VARIABLE PERFORATION LIQUID-LIQUID EXTRACTION COLUMN

Abstract

Liquid-liquid extraction column (1) comprising points for feedstock injection (2), washing (3) and backwashing (4), two withdrawal points (5, 6), trays (P.sub.i) located along the column and defining 2 to 30 zones each comprising at least two trays, the n zones comprising: at least one extraction zone Z.sub.i comprising the zones from Z.sub.1 to Z.sub.x, x being greater than or equal to 1, and at least one backwash zone comprising the zones from Z.sub.x+1 to Z.sub.n, n being greater than x; in which the trays of the same zone have the same number of holes per perforated tray; in which, when x>1, the number of holes per perforated tray of the zones Z.sub.i increases when i increases; and when x=1, said number of holes of the backwash zone is less than/greater than than that of the zone Z.sub.1.

Claims

1. Liquid-liquid extraction column (1) comprising the following elements: a first injection point for a first phase (2) located at an intermediate position between the top and the bottom of the column (1); a second injection point for a second phase (3) and a third injection point for a backwash liquid (4), one being located at the top of the column (1) and the other being located at the bottom of the column (1); a first withdrawal point for an extract (5) and a second withdrawal point for a raffinate (6), one being located at the bottom of the column (1) and the other being located at the top of the column (1); a plurality of trays (P.sub.i) located from the top of the column (1) to the bottom of the column (1) and defining n zones, each zone comprising at least two trays (Pi), n being between 2 and 30; in which the n zones comprise: at least one extraction zone Z.sub.i located between a zone Z.sub.i comprising the second injection point for the second phase (3), and a feed zone Z.sub.x comprising the first injection point for the first phase (2), x being greater than or equal to 1; and at least one backwash zone located between a zone Z.sub.x+1 and a zone Z.sub.n comprising the third injection point for the backwash liquid (4), n being greater than x; in which the trays (Pi) of the same zone have the same number of holes (9); and in which: when x is greater than 1, the number of holes (9) per perforated tray (P.sub.i) of the zones Z.sub.i increases as the value i increases; and when x is equal to 1, the number of holes (9) per perforated tray (P.sub.i) of the at least one backwash zone is less than or greater than than the number of holes (9) per perforated tray (P.sub.i) of the zone Z.sub.i.

2. Liquid-liquid extraction column (1) according to claim 1, in which, when x is 1, the ratio of the number of holes per perforated tray of the at least one backwash zone to the number of holes per perforated tray of the zone Z.sub.1 is between 0.40 and 0.95, or the ratio of the number of holes per perforated tray of the zone Z.sub.1 to the number of holes per perforated tray of the at least one backwash zone is between 0.40 and 0.95.

3. Liquid-liquid extraction column (1) according to claim 1, in which n is between 3 and 30, et x is greater than 1.

4. Liquid-liquid extraction column (1) according to claim 3, in which, in the extraction zones Z.sub.i where i ranges from 1 to x, the ratio of the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.i to the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.i+1 is between 0.40 and 0.95.

5. Liquid-liquid extraction column (1) according to claim 3, in which, in the extraction zones Z.sub.i where i ranges from 1 to x, the ratio of the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.i to the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.i+1 is between 0.80 and 0.95.

6. Liquid-liquid extraction column (1) according to claim 1, in which the at least one backwash zone is a plurality of zones, from a zone Z.sub.x+1 to zone Z.sub.n, and in which the number of holes (9) per perforated tray (P.sub.i) is constant from zone Z.sub.x+1 to zone Z.sub.n.

7. Liquid-liquid extraction column (1) according to claim 1, in which the at least one backwash zone is a plurality of zones, from a zone Z.sub.x+1 to zone Z.sub.n, and in which the number of holes (9) per perforated tray (P.sub.i) increases from zone Z.sub.x+1 to zone Z.sub.n.

8. Liquid-liquid extraction column (1) according to claim 1, in which: the at least one backwash zone is a plurality of zones Z.sub.j between zone Z.sub.x+1 and zone Z.sub.n, and in the zones Z.sub.j where j ranges from x+1 to n, the ratio of the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.j to the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.j+1 is between 0.70 and 0.95.

9. Liquid-liquid extraction column (1) according to claim 8, in which, in the zones Z.sub.j where j ranges from x+1 to n, the ratio of the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.j to the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.j+1 is between 0.70 and 0.90.

10. Liquid-liquid extraction column (1) according to claim 8, in which, in the zones Z.sub.j where j ranges from x+1 to n, the ratio of the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.j to the number of holes (9) per perforated tray (P.sub.i) of a zone Z.sub.j+1 is between 0.80 and 0.90.

11. Liquid-liquid extraction column (1) according to claim 3, in which the number of zones Z.sub.i is between 2 and 10 and/or the number of backwash zones is between 1 and 10.

12. Liquid-liquid extraction column (1) according to claim 1, in which the number of holes (9) in the perforated trays (P.sub.i) of the backwash zone Z.sub.x+1 is less than, equal to or greater than the number of holes (9) in the perforated trays (P.sub.i) of the extraction zone Z.sub.x.

13. Liquid-liquid extraction column (1) according to claim 1, in which each perforated tray (P.sub.i) has a number of holes (9) of between 3000 and 20000.

14. Liquid-liquid extraction column (1) according to claim 1, in which the ratio of the area of the holes (9) of each tray (Pi) to the total area of each tray (Pi) is less than 0.5.

15. Liquid-liquid extraction column (1) according to claim 1, in which the diameter of the holes (9) of the trays (Pi) is between 2 mm and 20 mm.

Description

LIST OF FIGURES

[0057] The FIG. 1 schematically shows a cross-sectional view of a liquid-liquid extraction column according to the present invention.

[0058] The FIG. 2 schematically shows a cross-sectional view of the flow of the dispersed phase and of the continuous phase in a liquid-liquid extraction column according to the present invention.

[0059] The FIG. 3 schematically shows a cross-sectional view of a liquid-liquid extraction column according to the present invention defined by a plurality of zones Z.sub.i located between the column head zone 1 and the feed zone Z.sub.x, and a plurality of zones Z.sub.j located between the zone Z.sub.x+1 and the column bottom zone Z.sub.n.

[0060] The FIG. 4 is a graph showing the change in the thickness of the coalesced layer along a liquid-liquid extraction column according to the present invention, in which the number of holes in the trays P.sub.l is variable.

[0061] The FIG. 5 is a graph showing the change in the thickness of the coalesced layer along a reference liquid-liquid extraction column, in which the number of holes in the trays P.sub.l is constant.

DESCRIPTION OF THE EMBODIMENTS

[0062] Embodiments of the invention will now be described in detail. In the following detailed description, many specific details are presented in order to provide a deeper understanding of the invention. However, it will be apparent to a person skilled in the art that the invention can be performed without these specific details. In other cases, well-known characteristics have not been described in detail in order to avoid unnecessarily complicating the description.

[0063] In the present description, the term comprise is synonymous with (means the same as) include and contain, and is inclusive or open-ended and does not exclude other unrecited elements. It is understood that the term comprise includes the exclusive and closed term consist. In addition, in the present description, the term substantially corresponds to an approximation of 10%, preferably of 5%, very preferably of 2%, of a reference value such as a distance, velocity, flow rate, compound content, temperature, pressure, etc.

[0064] With reference to FIG. 1, a liquid-liquid extraction column 1 comprises the following elements: [0065] a first injection point for a first phase 2 (or liquid to be separated), such as a feedstock (e.g. a mixture of aromatic and non-aromatic C6-C11 compounds), located at an intermediate position between the top and bottom of column 1; [0066] a second injection point for a second phase 3 (or separation liquid), such as a solvent (e.g. sulfolane), located at the top of column 1; [0067] a third injection point for a backwash liquid 4, such as a recycle (e.g. a mixture comprising at least 50% by weight of light compounds, (i.e., C5-C8, preferably C5-C6, compounds), located at the bottom of column 1; and [0068] a first withdrawal point for an extract 5 (in liquid phase), such as a solvent enriched in extracted compounds (e.g. aromatic compounds), located at the bottom of column 1; and [0069] a second withdrawal point for a raffinate 6 (in liquid phase), such as a feedstock depleted in extracted compounds, located at the top of column 1.

[0070] In addition, in order to increase the yield and purity, two distinct operating zones are defined opposite the injection point of the liquid 2 to be separated: [0071] an extraction sector 7, extending substantially from the first injection point for the first phase 2 to substantially the second injection point for the second phase 3, notably allows the extraction of compounds (e.g. aromatics) from the liquid 2 to be separated by contact with the separation liquid 3 counter-currentwise (the yield zone), and [0072] a backwash sector 8, adjacent to the extraction sector 7 and extending substantially to the third injection point for the backwash liquid 4, notably makes it possible to backwash undesired compounds (e.g. heavy non-aromatic compounds) contained in the extract 5 by the backwash liquid 4 in order to ensure a high level of purity.

[0073] Specifically with reference to FIG. 1, the separation liquid exits column 1 entraining compounds of interest to be separated (e.g. aromatic compounds) to form the extract 5. The extract may also contain undesired compounds (e.g. light non-aromatic compounds, such as C6-C7 compounds) which can be separated downstream (e.g. by distillation and/or stripping). Advantageously, the extract 5 contains no (or very few) undesired compounds that are difficult to separate (e.g. heavier non-aromatic compounds, such as C8+ compounds), which are separated from the extract in the backwash sector 8. With reference to FIG. 1, the separation liquid 3 is heavier than the liquid 2 to be separated and is injected at the top of column 1 while the backwash liquid 4 is injected at the bottom of column 1. It is understood that the present invention also relates to liquid-liquid extraction columns, in which the separation liquid is lighter than the liquid 2 to be separated, the point of injection of the separation liquid 3 is at the bottom of column 1 and the point of injection of the backwash liquid 4 is at the top of column 1.

[0074] With reference to FIG. 2, a two-pass liquid-liquid extraction column 1 comprises n perforated trays P.sub.i, i being between 1 and n. Each perforated tray P.sub.i is arranged so that the dispersed phase (i.e. the separation liquid 3 heavier than the liquid 2 to be separated) flows through the holes 9 in the perforated tray P.sub.i, the droplets of the dispersed phase recoalescing on the next perforated tray P.sub.i+1 to form a liquid volume preventing the passage of the continuous phase (i.e. the liquid 2 to be separated lighter than the separation liquid 3) through the perforated tray P.sub.i+1. The liquid 2 to be separated flows counter-currentwise relative to the separation liquid 3, i.e. upwards through central weirs 11 and peripheral weirs 12 of cross-section S.sub.C and S.sub.P, respectively, and transversely through an inter-tray space 10 of height H. With reference to FIG. 2, the heavy phase is the dispersed phase and the light phase is the continuous phase. It is understood that a liquid-liquid extraction column 1 may comprise perforated trays which are adapted so that the dispersed phase is the light phase and the continuous phase is the heavy phase.

[0075] According to one or more embodiments, the perforated trays P.sub.i are one-pass (e.g. one type of weir) or two-pass (e.g. two types of weirs) or multi-pass trays.

[0076] The Applicant has identified that the operation of a liquid-liquid extraction column can give rise to significant variations in flow rate and in the physicochemical properties of the phases circulating in the column, and that the use of trays which differ according to their position in the column can lead to ensuring homogeneous efficiency of the column, contrary to the prior art.

[0077] According to the invention, with reference to FIG. 3, a liquid-liquid extraction column 1 is also defined by: [0078] at least one extraction zone Z.sub.i, and preferably a plurality of extraction zones Z.sub.i defining the extraction sector 7, i.e. the extraction zone(s) Z.sub.i are located between the column head zone Z.sub.1 comprising the second injection point of the second phase 3, and the feed zone Z.sub.x comprising the first injection point of the first phase 2, x being greater than or equal to 1, preferably x being greater than 1; and [0079] at least one backwash zone defining the backwash sector 8, i.e. the backwash zone(s) are located between the zone Z.sub.x+1 and the column bottom zone Z.sub.n comprising the third injection point for the backwash liquid 4, n being greater than x.

[0080] According to the invention, each extraction and backwash zone comprises at least two trays, each extraction and backwash zone defining the structural characteristics of the perforated trays P.sub.i present in said extraction and backwash zones. Thus, according to the invention, the perforated trays P.sub.i of the same extraction or backwash zone have substantially the same number of holes 9 per perforated tray P.sub.i.

[0081] According to one or more embodiments, with reference to FIG. 3, the at least one backwash zone is a plurality of zones subdivided into a plurality of zones Z.sub.j located between the zone Z.sub.x+1 and the column bottom zone Z.sub.n.

[0082] Advantageously, the number of zones Z.sub.i and Z.sub.j can be defined relative to the flow rate variability and the physicochemical properties of the phase passing through said zones Z.sub.i and Z.sub.j.

[0083] According to one or more embodiments, the total number n of zones is between 2 and 30, preferably between 3 and 30, very preferably between 4 and 24, such as between 4 and 18, in particular between 4 and 8.

[0084] In the present description, i, j, x and n are natural integers.

[0085] The number of extraction zones Z.sub.i can be defined relative to the phase that has the most flow rate variability in the column. According to one or more embodiments, the number of zones Z.sub.i (number of zones Z.sub.1 to Z.sub.x) is between 1 and 10, preferably between 2 and 10, very preferably between 2 and 6, such as between 2 and 4.

[0086] The number of backwash zones Z.sub.j can be defined relative to the phase that has the most flow rate variability in the column. According to one or more embodiments, the number of backwash zones (number of zones Z.sub.x+1 to Z.sub.n) is between 1 and 10, preferably between 1 and 6, very preferably between 1 and 4. According to one or more embodiments, the number of backwash zones (number of zones Z.sub.x+1 to Z.sub.n) is greater than or equal to 2.

[0087] According to one or more embodiments, the number of trays per zone Z.sub.i and Z.sub.j can be determined by the number of actual stages required for the separation divided by the number of zones Z.sub.i and Z.sub.j.

[0088] Control of the flow rate variation of the dispersed phase Advantageously, the liquid-liquid extraction column 1 according to the invention comprises perforated trays Pi comprising a variable number of holes 9 so that the axial velocity (parallel to the central axis Z of the column) of the dispersed phase through the perforations remains substantially constant in the column. Indeed, due to the fluctuation of the flow rate of the dispersed phase and the physicochemical properties of the dispersed phase as it passes through the column, the variation of the number of holes in the perforated trays P.sub.i ensures that a minimum thickness of coalesced layer is maintained and that the hydraulic injection conditions in the column are identical. This solution also reduces the axial mixing of the continuous phase.

[0089] Specifically, in order to keep the axial velocity of the dispersed phase through the perforations substantially constant in the column, the liquid-liquid extraction column 1 according to the invention is divided into: [0090] x zones Z.sub.i, of which zones Z.sub.1 to Z.sub.x are located from the second withdrawal point for the raffinate 6 to the first injection point for the first phase 2 (phase to be separated), [0091] n-x zones Z.sub.j, of which zones Z.sub.x+1 to Z.sub.n are located from the point below the first phase 2 (phase to be separated) to the extract 5 withdrawal point at the bottom of the column.

[0092] According to the invention, when x is equal to 1, the number of holes in the perforated trays P.sub.i of the at least one backwash zone (e.g. zone Z.sub.2) is greater or less than the number of holes in the perforated trays P.sub.i of zone Z.sub.1. According to one or more embodiments, when x is equal to 1, the ratio of the number of holes in the perforated trays P.sub.i of the at least one backwash zone (e.g. the zone Z.sub.2) to the number of holes in the perforated trays P.sub.i of the zone Z.sub.1 is between 0.40 and 0.95, preferably between 0.60 and 0.95, very preferably between 0.80 and 0.95. According to one or more embodiments, when x is equal to 1, the ratio of the number of holes in the perforated trays P.sub.i of the zone Z.sub.1 to the number of holes in the perforated trays P.sub.i of the at least one backwash zone (e.g. the zone Z.sub.2) is between 0.40 and 0.95, preferably between 0.60 and 0.95, very preferably between 0.80 and 0.95.

[0093] According to the invention, when x is greater than 1, in the extraction zones Z.sub.i where i ranges from 1 to x (in the extraction sector 7), the number of holes in the perforated trays P.sub.i increases as the value i increases. According to one or more embodiments, in the zones Z.sub.i where i ranges from 1 to x, the ratio of the number of holes in the perforated trays P.sub.i of a zone Z.sub.i to the number of holes in the perforated trays P.sub.i of a zone Z.sub.i+1 is between 0.40 and 0.95, preferably between 0.60 and 0.95, very preferably between 0.80 and 0.95.

[0094] According to one or more embodiments, in the zones Z.sub.j where j ranges from x+1 to n (in the backwash sector 8), the number of holes 9 in the perforated trays P.sub.i is substantially constant.

[0095] According to one or more embodiments, in the zones Z.sub.j where j ranges from x+1 to n, the number of holes 9 in the perforated trays P.sub.i increases as the value i increases. According to one or more embodiments, in the zones Z.sub.j where j ranges from x+1 to n, the ratio of the number of holes 9 in the perforated trays P.sub.i of a zone Z.sub.j to the number of holes 9 in the perforated trays P.sub.i of a zone Z.sub.j+1 is between 0.70 and 0.95, preferably between 0.70 and 0.90, very preferably between 0.80 and 0.90.

[0096] According to one or more embodiments, the number of holes 9 in the perforated trays P.sub.i of the backwash zone Z.sub.x+1 is less than, equal to or greater than the number of holes 9 in the perforated trays P.sub.i of the extraction zone Z.sub.x. According to one or more embodiments, the ratio of the number of holes in the perforated trays P.sub.i of the backwash zone Z.sub.x+1 to the number of holes in the perforated trays P.sub.i of the extraction zone Z.sub.x is between 0.40 and 0.95, preferably between 0.60 and 0.95, very preferably between 0.80 and 0.95. According to one or more embodiments, the ratio of the number of holes in the perforated trays P.sub.i of the extraction zone Z.sub.x to the number of holes in the perforated trays P.sub.i of the backwash zone Z.sub.x+1 is between 0.40 and 0.95, preferably between 0.60 and 0.95, very preferably between 0.80 and 0.95.

[0097] According to one or more embodiments, each perforated tray P.sub.i has a number of holes of between 3000 and 20 000. According to one or more embodiments, for each tray P.sub.i, the ratio of the hole area to the total tray area is less than 0.5. According to one or more embodiments, the ratio of the hole area to the total tray area is between 0.005 and 0.2, such as between 0.01 and 0.1. According to one or more embodiments, the hole size of all the trays in the column is substantially constant. According to one or more embodiments, the diameter of the holes 9 in the trays P.sub.i is between 2 mm and 20 mm, preferably between 4 mm and 15 mm, very preferably between 5 mm and 10 mm.

EXAMPLES

Example 1: Liquid-Liquid Extraction Column with a Variable Number of Holes

[0098] This example is directed towards describing the effect of adjusting the number of holes on the homogeneity of the axial velocities of the dispersed phase and the height of the coalesced layer.

[0099] The column has a diameter of 2.9 m and comprises a succession of 108 perforated trays. The feedstock is injected at intermediate tray No. 47. The heavy solvent is injected at the top of the column at tray 1. The counter solvent is injected at the bottom of the column at tray 108.

[0100] Three zones are defined to adjust the number of holes to the variations of flow rate of the dispersed phase along the column: [0101] the first zone, Z1, is between trays 1 and 27: the number of holes is 3300, the rest of the geometry being unchanged; [0102] the second zone, Z2, is between trays 28 and 47: the number of holes is increased to 4000, in view of the increase in the flow rate of the dispersed phase, zones Z1 and Z2 correspond to the extraction sector 7; and [0103] the third zone, Z3, is between trays 48 and 108: this zone corresponds to the backwash sector 8; the number of holes is 5012.

[0104] In the extraction sector 7 between the head tray X=1 and the feed tray X=47, the number of holes in zone Z1 is less than the number of holes in zone Z2: the ratio of the number of holes in zone Z1 to the number of holes in zone Z.sub.2 is 0.83.

[0105] In the backwash sector 8 between the tray x=48 and the bottom tray x=108, the number of holes in zone Z2 is less than the number of holes in zone Z3: the ratio of the number of holes in zone Z2 to the number of holes in zone Z3 is 0.80.

[0106] FIG. 4 illustrates the technical effect of this adjustment: it ensures homogeneity of hydraulic functioning along the column. Firstly, this adjustment makes it possible to obtain a coalesced layer thickness above the tray which is always within an optimal range so as to prevent the continuous phase from passing through the tray. Secondly, this adjustment keeps the axial velocity of the dispersed phase through the tray perforations substantially constant throughout the column: thus, in this example, the velocity distribution is centred around a reference velocity with a standard deviation of 0.02.

[0107] The adjustment of the number of perforations thus ensures constant performance regardless of variations in the flow rate of the dispersed phase along the column.

Counter-Example 2: Liquid-Liquid Extraction Column with a Constant Number of Holes

[0108] The column has a diameter of 2.9 m, a height of 42 m and is composed of a succession of 108 perforated trays. The feedstock is injected at intermediate tray No. 47. The heavy solvent is injected at the top of the column at tray 1. The counter solvent is injected at the bottom of the column at tray 108.

[0109] No adjustment is made: the number of holes is constant (5012 holes) and the characteristics of the trays are identical in all points.

[0110] FIG. 5 illustrates that, without adjustment of the number of holes, the thickness of the coalesced layer is too low for trays No. 1 to 40, i.e. more than 35% of the trays, which can lead to hydraulic malfunction and thus degrade the efficiency of the extraction sector.

[0111] Moreover, the non-adjustment of the number of holes leads to velocity heterogeneities through the perforations of the trays: the velocity distribution along the column can vary between 60% and 115% of the reference velocity (standard deviation=0.15). Such functioning leads to a decrease in the interfacial area of more than 50% in the low velocity zones, and thus to degradation of the column efficiency.