EP2531283B1

EP2531283B1 - Apparatus and method for mixing dispersions and gases

Info

Publication number: EP2531283B1
Application number: EP11702438.0A
Authority: EP
Inventors: Dirk Achten; Herbert Saal; Fred DAHMS
Original assignee: Carbo Kohlensaurewerke & Co KG GmbH; Covestro Deutschland AG
Current assignee: Covestro Deutschland AG
Priority date: 2010-02-03
Filing date: 2011-01-31
Publication date: 2016-04-27
Anticipated expiration: 2031-01-31
Also published as: EP2353705A1; WO2011095454A1; TW201143883A; EP2531283A1

Description

The present invention relates to a method for mixing dispersions with gases. In particular, the present invention relates to a method for mixing a polymer dispersion with a gas.
Various mixing apparatuses for mixing dispersions with gases are known from the prior art. It is known to carry out the gassing of dispersions by means of stirring, by passing a gas over or through the dispersion, by means of static mixers or by membrane processes.
In the case of batch processes, such as stirring or passing a gas over or through a dispersion, the above-mentioned methods require a relatively long contact time, on account of small dispersion/gas interfaces, until adequate gassing of the dispersion has taken place. Such methods are therefore generally not suitable for use in continuous processes.
Known continuous mixer processes, such as static mixers or membrane processes, permit rapid mixing of gases with dispersions. However, the use of such mixers is problematic in the case of sensitive dispersions which have a tendency to coagulate, for example in the case of local overdosing of the gas and/or high shear, as is the case, for example, with some polymer dispersions when they are gassed with a gaseous acid.
In that case, the mixing apparatuses may become clogged and thus quickly lose their operating properties. They must therefore frequently be replaced, cleaning and subsequent re-use of the mixers generally not being possible.
The object of the present invention is, therefore, to provide a method for the efficient mixing of dispersions with gases, which is capable of overcoming the disadvantages known from the prior art.
The object is achieved by a method according to claim 1.
By means of the design according to the apparatus used in the method of the invention for mixing, the occurrence of high shear forces in the dispersion during the mixing operation is avoided on the one hand and, in addition, local overdosing of the gas can be prevented.
The idea underlying the invention is to provide in a receiver, which can be formed, for example, by a tube, a number of loose perforated plates which are spaced apart from one another by means of spacers and can be stacked freely in the receiver. As the support for the lowermost perforated plate it is possible, for example, to provide the receiver at its outlet side with a thread, by means of which a suitable closing element can be fastened to the receiver, on which closing element the lowermost perforated plate is supported. Alternatively, other types of fastening for such a closing element can be provided, such as, for example, flanges or the like.
An inlet element can be provided in the same manner at the upper end of the receiver.
In an embodiment, the receiver has a tapered cross-section at its inlet and/or outlet side. Such a tapered cross-section can be formed, for example, by the above-described outlet or inlet element. In a preferred embodiment of the invention, the cross-section tapers at the inlet and/or outlet side by ≥ 10% compared with the mean cross-section of the receiver. As a result of the tapered cross-section it is possible to adjust the flow velocity and accordingly the residence time of the dispersion and of the gas in the mixer.
According to the invention, the total perforated area per perforated plate is from 1% to 20% of the total plate area. It has been shown that, with such a perforated area or passage area, adequate mixing can be achieved without the hydraulic resistance through the perforated plates becoming too high.
According to a further embodiment, the perforation diameters of the individual perforations of a perforated plate are from 1% to 15% of the diameter of the perforated plate. The perforations can be arranged in a randomly or uniformly distributed manner over the entire surface of the perforated plate, preference being given to a uniform distribution. According to a further preferred embodiment, the perforations are formed in the perforated plates without burrs. As a result, the shear forces occurring at the perforations can be reduced further and coagulation of the dispersion caused by shear can be avoided.
According to a further embodiment, the perforated plates can have a thickness of from 1 mm to 10 mm. With such an embodiment, the perforated plates advantageously have sufficient stability, even with large perforation diameters and a high total perforated area, to withstand the mechanical forces occurring in the apparatus.
According to a further embodiment, the spacing between the individual perforated plates can be from 5 to 100 times the thickness of the perforated plates. It has been shown that, with such a mutual spacing of the perforated plates, thorough mixing of the gas with the dispersion can be achieved.
In a further embodiment, the longitudinal extent of the apparatus is from 5 to 50 times the diameter of the apparatus. With such a length/cross-section ratio, an optimised flow velocity is achieved within the apparatus.
According to a further embodiment, the apparatus can have from 3 to 100 perforated plates. The individual perforated plates can be arranged at equal distances or at different distances from one another. Furthermore, the perforated plates can have the same perforation diameters and/or total perforated areas or different perforation diameters and/or total perforated areas.
In embodiments, the apparatus used in the method of the invention for mixing a dispersion with a gas can additionally comprise further devices, such as, for example, devices for controlling the volume flow or the polymer dispersion and/or gas pressure with which the dispersion and/or gas is fed to the device. Furthermore, other devices such as pumps and the like can be provided in the disclosed apparatus.
According to the invention, a gaseous acid and a polymer dispersion flow through the apparatus disclosed apparatus for mixing a dispersion with a gas from top to bottom in the direction of gravity. Alternatively, the apparatus can also be arranged horizontally. According to the invention, the gas and the dispersion are introduced under volume and/or pressure control. In the case of volume control, preset volumes of gas and dispersion can be metered into the apparatus. In the case of pressure-controlled metering, the flow rate of the dispersion is specified, for example, by an inlet pressure and the gas is metered by means of a valve which ideally is open in only one direction and can be, for example, in the form of a swing check valve or Goodyear valve, or by means of a valve which is open in two directions, for example a ball valve. The inlet pressure of the dispersion in the case of pressure control can be controlled hydrostatically or, for example, by means of a membrane pump, a hose pump or the like. The gas inlet pressure can be set, for example, by means of a reducing valve which is arranged downstream of a pressure cylinder. If volume metering of the dispersion and the gas is carried out, it can be effected by known processes for the volume metering of liquids and gases. In a preferred embodiment, the dispersion and the gas can be combined upstream of the apparatus in a simple Y- or T-piece.
The receiver and the perforated plates can have any desired shape, preference being given to a tubular shape for the receiver and a round shape for the perforated plates.
The receiver and/or the perforated plates, and any further devices that may be provided, of the apparatus used in the method of the invention can preferably be produced from an inert material such as, for example, a stainless steel or a suitable plastics material. In an embodiment of the invention, the receiver, the spacers and the perforated plates can be produced from different materials.
Owing to the loose arrangement of the perforated plates and spacers inside the receiver, they can easily be removed from the receiver and thus, for example, conveyed to a cleaning operation if required.
The spacers between the perforated plates can have any desired geometrical shape, but tubular spacers are preferred. This facilitates cleaning of the spacers if required. In a further preferred embodiment, the spacers and the perforated plates have a diameter which is only negligibly smaller than the inside diameter of the receiver. The relative diameters are thereby so chosen that the spacers and the perforated plates can easily be introduced into the receiver but there is no or only a small gap between the inside wall of the receiver and the outside edges of the spacers and/or perforated plates.
In a form of the process, the dispersion and the gas are fed to the apparatus with a pressure of from 0.2 bar to 60 bar, preferably from 0.5 bar to 10 bar. The pressure can be adjusted by means of suitable devices, such as, for example, pumps and/or valves.
According to the invention, the dispersion is a polymer dispersion and the gas is a gaseous acid. In such a form, the polymer dispersion is, for example, to be acidified by introducing the gaseous acid. In a particularly preferred embodiment of the invention, the polymer dispersion is a dispersion of a polymer from the group consisting of polychloroprene, polystyrene-butadiene, polyurethane, polyacrylate and polysiloxane. The gas is preferably a gas from the group comprising CO₂, SO₂, SO₃, NO₂, COCl₂ and/or HCl.
The polymer dispersion can have a viscosity of, for example, < 100,000 mPas, preferably < 10,000 mPas and most particularly preferably < 1000 mPas.
More preferably, the polymer dispersion has a lower viscosity at the end of the mixing operation, that is to say at the outlet of the mixing apparatus according to the invention, than at the inlet.
In a form of the process described herein, more than 20%, preferably more than 40% and most particularly preferably more than 60% of the gaseous acid that is introduced is used in the apparatus to change or adapt the pH value.
The polymer dispersion to be acidified by the gaseous acid can have a pH value of, for example, > pH 6, preferably > pH 8 and particularly preferably > pH 9. Furthermore, the polymer dispersion can have a solids content of > 20%, preferably > 30% and particularly preferably > 40%.
In addition to organic or inorganic polymers and/or mixtures thereof, the dispersion can contain other ingredients, such as, for example, plasticisers, thickeners, antioxidants, pigments and the like. These can, for example, be dispersed in the aqueous phase or they can be present in the form of a suspension.
In a preferred form of the process, the polymer dispersion is a substantially anionically stabilised polymer dispersion which can also contain non-ionic emulsifiers in addition to anionic emulsifiers.
With the aid of the process according to the invention using the disclosed apparatus, the apparatus exhibits, under typical use conditions, a volume throughput prior to necessary maintenance, which can be caused, for example, by means of a blockage, of > 1000, preferably > 10,000, based on the volume of the device.
In an embodiment, the process exhibits a volume throughput per minute which is greater than 5 times the volume of the apparatus, preferably greater than 10 times the volume of the apparatus and yet more preferably greater than 20 times the volume of the apparatus. This gives a residence time of the polymer dispersion and of the gas, or of the mixture thereof, in the apparatus of < 12 seconds, preferably < 6 seconds, more preferably < 3 seconds.
The process according to the invention can be used, for example, in the field of the production of coating, dipping and/or rubber latex foam or in the production of adhesives.
The invention is explained in greater detail hereinbelow by means of figures and exemplary embodiments, but without being limited to those exemplary embodiments.
Figure 1 shows a schematic representation of an apparatus used in the method of the invention for mixing a dispersion with a gas.
Figure 1 shows an apparatus 1 used in the method of the invention for mixing a dispersion, for example a polychloroprene dispersion, with a gas, for example CO₂. The apparatus 1 has a receiver 2 in which perforated plates 3 are arranged. The perforated plates 3 are loosely arranged in the receiver 2 by means of spacers 4. The receiver 2 has an inlet 5 and an outlet 6. In the region of the inlet 5 and the outlet 6, the cross-section of the receiver tapers compared with the average cross-section. A volume stream 7, which comprises the polymer dispersion and CO₂, is fed to the apparatus 1 via the inlet 5. The polymer dispersion and the CO₂ can be premixed in a T-piece 8. The CO₂ can be fed to the apparatus 1 from a gas cylinder 9 by way of the T-piece 8 via a pressure reducer 11 and a connecting pipe 12. The polymer dispersion can be fed to the apparatus 1 from a storage tank 10 via a polymer pipe 13 by means of a pump 14. In the apparatus 1, the polymer dispersion and the gas are mixed sufficiently so that acidification of the polymer dispersion to the desired pH range can be ensured. The mixture of polymer dispersion and gas leaves the apparatus 1 via the outlet pipe 15.

Example 1:

Spacers having an outside diameter of 9.9 mm, an inside diameter of 8 mm and a length of 15 mm and perforated plates having an outside diameter of 9.9 mm and a thickness of 1 mm are arranged alternately in a polypropylene receiver having an outside diameter of 12 mm, an inside diameter of 10 mm and a length of 200 mm. The perforated plates have four perforations distributed in a square, each perforation having a diameter of 1.4 mm. 13 perforated plates, which are spaced apart from one another by means of 12 spacers, are arranged alternately in the receiver. The perforated plates and the spacers are held only by gravity, or by the pressure of the dispersion flowing from top to bottom. At the outlet of the apparatus there is arranged a reducing element by means of which the rate of flow can be regulated.

Example 2:

In a mixing apparatus as described in Example 1, a polychloroprene latex dispersion having a solids content of 55%, a viscosity of 100 mPas and a pH value of pH 12.5 is fed to the apparatus at an inlet pressure of 0.5 bar. CO₂ from a gas cylinder is adjusted by means of a reducing valve to a pressure of 2 bar and a rate of flow of 0.5 litre per minute and is fed to the apparatus. The rate of flow at the outlet of the apparatus was 0.5 litre per minute. The pH value of the latex dispersion at the outlet of the apparatus was pH 9. A useful life of > 120 litres was achieved.

List of reference numerals:

1: Apparatus
2: Receiver
3: Perforated plate
4: Spacer
5: Inlet
6: Outlet
7: Volume flow
8: T-piece
9: Gas cylinder
10: Storage container
11: Pressure reducer
12: Connector
13: Polymer pipe
14: Pump
15: Outlet pipe

Claims

Process for mixing a dispersion with a gas, wherein
a) the dispersion and the gas are fed with substantially the same pressures to a mixing apparatus (1) and are mixed therein, wherein the dispersion is a polymer dispersion and the gas is a gaseous acid;

b) the dispersion and the gas are fed to the apparatus (1) with a pressure of from 0.2 bar to 60 bar;

c) the mixing apparatus (1) having an external receiver (2) and at least two perforated plates (3) which are arranged in the receiver (2) spaced apart from one another by means of a spacer (4), the receiver (2) having a support for receiving a first perforated plate, the perforated plate being loosely arranged on the support and the spacer being loosely arranged on that first perforated plate, the second perforated plate in turn being arranged on the spacer;

d) the perforated plates (3) have a total perforated area per plate of from 1 % to 20% of the total area.
Process according to claim 1, wherein the dispersion and the gas are fed to the apparatus (1) with a pressure of from 0.5 bar to 10 bar.
Process according to claim 1 or 2, wherein the dispersion is a dispersion of a polymer from the group consisting of polychloroprene, polystyrene-butadiene, polyurethane, polyacrylate and polysiloxane.
Process according to one or more of claims 1 to 3, wherein the gas is a gas from the group comprising CO₂, SO₂, SO₃, NO₂, COCl₂ and/or HCl.
Process according to one or more of claims 1 to 4, wherein the residence time of the dispersion/gas mixture in the apparatus (1) is less than 12 seconds.
Process according to one ore more of claims 1 to 5, wherein the residence time of the dispersion/gas mixture in the apparatus (1) is less than 6 seconds.
Process according to one ore more of claims 1 to 6, wherein the residence time of the dispersion/gas mixture in the apparatus (1) is less than 3 seconds.
Process according to one or more of the preceding claims, wherein the receiver (2) has a tapered cross-section at its inlet and/or outlet side (5, 6) in a dimension of ≥ 10% compared with the mean cross-section of the receiver (2).
Process according to one or more of the preceding claims, wherein the perforation diameter is from 1% to 15% of the perforated plate diameter.
Process according to one or more of the preceding claims, wherein the perforated plates (3) have a thickness of from 1 mm to 10 mm.
Process according to one or more of the preceding claims, wherein the spacing between the perforated plates (3) is from 5 times to 100 times the thickness of the perforated plates.
Process according to one or more of the preceding claims, wherein the longitudinal extent of the apparatus (1) is from 5 to 50 times the diameter of the apparatus (1).
Process according to one or more of the preceding claims, wherein the apparatus (1) has from 3 to 100 perforated plates (3).