EP2608875B1 - Device and method for gas dispersion - Google Patents

Description

The invention relates to an apparatus and a method for dispersing gas in a liquid.

The dispersion of gases in liquid media is widely used in the chemical industry, for example in hydrogenations, chlorinations or oxidations. In fermentations and aerobic wastewater treatment, the oxygen input is essential. Also in the foam generation, a dispersion of gas takes place in a liquid medium. In food technology, gases are dispersed in highly viscous liquids to produce, for example, creams, foam rubber or chocolate with an air-filled porous structure (eg described in US Pat WO02 / 13618A2 ).

The goal of a gas dispersion is the introduction of gas into a fluid, preferably in the form of small bubbles as possible, to produce the largest possible interface between gaseous and liquid phase. The larger the phase interface, the higher the mass transfer between gas and liquid according to the first Fick's law.

1. 1. Einbringen des Gases in die Flüssigkeit in Form von Blasen
2. 2. Zerteilen der Blasen

Gas dispersion often takes place in two steps:

1. 1. introduction of the gas into the liquid in the form of bubbles
2. 2. Parting the bubbles

The type of introduction, generally via nozzles, frits or perforated plates, determines the size distribution of the primary bubbles. In the article " Gas dispersion in liquids through nozzles at high throughputs "from Chemie-Ingenieur-Technik, 28th year 1956, No. 6, pages 389-395 For example, the influence of parameters such as nozzle width, gas flow rate, viscosity and interfacial tension on the size distribution of gas bubbles created by injecting a jet of gas into a liquid from a nozzle is described.
The division of the bubbles can be done for example by means of a dynamic or static mixer. While in dynamic mixers, the homogenization of a mixture is achieved by moving organs such as stirrers, is static The feed fluid (eg a pump) pushes the liquid through a tube provided with static mixer inserts, wherein the liquid following the main flow axis is divided into partial flows which, depending on the type of internals, are stretched, sheared, intermixed and mixed become. The advantage of using static mixers is that there are no moving parts.

For an overview of different types of static mixers, see for example the article " Static mixers and their applications ", MH Pahl and E. Muschelknautz, Chem. Ing. 52 (1980) No. 4, pp. 285-291 , Examples of static mixers are SMX mixers (cf. US4062524 ) or SMXL mixer (cf., for example, patent specification US5520460 ) called. They consist of two or more mutually perpendicular lattices of parallel metal strips which are interconnected at their crossing points and employed at an angle to the main flow direction of the mixed material to divide the liquid into partial streams and mix. A single mixing element is unsuitable as a mixer, since a mixing takes place only along a preferred direction transverse to the main flow direction. Therefore, usually several mixing elements, which are each rotated by 90 ° to each other, arranged one behind the other.

The use of static mixers to disperse gas in a liquid is known. In WO2005 / 103115A1 For example, the use of a static mixer in a process for producing polycarbonate by the transesterification process will be described. To remove monomers and other volatiles from the polycarbonate, a foaming agent is added to the polymer melt. By subsequent reduction in pressure, the foaming agent escapes with foaming of the melt. The foam causes a strong surface enlargement, which is advantageous for the degassing, ie the removal of volatile components. The foaming agent used is preferably an inert gas such as nitrogen, which is introduced into the melt by means of a static mixer, for example an SMX mixer, and dispersed.
In US2005 / 0094482A1 and US5480589 Static mixers for the dispersion of gases for the production of closed-cell foams are described. A step-shaped structure for increasing the effectiveness of the gas dispersion is not described.

When dispersing gas into a liquid, generally longer mixer lengths are required than with the dispersion of liquids.

document US-A-5,605,399 discloses a device according to the preamble of claim 1.

Starting from the prior art, the object is to provide an apparatus and a method for dispersing gas in a liquid, to enable a more effective gas dispersion than described in the prior art. Compared to the prior art, a smaller average bubble size should be achieved at the mixer outlet with the same mixer length. Alternatively, a smaller average bubble size should be achieved at the mixer outlet with the same pressure loss over the entire mixer.

Surprisingly, it has been found that a static mixer in which there is an increasing specific energy input in the direction of flow has a particularly effective dispersing effect. With the aid of such a mixer, with comparable total pressure loss, smaller gas bubbles can be produced than with a static mixer in which the energy input over the length of the mixer is constant. With the aid of such a mixer, smaller gas bubbles can also be produced with the same overall mixer length than with a static mixer in which the energy input over the length of the mixer is constant.

A first subject of the present invention is therefore an apparatus according to claim 1 for the dispersion of gas into a liquid having a number n of successive zones Z ₁ , Z ₂ , ..., Z _n with static mixing elements, each zone Z _i a Length L _i and an effective diameter D _i , characterized in that the individual zones are designed so that the normalized to the respective ratio L _i / D _i mechanical energy input E _i , which acts on the gas-liquid mixture, in Flow direction increases from zone to zone, where n is an integer greater than or equal to 3 and i is an index that passes through the integer from 1 to the number n of the zones.

Another object of the present invention is a method for dispersing gas into a liquid in which gas and liquid are conveyed together by a mixing device and thereby a number n of successive Z zones Z ₁ , Z ₂ , ..., Z _n flow through with static mixing elements, each zone Z _{i has} a length L _i and an effective diameter D _i , characterized in that normalized to the respective ratio L _i / D _i mechanical energy input E _i , which acts on the gas-liquid mixture increases in the flow direction from zone to zone, where n is an integer greater than or equal to 3 and i is an index which is the integer from 1 to the number n of Passes through zones.

Liquid is generally understood here to mean a medium which can be conveyed through the device according to the invention. This may, for example, also be a melt or a dispersion (eg emulsion or suspension). In the following, the term fluid is used. The fluid is preferably of higher viscosity, ie it has a viscosity between 2 mPas and 10,000,000 mPas, more preferably between 1,000 mPas and 1,000,000 mPas (measured in a cone-plate viscometer according to DIN 53019 at a shear rate of 1 s ^-1 ).

In order to disperse a gas or gas mixture in the fluid, mechanical energy is introduced into the mixture. This energy input is realized by static mixing elements. In the mixing technique, the use of modular systems is common. A mixer consists of a series of modular mixing elements. To increase the mixing effect, the number of mixing elements in a mixer can be increased. Usually, the mixing elements are introduced into a tube to form a static mixer. It should be noted that the present invention is not limited to mixers constructed from an array of modular mixing elements but is also applicable to mixers in a compact design.

The device according to the invention is characterized in that it has a number n of contiguous zones, where n is an integer greater than or equal to 3. There are static mixing elements in each zone. Each zone Z _i has a length L _i and a cross-sectional area A _i . Where i is an index that traverses the integer from 1 to the number n of zones. The length L _{i of} a zone Z _i corresponds to the length of the mixing elements arranged one behind the other in this zone; the cross-sectional area A _i corresponds to the cross-sectional area of the mixing elements present in the zone Z _i .

From the cross-sectional area A _i , an effective diameter D _i according to equation 1 can be calculated: $$D_i=\sqrt{\frac{4A_i}{\pi }}$$

The effective diameter D _i corresponds in a circular cross section to the diameter of the circle. For a non-circular (eg, rectangular) cross-section, the effective diameter D _i corresponds to the diameter of a circle having an area corresponding to the cross-sectional area.

The ratio L _i / D _i is a characteristic characteristic of the respective zone Z _i .

A mixing element has internal structures and channels between these structures. If a fluid is conveyed through a mixing element, the structures and channels cause the fluid to be divided, distributed, sheared and, if necessary, fluidized into partial flows, thus mixing the partial flows with one another. The mean diameter of a channel is subsequently abbreviated by the letter d _i . A mean channel diameter d _i is understood to mean the channel diameter arithmetically averaged over all channels, whereby the effective channel diameter can be calculated analogously to the effective diameter of a zone Z _i according to equation 1. $$d_i=\sqrt{\frac{4a_i}{\pi }}$$

The ratio d _i / D _i between the mean channel diameter d _i and the effective diameter D _{i of} the mixing elements in a zone Z _i is also a characteristic index for the respective zone Z _i . The parameter a _i denotes the open cross-sectional area, more precisely the projection surface of the free cross-section. This results, for example, from Fig. 1a the open cross-sectional area a _i of the sum of the projection areas of the individual free cross-sectional areas of the open channels, through which the fluid can flow (equation 3). $$a_i={\displaystyle \underset{m=1}{\overset{N}{\Sigma }}}{b}_{i.m}\cdot w_{i.m}$$

The parameter m is a counting parameter, N is the number of individual free cross-sectional areas.

The static mixers used in the prior art for gas dispersion have mixing internals that are consistent throughout the length of the mixer. Here there is only a single zone whose length L corresponds to the length of the mixer and whose effective diameter D corresponds to the effective diameter of the mixer. In order to increase the dispersing effect of such a mixer, for example, the length L can be increased. With the length of the mixer, the pressure drop Δ p increases linearly over the mixer. The mechanical energy input E _abs is according to equation (4) proportional to the pressure loss, where V̇ is the volume flow of the fluid. $$e_{\mathit{Section}}=\mathrm{\Delta }p\cdot \dot{V}$$

The pressure loss Δ p and thus the mechanical energy input can be increased in the same way by reducing the effective diameter D.

The device according to the invention is characterized by a number n of zones. Each zone Z _i is characterized by a specific mechanical energy input E _i , which is registered in a fluid flowing through the respective zone. The specific mechanical energy input E _i is the mechanical energy input E _abs normalized to the characteristic L _i / D _i . In this case, according to the invention E ₁ <E ₂ < ... <E _n . $$e=\frac{e_{\mathit{Section}}\cdot D}{L}$$

The number n of zones in a device according to the invention is not limited. It can run to infinity if the zones become infinitesimally small and there is a continuously increasing specific energy input along the length of the device, as might be the case, for example, with a conically narrowing tube.

It is conceivable that further zones exist in front of or behind the zones Z ₁ to Z _n , which have freely selectable specific energy inputs.

Thus, a particularly preferred embodiment of the device according to the invention is characterized in that there is a first zone Z ₀ , which makes a higher specific energy input than the subsequent in the flow direction zone Z ₁ ( E ₀ > E ₁ ). According to the invention, further zones Z ₂ to Z _n follow on the zone Z ₁ , with the following for the corresponding specific energy inputs E ₁ to E _n : E ₁ <E ₂ < ... <E _n .

Surprisingly, it has been found that such an arrangement of zones through the zone Z ₀ produces primary bubbles which are less prone to coalescence in the subsequent zones and thus a more effective dispersion is achieved.

In a preferred embodiment, the device according to the invention has a number n of mixing zones, which are arranged one behind the other, wherein the mean channel diameter d _i in the mixing zones in the flow direction is smaller. Smaller channels produce a higher pressure loss per length, which is equivalent to increasing specific energy input.

This embodiment preferably comprises a cylindrical tube into which mixing elements are introduced. The effective diameter D _{i of} the mixing elements is preferably constant over the entire tube length, while the mean channel diameter d _i in successive zones in the flow direction is smaller. D ₁ = D ₂ = ... = D _n and d ₁ > d ₂ >...> d _n .

Preferably, mixing elements of the same type are used, eg SMX mixers with different characteristic numbers d / D.

und D ₁>D ₂>...>D_n .The inventive device has an array of mixing elements which have a progressively lower effective diameter D _i in the flow direction at a constant ratio d _i lD _i. It applies $\frac{d_1}{{\mathrm{<figure-callout\; id="1"\; label="D"\; filenames="imgf0001.png"\; state="\{\{state\}\}">D</figure-callout>}}_1}=\frac{d_2}{{\mathrm{<figure-callout\; id="2"\; label="D"\; filenames="imgf0001.png"\; state="\{\{state\}\}">D</figure-callout>}}_2}=\frac{d_i}{D_i}...=\frac{d_n}{D_n}$

and D ₁ > D ₂ >...> D _n .

Preferably, this embodiment comprises a cylindrical tube into which mixing elements are introduced, which have an increasingly smaller effective diameter D _i in the flow direction.

The mixing elements whose outer diameter is smaller than the inner diameter of the tube are preferably enclosed by a jacket tube whose outer diameter corresponds approximately to the inner diameter of the tube in order to be able to bring it into the tube. At the transition points from a mixing element with a large diameter to a mixing element with a small diameter, transition jacketed pipes are preferably present, which have a conically tapering in the direction of the mixing element with a small diameter inner diameter. These transition jacketed tubes can be integrally connected to the jacket pipes or executed separately.

In a further preferred embodiment, the device according to the invention has in each zone Z _i an arrangement of mixing elements of different types, which cause an increasing pressure loss in the same direction in the flow direction in each zone Z _i for the same ratio L _i / D _i .

Preferably, the mixing elements are introduced into a cylindrical tube. They preferably have the same effective diameter D _i .

If the outer diameters of the types of mixing elements vary, it is conceivable to surround those mixing elements whose outer diameter is smaller than the inner diameter of the tube with a jacket tube or ring whose outer diameter corresponds approximately to the inner diameter of the tube to fit into the To bring in pipe. The above-described use of transition jacketed pipes is advantageous here.

It is conceivable to combine the various embodiments listed.

The device according to the invention is suitable for the dispersion of gas in a liquid, for example for introducing a towing gas into a polymer melt or for foaming liquid media.

The gas may be added with tubes or thin capillaries, preferably in the flow direction before the static mixer cascade. Furthermore, the gas may also be added through a porous body. A porous body may comprise, for example, the following geometries: a frit and / or a porous, sintered body and / or a single or multi-layer sieve.

The porous body may, for example, be in the form of a cylinder, in the form of a cuboid, a sphere or a cube or in conical form, e.g. as a cone, be formed. These devices ensure a fine predispersion of the gas and possibly also for a distribution of the gas over the cross section.

The capillary or the porous body has a mean effective internal hole diameter of preferably 0.1-500 .mu.m, preferably 1 - 200 .mu.m, particularly preferably 10-90 .mu.m.

Porous bodies which may be used are, for example, porous sintered bodies of metal, such as frit bodies used in chromatography, for example the sintered bodies of Mott Corporation (Farmington, USA). Furthermore, wound wire mesh can be used, for example, the wound wire mesh from Fuji Filter Manufacturing Co., Ltd. (Tokyo, Japan), trade name: Fujiloy ^®. Furthermore, sieves or multi-layer fabrics can be used, such as the metal wire mesh composite panels of the company Haver & Boecker wire weaving (Oelde, Germany), trade name: Haver Porostar.

These devices serve the distribution of the gas over the pipe cross-section and a predispersion for the gas dispersion over the narrow pores. The effective diameter D _{i of} the holes inserted in the porous sintered bodies or wires or wound wire meshes is preferably 1-500 μm, more preferably 2-200 μm, most preferably 10-90 μm.

The invention is explained in more detail below by means of examples without, however, limiting them thereto.

Fig. 1 shows examples of three different static mixers (No.1, No. 2 and No. 3): Fig. 1 (a) from above, Fig. 1 (b) from the side (sectional drawing) and Fig. 1 (c) in the arrangement after installation in a pipe or housing. The indications for wi and bi denote the length and width of the projected cross section of the free flow channels. Di denotes the clear diameter and DM the outer diameter of the static mixing elements. Li denotes the entire length of a geometrically uniform mixer section and li the length of a single mixing element.

No. 1 illustrates a Kenics mixer. No.2 shows a commercial SMX static mixer with or without an outer ring. No. 3 shows a mixer with web structure and outer ring (FIG. DE 29923895U1 and EP1189686B1 ).

Fig. 2 shows three different examples (A, B and C) of variants static mixer, with individual zones (characterized by the lengths L ₁ , L ₂ , L ₃ ), characterized in that on the respective ratio L _i / D _{i of} the individual zones normalized mechanical energy input E _i increases in the flow direction of a fluid which flows through the respective zone Z _i . The flow direction is indicated by the thick arrow.

und D ₁ > D ₂ > D ₃. Fig. 2A shows a sequence of mixers according to the invention with geometrically similar structure and an array of mixing elements having an increasingly smaller effective diameter D _i in the flow direction at a constant ratio d _i / D _i . It applies $\frac{d_1}{{\mathrm{<figure-callout\; id="1"\; label="D"\; filenames="imgf0001.png"\; state="\{\{state\}\}">D</figure-callout>}}_1}=\frac{d_2}{{\mathrm{<figure-callout\; id="2"\; label="D"\; filenames="imgf0001.png"\; state="\{\{state\}\}">D</figure-callout>}}_2}=\frac{d_3}{{\mathrm{<figure-callout\; id="3"\; label="D"\; filenames="imgf0001.png"\; state="\{\{state\}\}">D</figure-callout>}}_3}$

and D ₁ > D ₂ > D ₃ .

Fig. 2B shows an embodiment with a cylindrical tube, in which mixing elements are introduced, in which the effective diameter D _i over the entire tube length is constant, while the mean channel diameter d _i in successive zones in the flow direction is smaller. D ₁ = D ₂ = D ₃ and d ₁ > d ₂ > d ₃ . Mixing elements of the same type are used, eg SMX mixers with different characteristic numbers d / D.

Fig. 2C shows an arrangement of mixing elements of different types, which cause an increasing pressure loss at the same ratio L _i / D _i in the flow direction in each zone Z _i . As an example, here in the first zone of length L1 is a Kenics mixer shown. In the second zone of length L2 is an SMX mixer. In the third zone of length L3 there is also an SMX mixer with smaller effective diameters D _i compared to the mixer in the second zone.

Fig. 3A shows a device according to the invention with three zones and a premixer and a gas metering via a capillary. Before the premixer is the area where the fluid is metered (L) and a device for metering gases (G) via a capillary (Ca).

Fig. 3B shows a gas metering by means of porous sintered body (the mixer behind it is not shown here). In front of the premixer is the area where the fluid is metered (L) and a device for gas metering (G) via a porous sintered body (PS), which is located within the flow cross section.

Claims (8)

1. Device for dispersing gas in a liquid with a number n of successive zones Z₁ , Z₂ , ..., Z_n with static mixing elements, each zone Z_i having a length L_i and an effective diameter D_i, the individual zones being constructed such that the mechanical energy input E_i normalized to the respective ratio L_i /D_i increases from zone to zone in the direction of flow, wherein n is an integer greater than or equal to 3 and i is an index which runs through the integers from 1 to the number n of zones, characterized in that the mixing elements present in the zones Z₁ to Z_n have the same ratio d_i /D_i, where d_i is the average diameter, and an effective diameter D_i which becomes increasingly smaller from zone to zone in the direction of flow.
2. Device according to Claim 1, characterized in that the average channel diameter d_i becomes smaller in the zones Z₁ to Z_n succeeding one another in the direction of flow.
3. Device according to Claim 1 or 2, characterized in that the zones Z₁ to Z_n have mixing elements of different types, which at the same ratio L_i /D_i cause an increasing pressure drop from zone to zone in the direction of flow.
4. Device according to one of the preceding claims, characterized in that there is a first zone Z₀ , which achieves a higher specific energy input Eo than the next zone Z₁ in the direction of flow.
5. Device according to one of Claims 1 to 4, further comprising a tube or a thin capillary for feeding gas into the device, characterized in that the tube or the thin capillary is mounted upstream of the arrangement of mixing elements.
6. Device according to one of Claims 1 to 4, further comprising a porous or screen-like body for feeding gas into the device, characterized in that the body is mounted upstream of the arrangement of mixing elements.
7. Method for dispersing gas in a liquid, in which gas and liquid are conveyed jointly through a mixing device according to one of Claims 1 to 6 and, in the process, flow through a number n of successive zones Z₁ , Z₂ ,..., Z_n with static mixing elements, each zone Z_i having a length L_i and an effective diameter D_i , characterized in that the mechanical energy input E_i acting on the gas/liquid mixture and normalized to the respective ratio L_i /D_i increases from zone to zone in the direction of flow, wherein n is an integer greater than or equal to 3 and i is an index which runs through the integers from 1 to the number n of zones.
8. Method according to Claim 7, characterized in that the liquid has a viscosity of between 2 mPa·s and 10,000,000 mPa·s, particularly preferably between 1,000 mPa·s and 1,000,000 mPa·s.

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