MXPA00012884A

MXPA00012884A - Method for isolating polymers from solutions

Info

Publication number: MXPA00012884A
Application number: MXPA/A/2000/012884A
Authority: MX
Inventors: Clemens Casper; Jorgen Weinschenck; Klemens Kohlgruber; Jurgen Heuser; Christian Kords; Thomas Elsner
Original assignee: Bayer Ag
Priority date: 1998-06-23
Filing date: 2000-12-20
Publication date: 2001-09-07

Abstract

The invention relates to the evaporation of a viscous polymer solution having at least 30 wt.%, preferably at least 50 to 70 wt.%, solvents and monomers. According to the inventive method, the polymer solution to be evaporated is guided through a heated helical tube (5) as a film flow with a vapor exit velocity ranging from 200 to 300 m/s and flows into a heated separator (6) afterwards. Heating of the helical tube (5) and of the separator (6) is realized in such a way that the temperatures of the heating means in the helical tube (5) and in the separator (6) are above the softening point of the polymer. An additional important method criterion is characterized in that the two-phase mixture which is made of a polymer melt,solvent vapors, and monomer vapors and which is formed in the helical tube (5) is relieved to a pressure ranging from 10 mb to 800 mbabs in the heated vapor separator (6).

Description

PROCESS FOR THE INSULATION OF POLYMERS TO START OF SOLUTIONS Description of the Invention The invention is based on a process for the evaporation of a polymer solution, wherein the polymer solution flows through a heated helical tube and a vapor separator that is connected to it. In the area of manufacturing polymers in solution, a solution is obtained after the reaction. While maintaining high product quality, the polymer must also be isolated from this solution in a manner that minimizes the residual solvent content of the final product. This needs a solution to a variety of partial problems, which usually demand diametrically opposed measures. The pure polymer has a softening temperature that is usually well above the boiling temperature of the solvent. While the boiling temperature of the solution is close to the boiling temperature of the solvent up to high REF: 125655 polymer concentrations due to the very large difference in molecular weight between the polymer and the solvent, the viscosity of the solution during the process initially increases sharply and can reach values well above 1000 Pas, which results in low heat transfer, requires large mixing forces and can lead to lower temperature in relation to the glass transition temperature . Only in the final phase of the process, when the proportion of the polymer has a substantial influence on the vapor pressure of the solution, does the temperature of the solution rise abruptly and exceed the softening temperature of the polymer, which becomes appreciable to way of a decrease in viscosity. Towards the end of the process, the convective evaporation process changes to a diffusion process. This phase demands a prolonged residence time, a large exchange area, a thorough mixing of the viscous product and a high partial pressure gradient. Most products are very thermosensitive, meaning that from this point of view, they require low temperature and short residence times. So far there have been two basic processes to achieve the objective: 1) The solution is heated during spray drying and then sprayed to a drying tower or agglomeration tube with the help of extraction steam. The solvent is evaporated during this process to produce a powder that is substantially free of solvents but is enriched with the condensate. Normally this requires a subsequent drying. The main disadvantage of this process is the high cost in terms of energy and equipment. 2) The evaporation process uses a combination of types of evaporators connected in series. An example of a known combination is: descending film-type evaporator (see evaporation at 20% polymer weight) + film-type evaporator (main evaporation at 50% polymer weight) + double-shaft threading machine (residual evaporation at 500 μg / g of the residual solvent).

To form the combination of the multiphase helical duo + double shaft threading machine has been used frequently in recent times. The multiphase helical tube, which is described in greater detail in DE 1667051 and DE 1921045, is essentially based on rapid evaporation and downstream evaporation in the indirectly heated helical tube. A steam separator is added later on the helical tube. To avoid excessively high viscosities, which can result in an obstruction of the tube, the process is carried out under a positive pressure, which raises the temperature level and therefore decreases the viscosity to the point where the danger of obstruction is avoided . On the other hand, the residence time of the product in the flow tube is so short that no damage to the product occurs. In this way the initial solution can usually be concentrated at 70 to 90% weight in one step. The molar ratio of the solventeven at high concentrations of the polymer near the end of the tube, it is still large enough to dominate the convection and not diffuse the evaporation process at this point. The high degree of concentration frees the threading machine that is found later, which results in a decrease of the residual content of the solvent of the final product. The main disadvantages of this process are the high investment costs for the threading machine, the wear on the rotating parts, the diversity of hermetic seals, especially those of the steam domes, and their susceptibility to failure due to the rotating parts, and the handling of parts of heavy machinery. The aim of the invention is, during the concentration of the polymers, to reduce the residual solvent or monomer content to at least the limit value of less than 1% by weight permitted by law, and at the same time to replace the expensive engineering process which is applied so far with simpler methods that can be carried out with economic equipment. This object is achieved according to the invention by a process for the evaporation of a polymer solution, wherein the polymer solution, which in the initial composition contains at least 30% by weight of solvents and monomers, preferably 50 to 70% by weight , HE - & &. * • '^ £ ¿¡¡¡¡¡¡¡_ _ _. it passes to a high velog of flow through a heated helical tube and a heated vapor separator that connects to it, characterized in that the temperatures of the heating medium in the helical tube and 5 in the separator are above the temperature of softening of the polymer and in that the two-phase mixing of the molten state of the polymer and of the vapors of the solvent and the vapors of the monomer, which are formed in the helical tube, expand in the separator heated vapors at an absolute pressure in the range of 10 mbar to 800 mbar. Advantageously, the temperature of the heating instrument in the helical tube and in the separator is maintained at a value which is 10% up to 30% above of the vitrea transition temperature of the polymer. Advantageously, the process is also carried out in such a way that the polymer solution is at a pressure between 4 bar and 20 bar when entering the helical tube. An advance of the invention to achieve even lower contents of residual solvent consists in evaporating the polymer solution, even more, until ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^^^^^^ g * ^^^^^^^^ »^^^^^^^^ a residual content of -solvent of 5 μg / g up to 100 μg / g , in a chain evaporator located further from the separator. If the proportion of the polymer in the initial solution is very low (<30% weight), two or more stages of a multiphase helical tube can be connected in series. If the specific performance is very high (> 5t / h of polymer), two or more phases of a multiphase helical tube can be connected in parallel, being necessary for each stage that is equipped with its own pumping system. For further processing of the concentrated polymer, the product at the bottom of the separator, which consists of the molten state of the polymer, is conveniently removed by means of a wide-mouth gear pump or screw conveyor and then converted into a form solid granulate in a cooling device. The process according to the invention is suitable for thermoplastics such as polycarbonates, polyester-carbonates, polyalkylene terephthalates, polymers EPDM, polystyrene and copolymers and copolymer grafts which are prepared on the basis of styrene, acrylonitrile and .. - ^ * * ^ ~ * --- ^ .___ «¿___-- ^ -.; _» _ • __--. It is also suitable for other polymers, such as inorganic silicones, for example, ABS, but it is also suitable for other polymers such as inorganic silicones. The great advantage of the process according to the invention is that the investment and operation costs of the machinery are exempted in another conventional and expensive manner. Associated with this are not only the lower operating costs in investment, but also a reduction in thermal and mechanical stress on the product, which results in a higher quality in the product. If the specification is above 0.1% weight, the multiphase helical tube together with the separator is sufficient to achieve the objective. However, if the required specification is below 0.1% weight, the extraction evaporator is added later from the set of connected devices. The invention is based on the following insights: In several experimental studies it is surprisingly discovered that polymer solutions can be concentrated in the multiphase helical tube to residual solvents and monomer contents of less than 1% by weight without obstructing or damage occurs in the ..___._._________.-_____ __l____i______2_1____ _i__________a______ product. This is achieved by high temperatures of a heating medium above the softening temperature of the polymers, which prevent the polymers from sticking to the wall and possibly initiate clogging in the pipe and damage the product, due to high speeds flow in the helical tube, which can reach the speed of sound and thus ensure a high level of pressure and large wall forces of wear, and by a subsequent expansion of evaporation in the evacuated separator. The temperature of the heating medium in the separator, above the glass transition temperature, ensures a reliable discharge of the viscous product out of the system when a heated discharge pump or a screw conveyor is used as a discharge system, it is also It is necessary that the temperatures of the heating medium be above the glass transition temperature in the discharge system. Under these conditions the residual solvent content of the polymer in the multiphase helical tube can be reliably and reproducibly reduced below 1% weight. ^ &^^^^^^^^^^^^^^^^ The invention is described in greater detail below with the help of the Examples and Figures. Figure 1 shows a flow diagram for a continuous evaporation of polymer of a single stage by using a helical tube with a steam separator located further from the set of connected apparatuses and a discharge system; Figure 2 shows the experimental residual chlorobenzene content of the concentrated polycarbonate for different series of experiments and, as a comparison, the residual chlorobenzene contents established under the experimental conditions in the phase equilibrium state as a function of the pressure in the separator; and Figure 3 shows a flow chart for a three-phase continuous evaporation of a polymer using two helical tube phases with steam separators and a later extraction evaporator.

The process is illustrated here using the evaporation of a polycarbonate solution (PC solution) by way of example. As shown in Figure 1, the solution PC 1 to be concentrated is pumped by pump 2 through a heat exchanger 3, expanded in valve 4 and then flows through helical tube 5 and a vapor separator 6 located further on which consists of a commercially available centrifugal separator . Both the helical tube 5 and the steam separator 6 are provided with an outer heating jacket through which a heating medium or a heat transfer medium, for example Diphyl THT, flows. A film flow is formed in the helical tube 5 due to the composition of the vapor / liquid mixture, the pressure gradient and the high temperatures, it being possible for the vapor at the end of the tube to reach flow rates of 200 m / s until 300 m / s. The vapors which are separated in the vapor separator 6 are condensed in the condenser 7 and 15 are extracted as a distilled product 9 by means of the pump 8. The concentrated molten state of the polycarbonate 10 remains in the steam separator 6 a The liquid phase is fed through the discharge pump 11, also heated, into a separating container or optionally into further processing steps. The discharge pump may consist of ^^^ jg ^^^^ ____ ^^^ ___ ^ _____ ^^^^^ ___________ ^ ______ ^^^^^^^^^^^^^^ _ ^^^^^^^^^^^^ ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ screw conveyor. The helical tube 5 has an internal diameter of 15 mm, a length of 6 m, an average helical diameter of 284 mm and a helical separation of -5.7. The internal surface area of the helical tube is 0.29 m2. This configuration is used to carry out the experiments with specific performance of 35 to 85 kg / h of a PC solution. The temperature of the PC solution located further from the heat exchanger is 180 ° C at a pressure of 19 bar. Upon entering the helical tube 5, the PC solution expands from 3 to 8 bar by means of the valve 4. The temperature at this point is 150 ° C to 180 ° C. Upon entering the heated vapor separator 6, the two-phase mixture consisting of the molten state of PC and a solvent vapor, which is formed in the helical tube 5, expands to an absolute pressure of 10 mbar to 800 mbar. In this expanding evaporation, large amounts of vapor are released abruptly. By leaving the vapor separator 6, the molten state of PC has a concentration of 98.1 to 99.85% by weight at a temperature of 240 ° C to 260 ° C. The temperature of the heating medium (i.e., the temperature of the thermal transfer medium in the heating circuit of the helical tube 5 and the steam separator 6) is 300 ° C. The boundary conditions for the three series of experiments using this configuration are compared in the form of a Table below and the results are shown in the Graph of Figure 2. The individual series of experiments differ in the composition of the initial solution. In each series of experiments, the system pressure in separator 6 varies between 10 and 1000 mbar. The pressure behind the nozzle is 19 bar and the pressure located further from the nozzle is 3 to 5 bar. The pressure losses in the tube are given by the difference between the pressure located farther from the nozzle and the pressure of the system. First Experiment Series PC Solution: Temperatures: Heating of | 200 ° C (transfer medium Thermotransfer 3 thermal: steam) Tube Heating 300 ° C (transfer medium Helical 5 thermal: Diphyl THT) Heating Separator 6 290 ° C (transfer medium and Thermal Discharge Pump 11: Diphyl THT) Second Series of Experiments PC Solution: Temperatures: Heating of 200 ° C (transfer medium Thermotransfer 3 thermal: steam) Tube Heating 300 ° C (transfer medium Helical 5 thermal: Diphyl THT) Heating Separator 6 290 ° C (transfer medium and Thermal Discharge Pump 11: Diphyl THT) Third Series of Experiments PC Solution: Temperatures Figure 2 shows the residual contents of the solvent (chlorobenzene) in the bottom product that is discharged from the helical tube 5, these contents are quantified with the help of an analysis. The theoretically calculated equilibrium values, that is, the values below which the contents of the solvent do not ^ g ^^^ SS ^ Sfeá ^ j &agSÍ ^^ may fall under the given boundary conditions, are also plotted on this graph. If even more stringent demands are made in the absence of solvent in the polycarbonate, an extraction evaporator located further from the separator 6 or the discharge pump 11 can be added as another evaporator stage. Residual solvent contents of less than 0.1% weight can be achieved with such a combination. Figure 3 shows a flow diagram of a configuration of a multi-stage helical tube evaporator with steam separators in combination with an extraction evaporator as the last stage. The two successive stages of multiphase helical tube 12 and 13 are constructed completely and analogously to the configuration of Figure 1 and consist of the pumps 2? and 22, heat exchangers 3? and 32, valves 4? and 42, the multiphase 5 helicoids? and 52 and steam separators 6? and 62. The outlet connection of the second steam separator 62 is connected by means of the discharge pump 14 to an extraction evaporator 15 belonging to advanced technology. The extraction evaporator is a diffusion device capable of producing a large exchange area for the product. It consists substantially of an evacuated container 16 equipped with a flat nozzle 17 at the upper end and a de-filing system at the lower end. The flat nozzle 17 produces strands of the molten state of the product, which fall through the container 16 and are then removed through the discharge system 18 in the manner of the molten state of the product. While the strands are falling, the residual solvent evaporates as it diffuses out of the product. The content of the residual solvent, therefore, can be reduced by one or more orders of magnitude. Upon entering the first helical tube 5_., The PC solution is at a temperature of 200 ° C. 15 Does a pressure of 2 bar prevail in separator 6? . The concentration of polycarbonate cp is 30% by weight. In the second stage the temperature in the helical tube 52 is 300 ° C and the pressure in the separator 62 is less than 0.1 bar. The value of cp is more than 99% weight. In the third stage 20 a vacuum of 1 mbar and a temperature of 300 ° C prevail in the extraction evaporator 15. At a point located further from the discharge system 18, the value ¡^^ l ^^^^^^^^^^^^^^^^ g ^^^^^^^^^^^ g ^^^^^^^^^^^ - ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ The final concentration is more than 99.9% weight. The concentration in the first stage of the helical tube 12, in the second stage of the helical tube 13 and in the extraction evaporator stage normally gives 5 cp values of 25 to 30% weight, more than 99% weight, and more than 99.99 % weight, respectively. In case there are very low specific yields per nozzle diameter, for example below 100 g / 1, the residual solvent contents of less than 10 ppm are achieved in the extraction evaporator. The evaporation or concentration is usually followed by even another processing step to cool and solidify the molten state of the polymer. For this purpose the molten and hot state of the The polymer is withdrawn from the vapor separator 6 in the embodiment of FIG. 1, and from the extraction evaporator 15 in the embodiment of FIG. 3, by means of a gear pump with a large feed area or by means of a conveyor. of screw that is built to way of a collecting pump, and then transferred as a thin layer to a conveyor belt. cooling or to a cooling roller, where it solidifies into a film or a sheet of the product.

Another Example The same experimental configuration is used to study the concentration in a solution of cyclopropanecarboxamide (CP-amide) / isobutanol. The limiting conditions are as follows: Specific yield: 15.9 kg / h Proportion of isobutanol in the initial solution: 44% weight. CP-amide melting temperature: 120 ° C. Half heating: steam. Temperature of the heating medium in the heat exchanger 3: 150 ° C. Temperature of the heating medium in helical tube 5: 150 ° C. Temperature of the heating medium in separator 6: 150 ° C. Pressure at a point located before the nozzle 4: 11 barabs • Pressure in the separator 6: 10 barabs -Result: 0.1% weight of isobutanol in the discharge 10.

It is noted that in relation to this date, the best known method for the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers.

Claims

CLAIMS Having described the invention as above, the content of the following claims is claimed as property:

1. A process for the evaporation of a viscous polymer solution containing at least 30% by weight of solvents and monomers, preferably 50 to 70% by weight, wherein the solution of the polymer to be evaporated is passed as a film flow , with a steam output velocity of 200 to 300 m / s, through a heated helical tube and then flows through a heated separator, characterized in that the temperatures of the heating medium in the helical tube and the separator are found by above the softening temperature of the polymer, and because the two-phase mixing of the molten polymer state of the solvent vapors and monomer vapors, which are formed in the helical tube, are expanded in the steam separator heated to a pressure absolute in the range of 10 mbar up to 800 mbar.

2. The process according to claim 1, characterized in that the temperature of the heating medium in the helical tube and in the separator is maintained at a value that is 10% up to 30% above the glass transition temperature of the polymer.

3. The process according to any of claims 1 to 2, characterized in that the polymer solution is maintained at a pressure between 4 bar and 20 bar when entering the helical tube.

4. The process according to any of claims 1 to 3, characterized in that the polymer solution is further concentrated, to a residual solvent content of 5 μg / g to 500 μg / g, in an extraction evaporator located in a point further on the separator.

5. The process according to any of claims 1 to 4, characterized in that if the proportion of the polymer in the initial solution is less than 30% weight, several stages of multiphase helical tube are connected in series.

6. The process according to any of the claims 1 to 5, characterized in that if the specific performance is more than 5t / h, several stages of the multiphase helical tube are connected in parallel, each stage is equipped with its own pumping system.

7. The process according to any of claims 1 to 6, characterized in that the product of the lower part in the separator is extracted by means of a discharge pump and then converted to a granular solid form in a device. 15 refrigerant. twenty ______________ ...., .V __.___-_. "* - * • J« - * • **** • ^ - * --- ^ - afe *.