DE3230069A1 - Process for the recovery of unpolymerised vinyl chloride from polymerisation mixtures - Google Patents

Abstract

VC-containing waste gas from polymerisation batches can be recovered in a quality corresponding to fresh VC by a combination of the following steps: a) compressing the waste gas by means of a liquid-seal compressor, b) condensing and drawing off the condensate at the boiling temperature, c) transferring the condensate to a sealed separating tank, d) removing the separated water. m

Description

Process for the recovery of unpolymerized

Vinyl chloride from polymerisation mixtures.

In the production of (co) polymers of vinyl chloride (VC) is usually to keep the polymerization time within economically justifiable limits to keep only polymerized up to conversions of up to 95%. This applies to everyone Types of polymerization, be it after the mass, but preferably after the suspension, Microsuspension or emulsion processes. If the reaction was canceled earlier the overpressure is simply released into the atmosphere. However, this is due to environmental and for reasons of workplace hygiene no longer justifiable and permissible. That VC discharged from the reaction vessel is therefore collected today and again used.

In addition, the VC (co) polymers, the simplifying general as polyvinyl chloride (PVC) are also subjected to a treatment, because although they are insoluble even in the monomeric VC, they themselves still contain such amounts of the VC contain that this is in further processing or at the end user due to its diffusion from the polymer still to the now well-known health Could lead to impairment. In this common and well-known treatment is the VC residual monomers except for the slightest, z. T. (especially with suspension PVC) just barely detectable residues usually by thermal means and expelled from the polymer under vacuum, if necessary by blowing in steam.

This VC, which originates from the residual polymer degassing, must also be returned to be brought into a reusable form, as it generally contains a lot of water is. It was found that in this water a large part of the im so-called reverse VC (RVC) contained by-products are dissolved.

If such RVC, which is usually water-containing, is used in the VC polymerization together with fresh VC also only in a relatively small amount Amounts, it occurs, presumably caused by the above-mentioned by-products significant deterioration in product properties, e.g. B.

in terms of thermal and light stability. This expresses discoloration of the freshly produced polymer.

In the work-up of the polymerization mixtures also occur problems that are practically inevitable, be it through leaks the equipment or substances already contained in the mixture due to decomposition, z. B. of initiator residues, oxygen in more or less small amounts is contained in the exhaust gas containing RVC. This can, however, become highly explosive Form VC peroxides, which unfortunately have already triggered explosions with serious consequences.

Many attempts have already been made from these foundations Problems to master. For example, it is proposed in DE-C 25 14 126, the gaseous and / or vaporous monomers such as VC with a at 50 "C and atmospheric Pressure to treat liquid substance with and / or in which said monomers subsequently can be implemented. Monomers such as styrene, vinyl esters and acrylic esters are used for this purpose and suggested inert solvents suitable for solution polymerization, z. B.

Benzene, hexane, cyclohexane, acetone, ethyl acetate, dioxane, methyl ethyl ketone. On the one hand, this process is already based on certain polymerization processes which must be carried out at least as a coupled process to others will have the problem that toxic or more generally environmentally harmful substances are made to be deposited in the exhaust gas, not dissolved, since such a substance is in the exhaust gas is only replaced by another It is only to the "maximum workplace concentration" remembered for benzene. This so-called MAK value is zero.

DE-A 27 04 065 proposes the exhaust gases in several stages to clean. The exhaust gas is initially compressed to approx. 4.5 bar in one Liebig cooler cooled to temperatures between +4.5 and -9.5 ° C, especially approx. -7 ° C, to separate the water. The exhaust gas is then accordingly via a molecular sieve absorber, to separate water residues and carbon dioxide, and finally through a heat exchanger by cooling it to temperatures below -129 ° C. The one that has cooled down in this way Mixture of liquid VC and associated gases such as nitrogen and oxygen is then fed into a separator in which the VC is to settle. The associated gases should be blown off after passing through activated charcoal.

Finally, the RVC still has to be worked up by distillation. Around Preventing explosions is also intended in the exhaust gas flow additional Nitrogen can be blown in.

Despite extensive recovery of the "cold energy" prevails in this Processes that are already very complex in terms of apparatus due to the large temperature jumps (from +15 ...

+38 "C down to -129 ... -151" C), the necessary regeneration of the molecular sieve and the cooling or liquefaction of the coolants (brine and nitrogen) requires a lot of energy.

One task, therefore, was a simpler yet safe one To develop processes that enable an RVC to be freed from impurities, to prevent or at least largely reduce peroxide formation and finally to separate the water reliably and without any technical effort.

This problem is solved by a process for separation and purification of vinyl chloride from exhaust gases originating from vinyl chloride polymerizations and in addition to gaseous vinyl chloride, water vapor and possibly other gaseous forms Contain substances by condensing out of the previously compressed exhaust gas flow, Separation of remaining gaseous substances, collection of the liquid phase in one Separation vessel and separation of the water, characterized in that a) the exhaust gas after Extraction from the polymerization and / or degassing tank by means of a liquid ring compressor, preferably by means of a water ring compressor with a preferably alkaline or neutral one Water circuit, to condensation pressure, preferably to 3 to 5 bar (absolute), compressed so that the gas leaves the compressor preferably at 25 to 50 "C, b) that compressed gas mixture is condensed in a condenser (cooler) in such a way that that the condensate at the boiling point or just below the boiling point from the cooler liquid is withdrawn, c) the condensate after leaving the cooler in an im essential only under the autogenous pressure of vinyl chloride and (in practically too negligible amount) of the water standing separation tank is transferred, in which it can cool down further if necessary, d) the water from the separation tank, that is not dissolved in the vinyl chloride and is removed preferably by gravity has separated, preferably through a drain line at the bottom of the container preferably is discharged cyclically, and e) the liquid vinyl chloride optionally in a Storage container is transferred.

The inventive method is particularly suitable for a continuous Implementation, but it can also be carried out batchwise.

It has been shown that RVC, which in this way according to the invention has been separated and purified without further distillation, rectification or different Cleaning steps directly again during the polymerization can be used and surprisingly results in products that are qualitatively comparable on the basis of VC (fresh VC) used for the first time.

It was already mentioned at the beginning that RVC consists of such a VC that either drained (distilled off) from the reactor at the end of the polymerization or by so-called degassing, often with the support of blown gas Water vapor, from the polymer or from polymer dispersions or slurries is recovered.

However, this RVC contains when leaving the polymerization or. Degassing tank not just water, but also gaseous decomposition products from the polymerization mixture and oxygen, which either penetrated through leaks in the apparatus may or may also come from the peroxidic initiators. This gas mixture, which is to be referred to here as exhaust gas for the sake of simplicity, must in order to be able to condense the RVC at easily manageable temperatures, in general still to be condensed. Pressures of 3 to 6 bar (absolute) have proven to be advantageous here. proven, since at these pressures VC boils at about + 15 to +35 "C. Around the RVC practical to condense quantitatively, a cooling medium, e.g. B. cooling water or preferably Brine, preferably at +15 to -20 ° C, especially at +10 to -15 ° C, especially preferably used at most -5 ° C, since lower temperatures are not special Bring more advantages with regard to the VC degree of separation, however on the one hand greater effort is required in terms of the cooling capacity for the cooling medium and, on the other hand, there is a risk of the cooler icing up, it unless you want the additional effort of water removal according to DE-A 27 04 065 drift. As this publication can be seen, you can because of the However, environmental protection requirements do not apply to a downstream activated carbon filter forego the uncondensed exhaust before blowing it off into the atmosphere still to be freed from the last VC traces.

It has been shown that the one used for compression Device has an influence on the quality of the RVC. So it turned out that when using conventional reciprocating compressors, the content of undesirable contaminating the RVC By-products is even increased.

Surprisingly, this adverse effect does not occur when instead, within the method according to the invention, liquid ring compressor, In particular, water ring compressors with preferably an alkaline or neutral water cycle can be used. Used alone, this measure results in comparison with a piston compressor Densified VC gives better, but still inadequate results. The washing too the RVC with antioxidants, reducing agents and / or alkalis were not viable Path.

The compressed exhaust gas, which corresponds to the temperature of the preferred The water circuit used is saturated with water vapor, leaves the compressor preferably with about a pressure of 2 to 5 bar (overpressure) and Temperatures of about 25 to 50 "C, i.e. the gas has temperatures which are preferred about 15 "C above the boiling point of the VC corresponding to this pressure.

The compressed, slightly superheated gas stream is then placed in a cooler (Condenser), in which the RVC and the water as well as possibly knock down some by-products. According to the invention, this condensate is at the boiling point of the VC or just below this temperature is withdrawn from the cooler, whereby the Discharge temperature preferably not more than 10 degrees Celsius lower than the boiling point of the VC.

This can be achieved by regulating the coolant temperature and quantity and by introducing coolant and exhaust gas into a Liebig cooler in synchronism will.

However, a vertical tube bundle cooler is preferred the condensate and gas are fed in countercurrent. The easiest and therefore preferred this is achieved in that the exhaust gas is introduced into the cooler from below. The condensate therefore runs due to gravity in the ascending exhaust gas flow counter and leaves the cooling surface at the desired temperature. Advantageous has also proven itself when the coolant temperature and amount are dependent can be regulated by the temperature of the withdrawn condensate. Moreover, will the coolant is introduced into the top of the cooling jacket in the preferred upright cooler and withdrawn at the bottom, i.e. also guided in countercurrent to the gas flow. It became found that up to a coolant inlet temperature of at least -200C surprisingly no icing problems occur.

The non-condensed exhaust gas components are at the top of the cooler drained through the usual activated carbon filter.

The condensate is withdrawn from the cooler at about the boiling point and transferred to the separation tank or pumped out.

The condensate is temporarily stored in the separation tank and the water, that is contained in the VC beyond the solubility, separates under the influence the force of gravity. It turned out that this was the largest proportion the VC impurities are separated from the VC with the water and removed with this In addition, it was found that the VC after the separation of the aqueous Phase practically no longer contains any oxygen and, accordingly, no peroxides can make more.

It is therefore extremely important that the deposited water layer is regularly separated as completely as possible from the VC layer. The draining the water phase can in principle be carried out manually, but the Boundary layer often not clearly recognizable.

Therefore, ways have been sought to circumvent these difficulties. A surprisingly simple and yet safe and therefore preferred method with extremely short dead time (with practically no VC losses in the wastewater) is the control of the water drain valve with the device described below: The preferred embodiment of the water drainage device consists of one in the ground (Sump) of the separating tank into the drain pipe before the shut-off valve a throttle point, e.g. B. in the simplest case a pinhole, as well as in front of and behind A temperature sensor is installed in each of this throttle point. As soon as when draining of the water vinyl chloride passes through the throttle point, this evaporates VC due to relaxation. This causes a lowering of the temperature in the direction of flow seen second temperature sensor. The temperatures at both temperature sensors are measured continuously at least during the deflation and automatically in compared to a control device. As soon as there is a temperature difference between the two Sensors occurs or a certain, easy to determine and set If the limit or setpoint value (e.g. 5 to 15 "C) is exceeded, the shut-off valve is activated by the Controller closed automatically, preferably before the VC reaches the valve or even happened.

After all it is usually sufficient for several polymerizers to provide a device for performing the method according to the invention, can the inventive method can be carried out continuously, even if the polymerizations run discontinuously will. With such continuous implementation of the process according to the invention has proven to be special proved advantageous to automate the drainage of the water even further. as The best and easiest method has proven to be to open the water drain valve by causing a timer while the valve is closing by the method described above is controlled. The frequency of drainage of the water depends to a large extent on the flow rate of the exhaust gas, so that no absolute limits can be given here. For example, the water can drained every 1 to 6 hours.

Apart from the preferred timing triggered deflation but also, of course, other methods of determining the best moment to let go of the water possible. Here are only methods for determining the height of the VC-water interface listed, e.g. B. on the basis of the different densities (floats, light refractive index measurement), Conductivity measurement (electrical and thermal conductivity), pH measurement and Measurement of other physical properties. The same measurement methods are natural Also possible in principle instead of measuring the temperature difference in the discharge line, in addition z. B.

also recording of the differential pressure. As already stated above, the methods described in more detail above are preferred because they are simpler and are safer and react faster.

The VC can be reused immediately after the water has been separated, However, if desired, it can also be in separate storage containers be pumped out.

A preferred embodiment of the method is based on the as Figure attached flow sheet, which is also a preferred embodiment of the The device used for this purpose is explained in principle: The RVC-containing Exhaust gas is from the polymerization or polymer processing plant, not shown here via line (1) to the liquid ring compressor (2), its liquid circuit is not shown, supplied, there to the required condensation pressure brought and superheated-n fed into a tube bundle condenser (3) from below. The condensed RVC / water mixture and that entering the condenser were superheated Gas are passed in countercurrent, so that the condensate removed in (4) is practically Has boiling point. The non-condensable parts of the exhaust gas, e.g. B. nitrogen and oxygen, are discharged via line (10) and then the usual final cleaning subjected. At (15) the coolant is pumped into the cooling jacket The foot of the cooling tubes is pulled off again (horizontal arrow).

The condensate is pumped (5) into a storage tank (6), in which the RVC-contained water settles under the action of gravity. The water is collected in a sump (7) and drawn off via line (8).

An automatic valve is used for this purpose by means of a time control (14) (12) open.

In the line (8) there is a throttle position (10) via the the separated water in a pressureless collecting device (not shown) relaxed 1.

The temperature difference measuring device records during the discharge process of the water (13) ideally no temperature difference between the two marked with (T) Sensors. As soon as VC passes through the throttle (10), the measuring point cools down in the direction of flow behind the throttle and the temperature difference measuring device (13) closes the valve (12) when a limit value is exceeded. A sight glass (11) can be used for visual control of the process. The RVC is over a line (9) back to the polymerisation system (not shown) or into a storage container convicted.

Surprisingly, it is unnecessary to distill the VC again, to rectify or otherwise purify before re-polymerizing can be used. The content of peroxides is also practically negligible, like a distillation residue from a distillation carried out for verification.

It goes without saying that by selecting some features of the Process according to the invention, albeit with a loss in quality of the RVC, a work-up is possible, e.g. B. by replacing the liquid ring compressors with reciprocating compressors, without d-aß, the inventive concept of the present application is thereby abandoned.

A test apparatus was used to assess the quality of the RVC * standardized Polymerizations carried out with Frisch-VC and RVC, the polymers uniform Refurbished and as characteristic in terms of heat stability and initial color Sizes for the product and thus for the RVC quality examined.

The thermal stability was tested according to the following procedure: 100 g PVC were made with 30 g of dioctyl phthalate, 1.5 g of commercially available Ba / Cd / Zn stabilizer (Irgastab (R) BC 206, Fa..Ciba-Geigy Marienberg GmbH) and 1 g of a commercially available Lubricant (Loxiol (R) G 10, Fa. Henkel KGaA, Düsseldorf) mixed and on one Roller mill rolled at 1700C for 10 minutes. The thickness of the rolled head was 1.0 mm.

The rolled sheet became a 15 mm wide and 170 mm long strip punched out and attached to the carrier plate of a Metrastat. The carrier plate was then pushed into the oven heated to 1900C and the test process was carried out Start of the extension mechanism of the metrastata started. The carrier plate will moved out of the furnace at a defined slow speed.

The length of the portion of the test strip that extended out of the oven is is directly proportional to the thermal load. As a measure for judging the Thermal stability is the time required during the test until the color of the test strip changes significantly from yellow to black.

To test the initial color of the PVC, a mill sheet was used as above described, manufactured. For this rolling * each fur and if necessary rectangles were cut out of standard patterns and placed in a frame, that this was filled out. By then pressing in a heated press at 185 "C and a pressure of 250 kg / cm (4.9 MPa) a test plate was produced in 30 minutes which was then cooled in a cooling press at 75 ° C. for 10 minutes.

Against a white background, the color can be assessed, whereby the individual samples can also be compared with one another, or against one Standard can be split.

The PVC should not show any initial discoloration, in particular no yellow tinge.

The vinyl chloride was also checked for active oxygen content, acid content and evaporation residue examined.

Comparative experiment A Originating from a suspension polymerization plant Exhaust gas (RVC) was compressed to 8 bar with a reciprocating compressor in a tube bundle condenser (whose condensation tubes had an inside diameter of 20 mm) with cooling water of 15 "C. The condensate was at approx. 20" C in a separating container, which was connected to the cooler via a gas displacement line. The container was drained manually every four hours.

The active oxygen content of the RVC was approx. 15 mg / kg VC, the Acidity approx. 10 mg / kg VC and the evaporation residue approx. 100 mg / kg VC. With increasing storage time the active oxygen content of the RVC increased.

The thermal stability of PVC made therefrom was 90 min The color of the press plate was brown. The product was insufficient.

Comparative experiment B Comparative experiment A was repeated, however was the compressed exhaust gas between the compressor and condenser through a gas scrubber, which was treated with alkaline sulfite solution, passed.

The active oxygen content was 10 mg, as was the acid content 10 mg and the evaporation residue 50 mg, each per kg of VC. With increasing storage time the active oxygen content of the VC increased.

The thermal stability of the PVC was 96 mini, the color of the press plate was dark yellow. The product was insufficient.

Comparative experiment C Comparative experiment A was carried out replacing the reciprocating compressor repeated by a water ring compressor with an alkaline water cycle. That had compressed exhaust gas a pressure of 6 bar.

The active oxygen content was 2 mg / kg VC, the evaporation residue 10 mg / kg VC. The active oxygen content increased with increasing storage time.

The heat stability of the PVC was 100 minutes, the color of the press plate was light yellow. The product was not generally applicable and therefore still unsatisfactory.

Example 1 Comparative Experiment C was modified so that the still low-boiling condensate from the condenser into a closed separation tank was pumped out, from which the water was automatically drained every 4 h until VC entered through a throttle of 4 mm and the 2nd temperature sensor a 10 ° C lower one Temperature as the first measured. There was no gaseous VC in the water tank the end. The active oxygen content, which was constant even during storage, was below 1 mg / kg VC dropped, the evaporation residue was below 3 mg / kg VC, the VC was neutral.

The thermal stability of the PVC was 112 minutes and thus corresponded to the of PVC from fresh VC. The press plate showed no discoloration.

Example 2 Example 1 was carried out with practically the same success as the modification repeated that a brine of approx. -10 ° C. was used as the coolant. Based the change in the loading of the activated carbon filter in the exhaust gas flow over time There was a slight improvement in the degree of separation of the VC in the condenser established.

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Claims (7)

Process for the separation and purification of vinyl chloride from exhaust gases that originate from vinyl chloride polymerizations and in addition to gaseous ones Vinyl chloride contain water vapor and other gaseous substances through condensation from the previously compressed exhaust gas flow, separation of remaining gaseous substances, Collecting the liquid phase in a separation vessel and separating the water, d a it is indicated that a) the exhaust gas after it has been withdrawn from the polymerization and / or degassing tank to condensation pressure by means of a liquid ring compressor is compressed, b) the compressed gas mixture is condensed in a cooler in the manner is that the condensate at boiling point or just below the boiling point liquid is withdrawn from the cooler, c) the condensate after leaving the cooler in one essentially only under the autogenous pressure of vinyl chloride and water standing separation tank is transferred, d) the water is drawn off from the separation tank and e) the liquid vinyl chloride is optionally transferred to a storage container will. 2. The method of claim 1, d a d u r c h g e -k e n n z e i c n e t that the exhaust gas is compressed in stage a by means of a water ring compressor, whose The water cycle is preferably set to be alkaline or neutral is. 3. The method according to claim 1 or 2, d a d u r c h g e k e n n z e i c h n e t that the exhaust gas in stage a to a pressure of 3 to 6 bar (absolute) is compressed and the temperature of the gas leaving the compressor at 25 - 50 "C. 4. The method according to any one of the preceding claims, d a -d u r c h it is noted that in stage b, the inlet temperature of the coolant by means of a coolant is at +15 to -20 ° C, cooled, the condensate in countercurrent to the in the cooler, preferably a standing tube bundle cooler, inflowing gas and led to the chosen Pressure corresponding to the boiling point of the vinyl chloride or at a maximum 10 Celsius degrees lower temperature is withdrawn from the cooler. 5. The method according to any one of the preceding claims, d a d u r c h It is not noted that the separating container in which the liquid vinyl chloride-water mixture is placed transferred: is formed so that the water not dissolved in the vinyl chloride separates by gravity from the vinyl chloride and through a drain line on The sump of the container is drained cyclically. 6. The method according to claim 5, since d u r c h g e -k e n n z e i c h n e t that the drain line for the water with a throttle and before and is provided with a temperature sensor behind this throttle point, so that a valve located behind the second temperature sensor from a timer opened and when a predetermined temperature difference is exceeded between the sensors are closed again. 7. Apparatus for performing the method according to claim 1, consisting from a) a water ring compressor (2) with an alkaline or neutral water circuit, in which the exhaust gas is fed in through line (1) and its pressure side with the The bottom part of a standing tube bundle condenser (3) is connected, b) the tube bundle condenser (3) with coolant circuit (15), exhaust gas discharge line (10) located on the head, with Gas inlet and condensate discharge (4) on the bottom part of the condenser, c) storage tank (6) with condensate supply line (4), vinyl chloride discharge line (9), sump (7), and d) with Sump (7) connected water drainage (8), which is provided with a throttle point (10), in front of and behind the throttle point, each with a temperature sensor and shut-off valve (12), which is opened by a timer (14) and by a temperature difference measuring device (13) is closed when the temperatures measured by the two temperature sensors exceed a certain set difference.