Waste stoichiometric equations

If the composition of the waste stream is known, then the theoretical oxygen demand can be calculated from the appropriate stoichiometric equations. As a first level of approximation, we can assume that this theoretical oxygen demand would be equal to the COD. Then, experience with domestic sewage indicates that the average ratio of COD to BOD will be on the order 1.5 to 2. The following example will help to clarify these relationships. 

The atom utilization or atom efficiency concept is a useful tool for rapid evaluation of the amount of waste that will be generated by alternative routes to a particular product. It is calculated by dividing the molecular weight of the desired product by the sum total of the molecular weights of all the substances produced in the stoichiometric equation of the reaction(s) in question. The comparison is made on a theoretical (i.e. 100% chemical yield) basis. Fig. 2.8 shows a simple illu.stration of the concept for ethylene oxide manufacture. 

As noted above, a knowledge of the stoichiometric equation allows one to predict the theoretical minimum amount of waste that can be expected. This led to the concept of atom economy [8] or atom utilization [9] to quickly assess the environmental acceptabihty of alternatives to a particular product before any experiment is performed. It is a theoretical number, that is, it assumes a chemical yield of 100% and exactly stoichiometric amounts and disregards substances which do not appear in the stoichiometric equation. 

Two useful measures of the potential environmental acceptability of chemical processes are the E factor [12-18], defined as the mass ratio of waste to desired product and the atom efficiency, calculated by dividing the molecular weight of the desired product by the sum of the molecular weights of all substances produced in the stoichiometric equation. The sheer magnitude of the waste problem in chemicals manufacture is readily apparent from a consideration of typical E factors in various segments of the chemical industry (Table 1.1). 

Where T)is flame temperature in K MC is moisture content of the waste, expressed on a total weight basis SR is defined as stoichiometric ratio or moles O2 avadable/moles O2 required for complete oxidation of the carbon, hydrogen, and sulfur in the fuel, ie, 1/SR = equivalence ratio and is temperature of the combustion air, expressed in K. In Fnglish units, this equation is as follows ... 

The total quantity of reactant is limited to 5.000 g. If either reactant is in excess, the amount in excess will be wasted, because it cannot be used to form product. Thus, we obtain the maximum amount of product when neither reactant is in excess (i.e., when there is a stoichiometric amount of each present). The balanced chemical equation for this reaction, 2 KI + Pb (N03 )2 -> 2 KN03 + Pbl2, shows that... 

This parameter is equivalent to "atom utilization suggested by Sheldon [56] However, one category of selectivity is largely ignored by organic chemists what I call the atom selectivity or atom utilization. The complete disregard of this important parameter is the root cause of the waste problem in fine chemical manufacture. The parameter is a constant of the synthesis route and is a measure of material utilization. As BA, increases, the smaller the amount of joint products theoretically produced in the synthesis route chosen. BA, is thus the maximum possible value of material planning productivity, because it is derived from the theoretical reaction scheme of the process, i.e., based on the stoichiometric reaction equation. 

The conversion of carboxylic acids to aldehydes is typically conducted by reduction to the alcohol with main group metal hydride reagents, followed by oxidation to the aldehyde, or by reaction of fhe corresponding ester with stoichiometric amounts of the the aluminum hydride DIBAL. Thus, development of a selective hydrogenation of carboxylic esters to aldehydes would be valuable and would generate less waste. To overcome the typically lower reactivity of fhe acid reagent versus the aldehyde product, Yamamoto developed a system in which ihe acid is converted to an anhydride, and fhe anhydride is hydrogenated to the aldehyde (Equation 15.118). 

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