Processes with multiple impurities

The framework developed above can be extended to the case in which multiple impurities are present in the system. It is straightforward to show that, in such circumstances, the dimension of the state-space realization of the model of the slow dynamics (Equation (4.34)) will be equal to the number of components whose rate of input to the process (either as feed impurities or as reaction byproducts) is small. 

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A closer inspection of this system revealed three sites with a relatively low pATai Nl-H, C5 -H, and CS -CHa. In considering the stoichiometry of the metalation event, the first equivalent of base should selectively deprotonate the desired Nl-H, and the second equivalent absorbs the HCl generated by the acylation, returning a salt form of the product. The observation of multiple impurities being generated as the reaction progressed was attributed to the competitive deprotonation at either C5 -H and/or CS -CHs, followed by the subsequent acylation of these two undesired sites. The unique reactivity of magnesium in this process was proposed to be associated with the formation of complex 107, which served as a stabilized intermediate in the desired C3-acylation. 

Crude oil is a mixture of many different hydrocarbons and small amounts of impurities. The composition of crude oil can vary significantly depending on its source. Petroleum refineries are a complex system of multiple operations and the operations used at a given refinery depend upon the properties of the crude oil to be refined and the desired products. For these reasons, no two refineries are alike. Portions of the outputs from some processes are re-fed back into the same process, fed to new processes, fed back to a previous process, or blended with other outputs to form finished products. The major unit operations typically involved at petroleum refineries are described briefly below. In addition to those listed below, there are also many special purpose processes that cannot be... 

Liquid-liquid extraction (also called solvent extraction) is the transfer of a substance (a consolute) dissolved in one liquid to a second liquid (the solvent) that is immiscible with the first liquid or miscible to a very limited degree. This operation is commonly used in fine chemicals manufacture (I) to wash out impurities from a contaminated solution to a solvent in order to obtain a pure solution (raffinate) from which the pure substance will be isolated, and (2) to pull out a desired substance from a contaminated liquid into the solvent leaving impurities in the first liquid. The former operation is typically employed when an organic phase is to be depleted from impurities which are soluble in acidic, alkaline, or neutral aqueous solutions Water or a diluted aqueous solution is then used as the solvent. The pure raffinate is then appropriately processed (e.g. by distillation) to isolate the desired consolute. In the latter version of extraction impurities remain in the first phase. The extract that has become rich in the desired consolute is then appropriately processed to isolate the consolute. Extraction can also be used to fractionate multiple consolutes. 

Modern SMR plants (Figure 2.5b) incorporate a PSA unit for purifying hydrogen from C02, CO, and CH4 impurities (moisture is preliminarily removed from the process gas). The PSA unit consists of multiple (parallel) adsorption beds, most commonly filled with molecular sieves of suitable pore size it operates at the pressure of about 20 atm. The PSA off-gas is composed of (mol%) C02—55, H2—27, CH4—14, CO—3, N2—0.4, and some water vapor [11] and is burned as a fuel in the primary reformer furnace. Generally, SMR plants with PSA need only a HT-WGS stage, which may somewhat simplify the process. 

A pivotal step in the analytical process is sample preparation. Frequently liquid-liquid extractions (LLEs) are used. Solvents, pH, and multiple back extractions are all manipulated to increase selectivity and decrease unwanted contaminants before injection on the GC system. Solid phase extraction (SPE) is more convenient than it used to be because of an increase in commercially available SPE columns. SPE columns are packed with an inert material that binds the drug of interest, allowing impurities to pass through. As with LEE, solvent choices and pH affect retention and recovery. There are three commercially available types of SPE columns, diatomaceous earth (which uses the same principles as LLE), polystyrene-divinylbenzene copolymer, and mixed mode bonded silica (Franke and de Zeeuw, 1998). 

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