Explosive Limits Vent

Some vent streams, such as light hydrocarbons, can be discharged directly to the atmosphere even though they are flammable and explosive. This can be done because the high-velocity discharge entrains sufficient air to lower the hydrocarbon concentration below the lower explosive limit (API RP 521, 1997). Toxic vapors must be sent to a flare or scrubber to render them harmless. Multiphase streams, such as those discharged as a result of a runaway reaction, for example, must first be routed to separation or containment equipment before final discharge to a flare or scrubber. 

As vent collection systems normally contain vapor/air mixtures, they are inherently unsafe. They normally operate outside the flammable range, and precautions are taken to prevent them from entering it, but it is difficult to think of everything that might go wrong. For example, an explosion occurred in a system that collected flarmnable vapor and air from the vents on a number of tanks and fed the mixture into a furnace. The system was designed to run at 10% of the lower explosion limit, but when the system was isolated in error, the vapor concentration rose. When the flow was restored, a plug of rich gas was fed into the furnace, where it mixed with air and exploded [17]. Reference 34 describes ten other incidents. 

Following fires in which endotracheal tubes became ignited by surgical lasers or electrocautery in atmospheres enriched by oxygen and/or nitrous oxide, the flammability of PVC, silicone rubber and red rubber tubes in enriched atmospheres was studied [1], Ozonised oxygen was reacted with hydrogen at low pressure to generate hydroxyl radicals. Pressure in the apparatus was maintained by a vacuum pump protected from ozone by a tube of heated silver foil. On two occasions there was an explosion in the plastic vent pipe from the vacuum pump. The vent gas should have been outside explosive limits and the exact cause is not clear the editor suspects peroxide formation. 

Some of the limitations of explosion relief venting include ... 

Care must be used in all of these oxidations to avoid fires and explosions. This means operating outside the explosive limits of the system. It also means testing for peroxides before workup. A common method is to see if a sample liberates iodine from a solution of potassium iodide. Chemists at Praxair Inc. have devised a reactor that uses pure or nearly pure oxygen below a gas containment baffle with nitrogen above.207 The oxygen efficiency is about 99%, which allows increased productivity and selectivity at lower-operating pressures or temperatures. The rate of reaction is increased. The amount of vent gas, lost solvent, and lost reactant are reduced. In the oxidation of p xylene, 25-62% less loss to carbon monoxide and dioxide was found together with less color. 

The continuous flow reaction was carried out with a fixed bed tubular metal reactor (18 mm (i.d.) X 100 cm) at atmospheric pressure. The catalyst was packed and heated with shell furnace. Air and ethanol (rectified spirit containing 88% ethanol) were introduced from the top of the reactor at desired rates (considering lower explosive limit). The reaction being exothermic, was started at 50-60° lower than the desired temperature. Acetic acid was condensed and collected from the bottom of the reactor. The vent gas contained air, carbondioxide, moisture, acetaldehyde and uncondensed acetic acid. 

ACRYLIC ACID, 2-ETHYLHEXYL-ESTER (103-11-7) CnHjoOj Combustible liquid. Forms explosive mixture with air [explosion limits in air (vol %) 0.8 to 6.4 flashpoint 180°F/82°C oc autoignition temp 496°F/258°C Fire Rating 2]. Unless inhibited, contact with heat, sunlight, contaminants, or peroxides may cause hazardous polymerization. Reacts violently ivith strong oxidizers, with risk of fire and explosions. Incompatible with strong acids, alkalis, aliphatic amines, alkanolamines, nitrates. The uninhibited monomer vapor may block vents and confined spaces by, forming a solid polymer material. On small fires, use AFFF, foam, dry chemical, or COj extinguishers. 

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