NL8303241A - METHOD AND APPARATUS FOR PREPARING STYRENE / ALKENYL NITRILE COPOLYMERS. - Google Patents

Description

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Process and apparatus for the preparation of styrene / alkenyl nitrile copolymers.

The present invention relates to a system for the continuous mass polymerization of SAN (styrene-alkenyl nitrile) copolymer, and more particularly to both a method and an apparatus for continuous mass polymerization of SAN,

The copolymerization of vinyl monomers, especially styrene and acrylonitrile, as used herein, is well known, as are the processes for their preparation. However, there are still many inherent problems, which are either unsolved or, in many cases, unsatisfactorily solved.

In the case of SAN polymerization, it is known that precise control is essential for the preparation of a product with acceptable properties. For example, when the acrylonitrile content of copolymer in a single product varies more than about 4%, the different copolymers become incompatible with each other. resulting in an unacceptable, hazy product. Since styrene and acrylonitrile monomers polymerize at different rates, careful control is necessary not only during the polymerization step, but also during the subsequent purification steps. maintain essential uniformity of the product, because even small amounts of copolymer product having a high acrylonitrile content can cause yellowing of the entire product due to ring closure of adjacent pendant acrylonitrile groups upon heating the copolymer, eg during subsequent heat-forming steps.

This is an important factor in the post-polymerization treatment of the product in a SAN polymerization process, e.g. the removal of residual monomer from the polymer, known as non-volatilization of the polymer. Once the polymer / monomer mixture leaves the reactor, there is a high risk of polymer having high acrylonitrile content due to the uneven polymerization rate. today for the two different monomer species and the high temperatures used for non-volatilization. ~. ï i * * - 2 - In conventional processes, the non-volatilization is typically performed with thin layer of non-volatilizing equipment, such as the so-called "Film Truder", which evaporates the liquid monomer very quickly to further polymerize as low as possible. However, this thin film equipment is relatively expensive and requires an inordinate amount of maintenance, eg, at least once a day, due to its many moving parts and its extensive seals. This maintenance therefore requires that the polymerization line is stopped or some measure is taken to keep material upstream during maintenance, resulting in uneven product quality and added equipment and operating costs.

It is therefore an object of the present invention to provide a continuous mass polymerization process for SAN copolymer as well as an apparatus for carrying out this process.

Moreover, it is an object of the present invention to provide a simple method for the continuous mass polymerization of SAN in that it can be carried out with relatively straightforward and inexpensive equipment and does not require highly skilled personnel.

Another object of the invention is to provide an improved method and apparatus for finishing the raw SAN copolymer when it comes out of the polymerization stage, to provide a high quality product while at the same time providing a method which efficient in energy consumption.

By accomplishing the above objects, according to one aspect of the present invention, a process for the continuous mass polymerization of styrene has provided an alkenyl nitrile monomers for the preparation of a styrene / alkenyl nitrile copolymer, which involves the steps of continuously feeding a feed containing a it contains predetermined ratio of styrene and alkenyl nitrile monomers in a reaction vessel to form a reaction mixture; subjecting the reaction mixture to temperature and pressure conditions, under which these monomers copolymerize to form a styrene / alkenyl nitrile copolymer; subjecting the reaction mixture to sufficient stirring to maintain a substantially uniform distribution of composition and a substantially uniform temperature distribution throughout the reaction mixture; continuously withdrawing styrene / alkenyl nitrile copolymer from the reaction vessel? and remove volatiles from the styrene / alkenyl nitrile copolymer withdrawn from the reaction vessel, which preferably comprises the steps of restoring the styrene to alkenyl nitrile monomer ratio in the withdrawn copolymer solution to about the same ratio as the previously certain styrene-alkenyl nitrile ratio of the feed to the reactor (eg, by withdrawing an amount of alkenyl nitrile monomer)? and in a second step, heating the withdrawn styrene / alkenyl nitrile copolymer to a temperature sufficient to evaporate substantially all of the volatile components therein and separate the volatilized components from the non-volatilized copolymer. Most preferably, the heating step in the second stage passing the withdrawn styrene / alkenyl nitrile copolymer through a heat exchanger in an amount sufficient to cause significant volatilization of the volatiles in the copolymer sufficient to cause a foam type two phase flow through the heat exchanger .

In a preferred embodiment, the non-volatilization step further comprises the steps of withdrawing the volatilized components from the second stage separation vessel, condensing the withdrawn volatilized components to form a liquid condensate of recovered monomers, and recirculating the recovered monomer liquid to the reaction vessel as part of the monomer feed. The method is further characterized by the step of maintaining an equilibrium concentration of water in the reaction vessel.

According to another aspect of the present invention, there is provided an apparatus for the continuous mass polymerization of styrene and alkenyl nitrile monomers to form a styrene / alkenyl nitrile copolymer equipped with a reaction vessel; means for continuously feeding in a supply! containing a predetermined ratio of styrene and alkenyl-40-trile monomers in said reaction vessel to provide a reaction ♦ * - 4 - form mixture; means for subjecting the reaction mixture containing styrene and alkenyl nitrile monomers to temperature and pressure conditions, including copolymerizing the monomers to form a styrene / alkenyl nitrile copolymer; stirring means for reacting the reaction mixture sufficiently to maintain a substantially uniform distribution of composition and a substantially uniform temperature distribution throughout the reaction mixture; means for continuously withdrawing styrene / alkenyl nitrile copolymer from the reaction vessel; and a volatiles removal device for removing volatiles from styrene / alkenyl nitrile copolymer withdrawn from the reaction vessel. Volatile components preferably comprise a first stage, wherein means for restoring the ratio of styrene to alkenyl nitrile monomers in the withdrawn copolymer solution to substantially the same ratio as the predetermined styrene: alkenyl nitrile ratio from the feed to the polymerization reactor, at preferred means for extracting an amount of alkenyl nitrile monomer, and a second stage including means for heating the withdrawn styrene / alkenyl nitrile copolymer to a temperature sufficient to volatilize all volatiles therein and means to volatilize the volatilized Separate 25 constituents from the non-volatilized copolymer. Most preferably, the second stage heaters include a heat exchanger, means for passing the withdrawn styrene / alkenyl nitrile copolymer through said heat exchanger, and means to reduce the pressure in said heat exchanger to an extent sufficient to substantially complete volatilization of the volatile constituents in the copolymer sufficient to cause a foam type two phase flow through said heat exchanger.

Further objects, features and advantages of the present invention will become apparent from the detailed description of preferred embodiments which follow, when considered together with the accompanying figures of the drawings.

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Fig. 1 shows a schematic view of a suitable device for carrying out the method according to the present invention;

Fig. 2 is a more detailed schematic view of another device for carrying out the controlled temperature polymerization according to the invention; and

Fig. 3 shows a schematic view explaining a preferred device for condensing and recirculating recovered monomers according to the invention.

The present invention is directed to an improved continuous mass polymerization process for the preparation of copolymers using a starting monomer composition containing at least one monoalkenyl aromatic (styrene) compound and at least one olefin nitrile compound.

The alkenyl nitrile compounds are characterized by the general formula 1, wherein R represents hydrogen or an alkyl group of 1-4 carbon atoms.

The monoalkenyl aromatic (styrene) compounds are characterized by the general formula 2, wherein Ar represents a phenyl group, an alkaryl group with 6-9 carbon atoms, a monoehloarphenyl group, a dichlorophenyl group, a monobromophenyl group or a dibromophenyl group, and X represents hydrogen or an alkyl group with less than three carbon atoms. The preferred alkenyl nitrile is acrylonitrile and the preferred styrene compound is styrene itself.

The invention provides a process and apparatus which enable the efficient preparation of a wide range of SAN copolymer products under a wide variety of operating conditions. For example, according to the invention, SAN copolymer can be prepared with a proportion of copolymerized acrylonitrile in progress of only about 10 wt.% to a maximum of about 60 wt.%. Also a wide range of molecular weights is possible, e.g., ranging from polymers with a melt flow index (ASTM 1238, condition I) of only about 1 g / 10 min. to 80 g / 10 min.

This wide range of values can be obtained without the use of internal lubricants in the polymer compositions. The polymers are characterized by a Vicat softening point (ASTM 1525 speed A) that is in the range 40 from 104-121 ° C.

SAN copolymers of excellent purity and quality can be prepared according to the invention. These compositions have a residual acrylonitrile monomer content of less than about 100 ppm, a residual styrene mon * 5 content of less than about 1000 ppm and a residual diluent content of less than 1000 ppm,

However, the process of the invention is also capable of achieving very high reaction rates and rates of conversion. It is possible to achieve a rate of reaction as high as 40% / 10 hours for the SAN copolymers and rates of conversion between 30 and 90%, typically up to about 60 %, A, Reactor

With reference to Fig. 1 of the drawings, the system uses a polymerization reactor vessel 1, the confi. 15 guration is not necessarily new. The configuration can vary widely, for example from a configuration which is generally horizontal to one which is substantially vertical. The preferred design includes a reactor of the vertical type, preferably a vertical cylinder, as found there. that a vertical reactor allows the preparation of a wider range of SAN products. In the past, horizontal type reactors for SAN have been preferred due to the difficulty of uniform temperature and concentration control from top to bottom of a vertical reactor 25, See eg, U.S. Patent No. 3,813,369, Many types and designs of reactors are known for the polymerization of SAN in the art, It is possible to use any of these known reactors within the scope of the present invention. can be jacketed to promote heat regulation, especially at start-up heat exchange medium, hot oil is used and it is circulated by the external circuit 6. However, in the present invention, it usually does not play an important role in temperature control during use under constant conditions. After start-up, the temperature of the jacket becomes characteristic of the desired reaction. temperature in order to minimize temperature gradients, 40 In use, a mixture of styrene and acrylic is used · ·· "* ·>» 4

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* 4 - 7 - nitrile monomers fed into the reactor through line 7, with original feed from a supply source 4 and passing through a preheater 8. The ratio of, for example, styrene and acrylonitrile in the reaction mixture is chosen in accordance with the desired monomer composition. of the copolymer, Compositions typically contain about 60-70 wt% styrene and about 30-40 wt% acrylonitrile. The reaction mixture typically also contains an inert diluent, for example, from about 2 to 50 wt% and character-10 about 15 to 50% by weight of the total reaction mixture? preference is given to about 25%. The percentage may be as low as 2%; clogging below this level may become a problem due to the lack of diluent. The diluent is preferably selected from the following compounds: ethylbenzene, butylbenzene, benzene, toluene, xylene (all isomers) and cumene ?; ethylbenzene is preferred. The reactor is usually used at a temperature between about 115 and 180 ° C and 2 at a pressure of between about 2.79 and 5.58 kg / cm.

Referring now to Fig. 2, a preferred reactor design for the present invention is explained. The reactor 1 contains a combination of measures to assist in obtaining a uniform temperature and composition therein. The reactor contains a rotary mixer 3 25 The mixer has at least one blade or paddle 4, which rotates in a horizontal plane. The mixer can be of any conventional design, ensuring lateral uniformity in the reaction mixture. The mixer is driven by a motor 2, which has a rotating shaft 5 which runs through the top of the reactor and down into the reaction mixture. The mixer is typically used at speeds between about 20 and 60 rpm.

Lateral uniformity of the reaction mixture is obtained by the rotary mixer 3, Vertical uniformity of the reaction mixture is achieved according to the invention by extracting the reaction mixture from the bottom of the reactor and the mixture via a pump 10 through an external recirculation line 9 pump in an external loop and then back into the top of the reactor 1. The reaction mixture, as it passes through the loop, is preferably homogenized in a mixer 11, preferably a static mixer of the type referred to as a "boundary developer". A "boundary developer" is an inline motionless mixer, the mixing mechanism of which is generally unrelated to the flowthrough if the flowthrough flows in the region of streamline flow. Such mixers can be considered as low-forming mixers, in which the flowing flow is distributed and two constituent parts are deformed and bonded together n united in such a way that the interface between the original elements of the stream is greatly increased. Such mixers are well known in the art, and some of them and their limitations are described in the following United States Patents: Nos. 3,051,542; 3,051,453; 3,195,865; 3,206,170; 3,239,197; 3,286,992; 3,328,003; 3,358,749; 3,373,534; 3,394,924; 3,404,869; 3,406,947; and 3,506,244. This in-line static mixer can be of any standard design. Suitable mixers are sold by various suppliers under the trade names Kenics, Koch and Lightning.

Fresh monomer is introduced into the recirculation loop through line 13, upstream of the static mixer 11, as necessary to maintain a constant monomer ratio in the reactor. As will be discussed further below, condensed monomer vapor is also returned to the polymerization vessel by introduce upstream of the static mixer 11.

The use of the external pump plus 9 and static mixer 11 in combination with the rotary mixer 3 provides surprising uniformity of temperature and composition 30 throughout the reaction mixture, even in the case of a vertical reactor. reactor can be controlled to within ± 1 ° C if the reaction mixture is circulated through the external loop only 1.2 times per hour. Similarly, the monomer composition of the reaction mixture is kept uniform to within 1%. 15,000 ton factory per year in which the present design is used requires only about 200 hp for the rotary mixer 3 and a pump of about 60 hp for the external loop, 40 B. Temperature control by evaporation - 9 -

Under constant operating conditions, almost all cooling for reactor 1 is provided by an external monomer cooler 15. Hot monomer vapors are withdrawn from reactor 1 through line 17 and condensed in cooler 15, Cow 5 may have either horizontal or vertical construction, and in Fig. 1 shows a vertical construction. In the cooler there are a number of pipes that are cooled by a cooling fluid, such as water. The monomer vapor can be passed on the house side or on the jacket side; in a vertical construction 10, however, it is preferably passed through on the housing side. On contact with the cold tubes, the hot vapor from the reactor is condensed into liquid monomer, which is collected in cooler 15. Also, an additional amount of the liquid is preferably collected in collector 19. The cooled liquid in collector 19, which is is essentially monomer, it is passed through line 20 to a three-way valve 21, from which a portion of the condensed monomer is returned via line 22 to monomer feed line 13, upstream of static mixer 11, after which the condensed monomer is returned to polymerization reactor 1 lined.

Control of this cooling system by evaporation is obtained as follows: a temperature controller 25 reads directly the temperature in reactor 1 and operates in cascade 25 with level controller 24 connected to cooler 15. The amount of cooling provided to the reactor in a function of the level of condensed monomer in cooler 15, ie, the lower the level of the fluid, the larger cooling surface comes into contact with the monomer vapors. Therefore, if more cooling capacity is required for the reactor, the liquid level in cooler 15 becomes and vice versa. Adjustment of the liquid level in cooler 15 can be accomplished in a number of ways, for example, by providing a feedback signal to three-way valve arrangement 21, 35. The evaporative cooling system of the invention in fact provides a dual mechanism to obtain rapid response to temperature changes in the polymerization reactor. As noted above, the cooling power of cooler 15 immediately increases as a result of a temperature rise in the reactor. This is accomplished by lowering the level of condensed monomer in the cooler and as a result the volume of concentrated monomer removed from the cooler is injected back into the reactor, providing a secondary and instantaneous cooling effect. In addition, the system of the invention provides by maintaining a constant stock of condensed monomer in collector 19, also the ability to immediately inject the entire amount of condensed monomer collected therein into the reactor in case a substantial immediate cooling capacity is required.

Prior art methods using an external cooler for temperature control in mass polymerization processes, such as mass SAN processes, typically suffer from the serious problems of plugging and vapor blockage in line 17 between reactor and cooler for the reason that the cooling capacity is typically controlled by a control valve in line 17 to control the vapor flow in the cooler, ie this valve was used as the main temperature 20 controller. Line 17 tends to clog at this valve within a short time, eg after only one or two days of operation. Clogging at the valve results in the loss of cooling power for the polymerization reactor. It was found that the clogging at the valve is due to the entropy change there due to the slight pressure drop Another of the problems which plagued prior art systems using cooling using an external vapor cooler was vapor barrier in vapor conduit 17, which was found to be caused by the accumulation of non-condensable components in the system, e.g. sky.

These problems are solved, in accordance with the present invention, by omitting the valve in conduit 17. This was particularly the case for the valve clogging problem, since the removal of the valve eliminated the source of the problem. On the problem of vapor barrier, the removal of the valve eliminated a source of pressure change and therefore vapor barrier.

Also, the inlet to cooler 15 must be located above the reactor 40 to exclude vapor traps.

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As mentioned, part of the cause of vapor barrier in such a system was found to be the presence of oxygen and other non-condensable components. To shut down non-condensable components, the cooler has a pressure control valve 31 on the top of it. regulator uses a trap 3Q for non-condensable components and controls the pressure just above the vapor pressure of the condensed liquid, so if something non-condensable builds up in trap 30, the pressure will increase slightly and automatically released by pressure control valve 31.

The operation of a preferred means of excluding non-condensable components is as follows: pressure control valve 31 is fitted at the top of cooler 15 above a trap 30 for non-condensable components, so that any accumulation of non-condensable components occurs in drop 30. When the pressure control valve is set to a vent pressure slightly above the vapor pressure of the condensed liquid, the non-condensable components 20 are vented automatically.

The non-condensable components can also be vented in another embodiment, since these stationary non-condensable components have a lower temperature than the hot vapor, the interface between the 25 non-condensable components and the hot vapor can be sensed by a temperature transducer. this interface descends beyond a first pair of temperature transducers, indicating an accumulation of non-condensable components, the temperature change is observed and the non-condensable components are vented. This embodiment can be incorporated in the first embodiment and in such a combined system the pair of temperature transmitters can act as a pressure regulator domination,

As a further means of protecting against condensation, the vapor line 17 is insulated to ensure that no cooling occurs in the line,

Another problem found in evaporative cooling systems known in the art is clogging the cooler. While virtually no vapor phase polymerization occurs, polymerization of hot styrene-12 and acrylonitrile monomer is easy, which is where it passes. because the superheated vapor condenses into very hot liquid on the pipes, this liquid can polymerize easily, The design according to the present invention has eliminated this problem and at the same time allows even more precise temperature control than according to the prior art was possible, This is achieved by recirculating part of the mixture of cooled monomer and diluent to the cooler from collector 19, The condensed monomer mixture runs from 19 - through line 20 through three way valve 21, This valve splits the monomer stream in a stream 22 which recirculates to reactor 1, and ee n second stream 23 running in cooler 15. This recirculation through line 23 solves the problem of cooler polymerization. The condensate is injected into cooler 15 at the entry point of the superheated vapor stream from the reactor, It is injected in the form of a spray curtain, through which the vapor passes. acts as a rapid cooler for the hot vapors and reduces the heat of the liquid monomers on the cold stores, reducing polymerization. In addition, the cooling spray is directed to continuously wash the cold stores, thereby condensing the liquid monomer on the tubes. cooling is continued and the residence time of condensed monomer on the cold stores is reduced.

By using the rapidly cooling spray, polymerization in the cooler is completely turned off. In a vertical heat exchanger, the entering hot vapor stream passes through the cooling spray. The amount of cooled monomer passing through line 23 is controlled to be greater than the amount of monomer condensing from the vapor on the cold stores. The cooled monomer recycled in the cooler thus forms a film cool monomer, which continuously runs off the tubes. The cool monomer is favorably introduced with 180 ° coverage. In a horizontal cooler the recycled condensate is conveniently fed through a pipe that runs the entire length of the top of the cooler. This pipe has a number of holes on nozzles along its length to allow washing the cooler over the entire length of the cold stores in 40 .

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Fig. 2 shows another embodiment of the reactor and the temperature control system in which a horizontal cooler is used. Reactor 1 comprises an external recirculation loop 9 including a static mixer 11. Cooler 5 is horizontally oriented and contains a number of pipes 14 through which a cooling medium flows, for example cooling water, which enters through line 16 and discharges through line Μδ, Μοηο-containing vapors from reactor 1 through line 17 enter cooler 15 and condenses upon contact with the outer surfaces of Tubes 14. The surface area of Tubes 14 available for contact with the monomer-containing vapors is controlled by the level of condensed liquid held in cooler 15. This liquid level is controlled by level regulator 24 in response to temperature in reactor 1 as followed with temperature controller 25. Control is effected by adjusting three-way valve 21, which reacts the quantity to or 1 condensed liquid returned through line 22 and the amount recycled through line 23 to cooler 15 re-goat.

Liquid recycled to cooler 15 is fed into a pipe 27, which extends near the top of cooler 15 and along its entire length. Pipe 27 contains a number of exhaust vents through which the condensed liquid is sprayed over all pipes 14 to prevent accumulation of polymer thereon. In addition, the monomer-containing vapors are forced to pass through a curtain of curtains when entering cooler 15 through line 17, thereby suddenly cooling the vapors.

Decondensed monomer and diluent are preferably fed via line 64 from cooler 15 to a collection tank 19, in which a significant volume of condensed cooled monomer is kept in stock. Typically, the stored voltime condensate is between about 5 and 30% by volume based on the volume of the reaction mixture in reactor 1, although condensate need not be stored This characteristic is only to increase the response and adaptability of the system, as described above. The liquid in collector 19 is passed through line 40 20 to three-way valve 21 pumped by pump 29 for the return of condensate, Any water that collects in the system t is collected in trap 32 and extracted to keep the water at its equilibrium concentration in the system, The pressure between cooler 15 and collector 19 is equated by means of line 62.

A trap 30 for non-condensables and pressure control valve 31 are provided to continuously rid the system of non-condensables, especially oxygen. The other system described above can also be used here.

The mass polymerization system as described so far provides another very important advantage over the prior art. State of the art systems for the preparation of SAN required the reactor to be made of stainless steel. A reactor of less expensive carbon steel would corrode quickly, causing black speckles in the polymer. In addition, carbon steel was believed to pop SAN polymer, which would clog the transfer lines and equipment. This problem has been solved by the design of the invention, which also allows for very significant cost savings. In the system of the invention, as mentioned above, the non-condensable components are continuously the reactor is flushed. This precludes corrosion due to the presence of oxygen in the reactor. In addition, it is also a feature of the invention to keep the amount of water in the system at or below its equilibrium concentration, ie about 5-6%. This prevents any liquid water in the reaction mixture from coming into contact with the r reactor 30 'walls by keeping the water in solution. Excess water is removed in the cooler system via water extractor 32, which is of conventional construction and located in a trap at the bottom of collector 19. Water separates from the relatively cool monomer mixture in collector 35 19 because the solubility of water in acrylonitrile decreases when acrylonitrile is cooled. Thus, by removing water from the monomer mixture at this point, the water concentration in the reactor is maintained at a low enough level to ensure that it is completely dissolved. -40% removal of water and non-condensable constituents - 15 -% offers significant cost advantages over the prior art systems by enabling the use of carbon steel reactors, The problem prior art swollen polymer is also eliminated.

The system of the present invention allows the preparation of SAN copolymers using a reaction temperature from 115 ° to about 180 ° C, preferably 130 ° to 155 ° C, while the system also operates within a pressure range from 2.79 to 5.58 kg / cm 2, C. Polymer work-up

Part of the reaction mixture from reactor 1 is continuously withdrawn through line 33 for further processing. For a SAN polymerization process it is necessary to remove the remaining volatiles (styrene, acrylonitrile and ethylbenzene) and it is preferable to recirculate them to the reactor. The volatilized polymer is then granulated or other Processed for commercial purposes after exiting the volatiles expulsion stage through line 34. *

As noted above, the unpleasant yellow color in the SAN polymer is formed when acrylonitrile groups polymerize in series and then cyclize upon exposure of the polymer to higher temperatures. Under polymerization conditions in the reactor, styrene polymerizes faster. Therefore, there is an excess of acrylonitrile monomer in the monomer-polymer mixture withdrawn from the reactor for further processing. This increases the probability of nearby polymerization of acrylonitrile in the copolymer chains, thereby altering the monomer ratio in the final product. yellowing of the polymer product, As also noted above, the presence of SAN polymers with different monomer ratios causes cloudiness of the product. This problem is addressed by the prior art by removing the volatiles from the polymer so quickly possible and at the lowest possible temperature. Most production plants use a complicated and very expensive thin 40 film evaporator sold under the trademark FILM; \ «· - · #; - 16 - TRUDER. The FILM TRUDER heats and strips a thin polymer film of volatiles very quickly, in less than 1 or 2 minutes. The disadvantage of the FILM TRUDER is that it. has a complex system of mechanical seals and rotating equipment that require daily maintenance and associated equipment shutdown or bypass. The system provided in accordance with the present invention eliminates the disadvantages of the prior art processes for an equipment cost of approximately one fourth of that for the 10 FILM TRUDER.

The reactor 1 operates at above atmospheric pressure. A polymer solution is withdrawn from the reactor through line 33 at a temperature from about 115 ° C to about 180 ° C, preferably about 140 ° C. The polymer mixture is placed in a first volatilization unit. lined without preheating. Volatilization unit 35 operates at a pressure above atmospheric pressure, but at a lower pressure than reactor 1. The pressure drop from line 33 to volatilization unit 35 causes part of the acrylonitrile and styrene 20 monomers to be evaporated in the hot polymer solution, where acrylonitrile has a higher vapor pressure than styrene, the amount of acrylonitrile removed can be controlled by controlling the pressure of volatilization unit 35. volatilization unit 35 is controlled such that acrylonitrile is evaporated in sufficient amounts to provide a monomer ratio in the solution, which upon polymerization will provide a SAN polymer having substantially the same ratio of styrene to acrylonitrile monomer units as the SAN copolymer leaving the reactor, Norma-30 liters, this would involve restoring the monomer ratio to the same ratio as originally fed into the polymerization reactor. With the monomer balance thus restored, small amounts of additional polymerization in the subsequent processing steps will reduce the copolymer. 35 Do not disturb ratio (or homogeneity) in the final product. Volatilization unit 35 can be of any construction suitable for controllable evaporation of acrylonitrile. In one embodiment, the volatilization unit is a simple evaporation pot or one-stage distillation unit. The volatilization unit 35 instead da arvan contain a barrel with a

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- 17 - nozzle to inject the hot polymer solution into. its interior, with provision at the top for vapor removal and provision at the bottom for liquid removal. The temperature in volatilization unit 5 is preferably about 115 and 190 ° C and the pressure 2 between about 1.74- 2.09 kg / cm.

The acrylonitrile-rich overflow product is removed from volatilization unit 35 through conduit 37 for recirculation into the reactor through a cooler unit, which will be described in more detail below. The polymer / monomer solution is removed from the first volatilization unit 35 through conduit 39 and then transferred into preheater 41. Preheater 41 is a heat exchanger capable of heating the polymer solution from a temperature of about 70 ° C (to where the temperature may drop in the first volatilization unit 35) to a temperature of about 190 ° to 260 ° C. The heated polymer mixture then moves from preheater 41 through line 43 to a second volatilization unit 45, which preferably operates within this latter temperature range, even though the solution now has a monomer composition similar to that fed to the reactor, the polymerization reaction should not be allowed to complete because a SAN copolymer having a molecular weight lower than that of the product removed from the reactor would be formed, which would cause a deterioration in the quality of the product. It is therefore essential that the residence time in preheater 41 be as short as possible. there is a low heat history in the preheater and also that the volatiles are removed as soon as possible in the subsequent volatilization unit 45. For this purpose, line 43 has been made with a relatively large diameter sufficient to withstand the absolute pressure of volatilization unit 45 , which is typically between about 0.014 and 0.021 kg / cm, about the same in preheater 41. In addition, the tubes 42 themselves in the heat exchanger of the preheater are designed sufficiently large that the polymer / monomer solution therein evacuates the vacuum of the second evaporator 45 is undergoing.

Using preheater 41 under partial vacuum provides surprising and favorable results. In the first - 18th of tubes 42 in preheater 41, the monomer in the polymer solution is heated to boiling temperature. The boiling temperature is very low because of the low pressure in the heat exchanger. The pressure in the heat exchanger allows boiling at temperatures of only about 150 ° C. The polymer / monomer solution now becomes a two-phase fluid, advantageously a foam, and in the remainder from the heat exchanger quickly to the temperature required for proper extrusion of volatiles, approximately 230 ° C. It has surprisingly been found that the two phase liquid has a heat transfer coefficient of about 125 BTU / hr ° C m 2, which is generally about three times higher than that of the liquid polymer solution, because turbulent flow of the two phase fluid that allows it to absorb hit-15 too much faster than a laminar flow of a viscous polymer solution, it is possible to heat the two-phase fluid to the proper volatilization temperature in about one-third of the time it takes to get a No. ». : phase polymer / monomer liquid to be heated. It is important that the preheater is designed with a relatively large number of tubes with a diameter large enough so that the vacuum can be drawn back through the tubes and so that the two -phase, preferably foamed fluid is formed. Preferably, the length and diameter of tubes 42 are chosen to allow this two-phase flow,

In addition to reducing the residence time of the polymer in the heat exchanger or preheater 41, early boiling of the volatiles to form the two-phase fluid has the additional benefit that the acrylonitrile is now in the vapor phase. Acrylonitrile monomer does not readily vaporize. This further reduces the possibility of the formation of nonuniform molecular weight SAN (and thus a discolored copolymer).

A third advantage is realized when the two phase fluid is introduced into volatilization unit 45. In the prior art, if a pressurized liquid polymer solution of the appropriate temperature was introduced into 40, the volatilization unit would remove ~ -: Λ * ..1 - ..v .1 -} 3 - 19 - volatile components, boiling the monomer at the injection nozzle. This boiling would lower the polymer temperature by as much as 25 ° C as the volatile components absorb their latent heat of evaporation This phenomenon would lead to a colder polymer, which is therefore more difficult to pump and therefore requires more energy. In addition, the temperature of the melt at the die fluctuates by as much as 25 ° C, causing problems with granulation, eg increasing and incorrect temperature control 10 leads to granules with poor properties,

In contrast to the prior art, in the present system, the second volatilization unit acts essentially as a phase separation chamber. A polymer of constant temperature is formed, and little cooling occurs when the polymer falls through the second volatilization unit. Volatilized polymer is withdrawn through line 34 and has a purity of about 99.80%. The remaining 0.20% consists mainly of volatiles. If a particularly high purity SAN copolymer is desired, another volatilization unit of the same type as volatilization unit 45 are positioned downstream of volatilization unit 45. The overflow product of diluent, acrylonitrile monomer and styrene monomer is withdrawn through line 49 for further processing and / or injection into reactor 1.

As indicated above, a vacuum is drawn in the second volatilization unit 45 and in part also in the first stage volatilization unit 35 to extract diluent and unpolymerized monomer, which are then condensed and preferably recycled to the polymerization reactor. conventional vacancy source, indicated by reference 51, can be used in the system of the present invention. For example, a conventional steam jet vacuum source can be used, however, it is also possible to use a relatively simple low pressure system. using rotary blowers as the source of negative pressure. The system of the invention does not require refrigeration, providing costly equipment and considerable

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- 20 - operating cost factor are disabled. It is also. possibly use a liquid-sealed vacuum pump; however, it produces a large amount of contaminated water, an environmental problem, which is also avoided by the preferred system according to the invention. The volatiles from the devolatilization system are drawn through the vacuum of vacuum source 51 into recirculation cooler 53, which is cooled with cooling water, condensed monomers and diluent pass from recirculation cooler 53 to recycle circulation drum 55, from where they are pumped back through recycle pump 57 through line 59 to reactor 1. These recycled components are preferably introduced upstream of preheater 8.

In Fig. 3, a preferred recirculation cooler system according to the invention is illustrated. Recirculation cooler 53 is supplied with cooling water through line 61 and the cooling water leaves the cooler through line 63. The cooler 53 receives the volatilized components (diluent and monomers) from the volatilization system, and after the volatile components have condensed, they are transferred in liquid form to recycle chasing drum 55 through line 65. Chasing drum recirculation is provided with a trap 56 to remove any water present in the system.

Cooler 53 is connected to the vacuum supply through line 67, and the pressure between the cooler and chasing drum 55 is equalized by providing interconnection through line 69. The vacuum in the recirculation cooler system is limited by. hydrocarbon vapor pressure in chasing drum 55, and it is therefore desirable to keep the temperature as low as possible. Typically, the temperature is kept between about 27 and 38 ° C.

Liquid monomer and diluent are withdrawn from chasing drum 55 and pumped back through recirculation pump 57 through line 59 to the polymerization reactor, as described above. Provisions are also made for selective recirculation of a portion of the condensate through line 58 back to chasing drum 55 and / or a part thereof through line 60 back to the cooler 53. In the latter case, the recycled liquid is advantageously injected through a nozzle in the incoming flow of volatiles. λ ^ ^ λ 7 | ": - 21 -

Cooler 53 is designed to provide a pressure drop thereon of less than about 2 mm Hg, and its dimensions are such that there is less than about 2 ° C temperature difference between the cooling water and the leaving hydrocarbon. cooling water has a temperature between about 16 and 29 ° C and the leaving water is between about 20 and 32 ° C. The pressure in the cooler system is typically between 0.014 and 0.021 kg / cm.

The following example serves to further illustrate the invention; it will be understood that the scope of the invention is not limited by this example.

Example I

In a vertical stirred tank reactor with a volume of 25,200 l, a mixture of styrene and acrylonitrile 15 is copolymerized. A continuous stream of reaction mixture is withdrawn from the bottom of the reactor, passed through an external loop, which includes a static mixing area, and is fed again in the top of the reactor at a speed sufficient to pass the reaction mixture about 1.2 times per hour. The mixture is conveyed by a positive mammoth pump operating at a constant speed. A continuous flow of liquid monomer composed of about 70 wt% styrene and 30 wt% acrylonitrile is introduced into the circulating reaction mixture stream just before the point where it enters the static mixing area at a rate of about 18,000 kg / hr. An additional amount of recycled monomers composed of about 28 wt% styrene, 34 wt% acrylonitrile and 38 wt% diluent are continuously fed at a rate of about 1,980 kg / h in the external recirculation loop at the same point as the fresh feed stream. The combination of the recycle and feed monomers is thoroughly mixed with the reaction mixture via the static mixing area. This mixture is then recycled into the reactor .

A thoroughly homogeneous composition is maintained in the reaction area by a combination of the external recirculation loop and stirring with a stirrer suspended from the top of the reactor. The stirrer is equipped with both anchor and turbine-type blades rotating with a speed - 22 - of 30 rpm. This combined stirring is sufficient to ensure a maximum temperature gradient of ± 1 ° C from the top of the reaction mixture to the bottom.

The polymerization reactor is jacketed u. 5 and heat transfer oil is circulated through the jacket.

The temperature of the oil is maintained by an automatic control at a constant temperature of about 144 ° C.

When equilibrium is reached, the liquid contents of the reactor are kept at a constant temperature 2 of about 144 ° C and a pressure of about 27.9 kg / cm. A constant volume of liquid is maintained in the reactor by continuously part of the Withdraw partially polymerized reaction mixture from the bottom at a rate equal to that of the combined feed and recycle streams entering the reactor. This stream is composed of approximately 48 wt% styrene / acrylonitrile copolymer, 14 .5 wt.% Styrene monomer, 17.5 wt.% Acrylonitrile lemonomer and 20 wt.% Diluent.

The copolymer contains about 29% by weight of acrylonitrile and 71% by weight of styrene and is of homogeneous composition. There is no color and the polymer is substantially clear.

The heat of polymerization is removed by boiling monomers from the reaction zone at a set rate. A jacket and tube bundle type vertical heat exchanger located adjacent to the reactor is used to condense the hot vapors, the vapors being condensed on the inside of the tubes and circulating cooling water through the jacket.

30 The liquid level in the cooler tubes is maintained, while the monomers are immediately returned to the reactor at the same entry point as the feed and recycle flows. The cooling rate is controlled by the level of liquid monomer in the tubes of the cow at to adjust the surface area available for condensation. A side stream of cooled monomers is sprayed into the top of the cooler above the point where the vapors enter the reactor. The vapor stream consists of approximately 8 wt% styrene, 78 wt .% acryloni-40 trile and 14% by weight diluent enters the cooler at ~ 23 - about 144 ° C. A volume of condensate from the cooler is stored in a collection tank placed between the cooler and the condensate recirculation pump This volume comes to about 2,100 1.

5 The copolymer / monomer mixture leaving the reactor is transported to the volatilization section by a variable speed gear pump. This pump maintains a constant level in the reaction zone. This mixture is passed through a jacketed pipe in which heat transfer oil is circulated, which is used at the same temperature as that in the reactor jacket and enters the first of the two volatilization vessels. The liquid mixture is at a temperature of about 144 ° C. The first vessel is operated at a pressure of about 2.0 kg / cm 2. held by an automatic pressure control. Vapors composed of about 19 wt% styrene, 51 wt% acrylonitrile and 30 wt% diluent are boiled from the copolymer mixture as it enters the vessel at a rate of about 990 kg / hr - The resulting polymer mixture, which is composed of about 13 wt% styrene, 5.5 wt% acrylonitrile, 16.5 wt% diluent and about 65 wt% styrene / acrylonitrile copolymer sinks to the bottom of the vessel at about 77 ° C and is conveyed out by a pump. The ratio of styrene and acrylonitrile monomers is about the same as in the feed ratio to the polymerization reactor.

From the bottom, the copolymer solution enters the housing side of a vertical jacket and tube-type heat exchanger, in which it is heated to a temperature of about 230 ° C by heating jacket-circulating oil. The design of this preheater is such that the monomer evaporates in the tubes and a foaming monomer / polymer mixture is formed. The flow characteristics of this foamed mixture is such that it allows much better heat transfer than with a conventional exchanger of this type allowing for a much smaller heating area. The tubes are large enough to allow the vacuum of the second stage of the volatilization unit to flow therein and allow foaming of the monomer / polymer mixture therein. The length - 24 - of each tube is 360 cm and the diameter is 3.18 - cm. - -

The two-phase mixture leaves the preheater at about 233 ° C and passes the second volatilization vessel inside a dividing head. where it is separated into small strands. The vessel is used at a pressure of about 0.02 kg / cm 2 and at 230 ° C. A mixture of about 37 wt% styrene, 16 wt% acrylonitrile and 47 wt% diluent is withdrawn as a vapor, condensed and recirculated and returned to the reaction vessel. The volatilized decomposed copolymer composed of about 0.15 wt% styrene, 0.01 wt% acrylonitrile, 0.05. 05 wt% diluent and 99/79 wt% copolymer sinks to the bottom of the vessel and is pumped to the granulating dies by a pump.

Examples II-VI

The method of Example I is repeated with several parameter changes. The parameters for these additional Examples, as well as Example I, are summarized in the following Table.

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Claims (2)

Process for the continuous mass polymerization of styrene and alkenyl nitrile monomers for the preparation of a styrene / alkenyl nitrile copolymer, characterized in that a feed with a predetermined ratio of styrene and alkenyl nitrile monomers is fed into a reaction vessel to form a reaction mixture to shape; the reaction mixture containing styrene and alkenyl nitrile monomers is subjected to temperature and pressure conditions under which said monomers copolymerize to form a styrene / alkenyl nitrile copolymer; the reaction mixture is subjected to sufficient stirring to maintain a substantially uniform distribution of the composition and a substantially uniform temperature distribution throughout the reaction mixture; continuous styrene / alkenyl nitrile copolymer is withdrawn from the reaction vessel, and the volatilization ± ng. stage consists of a first stage in which the ratio of styrene to alkenyl nitrile monomer in the withdrawn copolymer is adjusted to a ratio chosen to be polymerization to provide a styrene / alkenyl nitrile copolymer having substantially the same ratio of styrene: alkenyl nitrile monomer units as the styrene / alkenyl nitrile copolymer withdrawn from the reaction vessel; and in a second step, the withdrawn styrene / alkenyl nitrile copolymer is heated to a temperature sufficient to volatilize substantially all of the volatiles therein and separate the volatilized components from the non-volatilized copolymer. 2. Process according to claim 1, characterized in that in the adjusting step the monomer ratio is restored to the same ratio as the predetermined feed ratio, and in the first step recovery step an amount of alkenyl nitrile monomer becomes withdrawn, which is sufficient to restore said ratio, 3. A method according to claim 2, characterized in that upon withdrawal, the pressure of the first stage volatilization unit is maintained at a level sufficient to be sufficient. to provide a monomer-containing vapor phase and alkenyl nitrile monomer-rich vapors are selectively vented. - 27 - a Method according to one or more of the preceding 5 claims, characterized in that the heating step in the second step consists of passing the withdrawn styrene / alkenyl nitrile copolymer through a heat exchanger in an amount sufficient to to cause a significant volatilization of the volatiles in the copolymer sufficient to cause a foam-type two-phase flow through the heat exchanger, 5. A method according to claim 4, characterized in that the separation step in the second stage consists of passing the two-phase flow of copolymer / volatilized components from the heat exchanger through a separation vessel and reducing the pressure in the separation vessel to a level sufficient to keep the volatile * · components in vapor form. 6. A method according to claim 1, 3 or 4, characterized in that the volatilization step further comprises the steps of extracting the volatilized components from the separating vessel in the second stage, condensing the withdrawn volatilized components around a liquid condensate of recovered monomers, and recirculating the recovered monomer liquid to the reaction vessel as part of the monomer feed. Process according to one or more of the preceding claims, characterized in that the feed contains 30 to 60% by weight of acrylonitrile and 40-90% by weight of styrene, based on the total monomer content. 8. Process according to one or more of the preceding claims, characterized in that the feed further contains 2-50 wt.% Inert diluent for the monomers. m - 28 - Process according to claim 8, characterized in that the inert diluent is ethylbenzene, butylbenzene, benzene, toluene, xylene or cumene. Process according to one or more of the preceding 5 claims, characterized in that the copolymerization conditions are such that a reaction rate of up to 40% per hour and a conversion to copolymer of up to 90% are obtained. 11. Process according to one or more of the preceding claims, characterized in that it further comprises the step of cooling the reaction mixture by withdrawing vapor phase containing evaporated styrene and alkenyl nitrile monomers from the reaction vessel. 12. A method according to claim 11, characterized in that the cooling step further comprises condensing the evaporated styrene and alkenyl nitrile withdrawn from the reaction vessel by contacting a cooled condensation surface to form a condensed monomer-containing liquid and return condensed monomer-containing liquid 20 to the reaction vessel. 13. A method according to claim 12, characterized in that the cooling step further comprises collecting a volume of the condensed monomer-containing liquid and storing it for selective addition to the reaction vessel to produce an immediate cooling effect. . 14. A method according to claim 12, characterized in that the cooling step further comprises the step of recycling part of the condensed monomer-containing liquid to the condensing step and suddenly cooling the vapor withdrawn from the reaction vessel monomer by passing the withdrawn evaporated monomer through a stream of recycled condensed monomer-containing liquid. A method according to claim 14, characterized in that the stream of recycled condensed monomer containing liquid consists of a stream distributed as a curtain, 16. A method according to claim 14, characterized in that the cooling step further comprises the step of impacting the recycled condensed monomer-containing liquid against the cooled surface to prevent accumulation of polymerized monomer on that surface, A method according to claim 12, characterized in that the cooling step further comprises contacting the walls of the reaction vessel with a liquid cooling medium. 18. Apparatus for the continuous mass polymerization of styrene and alkenyl nitrile monomers to prepare a styrene / alkenyl nitrile copolymer, characterized in that it consists of a reaction vessel, means for continuously feeding a feed containing a predetermined ratio of styrene and alkenyl nitrile monomers contained in the reaction vessel 20 to form a reaction mixture; means for subjecting the reaction mixture containing styrene and alkenyl nitrile monomers to temperature and pressure conditions wherein these monomers copolymerize to form styrene / alkenyl nitrile copolymer? means to subject the reaction mixture to sufficient stirring to maintain a substantially uniform distribution of the composition and a substantially uniform temperature distribution throughout the reaction mixture means for continuously withdrawing styrene / alkenyl nitrile copolymer from the reaction vessel, and a volatilization unit for removing volatiles from the styrene / alkenyl nitrile copolymer withdrawn from the reaction vessel, the volatilization unit being provided with a first stage provided with means for restoring the ratio of styrene to alkenyl nitrile monomers in the withdrawn Copolymer to substantially the same ratio as the predetermined styrene: alkenyl nitrile ratio of the feed; and a second stage, comprising means for heating the extracted styrene / alkenyl nitrile copolymer to a temperature sufficient to volatilize substantially all of the volatile components therein, and means for separating the volatilized component of the non-volatilized copolymer. 19. Device according to claim 18, characterized in that the first stage recovery member is a member for extracting an amount of alkenyl nitrile monomer sufficient to restore said ratio, 20. Device according to claim 19, characterized in that the extracting means comprises a means for maintaining the pressure of the first stage devolatilization unit at a level sufficient to produce a monomer-containing vapor phase and a means for selectively venting alkenyl nitrile monomer-rich vapors, Device according to claim 18, characterized in that the heating member in the second stage 20 consists of a heat exchanger, means for passing the withdrawn styrene / alkenyl nitrile copolymer through the heat exchanger and means for reducing the pressure in the heat exchanger to an extent sufficient to cause almost complete volatilization of the volatiles in the copolymer, and sufficient to cause a foam-type two phase flow through the exchange device. 22. Device according to claim 21, characterized in that the heat exchange apparatus is provided with a number of tubes for the copolymer, which tubes have a diameter large enough to allow the two-phase flow of the foam type, 23. Device according to claim 18, characterized in that the separating member in the second stage 35 'consists of a separating vessel, means for feeding the two-phase - 31 - u flow of copolymer / volatilized components of the heat exchanger into the separating vessel. and means for reducing the pressure in the separation vessel to a level sufficient to keep the volatilized components in vapor form, 24. Apparatus according to claim 23, characterized in that the devolatilization unit is provided with means for extracting the volatilized components from the second stage desiccation vessel, a cooler for condensing the extracted volatilized components around a liquid condensate of recovered monomers, and means for recycling the recovered monomer liquid to the reaction vessel as part of the monomer feed. 25. Apparatus according to claim 18, further comprising means for cooling the reaction mixture by withdrawing a vapor phase containing evaporated styrene and alkenyl nitrile monomer from the reaction vessel, 26. An apparatus according to claim 25, characterized in that the cooling member further comprises a cooler for condensing the evaporated styrene and alkenyl nitrile withdrawn from the reaction vessel to prepare a found monomer-containing liquid, and to return the condensed monomer-containing liquid to the reaction vessel. 27. Device according to claim 26, characterized in that the cooling member further comprises a vessel for collecting a volume of condensed, monomer-containing liquid, and means for selectively introducing the collected monomer-containing liquid into the reaction vessel to effect instant cooling. 28. Device according to claim 26, characterized in that the cooling member further comprises means for recirculating part of the condensed monomer-containing liquid to the condensation step and means for sudden cooling of the evaporated monomer withdrawn from the reaction vessel by passing the withdrawn vaporized monomer through a stream of recycled condensed monomer-containing liquid. 29. An apparatus according to claim 28, characterized in that the sudden cooling means consists of means for forming a stream of condensed monomer-containing liquid distributed as a curtain. 30. An apparatus according to claim 26, characterized in that the cooling member further comprises means for causing the recycled, condensed, monomer-containing liquid to collide with the cooled surface to prevent accumulation of polymerized monomer on that surface. 31. An apparatus according to claim 26, characterized in that the cooling member is further provided with means for contacting the walls of the reaction vessel with a liquid cooling medium. 32. An apparatus according to claim 18, characterized in that it further comprises a preheater for the supply upstream of the means for introducing the supply. m * _ Sheet 1 of 1 lCH- = CCN 2 I R X C = CH0 I 2 Ar 2