FR2609023A1 - PROCESS FOR THE SELECTIVE HYDROGENATION OF ACETYLENES - Google Patents

Abstract

METHOD FOR SELECTIVE HYDROGENATION OF ALKYNES IN HYDROCARBON COMPOUND FEEDING CHARGES IS DESCRIBED, MORE PREFERABLY IN 1,3-BUTADIENE RICH C FRACTIONS, INCLUDING THE DROPPING FLOW THROUGH FLOW THROUGH A CATALYST BED BASED ON PALLADIUM IN THE PRESENCE OF HYDROGEN.

Description

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  avoided or inhibited to the extent possible. These

  reactions obviously include the hydrogenation of 1,3-

  butadiene and butenes, but also the reactions of

  polymerization which reduce the longevity of the catalyst.

  Regeneration of the catalyst is possible, but their frequency is an important economic factor, besides being at the origin of modifications of the catalyst and, where appropriate, of mechanical breaks of the catalyst pellets, leading to an increase in the fall of

pressure through the bed.

  It has been known for a long time to proceed with the hydroeneral. Selective at a temperature in the gas phase, in the presence of a tatayvseur this copper-nickei on a supporz de.SiO / A] Ot. iQuteIfis. more and more of these products are being abandoned, since the catalyst must be replaced or regenerated frequently,

  1, $ - butadiene and the residue concentration of carbide-

  acetylenics are now considered to be too important. US Pat. No. 4,449 describes a process for the removal of acetic acid carbides from liquid hydrocarbon streams, such catalyst being essentially comprised of finely divided methanol. diserse on a well-defined gamma-alumina which can contain up to 35% by weight alphosphine alumina). Gamma-gamma used at a surface area of 68 to 350 mI / g; 40 to 9 "of the pores have a diameter of 4 to 12 nm and 2 to 25% have a diameter of between 100 and 1000 ns.The support has a high purity, with less than 0.15% by weight of silicon S0 in the form of of SiO and less than 0.15% by weight Na in the form of Na 2 O. It is claimed that the catalyst of US Patent 4,493,905 leaves O ppm of acetylenic carbides when used at about 6 ° C and with LHSV

  (hourly space velocity of the liquid) less than 1.

  However, the corresponding cycle time is only 51-

  days; after about 10 days, about 10D ppm: acetylenic cracks are detected in the effluent. It is obvious that with higher values of LHSV, the cycle time would be even shorter or the elimination of

  acetylenic carbides would not be correct.

  Another type of palladium-based catalysts. Palladium is, among the metals of the Vili group, the most active and selective metal for the hydro- cation of acetylenic carbides. However, it is known in the art that two types of operational difficulties are encountered: losses of 6-bouadene are observed, even at moderate conversions. dce acyne-; and - losses of paiadiu = reduisen the catalyst,

  according to this type of information.

Process.n, March 19ó, D. 52.

  As the situation changes, the severity of

  from steaming to dry steam at the same time; and watering, brutes in C cont. There are, therefore, increasing concentrations of acynes in, or in, or even more. On the other hand, the existence of the concentration of acetylenic carbides in the eifluent

  the hydrogenaticn. sective become more and more

  strictest. There is therefore a need in the tecnuAcue of a

  improved process for removing alkyne

  of liquid hydrocarbons, with a minimum of loss of

  conjugated dienes which are present there.

  The process of the present invention for the selective hydrogenation of alkynes present in 1,3-butadiene-rich Ce moieties in the presence of a palladium catalyst comprises the steps of:

  (i) to obtain a fraction in Frequency 1.3

  butadienrie; (ii) passing this fraction through a trickle flow through the catalyst bed; p-sernce of hydrogen; (iii) separating the residual hydrogen from the remainder of the effluent from phase (ii); and

  (iv) to recover a high starting load of 1.3-

  butadiene. The palladium catalysts that can be used in the process of the invention are well known in the art. A catalyst is commonly used to form active palladium palladium on a suncrylate.

of high purity alumina.

  The amount of palladium in the catalyst comprises a high pure alumina carrier, preferably between 0.35 and 0.35% by weight, and in the range of 0.2% by weight. . It is advantageous: that the alumina be very pure and that the concentration of metau:

  heavy other cd Fd soilt less than 0.05_ in roidz.

  The surface area of the catalyst is 2 g and preferably 50 g / cm.sup.2 / g, in particular between 0 and 1.sup.5 m.sup.2. The pore volume is preferably 0.5 to 0.6 cm 3 / g. The catalyst is preferably under the

  shape of spheres of 2 to 4 xm in size.

  It has already been said that the acidity of alumina affects the undesirable reactions of oligomerization and that consequently gamma-alumina should be preferred to a conventional alumina. But this is not necessary with the process of the invention, since this only concerns the long-term stability, and not the activity of the fresh or regenerated catalyst. Other types of alumina may also be used, such as Q-type alumina, which has been

  described in Japanese Patent Application JB-580/7835.

  Palladium-based catalysts known to be stabilized or activated are known, such as the supported palladium-gold catalysts which are described in the European patent EP-25. The activity of these catalysts is as follows: lower than that of palladium catalysts. Without wishing to focus on a theory, this could be explained by a less homogeneous dispersion of metals on the support, because it is almost impossible to obtain a bimetallic catalyst

  fixed to a support in a controlled way, with low level.

  which are used for industrial catalysts based on precious metals. Indeed, the support must be suitable for an appropriate interaction between the metals and a well-dispersed bimetallic species must be obtained, and then manintenue. The use of catalysts which are stable or active in the process of the invention is optional and will therefore depend on the activity requirements. of

long-term stability.

  Activation, regeneration and regeneration. these palladium catalysts are known in the art. The activation consists in: (i) purging the oxygen in the medium of nitrogen and causing the hydrogen to drop at atmospheric pressure, while heating 2C progressively to a level of The initiating operation then slowly increases the hydrogen pressure, then the flow rates of the feedstock and the hydrozene, and finally the temperature. This regeneration is intended to pass water vapor at atmospheric pressure, while progressively raising the temperature to about 400 ° C., and then to continue to pass water vapor at atmospheric pressure at a temperature of 25.degree. approximately 400 ° C. for approximately 2 hours, and finally to gradually add to this water vapor up to several mol% of air During the regeneration operation, the catalyst temperature should not exceed about 500 ° C. The regeneration is completed when

  the COa content at the outlet is low enough.

  The state of the art and the manufacturers of cataiyeurs recommend the following typical operating conditions.

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  for the selective hydrogenation of vinvi- and ethylacetylene in a liquid fraction of 1,3-butadiene-rich, using palladium-based catalysts: Temperature: 15-20 ° C (at the inlet) Pressure: 0.5 MPa (5 bar) - VSHL: 30 l / lh- '

- Hz / alkynes molar ratio: 2: 1.

  The typical results obtained under these conditions are: - Feedstock: 1,3-butadiene 50. by volume BthyLacetyiene b,> 2, voluine Vin ylazetyeiene i, 2 by weight Rest = Butenes - Effiuent puriiie: 500 pp-Jd alcyneE totau :: 3, butadieene loss

- Cycle time: 8-10 months.

  It has been widely discovered that when used in the trickle-down mode, the hydrolysis catalyst is used. are much more selective than in the case of homozene liquid phase. The term "increased selectivity" as it is here

  uses, means that for a load of aiime.nticn ..

  given, it is lost less than 1,3-butadiene for a given level of hydrogenation of alkynes. The term "drip flow moae" as used herein is defined as the procedure under conditions of temperature and pressure such that the feedstock passes as a gaseous-mixed liquid phase. through the catalyst. Since the hydoragenation reaction is exothermic, it is ordinarily more common to introduce the feedstock in the liquid state.

  under conditions very close to the gas equilibrium-

  liquid. The reactor may be either an isothermal reactor or an adiabatic reactor. In the latter case, the

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  heat liberated by the hydrogenation reaction is compensated by the vaporization of part of the liquid phase. It is therefore very desirable, in the case of an adiabatic reactor, to have sufficient starting charge in the liquid phase for the entire heat released by the hydrogenation reactions to be absorbed and in addition, to inject a portion of the feedstock in liquid form along the a :: e of the reactor. According to one embodiment of the invention, a sufficient proportion of the feedstock, preferably up to about 2% c, is injected or eluted.

  liquid, in one or more places, are false, -

  The distance of the catalyst bed from a reactor to T5 Without attaching itself to a theory, it is believed that these injections serve perhaps to constant in the whole volume of the reactor adiabratioue lez.

  coniions- the trickle flow mode.

  In view of the fact that the gas phase is produced in part by the vaporization of the charging charge, the trickle flow mode is usually applied co-currently. Although it is possible to operate in the down-current mode, the bemanderese notes that it is clearly preferable to operate in the

down current mode.

  Hydrogen can be injected with the feedstock. However, it has also been found that it is very desirable to restart a portion of the hydrogen injection along the axis of the reactor, for example in one or more places, about half way to the bed. of catalyst. According to one embodiment of the invention, up to 30%, preferably up to about 15% of the total hydrogen stream is injected at one or more locations, about half way to the i, T of catalyst in an adiabatic reactor. In order to be attached to a theory, the Applicant considers that these injections may serve to keep the conditions of the adiabatic reactor constant throughout the volume of the adiabatic reactor.

drip flow mode.

  With a flow of hydroene at 100%, the total pressure should preferably be between 0.4 and 0 MFpa, and especially between 0.6 and 0, e XFPa. Therefore, in case of using refinery hydrogen in the process of the invention, this refinery hydrogen usually containing about 75% byrrogen and about 25% methane, the total pressure ddit of prefexence being a little

higher.

  The reaction temperature (or the input range when an adiabatic reagent is used) is adjusted to the maximum pressure, so that it can not be maintained.

  the desired mode of deafness drip. In the.

  the limits of the p-refereed ranges, the higher values of the press-on and the tereperai: re on. tendency to give a

greater catalyst activity.

  The VSHL may adopt easily. determined by

  the man of the art by holding, onp re e s.

  for acetylenic resins and / or loss of 1,3-butadiene in the case of 1,3-butadiene-rich fractions). For example, almost complete alkyne hydrogenation at a concentration of 1% in 1,3-butadiene-rich C.sub.1 fractions using palladium-based catalysts requires

  usually a LHSV of less than 10, but the loss of 1.3-

  corresponding butadiene is about 8% or more; lower losses of 1,3-butadiene can be achieved with higher WHSV, but hydrogenation of alkynres

may not be complete.

  The hydrogen / alkynes molar ratio is ordinarily from 2: 1 to 20: 1, preferably from 4: 1 to 10: 1,

  being in particular of the order of 6t: 1.

  The C feedstocks which can be used in the process of the invention are

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  usually with a mixture of gaseous hydrocarbons: - 1,3-butadiene 30-55%, typically 40-50% - 1,2-butadiene up to 2%, typically about 0.2% - Alkynes (mainly ethyl) - and vinylacetylenes) up to 5%, typically up to 1.5 "- Cr hydrocarbons and trace heavy products - Butanes up to 10%, typically up to 5%

- Butenes the rest.

  The feedstocks usually provide the steam cracking unit. However, other feedstocks or feedstocks from other sources may also be envisaged, such as those feedstocks rich in propylene and containing methylacetylene as an impurity, without

  however, it departs from the scope of the invention.

  The invention will be illustrated by the examples which follow and which should not be interpreted as

liritatifs.

  Exep ie i a. Preparation of the catalyst The alumina support chosen was in the form of spheres having a dianetre of 2 to 4 mm and a density

apparent 0.72 g / cm3.

  The carrier was contacted with a solution of palladium acetylacetonate in benzene. The carrier / solution weight ratio was 10: 15. The weight concentration of Pd in the solution was 1350 ppm before contacting the solution with the support and 100

ppm after 8 hours of impregnation.

  The impregnated support was filtered and dried at 120.degree. C. for 6 hours in a stream of air. It was then heated at 300.degree. C. in a tubular furnace, first for 2 hours in a stream of air and then, after a nitrogen sweep, for 2 hours.

again in a hydrogen stream.

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  After cooling, the catalyst contained 0.2%

weight of palladium.

  b. Activation and triggering of the catalyst The catalyst was purged for 1 hour with nitrogen at a space velocity of 333 l / hr. This hydrogen was then passed at atmospheric pressure through the catalyst at a space velocity of 200 l / 1 hr; the catalyst was then heated at 66 ° C for 0.5 h, then

  at 93 ° C for 2 hours and finally cooled to 20 ° C.

  The hydrogen flow rate was then increased to 333 l / lh at a temperature of 25 ° C for 5 minutes. The hydrogen pressure was slowly increased from atmospheric pressure to 0.05 (0.2 kg / cm 2) and held for a long time. 45 minutes. The feedstock and hydrogen feed rates were then increased to a quarter of the nominal value, maintained for 50 minutes, increased to half of the nominal value, maintained for 15 minutes, and finally increased to the nominal value, while that the temperature was raised to 57 ° C at a heating rate of

10o-C / h.

  c. Selective Hydrogenation of Alkynes The operating conditions were the following: - inlet temperature 57.5 ° C. - Pressure (gauge) 0, t1 KFa (e, 2 kg / cIn, - VSHL of the feedstock 14, Hydrogen Molecular Ratio / Feedstock 1:20 - Adiabatic Reactor, operated in the downflow mode The feedstock and treated effluent compositions after 24 and 44 hours were as follows: Binding load Aores 24 n Aors 44 h -1,3,3-butadiene (wt.%) 44,63 41,82 42,0i - Vinylacetylene (pp ") 7768 <5 5 - Ethylacetylene (ppm ') t 82 10 10 - Butanes (in ponds) 3.92 4.03 4.04 - Butenes (") 49.89 53.11.53.6 - Other hydrocarbons remainder remainder Rxe np 1 ç_à The procedure described in Example 1 It was applied continuously for 438 hours from the loading stage, with feedstocks having a composition similar to that of Example 1 and weighing between 7140 and 77 ppm vinylacetyler by weight. .ne e between ieO and -12 ppr ern poidc

  of eéhv lacetyene. Operational conditions etaier.

  also similar to those of the example.

  After 43.5 k of continuous operation, onr. has the following compositions:

  -, 3-buta iene 46.95 44.34 X in p,: .-

  Vinylacetylee 725C Primer p: - Ethylacetylene 1710 19 ppM in pl: Butanes 4.08 4.03 in pol - Butenes 47.72 50.90) in polyethylene - Other hydrocarbons remainder

Example 3

  The selective hydrogenation experiment of Example 1 was repeated with a feedstock having a somewhat different composition. All experimental conditions were identical except for

is mentioned in Table 1.

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  Table 1 Selective Hydrogenation of Alkynes at a LHSV of 14.2, at a Pressure of D, IIaFa (6.2 Kg / cc) and a Hydrogen Alanine / Charge Factor of 1:20 Fg Charge Effluent Effluent B Effluent C Effluent C allylate LLn Temorature 5, - 57, S 70 'C largeliquid glyceride 1,3-butadiene 46,99 44,23 44,47 43,50 I polis Polyacetylene 7140 109 S 4260 ppN by weight 1 Ethylacetylan lEO 2U2 7E 132 ppM in poics Butanes 3,7e 3,7j 3,72 3,51 I in peas Butenes 47, 9 51,6 s, 41 1 SE Er pl .: Other hydrocarbons remainder remainder remainder It is important that the use of the process of invention that the hydroenr.

  is the most selective; that is, the loss of l-.

  Butadiene and the residual concentration of acetylenic carbides are minimal for this type of pressure-ion, when the temperature is such that the hydrogenation is very low.

  driving in the drip mode.

Example 4

  The selective hydrogenation experiment in Example I was repeated (all conditions being identical, except for what is mentioned in Table 2) with

  the same feedstock as in the e: empie.3.

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  Example 52 The experiment of selective hydrogenation of Example 1 was repeated under all conditions, except for that mentioned in Table 4 with two slightly different feedstocks and different values of 1l VSHL, as indicated in the

table 4.

Table 4

  Selective hydrogenation of alkynes At a temperature of 57.8.degree. C., at a pressure of 0.61 MPa, and with 1 g, a molar hydrogen loading ratio of 1:20 CH.sub.n (Effluent Effluent Effluent E2 VSL). 2, I, C] 0.% I / lr, Cor., 1-butadine, 4E6, 6: 44.6, 4.2, 42.4, t%, bone, acetyl, 729C, S, 7-2. <5 <56, in pounds Ethylacetylene 177i 64 1Ei 24 20 pDR in po1s Butanes 3,66 3,71 3,6 3, 6 x ten poics Butines 48,31 51,66 49,76 53,58% Sen Weight Others This example shows that, under otherwise similar conditions, a lower WHSV gives a residual residual concentration of vinyl and ethylacetylenes at the same time.

  price of a higher loss of 1,3-butadiene.

  Two selective hydrogenations were carried out in the same reactor with the same feedstock, this reactor being operated (i) in the upflow mode and <ii) in the reactor. downflow mode (results from Table 4). All other conditions were identical to those of Example 1, a

  except for what is mentioned in Table 5.

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  Selective hydrogenation of alkynes under a pressure of 0.61 MPa (6.2 kglt) and with a solar hydrogen / feedstock ratio of 1:20, Charge Effluent in current Flowing in ascending alienation flow of eegeculation 58 57, 8 'C VHL 14.3 14.0 Illh 1,3butadiene 46,64 43,61 44,81 S by weight Vinylacetylene 72D50 69 c 5 ppi in polys 1 G Ethylacetylene 1770 283 42 rp? In po Butans 3,66, 91t, 76 Z in oo; c Butenes 48,31 52,22 51,62 X ei pcie Other hydrocarbons rest remainder xezsole 6 The selective hydrogenation experiment of the example was repeated (all conditions being ident.ques with the exception of what is mentioned in Table tj with a Ca feedstock containing quartitee

  significant amounts of methylacetylene (KAC).

Table g

  Selective hydrogenation of aicyne; at a temperature of 57.5 ° C, under a pressure of 625 ta, with a LHSV of 14.2 and with a hydrogen / feedstock molar ratio of 0.059 Coeomoition Alilentation charge Effluent 1,3-butadiene 44.63 42.0 wt.% Ethyltetylene 1140 <5 (limit of detection) ppm by weight Vinylacetylene 7768 (5 (limit of detection) Ethylacetylene 1182 26 wt. P. Butanes 3.92 nd, S wt. Butenes 49.89 nd , S in weight Other hc in C3 0,13 nd Z in weight Hc in Cs and + remainder (nd, = Not determined) This example shows that hydrogenation is selective

  for. all acetylenic carbides.

Example g

  The selective hydrogenation experiment of Example 1 was repeated (all conditions being the same, with the exception of what is mentioned in Table 7) with a feedstock of similar composition. Table 7 Selective Hydrogenation of Alkynes at a Temperature of 66 ° C and a Pressure of 0.8 MPa, with

  use of refinery hydrogen having a 74X Ha content.

  Effluent A Effluent 10 Total LHSV 9, t 10.2 I / lh Injection of the altes- ticant charge (.) By the main route 1cIt 91.2: (i) by a lateral adi-mum of the bed of catalyst 0: 8, total H2 / total load of total 0,072 C, 075 good ratio of hyorogen (i) by the main inlet 100D f6,71 (ii) by a lateral inlet at 1m-chelirn catalyst bed 0C 13 Vinylacetylene <5 <pp in pcc; Ethylacetylene 47 20 ppoen pOlcs Loss of 1,3-butadiene 6.2 6.4 l of Quantity Initiate This example shows that the injection of small portions of the feedstock and hydrogen is approximately half way of the catalyst bed is beneficial for the

method of the invention.

  at. Catalyst Preparation A palladium catalyst was prepared as described in Example 1, paragraph a. It was then contacted with an aqueous solution of HAuCli. The carrier / solution weight ratio was 10:18. The concentration of Au in the solution was 120O ppm by weight before contacting the solution with the carrier, and less than 10 ppm by weight after impregnation.

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  The impregnated catalyst was filtered, dried, calcined and reduced as described in Example 1, paragraph a for the palladium catalyst. After cooling, the catalyst was washed with a normal aqueous solution of NH 4 OH until the presence of chloride ions in the solution could no longer be detected, then washed with water and dried at room temperature. 120 ° C for 1 hour. The resulting catalyst contained 0.2% by weight of palladium and 0.02% by weight of gold. 1C b. Activation and triggering of the catalyst The operating mode of Example 1 was used to

  activation and triggering of the Pd-Au catalyst.

  c. Hydrogenaticn selectve of alkynes The operating concitons and the resuits and

presented in table e.

  Tabl'aa. Hydrocatalyst selected from alkynes in the presence of Pd-Au catalysis, in a lazatize mode operated in the downflow mode with a hydrogen / alkali releasing ratio of 1: 2 to 2 C Aras. Wt. O9.0 57.5SS, 1 57.8 ° C Pressure 0.62 0, 61 O, 61 0.61 M Loss of 1,3-butadiene -6.61 -6.81 -7.31 -9.2% of 46.95 * e po01s Vinylacetylene 7140 <5 (S '5 (5ppm in chids Ethylacetylene 1680 140 75 34 10 ppq in poics

  This example shows that although the catalysts of Pd-

  With the elimination of acetylenic carbides up to less than 30 ppm by weight, they have a lower selectivity than Pd catalysts, as shown by the greater losses of 1,3-butadiene observed with

Pd-Au catalysts.

Example 1-1

  A commercial palladium catalyst sold as Catalyst type GiRULER -S-G (United Catalysts, Louisville, Kentucky; Ud-C.hemie, Munich, Germany) was used. According to the manufacturer's indications, it had the following properties which fall within the scope of

the present invention.

  - Palladium: 0.2 0.02% by weight on alumina - Particles: Spheres 24 mm in diameter - Bulk density: 0.7 kg / i - Surface area: 65-95 m2 / g

- Pore volume: 0.5-0.6 ml / g.

  Activation and triggering were performed from

  the manner described in Example 1.

  The operating conditions for the selective hydration were as follows: - Absorbent reactor, operated in the downflow mode - Inlet temperature: 57.7 ° C - Pressure: 0.61 Ya (6.24 kg / cm 2) - VSHL of the feedstock: 14.3 1 / 1.h - Molar ratio hydrogen / feedstock: 0.050 The feedstock and treated effluent had the following compositions: Feeding load

  1,3-butadiene 45,22 42,38 a. in weight--

  Vinylacetylene 7150 <5 ppm by weight - Ethylacetylene 1750 <5 ppm by weight - Butanes 3.58 3.84% by weight - Butenes 49.79 53.28% by weight Other hydrocarbons remainder The preferred embodiments of the present invention The present invention provides an improved process for removing alkynes from hydrocarbon streams, as well as a method for removing alkynes from hydrocarbon streams.

Up to less than 30 ppm.

  The preferred embodiments of the present invention also provide a process for removing alkynes from hydrocarbon streams.

  longevity of the catalyst is greatly increased.

  The preferred embodiments of the present invention further provide an improved process for removing alkynes from hydrocarbon streams with

  minimum loss of other components of this current.

  In addition, the preferred embodiments of the present invention provide an improved method for the selective hydrogenation of the alkynes present in the 3,4-butadiene rich C. moieties from the steam cracking units and used therein. 0 mainly for the production of synthetic rubber.

2 O

Claims (16)

REVENDICATI0NS   1.- Proceeds for the selective hydrogenation of the drugs   present in 1,3-butadiene-rich fractions in the presence of a palladium catalyst, comprising the steps of:   <i) to obtain a 3,4-rich fraction of   butadiene, (ii) passing this fraction through a trickle flow through the catalyst bed in the presence of hydrogen, (iii) separating residual hydrogen from the remainder of the effluent from phase (ii), and   (iv) recovering a 1,3-rich raw material   butadiene. 2. A process according to claim 1, wherein the molar ratio of hydrogen to alkynes is from 2: 1 to 1. 3. The process according to claim 2, wherein the molar ratio of hydrogen to alkynes is between 4: 1 and 10: 1.   4. The process according to claim 3, wherein the   Hydrogen / alkynes molar ratio is about 6: 1.   5. Process according to any one of the claims   1 to 4, wherein the pressure in the phase (ii> is between 0.4 and 0.9 MPa when using a 100% hydrogen stream. 21 2609023   6. A process according to claim 5, wherein the pressure in the phase (ii) is between 0.6 and 0.8 NPa.   7. Process according to any one of the claims   1 to 6, wherein the phase (Ii) is conducted in uni adiabatic reactor.   8. Process according to any one of the claims   1 to 7, wherein a sufficient proportion of the feedstock is injected in liquid form into one or   several points along the catalyst bed.   9. Process according to any one of the claims   1 to 8, wherein up to 20% of the feedstock is injected in liquid form at one or more points   about half way to the catalyst bed.   10. A process according to any one of the claims   1 to 9, wherein up to 30% of the total hydrogen stream is injected at one or more points along the catalyst bed.   11. A process according to any one of the claims   1 to 10, wherein up to 15% of the total hydrogen stream is injected at one or more points along catalyst bed.   12. Process according to any one of the claims   1 to 11, wherein the phase (ii) is conducted in the down current.   13.- Method according to any one of the claims   1 to 12, in which the catalyst contains 0.1 to 0.35% by weight of active palladium metal, deposited on an alumina of high purity.   14. The process according to claim 13, wherein the catalyst contains about 0.2% by weight of palladium. active metal.   15. A process according to claim 13 or 14, wherein the catalyst is stabilized by the use of a bimetallic catalyst. 22 2609023   16. The process according to claim 15, wherein the catalyst is made of a platinum-gold alloy deposited on a high purity alumina.