WO2012052042A1

WO2012052042A1 - Process for hydrocracking a hydrocarbon feedstock

Info

Publication number: WO2012052042A1
Application number: PCT/EP2010/006411
Authority: WO
Inventors: Gordon Gongngai Low; Michael Glenn Hunter
Original assignee: Haldor Topsoe AS
Current assignee: Topsoe AS
Priority date: 2010-10-20
Filing date: 2010-10-20
Publication date: 2012-04-26
Anticipated expiration: 2013-04-20

Abstract

A process for hydrocracking a hydrocarbonaceous feedstock comprising the steps of : (a) combining the hydrocarbonaceous feedstock with a hydrogen-rich gas to obtain a mixture comprising hydrocarbonaceous feedstock and hydrogen; (b) catalytically hydrocracking the mixture comprising hydrocarbonaceous feedstock and hydrogen in a hydrocracking zone to obtain a hydrocracked effluent; (c) separating all the hydrocracked effluent into a first vapour portion and a first liquid portion in a separation zone; (d) heating the first liquid portion to form a vapourised first liquid portion; (e) feeding the vapourised first liquid portion to a fractionation section to produce individual product fractions including a heavy bottom fraction comprising unconverted oil at the bottom zone of the fractionation section; (f) withdrawing from the fractionation column the heavy bottom fraction (g) directing a stream of heavy bottom fraction for stripping (h) stripping the stream of heavy bottom fraction for stripping with steam, in a counter current stripping column to form an overhead vapour of the heavy bottom fraction and a stripped liquid portion of the heavy bottom fraction; (i) feeding the overhead vapour of the heavy bottom fraction to the fractionation section; and (j) removing the stripped liquid portion of the heavy bottom fraction from the bottom of the counter current stripping column as a net purge of unconverted oil.

Description

Title: Process for hydrocracking a hydrocarbon feedstock

The invention relates to a process for hydrocracking a hydrocarbon feedstock to obtain a more valuable lower boiling products such as liquefied petroleum gas ( LPG ) , naphtha, kerosene, and diesel. In particular, the invention concerns a process whereby heavy polynuclear aromatic compounds are concentrated in a portion of the unconverted oil so they can be removed resulting in increased conversion and yield of products.

The complete conversion of petroleum or synthetic heavy gas oils to distillate products such as gasoline, jet and diesel fuel in a hydrocracker is practically limited by the formation of .heavy polynuclear aromatic ( HPNA ) compounds. These compounds, formed by undesired side reactions, are stable and virtually impossible to hydrocrack. HPNA are fused polycyclic aromatic compounds having 7+ rings for example coronenes C₂ H₁₂, benzocoronenes C₂₈Hi , dibenzocorone- nes C₃₂Hi₆ and ovalenes C₃₂Hi₄ .

HPNA with 7+ aromatic rings are byproducts of hydrocracking reactions that can potentially cause significant problems in hydrocracking units. When the solubility limit for the HPNA is exceeded, solids form in transfer lines, valves and on heat exchanger surfaces. Furthermore the HPNA can contribute to catalyst deactivation by reversible inhibition and coke formation. HPNA problems particularly occur when processing heavy feedstocks with high distillation end- points and more aromatic cracked stocks in high conversion recycle units. Consequently, HPNA build up to high levels in the recycle streams normally employed in high conversion processes, resulting in fouling of the catalysts and equipment. The conventional solution to this problem is to remove a portion of the recycle oil stream as an unconverted oil stream to purge the HPNA compounds from the system, effectively balancing the HPNA purge rate . Ah the rate of their formation by reactions. This approach limits the total conversion level achievable in the hydrocracker .

In a conventional high conversion hydrocracking process, a hydrocarbonaceous heavy gas oil feedstock is combined with a hydrogen-rich gas and reacted over catalyst to obtain a hydrocracked effluent comprising of less dense, lower molecular weight products. The hydrocracked effluent from the reactor is condensed and separated in a separation zone into a liquid portion comprising hydrocarbons and a vapour portion comprising un-reacted hydrogen. The vapour from this separation may be combined with hydrogen makeup to account for hydrogen consumed by reaction and may then be compressed and re-circulated back to the reactor vessel. The first liquid portion from the separation zone is then directed to a fractionation section where the lighter prod- ucts are distilled from the heavy unconverted products in a fractionation section e.g. a fractionation tower or a series of fractionation towers. Heat is normally input to this recovery operation in order to provide the necessary energy for separation.

The conventional approach to controlling the buildup of HPNA compounds in the recycle oil is to withdraw a purge of recycle oil product from the unit as unconverted oil. The purge rate may be adjusted so as to balance the rejection of HPNA with the net production. Such a purge essentially reduces the achievable total conversion level by hydro- cracking to less than 100 percent. Depending on the feed quality and process conditions the purge rate can be from one or two percent up to as high as 10 percent of the fresh feed rate. The yield of valuable distillate products are correspondingly reduced at substantial economic loss to the refiner.

U.S. patent no. 6,361,683 discloses a hydrocracking process whereby the hydrocracked effluent is hydrogen stripped in a stripping zone to produce a gaseous hydrocarbonaceous stream which is passed through a post-treatment hydrogena- tion zone to saturate aromatic compounds. The fractionation zone is associated with a stripping zone which is fed with stripped hydrocarbonaceous liquid obtained by stripping the hydrocracked effluent. Stripping to remove HPNA is also considered.

U.S. patent no. 6,858,128 discloses a hydrocracking process which utilises a fractionation zone that is divided to include sections suitable for steam stripping to concentrate HPNA.

U.S. patent no. 5,120,427 discloses a hydrocracking process in which a stripping column is provided as a stub column at the bottom of the fractionation zone, for recovering a ma- jority of light hydrocarbons, while enabling a purge of a liquid hydrocarbonaceous stream rich in HPNA. There is substantial economic incentive to maximize the conversion of the heavy feed and a key feature of most such processes is the recycle of unconverted oil back to the reaction system thereby controlling the cracking severity and improving the selectivity of the hydrocracking reactions to more desirable end products such as gasoline, jet fuel and diesel fuel. All known hydrocracking processes and catalysts are, however, subject to undesirable side reactions leading to the formation of heavy poly-nuclear aromatic (HPNA) compounds which accumulate in the unconverted oil recycle stream. These compounds are virtually impossible to convert by hydrocracking reactions and show a strong tendency to build up to high concentration levels in the recycle oil stream. As the concentration builds up, the performance of the reactor system is continuously degraded leading to uneconomic conditions.

It is an objective of the invention to provide a hydrocracking process whereby conversion of the heaviest and highest molecular weight materials into products is increased, resulting in reduced net yield of unconverted oil.

It is a further objective of the hydrocracking process to minimize the need for purge by concentrating the HPNA compounds in a portion of the unconverted oil stream.

These objectives are achieved by a hydrocracking process comprising the steps of:

(a) combining the hydrocarbonaceous feedstock with a hydrogen-rich gas to obtain a mixture comprising hydrocarbonaceous feedstock and hydrogen; (b) catalytically hydrocracking the mixture comprising hy- drocarbonaceous feedstock and hydrogen in a hydrocracking zone to obtain a hydrocracked effluent;

(c) separating the hydrocracked effluent into a first va- pour portion and a first liquid portion in a separation zone ;

(d) heating the first liquid portion to form a vapourised first liquid portion;

(e) feeding the vapourised first liquid portion to a frac- tionation section to produce individual product fractions including a heavy bottom fraction comprising unconverted oil at the bottom zone of the fractionation section;

(f) withdrawing from the fractionation section the heavy bottom fraction;

(g) directing a stream of the heavy bottom fraction for stripping;

(h) stripping the stream of the heavy bottom fraction in a counter current stripping column to form an overhead vapour of the heavy bottom fraction and a stripped liquid portion of the heavy bottom fraction;

(i) feeding the overhead vapour of the heavy bottom fraction to the fractionation section; and

(j) removing the stripped liquid portion of the heavy bottom fraction from the bottom of the counter current strip- ping column as a net purge of unconverted oil.

In a further embodiment, the stream of heavy bottom fraction for stripping is heated prior to the stripping process to raise its temperature above its bubble point concentrat- ing the HPNA even further, by facilitating the evaporation of other constituents. In an embodiment the first vapour portion comprises lighter low molecular weight products and unconverted hydrogen.

Another embodiment provides as the heavy bottom fraction the highest normal boiling fraction from the fractionation section, comprising hydrocarbonaceous material

In one embodiment improved separation is obtained in the counter current stripping column as it comprises multiple equilibrium stages in the form of trays or packing material .

In a further embodiment a part of the heavy bottom fraction is directed into a stream of heavy bottom fraction for re- cycling being combined with the hydrocarbonaceous feedstock for being input to the hydrocracking zone, to provide hy- drocracking of unconverted oil.

In an embodiment the flow rate of the stream of heavy bot- torn fraction for stripping is controlled by a flow control unit according to a desired flow rate of the net purge of unconverted oil, such that the net purge flow may be optimised. The hydrocarbonaceous feedstock may be hydrotreated prior to hydrocracking.

In an embodiment the energy for heating of the heavy bottom fraction for stripping is provided from heat exchange with one or more streams from the hydrocracking process e.g. a reactor effluent, or from heat exchange with an external source of heating medium such as high pressure steam, hot flue gas from a fired heater, or by electrical heating.

An embodiment involves a process wherein the stripped liq- uid of heavy unconverted oil comprises heavy polynuclear aromatic compounds in an amount larger than the amount comprised in the heavy bottom fraction withdrawn from the fractionation column, thus reducing the share of unconverted oil in the net purge stream.

In a further embodiment steam output from the stripper may be added to the fractionation section, resulting in a saving of steam consumption. Fig. 1 illustrates an embodiment of the process according to the invention in which flow control is employed on a heavy bottom fraction for stripping and a part of the heavy bottom fraction is recycled. The disclosed process utilizes specific process steps to reduce the net purge of unconverted oil from a hydro- cracker. This reduction may be accomplished by taking the bottom fraction stream from the bottom of the product fractionation section such as a fractionation column, heating it substantially above its bubble point and then stripping with steam in a counter-current column with fractionating trays or packing material. The stripping step at elevated temperature vapourizes a substantial amount of the bottom fraction stream compared to simply stripping the heavy bot- torn fraction at its bubble point without heating. The overhead vapour of the heavy bottom fraction may be returned to the fractionation section e.g. at the bottom. The stripped part of the heavy bottom fraction remains a liquid and is collected in the bottom of the stripping tower. This stream is having a substantially higher boiling point than the original unconverted oil and therefore HPNA is concentrated in the heavier bottoms liquid which may then be removed as net purge from the hydrocracker.

The higher concentration of HPNA in the stripped liquid allows the removal of the required HPNA at lower purge rate. The reduced purge rate results in higher total conversion in the hydrocracker together with increased yields of valuable distillate products.

This disclosure provides a simple process for concentrating the HPNA compounds in a portion of the unconverted oil stream and thereby minimizing the required purge flow rate. The required purge flow rate is reduced substantially leading to higher conversion and better yields of final products .

The disclosure utilizes specific process steps to reduce the required purge of unconverted oil from the hydrocracker substantially, such as at least 25 percent and preferably by 50 percent or more. This reduction is accomplished by withdrawing a bottom fraction comprising unconverted oil in a first purge stream from the fractionation section, heating it substantially above its bubble point and then stripping with steam in a counter-current column with fractionating trays or packing material. The stripping step va- pourizes a substantial amount, such as at least 25 percent and preferably by 50 percent or more of the bottom fraction stream returning the vapour to the bottom of the fractiona- tion section. The remainder of the bottom fraction stream remains as liquid and is collected in the bottom of the stripping tower. This liquid is substantially higher boiling than the original unconverted oil and as a consequence of the very high normal boiling point of the HPNA compounds, the physical separation concentrates the HPNA in the heavier bottoms liquid which is then removed as net purge from the hydrocracker . The higher concentration of HPNA in the stripped liquid allows the removal of the required HPNA at lower purge flow rate. The reduced purge rate results in higher total conversion in the hydrocracker together with increased yields of valuable distillate products .

By providing the stripping of the unconverted oil in a separate process step, multiple advantageous effects are obtained. An independent temperature and flow control is made possible, which allows an optimisation of the stripping conditions, and counter current flow is enabled, which has a better stripping efficiency compared to co-current flow .

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

Fresh feedstock consisting of a hydrocarbonaceous feed, such as petroleum or synthetic heavy gas oils of mineral or biological origin 1 is combined with hydrogen rich gas 2 and an optional recycle stream of unconverted product 16 and fed to a hydrocracking zone 3 consisting of one or more catalysts contained in one or more reaction vessels. The catalysts promote the hydroconversion of the hydrocarbona- ceous feedstock, which may include hydrogenation to a lighter hydrocracked effluent. The hydrocracking effluent, comprising hydrocarbon products together with excess hydrogen not consumed by the reaction exits the hydrocracking zone 4 and enters a separation zone 5 consisting of one or more vessels that perform separation into a first vapour portion and a first liquid portion. The first vapour portion 6 from the separation zone may be combined with makeup hydrogen 7 to replenish the hydrogen consumed by reaction. The hydrogen rich stream may then be compressed in compressor 8 for recycle back to the hydrocracking zone.

The first liquid portion 9 from the separation step passes to a process heater 10 supplying energy for substantially vapourizing the fluid 11 before feeding the product fractionation section 12. The fractionation section consists of one or more towers or columns with multiple equilibrium stages in the form of trays or packing material which may be operated in counter-current. The towers are normally stripped with steam or reboiled to facilitate vapourization of the products. The fractionation section performs the separation of individual product and intermediate fractions 13, 14 such as gasoline, jet fuel and diesel fuel according to differences in their normal boiling points. At the bottom zone of the fractionation section the heaviest bottom fraction, i.e. unconverted oil 15, may be collected and withdrawn as an unconverted oil product or returned to the reactor in line 16 as a recycle oil stream for further conversion . The aim of a hydrocracking process is to convert all or as much of the heaviest and highest molecular weight materials into products resulting in no or a minimal net yield of unconverted oil 15. However, a first purge of unconverted oil or heavy bottom fraction 17 must be withdrawn from the hy- drocracker possibly on flow control 18 in order to avoid a build-up of HPNA within the reaction system. In a heavy bottom fraction stripping system, the heavy bottom fraction stream for stripping is routed to a process heater 19 such that the temperature of the heated heavy bottom fraction stream for stripping 20 is raised substantially above the bubble point of the heavy bottom fraction stream for stripping and of the temperature of the fractionation section bottom. This heated heavy bottom fraction stream for strip- ping is then fed to the top of a counter-current stripping tower 21 consisting of multiple equilibrium stages in the form of trays or packing material. Steam is added to the bottom of the stripping tower 22 to facilitate vapouriza- tion of the unconverted oil. The overhead vapour from the top of the stripping tower 23 is routed to the bottom of the fractionating column 12. The stripped heavy bottom fraction stream for stripping which is not vapourized in the stripper flows to the bottom of the tower and is then removed from the hydrocracker as a net purge (a necessary purge) of unconverted oil 24.

The operating conditions in the heavy bottom fraction stripping system are established such that the net purge of unconverted oil 24 from the bottom of the stripper is sub- stantially less than the heavy bottom fraction, i.e. unconverted oil 17 removed from the heavy bottom fraction stream for stripping, while sufficiently removing the undesired HPNA.

EXAMPLES

EXAMPLE 1

In order to test the potential split of HPNA in the proposed invention, a sample of hydrocracked unconverted oil obtained from a commercially operating hydrocracking plant with the properties shown in Table 1 was distilled in an

AST D-1160 apparatus. Since this apparatus does not utilize reflux it generates a physical separation with substantial overlap between the overhead and bottoms product and correspond well to the vapour/liquid separation in a simple steam stripper.

Table 1

Properties of Unconverted Oil Sample

Specific Gravity 0.844

Heavy Poly-Nuclear Aro- matics

Coronene wtppm 394

1-MethylCoronene wtppm 132

NaphCoronene wtppm 127

Ovalene wtppm 91

Total HPNA wtppm 744

Distillation

Initial Boiling Point °C 342

10% °C 397

50% °C 451

90% °C 513

Final Boiling Point °C 572

Two laboratory distillations were performed using the ASTM D-1160 method and apparatus, the first yielding a bottoms fraction of 50 volume percent of the initial charge and a second yielding a bottoms fraction of only 20 volume percent of the charge, to document how the HPNA would partition in the overhead and bottoms fractions. The results of HPNA analysis and distillation analysis on both the bottom fraction and the overhead vapour fractions are summarized in Table 2. Table 2

Properties of Distilled Fractions

Case I II

Fraction BotDistil BotDistil toms late toms late

Yield %vol. 50 50 20 80

Specific Gravity 0.849 0.838 0.855 0.840

Heavy Poly-Nuclear

Aromatics

Coronene wtppm 650 105 775 245

1-MethylCoronene wtppm 240 20 385 55

NaphCoronene wtppm 235 <5 565 <5

Ovalene wtppm 175 <5 475 <5

Total HPNA wtppm 1300 130 2200 305

Initial Boiling Point °C 406 288 440 338

10% °C 439 380 473 391

50% °C 479 426 510 441

90% °C 531 463 550 483

Final Boiling °C 583 511 596 529

Point

These results clearly show that the ASTM distillation has achieved a substantial separation of HPNA between the overhead distillate and bottoms fraction. This is a conse^¬ quence of the very low volatility of the HPNA compounds. In a hydrocracker , it is necessary to purge sufficient HPNA from the system to balance the net production of HPNA by reaction. In this example, Case I results in an increase of the total HPNA concentration by a factor of from 744 ppmwt to 1300 ppmwt or 175 percent. Case II results in an increase of total HPNA by a factor of from 744 ppmwt to 2200 ppmwt or 295 percent.

EXAMPLE 2

Performance of the invention was evaluated based on a steam stripper under the conditions shown in Table 3 below.

Table 3

Process Conditions for Steam Stripping Column

Theoretical Trays 4

Stripping Steam Rate kg/hr 3243

(22)

Column Top Pressure barg 1.30

Column Bottom Presbarg 1.36

sure

Process experiments were performed at two different stripper feed temperatures, 350°C and 380°C to illustrate the split of overhead vapour and bottoms liquid products.

Coronene HPNA molecule was also included in the experiment to show how the vapour-liquid equilibria would predict the distribution of the lightest HPNA species. The results based on 350°C stripper feed temperature are presented in Table 4 below. At this feed temperature, 50 weight percent is distilled overhead and 50 percent is recovered in the bottoms liquid product. The coronene component has been concentrated in the stripper bottoms from 461 ppmwt in the feed to by 691 ppmwt in the bottoms corresponding to 150 percent .

Table 4

Stripper Feed and Product Rates and Properties

Stream Description Feed Bottoms Distillate

Stream No. 20 24 23

Stream Temperature °C 350 209 312

Yield (% of Feed) %wt . 100 50 50

Heavy Poly-Nuclear

Aromatics

Coronene t ppm 461 691 231 Distillation

IBP °C 300 340 282

10% °C 360 393 344

50% °C 427 447 407

90% °C 483 505 455

FBP °C 560 563 511

The stripper results based on 380°C stripper feed temperature are presented in Table 5 below. At this feed temperature, 64 weight percent is distilled overhead and 36 percent is recovered in the bottoms liquid product. The coronene component has been concentrated in the stripper bottoms from 466 ppmwt in the feed to 727 ppmwt in the bottoms corresponding to 156 percent. Most of the HPNA molecules of concern in hydrocracker are in fact heavier and less volatile than coronene and can be expected to further concentrate in the stripper bottoms stream.

Table 5

Stripper Feed and Product Rates and Properties

Stream Description Feed Bottoms

late

Stream No. 20 24 23

Stream Temperature 380 195 325

Yield (% of Feed) %wt 100 36 64

Heavy Poly-Nuclear

Aromatics

Coronene Wt 466 727 319 ppm

Distillation

IBP °C 300 346 288 10% °C 360 398 350 50% °C 427 454 414 90% °C 483 515 462 FBP °C 560 554 524

These results demonstrate that under reasonable and practical conditions of temperature, pressure and flow rate, the unconverted oil stream can be split by steam stripping and result in the concentration of HPNA compounds in a bottoms liquid stream. This concentration will lead to decreased net purge rates from the hydrocracker and corre- sponding increased conversion and yields of distillate products .

Claims

1. A hydrocracking process comprising the steps of:

(a) combining the hydrocarbonaceous feedstock with a hy- drogen-rich gas to obtain a mixture comprising hydrocarbonaceous feedstock and hydrogen;

(b) catalytically hydrocracking the mixture comprising hydrocarbonaceous feedstock and hydrogen in a hydrocracking zone to obtain a hydrocracked effluent;

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

(d) heating the first liquid portion to form a vapourised first liquid portion;

(e) . feeding the vapourised first liquid portion to a fractionation section to produce individual product fractions including a heavy bottom fraction comprising unconverted oil at the bottom zone of the fractionation section;

(f) withdrawing from the fractionation section the heavy bottom fraction;

(g) directing a stream of the heavy bottom fraction for stripping;

(j) removing the stripped liquid portion of the heavy bot- tom fraction from the bottom of the counter current stripping column as a net purge of unconverted oil.

2. Process according to claim 1, wherein the stream of heavy bottom fraction for stripping is heated prior to the stripping process to raise its temperature above its bubble point .

3. Process according to claims 1 or 2, wherein the first vapour portion comprises lighter low molecular weight products and unconverted hydrogen.

4. Process according to claims 1, 2 or 3, wherein the heavy bottom fraction is the highest normal boiling fraction from the fractionation section, comprising hydrocarbonaceous material

5. Process according to anyone of claims 1 to 4, wherein the counter current stripping column comprises multiple equilibrium stages in the form of trays or packing material .

6. Process according to anyone of claims 1 to 5, wherein a part of the heavy bottom fraction is directed into a stream of heavy bottom fraction for recycling to be combined with the hydrocarbonaceous feedstock for being input to the hydrocracking zone.

7. Process according to claim 6, wherein the flow rate of the stream of heavy bottom fraction for stripping is controlled by a flow control unit according to a desired flow rate of the net purge of unconverted oil.

8. Process according to anyone claims 1 to 7, wherein the hydrocarbonaceous feedstock hydrotreated prior to hydrocracking .

9. Process according to anyone of claims 2 to 8, wherein the heating of the heavy bottom fraction for stripping is provided from heat exchange with one or more streams from the hydrocracking process

10. Process according to anyone of claims 2 to 8 wherein the heating of the heavy bottom fraction for stripping is provided from heat exchange with a reactor effluent, an external source of heating medium, high pressure steam, hot flue gas from a fired heater, or by electrically heating.

11. Process according to anyone of claims 1 to 10, wherein steam output from the stripping is added to the fractionation column.