Process explosion limits

As for the selectivity of DBO, the higher the reaction pressure and the lower the reaction temperature, the higher the selectivity. As for the reaction rate, the higher the reaction temperature, the larger the rate. Therefore, the industrial operation of the process is conducted at 10—11 MPa (1450—1595 psi) and 90—100°C. In addition, gas circulation is carried out in order to keep the oxygen concentration below the explosion limit during the reaction, and to improve the CO utili2ation rate and the gas—Hquid contact rate. 

Processes involving oxygen and nitrogen oxides as catalysts have been operated commercially using either vapor- or Hquid-phase reactors. The vapor-phase reactors require particularly close control because of the wide explosive limit of dimethyl sulfide in oxygen (1—83.5 vol %) plants in operation use Hquid-phase reactions. Figure 2 is a schematic diagram for the Hquid-phase process. The product stream from the reactor is neutralized with aqueous caustic and is vacuum-evaporated, and the DMSO is dried in a distillation column to obtain the product. 

The DMR process has no aqueous effluent, gives high purity products, and is less expensive. However, if hydrogen is produced, it has to be removed carefully and should not reach explosive limits. Not all metals are sufftcientiy reactive to be suitable for the DMR process. 

Low flow, low concentration streams are best handled by a catalytic recuperative oxidizer. When the concentration of the stream is between 15% to 20% LEL (Lower Explosion Limit) then both a catalytic recuperative or thermal recuperative is the best technologies. For process streams between 20% to 25% LEL then thermal recuperative is the preferred solution. 

In catalytic incineration, there are limitations concerning the effluent streams to be treated. Waste gases with organic compound contents higher than 20% of LET (lower explosion limit) are not suitable, as the heat content released in the oxidation process increases the catalyst bed temperature above 650 °C. This is normally the maximum permissible temperature to which a catalyst bed can be continuously exposed. The problem is solved by dilution-, this method increases the furnace volume and hence the investment and operation costs. Concentrations between 2% and 20% of LET are optimal, The catalytic incinerator is not recommended without prefiltration for waste gases containing particulate matter or liquids which cannot be vaporized. The waste gas must not contain catalyst poisons, such as phosphorus, arsenic, antimony, lead, zinc, mercury, tin, sulfur, or iron oxide.(see Table 1.3.111... 

A silver-gauze catalyst is still used in some older processes that operate at a relatively higher temperature (about 500°C). New processes use an iron-molyhdenum oxide catalyst. Chromium or cohalt oxides are sometimes used to dope the catalyst. The oxidation reaction is exothermic and occurs at approximately 400-425 °C and atmospheric pressure. Excess air is used to keep the methanol air ratio helow the explosion limits. Figure 5-6 shows the Haldor Topsoe iron-molyhdenum oxide catalyzed process. 

The composition of the gas mixture, which is introduced into the tube bundle reactor (tubes of 6-12 m length and 20-50 mm diameter, filled with the Ag catalyst) consists of 15-50 vol % ethylene, 5-9% oxygen, as much as 60% methane as dilution gas, and 10-15% carbon dioxide. The reaction therefore proceeds above the upper explosion limit. The ethylene conversion runs up to 10% per cycle through the reactor. The ethylene oxide selectivity amounts to 75-83 % maximum. The formed ethylene oxide is recovered by scrubbing with water and the newly formed carbon dioxide is separated from the cycle gas, e.g., by hot potash washing process. 

GP 11] [R 19] The third explosion limit is discussed in detail in [9] as it is important from both practical and mechanistic viewpoints (230-950 °C 10-10 Pa). This limit is normally responsible for the occurrence of explosions imder ambient pressure conditions. In addition, these explosions are known to be kinetically induced by radical formation. The formation of these species is sensitive to size reduction of the processing volume owing to the impact of the wall specific surface area on radical chain termination. It turns out that the wall temperature has a noticeable, but not decisive influence on the position of the third limit The thermal explosion limit lies below the kinetic limit for all conditions specified above (Figure 3.50) [9]. 

For processes under development, the most cost-effective means of avoiding potential risk is to eliminate those materials that are inherently unsafe that is, those materials whose physical or physico-chemical properties lead to them being highly reactive or unstable. This is somewhat difficult to achieve for several reasons. First, without a full battery of tests to determine, for example, flammability, upper/lower explosivity limits and their variation with scale, minimum ignition temperatures, and so on, it is almost impossible to tell how a particular chemical will behave in a given process. Second, chemical instability may make a compound attractive to use because its inherent reactivity ensures a reaction proceeds to completion at a rapid enough rate to be useful that is, the reaction is kinetically and thermodynamically favoured. 

Because there are two positive terms in the denominator of equation 4.2.85 (either of which may be associated with the dominant termination process), this equation leads to two explosion limits. At very low pressures the mean free path of the molecules in the reactor is quite long, and the radical termination processes occur primarily on the surfaces of the reaction vessel. Under these conditions gas phase collisions leading to chain breaking are relatively infrequent events, and fst fgt. Steady-state reaction conditions can prevail under these conditions if fst > fb(a — 1). 

As the pressure in the reaction vessel increases, the mean free path of the gaseous molecules will decrease and the ease with which radicals can reach the surfaces of the vessel will diminish. Surface termination processes will thus occur less frequently fst will decline and may do so to the extent that fst + fgt becomes equal to fb oc — 1). At this point an explosion will occur. This point corresponds to the first explosion limit shown in Figure 4.1. If we now jump to some higher pressure at which steady-state reaction conditions can again prevail, similar... 

There is a third explosion limit indicated in Figure 4.1 at still higher pressures. This limit is a thermal limit. At these pressures the reaction rate becomes so fast that conditions can no longer remain isothermal. At these pressures the energy liberated by the exothermic chain reaction cannot be transferred to the surroundings at a sufficiently fast rate, so the reaction mixture heats up. This increases the rate of the process and the rate at which energy is liberated so one has a snowballing effect until an explosion occurs. 

It should be evident from this discussion that the first explosion limit will be quite sensitive to the nature of the surface of the reaction vessel and its area. If the surface is coated with a material that inhibits the surface chain termination process, the first explosion limit will be lowered. Inert foreign gases can also have the effect of lowering the first explosion limit, since they can hinder diffusion to the surface. If something like spun glass or large amounts of fine wire are inserted, one can effect an increase in the first explosion limit by changing the surface/ volume ratio of the system. 

Explosive Limits for Common Constituents in Process Industries... 

A mixture of hydrogen and chlorine gas, eventually in combination with air, can be very explosive if one of the components exceeds certain limits. In chlorine production plants, based on the electrolysis of sodium chloride solutions, there is always a production of hydrogen. It is, therefore, essential to be aware of the actual hydrogen content of chlorine gas process streams at any time. There are several places in the chlorine production process where the hydrogen content in the chlorine gas can accumulate above the explosion limits. Within the chloralkali industry, mainly two types of processes are used for the production of chlorine—the mercury- and the membrane-based electrolysis of sodium chloride solutions (brine). 

Pellistors are used to detect flammable gases like CO, NH3, CH4 or natural gas. Some flammable gases, their upper and lower explosion limits and the corresponding self-ignition temperatures are listed in Tab. 5.1. This kind of gas sensor uses the exothermicity of gas combustion on a catalytic surface. As the combustion process is activated at higher temperatures, a pellistor is equipped with a heater coil which heats up the active catalytic surface to an operative temperature of about 500 °C. Usually a Platinum coil is used as heater, embedded in an inert support structure which itself is covered by the active catalyst (see Fig. 5.33). The most frequently used catalysts are platinum, palladium, iridium and rhodium. 

UOP FCC unit, 11 700-702 UOP/HYDRO MTO process, 18 568 UOP Olex olefin separation process, 17 724 Up-and-Down Method, 25 217 U/Pb decay schemes, 25 393-394 Updraft sintering, 26 565 Upflow anaerobic sludge blanket (UASB) in biological waste treatment, 25 902 Upgraded slag (UGS), 25 12, 33 Upland Cotton, U.S., 8 13 U-Polymer, 20 189 Upper critical solution temperature (UCST), 20 320, 322 Upper explosive limit (UEL), 22 840 Upper flammability limit, 23 115 Upper flammable limit (UFL), 22 840 Upper Freeport (MVB) coal... 

Many of the early contributions to the understanding of hydrogen-oxygen oxidation mechanisms developed from the study of explosion limits. Many extensive treatises were written on the subject of the hydrogen-oxygen reaction and, in particular, much attention was given to the effect of walls on radical destruction (a chain termination step) [2], Such effects are not important in the combustion processes of most interest here however, Appendix C details a complex modem mechanism based on earlier thorough reviews [3,4],... 

Mixtures of phosphine and oxygen, both above and below the explosion limits, subjected to flash photolysis show, in the spectra, the presence of PH-, OH- and PO-radicals as well as the PH2-radical Eiuiing the reaction of atomic oxygen with phosphine visible luminescence up to 3600 A and UV emission were observed, which were attributed to the partial processes ... 

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