Residence times, atmospheric

Residual moisture moisture remaining in the sample after air drying assayed by determining the mass lost from drying the sample at 104 to 110°C (219 to 230°F) at specified conditions of residence time, atmosphere, particle size, sample mass, and equipment configuration (ASTM D-3173 ASTM D-3302). 

The StabUity or persistence of a poUutant in the atmosphere depends on the poUutant s atmospheric residence time. Mean residence times and principal atmospheric sinks for a variety of species are given in Table 2. Species like SO2, (NO and NO2), and coarse particles have lifetimes less than... 

Table 2. Mean Atmospheric Residence Times (t) and Dominant Sinks of Air Pollutants...

Gas CAS Registry Number 1990 Concentrations Concentration increases, %/yr Greenhouse efficiency Atmospheric residence times, c yrs... 

The majority of thermal polymerizations are carried out as a batch process, which requires a heat-up and a cool down stage. Typical conditions are 250—300°C for 0.5—4 h in an oxygen-free atmosphere (typically nitrogen) at approximately 1.4 MPa (200 psi). A continuous thermal polymerization has been reported which utilizes a tubular flow reactor having three temperature zones and recycle capabiHty (62). The advantages of this process are reduced residence time, increased production, and improved molecular weight control. Molecular weight may be controlled with temperature, residence time, feed composition, and polymerizate recycle. 

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

Both vapor-phase and Hquid-phase processes are employed to nitrate paraffins, using either HNO or NO2. The nitrations occur by means of free-radical steps, and sufftciendy high temperatures are required to produce free radicals to initiate the reaction steps. For Hquid-phase nitrations, temperatures of about 150—200°C are usually required, whereas gas-phase nitrations fall in the 200—440°C range. Sufficient pressures are needed for the Hquid-phase processes to maintain the reactants and products as Hquids. Residence times of several minutes are commonly required to obtain acceptable conversions. Gas-phase nitrations occur at atmospheric pressure, but pressures of 0.8—1.2 MPa (8—12 atm) are frequentiy employed in industrial units. The higher pressures expedite the condensation and recovery of the nitroparaffin products when cooling water is employed to cool the product gas stream leaving the reactor (see Nitroparaffins). 

The reaction rates for oxidation of atmospheric SO2 (0.05-0.5 d ) yield a sulfur residence time of several days, at most this corresponds to a transport distance of several hundred to 1000 km. The formation of HNO by oxidation is more rapid and, compared with H2SO2P results in a shorter travel distance from the emission source. H2SO4 can also react with NH to form NH HSO or (NH2 2S04 aerosols. In addition the NH NO aerosols are in equihbrium with NH (g) and HNO (g). 

Pollutants have various atmospheric residence times, with reactive gases and large aerosols being rapidly removed from air. In the London air pollution episode of December 1952, the residence time for sulfur dioxide was estimated to be five hours daily emissions of an estimated 2,000 tons of sulfur dioxide were balanced by scavenging by fog droplets, which were rapidly deposited. Most relatively inert gases remain in the atmosphere for extended periods. Sulfur hexafluoride, used extensively in the electric power industiy as an insulator in power breakers because of its inertness, has an estimated atmospheric lifetime of 3,200 years. 

On an industrial scale, PA-6 is synthesized from e-caprolactam with water as the initiator. The process is very simple if the reaction is earned out at atmospheric pressure. The polymerization is earned out in a VK-reactor (Fig. 3.23), which is a continuous reactor without a stirrer, with a residence time of 12-24 h at temperatures of 260-280°C.5,28 Molten lactam, initiator (water), and chain terminator (acetic acid) are added at the top and the polymer is discharged at the bottom to an extruder. In this extruder, other ingredients such as stabilizers, whiteners, pigments, and reinforcing fillers are added. The extruded thread is cooled in a water bath and granulated. The resultant PA-6 still contains 9-12%... 

Transformation of parent contaminants into secondary products may occur during the processes of atmospheric diffusion and transport as a result of physical, chemicjd, and photochemical processes (22). Chemical conversion within the atmosphere may also change the physico-chemical characteristics of contaminants, dramatically altering their atmospheric residence times and fates from those of the parent contaminants. The complex reactions within the atmosphere that are driven by chemical processes such as hydroxyl scavenging... 

Fluxes are linear functions of reservoir contents. Reservoir size and the residence time of the carbon in the reservoir are the parameters used in the functions. Between the ocean and the atmosphere and within the ocean, fluxes rates are calculated theoretically using size of the reservoir, surface area of contact between reservoirs, concentration of CO2, partial pressures of CO2, temperature, and solubility as factors. The flux of carbon into the vegetation reservoir is a function of the size of the carbon pool and a fertilization effect of increased CO2 concentration in the atmosphere. Flux from vegetation into the atmosphere is a function of respiration rates estimated by Whittaker and Likens (79) and the decomposition of short-lived organic matter which was assumed to be half of the gross assimilation or equal to the amount transferred to dead organic matter. Carbon in organic matter that decomposes slowly is transferred... 

Gas Principal biological source Residence time in the atmosphere... 

The deposition velocities depend on the size distribution of the particulate matter, on the frequency of occurrence and intensity of precipitation, the chemical composition of the particles, the wind speed, nature of the surface, etc. Typical values of and dj for particles below about 1 average residence time in the atmosphere for such particles is a few days. 

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