Title:
Process for the selective desulphurization of olefinic gasolines, comprising a hydrogen purification step
Kind Code:
A1


Abstract:
The invention concerns a process for selective hydrodesulphurization of an olefinic gasoline, in which makeup hydrogen and/or the overall feed at the inlet to the catalytic reactor has a CO content of less than 50 ppmv of even less, but a COx(═CO+½CO2) content of more than 120 ppmv. The invention allows diversification as regards the sources of makeup hydrogen and/or can simplify the purification treatment for said hydrogen, and/or can reduce the purge of hydrogen from the hydrodesulphurization unit.



Inventors:
Bournay, Laurent (Chaussan, FR)
Baudot, Arnaud (Lyon, FR)
Application Number:
11/236792
Publication Date:
04/13/2006
Filing Date:
09/28/2005
Primary Class:
Other Classes:
208/216R, 208/217
International Classes:
C10G45/02; C07C67/03; C07C67/56; C11C3/10
View Patent Images:



Primary Examiner:
SINGH, PREM C
Attorney, Agent or Firm:
MILLEN, WHITE, ZELANO & BRANIGAN, P.C. (ARLINGTON, VA, US)
Claims:
1. A process for hydrodesulphurization of a hydrocarbon cut HC comprising at least 5% by weight of olefins, in which said hydrocarbon cut, a stream HYD of makeup hydrogen and optionally a stream REC of recycle hydrogen are mixed to form an overall feed which is supplied to the inlet to at least one reactor comprising a desulphurization catalyst, under operating conditions that can transform the organic sulphur-containing compounds of the HC cut into H2S, in which the source or sources of hydrogen forming the stream HYD is selected, and optionally at least one hydrogen purification treatment is carried out on HYD, REC or a mixture thereof such that said overall feed comprises at most 50 ppmv of CO and at least 120 ppmv of COx, where COx═CO+½CO2.

2. A process according to claim 1, in which the source or sources of hydrogen forming the stream HYD is selected and at least one hydrogen purification treatment is carried out on at least a portion of HYD so that HYD comprises at most 50 ppmv of CO and at least 120 ppmv of COx.

3. A process according to claim 1, comprising at least one hydrogen purification treatment T1 carried out on HYD, REC or a mixture thereof, said treatment T1 comprising carrying out limited CO2 elimination to obtain at least 200 ppmv of CO2 in the overall feed.

4. A process according to claim 1, comprising at least one treatment T2 for the purification of hydrogen carried out on HYD, REC or a mixture thereof, said treatment T2 comprising carrying out catalytic oxidation of CO by O2 and/or H2O to obtain at most 50 ppmv of CO in the overall feed.

5. A process according to claim 1, comprising at least one treatment T3 for the purification of hydrogen carried out on HYD, REC or a mixture thereof, said treatment T3 comprising carrying out catalytic methanation of CO by H2 to obtain at most 50 ppmv of CO in the overall feed.

6. A process according to claim 4, comprising a hydrogen purification treatment carried out on HYD, and optionally on REC or a mixture thereof, said treatment comprising a treatment T2 of carrying out catalytic oxidation of CO by O2 followed directly, and with no secondary elimination of CO2, by desulphurization of the HC cut in the presence of a stream of the purified hydrogen.

7. A process according to claim 5, comprising a hydrogen purification treatment carried out on HYD, and optionally on REC or a mixture thereof, said treatment T3 comprising carrying out methanation of CO by H2 followed directly, and with no secondary elimination of CO2, by desulphurization of the HC cut in the presence of a stream of the purified hydrogen.

8. A process according to claim 6, comprising a conducting said treatment T1 for eliminating CO2 on HYD upstream of T2 or T3.

9. A process according to claim 1, comprising a hydrogen recycle REC ard a purge of recycled hydrogen WGAS in which the flow rate of the purge WGAS is controlled so that the CO content in the overall feed is less than 50 ppmv but that the COX content in the overall feed is more than 120 ppmv.

10. A process according to claim 1, for hydrodesulphurization of an olefinic gasoline cut, carried out on a catalyst which comprises at least one element from group VIII and optionally an element from group VIB, the element from group VIII being selected from the group constituted by nickel, cobalt and iron, the optional group VIB element is molybdenum or tungsten, in which the reactor pressure is in the range 0.5 MPa to 5 MPa, the ratio of the flow rate of hydrogen in normal litres of hydrogen per hour to the flow rate of hydrocarbons in litres per hour is in the range 50 to 800, and the temperature is in the range 200° C. to 400° C.

11. A process according to claim 10, in which the catalyst comprises cobalt and molybdenum.

12. A process according to claim 2, comprising at least one treatment T2 for the purification of hydrogen carried out on HYD, REC or a mixture thereof, said treatment T2 comprising carrying out catalytic oxidation of CO by O2 and/or H2O to obtain at most 50 ppmv of CO in the overall feed.

13. A process according to claim 3, comprising at least one treatment T2 for the purification of hydrogen carried out on HYD, REC or a mixture thereof, said treatment T2 comprising carrying out catalytic oxidation of CO by O2 and/or H2O to obtain at most 50 ppmv of CO in the overall feed.

14. A process according to claim 2, comprising at least one treatment T3 for the purification of hydrogen carried out on HYD, REC or a mixture thereof, said treatment T3 comprising carrying out catalytic methanation of CO by H2 to obtain at most 50 ppmv of CO in the overall feed.

15. A process according to claim 3, comprising at least one treatment T3 for the purification of hydrogen carried out on HYD, REC or a mixture thereof, said treatment T3 comprising carrying out catalytic methanation of CO by H2 to obtain at most 50 ppmv of CO in the overall feed.

16. A process according to claim 12, comprising a hydrogen purification treatment carried out on HYD, and optionally on REC or a mixture thereof, said treatment comprising a treatment T2 of carrying out catalytic oxidation of CO by O2 followed directly, and with no secondary elimination of CO2, by desulphurization of the HC cut in the presence of a stream of the purified hydrogen.

17. A process according to claim 13, comprising a hydrogen purification treatment carried out on HYD, and optionally on REC or a mixture thereof, said treatment comprising a treatment T2 of carrying out catalytic oxidation of CO by O2 followed directly, and with no secondary elimination of CO2, by desulphurization of the HC cut in the presence of a stream of the purified hydrogen.

18. A process according to claim 14, comprising a hydrogen purification treatment carried out on HYD, and optionally on REC or a mixture thereof, said treatment T3 comprising carrying out methanation of CO by H2 followed directly, and with no secondary elimination of CO2, by desulphurization of the HC cut in the presence of a stream of the purified hydrogen.

19. A process according to claim 15, comprising a hydrogen purification treatment carried out on HYD, and optionally on REC or a mixture thereof, said treatment T3 comprising carrying out methanation of CO by H2 followed directly, and with no secondary elimination of CO2, by desulphurization of the HC cut in the presence of a stream of the purified hydrogen.

Description:

FIELD OF THE INVENTION

The present invention relates to a process for producing low sulphur content hydrocarbons. This invention is principally applicable to mixtures of hydrocarbons which contain a fraction of olefins which is generally over 5% by weight and usually over 10% by weight, and at least 50 ppm by weight of sulphur. The process allows hydrogen with very low CO contents, but with relatively high CO2 contents to be used without significantly affecting the performance of the catalysts employed during the hydrodesulphurization step. This allows diversification as regards the sources of possible makeup hydrogen and/or can simplify the treatment of the hydrogen without resorting to eliminating a lot of CO2.

PRIOR ART

Future specifications regarding vehicle fuels envisage a substantial reduction in the sulphur content of such fuels, in particular gasoline. In Europe, specifications regarding sulphur contents are at 150 ppm by weight and will reduce in years to come to levels below 10 ppm after passing through a 50 ppm by weight level. The change in sulphur content specifications thus necessitates the development of novel processes for deep desulphurization of gasoline.

The principal source of sulphur in bases for gasoline is cracked gasoline, and principally the gasoline fraction derived from a process for catalytically cracking an atmospheric distillation residue or a crude oil vacuum distillate. The fraction of gasoline from catalytic cracking, which represents on average 40% of the gasoline base, contributes by more than 90% to the amount of sulphur in the gasoline. As a result, the production of low sulphur gasoline necessitates a step for desulphurizing catalytically cracked gasoline. That desulphurization is conventionally carried out using one or more steps for bringing the sulphur-containing compounds contained in said gasoline into contact with a catalyst in the presence of a hydrogen-rich gas in a process termed hydrodesulphurization.

Further, the octane number of such gasoline is very high because of their high olefin content. Preserving the octane number of such gasoline necessitates limiting reactions in which olefins are transformed into paraffins. Such hydrogenation reactions are inherent to hydrodesulphurization processes, which causes a loss of octane number which may be as high as 5 to 10 points, principally due to a reduction in the olefin content.

Further, in refineries, the gasoline hydrodesulphurization process is often installed on the gasoline cut, directly at the outlet from cracking units such as catalytic cracking units which usually operate continuously for several years. The hydrodesulphurization process must thus be operated in an uninterrupted manner for 3 to 5 years. The activity and quality of the catalysts used to transform sulphur into H2S must be high in order to be able to operate continuously for several years.

In order to be competitive, hydrodesulphurization processes must satisfy two principal constraints, namely:

    • limited olefin hydrogenation;
    • good stability of the catalytic system and continuous operation for several years.

Hydrodesulphurization processes are based on treating hydrocarbon cuts over a catalyst containing sulphurized non noble metals and supported. on a mineral support in the presence of hydrogen. The metals used generally contain at least one metal from group VIII (for example cobalt) and possibly a metal from group VIB (for example molybdenum) of the periodic table. The most usual catalytic formulations which are encountered are based on Co and Mo or Ni and Mo deposited on alumina. In the case of treatment of olefinic gasoline from cracking units, the catalyst and operating conditions are optimized to limit the degree of olefin hydrogenation while maximizing the transformation of organic sulphur-containing compounds to H2S. Such processes have been described in particular in European patents EP-A-0 1 031 622 and EP-A-0 1 250 401.

Hydrodesulphurization processes may use hydrogen from a number of sources. The principal source of hydrogen in the refinery is catalytic reforming. The catalytic reforming unit produces hydrogen during the dehydrogenation of naphthenes to aromatics and dehydrocyclization. That hydrogen is generally 60% to 90% pure, but is substantially free of CO and CO2.

Depending on refinery requirements, hydrogen may also be produced by steam reforming light hydrocarbons or by partial oxidation of various hydrocarbons, in particular heavy residues. Steam reforming consists of transforming a light hydrocarbon feed into synthesis gas (mixture of H2, CO, CO2, CH4, H2O) by reaction with steam over a nickel based catalyst. The production of hydrogen by partial oxidation consists of treating a hydrocarbon fraction by high temperature oxidation with oxygen to produce a synthesis gas constituted by CO, CO2, H2 and H2O. In the last two cases the production of hydrogen is accompanied by a production of oxides of carbon which are generally substantially eliminated either by methanation or by adsorption. However, the residual amount of oxides of carbon (CO and CO2) may in some cases be more than 0.50 ppmv, or 100 ppmv or even more. Other sources of hydrogen are also occasionally used, such as hydrogen derived from catalytically cracked gas, which contains substantial quantities of CO and CO2. Finally, CO and CO2 may be supplied in some cases by the hydrocarbon feed itself in the form of dissolved gas if the feed has been in contact with traces of those gases upstream.

Hydrogen from the refinery and hydrogen from the reaction zone of a hydrotreatment step may thus contain varying quantities of CO and CO2. The most widely used technique when hydrogen containing CO and CO2 is used or produced is to completely eliminate those impurities, typically by pressure swing adsorption, PSA. That technique is expensive, however, and consumes some of the available hydrogen.

One of the aims of the invention is to operate hydrotreatment steps well, in particular hydrotreatment steps for selective desulphurization of olefinic cuts (typically gasoline), while using more diversified sources of hydrogen, and typically milder purification treatments.

The invention also aims to reduce the consumption of hydrogen by reducing the hydrogen purge flow rate in the hydrotreatment step (purge of a portion of the recycle gas around the hydrodesulphurization reactor).

BRIEF DESCRIPTION OF THE INVENTION

During the course of studies carried out by the Applicant, it was discovered that the presence of CO in the hydrogen, even in amounts of the order of 100 ppmv (parts per million by volume) or even 50 ppmv or even 20 ppmv caused a significant reduction in the activity of the hydrodesulphurization catalysts. Further, it was determined that the olefin hydrogenation reaction rate was little affected by the presence of CO. The presence of CO in the hydrogen thus caused a reduction in catalytic activity and a greater loss of octane number during the hydrodesulphurization step if the catalytic volume was increased to maintain the degree of desulphurization. The reduction in activity may be compensated for by an increase in temperature, but in that case, the catalyst service life is affected. Similar observations were also reported in U.S. patent application 2003/0221994. According to that patent, it is recommended to use, for the selective hydrodesulphurization step, hydrogen containing oxides of carbon in amounts such that the sum CO+½CO2 (hereinafter designated COx) must not exceed 100 ppmv in the mixture of hydrocarbons and hydrogen.

However, the Applicant has surprisingly discovered that while the CO content is a parameter directly linked to substantial inhibition of the desulphurization catalyst, the amount of CO2 may in contrast vary widely without making a significant impact. It was also discovered that a hydrogen pre-treatment step consisting of oxidizing CO to CO2 without extracting the CO2 thus formed can overcome the deleterious effect of oxides of carbon even for CO2 contents of above 200 ppmv. Saving the cost of an expensive step for almost complete elimination of CO2 is a very significant advantage as well as opening the possibility of using less pure sources of hydrogen. Thus, the present invention proposes a process for desulphurizing hydrocarbon cuts compatible with a very low CO content but with a substantial COx content. This process preferably comprises a step for selective oxidation of the CO contained in the hydrogen to CO2 and a hydrodesulphurization step, the two steps being carried out in succession, typically with no intermediate extraction of the CO2 formed. This approach has the advantage over prior art processes of using a simple, cheap solution to overcome problems regarding the inhibition of hydrodesulphurization catalysts by oxides of carbon.

The principal processes for substantially eliminating CO are processes for oxidizing CO to CO2, which are preferred in the present invention, and processes for methanation of CO (to methane).

The principal processes for oxidizing CO to CO2 are the steam conversion reaction which can transform CO to CO2 by reaction with steam carried out over a nickel based catalyst, for example, or selective oxidation of CO to CO2 using oxygen. This second option (the most preferred in the present invention) is described in more detail in the present application.

Methods for selectively oxidizing CO to CO2 using oxygen have been described in the literature. As an example, International patent application WO-A-01/0181242 may be cited, which proposes a method for purifying hydrogen based on oxidizing CO to CO2 using a material having a thermal conductivity of more than 30 W/m.K, to improve the selectivity of the reaction. U.S. Pat. No. 5,789,337 describes a method for synthesizing catalysts containing finely dispersed gold on a support having increased activity. WO-A-00/17097 recommends the use of catalysts containing ruthenium or platinum or a mixture of said two elements deposited on a support based on a alumina.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for the hydrodesulphurization of hydrocarbon fractions using a source of makeup hydrogen, and generally recycled hydrogen, so that at the inlet to the hydrodesulphurization reactor, the CO content in the overall feed is at most 50 ppmv, preferably at most 20 ppmv and preferably at most 10 ppmv or less, while the amount of COx═CO+½CO2 is more than 120 ppmv for at least part of the time (for example at least 30% or 50% of the time, or preferably 100% of the function time). Usually, the amount of COX is below 10000 ppmv, and normally below 5000 ppmv. Typically, the COx content is in the range 120 to 1000 ppmv, and more generally in the range 120 to 500 ppmv. The process of the invention does not exclude a function in which for a part of the time the COX content is below 120 ppmv, or 50 ppmv or still less. This may, for example, occur when “clean” hydrogen sources, substantially containing neither CO nor CO2, are available in sufficient quantities to supply the various consuming units (which depends on the nature of the crude oil being processed).

More precisely, the invention proposes a process for hydrodesulphurization of a hydrocarbon HC cut comprising at least 5% by weight of olefins, in which said hydrocarbon cut, a stream HYD of makeup hydrogen, and generally a stream REC of recycle hydrogen are mixed to form an overall feed which is supplied to the inlet to at least one reactor comprising a desulphurization catalyst, under operating conditions that can transform the organic sulphur-containing compounds of the HC cut into H2S, in which the source or sources of hydrogen forming the stream HYD is/are selected, and optionally, at least one hydrogen purification treatment is carried out on the HYD stream, REC stream (or a fraction of REC) or a mixture thereof, so that said overall feed comprises at most 50 ppmv of CO, and in that it comprises at least 120 ppmv of COX for at least a substantial fraction of the time.

Preferably, said purity conditions: (at most 50 ppmv of CO [or 20 or 10 ppmv] and at least 120 ppmv of COX for at least a substantial fraction of the time) are, in accordance with the invention, also obtained for the stream HYD of makeup hydrogen. The best quality makeup hydrogen in the present invention is that in which the CO content is very low (less than 10 ppmv, preferably less than 5 ppmv) and in which the ratio CO2/CO is high (for example more than 5 or 10, for example in the range 5 to 60).

Further, when using a hydrogen recycle stream REC around the hydrodesulphurization unit, a purge stream WGAS is also used, to prevent impurities from accumulating. The hydrogen loop tends to concentrate the CO and CO2. The present invention, which results in the acceptance of large quantities of CO2, can thus increase the REC/HYD recycle ratio, which may exceed 4, or may be in the range 6 to 30. This results in a reduction in the required hydrogen purge WGAS. Advantageously, the purge rate is controlled so that the CO content in the overall feed is less than 50 ppmv, and preferably less than 20 ppmv, but the COX content in the overall feed is more than 120 ppmv.

The process typically comprises at least one treatment T1 for purifying hydrogen carried out on HYD, REC or a mixture thereof, said treatment T1 carrying out limited CO2 elimination leading to a content of at least 200 ppmv of CO2 in the overall feed.

Preferably, the process comprises at least one treatment T2 for purifying hydrogen carried out on HYD, REC or a mixture thereof, said treatment T2 carrying out catalytic oxidation of CO by O2 and/or H2O to obtain at most 50 ppmv of CO (and preferably at most 20 ppmv) in the overall feed.

Oxidation may be carried out by steam CO conversion, which is known as shift conversion, and which may be carried out in one or 2 stages.

Preferably, the process comprises a hydrogen purification treatment carried out on HYD, and optionally on REC (or a portion of the REC) or a mixture thereof, said treatment comprising a treatment T2 carrying out catalytic oxidation of CO by O2 (preferential oxidation of CO over the hydrogen present), directly followed and without secondary CO2 elimination by desulphurization of the HC cut in the presence of a stream of the purified hydrogen. It is also possible to combine steam conversion, typically at low temperature, and final preferential oxidation.

In a variation, the process may comprise at least one treatment T3 for hydrogen purification carried out on HYD, REC or a mixture thereof, said treatment T3 carrying out catalytic methanation of CO by H2 to obtain at most 50 ppmv of CO in the overall feed. In this case, the process usually comprises a hydrogen purification treatment carried out on HYD and optionally on REC or a mixture thereof, said treatment comprising T3 producing methanation of CO by H2, directly followed without secondary CO2 elimination by desulphurization of the HC cut in the presence of a stream of the purified hydrogen. While methanation also tends to eliminate the CO2, the methanation conditions may be such and/or associated with the presence of CO2 dissolved in the feed, that the overall feed nevertheless contains substantial quantities of CO2 (and COX possibly over 120 ppmv).

Preferably, the process comprises a hydrogen purification treatment carried out on HYD, and optionally on REC (or a portion of the REC) or a mixture thereof, said treatment comprising a treatment T2 resulting in the catalytic oxidation of CO by O2, directly followed and with no secondary elimination of CO2 by desulphurization of the HC cut in the presence of the purified hydrogen stream.

Frequently, the process also comprises a preliminary treatment T1 for eliminating CO2 carried out on HYD upstream of T2 or T3, to eliminate the major portion of the CO2.

As an example of the production of makeup hydrogen, methane may be treated by steam reforming followed by one or two steam CO conversion steps and a CO2 elimination step T1, for example by washing with a methyldiethanolamine solution, to obtain hydrogen with a residual CO low content, for example 2000 to 5000 ppmv, and with a low CO2 content, in the range 50 to 1000 ppmv. This hydrogen may then be treated by a preferential oxygen oxidation treatment T2 (or by steam conversion followed by preferential oxidation), preferably mixing it with another source of very pure hydrogen (hydrogen from catalytic reforming, substantially free of CO and CO2), at a flow rate suitable to obtain a final makeup hydrogen with a CO content of 10 or less, or even less than 5 ppmv, and a CO2 content in the range 120 to 1000 ppmv.

Complementary technical features concerning the steam conversion, methanation and CO2 elimination by amine washing treatments (or other absorption liquids) may be found in the reference text “Procédés de transformation” [“Transformation processes”], 1998, by P LEPRINCE, Technip, publishers (Paris), pages 476-490.

Description of Preferred Preferential CO Oxidation Treatment Using Oxygen:

Many catalysts based on supported or unsupported noble metals may catalyze the oxidation of CO to CO2 in the presence of oxygen. In the presence of hydrogen, however, a catalyst has to be used which does not transform too much of the hydrogen into water. The use of a selective catalyst for carrying out the preferential oxidation of CO is thus a very important solution to hydrogen purification problems. A very high degree of selectivity is not necessary, however, in the context of the present invention; the presence of a little steam in the hydrogen that has been purified over the preferential oxidation catalyst does not completely negate the use of hydrogen in a selective olefinic gasoline hydrodesulphurization process. The quantity of H2S contained in the hydrogen must not generally exceed 10 ppmv (ppm by volume), and preferably 1 ppmv before the preferential oxidation step. The copper strip test, which is well known to the skilled person, must be negative. Thus, the hydrogen may optionally be purified of hydrogen sulphide using any method which is well known to the skilled person. Examples which can be cited are absorption, extraction or amine washing treatments or treatments for chemical conversion of the H2S; this list does not in any way limit the treatments which may be used in the present invention.

The step for preferential oxidation of CO to CO2 of the present invention may, for example, be carried out on a selective catalyst in the presence of hydrogen. The metals which may be used to carry out this reaction may be selected from the group formed by the noble metals Pt, Pd, Ru, Rh, Ir, Au or Cu, Cr, V, Mn or Ce. The metals may be used alone or in association with other metals, or they may form alloys. They may be used in the bulk metallic form (filaments, foam, sponge, etc) or supported on porous refractory oxides such as alumina, cerine, anatase or rutile, zirconia, silica, ferric oxide (α-Fe2O3) or zinc oxide. Without in any way limiting the scope of the invention, the preferential CO oxidation step of the present invention may be carried out on a catalyst based on finely divided gold on ferric hydroxide. Such a catalyst may be prepared using the method described in the publication by Haruta et al, J Catal 1993, 144, p 175 but it may also be prepared using any other protocol described in the literature.

The catalyst is, for example, prepared by co-precipitation of a solution containing HAuCl4.3H2O and Fe(NO3)3.9H2O and a solution containing sodium carbonate. These two solutions are gradually added then stirred vigorously in a precipitation reactor containing distilled water. The reaction mixture is maintained at 80° C. while the two solutions are added; throughout the operation, the pH is kept between 8 and 8.5. After filtration, the precipitate is washed with hot water until the washing water contains no more chlorine (monitored by the silver nitrate reaction) then dried at 40° C. in a vacuum oven for 12 h. The powder obtained is then calcined in dry air at 400° C. for 2 h with a flow rate of air of 0.5 l/g catalyst/h. After milling, a powder with a mean granulometry of close to 20 μm and with a surface area of 60 m2/g is obtained. The catalyst contains 3% by weight of Au.

The catalyst may be formed using any of the methods which are known to the skilled person; non limiting examples which may be cited are deposition onto a monolith using a wash-coat (coating deposited in the liquid phase), granulation, extrusion, etc.

Description of Hydrodesulphurization Step

The hydrodesulphurization step is carried out on a catalyst which comprises at least one element from group VIII and preferably an element from group VIII and an element from group VIB. The element from group VIII is selected from the group constituted by nickel, cobalt and iron. The element from group VIB, if present, is preferably molybdenum or tungsten. The metals are deposited onto an amorphous solid support selected from the group constituted by silica, silicon carbide and alumina, formed into beads or extrudates. To selectively hydrodesulphurize carbonaceous fractions containing olefins, it is preferable to use catalysts containing cobalt and molybdenum on a support based on alumina.

The hydrodesulphurization step may advantageously be carried out in two steps, a first hydrodesulphurization step which can transform more than 50% of the sulphur present in the feed to H2S, and a finishing step constituted, either by a step for hydrogenolysis of the saturated sulphur-containing compounds over a catalyst containing a metal from group VIII or by a step for hydrodesulphurization over a catalyst having an activity which is lower than the catalyst for the first step. This type of concatenation may improve the selectivity of the hydrodesulphurization step.

The catalyst or catalysts employed during this step are in the sulphurized form. The sulphurization procedure may be carried out in situ or ex situ. In the first case, the catalyst is sulphurized before loading into the reactor, while in the second case, the catalyst is loaded into the reactor in the form of metallic oxides, sulphurization is carried out in the reactor by injecting H2S or compounds which may decompose to H2S such as DMDS and hydrogen. Any conventional sulphurization method used by the skilled person which can sulphurize at least 50% and preferably 70% of the metallic oxides deposited on the support may be employed.

The reactor pressure is generally in the range 0.5 MPa to 5 MPa, the hydrogen flow rate is such that the ratio of the hydrogen flow rates in normal litres per hour to the flow rate of hydrocarbons in litres per hour is in the range 50 to 800, preferably in the range 60 to 600. The temperature is in the range 200° C. to 400° C., preferably in the range 230° C. to 350° C. depending on the amount of sulphur in the hydrocarbon fraction to be desulphurized.

EXAMPLES

Example 1

Comparative

A pilot unit constituted by a reactor with a capacity of 200 ml was charged with 100 ml of commercially available HR806S catalyst sold by AXENS. This catalyst is based on cobalt and molybdenum deposited on alumina and was supplied in the pre-sulphurized form and thus did not require a subsequent sulphurization step before contact with the feed. The treated feed was gasoline A from a catalytic cracking unit. This gasoline was depentanized to treat only the C6+ fraction by hydrodesulphurization. This feed contained 425 ppm of sulphur with 6 ppm of sulphur in the form of mercaptans and with a bromine index, measured using the ASTM D1159-98 method, of 49 g/100 g. The cut points for this gasoline A were determined by simulated distillation: gasoline A had 5% by weight and 95% by weight cut points of 61° C. and 229° C. respectively.

Gasoline A was mixed with pure hydrogen and injected into the reactor. The pressure was kept at 2.1 MPa, the feed flow rate was 400 ml/h, representing an hourly space velocity (HSV) of 4 h−1, the hydrogen flow rate was 120 litres per hour, representing a flow rate of 300 (normal) litres of hydrogen per litre of feed. Three different temperatures were tested.

The apparent selectivity of the catalyst was calculated for each point as being the ratio of the apparent first order rate constants between the rate of desulphurization and the olefin hydrogenation rate.

The sulphur contents and the olefins contents, measured by the bromine index, and the selectivities are shown in Table 1.

TABLE 1
Temperature° C.260280300
Sulphur, testPpm64189
IBr, testg/100 g37.931.422.4
Selectivity7.47.14.9

Example 2

Comparative

In order to measure the influence of CO and CO2 on catalyst performance, a cylinder of hydrogen containing 100 ppmv of CO and 350 ppmv of CO2 was used. This hydrogen was mixed with gasoline at flow rates identical to those of Example 1. The mixture thus formed had a CO content of 65 ppmv and a CO2 content of 228 ppmv. The operating conditions were identical to Example 1. Table 2 shows the results of the test.

TABLE 2
Temperature° C.260280300
Sulphur, testppm1032513
IBr, testg/100 g38.432.323.6
Selectivity5.86.84.8

The presence of CO and CO2 in respective amounts of 65 ppmv and 228 ppmv in the mixture of hydrogen and gasoline A degrades the hydrodesulphurization activity of the catalyst. In contrast, the hydrogenating activity is virtually unaffected, causing a fall in selectivity.

Example 3

In Accordance with the Invention

Example 3 was in accordance with the invention, i.e. the hydrogen containing CO and CO2 used in Example 2 was pre-treated to oxidize the CO to CO2. Oxidation was carried out by mixing hydrogen and oxygen and treating the mixture over an oxidation catalyst. The hydrogen was mixed with a stream of pure oxygen, the flow rate of which was adjusted so that the molar ratio between the oxygen and the CO was 1.1. The reactor was operated at a temperature close to ambient temperature (50° C.), at a pressure of 2.1 MPa.

The catalyst was prepared, for example, by co-precipitation of a solution containing HAuCL4.3H2O and Fe(NO3)3.9H2O and a solution containing sodium carbonate. These two solutions were gradually added to a precipitation reactor containing distilled water then vigorously stirred. The reaction mixture was maintained at 80° C. throughout addition of the two solutions, and during the entire operation the pH was maintained between 8 and 8.5. After filtration, the precipitate was washed with hot water until the washing water contained no more chloride (monitored by reaction with silver nitrate) then dried at 40° C. in a vacuum oven for 12 h. The powder obtained was calcined in dry air at 400° C. for 2 h with an air flow rate of 0.5 l/g of catalyst/hydrogen. After milling, a powder with a mean granulometry of close to 20 μm and with a specific surface area of about 60 m2/g was obtained. The catalyst contained a quantity of 3% by weight of Au. Using the Au[111] peak, X ray diffraction analysis allowed a gold particle side of 60 Å to be determined. The catalyst (100 mg, diluted in a ratio of 1:20 with α-Al2O3) was then disposed in a stainless steel reactor with an internal diameter of 10 mm and inserted into a jacketed tubular oven heated to 50° C. The hydrogen flow rate was adjusted to 5×104 Nml/h/g of catalyst, so that the hourly space velocity was 5×105 h−1. The density of the catalyst was 1 g/cm3. An analysis of the gas entering the reactor and that of the effluents (CO, —CO2, H2O, O2) was carried out by gas chromatography having two catharometric detectors.

After passing hydrogen over the preferential oxidation catalyst under the conditions described, the stabilized CO and CO2 content in the treated hydrogen was 17 ppmv and 430 ppmv respectively.

A cylinder of pressurized hydrogen gas containing about 17 ppmv of Co and 430 ppmv of CO2 was produced following these results. The gas it contained was then mixed with gasoline A and sent to the reactor used in Examples 1 and 2 under the same operating conditions. The mixture thus constituted had a CO content of 11 ppmv and a CO2 content of 283 ppmv. Table 3 shows the results of the tests.

TABLE 3
Temperature° C.260280300
Sulphur, testppm712010
IBr, testg/100 g38.131.822.9
Selectivity7.17.14.9

Carrying out an oxidation step to pre-treat the hydrogen significantly improved the hydrodesulphurization activity of the catalyst and produced activities and selectivities close to those of the tests carried out with hydrogen free of CO and CO2 described in Example 1.

As a result, the use of an oxidizing pre-treatment step for CO can, by means of a simple device, considerably limit the deleterious effect of the presence of CO in hydrogen on the performance of hydrodesulphurization catalysts, without the need to eliminate CO2 to any great extent.