Liquid-liquid extraction column using plates equipped with a pressure drop generator element

Abstract

The present invention describes a liquid-liquid extraction column with perforated plates and overflows, the plates also being equipped with an additional friction element which makes it possible to increase the thickness of the layer of coalesced matter and to guarantee a counter-current flow of the continuous and dispersed phases.

Claims

1. A liquid-liquid counter-current extraction column with perforated plates having overflows for bringing into contact a light dispersed phase and a heavy continuous phase, wherein the density of the light dispersed phase is less than 750 kg/m.sup.3 and the difference in density between the heavy continuous phase and the light dispersed phase is greater than 300 kg/m.sup.3, the solute being transferred from the dispersed phase to the continuous phase, wherein said column is provided with open plates with perforations and a degree of perforation of between 2% and 6% which are completed by additional friction elements of a porosity strictly less than that of the open plate, each friction element being a perforated plate placed below each open plate at a distance of between 0.5 cm and 5 cm below said open plate, and the reduction factor in the porosity of each friction element being between 0.6 and 0.8 with respect to the porosity of an open plate and wherein the additional friction elements have their orifices offset with respect to the perforations of the open plate which it is below, such that the orifices of the additional friction element are in an opposite relationship with the perforations of the open plate.

2. A liquid-liquid counter-current extraction column with perforated plates having overflows for bringing into contact a heavy dispersed phase and a light continuous phase, wherein the density of the light continuous phase is less than 750 kg/m.sup.3 and the difference in density between the light continuous phase and the heavy dispersed phase is greater than 300 kg/m.sup.3, the solute being transferred from the dispersed phase to the continuous phase, wherein said column is provided with open plates with perforations and a degree of perforation of between 2% and 6% which are completed by additional friction elements of a porosity strictly less than that of the open plate, each friction element being placed above each open plate at a distance of between 0.5 cm and 5 cm above said open plate, and the reduction factor in the porosity of each friction element being between 0.6 and 0.8 with respect to the porosity of an open plate and wherein the additional friction elements have their orifices offset with respect to the perforations of the open plate which it is above, such that the orifices of the additional friction element are in an opposite relationship with the perforations of the open plate.

3. A liquid-liquid extraction column according to claim 1 wherein each open plate has an overflow with a passage section of between 6% and 20% of the free section of the column.

4. A liquid-liquid extraction column according to claim 1 wherein the number of open plates is between 4 and 25.

5. A method for the extraction of sulphur compounds from a hydrocarbon phase by an aqueous solution of sodium hydroxide representing from 10% to 20% by weight, comprising passing the aqueous solution as the heavy continuous phase and the hydrocarbon phase as the light dispersed phase through the liquid-liquid extraction column according to claim 1, wherein the density of the light dispersed phase is less than 750 kg/m.sup.3 and the difference in density between the heavy continuous phase and the light dispersed phase is greater than 300 kg/m.sup.3.

6. A method for liquid-liquid extraction which comprises contacting a light dispersed phase and a heavy continuous phase in an extraction column according to claim 1, where the difference in mass per unit of volume between the heavy continuous phase and the light phase is greater than 300 kg/m.sup.3 and the mass per unit of volume of the light dispersed phase is less than 750 kg/m.sup.3.

7. A liquid-liquid extraction column according to claim 2 wherein each open plate has an overflow with a passage section of between 6% and 20% of the free section of the column.

8. A liquid-liquid extraction column according to claim 2 wherein the number of open plates is between 4 and 25.

9. A method for the extraction of sulphur compounds from a hydrocarbon phase by an aqueous solution of sodium hydroxide representing from 10% to 20% by weight, comprising passing the aqueous solution as the heavy dispersed phase and the hydrocarbon phase as the light continuous phase through the liquid-liquid extraction column according to claim 2, wherein the density of the light continuous phase is less than 750 kg/m.sup.3 and the difference in density between the heavy dispersed phase and the light continuous phase is greater than 300 kg/m.sup.3.

10. A method for liquid-liquid extraction which comprises contacting a light continuous phase and a heavy dispersed phase in an extraction column according to claim 2, where the difference in mass per unit of volume between the heavy phase and the light phase is greater than 300 kg/m.sup.3 and the mass per unit of volume of the light continuous phase is less than 750 kg/m.sup.3.

11. A liquid-liquid extraction column according to claim 1 wherein the number of open plates is between 6 and 20.

12. A liquid-liquid extraction column according to claim 2 wherein the number of open plates is between 6 and 20.

13. A liquid-liquid extraction column according to claim 1, wherein the column has an equivalent number of theoretical stages of 2.8.

14. A liquid-liquid extraction column according to claim 2, wherein the column has an equivalent number of theoretical stages of 2.8.

15. A liquid-liquid extraction column according to claim 1, wherein the plates have an efficiency of 18.7%.

16. A liquid-liquid extraction column according to claim 2, wherein the plates have an efficiency of 18.7%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a liquid-liquid extraction column with plates and overflows according to the prior art,

(2) FIG. 2 shows the liquid-liquid extraction column according to the present invention in which each plate is equipped with an additional friction element which here is a plate of a porosity strictly less than the porosity of the open plate, and

(3) FIG. 3 shows a plan view of a plate according to the invention equipped with an additional friction element which is a plate of strictly lower porosity with its orifices offset with respect to those of the open plate.

BRIEF DESCRIPTION OF THE INVENTION

(4) The present invention can be defined as a liquid-liquid counter-current extraction column with perforated plates having overflows for bringing into contact a light dispersed phase and a heavy continuous phase, the solute being transferred from the dispersed phase to the continuous phase, said column being provided with open plates with a degree of perforation of between 2% and 6% and each open plate being completed by an additional friction element of a porosity strictly less than that of the open plate, each friction element being placed below each open plate at a distance of between 0.5 cm and 5 cm below said plate.

(5) When the dispersed phase is the heavy phase and the continuous phase is the light phase the definition of the extraction column according to the present invention becomes: a liquid-liquid counter-current extraction column with perforated plates having overflows for bringing into contact a heavy dispersed phase and a light continuous phase, the solute being transferred from the dispersed phase to the continuous phase, said column being provided with open plates with a degree of perforation of between 2% and 6% which are completed by additional friction elements of a porosity strictly less than that of the open plate, each friction element being a perforated plate placed above each open plate at a distance of between 0.5 cm and 5 cm above said plate.

(6) The porosity expressed in % represents the empty fraction of the plate with respect to the free section of the column.

(7) In a first variant of the present invention the additional friction elements are perforated plates disposed below (or above if the dispersed phase is the heavy phase) each open plate having a porosity equal to p times that of the open plate, p being between 0.6 and 0.9.

(8) In a preferred feature of the present invention the additional friction elements, when they are plates, have their orifices offset with respect to those of the open plate to which it is attached.

(9) In another variant of the present invention the additional friction elements are formed by one or more layers of a material of woven fibres.

(10) According to a preferred feature of the present invention the overflows of each plate have a passage section of between 6% and 20% of the free section of the column.

(11) According to another preferred feature of the present invention the number of plates of the extraction column according to the invention is between 4 and 25 and preferably between 6 and 20.

(12) The extraction column according to the present invention is applied in particular to the extraction of sulphur compounds from a hydrocarbon phase by an aqueous solution of sodium hydroxide representing from 10% to 20% by weight, the hydrocarbon phase being the dispersed phase.

(13) Finally the extraction column according to the present invention is particularly well applied to the case where the difference in density between the heavy phase and the light phase is greater than 300 kg/m.sup.3 and the density of the light phase is less than 750 kg/m.sup.3.

DETAILED DESCRIPTION OF THE INVENTION

(14) The invention concerns a liquid-liquid counter-current extraction column with perforated plates and overflows, involving a heavy phase and a light phase, between which a solute is exchanged, the difference in mass per unit of volume between the two liquids being high (greater than 300 kg/m.sup.3) and the mass per unit of volume of the light phase being low (less than 750 kg/m.sup.3).

(15) According to the case involved the light phase can be the continuous phase or the dispersed phase.

(16) Hereinafter in this specification for the sake of clarity we shall adopt the case of a dispersed light phase but the column according to the invention is equally well applied to the case of a heavy dispersed phase.

(17) The essential difference between the two cases represented is that, in the case of a light dispersed phase, the coalescence layer is below the plate while in the case of a heavy dispersed phase it is above the plate.

(18) The object of the invention involves ensuring good functioning of the extraction column by guaranteeing a minimum coalesced layer thickness (that is to say greater than 7 cm) and an optimum speed in terms of flow through the orifices of each plate to generate drops having a very tight size distribution.

(19) The invention cleverly combines a perforated plate referred to as open (Po) having a substantial perforated surface area so as to guarantee an optimum speed in terms of flow through the orifices, corresponding to a continuous jet of dispersed phase of maximum size at the orifice outlet, and an arrangement which we shall refer to hereinafter as the additional friction element (Pad) making it possible to ensure a coalesced layer thickness greater than 7 cm.

(20) The friction element (Pad) is disposed within the coalesced layer of dispersed phase and at a distance of between 0.5 cm and 5 cm from the open plate with which it is associated.

(21) It generates pressure drops in respect of the dispersed phase, such that the coalesced layer thickness is greater than 7 cm.

(22) According to a first variant of the column according to the invention the friction element (Pad) making it possible to generate the additional pressure drop is a second perforated plate.

(23) If the dispersed phase is the light phase the additional friction element (Pad) is therefore disposed beneath the open perforated plate (Po).

(24) If the dispersed phase is the heavy phase the additional friction element (Pad) is disposed above the open perforated plate (Po).

(25) The degree of perforation of the additional friction element (Pad) is strictly less than that of the open perforated plate (Po) in a ratio p of between 0.6 and 0.9 and preferably between 0.6 and 0.8.

(26) The degree of perforation of the open plate is between 2% and 6% with respect to the section of the empty column. To calculate the degree of perforation of the friction element the ratio p is therefore applied to the degree of perforation of the open plate.

(27) The perforations of the friction element (Pad) are so positioned that they are not on the same vertical line as the perforations of the open perforated plate (Po), as is shown in FIG. 3.

(28) In a preferred variant of the present invention the perforations of the friction element (Pad) can be offset so as to be in opposite relationship with respect to the perforations of the open plate (Po) as is shown in FIG. 3. It is very clear that this offset arrangement of the friction element with respect to the open plate maximises the pressure drop caused by said element.

(29) According to a second variant of the process according to the invention the friction element (Pad) permitting generation of the additional pressure drop is an assembly of woven fibres. That friction element thus facilitates coalescence of the dispersed phase and make it possible to maintain the thickness of the coalesced layer (Ch) in the desired range.

(30) More precisely FIG. 1 according to the prior art describes a liquid extraction column fed with a heavy phase by the conduit 2 and a light phase by the conduit 3.

(31) The heavy phase which is the continuous phase issues at the base of the column by way of the conduit 5 and the light phase which is the dispersed phase issues at the head of the column by way of the conduit 4.

(32) The column is equipped with eight overflow-equipped plates of the open type numbered P1 to P8.

(33) Each open plate is composed of a perforated plate (6) and an overflow (7).

(34) The pressure drop generated on passing through the plates induces the presence of a coalesced dispersed phase layer (8) under each plate.

(35) The section and the shape of the overflow of the closed plate can be constant over its entire length or variable as shown in FIG. 1 by the elements of different sections (10) and (12).

(36) FIG. 2 according to the invention shows more precisely the association of a plate referenced Po with its friction element which here is a plate referenced Pad. The coalesced layer under the plate is referenced Ch.

(37) D denotes the overflow whose section is between 6% and 20% of the free section of the column.

(38) FIG. 3 according to the invention shows a plan view of an open plate associated with its additional perforated plate disposed below (or above if the dispersed phase is the heavy phase), the additional perforated plate having a porosity equal p times that of the open plate, p being between 0.6 and 0.9. FIG. 3 shows the offset between the orifices (To) of the open plate and those (Tad) of the closed plate disposed below it.

(39) The section (D) of the overflow representing about 8% of the free section of the column is also illustrated.

(40) 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.

(41) 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.

(42) The entire disclosures of all applications, patents and publications, cited herein and of corresponding application No. FR 12/03.121, filed Nov. 20, 2012 are incorporated by reference herein.

Comparative Example According to the Invention

(43) The comparative example which follows is obtained by simulation.

(44) The liquid charge to be treated is an LPG (liquefied petroleum gas) containing 100 ppm of methanethiol.

(45) The solvent used to extract the methanethiol is a 20% solution of sodium hydroxide.

(46) The dispersed phase is the hydrocarbon phase, the continuous phase is the sodium hydroxide solution.

(47) The flow rates of LPG and sodium hydroxide are 165 and 4.3 m.sup.3/h respectively.

(48) The extraction operation is performed at 30 C. and under 10 bars absolute.

(49) Three separate devices are used to perform the extraction operation.

(50) 1) The device 1 referred to as an open plate device is according to the prior art.

(51) It is formed by an extraction column equipped with 15 perforated plates with overflows, spaced at 1 m, and 2.9 m in diameter. The perforations are 5 mm in diameter and the perforated surface area is equal to 2.9% of the column section so as to have a speed through the orifices of 0.24 m/s, as recommended in the prior art.

(52) The overflows have a surface area equal to 7.3% of the column section. The thickness of a coalesced layer under a plate is then 1 cm.

(53) 2) The device 2, still according to the prior art, is referred to as being a closed plate device. It is similar in every point to the device 1 but the perforated surface area is equal to 0.7% of the column section.

(54) The speed through the orifices is then 1.01 m/s.

(55) The coalesced layer thickness beneath the plate is then 10 cm.

(56) 3) The device 3 is according to the invention. It is formed by an extraction column equipped with 15 plates of open type, such as those equipping the device 1, each plate according to the device 1 being associated with a closed plate according to the device 2, the additional closed plate being located below the open plate. The assembly of open plate (Po)/closed plate (Pad) forms the plate according to the invention.

(57) Each additional closed plate is located below an open plate at a distance of 0.8 cm from the latter.

(58) The perforations in each additional closed plate are so positioned as not to be on the line of the perforations of the open plate disposed just above it. The open plates are spaced at 1 m.

(59) The speed through the orifices is then 0.24 cm/s.

(60) The thickness of the coalesced layer resulting from the plate according to the invention is 10 cm.

(61) A comparison between the three devices can be summarised in the following fashion:

(62) 1) The device formed by open plates does not make it possible to ensure a sufficient coalesced layer thickness over the whole of the section of the column to avoid the problems of retro-mixing of the sodium hydroxide which is the continuous phase. The small thickness of the coalesced layer of the hydrocarbon phase permits a fraction of the continuous phase of sodium hydroxide to pass through the perforated plate, giving rise to the bleeding phenomenon and ruling out the desired plug type flow.

(63) That device therefore no longer makes it possible to guarantee a suitable counter-current flow.

(64) The result of this is that the assembly of the open plates behaves like a single theoretical stage.

(65) 2) The device formed by closed plates ensures a substantial coalesced layer thickness but, by virtue of the diameter of the orifices, the speed through the orifices is so high that it involves the formation of fine droplets of dispersed phase. Those fine dispersed phase droplets give rise to coalescence problems below the plate, which induces retro-mixing of the sodium hydroxide, as well as a problem of retro-mixing of the hydrocarbon phase due to entrainment of the finest droplets into the overflows by the continuous sodium hydroxide phase.

(66) That device therefore no longer makes it possible to guarantee a suitable counter-current flow.

(67) The result of this is that the assembly of the closed plates behaves like a single theoretical stage.

(68) 3) Finally the device according to the invention ensures a good counter-current flow while maintaining a large coalesced layer thickness and generating a tight drop size distribution. The plates then have an efficiency of 18.7%. The device therefore has an equivalent number of theoretical stages of 2.8.

(69) 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.

(70) 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.