DE68920377T2 - Process for hydrocracking hydrocarbon feeds. - Google Patents

Description

The present invention relates to a process for hydrocracking a hydrocarbon-containing starting material, in which several reaction stages are carried out.

Hydrocracking is an established process in which heavy hydrocarbons are contacted with a hydrocracking catalyst in the presence of hydrogen. Temperature and pressure are relatively high so that the heavy hydrocarbons are cracked to lower boiling point products. Although the process can be carried out in one stage, it has been found to be advantageous if it is carried out in several stages. In a first stage, the feedstock is subjected to denitration, desulfurization and hydrocracking to some extent, and in the second stage the majority of the hydrocracking reactions take place.

The presence of nitrogen compounds in the feedstock can cause problems, since amorphous hydrocracking catalysts are generally sensitive to nitrogen and can be deactivated by nitrogen compounds. A solution to this problem was seen in the use of a hydrocracking catalyst with hydrodenitrating activity in a first reaction stage and in the use of a zeolitic hydrocracking catalyst, which is less sensitive to nitrogen, in a second reaction stage. However, these zeolitic catalysts have the disadvantage that "bulky" molecules with multiple ring structures could not be adequately cracked unless very high temperatures were used.

US Patent No. 3,702,818 describes a proposed solution to this problem, according to which two reactors are used in the second process stage, one of which has a zeolite catalyst and the other contains an amorphous base catalyst. Although bulky ring structures can be cracked by the amorphous catalyst, the recirculated stream still contains a significant amount of intermediate products, resulting in an unacceptably high catalyst loading.

The present invention provides a solution to the problem of nitrogen sensitivity and inadequate hydrocracking of bulky materials.

The present invention therefore provides a process for hydrocracking a hydrocarbon-containing starting material in several reaction stages, which comprises contacting the starting material with a first hydrocracking catalyst at elevated temperature and pressure in the presence of hydrogen in a first reaction stage to obtain a first effluent, mixing at least a liquid portion of the first effluent with a second effluent originating from a second reaction stage, contacting the resulting mixture in a third reaction stage with a third hydrocracking catalyst at elevated temperature and pressure in the presence of hydrogen to obtain a third effluent, passing the third effluent to a separation stage in which at least one top fraction and one residual fraction are obtained, and passing the residual fraction to the second reaction stage where it is contacted with a second hydrocracking catalyst at elevated temperature and pressure in the presence of hydrogen to obtain the second effluent. is, includes.

If the first hydrocracking catalyst has a denitrating effect, the third catalyst is relatively insensitive to nitrogen and the second hydrocracking catalyst is able to hydrocrack bulky molecules, then the problems mentioned are solved.

The present invention can be embodied in various ways which are variations of a series-connected hydrocracking system and a 2-stage hydrocracking system. For the flow diagram of these known processes, please refer to the literature "The Petroleum Handbook", 6th edition, Elsevier, Amsterdam, 1983, pages 294-300.

A first embodiment is based on a series-connected hydrocracking system. In a conventional series-connected hydrocracking system, the effluent from a first reaction stage is passed to a second and the effluent from the second reaction stage is subjected to a processing process including fractionation. Fractionation results in a residue fraction which is combined with the first effluent to be passed to the second reaction stage. The third stage according to the invention corresponds to the second stage in conventional processes.

According to the present invention, the combination of at least a part of the first effluent with the second effluent is passed to the third reaction stage. The third effluent from the third reaction stage is subjected to the processing process in which the product distillation fractions are recovered and a residue fraction is obtained. The residue fraction from the fractionation is recycled to the second stage to obtain the second effluent. It is possible to subject the first effluent to a gas purification stage in order to remove ammonia and/or hydrogen sulphide formed in the first reaction stage by treatment with, for example, an aqueous solution of, for example, a mineral acid. Suitably, however, the entire first effluent is combined with the second effluent. This has the advantage that not much has to be changed in the conventional series-operating process. In a further preferred embodiment, the first effluent is subjected to a separation into a liquid and a gaseous phase. The gaseous phase is combined with the third effluent and subjected to the processing process. This has the advantage that the gaseous phase has the greatest

part of the ammonia, the hydrogen sulphide and also the cracked products.

There is therefore no risk of over-cracking the desired products into undesirable C₁₋₄ hydrocarbon products. The liquid phase, which contains only a very small proportion of the ammonia in the first effluent, is combined with the second effluent to be hydrocracked in the third stage. It is of course possible to separate the first effluent under different temperature and pressure conditions. The effluent can thus be cooled to a temperature below the temperature prevailing in the first reaction stage. Preferably, however, this separation is carried out at substantially the same temperature and pressure as prevailing in the first reaction stage. This is advantageously achieved by avoiding heating or cooling the first effluent. In an even more preferred embodiment, this separation is carried out in the same reaction vessel in which the first reaction stage is carried out. This can be achieved by creating a settling area in the bottom of the reaction vessel from which the gas phase and the liquid phase are withdrawn through different openings. Of course, slight fluctuations can occur in the first reaction stage. In this case, it is preferable to carry out the separation at essentially the same temperature and pressure as prevail in the initial section of the first stage.

Another embodiment of the present invention is derived from a conventional two-stage hydrocracking system. In a conventional hydrocracking apparatus, the effluent from a first reaction stage is combined with the effluent from a second reaction stage and the combined effluents are subjected to a processing procedure including fractionation which results in the desired distillate products and a residue fraction which must be passed to the second reaction stage.

According to the above-mentioned embodiment of the present invention, the residual fraction of the fractionation is mixed with the effluent of the second stage and the mixture is passed to the third reaction stage. The effluent of this third stage is separated into a gaseous top fraction and a liquid bottom fraction. Preferably, the top fraction obtained after separation of the third effluent is combined with the first effluent and the combination is subjected to a work-up process from which the products are recovered by fractionation. The liquid bottom fraction from the first effluent is passed to the second reaction stage.

Accordingly, in this embodiment, one or more distillate fractions are obtained by fractionation from the first effluent and recovered as products and further a residual liquid portion is obtained which is mixed with the second effluent. The top fraction obtained in the separation stage can be processed separately. Preferably, in this embodiment, the top fraction obtained in the separation stage is combined with the first effluent to be fractionated.

Advantageously, the separation of the third effluent is carried out in the manner described above for the first effluent of the system based on a series hydrocracking process. Although it is possible to separate the third effluent under different temperature and pressure conditions, e.g. by cooling the effluent to a temperature below that prevailing in the third reaction stage, it is preferred to carry out this separation at substantially the same temperature and pressure as prevailing at the exit of the third reaction stage. This is advantageously achieved by refraining from heating or cooling the third effluent. In an even more preferred embodiment, this separation is carried out in the same reaction vessel in which the third reaction stage. This can be achieved by creating a settling area at the bottom of the reaction vessel from which the gas phase and the liquid phase are withdrawn through different openings.

The second reaction stage and the third reaction stage can be carried out in different reactors. Preferably, however, both stages are carried out in a single reaction vessel. Usually, not too many technically difficult adjustments are necessary for this, since most conventional hydrocracking plants comprise a large number of catalyst beds, and by changing the feed lines and by loading the catalyst beds differently, conventional hydrocracking plants can be adapted to the process according to the invention.

As described above, the first hydrocracking catalyst is preferably a catalyst having hydrodenitration and/or hydrodesulfurization activity. Suitable catalysts include first hydrocracking catalysts containing at least one component of a metal from Group 8 and/or 6b of the Periodic Table of the Elements on an amorphous support. Advantageously, the support is selected from the group consisting of silica, alumina, thoria, titania, magnesium oxide, zirconia and mixtures thereof. More preferably, the support contains alumina. Reference is made to the Periodic Table of the Elements published in the reference "Handbook of Chemistry and Physics", 55th edition (1975), CRC Press, Ohio, USA.

In the second and third stages, the same or different hydrocracking catalysts can be used. Mixtures or combinations of different catalysts can be used in each stage; such mixtures or combinations can be the same or different for each stage. Preferably, however, the catalyst, catalyst mixture or catalyst combination used in the second stage is different from that used in the third stage.

The second hydrocracking catalyst is preferably a catalyst with good performance in cracking bulky molecules with a large number of rings. A very good catalyst in this respect is a second hydrocracking catalyst containing at least one component of a Group 8 and/or 6b metal on a silica-alumina-containing support. This catalyst tends to be very nitrogen sensitive. Since the hydrocarbonaceous feedstock to be cracked by this silica-alumina catalyst has already been subjected to two hydrocracking processes in which nitrogen was also removed, the disadvantages of using a nitrogen sensitive catalyst do not appear here. It is emphasized that other catalysts can also be used in the second stage, e.g. catalysts based on aluminum oxide, silicon dioxide, titanium oxide, magnesium oxide, zirconium oxide or mixtures thereof. The use of zeolitic catalysts in the second stage is also possible, but less preferred because although the catalysts are very effective, their effect with regard to the conversion of bulky molecules needs to be improved.

The third hydrocracking catalyst is preferably a zeolitic catalyst, i.e. a catalyst having at least one component of a Group 8 and/or 6b metal on a zeolite-containing support. The zeolite in the third hydrocracking catalyst is preferably a faujasite-type zeolite, and in particular zeolite Y. The molar ratio of silica to alumina in such zeolites may vary from 4:1 to 25:1, and in particular is 6:1 to 15:1. The unit cell size of zeolite Y may vary from 2.419 to 2.475 nm, in particular from 2.425 to 2.460 nm. In addition to the zeolite, the third hydrocracking catalyst suitably also contains at least one binder. The binder is suitably selected from Silica, alumina, silica-alumina, thoria, titania, zirconia, magnesia and mixtures thereof. Alumina is particularly preferred, optionally in combination with silica-alumina.

In all three hydrocracking catalysts, the catalytically active metal from Groups 8 and 6b is preferably selected from cobalt, nickel, platinum, palladium, tungsten and molybdenum. Preferably, non-noble metal mixtures are used, such as Ni-W, Ni-Mo, Co-Mo and Co-W. The catalysts may also contain phosphorus and/or fluorine to increase their activity. When the catalytically active metals are non-noble metals, they are preferably present on the hydrocracking catalysts in an amount of from 1 to 16% by weight with respect to a Group 8 metal and/or from 6 to 24% by weight with respect to a Group 6 metal, the weight percentages being based on the total catalyst. Noble metals are suitably present in smaller amounts, e.g. in an amount of from 0.2 to 2% by weight. Precious metals can be used primarily on the third and/or second hydrocracking catalyst. Especially if the catalytically active metals are not noble metals, they are preferably present in oxide or, even more preferably, in sulfide form. The production of the hydrocracking catalysts is state of the art.

Hydrocarbonaceous feedstocks which may be used in the present process include gas oils, vacuum gas oils, deasphalted oils, long residues, short residues, catalytically cracked cycle oils, thermally cracked gas oils and synthetic crude oils, which oils may be derived from tar sands, shale oils, residue refining processes or from biomass. Combinations of various hydrocarbonaceous feedstocks may also be used. The hydrocarbonaceous feedstock is generally such that a major portion, ie more than 50% by weight, has a boiling point greater than 370°C. The present process offers the most advantages when the Starting material contains nitrogen. Typical nitrogen contents are in the range up to 5000 parts by weight pM. Nitrogen contents can start at 50 parts by weight pM. The starting material generally also contains sulphur compounds. The sulphur content is usually in the range of 0.2 to 6 percent by weight.

As regards the process conditions in the reaction steps, the temperature in each reaction step is preferably 300 to 450ºC, in particular 350 to 420ºC, the pressure is in the range of 50 to 250 bar, in particular 75 to 150 bar, the space velocity is in the range of 0.1 to 10 kg/l/h, in particular 0.2 to 5 kg/l/h, and the hydrogen to oil ratio is in the range of 500 to 5000 Nl/kg, in particular 1000 to 2500 Nl/kg.

The process is explained in more detail using the following drawings. For the sake of simplicity, various parts of the device that are not essential to the invention are not shown. The drawings show only one catalyst bed in each reaction stage. Of course, several catalyst beds can also be used instead of one catalyst bed.

Fig.1 shows an embodiment based on a series-connected hydrocracking system in which the second and third reaction stages are carried out in a single reaction vessel.

Fig.2 shows an embodiment based on a series-connected hydrocracking system in which, after the first stage, the first effluent is subjected to a gas-liquid separation.

Fig.3 shows an embodiment based on a two-stage hydrocracking system in which the third effluent is subjected to a gas-liquid separation which takes place in the same reaction vessel in which the second and third reaction stages are also carried out.

In Fig. 1 a reaction vessel is shown which contains a catalyst bed 2. The catalyst bed 2 contains the first hydrocracking catalyst. The drawing also shows a second reaction vessel which comprises a catalyst bed 4 with the second hydrocracking catalyst and a catalyst bed 5 with the third hydrocracking catalyst. Also shown are a gas purification zone 6 and a fractionating device 7. During operation, a hydrocarbon-containing feedstock is passed to the reaction vessel 1 via line 8 together with a hydrogen-containing gas which is fed via line 9. After being passed through the catalyst bed, the mixture of hydrocarbons and hydrogen is withdrawn from the reaction vessel 1 via line 10. Additional hydrogen is added to the mixture via line 11 and the resulting mixture is fed to the reaction vessel 3 at a location between the catalyst beds 4 and 5. The mixture is then combined with an effluent from the catalyst bed 4, i.e. the second effluent, and the mixtures are passed together over the catalyst bed 5 to obtain the third effluent which is withdrawn via line 12. The effluent in line 12 is passed to the gas purification zone where it is freed from ammonia and/or hydrogen sulphide which is withdrawn from the system via line 13. The operation of the purification zone is state of the art. Various gas-liquid separators at different temperatures and/or pressures may be used to separate a hydrogen-containing gas from the desired products and a scrubbing zone may also be used to remove ammonia and/or hydrogen sulphide. The hydrogen-containing gas (recycled hydrogen) may contain some light (C₁₋₂) hydrocarbons and/or hydrogen sulphide. Hydrogen produced in zone 6 is recycled to the system via line 14 and hydrocarbon products are recycled via line 15 to the fractionator. There they are separated to obtain product streams 16,17 and 18 and a residue fraction is withdrawn from the fractionator via line 19. The residue fraction is recycled to reaction vessel 3 and fed to vessel 3 after being mixed with hydrogen fed from line 20 at a point above catalyst bed 4. The residue fraction is hydrocracked in catalyst bed 4 to obtain the second effluent. The hydrogen for lines 9,11 and 20 comes from a feed line 21 which is replenished with hydrogen by the recycled hydrogen in line 14 and the additional hydrogen fed via line 22.

In Fig. 2, a reaction vessel 31 is shown which contains a catalyst bed 32 in which the first reaction stage is carried out. Also shown are a reaction vessel 33 with a catalyst bed 34 for the third reaction stage and a reaction vessel 35 with a catalyst bed 36 for the second reaction stage. In addition, the illustration shows a gas purification zone 37, a fractionating device 38 and a separation device 39. In operation, a hydrocarbon-containing feedstock from line 40 is mixed with a hydrogen-containing gas from line 41 and these are fed together into the reaction vessel 31 where the mixture is hydrocracked over the catalyst bed 32 containing the first hydrocracking catalyst. The first effluent is withdrawn from the reaction vessel 31 via line 42 and fed to the separation device 39. The separation device 39 can be any device known in the art suitable for carrying out a gas-liquid separation. Preferably, the same temperature and pressure are present in this device as in the reaction vessel 31. A gas phase is withdrawn from the separating device 39 via line 43 and a liquid phase via line 44. The liquid phase is mixed with a second effluent in line 45, which originates from the reaction vessel 35, and the mixture is fed via line 46, into which via line 47 Hydrogen is fed to reaction vessel 33. In reaction vessel 33, the mixture from line 46 is hydrocracked over the third hydrocracking catalyst in catalyst bed 34 to give the third effluent which is withdrawn from reaction vessel 33 via line 48. To the third effluent is fed the gas phase of the first effluent in line 43 and the combined streams are fed to gas purification zone 37 to give a waste stream of ammonia and/or hydrogen sulphide which is disposed of via line 49, recycled hydrogen which is withdrawn via line 50 and the hydrocarbon product which is withdrawn via line 51. Line 51 opens into fractionator 38 where product streams 52, 53 and 54 are recovered. Distillation in fractionator 38 further produces a residue fraction which is passed to reaction vessel 35 via line 55. In the presence of hydrogen from line 56, the residue fraction is hydrocracked over the second hydrocracking catalyst in catalyst bed 36. The second effluent resulting from hydrocracking over catalyst bed 36 is withdrawn via line 45 and mixed with the liquid phase in line 44. Hydrogen is introduced into the process via line 57 which is fed by the additional hydrogen from line 58 and the recycled hydrogen from line 50.

Of course, the separation device 39 can be integrated into the reaction vessel 31 as a settling zone. This could save the costs of a separate vessel. It is also possible to arrange catalyst beds 36 and 34 in a single reaction vessel, as shown in Fig. 1 with the catalyst beds 4 and 5.

Fig.3 shows a reaction vessel 61 with a catalyst bed 62 comprising the first hydrocracking catalyst, a gas purification zone 63, a fractionating device 64, and a second reaction vessel 56 comprising a catalyst bed 66 for the second reaction stage and a catalyst bed 67 for the third reaction stage The reaction vessel 56 further includes a settling zone 68 in which gas-liquid separation takes place. During operation, a hydrocarbon-containing feedstock in line 69 is combined with hydrogen from line 70, fed to the reaction vessel 61 and hydrocracked over the first hydrocracking catalyst in catalyst bed 62. The resulting effluent is withdrawn from the reaction vessel 61 via line 71. It is mixed with a gaseous overhead fraction in line 72 and the mixture is fed to the purification zone 63 where ammonia and/or hydrogen sulfide are withdrawn from the system via line 74, a hydrogen-containing gas is returned to the system via line 75, and hydrocarbon products are separated and passed to the fractionator 64 via line 76. Product streams 77, 78 and 79 are obtained in the fractionator 64 and a residue fraction is withdrawn via line 80. Hydrogen from line 81 is added to the residue fraction and the mixture is fed to reaction vessel 65 at a location between catalyst beds 66 and 67. Between beds 66 and 67, the residue fraction from line 80 mixes with the effluent from catalyst bed 66 and together they are passed over catalyst bed 67 to form the third effluent. The third effluent passes to a settling zone 68 where gas-liquid separation takes place. The gaseous overhead fraction is withdrawn via line 72 and mixed with the first effluent and the liquid residue fraction is withdrawn from reaction vessel 65 via line 82, combined with hydrogen from line 83 and recycled to reaction vessel 65 but introduced at a location above catalyst bed 66. The liquid residue fraction is hydrocracked over the second hydrocracking catalyst in catalyst bed 66 to obtain the second effluent. The hydrogen required for the process is supplied via line 84 which is fed by the additional hydrogen from line 85 and the recycled hydrogen from line 75.

Claims (11)

1. A process for hydrocracking a hydrocarbon-containing feedstock in a plurality of reaction stages, comprising contacting the feedstock with a first hydrocracking catalyst at elevated temperature and pressure in the presence of hydrogen in a first reaction stage to obtain a first effluent, mixing at least a liquid portion of the first effluent with a second effluent originating from the second reaction stage, contacting the resulting mixture in a third reaction stage with a third hydrocracking catalyst containing at least one component of a Group 8 and/or 6b metal on a support containing a faujasite-type zeolite at elevated temperature and pressure in the presence of hydrogen to obtain a third effluent, passing the third effluent to a separation stage in which at least a top fraction and a bottom fraction are obtained, and Passing the residue fraction to the second reaction stage where it is contacted with a second hydrocracking catalyst containing at least one Group 8 and/or 6b metal component on a silica-alumina or faujasite-type zeolite support at elevated temperature and pressure in the presence of hydrogen to obtain a second residue. 2. A process according to claim 1, in which the complete first effluent is mixed with the second effluent. 3. A process according to claim 1 or 2, in which the first effluent is subjected to a separation into a liquid and a gaseous phase, the gaseous phase is mixed with the third effluent and the liquid phase are combined with the second effluent, which is to be passed to the third reaction stage. 4. A process according to claim 3, in which the first effluent is separated into a liquid and a gaseous phase at substantially the same temperature and pressure as those present in the first reaction stage. 5. A process according to any one of claims 1 to 4, in which the head fraction(s) obtained in the separation step is (are) recovered as product(s). 6. A process according to claim 1, in which one or more distillate fractions are obtained from the first effluent by fractionation and recovered as products and a liquid residue portion is also obtained which is mixed with the second effluent. 7. A process according to claim 6, in which the overhead fraction obtained in the separation step is combined with the first effluent to be fractionated. 8. A process according to any one of claims 1 to 7, in which the first hydrocracking catalyst contains at least one component of a group 8 and/or 6b metal on an amorphous support. 9. The method of claim 8, wherein the support is selected from the groups consisting of silica, alumina, thoria, titania, magnesia, zirconia and mixtures thereof. 10. The method of claim 9, wherein the support comprises alumina. 11. A method according to any one of claims 1 to 10, in which the the first, second and third hydrocracking catalysts comprise at least one hydrogenating component of a metal selected from the group consisting of nickel, cobalt, platinum, palladium, tungsten and molybdenum.