RU2152406C1 - Method of synthesis of ethylene homopolymers and copolymers of ethylene with vinyl acetate - Google Patents

Abstract

FIELD: organic chemistry, chemical technology. SUBSTANCE: process of (co)polymerization is carried out under pressure 1 400-3 000 atm, at flow rate of reaction mass 5-10 m/s using periodic drops of pressure in reactor: drop value pressure in beginning of reactor is 70-150 atm, period between drops is 100-200 s in the presence of homogenizing hydrocarbon addition. Addition is branched acyclic compounds of paraffin order at medium-measured molecular mass M 300-3 000 Da and 230-400 CH₃-groups per 1 000 hydrocarbon atoms at mass ratio to homogenizing addition = (1:0.0003)-(1:0.002). Process of (co)polymerization at beginning of a single-zone reactor or at beginning of the first zone of multi-zone reactor is carried out at 160-200 C and at the next sites of a single-zone or multi-zone reactors process is carried out at 200-320 C and value of specific heat loading for reactor zone site at 160-200 C is maintained at the range 30 000-50 000 kkal/m² x h. Method provides the necessary stability of the process of synthesis of (co)polymers that ensures to diminish the length of tube reactor up to 225-300 m. Invention can be used in chemical and petrochemical industry. EFFECT: improved method of synthesis, decreased material consumptions. 2 cl, 1 tbl

Description

 The invention relates to a technology for the production of homopolymers or copolymers of ethylene with vinyl acetate by the high pressure method in a tubular reactor and can be used in the chemical and petrochemical industries.

Known (USSR Patent N1838330, class C 08 F 110/02, 2/38, publ. 30.08.93) a method of producing polyethylene by polymerization of ethylene in a tubular reactor or reactor with a stirrer in the presence of oxygen and (or) organic peroxides at a pressure of 1350- 2270 atm (134-226 MPa) and a temperature of 145-317 ^o C (418-530 K). To increase the homogeneity and purity of the final product, a homogenizing additive is introduced before and (or) after the reactor, which is used as a mixture of normal acyclic compounds of the paraffin series, consisting of 2.5-10.0 wt.% N-paraffins C ₁₁ -C ₁₂ , 80-90 wt.% N-paraffins C ₁₃ -C ₁₇ and 0.5-10.0 wt.% N-paraffins C ₁₈ -C ₂₀ , and n-paraffins are introduced in an amount of 0.02-0.30 wt. % of the initial reaction mass. The introduction of a homogenizing additive into the reaction mass helps to clean the reactor wall of high molecular weight deposits, which improves the conditions of heat transfer, increases the conversion of the reaction mass and reduces the number of gel-like inclusions in polyethylene. However, the conduct of the polymerization process in the presence of normal acyclic compounds, in particular dodecane, leads to a deterioration in the optical properties and strength characteristics of the film made from polyethylene obtained in their presence (see control example 30). Apparently, the marked deterioration of the properties of the polyethylene film is also observed when the above mixture of n-paraffins is introduced into the reaction mass, since n-dodecane and the mixture of n-paraffins are similar in molecular weight and physicochemical properties. Correct comparison of the properties of polyethylenes obtained in the conditions of USSR patent N 1838330 and in the conditions of this application is difficult, since the patent does not contain data on the optical properties and strength characteristics of the polyethylene film, and the experiment with the use of a mixture of n-paraffins in the ethylene polymerization was not carried out due to lack of industrial production of this product.

There is also a method of producing polyethylene, described in the book "High-pressure polyethylene. Scientific and technical foundations of industrial synthesis" / A.V. Polyakov, F.I. Duntov, A.E.Sofiev and others. -L .: Chemistry, 1988 ., 13-37 p. , according to which polyethylene is obtained by radical polymerization of ethylene in bulk in a heated single or multi-zone tubular reactor at a temperature of 200-320 ^o C, a pressure of 1500-3500 atm and a low flow rate of the reaction mass - 5-10 m / s, using periodic pressure relief with a depth of pressure relief at the beginning of the reactor 200 to 300 atm and a period between releases of 60-100 s, initiating the polymerization reaction with oxygen supplied in a mixture with ethylene to each zone of the reactor. The polyethylene formed in the reactor together with unreacted ethylene is continuously removed from the reactor, cooled and the polymer is separated from ethylene. Unreacted ethylene is cooled, purified and again fed into a reactor in a mixture with fresh ethylene. Due to the low flow rate of the reaction mixture, the method allows efficient use of the reactor volume, but the high pressure relief depth (200-300 atm at the beginning of the reactor and, accordingly, 300-400 atm at the end of the reactor) with a small (60-100 s) period between discharges leads to a violation of the homogeneity of the reaction mass and, as a consequence, to a deterioration in the quality indicators of polyethylene — increased turbidity of a film made from such a polymer and an increase in the number of gel-like inclusions (see control example 22).

Closest to the claimed method for the totality of the claimed features is a method for producing homopolymers of ethylene or copolymers of ethylene with vinyl acetate, according to ed. testimonial. USSR N475861, (M cl. C 08 F 110/02, C 08 F 210/02, publ. 05.03.76). In accordance with this method, ethylene homopolymers or ethylene vinyl acetate copolymers are obtained by (co) polymerizing ethylene in a heated single or multi-zone tubular reactor at a pressure of 1200-4000 atm and a temperature of 100-350 ^o C with initiation of the reaction (co) polymerization with oxygen supplied to mixtures with ethylene in each zone of the reactor, introducing a hydrocarbon additive (propane) into the reaction mass in front of the reactor and then separating the obtained (co) polymer from unreacted ethylene or a mixture of ethylene and vinyl acetate. (Co) polymerization is carried out at a flow rate of the reaction mass in the reactor of more than 10 m / s (but not higher than 20 m / s). The pressure in any section of the reactor is kept constant over time.

 The specified method provides a high yield of the final product, obtaining a high-quality polymer suitable for producing transparent films and ease of process control, however, the implementation of the method involves high material costs, since, as you know, when the reaction mass is in the reactor for 60-300 s and speed 14 -20 m / s (see. High-pressure polyethylene. Scientific and technical fundamentals of industrial synthesis / A.V. Polyakov, F.I. Duntov, A.E. Sofiev, etc. -L. Chemistry, 1988, 29 sec. .) a reactor with a minimum length of 840 m is required.

 The technical result, the achievement of which the claimed method provides, is to reduce material costs in the implementation of the process of (co) polymerization through the use of a relatively small reactor.

The specified technical result is achieved due to the fact that in the method for producing ethylene homopolymers or copolymers of ethylene with vinyl acetate, comprising (co) polymerizing ethylene in a heated single or multi-zone tubular reactor at a pressure and temperature, initiating the reaction (co) polymerization with oxygen supplied in the mixture with ethylene in each zone of the reactor, introducing additives into the reaction mass in front of the reactor and separating the obtained (co) polymer from unreacted ethylene or a mixture of ethylene with vinyl acetate, (co) polymer They are carried out at a pressure of 1400-3000 atm, a flow rate of the reaction mixture of 5-10 m / s using periodic pressure drops at the beginning of the reactor 70-150 atm with a period between releases of 100-200 s, and a homogenizing hydrocarbon additive representing are branched acyclic compounds of a paraffin series with a number average molecular weight (M _n ) of 300-3000 and an amount of 230-400 CH ₃ groups per 1000 carbon atoms with a mass ratio of the reaction mass to the homogenizing additive equal to 1: 0,0003-1: 0.002, at the same time d separating the resultant (co) polymer, the reaction mixture was cooled.

The process of (co) polymerization at the beginning of a single-zone reactor or at the beginning of the first zone of a multi-zone reactor is carried out at a temperature of 160-200 ^o C, and in subsequent sections of a single-zone and multi-zone reactor is carried out at a temperature of 200-320 ^o C, while the specific heat load for the zone the reactor with a temperature of 160-200 ^o C support in the range of 30000-50000 kcal / m ² • hour.

The process of (co) polymerization of ethylene in a tubular reactor, initiated by oxygen, can be divided into two stages:
1. Stage sluggish (co) polymerization, proceeding at temperatures 160-200 ^o C;
2. The stage of intensive (co) polymerization, proceeding at temperatures above 200 ^o C.

The polymer obtained under conditions of a mode of slow flow (co) polymerization has a high molecular weight (M _n more than 100,000).

 Such a polymer even at pressures up to 3000 atm in a mixture with ethylene (or ethylene with vinyl acetate) forms a heterogeneous system consisting of a polymer phase with a small amount of ethylene (or ethylene with vinyl acetate) dissolved in it, and a gaseous phase, which is a solution of ethylene (mixture ethylene with vinyl acetate) with a small amount of high molecular weight (co) polymer. At high (more than 10 m / s) velocities of the reaction medium, the polymer phase breaks off the walls of the tubular reactor and is transferred by the reaction stream to the site of intense (co) polymerization, where it is mixed with a relatively low molecular weight polymer and forms a single-phase polymer-ethylene or copolymer-ethylene-polymer system vinyl acetate. At low (5-10 m / s) speeds of the reaction mass, the high molecular weight (co) polymer precipitates on the walls of the reactor, worsens the conditions for heat transfer from the coolant to the reaction mass, and subsequently, as it accumulates, and travels to the intense ( co) polymerization creates the conditions for heterogeneous (co) polymerization. (Co) polymerization under heterogeneous conditions leads to a deterioration in the quality of the product and to periodic thermal decomposition of the reaction medium in the reactor. To increase the stability of the polymerization process at low speeds of the reaction mixture, a pulsating pressure maintenance mode is used - periodically, after 60-100 s, the pressure at the beginning of the reactor is sharply reduced by 200-300 atm for a short time. This corresponds to a pressure drop at the end of the reactor to 300 - 400 atm. In this case, due to an increase in the speed of the reaction mixture, the polymer melt from the walls of the reactor breaks down and is transferred to the site of intense (co) polymerization. This technique makes it possible to increase the stability of the process, but leads to a deterioration in the properties of the obtained product, insofar as pressure drop by 200-300 atm and, associated with the discharge, the effect of lowering the temperature causes the reaction mixture to stratify and transfer the synthesis reaction to heterogeneous conditions. In addition, such deep pressure drops with a short period between the discharges adversely affect the life of the reaction equipment.

 The introduction of acyclic compounds into the reaction mass promotes homogenization of the reaction mixture and improves heat transfer conditions, as is rightly noted in the description of USSR patent N 1838330. However, the addition of n-paraffins also has negative consequences: a film made from polyethylene obtained in the presence of n -paraffins, has increased turbidity and low strength. This is due to insufficient homogenizing ability and reactive inertness of n-paraffins. More effective is the use of branched acyclic compounds that have a relatively mobile hydrogen atom at the tertiary carbon atom and, therefore, under the conditions of (co) polymerization of ethylene are chain transfer agents. The homogenizing ability of such compounds with respect to the ethylene-polyethylene system is also higher.

As branched acyclic compounds having a number average molecular weight of 300-3000 and 230-400 CH ₃ groups per 1000 carbon atoms, hydrocarbons obtained by thermal destruction of ethylene-propylene copolymers can be used. The introduction of branched acyclic compounds into the reaction mass of a homogenizing additive makes it possible, along with stabilization of the process, which is expressed in a decrease in the depth of discharges and an increase in the period between discharges, to improve the quality parameters of the resulting product — turbidity and film strength in the longitudinal and transverse directions.

In addition, the introduction of branched acyclic hydrocarbons into the reaction mass allows increasing the specific heat load up to 30000-50000 kcal / m ² • hour in the reactor section corresponding to the mode of slow-flowing (co) polymerization, which is impossible for modern tubular reactors having a diameter of 16-70 mm (specific heat load for them is in the range of 15000-28000 kcal / m ² • hour). An increase in the specific heat load for the sluggish (co) polymerization section also helps to stabilize the process, reduce the residence time of the reaction mixture in this section to 5-10 s and improve the quality of the product.

 The invention is illustrated by the following examples.

 Example 1

At the entrance of a single-zone tubular reactor with a length of 225 m and with an inner diameter of 16 mm, a stream of ethylene in the amount of 2700 kg / h containing 0.00027 wt.% Oxygen and 0.1 wt.% Homogenizing is preheated to 160 ^o C under a pressure of 2000 atm. hydrocarbon additives, representing a mixture of branched acyclic compounds of the paraffin series with a number average molecular weight (M _n ) of 1,500 and 300 CH ₃ groups per 1000 carbon atoms. A homogenizing additive is prepared by thermal degradation of a high molecular weight ethylene-propylene copolymer at a degradation temperature of 390 ^° C. and a residence time of the copolymer in the degradation zone of 10 s.

The mass ratio of the reaction mass to the homogenizing hydrocarbon additive is 1: 0.001. The additive is introduced into the reaction mass at the stage of ethylene compression by a special pump, while the additive performs the function of lubricating the pump plunger pair. The average speed of the reaction mass in the reactor is 7.5 m / s; its residence time in the reactor is 30 s. Using a control valve located at the end of the reactor, the depth of periodic pressure drops at the beginning of the reactor is set to 100 atm with a period between releases of 130 s. The specific heat load in the reactor section corresponding to slow polymerization is 40,000 kcal / m ² • h, the maximum temperature in the reactor is 300 ^o C, the ethylene conversion in one pass is 15.4 wt.%. The mixture of the formed polyethylene and unreacted ethylene is cooled to a temperature of 280 ^o C and through the control valve out of the reactor into the system for separating polyethylene from ethylene.

 The properties of the polyethylene obtained in example 1, as well as the (co) polymers obtained in the following examples, are shown in the table.

 Examples 2-3.

 The experiments are carried out under the conditions of example 1, but in example 2 the speed of the reaction mass in the reactor is 5 m / s, and in example 3 - 10 m / s.

 Examples 4-5.

 The experiments were carried out under the conditions of example 1, but the mass ratio of the reaction mass to the homogenizing hydrocarbon additive in example 4 was 1: 0.0003, and in example 5 it was 1: 0.002.

 Examples 6-7.

The experiments are carried out under the conditions of example 1, but the pressure in the reactor is 2500 atm, the number average molecular weight (M _n ) of the homogenizing hydrocarbon additive in example 6 is 300, and in example 7 to 3000.

 Examples 8-9.

The experiments were carried out under the conditions of example 1, but in example 8, acyclic compounds contain 230 CH ₃ - groups per 1000 carbon atoms, and in example 9 - 400.

 Examples 10-11.

 The experiments are carried out under the conditions of example 1, but the depth of periodic pressure drops in example 10 is 70 atm, and in example 11 - 150 atm.

 Examples 12-13.

 The experiments are carried out under the conditions of example 1, but the period between discharges in example 12 is 100 s, and in example 13 - 200 s.

 Examples 14-15.

The experiments are carried out under the conditions of example 1, but the specific heat load on the reactor section corresponding to the mode of slow polymerization in example 14 is 30,000 kcal / m ² • hour, and in example 15 - 50,000 kcal / m ² • hour.

 Example 16

The experiment is carried out under the conditions of example 1, but the reaction mass additionally contains 5 wt. % vinyl acetate, the pressure in the reactor is 1800 atm, the maximum temperature in the reactor is 268 ^o C, the depth of periodic pressure drops is 70 atm, the mass ratio of the reaction mass to the homogenizing additive is 1: 0,0003.

 Example 17 (control).

 The experiments are carried out under the conditions of example 1, but the speed of the reaction mass is beyond the claimed limits and is 3 m / s. At this speed, the process becomes unstable, explosive decomposition of the reaction mixture periodically occurs.

 Example 18 (control).

The experiment is carried out under the conditions of example 1, but the speed of the reaction mass is outside the claimed limits and is 10 m / s. At this speed, the temperature maximum shifts to the end of the reactor, as a result of which the temperature of the reaction mass removed from the reactor reaches 310 ^o C. Since the throttling of the reaction mass from 2000 atm to 250 atm due to the throttle effect increases the mass temperature by ~ 20 ^o C, this leads to to its thermal decomposition in the system for separating polyethylene from unreacted ethylene.

 Examples 19-20 (control).

 The experiments are carried out under the conditions of example 1, but the mass ratio of the reaction mass to the homogenizing hydrocarbon additive is outside the claimed limits and is in example 19 - 1: 0,0002, and in example 20 - 1: 0,003. The polyethylene obtained in example 19 has an increased amount of gel-like inclusions (see table), and the polyethylene obtained in example 20 has an increased content of extractable substances in the polymer, which leads to a deterioration in its sanitary and hygienic properties and processing conditions.

 Examples 21-22 (control).

The experiments are carried out under the conditions of example 1, but the amount of CH ₃ groups in the homogenizing additive is outside the declared limits and in example 21 is 190 per 1000 carbon atoms, and in example 22 is 500 CH ₃ groups per 1000 carbon atoms. In example 21, the process is unstable, the temperature of the reaction mixture increases periodically, leading to its thermal decomposition, and the polyethylene obtained under the conditions of example 22 has an increased melt flow rate and a lower density.

 Example 23 (control).

The experiment is carried out under the conditions of example 1, but the number average molecular weight (M _n ) of the homogenizing hydrocarbon additive is outside the declared limits and is 200. The polyethylene obtained using such an additive has an increased amount of gel-like inclusions (see table), due to its insufficient homogenizing ability.

 Examples 24-25 (control).

 The experiments are carried out under the conditions of example 1, but the depth of the periodic pressure drops beyond the declared limits and in the example is 24 - 200 atm, and in example 25 - 50 atm. The polyethylene obtained in example 24 has deteriorated quality indicators for turbidity and the number of gel-like inclusions (see table), and the experience in the conditions of example 25 is characterized by an unstable synthesis process, a rise in temperature at the beginning of the section corresponding to the intensive polymerization mode, which leads to subsequent explosive decomposition of the reaction mixture.

 Example 26-27 (control).

 The experiment is carried out under the conditions of example 1, but the period of periodic pressure drops goes beyond the declared limits and in example 26 - 80 s, and in example 27 - 250 s. The polyethylene obtained in example 26 has a deteriorated rate of turbidity and the number of gel-like inclusions. The experiment conducted in the conditions of example 27, is characterized by an unstable synthesis process, leading to explosive decomposition of the reaction mixture.

 Example 28 (control).

The experiment is carried out under the conditions of example 1, the heat load on the site of the reactor, corresponding to the mode of slow polymerization, goes beyond the stated limits and is 15,000 kcal / m ² • hour. At this load, the length of the reactor section corresponding to the mode of slow polymerization increases, the maximum temperature shifts to the end of the reactor, which leads to its thermal decomposition in the system for separating polyethylene from unreacted ethylene.

 Example 29

At the entrance to the first zone of a two-zone tubular reactor having a first zone length of 150 m and an inner diameter of 16 mm, the first part of the ethylene stream, preheated to 160 ^° C., is supplied at a pressure of 2200 atm in the amount of 1800 kg / h. The ethylene stream contains oxygen in an amount of 0.0026 wt.% And a homogenizing hydrocarbon additive in an amount of 0.1 wt.% (The mass ratio of the reaction mass to the homogenizing additive is 1: 0.001, the amount of CH ₃ groups is 300 per 1000 carbon atoms, viscosity additives 130 cP). The homogenizing additive is introduced at the stage of ethylene compression with a special pump. The second part of the ethylene stream in an amount of 1800 kg / h and a temperature of 100 ^o C serves under the same pressure in the second zone of the reactor, which has a length of 150 m and an inner diameter of 20 mm The temperature of the reaction mass after mixing the ethylene flows is 200 ^° C. The oxygen content in the second part of the ethylene flow is 0,00033 wt.%, The content of the homogenizing additive is 0.1 wt. % The speed of the reaction mass in the first zone is 5 m / s, in the second zone 6.4 m / s (respectively, the residence time in the first zone is 30 s, and in the second zone 23 s). The depth of periodic pressure drops at the beginning of the reactor is set to 100 atm with a period between releases of 130 s. Using a heat carrier, a specific heat load is established in the area of the first reactor zone corresponding to the slow-flow polymerization mode, 35,000 kcal / m ² • hour. The maximum temperature in the first zone of the reactor is 310 ^o C, in the second zone - 300 ^o C. The conversion of ethylene in one pass through the reactor is 20.6 wt.%. The properties of the obtained polyethylene are given in the table.

 Analogously to example 29, it is possible to carry out the process of (co) polymerization in a tubular reactor consisting of three or more zones.

 Example 30 (control).

 The experiment is carried out under the conditions of example 1, but n-dodecane is used as a homogenizing hydrocarbon additive. Polyethylene obtained in the presence of dodecane has a deteriorated rate of turbidity and tensile strength in the longitudinal and transverse directions.

 Example 31 (control).

 The experiment is carried out under the conditions of example 1, but a homogenizing hydrocarbon additive is not introduced into the ethylene stream. The depth of the periodic pressure drops at the beginning of the reactor is set to 250 atm with a period between releases of 60 s.

The specific heat load at the reactor site, corresponding to the mode of slow flow polymerization, is 15,000 kcal / m ² • hour.

The conversion of ethylene in one pass is 15.4 wt.%. The polyethylene obtained in this example (see table) has poor optical performance (film haze of 39%), reduced film strength and an increased number of gel-like inclusions (3 pcs per m ² ), which reduces the consumer properties of products made from it.

It should be noted that the following experiments were not carried out with process parameters that went beyond the claimed limits:
- experience using hydrocarbons with a molecular weight of more than 3000 as a homogenizing additive, since special equipment is required to supply such an additive to the reactor;
- experience with a heat load in the slow polymerization section above 50,000 kcal / m ² • hour, since it requires a coolant with temperatures above 230 ^o C. Special equipment is also required to prepare a coolant with such a high temperature.

 Thus, the proposed method for the production of ethylene polymers and its copolymers with vinyl acetate provides the necessary stability of the synthesis process of (co) polymers, which makes it possible to reduce the length of the tubular reactor to 225-300 m, and therefore, to reduce material costs when implementing the method compared to the prototype.

Claims (2)

1. A method of producing ethylene homopolymers or copolymers of ethylene with vinyl acetate, comprising (co) polymerizing ethylene in a heated single or multi-zone tubular reactor at a pressure and temperature, initiating a reaction (co) polymerization with oxygen supplied in a mixture with ethylene to each zone of the reactor, introducing in the reaction mass in front of the reactor additives and separation of the obtained (co) polymer from unreacted ethylene or a mixture of ethylene with vinyl acetate, characterized in that (co) polymerization is carried out at a pressure of 1400 - 3000 atm, soon the flow rate of the reaction mixture is 5–10 m / s using periodic pressure drops in the solution at a pressure drop depth at the beginning of the reactor of 70–150 atm and a period between 100–200 s of discharge, and a homogenizing hydrocarbon additive, which is a branched acyclic, is used as an additive compounds of the paraffin series with a number average molecular weight (M _n ) of 300 to 3000 and an amount of 230 to 400 CH ₃ groups per 1000 hydrocarbon atoms with a mass ratio of the reaction mass to the homogenizing additive of 1: 0,0003 - 1: 0.002, while By separating the obtained (co) polymer, the reaction mixture is cooled. 2. The method according to p. 1, characterized in that the process of (co) polymerization at the beginning of a single-zone reactor or at the beginning of the first zone of a multi-zone reactor is carried out at 160 - 200 ^o C, and in subsequent sections of the single-zone and multi-zone reactors is carried out at 200 - 320 ^o C, while the specific heat load for the portion of the zone of the reactor with a temperature of 160 - 200 ^o C support in the range of 30,000 - 50,000 kcal / m ² • h.

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