RU2588121C2 - Method of hydrocracking hydrocarbon feedstock - Google Patents

Abstract

FIELD: oil industry.

SUBSTANCE: invention relates to a hydrocracking method, comprising steps of: (a) combining a hydrocarbonaceous feedstock and a heavy bottom fraction recycle stream with a hydrogen-rich gas to obtain a mixture comprising hydrocarbonaceous feedstock and hydrogen; (b) catalytically hydrocracking mixture comprising hydrocarbonaceous feedstock and hydrogen in a hydrocracking zone to obtain a hydrocracked effluent; (c) separating hydrocracked effluent into a first vapour portion and a first liquid portion in a separation zone; (d) heating first liquid portion to form a vapourised first liquid portion; (e) feeding vapourised first liquid portion to a fractionation section producing individual product fractions including a heavy bottom fraction comprising unconverted oil at bottom zone of fractionation section; (f) withdrawing from fractionation section heavy bottom fraction; (g) splitting heavy bottom fraction in a stream for stripping and a heavy bottom fraction recycle stream; (h) stripping stream for stripping, with a stripping medium, in a counter current stripping column to form an overhead vapour and a stripped liquid; (i) feeding overhead vapour to fractionation section, to a recycle stream or to a position upstream fractionation section; and (j) removing at least a part of stripped liquid from counter current stripping column as a net purge of unconverted oil. Method includes a step (k) for heat energy transfer to one of said first stream, second stream or an optional third stream before feeding said stream into counterflow stripping column.

EFFECT: use of invention allows to increase conversion of heaviest and highest molecular weight materials into products, and to minimise need for purging by concentration of heavy polynuclear aromatic compounds into portion of unconverted oil stream.

14 cl, 3 ex, 7 tbl, 4 dwg

Description

The invention relates to a method for hydrocracking a hydrocarbon feedstock to produce more valuable low boiling products such as liquefied petroleum gas (LPG), naphtha (naphtha), kerosene and diesel fuel. In particular, this invention relates to a process by which heavy polynuclear aromatic compounds are concentrated in a portion of unconverted oil so that they can be removed, resulting in increased conversion and yield of products.

The complete conversion of oil or synthetic heavy gas oils to distilled products such as gasoline, jet and diesel fuels in a hydrocracker is actually limited to the formation of heavy polynuclear aromatic compounds (TPNA). These compounds, formed by undesirable side reactions, are stable and virtually inaccessible for hydrocracking. TPNA are condensed polycyclic aromatic compounds having a ring number of more than 7 (7+), for example, C ₂₄ H ₁₂ coronene, C ₂₈ H ₁₄ benzorone, C ₃₂ H ₁₆ dibenzo and C ₃₂ H ₁₄ oval.

TPNA with more than 7 aromatic rings are by-products of hydrocracking reactions that can potentially cause significant problems in hydrocracking units. When the solubility limit for TPNA is exceeded, solid particles form in the transport lines, valves and on the surfaces of the heat exchangers. In addition, TPNA can contribute to catalyst deactivation by reversible inhibition and coke formation. TPNA problems arise especially often when processing heavy types of feedstock with high end distillation points and a high aromatic content in the feedstock subjected to cracking in high conversion recycling plants.

Consequently, the TPNA content rises to high levels in the recycle streams commonly used in high conversion processes, which leads to clogging of the catalysts and equipment.

The usual solution to this problem is to remove part of the oil recirculation stream in the form of an unconverted oil stream in order to remove the TPJA compounds from the system, effectively balancing the speed of blowing TPNA with the rate of their formation during the reaction. This approach limits the level of total conversion achieved in a hydrocracker.

In a typical high conversion hydrocracking process, the heavy gas oil feedstock is combined with hydrogen-rich gas and reacts in the presence of a catalyst to produce hydrocracking effluents containing less dense, lower molecular weight products. Hydrocracking products leaving the reactor are condensed and separated in the separation zone into a liquid part containing, first of all, hydrocarbons, and a vapor part, containing, first of all, unreacted hydrogen. The vapor portion of this separation can be combined with make-up hydrogen to compensate for the hydrogen consumed by the reaction, and it can then be compressed and recycled to the reactor tank. The first liquid portion from the separation zone is then sent to the fractionation section, where lighter products are separated from the heavy unconverted products in the fractionation section, for example in a distillation column or a series of distillation columns. Heat is usually added to this recovery operation to provide the necessary energy for separation.

A common approach to controlling the release of TPNA compounds in recirculated oil is to extract the blown product from the recirculated oil in the unit in the form of unconverted oil. The purge rate can be adjusted to balance the retraction TPNA with clean products. Such a purge substantially reduces the total hydrocracking conversion achieved to less than 100 percent. Depending on the quality of the raw materials and the process conditions, the purge rate can be from one or two percent to as high as 10 percent of the feed rate of fresh raw materials. The yield of valuable distillation products, respectively, decreases with a significant economic loss for the refinery.

U.S. Patent No. 6,361,683 discloses a hydrocracking process in which the hydrocracking effluent is hydrogen distilled off in a stripping zone to produce a stream of gaseous hydrocarbons that is passed through a hydrogenation post-treatment zone to saturate the aromatics. The fractionation zone is associated with the distillation zone, into which distilled liquid hydrocarbons obtained by distillation of the hydrocracked waste products are fed. Distillation is also considered to remove TPNA.

US Pat. No. 6,858,128 discloses a hydrocracking method that uses a fractionation zone having a lower section with a baffle to include sections suitable for stripping steam for the concentration of TPNA.

U.S. Patent Numbers 4,961,839 and 5,120,427 disclose a hydrocracking process in which the entire lower fraction is fed to a stripping column represented by a shortened column at the bottom of the fractionation zone. An evaporated stream is fed into the fractionation zone to extract most of the light hydrocarbons, while at the same time allowing the total liquid bottom stream enriched with TPNA to be purged. This patent uses a high degree of evaporation of the fractionation fed to minimize the purge flow and to ensure that only the TPNA-free fraction is recirculated, but this high degree of evaporation is associated with an undesired energy consumption.

There is a real economic incentive to maximize the conversion of heavy feedstock, and a key feature of most of these processes is the recycling of unconverted oil back to the reaction system, thereby controlling the severity of cracking conditions and improving the selectivity of hydrocracking reactions for the most desirable end products, such as gasoline, jet fuel and diesel fuel. All known hydrocracking processes and catalysts are, however, the cause of undesirable side reactions leading to the formation of heavy polynuclear aromatic compounds (TPNA), which accumulate in unconverted oil in a recycle stream. These compounds are virtually impossible to convert by hydrocracking reactions, and they show a strong tendency to give high degrees of concentration in the recycle oil stream. When their concentration increases, the characteristics of the reaction system continuously decrease, leading to economically unjustified conditions.

The objective of the invention is to provide a hydrocracking method by which the conversion of the heaviest and highest molecular weight materials to products increases, leading to a reduced mass yield of unconverted oil.

The next objective of the hydrocracking method is to minimize the need for purification by concentrating with a TNRN of the compounds in the portion of the unconverted oil stream.

These tasks are achieved by a hydrocracking method comprising the steps of:

(a) combining the hydrocarbon feedstock and a heavy bottoms fraction of the distillation of the recycle stream with hydrogen-rich gas to form a mixture containing the hydrocarbon feedstock and hydrogen;

(b) catalytic hydrocracking of a mixture containing hydrocarbon feedstock and hydrogen in a hydrocracking zone to produce a hydrocracking effluent;

(c) separating the hydrocracking effluent into a first vapor portion and a first liquid portion in a separation zone;

(d) heating the first liquid portion to form an evaporated first liquid portion;

(e) feeding the vaporized first liquid portion to the fractionation section, producing individual product fractions including a heavy lower fraction containing unconverted oil in the lower zone of the fractionation section;

(f) recovering a heavy bottom fraction from the fractionation section;

(g) splitting the heavy bottom fraction into a stripping stream and a recycle stream of the bottom bottom fraction;

(h) stripping the stripping stream using stripping means in a countercurrent stripping column to form overhead vapor and distilled liquid;

(i) supplying overhead steam to the fractionation section, to the heavy bottom fraction recirculation stream, or to a location upstream of the fractionation section; and

(j) removing at least a portion of the distilled liquid from the countercurrent stripping column as a total purge of unconverted oil.

In one embodiment, the vaporized first liquid portion is at least 50%, preferably at least 75%, even more preferably at least 85%, and most preferably at least 90%, and a maximum of 95%, preferably a maximum of 90% , even more preferably, a maximum of 85% and most preferably a maximum of 75% evaporated, with associated effects of increasing separation of TPNA and the product in the fractionation zone with an increase in the degree of evaporation and increased energy efficiency and reduction of evaporation, since any recirculated vaporized fractions will undergo an additional phase transition before recirculation.

In one embodiment, a portion of the distilled liquid is recirculated by combining with the stripping stream and directed to the inlet of the countercurrent stripping column, which leads to an increased concentration of TPNA in the total purge.

In one embodiment of the invention, the recirculated portion of the distilled liquid and / or the stripping stream is heated by heat exchange with a heavy bottom fraction, successfully increasing the recovery of waste heat and improving the flow and separation of the liquid in the stripping device.

In a further embodiment of the invention, the stripping stream is heated before the stripping process in order to raise its temperature above its boiling point, in particular above 300 °, preferably above 320 ° C and most preferably above 330 ° C, which further affects the concentration of TPNA, facilitating evaporation of other components.

In a further embodiment of the invention, heat energy is transferred from the heavy bottom fraction to a heat-stripping medium, which allows heat exchange with streams that were not further concentrated in heavy non-converted oil during stripping.

In a further embodiment, the stripping medium is water vapor, preferably medium pressure steam, having a pressure between 1 and 20 bar (excess), more preferably between 3.5 and 10 bar (excess), and most preferably between 3.5 and 6 bar (redundant).

In an embodiment of the invention, the first portion of the vapor contains lighter low molecular weight products and unreacted hydrogen.

Another embodiment of the invention provides, as a heavy bottom fraction, the highest normally boiling fraction from the fractionation section containing hydrocarbon material.

In one embodiment of the invention, improved separation is obtained in a countercurrent stripping column when it contains several equilibrium steps in the form of plates or packing material.

In a further embodiment of the invention, a portion of the heavy bottom fraction is sent to the heavy bottom stream for recycling and combined with the hydrocarbon feed to enter the hydrocracking zone to provide hydrocracking of the unconverted oil.

In an embodiment of the invention, the flow rate of the stripping stream is controlled by a flow control device according to the desired flow rate of the total purge of unconverted oil so that the total purge stream can be optimized.

The hydrocarbon feed may be hydrotreated prior to hydrocracking.

In an embodiment of the invention, part or all of the energy for heating the stripping stream is provided by heat exchange with one or more streams from the hydrocracking process, for example, by reactor waste products, or by heat exchange with an external source of a heating medium, such as high pressure water vapor , hot flue gas from a fire heater, or electric heating.

An embodiment of the invention includes a method where the distilled liquid contains heavy polynuclear aromatic compounds in an amount greater than the amount contained in the heavy bottom fraction recovered from the distillation column, thereby reducing the proportion of unconverted oil in the total purge stream.

In a further embodiment of the invention, the distillation medium from the distillation unit can be added to the fractionation section, which leads to savings in the consumption of the distillation means.

In a further embodiment of the invention, the method further comprises the step of recirculating a portion of the distilled liquid from the countercurrent stripping column and mixing it with a stripping stream to supply it to a countercurrent stripping column with the associated effect of providing an even higher concentration of TPNA in unconverted oil. In this case, it may be necessary to further supply heat to the countercurrent distillation process in order to guarantee the temperature of the liquid above its boiling point during distillation.

In a further embodiment, the TPNA is extracted from the total purge when absorbed by the adsorbent in order to facilitate the return of the total purge to the process with the advantage of increased yield.

Figure 1 illustrates an embodiment of the method according to the invention, in which flow control is used in the distillation stream and a portion of the heavy bottom fraction is recycled.

The disclosed method uses specific process steps to reduce the total purge of unconverted oil from a hydrocracking unit. This reduction can be accomplished by withdrawing the bottom fraction stream from the bottom of the product fractionation section, such as a distillation column, heating it to a temperature substantially above its boiling point, and then stripping it with steam in a countercurrent column with distillation column plates or packing material. In the distillation step at elevated temperature, a substantial amount of the lower fraction stream is vaporized compared to simply distilling the heavy lower fraction at its boiling point without heating. The overhead steam of the heavy bottom fraction can be returned to the fractionation section, for example, to its lower part. The distilled portion of the heavy lower fraction remains liquid, and it is collected in the lower part of the distillation column. This stream has a substantially higher boiling point than the original unconverted oil, and therefore TPNA are concentrated in the heavier liquid in the lower parts, which can then be removed as a total purge from the hydrocracker.

A higher concentration of TPJA in the distilled liquid allows you to remove the desired amount of TPJA at a lower purge rate in the total purge stream. The reduced rate of total purge leads to a higher total conversion in the hydrocracking unit along with an increased yield of valuable distilled products.

The concentration of TPNA in the total purge can even be further increased by recirculation of part of the distilled liquid of the heavy lower fraction to the inlet of the stripping section. The recycled stream may be heated by heat exchange, for example, with a heavy bottom fraction, in order to optimize the heat consumption of this process.

This publication uses a simple method for concentrating TPNA compounds in a portion of an unconverted oil stream and thereby minimizing the required purge flow rate. The required purge flow rate is reduced, essentially leading to a higher conversion and better yields of the final products.

This publication uses specific process steps to reduce the required purge of unconverted oil from a hydrocracker by substantially at least 25 percent and preferably 50 percent or more. This reduction is achieved by extracting the bottom fraction containing unconverted oil in the first purge stream from the fractionation section, heating it substantially above its boiling point and then stripping it with steam in a countercurrent column with trays (distillation column) or packing material. The stripping step vaporizes a substantial amount, such as at least 25 percent and preferably 50 percent or more, of the bottom stream, returning this overhead vapor to the bottom of the fractionation section. The residual stream of the lower fraction remains as a distillation liquid, and it is collected in the lower part of the distillation column. This liquid essentially has a boiling point higher than the original unconverted oil, and due to the very high normal boiling point of the TPJA of the compounds, the physical separation concentrates the TPJA in the heavier bottom liquid, which is then removed as a total purge from the hydrocracker. A higher concentration of TPJA in the distilled liquid helps to remove the desired TPJA at a lower purge flow rate. A reduced purge rate leads to a higher total conversion in the hydrocracking unit along with higher yields of valuable distilled products.

By distillation of the unconverted oil in a separate process step, multiple beneficial effects are obtained. Independent control of temperature and flow becomes possible, which allows optimization of the distillation conditions and enables countercurrent flow, which improves the efficiency of distillation compared to parallel flow.

Reference is made to FIG. 1, which schematically illustrates process flows and equipment configuration, as embodied in this invention.

Fresh feedstocks consisting of hydrocarbon feedstocks, such as petroleum or synthetic heavy gas oils of mineral or biological origin 1, are combined with hydrogen-rich gas 2 and a possible recycle stream of unconverted product 16 and fed to hydrocracking zone 3, consisting of one or more catalysts contained in one or more vessels of the reactor vessels. The catalysts activate the hydroprocessing of the hydrocarbon feed, which may include hydrogenation to slightly hydrocracked off-products. These hydrocracking effluents containing hydrocarbon products, together with excess hydrogen not used in the reaction, exist in hydrocracking zone 4 and are included in separation zone 5, consisting of one or more tanks that separate into a first vapor part and a first liquid part. The first vapor portion 6 from the separation zone can be combined with make-up hydrogen 7 to replenish the hydrogen spent in the reaction. The hydrogen-rich stream may then be compressed in compressor 8 to return back to the hydrocracking zone.

The first liquid part 9 from the separation stage enters the industrial heater 10, which provides energy mainly for the evaporation of the liquid 11 before the product of the fractionation section 12. The fractionation section consists of one or more towers or columns with many equilibrium stages in the form of plates or packing material, which can be operated in counterflow mode. These towers are usually distilled with steam or by boiling again to facilitate evaporation of products. The fractionation section separates the individual product and intermediate fractions 13, 14, such as gasoline, jet fuel and diesel fuel, according to the difference in their normal boiling points. In the lower zone of the fractionation section, the heaviest lower fraction, that is, unconverted oil 15, can be collected and removed from it as an unconverted oil product or returned to the reactor in line 16 as an oil recycle stream for further conversion.

The purpose of the hydrocracking process is to convert all or as much as possible the heaviest and highest molecular weight materials into products, which results in the absence or minimum yield of unconverted oil 15. However, the first purge of the unconverted oil or heavy bottom fraction 17 must be removed from the hydrocracking unit if possible, taking into account flow control 18, in order to avoid an increase in the content of TPNA in the reaction system. In the heavy bottom fraction distillation system, the heavy distillation bottoms stream is directed to an industrial heater 19 so that the temperature of this distillation stream 20 is raised substantially above the boiling point of the distillation stream and the temperature of the bottom fractionation section. This heated stripping stream is then fed to the top of the countercurrent stripping column 21, consisting of a plurality of equilibrium steps in the form of plates or packing material. Water vapor is added to the bottom of the stripping column 22 to facilitate evaporation of the unconverted oil. The steam of the overhead from the top 23 of the stripping column is sent to the lower part of the distillation column 12. The distilled liquid part of the stripping stream, which remains unevaporated in the stripping section, flows into the lower part of this column, and then it is removed from the hydrocracking unit as a total purge of the non-converted oil 24.

The operating conditions in the heavy bottom fraction of the stripping system are set so that the total purge of unreacted oil 24 from the bottom of the stripping section is substantially less than the heavy bottom fraction, that is, the unconverted oil 17 is removed from the stream of the heavy bottom fraction for stripping, while while sufficiently removing the unwanted TPNA.

Reference is made to FIG. 2, which schematically illustrates the processes and equipment configuration of a preferred embodiment of the invention, using the same reference numbers as FIG. 1 for similar elements in a similar function.

Figure 2 shows the flow diagram of the output section of the fractionation. The earlier process elements correspond to those in FIG. 1, as described above.

As indicated, the purpose of the hydrocracking process is to convert all or as much as possible the heaviest and highest molecular weight materials into products, which leads to the absence or minimum total yield of unconverted oil 15. However, the first purge of the unconverted oil or heavy bottom fraction 17 should be removed from the hydrocracking unit, if possible, taking into account flow control 18, in order to avoid an increase in the content of TPNA in the reaction system. In the heavy bottom fraction of the stripping system as described herein, the removed heavy bottom stream is directed as a stripping stream and can be directed to an industrial heater 19 so as to substantially raise the temperature of this stripping stream 20 above the boiling point of the heavy bottom stream fractions for distillation and temperature of the lower fractionation section. This heated stripping stream is then fed to the top of the countercurrent stripping column 21, which consists of a plurality of equilibrium stages in the form of plates or packing material. Water vapor is added to the bottom of the stripping column 22 to facilitate evaporation of the unconverted oil. The overhead steam from the top of the stripping column 23 is sent to the lower part of the fractionation section 12. The liquid to be distilled off from the stripping stream, which remains unevaporated in the stripping section, will flow to the bottom of the column. Part of this distilled liquid is removed from the hydrocracking unit in the form of a total purge of unconverted oil 24, and the other part 25 is recycled to the inlet to the stripping column 22, which can be either the same or different from the inlet through which the stream for distillation from the fractionation section is supplied. 2, the recirculated liquid 27 is heated by heat exchange 26 with a heavy bottom fraction 15 of the fractionation section.

The operating conditions in the heavy bottom fraction of the stripping system are set so that the total purge of unconverted oil 24 from the bottom of the stripping column is substantially less than the heavy bottom fraction, that is, the unconverted oil 17 is removed from the stream of heavy bottom fraction for distillation, while while sufficiently removing the unwanted TPNA.

In the alternative embodiment of FIG. 3, a portion 25 of the stripping liquid 24 is recycled and fed to the top of the stripping column 21 after heating by heat exchange with a heavy bottom fraction stream 24. Heating of this recycled stripping liquid is required due to a temperature drop caused by contact with a large volume of driving away steam. Essentially, thermal energy can be directed to the distilled liquid and unconverted oil without excessive temperature increase above the temperature directed to the distillation column. The advantage of this is the reduction in thermal degradation of unconverted oil compared to the supply of a heavy lower fraction to the distillation column at a higher temperature. Further, in the embodiment of the invention in FIG. 3, overhead steam 23 is sent to a position upstream of the fractionation section 12 and not directly to the fractionation section, which may require less modification in case of retrofitting an existing installation compared to embodiments of the invention, where the upper steam epaulettes are sent directly to the fractionation section 12.

Under certain process conditions, it may be advantageous to avoid directing a high boiling recirculated distilled liquid into the heat exchanger. Therefore, under such process conditions, it may be preferable to use the embodiment of FIG. 4, in which the heat of the heavy bottom fraction 15 is recovered by heat exchange in a heat exchanger 30 with a steam line 22, providing superheated steam 31 that is supplied to the stripping column 21. In this In this case, the residual amount of low-pressure water vapor with a temperature of 170 ° C can be heated to superheated steam with a temperature of 330 ° C while lowering the temperature of the heavy lower fraction by only about 5 ° C.

Depending on the configuration of the hydrotreatment unit and fractionation section, alternative distillation column configurations exist.

In alternative cases, where fractionation section 12 is a vacuum distillation column or is a main fractionation column with a fire reboiler such that it does not work with steam, the TPNA concentrator is not configured to return the produced steam to the fractionation column. In these cases, the TPNA concentrator can be configured with a condenser to condense water vapor and higher hydrocarbons. The overhead water from steam can be reused as wash water, and higher hydrocarbons can be fed to the fractionation column, to a recycle stream, or to a place higher upstream of the fractionation column, such as equalization capacity of the feedstock.

In such alternative embodiments of the invention, the heavy fraction from the fractionation column can still be used to preheat the recirculated distilled liquid stream.

The pressure mode of the stripping column can be configured appropriately, for example, to operate under vacuum or low pressure if necessary by connecting to a vacuum system and using only a small amount of low pressure water vapor to distill off the unconverted oil.

In alternative embodiments of the invention, methane or other gases can also be used as an alternative to steam as a stripping means.

Further alternative supply points for the overhead steam from the stripping column may include any position upstream of the fractionation section, including entering the process heater 10.

To further optimize the yield, it is also possible to recover TPNA by absorbing a layer of activated carbon or another absorbent, as disclosed in US Pat. No. 4,447,315. Such a layer will work especially well in the case of a high concentration of TPNA in the purge stream, since the size of the layer may be smaller. The operation may include the operation of two alternating parallel layers so that one layer can be updated or replaced without interrupting the installation.

Examples

Example 1

To test the potential TPNA cleavage in the proposed invention, a hydrocracked unconverted oil sample obtained from a commercially available hydrocracker with the properties shown in Table 1 was distilled in ASTM D-1160. Since this unit does not use a reverse flow, it generates a physical separation with a substantial overlap between the overhead and the bottom product and corresponds well to the vapor / liquid separation in a simple steam distillation unit.

Table 1 Convert Oil Sample Properties Specific gravity 0.844 Heavy Polynuclear Aromatic Compounds Coronen the weight. million shares 394 1-methylcoronene the weight. million shares 132 Naftokoronen the weight. million shares 127 Ovalen the weight. million shares 91 Total TPJA the weight. million shares 744 Distillation Boiling point ° C 342 10% ° C 397 fifty% ° C 451 90% ° C 513 Boiling point ° C 572

Two laboratory distillations were carried out using the ASTM D-1160 method and apparatus, the first yield of the lower fraction was 50 percent by volume of the initial charge, and the second yield of the lower fraction was only 20 percent by volume of this charge, in the document how TPNA could be distributed in the upper and lower fractions. The results of the analysis of TPNA and analysis of distillation for both the lower fraction and the vapor fractions of the overhead are shown in table 2.

table 2 Distilled fraction properties Happening I II Fraction Lower Distillate Lower Distillate Exit about.% fifty fifty twenty 80 Specific gravity 0.849 0.838 0.855 0.840 Heavy Polynuclear Aromatic Compounds Coronen v.m.d 650 105 775 245 1-methylcoronene n.m.d. 240 twenty 385 55 Naftokoronen n.m.d. 235 <5 565 <5 Ovalen n.m.d. 175 <5 475 <5 Total TPJA n.m.d. 1300 130 2200 305 The beginning Boiling point ° C 406 288 440 338 10% ° C 439 380 473 391 fifty% ° C 479 426 510 441 90% ° C 531 463 550 483 The end. the point is boiled. ° C 583 511 596 529

These results clearly show that the distillation of ASTM achieves a significant separation of TPNA between the overhead distillate and the bottom fraction. This is a consequence of the very low volatility of TPNA compounds. In a hydrocracking unit, it is necessary to blow enough TPNA from the system in order to maintain the equilibrium of the total TPNA production by the reaction. In this example, case I leads to an increase in the total concentration of TPNA by a factor from 744 million shares by weight to 1300 million shares by weight or 175 percent. Case II leads to an increase in the total TNLA by a factor of 744 million shares by weight to 2,200 million shares by weight or 295 percent.

Example 2

The implementation of the invention was evaluated based on the installation for stripping with steam under the conditions shown in table 3 below.

Table 3 Process conditions for a steam stripping column Perfect plates four Steam Stripping Speed (22) kg / h 3243 Pressure at the top of the column bar hut 1.30 Bottom pressure bar hut 1.36

The experiments of this method were carried out at two different feed temperatures to the stripping column, 350 ° C and 380 ° C, to show the separation of the overhead vapor and the lower liquid products. The TPNA coronene molecule was also included in the experiment to show how the equilibrium of vapor and liquid could determine the distribution of the lightest TPNA samples. The results are presented based on the temperature of the feed to the distillation column 350 ° C in table 4 below. At this feed temperature, 50 percent by weight of the overhead is distilled and 50 percent by weight is returned to the lower liquid product. The coronene component was concentrated in the lower part of the stripping column from 461 million shares by weight with a feed of up to 691 million shares by weight in the lower parts, which corresponds to 150 percent.

Table 4 Submission to the distillation column and product speed and properties Flow description Flow to distillation columns Stripping liquid Overhead steam Stream number twenty 24 23 Flow temperature ° C 350 209 312 Yield (% of feed) The weight.% one hundred fifty fifty Heavy Polynuclear Aromatic Compounds Coronen The weight. million d 461 691 231 Distillate Temp. boiling ° C 300 340 282 10% ° C 360 393 344 fifty% ° C 427 447 407 90% ° C 483 563 455 Pace. end boiling ° C 560 563 511

The results of the stripping column based on a feed temperature of 380 ° C are presented in table 5 below. At this feed temperature, 64 percent by weight of the overhead is distilled off and 36 percent is returned to the liquid bottom. The coronene component was concentrated in the lower part of the distillation column from 466 million shares by weight with a feed of up to 727 million shares by weight in the lower part, which corresponds to 156 percent. Most TPNA molecules in a hydrocracking unit are actually heavier and less volatile than coronene, and it can be expected that they will further be concentrated in the flow of the lower part of the stripping section.

Table 5 Submission to the stripping section, speeds and properties of the product Flow description Flow to the stripping section Stripping liquid Overhead steam Stream number twenty 24 23 Flow temperature ° C 380 195 325 Yield (% of feed) The weight.% one hundred 36 64 Heavy Polynuclear Aromatic Compounds Coronen The weight. million d 466 727 319 Distillate Pace. beg. boiling ° C 300 346 288 10% ° C 360 398 350 fifty% ° C 427 454 414 90% ° C 483 515 462 Pace. end boiling ° C 560 554 524

Example 3

An embodiment of the invention based on recirculation of the bottom of the stripping column in the same amount as the feed stream and heating to the same temperature of 350 ° C are shown in Table 6. A comparison of the distillation curve of the total purge stream 24 in Table 4 and Table 6 shows that recirculation of the part of the outlet from the stripping column increases the amount of high-boiling products in the total purge, that is, the temperature of 10% of the highest-boiling product increases from 505 ° C to 527 ° C. With this higher degree of concentration, it can be understood from Table 6 that the concentration of coronene in the overhead pair 23 is only slightly lower than that in the heavy bottom fraction 15, which indicates that most of these traces of TPNA evaporate into the overhead vapor fraction. However, other TPNA compounds, which are heavier and boiling at a higher temperature than coronene, would be predominantly concentrated in the heavy fraction of the lower part and would be removed from the system.

Table 6 Feeding to the stripping section, speeds and properties of the product with an alternative configuration of lower parts recycling Flow description Flow to the stripping section Stripping liquid Overhead steam Stream number twenty 27 24 23 Flow temperature ° C 350 350 254 326 Yield (% of The weight.% one hundred one hundred twenty 80 filing) Heavy multi-core aroma Coronen The weight. million d 470 720 720 408 Distillate N.t.k. ° C 301 376 376 295 10% ° C 361 415 350 355 fifty% ° C 428 472 472 419 90% ° C 484 527 527 465 K.t.k. ° C 527 554 554 488

These results demonstrate that under acceptable and practical conditions of temperature, pressure and flow rate, the unconverted oil stream can be split by steam stripping and reduced to the concentration of TPNA compounds in the lower liquid stream. This concentration will lead to reduced rates of total purge from the hydrocracking unit and corresponding increased conversions and yields of distilled products.

An example of improved conversion compared to the case with a total purge equal to three volume percent of the hydrocarbon supply, compared with the case with a total purge equal to 0.6 volume percent of the hydrocarbon supply, is shown in Table 7. The production of naphtha, kerosene and diesel fuel increases from 107 , 45 to 109.84 percent by volume of hydrocarbon supply.

Table 7 Improved yield due to purge stripping Yields in vol.% Of feed No purge stripping With distillation total. purge Naphtha 23.42 23.94 Kerosene 54.42 55.63 Diesel fuel 29.61 30.27 Total purge of unconverted oil 3.0 0.60 Ligroin + kerosene + diesel 107.45 109.84

Claims (14)

1. A hydrocracking method comprising the steps of:
(a) combining the hydrocarbon feedstock and a heavy bottoms fraction of the distillation of the recycle stream with hydrogen-rich gas to form a mixture containing the hydrocarbon feedstock and hydrogen;
(b) catalytic hydrocracking of a mixture containing hydrocarbon feedstock and hydrogen in a hydrocracking zone to produce a hydrocracking effluent;
(c) separating the hydrocracking effluent into a first vapor portion and a first liquid portion in a separation zone;
(d) heating the first liquid portion to form an evaporated first liquid portion;
(e) feeding the vaporized first liquid portion to the fractionation section, producing individual product fractions including a heavy lower fraction containing unconverted oil in the lower zone of the fractionation section;
(f) recovering a heavy bottom fraction from the fractionation section;
(g) separating the heavy bottom fraction into a stripping stream and a bottom fraction recycle stream;
(h) flow directions for stripping as a first stream, stripping means as a second stream and a recirculated portion of the distilled liquid as an optional third stream to a countercurrent stripping column and recovering overhead vapor and distilled liquid from said stripping column;
(i) supplying overhead steam to the fractionation section, to the heavy bottom fraction recirculation stream, or to a location upstream of the fractionation section; and
(j) removing at least a portion of the distilled liquid from the countercurrent stripping column as a total purge of unconverted oil,
characterized in that it further comprises a step
(k) transferring thermal energy to one of said first stream, second stream or optional third stream before directing said stream to a countercurrent stripping column. 2. The method of claim 1, wherein the vaporized first liquid portion is at least 50% vaporized, preferably 50-95% vaporized, even more preferably 75-95% vaporized, and most preferably 75-90% vaporized. 3. The method according to p. 1, where part of the distilled liquid is recycled, combined with the flow for distillation, and sent to the entrance to the countercurrent stripping column. 4. The method according to p. 3, where the recirculated portion of the distilled liquid and / or stream for distillation is heated by heat exchange with a heavy bottom fraction. 5. The method of claim 1, wherein the stripping stream is heated prior to the stripping process to raise its temperature above its boiling point, in particular above 300 °, preferably above 320 ° C and most preferably above 330 ° C. 6. The method according to p. 1, where the thermal energy is transferred from the heavy lower fraction to the distillation means through heat transfer. 7. The method of claim 1, wherein the stripping means is water vapor, preferably medium pressure steam, having a pressure between 1 and 20 bar (excess), more preferably between 3.5 and 10 bar (excess), and most preferably between 3, 5 and 6 bar (excess). 8. The method according to p. 1, where the countercurrent stripping column contains many stages of equilibrium in the form of plates or packing material. 9. The method according to claim 1, where the flow rate for distillation is controlled by a flow control device according to the desired flow rate of the total purge of unconverted oil. 10. The method according to p. 1, where the hydrocarbon feedstock is subjected to hydrotreatment before hydrocracking. 11. The method of claim 1, wherein part or all of the energy for heating the distillation stream is provided by heat exchange with one or more streams from a hydrocracking process. 12. The method of claim 1, wherein the heating of the stripping stream is provided from one or more heat sources taken from the group consisting of a stream exiting the reactor, an external source of heating medium, high pressure water vapor, hot flue gas from a fire heater, and electric heating up. 13. The method of claim 1, wherein the output of the stripping means from the stripping unit is added to the fractionation section. 14. The method according to any one of paragraphs. 1-13, where heavy polynuclear aromatic compounds are extracted from the total purge when absorbed by the adsorbent.

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