EP0347653B1 - Mixing receptacle with radial transport stirrer and at least one flow breaker as well as process for liquid mixing using this mixing receptacle - Google Patents

Abstract

It was the object of the invention to develop mixing systems which allow rapid axial mixing of liquids of different densities without foam formation. <??>The mixing receptacle 1 according to the invention is characterised in that it is fitted with a radial-transport stirrer 3 and at least one flow breaker 2, the latter having the shape of an aerofoil profile. <??>The mixing receptacle is particularly suitable for homogeneous mixing of liquids in large reactors.

Description

The homogeneous mixing of liquids in reactors, especially in large reactors, is by no means unproblematic. Not only should a good mixing effect be achieved, there should also be no foaming. The liquid, for example a polymerization batch, should not form a vortex and in particular should not draw in any gas. Caking should be avoided since it often leads to product damage and inhomogeneities. As is known, it is particularly difficult to achieve homogeneous mixing in liquids which consist of two or more liquid components of different densities.

In all of these cases, conventional stirred kettles lead to unsatisfactory results.

If you set a common stirrer type, e.g. B. a disc stirrer (Rushton turbine), one achieves a good horizontal, but an extremely poor axial mixing. The easiest way to avoid the formation of drums is to install additional baffles. In a common embodiment, a stirred tank (diameter D) and a Rushton turbine are used and four baffles approximately D / 10 wide are installed along the wall of the stirred tank. The disadvantage is that these baffles are not adjustable and can therefore not be adapted to the respective stirring problems. The formation of caking can usually not be avoided on the baffles, which require a considerably increased power input.

According to the state of the art, very good mixing results are achieved with the Pfaudler system (cf. prospectus of the Pfaudler works, D-6830 Schwetzingen, 1978). A Pfaudler impeller stirrer is usually used as the stirrer. Between the stirrer diameter d and the height The stirring blade b has the relationship d / 10 = b. The rotating baffles, usually with three fingers (Mixing in the Chemical Industry, Pergamon Press, I. Sterbacek, Tausk, 1965, p. 281), the ends of which can be connected, are usually located directly above the stirrer. While the mixing in the lower region of the stirred tank is satisfactory, there are problems with the axial mixing in particular in the case of large and slim stirred tanks, unless the stirring power is increased considerably, which in turn leads to the undesirable formation of droplets.

DE-A-2933421 describes a device for mixing a gas with a liquid. The baffles are designed as concave blades; the ratio of the height of each blade to its length in a horizontal plane is less than 1, so the blades are narrow. A liquid is gassed in this device; the gas not yet taken up by the liquid is directed into the circulating liquid by means of the blades
DE-A-32 02 159 discloses a process for the preparation of styrene polymers with the addition of low-boiling propellants, in which it is important to obtain a surface that is as free as possible from pellets while stirring. It is proposed, inter alia, to use a stirring system which consists of a stirred kettle, an axially arranged stirrer, preferably an impeller stirrer and a Pfaudler-type finger flow disturbance, which is arranged in the vicinity of the stirrer. Such a stirring system can be used above all if the filling height is not greater than the bowl diameter. Problems arise with stirred tanks that are filled higher. It can be seen from the examples that the product quality deteriorates considerably if the work is not carried out free of leaks.

• 1. Sie sollten eine schnelle axiale Durchmischung von Flüssigkeiten unterschiedlicher Dichten ermöglichen.
• 2. Beim Rühren sollte sich kein Schaum entwickeln.
• 3. Beim Rühren sollte sich keine oder nur eine unwesentliche Trombe bilden.
• 4. Beim Rühren sollte kein Gas eingezogen werden.
• 5. Das Rührsystem sollte auch bei großen Füllhöhen H_L zuverlässig arbeiten ${\text{(H}}_{\text{L}}\text{/D > 1)}$
  

Es wurde jetzt ein Rührkessel 1 gefunden, der mit einem radial fördernden Rührer 3 und mindestens einem verstellbaren Stromstörer 2 ausgestattet ist und durch folgende Merkmale gekennzeichnet ist:

• Der radial fördernde Rührer 3, vorzugsweise ein Impellerrührer, ist maximal in einem Abstand D/2 vom Rührkesselboden entfernt angebracht.
• Der oder die Stromstörer 2 befinden sich in der oberen Hälfte des Rührkessels.
• Der bzw. die Stromstörer 2 haben die Form eines Tragflächenprofils, d. h. sie sind in Form einer archimedischen Spirale oder in Form eines Teiles oder Teilen eines Kreisbogens gekrümmt.
• Der bzw. die Stromstörer 2 sind in einem Abstand R₁ mit D/8 < R1 < D/3 von der Wand entfernt angebracht.
• Der bzw. die Stromstörer besitzen die Höhe h_s, für die folgende Beziehung gilt:
  
  ${\text{0,2 < h}}_{\text{s}}\text{/D < 0,4}$
  
  
  
  
• Der Ausströmquerschnitt des bzw. der Stromstörer ist vorzugsweise rechteckig.

The aim of the present invention was therefore to develop stirring systems which have the following properties:

• 1. They should enable rapid axial mixing of liquids of different densities.
• 2. No foam should develop when stirring.
• 3. No stirring or only an insignificant drum should form when stirring.
• 4. No gas should be drawn in when stirring.
• 5. The stirring system should work reliably even at high filling levels H _L ${\text{(H}}_{\text{L}}\text{/ D> 1)}$
  

A stirred tank 1 has now been found, which is equipped with a radially conveying stirrer 3 and at least one adjustable baffle 2 and is characterized by the following features:

• The radially conveying stirrer 3 , preferably an impeller stirrer, is mounted at a maximum distance of D / 2 from the bottom of the stirred tank.
• The baffle 2 is located in the upper half of the stirred tank.
• The baffle or baffles 2 have the shape of an airfoil profile, ie they are curved in the form of an Archimedean spiral or in the form of a part or parts of a circular arc.
• The or the baffle 2 are attached at a distance R₁ with D / 8 <R1 <D / 3 from the wall.
• The baffle ( _s) have the height h _s , for which the following relationship applies:
  
  ${\text{0.2 <h}}_{\text{s}}\text{/ D <0.4}$
  
  
  
  
• The outflow cross section of the baffle or baffles is preferably rectangular.

The invention further relates to a method for mixing liquids using the stirred tank just described.

• 1. Die gesamte Flüssigkeitsmenge wird homogen durchmischt.
• 2. Es wird eine hohe axiale Umwälzung erreicht.
• 3. Die Form der Stromstörer 2 garantiert einen erstaunlich geringen Leistungsbedarf.
• 4. Die Flüssigkeit wird trombenfrei gerührt.
• 5. Die Stromstörer 2 können sowohl in ihrer Höhe als auch in ihrem Winkel gegenüber der zu rührenden Flüssigkeit verstellt werden und damit auch sehr unterschiedlichen Rührproblemen angepaßt werden.
• 6. Bei Verwendung eines radial fördernden Rührers mit Untenantrieb kommt man mit einer sehr kurzen Rührerwelle aus.

The following advantages are achieved with the present invention:

• 1. The entire amount of liquid is mixed homogeneously.
• 2. A high axial circulation is achieved.
• 3. The shape of the baffle 2 guarantees an astonishingly low power requirement.
• 4. The liquid is stirred free of bubbles.
• 5. The baffle 2 can be adjusted both in height and in their angle with respect to the liquid to be stirred and thus very different stirring problems can be adapted.
• 6. When using a radially conveying stirrer with bottom drive, a very short stirrer shaft is sufficient.

The stirred kettle used according to this invention is a standing stirred kettle 1 with a usually vertical axis and cylindrical shape. The ratio between its height H and its diameter D is usually greater than 1, in particular it is between 1.5 and 2.5. The stirred tank 1 usually has a wicker, ball or dished bottom. The latter is preferred. The total volume of the stirred tank is not critical.

It is usually equipped with a single-stage stirrer 3 near the bottom. It is therefore not necessary, as is often the case with other types of stirrer in difficult cases, to equip it with a multi-stage stirrer or with several stirrers. The shape of the stirrer 3 corresponds to that of an impeller stirrer. Its diameter d is 0.5 D to 0.7 D, the leaf height is 0.2 d. The angle with respect to the horizontal is usually adapted to the shape of the bottom of the stirred tank.

The stirred tank 1 contains at least one rotatable baffle 2 ; preferably there are two. These are expediently attached in such a way that they are immersed in the upper third of the liquid in the reactor and the distance between their pivot point and the boiler wall is between 0.1 D and 0.2 D. The angle of attack alpha is preferably between 0 and 60 ° (see Fig. 1).

The cross-sectional area of the baffle 2 resembles an airfoil profile, the center line of which is preferably designed as a piece of an Archimedean spiral. For technical reasons, this is often replaced by an arc. The radius of this circle is between about 0.15 D and 0.3 D. In the simplest case, the baffle 2 can have the shape of a sheet whose cross-section corresponds to the center line described. The height of the baffle 2 is usually between 0.2 D and 0.5 D.

The stirred tank 1 according to the invention is used particularly advantageously when dispersions of two different, immiscible liquids of different densities have to be maintained.

Such a problem arises, for example, in the production of foam polystyrene. An aqueous suspension containing styrene polymers and unreacted monomeric styrene is mixed with blowing agent, for. B. pentane. DE-OS 32 02 159 shows that the product properties of foam polystyrene are significantly influenced by the stirring process. The good axial mixing is particularly noticeable here.

Another area of application relates to the polymerization of suspension PVC. Under certain circumstances, foam forms on the surface of the reaction mixture during the polymerization. In the case of reactors equipped with reflux coolers, this foam also settles in the lower part of the cooler. The result is relocation, material damage and failure of the cooler. If the stirring system just described is used in this critical phase of the polymerization, the formation of foam can in many cases be avoided entirely.

The following experiments relate to the process described in DE-OS 32 02 159 for the production of finely divided, expandable styrene polymers. Fig. 1 shows a stirred tank according to the invention. The direction of rotation of the stirrer is designated 4 .

Comparative Example A

Example 8 of DE-OS 32 02 159 shows that, using the stirred kettle shown in Fig. 1 of this document, stirring with the DAT stirring system can be carried out without dripping and good product properties can be obtained. In this case, however, you have to rely on a stirred tank with two independent stirrers. This is comparatively complex.

Comparative Example B

Example 11 of DE-OS 32 02 159 shows that using the stirred kettle shown in Fig. 1 of this document with a Pfaudler stirring system at low fill levels, products of unsatisfactory quality are obtained.

Comparative Example C

Example 1 of DE-OS 32 02 159 shows that using the stirred kettle shown in Fig. 2 of this document with a Pfaudler stirring system at a fill level of 90%, products of unsatisfactory quality are also obtained.

example 1

Instead of the stirred tank described in Example 1 of DE-OS 32 02 159 with the Pfaudler stirring system, the stirred tank according to the invention is used. It is an impeller stirrer that is completely immersed in the liquid. Two baffles with a circular wing profile are used. The following dimensions were determined: V = 40 l D = 350 mn d = 250 mm n = 120 min⁻¹ R₁ = 50 mm alpha = 20 °

Even with a filling level of 90% and correspondingly high stirring speeds, no drum is obtained during stirring. The product quality corresponds to that of Example 1 (see Table 1 on page 14 of DE-OS 32 02 159).

Claims (5)

1. A stirred vessel (1) for homogeneous mixing of liquids, having a diameter D,fitted with at least one adjustable baffle (2) and a radially delivering stirrer (3) which is fitted at a distance of at most D/2 from the bottom of the stirred vessel, characterised in that the baffle or baffles (2)

   1. is or are located in the upper half of the stirred vessel,

   2. is or are fitted at a distance R₁ from the wall of the stirred vessel, the relationship
   
   $\text{D/8 < R₁ < D/3}$

   

   
   
   applying,

   3. has or have a height h_s, with ${\text{0.2 < h}}_{\text{s}}\text{/D < 0.4}$

   

   , and

   4. has or have the form of an aerofoil profile, i.e. is or are curved in the form of an Archimedean screw or in the form of part of a circular arc.
2. A stirred vessel according to claim 1, characterised in that the stirrer is a bottom-driven impeller stirrer.
3. A stirred vessel according to either of claims 1 and 2, characterised in that the number of baffles (2) is 1 to 4, preferably 2.
4. A stirred vessel according to any of claims 1 to 3, characterised in that the flow impact cross-section of the baffle or baffles (2) is rectangular in the direction of rotation (4).
5. A process for mixing liquids, using a stirred vessel according to any of claims 1 to 4, the ratio of the filling height H_L and diameter D being greater than 1.

Images