DE69007104T2 - MULTI-STAGE HYDRODESULFURING PROCESS. - Google Patents

Description

The invention relates to a process for hydrodesulfurization of a hydrocarbon feedstock.

Crude oils, their direct distillate and cracked fractions and other petroleum products contain sulphur in varying amounts depending on the origin of the crude oil and the subsequent treatment to which it is subjected. In addition to elemental sulfur, numerous sulfur compounds have been identified in crude oil, including hydrogen sulfide (H2S), primary C1 to C5 alkyl mercaptans, secondary C3 to C8 alkyl mercaptans, tertiary C4 to C6 alkyl mercaptans, cyclic mercaptans (such as cyclopentanethiol, cyclohexanethiol and cis-2-methylcyclopentanethiol), open-chain sulfides of the formula R-S-R', where R and R' are C1 to C4 alkyl groups, mono-, bi- and tricyclic sulfides, thiophene, alkyl-substituted thiophenes, condensed thiophenes (such as benzo(b)thiophene, isothionapthene, dibenzothiophene and Benzo(b)naphtho(2,3-d)thiophene), thienothiophenes, alkylcycloalkyl sulfides, alkylaryl sulfides, 1-thiaindanes, aromatic thiols (such as thiophenol) and cyclic thiols, such as cyclohexanethiol.

In general, low API gravity crude oils usually contain more sulfur than high API gravity crude oils, although there are some exceptions. In addition, the distribution of sulfur compounds in the various petroleum fractions varies mainly with the boiling range of the fractions. Thus, the lighter fractions, such as naphtha, contain fewer sulfur compounds, while the content of sulfur compounds also increases as the boiling point, density or molecular weight of the fraction increases. Most of the sulfur compounds actually identified as crude oil constituents boil below about 200ºC. Many other high molecular weight and high boiling point sulfur compounds remain unidentified in crude oil.

For various reasons, it is necessary to subject crude oil and the petroleum fractions derived from it to a treatment in order to remove the sulphur components present therein. Otherwise, subsequent processing could be hindered, eg because the sulphur compounds could adversely affect the performance of a catalyst. If the hydrocarbon fraction is to be used as fuel, combustion of the fuel will result in all sulphur compounds present in it being converted to sulphur oxides, which are harmful to the environment.

For these reasons, it is necessary to remove as much of the sulfur content as possible from the hydrocarbon fractions derived from crude oil, such as gasoline fractions, diesel oil, gas oils and the like. Typically, this sulfur removal is carried out by a process commonly known as hydrodesulfurization. In this process, the hydrocarbon fraction is mixed with hydrogen and passed over a hydrodesulfurization catalyst under suitable temperature and pressure conditions. The aim of this process is to break the carbon-sulfur bonds present in the feedstock and to saturate the resulting free valences or the olefinic double bonds formed during such cleavage with hydrogen. The aim of this process is to convert as much of the organic sulfur as possible to hydrocarbons and H₂S. The typical equations for the main types of sulfur compounds to be hydrodesulfurized are given below:

1. Thiols:

RSR + H₂T RH + H₂S

2. Disulfides:

RSSR' + 3H&sub2; T RH + R'H + 2H₂S

3. Sulfides a. Open chain

RSR' + 2H2 T RH + R'H + H₂S b. Cyclic c. Bicyclic 4. Thiophenes 5. Benzothiophenes: 6. Dibenzothiophenes:

In general, cyclic sulfur-containing compounds are more difficult to hydrogenate than open-chain compounds, and within the class of cyclic sulfur-containing compounds, the greater the number of rings present, the greater the difficulty of breaking the carbon-sulfur bonds.

In addition to the sulphur oxides present in the combustion gases of hydrocarbon fuels, other environmentally undesirable components of these combustion gases are typically aromatic hydrocarbons, which may be present as a result of incomplete combustion, and carbon-containing particulate matter, which frequently includes polycyclic aromatic hydrocarbons, metal compounds, oxidised organic materials.

Because of the current concern about impurities, various national legislations around the world have set increasingly strict limits on the permissible levels of impurities in hydrocarbon fuels such as diesel oil. In particular, the United States Environmental Protection Agency has recently proposed regulations that would limit the sulfur content to 0.05% by weight and the aromatics content to 20% by volume in road diesel fuels (see, for example, the article "Higher diesel quality would restrict refining" by George H. Unzelman, Oil and Gas Journal, July 19, 1987, pages 55 to 59). These regulations require the refiner to comply with additional diesel treatment requirements. and rising investment and operating costs. Further reductions in the permissible values of sulphur and aromatics content cannot be ruled out in the future.

When a hydrocarbon feedstock is treated with hydrogen in the presence of a suitable catalyst for the purpose of hydrodesulfurization, other reactions may also occur. Therefore, hydrotreating is often used as a more general term to encompass not only the hydrodesulfurization reactions but also other reactions that occur, such as hydrocracking, hydrogenation and other hydrogenolysis reactions. The term "hydrotreating" is further explained in the article "Here is a Nomenclature System Proposed for Hydroprocessing," The Oil and Gas Journal, October 7, 1968, pages 174 to 175.

There are four main hydrogenolysis reactions, of which hydrodesulfurization (HDS) is probably the most important, followed by hydrodenitration (HDN), hydrodeoxidation (HDO) and hydrodemetallation (HDM). Among the catalysts proposed for these hydrotreating reactions are molybdenum disulfide, tungsten sulfide, sulfided nickel molybdate catalysts (NiMoSx) and cobalt-molybdenum-alumina-sulfide (co-Mo/alumina).

Although in the prior art the simultaneous occurrence of several hydrogenation reactions, such as the hydrogenation of olefins and aromatic hydrocarbons, in a hydrodesulfurization process is considered disadvantageous because the aromatics content of the product was within the required specification and the use of valuable hydrogen for unnecessary hydrogenation reactions was considered disadvantageous, there is a growing shortage of light crude oil. Therefore, the current and future trend towards the use of middle distillates and heavier petroleum fractions in conjunction with increasingly stringent specifications mean that aromatics hydrogenation will become an increasingly necessary part of refinery operations. Therefore, under present conditions and to an increasing extent in the future, it is desirable to combine hydrodesulfurization and aromatics hydrogenation.

In contrast, with the exception of high molecular weight residue processing, the extensive hydrocracking reactions in most refinery hydrotreating processes should be avoided as much as possible because they are highly exothermic and can cause thermal damage to catalysts and reaction vessels, as well as lead to the deposition of carbonaceous materials that cause loss of catalyst activity. For example, in the article "Refiners Seek Improved Hydrogen Production," Oil and Gas Journal, July 20, 1987, pages 48 and 49, a hydrodesulfurization plant operator reported that reactors in operation had become severely overheated, one of them to the point of rupture, as a result of the undesirable hydrocracking reactions that occurred.

The risk of these hydrocracking reactions occurring can be minimized by ensuring that the catalyst remains sufficiently sulfided.

A number of articles have appeared in the literature related to hydrodesulfurization technology, including:

(a) "Kinetics of Thiophene Hydrogenolysis over a Cobalt Molybdate Catalyst", by Charles N. Satterfield et al. AIChE Journal, Vol. 14, No. 1 (January 1968), pp. 159-164;

(b) "Hydrogenation of aromatic hydrocarbons catalyzed by sulfided CoO-MoO3/gamma-Al2O3. Reactivity and reaction cross-links" by Ajit V. Sapre et al, Ind. Eng. Chem Process Des. Dev., Vol. 20, No. 1, 1981, pp. 68 to 73;

(c) "Hydrogenation of diphenyl catalyzed by sulfided CoO-MoO₃/gamma-Al₂O₃. The reaction kinetics", by Ajit V. Sapre et al, In. Eng. Chem. Process Des. Dev, Vol. 21, No. 1, 1982, pages 86 to 94;

(d) "Hydrogenolysis and hydrogenation of dibenzothiophene catalyzed by sulfided CoO-MoO3/gamma-Al2O3: the reaction kinetics" by D.H. Broderick et al. AIChE Journal, Vol. 27, No. 4, July 1981, pp. 663-672; and

(e) "Hydrogenation of aromatic compounds catalyzed by sulfided CoO-MoO3/gamma-Al2O3" by D.H. Broderick et al, Journal of Catalysis, Vol. 73, 1982, pages 45 to 49.

A review of the reactivity of hydrogen in sulfide catalysts, such as those used as hydrotreating catalysts, appears on pages 584 to 607 of the book "Hydrogen Effects in Catalysis" by Richard B. Moyes, published by Marcel Dekker, Inc. (1988).

A review of industrially practiced hydrotreatment processes is published every year in the journal "Hydrocarbon Processing", usually in the September issue. For example, reference can be made to "Hydrocarbon Processing", September 1984, page 70 ff and to "Hydrocarbon Processing", September 1988, pages 61 to 91.

A review of three known hydrotreating processes appears in Hydrocarbon Processing 1988 Refining Handbook, pages 78 and 79 of Hydrocarbon Processing, September 1988. In the Chevron RDS/VRDS hydrotreating process, a mixture of fresh liquid hydrocarbon feed, make-up hydrogen and recycled hydrogen is fed to a reactor in a single pass operation. As shown, the reactor has three beds and interbed cooling is provided by introducing additional amounts of recycled hydrogen. The recycled hydrogen is passed through a H2S scrubber. The HYVAHL process also uses a single pass operation for the liquid feed. Again, an amine scrubber is used. to remove H₂S from the recycle hydrogen. The Unionfining process also uses a single pass for the liquid feed. A co-current flow of hydrogen and liquid is provided. Unreacted hydrogen is recycled.

in all three processes, gas recirculation serves to cool the catalyst bed and to minimize the risk of thermal runaway that occurs as a result of significant hydrocracking. Gas recirculation means that inert gases tend to accumulate in the circulating gas, which in turn means that to maintain the desired hydrogen partial pressure, the total operating pressure to accommodate the circulating inert gases must increase and the size and cost of the gas recirculation compressor must increase and increased operating costs must be tolerated.

The use of the trickle technique is described in the article "New Shell Hydrodesulfurization Process Shows These Features", Petroleum Refiner, Vol. 32, No. 5 (May 1953), page 137 ff. Figure 1 of this article shows a reactor with four catalyst beds with introduction of a mixture of hot gas and gas oil at the inlet end of the first bed and the use of cold gas oil injections between successive beds.

In these hydrodesulfurization processes, the conditions at the inlet end of the catalyst bed are critically important, because this is where the risk of hydrocracking is greatest, especially if catalyst sulfurization should drop. This can occur, for example, when a low sulfur feedstock is fed to the plant or a feedstock is used in which sulfur-containing impurities are predominantly polycyclic compounds.

Hydrorefining of a naphtha feedstock is described in US-A-4243519. It appears to be an essentially entirely vapor phase process.

Multi-stage hydrodesulfurization of residue with catalyst movement between stages in the opposite direction to the movement of gas and liquid is described in US-A-3809544.

US-A-3847799 describes the conversion of black oil into fuel oil with a low sulfur content in two reactors. The make-up hydrogen is fed to the second reactor, but in a mixture with hydrogen exiting the first reactor which has been purified therefrom by hydrogen sulfide removal. As a result, hydrogen is recovered from the first reactor and returned to the second reactor in a mixture with inert gases which consequently tend to enrich in the gas recycle loop. Any condensate obtained from the first reactor is mixed with product from the second reactor.

US-A-4051020 describes the hydrodesulfurization of a residual fuel oil in a reactor vessel divided into a number of stages by horizontal perforated support plates. Liquid flows downwards through the reaction vessel from stage to stage through drain nozzles, while gas flows upwards through the holes in the horizontal plates and through the liquid at each stage. Catalyst particles can move upwards from stage to stage by means of eductors. The flat perforated plates cannot prevent hot spot formation.

In a hydrodesulfurization plant with gas recycle operation, some of the H₂S produced, usually a minor proportion, remains in the liquid phase after product separation, while the rest of the H₂S, usually a major proportion, remains in the gas phase. Even in plants where interbed cooling is practiced with "cold shots" of recycle gas, the released H₂S remains in the gas/liquid mixture as it passes through the catalyst bed. Therefore, the H₂S partial pressure at the outlet end of the The catalyst activity is usually highest at the end of the catalyst bed or, if more than one bed is used, the last bed. Since the catalyst activity for hydrodesulfurization decreases with increasing H₂S partial pressure, the catalyst activity is lowest at the outlet end of the bed, which is actually where the highest activity is needed to hydrodesulfurize the least accessible polycyclic organic sulfur compounds.

The catalysts used for hydrodesulfurization are usually also capable of effecting the hydrogenation of aromatic compounds, provided that the sulfur content is low. The conditions required to effect the hydrogenation of aromatic compounds are generally similar to those required for hydrodesulfurization. However, since the reaction tends toward an equilibrium which is not favored by the use of high temperatures, the conditions required for the hydrodesulfurization of cyclic and polycyclic organic sulfur compounds in a conventional plant do not favor the hydrogenation of aromatic compounds. Furthermore, since the design of conventional hydrodesulfurization plants results in high H2S partial pressures at the downstream end of the plant, the catalyst activity is correspondingly reduced and the conditions do not result in a significant reduction in the aromatic content of the feedstock being treated. Accordingly, in an article entitled "Committee Provides Hydrotreatment Guidelines" Hydrocarbon Processing, March 1989, pages 113 to 116, on page 115 it is stated:

"It is a fundamental kinetic fact that at pressures for normal middle distillate desulfurization plants (35.5 to 56.2 bar) it is difficult to obtain any significant aromatic saturation. Therefore, if the aromatics content of the feedstock is much above 20%, they can be used with typical hydrotreatment plants and all the catalysts known to us cannot do much to significantly reduce the aromatics.

They are then forced to rely on the unpleasant alternatives of higher pressure systems, aromatics extraction and all the other alternatives."

The removal of H2S from a hydrodesulfurization plant with a recycle gas system is normally accomplished by scrubbing the recycle gas with an amine. Since the scrubbing section must be large enough to handle the highest levels of sulfur-containing contaminants likely to be present in the feedstocks being treated, the scrubbing plant must be designed with adequate capacity, even though the plant will most often operate with low sulfur feedstocks. The capital cost of such a scrubbing facility is significant.

It would be desirable to provide a more efficient process for hydrodesulfurizing liquid hydrocarbon feedstocks, particularly a process which substantially eliminates the risk of a hydrocracking reaction occurring. It would also be desirable to provide a hydrodesulfurization process in which the activity of the catalyst throughout the reactor is controlled so that improved hydrodesulfurization levels can be achieved at a given operating pressure as compared to a conventional process. It would also be desirable to provide a hydrodesulfurization process which allows operation to achieve a simultaneous significant reduction in the aromatics content of the feedstock being treated, particularly those feedstocks in which the aromatics content exceeds about 20%.

The invention accordingly aims to provide a process in which hydrodesulfurization can be carried out more effectively than in a conventional hydrodesulfurization process. It also aims to provide a hydrodesulfurization process in which the catalyst activity in the entire reactor is controlled in a favorable manner so that improved hydrodesulfurization values of the feedstock can be achieved. It also aims to create a hydrodesulfurization process which, at the same time as hydrodesulfurization, also allows a significant reduction in the aromatics content of the feedstock.

The present invention accordingly provides a hydrodesulfurization process for the continuous hydrodesulfurization of a liquid, sulfur-containing hydrocarbon feedstock, which comprises

(a) providing a hydrodesulfurization zone maintained under hydrodesulfurization conditions and having a column reactor having a plurality of reaction trays disposed one above the other therein, each reaction tray defining a reaction stage capable of holding a given volume of liquid and a charge of finely divided sulfided solid hydrodesulfurization catalyst suspended therein, and each reaction tray having a tray at least partially inclined at an angle equal to or greater than the angle of repose of the catalyst particles beneath the liquid, the column reactor further having liquid drain ports in communication with each reaction tray through which the liquid from the reaction tray can pass downwardly of the column reactor while the solid catalyst is maintained thereon, and gas rise ports in communication with each reaction tray allowing gas to enter the tray from below and Mixture of liquid and catalyst is stirred on the reaction tray.

(b) supplying liquid sulphur-containing hydrocarbon feedstock to the uppermost of said plurality of reaction trays,

(c) Hydrogen-containing gas below the lowest of the mentioned plurality of reaction trays,

(d) allowing the liquid to flow downwards from reaction tray to reaction tray through the column reactor,

(e) allowing the hydrogen-containing gas to rise from reaction tray to reaction tray upwards through the column reactor,

(f) recovering from the uppermost of said plurality of reaction trays an exhaust gas containing H₂S formed by hydrodesulfurization, and

(g) recovering a liquid hydrocarbon product having a reduced sulphur content from the lowest of said plurality of reaction trays.

Normally, the gas entering the top tray contains sufficient H₂S and/or the liquid feed contains sufficient sulfur-containing material selected from H₂S and active sulfur-containing materials to maintain the catalyst feed on the tray in sulfided form.

The term "active sulfur-containing material" is to be understood as meaning material which, under hydrodesulfurization conditions in the presence of a hydrodesulfurization catalyst, very rapidly forms H₂S. Examples of such materials are, for example, CS₂, COS, alkyl mercaptans, dialkyl sulfides and dialkyl disulfides.

The solid sulfided catalysts in the process of the present invention are preferably selected from molybdenum disulfide, tungsten sulfide, cobalt sulfide, nickel/tungsten sulfide, cobalt/tungsten sulfide, sulfided nickel molybdate catalysts (NoMoSx), a sulfided CoO-MoO₃/gamma-Al₂O₃ catalyst and mixtures thereof.

Typical hydrodesulfurization conditions include the application of a pressure in the range of 20 bar to 150 bar and a temperature in the range of 240 ºC to 400 ºC. Preferred conditions are the application of a pressure of 25 bar to 100 bar and a temperature of 250ºC to 370ºC.

The liquid sulfur-containing hydrocarbon feedstock may be a mixture of saturated hydrocarbons, such as

n-paraffins, iso-paraffins and naphthenes in variable proportions. It may also contain one or more aromatic hydrocarbons in amounts of, for example, about 1% by volume to about 30% by volume or more. If the feedstock has a low aromatic hydrocarbon content, hydrodesulfurization will be the predominant reaction occurring. However, if the feedstock has a significant aromatic hydrocarbon content, at least some hydrogenation of these aromatics to partially or fully saturated hydrocarbons may occur simultaneously with the hydrodesulfurization. In this case, hydrogen consumption increases accordingly. The extent of this hydrogenation of aromatic hydrocarbons is influenced by the choice of reaction conditions, and consequently the degree of dearomatization of the feedstock achieved can be influenced by the choice of reaction conditions.

In the process of the invention, the stoichiometric hydrogen demand can therefore be a function not only of the sulfur content of the feedstock, but also of the aromatics content of the feedstock. The actual hydrogen consumption is a function of the severity of the reaction conditions chosen, i.e. the operating temperature and pressure chosen. For example, very severe conditions are understood to mean the use of a high operating pressure, a high operating temperature, or a combination of both. In general, the higher the temperature to which the hydrocarbon feedstock is exposed during hydrodesulfurization at a given hydrogen partial pressure, the closer the degree of aromatics hydrogenation (or dearomatization) is to that corresponding to the theoretical equilibrium concentration achievable. Therefore, the amount of hydrogen consumed by the process of the invention depends not only on the type of feedstock, but also on the severity of the reaction conditions used.

When the feedstock is a diesel fuel feedstock, the reaction conditions used in the process of the invention are typically selected to reduce the residual sulfur content to about 0.5 wt.% S or less, e.g. 0.3 wt.% S or less, even down to about 0.05 wt.% S or less, and the aromatics content to about 27 vol.% or less, e.g. about 20 vol.% or less. When the desired product is a "technical grade" white oil, the production conditions are selected with the goal of reducing the sulfur content to very low levels and the aromatics content as much as possible. Typically, the goal will be to reduce the aromatics content sufficiently to provide a white oil which is a colorless, substantially non-aromatic mixture of paraffinic and naphthenic oils meeting the following specification:

Saybolt Color +20

UV absorbance limits

Maximum absorbance

per cm

280-289 mu 4.0

290-299 mu 3.3

300-329 mu 2.3

π330-350 μm 0.8

If the desired end product is a medicinal grade white oil that complies with current U.S. Department of Food and Drug Administration regulations, the goal is to produce a product with a maximum UV absorbance per cm at 260-350 nm of 0.1 as measured on a dimethyl sulfoxide extract using the procedure listed in the U.S. Pharmacopoeia. Other specifications require that a sample should show no more than a faint color in a hot acid test using sulfuric acid and no reaction in the sodium plumbite test. In order to effectively meet these stringent requirements, the aromatic hydrocarbons present in the feedstock must be hydrogenated.

In the process according to the invention, an amount of hydrogen is used which is at least equivalent to the stoichiometric amount of hydrogen required to desulfurize the feedstock and achieve the desired degree of dearomatization. Normally, at least about 1.05 times this stoichiometric amount of hydrogen is preferably used. In addition, the hydrogen dissolved in the treated feedstock obtained must be taken into account.

In the process of the invention, the feed rate of the hydrogen-containing gas typically corresponds to a H2:feed molar ratio of from about 2:1 to about 20:1. Preferably, this ratio is from about 3:1 to about 7:1.

The hydrogen-containing gas can be obtained in a known manner, e.g. by steam reforming or partial oxidation of a hydrocarbon feedstock such as natural gas, followed by conventional steps such as water gas shift reaction, CO2 removal and pressure swing adsorption.

Different hydrodesulfurization conditions may prevail on the different reaction trays. For example, the temperature on the top reaction tray, which forms a first hydrodesulfurization zone, may be lower than on the next lower tray, which in turn may be lower than the temperature on the next lower tray, etc.

It is also envisaged that the temperature can increase from the top tray to a middle lower tray, but the temperature is then reduced from tray to tray on the subsequent lower trays. It is thus possible to operate the process in such a way that as the liquid passes downwards through the column reactor, the temperature from the top tray to the middle tray increases from tray to tray, but then decreases from tray to tray. In this system, the feedstock encounters increasingly hotter conditions and increasingly lower H₂S partial pressures under essentially the same pressure as it passes downward through successive reaction trays. Since the H₂S partial pressure is lower on the second tray and on wet trays than on the top tray, the catalyst on the deeper trays is actually less sulfided and therefore more active than on the top tray. In this way, the effectiveness of the hydrodesulfurization is enhanced because the conditions on the deeper trays become more favorable for the conversion of the remaining sulfur-containing compounds, which tend to be the least reactive compounds, such as polycyclic sulfur-containing compounds. Furthermore, by reducing the temperature on the lower trays and also increasing the catalyst activity due to the lower H₂S partial pressure on these trays, conditions become more favorable for the hydrogenation of aromatic components of the feed, a reaction which, although favored by an increase in the hydrogen partial pressure, is limited by equilibrium at high temperatures.

Since the hydrogen-containing gas flowing to the top tray comes from the next lower tray, it normally contains a proportion of H₂S. Since the make-up gas is introduced below the lowest tray, the H₂S concentration in the gas is highest in the gas leaving the top tray. The content of organic sulfur-containing compounds is lowest in the liquid on the lowest tray, but these compounds are the least reactive. While sufficient H₂S partial pressure should be maintained on the lowest tray to keep the catalyst on the tray in a sufficiently sulfided form and prevent the danger of hydrocracking on the tray, catalyst activity tends to be highest on this tray, so that conditions on this soil are favourable not only for hydrodesulfurization but also for the hydrogenation of aromatic compounds. Therefore, under suitable operating conditions, a significant reduction in the aromatic content of the feedstock can be effected while at the same time achieving effective removal of the less easily removed sulfur-containing materials such as cyclic and polycyclic organic sulfur compounds.

It is also envisaged that the process according to the invention can use different catalysts on different trays. In this case, on the top tray or on some of the top trays, a catalyst can be used which favours hydrodesulfurization more than the hydrogenation of aromatic compounds, while on the lower trays a catalyst is used which has a greater activity for the hydrogenation of aromatic compounds.

Means may be provided for withdrawing a mixture of catalyst and liquid from one or more trays. If suitable valves and pumps are provided, the same withdrawal means may also be used to feed each tray with fresh catalyst, either to vary the amount of catalyst according to changes in the feedstock or operating conditions, or to replenish the catalyst charge.

The sulfur levels of the gas and liquid feeds to the top tray are preferably monitored to ensure that sufficient H₂S is present to maintain the catalyst in sulfided form. Most often, the feed contains sufficient active sulfur-containing material or the hydrogen-containing gas feed contains sufficient H₂S, or both, to maintain the catalyst in sufficiently sulfided form. However, if for any reason there should be a dangerously low level of H₂S or active sulfur-containing material on the top tray, sufficient additional amount of H₂S or an active sulfur compound such as CS₂, COS, alkyl mercaptan, dialkyl sulfide or dialkyl disulfide is added to the feedstock fed to the topsoil to restore a safe sulfur content to that soil.

Normally it is sufficient to provide a sulfur concentration in the form of H₂S or an active sulfur material on the topsoil of about 1 ppm, preferably at least about 5 ppm to about 1000 ppm. Typically the sulfur concentration may range from about 10 ppm upwards, e.g. from about 40 ppm to about 100 ppm.

Preferably, the sulfur concentration is monitored on at least one lower tray, e.g. on the lowermost tray, possibly on each lower tray below the uppermost tray, and if necessary, sufficient H2S or additional active sulfur-containing material, such as CS2, COS or an alkyl mercaptan, a dialkyl sulfide or a dialkyl disulfide, is fed into the feed to that tray to maintain the sulfur concentration in the range of about 1 ppm to about 1000 ppm, e.g. at about 5 ppm to about 1000 ppm.

The liquid hydrocarbon feedstock can be selected from, for example, petroleum oils, kerosenes, middle distillates, vacuum gas oils, lubricating bright stock oils, diesel fuels, atmospheric gas oils, light cyclo oils, light fuel oils and the like.

In the reactor used in the process of the invention, each reaction tray comprises a tray at least a portion of which is inclined at an angle equal to or greater than the angle of repose of the catalyst particles under the liquid. Thus, each reaction tray may have a frustoconical tray whose inclination is equal to or greater than the angle of repose of the catalyst particles under the liquid present on the trays. Alternatively, each reaction tray may have a tray and a surrounding wall, the tray having an inner Tray portion inclined downwards and inwards from a central portion and an outer tray portion inclined downwards and inwards from the surrounding wall, and wherein the inclinations of the inner and outer tray portions are equal to or greater than the angle of repose of the catalyst particles beneath the liquid on the tray.

The gas riser means associated with each reaction tray may comprise one or more bells of conventional form. In a particularly preferred arrangement, each such bell is associated with a relatively slender riser tube, the height of which is sufficient to prevent the liquid from draining off the tray if, for any reason, the upward flow of gas is temporarily interrupted. This relatively slender riser tube is covered by a corresponding inverted, relatively slender bell. This riser tube may also have a non-return valve fitted therein.

If the desired degree of desulfurization or dearomatization is not achieved in a single pass through the column reactor, the treated material can be further treated in a subsequent column reactor or a hydrodesulfurization reactor of a different type, if necessary under more severe conditions.

For a clear understanding and easy practical implementation of the invention, a preferred method of the invention will now be described by way of example only with reference to the accompanying schematic drawing.

Figure 1 is a flow diagram of a hydrodesulfurization plant designed for operation according to the process according to the invention,

Figure 2 shows a vertical section through a bottom of the column reactor of the hydrodesulfurization plant of Figure 1,

Figure 3 shows a similar vertical section of a bottom of a modified construction of the column reactor,

Figure 4 is a flow diagram of an experimental apparatus, Figure 5 is a diagram of a reaction tray of the apparatus of Figure 4, and

Figure 6 is a diagram showing the relationship between the aromatic content of the product and the operating temperature.

It will be appreciated by those skilled in the art that Figure 1 is a schematic representation and that other equipment such as heaters, coolers, temperature sensors, temperature controllers, pressure sensors, pressure relief valves, control valves, level controllers and the like would be additionally required in a commercial plant. These minor pieces of equipment are not part of the invention and would be consistent with conventional chemical engineering practice.

Referring to Figure 1, a hydrodesulfurization line comprises a column reactor vessel 1 provided with a plurality of reaction trays 2, each containing a charge of a particulate sulfided hydrodesulfurization catalyst and a certain volume of liquid. A hydrogen-containing gas is fed to the reactor vessel 1 through line 3. Distributors 4 are mounted in each tray 2 and allow the upward flow of gas through the liquid on each tray 2. A sulfur-containing hydrocarbon feedstock to be treated is fed to the plant through line 5 and mixed with liquid returned through line 6 to form a feed mixture in line 7 which is fed to the top tray 2 of the reactor vessel 1. Downcomers 8 allow the liquid to flow from one tray 2 to the next lower tray through the column reactor vessel 1 and finally collect in the sump 9 of the column reactor 1. Further details of one of the trays 2 with two different arrangements of the distributors 4 and downcomers 8 are described below with reference to Figures 2 and 3.

It is thus evident that in the column reactor vessel 1 gas and liquid flow in countercurrent.

Each tray 2 is provided with a respective heat exchange coil 10. A temperature controller 11 is provided for each tray 2 and controls a respective control valve 12 which controls the flow of a heat exchange medium supplied through line 13 to an inlet manifold 14 to the respective heat exchange coil 10. (For the sake of simplicity, only one temperature controller 11 is shown in Figure 1 for the top tray 2; in practice, each other tray 2 has its own temperature controller 11). Reference numeral 15 designates the return manifold and reference numeral 16 the return line for the heat exchange medium. This arrangement allows individual temperature control on each tray 2.

The off-gas from the top of the column reactor vessel 1 is recovered through line 17. This contains vapors of the liquid feedstock, gaseous products of the hydrodesulfurization reaction including H2S, and intergases contained in the feed gas in line 3. The vaporous materials are condensed as they pass through condenser 18 and a two-phase mixture of gas and condensate passes through line 19 to a gas/liquid separator 20 provided with a droplet separator 21.

The condensate is withdrawn from the separator 20 by means of pump 24 via lines 22 and 23 and returned to the separator 20 via lines 25 and 25 and through the flow restrictor 27, whereby the restrictor in lines 25 and 26 creates a pressure that is greater than that in the separator 20. Part of the liquid flowing through line 25 is returned to the column reactor 1 via lines 28, 29 and 6, controlled by valve 30, which in turn is controlled by the level controller 31 connected to the separator 20. A certain liquid flow is separated from line 28 and fed to a selected lower tray 2 via line 32, valve 33 and flow sensor 34. In Figure 1, it is the lowest tray 2 to which line 32 is connected. but could be a higher tray than the lowermost tray 2. The flow through the valve 33 is controlled by a flow controller 35 connected to the flow sensor 34. This liquid supply through the line 32 to a lower part of the column reactor 1 provides a path through which reactive sulfur compounds can be supplied to the lower trays 2, whereby the activity of the catalyst on the lower trays 2 can be controlled, as described in more detail below.

The gas from the separator 20 is expelled from the plant through line 36 and passes through the pressure relief valve 37 into lines 38 and 39, through which it leaves the plant. This exhaust gas contains H₂S, which is a product of the hydrodesulfurization treatment, and can be further treated (e.g. H₂S removal and subsequent conversion by partial oxidation to elemental sulfur).

The liquid collecting in the sump 9 is recovered through the line 40 and passes through the flow control valve 41 to a cooler 42. The flow regulator 43 controls the valve 41. The cooled liquid passes together with desorbed hydrogen (which is less soluble in cold liquid hydrocarbons than in hot liquid hydrocarbons) through the line 44 to a gas/liquid separator 45. This is equipped with a droplet separator 46 and a gas return line 47 which leads back to the sump of the column reactor 1. The liquid collected in the separator 45 flows through the line 48 and through the valve 49, which is controlled by the level controller 50, to the gas/liquid separator 51. This has a droplet separator 52 and a gas rejection line 53 as well as a product recovery line 54.

Reference numeral 55 designates a line through which a controlled amount of H₂S or an active sulfur-containing material, such as CS₂, COS, alkyl mercaptan of the formula RSH, alkyl sulfide of the formula RSR or dialkyl disulfide of the formula RS-SR, wherein R is an alkyl group such as n-butyl, can be fed to the hydrodesulfurization plant, conveniently if necessary in the form of a solution in a hydrocarbon solvent as described below.

In operation of the plant of Figure 1, the liquid feedstock fed through line 5 passes through reactor 1 and ultimately exits the plant through line 54. In passing through reactor 1, the organic sulphur compounds are largely converted to H₂S, some of which leaves the plant through line 54 dissolved in the liquid product. Separation of the H₂S from the liquid product may be accomplished in a known manner, e.g. by stripping in a downstream processing plant (not shown). Although the hydrogen-containing make-up gas in line 3 is substantially free of sulphur, the liquid flowing onto the lowermost tray 2 of reactor 1 will normally contain sufficient H₂S to ensure that the hydrodesulfurization catalyst thereon remains adequately sulphided, thus minimizing any risk of hydrocracking reactions on the lowermost tray 2 of reactor 1. On the higher trays 2 the gas feed comes from a lower tray and therefore contains H₂S from contact with the liquid phase in that tray. Therefore, there is normally sufficient H₂S partial pressure on each tray 2 of the reactor to ensure adequate sulphidation of its catalyst charge. However, if for any reason the H₂S partial pressure on any tray 2 of the reactor 1 should fall below a safe level, a suitable amount of sulphur-containing material, preferably H₂S, CS₂, COS or an active organic sulphur-containing material such as an alkyl mercaptan (e.g. n-butyl mercaptan), dialkyl sulphide (such as di-n-butyl sulphide), or a dialkyl disulfide (e.g. di-n-butyl disulfide) is fed through line 55, conveniently as a solution in a hydrocarbon solvent, to enhance the sulphur content of the feed to the tray 2 in question. Since CS₂, COS, Alkyl mercaptans, dialkyl sulfides and dialkyl disulfides are easily and rapidly converted to H₂S, one can be assured that the catalyst charge on each tray 2 of the reactor 1 will remain sufficiently sulfided so that essentially all risks of hydroracking occurring in the reactor 1 are eliminated.

Figure 2 shows a design of the tray 2 which is suitable for use in a relatively small scale reactor 1. A frustoconical partition or septum 70 extends in this case within the walls 71 of the reactor 1 and completely closes off the cross section of the reactor 1 with the exception of a downcomer 72 for the liquid and a gas riser 73. The inclination of the frustoconical partition 70 is equal to or greater than the angle of repose of the solid, particulate hydrodesulfurization catalyst under the liquid present on the tray 2.

The gas riser 73 comprises an axial tube 74 which is open at its upper end and covered with a bell 75. An annular mesh body 76 prevents catalyst particles from being sucked back into the bell 75. A cylindrical baffle 77 surrounds the bell 75 symmetrically and is arranged so that it lies on the tray 2 below the liquid level 78, the height of the liquid level being determined by the height of the upper end of the downpipe 72. The upper end of the downpipe 72 is fitted with a sieve 79 to retain catalyst particles on the tray 2. The reference number 80 designates the downpipe from the next higher tray 2 (not shown).

The baffle 77 promotes the stirring of the suspension of liquid and catalyst by the rising gas. The vertical extent of the baffle 77 is not critical, but should generally be between one third and three quarters of the vertical height between the septum 70 and the liquid surface 78. The baffle 77 should preferably be arranged in a symmetrical or nearly symmetrical vertical position. In the zone within the baffle 77, the Liquid flow is generally upward, while outside the baffle 77 the liquid flow is generally downward. Preferably, the area of the zone inside the baffle 77 is approximately equal to the total area outside the baffle 77.

The gas riser pipe 73 is equipped with a check valve 81.

The temperature of the liquid and the catalyst on the tray 2 can be controlled by the heat exchanger coil 82.

During operation, the rising gas bubbles through the liquid on the tray 2 and agitates the liquid-catalyst mixture so that the catalyst particles are kept in suspension. The baffle 77 promotes good liquid circulation on the tray 2. With a suitable strength of the upward gas flow, the gas bubbles, generally designated 83, keep the majority of the catalyst particles (designated 84) in suspension. (Note that only some bubbles 83 and some catalyst particles 84 are shown so as not to make the drawing too complicated.)

Figure 3 shows another construction of the tray 2 of the reactor 1 of the plant of Figure 1. A horizontal partition or dividing wall 100 extends within the wall 101 of the reactor 1 and completely closes off the cross-section of the reactor 1 except for the liquid downcomer 102 and a gas riser 103. The dividing wall 100 has an axial frustoconical part 104 surrounding the gas riser 103 and an annular inclined part 105 on the wall 101. The tray 2 can therefore retain a volume of liquid, the surface area of which is indicated at 106 and which is determined by the height of the overflow level of the downcomer 102 above the dividing wall 100. Each tray 2 also carries a charge of a solid sulphided hydrodesulfurization catalyst, the particles of which are schematically indicated at 107. These particles 107 are on the tray 2 is kept in suspension due to the agitation caused by the rising gas in the liquid, as described below. To prevent the escape of particles 107 with the liquid overflowing through the downpipe 102, the upper end of the downpipe 102 is provided with a sieve 108. The inclination of the frustoconical part 104 and the inclination part 105 is equal to or greater than the angle of repose of the solid particulate catalyst under the liquid on the tray 2.

The gas riser pipe 103 leads the rising gas via star-shaped pipe 110 to a circular distributor 109, which surrounds the frustoconical part 104. The back-suction of liquid downwards through the riser pipe 103 is prevented by a check valve 111.

Annular draft plates or baffles 112 and 113 are arranged within the liquid body on the plate 2, one inside and one outside the circular distributor 109, to promote agitation of the liquid/catalyst suspension by the rising gas. The vertical extent of the plates 112 and 113 is not critical, but should generally be between one-third and three-quarters of the vertical height between the partition 100 and the liquid surface 106. Preferably, the plates 112 and 113 should be arranged in a symmetrical or nearly symmetrical vertical position. In the annular zone between the plates 112 and 113, the liquid flows generally upwards, while inside the plate 112 and outside the plate 113, the general direction of liquid flow is downwards. Preferably, the area of the ring zone between the sheets 112 and 113 is approximately equal to the sum of the areas inside the sheet 112 and outside the sheet 113.

The reference number 114 designates a downpipe from the next tray above the tray shown in Figure 3. The liquid level in the downpipe 114 is shown at 115, the height H of this liquid level above the liquid level 116 on the tray 2 being determined by the liquid level on the tray containing the downpipe 14. applied (ie the floor above the illustrated floor 2), plus the pressure drop in the manifold 109 on that floor (ie the floor above the illustrated floor 2) and the frictional pressure loss.

The temperature on the tray 2 of Figure 3 can be regulated by a heat exchanger coil 117.

In another embodiment (not shown), the heat exchanger 117 is omitted, but an external heat exchanger is connected to the column reactor, through which liquid withdrawn from the tray 2 is pumped for temperature control.

The invention is further explained in the following example.

Example

The hydrodesulfurization of a synthetic mixture of an organosulfur compound (dibenzothiophene) dissolved in a saturated hydrocarbon solvent (n-hexadecane) is studied in the apparatus shown in Figure 4.

The liquid feedstock consists of a solution of dibenzothiophene (DBT) in normal hexadecane (114 grams of DBT per liter of solution) stored in a feed tank 201. The feed tank 201 is fed with a previously prepared solution via line 202 and then purged with dry nitrogen via lines 202 and 203. The feed tank 201, the feed line 204, the metering pump 205 and the transfer line 206 are arranged in a circulating hot air chamber (not shown) thermostatted at 50ºC +/- 5ºC to avoid any risk of solid formation (since the melting point of n-hexadecane is about 20ºC). The flow of liquid to the column reactor 207 is carried out by the metering pump 205 and is checked from time to time by a burette in the hot air chamber (also not shown). The transfer line 206 is wound with an electrical resistance heater 208 so that the feed liquid is heated before entering the top of the column reactor 207 can be preheated. The column reactor 207 consists of a vertical metal cylinder 2.0 m high and 7.5 cm inside diameter containing eight reaction tray sections 209 through 216. (The construction of one of the trays 209 through 216 is shown in detail in Figure 5 and further described below). Hydrogen gas is supplied through line 217 to the bottom of the reactor 207 (above the liquid level in its sump 218) and the desired hydrogen flow is adjusted by a mass flow controller 219.

The entire reactor 207 is surrounded by an electrically heated forced-circulation hot air bath (not shown).

The liquid flows from the uppermost reaction tray 209 to the lowermost reaction tray 216 by sequential overflow from one tray to the next tray, and at the same time the hydrogen-containing gas flows upward through the trays 216 to 209, bubbling through the liquid at each tray. The liquid collects in the sump 218 at the bottom of the reactor 207, is withdrawn from the system through line 220 by the level controller 223 under control of the valve 222, and is cooled by the cooler 221 through which water at about 45°C flows. Liquid samples are taken from line 224 for analysis.

A gas phase leaves the reactor 207 through line 225 and is cooled by the condenser 226 which is fed with water at 40°C and the cooled gas and condensate passes through line 227 to a drum 228. The gas phase then flows through line 229 and the face pressure control valve 230 which sets the operating pressure throughout the system. The gas flows on to an analytical flow meter and a flare (not shown). The liquid collected in the drum 228 can return to the tray 209 through line 231 under control of the valve 232 which is operated by a level sensor 233 on the drum 228 or a Sample can be withdrawn through line 234 and manual valve 235.

An example of a structural design of one of the reaction trays 209 to 216 is shown in Figure 5. The vertical walls of the reactor column are designated by the number 240. The tray is also equipped with a feed/discharge pipe 241 with a compartment valve 242, which serves to supply/discharge the catalyst suspension to the reaction tray in question.

The horizontal cross-section of the reactor column 207 is closed by a frustoconical partition, which is designated in the vertical section by the reference numerals 243 and 244. The inclined part of the partition 243 has an angle of 30º to the horizontal. The horizontal part of the partition 244 is 4 cm in diameter and pierced at two points by vertical tubes of 4 mm internal diameter.

The tube 245 penetrates the partition wall in the vertical axis of the reactor and projects upwards for a distance of 22 cm into a bell 246. The projection of the tube 245 below the bottom of the horizontal wall part 244 is small and can be zero. The bell 246 has an outside diameter of 20 mm. A gap of 2 mm between the bottom edge of the bell 246 and the top of the horizontal wall part 244 is closed by a sieve material 247 made of a fine stainless steel in order to prevent catalyst particles from entering the interior of the bell 246.

The tube 248 begins at a level 9 cm above the horizontal wall portion 244 and extends downward to an end in the space below the portion 244. It is covered by a screen 249 to prevent the entry of catalyst particles. The level of the upper end of the tube 248 determines the liquid level 250 on the tray. The lower end of the tube 248 is below the liquid level on the tray below (or in the case of tray 216, the lower end of the tube 248 is below the liquid surface in the sump 218 of reactor 207).

In operation, the tray, i.e. one of the trays 209-216, receives liquid from the tray above through a tube 248 as described above (or in the case of tray 209 from the liquid supply line 206); liquid overflows through the catalyst screen material 249 and flows downward to the next tray below. Gas flows through tube 245, the inner part of the bell jar 246, the screen 247 and bubbles through the liquid on the tray. The agitation created by the bubbles keeps the catalyst particles 252 in turbulent suspension. The gas phase escaping through the liquid surface 250 flows through the axial tube of the tray above (or in the case of tray 209 into the line 225). In this way, the liquid phase moves downward from tray 209 to tray 210 and so on down to tray 216 and into sump 218, coming into countercurrent contact with the gas phase flowing up through tray 216 to tray 215 and so on up to tray 209 and into the conduit.

After the apparatus is purged with nitrogen and the feed tank 201 is charged with the feed solution, the vent line is opened to atmosphere by opening the bypass line around the valve 230 (not shown).

With valve 242 open, each tray 209-216 is charged with a catalyst slurry through line 241. In this way, each of the trays 209-216 is charged with 35 cc of catalyst in the size range of 0.1 to 0.5 mm in 290 cc of n-hexadecane. The catalyst is sulfided CoO-MoO3/gamma alumina which has previously been reduced with hydrogen, cooled and immersed in n-hexadecane. 45 cc of n-hexadecane is used to flush all of the catalyst to the tray through line 241. Then valve 242 is closed. A stream of nitrogen is passed through line 217 and up through column 207 and vented to atmosphere through line 229. The nitrogen stream is converted into hydrogen and the apparatus is slowly pressurised to 35 bar (absolute), the column heater serving to raise the temperature from 30 ºC to 325 ºC in four hours.

Once operating pressure is reached after about 45 minutes, the hydrogen feed rate is increased to 465 liters/hr (NTP). At this point, a solution of 10 g/liter CS₂ in n-hexadecane is pumped through line 241 at 500 cm³/hr to tray 216 to ensure that the catalyst is sulfided. After 3 hours, the feed of CS₂ solution to tray 216 is discontinued and the liquid feed to tray 209 is turned on and adjusted to 587 cm³/hr. During this time, the liquid level in sump 218 rises and then liquid leaves the apparatus under control of level sensor 223 and control valve 222.

Samples are taken from time to time from the product liquid in line 224 and analyzed for dibenzothiophene by gas-liquid chromatography. After twelve hours of steady-state operation, the analysis shows that 99.1% of the dibenzothiophene has been converted to sulfur-free products and only traces of the tetrahydro and hexahydro derivatives of dibenzothiophene, which cannot be determined quantitatively, can be seen on the chromatographic image. During the next three days of operation, the dibenzothiophene conversion increases continuously to 99.5% and stabilizes.

Comparison example

In a comparative test, 280 cm³ of the same catalyst in the form of an extrudate of 1 mm diameter and 1 to 3 mm length are packed into a reactor of 25 mm internal diameter to form a 57 cm deep catalyst bed. When this conventional reactor is operated in gas and liquid cocurrent downward flow using the same feed mixture and at the same temperature, pressure and gas and liquid flow rates as in the example, only 96.4% conversion of the dibenzothiophene is achieved, which is a shows considerably lower performance compared to operation in the countercurrent column (more than seven times the amount of dibenzothiophene remains unreacted).

The hydrogenation of aromatic compounds in the presence of a hydrodesulfurization catalyst depends on a number of factors including thermodynamic and kinetic factors as well as on the catalyst activity and its performance.

From the thermodynamic point of view, the hydrogenation of an aromatic compound, e.g. an aromatic hydrocarbon, is an exothermic process. In addition, the extent to which the reaction will proceed under certain conditions is limited by considerations such as equilibrium under those conditions. In general, equilibrium is less favorable at higher temperatures. Therefore, it is advantageous to operate at lower reaction temperatures if possible.

The kinetics of aromatic hydrogenation reactions are favored by the use of high temperatures. Therefore, the rate of aromatic hydrogenation at a certain fixed hydrogen partial pressure increases sharply with increasing temperature, provided that the aromatic concentration in the reaction mixture is below the equilibrium limit at the temperature in question.

The ability of a given catalyst mass of defined particle size range to carry out aromatic hydrogenation is a function of the spray intensity applied to the catalyst particles, the degree of sulfidation of the catalyst, and the rates of mass transfer of H2 and H2S to and from the catalyst surface. In general, the greatest tendency toward aromatic hydrogenation is exhibited by a catalyst with a low degree of sulfidation subjected to turbulent two-phase (gas/liquid) mixture flow.

Figure 6 is a graphical representation schematically showing the effect of these various factors on aromatic hydrogenation Figure 6 plots the percentage of aromatics in the product versus temperature at a given hydrogen partial pressure. Line AA' in Figure 6 shows the variation with temperature of the kinetically limited aromatics content of the product from a given feedstock of a certain aromatics content using a fixed amount of catalyst at a fixed hydrogen partial pressure. Line BB' shows the equilibrium limited aromatics content in the product from the same reaction system as a function of temperature. Line XY (or X'Y') represents the excess aromatics content of the product at any given temperature and therefore provides a measure of the driving force required of the catalyst. Point O represents the lowest aromatics content that can be obtained in a given system and can only be reached by choosing a combination of the most favorable kinetics and the less favorable equilibrium as the temperature increases.

If the catalyst activity can be increased in some way, e.g. by controlling the degree of sulfidation, a new curve, such as C-C', with a new lower optimal aromatics level (point O') can be obtained.

In practice, the crude oil feedstocks contain many different aromatic and sulfur compounds, each of which has its own hydrogenation and hydrodesulfurization kinetics. The prior removal of the less refractory materials and the removal of the bound H2S from the sulfur compounds, which is possible according to the teaching of the invention, makes it possible to achieve significant advantages according to the process of the invention in comparison to the conventional hydrodesulfurization processes.

Claims (8)

1. A hydrodesulfurization process for the continuous hydrodesulfurization of a liquid sulfur-containing hydrocarbon feedstock, which comprises: (a) providing a hydrodesulfurization zone maintained under hydrodesulfurization conditions and having a column reactor having a plurality of reaction trays disposed one above the other therein, each reaction tray defining a reaction stage capable of holding a given volume of liquid and a charge of a finely divided sulfided solid hydrodesulfurization catalyst suspended therein, and each reaction tray having a tray at least partially inclined at an angle equal to or greater than the angle of repose of the catalyst particles beneath the liquid, the column reactor further having liquid drain ports associated with each reaction tray through which the liquid from the reaction tray can pass downwardly of the column reactor while the solid catalyst is maintained thereon, and gas rise ports associated with each reaction tray allowing gas to enter the tray from below and to cool the mixture of liquid and catalyst is stirred on the reaction tray, (b) supplying liquid sulfur-containing hydrocarbon feedstock to the uppermost of said plurality of reaction trays, (c) supplying hydrogen-containing gas beneath the lowest of said plurality of reaction trays, (d) allowing the liquid to flow downwards from reaction tray to reaction tray through the column reactor, (e) allowing the hydrogen-containing gas to rise from reaction tray to reaction tray upwards through the column reactor, (f) from the uppermost of said plurality of reaction trays a H₂S-containing reaction tray formed by hydrodesulfurization Exhaust gas wins, and (g) recovering a liquid hydrocarbon product having a reduced sulphur content from the lowest of said plurality of reaction trays. 2. Process according to claim 1, in which the solid sulfided catalyst used is selected from molybdenum disulfide, tungsten disulfide, cobalt sulfide, sulfided nickel molybdate catalysts (NiMoSx), a sulfided CoO-MoO₃/gamma-Al₂O₃ catalyst and mixtures thereof. 3. A process according to claim 1 or claim 2, wherein the hydrodesulfurization conditions comprise the application of a pressure in the range of 20 bar to 150 bar and a temperature in the range of 240ºC to 400ºC. 4. A process according to claim 3, wherein the hydrodesulfurization conditions comprise the application of a pressure of 25 bar to 100 bar and a temperature of 250ºC to 370ºC. 5. A process according to any one of claims 1 to 4, wherein each reaction tray has a tray of frustoconical shape, the inclination of which is equal to or greater than the angle of repose of the catalyst particles under the liquid present on the tray. 6. A process according to any one of claims 1 to 4, wherein each reaction tray has a tray and a surrounding wall, the tray comprises an inner tray portion inclined downwardly and inwardly from a central portion and an outer tray portion inclined downwardly and inwardly from the surrounding wall, and the inclination of the inner and outer tray portions is equal to or greater than the angle of repose of the catalyst particles under the liquid on the reaction tray. 7. A process according to any one of claims 1 to 6, wherein each reaction tray is provided with a heat exchanger for controlling the temperature of the mixture of liquid and catalyst on the reaction tray. 8. A process according to any one of claims 1 to 7, wherein the temperature of the mixture of liquid and catalyst increases from the uppermost reaction tray to a middle lower reaction tray and decreases from tray to tray on the reaction trays following said middle lower reaction tray.