OA16783A - Liquid extraction column comprising two types of distinct plates. - Google Patents

Abstract

The present invention describes a liquid extraction column with perforated trays and weirs, having a majority of so-called “open” trays and a minority of so-called “closed” trays, the two types of trays differing in the degree of perforation. Application of said column to the treatment of a liquid system with variable coalescence over time.

Description

The principle of solvent extraction between two partially immiscible liquid phases has been known for several decades. If a raffinai phase containing the solute is brought into contact with an extracted phase containing none (or little), a transfer of the solute takes place from one phase to another until a thermodynamic equilibrium between both phases.

The phenomenon is greatly facilitated if, at thermodynamic equilibrium, the solute concentration in the extracted phase is greater than that in the raffin phase. The concentration ratio between its two phases at equilibrium is called the partition coefficient.

A usual implementation of liquid-liquid extraction consists in bringing the two liquids into contact in counter-current, that is to say in opposite directions of flow. In the case of extraction in a column positioned vertically, we speak of a light phase and a heavy phase.

A very common case is that of systems composed of an organic or hydrocarbon phase and an aqueous phase. In this case, the aqueous phase is very often the heavy phase,

In liquid liquid extraction, a distinction must also be made between the continuous phase and the dispersed phase.

The continuous phase follows a flow as close as possible to the plug flow, that is to say without back-mixing.

The dispersed phase follows a flow generally countercurrent to that of the continuous phase and passes through a system of perforated plates which allows the formation of droplets, droplets which coalesce to form a coalesced layer located under each perforated plate when the dispersed phase is at, light phase (above each tray when the dispersed phase is the heavy phase). The exchanges of material between continuous phase and dispersed phase essentially take place between the droplets of the dispersed phase and the continuous phase flowing above each plate.

The counter-current implementation makes it possible to constitute several successive theoretical stages in the column and thus to achieve an extraction efficiency much higher than that obtained with a single stage corresponding to thermodynamic equilibrium.

A more precise description of liquid-liquid extraction operations is provided in numerous dedicated books, including Handbook of Solvent Extraction by Lo, Baird & Hanson, Krieger Publishing Company, Malabar, 1991, or Transport Phenomena in Liquid Extraction by Laddha & Degaleesan , Tata McGraw-Hill Publishing Co Ltd, 1976.

The concepts of theoretical stages are described therein in particular, which is an important point for understanding the value of the invention.

Many extraction technologies are based on the dispersion of one phase in the other, and we speak of dispersed phase and continuous phase. The dispersed phase can be the heavy or the light phase.

When the partition coefficient of a solute to be extracted is very high, i.e. the solute is very easily transferred into the extracted phase, the liquid-liquid extraction columns against the current are operated with a phase flow rate very low extract compared to the raffin phase flow rate. As a result, one of the two liquid phases moves through the column at a very low average speed compared to the other phase. In this case, very particular attention must be paid to the hydrodynamics in the column, because there is a great risk of not operating the column efficiently, and of not reaching more than one theoretical stage.

The benefit of the countercurrent operation may be reduced due to lower than expected extraction efficiency.

For example, when the dispersed phase is the phase which has the greatest flow, it is possible that the displacement of the droplets causes a little continuous phase in their wakes, which tends to displace said continuous phase in the direction opposite to its wake. average movement.

In the extreme case, the flow of the continuous phase becomes comparable to that of a single perfectly stirred reactor, and the solute concentrations in the continuous phase are homogeneous in the column. The theoretical number of stages of the device is then 1.

t t

When the ratio of volume flow rates between extract (E) and raffinate (R) phases, denoted E / R, exceeds 5 or is less than 0.2, a plate and weir extraction column technology is often used.

This technology is based on the division of the column into compartments separated two by two by a perforated plate equipped with a weir. The function of the plate is to coalesce the dispersed phase on one side of the plate and to redisperse this same phase on the other side, through the perforations of the plate.

This ensures a mono-directional flow of the dispersed phase between one compartment and the next compartment. Likewise, the fact of forming a coalesced layer of phase initially dispersed on one side of the plate makes it possible to prevent the passage of the continuous phase through the perforations of the plate.

The number and size of the perforations in the tray are chosen to generate a pressure drop which will induce the formation of a coalesced layer of dispersed phase having a desired thickness under given operating conditions.

For example, in the case of a light dispersed phase, the perforations can be chosen to form a layer of coalesced light phase under each plate, a layer with a thickness typically between 40 mm and 100 mm.

The coalesced layer should be thick enough over the entire section of the column to prevent passage of the continuous phase through the perforations of the tray. However, too thick a layer is detrimental to the efficiency of extraction, because it decreases the volume in which the transfer between liquid phases takes place, that is to say the volume in which the dispersed phase droplets are present.

The tray of a liquid liquid extraction column is generally equipped with a weir whose geometry and location are chosen to allow the movement of the continuous phase from one compartment to another, in the direction opposite to the movement. of the dispersed phase.

In addition, the geometry of the weir is chosen to limit the entrainment of dispersed phase droplets, such entrainment having the consequence of degrading the overall performance of the extraction by reducing the number of associated theoretical stages. A maximum speed criterion in the weir is therefore often imposed so as not to entrain droplets of dispersed phase.

The successive trays of the extraction column are staggered so that the continuous phase passes through the compartment in its entire width before entering a new weir.

D-shaped weirs, more generally in the form of an arc, are often used because of their simplicity and the fact that they make it possible to cover a large fraction of the diameter of the column, thus minimizing dead volumes. The chord of the weir is conventionally fixed between 0.3 and 0.8 times the diameter of the column.

Several problems can be encountered during the sizing and operation of extraction columns with perforated plates and weirs.

If the dispersed phase flow rate is greater than ten times the continuous phase flow rate, it becomes difficult to design weirs allowing both to prevent the entrainment of droplets, having a chord representing a large fraction of the column diameter, and allowing sufficient compartmentalisation of the flow of the continuous phase to prevent re-mixing of the latter in the column.

It is then sometimes suggested to use trays having very small weir sections, as mentioned in patents US4207174 and US6749741B1.

To limit the entrainment of droplets, the compartments are equipped with baffles and internal partitions which limit the area of the compartment in which contact between the liquid phases is possible. As a result, this type of plate offers a small contact surface between phases, which is detrimental to the efficiency of the extraction.

The interfacial tension of the liquid-liquid system treated is an important parameter because it conditions the size of the droplets formed and therefore their terminal velocity. This parameter therefore conditions the maximum speed allowed in the weirs without entrainment of the dispersed phase, and therefore conditions the minimum section of the weirs.

Another problem encountered is linked to the ease of coalescence of the dispersed phase droplets. If the system is very coalescing, a small thickness of dispersed phase is necessary to recoalesce the drops and form a continuous layer before passing through the plate.

On the other hand, if the coalescence between droplets is slow, it is necessary to ensure a certain contact time between the droplets before they coalesce. Therefore the trays are then effective only if the dispersed phase layer is significant under or on the trays, depending on whether the dispersed phase is light or heavy respectively.

It is then necessary to reduce the total section of the perforations in order to increase the pressure drop linked to the crossing of the plates and thus increase the thickness of the coalesced phase. This often requires, as a consequence, to increase the distance between plates.

It is easily understood that, depending on the coalescing or slightly coalescing nature of the system, and depending on its interfacial tension, the optimal plate will not have the same geometry.

A plate defined to mainly treat a very coalescent system with high interfacial tension can have a very degraded operation in the event of a drop in interfacial tension due to the entrainment of droplets in the weirs, or in the event of slower coalescence due to continuous phase passage through the plates.

The present invention describes a liquid liquid extraction column equipped with trays and weirs, which makes it possible to maintain the extraction performance despite any changes in interfacial tension, and of a more or less coalescing nature of the system.

Document US Pat. No. 4,081,354 teaches a liquid liquid extraction process exhibiting variable flow rates of dispersed phase. It does not specify any variation in the degree of opening of the trays used.

Document US Pat. No. 2,794,711 teaches a device allowing the intimate contact of two liquids by means of contact plates and dispersion baffles. No information on a possible difference in the opening degree of the trays is provided in this document.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 represents an overall view of an extraction column according to the invention in the case of a dispersed light phase.

FIG. 2 represents an overall view of a column according to the invention in the event of modification of an existing column initially comprising perforated plates and conventional weirs.

FIG. 3 represents a detailed view of a closed plate with a through weir according to the invention in the event of modification of an existing column.

Figure 4 shows a detailed view of the column according to the invention with the addition of filter media under the closed plate.

"

I

FIG. 5 represents an overall view of the column according to the invention in the case of a dispersed heavy phase,

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to a countercurrent liquid-liquid extraction column with perforated plates with weirs requiring a small number of theoretical stages (2 <NET <4), and within which the partition coefficient of the solute to be extracted is very high so that the extracted phase flow rate is very low compared to the raffinate phase flow rate.

The partition coefficient is defined as the ratio of the molar concentration of solute in the extract to the molar concentration of solute in the raffinate at equilibrium.

In the context of the present invention, this partition coefficient is> 50.

The object of the invention is to ensure proper operation of the extraction column even in the event of variations in the properties of the liquid-liquid system treated (interfacial tension, coalescing nature of the dispersed phase, etc.).

The device according to the invention combines in an innovative way 2 types of perforated plates with weirs having very distinct geometries and functions.

The two types of trays differ in both the perforation section and the weir section. We thus define in the remainder of the text the so-called open plate as having a perforation section and a given weir section, and the so-called closed plate as having a lower perforation section and a weir section than those of the open plate.

The invention is based on the use of a majority of open plates in the column, and a small number, or minority, of closed plates distributed within the open plates. A closed plate is always included between two open plates.

The number of open trays is between 4 and 25, preferably between 6 and 20, and that of closed trays is between 1 and 3 and preferably between 1 and 2.

The open plates are defined to obtain optimum operation in the presence of a coalescing system with high interfacial tension, i.e. having a coalescence time between two drops in contact of less than 2 seconds, and an interfacial tension greater than mN / m.

As a result, the thickness of the coalesced layer is maintained between 50 mm and 100 mm.

The weir of an open plate is chosen so that the speed of the dispersed phase is not greater than a value between 5 mm / s and 60 mm / s. The maximum value not to be exceeded, located in the range of 5 to 60 mm / s, depends on the physico-chemical properties of the liquid liquid system, and in particular on its surface tension.

The closed plate has a perforation section smaller than that of an open plate, the perforation reduction coefficient being between 0.2 and 0.8 and preferably between 0.3 and 0.5 times the perforation of an open tray.

The perforation of an open plate is typically between 2% and 6% relative to the empty section of the column, and preferably between 3% and 5% of the empty section of the column.

As a result, the layer of dispersed phase coalesced under a closed plate has a thickness of between 100 mm and 1000 mm, preferably between 300 mm and 800 mm.

The section of the weir of a closed tray is chosen so as to be between 0.25 and 1 times the section of the weir of the open trays.

The invention is particularly well suited to the case of a liquid liquid system the coalescing character of which changes over time. This type of variation in properties can be linked to various phenomena: a change in load, the use of a discontinuous pretreatment, etc. More precisely, the invention applies particularly well to systems which coalesce rapidly most of the time, but which from time to time become not very coalescent, or even foaming.

In this case, a device according to the prior art equipped with trays only of the open type may have greatly reduced separation performance due to the poor coalescence of the dispersed phase, and due to the entrainment of the continuous phase through the perforations of the tray. As explained in the paragraph devoted to the examination of the prior art, these two phenomena reduce the number of theoretical stages of the device.

When the system is not very coalescent, the simplest solution that a person skilled in the art would think of would be to implement a device equipped only with more closed plates, generating a greater contact time of the phase to be coalesced before passing through. the perforated tray. But such a device would then be inefficient when the liquid-liquid system would at times become rapidly coalescing.

Starting from a rapidly coalescing system, the invention makes it possible to maintain good performance, even when the system intermittently becomes less coalescent.

Likewise, starting from a slowly coalescing system, the invention makes it possible to maintain good performance even when the system intermittently becomes rapidly coalescing.

DETAILED DESCRIPTION OF THE INVENTION

More precisely, the invention can be defined as a countercurrent liquid liquid extraction column with perforated trays with weirs requiring a limited number of theoretical stages (2 <NET <4), and within which the partition coefficient of the solute to be extracted is greater than or equal to 50.

Said extraction column is dimensioned for loads which coalesce easily most of the time, but also allows the treatment of weakly coalescent load, or of easily coalescent loads, but which, intermittently, may become less coalescent, or even weakly coalescent. It therefore has a very wide field of application with regard to the coalescing nature of the liquid liquid system used, and this clearly distinguishes it from the extraction columns of the prior art.

Said column has two distinct types of plates; a majority of open plates defined by a degree of perforation p% between 2% and 6%, and a minority of closed plates defined by a degree of perforation between 0.2 and 0.8 times, and preferably between 0, 3 and 0.5 times that of open trays. Percentage of perforation is related to the empty section of the column. Preferably, the degree of perforation of the open trays is between 3% and 5%.

Each closed plate is located between two open plates, and the minority of closed plates is located in the middle part of the column, that is to say by dividing this one into 2 substantially equal portions, numbered l and 2 from top to bottom, in the lower part of portion 1 or the upper part of portion number 2.

According to a preferred variant of the present invention, the weirs associated with the closed trays have sections of between 0.25 and 1 times that of the weirs of the open trays.

According to another preferred variant, the weirs of the closed plates have a variable section in the sense that they have a first section followed, in the direction of the flow of the liquid, by a second section of larger size than the first.

According to an equally preferred variant of the present invention, a closed plate occupies the space corresponding to two open plates, and in this case, the weir associated with said closed plate comprises a part passing through the column substantially along a diameter, so that the outlet of said weir is located opposite its inlet.

According to another variant of the present invention, under certain closed trays there is a lining layer intended to promote coalescence, the thickness of said layer being less than the height of the weir of the closed tray considered.

Preferably, the number of open trays is between 4 and 25, and very preferably, it is between 6 and 20. The number of closed trays is preferably between 1 and 3, and very preferably included. between 1 and 2.

In general, the extraction column according to the present invention makes it possible to treat liquid liquid systems in which the dispersed phase can be either the light phase or the heavy phase.

The extraction column according to the present invention applies particularly well to the extraction of sulfur compounds from a hydrocarbon phase with an aqueous sodium hydroxide solution representing from 10% to 20% by weight, the hydrocarbon phase being the dispersed phase.

Preferably, the liquid liquid extraction column according to the present invention is applied to systems in which the flow rate ratio between the extracted phase (E) and the raffinate phase (R) is between 5 and 50 or (at exclusive direction) between 0.02 and 0.2.

The rest of the detailed description is made on the basis of the figures.

FIG. 1 describes a liquid liquid extraction column 1 according to the invention, supplied with a heavy phase via line 2, and with a light phase via line 3.

The heavy phase, which is the continuous phase, exits at the bottom of the column via line 5, and the light phase, which is the dispersed phase, exits at the top via line 4.

The column is equipped with 7 open type plates and a closed type plate located halfway down the column. By numbering the boards from 1 to N from top to bottom, the closed board is No. 4, and the open boards are the boards 1 to 3 and 5 to 8.

Each open tray consists of a perforated plate (6) and a weir (7).

The pressure drop generated when crossing the plates induces the presence of a layer of coalesced dispersed phase (8) under each plate.

The closed-type tray consists of a perforated plate (9) and a weir (10).

ίο

The perforations of the closed tray are defined by a degree of perforation relative to the perforations of an open tray. The degree of perforation of a closed plate is between 0.2 and 0.8 times that of an open plate, and preferably between 0.3 and 0.5 times said perforation of an open plate.

The section and shape of the weir of the closed tray can be constant over its entire length, or variable as shown in Figure 1.

Constant or variable, the section of the weir of a closed plateau is between 0.25 and

I times the weir section of an open plateau.

The coalesced layer (11) generated under the closed plate is then greater than the coalesced layer (8) under the open plates, which may require a spacing between a closed plate and the open plate located below, greater than the spacing between two successive open trays.

FIG. 2 illustrates the invention in the case of an existing extraction column initially equipped with trays only of the open type. This case corresponds to the remodeling of an existing column to cope with liquid liquid systems which may be weakly coalescent at times.

In this case, the implementation of the invention consists in modifying an open plate by plugging a fraction of the perforations so as to reach the range of 0.2 to 0.8 times the initial perforation, and in removing the lower open plate. so as to leave sufficient free space for the coalesced layer (11) of dispersed phase which will become larger as a result of the reduction in the degree of perforation.

It is then also necessary to modify the weir (10) of the closed plate thus formed in order to reduce its section in a ratio which is between 0.25 and 1 times the section of the weirs of the open plates.

To respect the orientation of the existing open plates, the weir (10) uses bends so as to lead the liquid phase to the side opposite that of said weir (10) as shown in FIG. 2. We then speak crossing weir, since the weir (10) has a slightly inclined portion extending substantially over the width of the column. The weir (10) of the closed tray No.4 communicates with the weir of the open tray located below (tray No.5) as shown in Figure 2.

The weir (12) of the open plate located below the closed plate No. 4 can thus be preserved.

Figure 3 shows in more detail the perforated plate (9) of the upper closed plate, the weir of this plate with a first part (10) and a second substantially horizontal part, and the connection of the weir with the open plate located below ( lower plate) by means of a third vertical part (12) with a section greater than that of the section of the first part (10).

FIG. 3 also makes it possible to see that the thickness of the coalesced layer (11) under the upper closed plate is greater than the thickness of the coalesced layer (8) under the open plate located below (lower plate).

FIG. 4 illustrates a particular implementation of the invention. To facilitate the coalescence between the droplets, a porous filter medium (13) is introduced under the closed plate (9), which makes it possible to improve the performance of the invention in the case of a very low coalescing system.

The weir (10) then passes through the porous medium (13) and is then widened in an end part (12) with a view to supplying the lower open plate (6).

The coalesced layer is normally located below the plate (9), and the interface (11) between this coalesced layer and the liquid phase with dispersed drops can be located below the packing (13).

FIG. 5 illustrates the invention in the case where the heavy phase is the dispersed phase.

The light phase is introduced through the lower tubing (2) and exits through the upper tubing (5).

The heavy phase is introduced through the upper tube (3) and leaves through the lower tube (4).

The coalesced layer (8) is located above each open plate such as (6).

The coalesced layer (11) is also located above the closed plate (9), but is larger than the coalesced layer corresponding to an open plate.

The operation of the extraction column according to the invention is the same as in the case where the dispersed phase is the light phase.

The two technological differences compared to the case of the light dispersed phase are:

- The weirs (10), (12) are then oriented upwards.

- The possible coalescing media (13) is positioned above the closed plate.

COMPARATIVE EXAMPLE

The following comparative example is obtained by simulation.

The feed to be treated is a hydrocarbon containing 100 ppm of methanethiol.

The solvent used to extract the methanethiol is a 20% sodium hydroxide solution.

The dispersed phase is the hydrocarbon phase.

The hydrocarbon and sodium hydroxide flow rates are 150 and 10 m3 / h respectively.

The extraction is carried out at 30 ° C. and under absolute 10bars.

Three separate devices are used to perform the extraction.

The device 1 said to have open trays is according to the prior art. It consists of an extraction column equipped with 15 perforated trays with weirs, 1m apart, and 3m in diameter.

The perforations are 5mm in diameter and the perforated area is equal to 3% of the column section.

The weirs have an area equal to 20% of the column section.

The device 2, still according to the prior art, is said to have closed plates. It is in all respects similar to device 1, but the perforated area is equal to 1% of the column section, the weir area is equal to 10% of the column section.

The device 3 is according to the invention. It consists of an extraction column equipped with 14 open-type trays, such as those fitted to device 1, and 1 closed-type tray such as those fitted to device 2.

The closed plate is positioned at mid-column height, ie positioned above the first 7 open plates and below the 7 other open plates.

When the liquid-liquid system exhibits rapid coalescence, fixed at 2s, the efficiency of an open type plate is 20% compared to a theoretical stage, that of a closed plate is 10%.

When the liquid-liquid system presents a slow coalescence, considered here at 20 seconds, the efficiency of a closed plate is always 10%, on the other hand that of an open plate is greatly modified. In fact, the open plates then no longer make it possible to ensure a counter-current flow because the heavy phase undergoes a very strong back-mixing through the plates. It follows that a series of successive open plates of at least 5 plates then behaves like a single theoretical stage.

I3

The presence of a judiciously positioned closed type plate makes it possible to compartmentalize the flow of the heavy phase.

The equivalent theoretical number of stages resulting from the 3 devices, depending on the system considered, is given in table l below:

Device 1 Device 2 Device 3 Coalescing system 2.8 1.4 2.68 Low coalescing system 1 1.4 2

Table l: Number of equivalent theoretical stages for devices l to 3

The advantage of the invention appears clearly on reading Table 1. The performances of the column according to the invention (device 3) are almost identical to that of the column with open plates (device 1) in the case of a coalescing system, and much higher than the other two columns (devices 1 and 2) in the case of a weakly coalescing system.

Claims (9)

1) Counter-current liquid liquid extraction column with perforated trays with weirs requiring a limited number of theoretical stages between 2 and 4, and within which the partition coefficient of the solute to be extracted is greater than or equal to 50 , the liquid liquid system possibly having a variable coalescence over time, said column comprising two types of distinct plates; a number of open trays between 1 and 25, defined by a degree of perforation p% between 2% and 6%, and a number of closed trays between 1 and 3, the column being characterized by: - each closed tray is defined by a degree of perforation between 0.3 and 0.5 times that of open trays, - each closed tray is located between two open trays, and - the closed trays are located in the middle part of the column, that is to say by dividing it into 2 substantially equal portions, numbered 1 and 2 from top to bottom, in the lower part of portion 1 or the upper part portion number 2, - the weirs associated with the closed plates have sections between 0.25 and 1 times that of the weirs of the open plates. 2) Liquid liquid extraction column according to claim 1, wherein a closed plate occupies the space corresponding to two open plates, and in this case, the weir associated with said closed plate comprises a portion passing through the column substantially along a diameter, so that the outlet of said weir is located opposite its inlet. 3) Liquid liquid extraction column according to claim 1, wherein under some closed trays is a packing layer intended to promote coalescence, the thickness of said packing layer being less than the height of the weir of the closed tray considered . 4) Liquid liquid extraction column according to claim 1, wherein the number of open trays is between 6 and 20, and the number of closed trays is between 1 and 2. 5) Application of the extraction column according to one of claims 1 to 4, to the extraction of sulfur compounds from a hydrocarbon phase with an aqueous sodium hydroxide solution representing 10 to 20% by weight, the hydrocarbon phase being the dispersed phase, 6) Application of the extraction column according to one of claims 1 to 4, wherein the flow rate ratio between the extracted phase (E) and the raffinai phase (R) is between 5 and 50. 7) Application of the extraction column according to one of claims 1 to 4, wherein the flow rate ratio between the extracted phase (E) and the raffinate phase (R) is between 0.02 and 0.2. 8) Application of the extraction column according to one of claims 1 to 4, wherein the dispersed phase is the light phase. 9) Application of the extraction column according to one of claims 1 to 4, wherein the dispersed phase is the heavy phase.