CS204533B1 - Process for demonomerisation of vinyl chloride polymers or copolymers suspensions - Google Patents

Description

The invention relates to a process for the continuous removal of monoraers from aqueous dispersions. polymers, in particular removing monomeric vinyl chloride from aqueous dispersions! homo-, respectively. copolymers of vinyl chloride as well as apparatus for this purpose.

It has been found that monomeric vinyl chloride is a carino-ionic substance which, upon prolonged exposure to concentrations of several tens of ppm in air, may cause cancer prooes. The production of polyvinyl chloride, as one of the most important plastics, is steadily increasing from year to year, and is widely used in the most diverse areas of our lives. At the same time, however, the extent of the threat to human health caused by vinyl chloride monomer is increasing. In order to prevent detrimental effect of monomeric vinyl chloride on the organism, it is necessary to remove residual monomeric vinyl chloride from the polyvinyl chloride before its treatment to a minimum. Hygiene regulations for direct contact with foodstuffs indicate a permitted level of 1 ppm vinyl vinyl chloride monomer. Because the monomer content of the dispersion! of the polymer is 2 to 4% by weight. (per dry product), the regeneration of the residual monomer is also an economic benefit.

A large number of different processes are used to remove residual vinyl chloride from vinyl chloride homo- or copolymers which can be distinguished from each other by the nature of the starting material to be treated.

One of the most commonly used processes for reducing vinyl chloride monomer content in polymer and vinyl chloride base is thermal spraying of polymer suspensions. directly in an auto key (NSR 2,500,765) or in a separate attachment (NSR 2,565,665) by heating the suspension to 70-90 ° C for several hours. The suspension can be thermally demonomerized (NSR 2,429,777 and NSR 2,439,203) such that a pressure is set in the desorption device such that the suspension boils at 7 ° to 90 ° C. The advantages of these processes are the relatively good handling of the slurries to be processed, the desirability of the desorption equipment and the relatively good desorption efficiency. The disadvantage is especially high energy consumption.

A highly efficient method of removing residual monomer is the heat treatment of the centrifuged polymer cake. The wet cake is treated with saturated or overheated water vapor (NSR 2,541,809 and NSR 2,534,794), which acts as both a heat transfer medium and a mass transfer agent (vinyl chloride). Also, dielectric high-frequency heating (NSR 2,556,435) can be used to heat the skimmed cake. Despite the high efficiency, these processes are practically inapplicable because the impaired maneuverability places very high demands on the construction (especially continuous) devices. These processes also show a greater likelihood of polymer degradation due to local overheating of the polymer on contact with the hot wall of the device.

Thermal spraying of powder, resp. The granulate by means of heated gases and vapor in the fluidized bed is cited by NSR 2,451,988. A disadvantage is in particular the high constructional and energy demands.

Procedures are also known in which, in addition to a stream of inert gas (N ₂ , air, CO ₂ , CH 2, natural gas * 1), a heated suspension is entrained to entrain the released vinyl alloy (Belg. 839 320, Jap. Kokai). 76,052,486). These processes have at least a substantially lower efficiency compared to the above-mentioned disadvantage.

Recently, several patent applications have appeared that solve the demonomerization of a polyvinyl chloride suspension. in a desorption device of the column type. The demonometization column according to NSR 2,441,304 contains a series of plate-shaped surfaces within a 45 to 80 ^° inclination from the vertical. The process slurry is fed to the top of the column and passed through the layer in a coarse 0.5 to 5 bed thickness. The contact between the slurry and the water vapor of this type of column is relatively small and a significant high column is required to achieve the desired efficiency.

Improved contact is achieved in a slatted column (Germany 2 714 438), where the vapor entering the bottom of the column is bubbled through the overflow of the polyvinyl chloride chloride. According to the example in a 25-stage column, 200-400 ppm of vinyl chloride monomer are obtained in a demonomerized slurry, which is inefficient due to the need to extend the drying stage in the final product to achieve 1 ppm of vinyl chloride monomer in the final product. a nuany operation may encounter considerable problems.

Better efficiency results (100 to 200 ppm) are reported in NSR 2,714,685, wherein the column consists of a valoidal vessel equipped with sieve bottoms with a 5-15% gap and a 4 to 30 mm aperture size on the surface of which is a cyclone-type droplet separator. The slurry flow is provided by leakage through the holes and the gap between the inner wall of the column and the suture.

The most effective dsmonomerization devices hitherto known are the screen column of Httohst AG (NSR 2,550,023 and NSR 2,640,546). It is a sieve column which according to the examples has 14 to 17 trays with sieves (gap <% _ / 6%, 0 apertures 2 mm) provided with 1 to 3 overflow tubes and an overflow height of 12 µm. Amount of residual monomeric vinyl chloride in suspension! treated in said column is about 10 ppm.

According to the invention, the suspension is demonomerized. of polymers or copolymers of vinyl chloride by the action of gases and / or vapors at elevated temperature, such that the suspension is fluidized by a stream of saturated water vapor at a pressure of 0.15 to 0.5 MPa, preferably 0.3 to 0.5 MPa, the diameter of the individual streams is 0.5 to 4 mm, preferably 1.5 to 2.5 mm to 0.1 to 4 µ, preferably 0.5 to 2.5% of the lower surface of the layer, the linearity of the steam at the inlet stream is from 20 to 35 m (s), preferably 25 to 30 m / s, and the contact time of the fluidized suspension vapor is from 40 to 250 s, preferably from 60 to 120 s. Quantities supplied

The steam is 30 to 100 kg, preferably 40 to 7θ kg per 1 m ^J fluidized suspension and the suspension is sent to the next step, and when the required konoentráoie EA monomers in addition to other spraoovanie.

Among the main advantages of the process according to the invention is, in particular, the fact that the process makes it possible to process suspensions with a high monomer content (about

204 533

000 ppm), while in suspension! leaving the column, the desired concentration of monomeric vinyl chloride (e.g., 3 to 25 ppm) is achieved. The degree of monomer separation achieved is easily controlled by the choice of column efficiency (number of trays). The process allows for better utilization of the column throught / v t Q when 30 to 60 m suspension can be demonomerized in 1 m column per hour.

Even with such a load under the conditions of the invention, the operation of the column is stable with stable efficiency and, due to the high turbulence, a high efficiency of demonomerization is achieved for one tray. The process also appears to be favorable in terms of power engineering, when the steam consumption per kg of final product is 0.15 to 0.3 kg steam.

By the process and apparatus of the present invention, all types of suspension polyvinyl chloride and copolymers of vinyl chloride with vinyl acetate or lower olefins, as well as suspensions of tough polyvinyl alcohol, chlorinated polyvinyl chloride and also suspensions of copolymers of vinyl chloride with acrylic acid and / or its acrylic acid and / or acrylic acid and / or acrylic acid. The suspended suspensions generally contain from 10 to 50% by weight. dry matter; however, the values given are not linear. Prior to entering the column, the slurry was heated by direct and quenching, respectively. by indirect heating to a temperature close to the boiling point (under given pressure conditions).

The heated slurry is injected onto the top of a multi-stage sieve column, wherein a multi-stage sieve column is assumed to be at least a two-stage sieve column. The trays are specially adapted such that each tray consists of a sewn bottom having a gap of 0.1 to 4 µl, preferably 0.5 to 2 µl, through which one or more overflows (usually overflow tubes) pass, the total cross-sectional area of the overflows is 2 to 10%, preferably 4 to 7% of the cross-sectional area of the column.

The suspension is maintained in a fluidized state on a sieve tray by treatment with a saturated or saturated suspension. superheated water vapor that passes through the suture holes. The linear velocity of the steam in the suture openings is 20 to 35, preferably 25 to 30 m / s. The height of the overflows ensures the required contact time of the fluidized suspension steam in the range of 40 to 250 s, preferably in the range of 60 to 120 s. The height of the overflows above the screen is usually in the range of 200 to 500 mm. Overflows suspend the suspension from the higher level to the lower, respectively. if the monomer concentration reaches the desired value, the suspension is removed for further processing. The overflow pipe extends to a height of 40 to 100 mm above the lower tray, whereby to create hydraulic and hydrostatic conditions, the unbored area of the seam below the overflow pipe is equal to or heavier than the overflow area of the overflow.

The overall efficiency of the process is increased if the suspension contains 0.0001-0.5% by weight. alkali or ammonium persulphate, and at the same time it is ensured that the oxygen or oxygen-containing inert gas content of the supplied steam is in the range of 0.1 to 10%, preferably 1 to 5%

Example 1

The suspensions contain about 25% dry matter and about 23,000 ppm residual vinyl chloride

204 533 * 1 (calculated on dry weight) was boiled in an amount of about 1.2 m ^J / h into a preheater (steam injector) where it was continuously heated to 98 ° C. The preheated slurry is then sprayed onto the sieve column head with the following parameters:

column diameter 325 mm number of floors 4 distance between sieves 750 mm height of overflows 200 mm porosity 2% ratio of weirs to areas cross section of the column 0,024 hole diameter 1,9 Bm

The inner surface of the column (especially the sieve) is highly polished. The whole equipment is made of stainless steel.

Saturated water vapor (0.5 MPa) at a rate of 80 kg / h was fed under the lower deck. Against the streams of both media, perfect contact and complete desorption of residual vinyl chloride from the polymer particles of polyvinyl chloride occurs. The temperature at the top of the column is 98 ° C, at the bottom of the column it is 100 ° C. The pressure drop across the column is 7θ om water column. The slurry discarded from the column still contained 30 ppm of vinyl alcohol (calculated on the dry product). The monomeric vinyl chloride along with the water vapor is discharged from the top of the column through the separator to the cooler.

Example 2

A polyvinyl chloride chloride slurry (approximately 25% dry matter) containing about 23,500 ppm of residual vinyl chloride is next to a preheater (steam injector at about 1.2 m ^J / h where it is heated to 98 ° C). the following design parameters:

0 columns 325 mm number of floors 4 distance between sieves 750 mm overflow height 400 mm porosity 2% ratio of overflow to surface cross section of the column 0,024 hole diameter 1.9 mm

The inner surface of the column (especially the sieve) is highly polished. The whole device is made of stainless steel.

Saturated water vapor (0.5 MPa) at a rate of 80 kg / h was fed under the lower tray. Against the current flow, both media, i. of the slurry and water vapor, perfect contact and desorption of the residual vinyl chloride from the polymeric polyvinyl chloride chloride occurs. The temperature at the top of the column is 99 ° C, at the bottom of the column 102 ° C. The pressure drop on the column is 130 µm water column. The slurry leaving the column contains 20 ppm of residual vinyl chloride (calculated on dry powder). The monomer vinyl chloride together with the defective steam is discharged from the top of the column to the cooler.

Example '

A heated polyvinyl chloride suspension of the same quality as in Example 2 and in the same amount is sprayed onto the sieve column head of the following design parameters:

ff columns 325 mm number of trays 4 distance between sieves 750 mm height of overflows 400 mm gap 1% ratio of overflow area to flat cross-section 0.024 aperture diameter 1.9 mm

The inner surface of the column is high-alloyed. The whole device is made of stainless steel.

A low pressure saturated steam (0.4 MPa) at a rate of 50 kg / h was fed under the lower stage. Counterflow flows of both media i.e. of the suspension and water vapor, a perfect desorption of monomeric residual vinyl chloride from the polymer particles of polyvinyl chloride occurs. The temperature at the top of the column is 98 ° C, at the bottom of the column 101 ° C. Pressure drop on. the column is 120 µm. The suspensions are discarded from the column containing 25 ppm of residual vinyl chloride. The monomethylene vinyl chloride together with the water vapor is discharged from the top of the column through the separator into the cooler.

Example 4

Suspensions having a dry matter content of about 25% and a residual vinyl chloride content of about 20,000 ppm of monomeric vinyl chloride are next to the preheater (steam injector) in an amount of about 1.5 m ^ / h. Here it is heated to a temperature of 99 ° C. The preheated suspension is then injected onto the sieve column head with the following parameters:

0 columns 325 mm number of floors 8 distance between sieves 750 mm overflow height 200 mm medzemitosť 1 %

ratio of overflow area to column cross-section area

0,046

204 533 hole diameter 1.9 mm

The inner surface of the column is high-wet. The whole device is made of stainless steel.

A low pressure saturated steam (0.4 MPa) is fed below the lower deck aa at a rate of 60 kg / h. The countercurrent flow is intensive contact of suepension and steam and perfect desorption of residual vinyl chloride from polymeric polystinyl chloride. The temperature at the top of the column is 98 ° C, at the bottom of the column 102 ° C. The pressure drop on the column is 150 cm water column. The slurry leaving the column still contained 10 ppm residual vinyl chloride. The vinyl chloride chloride monomers along with the water vapor are discharged from the column head to the smoother.

Example 5

The polyvinyl chloride slurry contains, per 25% pulp, containing 20,000 ppm monomer vinyl chloride, at a rate of 1.2 m m / h continuously fed through a preheater (steam injector) to the head of the sewing column of the following parameters:

ff columns 325 mm number of trays 8 distance between sieves 750 mm overflow height 400 mm spacing 1% ratio of overflow area to flat column cross-section 0.046 ff holes 1.9 mm

The pre-heated slurry (98 ° C) passes a successive column in countercurrent low pressure steam (0.4 MPa) at 50 kg / h, which is fed under the lower tray. The inner surface of the column is high-pitched. The countercurrent flow achieves a perfect contact of the foils and thereby ensures perfect absorption of the residual monomeric vinyl chloride from the polymer particles of polyvinyl chloride. The temperature at the top of the column is 98 ° C and at the bottom of the column 105 ° C. The pressure drop is 240 om water column. The slurry leaves the column containing 2 ppm of monomeric vinyl chloride (calculated on dry strand). Monomemes of vinyl chloride along with water vapor are thrown from the column head into the smoother.

Example 6

The slurry as in the foregoing example is fed to a preheater (steam injector) in an amount of 1.9 m ^ / h where it is heated to 98 ° C. The heated slurry enters the head of the sewing column with the following parameters:

ff columns 325 mm number of trays 8 distance between sieves 750 mm

204 533 overflow height 200 mm gap 1% ratio of overflow area to column cross-sectional area 0.046 fS of apertures - 1.9 mm

The inner surface of the column is highly polished. At the bottom of the column, 70 kg / h of low pressure steam (0.45 MPa) was introduced. The countercurrent movement of both media ensures perfect contact and high desorption of free monomeric vinyl chloride from the polyvinyl chloride particles is achieved. The temperature at the top of the column is 9 ° C ^and at the bottom of the column 102 ° C. The pressure drop in the column is 130 om water column. From the bottom of the column, the slurry contains 20 ppm of residual vinyl chloride monomer, while from the top of the column a gaseous mixture consisting of desorbed vinyl chloride and water vapor is discharged to a condenser.

Claims (3)

OBJECT OF THE INVENTION 1. A method for demonomerizing a suspension! of polymers or copolymers of vinyl chloride by the action of gases and / or vapors at elevated temperature, characterized in that the slurry is fluidized by a stream of saturated water vapor at a pressure of 0.15 to 0.5 MPa, preferably 0.3 to 0.5 MPa, the radius of the individual the flow rates are 0.5 to 4 mm, preferably 1.5 to 2.5 mm, to 0.1 to 4%, preferably 0.5 to 2% of the lower surface area of the layer, the line and the vapor velocity at the stream inlet is from 20 to 4% 35 m / s, preferably 25 to 30 m / s, and the contact time of the fluidized suspension steam is from 40 to 250 s, preferably from 60 to 120 s, the amount of steam introduced being 30 to q 100 kg, preferably 40 to 70 kg per suspension, and the fluidized suspension is discharged to the next stage and, after reaching the desired monomer concentration, is further processed. 2. A method according to claim 1, wherein the supplied steam contains 0.1 to 10, preferably 1 to 5, of oxygen or oxygen-containing gas and the feed suspension contains 0.0001 to 0.5% by weight. persulfate. Multi-level sieve column for carrying out the process according to Claims 1 and 2, characterized in that the sutures have an interstices of 0.1 to 4 ¢, preferably of 0.5 to 2,, an overflow height of 200 to 500 mm and a ratio of the total cross-sectional area of the weirs to the cross-sectional area of the column is 0.02 to 0.1, preferably 0.04 to 0.07.