METHOD FOR THE VACUUM DISTILLATION OF A HYDROCARBON FEEDSTOCK AND ASSOCIATED FACILITY

Abstract

The method of vacuum distillation of a hydrocarbon feedstock comprises the following steps: heating the feedstock (12); introducing the feedstock into a flash zone of a vacuum distillation column; removal of at least one distillate stream from the vacuum distillation column at an intermediate level; withdrawing a column head stream at the head of the vacuum distillation column; partially condensing the column head stream to recover a liquid flow and a gaseous flow; passing the gaseous flow through a vacuum generator and recovering at least one condensate and a gaseous fraction of non-condensable gas. It is characterized in that the method comprises introducing into the vacuum distillation column a flow of a feedstock-stripping fluid, wherein the stripping fluid comprises a hydrocarbon mixture having a weighted average boiling temperature (Tmav) of between 150.degree. C. and 250.degree. C., preferably between 190.degree. C. and 210.degree. C.

Description

[0001] The present invention relates to a method for the vacuum distillation of a hydrocarbon feedstock, comprising the following steps:

[0002] heating the feedstock;

[0003] introducing the feedstock into a flash zone of a vacuum distillation column;

[0004] removing at least one distillate stream at an intermediate level from the vacuum distillation column;

[0005] drawing a column head stream at the top of the vacuum distillation column;

[0006] partially condensing the column head stream to recover a liquid flow and a gaseous flow;

[0007] conveying the gaseous flow in a vacuum generation unit and recovering at least one condensate and a non-condensable gas.

[0008] Such a method is intended, in particular, for the distillation of a hydrocarbon feedstock comprising heavy compounds having high boiling points. In particular, the method is intended for the distillation of a feedstock resulting from the atmospheric distillation of crude oil.

[0009] Crude oil refining generally comprises atmospheric distillation in which temperatures are maintained below 370.degree. C.-380.degree. C. to prevent high molecular weight components from being thermally cracked and forming petroleum coke.

[0010] The formation of coke is particularly undesirable and results, in particular, in the fouling of the tubes in the furnace that is used to heat the feedstock of the distillation column.

[0011] Due to the limitation of the heating temperature, atmospheric distillation produces a residual oil which is collected at the lower part of the atmospheric distillation column. This oil comprises hydrocarbons which generally have boiling points above 350.degree. C.

[0012] This oil is then distilled under vacuum to remove recoverable distillates. For this purpose, the vacuum distillation is operated at very low pressures lying generally between 13 mbar and 133 mbar (10 mmHg to 100 mmHg) in order to limit the exit temperature of the furnace and, consequently, to limit the risk of cracking and coke formation upon lowering the required exit temperatures.

[0013] To achieve this level of vacuum, a vacuum generation unit comprising several stages of steam jet ejectors installed in series may be used. These ejectors represent a significant consumption of motive steam.

[0014] Moreover, at this level of vacuum, the condensation of light hydrocarbons and water vapor recovered at the head of the column represents a very high consumption of cooling water.

[0015] To facilitate distillation, a steam-based stripping flow is introduced into the distillation column under the feedstock at the bottom of the distillation column. This stripping flow is recovered at the top of the column in the form of steam, mixed with the residual hydrocarbons, and then condensed before being treated in a downstream unit.

[0016] Finally, the highest drawing in the column where the light vacuum distillation oils are removed, has a not inconsiderable viscosity at ambient temperature, which is disadvantageous for the design of the air exchangers.

[0017] An object of the invention is to obtain a vacuum distillation method which offers decreased utility consumption and a reduction in the size of certain equipment, while maintaining release performance that is at least as good.

[0018] For this purpose, the subject-matter of the invention is a method of the aforementioned type, characterized in that the method comprises introducing into the vacuum distillation column a flow of a fluid for the stripping of the feedstock, wherein the stripping fluid is a mixture of hydrocarbons having a weighted average boiling temperature of between 150.degree. C. and 250.degree. C., preferably between 190.degree. C. and 210.degree. C.

[0019] According to particular embodiments, the method according to the invention comprises one or more of the following characteristics, taken separately or in any technically feasible combination:

[0020] the stripping fluid consists of kerosene,

[0021] it comprises a step of forming a recycling stream of the stripping fluid, from the liquid flow;

[0022] it comprises the removal, in the stripping fluid recycling stream, of a purging stream of a portion of the stripping fluid, and the introduction of a supplementary stream of stripping fluid downstream of the drawing of the purging stream, wherein the recycling stream of the stripping fluid forms at least a portion of the stripping fluid flow;

[0023] the stripping fluid consists entirely of a supplementary stream, wherein no stream coming from the liquid flow is recycled into the stripping fluid flow introduced into the vacuum distillation column;

[0024] it comprises introducing the heated stripping fluid flow into the vacuum distillation column at a level located below the feedstock introduction level;

[0025] it comprises the derivation of a part of the stripping fluid flow, before its introduction into the vacuum distillation column, and its introduction into the feedstock;

[0026] the condensation step of the column head stream is carried out in a first downstream heat exchanger, advantageously a plate heat exchanger, in particular with low feedstock loss, wherein the first downstream heat exchanger is arranged in the distillation column under vacuum, or outside the vacuum distillation column;

[0027] the condensation step of the column head stream is implemented in a first downstream heat exchanger, advantageously a plate heat exchanger, in particular with a low feedstock loss, wherein the method comprises a step of passing the gaseous flow from the first downstream heat exchanger to a second downstream heat exchanger in order to obtain an additional condensate;

[0028] the step of passing the gaseous flow through the second downstream heat exchanger comprises spraying a stream of liquid hydrocarbons into the gaseous flow;

[0029] it comprises a step of recovering the, or each, condensate in a tank and the recovery of a stream of water to be treated and a stream of recovered hydrocarbons in the tank;

[0030] the step of passing the gaseous flow into a vacuum generation unit comprises introducing the gaseous flow into at least one steam ejector in order to form an ejected stream, and partially condensing the ejected stream produced by each steam ejector in a condenser;

[0031] it comprises the recovery of a net bottom stream at the bottom of the vacuum distillation column, wherein the ratio between the mass flow rate of the stripping fluid introduced into the vacuum distillation column and the mass flow rate of the net bottom stream that is recovered at the bottom of the vacuum distillation column is greater than 80 kg of stripping fluid per 1000 kg of net bottom stream recovered at the bottom of the vacuum distillation column, and lies, in particular, between 80 kg and 800 kg of stripping fluid per 1000 kg of net bottom stream recovered at the bottom of the vacuum distillation column;

[0032] more than 95% by weight of the stripping fluid introduced into the vacuum distillation column is extracted from the vacuum distillation column via the column head stream.

[0033] The invention also relates to a vacuum distillation plant, comprising:

[0034] a feedstock heating assembly;

[0035] a vacuum distillation column and an assembly for feedstock introduction in a flash zone of the vacuum distillation column;

[0036] an assembly for removal of at least one distillate stream at an intermediate level of the vacuum distillation column;

[0037] a assembly for drawing a column head stream at the head of the vacuum distillation column;

[0038] a unit for partial condensing of the column head stream to recover a liquid flow and a gaseous flow;

[0039] a vacuum generation unit, an assembly for the passage of a gaseous flow in the vacuum generation unit and an assembly for recovery from the gaseous flow of at least one condensate and a gaseous fraction of combustible gas produced in the vacuum generation unit;

[0040] characterized by an introduction into the vacuum distillation column of a feedstock stripping fluid flow, wherein the stripping fluid has a weighted average boiling temperature of from 150.degree. C. to 250.degree. C.

[0041] According to particular embodiments, the installation according to the invention comprises one or more of the following characteristics, taken in isolation or in any technically feasible combination:

[0042] it comprises a stripping fluid recycling element, wherein the installation comprises an assembly to form the stripping fluid flow at least partly from the stripping fluid recycling element;

[0043] the condensing unit of the column head stream comprises a first downstream heat exchanger, advantageously a plate heat exchanger, preferably with low feedstock losses, arranged in the vacuum distillation column, or installed outside the vacuum distillation column;

[0044] the condensing assembly of the column head stream comprises a first downstream heat exchanger, advantageously a plate heat exchanger, preferably with a low feedstock losses, and a second downstream heat exchanger, wherein the installation comprises an assembly for passage of the gaseous flow in the second downstream heat exchanger in order to obtain additional condensate.

[0045] The invention will be better understood upon reading the description which follows, given solely by way of example, and with reference to the appended drawings, wherein:

[0046] FIG. 1 shows a block diagram of a first vacuum distillation installation intended for the implementation of a first method according to the invention;

[0047] FIG. 2 shows a view similar to FIG. 1, of a second vacuum distillation installation intended for the implementation of a second method according to the invention;

[0048] FIG. 3 shows a view similar to FIG. 1, of a third vacuum distillation installation intended for the implementation of a third method according to the invention.

[0049] In all that follows, we will designate by the same references a stream flowing in a conduit and the conduit that carries it.

[0050] In addition, unless indicated otherwise, the flow rates quoted are mass flow rates, wherein the pressures are given in absolute millibars.

[0051] A first installation 10 according to the invention is illustrated in FIG. 1. This installation 10 is intended for the implementation of a first method according to the invention for the vacuum distillation of feedstock 12.

[0052] The installation 10 comprises a furnace 14 for heating the feedstock 12 and a stripping fluid, a vacuum distillation column 16, and bottom heat exchangers 18.

[0053] The installation 10 further comprises lateral sidestreams 20, 22, 24, and heat exchangers 26, 28 and 30 respectively associated with each lateral sidestream 20, 22, 24. In variants, the installation 10 comprises other types of vacuum distillation arrangements (sidestream variable numbers 20, 22, 24, variable number of packing beds 43, variable number of packing beds 41).

[0054] The installation 10 comprises a downstream heat exchanger 32, a vacuum generation unit 34, and a condensate recovery tank 36.

[0055] In this example, the installation 10 comprises an optional downstream separator 38 to recover the stripping fluid and a pump 40 for recycling the stripping fluid.

[0056] In this example, the vacuum distillation column 16 has a first packing bed 43 whose role is to ensure heat exchange over each sidestream 20, 22, 24, and whose liquid supply is provided by a first diffuser 44. The vacuum distillation column 16 has, below each sidestream 20, 22, 24, a second packing bed 41 whose role is to ensure fractionation and whose supply is provided by a second liquid diffuser 42.

[0057] In the example shown in FIG. 1, the heat exchangers 26 and 28 are heat exchangers enabling the cooling of a part of the sidestreams 58 and 64. The downstream heat exchanger 32 is a water exchanger supplied with a flow of water at room temperature. The heat exchangers 30 are air heat exchangers.

[0058] The vacuum generation unit 34 comprises a plurality of steam jet ejectors 45, connected in series with each other, and with a downstream water condenser 46 for each ejector 45. The number of ejector stages in series may vary as a function of the desired vacuum level.

[0059] In the example shown in FIG. 1, the unit 34 comprises three ejectors 45 in series and three condensers 46 interposed between the ejectors 45.

[0060] A first method of vacuum distillation of the feedstock 12 in the installation 10 will now be described.

[0061] Initially, the feedstock 12 is provided. This feedstock 12 is, for example, a feedstock of liquid hydrocarbons, such as a residual oil feedstock resulting from atmospheric distillation.

[0062] The mass flow rate of the feedstock 12 is generally between 100 t/h and 1000 t/h.

[0063] The feedstock 12 is output either directly from the bottom of a crude oil atmospheric distillation column (generally of the order of 350.degree. C.) or from a storage tank (at a temperature, for example, of the order of 80.degree. C.) after reheating to a temperature of the order of 300.degree. C. The feedstock 12 is first introduced into the furnace 14 to be heated and advantageously vaporized.

[0064] The temperature of the heated feedstock 50 at the output of the furnace 14 generally lies between 380.degree. C. and 420.degree. C. as a function of the desired distillation performance and the TBP (True Boiling Point) cut point between the vacuum residue 79 and the vacuum distillate 20. The cut points represent the distilled fraction of the crude oil at the indicated temperatures.

[0065] The partially vaporized feedstock 50 is then introduced into the vacuum distillation column 16 at a level N1 located in the flash zone above the bottom of the vacuum distillation column 16.

[0066] Simultaneously, according to the invention, a stripping fluid flow 52, consisting of hydrocarbons having a mean average boiling point (Tmav) temperature between 150.degree. C. and 250.degree. C. (typically of the order of 200.degree. C.), is introduced into the furnace 14 in order to be reheated and advantageously vaporized. This temperature Tmav is defined in the "Databook on hydrocarbons" written by J. B. Maxwell by the term "mean average boiling point". The calculation of the Tmav temperature according to J. B. Maxwell's method is also detailed in Pierre Wuithier's book "Le Petrole--Refining et Genie Chimique", Volume 1.

[0067] A kerosene obtained from an atmospheric distillation of crude oil having an initial TBP cut point of between 145.degree. C. and 180.degree. C. and a final TBP cut point of between 220.degree. C. and 250.degree. C., advantageously constitutes the stripping fluid.

[0068] The stripping fluid 52 is completely vaporized and superheated before being introduced at the bottom of column 16. Part of this stripping fluid is injected in liquid form into the vacuum furnace radiation beam as an accelerating fluid in order to limit the film temperatures in the tubes.

[0069] The overheated stripping fluid flow 54 is introduced into the vacuum distillation column 16 at a level N2 located below the last tray of a stripping zone 56 of the vacuum distillation column 16.

[0070] The heated stripping fluid stream 54 rises through the trays of the stripping zone 56 between the N1 and N2 levels and vaporizes the lighter fractions of the vacuum residue. In the vacuum distillation column 16, the vacuum level at the top of the column lies advantageously between 13 mbar and 40 mbar (10 mmHg and 30 mmHg), in this case substantially around 27 mbar (20 mmHg).

[0071] A first stream 58 of heavy vacuum distillate (HVGO--Heavy Vacuum Gas Oil) is taken laterally at a first sidestream 20 at a lower level N3 located above the level N1.

[0072] A first fraction 60 of the vacuum heavy distillate stream 58 is reintroduced into the column 16 through the second diffuser 42 associated with the sidestream 20. The remainder of the vacuum heavy distillate stream 58 passes into a heat exchanger 26, while a second fraction 62 of the heavy distillate stream 58 issuing from the heat exchanger 26 is reintroduced into the vacuum distillation column 16 through the first diffuser 44 associated with the sidestream 20. The remainder of the stream constitutes the production 180 of vacuum heavy distillate from the unit.

[0073] A second stream 64 of optional medium vacuum distillate (MVGO--Medium Vacuum Gas Oil) is removed at a second sidestream 22 at an average level N4 above the level N3.

[0074] A first fraction 66 of the optional medium vacuum distillate stream 64 is reintroduced into the vacuum distillation column 16 through the second diffuser 42 associated with the second sidestream 22. The remainder of the medium vacuum distillate stream 64 passes into a heat exchanger 28. A second optional fraction 64 of the medium vacuum distillate stream 64 from the heat exchanger 28 is reintroduced into the vacuum distillation column 16 through the first diffuser 44 of the second sidestream 22. The remainder of the stream constitutes the production 170 of medium vacuum distillate from the unit.

[0075] A third stream 70 of light vacuum distillate (LVGO--Light Vacuum Gas Oil) is removed at a third sidestream 24 at a high level N5 located above the level N4 in the vicinity of the head of the vacuum distillation column 16.

[0076] A first fraction 72 of the light vacuum distillate stream 70 is reintroduced into the vacuum distillation column 16 through the second diffuser 42 associated with the third sidestream 24. The remainder of the stream 70 passes into an air exchanger 30. A second fraction 74 of the light vacuum distillate stream 70 issuing from the heat exchanger 30 is reintroduced into the distillation column 16 through the first diffuser 44 of the third sidestream 24. The remainder of the stream constitutes the production 160 of light distillate vacuum from the unit.

[0077] A bottom stream 79 is recovered at the bottom of the vacuum distillation column 16 and passes through heat exchangers at the bottom.

[0078] A head stream 80 is drawn at the top of the column, under the effect of the suction produced by the vacuum generation unit 34.

[0079] The column head stream 80 has a pressure equal to the column head operating pressure 16 and has a temperature generally between 60.degree. C. and 100.degree. C.

[0080] The column head stream 80 is then introduced into the downstream heat exchanger 32, to be partially condensed by heat exchange with the water circulating in the downstream heat exchanger 32. The column head stream 80 is thus separated into a gaseous head flow 82 and a liquid foot flow 84.

[0081] The column head gaseous flow 82 is fed to the vacuum generation unit 34. It is introduced into a first ejector 45 where it is driven by a motive steam flow 150. The mixture thus formed is introduced into the first condenser 46, to form a first condensate 86 and a first gaseous flow 88.

[0082] The first gaseous flow 88 is introduced successively into a second ejector 45, then into a second condenser 46 in order to form a second condensate 90 and a second gaseous flow 92.

[0083] The second gaseous flow 92 is then introduced into a third ejector 45, then into a third condenser 46 in order to form a third gaseous flow 94 of non-condensable gas and a third condensate 96 that is available at a pressure slightly above atmospheric pressure.

[0084] The condensates 86, 90, 96 are recovered in a tank 36 and are separated into a flow of condensed hydrocarbons 98 and a stream of water to be treated 100.

[0085] The liquid bottom flow 84 contains the majority of the stripping fluid introduced into the stripping fluid flow 52 at the bottom of the vacuum distillation column 16. In fact, the boiling point of the stripping fluid 52 is lower than that of the light vacuum distillate stream 70 and is greater than that of water vapor.

[0086] The stripping fluid is however more easily condensable than water vapor. This makes it possible to operate the column at a low pressure, as described above.

[0087] The liquid bottom stream 84 is introduced into the optional downstream separator 38 in equilibrium with the gaseous flow 82.

[0088] The bottom fraction 112 contains the majority of the stripping fluid. It is pumped into the pump 40 to form a stream 114 for recycling the stripping fluid.

[0089] A purging stream 116 is removed from the recycling stream 114 in order to decrease the quantity of impurities and to maintain a constant quality of the recycling stream 114 that will be as close as possible to the supplementary stripping fluid flow 118. The purging stream 116 generally represents between 5% by weight and 20% by weight of the bottom fraction 112 introduced into the pump 40. Alternatively, the method may be implemented without total purging recycling (the absence of recycling flow being compensated by an additional supplementary flow 118).

[0090] Referring to FIG. 1, the remainder of the recycling stream 114 is returned to the furnace 14 to form, with a supplementary stripping fluid supply stream 118, the stripping fluid flow 52.

[0091] The removal of the purging stream 116 and the supply provided by the stripping fluid flow 118 make it possible to renew the stripping fluid circulating in the method according to the invention. This ensures the maintenance of a good quality of distillation, and a good ability to condense the stripping fluid by eliminating the slightest fractions possibly accumulated.

[0092] Moreover, the flow rate of the stripping fluid flow 52 introduced into the distillation column 16 may be controlled by adjusting the respective flow rates of the purging stream 116 and the supply stream 118.

[0093] Table 1 below illustrates the results obtained by numerical simulation for the first method according to the invention in the context of the treatment of hydrocarbon feedstock 12 with a mass flow rate of 666.7 t/h resulting from the atmospheric distillation of a crude oil of the "URAL" type. The stripping fluid is kerosene from an atmospheric distillation having a TBP cut point of 145.degree. C.-230.degree. C., a molecular weight of 152 g/mol, and a density at 15.degree. C. of 0.781.

[0094] The results obtained are compared with those of a prior art method, wherein the stripping fluid is water vapor and the installation is devoid of a downstream separator 38 and a recycling pump 40.

TABLE-US-00001 TABLE 1 Prior art FIG. 1 Yield Operating conditions Column operating pressure 16 mbars (mmHg) 72 (54) 27 (20) Introduction pressure of the mbars (mmHg) 105 (79) .sup. 50 (37.5) flow 50 in the flash zone Pressure at the output of mbars (mmHg) 60 (45) 15 (11) the exchanger 32 Temperature at the output of .degree. C. 33.5 33.5 the exchanger 32 Balance of materials Stripping steam flow in the t/h 33.5 -- column 16 Total kerosene flow in the t/h -- 115 flow 52 + 119 Gaseous flow 82 t/h 8.3 5.7 -31% Flow of stream 160 t/h 46.9 46.9 = Flow of stream 170 t/h 302.2 302.2 = Flow of stream 180 t/h 115.7 115.7 = Equipment design Area of exchangers 30 m2 3480 1360 -61% Area of exchangers 32 m2 4 .times. 1220 2 .times. 1150 -53% Cooling water flow t/h 4100 3545 -13% Water flow for production t/h 53.2 22.0 -59% of steam Combustible gas (furnace 99.6 102.1 +2.5% heating and steam production) Final processing Flow of water to be treated 100 t/h 53.3 22.4 -62%

[0095] The presence of a readily-condensable stripping fluid ensures a flow to the vacuum group 34 that is much less utility consuming than motive steam 150, which greatly limits the overall consumption of water steam.

[0096] Similarly, the temperatures are higher at the head of column 16, which allows a higher flow temperature 74 than in a conventional arrangement. The viscosity of the flow 74 is then reduced, thus reducing the size of the first air heat exchanger 30.

[0097] Similarly, the size of the downstream heat exchanger 32 may be reduced because there is no more water steam to be condensed.

[0098] Moreover, the use of a stripping fluid other than water steam limits the amount of water to be treated 100 and recovered in the tank 36.

[0099] A portion 119 of the flow of the motive fluid 52 is derived in the feedstock 12 before it passes into the furnace 14, or during this passage.

[0100] A second installation 130 according to the invention is illustrated in FIG. 2. The second installation 130 is intended for the implementation of a second vacuum distillation method according to the invention.

[0101] Unlike the first installation 10, the downstream heat exchanger 32 of the second installation 130 is arranged directly in the distillation column 16 at the head of the column and above the upper bed 43 associated with the upper sidestream 30. The downstream heat exchanger 32, in this case, is a plate heat exchanger with low pressure loss (typically less than 7 mbar (5 mmHg), in particular of the order of 1.3 mbar (1 mmHg)).

[0102] Unlike the method shown in FIG. 1, the column head steams forming the column head stream 80 coming from distillation in the vacuum distillation column 16, enter the heat exchanger 32 within the vacuum distillation column 16 from the top. The column head stream 80 condenses in the heat exchanger 32.

[0103] As before, a gaseous flow 82 is extracted from the heat exchanger 32 in order to be supplied to the vacuum generation unit 34, and a liquid bottom flow 84 is recovered at the bottom of the heat exchanger 32 in order to be supplied to the downstream separator 38.

[0104] The table below illustrates the results obtained for the second method according to the invention in the context of the processing of a hydrocarbon feedstock with a mass flow rate equal to 666.7 t/h, resulting from the atmospheric distillation of a crude oil of the "URAL" type. The stripping fluid is kerosene from an atmospheric distillation having a TBP cut point of 145.degree. C.-230.degree. C., a molecular weight of 152 g/mol, and a density at 15.degree. C. of 0.781.

[0105] The results obtained are compared with a method of the prior art, wherein the stripping fluid is steam and the installation is devoid of a downstream separator 38 and a recycling pump 40.

TABLE-US-00002 TABLE 2 Prior art FIG. 1 Yield Operating conditions Column operating pressure 16 mbars (mmHg) 72 (54) .sup. 17 (12.5) Introduction pressure of the mbars (mmHg) 105 (79) 40 (30) flow 54 in the flash zone Pressure at the output of mbars (mmHg) 60 (45) 15 (11) the exchanger 32 Temperature at the output of .degree. C. 33.5 30 the exchanger 32 Balance of materials Stripping steam flow in the t/h 33.5 -- column 16 Total kerosene flow in the t/h -- 65 flow 52 + 119 Gaseous flow 82 t/h 8.3 5.2 -37% Flow of stream 160 t/h 46.9 46.9 = Flow of stream 170 t/h 302.2 302.2 = Flow of stream 180 t/h 115.7 115.7 = Equipment design Area of exchangers 30 m2 3480 1683 -52% Area of exchangers 32 m2 4 .times. 1220 2475 -49% Cooling water flow t/h 4100 2423 -41% Water flow for production t/h 53.2 21.8 -59% of steam Combustible gas (furnace 99.6 95.5 -4% heating and steam production) Final processing Flow of water 100 t/h 53.3 22.2 -63%

[0106] Unlike the method shown in FIG. 1, the method illustrated in FIG. 2 has an even lower pressure at the head of the column, for example less than 27 mbar (20 mmHg), and, in particular, between 13 mbar (10 mmHg) and 20 mbar (15 mmHg).

[0107] With performance identical to the performance of FIG. 10 (recovery rate of the identical heavy vacuum distillate), this arrangement reduces the required flow of stripping fluid 52, while maintaining a reduced heat exchange area in the downstream heat exchanger 32, and in the exchangers 30.

[0108] As the quantity of stripping fluid injected into the vacuum distillation column 16 is reduced, the consumption of combustible gas intended for the vaporization of the stripping fluid is thus reduced, which significantly reduces the overall consumption of combustible gas.

[0109] Furthermore, the consumption of cooling water is reduced, taking into account the smaller amount of stripping fluid to be condensed in the heat exchanger 32.

[0110] In a variant (not shown), the exchanger 32 is located outside the distillation column 16. The performance of this variant is between the performance obtained with the installation shown in FIG. 1 and the performance obtained with the installation shown in FIG. 2.

[0111] A third installation 140 according to the invention is illustrated in FIG. 3. Unlike the second installation 130 shown in FIG. 2, the installation 140 comprises a second downstream heat exchanger 142 arranged downstream of the first downstream heat exchanger 32 located in the vacuum distillation column 16. The second downstream heat exchanger 142 is arranged outside the vacuum distillation column 16, and receives the gaseous flow 82 from the first downstream heat exchanger 32.

[0112] The second downstream heat exchanger 142 is a plate heat exchanger with low pressure drop (typically less than 7 mbar (5 mmHg), in particular of the order of 1.3 mbar (1 mmHg)). It is here provided with a boom 143 for spraying a stream of liquid hydrocarbons 144, typically a stream coming from the atmospheric distillation plant. The liquid hydrocarbon stream 144 is, for example, an atmospheric heavy distillate stream (HAGO--Heavy Atmospheric Gas Oil).

[0113] The third vacuum distillation method according to the invention differs from the second method in that the head stream 82 produced in the first downstream heat exchanger 32 is introduced into the second heat exchanger 142. The head stream 82 is at least partially condensed. on the one hand, as a result of the absorption generated by the introduction of the fluid 144, and, on the other hand, by heat exchange with the water.

[0114] An additional condensate 146 is produced at the bottom of the second downstream heat exchanger 142, wherein the remainder of the gaseous flow 82 is introduced into the vacuum generation unit 34, as previously described. The additional condensate is collected in the tank 36.

[0115] The table below illustrates the results obtained for the third method according to the invention in the context of the processing of a hydrocarbon feedstock 12 with a mass flow rate equal to 666.7 t/h resulting from the atmospheric distillation of a crude oil of the "URAL" type. The stripping fluid is kerosene.

[0116] The results obtained are compared with a method of the prior art, wherein the stripping fluid is steam and the installation is devoid of a downstream separator 38 and a recycling pump 40.

TABLE-US-00003 TABLE 3 Prior art FIG. 1 Yield Operating conditions Column operating pressure 16 mbars (mmHg) 72 (54) .sup. 17 (12.5) Introduction pressure of the mbars (mmHg) 105 (79) 40 (30) flow 50 in the flash zone Pressure at the output of mbars (mmHg) 60 (45) 15 (11) the exchanger 32 Temperature at the output of .degree. C. 33.5 30 the exchanger 32 Balance of materials Stripping steam flow t/h 33.5 -- Total kerosene flow in the t/h -- 65 flow 52 + 119 Gaseous flow 82 t/h 8.3 3.3 -60% Flow of stream 160 t/h 46.9 46.9 = Flow of stream 170 t/h 302.2 302.2 = Flow of stream 180 t/h 115.7 115.7 = Equipment design Area of exchangers 30 m2 3480 1683 -52% Area of exchangers 32, 142 m2 4 .times. 1220 2475 + 302 -42% Cooling water flow t/h 4100 2493 -39% Water flow for production t/h 53.2 18.7 -65% of steam Combustible gas (furnace 99.6 93.7 -6% heating and steam production) Final processing Flow of water 100 t/h 53.3 19.1 -68%

[0117] The third method of the invention further reduces the steam consumption of the vacuum generation unit 34. Thus, the overall fuel consumption in the method is further reduced, as well as the amount of water to be treated 100 that is produced.

[0118] As indicated above, the methods according to the invention considerably reduce the consumption of utilities, in particular cooling water and water stream, which reduces the operating costs of the installation, while maintaining a method allowing the achievement of ambitious vacuum distillate recovery rates.

[0119] In the method according to the invention, and as indicated above, the supply flow rate 118 for the stripping fluid may be controlled independently of the nature and the flow rate of the feedstock 12, and independently of the flow rate of the recycling stream 114. The latter makes it possible to supply the quantity and quality of stripping fluid 52 necessary for the distillation quality.

[0120] Advantageously, the ratio between the mass flow rate of the stripping fluid 52 introduced into the vacuum distillation column 16 and the mass flow rate of the net bottom stream 190 recovered at the bottom of the vacuum distillation column 16 (after derivation of a quenching stream returned to the vacuum distillation column 16) is greater than 80 kg of stripping fluid per 1000 kg of net bottom stream recovered at the bottom of the vacuum distillation column 16 and is, in particular, between 80 kg and 800 kg of stripping fluid per 1000 kg of net bottom stream recovered at the bottom of the vacuum distillation column 16.

[0121] In addition, the quality of the stripping fluid 52 used, consisting of a mixture of hydrocarbons having a weighted average boiling temperature (Tmav) of between 150.degree. C. and 250.degree. C., advantageously between 190.degree. C. and 210.degree. C., is perfectly mastered in the method according to the invention.

[0122] Such a fluid is optimal since it ensures effective stripping of the feedstock 12, while being easily condensable at very low pressure at the head of the vacuum distillation column 16, and is, therefore, recyclable through the liquid stream 114.

[0123] The fluid 52 has the advantage of being almost completely recovered in the column head stream 80. This allows the recycling and avoids the extraction of stripping fluid 52 in the distillate streams 160, 170, 180, particularly in the stream 160 of light distillate under vacuum.

[0124] Advantageously, more than 95% by weight of the stripping fluid 52 introduced into the vacuum distillation column 16 is recovered from the vacuum distillation column 16 in the flow 112 via the column head stream 80 after condensation.

[0125] Thus, it is not necessary to introduce additional steam into the stripping fluid 52, which limits the pressure in the vacuum distillation column 16, thereby reducing utility consumption and reducing the size of certain equipment while obtaining an optimal stripping of the feedstock 12.

Claims

1. A method for the vacuum distillation of a hydrocarbon feedstock comprising: heating a feedstock; introducing the feedstock into a flash zone of a vacuum distillation column; removing at least one distillate stream at an intermediate level from the vacuum distillation column; withdrawing a column head stream at a head of the vacuum distillation column; partially condensing the column head stream to recover a liquid flow and a gaseous flow; passing the gaseous flow through a vacuum generator and recovering at least one condensate and a gaseous fraction of non-condensable gas; introducing into the vacuum distillation column a flow of a feedstock-stripping fluid, the stripping fluid comprising a hydrocarbon mixture having a weighted average boiling temperature (Tmav) of between 150.degree. C. and 250.degree. C.

2. The method according to claim 1, wherein the stripping fluid is kerosene.

3. The method according to claim 1, comprising forming a stripping fluid recycling stream from the liquid flow.

4. The method according to claim 3, comprising removing a purging stream from a portion of the stripping fluid in the stripping fluid recycling stream at a the purging stream removal point, and introducing a supplementary stripping fluid flow downstream of the purging stream removal point, the stripping fluid recycling stream forming at least a portion of the stripping fluid flow.

5. The method according to claim 1, wherein the stripping fluid consists entirely of a supplementary stream, no stream coming from the liquid flow (84) being recycled into the stripping fluid flow introduced into the vacuum distillation column.

6. The method according to claim 1, comprising introducing the heated stripping fluid flow into the vacuum distillation column at a level located below a level of the introduction of the feedstock.

7. The method according to claim 6, comprising deriving a portion of the stripping fluid flow, before introducing the heated stripping fluid flow into the vacuum distillation column, and introducing the portion of the stripping fluid flow into the feedstock.

8. The method according to claim 1, wherein condensing the column head stream is implemented in a first downstream heat exchanger, the first downstream heat exchanger being arranged in the vacuum distillation column, or outside the vacuum distillation column.

9. The method according to claim 1, wherein condensing the column head stream is implemented in a first downstream heat exchanger (32), the method comprising passing the gaseous flow from the first downstream heat exchanger to a second downstream heat exchanger in order to obtain an additional condensate.

10. The method according to claim 9, wherein passing the gaseous flow into the second downstream heat exchanger comprises spraying a liquid hydrocarbon stream into the gaseous flow.

11. The method according to claim 1, comprising recovering the at least one condensate (86, 90, 96) in a tank, and recovering a stream of water to be treated and a recovered hydrocarbon stream in the tank.

12. The method according to claim 1, wherein passing the gaseous flow through a vacuum generator comprises introducing the gaseous flow into at least one steam ejector to form an ejected stream, and partially condensing the ejected stream produced by the at least steam ejector (45) in a condenser (146).

13. A vacuum distillation installation comprising: a furnace for heating a feedstock; a vacuum distillation column and an inlet for the introduction of the feedstock in a flash zone of the vacuum distillation column; an outlet for removal of at least one distillate stream at an intermediate level of the vacuum distillation column; an outlet for withdrawal of a column head stream, at a head of the vacuum distillation column; a condenser for partial condensing of the column head stream, the condenser having outlets for recovering a liquid flow and a gaseous flow; a vacuum generator, an inlet for passage of a gaseous flow in the vacuum generator, and an outlet for recovery of at least one condensate and a gaseous fraction of combustible gas produced in the vacuum generator from the gaseous flow; an inlet for introduction of a flow of the feedstock-stripping fluid into the vacuum distillation column, the stripping fluid having a weighted average boiling temperature (Tmav) between 150.degree. C. and 250.degree. C.

14. The installation according to claim 13 comprising a stripping fluid recycler, and an outlet for forming a stripping fluid flow at least partially from the stripping fluid recycler.

15. The installation according to claim 13, wherein the condenser comprises a first downstream heat exchanger, arranged in the vacuum distillation column, or arranged outside the vacuum distillation column.

16. The installation according to claim 13, wherein the condenser comprises a first downstream heat exchanger, a second downstream heat exchanger, and an inlet for passing the gaseous flow into the second downstream heat exchanger in order to obtain an additional condensate.