Liquid double distribution device of use in particular in an apparatus in which a liquid phase flows under gravity

Abstract

A double liquid distribution device (1) suitable for use in in a fractionating or wash column (10) including a high collector tray (2) connected to a manifold support (7) via at least two longitudinal liquid downflow ducts (5, 6), the manifold support (7) supporting at least two series of transverse tubular manifolds (8, 8a 8b) and serving to feed liquid respectively to the first series of manifolds (8a) via a first longitudinal duct (5) and to the second series of manifolds (8b) via a second longitudinal duct (6), each manifold (8, 8a 8b) including distribution orifices (8c) in its under face that are suitable for distributing the liquid onto the top face of the packing bed (9). The two longitudinal liquid downflow ducts (5, 6) are connected together in a low portion (5b, 6b) by a communication device (11) fitted with a valve (12) having controlled opening that is suitable for allowing the liquid to be transferred between the two longitudinal ducts in a controlled manner.

Claims

1. A double liquid distribution device suitable for use in any type of vertical apparatus that is a seat of gravity flow of a liquid phase that needs to be distributed in uniform manner over at least one zone arranged under said double liquid distribution device and extending over at least one section orthogonal to a vertical axis of said vertical apparatus, the double liquid distribution device comprising a high collector tray connected to a manifold support via at least two longitudinal liquid downflow ducts, said manifold support supporting at least two series of transverse tubular manifolds and serving to feed liquid respectively to a first series of manifolds of the at least two series of transverse tubular manifolds via a first longitudinal duct of the at least two longitudinal liquid downflow ducts and to the second series of manifolds of the at least two series of transverse tubular manifolds via a second longitudinal duct of the at least two longitudinal liquid downflow ducts, each manifold having distribution orifices in its under face suitable for distributing the liquid over said at least one zone situated under said manifolds of the double liquid distribution device, a shape of the high collector tray being suitable for channelling a liquid on the high collector tray towards a top opening of the first longitudinal duct, and a top opening of the second longitudinal duct reaching a level that lies above said high collector tray; wherein the at least two longitudinal liquid downflow ducts are connected together in a low portion by a communication device fitted with a valve having controlled opening that is suitable for allowing the liquid to be transferred between the at least two longitudinal liquid downflow ducts in a controlled manner, opening and closing of said valve being triggered solely on a basis of measuring a liquid level in said first and said second longitudinal ducts.

2. The double liquid distribution device according to claim 1, wherein the communication device opens out into the respective low portions of each of the first and the second longitudinal ducts at their bottom ends.

3. The double liquid distribution device according to claim 1, wherein triggering of the opening or the closing of the valve is based solely on measuring the liquid level in said first and said second longitudinal ducts, such that: a) with flow rate increasing in said first longitudinal duct only, said valve opens automatically when the liquid level in the first longitudinal duct reaches a maximum threshold value Hmax; and b) with flow rate decreasing in both of said first and said second longitudinal ducts, the valve closes automatically when an identical liquid level in the first and the second longitudinal ducts that are in communication via the open valve drops down to a minimum threshold value greater than or equal to Hmin, Hmin being a minimum height required for liquid in said first and said second longitudinal ducts while they are in communication with each other via said valve in order to obtain uniform distribution of liquid among various orifices of said first and said second series of manifolds.

4. The double liquid distribution device according to claim 3, further comprising first and second distributors configured in such a manner that a) Q.sub.2min is less than or equal to Q.sub.1max-Q.sub.1min; the first and second distributors being defined as consisting of: the first longitudinal duct and the orifices of the first series of manifolds, for the first distributor; and the second longitudinal duct and the orifices of the second series of manifolds, for the second distributor; and Q.sub.1 min and Q.sub.2min being defined respectively as combined minimum flow rates through the orifices of the first and the second series of manifolds respectively, that enable liquid to be distributed uniformly by said first and second series of manifolds; and Q.sub.1max being defined as the maximum flow rate of the first distributor when the liquid height in the first longitudinal duct is of maximum value Hmax; and b) when said valve is opened and the at least two longitudinal liquid downflow ducts are put into communication with each other, the level of the liquid in the first longitudinal duct that was at a value Hmax moves down to a liquid level H.sub.0 in the second longitudinal duct that is identical to the level of the first longitudinal duct, H.sub.0 being greater than or equal to Hmin, Hmin being a common minimum height required for liquid in said first and said second longitudinal ducts in communication via said valve for obtaining uniform distribution of liquid among the various orifices of said first and said second series of manifolds.

5. The double liquid distribution device according to claim 1, further comprising first and second distributors configured in such a manner that a flow rate corresponding to a threshold for triggering opening of said valve while flow rate is increasing in order to pass from a mode of operation with only the first distributor in action to a double distribution mode of operation with both the first and the second distributors in action is greater than the flow rate corresponding to the threshold for triggering closing of the valve while flow rate is decreasing in order to pass from the double distribution mode of operation with both the first and second distributors in action to the mode of operation with only the first distributor in action.

6. The double liquid distribution device according to claim 4, wherein the first and second distributors are configured in such a manner that: a) Q.sub.2min is less than Q.sub.1max-Q.sub.1min; and b) H.sub.0 is greater than Hmin but less than Hmax.

7. The double liquid distribution device according to claim 1, wherein the transverse tubular manifolds extend in parallel in a transverse direction perpendicular to an axial longitudinal direction of the double liquid distribution device, and both series of manifolds are arranged at a same level in the axial longitudinal direction of the double distribution device, the manifolds of the first series being interleaved in parallel between the manifolds of the second series.

8. A vessel or floating support including a vertical apparatus that is the seat of a gravity flow of a liquid phase and including at least one double liquid distribution device according to claim 1 arranged coaxially inside a cylindrical wall of said vertical apparatus above a zone needing to be sprayed in uniform manner with the liquid, said zone extending over a cross section of said vertical apparatus perpendicularly relative to an axial longitudinal direction of said cylindrical wall, said high collector tray being arranged transversely and coaxially relative to said cylindrical wall and adjacent to said cylindrical wall.

9. The vessel or floating support according to claim 8, wherein said vertical apparatus has a plurality of zones extending over the cross section of said vertical apparatus and spaced apart from one another in the axial longitudinal direction of said cylindrical wall, with a plurality of said double liquid distribution devices, each interposed between two packing beds.

10. The vessel or floating support according to claim 8, wherein said vertical apparatus is a fractionating or wash column containing at least one packing bed extending in a zone over a cross section of said column perpendicularly to an axial longitudinal direction of said column.

11. A method of distributing liquid by using a double liquid distribution device according to claim 1, receiving a liquid on said high collector tray, the method being wherein the following steps are performed: e.1) filling the first longitudinal duct with liquid; and e.2) when the liquid level in the first longitudinal duct reaches a maximum threshold value, opening said valve and putting the at least two longitudinal liquid downflow ducts into communication with each other, thereby filling the second longitudinal duct to an identical liquid level as the first longitudinal duct; and e.3) re-closing said valve if said identical liquid level of said first and said second longitudinal ducts decreases below a minimum threshold value.

12. The method according to claim 11, wherein the opening or the closing of the valve is triggered automatically as a function of measuring the liquid level in said first and the second longitudinal ducts, in such a manner that: a) in step e.2), with flow rate increasing in said first longitudinal duct only, said valve opens automatically when the liquid level in the first longitudinal duct reaches a maximum threshold value Hmax; and b) in step e.3), with flow rate decreasing in both of said first and said second longitudinal ducts, the valve closes automatically when the identical liquid level in the first and second longitudinal ducts that are in communication via the open valve drops down to a minimum threshold value greater than or equal to Hmin, Hmin being a minimum common height required for liquid in said first and said second longitudinal ducts while they are in communication with each other via said valve in order to obtain uniform distribution of liquid among various orifices of said first and second series of manifolds.

13. The method according to claim 12, wherein the double liquid distribution device comprises first and second distributors and, wherein in step e.2), said valve is opened and the at least two longitudinal liquid downflow ducts are put into communication with each other such that the level of the liquid in the first longitudinal duct, which was of maximum value Hmax, drops down to a liquid level H.sub.0 in the second longitudinal duct that is identical to the level of the first longitudinal duct, H.sub.0 being greater than or equal to Hmin and corresponding to a flow rate of the second distributor that is greater than or equal to Q.sub.2min; the first and second distributors being configured in such a manner that: a) Q.sub.2min is less than or equal to Q.sub.1max-Q.sub.1min; the first and second distributors being defined as consisting of: the first longitudinal duct and the orifices of the first series of manifolds, for the first distributor; and the second longitudinal duct and the orifices of the second series of manifolds, for the second distributor; and Q.sub.1min and Q.sub.2min being defined respectively as combined minimum flow rates through the orifices of the first and the second series of manifolds respectively, that enable liquid to be distributed uniformly by said first and said second series of manifolds; and Q.sub.1max being defined as the maximum flow rate of the first distributor when the liquid height in the first longitudinal duct is of maximum value Hmax; and b) on opening said valve and putting the at least two longitudinal liquid downflow ducts into communication with each other, the level of the liquid in the first longitudinal duct, which is of maximum value Hmax, drops down to a liquid level H.sub.0 in the second longitudinal duct that is identical to the level of the first longitudinal duct, H.sub.0 being greater than or equal to Hmin.

14. The method according to claim 13, wherein in step e.2), said valve is opened and the at least two longitudinal liquid downflow ducts are put into communication with each other such that the level of the liquid in the first longitudinal duct, which was of maximum value Hmax, drops down to a liquid level H.sub.0 in the second longitudinal duct that is identical to the level of the first longitudinal duct, H.sub.0 being greater than Hmin and corresponding to a flow rate of the second distributor that is greater than Q.sub.2min; Q.sub.2min being greater than Q.sub.1max-Q.sub.1min, the first and second distributors being configured in such a manner that: a) Q.sub.2min is less than Q.sub.1max-Q.sub.1min; and b) on opening said valve and putting the at least two longitudinal liquid downflow ducts into communication with each other, the level of the liquid in the first longitudinal duct, which is of maximum value H.sub.1max, drops down to a liquid level H.sub.0 in the second longitudinal duct that is identical to the level of the first longitudinal duct, H.sub.0 being greater than or equal to Hmin.

15. The method according to claim 11, wherein a flow rate corresponding to the threshold for triggering opening of said valve, while flow rate is increasing, in order to pass from a mode of operation with only the first distributor in action to a double distribution mode of operation with both the first and the second distributors in action is greater than the flow rate corresponding to the threshold for triggering closing of the valve, while flow rate is decreasing, in order to pass from the double distribution mode of operation with both the first and second distributors in action to the mode of operation with only the first distributor in action.

16. The method according to claim 14, wherein it comprises the following steps: 1) increasing the liquid level in the first longitudinal duct only; and 2) opening said valve when the liquid level reaches the maximum height threshold Hmax in the first longitudinal duct corresponding to a maximum uniform flow rate Q.sub.1max of the first distributor and putting the at least two longitudinal liquid downflow ducts into communication so that the level of the liquid in the first longitudinal duct, which was of maximum value Hmax, drops down to a liquid level H.sub.0 in the second longitudinal duct that is identical to the level of the first longitudinal duct, H.sub.0 being greater than said value Hmin; and 3) the liquid level increases from H.sub.0 in both the first and the second longitudinal ducts, the device now being in a double distribution mode of operation with both the first and second distributors in action, the combined flow rate of both distributors increasing from Q.sub.1max to Q.sub.1max+Q.sub.2max, Q.sub.2max being defined as the maximum flow rate of the second distributor when the identical liquid height in the first and second longitudinal ducts is at a maximum Hmax; and 4) when the device is in the double distribution mode of operation with both the first and second distributors in action, the flow rate lying in a range Qimax to Q.sub.1max+Q.sub.2max, if the flow rate decreases, said valve is closed only when the flow rate reaches a minimum threshold value Q.sub.0 in a range Q.sub.1min to Q.sub.1max with a level Hmin of the liquid in both the first and the second longitudinal ducts, Q.sub.0 being greater than or equal to Q.sub.1min+Q.sub.2; and 5) after closing said valve, the liquid level in the first longitudinal duct rises to a value H.sub.1 greater than Hmin but less than Hmax with a flow rate in the range Q.sub.1min and Q.sub.1max, the device remains in a single distribution mode of operation with only the first distributor in action so long as the liquid level does not exceed Hmax in the first longitudinal duct.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the present invention appear from the following description made with reference to the accompanying drawings, which show an embodiment having no limiting character. In the figures:

(2) FIG. 1 is a view of a fractionating column 10 having a plurality of liquid distributors 1 cooperating with a plurality of packing beds 9; in FIG. 1, the packing bed 9 extends from the top dashed line under a top distributor to the pair of dashed lines situated lower down (above the chimney tray of the lower distributor);

(3) FIGS. 2A, 2B, and 2C are views of a prior art double liquid distributor device with overflow having two vertical ducts 5 and 6 arranged side-by-side in the proximity of the central axis ZZ′ and symmetrically relative thereto, shown in a perspective view (FIG. 2A), in a side view (FIG. 2B), and in a plan view (FIG. 2C);

(4) FIGS. 3A and 3B-3C are diagrammatic views of a double liquid distributor showing the cooperation between the two distribution systems having two different arrangements for the two series of manifolds 8a and 8b relative to one another;

(5) FIGS. 4A and 4B are views of a double liquid distributor of the invention with a device 11 for controlled communication between the two ducts 5 and 6, shown in side view (FIG. 4A) and in plan view (FIG. 4B);

(6) FIGS. 5A and 5B are graphs showing how the uniformity parameter k varies as a function of flow rate in the first duct in a double distributor of the prior art (FIG. 5A) and of the invention (FIG. 5B); and

(7) FIG. 6 shows graphs plotting the variation in the liquid height as a function of flow rate, illustrating the operation of a device of the invention, both with a single valve opening or closing threshold (without hysteresis: curves A, B, and C), and with two distinct valve thresholds respectively for opening and for closing (with hysteresis: curves A′, B′1/B′2, and C′).

DETAILED DESCRIPTION OF THE INVENTION

(8) FIG. 1 shows a fractionating or phase separating column 10 having a cylindrical wall 10a and containing three liquid distribution devices 1 arranged axially above three respective packing beds 9, each occupying the entire circular section of the column. The packing of these beds may be of the “structured” type such as “Mellapak™” structured packing from the supplier SULZER (CH) or “FLEXIPAC®” packing from the supplier KOCH GLITSCH (USA) or “INTALOX®” or “IMTP®” random packing from the supplier KOCH GLITSCH (USA).

(9) FIGS. 2A-2C, and also FIGS. 3A-3C and 4A-4B, show a double liquid distribution device 1 suitable for use in a fractionating or wash column 10 containing at least one packing bed 9 extending over the cross section of said column perpendicularly to the axial longitudinal direction ZZ′ of said column. From bottom to top, the liquid distribution device 1 comprises:

(10) a high collector tray 2 of circular outline and of diameter equal to the diameter of the cylindrical wall 10a, the tray 2 having a horizontal plane surface (perpendicular to ZZ′) including two channels 3a and 3b with sloping bottoms that are arranged diametrically in a cross configuration to trap the liquid and direct it down towards a first liquid downflow duct 5, the tray also supporting chimneys 4 allowing the vapor phase to rise (which chimneys have hats that are not shown in the figures);
two parallel longitudinal liquid downflow ducts 5 and 6 arranged symmetrically in the proximity of the axis ZZ′ of the tray 2 and of the column 10, connecting said tray 2 to a manifold support 7, the top opening 5a of the first longitudinal duct 5 being level with the bottom of the channels 3a-3b, while the top opening 6a of the second duct reaches a level that is at a height h0 above said collector tray 2;
said manifold support 7 being arranged diametrically and supporting two series of transverse tubular manifolds 8, 8a-8b arranged perpendicularly to the axis ZZ′ (horizontally) and parallel to one another; and
a first series of manifolds 8a is fed solely by the first longitudinal duct 5 and a second series of manifolds 8b is fed exclusively respectively by the second longitudinal duct 6, each manifold 8, 8a-8b having distribution orifices 8c arranged in its under face and suitable for spraying the liquid 8d onto the top face of the packing bed 9 under the double liquid distribution device.

(11) In FIG. 3A, in a preferred embodiment, the manifolds 8a of the first duct 5 are interposed on a common level between the empty manifolds 8b of the second duct 6 that extends to a greater height than the first duct.

(12) FIGS. 3B and 3C show the manifolds 8a of the first duct 5 interposed in staggered manner between the manifolds 8b of the second duct 6 and arranged at a level that is just a little higher than the manifolds 8b so as to show more clearly the principle of operating two series of manifolds and so as to identify better which manifolds correspond with which ducts, each manifold being fed exclusively via only one of the vertical ducts. However, in practice, the manifolds of a double distributor are all at the same height.

(13) In FIG. 3B, the device is operating in single or “mono-distributor” mode, with only the first duct 5 and the first manifolds 8a being full of liquid 8d for spraying through the orifices 8c. In FIG. 3C, the device is operating in double mode, with both of the ducts 5 and 6 and both series of manifolds 8a and 8b being shown completely full of liquid and with maximum flow rates.

(14) FIGS. 4A and 4B show a double liquid distributor of the invention identical to that shown in FIGS. 2 and 3, but also fitted with a communication device 11 providing communication between the two ducts 5 and 6 and including a valve 12 with controlled opening and closing. The communication device 11 comprises two angled ducts with multiple bends 13 and 14, together with a valve 12. The two angled ducts 13 and 14 provide connections between the valve 12, which is located outside the wall 10a, and the bottom portions respectively of the first duct 5 and of the second duct 6. The two angled ducts 13 and 14 are arranged symmetrically relative to the valve 12, with portions 13a and 14a that are connected to the ducts 5 and 6 being arranged inside the wall 10a, and with portions 13b and 14b that are connected to the valve 12 being arranged outside of the wall 10a.

(15) The valve 12 is positioned outside the wall 10a for practical maintenance reasons. However, it is entirely possible to envisage said valve remaining inside the wall 10a. Furthermore, the ducts 13 and 14 could be of some other shape without changing the principle of the invention. In contrast, the valve 12 must be positioned at the low point, without any “pockets” along the paths followed by the elements of the ducts 13 and 14 to ensure that, in the event of the distributor being used with cryogenic liquids, the vapor bubbles produced by the liquid evaporating in the dead arms can be discharged naturally towards the inside of the column. Furthermore, in order to avoid risking un-priming the flow, given that the liquid is at its bubble point, the elements of the ducts 13 and 14 must remain at heights that are lower than Hmin, i.e. in this example that the portions 13a and 14a must follow paths that remain at a height that is lower than the height corresponding to the liquid level Hmin.

(16) The height H3 of the second duct is equal to the height H2 of the first duct+h0, where h0 is the extra height of the second duct and is equal in practice to about 10% of H2.

(17) The minimum liquid height Hmin required in both ducts 5 et 6 for feeding all of the orifices in the two series of manifolds 8a and 8b depends on the amplitude of the tilting, in practice about 5° to 20°, on the dimensions (length and diameter) of the manifolds, and on the range or ratio that is required between the minimum flow rate Qmin and the maximum flow rate Qmax. Thus, the number and the size of the holes 8c and the heights of the first and second ducts 5 and 6 are determined so as to deliver the desired minimum uniform flow rates Q.sub.1min, Q.sub.1min at said minimum liquid height Hmin and the desired maximum uniform flow rates Q.sub.1max, Q.sub.2max at a maximum liquid height Hmax.

(18) The minimum height Hmin can be calculated from the diameter D of the column, which determines the lengths of the manifolds, from a criterion E for the relative difference in the flow rates due to the movement of the float, this flow rate difference being the difference between a central orifice and an “extreme” orifice of a manifold, and from an angle “dynamic alpha” defined by the angular sector formed between the acceleration vector and the axis ZZ′ in the reference frame of the vessel. Thus, Hmin=D×0.5×tan(alpha)×(1/((1+E).sup.2−1). The number and the size of the orifices are calculated so that the distributor can pass the flow rate Qmin at a height Hmin while guaranteeing sufficient uniformity. The intended uniformity (factor k.sub.0) thus determines the required ratio Hmin/D.

(19) The maximum liquid height Hmax in the distributor results from the minimum flow rate Qmin and from the ratio Qmax/Qmin, i.e. Hmax/D=(Qmax/Qmin).sup.2×Hmin/D). Typically, the ratio Qmax/Qmin is 100/40. By way of illustration, for a maximum difference E of 8.5% and for an angle alpha of 10°, Hmin is 0.5×D. Typically Hmax/D is in the range 0.5 to 1.5. Typically D is 4 m to 5 m and E=5% to 15%.

(20) The intended uniformity (factor k.sub.0) determines the required ratio Hmin/D. However, the effective ratio Hmin/D depends subsequently on the effective flow rate at which the column is to be operated, it being possible for the uniformity criterion to be in compliance for the tilt angle taken into consideration during design only for a flow rate that is greater than or equal to the minimum flow rate Qmin that was taken into account during said design.

(21) In FIGS. 5A and 5B, the distributors are calibrated for the manifolds having a maximum length of 4.15 m and Q.sub.1min/(Q.sub.1max+Q.sub.2max)=40% so that k.sub.0 is 4.5%.

(22) In FIG. 5A, it can be seen that in a prior art double distributor with overflow, when in a maximum tilt situation, and when the liquid height in the first duct is increased, the following various successive stages take place:

(23) curve A: the flow rate of the first distributor goes from Q.sub.1min at 42% to Q.sub.1max at 65% when the liquid level in the first duct goes from Hmin to Hmax, with the uniformity of distribution among the various orifices of the distributor increasing since k decreases to less than k.sub.0; then
curves B and C: the liquid overflows from the first duct and pours into the second duct, with the combined flow rate through both the distributors increasing up to a range (curve C) from Q′=66.58% to Q″=75.7%, in which range the required minimum height Hmin is not reached in the second duct, and as a result the distribution is not sufficiently uniform, with nonuniformity increasing, and k exceeding k.sub.0=4.5% up to 7%; then
curve D: the liquid height in the second duct exceeds Hmin and the combined flow rate increases, with k decreasing to a minimum value of 1.5% when the combined maximum flow rate reaches (Q.sub.1max+Q.sub.2max).

(24) In that prior art embodiment, over an operating flow rate lying in the range Q.sub.1min to Q.sub.1max+Q.sub.2max, there is thus a flow rate bandwidth between Q′ and Q″ of almost 10% (assuming Q.sub.1max+Q.sub.2max=100%) right in the middle of the operating flow rate range and that lies outside the required uniformity tolerance.

(25) In FIG. 5B and in FIG. 6 (curves A, B, and C without hysteresis drawn in fine lines), it is shown that in a double distributor of the invention with a communication device 11 having a valve 12 with controlled opening, and in a situation of maximum tilt, when the liquid height in the first duct is increased, with the second distributor being hydraulically calibrated to deliver the flow rate difference Q.sub.2min=Q.sub.1max−Q.sub.1min with a liquid height Hmin, the various following stages take place:

(26) curve A: the flow rate goes from 0 to Q.sub.1max, with the liquid level going from 0 to Hmax.sup.1 (passing via Hmin for Q.sub.1min) in the first distributor operating as a mono distributor, and the uniformity of distribution among the various orifices of the first distributor increases, since k decreases below k.sub.0, and then at point P1;
curve B: the low communication valve 12 between the two ducts is opened, with the liquid passing through this valve from the first duct to the second duct until, at point P2, the liquid height changes almost instantaneously to be identical in both ducts at H.sub.0=Hmin, with the flow rate going from Q.sub.1max=Q.sub.1max.sup.1 to Q.sub.1min in the first distributor and from 0 to Q.sub.2min=Q.sub.1max.sup.1−Q.sub.1min in the second distributor, such that the combined flow rate through both distributors remains unchanged at P2 and practically equal to Q.sub.1max=Q.sub.1max.sup.1; and then
curve C: when the combined flow rate through both distributors increases from Q.sub.1max.sup.1 to (Q.sub.1max+Q.sub.2max), the liquid height in both ducts increases identically and simultaneously from Hmin to Hmax.sup.1, with k decreasing down to a minimum value of 1.5% when the combined maximum flow rate is reached.

(27) Hmax=H2+h1, where h1 is the maximum liquid height allowed on the collector tray 2, h1 being less than h0 to avoid untimely overflowing of liquid into the second duct before the valve 12 is opened.

(28) In this embodiment in accordance with the invention, the cycle of opening and closing the valve respectively for increasing and decreasing flow rates is without hysteresis, the trigger threshold for the valve being identical for opening the valve at P1 while the flow rate is increasing in the direction B1 to C1, and for closing the valve at P2 while the flow rate is decreasing in the direction C2 to B2.

(29) FIG. 6 also shows the advantageous operation of a variant calibration for a double distributor of the invention that presents hysteresis. In this advantageous variant, in order to avoid any risk of operating instability in the vicinity of the flow rate for opening/closing the valve to switch between the “mono-distributor” mode and the “double distributor” mode, the device is configured so as to be capable of performing hysteresis so that the flow rate Q.sub.1max=Q.sub.1max.sup.2 corresponding to the trigger threshold Hmax.sup.2 for opening the valve while flow rate is increasing is greater than the flow rate (Q.sub.1min+Q.sub.2min) corresponding to the trigger threshold Hmin for closing the valve while flow rate is decreasing. The fact that these two thresholds Hmin and Hmax for controlling the communication valve correspond to two different flow rates Q.sub.1max and (Q.sub.1min+Q.sub.2min) serves to avoid phenomena of untimely oscillation between the “mono-distributor” mode of operation and the “double distributor” mode of operation.

(30) To do this, the second distributor is hydraulically calibrated so as to guarantee a liquid level Hmin not for a flow rate Q.sub.2min=Q.sub.1max−Q.sub.1min but for a fraction of Q.sub.1max−Q.sub.1min, i.e. Q.sub.2min=r×(Q.sub.1max−Q.sub.1min) where r is less than 1.

(31) In FIG. 6, the following stages take place:

(32) curve A′ in the direction A′1: the flow rate goes from 0 to Q.sub.1max.sup.2, with the liquid level going from 0 to Hmax.sup.2 (passing via Hmin for Q.sub.1min) in the first distributor operating as a mono distributor, and the uniformity of distribution among the various orifices of the first distributor increases, since k decreases below k.sub.0, and then at point P′1;
curve B′1: the low communication valve 12 between the two ducts 5 and 6 is opened, and the liquid flows through the valve from the first duct to the second duct, until the liquid height becomes identical almost instantaneously in both ducts at H.sub.0 greater than Hmin, the flow rate passing from Q.sub.1max.sup.2 to Q.sub.1(H.sub.0) with Q.sub.1(H.sub.0)>Q.sub.1min in the first distributor and passing from 0 to Q.sub.2(H.sub.0)=(Q.sub.1max.sup.2−Q.sub.1(H.sub.0)) with (Q.sub.1max.sup.2−Q.sub.1(H.sub.0))<(Q.sub.1max.sup.2−Q.sub.1min) in the second distributor, such that the combined flow rate through both distributors is initially unchanged and substantially equal to Q.sub.1max=Q.sub.1max.sup.2, and then at point P′2;
curve C′ in the direction C′1: the device operates in double distribution mode with both distributors. When the combined flow rate through both distributors increases from Q.sub.1max.sup.2 to (Q.sub.1max+Q.sub.2max), the liquid height in both ducts increases identically and simultaneously from H.sub.0 to Hmax.sup.2, with k decreasing down to a minimum value of 1.5% when the combined maximum flow rate is reached.
curve C′ in the direction C′2 and then B′2: while the device is operating in the double distribution mode of operation with both the first and second distributors in action, with the flow rate lying in the range Q.sub.1max.sup.2 to Q.sub.1max+Q.sub.2max, if the flow rate decreases, then said valve 12 is closed at P3 only when the liquid level reaches Hmin with a flow rate Q.sub.0 in both ducts 5 and 6; and
curve B′2: on closure of the valve 12, the device passes at P′3 into the single distribution mode of operation with only the first distributor in action, the liquid level in the first duct rises to a value H.sub.1 less than Hmax.sup.2 with a flow rate Q.sub.0=(Q.sub.1min+Q.sub.2min) lying in the range Q.sub.1min to Q.sub.1max.sup.2; and
curve A′ and direction A′2: the first distributor remains in operation in mono-distributor mode if the flow rate decreases further from Q.sub.0 to 0, or so long as the flow rate remains less than Q.sub.1max.sup.2.

(33) In both embodiments shown in FIG. 6 (with and without hysteresis), the device is calibrated for the same maximum saturation flow rate Q.sub.1max.sup.1+Q.sub.2max.sup.1 (without hysteresis)=Q.sub.1max.sup.2+Q.sub.2max.sup.2 (with hysteresis). Furthermore Q.sub.1max.sup.2>Q.sub.1max.sup.1 and Q.sub.2max.sup.2<Q.sub.2max.sup.1.

(34) Nevertheless, when there is hysteresis, the first distributor must be made to operate with a greater liquid load prior to opening the valve (Hmax.sup.2 is greater than Hmax.sup.1). Also, the second distributor is calibrated for a flow rate that is lower in the “without hysteresis” situation: given that r is selected so that 0<r<1, then Q.sub.2min=r×(Q.sub.1max−Q.sub.1min) with hysteresis<Q.sub.2min=Q.sub.1max−Q.sub.1min without hysteresis. Specifically, when the total flow rate decreases starting from 100%, i.e. (Q.sub.1max+Q.sub.2max), the flow rate threshold at P3, which corresponds to closing the valve (for a height Hmin), must be less than in the “without hysteresis” situation at P2.

(35) In both of the embodiments of FIG. 6 (with and without hysteresis), the uniformity of the distribution remains in compliance with the required uniformity tolerance, with k being less than k.sub.0 over the entire operating flow rate range from Q.sub.1min to (Q.sub.1max+Q.sub.2max). For the same saturation flow rate (Q.sub.1max+Q.sub.2max), the height Hmax is slightly greater than the same height for a double distributor with overflow. Specifically, for a double distributor with controlled valve opening in accordance with the invention and calibrated in this way, the ratio Hmax/Hmin is equal to the flow rate ratio Qmax/Qmin, whereas for a mono distributor the same ratio Hmax/Hmin is equal to the square of the ratio of the maximum and minimum flow rates.

(36) In FIG. 6, for the respective configurations “with” and then “without” hysteresis, it is the length of the ducts 5 and 6, and thus the height of the collector tray 2 above the manifolds 8 that changes between the two configurations; however the thickness h1 of the sheet of liquid on the tray 2 is the same in both situations, say 0.2 m. In a device with controlled opening, the sheet of liquid rises up to the top of the overflow 6a only in the event of the valve malfunctioning. In the invention, the overflow 6a is an emergency device.

(37) Under all circumstances, opening or closing of the valve 12 is triggered solely on the basis of measuring the level of the liquid (and not on measuring flow rate). When the liquid level in the duct 5 reaches the threshold Hmax the valve 12 opens, and when the liquid level in both ducts 5 or 6 drops down to the threshold Hmin the valve 12 closes.

(38) The specific nature of a system with hysteresis lies firstly in the ducts 5 and 6 being lengthened in order to increase the maximum liquid height of the first distributor and thus obtain a value Q.sub.1max.sup.2 greater than Q.sub.1max.sup.1, and secondly in how the series of manifolds in the second submitter is calibrated: this series is calibrated so that the flow rate Q.sub.1max.sup.2 reached in the first distributor on its own before opening the valve leads subsequently to a liquid height H.sub.0 (identical in both ducts 5 and 6) that is greater than Hmin once the valve is open. In a system without hysteresis, the level H.sub.0 reached immediately after opening the valve is equal to Hmin.

(39) If H0 is greater than Hmin, the device operates de facto with hysteresis, which is characterized by the fact that the flow rate Q.sub.1max.sup.2 corresponding to the threshold Hmax.sup.2 for opening the valve 12 is different and of value greater than the flow rate Q.sub.1min+Q.sub.2min corresponding to the threshold for re-closing the same valve 12 at the level Hmin in order to avoid untimely oscillation phenomena between the “mono-distributor” mode of operation and the “double distributor” mode of operation.