DE2814650C2 - Production of homo- or copolymers of ethylene by the high pressure process in a stirred autoclave with practically complete backmixing - Google Patents

Description

a) two such autoclaves are connected in series in such a way that the entire reaction product of the first autoclave in the second Is transferred to an autoclave,

b) the monomer (s) feed stream is divided between the two autoclaves in such a way that

(1) in the case of an operating mode that is different from one another in the two autoclaves Temperature, concentration and / or pressure conditions of the monomer (s) feed stream to the first autoclave is equal to or greater than the monomer (s) feed stream to the second autoclave or

(2) in the case of the same operating conditions in both autoclaves, the monomer (s) feed stream can be divided as required,

c) d ^: e reaction temperatures in both autoclaves are set so that

(1) in the case of an operating mode that is different from one another in the two autoclaves Temperature and concentration conditions, the reaction temperature in the first autoclave is lower than the reaction temperature in the second autoclave or

(2) in the case of the same operating conditions in both autoclaves, the reaction temperature in the first autoclave equal to the reaction temperature in the second autoclave is,

d) the initiator for the first autoclave is the same or different from the initiator for the second autoclave and the amount of initiator used in each autoclave is greater than 0.

2. The method according to claim 1, characterized in that in case b) (l) the reaction temperature in autoclave 2 is greater than the reaction temperature in autoclave 1.

3. The method according to claim 1, characterized in that in case b) (l) the monomer (s) feed stream to the first autoclave is 50-99% of the total amount.

4. The method according to claim 1, characterized in that in case b) (l) the reaction pressure im Autoclave 2 is equal to or lower than the reaction pressure in the autoclave 1.

5. The method according to claim 1, characterized in that in case b) (2) the ratio of Monomer (s) feed flows to the first and second autoclaves equal the ratio of the reaction volumes from the first and second autoclaves.

The large-scale processes for the production of high pressure polyethylene differ essentially only in the design of the reactors. The reactors are generally divided into tubular reactors according to their construction principle and (stirring) autoclaves.

Tube reactors are characterized by a very large L / D ratio (L = length, D = diameter), with values between 10,000 and 50,000 being the rule. As a result of this geometry, the backmixing of the reaction mixture in such reactors is extremely low, ie a very close approximation of a plug flow is achieved.

The stirred autoclaves in high pressure polyethylene processes generally have an L / D ratio of <20. Backmixing in such autoclaves is significantly influenced by the L / D ratio. With! / © ratios <3-4 and with an appropriate design of the stirrer, practically complete backmixing is achieved. On the other hand, backmixing can be deliberately limited in the case of larger L / D ratios and with suitable internals. Such autoclaves and their backmixing characteristics are known from Ullmann's Enzyklopadie der Technisehen Chemie, 4th Edition, Volume 2, page 263 (1972) and Volume 3, page 323 (1973).

The degree of backmixing is of great importance from two points of view: Operational safety and product properties.

Because of the extreme reaction conditions in the production of high pressure polyethylene (pressure range normally 150-30OMPa, temperature range 150-300 ⁰ C), the high heat of polymerization of ethylene (approx. 3.3 MJ / kg) and its tendency to be at high pressures and temperatures above To decompose 300 ⁰ C explosively (see Chemical Engineering Science. 1973, Vol. 28, 1505-1514), the operational safety in high pressure ethylene processes is of paramount importance.

The good mastery of such extreme reaction conditions is therefore an essential contribution to Operational safety. High operational reliability also means, for example, that the number of note voltages into the atmosphere and the associated dangers and consequences, such as ignition of the ethylene-air mixture, The environmental impact of noise and pollutants can be significantly reduced. In addition to effective control devices, like sophisticated temperature and pressure control circuits, is primarily an intensive one Mixing of the reaction space helps to control the reaction because it is fast and even Distribution of all reactants causes. Thus, possible inhomogeneities in the reaction space that in the case of incomplete mixing caused by metering fluctuations and / or flow conditions can be switched off from the start, and the formation of so-called hot zones, the starting point for the explosive decomposition of ethylene, avoided.

According to the prevailing opinion, a reaction system exhibits practically complete backmixing when the mixing time, ie the mean duration of a mixing cycle, is one tenth or less of the mean residence time of the reaction mixture. In US Pat. No. 2,897,183, a reaction system is considered to be completely backmixed (constant environment reaction system) if the average number of cycles of the reaction mixture N _c according to the equation N _r

= GJG) is between 30 and 200, where G _{r is} the circulated amount (kg / h) and G _{is the amount of monomer (s) used (kg / h).

It is known that the application properties of a high-pressure polyethylene, apart from melt index and density, which are normally set by the reaction parameters pressure, temperature and molecular weight regulator concentration, also depend on molecular parameters such as long chain branching λ and

Inconsistency U = - -1 (Mw = weight. Mn

Mn

= Number average molecular weight) are significantly influenced for the other than the reaction parameters mentioned Reaction states are responsible that the homogeneity or inhomogeneity of the Reaction system are to be assigned (cf. Die Angewandte Makromolekulare Chemie 40/41 [1974] 361-389 [No. 586]). Such reaction states are characterized by the presence or absence of temperature, concentration and pressure profiles during the course of the reaction. Temperature- and concentration profiles are mutually dependent in an adiabatic reaction and the consequence of an incomplete one Backmixing. The pressure profile is usually determined by the design of the reaction system given and inevitably results in tube reactions. With complete back-mixing (homogeneous Reaction system), none of the three profiles is possible.

The relationship between temperature, concentration and pressure profile on the one hand and the product properties on the other hand is given by the fact that all reaction steps involved in the polymerization, such as start, growth, transfer and termination reactions, are temperature, concentration and pressure dependent in different ways . For example, the long-chain branch λ, which requires the interaction of a radical with a polymer chain, is favored by a high polymer concentration. Consequently, under comparable reaction conditions (pressure, temperature, same final conversion) in a (homogeneous) reaction system with constant polymer concentration (= final concentration), more long chain branches are generated than in an (inhomogeneous) reaction system with a polymer concentration increasing from a (lower) initial value to the final value. Despite the same Schmelündex, both products differ in their rheology, crystallization ability, melt relaxation and other properties.

For certain areas of application, e.g. B. melt coating of paper, cardboard, metal foils, etc., products with high values for λ and U are advantageous because their relaxation behavior results in low values for the constriction of the melt tail after leaving the extrusion plant (so-called "neck-in" values). For this purpose, a practically complete backmixing through the use of autoclaves with a small L / D ratio as in US Pat. No. 2,897,183 or other supporting measures, such as a turbulent flow in a "slim" autoclave through ethylene flowing in from below, as in US Pat 36 92 763, required. For other areas of application, e.g. B. Bubbles of film, products with low λ and t / values are advantageous because they have good theological properties, pull-out capabilities and ι optical properties.

This comparison shows the influence of backmixing in the reaction system on the product properties. As a result of the embodiment, backmixing is practical in tubular reactors prevented. In the case of autoclaves, the extent of backmixing depends essentially on their geometry and their internals.

Albeit a practically completely backmixed reaction system from the point of view of operational safety deserved preference, the arguments for

ι a non-backmixed reaction system.

A number of proposals result from this knowledge (US-PS 3178 404, 35 36 693, 35 75 950, 36 92 763,38 75 128, BR-PSlO 71 305, BE-PS 71 03 92), which aim at certain product properties

ι to improve that the polymerization in two or more reaction zones with different (increasing) temperature (temperature profile) carried out will. This is mainly due to the embodiment of the reactor, e.g. B. with the help of internals and achieved special stirrer constructions that create individual reaction zones between which no backmixing takes place is allowed. In US-PS 31 78 404 is to achieve the temperature zones a two-zone reactor without backmixing

ι proposed as a preferred solution between the zones and also indicated the possibility of using an elongated tubular reactor or separate reactors. The use of initiators with different levels of activity and adapted to the respective temperatures for the various reaction zones is expedient. In US-PS 31 78 404 the use of caprylyl peroxide is claimed for the reaction zone with a lower temperature (130 to 190 ° C). In the BR-Pi; 10 71 305 describes the combination of two slim autoclaves (L / D 11: Ibis 20: 1) which are operated under very different reaction conditions in such a way that the (high molecular weight) reaction mixture of the first autoclave enters the lower zone (mixing zone) of the second autoclave ("Wax reactor") is performed. In US-PS 38 75 128 (equivalent to DE-OS 23 22 554) the combination of two or more slim autoclaves (L / D 5 -20) with cooling units located between the autoclaves is claimed in order to increase the conversion of the polymerization. The resulting considerable pressure losses and deposits in the coolers are accepted.

However, the advantages of these proposals are offset by two major disadvantages, those of the loss are caused by practically complete back-mixing:

1. Loss of operational safety, for example due to the formation of hot spots in the polymerization mixture;

2. Loss of the ability to manufacture products whose optimum properties are practically complete Back mixing of the reactor is achieved.

It has now been found that both the desirable product properties, such as those with tubular reactors e. are targetable, as well as the process and product-specific advantages of a process with practical Complete backmixing can be achieved if you have two autoclaves of a special design and operates in series with practically complete backmixing.

The present invention is a continuous

A detailed process for the production of polymers of ethylene in an autoclave with practically complete backmixing due to a length-to-diameter ratio (L / D) between 1: 1 and 3: 1 at pressures of 80 to 300 MPa in the presence of free radical initiators, which is characterized by this , that

a) two autoclaves are connected in series in such a way that the entire reaction product of the first autoclave is transferred to the second autoclave,

b) the monomer (s) feed stream is divided between the two autoclaves in such a way that

(1) in the case of an operating mode with different temperature, Concentration and / or pressure conditions of the monomer (s) feed stream to the first autoclave equal to or greater than is the monomer (s) feed stream to the second autoclave or

(2) in the case of the same operating conditions in both autoclaves, the monomer (s) feed stream can be divided as required,

c) the reaction temperatures in both autoclaves are set so that

(1) in the case of an operating mode with different temperature values in both autoclaves and concentration conditions, the reaction temperature in the first autoclave is lower than the reaction temperature in the second autoclave or

(2) in the case of the same operating conditions in both autoclaves, the reaction temperature in the first autoclave is the same as the reaction temperature in the second autoclave,

d) the initiator for the first autoclave is identical to or different from the initiator for the second Autoclave and the amount of initiator used for each autoclave is greater than 0.

This reaction system allows, depending on the objective, to set operating conditions, either one Temperature, concentration and pressure profile or a reaction system with practically complete Backmixing. In addition, it is possible to switch from one operating status to another, d. H. from the operating conditions with temperature, concentration and pressure profile to the operating conditions a practically completely backmixed reaction system or vice versa pass over without having to shut down the system or interrupt the reaction.

It has been shown that the reaction state corresponding to practically complete backmixing can also be achieved without alternative piping for parallel connection of the autoclaves can; this leads to disadvantages and risks, such as the installation of valves and the temporary shutdown of parts of the pipeline and the associated dangers through dead spaces, cumbersome manipulations during changes and the associated malfunctions, eliminated from the start.

The high operational reliability of the reaction system described in comparison to multi-chamber or tubular reactors is due to both the practically complete backmixing and the individual Control and protection devices of each of the two autoclaves are guaranteed and are available to those of one well mixed single autoclaves.

The method according to the invention is thus characterized by a high degree of flexibility with regard to the properties that can be produced therewith Product quality, high operational safety and thus environmental friendliness, as well as good handling the end. Its mode of operation will be explained in more detail with reference to FIG.

ίο The use of ethylene E in Fig. 1, which can contain comonomers such as vinyl acetate, butene-1 and others, and molecular weight regulators such as hydrogen, ethane, propane and others, and depending on the reaction requirements, a pressure of 80 to 300 MPa and a temperature

π is from -20 ⁰ C to 100 ° C, is divided by a ratio control 3 in the partial flows Q 1 and Q that go to the autoclave. 1 and 2 (Λ / denotes the stirrer drives.) The reaction mixture from autoclave 1 goes via connecting line 4 into autoclave 2, the reaction mixture from autoclave 2 via pressure regulator 5 into separating devices for separating the reaction mixture. The initiators / 1 and / 2, which are fed into the substreams Q 1 and Q 1 immediately before they enter the autoclaves 1 and 2, respectively, are adapted in terms of type and amount to the respective reaction requirements.

Depending on the desired product properties, the entire reaction system can be operated either in the sense of a temperature and concentration profile or in the sense of a practically completely backmixed reaction system. For a reaction system with a temperature and concentration profile, the substreams Q 1 and Q 2 are set so that Q1> Q 2, and the reaction temperatures Tl and Tl (in autoclave 1 and 2) are selected according to the requirements, where Tl> Tl is. The type and amounts of initiators / 1 and II are adapted to the reaction temperatures T1 and T2, respectively. The product property changes that can be achieved through the temperature and concentration profile

4 (1 ments are more pronounced, the greater the difference between the reaction temperatures 7Ί and Tl and the smaller the ratio QlIQl of the two substreams. This relationship and the influence of the conversion profile on the product properties is illustrated in Fig. 3. Preferred temperature ranges for Tl and Tl are 120 ⁰ C to 220 ° C and 190 ⁰ C to 280 ⁰ C. A change in the temperature difference between Tl and Tl or a change in the ratio of the partial flows affects the property

ΐη image in different ways. For a reaction system with practically complete backmixing, the reaction temperatures F1 and Tl are brought to the same value. The division of the partial flows Q 1 and Q1 is zwarunkritisch in this case, it is preferable in view of a maximum initiator use, however, selected so that the ratio Q / Q is equal to the volume ratio Vl / Vl is the autoclave. The initiators / 1 and II are then normally chemically identical.

The flexibility of the method according to the invention can still be achieved through numerous possible variations be expanded. For example, the substreams Q1 and Q2 can be cooled differently or be heated in order to additionally influence the turnover profile. Through such measures, the Differences in properties in autoclave 1 and autoclave which can be influenced with temperature and conversion profile 2 resulting products or their proportions

nis, can also be varied. To reinforce the differences in properties in the two autoclaves The method according to the invention can also be produced with an adjustable pressure gradient between two autoclaves (pressure profile) are operated (not shown in Fig. 1, the pressure in first autoclave is higher than the pressure in the second autoclave. All possible variations mentioned can be used either on their own or in any combination with one another.

The following examples are intended to explain the process according to the invention in more detail. For all examples will be two practically completely mixed stirred autoclaves are used, which correspond to that shown in FIG Scheme are interconnected.

In Examples 1-5 and in Comparative Experiment A, two autoclaves of the same size with an L / D ratio of 2.6 are used. In Examples 6-10 and in Comparative Experiment B, the first autoclave has a volume of 0.725 1 and an L / D ratio of 1.1, the second autoclave has a volume of 0.325 1 and an L / D ratio of 1.0 . Only the first autoclave is used in each of the two comparative tests. The amount of ethylene E used is kept constant and is 120 kg / h in Examples 1-5 and in Comparative Experiment A, and 13 kg / h in Examples 6-10 and in Comparative Experiment B. The other test data and the results are shown in the following tables.

example 1

This example shows that the method according to the invention can be operated in the sense of a practically completely backmixed reaction system. A comparison of the product properties listed in the summary table of Example 1 and the comparative experiment A confirms the equivalence of the example 1 and comparative experiment A with products manufactured by different technologies.

Examples 2-5

ίο These examples show the influence of different ones Reaction temperature gradations and feed partial flow ratios on the product properties.

Examples 6-10

These examples show that with the method according to the invention under operating conditions that a Temperature / conversion profile result, certain product properties are obtained, which are significantly of

> o differ from those with only one autoclave can be achieved and that these product properties are surprisingly close to those with a Tube reactor can be achieved. In Fi g. Figure 2 illustrates this using the relationship between extrusion pressure and melt index in which the values of commercial products manufactured in tubular reactors are compared with the values of commercial products that come from practically completely backmixed single-autoclave processes originate as well as with the values of the

jo Examples 6-10 and 5. As you can see, the values are of these examples in line with the values of the products from tubular reactors.

Response parameters ⁰ C ° C Tr ₂ ') pressure Ql ² )
E. Initiator') / 2 sales
share R 1 MG controller
conc. ⁴ ) Example or T _F ^X )
Comparative experiment 20th 260 ° C MPa /1 - % Mol% 20th 260 - 180 1.0 PN PN 100 0.9 Pr Comparison
attempt A 20th 260 260 180 0.5 PN PN 50 0.9 Pr 1 20th 210 260 180 0.6 PN PN 60 0.5 Pr 2 20th 160 260 180 0.6 PO PN 48 1,4Pr 3 20th 160 26Q 180 0.6 PD PN 35 1.9 Pr 4th 40 160 260 180 0.95 PD PZ 55 2.7 Pr 5 40 160 250 145 1.0 PL PZ 57 1.5 Cy 6th 40 160 250 186 1.0 PL PZ 57 1.1 Pr 7th 85 170 250 186 1.0 PL PZ 57 2.5 Cy 8th 43 160 250 186 1.0 PL PZ 52 0.7Pr 9 31 217 280 186 1.0 PL - 49 UCy 10 - 186 1.0 PB 100 UCy Comparison
attempt B

') Te, Tm, T _R 2 mean use or reaction temperatures in autoclave 1 and 2, respectively.

² ) Ql = partial flow of feed to autoclave 1; E = total input flow.

³ ) PN = t-butyl mononate; PO = t-butyl peroxtoate; PD = t-butyl perneodecanoate; PL = t-butyl perpivalate; PZ = di-t-butyl peroxide; PB = t-butyl perisobutyrate

⁴ ) Pr = propylene, Cy = cyclohexane; External regulator content of the feed stream 0.05 mol%.

ίο

Product features

Example or
Comparative experiment

density

g / 10 min. g / cm ³

Expansion of the melt strand ⁵ )

Extrusion pressure ⁶ )

shine
at 20 °

Extensibility

μηι ⁹ ) or
m / min

λ ¹ )

Surface roughness ⁸ )

μΠΊ

Comparative experiment A

1
2
3
4th
5
6th
7th
8th
9
10

Settlement "request B

5.1

5.0

1.79

1.81

1.80

1.81

0.36

0.63

1.95

0.19

0.23

2.0

0.917

0.917 0.917 0.919 0.919 0.921 0.923 0.924 0.929 0.922 0.923 0.928

136

137 133 130 128 125 114 114 116 103 99 114

0.186

12.6 - - 16.4 - 90 15.2 10 45 14.0 23 30th 13.8 48 25th 19.8 1.5 4.6 18.3 25th 7.6 14.5 21 15th 22.8 16 4.6 18.6 - 3.7 16.9 42 12th

1.6

0.186 1.5 0.213 2.2 0.190 1.2 0.178 0.7 0.155 0.5

"Ί With A // measurement (» melt-swell «, large values mean small constriction values (» neck-in «) for flat film extrusion).

⁶ ) Measured under standard conditions in an extrusion meter in Examples 1-5 and during the manufacture of blown film in Examples 6-10.

⁷ ) long chain branching per 1000 carbon atoms.

^s ) Measured under standard conditions on a string from the high pressure capillary viscometer.

^g ) In Examples 2-5, the film pull-out capacity was measured in µm; In Examples 6-10 and Comparative Experiment B, the take-off speed in m / min up to the break of the strand was measured in the extrusion meter test.

For this purpose 3 sheets of drawings

Claims (1)

Claims; 1. Continuous process for the production of homo- or copolymers of ethylene in autoclaves with practically complete backmixing due to a length-to-diameter ratio (L / D) between 1: 1 and 3: 1 at pressures of 80-300 MPa in the presence of free radical initiators , characterized in that