I. CHEM. E. SYMPOSIUM SERIES NO. 85. REDUCTION of EXPLOSION RISKS SUBSEQUENT to ETHYLENE DECOMPOSITION on HIGH

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1 REDUCTION of EXPLOSION RISKS SUBSEQUENT to ETHYLENE DECOMPOSITION on HIGH PRESSURE POLYETHYLENE PLANTS B. MARTINOT (CdF Chimie, France) Emergency releases from polyethylene reactors or separators are liable to cause aerial explosions when ignition occurs after a delay. Delayed ignition may be spontaneous or transmitted from a neighbouring burning discharge. CdF have developed a system of preventing this, which is described. It entails containment of the emergency release under inert gas, accompanied by automatic quenching with water, prior to ultimate discharge to atmosphere. I INTRODUCTION The main danger in making polyethylene lies in the size of ethylene/air mixtures explosiveness range. Any emission of ethylene to the atmosphere thus brings forth some risks. Ethylene decompositions, which sometimes take place in vessels under high or medium pressure, actually lead to important gas release to the atmosphere. With regard to their consequences, decompositions fall into several categories: First type : Decompositions not followed by inflammation at the stack outlet. A gas stream escapes from the stack and vanishes without apparent consequences. No mechanical effect results for the plant. Second type : Decompositions followed by a mild inflammation. A very short flare takes place at the stack outlet. No mechanical effects are recorded. Third type : Decompositions followed by a violent inflammation. A flare takes place as for the second type. In addition the shock waves resultting from the flare cause important mechanical effects. This third type inflammation is thought to result from a delayed self ignition of the gas stream or from ignition by an inflamed jet coming from a neighbouring stack. Such type of decompositions seems rather to occur with high pressure bursting discs either when a notable difference exists between the bursting pressure of the disk and operating pressure. 315

2 II CAUSES of AERIAL EXPLOSIONS a) Ethylene decompositions Decomposition of ethylene may occur mainly in two high exothermic chemical ways leading to formation either of : hydrogen C 2H 4 2 C + 2 H 2 (A) or methane C 2H 4 C + CH 4 (B). Total decomposition of ethylene as shown in (A) and (B) reactions brings the resulting products to an equilibrium temperature of 700 or 1500 C. Experimental studies about decomposition gas recovery, have shown that, for 200 2,000 bars ethylene pressures, decomposition is incomplete and leads essentially to methane formation. b) Inflammation extension and explosion risk Increase of temperature causes sudden pressure increase. Rupture of one or more safety discs protecting the (reactor or separator) vessels from overpressure results in the quick relief of related gas volumes to the atmosphere. Once inflammation has started in a point of the stream where suitable temperature and concentration conditions are realized, it spreads through the gas mass, the concentration of which lies between the lower and the upper inflammability limit of the mixture. Such propagation may occur in two ways: deflagration or detonation. The type of propagation depends on both decomposition and temperature of the mixture as well as on its confinement degree and the inflammation onset conditions. Despite the magnitude of recorded damage in certain cases, inflammations appear to be just deflagrations, the flame of which has a propagation velocity fast enough to bring forth destructive over-pressure. Should this be the case, however, any inflammation followed by a detonation in which huge gas volumes are involved, would cause very important damage. Such a possibility cannot be fully discarded. c) Formation of the inflammable gas mixture Once a safety disc in a pressurized vessel gets broken, contact is at once established between the air and the exhausting gas. Changes in the stack section and direction, diffusion phenomena, differences in velocity of the gas flow between center and wall areas of the stack, locally contribute to the formation of pre-mixed clouds of gas and air. Such clouds disperse rapidly as the pressure relief device carries them from the stack outlet. Once out, the jet stream gets mixed with the surrounding air. The moving gas mass raises in volume, the jet gets wider and slows down. If the mixture is formed essentially in free air, pre-mixed clouds are also formed as the jet is released to the atmosphere. Such areas disappear as soon as air is replaced by the exhaust gas. 316

3 d) Inflammation onset and consequences Inflammation takes place as a part of the flammable mixture is brought to a sufficient temperature. For each flammable mixture, one can roughly define a temperature limit, called ignition point, so that the mixture could inflame when it reaches such temperature. The ignition point is mainly determined by the mixture pressure and composition. The ignition point of air mixtures with ethylene, hydrogen and methane, under standard atmospheric conditions, is respectively 450, 500 and 600 C. Let us consider the different initiation types of this mixture inflammation and the subsequent explosion hazards. 1. Spontaneous ignition A volume of hot gas, dispersing into the air and coming from a vessel in which decomposition has occured, is bound to undergo gradual concentration and temperature decrease. If the gas concentration of the mixture is near upper flammability limit and if its temperature is still high enough - at least equal to the ignition point - the mixture undergoes spontaneous ignition. It is then possible to determine, according to the flammable gas concerned, a minimum temperature limit to be reached by such gas in air-free conditions so as to allow spontaneous inflammation when the gas gets mixed with air under favourable concentration conditions. Such minimum temperature is 1,200 C for methane and 650 C for ethylene. Now, 800-1,000 C temperatures have been observed in structures undergoing decomposition. And as previously noticed, total decomposition of ethylene into methane brought the reaction products to 1500 C. Thus, it is possible that spontaneous ignition of a hot gas part is the cause of the jet inflammation. We must notice that such inflammation type may occur not only in the gas stream leaving the stack outlet but also into the stack itself, in pre-mixture areas. As inflammation takes place in the very first moments of gas relief, either into the stack or at its outlet, it has been found likely that no destruction effects occurred : as the inflammable gas mixture got burned up as soon as formed. On the other hand, if such inflammation type occurs somewhat late after emergence of the jet at the stack outlet, one can expect a destructive ignition of the inflammable mixture formed before inflammation. 317

4 Once the decomposition products have changed temperature and composition, it becomes possible to admit as the relief starts, that a gas may be released, the temperature of which is not sufficient to bring forth inflammation. Such emission is then followed by a second one, the composition and temperature of which initiate a spontaneous inflammation, which is transmitted, with destructive effects, to the first emission. The latter, in the meantime, was forming with air an inflammable mixture. Some observed explosions are thought to be caused by self-ignition of a hot gas part which would induce deflagration in a pre-existent inflammable mixture. Let us notice that independant emissions of gas, capable or not of spontaneous inflammation, may quite as well, come from different streams or stacks. 2. Inflammation by shock waves This type of inflammation characterizes inflammations subsequent to mere discs rupture, out of decomposition emergence, that is in presence of relatively cold ethylene (150 to 300 C). Such inflammation occurred on various occasions, during disc rupture tests performed while starting a new unit and twice during water injection tests which took place on a small industrial unit. During this last testing programe, we were able to observe at a rate of 1,000 images/sec., the inflamed mixture leaving the stack outlet. Undoubtedly ethylene inflammation occurred into the stack. Absence of decomposition, that is absence of hot points, cannot lead us to consider self-ignition of the mixture as the inflammation cause. Various hypotheses can be put forward as to the cause of this inflammation: - electrostatic discharges subsequent to gas friction against the stack walls, - successive reflections of the ruptured disc on the disc holder and/or the stack walls, - induced heating by shock wave of the air column previously located in the stack. This last hypothesis appears to be the most likely cause. As the disc is ruptured, indeed, the previously stable air column of the stack is flashed through by a shock wave which compresses and heats the air up, so as to bring its temperature sometimes over 1000 C. This pressure wave moves at a velocity which depends on its intensity and is superior to the sound velocity in the milieu. The various testing programmes carried out in two plants have shown existence of a shock wave moving at m/s. 318

5 The air thus compressed and heated is gradually put into motion by the gas relived from the pressurized vessel. Inflammation is initiated at the hot air-ethylene interface (which moves at a lower speed than the shock wave and thus succeeds to the latter), and especially in pre-mixture areas, the formation of which has been previously described. The flame is then carried by the flow to the stack outlet section where it remains until the end of the whole gas relief. Emergence of such type of inflammation is made easier by: - high calibrating pressure of the disc, - a low ratio between stack and disc diameters, - a high temperature of the gas to be relieved (temperature increase of the gas-hot air mixture). Hot gas resulting from decomposition is thus more likely to inflame under action of a shock wave than ethylene at 300 C. Emergence of a gas stream inflamed by shock wave at the stack outlet may induce the inflammation, possibly delayed, of a neighbouring jet. Such inflammation may generate destructive effects. The delayed ignition, either by spontaneous inflammation or by shock wave, of a gas jet induced by the inflamed stream of another stack, is thought to be the most common cause of aerial explosions. Our attention has been drawn to the following conclusions: - inflammation of one jet by another could be the most frequent cause of aerial explosions; - the other cause could be : delayed self-ignition of the jet; - the initial inflammation could be originated either by: a spontaneous inflammation, or ignition induced by shock wave. So we decided in order to prevent any such inflammations, to study, design and operate a new system which would answer each of the different causes of inflammation. Figure I describes such a system for new units and Figure II a system to protect existing units. III CdF CHIMIE NEW SYSTEMS On Figure I, the high pressure enclosure (reactor or separator) 1, is provided with two safety discs, 2 and 2A (these are made inside CdF Chimie and for a given temperature, very often 200 C, their bursting pressure is within a 2% precision range). The number of safety disks is designed according to geometry and volume of the reactor vessel, and can be more numerous without any problem. 319

6 17 and 17A are disk holders which catch the ruptured disc, Gas mixtures escape into the conduits 3 and 3A through numerous slits. The upper sections 4 and 4A of these conduits are perforated with slits and at their tops include spreader and deflector means 6 and 6A respectively in the shape of inverted cones. The conduits 3 and 3A enter the oblong and horizontal reservoir 7 at its bottom. The bottom of reservoir 7 is filled with a volume of water 8 and it will be noted that the tops of the conduits 3 and 3A are above the level of the water surface. Valves 9 and 9A also are provided in the bottom of the reservoir to evacuate part of the water into an overflow 10 when required. The single stack 11 is located at one of the ends of the reservoir 7 and permits evacuating the contents of enclosure 1 to the atmosphere. The reservoir 7 furthermore comprises water injection equipment, 12 and 12A located straight above the gas spreaders 6 and 6A. The water injection devices are connected by conduits 13 and 13A to water reservoirs 14 and 14A which are pressurized with inert gas, for instance nitrogen. The pressure and volume of nitrogen and the volume of water are determined according to the curve of depressurization of the high pressure enclosure, to be able to achieve a minimum ratio between the flow of water and the flow of gas all along the depressurization. The calculations are based on decomposition, which is the most critical case. The operation of the water devices is controlled by opening of explosive valves 16 and 16A, which opening in turn is controlled by detectors 17 and 17A located in the immediate vicinity of the discs. Electronic equipment avoids operating the system in case of an untimely signal. To be sure to get the minimum response time for water injection, pyrotechnic valves 16 and 16A are used, which are completely open in less than 5 milliseconds. The conduits 13 and 13A are as short as possible with the maximum possible diameter suitable with the different cross section areas of each part of the water circuit so as to keep at reasonable level the shock wave when water is arriving on spreaders 12 and 12A. The normal response time is in the range of 25 milliseconds for the total system. To put the gas conduits and the reservoir 7 under inert atmosphere with a small consumption, the single stack 11 is equipped with a sealing device taking up the entire cross-section of the stack, for instance a rubber balloon which is inflated by inert gas at a pressure exceeding that in the reservoir (a pressure of 50 to 100 millibars is often used) and the balloon bursts if the pressure in the reservoir becomes higher than about 250 to 500 millibars. But to calibrate such jet flow structure and its supports, it was necessary to make a large survey including: - a series of blast tests with reactor stacks models of several units, as well as with separator stacks models, - a theoretical study of flow in the stacks under quasi non steady conditions, based on the experimental results obtained during testing operations on the models. 320

7 With this survey, we were able to confirm the soundness of the calculation methods we used in order to determine the pressure and stress levels. This survey also allowed us to determine the conditions of gas release : velocity and pressure characteristics at the outlet section. Thus it appears that the flow on leaving the stack has a sonic velocity and a pressure value which is slightly superior to the atmospheric pressure. When designing and building new units, the above described system is preferred because it offers following advantages: - suppression of the possibility for one jet to inflame another, through delayed ignition by adapting one stack per vessel, - limitation of the delayed self ignition hazard in the jet (by maintaining the gas during a longer period into the pressure relief device, thereby obtaining better water vaporization). But it would be either impossible, or too expensive to install this complete system on some existing units which are equipped with several stacks per vessel. In that case, a system based on similar principles is used. Such second type of system is shown in Figure II. Pressurization of each stack under inert gas is provided by sealing with a balloon or top cover, and quick injection of water with a pyrotechnic valve. The water injection device is located on part of the circumference of the stack, near the bottom. But, because there is no reservoir to avoid contact between water and rupture disc, in case of untimely operation of the pyrotechnic valve, it was necessary to study how to prevent that dangerous hazard. Two systems have been adopted: 1. Water injection device is located above the balloon. Tightness between balloon and stack wall prevents water to reach the rupture disc and to fill in the stack; water then would be removed through slits performed in the stack wall above the balloon, leading to a recovery volume which consists of a jacket around and outside the stack. 2. A preferred system is to install on the pressurized water tank another pyrotechnic valve, which leads to the atmosphere. An interlock checks if the operation of pyrotechnic valve on water injection is actually caused by a rupture disk. If it is not the case, the interlock operates the second pyrotechnic valve and allows the quick expansion of the inert gas pressurizing the water tank. The size of the opening has been calculated so that the water volume which is then introduced in the stack is very small and can be removed easily at the bottom of the stack without contact with the bursting disc. That system has been tested under real conditions several times, and the results are very good. 321

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