Evaporation ratio

Establish the maximum evaporation ratio (i.e., evaporated quantity/feed quantity) without compromising the pumpability of the bottoms discharge (or impurity) stream and without excessive decomposition of the bottoms stream while minimizing the volatility of any bottoms components (such as HF or HC1) or decomposition of the overheads components. 

Efficient purification is achieved by selective evaporation and condensation. This technique is applicable to actinides of medium volatility i.e. Am or Cm The volatile impurities are eUminated by selective condensation of the actinide metal, less volatile impurities are left in the crucible. The efficiency of this refining method is determined by the relative evaporation ratio, which for two elements A and B equals the ratio of their activities at a given temperature. 

Precipitation/Evaporation ratio (P/E) The ratio between water supplied by precipitation (rainfall) and that lost by evaporation. In lakes, the P term is normally taken to be all inputs to the waterbody (i.e., runoff, groundwater flow and direct precipitation on the lake surface), E is water lost by evaporation from the lake surface. 

Here, the term is the evaporation ratio which is zero for cooling-crystal-... 

The aforementioned scale-up procedure was used to design a series of jet-spouted bed dryers based on experiments performed on a 0.5 m unit. Field trials on a 1.5-m dryer filled with 30 kg of Teflon cubes 4 mm in equivalent diameter have shown that the volumetric evaporation ratio varied from 55.4 kg/(m%) to 120.6 kg/(m h) against 100 kg/(m h) taken for design calculations. Also, the heat utilization factor from 0.192 to 0.335 corresponded well to the assumed value of 0.25 (Strumillo et al., 1980). 

Not all molecules striking a surface necessarily condense, and Z in Eq. VII-2 gives an upper limit to the rate of condensation and hence to the rate of evaporation. Alternatively, actual measurement of the evaporation rate gives, through Eq. VII-2, an effective vapor pressure Pe that may be less than the actual vapor pressure P. The ratio Pe/P is called the vaporization coefficient a. As a perhaps extreme example, a is only 8.3 X 10" for (111) surfaces of arsenic [11]. 

In a typical experiment 105 mg (0.50 mmol) of 3.8c, dissolved in a minimal amount of ethanol, and 100 mg (1.50 mmol) of 3.9 were added to a solution of 1.21g (5 mmol) of Cu(N03)2 BH20 and 5 mmol of ligand in 500 ml of water in a 500 ml flask. -Amino-acid containing solutions required addition of one equivalent of sodium hydroxide. When necessary, the pH was adjusted to a value of 5 ( -amino acids) and 7.5 (amines). The flask was sealed carefully and the solution was stirred for 2A hours, followed by extraction with ether. After drying over sodium sulfate the ether was evaporated. Tire endo-exo ratios were determined from the H-NMR spectra of the product mixtures as described in Chapter 2. 

Endo-exo ratios of the micelle-catalysed reactions have been determined by adding 0.25 mmol of 5.1c and 0.5 mmol of 5.2 to a solution of 5 mmol of surfactant and 0.005 mmol of EDTA in 50 ml of water in carefully sealed 50 ml flasks. The solutions were stirred for 7 days at 26 C and subsequently freeze-dried. The SDS and CTAB containing reaction mixtures were stirred with 100 ml of ether. Filtration and evaporation of the ether afforded the crude product mixtures. Extraction of the Diels-Alder adducts from the freeze-dried reaction mixture containing C12E7 was performed by stirring with 50 ml of pentane. Cooling the solution to -18 C resulted in precipitation of the surfactant. Filtration and evaporation of the solvent afforded the adduct mixture. Endo-exo ratios... 

A further consequence of the high temperatures is that much of the sample is simply evaporated without producing isolated positive ions. There is a competition between formation of positive ions and the evaporation of neutral particles. Since the mass spectrometer examines only isolated charged species, it is important for maximum sensitivity that the ratio of positive ions to neutrals be as large as possible. Equation 7.1 governing this ratio is given here. 

In Equation 7.1, n+/n is the ratio of the number of positive ions to the number of neutrals evaporated at the same time from a hot surface at temperature T (K), where k is the Boltzmann constant and A is another constant (often taken to be 0.5 see below). By inserting a value for k and adjusting Equation 7.1 to common units (electronvolts) and putting A = 0.5, the simpler Equation 7.2 is obtained. 

As ions and neutrals evaporate from a heated filament surface, the amount of sample decreases and the surface densities (C, Cq) must decrease. Therefore, Equation 7.1 covers two effects. The first was discussed above and concerns the changing value for the ratio n+/n° as the temperature of the filament is varied, and the other concerns the change in the total number of ions desorbing as the sample is used up. The two separate effects are shown in Figure 7.8a,b. Combining the two effects (Figure 7.8c) reveals that if the temperature is increased to maintain the flow of ions, which drops naturally as the sample is used up (time), then eventually the flow of ions and neutrals becomes zero whatever the temperature of the filament because the sample has disappeared from the filament surface. 

Figure 7.9 shows a schematic representation of this effect, in which the ratio of the two isotopes changes with time. To obtain an accurate estimate of the ratio of ion abundances, it is better if the relative ion yields decrease linearly (Figure 7.9) which can be achieved by adjusting the filament temperature continuously to obtain the desired linear response. An almost constant response for the isotope ratio can be obtained by slow evaporation of the sample, viz., by keeping the filament temperature as low as is consistent with sufficient sensitivity of detection (Figure 7.9). 

The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

Schematic illustrations of the effect of temperature and surface density (time) on the ratio of two isotopes, (a) shows that, generally, there is a fractionation of the two isotopes as time and temperature change the ratio of the two isotopes changes throughout the experiment and makes difficult an assessment of their precise ratio in the original sample, (b) illustrates the effect of gradually changing the temperature of the filament to keep the ratio of ion yields linear, which simplifies the task of estimating the ratio in the original sample. The best method is one in which the rate of evaporation is low enough that the ratio of the isotopes is virtually constant this ratio then relates exactly to the ratio in the original sample.

The small differences in physical properties of substances containing elements with isotopes are manifested through mea.surement of isotope ratios. When water evaporates, the vapor is richer in its lighter isotopes ( Hj O) than the heavier one ( Hj O). Such differences in vapor pressures vary with temperature and have been used, for example, to estimate sea temperatures of 10,000 years ago (see Chapter 47). 

Calcium Chloride. Distiller waste Hquor from synthetic plants can be evaporated in multiple effect evaporators, precipitating residual sodium chloride. The resulting mother Hquor is then further evaporated to a molar ratio of lCaCl2 2H20 and cooled to produce flakes that are dried in rotary or... 

To produce a spandex fiber by reaction spinning, a 1000—3500 molecular weight polyester or polyether glycol reacts with a diisocyanate at a molar ratio of about 1 2. The viscosity of this isocyanate-terrninated prepolymer may be adjusted by adding small amounts of an inert solvent, and then extmded into a coagulating bath that contains a diamine so that filament and polymer formation occur simultaneously. Reactions are completed as the filaments are cured and solvent evaporated on a belt dryer. After appHcation of a finish, the fibers are wound on tubes or bobbins and rewound if necessary to reduce interfiber cohesion. 

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