GB2199588A - Process for the selective hydrogenation of acetylenes - Google Patents

Description

I- 2199588 PROCESS FOR THE SELECTIVE HYDROGENATION OF ACETYLENES The

present invention relates to an improved process for removing alkynes from liquid hydrocarbon streams with a minimum loss of conjugated dienes present therein. More specifically, the invention relates to the selective hydrogenation of the alkynes present in the 1,3-butadiene-rich G4 cuts from the steam cracking units and mainly used for the production of synthetic rubber.

The polymerization of 1,3-butadiene to produce synthetic rubber is an important industrial process, some 10 million tons being produced annually. The typical feedstocks.used contain a major proportion of 1,3butadiene and butenes, but they also contain significant amounts of alkynes (also called acetylenes),',mainly vinylacetylene. As acetylenes act as a catalyst poison in the polymerization, they should be removed as completely as possible. Accordingly, it is usual to selectively hydrogenate the acetylenic compounds while trying to avoid or to limit losses in 1,3-butadiene.

The selectivity requirements for the process are high, since all other reactions should be avoided or prevented as much as possible. Said reactions obviously include the hydrogenation of 1,3-butadiene and butenes, but also polymerization reactions which reduce the catalyst life. Regenerations of the catalyst are possible, but the frequence thereof is 2 economically important, while they induce catalyst modifications and eventually mechanical breakdown of the catalyst pellets which results in higher pressure drops across the bed.

It has long been known to selectively hydrogenate at high temperature in vapour phase over a copper-nickel catalyst on a Si02/Al2O3 support. However, such processes are increasingly being abandoned because the catalyst has to be replaced or regenerated frequently, while the 1,3butadiene loss and the residual acetylenes concentration are presently considered as being too important.

U.S. Patent 4,493,906 discloses a catalyst for the removal of acetylenes from liquid hydrocarbon streams, said catalyst consisting essentially of finely divided Cu metal dispersed on a well-defined gamma alumina (which may contain up to 35 wt % of alpha alumina). The gamma alumina used has a surface area of 68-350 m2/g; 40-98% of the pores have a pore diameter of 4-12 nm, and 2-25% have a pore diameter between 100 and 1000 nm. The support is of high purity, with silicon less than 0.15 wt % as Si02 and Na < 0.15 wt % as Na20. The catalyst of U.S. Patent 4,493,906 is claimed to leave 0 ppm acetylenics when used at about 68'C and with a LHSV (liquid hourly space velocity) lower than 1. However, the corresponding cycle life is of only 5 1/2 days; after 6 days, about 100 ppm acetylenes are detected in the effluent. Obviously, at higher values of LHSV, either the cycle life would be still shorter, or the acetylenes removal would not be complete.

Another type of catalysts is palladium-based. Palladium is, among the metals of Group VIII, the most active and selective metal for the hydrogenation of acetylenics. However, it is known in the art that two types of operating troubles are encountered:

1,3-butadiene losses are observed even at moderate conversion of alkynes; T and - Palladium losses often reduce the catalyst life, as clearly disclosed in Hydrocarbon Processing, March 1985, p. 52.

As time goes by, the severity of the steam cracking increases, and the C4 raw cuts thus contain increasing concentrations of alkvnes, up to 1 wt % or even higher. On the other hand, the requirements for the acetylenics concentration in the effluent from the selective hydrogenation are becoming more and more severe. There is accordingly a need in the art for an improved process for removing alkynes from liquid hydrocarbon streams with a minimum loss of conjugated dienes present therein.

The process of the present invention for the selective hydrogenation of the alkynes present in 1,3-butadiene-rich C4 cuts over a palladium-based catalyst comprises the steps of (i) providing a 1,3-butadiene-rich C4 cut; (ii) passing said cut in trickle mode over the catalyst bed in the presence of hydrogen; (M) separating the residual hydrogen from the remainder of the ef fl uent f rom step (i i); and (iv) recovering a 1,3-butadiene-rich feedstock.

Palladium-based catalysts which may be used in the process of the invention are well known in the art. A particularly preferred catalyst consists of active palladium metal desposited on a high purity alumina support.

The amount of palladium in the catalyst which includes a high purity alumina support is preferably 0.1 to 0.35 wt more preferably about 0.2 wt %. Advantageously, the alumina is of high purity and the concentration of heavy metals other than Pd is less than 0.05 wt %.

The surface area of the catalyst is oreferably 50-110 m2/g,mcre preferably 65-95 m2/g. The volume of the pores is Qreferablv 0.5 - 0.6 cm3/g. The catalyst is preferably in the form of Spheres of 2-4 rim size.

It has already been said that the acidity of the alumina influences the undesired oligomerization reactions, and that gamma-alumina should therefore be preferred to a conventional eta-alumina. However, this is not required by the process of the invention, because this concerns only the long-term stability, not the activity of the fresh or regenerated catalyst. Other types of alumina may also be used, such as Q-type alumina, that has been disclosed in Japanese Patent Application JP-58017835.

So-called stabilized or promoted palladium-based catalysts are well known, as e.g. the palladium-gold supported catalysts disclosed in European Patent EP-89,252. However, the activity of these catalysts is usually lower than that of palladium-based catalysts. Although not - i 1 wishing to be bound by a theory, this could be explained by a less homogeneous dispersion of the metals on the support, because it is almost impossible to obtain a controlled supported bimetallic catalyst at the low loading-levels used for industrial precious metals catalysts. Indeed, the carrier must be suitable for appropriate interaction between the metals, and a well dispersed bimetallic species must be obtained then maintained. The use of stabilized or promoted catalysts in the process of the invention is optional and will therefore depend on the activity and long-term stability requirements.

The activation, start-up and regeneration procedures of the palladiumbased catalysts are known in the art. The activation consists of (i) purging the oxygen using.nitrogen, and (ii) passing hydrogen under atmospheric pressure while gradually heating up to a level of about WC then cooling. The start-up procedure consists of slowly increasing the hydrogen pressure, then the feed and hydrogen flow rates, and finally the temperature. The regeneration procedure consists of passing steam under atmospheric pressure whilegradually increasing the temperature to about 400% then continuing to pass steam under atmospheric pressure at a temperature of about 400C during about two hours thereafter, and finally gradually adding upto several mol percent air to said steam. During the regeneration procedure, the catalyst temperature should not exceed about SOWC. The regeneration is complete when the C02 content at the exit is sufficiently low.

The prior art and the catalysts manufacturers recommend the following typical process conditions for the selective hydrogenation of vinyl and ethyl acetylene in a liquid 1,3butadiene-rich C4 cut, using palladiumbased catalysts - temperature;15-10C (inlet), - pressure: 0.5 MPa (5 bar) LHSV: 30 1/l.h-1 Fi/alkvnes molar ratio: 2:1 The typical results obtained with these conditions are - feed: 1,3- butadiene 50 vol. Z ethyl acetylene 0.2 vol. Z vinyl acetylene 1.2 vol. % balance = butenes - purified effluent: 500 ppm total alkynes 37 butadiene loss cycle life: 8-10 months The Applicant has unexpectedly found that the known palladiumbased catalysts are much more selective when used in trickle mode than in homogeneous liquid phase. The term 'Incre2sed selectivity", as used herein, means that, for a given feed, less 1,3-butadiene is lost for a given level of 21kynes hydrogen2tion. The term "trickle mode", 2S used herein, is defined as the operation under such conditions of temperature and pressure that cause the feed to pass as a mixed gaseous-liquid phase over the catalyst. The hydrogenation reaction being exothermal, it is usually more convenient to provide the feed in the liquid state, under conditions very near to the gaseous-liquid equilibrium.

The reactor may be either an isothermal reactor or an adiabatic reactor. In the latter case, the heat released by the hydrogenation reaction is compensated by the vaporization of part of the liquid phase. It is thus highly desirable in the case of an adiabatic reactor to have at the inlet enough feedstock in the liquid phase in order to absorb all the heat released by the hydrogenation reactions, and further to inject part of the feed in the liquid form along the axis of the reactor.

According-to an embodiment of the invention, a sufficient proportion of the feed, preferably up to about 20%,is injected in liquid form, at one or several places, preferably about half way, of the catalyst bed in an adiabatic reactor. Although not wishing to be bound by a theory, 'the Applicant believes that these injections possibly serve to maintain the trickle mode conditions constant throughout the adiabatic reactor.

t_ Considering that the gaseous phase is produced partially by the vaporization of the feed, the trickle mode is usually operated in cocurrent mode. Although it is possible to operate.in up-flow mode, the Applicant has found that it is highly preferable to operate in down-flow mode.

The hydrogen may be injected with the feed. However, it has also been found highly desirable to distribute part of the hydrogen injection along the axis of thareactor, e.g. at one or several places about half way of the catalyst bed. According to an embodiment of the invention, up to 30%, preferably up to about 15%, of the total hydrogen flow is injected at one or several places about half way of the catalyst bed in an adiabatic reactor. Although not wishing to be bound by a theory, the Applicant believes that these injections possibly serve to maintain the trickle mode conditions constantthroughout the adiabatic reactor.

With a 100% hydrogen flow, the total pressure should preferably be of from 0.4 to 0.9 MPa, most preferably of 0.6 to. 0.8 MPa. Thus, if refinery hydrogen is used in the process of the invention, said refinery hydrogen usually containing about 75% hydrogen and about 25% methane, the total pressure should preferably be slightly higher. - The reaction temperature (or inlet temperature if an adiabatic reactor is used) is adjusted with respect to the total pressure, in order to maintain the desired trickle mode operation. Within the preferred ranges, higher values of the pressure and temperature tend to impart a higher activity to the catalyst.

The LHSV to be used is easily determined by one skilled in the art, in view of the specifications for residual acetylenics (and/or 1,3-butadiene loss in the case of 1,3-butadiene-rich C4 cuts). For example, nearcomplete alkynes hydrogenation at the 1% concentration level in 1,3butadiene-rich C4 cuts using palladium-based catalysts usually requires a LHSV lower than 10, but the corresponding 1,3-butadiene loss is of about 8% or higher; lower 1,3-butadiene losses may be obtained with higher LHSV, but the hydrogenation of alkynes may not be complete.

The hydrogen/alkynes molar ratio is usually of from 2:1 to 20:1, preferably of 4:1 to 10:1, most preferably of about 6:1.

The C4 feeds that may be used in the process of the invention usually comprise a mixture of normally gaseous hydrocarbons - 1,3-butadiene 30-55%, typicaily 40-50% - 1,2-butadiene up to 2%, typically about 0.2 % alkynes (mainly ethyl up to 5%, typically up to 1.5% and vinyl acetylene) - C3 hydrocarbons and heavies - butanes - butenes traces up to 10%, typically up to 5% balance The feeds are usually obtained from the steam cracking unit. However, other feeds or feeds obtained from other sources may also be contemplated, as for example propylene-rich feeds containing methylacetylene as impurity, without departing from the scope of the invention.

The invention will now be illustrated by means of the following examples, which are not meant to be limitative.

$0 ExamDle 1 a. Catalyst preparation The aiumina support selected was under the form of spheres having a diameter of 2-4 mm, and a bulk density of 0.72 g/cm3.

The support was contacted with a solution of palladium acetylacetonate in benzene. The weight ratio support: solution was of 10:16. The Pd weight concentration in the solution was of 1350 ppmw before contacting the solution with the support, and of 100 ppmw after 8 hours of impregnation.

The impregnated support was filtered and dried at 120'C during 6 hours under an air flow. It was then heated at 300'C in a tubular over, first during 2 hours under an air flow, then, after a nitrogen flush, during a further 2 hours under an hydrogen flow.

After cooling, the catalyst contained 0.2 wt % of palladium.

b. Catalyst activation and start-up The catalyst was purged during one hour with nitrogen at a space velocity of 333 1/1.h. Hydrogen under atmospheric pressure was then passed over the catalyst at a space velocity of 200 1/1.h; the catalyst was then heated at WC during 0.5 hours, then at 93C during 2 hours, and finally cooled to 2WC.

The hydrogen flow was then increased to 333 1/1.h at a temperature of 26'C for 35 minutes. The hydrogen pressure was then slowly increased from atmospheric to 0.61 MPa (6.2 kg/cm2) and maintained for 45 minutes. The feed and hydrogen flow rates were then increased to one fourth of nominal, maintained for 50 minutes, increased to half of the nominal values, maintained for 15 minutes, and finally increased to nominal values while raising the temperature to 57'C at a heating rate of 10C/h.

11 - c. Alkynes selective hydrogenation The operating conditions were as follows - inlet temperature 57.5C - pressure (gauge) 0.61 MPa (6.2 kg/CM2) - feed LMV 14.2 1/1.h hydrogen: feed molar ratio 1:20 adiabatic reactor, operated in downflow mode.

The composition of the feed and of the treated effluent after 24 and 44 hours were as follows:

feed after 24 h after 44 h - 1,3-butadiene 44.63 41.82 42.01 wt vinylacetylene 7768 < 5 < 5 ppmw - ethyl acetylene 1882 10 10 ppmw butanes 3.92 4.33 4.04 wt % butenes 49.89 53.11 53.26 wt % - other hydrocarbons balance balance balance ExamDle 2 The process described in Example 1 was continuously operated during.for 438 hours since start-up, with feeds having a similar composition to that of Example 1 and containing from 7140 to 7768 ppmw of vinyl acetylene and from 1680 to 1882 ppmw of ethyl acetylene. The operating conditions were also similar to those of Example 1.

After 438 hours of continuous operation, the following compositions were determined feed effluent - 1,3-butadiene 46.95 44.34 wt % - vinyl acetylene 7250 < 5 ppmw - ethyl acetylene 1710 19 ppmw - butanes - butenes - other hydrocarbons Example 3 - 12 4.08 47.72 4.03 wt % 50.90 wt % balance balance The selective hydrogenation experiment of Example 1 was repeated with a feed having a slightly different composition. All experimental conditions were identical. except where mentioned in Table 1.

Table 1

Selective hydrogenation of alkynes at a MSV of 14.2, under a pressure of 0.61 MPa (6.2 kg/cm2) and with an hydrogen: feed molar ratio of 1: 20.

Feed Effluent A Effluent B Effluent C 50.8 57.5 70 liquid trickle gaseous Temperature Mode 0 C Composition 1,3-butadiene 46.99 44.23 44.47 43.50 wt % vinyl acetylene 7140 109 < 5 4260 ppMW ethyl acetylene 1680 282 78 1320 PPMW butanes 3.74 3.75 3.72 3.91 wt % butenes 47.89 51.60 51.41 51.58 wt % other hydrocarbons balance balance balance balance This example shows that the hydrogenation is most selective when using the process of the invention; i.e. the 1,3-butadiene loss and the residual acetylenes concentration are minimum for a certain pressure when the temperature is such that the hydrogenation is carried out in trickle mode.

C Example 4

The selective hydrogenation experiment of Example 1 was repeated (all conditions being identical, except where mentioned in Table 2), with the same feed as used in Example 3.

Table 2 Selective hydrogenation of alkynes at a temperature of 57% at a LH5V of 14.2 and with an hydrogen:feed molar ratio of 1:20.

Feed Effluent A Effluent B Effluent C Pressure 0.6 0.66 0.78 MPa Mode trickle trickle liquid ComiDosition 1,3-butadiene vinyl acetylene ethyl acetylene butanes 46.99 7140 1680 3.74 44.26 < 5 45 3.75 51.55 45.44 36 187 3.91 50.15 44.05 wt % 507 449 3.77 51.68 PPMW ppmw wt % butenes 47.89 wt % other hydrocarbons balance balance balance balance This example shows that, for a given temperature, the selectivity is improved when the pressure is such that the hydrogenation is carried out in trickle mode.

1:

E.xw..:-,le 5 The experiment of Example 4 was repeated at a lower temperature. The experimental data are shown in table 3. Table 3 Selective hydrogenation of alkynes at a temperature of 50.5'C, at a MSV of 14.2 and with an hydrogen:feed molar ratio of 1:20.

Feed Effluent A Effluent B Effluent C Effluent D Pressure Mode Composition 1,3-butadiene vinyl acetylene ethyl acetylene butanes 46.99 7140 1680 3.74 butenes 47.89 other hydrocarbons balance 0.48 trickle 44.36 < 5 26 3.75 51.54 balance 0.61 0.69 0.78 liquid liquid liquid 44.23 43.98 109 258 282 375 3.75 3.77 51.60 51.78 balance balance MPa 43.71 wt % 487 455 3.79 52.02 balance ppMW ppMW wt % wt % 1 c L, r Example 6

The selective hydrogenationexperiment of Example 1 was repeated (all conditions being identical, except where mentioned in Table 4) with two slightly different feeds at different values of MSV, as shown in Table 4.

Table 4 Selective hydrogenation of alkynes at a temperature of 57.8% under a pressure of 0.61 MPa, and with an hydrogen:feed molar ratio of 1:20.

Feed A Effluent A Feed B Effluent Bl Effluent B2 MSV 29.3 14.0 10.8 1/1.h Composition 1,3-butadiene vinyl acetylene ethyl acetylene butanes 46.64 7290 1770 3.66 44.06 < 5 84 3.71 3.56 3.56 butenes 48 31 51.86 49.76 53.58 other hydrocarbons balance balance balance balance not detd.) This-example shows that, under otherwise similar conditions, a lower MSV gives a lower residual concentration of vinyl and ethyl acetylenes at the eXDense of a greater loss of 1,3-butadiene.

45.21 7260 1810 42.48 wt % < 5 24 < 5 PPMW ppMW wt % wt % Exa=le 7 Two selective hydrogenations were carried out in the same reactor with the same feed, said reactor being operated (i) in the upflow mode and (ii) in the downflow mode (data from Table 4). All other conditions were identical to those of Example 1, except where mentioned in Table 5. Table 5 Selective hydrogenation of alkynes under a pressure of 0.61 MPa (6.2 kg/cm2), and with an hydrogen/feed molar ratio of 1:20.

Feed---- Upflow effluent Downflow effluent Temperature 58 57.8 'C MSV 14.3 14.0 1/1.h Composition 1,3-butadiene 46.64 43.61 44.31 wt % vinyl acetylene 7290 69 < 5 ppMW ethyl acetylene 1770 283 42 ppMW butanes 3.66 3.91 3.76 wt % butenes 48.31 52.22 51.52 wt % other hydrocarbons b alance balance balance lample 8 The selective hydrogenation experiment of Example 1 was repeated (all conditions being identical, except where mentioned in Table 6) with a C4 feed containing important amounts of methyl acetylene (MAC5.

- Table 6

Selective hydrogenation of alkynes at a temperature of 57.5% under a pressure of 0.625 MPa, with a LHSV of 14.2 and with an hydrogen feed molar ratio of 0.059.

Composition Feed Effluent 1,3-butadiene 44.63 42.00 wt % methyl acetylene 1140 < 5 (detection limit) ppMW vinyl acetylene 7768 < 5 (detection limit) ethyl acetylene 1182 26 PPMW butanes 3.92 n.d. wt % butenes 49.89 n.d. wt % other C3 h.c. 0.13 n.d. wt % C 5+ h.c. balance (n.d. = not determined) This example shows that the hydrogenation is selective for all acetylene. s.

- 18 Example 9

The selective hydrogenation experiment of Example 1 was repeated (all conditions being identical, except where mentioned in Table 7), with a feed of similar composition.

Table 7

Selective hydrogenation ol alkynes at a temperature of WC and under a pressure of 0.8 MPa, using refinery hydrogen having a H2 content of 74%.

Effluent A Effluent B Total-HSV Feed injection (i) a main inlet (ii) at side inlet half way of catalyst bed Total H2 = total feed Hydrogen injection i) at main inlet (ii) at side inlet half way of catalyst bed 0 % Vinyl acetylene < 5 Ethyl acetylene 47 1,3-butadiene loss 6.2 9.8 % 0% 0.072 % 10.2 1 of liquid feed per 1.h 91.2 % 8.8 % 0.075 molar ratio 86.7 % 13.3 % < 5 20 6.4 ppMW ppMW % of initial amount This example shows that the injection of small portions of the feed and of the hydrogen about half way of the catalyst bed is beneficial to the process of the invention.

1 Example 10 a. Catalyst preparation _A palladium catalyst was prepared as described in Example 1 under,a. It was then contacted with an aqueous solution of HAuC14. The weight ratio support: solution was of 10:18. The Au concentration in the solution was of 120 ppmw before contacting the solution with the support, and of less than.10 ppmw after 3 hours of impregnation.

The impregnated catalyst was filtered, dried, calcined and reduced as described in Example 1 under a for the palladium catalyst. After cooling., the catalyst was washed with a normal NH40H aqueous solution until no more chloride ions can be detected in the solution then rinsed with water and dried at 12WC during 1 hour. The resulting catalyst contained 0.2 wt % of palladium and 0.02 wt % of gold.

b. Catalyst activation and start-up The procedure used in Example 1 was used for activation and startup of the Pd-Au catalyst.

c. Alkynes selective hydrogenation The operating conditions and the results were as disclosed in Table 8---.

1 Table 8 Selective hydrogenation of alkynes over a Pd-Au catalyst, in an adiabatic reactor operated in down-flow mode with an hydrogen:feed molar ratio of 1:20.

Feed A B c D Temperature 59.0 57.5 58.1 57.8 OC Pressure 0.62 0.61 0.61 0.61 MPa MSV 14.1 13.8 13.7 13.6 1/1.h Loss of 1,3-butadiene -6.6% -6.8% -7.3% -9.2% of 46.99 wt % Vinyl acetylene 7140 <5 G <5 <5 PPMW Ethyl acetylene 1680 140 75 34 10 ppMW This example shows that, although the Pd-Anu catalysts allow to remove the acetylenes down to lower than 30 ppmw, they have a lesser selectivity than Pd catalysts as shown by the greater losses of 1,3butadiene observed with Pd-Au catalysts.

Example 11

A commercially available palladium-based catalyst, sold as Catalyst type GIRDLER G-68-G (United Catalysts, Louisville, Kentucky; SUd-Chemie, MUnchen, Germany) was used. It had the following properties as indicated by the manufacturer which are within the scope of this invention palladium - particles - bulk density surface area - pore volume in Example 1.

0.2 +/- 0.02 wt % on alumina 2-4 mm diameter spheres 0.7 kg/l 65 - 95 m2/g 0.5 - 0.6 ml/g The activation and start-up were carriedout as described The operating conditions for the selective hydrogenation were as follows adiabatic reactor, operated in downflow mode inlet temperature 57.7'C pressure - feed MSV 0.61 MPa (6.24 kg/cm2) 14.3 1/1.h - hydrogen:feed molar ratio 0.050 The feed and the treated effluent had the following composition feed effluent - 1,3-butadiene 45.22 42.38 wt % vinylacetylene 7.150 < 5 ppmw - ethylacetylene 1750 < 5 ppMW - butanes 3.58 3.84 wt % - butenes 49.79 53.28 wt % - other hydrocarbons balance balance The preferred embodiments of the present invention provide an improved process for removing alkynes from hydrocarbon streams, and also a process for removing alkynes from hydrocarbon streams down to less than 30 ppm.

The preferred embodiments of the present invention also provide a process for removing alkynes from hydrocarbon streams, wherein the catalyst life is greatly increased.

The preferred embodiments of the present invention further provide an improved process for removing alkynes from hydrocarbon streams with a minimum loss of the other components thereof.

The preferred embodiments of the present invention still further provide a process for the selective hydrogenation of the alkynes present in the 1, 3-butadiene-rich C4 cuts from the steam cracking units and mainly used for the production of synthetic rubber.

Claims (17)

1. Claims

   Process for the selective hydrogenation of the alkynes present in 1,3-butadiene-rich C4 cuts over a palladium-based catalyst, comprising the steps of (i) providing a 1,3-butadiene-rich C4 cut; (ii) passing said cut in trickle mode over the catalyst bed in the presence of hydrogen; (iii) separating the residual hydrogen from the remainder of the effluent from step (ii); and (iv) recovering a 1,3.-butadiene-rich feedstock.
2. 2. Process according to Claim 1, wherein the hydrogen.:alkynes molar ratio is from 2:1 to 20:1.
3. 3. Process according to Claim 2, wherein the hydrogen:alkynes ratio is from 4:1 to 1M.
4. 4. Process according to Claim 3, wherein the hydrogen:alkynes ratio is about 6:1.
5. 5. Process according to any foregoing Claim, wherein the pressure in step (ii) is from 0.4 to 0.9 MPa when using a 100% hydrogen flow.
6. 6. Process according to Claim 5, wherein the pressure in step. (ii) is from 0.6 MPa to 0.8 MPa.
7. 7. Process according to any foregoing Claim, wherein step (ii) i carried out in a adiabatic reactor.

   !
8. S 8. Process according to any foregoing Claim, wherein a sufficient proportion of the feed is injected in liquid form at one or several places along the catalyst bed.
9. 9. Process according to any foregoing Claim, wherein up to about 20% of the feed is injected in liquid form at one or several places about half way of the catalyst bed.
10. 10. Process according to any foregoing Claim, wherein up to 30%, of the total hydrogen flow is injected at one or several places along the catalyst bed.
11. 11. Process according to Claim 10, wherein up to about 15% of the total hydrogen flow is injected at one or several places along the catalyst bed.
12. Process according to any foregoing Claim, wherein step (ii) is operated in down-flow mode.
13. 13. Process according to any foregoing Claim, wherein the catalyst contains 0.1 to 0.35 wt % of active palladium metal deposited on high purity alumina.
14. 14. Process according to Claim 13, wherein the catalyst contains about 0. 2 wt % active palladium metal.
15. 15. Process according to Claim 13 or Claim 14, wherein the catalyst is stabilized by the use of a bimetallic catalyst.
16. 16. Process according to Claim 15, wherein the catalyst is made of platinum-gold alloy deposited on high purity alumina.
17. 17. Process for the selective hydrogenation of the alkynes present in 1,3butadiene-rich cuts over a palladium-based catalyst substantially as described in any one of Examples 1 to 11.

    Published 19IR8 '. rhe Patent Office, State House. 6671 High Holborn, London WC1R 4TP Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BRS 3RD. Printed by Multiplex techniques ltd. St Maxy Cray, Kent. Con. 1/87.