The economics of water use and wastewater treatment in ammonia production



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A linear programming model is developed for evaluating the economic effects of increasingly restrictive wastewater effluent standards and higher costs of water withdrawals on production costs, effluent taxes, water use, and energy use in a representative ammonia plant. Both between and within class evaluations were made for Houston, Texas; Trenton, New Jersey; Saginaw, Michigan; and Sacramento, California. The models assumed the plant considered was large and a new build rather than an existing facility. Both naphtha and natural gas feedstocks were evaluated. Plans for extension of the models to include plant size and age are discussed. Further extension allows forecasting of national production costs and water use. The following results were obtained: (1) zero discharges of suspended solids, heat, and total dissolved solids increased production costs by approximately 1.5% using injection wells for ultimate disposal and 3.2% using evaporative disposal techniques; (2) zero discharges increase energy consumption by approximately 2.7%; (3) higher costs for water withdrawals do not significantly increase production costs; (4) effluent taxes required to achieve zero discharge vary with geographic location ranging from 20[cents]/lb. for Saginaw, Michigan, to 36[cents]/lb. for Trenton, New Jersey; (5) a water withdrawal price greater than 50[cents]/MGal, is necessary before air cooling becomes feasible in any location. The analysis indicates that zero discharge of ammonia production effluents can be achieved in a new plant through a combination of process design and waste treatment for a nominal increase in production costs. Technological implications for new plants include a trend toward larger, more efficient plants using higher boiler pressures and gas turbines. Economic implications include higher demand for capital, fuel, and feedstock.