EP1127609B1 - Method for processing a product in at least a mixing kneader - Google Patents

Abstract

Continuous evaporation or heat treatment of elastomers and thermoplastics is carried out in a mixer and kneader fitted with stirring shafts (7 - 10) on which mixing and kneading components (11) are mounted. New low viscosity solution is mixed with pre-evaporated viscous material, the viscosity and concentration of the solution being adjusted so that the energy input from mechanical kneading and heat transfer is maximised. An Independent claim is included for apparatus for carrying out the process with an inlet (5) for solution connected to a mixer/kneader (1) with good back mixing connected to a second mixer/kneader (2) with good forced flow.

Description

The invention relates to a process for the continuous evaporation or thermal treatment of viscous products, in particular elastomers and thermoplastics, in a mixing kneader with at least one stirrer shaft equipped with mixing or kneading elements.

The industrial production of viscous products, in particular elastomers and plastics is often carried out using solvents, these solvents must be removed by thermal separation (evaporation) at the end of the manufacturing process. Above all, it is in the present embodiment, the continuous evaporation and degassing of an elastomer solution, wherein a heat transfer limited section and a substance transfer limited section are to be distinguished.

The corresponding separation processes are usually carried out in stirred heat exchangers and so-called mixing kneaders in which the product is transported by appropriate kneading and transport elements from an inlet to an outlet and at the same time brought into intensive contact with the heat exchange surfaces. Such mixing kneaders are described, for example, in DE-PS 23 49 106, EP 0 517 068 A1 and DE 195 36 944 A1.

From US-A-3,357,479 an improved evaporator is known in which viscous material is processed by a mixer shaft equipped with mixing or kneading elements. There, it is all about the reduction of energy consumption by cleaning the kneading surfaces of the stirring elements.

Also known from US-A-3,334,680 is an evaporator in which an inner rotor rotates with a plurality of kneading elements in an outer casing. The housing is heated or cooled by water vapor or other medium. Again, it is essentially about the cleaning of coming into contact with the kneading surfaces.

The present invention has for its object to optimize the heat transfer limited portion of the evaporation.

To achieve this object, that in a mixing kneader, the solvent by energy input consisting of mechanical kneading energy and heat transfer via the contact with the kneader heat exchange surface is evaporated to near the breaking point of the evaporation rate and in the so pre-evaporated viscous product bed continuously new low-viscosity product solution is mixed in such a way that the evaporation rate remains above the breaking point.

This is preferably done in a mixing kneader, which has a good backmixing. This is preferably a so-called LIST-DTB mixer, which is also shown in the above-mentioned DE-PS 23 49 106. In this mixer, the product is not only transported from the inlet to the outlet, but also homogeneously mixed again by backmixing with a new product throughout the apparatus.

For the product itself, it is determined beforehand at which percentage of existing liquid constituent the sum of heat transfer by contact heat and heat input by shearing heat is maximum and thus the evaporation rate is highest. Accordingly, it is then attempted to solvent-hold the product at that percentage in the compounding kneader with good back-mixing, which is done by metered continuous addition of new product, which is always thinner and contains a higher percentage of a solvent. In this way, a concentration can be set at which the required evaporation energy consisting of contact heat and shear heat is optimized.

If the product is initially very liquid at high solvent content, the evaporation takes place essentially via the heat of contact, which comes from a heated housing jacket, a heated shaft and / or heated kneading counterparts. The more viscous the product is by evaporation, the more the contact heat transfer decreases and the more shear heat is added, with the sum of both going through a maximum. From a certain reduction of the percentage of liquid components, however, the evaporation rate decreases considerably, since not enough solvent can be replenished from Gutsinneren to the surface. According to the invention, therefore, the constituent is to be kept at, for example, a solvent in a percentage range which, close to the maximum, is necessarily above this break-in point of the evaporation rate.

The inventive method also has the advantage that at the increased work intensity foaming, which results from the flash metering of the product in the mixing kneader, is broken, whereby the heat energy input is further improved. The evaporation energy required for evaporation is accordingly maximized by the combination of contact heat and shear heat. The possibility of keeping the product temperature constant through the evaporation of solvent allows a high degree of freedom with regard to the regulation of the shear heat via the speed (shear rate) and the degree of filling of the mixing kneader.

In the downstream second mixing kneader, the product is subjected to a plug flow by a corresponding geometry of the kneading elements. In this second mixing kneader, the material transfer limited evaporation takes place, which is why preferably two-shaft mixing kneaders are used, which are commercially available under the name LIST-ORP (EP 0 517 068 A1) and LIST-CRP (DE 195 36 944 A1). In this mixing kneader with plug flow, it is essential that the surface of the product be renewed as quickly as possible, since the liquid evaporates from this surface. Since the evaporation points retreat more and more into the interior of the product, the product surface must be permanently renewed by intensive kneading. Furthermore, a good product temperature control is necessary.

• Figur 1 eine Seitenansicht einer erfindungsgemässen Vorrichtung zum Behandeln eines Produktes in zumindest einem Mischkneter;
• Figur 2 eine Frontansicht eines Teils der Vorrichtung gemäss Figur 1.

Further advantages, features and details of the invention will become apparent from the following description of preferred embodiments and from the drawing; this shows in

• Figure 1 is a side view of an inventive device for treating a product in at least one mixing kneader;
• FIG. 2 shows a front view of a part of the device according to FIG. 1.

In Figure 1, two mixing kneaders 1 and 2 are interconnected. The mixing kneader 1 is a kneader with a good back-mixing, as it is, for example, on the market under the name LIST-DTB. DE-PS 23 49 106 refers to this DTB. A mixing kneader with good back-mixing is characterized in that the product is homogeneously mixed with one another throughout the mixing kneader.

In contrast, the mixing kneader 2 is a kneader with a so-called plug flow, as it is, for example, on the market under the name LIST-CRP or LIST-ORP. The corresponding industrial property rights are published in EP 0 517 068 and DE 195 36 944 A1. In a mixing kneader with good plug flow, the product is transported from the inlet to the outlet without axial mixing with the subsequent product. Therefore, there is usually an inlet 3 for the product at one end of the mixing kneader and an outlet 4, which in the present case is designed as a discharge twin screw, at the other end of the mixing kneader 2.

In contrast, the mixing kneader 1 has an inlet 5 approximately in the middle of the mixing kneader and a discharge twin screw 6 at the end of the mixing kneader 1. In both mixing kneaders are each two shafts 7 and 8 or 9 and 10 indicated, in which mixing, transport or Kneading elements 11 are arranged, which rotate with the waves, as in the above-mentioned Protected rights is described.

In a supply line 12 to the inlet 5, a pump 13, a heat exchanger 14 and a pressure-holding valve 15 is turned on. This device, referred to as a flash device, heats the product.

In the mixing kneader 1, a large part of the liquid component of the product is evaporated, wherein the mixing kneader 1 heat is supplied. For this example, a housing shell 16, the kneader shafts 7 and 8 and the kneading elements 11 are heated. The evaporated component leaves the mixing kneader 1 via a Brüdendom 17, wherein in a corresponding derivative 18, a capacitor 19 and an inert gas pump 20 are turned on.

In addition, FIG. 2 shows that the mixing kneader 1 can also have only one shaft 21.

The largely freed of the liquid component product is discharged from the mixing kneader 1 through the discharge double screw 6 and fed via a line 22 and a pressure-maintaining valve 23 to the inlet 3 of the mixing kneader 2. Also, this mixing kneader 2 has a Brüdendom 24, which merges into a derivative 25, in which a capacitor 26 and an inert gas pump 27 are turned on.

The operation of the present invention will be explained in more detail with reference to an example:

In 77% cyclohexane, for example, an elastomer, namely styrene-butadiene-styrene (SBS), dissolved. In a batch experiment, the evaporation and degassing behavior of this dissolved elastomer was investigated and the evaporation rate was shown over time. From 77% to 40% solvent concentration, the rate of evaporation is mainly dependent on the contact heat of the product with the heated shell, the heated shafts 7 and 8 and the heated kneading elements 11.

In the range of 40% to 25% of the solvent, the kneading energy increases considerably as a result of the increasing viscosity of the product (up to 1300 Pas at 80 s ^-1 shear rate), without the contact heat transfer significantly decreasing. This means that a considerable amount of shear heat is added to the contact heat. At the same time, however, the solvent concentration is still high enough so that the entire energy input for the evaporation of the solvent can be used, so that the product temperature remains constant. This maximizes the effectiveness of the kneader in this area.

Below 23% solvent, the rate of evaporation slows dramatically. This is because there is not enough solvent left on the product surface and the solvent replenishment from inside is slowed down. This slows down the Drying speed considerably. The beginning of this decrease in speed is characterized by a sharp bend in the drying curve.

According to the invention, this product behavior is utilized in the mixing kneader 1 with good back-mixing by keeping the concentration of the solvent always at about 25% cyclohexane. This is done by controlling the feed amount of fresh product and by regulating the kneading energy by means of the shaft speed and the degree of filling.

As a further advantage, it was observed that no foam formation occurs when this concentration is maintained, resulting in a better heat transfer coefficient. The procedure according to the procedure of the mixing kneader 1 is of great influence on the "scale-up" of the here carried out heat transfer controlled drying section.

The thus evaporated solution is discharged continuously by means of the discharge double screw 6 from the mixing kneader 1, simultaneously heated by shearing in the screw and flashed into the mixing kneader 2. In this mixing kneader 2, a substance transfer limited drying section is performed in which substantially no surface evaporation takes place. The evaporation points, whose entirety represents the so-called drying mirror, withdraw more and more into the interior of the interior. The liquid must diffuse.

Due to the wave geometry and the kinematics, the mixing kneader 2 is equipped with maximum surface renewal, plug flow and good product temperature control capability for this process section.

The degassed elastomer is discharged from the mixing kneader 2 at the end with the second discharge twin screw 4. The elastomer still contains a residual solvent content below 1000 ppm. Position Number List 1 mixing kneader 34 67 2 mixing kneader 35 68 3 inlet 36 69 4 Austragdoppelschnecke 37 70 5 inlet 38 71 6 Austragdoppelschnecke 39 72 7 waves 40 73 8th waves 41 74 9 waves 42 75 10 waves 43 76 11 kneading 44 77 12 supply 45 78 13 pump 46 79 14 heat exchangers 47 15 Pressure holding valve 48 16 housing jacket 49 17 vapor dome 50 18 derivation 51 19 capacitor 52 20 pump 53 21 wave 54 22 management 55 23 Pressure holding valve 56 24 vapor dome 57 25 derivation 58 26 capacitor 59 27 pump 60 28 61 29 62 30 63 31 64 32 65 33 66

Claims (8)

1. Method for the continuous thermal treatment of viscous, solvent-containing products, more especially elastomers and thermoplastic materials, in mixing kneaders (1, 2) having at least one agitator shaft (7-10, 21) provided with mixing and/or kneading members (11), wherein the solvent is evaporated in a mixing kneader (1) by the introduction of energy, comprising mechanical kneading energy, and by the transmission of heat, through the contact with the heat-exchange face of the kneader, up to just above the surge point of the evaporation speed, and new low-viscous product solution is mixed continuously into the viscous product bed, thus previously evaporated, in such a manner that the evaporation speed remains above the surge point.
2. Method according to claim 1, characterised in that a kneading energy is influenced by changing the rotational speed and/or the degree of filling of the mixing kneader (1).
3. Method according to claim 1 or 2, characterised in that the product in the mixing kneader (1) is continuously and repeatedly mixed.
4. Method according to one of claims 1 to 3, characterised in that the product is continuously removed from the mixing kneader (1) and is introduced into a second mixing kneader (2).
5. Method according to claim 4, characterised in that the product is heated-up during removal from the mixing kneader (1) before it passes into the mixing kneader (2).
6. Method according to claim 4 or 5, characterised in that the product in the second mixing kneader (2) is subjected to an air-pressure flow.
7. Method according to claim 6, characterised in that the product in the second mixing kneader (2) is subjected to a considerable surface renewal as well as to good product temperature control.
8. Method according to at least one of claims 1 to 7, characterised in that an elastomeric solution, containing a solvent, is treated as the product.

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