DD152795A1 - METHOD FOR RELAXING HIGH PRESSURE SYSTEMS IN AETHYLENE POLYMERIZATIONS - Google Patents

Abstract

The invention relates to a process for the safe relaxation of reaction systems in the homo- and copolymerization of the ethylene under pressures above 50 MPa as a result of exceeding predetermined pressure and / or temperature limits. The training imweltbelastender and explosionsfaehiger relaxation mixtures should be avoided and an economic utilization of the raw materials can be achieved. The resulting in the Notentspannungen of such reaction systems flash mixture is initiated within 2 to 15 seconds to 5 to 50% of the reaction pressure in a container opposite to the atmosphere completed, where it is divided into a directly in the free volume of the container expanding polymer low expansion gas content and a polymer-rich gas content so that the mitentspannte polymer content is deposited and discharged and the gas fraction is initially stored and finally a recovery or torch supplied. This process avoids gas emissions into the atmosphere, which can lead to the formation of explosive gas / air mixtures, thus contributing to the increase of plant safety. In addition, the expansion gas collected in the flash tank can be worked up and reused.

Description

VBB Leuna-Werke. Leuna, 11 "06.1980

HWaIter UlbrichtIl

LP 8015

Title of the invention

Method for the expansion of high-pressure systems in Aethylenpolyraeri sat ions

Field of application of the invention

The invention relates to a process for the safe relaxation of reaction systems in the homo- and copolymerization of ethylene at pressures above 50 MPa, in particular in tubular reactor plants.

Characteristic of the known technical solutions

The radical homo- and copolymerization of the ethylene is carried out industrially at temperatures above 373 K and pressures above 50 MPa both in stirred autoclaves and in tube reactors.

It is known ö.ass due to the high Reaktionswaerme the Polymerisaticnereaktion (ca _o 3000 kJ / kg Polyethylene) low Unregelmaescigkeiten in the Litigation, such as initiator overdose, reduced by .the Waerraeabfuhr Reaktorvv'and as a result of 'Wandbelaegen or insufficient throughput

223 50 1

Mixing of the reaction system to exceed these limits and thus, lead to the release of decomposition reactions; , · ·

Puer Roiir reactors are known to realize the required emergency releases so quickly that material-causing effects are achieved with a special arrangement of pressure relief devices in the form of programmable fast voltage valves along the reactor, correspondingly short response times and a certain cross section of the purge effluents relative to the reactor volume in the event of decomposition reactions due to high pressure and temperature increases do not occur and, due to the fast Entsp.annungsVorgang immediately after exceeding predetermined pressure and temperature limits in the reactor by means of these devices, the attack of decomposition products, which lead to contamination of the reaction system is effectively prevented (DD -PS 118 288).

By means of this procedure, although the dangers resulting directly from the occurrence of decomposition reactions are avoided with certainty for the reaction system, the possibility of premelting of the expansion gas cloud in the atmosphere remains as a safety risk.

As a source of ignition for the Fremdzuendung after sedimentation of the flue gases mat air to an ethylene content in the field of explosion limits, the electrostatic charge entrained in the relaxation Schmelsetroepfchen, lightning and in petrochemical plants located in the surrounding pack flames into consideration. This endangerment is further increased by the fact that the expansion gas cloud, which mainly consists of ethylene, can remain in the explosive region of the ethylene-air mixture for a relatively long time and uncontrollably due to the small difference in density between ethylene and air in corresponding weather conditions.

In addition, the relaxation of large amounts of ethylene into the atmosphere creates a significant environmental impact

the area surrounding the plant dar.

It is more well known that the partial tension of the hot reaction mixture of tube reactors is filled into a container partially filled with water for the separation of the polymer fraction contained in the form of melt. Cooling of the hot flue gas takes place below 373 K and so cooled, only. still residual amounts of polymer fractions containing gas is released into the atmosphere (DE-OS 2,631,834). This procedure represents an improvement over the direct release into the atmosphere in that the polymer portion is separated from the expansion gas. However, the danger remains here that the expansion gas cloud forms an explosive mixture in the course of mixing with the air can be reacted

Similarly, the environmental impact of relaxing in the atmosphere can not be avoided «

This residual hazard is specific for flash gas clouds at tentο tentspannungen of tube reactor systems, since these largely consist of undecomposed ethylene, in contrast to those of stirred reactor plants.

Object of the invention

The object of the invention is to develop a process which permits safe emergency tapping of reaction systems for the homo- and copolymerization of the ethyl ether under high pressure, ie "above 30 MPa, in which the formation of an environmentally polluting and explosive mixture from flash gas and air in the atmosphere is avoided and the "recovery" mixture may optionally be recycled ",

Explanation of the nature of the invention

The object of the invention is to run through such Notentspannungen so that electrostatic charge and Fremdzuendung of such relaxation mixtures and agglomeration of the Polymeranteils.verhindert can be prevented.

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This object is achieved by the fact that the reaction mixture, which is depressurized by means of one or more depressurization lines, releases the pressure within 2 to 15 seconds to 5 to 50 % of the reaction pressure directly into the free volume of a container closed off from the atmosphere with separation is expanded in a specific light polymer low gas content and a specific heavy polymer-rich gas content, the polymer-rich gas fraction is fed to a limited formed in separator form part of the container and there is the polymer deposition.

Preferably, the polymer deposition takes place in such a way that the polymer-rich gas fraction meets the surface of an inert liquid, the. is presented in the demarcated formed in Abscheiderform area of the container.

Since in each case only the previously separated polymer-rich portion of the flash mixture is brought directly into contact with the liquid and. the polymer-poor fraction of the expansion mixture entering directly into the flash tank is cooled off under heating of the steel mass of the flash tank, a quantity of liquid of <50 % of the mass of the entire flash mixture is sufficient for the separation of solids. "

After the emergency release, the polymer fraction is separated with the liquid from the flash tank. The liquid is fed by means of a pump via a sprinkling ring again the flash tank and submitted to the polymer separation.

The inventive method of emergency relaxation in a closed relaxation system will be explained in more detail below with reference to the figure in its preferred embodiment as Kugeibehaelter and the polymer separation into a liquid. ,

The emergency release of the reaction system takes place via

or several program-controlled expansion valves 1. The expansion mixture is fed via one or more expansion lines 2 into the interior of the expansion tank 3. Due to the beam expansion and a special design of the inlet opening 4 in the filled with liquid cylindrical separator 5, the expansion mixture is slowed down by using gas-dynamic effects supersonic to subsonic speed and simultaneously in a polymer poor directly in the free volume of the container 3 expanding gas content and a polyme. rreichen in the separation device 5 entering gas fraction separated. The polymer fraction separated in the liquid is separated off via a separating device 6 and the liquid is returned to the container by means of the pump 7 via the sprinkling ring 8. The short-term stored in the container expansion gas is removed via the line 9 from the container and, depending on the circumstances of a refurbishment and re-evaluation or a torch supplied,

Ausfuehrungsbeispiele

example 1

Polyethylene is continuously produced in a tubular reactor at a pressure of 240 MPa

On the reactor, whose volume including the preheating and cooling zones 3.55 w. are four program-controlled. Quick-release valves are arranged to lower the pressure in the reactor in the event of inadmissible pressure and / or temperature increases., The temperature limits of the temperature measurements triggering the emergency release are set to 603 K and the pressure limit is set to 250 MPa. It is polymerized at maximum temperatures of 593 K. By an initiator overdosage, the maximum temperature of the second reaction zone exceeds the value of 603 K and triggers an emergency release.

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The four quick-release valves 1 open and within 2 s the pressure 'in the reaction system is lowered to 100 MPa. The emergency release temperature drops below the temperature limit of 603 K and the program-controlled "relaxation valves 1 close." In the course of the emergency release, a total of 373 kg of expansion mixture with an average polymer content of 15% by mass is introduced into a sealed container 3 via four expansion lines 2. The volume of the inserted ball-shaped Eatspannungsbehaelter 3 entraegt

800 m, The relaxation jets exit the expansion lines at a speed of 465 m / s and a temperature of 598 K. As a result of the beam expansion, 27 % of the gas expand directly into the gas space of the container 3, while the polymer-rich core jet is decelerated to a speed of 130 m / s, with a further 51 % of the gas fraction expanding into the flash tank.

The now still 22 % of the total amount of flash contained residual jet impinges on the surface of the water introduced in the separator 5 and the polymer precipitates as a finely divided pesticide in the water. The pressure in the expansion tank is 0.23 MPa at the end of the expansion process.

After completion of the relaxation, 56 kg of polyethylene are discharged from the container with the water. The amount of gas collected in the container of 315 kg consists of 91 % ethylene, 5 % propane, 3.5% methane and traces of hydrogen, ethane and other hydrocarbons. This gas is fed to a treatment plant.

Example 2

It is polymerized under the same conditions as in Example 1.

2 2 3 5 0 1

In contrast to Example 1, the emergency release is triggered simultaneously by exceeding the temperature limit value of 603 K in the "2nd reaction zone and the predetermined temperature limit value of 583 K at the inlet of the intermediate pressure product separator _β

In addition to the four expansion valves 1 of the reaction system open the arranged after the reactor pressure control valve three expansion valves.

The pressure in the reaction system and in the intermediate pressure product separator is lowered to a pressure of 10 MPa within 7 s. «

The emergency release temperatures drop below the limits and the expansion valves 1 close. During the emergency release, a total of 2,119 kg of expansion mixture having an average polymer fraction of 10% by mass is introduced into the closed expansion tank 3 via seven expansion conduits. As a result of the beam expansion, 75 % of the initial gas quantity enters directly into the expansion tank. The polymer-rich residual jet enters into the water-filled separator 5. There are 212 kg of polyethylene in finely divided form to ' _f discharged after the completion of the relaxation process with the water and separated. The pressure in the flash tank after relaxation is O _$ 54 MPa.

The 1 900 kg flash gas stored in the flash tank has a composition of 89 % ethylene ₅ 5% propane, 4 % methane, 0 _? 5 % hydrogen and traces of ethane and other hydrocarbons. This gas is recycled.

Claims (2)

20 invention claim 1 · Process for the expansion of high-pressure systems in Äethylenpolymerisationen, in particular tubular reactors in which ethylene or ethylene and Comonoinere be polymerized at pressures above 50 MPa and temperatures above 373 K, wherein as a result of exceeding predetermined pressure and / or temperature limits an emergency relaxation is carried out,. .Dadurch characterized in that the expansion mixture under pressure reduction within 2 to 15 see to 5 to 50% of the reaction pressure directly into the free volume of a closed Atrnosphaere container under separation into a specific light polymer low gas content and a specific heavy polymer-rich gas portion expands is, the polymer-rich gas fraction is fed to a limited formed in separator form part of the container and there is the polymer deposition. 2. The method of item 1, characterized _s that the polymer separation from the polymer-rich gas fraction is carried out in such a way that the polymer-rich gas portion is incident on the surface of an inert Pluessigkeit, which is presented in the formed in Abscheiderform portion of the container. This includes 1 sheet of drawings. -