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Class 4 - Class 4 Heuristics for Process Synthesis(Rules of...

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Heuristics for Process Synthesis (“Rules of Thumb” - either gained through experience or common sense engineering) Class 4
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Heuristics for Raw Materials and Chemical Reactions Heuristic 1: Select raw materials and chemical reactions to avoid, or reduce, the handling and storage of hazardous and toxic chemicals 1
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- Hydrochlorination of Acetylene C 2 H 2 + HCl ==> C 2 H 3 Cl Advantages: High Conversion of C 2 H 2 to C 2 H 3 Cl using Catalyst Vinyl Chloride ($0.35/lb VC)(1 lb VC/lb VC) – ($0.25/lb HCl)(0.5834 lb HCl/lb VC) -($0.80/lb C 2 H 2 )(0.4166 lb C 2 H 2 /lb VC) = -$0.13/lb VC -Eliminated from consideration because of poor gross profit -Also can be eliminated because of high reactivity of acetylene & difficulty in assuring safe operation in the face of unanticipated disturbances 1
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Raw Materials and Chemical Reactions Select raw materials and chemical reactions to avoid or reduce the handling and storage of hazardous and toxic chemicals phosgene mono- methyl- amine methylisocyanate storage chlorine carbon monoxide MIC Sevin pesticide Temik pesticide α-naphthol modified process: no MIC storage MIC kept in vapor form to minimize mass present "WW1 nerve gas" 5x deadly than phosgene 1
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Heuristics for Distribution of Chemicals Heuristic 2: Use an excess of one chemical reactant in a reaction operation to consume a valuable, toxic, or hazardous chemical reactant Note: (a) For exothermic reactions, the excess chemical often serves the useful function of absorbing the heat of reaction and thereby maintaining more moderate temperatures. (b) An excess of one chemical reactant is also used to increase conversion of the other (limiting) reactant when the extent of reaction is limited by equilibrium. Also, side reactions can be minimized by using an excess of one reactant. 2
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Distribution of Chemicals Use an excess of one chemical reactant in a reaction operation to consume completely a valuable, toxic, or hazardous chemical reactant. 1:1 stoichiometric ratio impossible to meet exactly increase ethylene feed rate to make certain all chlorine converts chlorine the more valuable and hazardous reactant chlorine difficult to handle in product realistically, there will be other by-products that have to be considered, e.g., VCM chlorine ethylene dichloroethane unreacted ethylene separation scheme now required to pyrolysis dichloroethane recycle ethylene 2
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Heuristic 3: When nearly pure products are required, eliminate inert species before the reaction operations when the separations are easily accomplished and when the catalyst is adversely affected by the inert, but not when a large exothermic heat of reaction must be removed. 3 Balance the ease of separation of the inert(s) before/after the Reactor vs. the effect of the inert(s) on the reaction. For example, inerts may poison a catalyst.
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