Process Simulation and Economic Evaluation of an Ammonia Recovery Plant Using a Counter-current Air Stripping and Absorption Process
Etinyene Uko Ukpong Essien *
Centre for Gas, Refining and Petrochemicals, Institute of Petroleum Studies, School of Graduate Studies, University of Port Harcourt, Port Harcourt, Rivers State, Nigeria.
Ini-Obong S. Asuquo
Centre for Gas, Refining and Petrochemicals, Institute of Petroleum Studies, School of Graduate Studies, University of Port Harcourt, Port Harcourt, Rivers State, Nigeria.
*Author to whom correspondence should be addressed.
Abstract
Ammonia-laden effluent from fertiliser and urea processing facilities poses a significant environmental hazard, causing eutrophication, dissolved-oxygen depletion, and acute aquatic toxicity when discharged untreated. This study presents the process simulation, design, and economic evaluation of an ammonia recovery plant that combines counter-current air stripping with sulphuric-acid absorption to remove ammonia from urea-desorber effluent water while recovering ammonia into a sulphuric-acid absorption stream with potential for ammonium sulphate production. A steady-state flowsheet, comprising a stripping column, two centrifugal compressors, two centrifugal pumps, and an absorption column, was developed in ASPEN HYSYS® 8.6 using the Non-Random Two-Liquid (NRTL) property method. At the simulated operating conditions, the stripping column reduced the ammonia mass fraction in the treated water from 0.0593 to 0.0005, corresponding to a tray efficiency of 90%. Expressed on the same basis as common regulatory limits, this mass fraction corresponds to approximately 500 ppm by mass, which exceeds rather than satisfies the frequently cited 50 ppm ammonia discharge guideline, indicating that further polishing treatment would be needed to meet that standard. The absorption column recovered the stripped ammonia from the rich air stream into 98 wt% sulphuric acid, reducing the ammonia content of the exhaust air to a mass fraction of 0.0004 while recovering ammonia into a sulphuric-acid absorption stream with potential for ammonium sulphate production. A capital and operating cost evaluation, escalated to 2017 values using the Chemical Engineering Plant Cost Index, gave a total capital expenditure of $5.73 million against an annual operating cost of $58.55 million and annual product/avoided-cost revenue of $66.23 million. Discounted cash-flow analysis over a 20-year horizon, at a 15% discount rate, returned a net present value of $53.43 million, an internal rate of return of 58%, and a payback period of 1.6 years. These results indicate that counter-current air stripping coupled with acid absorption is both an environmentally effective and economically attractive route for ammonia recovery from fertiliser-industry effluent, converting a compliance liability into a profitable process unit.
Keywords: Ammonia recovery, air stripping, absorption, ASPEN HYSYS, process simulation, wastewater treatment, ammonium sulphate, techno-economic analysis