Liquid Redistributor Field of the Invention

The invention relates to a liquid redistributor, arranged in a packing column, comprising a liquid distributor tray; furthermore, the invention relates to packing columns which are equipped with at least one liquid redistributor. The term packing column here also comprises those columns which e.g. are designed as packing columns only in their lower part, while their upper part is designed e.g. as tray column. Furthermore, the invention relates to a method for the extraction of 1 ,3-butadiene from a gaseous C ₄ hydrocarbon mixture, in which such packing columns are used.

Prior art

To achieve a high separation efficiency of a packing column which is used in a thermal separation process, it is required to uniformly distribute the liquid phase over the column cross-section. When the liquid trickles through the packing, independent of whether the same consists of a random packed bed or whether it is a stacked or structured packing, an unequal distribution of the liquid occurs, which increases with the packing height trickled through. It therefore is required to limit the height of a packing and collect the liquid after passing through a packing and newly distribute the liquid over the next following packing.

The liquid dripping out of a packing can be collected by a column tray designed as liquid collector and can be passed from there into a column tray designed as liquid redistributor, cf. M. Baerns et al., Technische Chemie, 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, p. 334, Fig. 9.51 .

Depending on the application, it is also possible, however, to refrain from using a liquid collector and carry out the collection and redistribution of the liquid with the same column tray. The invention relates to such column tray designed as liquid redistributor.

Such column tray comprises a liquid distributor tray which is equipped with numerous openings distributed over its surface, wherein these openings are formed such that a liquid layer is formed on the tray, out of which the uniformly distributed liquid drips down onto the next following packing. The thickness of this liquid layer in most cases is 20 to 400 mm. It chiefly is dependent on the actual load of the column and on the size and number of the openings in the tray. In many cases, the liquid distributor tray is designed as perforated plate or channel distributor.

To prevent that the dripping down of the liquid through the openings in the distributor tray is impeded by the gas phase rising in the column, either the distributor tray is designed with a smaller diameter than the column inside diameter, so that the gas phase can rise at the column inner wall, or gas chimneys are inserted into the distributor tray. The same serve for leading the gas through the distributor tray and the liquid layer formed on the distributor tray. The chimneys can be formed for example cylindrical or rectangular. They reach through the liquid layer and generally are closed at the top, so that no liquid from the overlying packing drips into or through the same. For this reason, the gas chimneys are provided with lateral openings for the gas exit, wherein these openings are located with a safety distance above the liquid layer to be formed on the distributor tray. In many cases, the gas chimneys have a total height of up to 500 mm. Above the gas chimneys, up to the bottom side of the overlying packing, a free height of at least 800 mm generally is maintained, in order to enable the access for assembly, maintenance and cleaning tasks. From a height of more than 1 m, the liquid droplets therefore fall from the bottom side of the packing lying above the distributor tray into the liquid layer formed on the distributor tray. Due to the kinetic energy of the droplets resulting from the height of fall, the liquid layer is greatly perturbed or fluidized at least on its surface. This fluidization can result in the formation of foam on the liquid layer, whereby the flux of the process media through the column and hence the operability of the column is impaired. Furthermore, the whirling up of the liquid leads to the fact that gas already dissolved from the same is expelled again, which likewise leads to the formation of foam.

Therefore, it is the object of the invention to provide a liquid redistributor which avoids these disadvantages, in particular an excessive formation of foam inside the column.

Description of the invention, preferred aspects

The object is solved by a liquid redistributor according to claim 1 . The free distance between the liquid distributor tray and the droplet damping tray should be so large that a liquid layer can form on the distributor tray, which covers the tray in a uniform layer thickness, so that the liquid can uniformly be distributed over the column cross-section. However, the distance should not be larger than is required for the secure formation of this liquid layer, so that the height of fall of the droplets exiting from the droplet damping tray into the liquid layer is as small as possible. With reference to these specifications, the skilled person will be able to determine suitable distances, for example by routine experiments.

Alternatively, the droplet damping tray also can rest directly on the liquid distributor tray, wherein the liquid layer present on the distributor tray is formed within the droplet damping tray. In this way, a particularly compact design of the column is achieved.

In accordance with the present invention, gas phase also is understood to be a vapor phase in equilibrium with the corresponding liquid phase.

A particularly suitable aspect of the invention is characterized in that the droplet damping tray is formed by a random packed bed arranged between a supporting grid and a hold-down grid or by a stacked packing. This aspect has the advantage that the tray is constructed in the same or a similar way as the packings used in the column. In general, the suitability of the packing material for the respective process media hence is known, so that no unnecessary process risks are caused by the material selection. In addition, stock-keeping of the packing material therefore is less expensive.

A further aspect of the invention is characterized in that the void volume of the bed or packing is at least 50 vol-%. With a large void volume, the passage of the liquid through the droplet damping tray still is possible even with an advanced soiling of the tray, and the shut-down periods of the column for cleaning purposes thus are reduced.

A further preferred aspect of the invention is characterized in that the layer height of the bed or packing of the droplet damping tray lies between 10 and 800 mm. This layer height is dependent on the properties of the process media, the packing structure or the packing material, the column geometry and load. It should be chosen such that the liquid passes through the layer with sufficient speed, so that there is no impoundment of the liquid and no formation of a liquid layer on the upper side of the droplet damping tray.

A further preferred aspect of the invention is characterized in that the distance between the liquid distributor tray and the droplet damping tray lies between 0 and 500 mm. To avoid any height of fall of the droplets, the droplet damping tray accordingly can be placed directly on the liquid distributor tray. In general, however, it is expedient to maintain a free distance for the formation of a liquid surface on the distributor tray. A further preferred aspect of the invention is characterized in that the gas phase rising in the column is guided vertically through at least one, in general mostly a plurality of gas chimneys through the liquid distributor tray and the droplet damping tray. The liquid distributor tray thereby can be extended up to the column inner wall. Due to this design, liquid streams running along the column edge or in the edge region of the column cross- section also can be seized by the liquid redistributor. Furthermore, the invention also comprises packing columns or columns which partly are designed as packing columns, in case they are equipped with at least one liquid redistributor according to the invention. Furthermore, the invention also comprises the use of a packing column according to the invention for carrying out a thermal separation process, selected from distillation, rectification, extractive distillation and absorption.

Furthermore, the invention also comprises a method for the extraction of 1 ,3-butadiene from a gaseous C ₄ hydrocarbon mixture by using a liquid mixture of N-methylpyrrolidone (NMP) and up to 15 wt-% of water as selective solvent, wherein the extraction is carried out by means of a packing column according to the invention. The 1 ,3-butadiene, which is contained in the gaseous C ₄ hydrocarbon mixture with an amount of approximately 40 wt-%, is almost completely absorbed by the solvent. The gaseous C ₄ hydrocarbon mixture leaves the packing column as top product or refined product with a 1 ,3- butadiene content of, generally, below 0.2 wt-%.

When carrying out this method, without use of the droplet damping trays according to the invention, gamma scanning of the column provided indications that a formation of foam might have occurred on the liquid distributor trays. When using the trays according to the invention, the quantity ratio solvent/C ₄ hydrocarbon mixture could be reduced by approximately 10% with the product quality remaining constant, whereby the formation of foam could be reduced and the throughput of the column thus could likewise be increased by this percentage.

Exemplary Embodiments

Further developments, advantages and possible applications of the invention can also be taken from the following description and the drawing. All features described and/or illustrated form the subject-matter of the invention per se or in any combination, independent of their inclusion in the claims or their back-reference. With reference to the drawing, Figs. 1 , 2 and 3, the invention will now be explained in detail. In the drawing:

Fig. 1 shows the longitudinal section of a column equipped with liquid redistributors according to the invention for separating 1 ,3-butadiene from a C ₄ cut,

Fig. 2 shows the longitudinal section of a liquid redistributor according to the invention, which is mounted in a packing column,

Fig. 3 shows the cross-section of a liquid redistributor according to the invention.

In Fig. 1 , the column 1 in the lower part is designed as packing column with three packings 2 a, b and c, and in the upper part as tray column with four wash trays 21 a-d and a reflux condenser 22. The number of the packings and trays here is chosen by way of example, and it does not necessarily correspond to the number used in practice. Via conduit 3 the solvent as liquid phase and via conduit 4 the gaseous C ₄ -hydrocarbon mixture as gas phase is introduced into the column. Via conduit 5 the refined product, i.e. the C ₄ -hydrocarbon mixture liberated from 1 ,3-butadiene, and via conduit 6 the liquid product stream, i.e. the solvent loaded with 1 ,3-butadiene, is discharged from the column.

Via a feed system 7a, the solvent 3 is distributed on the liquid distributor 8 as uniformly as possible over the cross-section of the column 1 . In the example shown here, the liquid distributor comprises a perforated plate 9 and gas chimneys 10a. In this example, it is not equipped with a droplet damping tray. It can be omitted at this point, when either the feed system 7a is designed such that it gently distributes the liquid phase on the perforated plate 9 without a strong impulse, or when the added liquid phase, before loading with the gas phase, does not tend to foaming. The liquid phase drips from the perforated plate 9 onto the packing 2a, seeps through the packing and again drips out from the bottom side 1 1 of the packing 2a. The droplets impinge on the droplet damping tray 12b of the liquid redistributor 13b. The liquid seeps through the droplet damping tray 12b and drips out on its bottom side 14b onto the liquid distributor tray 15b of the liquid redistributor 13b. The gas phase rises through the packings 2a, 2b and 2c in the column. By means of the liquid distributor 8 and the liquid redistributors 13b and 13c it is guided through the gas chimneys 10a, 10b and 10c. In the upper part of the column equipped with wash trays, solvent passed over into the gas phase is washed out with the reflux 23 produced in the condenser 22.

Through the manhole 16 the access to maintenance and cleaning works in the interior of the column becomes possible. Fig. 2 shows a section of Fig. 1 , in order to represent the liquid redistributor 13b in more detail. There are shown the lateral openings 18 of the gas chimneys 10b and the flux of the gas phase 17 through the chimneys. There are also shown the supporting grid 19b and the hold-down grid 20b of the droplet damping tray 12b. Fig. 3 shows the rectangular shape of the gas chimneys 10b used in this example and also frequently used in practice.

Industrial Applicability

The invention provides a measure to be realized in a technically simple and inexpensive way, in order to increase the production capacity of a packing column which is used for the treatment of media tending to foaming.

List of Reference Numerals

1 column

2 a, b, c packings

3 inflow of liquid phase, solvent

4 inflow of gaseous phase, C ₄ -hydrocarbon mixture

5 outflow of refined product

6 outflow of liquid product

7 feed system

8 liquid redistributor

9 perforated plate

10 a, b, c gas chimney

1 1 bottom side of the packing

12 a, b, c droplet damping tray

13 b, c liquid redistributor

14 b bottom side of droplet damping tray

15 b, c liquid distributor tray

16 manhole

17 flow of the gas phase

18 opening in the gas chimney for the gas phase 19 b supporting grid

20 b hold-down grid

21 a-d wash trays

22 reflux condenser

23 reflux