Olefins such as ethylene are generally made by thermal cracking a hydrocarbon feedstock containing saturated hydrocarbons containing two or more carbon atoms. Typically the feedstock is ethane, LPG, or a naphtha cut from a refinery. The product from the cracker, herein termed cracked gas, generally contains a variety of materials including hydrogen, methane, saturated and unsaturated C2 hydrocarbons, saturated and unsaturated C3 hydrocarbons, and saturated and unsaturated higher hydrocarbons. As a result of the production process, the cracked gas generally contains small proportions of acetylene and other compounds that are more highly unsaturated than mono-olefins. It is normally desirable to remove the acetylene and such more highly unsaturated compounds. The removal of acetylene and other highly unsaturated components is normally effected by selective hydrogenation.

The cracked gas will generally also contain steam, carbon oxides, and sulphur compounds.

Normally the cracked gas is cooled and then compressed, e. g. to a pressure in the range 10-40 bar abs, and then subjected to fractional distillation to effect separation. Usually hydrogen sulphide and carbon dioxide are separated from the cracked gas, e. g. by means of a caustic wash, prior to the fractional distillation.

In some cases, hydrogen and methane are first separated from the cracked gas, before or after such a washing step. Subsequent acetylene removal by selective hydrogenation is termed "tail-end"selective hydrogenation and is normally effected after fractional distillation. Since the hydrogen in the cracked gas has been removed, it is necessary in"tail-end"selective hydrogenation to re-introduce hydrogen. The amount of hydrogen added is normally limited so that there is not a large excess over that required to hydrogenate the acetylene and any other components that are more highly unsaturated than mono-olefins that may be present in the stream being selectively hydrogenated. Normally the hydrogen content of the gas subjected to selective hydrogenation in a tail-end process is less than about 5% by volume, usually less than 3% by volume.

In other cases however, there is no initial separation of methane and hydrogen from the cracked gas, and the selective hydrogenation is effected on a gas stream containing the hydrogen <BR> <BR> <BR> <BR> and methane, possibly after some separation step to remove high boiling components, e. g. C5 and higher hydrocarbons. In this case the selective hydrogenation process is termed"front-end" selective hydrogenation and is thus effected on a gas stream containing a relatively large proportion of hydrogen, much larger than is required to effect hydrogenation of the acetylene and any other components that are more highly unsaturated than mono-olefins remaining in the gas stream.

Thus the selective hydrogenation may be effected on the"total cracked gas", generally after separation of C5 hydrocarbons and higher boiling components, or on a"light"stream produced by

distillation: the distillation stage is usually either"de-ethanisation"wherein a light stream containing C2 hydrocarbons and lighter materials is separated from a heavy stream containing C3 and higher hydrocarbons, or"de-propanisation"wherein a light stream containing C3 hydrocarbons and lighter <BR> <BR> materials is separated from a heavy stream containing C4 and higher hydrocarbons. In either case, the light stream contains hydrogen, carbon monoxide, methane, ethane, ethylene and acetylene.

Where the fractional distillation is de-propanisation, the light stream will also contain propane, propylene, methyl acetylene, and propadiene. The light stream may also contain small amounts of butadiene. In either case it is desirable to hydrogenate the more highly unsaturated components, i. e. acetylene and any methyl acetylene, propadiene and butadiene, to the corresponding olefin without effecting substantial hydrogenation of olefins present to the corresponding paraffin.

Typically the gas stream in a front-end selective hydrogenation process will contain more than 10% by volume of hydrogen but a total of less than 2% by volume of acetylene and other components more highly unsaturated than mono-olefins. The concentration of acetylene in the light stream is typically 2000 to 10000 ppm by volume. The concentrations of methyl acetylene, propadiene and butadiene are generally each less than 5000 ppm by volume.

As indicated above the selective hydrogenation is normally effected by passing the light stream, in some cases after passage through guard beds to remove contaminants such as arsenic and mercury compounds, at an inlet temperature in the range 60 to 90°C, through a fixed bed of a particulate selective hydrogenation catalyst, typically in the form of a support, e. g. alumina, impregnated with palladium and sometimes also silver or copper.

While the selective hydrogenation may be effected in a single stage using a reactor provided with cooling means, so that the process approximates to isothermal operation, normally it is effected in two or more stages using adiabatic beds of catalyst, with cooling between stages. The reason for such multistage operation with inter-stage cooling is that the hydrogenation processes are exothermic leading to a significant temperature increase: a series of beds with inter-bed cooling, rather than a single bed, is normally used to keep the temperature to a minimum since as the temperature increases, the hydrogenation becomes less selective and an increasing amount of the olefins are also hydrogenated. Furthermore as the temperature increases, because, in a front-end process, there is a large excess of hydrogen over that required for hydrogenation of the acetylene and any other more highly unsaturated components that may be present, there is a risk of run-away reactions occurring. While the temperature at which runaway reactions, involving substantial hydrogenation of the olefins, occurs is somewhat higher than the temperatures required for acetylene hydrogenation, the temperature rise resulting from hydrogenation of substantial amounts of acetylene may be such that temperatures where there is a risk of hydrogenation of substantial amounts of olefins may be reached. For example hydrogenation of each 1000 ppm by volume of acetylene, methyl acetylene, propadiene or butadiene in the light stream gives rise to a temperature

rise of about 4°C. In addition, at the temperatures at which the catalysts are effective for hydrogenation of the acetylene, a small proportion of the olefins present will normally also be hydrogenated. The hydrogenation of the olefin is also exothermic giving a temperature rise of about 3°C for each 1000 ppm hydrogenated. Consequently If the temperature of the catalyst bed becomes too high, hydrogenation of substantial amounts of the olefins present may also take place with the risk of runaway reactions. Therefore it is normal procedure to operate the selective hydrogenation under such conditions of inlet temperature and space velocity that only part of the acetylene is hydrogenated in the first stage, and so that the temperature rise is limited to less than about 20°C. The gas from the first stage is then cooled and fed to a second bed of catalyst to effect further selective hydrogenation. In some cases there may be three or four such stages with inter-stage cooling.

In US 5 488 024 there are described selective hydrogenation catalysts obtained by subjecting a supported palladium and silver material to a wet reduction step, preferably in the presence of alkali metal compounds, especially potassium fluoride, followed by calcination in an oxidising atmosphere. Prior to use, or during the first use, the catalyst is re-reduced with hydrogen. These catalysts are said to exhibit a relatively large difference between the"cleanup"temperature (the minimum temperature at which acetylene is substantially hydrogenated to ethylene so as to obtain a product containing less than about 10 ppm by weight of acetylene) and the"runaway"temperature (the minimum temperature at which a substantial portion of the ethylene is converted to ethane).

We have realised that by utilising catalysts with a large difference between the"cleanup"and "runaway"temperatures, it is possible to operate the selective hydrogenation of a gas stream containing substantial amounts of hydrogen, e. g. front-end selective hydrogenation, such that a substantial proportion, if not all, of the desired selective hydrogenation, is effected in a single adiabatic stage, with minimal hydrogenation of the olefins present.

The hydrogenation of a substantial amount, about 10000 ppm, of acetylene in the presence of ethylene in a single adiabatic bed is disclosed in GB 2 052 556, but there the process was a tail-end hydrogenation of a gas stream containing about 2.25% hydrogen. The composition quoted for the product indicates that essentially all the hydrogen reacted, resulting in hydrogenation not only of the acetylene but also hydrogenation of some of the ethylene, giving a net reduction in the amount of ethylene in the gas.

Accordingly the present invention provides a process for the selective hydrogenation of a feed gas stream containing at least 10% by volume of hydrogen, ethylene and a total of 4000 to 15000 ppm by volume of hydrocarbons containing 2 to 4 carbon atoms that are more highly unsaturated than mono-olefins, and including 2000 to 10000 ppm by volume of acetylene and not more than 5000 ppm by volume of each of methyl acetylene, propadiene and butadiene, by passing the gas stream adiabatically through a fixed bed of a particulate selective hydrogenation catalyst at

an inlet temperature in the range 60 to 90°C, characterised in that the inlet temperature and flow rate to the bed are such that at least 80% by volume, and not less than 4000 ppm by volume, of said more highly unsaturated compounds are hydrogenated in said bed, the outlet temperature is below the runaway temperature of the catalyst, and the difference between the outlet and inlet temperatures of said bed is not more than 80% of the difference between the clean-up and runaway temperatures of said catalyst.

The"clean-up"and"runaway"temperatures for the catalyst are determined by the following procedure. 20 ml of the catalyst is placed in a stainless steel reactor tube of about 12 mm internal diameter and about 46 cm length and reduced under flowing hydrogen at a pressure of about 14 bar g. at a temperature of about 50°C for about 16 hours. The reactor is then cooled to about 43°C and then a test gas containing 26.6% methane, 14.1% ethane, 39.2% ethylene, 2680 ppm acetylene, 210 ppm carbon monoxide, balance, i. e. almost 20%, hydrogen (% and ppm by volume) is introduced at 900 ml/minute. The reactor temperature is then increased gradually and samples of the exit gas are analysed. The"clean-up"temperature is that temperature at which the acetylene content of the product was 10 ppm by volume, and the"runaway"temperature is that temperature where the ethylene to ethane molar ratio of the product was 2.3 indicating that about 5% of the ethylene had been hydrogenated. In preferred catalysts the difference between the"clean-up"and "runaway"temperatures is at least 40°C. Preferred catalysts are catalysts E6, F3 to F12, F15 to F19, G2, G3, G5 to G7 and I of the aforementioned US 5 488 024. Other suitable catalysts include supported palladium/silver catalysts also containing alkali metal compounds, e. g. fluorides, possibly with additional fluorine compounds, as described in US 5 583 274 and US 5 587 348.

As mentioned above, in addition to hydrogenation of the more highly unsaturated compounds, some of the ethylene will also be hydrogenated. Thus hydrogenation of a feed gas containing 5000 ppm of acetylene (and no methyl acetylene, propadiene, or butadiene) to an acetylene content of below 2 ppm will give an exotherm due to the acetylene hydrogenation of about 20°C. At the same time typically about 2500 ppm of ethylene are also hydrogenated, giving a total exotherm, i. e. difference between the bed outlet and inlet temperatures, of about 28°C. Hence the catalyst employed should be such that the difference between the clean-up and runaway temperatures is at least about 35°C. Likewise hydrogenation of a feed gas containing 6000 ppm of acetylene and 1000 ppm of methyl acetylene to an acetylene content below 2 ppm and a methyl acetylene content of 100 ppm, will typically also result in the hydrogenation of about 3450 ppm of ethylene, giving a total exotherm of about 38°C and so the catalyst employed should be such that the difference between the clean-up and runaway temperatures is at least about 48°C.

It is preferred that the process is operated such that there is an increase in the olefin molar content of the gas amounting to at least half of the molar amount of acetylene, methyl acetylene, propadiene and butadiene hydrogenated.

In the process of the invention, the bulk, i. e. at least 80% by volume, of the more highly unsaturated hydrocarbons are hydrogenated during passage through the catalyst bed, subject to hydrogenation of at least 4000 ppm of the more highly unsaturated hydrocarbons. In many cases all the desired hydrogenation can be achieved in a single bed, but in some cases, where the feed contains a high proportion of the more highly unsaturated hydrocarbons, e. g. above 10000 ppm by volume, it may not be possible to select a catalyst having a large enough difference between the clean-up and runaway temperatures for hydrogenation of essentially all of the more highly unsaturated hydrocarbons without risk of hydrogenation of a substantial proportion of the ethylene present. In such cases the conditions should be selected so that hydrogenation of only part, but at least 80% by volume and at least 4000 ppm, of the more highly unsaturated hydrocarbons is effected in the bed and the remainder of the desired hydrogenation effected in a subsequent bed or series of beds. Also in some cases it may be desirable to employ a series of beds so that a large proportion of the hydrogenation is effected in the bed to give an effluent containing, for example, less than 2 ppm by volume of acetylene, and this effluent is subjected to a further selective hydrogenation stage to reduce the more highly unsaturated hydrocarbon content to below a somewhat lower desired level.

In operating the process, the inlet temperature employed is preferably the minimum consistent with achieving the desired acetylene content of the product gas. The reaction is preferably operated at a pressure in the range 10 to 40 bar abs. and at a space velocity (at NTP) of 3000 to 8000 h-'. The process is preferably operated on such a scale that the volume of catalyst employed is at least 0.5 m3, particularly at least 1 m3. Generally the process is controlled by control of the inlet temperature to the bed.

The process is preferably applied to the hydrogenation of total cracked gas or to light olefin-containing streams produced as aforesaid by subjecting cracked gas to de-ethanisation or de-propanisation. The process may however also be applied to other olefin-containing streams containing more than 10% by volume of hydrogen.

The invention is illustrated by the following examples.

Example 1 A catalyst A containing 0.02% by weight of palladium and 0.07% by weight of zinc supported on alumina was prepared by impregnating alpha-alumina spheres of 4-5 mm diameter and having a narrow, unimodal, pore size distribution and surface area no greater than 4 mol with a mixed aqueous solution of palladium and zinc nitrates. After impregnation, the spheres were calcined in air at 450°C for 4 hours. The"clean-up"and"runaway"temperatures were 53°C and 80°C respectively and so this catalyst could be used in processes where the temperature rise through hydrogenation was less than about 22°C.

For example computer modelling of the adiabatic selective hydrogenation of a feed gas containing 20 mol% methane, 35 mol% ethylene, 5 mol% ethane, 0.39 mol% acetylene, 0.02 mol% methyl acetylene, 0.02 mol% carbon monoxide and balance, i. e. about 39.5%, hydrogen at an inlet temperature of 60°C would give a product having an acetylene content below 1 ppm by volume and a methyl acetylene content below 20 ppm by volume at an outlet temperature of 78°C, representing a temperature rise of 18°C. The amount of ethylene hydrogenated is less than the amount produced through hydrogenation of the acetylene.

Example 2 A catalyst B also containing 0.02% by weight of palladium and 0.07% by weight of zinc supported on alumina was prepared in the same was as Catalyst A except that the alpha-alumina was in the form of pellets of 5 mm diameter and 3 mm height which again have a narrow, unimodal pore size distribution and surface area less than 4 m2/g, and the pellets were immersed in aqueous sodium hydroxide solution followed by drying at 150°C before impregnation with the palladium and zinc nitrates solution. The"clean-up"and"runaway"temperatures were 63°C and about 103°C respectively and so this catalyst could be used in processes where the temperature rise through hydrogenation was less than about 32°C.

For example computer modelling of the adiabatic selective hydrogenation of a feed gas containing 20 mol% methane, 35 mol% ethylene, 5 mol% ethane, 0.45 mol% acetylene, 0.3 mol% methyl acetylene, 0.02 mol% carbon monoxide and balance hydrogen at an inlet temperature of 62°C would give a product having an acetylene content below 1 ppm by volume and a methyl acetylene content below 300 ppm by volume at an outlet temperature of 91 °C, representing a temperature rise of 29°C. The amount of ethylene hydrogenated is less than the amount produced through hydrogenation of the acetylene.

Example 3 Similarly computer modelling shows that a catalyst such as Catalyst F18 described in the aforesaid US 5 488 024 having"clean-up"and"runaway"temperatures of about 72°C and about 146°C respectively could be employed for the adiabatic selective hydrogenation of a feed gas containing 20 mol% methane, 35 mol% ethylene, 5 mol% ethane, 0.6 mol% acetylene, 0.4 mol% methyl acetylene, 0.02 mol% carbon monoxide and balance, about 39%, hydrogen at an inlet temperature of 75°C to give a product having an acetylene content below 1 ppm by volume and a methyl acetylene content below 500 ppm by volume at an outlet temperature of 115°C, representing a temperature rise of 40°C. The amount of ethylene hydrogenated is less than the amount produced through hydrogenation of the acetylene.