Use of a bifunctional catalyst based on zeolite IZM-2 for the hydroisomerization of light paraffinic feedstocks resulting from Fischer-Tropsch synthesis

Abstract

A process is described for producing middle distillates from a paraffinic feedstock produced by Fischer-Tropsch synthesis and divided into a light fraction (cold condensate) and a heavy fraction (waxes). The process involves fractionation of the waxes to obtain a light fraction, the final boiling point of which is between 350 C. and 400 C., and a heavy fraction which boils above the light fraction. The light fraction is mixed with at least one portion of the cold condensate. The resultant mixture is hydrotreated in the presence of a hydrotreatment catalyst of at least one portion of the resultant effluent is hydroisomerized in the presence of a catalyst comprising at least one noble metal from Group VIII and at least one zeolite IZM-2. At least one portion of the heavy fraction is subjected to hydrocracking and hydroisomerization in the presence of a hydrocracking catalyst. The resultant effluents are fractionated to obtain at least one middle distillates fraction.

Claims

1. A process for producing middle distillates from a paraffinic feedstock produced by Fischer-Tropsch synthesis and divided into two fractions, a light fraction, known as cold condensate, and a heavy fraction, known as waxes, comprising at least the following steps: a) fractionation of said heavy fraction, known as waxes, so as to obtain a light fraction of the waxes, the final boiling point of which is between 350 C. and 400 C., and a heavy fraction of the waxes which boils above said light fraction, b) mixing of said light fraction of the waxes, the final boiling point of which is between 350 C. and 400 C. derived from step a) with at least one portion of said cold condensate fraction, c) hydrotreatment of the mixture derived from step b) in the presence of a hydrotreatment catalyst and which operates at a temperature of between 250 and 450 C., at a pressure of between 0.5 and 15 MPa, at a hydrogen flow rate adjusted in order to obtain a ratio of between 100 and 3000 standard litres per litre, and at an hourly space velocity of between 0.1 and 40 h.sup.1, d) hydroisomerization of at least one portion of the effluent derived from step c) in the presence of a catalyst comprising at least one noble metal from Group VIII of the Periodic Table and at least one zeolite IZM-2, step d) operating at a temperature of between 200 and 450 C., a pressure of between 1 and 15 MPa, a space velocity of between 0.1 and 10 h.sup.1 and a hydrogen flow rate adjusted in order to obtain a ratio of between 100 and 2000 standard litres of hydrogen per litre of feedstock, e) hydrocracking and hydroisomerization of at least one portion of the heavy fraction of the waxes derived from step a) in the presence of a hydrocracking catalyst and operating at a temperature of between 250 C. and 450 C., at a pressure of between 0.2 and 15 MPa, at a space velocity of between 0.1 h.sup.1 and 10 h.sup.1, and at a hydrogen flow rate adjusted in order to obtain a ratio of between 100 and 2000 standard litres of hydrogen per litre of feedstock, and f) fractionation of the mixture of the effluent derived from step e) and the effluent derived from step d) so as to obtain at least one petrol fraction and at least one middle distillates fraction.

2. The process according to claim 1, in which said light fraction of the waxes, separated in step a), is mixed with all of said cold condensate fraction.

3. The process according to claim 1, in which the process further comprises a step a) of fractionation of the cold condensate fraction so as to obtain a light fraction of said cold condensate fraction, the final boiling point of which is between 120 and 200 C., and a heavy fraction of said cold condensate fraction which boils above said light fraction, having an initial boiling point of between 80 C. and 200 C.

4. The process according to claim 3, in which said light fraction of said cold condensate fraction separated in said step a) is sent to a hydrotreatment step c) which operates at a temperature of between 150 and 430 C., at a pressure of between 0.5 and 15 MPa, at a hydrogen flow rate adjusted in order to obtain a ratio of between 150 and 1500 standard litres per litre, and at an hourly space velocity of between 0.2 and 20 h.sup.1.

5. The process according to claim 4, in which the effluent resulting from step c) is mixed with the petrol effluent resulting from the fractionation step f).

6. The process according to claim 1, in which said hydroisomerization step d) is carried out at a temperature of between 300 and 450 C., at a pressure of between 1 and 9 MPa, at a space velocity of between 0.5 and 5 h.sup.1 and at a hydrogen flow rate adjusted in order to obtain a ratio of between 150 and 1500 litres of hydrogen per litre of feedstock.

7. The process according to claim 1, in which all of the effluent resulting from step c) is sent to the hydroisomerization step d).

8. The process according to claim 1, in which the process further comprises a step a) of fractionation of the effluent resulting from the hydrotreatment step c) so as to obtain a light fraction of said effluent, the final boiling point of which is between 120 and 200 C., and a heavy fraction of said effluent which boils above said light fraction, having an initial boiling point of between 120 and 200 C. and a final boiling point of between 350 C. and 400 C.

9. The process according to claim 8, in which said light fraction of said effluent, the final boiling point of which is between 120 and 200 C., separated in step a), is mixed with the petrol effluent resulting from the fractionation step f).

10. The process according to claim 1, in which the process further comprises a step c) of hydrotreatment of said heavy fraction of the waxes resulting from step a), prior to said hydrocracking and hydroisomerization step e).

11. The process according to claim 1, wherein the final boiling point of the light fraction of the waxes is between 360 C. and 380 C.

12. The process according to claim 11, wherein the final boiling point of the light fraction of the waxes is between 360 C. and 370 C.

13. The process according to claim 1, wherein the hydrocracking and hydroisomerization in step e) is of the entire heavy fraction of the waxes derived from step a).

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 illustrates Embodiment No. 1 in which all of the light fraction of the cold condensates is sent, as a mixture with a portion of the light fraction of the waxes, to the hydrotreatment step c) then to step d) of hydroisomerization in the presence of the IZM-2-based catalyst.

(2) FIG. 2 and FIG. 3 illustrate Embodiments 2 and 3 in which the heavy fraction of the cold condensates, i.e. the middle distillates fraction, is sent, as a mixture with a portion of the light fraction of the waxes, to the hydrotreatment step c) then to step d) of hydroisomerization in the presence of the IZM-2-based catalyst.

(3) FIG. 4 illustrates the prior art in which the two effluents resulting from the Fischer-Tropsch synthesis, the light fraction, which is known as cold condenate, and the heavy fraction, which is known as waxes, are mixed, then simultaneously hydrotreated and, finally, simultaneously hydrocracked/hydroisomerized on the IZM-2-based catalyst C1. Reference will be made to FIG. 4 in Example 5.

(4) In FIG. 1, the synthesis gas 1, composed predominantly of carbon monoxide and of hydrogen, is directed to a Fischer-Tropsch synthesis unit (A1). At the outlet of the unit (A1), the effluent is divided into two streams: the hydrocarbon-based fraction 2 or cold condensate, which is sent to the hydrotreatment unit (C), and the heavy fraction 3 or waxes, which is separated, in a separation unit (A), into two fractions, a light fraction 4 having a final boiling point of less than 370 C. and a heavy fraction 8 having an initial boiling point of greater than 370 C. This light cut 4 derived from the heavy fraction 3 is mixed (B) with the cold condensates 2 resulting from the Fischer-Tropsch synthesis (A1). The effluent 5 is then directed to a hydrotreatment unit (C). The effluent 6 resulting from the hydrotreatment step (C) is sent to a hydroisomerization unit (D) after removal of the water not represented in FIG. 1. The effluent 7 produced by the sequence of the hydrotreatment step (C) and the hydroisomerization step (D) no longer contains olefinic compounds and oxygen-comprising compounds and is isomerized.

(5) The heavy fraction 8 resulting from the separation (A) of the waxes 3 produced by the Fischer-Tropsch synthesis can be directed to a hydrotreatment step (C). This hydrotreatment step (C) is optional as this portion of the effluent contains little in the way of olefins and little in the way of oxygen-comprising compounds, which compounds are mainly present in the light cuts produced by the Fischer-Tropsch synthesis (A1), 2 and 4. The effluent 9 is sent, after removal of the water not represented in FIG. 1, to a hydrocracking and hydroisomerization step (E). The effluent 7 resulting from the hydrotreatment (C) and the hydroisomerization (D) of the light condensates fractions produced by the Fischer-Tropsch synthesis, and the effluent 10 resulting from the hydrotreatment (C) and from the hydrocracking and the hydroisomerization (E), are mixed so as to obtain an effluent 11, before being sent to a separation step (F). On conclusion of this separation step (F), four main streams are produced: a light cut 12 similar to a naphtha, the maximum boiling point of which is less than 180 C., an intermediate cut 13, equivalent to a kerosene with an initial boiling point of greater than 120 C. and a final boiling point of less than 250 C., a heavy cut 14, equivalent of a gas oil composed of product with boiling points of between 120 C. and 370 C., and a very heavy cut 15 corresponding to the fraction not converted in the hydrocracking and hydroisomerization step (E). This effluent 15 perhaps is recycled to the hydrocracking and hydroisomerization step (E).

(6) In FIG. 2, the synthesis gas 1 is directed to a Fischer-Tropsch synthesis unit (A1). At the outlet of the unit (A1), the effluent is divided into two streams: the hydrocarbon-based fraction 2 or cold condensate, which is sent to a separation step (A), and the heavy fraction 3 or waxes, which is sent to a separation step (A). The separation (A) is carried out so as to obtain a light cut 4, the final boiling point of which is less than 150 C., and a heavy cut 5, the initial boiling point of which is greater than 150 C. The light cut 4 is sent to a hydrotreatment step (C).

(7) The heavy waxes fraction 3 is separated into two fractions in the separation step (A), a light fraction 6, the final boiling point of which is less than 370 C., and a heavy fraction 7, the initial boiling point of which is greater than 370 C. This light cut 6 resulting from the step (A) of separation of the heavy fraction 3 is mixed (B) with the stream 5 resulting from the separation (A), so as to form a stream 9. The stream 9 is then directed to a hydrotreatment unit (C). The effluent 10 resulting from the hydrotreatment step (C) is sent to a hydroisomerization unit (D) after removal of the water not represented in FIG. 2. The effluent 13 produced by the sequence of the hydrotreatment step (C) and the hydroisomerization step (D) no longer contains olefinic compounds and oxygen-comprising compounds and is isomerized. The heavy fraction of the waxes 7 resulting from the separation (A) can be directed to a hydrotreatment step (C). This hydrotreatment step (C) is optional as this portion of the effluent contains little in the way of olefins and little in the way of oxygen-comprising compounds, which compounds are mainly present in the light cuts produced by the Fischer-Tropsch synthesis (A1), 2 and 6. The effluent 11 resulting from (C) is sent, after removal of the water not represented in FIG. 2, to a hydrocracking and hydroisomerization step (E). The effluent 12 produced by the sequence of the hydrotreatment step (C) and the hydrocracking and hydroisomerization step (E) no longer contains olefinic compounds and oxygen-comprising compounds, is cracked and is isomerized. The effluents 12 and 13 are mixed in the stream 14 and sent to a separation step (F). On conclusion of this separation step (F), four main streams are produced: a light cut 15 similar to a naphtha, the maximum boiling point of which is less than 180 C., an intermediate cut 16, equivalent to a kerosene with an initial boiling point of greater than 120 C. and a final boiling point of less than 250 C., a heavy cut 17, equivalent of a gas oil composed of product with boiling points of between 180 C. and 370 C., and a very heavy cut 18 corresponding to the fraction not converted in the hydrocracking and hydroisomerization step (E). This effluent 18 perhaps is recycled to the hydrocracking/hydroisomerization step (E).

(8) The effluent 15 produced by the distillation is mixed with the stream 8 resulting from the hydrotreatment (C) so as to form the stream 19, similar to a naphtha.

(9) In FIG. 3, the synthesis gas 1 is directed to a Fischer-Tropsch synthesis unit (A1). At the outlet of the unit (A1), the effluent is divided into two streams: the hydrocarbon-based fraction 2 or condensates and the heavy fraction 10 or waxes, which is sent to a separation step (A). The heavy fraction 10 is separated into two fractions: a light fraction of the waxes 9, the final boiling point of which is less than 370 C., and a heavy fraction of the waxes 11, the final boiling point of which is greater than 370 C.

(10) This light cut 9 resulting from the heavy fraction of the waxes 10 is mixed (B) with the hydrocarbon-based fraction 2 or condensates resulting from the Fischer-Tropsch synthesis (A1), so as to obtain a stream 3.

(11) The stream 3 is then directed to a hydrotreatment unit (C). The effluent 4 resulting from the hydrotreatment step (C) is sent to a separation step (A). The separation (A) is carried out so as to obtain a light cut 5, the final boiling point of which is less than 150 C., and a heavy cut 6, the initial boiling point of which is greater than 150 C.

(12) The heavy fraction 6 is directed to a hydroisomerization step (D) after removal of the water not represented in the figure. The effluent 7 produced by the sequence of the hydrotreatment step (C), the separation step (A) and the hydroisomerization step (D) no longer contains olefinic compounds and oxygen-comprising compounds and is isomerized.

(13) The heavy cut 11 resulting from the separation (A) of the waxes 10 produced by the Fischer-Tropsch synthesis (A1) can be directed to a hydrotreatment step (C) which is optional as this portion of the effluent contains little in the way of olefins and little in the way of oxygen-comprising compounds, which compounds are mainly present in the light cuts produced by the Fischer-Tropsch synthesis (A1), 2 and 9. The effluent 12 is sent, after removal of the water not represented in FIG. 3, to a hydrocracking and hydroisomerization step (E). The effluent 13 produced by the sequence of the hydrotreatment step (C) and the hydroisomerization step (E) no longer contains olefinic compounds and oxygen-comprising compounds, is cracked and is isomerized. The effluents 7 and 13 are mixed in the stream 8 and sent to a separation step (F). On conclusion of this separation step (F), four main streams are produced: a light cut 14 similar to a naphtha, the maximum boiling point of which is less than 180 C., an intermediate cut 16, equivalent to a kerosene with an initial boiling point of greater than 120 C. and a final boiling point of less than 250 C., a heavy cut 17, equivalent of a gas oil composed of product with boiling points of between 180 C. and 370 C., and a very heavy cut 18 corresponding to the fraction not converted in the hydrocracking and hydroisomerization step (E). This effluent 18 perhaps is recycled to the hydrocracking/hydroisomerization step (E).

(14) The effluent 14 produced by the distillation can be mixed with the effluent 5 so as to form the stream 15, similar to a naphtha.

(15) Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The preceding preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

(16) In the foregoing and in the examples, all temperatures are set forth uncorrected in degrees Celsius and, all parts and percentages are by weight, unless otherwise indicated.

(17) The entire disclosures of all applications, patents and publications, cited herein and of corresponding application No. FR 1856669, filed Jul. 18, 2018 are incorporated by reference herein.

(18) The examples illustrate the invention without limiting the scope thereof.

Example 1: Preparation of the IZM-2-Based Catalyst C1

(19) The catalyst C1 is a catalyst containing a noble metal and the zeolite IZM-2. This zeolite IZM-2 was synthesized in accordance with the teaching of Patent Application FR 2 918 050. The crude synthesis IZM-2 zeolite then undergoes calcination at 550 C. for ten hours (temperature increase gradient of 5 C./min) in a traversed bed under dry air (2 standard litres per hour and per gram of solid). The solid obtained is refluxed for 4 hours in an ammonium nitrate solution (100 ml of solution per gram of solid, ammonium chloride concentration of 10M) so as to exchange the alkali metal cations with ammonium ions. This refluxing step is performed four times. The solid thus obtained has an Si/Al ratio (determined by X-ray fluorescence) of 53. This zeolite is blended with an alumina gel of SB3 type supplied by the company Conda-Sasol. The blended paste is extruded through a 1.4 mm die. After drying in an oven overnight at 110 C., the extrudates are calcined at 500 C. for two hours (temperature increase gradient of 5 C./min) in a traversed bed under dry air (2 standard litres per hour and per gram of solid). The extrudates are then dry-impregnated with an aqueous solution of tetraamineplatinum nitrate Pt(NH.sub.3).sub.4(NO.sub.3).sub.2, left to mature in a water maturator for 24 hours at ambient temperature, then calcined at 500 C. (temperature increase gradient of 5 C./min) for two hours in a traversed bed under dry air (2 standard litres per hour and per gram of solid). The weight contents of the zeolite IZM-2 and of the platinum on the finished catalyst after calcination are respectively 20% and 0.32% by weight.

Example 2 (in Accordance with the Invention)

(20) Example 2 illustrates the embodiment according to FIG. 1.

(21) The effluent resulting from the Fischer-Tropsch synthesis unit is, at the outlet of the Fischer-Tropsch synthesis unit, divided into two fractions: a light fraction, known as cold condensate, and a heavy fraction, known as waxes.

(22) The characteristics of the various fractions are given in Table 1 below:

(23) TABLE-US-00001 TABLE 1 composition of the various fractions Cold condensate Waxes C1-C4 [wt %] 1 0 C5-C9 [wt %] 35 1 C10-C22 [wt %] 61 26 C22.sup.+ [wt %] 3 73 Paraffins [wt %] 74 96 Olefins [wt %] 19 2 Oxygen-comprising [wt %] 7 2 compounds

(24) Said heavy fraction, known as waxes, is separated in a distillation column into a light fraction, the final boiling point of which is less than 370 C., and a heavy fraction which boils above said light fraction.

(25) Said light fraction, the final boiling point of which is less than 370 C., is mixed with all of said cold condensate fraction.

(26) The mixture is then sent to a step of hydrotreatment in the presence of a hydrotreatment catalyst in its reduced form comprising 11.4% by weight of NiO and 8% by weight of MoO.sub.3 on an alumina support and which operates at a temperature of 335 C., at a pressure of 0.65 MPa, at a hydrogen flow rate adjusted in order to obtain a ratio of 600 standard litres per litre, and at an hourly space velocity of 2 h.sup.1.

(27) The effluent at the outlet of the hydrotreatment step no longer contains oxygen-comprising compounds or olefins and is 100% paraffinic.

(28) All of the effluent resulting from step c) is sent to a step d) of hydroisomerization in the presence of the hydroisomerization catalyst C1, step d) being carried out at a temperature of 330 C., at a pressure of 0.65 MPa, at a space velocity of 600 h.sup.1, and at a hydrogen flow rate adjusted in order to obtain a ratio of 2 standard litres of hydrogen per litre of feedstock.

(29) The isomerized effluent is then sent to the fractionation step f) in order to obtain a petrol fraction, a middle distillates fraction (kerosene and gas oil) and an unconverted fraction.

(30) In parallel, the heavy fraction of the waxes which boils above said light fraction, that is to say at a boiling point of greater than 370 C., is sent to a step e) of hydrocracking and of hydroisomerization in the presence of a hydrocracking and hydroisomerization catalyst comprising 0.3% by weight of Pt on an SiAl support and which operates at a temperature of 360 C., at a pressure of 0.65 MPa, at a space velocity of 2 h.sup.1, and at a hydrogen flow rate adjusted in order to obtain a ratio of 600 standard litres of hydrogen per litre of feedstock.

(31) The effluent resulting from the hydrocracking and hydroisomerization step e) is subsequently also sent to the fractionation step f) in order to separate a petrol fraction, a middle distillates fraction (kerosene and gas oil) and an unconverted fraction which is sent back to step e).

(32) The material balance according to the scheme is given in Table 2. As the hydrogen consumption is very low, it is disregarded in this example.

(33) TABLE-US-00002 TABLE 2 Material balance according to the principle scheme 1, with reference to the streams of FIG. 1 Stream [] 2 3 4 5 7 8 10 12 13 + 14 Flow [t/h] 55 45 11 66 66 34 34 22 78 rate

(34) The flow rate of middle distillate is increased by 4 wt % relative to Example 5 illustrating a scheme not in accordance with the invention.

Example 3 (in Accordance with the Invention)

(35) Example 3 illustrates the embodiment according to FIG. 2.

(36) Example 3 is carried out like Example 2, the only difference being that it is the heavy fraction of the condensate fraction resulting from the Fischer-Tropsch process which is sent, as a mixture with the light fraction of the waxes, to the step of hydroisomerization on an IZM-2-based catalyst, and not all of the condensate fraction.

(37) The effluent resulting from the Fischer-Tropsch synthesis unit is, at the outlet of the Fischer-Tropsch synthesis unit, divided into two fractions: a light fraction, known as cold condensate, and a heavy fraction, known as waxes.

(38) The characteristics of the various fractions are given in Table 1 above.

(39) Said heavy fraction, known as waxes, is separated in a distillation column (step a) into a light fraction, the final boiling point of which is less than 370 C., and a heavy fraction which boils above said light fraction.

(40) The cold condensate fraction is separated in a distillation column (step a) into a light fraction of said cold condensate fraction, the final boiling point of which is less than 150 C., and a heavy fraction which boils above said light fraction, having an initial boiling point of greater than 150 C. and a final boiling point of less than 370 C.

(41) Said heavy fraction of the cold condensate fraction separated in step a) corresponds to a fraction which boils in the middle distillate temperature range (120-370 C.).

(42) Said light fraction of the waxes fraction (separated in step a), the final boiling point of which is less than 370 C., is mixed with the heavy fraction of said cold condensate fraction (separated in step a) corresponding to the middle distillates fraction (120-370 C.).

(43) The mixture is sent to a step c) of hydrotreatment in the presence of a hydrotreatment catalyst in its reduced form comprising 11.4% by weight of NiO and 8% by weight of MoO.sub.3 on an alumina support and which operates at a temperature of 335 C., at a pressure of 0.65 MPa, at a hydrogen flow rate adjusted in order to obtain a ratio of 600 standard litres per litre, and at an hourly space velocity of 2 h.sup.1.

(44) All of the effluent resulting from step c) is sent to a step d) of hydroisomerization in the presence of the hydroisomerization catalyst C1, step d) being carried out at a temperature of 330 C., at a pressure of 0.65 MPa, at a space velocity of 2 h.sup.1, and at a hydrogen flow rate adjusted in order to obtain a ratio of 600 standard litres of hydrogen per litre of feedstock.

(45) The isomerized effluent is then sent to the fractionation step f) in order to obtain a petrol fraction, a middle distillates fraction (kerosene and gas oil) and an unconverted fraction.

(46) The light fraction of the cold condensates fraction having a final boiling point of less than 150 C. (separated in step a) is mixed with the petrol cut separated in the fractionation step f).

(47) In parallel, the heavy fraction of the waxes which boils above said light fraction, that is to say at a boiling point of greater than 370 C., is sent to a step e) of hydrocracking and of hydroisomerization in the presence of a hydrocracking and hydroisomerization catalyst comprising 0.3% by weight of Pt on an SiAl support and which operates at a temperature of 360 C., at a pressure of 0.65 MPa, at a space velocity of 2 h.sup.1, and at a hydrogen flow rate adjusted in order to obtain a ratio of 600 standard litres of hydrogen per litre of feedstock.

(48) The effluent resulting from the hydrocracking and hydroisomerization step e) is subsequently also sent to the fractionation step f) in order to separate a petrol fraction, a middle distillates fraction (kerosene and gas oil) and an unconverted fraction which is sent back to step e).

(49) The material balance according to the scheme is given in Table 3. As the hydrogen consumption is very low, it is disregarded in this example.

(50) TABLE-US-00003 TABLE 3 Material balance according to the principle scheme 2, with reference to the streams of FIG. 2 Stream [] 2 3 4 5 6 7 8 9 12 13 15 16 + 17 19 Flow [t/h] 55 45 17 38 11 34 17 49 34 49 5 78 22 rate

(51) The flow rate of middle distillate is increased by 4 wt % relative to the scheme of Example 5 not in accordance with the invention.

Example 4 (in Accordance with the Invention)

(52) Example 4 illustrates the embodiment according to FIG. 3.

(53) The effluent resulting from the Fischer-Tropsch synthesis unit is, at the outlet of the Fischer-Tropsch synthesis unit, divided into two fractions: a light fraction, known as cold condensate, and a heavy fraction, known as waxes.

(54) The characteristics of the various fractions are given in Table 1 above.

(55) Said heavy fraction, known as waxes, is separated in a distillation column (step a) into a light fraction, the final boiling point of which is less than 370 C., and a heavy fraction which boils above said light fraction.

(56) Said light fraction, the final boiling point of which is less than 370 C., is mixed with all of said cold condensate fraction.

(57) The mixture is then sent to a step of hydrotreatment in the presence of a hydrotreatment catalyst in its reduced form comprising 11.4% by weight of NiO and 8% by weight of MoO.sub.3 on an alumina support and which operates at a temperature of 330 C., at a pressure of 0.65 MPa, at a hydrogen flow rate adjusted in order to obtain a ratio of 600 standard litres per litre, and at an hourly space velocity of 2 h.sup.1.

(58) The effluent resulting from the hydrotreatment step is then separated in a distillation column (step a) so as to obtain a light fraction, the final boiling point of which is less than 150 C., and a heavy fraction, the initial boiling point of which is greater than 150 C. and the final boiling point of which is less than 370 C., said heavy fraction corresponding to the middle distillates fraction.

(59) The heavy fraction of the effluent resulting from the hydrotreatment step c) corresponding to the middle distillates fraction is sent to a step d) of hydroisomerization in the presence of the hydroisomerization catalyst C1, step d) being carried out at a temperature of 330 C., at a pressure of 0.65 MPa, at a space velocity of 2 h.sup.1, and at a hydrogen flow rate adjusted in order to obtain a ratio of 600 standard litres of hydrogen per litre of feedstock.

(60) The isomerized effluent is then sent to the fractionation step f) in order to obtain a petrol fraction, a middle distillates fraction (kerosene and gas oil) and an unconverted fraction.

(61) In parallel, the heavy fraction of the waxes which boils above said light fraction, that is to say at a boiling point of greater than 370 C., is sent to a step e) of hydrocracking and of hydroisomerization in the presence of a hydrocracking and hydroisomerization catalyst comprising 0.3% by weight of Pt on an SiAl support and which operates at a temperature of 360 C., at a pressure of 0.65 MPa, at a space velocity of 2 h.sup.1, and at a hydrogen flow rate adjusted in order to obtain a ratio of 600 standard litres of hydrogen per litre of feedstock.

(62) The effluent resulting from the hydrocracking and hydroisomerization step e) is subsequently also sent to the fractionation step f) in order to separate a petrol fraction, a middle distillates fraction (kerosene and gas oil) and an unconverted fraction which is sent back to step e).

(63) The light fraction of the effluent resulting from the hydrotreatment step c) having a final boiling point of less than 150 C. (separated in step a) is mixed with the petrol cut separated in the fractionation step f).

(64) The material balance according to the scheme is given in Table 4. As the hydrogen consumption is very low, it is disregarded in this example.

(65) TABLE-US-00004 TABLE 4 Material balance according to the principle scheme 3, with reference to the streams of FIG. 3 Stream [] 2 10 9 3 5 6 11 7 13 15 16 + 17 Flow [t/h] 55 45 11 66 17 49 34 49 34 22 78 rate

(66) The flow rate of middle distillate is increased by 4 wt % relative to the scheme of Example 5 not in accordance with the invention.

Example 5 (not in Accordance with the Invention)

(67) Example 5 is not in accordance with the invention in that the two effluents resulting from the Fischer-Tropsch synthesis, the light fraction, known as cold condensate (2), and the heavy fraction, known as waxes (3), are mixed in a stream 4, then simultaneously hydrotreated in a zone E and, finally, simultaneously hydrocracked/hydroisomerized in a zone F via a stream 5 on the IZM-2-based catalyst C1 according to a scheme represented in FIG. 4.

(68) The effluent resulting from the hydrocracking/hydroisomerization step 6 is then sent to a fractionation step, from which a naphtha cut 7 and a middle distillates fraction 8 and 9 are obtained. The unconverted fraction 10 is recycled to the hydrocracking/hydroisomerization step F.

(69) The operating conditions and the catalysts used in Example 5 are identical to those used in Example 2 according to the invention in the hydrotreatment and hydrocracking/hydroisomerization steps.

(70) The material balance according to the scheme is given in Table 5. As the hydrogen consumption is very low, it is disregarded in this example.

(71) TABLE-US-00005 TABLE 5 Material balance according to the scheme of the prior art 4 with reference to the streams of FIG. 4 Description Middle [] Condensates Waxes Naphtha distillate Stream [] 2 3 7 8 + 9 Flow rate [t/h] 55 45 25 75

(72) The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

(73) From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.