DESULPHURISATION

Abstract: Desulphurisation process for feedstocks in which the sulphur compounds contained therein are first separated into a concentrate stream which is then hydro-desulphurised, and the resulting hydrogen sulphide removed using conventional sulphur removal techniques. The initial separation is preferably effected using membrane separation or temperature swing absorption processes.

Description: This invention relates to desulphurisation and in particular to the removal of organic sulphur compounds, such as mercaptans, organic sulphides, carbonyl sulphide, and carbon disulphide, as well as any hydrogen sulphide present, from a liquid or gaseous feedstock stream, particularly a feedstock stream comprising a hydrocarbon such as natural gas. Such hydrocarbon feedstock streams often contain substantial amounts of organic sulphur compounds and hydrogen sulphide as aforesaid; for example where the feedstock stream comprises a gaseous hydrocarbon, the amount of such sulphur compounds may be in an excess of 50 ppm by volume expressed as equivalent hydrogen sulphide. For many applications, before use of the feedstock stream, it is generally desirable to remove such sulphur compounds to a low level. Generally, the maximum acceptable sulphur content will depend on the intended use of the treated feedstock. Thus for example where the feedstock stream is to be subjected to some catalytic reactions, the maximum acceptable sulphur content may be below 1 ppm, or even below 0.1 ppm, by volume expressed as equivalent hydrogen sulphide. In other applications the specification may be, for example, a maximum sulphur content of 30 ppm, or a maximum hydrogen sulphide content of 3 ppm by volume. One method of sulphur compound removal that is particularly effective for the removal of hydrogen sulphide is to contact the feedstock stream with an absorbent liquid or with a bed of particles of a suitable particulate sorbent, such as zinc oxide. However, such techniques are not particularly effective for the removal of the aforesaid organic sulphur compounds. It has been proposed in DE-B-2941804 to remove hydrogen sulphide, and organic sulphur compounds, from a gas stream comprising carbon monoxide and hydrogen, by catalytic hydrogenation of the organic sulphur compounds to hydrogen sulphide, ie hydrodesulphurisation, followed by absorption of the hydrogen sulphide using a zinc oxide absorbent. Catalytic hydro-desulphurisation (hereinafter also referred to as HDS), can be effected by passage of the feedstock stream in the presence of hydrogen over a suitable catalyst, e.g. nickel and/or cobalt molybdate, often in admixture with alumina. However such HDS processes require an appreciable hydrogen concentration in order that the organic sulphur compounds are fully converted. In some cases, particularly where large volumes of feedstock stream are to be treated, an adequate supply of hydrogen may not be available or may be costly to provide. It has been proposed in JP-B-5433243 to extract organic sulphur containing components from a feedstock gas into a hydrocarbon oil stream; to regenerate the sulphur compound-loaded oil stream to give a regenerated oil stream which is recycled and a gas containing an increased concentration of organic sulphur compounds; and then to subject this sulphur compound-containing gas to reaction with hydrogen at high temperatures, followed by removal of the resultant hydrogen sulphide. In the process of the present invention the feedstock stream is subjected to a membrane separation stage to provide a product stream having a reduced content of sulphur compounds, and a concentrate stream containing a higher concentration of the sulphur compounds, and then the concentrate stream is subjected to HDS and sulphur removal. The concentrate stream can be relatively small proportion of the volume of the original feedstock stream, but may contain a high proportion of the sulphur compounds contained within the original feedstock stream. Since the concentrate stream is smaller in volume than the feedstock stream, the amount of hydrogen that is required in order to achieve a given concentration in the concentrate stream is less than that required to achieve the same concentration of hydrogen in the feedstock stream. The process of the present invention also has the advantage that, as the volume of the stream that has to pass over the HDS catalyst is decreased, the size of the bed thereof, and/or the space velocity therethrough may be decreased. This may be advantageous even where a supply of hydrogen adequate to subject all the feedstock stream to HDS is available. The present invention employs a membrane separation stage to separate the sulphur compounds from the feedstock stream as a concentrate stream which may represent only a small proportions of the feedstock stream. The concentrate stream is then subjected to the HDS and subsequent sulphur removal stages to produce an effluent stream of reduced sulphur compound content. The effluent stream is combined with the product stream from the membrane separation stage so that there is little or no loss of the useful feedstock stream. Accordingly the present invention provides a process for the production of a desulphurised product stream from a sulphur compound-laden feedstock stream comprising: a) subjecting said feedstock stream to a membrane separation stage to give a sulphur compound-laden concentrate stream and a product stream of reduced sulphur content; b) passing said concentrate stream over a hydro-desulphurisation catalyst, thereby converting sulphur in organic sulphur compounds in said concentrate stream to hydrogen sulphide and forming a hydrogen sulphide-containing stream; c) removing hydrogen sulphide from said hydrogen sulphide-containing stream, thereby producing an effluent stream of reduced sulphur compound content; and d) adding said effluent stream to the product stream; and in which process hydrogen, or a compound that decomposes thereto in the presence of the hydro-desulphurisation catalyst, is present in, or is added to, the concentrate stream before passage thereof through the hydro-desulphurisation catalyst. Since, in a membrane separation unit, a significant pressure drop may result in one or both of the streams, it may be necessary to compress one, or both, streams downstream of the separation stage. The HDS may be effected in known manner using, for example cobalt and/or nickel molybdate catalysts which are preferably sulphided prior to use. In order to effect HDS a minimum concentration of free hydrogen is required in the concentrate stream. The free hydrogen may already be present as such in some feedstock streams or may be added to the concentrate stream prior to passage over the HDS catalyst. Alternatively in some cases it may be possible to make the hydrogen in situ , as the concentrate stream contacts the HDS catalyst. Thus where the HDS catalyst shows activity for the shift reaction at the temperature at which the HDS is effected, and where both carbon monoxide and water, or steam, is present in the concentrate stream, passage of the concentrate stream through the HDS catalyst effects the shift of carbon monoxide in the concentrate stream to form carbon dioxide and hydrogen. In such cases it may be necessary to add carbon monoxide or water to the concentrate stream. In a further alternative, where the HDS catalyst has activity, at the temperature at which the HDS is effected, for the reforming of an organic compound to hydrogen, the addition of such an organic compound, e.g. an ester, alcohol or aldehyde e.g. methanol, formaldehyde, provides a source of hydrogen. Generally, the concentrate stream that is subjected to the hydro-desulphurisation is 5-30%, particularly 10-20%, of the volume of the feedstock stream. For satisfactory HDS the concentrate stream typically contains 1-10%, particularly 2-5%, by volume of free hydrogen. Where the free hydrogen is produced in situ the amount of the compound, or compounds, giving rise to the hydrogen should be such as to produce such proportions of hydrogen. After HDS, the hydrogen sulphide is removed. Such removal of hydrogen sulphide may be effected by sorption into a suitable regenerable solid or liquid sorbent, by conversion to sulphur, or by sorption with a particulate non-regenerable sorbent such as zinc oxide. The conditions employed for the hydrogen sulphide removal will of course depend on the type of removal technique adopted. With zinc oxide adsorbents, temperatures from ambient to about 400°C can be employed. Generally if the hydrogen sulphide removal is effected at low temperatures, i.e. below about 150°C, a high pore value, high surface area form of zinc oxide should be employed. Preferred zinc oxide sorbents comprise at least 60%, especially at least 80%, by weight of zinc oxide, calculated on the constituents of the particulate non-regenerable sorbent material non-volatile at 900°C. As used in the process the zinc oxide may be, at least initially, wholly or partly hydrated or in the form of a salt of a weak acid, e.g. a carbonate. Such sorbents are preferably in the form of porous agglomerates, as may be made, for example, by mixing a finely divided zinc oxide composition with a cement binder and a little water, insufficient to give a slurry, and then granulated or extruded. In order to aid access of the heated gas stream into the particles, the latter may be provided in the form of extruded pellets having a plurality of through passages. Typically the BET surface area of the particles is at least 20, preferably in the range 50 to 200, m².g⁻¹, and the pore volume of the particles is preferably at least 0.2 cm³.g⁻¹. Since the sorption efficiency and hence the life of a zinc oxide particulate bed depends on the rate of diffusion of the zinc sulphide formed by reaction of the zinc oxide with the hydrogen sulphide towards the interior of the particle, particularly at low sorption temperatures, it is preferable to employ zinc oxide particles having a high pore volume, above 0.2 cm³.g⁻¹ and high surface area, above 50 m².g⁻¹. Thus while zinc oxide particles having a lower pore volume and a surface area of the order of 25 to 30 m².g⁻¹ can be employed, the bed life at low sorption temperatures is relatively low, necessitating the use of large bed volumes to avoid premature break-through of the hydrogen sulphide into the effluent stream. By using a bed of particles of pore volume above, for example, 0.25 cm³.g⁻¹ and surface area above, for example, 70 m².g⁻¹, the bed volume can be markedly reduced, e.g. to about one third of that required with particles of low pore volume and surface area 25 to 30 m².g⁻¹. The particles employed thus preferably have a surface area above 50, particularly above 70, m².g⁻¹ and a pore volume above 0.25 cm³.g⁻¹. Preferred particulate non-regenerable sorbent materials for the process have a hydrogen sulphide sorption capacity of at least 20%, especially at least 25%, of the theoretical, at a temperature of 25°C, as determined in a standard test in which a mixture of hydrogen sulphide (2000 ppm by volume), carbon dioxide (4% by volume), and methane (balance) is passed through a bed of the particles at atmospheric pressure and a space velocity of 700 h⁻¹ using a bed of circular cross section having a length to diameter ratio of 5. A particularly suitable particulate non-regenerable zinc oxide material is that sold by Imperial Chemical Industries plc as "Catalyst 75-1". These particles are granules typically having a surface area of the order of 80 m².g⁻¹ and a pore volume of about 0.3 cm³.g⁻¹, and an sorption capacity of about 27% of theoretical when measured by the above procedure. Alternatively the particulate non-regenerable sorbent may comprise agglomerates of particles of an intimate mixture of oxides, hydroxides, carbonates and/or basic carbonates of copper, and zinc and/or at least one element such as aluminium as described in EP-A-243052. The feedstock stream typically contains hydrocarbons up to those containing six carbon atoms and may be gaseous or liquid. Usually it will contain, in addition to methane, one or more of ethane, propane, propene, butanes, and butenes. The invention is also of utility with other feedstock streams, for example naphtha, carbon dioxide, refinery off gas, or the product of fractionating a gas mixture produced by cracking or hydrocracking a normally liquid hydrocarbon feedstock, or the gaseous by-product of a zeolite-catalysed conversion of a feedstock such as methanol to gasoline. The composition of the feedstock stream, where the latter is a natural or oilfield associated gas, expressed by volume is typically: ethane 2 to 20 % propane plus propene 1 to 10 % butanes plus butenes 0.5 to 5 % higher hydrocarbons 0.2 to 2 % carbon dioxide 0 to 20 % nitrogen 0 to 20 % water up to saturation methane balance. The sulphur compounds initially present in the feedstock stream usually include hydrogen sulphide and/or carbonyl sulphide, and possibly carbon disulphide, methyl mercaptan, diethyl sulphide, and/or tetrahydrothiophene. The total initial concentration of sulphur compounds, expressed as sulphur equivalent hydrogen sulphide, is typically in the range 10 to 1000 ppm by volume of the feedstock stream when the latter is in the gaseous phase. The process can be conducted so that a substantial proportion, e.g. over 75% by volume of the sulphur content of to feedstock stream can be removed. Typically the sulphur compound content of the product is under 10, for example under 5, ppm by volume, expressed as above, but this is a matter of design, depending on the user's requirements. The invention is illustrated by reference to the accompanying drawing which is a diagrammatic flowsheet representing one embodiment of the present invention. In this system the feedstock stream fed, via line (20), to a membrane separation unit (21) from which there is obtained a permeate stream (22) and an impermeate stream (23). It is assumed for the purposes of this flowsheet that the sulphur compound-laden concentrate stream is the permeate stream (22) while the sulphur compound-depleted product stream is the impermeate stream (23). It will be appreciated that whether this is the case will depend on the nature of the membrane employed and on the constituents of the feedstock. The permeate, containing the sulphur compounds, represents only a minor proportion of the feedstock stream fed to the membrane separation unit (21). This permeate is compressed in compressor (24) and mixed with hydrogen supplied via line (5) and then fed through first and second heat exchangers (8) and (9) to effect heating of the permeate to the required HDS temperature. The heated permeate then is fed through a bed (12) of a HDS catalyst effective to convert the sulphur in the organic sulphur compounds in the concentrate stream to hydrogen sulphide, and then through a bed (14) of a material that is effective to absorb hydrogen sulphide from the concentrate stream at the temperature exit the HDS catalyst, before being used as the heating medium in heat exchanger (8) and return, via line (15), to the product impermeate stream (23). It will be appreciated that the hydrogen required for HDS may be added at any suitable stage. Where there is hydrogen in the feedstock, and this passes into the permeate stream containing the sulphur compounds, no addition of hydrogen may be necessary. Where the presence of hydrogen in the product stream is undesirable, any excess of hydrogen remaining after HDS may be removed by passage over a suitable catalyst, e.g. copper oxide, optionally stabilised with zinc oxide, to convert the hydrogen to water. This step is preferably effected after removal of the hydrogen sulphide in bed (14). The effluent stream, from bed (14), and after passage through any such bed for removing hydrogen, may be dried, if necessary, for example by means of a molecular sieve drier. Such drying may take place before or after return of the effluent stream to the product stream.

Claims: A process for the production of a desulphurised product stream from a sulphur compound-laden feedstock stream, wherein at least some of said sulphur compounds are organic sulphur compounds, comprising: a) subjecting said feedstock stream to a separation stage to give a sulphur compound-laden concentrate stream and a product stream of reduced sulphur content; b) converting sulphur in said organic sulphur compounds in said concentrate stream to hydrogen sulphide and forming a hydrogen sulphide-containing stream, there being hydrogen, or a compound that decomposes thereto during step (b), present in, or added to, the concentrate stream before step (b); and c) removing hydrogen sulphide from said hydrogen sulphide-containing stream, thereby producing an effluent stream of reduced sulphur compound content; and characterised in that a membrane separation step is employed in stage (a), the sulphur in the organic sulphur compounds is converted to hydrogen sulphide in step (b) by passing the concentrate stream over a hydrodesulphurisation catalyst, and the effluent stream is added to the product stream. A process as claimed in claim 1 wherein the hydrodesulphurisation catalyst has activity, at the temperature at which the hydrodesulphurisation is effected, for the shift reaction, and hydrogen is formed by the shift reaction between carbon monoxide and steam present in the concentrate stream fed to the hydrodesulphurisation catalyst. A process as claimed in claim 1 or claim 2 wherein the hydrodesulphurisation catalyst has activity, at the temperature at which the hydrodesulphurisation is effected, for reforming an organic compound to give hydrogen and such an organic compound is present in the concentrate stream fed to the hydrodesulphurisation catalyst. A process as claimed in any one of claims 1 to 3 wherein the hydrogen sulphide is removed from the hydrogen sulphide-containing stream by sorption with a sorbent comprising zinc oxide.

Inventors: CARNELL, PETER JOHN HERBERT, DENNY, PATRICK JOHN
Applicants: IMPERIAL CHEMICAL INDUSTRIES PLC
Application year: 1989
Main classification: B01D53
IPC codes: B01J23, C10G67, B01D53