Subatmospheric pressure

Most of the principal industrial processes that necessitate the use of subatmospheric pressures are listed here, together with the typical pressure range for each one. The processes involve the evacuation or removal of various... 

FIGURE 18.2 Superatmospheric and subatmospheric pressures below and above a free-water surface. 

Cole, R., 1967, Bubble Frequencies and Departure Volumes at Subatmospheric Pressures, AIChE J. /3(4) 779-783. (2)... 

Figure 2. Second-order kinetic plot for the reaction of CO at subatmospheric pressures with Mn(CO)k(CCHsOAlBrBr2) in toluene solution (19)...

B. Subatmospheric pressure The operation is pressurized, so no penalty is applied here. 

Low (subatmospheric) pressures are not in general as hazardous as the other extreme operating conditions. But a hazard, which does exist in low pressure plant handling flammables, is the ingress of air with consequent formation of a flammable mixture. Also steam explosions may take place if a volatile material (e.g. water) is fed to a low pressure system which results to a consequent vapourization. 

Osmotic dehydration, both at atmospheric pressure or preceded by the application of subatmospheric pressure for a short time, has been proposed in the production of minimally processed fruits and vegetables, which are convenient, ready-to-eat, high-moisture but ambient stable foods. The consumer prefers minimally processed foods, as these foods have appealing fresh-like characteristics and thus superior sensory quality. However, at the same time, these foods must be microbiologically safe and stable. These somewhat conflicting goals are achievable by the application of... 

The pressure in the ERH is continuously monitored at the vent gas exit duct of the ERH drum to ensure that it operates at subatmospheric pressure. In addition, the hydrogen gas concentration is measured in the ERH drum vent gas exit duct to ensure that hydrogen gas concentration remains below 50 percent of its LEL. Signals from these monitors are used to control the ventilation gas flow by changing the speed of the induced draft fan. 

Ehrett, W.E. and Weber, J.H. Vapor-liquid equilibria at atmospheric and subatmospheric pressures for system n-hexane-methylcyclopentane, /. Chem. Eng. Data, 4(2) 142-145, 1959. 

The direct use of ammonia or dinitrogen and hydrogen to synthesize imido amido complexes is essentially pioneers territory [58, 59]. Recently, we have formed, by surface organometaUic chemistry, a well-defined Ta amido imido species by reaction with either ammonia [9] or with dinitrogen and dihydrogen at subatmospheric pressure [60]. 

Sulfur vapor consists of several small molecules from Si through Si and cyclic Ss through Sio. At low pressures and temperatures above 1,500°C, sulfur occurs in vapor phase in monoclinic Si form. At temperatures between approximately 500 to 1,500°C and subatmospheric pressures, the diatomic oxygen-like S2 form predominates. The S3 and S4 molecules of uncertain structures and the Ss to Sio rings are usually minor components of sulfur vapors. 

Figure 15. Burning rate-pressure dependence at subatmospheric pressures for ammonium perchlorate with different binders...

Figure 16 shows our results for the effect of several binders on the burning rate at subatmospheric pressures. This set of curves represents propellants loaded as highly as practical with 5- mean particle size unimodal spherical AP. [This particular set of parameters was chosen... 

Burning at subatmospheric pressure is accompanied by the evolution of white fumes which solidify upon contact with a cold surface. For the LP3 propellant, it is larger in quantity and increases considerably as soon as the ash starts forming. This smoke was found to contain 25% ammonium perchlorate and 55% ammonium chloride. A trap was installed to determine the amount of smoke evolved per unit mass of propellant as a function of pressure and oxidizer particle size, but the results were inconclusive. The weight of sublimate caught was approximately 20% of the original weight of the propellant. 

Butadiene and Isoprene. Butane may be transformed directly to 1,3-buta-diene on chromia-alumina (Houdry Catadiene process).144-146 172 The most significant condition is operation under subatmospheric pressure (0.1-0.4 atm), which provides an improved yield of 1,3-butadiene. Operating at about 600°C, the process produces a mixture of butenes and 1,3-butadiene. After the removal of the latter, the remaining butane-butenes mixture is mixed with fresh butane and recycled. Extensive coke formation requires regeneration of the catalyst after a few minutes of operation. 1,3-Butadiene yields up to 63% are obtained at a conversion level of 30 40%. 

Two problems arose with the use of the barometric condenser. First, if the condenser operated at subatmospheric pressure, how can the water be drained out of the condenser That is easy. Set the condenser on a hill 34 ft high. Then drain the water down, through a barometric leg, to a seal pot. The pressure which a column of water 34 ft high exerts is equal to one atmosphere, or one bar. Hence the term, barometric leg. ... 

Conveying capacity expressed as vol % of solids in the stream usually is well under 5 vol %. From Table 5.1, for example, it is about 1.5% for alumina and 6.0% for polystyrene pellets, figured at atmospheric pressure at 12psig these percentages will be roughly doubled, and at subatmospheric pressures they will be lower. 

Vacuum pumps produce subatmospheric pressures in process equipment. Often they are compressors operating in reverse but other devices also are employed. Operating ranges of some commercial equipment are stated in Table 7.3. 

Application ranges of the various kinds of devices for maintenance of subatmospheric pressures in process equipment are shown in Table 7.3. The use of mechanical pumps—compressors in reverse— for such purposes is mentioned earlier in this chapter. Pressures also can be reduced by the action of flowing fluids. For instance, water jets at 40psig will sustain pressures of 0.5-2.0psia. For intermediate pressure ranges, down to O.lTorr or so, steam jet ejectors are widely favored. They have no moving parts, are quiet, easily installed, simple, and moderately economical to operate, and readily adaptable to handling corrosive vapor mixtures. A specification form is in Appendix B. 

Subatmospheric pressure usually is expressed in reference to perfect vacuum or absolute zero pressure, lake absolute zero temperature (the concept is analogous), absolute zero pressure cannot be achieved, but it does provide a convenient reference datum. Standard atmospheric pressure is 14.695 psi absolute, 30 inches of mercury absolute, or 760 mmHg of density 13.595 g/cm3 where acceleration due to gravity is g = 980.665 emir. ] mmHg. which equals 1 torr. is the most commonly used unit of absolute pressure. Derived units, the million or micrometer, representing 1/1000 of 1 mmHg or 1 torr, are also used for subtorr pressures. 

Lyophilization processes consist of the manipulation of environmental conditions of subambient temperatures and subatmospheric pressures. These extraordinary conditions are created by the lyophilization equipment. The success of the process, therefore, relies heavily on the operating performance of the lyophilizer. Confidence in the ability of the equipment to create these necessary environmental conditions is achieved through the successful completion of a comprehensive IQ and OQ. Without the proper performance of the equipment, there is limited opportunity for successful processing of materials. 

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