Microeconomic theory of chemical production processes: application to aqueous VOC air stripping operations

A novel microeconomic formalism for chemical processes is applied to air stripping of harmful volatile organic compounds (VOCs) from aqueous streams in a conventional packed tower. A simplified conceptual process model is combined with techno-economic aggregation rules to obtain an engineering production function, which relates process inputs (capital, material, labor, and energy) to output (stripped water flow rate). Process relationships determine column length and diameter required to achieve a specified VOC percent recovery from an aqueous stream of given composition and flow rate. Total capital is determined using a defined capital unit (1 capital unit=10 foot of 6-inch schedule 40 carbon steel welded pipe=$122, Jan. 1990) to put all installed capital equipment on the same basis and multiplying by 3.37 to account for the necessary plant infrastructure. Process labor is obtained from an empirical correlation, which relates workerhours to mass throughput. Simulations were made for four systems comprised from each of two representative random packings (i.e. 25-mm plastic Pall rings and 25-mm ceramic Raschig rings) and two model VOCs (i.e. 1,2-dichloroethane and trichloroethylene). Several examples are given, and process, technical and economic analyses of the results are presented. Two-dimensional (capital-energy) optimization was effective in determining the least cost combination of inputs for a plant to produce a specified output of stripped water. Furthermore, conventional economic models (e.g. Cobb-Douglas model) and associated methods (e.g. linear programming) were shown to be ineffective for air stripping plant design and optimization. Moreover, increasing returns to scale with economies of scale (declining long-run average cost) are prevalent.