Mixtures of Flammable Gases

Le Chatelier s Rule allows the calculation of the lower flammability (explosibility) limits for flammable mixtures  

Applied Process Design for Chemical and Petrochemical Plants 

Ethyl oxalate Diethyl oxalate (COOCjHs) C H NCaH CHaCHjOH  

Ethyl phthallyl ethyl giycoiate CaH.OCOCiH.COOCHaCOOC.H 365  

2-Ethyl-3-propylacrolein Ethyl n-propyl ether C.H CH C(CaHj)CHO CaH.OC.H. 155 

---

Lx = distance between adjacent vents, meters or feet LEL = lower explosive, or lower flammable limit, percent of mixture of flammable gases only in air Lj, Lj, etc = lower flammability limits, vol % for each flammable gas in mixture... 

Flammable limits for mixtures of flammable gases and vapors. For mixtures of several flammable gases and vapors, the flammable limits can be estimated by application of Le Chatelier s equation, if the flammable limits of the components are known 2... 

The addition of inert gases to a mixture of flammable gases and air affects flammability limits. Carbon dioxide causes a greater... 

An explosible atmosphere is defined as a mixture of flammable gases, vapours, mists or dusts and air or another oxidizer under atmospheric conditions in such a ratio that the reaction, once ignited, proceeds self-sustained, accompanied by increases in temperature and pressure. 

Mixtures of flammable gases and oxidant were treated in the preceding Section. In practice often mixtures have to be assessed which in addition contain an inert gas. The corresponding situation is represented by Fig. 2.4. It is subsequently described following [9]. 

In process plants apparatuses are connected by pipes. Therefore pressure waves generated by an explosion in an apparams can cause pressure loads in other apparatuses. In order to avoid this, isolation of apparatuses can be achieved by flame arresters (or other devices). They are described below following [14—16]. These impede the propagation of an explosion and hence reduce its consequences (level 4 of Table 4.1). They can be applied to mixtures of flammable gases or vapours with oxidants (usually the oxygen in air) or mixtures of dusts and oxidants. 

Mixtures of flammable gases with air (or other oxidizing gases like pure oxygen or chlorine) can explode if the concentration is within certain limits and sufficient energy is provided to initiate the reaction. 

Never conduct hot work in the presence of explosive atmospheres (mixtures of flammable gases, vapors, liquids, or dusts with air), or on a metal partition, wall, ceiling, or roof with a combustible cover. 

A liquefied gas or any mixture of liquefied gases, which is flammable in air and has a boiling point <0°C (measured at 1 bar absolute), when held under refrigeration or cooling at a pressure <1.4 bar absolute. 

A liquefied gas or any mixture of liquefied gases, which is flammable in air and 50 t... 

A criterion that is commonly used to estimate the potential hazard of a discharge is to compare the MIE of the fuel-air mixture to the equivalent energy of the discharge. A precise experimental determination of the MIE is often required under the specific conditions of the system. MIEs are shown in Table 6-4 for a number of flammable gases and in Table 6-8 for dusts. 

Flammable gas or vapor-air mixtures Due to their low minimum ignition energies (<1 mj), avoidance of effective ignition sources in flammable gas or vapor-air mixtures is in principle possible only in exceptional cases. For hot surfaces a maximum permissible surface temperature Tmax must be specified, with the help of the minimum ignition temperature (MIT) of flammable gases, vapors, or liquids, so that the temperature of all surfaces is not exceeded (Table 23-8). 

A calculation of the flammability limits of complex gas mixtures is carried out by the application of the mixture rule. Stated simply, the mixture rule is that if two limit mixtures of different gases are added together, the resulting mixture also will be a limit mixture. The mathematical statement of this law is as follows ... 

The objective of this postulate is to dilute the oxygen in the area of the fire with inert gases to such an extent that the ratio of this mixture with flammable gases emanating from the combustible substrate (fuel) is below the limits of combustion. Oxygen may be diluted by the evolution of nonflammable gases which result from the decomposition of the ingredients of the fire-resistant mastic under fire conditions. 

The basis of the hydrogen enrichment concept to provide low-temperature combustion stems from experimental observations that admixtures of hydrogen with HC/air mixtures so depress the lean flammability limit that burning can take place at ultralean combined fuel-to-air ratios. The potential reduction in lean limits provided by such tertiary mixtures can be illustrated by the use of Le Chatelier s formula which predicts the lean limit of any mixture of fuel gases from a knowledge of the lean limits for the individual fuel gases. This formula is (17) ... 

---