be achieved by means of control valves responsive to the monitored the hydrogen sulphide content of the feed.

When bed 3b becomes saturated with hydrogen sulphide, for example as detected by monitoring the hydrogen sulphide content of the effluent from bed 5b, the flow of feed is switched from line 2a to line 2b (see FIG. 6). Part of the effluent from bed 3b is passed through bed 4b to remove hydrogen sulphide and fed via lines 5b and 7b into the final product steam while the remainder of the effluent from bed 5b is taken via line 9b as the rest of the product stream. Beds 3a and 4a are thus off-line and can be replenished with fresh absorbent

After beds 3a and 4a have been replenished and before bed 4b is saturated with hydrogen sulphide, the flow from bed 4b is switched, as shown in FIG. 7, to line 6b and hence through beds 3a and 4a, and via lines 5a and 7a to the product stream 8. When bed 4b becomes saturated, bed 3a starts absorbing hydrogen sulphide and converting the variable valency metal compound therein to the corresponding sulphide. When bed 5a is saturated with hydrogen sulphide, and so ready to absorb mercury, the system is switched (see FIG. 8) with the feed to line 2a and bed 3a. Part of the effluent from bed 3a passes through bed 4a to absorb hydrogen sulphide and then passes via lines 5a and 7a into the product stream 8 while the remainder of the effluent from bed 3a is taken via line 9a to form the rest of product stream 8. Beds 5b and 4b are replenished and then the system switched back to the arrangement of FIG. 1 and the cycle repeated.

The beds are preferably sized so that the period between replenishment of the beds is typically in the range 1 week to 1 year.

In FIGS. 9 and 10 there is shown a preferred form of absorbent vessel for containing beds 3a and 4a. The vessel has an outer shell 10 and is provided with an inlet port 11 at the upper end and an outlet port 12 at the lower end. Port 11 is connected to line 2a and port 12 is connected to line 5a of FIGS. 1 to 4. Disposed across the interior of the shell 10 and out through the shell is a hollow header 13 which is connected to line 9a of FIGS. 1 to 4. Extending laterally from header 13 are a plurality of pipes 14. These pipes are closed at their outer ends but at their inner ends communicate with the Interior of header 13. Piper 14 have a plurality of perforations (not shown in FIGS. 9 or 10) therethrough. Surrounding each lateral pipe 14 is a mesh cage 15.

In use, the vessel is charged with absorbent through a manhole 16 at the upper end of the shell 10. The portion of the absorbent above header 13 and lateral pipes 14 forms the bed 3a while the portion of the absorbent below header 13 and pipes 14 forms the bed 4a. The mesh cages 15 serve to prevent the absorbent particles, e.g. agglomerates from blocking the perforations in pipes 14. Thus part of the fluid that has passed down through the upper portion of the absorbent from port 11 can enter cages 15 and then pass through the perforations in pipes 14 and flow through the header 13, while the remainder of the fluid passes between the cages 15 and passes through the absorbent in the lower part of the vessel and leaves via port 12. A manhole 17 is provided to permit the absorbent to be discharged.