Pressure elevation, flammable atmospheres

The catalytic vapor-phase oxidation of propylene is generally carried out in a fixed-bed multitube reactor at near atmospheric pressures and elevated temperatures (ca 350°C) molten salt is used for temperature control. Air is commonly used as the oxygen source and steam is added to suppress the formation of flammable gas mixtures. Operation can be single pass or a recycle stream may be employed. Recent interest has focused on improving process efficiency and minimizing process wastes by defining process improvements that use recycle of process gas streams and/or use of new reaction diluents (20-24). 

A process is described [224] in which an exothermic reaction takes place in a semi-batch reactor at elevated temperatures and under pressure. The solid and liquid raw materials are both toxic and flammable. Spontaneous ignition is possible when the reaction mass is exposed to air. Therefore, the system must be totally enclosed and confined in order to contain safely any emissions arising from the loss of reactor control, and to prevent secondary combustion reactions upon discharge of the materials to the atmosphere. Further, procedures and equipment are necessary for the safe collection and disposal of solid, liquid, and gaseous emission products. 

The behavior of flammability limits at elevated pressures can be explained somewhat satisfactorily. For simple hydrocarbons (ethane, propane,..., pentane), it appears that the rich limits extend almost linearly with increasing pressure at a rate of about 0.13 vol%/atm the lean limits, on the other hand, are at first extended slightly and thereafter narrowed as pressure is increased to 6 atm. In all, the lean limit appears not to be affected appreciably by the pressure. Figure 25 for natural gas in air shows the pressure effect for conditions above atmospheric. 

There are no reliable theoretical equations for the combined effects of pressure, temperature, oxidant type and concentration, and fuel mixture composition on the limits of flammability. However, chemical processes are often operated at elevated temperatures and pressures and at times in oxidant enriched atmospheres. Flammability limits should be measured at actual process conditions with adequate test methods. 

As for the pressure levels in the reaction operations, 1.5 atm is selected for the chlorination reaction to prevent the leakage of air into the reactor to be installed in the task integration step. At atmospheric pressure, air might leak into the reactor and build up in sufficiently large concentrations to exceed the flammability limit. For the pyrolysis operation, 26 atm is recommended by the B.F. Goodrich patent (1963) without any justification. Since the reaction is irreversible, the elevated pressure does not adversely affect the conversion. Most likely, the patent recommends this pressure to increase the rate of reaction and, thus, reduce the size of the pyrolysis furnace, although the tube walls must be thick and many precautions are necessary for operation at elevated pressures. The pressure level is also an important consideration in selecting the separation operations, as will be discussed in the next synthesis step. 

Flammability limits are dependent on both temperature and pressure. Values quoted in the literature are generally for normal atmospheric temperature and pressure with little information available for elevated conditions. 

Carbon monoxide is a colorless, odorless, flammable toxic gas. Liquid carbon monoxide is a cryogenic liquid, which exists at a temperature of -313°F (-192°C) and atmospheric pressure. It becomes a flammable vapor upon addition of heat. If inhaled, concentrations of 0.4 percent in air prove fatal in less than 1 hour, while inhalation of high concentrations can cause sudden collapse with little or no warning. Pure carbon monoxide has a negligible corrosive effect on metals at atmospheric pressures. Impure carbon monoxide, containing water vapor, sulfur compounds, or other impurities causes stress corrosion to ferrous metals at elevated pressures. 

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