Storage of Fischer-Tropsch effluents

Abstract

Process for the production of middle distillates from a paraffinic feedstock produced by Fischer-Tropsch synthesis comprising at least one light fraction, known as condensate, and a heavy fraction, known as waxes, in which: the said light fraction is stored in a vessel (B) maintained under an inert atmosphere and in which the temperature inside the vessel is maintained at a value of less than 20 C.; the said heavy fraction is stored in a vessel (C) maintained under an inert atmosphere and in which the temperature inside the vessel is maintained at a value of between 80 and 230 C.

Claims

1. A process for the production of middle distillates from a paraffinic feedstock produced by Fischer-Tropsch synthesis comprising at least the following stages: a) the said paraffinic feedstock resulting from a Fischer-Tropsch unit (A) is recovered, the said paraffinic feedstock comprising at least a light fraction (2), known as condensate, and a heavy fraction (3), known as waxes; b) a least a part of the said light fraction and at least a part of the said heavy fraction which are obtained on conclusion of stage a) are sent, as a mixture (4), to a hydrotreating unit (D) in the presence of hydrogen and a hydrotreating catalyst to obtain a first hydrotreated effluent (5); c) at least a part of the first hydrotreated effluent (5) obtained on conclusion of stage b) is sent to a hydrocracking/hydroisomerization unit (E) in the presence of hydrogen and of a hydrocracking/hydroisomerization catalyst to obtain a second effluent (6); d) the second effluent (6) resulting from the hydrocracking/hydroisomerization unit is separated in a fractionation unit (F) to obtain at least a naphtha cut (7) having a maximum boiling point of less than 180 C., a middle distillates fraction (8,9) and an unconverted heavy fraction (10); which process is characterized by periodic shutdowns of the hydrotreating unit and/or hydrocracking/hydroisomerization unit and when the hydrotreating unit (D) and/or the hydrocracking/hydroisomerization unit (E) is at shutdown, then: the said light fraction (2) obtained on conclusion of stage a) is stored in a vessel (B) maintained under an inert atmosphere and in which the temperature inside the vessel is maintained at a value of less than 20 C.; and/or the said heavy fraction (3) obtained on conclusion of stage a) is stored in a vessel (C) maintained under an inert atmosphere and in which the temperature inside the vessel is maintained at a value of between 80 and 230 C.

2. The process according to claim 1, in which the said light fraction (2) is stored in the vessel (B) at a temperature of less than 15 C.

3. The process according to claim 1, in which the vessel (B) is maintained under an inert atmosphere by flushing by an inert gas and by creating an excess pressure of at most 0.1 MPa with respect to the pressure within the said vessel (B).

4. The process according to claim 1 in which the said light fraction exhibits an initial boiling point T.sub.1 of between 15 and 50 C.

5. The process according to claim 1, in which the said heavy fraction exhibits an initial boiling point T.sub.2 of between 100 and 300 C.

6. The process according to claim 1, in which the hydrotreating unit is operated at a temperature of between 250 and 450 C., at a pressure of between 0.5 and 15 MPa, an hourly space velocity of between 0.1 and 40 h.sup.1 and a hydrogen flow rate adjusted to obtain a ratio of between 100 and 3000 standard litres per litre.

7. The process according to claim 1, in which the hydrocracking/hydroisomerization stage c) is carried out unit is operated at a temperature of between 250 C. and 450 C., at a pressure of between 0.2 and 15 MPa, at an hourly space velocity of between 0.1 h.sup.1 and 10 h.sup.1 and at a hydrogen flow rate adjusted to obtain a ratio of between 100 and 2000 standard litres of hydrogen per litre of feedstock.

8. The process according to claim 1, in which the hydrocracking/hydroisomerization catalyst comprises at least one hydro/dehydrogenating metal selected from the group consisting metals of Group VIb and of Group VIII of the Periodic Table and at least one Bronsted acid solid, and optionally a binder.

9. The process according to claim 8, in which the said metal from Group VIII is platinum or palladium, taken alone or as a mixture, which is/are active in its/their reduced form.

10. The process according to claim 1, in which the hydrotreating catalyst comprises at least one metal selected from the group consisting of nickel, molybdenum, tungsten, cobalt, ruthenium, indium, palladium and platinum, alone or as a mixture, and comprises at least one support selected from the group consisting of aluminas, boron oxides, magnesia, zirconia, titanium oxides and clays or a combination of these oxides.

11. A process for the storage of a light fraction of an effluent resulting from the Fischer-Tropsch synthesis, the said light fraction exhibiting an initial boiling point T.sub.1 of between 15 and 50 C., the said light fraction being stored in a vessel maintained under an inert atmosphere and in which the temperature inside the vessel is maintained at a value of less than 20 C.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates an embodiment of the process for the production of middle distillates according to the invention in which the hydrotreating unit (D) and the hydrocracking/hydroisomerization unit (E) are in operation. The vessels (B) and (C) are not used to store the effluents resulting from the Fischer-Tropsch unit.

(2) FIG. 2 illustrates an embodiment of the process for the production of middle distillates according to the invention in which the hydrotreating unit (D) is not operating. The vessels (B) and (C) are used to store the effluents resulting from the Fischer-Tropsch unit.

(3) In FIGS. 1 and 2, the same references denote identical or analogous elements.

(4) With reference to FIG. 1, the synthesis gas 1, a gas composed predominantly of carbon monoxide and of hydrogen, is directed to a Fischer-Tropsch synthesis unit (A). The gas fraction under the operating conditions of the Fischer-Tropsch synthesis (A) is composed of the unconverted gas fraction, of the light hydrocarbons part and of the water formed by the Fischer-Tropsch synthesis (A). These three phases are separated by cooling and then flash distillation. The gas fraction is preferably recycled with the synthesis gas 1 so as to increase the total conversion and the material yield. The aqueous fraction is treated before being returned to the process or to the outside. The light hydrocarbon fraction 2, or condensates, is sent to the hydrotreating unit (D) as a mixture 4 with the heavy fraction 3 resulting from the Fischer-Tropsch synthesis (A). This heavy fraction 4, or waxes, corresponds to the liquid fraction under the operating conditions of the reaction section. In normal operation, the condensates 2 and the waxes 4 are sent directly to the hydrotreating stage (D). The objective of the hydrotreating stage (D) is to reduce the content of olefins and to decompose the oxygen-comprising compounds. The hydrotreated effluent 5 resulting from the hydrotreating unit (D) is sent to a hydrocracking/hydroisomerization stage (E). The objective of the hydrocracking/hydroisomerization stage (E) is to increase the yield of middle distillates, kerosene and gas oil, by selective conversion of the fraction having a boiling point of greater than 370 C. to give a hydrocarbon fraction having a boiling point of between 120 C. and 370 C., and to improve the properties of the middle distillates produced by the isomerization of the effluent entering the stage and predominately composed of normal paraffins. The effluent 6 is subsequently sent to a fractionation stage (F) in order to recover, by way of example, a naphtha fraction 7, a kerosene fraction 8 and a gas oil fraction 9. A heavier fraction 10 can also be recycled or recovered in order to be upgraded to give lubricating base. Thus, in this embodiment, the vessels (B,C) for storage respectively of the condensates 2 and of the waxes 3 are not used.

(5) With reference to FIG. 2, in the case of shutdown of the hydrotreating unit (D) and/or of the hydrocracking/hydroisomerization unit (E), storage of the condensates 2 in a vessel (B) is necessary. This storage has to be carried out under the operating conditions as described above. Analogously, storage of the waxes 3 in a vessel (C) has to be provided under the operating conditions as described above. These storages make it possible to keep the Fischer-Tropsch synthesis unit in operation, which unit is more complex to restart than the hydrotreating and/or hydrocracking/hydroisomerization units. In the case of the hydrocracking/hydroisomerization unit (E), storage of the effluent 5 can also be provided in a storage unit (not represented in FIG. 2), which can be recycled upstream of the hydrotreating unit (D) (not represented in FIG. 2).

(6) The examples presented below illustrate the invention without limiting the scope thereof.

EXAMPLES

(7) The aim of the examples below is to demonstrate the importance of the operating conditions for storage of the light fraction 2 (or condensates) of the paraffinic feedstock resulting from the Fischer-Tropsch unit (A) in the vessel (B).

(8) In order to do this, the maleic anhydride value (MAV) was used. This value is related to the property which conjugated olefinic double bonds have of being able to add to maleic anhydride. The MAV value is expressed in milligrams of anhydride consumed per gram of product subjected to the test. The objective of the method in the case of the present invention is to quantitatively determine the conjugated diolefins which are precursors of rubbers. The results are presented in Table 1 below.

(9) 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.

(10) 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.

(11) The entire disclosures of all applications, patents and publications, cited herein and of corresponding French application No. 18/56.668, filed Jul. 18, 2018, are incorporated by reference herein.

Example 1 According to the Invention

(12) In this example, the condensates produced by Fischer-Tropsch synthesis are stored in a vessel (B) at a temperature of 8 C.

Example 2 (Comparative)

(13) In this example, the condensates produced by Fischer-Tropsch synthesis are stored in a vessel (B) at a temperature of 35 C.

(14) TABLE-US-00001 TABLE 1 MAV value of the condensates stored in the vessel according to the temperature Storage temperature MAV value ( C.) (mg/g) Example 1 (in 8 16.5 accordance with the invention) Example 2 35 42.0 (comparative)

(15) 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.

(16) 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.