Life Cycle Assessment technique coupled with simulation for enhanced sustainability of phosphoric acid production
Multi-scale and/or multi-disciplinary approach to process-product innovation
Integrated Methodologies for Process Development (T3-7)
Keywords: phosphoric acid, life cycle assessment, simulation
Phosphoric acid is the second largest mineral acid produced worldwide considering its volume and value. The production process is based on sulphuric acid lixiviation of apatite (Ca10P6O24F2) followed by filtration of gypsum and the subsequent concentration of the solution, yielding phosphoric acid in technical grade, or wet process phosphoric acid [1,2]. During the lixiviation of the mineral, controlling reactor temperature and P2O5 concentration allows to select which gypsum hydrate is formed: dihydrate at 70-80ºC for 26-32% P2O5 and hemihydrate at 85-95ºC for 40-52% P2O5. The phosphoric acid obtained through this method is suitable for fertilizer production, and that is the destiny of 80% of its production in Europe [3, 4].
To study environmental effects associated to the phosphoric acid related industry it is mandatory to adopt a Life Cycle Assessment (LCA) approach. LCA is an objective method to evaluate the environmental loads associated with a product, process or activity. It has a systematic procedure that includes the whole energy and material inputs/outputs to a given process and the impact produced by the process itself [5]. The framework includes the entire life cycle of the product, process or activity, encompassing extraction and processing of raw materials; manufacturing, transportation and distribution; re-use, maintenance recycling and final disposal. Most important a LCA involves a holistic approach, bringing the environmental impacts into one consistent framework, wherever and whenever these impacts have occurred or will occur [6].
A main limitation in the application of LCA technique is the availability of reliable data. Here a combined use of LCA and process simulation is proposed. Process simulation involves usage of computer software to help develop accurate and representative models of chemical processes aiming at understanding its behaviour [7]. Computer simulation allows to perform mass and energy balances promptly, and quick analysis of a variety of scenarios even with process scarce data which guarantees a more robust approach. The use of process simulators, in this case Aspen Plus to process the scarce data available guarantees a robust approach.
Prior LCA studies related to the fertilizer industry have shown that the most relevant environmental issues are those related with energy consumption and green house gas emissions, the emissions of fluoride (HF and SiF4 mixtures to air) and the management gypsum and spent process water. The last two wastes also contain trace metals found in the phosphate mineral and other Si and F compounds, and it is generally accepted that the biggest environmental problem in phosphoric acid industry is the destiny of phosphogypsum waste [2, 3].
LCA can be performed in different ways, by selecting the boundaries of the systems under study. In this paper, consideration of the impact of raw materials (rock and sulphuric acid) is intended, and no analysis of product destiny (grave) was studied given the many uses that phosphoric acid has, besides fertilizer production. So a cradle to gate approach is used to address each one of the above mentioned issues:
Energy Consumption for producing phosphoric acid should account for the energy consumed in phosphate raw material extraction and sulphuric acid production [8]. During sulphuric acid production energy consumption varies depending on the process and sulphur raw material used. Sulphur mining is energy intensive (2GJ/tn of produced acid), but generally sulphur is “mined” from natural gas. Given that the amount of sulphuric acid for phosphoric acid production is approximately from 3 to 5 ton H2SO4/ton P2O5 a more rigorous analysis is needed taking into account sulphur extraction in the required case, and possible integration of normal steam for phosphoric acid production.
According to net emission of green house gasses (GHG) from phosphate fertilizer manufacture is largely determined by the method of sulphuric acid production [9]. GHG emissions were primarily CO2 emitted during consumption of fossil fuels. It is also reported that transport comprised a considerable proportion of the emissions budget, for some studies it ranged from 20-33% according to different studies stating that overseas transport of raw phosphate rock was particularly important.
Regarding fluoride emissions, they can be reduced almost completely to zero, if a closed loop is accomplished and scrubber efficiency for its abatement is bigger than 99%. It is also reported the possibility of generation of a by-product: H2SiF6 up to 20-25% concentration, which can be sold [2]. So two scenarios will be analyzed: a plant with recovery and with out by-product recovery.
Finally, and because waste water treatment plant process data is difficult to obtain, the waste water treatment will be discussed as a moving boundary, also considering two possible scenarios: a plant with no treatment and with water treatment. Other scenarios will arise due to the different possibilities of waste management given to phosphogypsum [3].
Acknowledgements
This work has been funded by the ECOPHOS EU project (INCO-CT-2005-013359), and Aarón Bojarski wishes to express his gratitude for the financial support received from the Agencia de Gestió d’Ajuts Universitaris i de Recerca (AGAUR) from Generalitat de Catalunya and Fons Social Europeu (EU).
References
1. Becker P. Phosphates and Phosphoric Acid. Raw Material, Technology and Economics of the Wet Process. Marcel Dekker, INC. 2nd. 1989. Marcel Dekker, INC.
2. European Fertilizer Manufacturers' Association (EFMA). Best Available Techniques for Pollution Prevention and Control in the European Fertilizer Industry. Booklet No. 4 of 8: Production of phosphoric acid. 2nd. 2000. European Fertilizer Manufacturers' Association (EFMA).
3. van der Loo J.H.W and Weeda M. Dutch notes on BAT for the phosphoric acid industry. 2000. Ministry of Transport, Public Works and Water Managment. Institute for Inland Water Managment and Waste Water Treatment.
4. Wiesenberger H. State of the art for the production of fertilizers with regard to the IPPC directive. Umweltbundesamt.Federal Environment Agency, Austria. 2002.
5. Jensen A.A., Hoffman, L., Moller B.T., Schmidt A., Christiansen, K., and Elkington., van Dijk F. Life Cycle Assessment (LCA), A guide to approaches, experiences and information sources. European Environmental Agency. 1998. European Environmental Agency.
6. Guinée J.B., Gorreé M., Heijungs R, Huppes G, Kleijn R, de Konig A, van Oers L, Sleeswijk A W, Suh S, Udo de Haes H, de Brujin H, and Huijbregagts M. Life cycle assesment. An operational guide to the ISO standards. Guinée J.B. 2001.
7. Diwekar U. and Small M.J. Process analysis approach to industrial ecology. Ayres R.U. and Ayres L.W. A Handbook of Industrial Ecology. [11], 114-137. 2002.
8. Kongshaug G. Energy Consumption and Greenhouse Gas emissions in fertilizer production. Hydro Agri Europe. 1998.
9. Wood Sam and Cowie Annette. A review of greenhouse gas emission factors for fertilizer production. IEA Bioenergy Task 38. 5-1-2004.
Presented Thursday 20, 16:00 to 16:20, in session Integrated Methodologies for Process Development (T3-7).
