WO2002092205A1

WO2002092205A1 - Emulsion production with internal plate devices

Info

Publication number: WO2002092205A1
Application number: PCT/AU2002/000601
Authority: WO
Inventors: Geoffrey Michael Evans; Simon Richard Biggs; Stephen John Pratten; Kevin Patrick Galvin
Original assignee: The University Of Newcastle Research Associates Limited
Priority date: 2001-05-16
Filing date: 2002-05-16
Publication date: 2002-11-21
Also published as: AUPR503801A0

Abstract

Coarse emulsion suitable for further refining by in-line mixers or high rate air sparging, is formed from two immiscible liquids (2) and (3) placed within a container (1) and sparged with air through spargers (4). Efficiency is achieved by using a number of transverse mesh sheets (9) within the container which divide the container into stages. The mesh sheets are typically expanded metal mesh. An alternative method uses arrays of inclined plates in place of the mesh sheets.

Description

EMULSION PRODUCTION WITH INTERNAL PLATE DEVICES

FIELD OF THE INVENTION

This invention relates to emulsion production with internal plate devices and has been devised particularly though not solely for the efficient production of high internal phase emulsions.

BACKGROUND OF THE INVENTION

Emulsions are often manufactured using confined jet technology or other mixing technology. The resulting coarse emulsion may then be refined by pumping the emulsion through a static mixer to reduce the droplet size. The conventional wisdom of emulsion technology is that incremental addition of the internal phase is necessary when forming high internal phase ratio emulsions. This is because the incremental addition of the internal phase results in the internal phase liquid always "seeing" a relatively large proportion of the external phase. i the past, high internal phase emulsions have conventionally been formed by various methods of mechanical mixing requiring high energy inputs to achieve the desired shear and result in a homogenous emulsion. Various proposals have also been made to manufacture emulsions using air sparging such as the method described in International Patent Application PCT/AU00/00787 for the manufacture of emulsion explosives which describes an apparatus in which air bubbles added to a vessel containing a water and an oil phase, generates a water in oil emulsion.

Earlier researchers had thought that air sparging was not much more efficient than mechanical stirring unless extremely large volumes of gas were used, but more recent experience, including the method described in the aforesaid International Patent Application PCT/AU00/00787 shows that air sparging can provide an efficient method for emulsion production. However it has been found that air sparging is only effective within a narrow operating regime, and that this regime is sensitive to small changes in conditions, including system chemistry, temperature, and vessel design. For these reasons, air sparging is unreliable, the limited operating regime coincides with relatively low air addition rates, and hence the rate of emulsification is low leading to low production efficiencies. The desired emulsification can sometimes occur at high air rates, but the process is slow, unpredictable and unreliable, and the product of poorer quality. SUMMARY OF THE INVENTION

The present invention therefore provides a method of forming an emulsion from two immiscible liquids, comprising the steps of: placing the two liquids in a container having one or more transverse devices having apertures therein, such that the devices are immersed; and sparging gas into the container such that bubbles of gas are formed moving upwardly through the liquids and through the apertures in the devices. Preferably the devices are formed from mesh.

Preferably the mesh comprises expanded metal mesh.

Alternatively the devices comprise arrays of inclined plates.

Preferably the devices are arranged in layers, one above the other in the container. Preferably the gas comprises air.

Preferably the gas is sparged into the container adjacent the bottom of the container.

Preferably the gas is sparged into the container through an array of perforated sparger tubes in the bottom of the container. In a further aspect the invention provides an apparatus for forming an emulsion from two immiscible liquids, comprising a container having one or more transverse devices having apertures therein arranged within the container in locations adapted to be immersed within the liquids in use, and a sparging device arranged to sparge gas into the container such that bubbles of gas move upwardly within the liquids and through the apertures in the devices.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms that may fall within its scope, one preferred form of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Fig 1 is a diagrammatic cross-sectional elevation through apparatus used for forming an emulsion by the method according to the invention, and

Fig 2 is a similar view to Fig 1 showing the use of an alternative form of transverse devices within the container.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The invention is based on the use of air bubbles to form emulsions, the preferred embodiment described below we will refer to the production of a concentrated water in oil emulsion, the term "water" representing an aqueous phase, and the term "oil" representing a hydrocarbon phase, usually containing an emulsifier. It should be realised however that the present invention can be used for the formation of an emulsion from any two or more immiscible liquids and is not limited to the production of water in oil emulsions, normally used as emulsion explosives. The method could be used in the preparation of emulsion explosives, food products, pharmaceuticals, lotions, and other emulsion based products.

The basic mechanism of emulsion production by air sparging involves the transport of elements of the water into the oil by the rising bubbles. Being more dense, the water droplets tend to settle back through the oil, and down to the interface between the water and the oil where they concentrate. The emulsifϊer in the oil stabilises the droplets, preventing coalescence, and the oil between the droplets drains upwards. The coarse emulsion, which has the appearance of a foam, is initially very coarse, and spherical, and then becomes polyhedral in structure. This "foam-like" emulsion is displaced downwards into the water, gradually filling the whole space. Eventually, the entire contents of the vessel has the appearance of a "dry foam", with large polyhedral water droplets surrounded by thin films of oil. The oil is then well dispersed in the form of thin films throughout the vessel. The critical step in the overall process is the formation of this coarse emulsion. The emulsion which, at this time, has a viscosity moderately higher than the viscosity of water, is now ready for refining, either by continued air sparging at a very high rate, or by passing the emulsion through an in-line mixer. Either way, the final refining of the emulsion is relatively straightforward. So, the key to this technology is in achieving the coarse emulsion.

The present invention uses a container or vessel (1) for the production of the coarse emulsion. The vessel is filled with a given volume of the water (2), and a much smaller volume of oil (3) is added, or the oil (3) is added first, with the larger volume of the water (2) added progressively from below, or from above. Bubbles of air are sparged in from below, through an array of perforated sparger tubes (4) in the bottom of the vessel. Where desired, a packing block (5) is placed around the sparger tubes to ensure that there is no free water (unaffected by the sparging) in the vicinity of the tubes. As thus far described, the process is similar to that described in the aforementioned International Patent Application PCT/AU00/00787.

The effectiveness of the process is improved significantly by providing one or more transverse devices having apertures therein within the vessel. In one version, shown in Figure 1, the devices comprise substantially horizontal sheets of expanded metal mesh (9) placed across the vessel so as to form a number of stages within the vessel. The elevation between each layer in this example is 40 mm. This dimension is given by way of example only. The emulsion tends to form progressively in each stage at a time. The expanded metal mesh helps protect the coarse emulsion network from the potentially disruptive effectives of the airflow.

Bubbles rising through the vessel from the air sparger tubes (4) are forced by gravity to move up through the apertures in the mesh. Water is carried up by the rear of the bubble. This liquid is extended in length until it breaks-up into smaller droplets.

Any form of mesh or perforated sheet can be used as the effect is achieved over a wide range of sizes and thicknesses etc. The preferred form of mesh is that typically manufactured by forming an array of parallel slits in a metal sheet and pulling the edges of the sheet away from each other in a direction perpendicular to the slits to form an expanded metal mesh.

The more sheets (9) that are used, forming a larger number of stages, the less time required for a complete coarse emulsion to be formed. The number of sheets is typically limited by practical considerations including the necessity to drain the coarse emulsion from the vessel (1).

In some cases, the device will work well with only one sheet of mesh which may even be placed below the interface (8). The number of sheets needed for efficient operation also depends on the nature of the emulsifier used. A second transverse device arrangement is shown in Figure 2. Here arrays of plates (6 and 7) arranged in a parallel fashion at an angle of perhaps 60° are placed across the vessel. This gives a similar effect to the mesh sheets but has been typically found to be less efficient as well as more difficult to construct. EXPERIMENTAL FINDINGS

We prepared water-in-oil emulsions of 90% internal phase ratio by bubbling air through a 700 mm high, 150 mm x 150 mm cross-section vessel containing a water phase (90% by volume) and an oil phase (10% by volume). The water phase consisted of 25% by weight Ammonium Chloride dissolved in water. The oil phase consisted of 70% by weight diesel, and 30% by weight emulsifier, an ethanolamine adduct with polyisobutaline succinic anhydride. Of course, the invention may be applied to other compositions.

It will be appreciated that a variety of devices can be added to the vessel to improve the emulsification through various combinations of the mechanisms described. Some devices will tend to perform better than others. In addition to vertical and inclined plates, perforated plates can be used, or simple mesh-like structures inserted within the vessel. These will also assist droplet entrainment and break-up, and assist in supporting the emulsion structure. There may also be a reduction in the inefficient circulation, and improvement in drainage, depending on the exact arrangement. Prior Art - Without Plates

The passage of the bubbles tended to cause large circulation patterns, with oil droplets forming in the water phase. These droplets gradually became creamy in appearance indicating that water was being incorporated within the circulating oil droplets. Over time the droplet size decreased because of the shear generated by the rising bubbles. In general, these experiments failed to produce the desired water in oil emulsion. Sometimes the desired water in oil emulsion was eventually produced, almost by chance, requiring in excess of 120 minutes. Even at the end of the experiment, there was still some free water present.

However, by operating at an exceedingly low air rate, the large circulating patterns did not form. Instead, a coarse emulsion was produced near the interface. This emulsion had a "foam-like" appearance. More correctly, an emulsion of water droplets in the oil formed. These were spherical at first. The oil drained upwards, leaving an emulsion of polyhedral water droplets surrounded by thin films of oil.

This coarse emulsion was displaced downwards into the water. Gradually the entire contents of the vessel evolved into a very coarse emulsion. Despite the use of a low air rate, the emulsion was formed in about 30 minutes. So, by applying a low air rate, the air sparging method becomes more efficient. With Internal Plate Devices

By introducing a system of transverse devices of the type described above with reference to either Fig 1 or Fig 2, it is possible to improve the effectiveness of the bubbles. A variety of mechanisms are believed to be responsible for the improvement in the rate of emulsification. These are described below in turn: 1. The formation of the coarse emulsion state is essentially the critical step. This state, which consists of large polyhedral water droplets surrounded by thin oil films, accumulates over time. Its stability is important. Bubble motion can cause it to shear off, and produce a fragmented state. The emulsion can also breakdown. The placement of sets of internal devices within the vessel at various elevations helps provide a geometrical structure to support, protect, and contain the foam. High air rates can then be used, resulting in high rates of emulsification. 2. The internal devices result in higher shear rates and hence the potential for better refinement by the rising air bubbles.

3. The presence of internal plates at various elevations reduces the tendency of the oil to become circulated throughout the water phase. Thus the water phase is entrained by the air bubbles into the oil phase incrementally, and hence inefficient bulk mixing of the two phases is avoided.

The results of the experiments are summarised in Table 1, in terms of the devices used, the air rate applied, and the time required to produce the coarse emulsion, the absence of plates, the best result was obtained using a low air rate, with the time required only 25 minutes. However, it was only possible to consistently produce the required emulsion using air rates in the range 0.5 to 2.0 L/min. All attempts at rates below and above this range generally produced a failed emulsion product, even after sparging for more than 100 minutes.

Using inclined plates the system was much more robust, and hence emulsions were formed using air at all of the air rates tested, ranging from 1 to 50 L/min. The inclined plates did not substantially increase the rate of emulsification. However, the refining did lead to substantially higher product viscosities, about 50% higher than when no internal devices were used.

Using the expanded metal mesh, it was possible to generate the desired emulsion product at all of the air rates tested, from 1 to 50 L/min. At an air rate of 50 L/min, the desired water in oil emulsion formed in only 2 ^"minutes. Thus, the expanded metal mesh provided both a robust process, and very high rates of emulsification. At very low air rates the emulsification rates were relatively low due to increased drainage times and hence a greater tendency for coalescence. Above these low air rates, however, the rate of emulsification tended to increase with the air rate to remarkably high levels. Table 1. Summary of emulsification experiments using a vessel 150mmx 150mm in cross-section and 700 mm high. The emulsion produced contained 10.8 kg aqueous phase and 1.2 kg oil phase.

It should be noted that the "window" of non-failed results in the system with no internal devices (prior art) may move up or down within the table depending on the emulsifier used, but it typically remains a narrow "window" with significantly long times to produce the desired coarse emulsion. These experimental results show that using transverse devices within the container or vessel, in combination with air sparging, results in an increased rate at which the emulsion is formed. The use of the devices, whether in the form of inclined or vertical plates, or some form of mesh such as expanded metal mesh also results in:

(i) A significantly more robust emulsification process; (ii) Protection of the coarse network of droplets;

(iii) The opportunity to use significantly higher air addition rates and hence achieve significantly higher rates of emulsification;

(iv) The potential to generate higher viscosities when refining the coarse emulsion by air sparging;

(iv) A significant reduction in the tendency to cause inefficient bulk circulation of the two phases.

Claims

CLAIMS:-

1. A method of forming an emulsion from two immiscible liquids, including the steps of: placing the two liquids in a container having one or more transverse devices having apertures therein, such that the devices are immersed; and sparging gas into the container such that bubbles of gas are formed moving upwardly through the liquids and through the apertures in the devices.

2. A method as claimed in claim 1 wherein the devices are formed from mesh.

3. A method as claimed in claim 1 wherein the mesh comprises expanded metal mesh.

4. A method as claimed in claim 2 wherein the devices comprise arrays of inclined plates.

5. A method as claimed in any one of the preceding claims wherein the devices are arranged in layers, one above the other in the container, defining stages within the container.

6. A method as claimed in any one of the preceding claims wherein the number of transverse devices and the spacing therebetween are chosen to achieve a desired emulsification rate.

7. A method as claimed in any one of the preceding claims when used to form a coarse emulsion which is subsequently further refined.

8. Apparatus for forming an emulsion from two immiscible liquids, including a container having one or more transverse devices having apertures therein arranged within the container in locations adapted to be immersed within the liquids in use, and a sparging device arranged to sparge gas into the container such that bubbles of gas move upwardly within the liquids and through the apertures in the devices.

9. Apparatus as claimed in claim 8 wherein the devices are formed from mesh.

10. Apparatus as claimed in claim 9 wherein the mesh comprises expanded metal mesh.

11. Apparatus as claimed in claim 8 wherein the devices comprise arrays of inclined plates.