GB2093045A - Letting down high pressure polymerization systems - Google Patents

Abstract

In the event of a dangerous pressure build-up inside a polymerization reactor, a relief device, such as a rupture disc, bursts allowing gaseous reaction mixture into a stack. Water is then released from a vessel into the stack under pressure of a gas or vapour stream, said gas or vapour stream being connected to the water-containing vessel in response to the pressure rise in the stack resulting from the opening of the relief device. The stack may also be watered and purged with inert gas to discourage thermal decomposition of the reaction gases.

Description

SPECIFICATION Letting down high pressure polymerization systems This invention relates to a method of letting down the pressure in high pressure polymerization systems, and particularly in systems for polymerization of ethylene or a mixture of ethylene and one or more copolymerizable compounds.

Exothermic polymerization reactions carried out at high pressure are inherently hazardous since any failure in the normal system can lead to a runaway reaction and a danger of explosion. The reaction may be brought under control by letting down the pressure in the reactor, usually by venting. There is a further danger when the reaction is vented to the atmosphere: the vented gas may explode or catch fire, and this invention is also concerned with the avoidance of such, so-called "aerial decompositions." Conventionally excess pressures in a high pressure polymerization system are let down through a safety valve or rupture disc which vents the reactants when a predetermined "safe" pressure is exceeded. Aerial decomposition of the released gases are then traditionally dealt with by water-quenching, total containment or supercritical expansion.

In the water-quenching method water is mixed with the escaping gases. The water is vapourized and because of its relatively high latent heat of vapourization a great quantity of heat is absorbed from the gases. For example, the vent stack may be provided with a number of rupturable water-containing bags which break when hot gases enter the stack. Other methods use pressure detectors or rupture discs to release water into the vent stack. U.S.

Patent 3781256 claims a process in which let down gases are quenched with at least 0.3 kg of water per kg of reactor contents within 10 seconds of the venting. The problems with these known systems is obtaining a sufficiently rapid response to the venting of gases to avoid an explosion.

In total containment systems emergency releases are discharged into a vessel, optionally containing water, and this system has the advantage of reducing the noise and pollution normally associated with venting to the atmosphere. However, this equipment has to be extremely large to contain possible gas releases and is therefore very costly.

The technique of supercritical expansion relies on the expansion of the release gases through vents at very high velocities. The volume of explosive reactants within explosive concentration limits varies as the cube of the stack diameter so that rapid expansion through a narrow stack reduces the chance of an explosion. However, inflammable gas clouds may form at the stack exit, and this system is unsuitable for venting large quantities of gases.

The present invention provides a system of water-quenching which reacts rapidly to an emergency situation.

In one aspect the invention provides a method of letting down the pressure in a high pressure polymerization system, in which method when the pressure within the system exceeds a predetermined value a relief device is opened to transfer a part of the reaction mixture of the system to a stack, and water is released into the stack from a vessel under pressure of a gas or vapour stream, the gas or vapour stream being connected to the watercontaining vessel in response to the pressure rise in the stack resulting from the opening of the relief device.

The invention also provides a method of preventing thermal decomposition of gaseous reaction mixture vented from a high pressure polymerization system into a stack, in which method water is charged to the stack from a vessel by pressurizing the water-containing vessel with a gas or vapour stream connected to the vessel in response to the pressure rise resulting from the venting of the gaseous reaction mixture to the stack.

The invention further provides apparatus for letting down the pressure in a high pressure polymerization system, comprising a stack provided with a normally-closed relief device which when open connects the stack to the system, a vessel for containing water, means to connect the vessel to the stack to enable water in the vessel to be released into the stack when the pressure in the vessel exceeds a predetermined value, and means for connecting a gas or vapour stream to the vessel to raise the pressure in the vessel in response to a pressure increase within the stack.

The invention has particular application to systems for high pressure polymerization or ethylene or a mixture of ethylene with one or more copolymerizable compounds such as vinyl acetate. Typically such systems operate at from 500 to 5000 bars and at from 1 50 to 400"C. Batch and continuous processes using autoclave or tubular reactors are well-known in the art.

The relief device connecting the reactor with the stack may be a relief valve, but is preferably a rupture disc selected to burst when the reactor pressure exceeds the safety level.

The vessel containing water is preferably normally isolated from the stack by a second relief device, such as a rupture disc, which opens to release water into the stack when the vessel is pressurized by a gas or vapour stream. This stream is preferably high pressure stream, typically having a pressure of from 10 to 100 bar and preferably in the region of 20-50 bar, and most preferably from 20 to 30 bar. Of cours, the steam pressure must be in excess of the pressure within the stack so that the vessel may be pressurized to open the second relief device.

However, it is believed to be within the competence of one skilled in the art to select an appropriate steam pressure for a given system.

The stream used to pressurize the water vessel is triggered by the rise in stack pressure. Preferably a portion of the reaction mixture gases is bled from the stack (most preferably in the region of the first relief device where the pressure is relatively higher) and this bleed stream employed to operate means to connect the pressurizing stream to the vessel. Most preferably the high pressure bleed stream operates a normally-closed relief device in a pipe connecting a source of the pressurizing gas or vapour to the water vessel.

This relief device is preferably one or more rupture disc arranged so as to burst when struck by the shock wave in the bleed stream.

When the vessel is pressurized sufficiently to open the second relief device, the water from the vessel is released into the stack.

When all the water in the vessel is exhausted into the stack the pressurizing stream of gas or vapour passes directly into the stack. Particularly when this stream is steam, this has a further beneficial effect of diluting the contents of the stack and so reducing the likelihood of thermal decomposition.

By monitoring the pressure in the vessel the progress of a let-down may be monitored.

Preferably the apparatus incorporates means to cut off the feed to the polymerization system and the pressurizing stream in response to a sudden rise of pressure in the water-containing vessel which indicates that the water-quench has been initiated.

The invention may be used in conjunction with known techniques such as watering the stack, in which water is fed into the stack via an annular pipe at the head of the stack, and allowed to run down the stack under gravity.

The base of the stack may be provided with a sump so that the stack always contains a small amount of water. An overflow from the sump to a collector vessel prevents too much water accumulating in the stack. This collector vessel may be purged with an inert gas such as nitrogen, so that the stack is permanently fed with the inert gas where it will also help to reduce the explosive nature of the stack contents.

The invention will now be further described, though only by way of illustration, by reference to the accompanying drawing which show a schematic view of a reactor provided with a let-down system of the invention.

An autoclave reactor 1 is connected by a rupture disc 2 to a vent stack 3 provided with a water-ring 1 2 fed from a water vessel 8.

There is a supply of water to vessel 8 through a water supply pipe 14 provided with a nonreturn valve 16.

Vessel 8 is also connected to the stack 3 through a rupture disc 9, which keeps water in the vessel under normal (no let-down) conditions.

Steam may be passed to vessel 8 along a supply pipe 7 provided with a non-return valve 14 but pipe 7 is normally blocked by rupture disc 6. A bleed pipe 4 from the stack in the region of the rupture disc 2 joins the steam supply pipe by way of normally-closed rupture disc 5.

The stack 2 has a sump 10 fed by water from water-ring 1 2. The sump level is governed by an overflow to vessel 11, which is also provided with a nitrogen feed 1 3.

The water vessel 8 is provided with a pressure-sensitive switch 1 5 electrically con nscted to the steam supply, the high pressure compressor for the reactor and the feed values for the reactor.

In the event of a runaway reaction in reactor 1, the rupture disc 2 burst allowing gaseous reaction mixture into stack 3. A slip stream of these gases is taken from the rupture disc holder, which is of necessity at a relatively high pressure. This slip stream is passed via pipe 4 to rupture discs 5 and 6.

These are physically positioned, so that the space in between them always be filled with condensate from steam supply 7. The shockwave from the slip stream ruptures both discs to allow steam to flow to vessel 8, together with the slip stream of relieved gases; however, this slip stream is minor in amount because of small dimensioning of the inlet nozzle. Vessel 8 being brought under steam pressure, the rupture disc 9 at the outlet bursts when the pressure in the vessel is higher than in the stack. It is of course also possible for the shockwave in the stack to rupture the disc at the outlet of this vessel; but the steam will still force water into the stack. The water will evaporate and increase the stack pressure. As soon as the stack pressure reaches the stea, pressure, the inflow of water will be stopped. This device therefore limits the stack pressure to a maximum pressure close to the steam pressure, at the same time as reducing its temperature by quenching.

The rupturing of the discs and introduction of water from vessel 8 to the stack is, notwithstanding all precautions taken to start the water inflow as soon as possible, of necessity later than the outflow of gases. Therefore, a small amount of water is held at all times in a sump 10 at the bottom of the stack, which will be hit by the outflowing gases immediately on bursting of the reactor rupture disc.

The amount of this water must be limited to prevent dangerous build-up of pressure in the stack and its level is controlled by an overflow into vessel 11. The water in the stack is delivered by a water ring 1 2 at the top of the stack.

As the apparatus of the invention has to function in conditions varying from an overpressure in relatively cold conditions, caused by malfunction of the reactor pressure control system, to the conditions of a complete reactor decomposition, it is impossible to foresee all decomposition conditions in the stack. The introduction of the water from vessel 8 may not end at the moment the speed of gas flow into the stack drops to such a value that the water is not ejected from the stack. This excess water is caught in vessel 11 before it builds up a level sufficient to flow into the reactor.

When vessel 8 is empty, steam will be admitted to the stack and will help dilute the residual gases flowing out of the reactor and will maintain a sufficient gas speed to the top of the stack to prevent backwash of these gases to ground level.

The pressure is used through a pressure switch 1 5 to close, after an appropriate time, the steam supply to stop the HP compressor for the reactor (not shown), to close the feed valve to the reactor and to open the compressor bypass. One of the feed valves is closed only after a delay, so as to maintain a limited flow through the rupture disc, to allow time for the evacuation of excess water.

A surge check valve on vessel 11 will prevent outflow of gases through that vessel.

The surge valve will be calibrated so as to reopen, whatever the water level in drum 4, upon pressure drop in the stack to atmospheric.

Non-return valves 1 5 and 1 6 prevent outflow of gases along supply pipes 14 and 7 respectively.

A small flow of nitrogen 1 3 is fed to vessel 11 and will thus fill the stack which helps to prevent explosive conditions in the stack upon bursting of the rupture disc.

Claims (23)

1. A method of letting down pressure in a high pressure polymerization system, in which method when the pressure within the system exceeds a predetermined value a relief device is opened to transfer a part of the gaseous reaction mixture of the system to a stack, and water is released into the stack from a vessel under pressure of a gas or vapour stream, the gas or vapour stream being connected to the water-containing vessel in response to the pressure rise in the stack resulting from the opening of the relief device.

2. A method of preventing thermal decomposition of gaseous reaction mixture vented from a high pressure polymerization system into a stack, in which method water is charged to the stack from a vessel by pressurizing the water-containing vessel with a gas or vapour stream connected to the vessel in response to the pressure rise resulting from the venting of the gaseous reaction mixture to the stack.

3. A method as claimed in claim 1 or claim 2, in which the high pressure polymeri zation system is the polymerization of ethylene or a mixture of ethylene with one or more copolymerizable compounds.

4. A method as claimed in claim 1, in which the relief device is a rupture disc.

5. A method as claimed in any of the preceding claims, in which water-containing vessel is normally isolated from the stack by a relief device, which opens to release water into the stack when the water-containing ves sle sle is pressurized by a gas or vapour stream.

6. A method as claimed in any of the preceding claims, in which the gas or vapour is steam having a pressure of from 10 to 100 bar.

7. A method as claimed in claim 6, in which the steam pressure is from 20 to 30 bar.

8. A method as claimed in any of the preceding claims, in which the portion of the gaseous reaction mixture is bled from the stack and the this bleed stream operates means to connect the pressurizing stream to the vessel.

9. A method as claimed in claim 8, in which the connecting means comprises a nor mally-closed relief device in a pipe connecting a source of the pressurizing gas or vapour stream to the water vessel.

10. A method as claimed in claim 9, in which relief device comprises one or more rupture disc arranged so as to burst when struck by a shock wave in the bleed stream.

11. A method as claimed in any of the preceding claims, in which means are pro vided and the pressurizing gas or vapour stream in response to a sudden rise of pres sure in the water-containing vessel.

1 2. A method as claimed in any of the preceding claims, in which water is fed into the stack via an annular pipe at the head of the stack, and allowed to run down the stack under gravity.

1 3. A method as claimed in claim 12, in which the base of the stack is provided with a sump in which water running down the stack is collected.

14. A method as claimed in claim 13, in which the sump is provided with an overflow pipe connected to a collector vessel so that excess water drains from the sump to the collector vessel.

15. A method as claimed in claim 14, in which the collector vessel is purged with an inert gas which is also fed to the stack.

16. A method as claimed in claim 1 or claim 2 and substantially described herein with reference to and as illustrated in the accompanying drawing.

1 7. Aapparatus for letting down the pres sure in a high pressure polymerization system, comprising a stack provided with a normallyclosed relief device which when open connects the stack to the system, a vessel for containing water, means to connect the vessel to the stack to enable water in the vessel to be released into the stack when pressure in the vessel exceeds a predetermined value, and means for connecting a gas or vapour stream to the vessel to raise the pressure in the vessel in response to a pressure increase within the stack.

1 8. Apparatus as claimed in claim 17, in which the relief device is a rupture disc.

1 9. Apparatus as claimed in claim 1 7 or claim 18, in which the vessel for containing water is normally isolated from the stack by a second relief device which opens when the vessel is pressurized.

20. Apparatus as claimed in any of claims 1 7 to 19, which includes a bleed pipe connecting the interior of the stack to a normallyclosed relief device in a pipe for connecting a source of the pressurizing stream to the vessel.

21. Apparatus as claimed in any of claims 1 7 to 20, which includes means for cutting off feed to the polymerization system and the pressurizing stream in response to a sudden rise in pressure in the vessel.

22. Apparatus as claimed in any of claims 17 to 21, in which the head of the stack is provided with an annular pipe through which water may be introduced into the stack, and the base of the stack is provided with a sump to collect water running down the stack, the sump being provided with an outlet connected to a collector vessel for receiving excess water drained from the sump.

23. Apparatus as claimed in claim 1 7 and substantially as described herein with reference to and as illustrated in the accompanying drawing.