Sulfate residence time

This process yields satisfactory monomer, either as crystals or in solution, but it also produces unwanted sulfates and waste streams. The reaction was usually mn in glass-lined equipment at 90—100°C with a residence time of 1 h. Long residence time and high reaction temperatures increase the selectivity to impurities, especially polymers and acrylic acid, which controls the properties of subsequent polymer products. 

NMe is now commercially available and is prepd by the vapor phase nitration of methane at a ratio of 9 moles of methane to I mole of nitric acid at 475° and a residence time of 0.18sec (Ref 12) or by the similar nitration of aliphatic hydrocarbons (Ref 8). Other prepns are from Me sulfate and Na nitrite (Ref 26) by the oxidn of Me amine with dinitrogen trioxide in the gas phase or in methylene chloride, yield 27%... 

The definition of turnover time is total burden within a reservoir divided by the flux out of that reservoir - in symbols, t = M/S (see Chapter 4). A typical value for the flux of non-seasalt sulfate (nss-SOl"") to the ocean surface via rain is 0.11 g S/m per year (Galloway, 1985). Using this value, we may consider the residence time of nss-S04 itself and of total non-seasalt sulfur over the world oceans. Appropriate vertical column burdens (derived from the data review of Toon et ai, 1987) are 460 fxg S/m for nss-801 and 1700 jig S/m for the sum of DMS, SO2, and nss-S04. These numbers yield residence times of about 1.5 days for nss-S04 and 5.6 days for total non-seasalt sulfur. We might infer that the oxidation process is frequently... 

Figure 13-5 is the box model of the remote marine sulfur cycle that results from these assumptions. Many different data sets are displayed (and compared) as follows. Each box shows a measured concentration and an estimated residence time for a particular species. Fluxes adjoining a box are calculated from these two pieces of information using the simple formula, S-M/x. The flux of DMS out of the ocean surface and of nss-SOl back to the ocean surface are also quantities estimated from measurements. These are converted from surface to volume fluxes (i.e., from /ig S/(m h) to ng S/(m h)) by assuming the effective scale height of the atmosphere is 2.5 km (which corresponds to a reasonable thickness of the marine planetary boundary layer, within which most precipitation and sulfur cycling should take place). Finally, other data are used to estimate the factors for partitioning oxidized DMS between the MSA and SO2 boxes, for SO2 between dry deposition and oxidation to sulfate, and for nss-SO4 between wet and dry deposition. 

Although hydrogen sulfide does not react photochemically, it may be transformed to sulfur dioxide and sulfate by nonphotochemical oxidation reactions in the atmosphere. Its atmospheric residence time is typically less than 1 day (Hill 1973), but may be as high as 42 days in winter (Bottenheim and Strausz 1980). 

The ammoxidacion reaction is carried out at about 800°F and 30 psi. Because it is highly exothermic, heat is removed continuously from the reactor by hear exchangers. The residence time of the reactants is about three seconds. The reaction gases are cooled as they pass by the water-to-steam heat exchanger in the reactor. The effluent is treated for removal of ammonia by scrubbing it with water acidified with sulfuric acid, forming ammonium sulfate, a marketable commodity that can be recovered by crystallization. But that s another story. 

Calcium sulfate crystals were precipitated in a Continuous Mixed Suspension Mixed Product Removal (CMSMPR) crystallizer by mixing of calcium phosphate and sulfuric acid feed streams. The formed calcium sulfate hydrate (anhydrite, hemihydrate and dihydrate) mainly depends on the temperature and the solution composition. The uptake of cadmium and phosphate ions in these hydrates has been studied as a function of residence time and solution composition. In anhydrite, also the incorporation of other metal ions has been investigated. The uptake was found to be a function of both thermodynamics and kinetics. 

The level of impurity uptake can be considered to depend on the thermodynamics of the system as well as on the kinetics of crystal growth and incorporation of units in the growing crystal. The kinetics are mainly affected by the residence time which determines the supersaturation, by the stoichiometry (calcium over sulfate concentration ratio) and by growth retarding impurities. The thermodynamics are related to activity coefficients in the solution and the solid phase, complexation constants, solubility products and dimensions of the foreign ions compared to those of the ions of the host lattice [2,3,4]. 

The aim of this work is to study the incorporation of cadmium and phosphate in the three calcium sulfate modifications. The uptake of other metal ions in AH will also be described. Kinetic effects of operating conditions such as the residence time, sulfuric acid and phosphate concentration upon the phosphate and cadmium uptake has been investigated. In addition the influence of a growth retarding impurity, AIF3, on the cadmium and phosphate uptake will be given. 

Chemically pure reagents were used. Cadmium was added as its sulfate salt in concentrations of about 50 ppm. Lanthanides were added as nitrates. For the experiments with other metal ions so-called "black acid from a Nissan-H process was used. In this acid a large number of metal ions were present. To achieve calcium sulfate precipitation two solutions, one consisting of calcium phosphate in phosphoric acid and the other of a phosphoric acid/sulfuric acid mixture, were fed simultaneously in the 1 liter MSMPR crystallizer. The power input by the turbine stirrer was 1 kW/m. The solid content was about 10%. Each experiment was conducted for at least 8 residence times to obtain a steady state. During the experiments lic iid and solid samples were taken for analysis by ICP (Inductively Coupled Plasma spectrometry, based on atomic emission) and/or INAA (Instrumental Neutron Activation Analysis). The solid samples were washed with saturated gypsum solution (3x) and with acetone (3x), and subsequently dried at 30 C. The details of the continuous crystallization experiments are given in ref. [5]. 

General, in order to precipitate either AH, HH or DH the appropriate temperature and phosphate and/or sulfate concentration must be selected. In table 2 the operational conditions are listed for each experiment. In the experiments where DH is crystallized only the sulfuric acid concentration, the temperature and the residence time were varied. In case of HH crystallization various sulfuric acid and phosphoric acid concentrations were applied. AH crystallization has been studied at various residence times, in the presence of lanthanides or AIF3 and also in black acid from a Nissan-H process. 

To be able to interpret these results and to correct for the lower calcium concentrations at high sulfate and phosphate concentrations, the partition coefficients D have been determined. These values follow from the slopes of the curves in figure 7. For 5.5 and 6.0 M HjPO a D of about 1.5 10" is obtained. A similar D-value for both acid concentrations should indeed be obtained, when the activity coefficients of the ions in solution is not strongly affected by the acid concentration. The D-value for 6.5 M H PO lies somewhat higher. This could e.g. be caused by a higher activity coefficient of cadmium compared to calcium at this acid concentration. The thermodynamic D-value cannot be determined by increasing the residence time, because a residence time of 2400 seconds already caused anhydrite formation. 

The supersaturation is too low in all experiments to be measured accurately, but it seems reasonable to assume that the effect of residence time is imposed through the kinetics. Another observation is that the D-value for cadmium uptake in anhydrite is about ten times higher than in HH or DH. An explanation for this higher D seems to be related to the crystal structures of the calcium sulfates. Only the AH structure matches with an anhydrous CdSO phase, while no hemi- or dihydrate phase of CdS04 exists. 

In addition to the differences in geographical distribution of the greenhouse gases compared to the aerosol particles and the day-night differences, there are also differences in their temporal behavior. As discussed earlier, typical residence times for sulfate particles are about a week, whereas that of C02 is about 100 years. As a result, the impacts of sulfate aerosols are almost immediately manifested, whereas those due to C02 occur over decades to centuries (Schwartz, 1993). 

A micelle is a dynamic structure. Surfactants leave the micelle and go into solution while other surfactants enter the micelle from solution. The timescales involved depend critically on the specific structure of the surfactant, in particular on the length of the hydrocarbon chain. For example, the residence time of a single dodecylsulfate (CH3(CH2)h0S03 ) in a SDS micelle at 25° C is 6 /xs [525], If we reduce the chain length by two methylene units to decyl sulfate (CH3(CH2)g0S03 ) the residence time decreases to roughly 0.5 /us. Tetradecyl sulfate (CH3(CH2)i30S03 ), which has two methylene units more than dodecylsulfate, typically remains 83 /its in a micelle. 

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