Impurity ratios

We should add that while resolution and peak capacity are excellent criteria of merit for the separation of multicomponent mixtures into discrete zones, other criteria exist, some very general, for judging the efficacy of separation and purification in any separative operation (see Section 1.4). Various terms such as impurity ratio and purity index abound. Rony has developed a criterion termed the extent of separation [22]. Stewart, as well as de Clerk and Cloete, have shown that entropy can be formulated as a very general measure of separation power, as we might expect from the discussion of Section 1.6 [23,24]. An excellent discussion of separation indices, with an emphasis on non-Gaussian zones (below), is found in Dose and Guiochon [25]. 

An important aspect of dealing with chemical threats is the ability to trace their sources, which is addressed by the discipline of chemical forensics." With increasingly sophisticated instrnmenta-tion, chemical forensics nses information snch as the presence of impurities, ratios of carbon and oxygen isotopes, and ratios of stereoisomers (see the discussion of chirality in Chapter 14, Section 14.11, to trace sources of toxicants such as those that may be used in terrorist attacks). 

The subscript on the impurity ratio t/j refers to the phase (fraction or region) under consideration. Since phase 1 is supposed to contain primarily species 1 as a valid separation goal, species 2 in phase 1 is an impurity. Perfect separation therefore requires 1/1 = 0 and 1/2 = 0. For imperfect separation t/j > 0. Note, however, that 1/1 = 0 does not imply 1/2 = 0 or vice versa. 

With regard to the various indices of separation, the definitions of k li, Kji and Ki remain unaffected, regardless of whether / = 1, 2 or i = 1, 2,..., n. The impurity ratio pj defined earlier according to de Clerk and Cloete (1971) has to he modified for a multicomponent system impurity ratio for the ith species in the/th region ... 

Example 2.5.1 Consider the time-varying molar outputs of two species 1 and 2 from a chromatographic system (Figure 2.5.1(b)). Assume both outputs, which are overlapping each other, to be Gaussian and o,i = a. If the cut point is located such that the impurity ratio in each region is the same, and if it is known that = m, develop a relation between this impurity ratio and the resolution. Calculate the values of this impurity ratio for i , = 0.2, 0.6, 1.0 and 1.4. 

Solution The impurity ratios for regions 1 and 2 are defined by relation (1.4.3), where region 1 is from, say, to and region 2 is from to We have = nhxjmw and... 

This is an exothermic, reversible, homogeneous reaction taking place in a single liquid phase. The liquid butadiene feed contains 0.5 percent normal butane as an impurity. The sulfur dioxide is essentially pure. The mole ratio of sulfur dioxide to butadiene must be kept above 1 to prevent unwanted polymerization reactions. A value of 1.2 is assumed. The temperature in the process must be kept above 65°C to prevent crystallization of the butadiene sulfone but below lOO C to prevent its decomposition. The product must contain less than 0.5 wt% butadiene and less thM 0.3 wt% sulfur dioxide. 

The parameter r continues to measure the ratio of the number of A and B groups the factor 2 enters since the monofunctional reagent has the same effect on the degree of polymerization as a difunctional molecule with two B groups and, hence, is doubly effective compared to the latter. With this modification taken into account, Eq. (5.40) enables us to quantitatively evaluate the effect of stoichiometric imbalance or monofunctional reagents, whether these are intentionally introduced to regulate or whether they arise from impurities or side reactions. 

The ratio of reactants had to be controlled very closely to suppress these impurities. Recovery of the acrylamide product from the acid process was the most expensive and difficult part of the process. Large scale production depended on two different methods. If soHd crystalline monomer was desired, the acrylamide sulfate was neutralized with ammonia to yield ammonium sulfate. The acrylamide crystallized on cooling, leaving ammonium sulfate, which had to be disposed of in some way. The second method of purification involved ion exclusion (68), which utilized a sulfonic acid ion-exchange resin and produced a dilute solution of acrylamide in water. A dilute sulfuric acid waste stream was again produced, and, in either case, the waste stream represented a... 

For caustic crevice environment, a plant-specific chemical impurity molar ratio <0.5 is defined, eg, Na Cl molar ratio <0.5. ... 

Gas purification processes fall into three categories the removal of gaseous impurities, the removal of particulate impurities, and ultrafine cleaning. The extra expense of the last process is only justified by the nature of the subsequent operations or the need to produce a pure gas stream. Because there are many variables in gas treating, several factors must be considered (/) the types and concentrations of contaminants in the gas (2) the degree of contaminant removal desired (J) the selectivity of acid gas removal required (4) the temperature, pressure, volume, and composition of the gas to be processed (5) the carbon dioxide-to-hydrogen sulfide ratio in the gas and (6) the desirabiUty of sulfur recovery on account of process economics or environmental issues. 

Chemical precipitation and solvent extraction are the main methods of purifying wet-process acid, although other techniques such as crystallisa tion (8) and ion exchange (qv) have also been used. In the production of sodium phosphates, almost all wet-process acid impurities can be induced to precipitate as the acid is neutralized with sodium carbonate or sodium hydroxide. The main exception, sulfate, can be precipitated as calcium or barium sulfate. Most fluorine and siUca can be removed with the sulfate filter cake as sodium fluorosiUcate, Na2SiFg, by the addition of sodium ion and control of the Si/F ratio in the process. 

Iron Blocks. Chemically, iron blacks are based on the binary iron oxide, FeOFe2 O3. Although the majority is produced in the cubical form, these can also be produced in acicular form. Most of the black iron oxide pigments contain iron(III) oxide impurities, giving a higher ratio of iron(III) than would be expected from the theoretical formula. 

The devitrification rate is extremely sensitive to both surface and bulk impurities, especially alkah. Increased alkah levels tend to increase the devitrification rate and lower the temperature at which the maximum rate occurs. For example, a bulk level of 0.32 wt % soda increases the maximum devitrification rate 20—30 times and lowers the temperature of maximum devitrification to approximately 1400°C (101). The impurity effect is present even at trace levels (<50 ppm) and can be enhanced with the addition of alumina. The devitrification rate varies inversely with the ratio of alumina-to-alkah metal oxide. The effect is a consequence of the fact that these impurities lower glass viscosity (102). 

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