ENVIRONMENTAL ENGINEERING Introduction Technological development has increased the human impact on our environment and the ecosystem, but a new technology has also emerged simultaneously to solve environmental pollution problems. This discipline of science is environmental engineering, in which new methods of purification and recirculation of pollutants are developed and attempts are made to change existing technology to reduce pollution. Environmental technology involves the application of engineering principles to devise methods and processes to solve problems in air, water and solid waste pollution. The role of this technology is changing in two important areas: sustainable development which deals primarily with global problems, and preventive technology which is designed to reduce the environmental effects of processes, operations and products. Environmental engineering is one of the key research areas that the staff in the Thermal and Fluids Engineering Division, School of Mechanical and Aerospace Engineering, are pursuing in. Most of the equipment and facilities in this area are housed in the Thermal and Fluids Research Laboratory. Research and Development One of the leading research areas in environmental engineering currently engaged in is the development of activated carbons from biomass waste materials. Substantial research has already been carried out to prepare activated carbons from oil-palm shells to be used in the gaseous adsorption of pollutant gases such as sulphur dioxide, ammonia and nitrogen oxides. Traditionally, palm-oil mills generate a large amount of wastes which have to be disposed of in an environmentally friendly way. When oil is extracted from the fruit, a large amount of waste is generated as the shells and pulp fibres are left behind. For instance, in Malaysia which is the largest producer of palm oil, 6 million tonnes of waste fibre and 2.4 million tonnes of shell are generated annually. The technique first grinds the shells and fibres into smaller sizes of a few millimetres. These raw materials are heated in an inert atmosphere in a process called pyrolysis to release the volatile matters, thereby leaving rudimentary pores inside the char. The char is subsequently activated by burning some of the carbon contents to create a well developed network of pores of nanometre sizes. These highly porous structures yield a large pore surface area for trapping pollutants. One gram of this activated carbon has a pore area of 1,950 sq m, which is equivalent to 18 five-room Singapore Housing Board flats. Activated carbon is used in various industries and treatment plants to filter the air of poisons such as hydrogen sulphide, sulphur dioxide, nitrogen oxides and ammonia by an adsorption process. It can also be used to remove organics such as phenol and benzene; and heavy metals such as lead, cadmium and arsenic from raw water in the purification of drinking water. Activated carbons can also be used in gas masks to be worn by pump attendants in petrol stations to prevent them from inhaling noxious petrol vapours, which are suspected to be carcinogenic. They can also be put into gas masks in times of chemical or biological warfare to trap nerve agents (such as sarin) or viruses. Fig. 1 shows the progression of the oil-palm fruits to the final activated carbons. Oil is being squeezed out from the mesocarp (pulp) and the kernel of the fruit, producing waste residues of fibre and shell. The fibres and shells are carbonised in inert atmosphere to form chars which are then activated in a gas stream of CO2 to produce the final activated carbons. Fig. 1 Photographs of the oil-palm fruits, fibres, shells, chars and activated carbons The adsorptive capacity of activated carbon is related to its specific pore surface area, pore volume and pore size distribution. Generally, as the pore surface area of the activated carbon increases, its adsorptive capacity will also increase. These surface areas are generated gradually during the activation process. Activation temperature is an important parameter for the process. Figure 2 shows the effects of activation temperature on the pore surface area of activated carbons prepared from oil-palm shells pre-treated with different impregnating solutions for 24 hours. In spite of different impregnating solutions used, the pore surface area versus the activation temperature showed a similar trend. Increasing the activation temperature from 500oC up to an optimum value increased the pore surface area, beyond which the surface area decreased. For oil-palm shells impregnated with 20% ZnCl2, the largest pore surface area was obtained at 750oC while a higher temperature of 800oC was required for those impregnated with 40% H3PO4 and 10% KOH. For temperatures beyond these optimum values, the pore surface area decreased with increasing temperature. 2000 BET Surface Area, m2/g 1700 1400 1100 20% ZnCl2 800 40% H 3PO 4 10% KOH 500 500 600 700 800 900 o Activation Temperature, C Fig. 2 Pore surface areas of activated carbons prepared at different activation temperatures for 1 hour from oil-palm shells pre-treated with different impregnating solutions for 24 hours. The amount of gaseous pollutant adsorbed onto the activated carbon is a function of its pore surface area. Figure 3 shows the amounts of SO2 (a common gaseous pollutant) adsorbed onto the oil-palm-shell activated carbon versus the pore surface area. For the various SO2 concentrations shown, the amount of SO2 adsorbed increased progressively with increasing pore surface area. Amount of SO2 Adsorbed (mg/g) 80 SO2 : 500 ppm 64 SO2 : 1000 ppm SO2 : 2000 ppm 48 32 16 0 500 700 900 1100 1300 1500 BET Surface Area (m 2/g) Fig. 3 Amount of SO2 adsorbed for different SO2 concentrations versus the pore surface area of the oil-palm-shell activated carbon. Another biomass material that is also studied to be used as a precursor for preparing activated carbon is pistachio-nut shell. It is reported that the world production of pistachio nuts is 0.21 million tonnes and therefore a huge amount of nut shells are generated as waste materials. The raw shells first undergo a pyrolysis or carbonisation process, and thereafter the chars are impregnated with KOH solution and activated in N2 atmosphere. Figure 4 shows the effects of activation temperature on the pore surface area and pore volume of the chemically activated carbons. Increasing the temperature from 500 to 8000C progressively increased both the pore surface area and micropore volume. However, for further temperature increase from 800 to 9000C, both the pore surface area and micropore volume decreased due to excessive carbon “burn-off”, resulting in the widening of pores as could be seen in the increases in the non-micropore volume at 800 and 9000C. Non-micropore volume BET surface area Micropore volume percentage Micropore volume Micropore volume(cm3/g) 0.7 BET surface area(m2/g) 2000 0.6 1800 1600 0.5 1400 1200 0.4 1000 (a) 800 600 500 600 700 90 0.6 80 0.5 0.4 70 0.3 60 0.2 50 0.1 (b) 0.3 800 900 o Activation temperature( C) 0.0 500 600 700 800 40 900 o Activation temperature( C) Fig. 4 Effects of activation temperature on the (a) pore surface area and micropore volume, and (b) non-micropore volume and micropore volume percent of chemically activated carbons. Another research area in environmental engineering is the development of carbon molecular sieve membranes for gas separation. These membranes consist of thin carbon films with pore sizes similar to those of gas molecules. They are prepared either by the carbonization/pyrolysis of polymeric substrates or the carbonization of polymeric films deposited on porous supports. In a particular study, a commercial polyimide film, Kapton 100HN from DuPont, was used to prepare the carbon membranes. The effects of carbonization parameters, such as the carbonization atmosphere, the final temperature, the heating rate and the thermal soak time at the final temperature, on the permeation rates of He, CO2, O2 and N2 were studied. The permselectivities of the membranes for the gas pairs of He/ N2, CO2/N2, O2/N2 and CO2 /CH4 were also measured. Micropore volume pecentage(%) 2200 Non-micropore volume(cm3/g) 0.7 Permeability (Barrer) 1200 He CO2 O2 N2 900 600 300 0 CM1 CM2 CM3 CM4 CM5 CM6 CM7 CM8 Sample ID Figure 5. Effect of the carbonisation parameters on the transport properties of the Kapton® based carbon membranes. Figure 5 compares the average gas permeabilities for He, CO2, O2 and N2 for the various carbon membranes obtained at different preparation conditions. Samples CM1, CM5 and CM6 show large differences in the permeabilities of the different penetrating gases while samples CM2, CM3, CM4, CM7 and CM8 yield small permeability variations. Hence, the processing parameters during carbonization will determine the pore structure and characteristics of the membranes. Facilities The equipment available include the ultra-pycnometer (Fig. 6) to measure the solid density of sample, thermogravimetric analyser (Fig. 7) to determine the proximate analysis of sample, accelerated surface area and porosimeter (Fig. 8) to measure pore surface area and pore volume of porous material, a mercury intrusion porosimeter to measure the apparent density and pore surface area of porous material, SO2 and NOx analysers, and electrical furnaces. Fig. 6 Ultra-Pycnometer Fig. 7 Thermogravimetric Analyser Fig. 8 Accelerated surface area and porosimetry system Other equipment in the laboratory include gas chromatograph – mass spectrometry system, ion chromatograph and various gas analyzers. Publications 1. A.C. Lua and J. Guo, Preparation and characterization of chars from oil palm waste, Carbon, Vol 36, No 11, U.K., 1998, pp 1663-1670. 2. J. Guo and A.C. Lua, Characterization of chars pyrolyzed from oil palm stones for the preparation of activated carbons, Journal of Analytical and Applied Pyrolysis, Vol 46, No 2, The Netherlands, 1998, pp 113-125. 3. A.C. Lua and J.Guo, Activated carbons prepared from extracted-oil palm fibers for nitric oxide reduction, Energy & Fuels, An American Chemical Society Journal, Vol 12, No 6, U.S.A., 1998, pp 1089-1094. 4. A.C. Lua and J. Guo, Chars pyrolyzed from oil palm wastes for activated carbon preparation, Journal of Environmental Engineering, American Society of Civil Engineers, Vol 125, No 1, U.S.A., 1999, pp 72-76. 5. J. Guo and A.C. Lua, Textural and chemical characterisations of activated carbon prepared from oil-palm stone with H2SO4 and KOH impregnation, Microporous and Mesoporous Materials, Vol 32, The Netherlands, 1999, pp 111-117. 6. J. Guo and A.C. Lua, Kinetic study on pyrolysis of extracted oil palm fiber: Isothermal and non-isothermal conditions, Journal of Thermal Analysis and Calorimetry, Vol 59, The Netherlands, 2000, pp 763-774. 7. J. Guo and A.C. Lua, Effect of heating temperature on the properties of chars and activated carbons prepared from oil palm stones, Journal of Thermal Analysis and Calorimetry, Vol 60, The Netherlands, 2000, pp 417-425. 8. J. Guo and A.C. Lua, Textural characterization of activated carbons prepared from oil-palm stones pre-treated with various impregnating agents, Journal of Porous Materials, Vol 7, The Netherlands, 2000, pp 491-497. 9. J. Guo and A.C. Lua, Effect of surface chemistry on gas-phase adsorption by activated carbon prepared from oil-palm stone with pre-impregnation, Separation and Purification Technology, Vol 18, The Netherlands, 2000, pp 47-55. 10. A.C. Lua and J. Guo, Activated carbon prepared from oil palm stone by one-step CO2 activation for gaseous pollutant removal, Carbon, Vol 38, U.K., 2000, pp 1089-1097. 11. J. Guo and A.C. Lua, Preparation of activated carbons from oil-palm-stone chars by microwave-induced carbon dioxide activation, Carbon, Vol 38, U.K., 2000, pp 1985-1993. 12. J. Guo and A.C. Lua, Adsorption of sulfur dioxide onto activated carbons prepared from oil-palm shells impregnated with potassium hydroxide, Journal of Chemical Technology and Biotechnology, Society of Chemical Industry, Vol 75, U.K., 2000, pp 971-976. 13. J. Guo and A.C. Lua, Preparation and characterization of adsorbents from oil palm fruit solid wastes, Journal of Oil Palm Research, Vol 12, Malaysia, 2000, pp 64-70. 14. A.C. Lua and J. Guo, Preparation and characterization of activated carbons from oil-palm stones for gas-phase adsorption, Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol 179, Ireland, 2001, pp 151-162. 15. J. Guo and A.C. Lua, Experimental and kinetic studies on pore development during CO2 activation of oil-palm-shell char, Journal of Porous Materials, Vol 8, The Netherlands, 2001, pp 149-157. 16. J. Guo and A.C. Lua, Kinetic study on pyrolytic process of oil-palm solid waste using two-step consecutive reaction model, Biomass & Bioenergy, Vol 20, U.K., 2001, pp 223-233. 17. A.C. Lua and J. Guo, Adsorption of sulfur dioxide on activated carbon from oilpalm waste, Journal of Environmental Engineering, American Society of Civil Engineers, Vol 127, U.S.A., 2001, pp 895-901. 18. A.C. Lua and J. Guo, Microporous oil-palm-shell activated carbon prepared by physical activation for gas-phase adsorption, Langmuir, American Chemical Society, Vol 17, U.S.A., 2001, pp 7112-7117. 19. J. Guo and A.C. Lua, Microporous activated carbons prepared from palm shell by thermal activation and their application to sulfur dioxide adsorption, Journal of Colloid and Interface Science, Vol 251(2), UK, 2002, pp 242-247. 20. J. Guo and A.C. Lua, Characterization of adsorbent prepared from oil-palm shell by CO2 activation for removal of gaseous pollutants, Materials Letters, Vol 55(5), Ireland, 2002, pp 334-339. 21. J. Guo and A.C. Lua, Textural and chemical characterizations of adsorbent prepared from palm shell by potassium hydroxide impregnation at different stages, Journal of Colloid and Interface Science, Vol 254(2), UK, 2002, pp 227233. 22. J. Guo and A.C. Lua, Adsorption of sulphur dioxide onto activated carbon prepared from oil-palm shells with and without pre-impregnation, Separation and Purification Technology, Vol 30(3), Ireland, 2003, pp 265-273. 23. J. Guo and A.C. Lua, Textural and chemical properties of adsorbent prepared from palm shell by phosphoric acid activation, Materials Chemistry and Physics, Vol 80(1), Ireland, 2003, pp 114-119. 24. J. Guo and A.C. Lua, Surface functional groups on oil-palm-shell adsorbents prepared by H3PO4 and KOH activation and their effects on adsorptive capacity, Transaction IChemE: Chemical Engineering Research & Design, Vol 81, Part A, UK, 2003, pp 585-590. 25. T. Yang and A.C. Lua, Characteristics of activated carbons prepared from pistachio-nut shells by physical activation, Journal of Colloid and Interface Science, Vol 267(2), USA, 2003, pp 408-417. 26. T. Yang and A.C. Lua, Characteristics of activated carbons prepared from pistachio-nut shells by potassium hydroxide activation, Microporous and Mesoporous Materials, Vol 63(1-3), Netherlands, 2003, pp 113-124. 27. A.C. Lua and T. Yang, Properties of pistachio-nut-shell activated carbons subjected to vacuum pyrolysis conditions, Carbon, Vol 42(1), UK, 2004, pp 224226. 28. A.C. Lua and T. Yang, Effect of activation temperature on the textural and chemical properties of potassium hydroxide activated carbon prepared from pistachio-nut shell, Journal of Colloid and Interface Science, Vol 274(2), USA, 2004, pp 594-601. 29. A.C Lua and T. Yang, Effects of vacuum pyrolysis conditions on the characteristics of activated carbons derived from pistachio-nut shells, Journal of Colloid and Interface Science, Vol 276(2), USA, 2004, pp 364-372. 30. A.C. Lua, T. Yang and J. Guo, Effects of pyrolysis conditions on the properties of activated carbons prepared from pistachio-nut shells, Journal of Analytical and Applied Pyrolysis, Vol 72(2), Ireland, 2004, pp 279-287. 31. J. Guo, W.S. Xu, Y.L. Chen and A.C. Lua, Adsorption of NH3 onto activated carbon prepared from palm shell impregnated with H2SO4, Journal of Colloid and Interface Science, Vol 281(2), USA, 2005, pp 285-290. 32. A.C. Lua and T. Yang, Characteristics of activated carbon prepared from pistachio-nut shell by zinc chloride activation under nitrogen and vacuum conditions, Journal of Colloid and Interface Science, Vol 290(2), USA, 2005, pp 505-513. 33. A.C. Lua and J.C. Su, Isothermal and nonisothermal pyrolysis kinetics of Kapton polyimide, Polymer Degradation and Stability, Vol 91(1), UK, 2006, pp 144-153. 34. A.C. Lua, F.Y. Lau and J. Guo, Influence of pyrolysis conditions on pore development of oil-palm-shell activated carbons, Journal of Analytical and Applied Pyrolysis, in press. 35. J.C. Su and A.C. Lua, Influence of carbonization parameters on the transport properties of carbon membranes by statistical analysis, Journal of Membrane Science, in press. 36. T. Yang* and A.C. Lua, Textural and chemical properties of zinc chloride activated carbons prepared from pistachio-nut shells, Materials Chemistry and Physics, Vol 100(2-3), Ireland, 2006, pp 438-444. 37. A.C. Lua and J.C. Su*, Effects of carbonisation on pore evolution and gas permeation properties of carbon membranes from Kapton polyimide, Carbon, Vol 44(14), UK, 2006, pp 2964-2972. 38. J. Guo, Y. Luo, A.C. Lua, R.A. Chi, Y.L. Chen, X.T. Bao and S.X. Xiang, Adsorption of hydrogen sulphide (H2S) by activated carbons derived from oil-palm shell, Carbon, Vol 45(2), UK, 2007, pp 330-336. 39. A.C. Lua and Q.P. Jia*, Adsorption of phenol by oil-palm-shell activated carbons, Adsorption, Vol 13, USA, 2007, pp 129-137. 40. J.C. Su* and A.C. Lua, Effects of carbonisation atmosphere on the structural characteristics and transport properties of carbon membranes prepared from Kapton polyimide, Journal of Membrane Science, Vol 305, Ireland, 2007, pp 263-270. 41. Q.P. Jia* and A.C. Lua, Concentration-dependent branched pore kinetic model for aqueous phase adsorption, Chemical Engineering Journal, Vol 136(2-3), Ireland, 2008, pp 227-235. 42. J. Guo, B. Gui, S.X. Xiang, X.T. Bao, H.J. Zhang and A.C. Lua, Preparation of activated carbons by utilizing solid wastes from palm oil processing mills, Journal of Porous Materials, Vol 15(5), 2008, pp 535-540. 43. Q.P. Jia* and A.C. Lua, Effects of pyrolysis conditions on the physical characteristics of oil-palm-shell activated carbons used in aqueous phase phenol adsorption, Journal of Analytical and Applied Pyrolysis, Vol 83(2), Ireland, 2008, pp 175-179. 44. A.C. Lua and T. Yang*, Theoretical analysis and experimental study on SO2 adsorption onto pistachio-nut-shell activated carbon, American Institute of Chemical Engineers (AIChE) Journal, Vol 55(2), USA, 2009, pp 423-433. 45. A.C. Lua and J.C. Su*, Structural changes and development of transport properties during the conversion of a polyimide membrane to a carbon membrane, Journal of Applied Polymer Science, Vol 113(1), USA, 2009, pp 235-242. 46. A.C. Lua and Q.P. Jia*, Adsorption of phenol by oil-palm-shell activated carbons in a fixed bed, Chemical Engineering Journal, Vol 150(2-3), Ireland, 2009, pp 455-461. 47. J.C. Su* and A.C. Lua, Experimental and theoretical studies on gas permeation through carbon molecular sieve membranes, Separation and Purification Technology, Vol 69, Ireland, 2009, pp 161-167. 48. A.C. Lua and T. Yang*, Theoretical and experimental SO2 adsorption onto pistachio-nutshell activated carbon for a fixed-bed column, Chemical Engineering Journal, Vol 155, Ireland, 2009, pp 175-183. 49. Y. Shen* and A.C. Lua, Effects of membrane thickness and heat treatment on the gas transport properties of membranes based on P84 polyimide, Journal of Applied Polymer Science, Vol 116(5), USA, 2010, pp 2906-2912. 50. J. Guo, G. Hu, A.C. Lua and M.J. Heslop, Separation of ethane gas by adsorption onto various biomass-derived activated carbons, Advanced Materials Research, Vols 113-114, Switzerland, 2010, pp 1896-1899. 51. Y. Shen* and A.C. Lua, Structural and transport properties of BTDA-TDI/MDI copolyimide (P84)-silica nanocomposite membranes for gas separation, Chemical Engineering Journal, Vol 188, Ireland, 2012, pp 199-209. 52. Y. Shen* and A.C. Lua, Preparation and characterization of mixed matrix membranes based on poly(vinylidene fluoride) and zeolite 4A for gas separation, Polymer Engineering and Science, Vol 52, Issue 10, USA, 2012, pp 2106-2113. 53. Y. Shen* and A.C. Lua, Preparation and characterization of mixed matrix membranes based on PVDF and three inorganic fillers (fumed nonporous silica, zeolite 4A and mesoporous MCM-41) for gas separation, Chemical Engineering Journal, Vol 192, Ireland, 2012, pp 201-210. 54. A.C. Lua and Y. Shen*, Preparation and characterization of asymmetric membranes based on nonsolvent/NMP/P84 for gas separation, Journal of Membrane Science, Vol 429, Ireland, 2013, pp 155-167. 55. A.C. Lua and Y. Shen*, Preparation and characterization of polyimide-silica composite membranes and their derived carbon-silica composite membranes for gas separation, Chemical Engineering Journal, Vol 220, Ireland, 2013, pp 441-451. 56. A.C. Lua and Y. Shen*, Influence of inorganic fillers on the structural and transport properties of mixed matrix membranes, Journal of Applied Polymer Science, Vol 128, Issue 6, USA, 2013, pp 4058-4066. Contact Person Dr. Lua Aik Chong Associate Professor School of Mechanical and Aerospace Engineering Nanyang Technological University 50 Nanyang Avenue Singapore 639798 Tel: (65)-67905535 Fax: (65)-67924062 E-mail: [email protected]
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