Butane flammability limits

Air is compressed to modest pressures, typically 100 to 200 kPa ( 15-30 psig) with either a centrifugal or radial compressor, and mixed with superheated vaporized butane. Static mixers are normally employed to ensure good mixing. Butane concentrations are often limited to less than 1.7 mol 1 to stay below the lower flammable limit of butane (144). Operation of the reactor at butane concentrations below the flammable limit does not eliminate the requirement for combustion venting, and consequendy most processes use mpture disks on both the inlet and exit reactor heads. A dow diagram of the Huntsman fixed-bed maleic anhydride process is shown in Figure 1. 

Flammability limits for pure components and selected mixtures have been used to generate mixing rules. These apply to mixtures of methane, ethane, propane, butane. 

There is an alternative to using neat butane vapor, however, which overcomes the need for pipework heating. This is to use a gas-air mixture. A special gas-air mixer is used which ensures that a preset ratio is maintained at all demand rates. The ratio chosen must be well outside the flammable limits. A typical LPG-air-mixing plant is illustrated by Figure 20.7. 

Flammability. The flammability limits of mixts ofN trifluoride with gaseous fuels and the effect of N as a diluent are given in graphic form in Ref 18. Fuels examined are H, butane, and hexafluoroethane. The authors also report that... 

Given that the lower flammability limit of n-butane (w-C4H10) in air is 1.8 % by volume, calculate the adiabatic flame temperature at the limit. Assume the initial temperature to be 25 °C. Use Table 4.5. 

Heavy-gas behavior is often dominant close to the point of release and in the near field. It is particularly important when considering large releases of pressurized or refrigerated flammable materials, for which the value of the lower flammable limit is low. Typical hydrocarbons that fall into this grouping are ethane, 2.9% by vol LFL propane, 2.1% by vol LFL and the butanes, 1.8% by vol LFL (Gas Processors Suppliers Association, 1972). Farther downwind, after additional mixing with air, the concentration of the flammable material is less important because it is then less than the lower flammable limit. The other hazard of heavy gases is asphyxiation of personnel who may inadvertently enter or be surrounded by the cloud. 

Burning in other oxidizable atmospheres. Flames can propagate in mixtures of oxide of nitrogen and other oxidizable substances. For example, Bodurtha27 reports that the flammability limits for butane in nitric oxide are 7.5 percent (lower) and 12.5 percent (upper). 

Recent advances in fixed-bed-based MAN processes include the development of reactors that can operate in the fiammable regime and of the total butane recycle processes that can give overall process yields between 65% and 75%. The ability to safely use butane feed compositions that are above the lower flammability limit of approximately 1.8% in air can translate into increased production rate of MAN, which has obvious advantages from a process economics perspective. 

There are two composition limits of flammability for air and a gaseous fuel under specified conditions [76]. The lower limit is the minimum concentration of combustible gas that will support combustion, while the higher limit is the maximum concentration. Table 5.1 shows the lower and higher limits for pure hydrocarbons in air at room temperature and atmospheric pressure (RTP) [76]. For methane in air, the flammability limit is 5—15 mol%. For ethane in air, the limits are 2.9—13.0 mol%. The limits become lower with increasing molecular weight. It also is interesting to note that the limits are the same for n-pentane and isopentane, and also for -butane and isobutane. 

Clearly, heat transfer, flammability and catalyst stability are the major concerns of fixed-bed technology. In the early 1990s, tests were conducted at a commercial plant to increase MA productivity by feeding butane concentrations beyond the flammability limit (which is nominally 1.8 vol% butane in air). Apparently, the butane feed concentration in the vessel approached 2.3 vol% before the reactor head lifted off the body of the vessel (the reactor was designed to allow for such an event). Many patents have been issued that claim the interior vessel walls can be passivated to reduce the frequency of free radical formation (or quench free radicals). Free radicals initiate thermal events that lead to combustion or deflagration and even to detonation when the flame front exceeds the speed of sound. 

By injecting bntane and air separately, fluidized beds allow the use of an inlet concentration of butane that is higher than the flammable limit. The same... 

The limits of flammability for propane and butane are much narrower than most other gaseous fuels, making LPG safer in this respect. 

Similarly, Mota et al. [210] carried out the selective oxidation of butane to maleic anhydride over VPO mixed oxides-based catalysts enclosed in an MFI membrane. Different feed configurations of the zeohte-membrane reactor were tested to outperform the conventional co-feed configuration. The results achieved were rather similar however, the authors pointed out the possibility to take advantage of the O2 distribution, which limits the flammability problems and allows operation with higher butane concentrations than those used in conventional processes. 

Although hydrocarbon aerosol propellants are relatively inexpensive, nontoxic, and environmentally friendly (since they are not damaging to the ozone layer and are not greenhouse gases), their use is limited by their flammability. While hydrocarbon propellants are primarily used in topical aerosol formulations, it is possible that butane may also be useful in metered-dose inhalers as a replacement for chloro-fluorocarbons. 

Hazard TLV Asphyxiant (Proposed TWA 2500 ppm). Flammable, dangerous fire risk, explosive limits in air 2.4—9.5%. For storage, see butane (note). 

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