Abstract of Special Research Report (SRR-No.29)

National Institute of Occupational Safety and Health, Japan

A Study on the Prevention of Explosion and Fires for Recycle Industries (Midterm Report)


Takayuki ANDO

: With the recent promotion of the conservation of energy and resources and reduction of waste, more materials are being recycled. Some aspects of recycling, however, are dangerous, and frequent incidents of explosion, fire, and poisoning have been reported. These hazards are attributed largely to the increase in the amount and types of recycled substances, the replacement of chlorofluorocarbons with liquefied petroleum gas, which is done to protect the ozone layer and to ward off global warming, and the use of flammable cleaning liquids instead of nonflammable ones in an effort to reduce environmental pollution.
    The objective of this research is to present accident-prevention measures that are applicable to the industry by evaluating the fire and explosion hazards related to the recycling process, developing techniques to evaluate the risk of such hazards, and designing technology that is capable of controlling or preventing the development of explosions.
    The research subjects and study items of this research project are as follows:
(1) Study of the fire and explosion hazards resulting from mixing chemicals during the recycling process.
  * Study of scale effect, a method for the analysis of experimental results, and a method for the identification of reaction products for laboratory recycling simulations.
  * Evaluation of the hazard associated with the catalytic effect of metals on decomposition and development of a method to eliminate that effect.
  * Development of a classification system for incompatible hazard of materials.
(2) Study of explosibility and combustibility of discarded flammable substances.
  * Development of a simple test method for the explosibility and combustibility of waste oil.
  * Study of metal dust explosions under an atmosphere of oxy-compound vapor.
  * Study of vapor explosions related to the rupture of LPG containers.
(3) Development of an explosion mitigation system applied to crushing and milling equipment.
  * Determination of the extinction limit of burning industrial waste by extinguishers, such as those using water, inert gas, and dry powder.
  * Development of an explosion mitigation system applicable to crushing and milling equipment at a reasonable cost.

Measurements of Droplet Size for Calculating Reaction Heat in a Liquid-Liquid Heterogeneous Reaction Process


: Some explosions, which occurred in heterogeneous liquid-liquid reaction processes, have been reported. In such processes, reactants need to be mixed well by mechanical agitation for smooth progress of the reactions. And the unsuitable control for the mixing is the most frequent cause for runaway situation.
    Better understanding of the heat release behaviour under some condition of mixing would give better knowledge of the explosion stories. It could be quite useful to reduce the occurrence of same type of explosion.
    An aim of this study is to simulate accident stories, which might be caused by inappropriate mixing conditions in the reaction process. A typical explosion happened in a liquid-liquid two-phase heterogeneous reaction which rate is controlled by only agitation speed for mixing. That is, the faster speed, the faster reaction rate, and the slower, the slower. In this case, we guessed that the accident was brought on a runaway reaction triggered by the faster agitation speed than one in the latest successful batch. The reason of the modification of the control sequence of the agitation was to improve productivity by the faster reaction. As the result, the reaction temperature might finally achieve to the boiling point and the bubbling might start. After that, the slower agitation couldn't make the reaction rate smaller any more because of the bubbling would stir up the reactants instead of the mechanical stirrer.
    To simulate it, the heat release rate in the heterogeneous reaction must be estimated under any agitating speed.
    In the previous work, in order to predict the heat release rate on anytime, the method which could lead the equation to predict, is proposed using an empirical equation of time versus particle size with the experimental results from a reaction calorimeter RC1. The hydrolysis of anhydride is selected as a model reaction, which reaction rate is suitable.
    In this paper, some experiments to evaluate change of particle size distribution over reaction time are done with FBRM system, a laser reflection type in-situ probe. And one of the experiments is visualized with an in-situ fibre-probe. Modified sauter diameter which calculated from chord length data of FBRM measurement was fitted to the empirical equation.
    The experimental modified sauter diameter in the equilibrium state was observed to be the constant value in spite of proceeding and/or finishing heat generating reaction. Precipitation of sodium salt seems to occur.

The Dust Explosion of Aluminum Dust in Water Vapor

Teruhito OTSUKA and Haruhiko ITAGAKI

: The popular fire-extinguisher is the water. However, some kinds of metals, which have higher oxidation-reduction potential than hydrogen, can react with and get oxygen from water. In this study, aluminum powder is employed to evaluate the reactivity with water. Aluminum is one of the metal used most in the world, especially for electronic device's case as aluminum-magnesium alloy.
    The reaction of liquid phase aluminum and water is well studied as the trigger of phreatic explosion, which is physical explosion caused by rapid heat transfer from aluminum to water and vaporization of water. In former days, those accidents were described with only phreatic explosion and temperature of aluminum is the main factor. Recent investigation showed another cause of the explosion. The melted high temperature aluminum is easily distributed in air as droplet in response to the pressure rise of water vapor. As the consequence of mixing of air and aluminum droplet, that makes the surface oxidization of droplet. Therefore, inside of droplet was vaporized by the heat of the surface oxidization. Fog drip explosion, which is the phenomenon that increased inner pressure breaks up an surface aluminum oxide layer, is caused.
    This study gives an another scenario of the accident with aluminum and water. In this paper, the experimental result which was measured the explosion characteristic of the aluminum powder under water vapor atmosphere with the 1.0 L small-scale explosion container was reported. From this result, it was turned out that even if it was not under high temperature which melts aluminum, and even if oxygen was not contained in atmosphere, when there was sufficient ignition energy, the aluminum powder can cause the explosion with the reaction with water vapor. When this type explosion was occur, hydrogen would be produced. Leakage of this hydrogen could be also another accident.
    It is also reported that explosion properties of aluminum powder in air and water vapor mixture atmosphere, comparing to in air only. The big feature is that both the pressure and pressure rise rate are higher at the excess amount of aluminum powder.

Survey of Explosions / Fires by Portable Liquefied Gas Cylinder and the Ignition Experiment

Haruhiko ITAGAKI and Ou-sup HAN

: Concern for recycling of the people and the industry rises. But, explosions / fires often happened in the recycling industry due to the increase in the explosion / fire hazard of the recycle materials. This research paid attention to the explosion / fire which happened due to the portable liquefied gas cylinder and the spray can for the purpose of the safe handling in the people and the recycling industry.
    First, kind of materials and its containing rate about the spray gas of the spray cans and the occurrence of the explosion / fires were investigated, and it found out that, (1) 90% and more of the spray cans contain flammable gas. (2) The spray gas of most spray cans is LPG or LPG / DME. (3) Most of the spray cans of cosmetics contain alcohol in addition to the flammable gas. (4) The containing rate of the spray gas can be divided into less than 20 %, about 50 %, and more than 80 %. (5) More than 99 % of the contents of a spray can for the harmful insect was flammable gas. (6) Most of the explosions or fires at the recycling industry happened in burning or the dismantling process. (7) Most of the explosions or fires in the general factory and the consumer happened by the cause of lack of understanding of the flammability of spray gas or leaving to the high temperature environment. (8) The fire of the garbage truck is the most conspicuous about the number of explosions or fires. (9) Damage due to the explosion or fire at the garbage dump factory tends to grow big, and a casualty often occurs. (10) The long closedown of the garbage dump factory has a great influence to the people.
    Second, experiments were carried out about the observation of the leak gas when a spray can burst and the ignition area when flammable gas leaked out, and it found out that, (11) The diffusion of the leak gas when a vessel burst depend on the condition of the hole. (12) When flammable gas leaked out, the flame to the leak side propagated longer than flames to the other directions. (13) The maximum ignition distance from the leak point to the ignition source was 46 cm when leak time was 1 sec. When leak time was 2 sec, it was 46 cm.
    From now on, an actual scale explosion experiment will be done about the case that goods on the market burst with a viewpoint of the ignite area and the explosion scale.

Field Study on Explosion Protection
in the Bulky Refuse Crushing Process Facilities


: In a daily life, the disposal of garbage and industrial waste is a serious problem. The legal system for proper disposal of refuse and promotion of the recycling has been enforced in order to promote the formation of a recycling society in Japan. Though explosion or fire has often occurred in the bulk refuse crushing process facilities, the problem is caused by the stop of the machine which causes further accidents for the smooth citizen life. In addition, there can be the accidents involving a new type of explosion and fire which is related to the development of the new refuse resource recycling technology. The final objective of this study will be made to develop a practical explosion protection/suppression system, which can be applied to the bulk refuse crushing process facilities. In order to obtain a useful knowledge for the prevention of explosion and fire in the process facilities, a field study was carried out in 7 municipal facilities in the different district (Yamagata, Miyagi, Saitama, Tokyo, Gifu, Mie, and Nara) and 2 recycling facilities for household electric appliances (Tokyo and Chiba). Gas explosion and fire are, in the most cases, caused by spray can, gas cartridge, and small cylinder including liquefied petroleum gas. Hot surfaces in the crusher and sparks by the impact, etc. seem to be the ignition source. In most investigated facilities, injection systems of air or steam are adopted in order to prevent the explosion in the crusher. Attention is necessary for not only gas explosions but also dust explosions. These dangerous objects should be directly removed by visual observation and camera monitoring, so that dangerous objects such as the LP gas cylinder as a cause of the explosion may not enter the crusher. Automatic explosion suppression systems as an explosion protective measure have not introduced, since installation and maintenance costs are expensive, until it widely installed. To begin with, an appropriate explosion pressure relief vent should be installed, as an explosion protection system.

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