1.1 Background of the Research and Problem Statement
Since the industrial revolution commenced in the 18th century, fossil fuels in the form of coal, oil, and natural gas have powered the technology and transportation networks that drive society. But continuing to power the world from fossil fuels threatens our energy supply and puts enormous strains on the environment. The world's demand for energy is projected to double by 2050 in response to population growth and the industrialization of developing countries. The supply of fossil fuels is limited, global oil and gas reserves are concentrated in a few regions of the world, while demand is growing everywhere; as a result, a secure supply is increasingly difficult to assure. Moreover, the use of fossil fuels puts our own health at risk through the chemical and particulate pollution it creates. Carbon dioxide and other greenhouse gases emissions are associated with global warming, which is reflected on glaciers melting, rising sea levels, floods, hurricanes, droughts and a host of other environmental catastrophes.
1.1.1 Hydrogen economy
A worldwide energy crisis and increasing environmental concerns are strong incentives for using hydrogen as a sustainable and clean energy source. “Hydrogen economy” has been around since 1970, but it started to seem practicable only in recent years. Major industrialized countries have started to support a wide range of fundamental and applied research into to use hydrogen as the primary energy carrier. [Analysis of Effectiveness of the DOE Hydrogen Program, Hydrogen Technical Advisory Panel (HTAP), Dec. 1998].
Currently, the hydrogen is mainly used as:
· Fuel as an energy carrier or as a medium by which energy may be moved from the place of production to the place of use. [A.K. Avci, Z. I. Onsan, and D. L. Trimm, Appl Catal A-Gen 216 (1-2) (2001) 243]
* Petroleum refineries around the world consume large amounts (95%) of hydrogen during the hydrogenation, desulphurisation and denitrogenation of fuel. Also, hydrogen is required for ammonia and methanol manufactures.
* Coal-liquefaction, both direct and indirect, also requires large amounts of hydrogen.
· Aerospace use hydrogen as a fuel and it amounts is 0.1% [H. W. Pohl and V. Malychev, Int. J. Hydrogen Ener., 22 (10-11) (1997) 1061]
* Other industries used the hydrogen residual (4.9%).
In the long term, as fossil fuel prices rise due to depletion of the reserves and as global warming worsens, hydrogen may become more and more attractive as an alternative energy source. The trend in the future is to switch from using hydrogen as the basic raw material in the chemical industry to the energy carrier in the transportation and distributed energy industries because of the worldwide energy crisis and increasing environmental concerns. Hydrogen's long-term potential value lies primarily in serving as a versatile energy carrier such as a fuel which will be ultimately derived from renewable energy sources and consumed with little or no pollution.
There are two distinct ways in which hydrogen can be used as a fuel:
* Hydrogen can be used in transportation by mixing it with natural gas in modified internal combustion engines, which would increase engine performance and decrease pollution.
· The second approach is to use fuel cells. Fuel cells operate similarly to batteries when supplied with fuels to the anode and oxidants (e.g. air) to the cathode and electrochemically convert chemical energy directly to electrical energy [N.Q. Minh, J. Am. Ceram. Soc., 76, (1993) 563; A. J. Appleby, Energy, 21, (1996) 521; M. Ippommatsu, H. Sasaki, and S. Otoshi, Int. J. Hydrogen Energy, 21, (1996).125; L. Carrette, K. A. Friedrich, and U. Stimming, Chemphyschem, 1 (4): (2000)162; M. Prigent, Revue De L Institut Francais Du Petrole, 52 (3): (1997) 349]
A key requirement for successful realization of fuel-cell-powered vehicles is an infrastructure capable of supplying competitively priced hydrogen [A. L. Dicks, J. Power Sources, 61, (1996) 113].
As mentioned above hydrogen has a great potential as a fuel for various applications and all these emerging technologies will no doubt substantially increase the demand for hydrogen in 21st century. To meet the expected rising demand, hydrogen has to be generated in a more cost-effective manner. Although it is the universe's most abundant element, most of the Earth's hydrogen exists in chemical compounds.
1.1.2 Hydrogen production
Hydrogen must be extracted chemically from hydrogen-rich materials such as natural gas, water, coal or plant matters. Also, it is derived from fossil fuels through the following process:
· Steam reforming of natural gas
When steam and methane or other hydrocarbons are combined at high pressure and temperature, a chemical reaction converts them into hydrogen and carbon monoxide . The most common feedstock is natural gas, which consists primarily of methane.
The energy content of the hydrogen produced is higher than that of the natural gas consumed, but considerable energy is required to operate the reformer, so the net conversion efficiency is typically only 65 percent. Other feedstock can be steam reformed in essentially the same way after partial burning to convert them into gaseous forms. Sometimes additional procedures are required such as the removal of sulphur or other impurities to reforming natural gas.
· Partial oxidation of other fossil fuels
Methane and other hydrocarbons may be converted to hydrogen and carbon monoxide via partial oxidations [U. Balachandran, J. T. Dusek, S.M. Sweeney, R.B. Poeppel, R. L. Mieville, P. S. Maiya, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich, and A.C. Bose, Am.Ceram. Soc. Bull., 74, (1995) 71].
It should be noticed that this reaction produces less hydrogen per mole of methane than steam reforming reaction.
· Catalytic reforming of petroleum
Catalytic reforming of petroleum is a chemical process used to convert petroleum refinery naphtha, typically having low octane ratings, into high-octane liquid products (also known as petrol). Essentially, the process re-arranges or re-structures the hydrocarbon molecules in the naphtha feedstocks as well as breaking some of the molecules into smaller molecules. The overall effect is that the reformed product contains hydrocarbons with more complex molecular shapes having higher octane values than the hydrocarbons in the naphtha feedstock. Thus, the process separates hydrogen atoms from the hydrocarbon molecules and produces very significant amounts of byproduct hydrogen gas for use in a number of the other processes involved in a modern petroleum refinery. Other byproducts are small amounts of methane, ethane, propane and butanes. [Gary, J.H. and Handwerk, G.E. Petroleum Refining Technology and Economics (2nd Edition).,Marcel Dekker, Inc. (1984)]
1.1.3 Global warming or Big freeze
The use of fossil fuels for obtaining energies through combustion process raises carbon dioxide level in the atmosphere while oxygen has been reducing. Indeed, deficiency of oxygen in energy metabolism can produce chronic diseases such as cancer and death. In the atmosphere there is much more oxygen than carbon dioxide, i.e. 20.95 percent (209460 ppm of O2) compared with around 380 ppm of carbon dioxide. Currently, oxygen concentration is at 19.5 percent in enclosed spaces, below this figure, fainting and death may result. [W., Friederike, B. Aaby and H. Visscher, “Rapid atmospheric O2 changes associated with the 8,200-years-B.P.cooling event”. PNAS 99 (19) (2002)].
Oxygen deficiency in the atmosphere can provoke changes in the global climate. The simultaneous decrease in ocean oxygen is symptomatic of the slowdown in the circulation system of heat from the tropics to the poles. This dynamic system is highly nonlinear, and small changes could make it fail altogether, with disastrous runaway effects on the climate [M. Maslin, “ Global warming, A very short introduction” Oxford University Press, Oxford, Great Britain, 2004; B. N. Kursunoglu, S .L. Mintz, A. Permutter, “Global warming and Energy policy”, Kluwer Academic / Plenun Publishers, New York, USA, 2000]. On the other hand, high oxygen content in seawater enables much life in the oceans, "consuming" the greenhouse gas CO2, and subsequently leads to a cooling of the earth's surface.
Throughout history our climate has been dependent on balance between CO2 and atmospheric oxygen. The more CO2 and other greenhouse gases, the warmer the climate has been. However, it is not fully understood the process which drives the earth from a period with a warmer climate towards an "ice age" with colder temperatures.
1.1.4 Current oxygen separation process.
There are two fundamentally different approaches to oxygen separation:
1. Cryogenic distillation which uses ultra-low-temperature and elevate pressures, which makes it too expensive. This process is typically reserved for applications that require tonnage quantities of oxygen or nitrogen.
2. Non-cryogenic process, which carries out oxygen separation at ambient temperatures using either molecular sieve adsorbents via a process called pressure swing adsorption (PSA), or polymeric membranes.
More recently, a third category of oxygen separation has emerged. This novel alternative is mixed ionic-electronic conducting (MIEC) ceramic membranes that conduct oxygen ions at elevated temperatures. Successful application of MIEC technology will allow significant improvement in the performance of several large-scale industrial processes. The MIEC ceramic membranes are used to separate high-purity oxygen from air and has the potential for significant advantages when integrated with power generation cycles. In addition, the use of MIEC ceramic membrane in syngas process can reduce the capital investment for gas-to-liquid (GTL) plants and for distributing hydrogen, since it can be integrated oxygen separation and high-temperature syngas generation processes into a single compact ceramic membrane reactor.
Perovskite-type, MIEC within the system La1-xSrxCo1-yFeyO3-δ (LSCF) can deliver 100% pure oxygen under the oxygen partial difference. This material could be a good candidate to replace the current oxygen production technology. Certainly, these oxygen selective membranes can find wider applications in oxygen production since they can directly provide pure oxygen for many industrial processes where continuous supply of oxygen is needed [F.T. Akin, Y.S. Lin, “Selective oxidation of ethane to ethylene in a dense tubular membrane reactor”, J. Membr. Sci. 209 (2002) 457; S. Liu, X. Tan, K. Li, R. Hughes, “Methane coupling using catalytic membrane reactors”, Catal. Rev. 43 (1-2) (2001) 147]:
· Oxidation of hydrocarbons such as oxidative coupling of methane to C2 (OCM)
· Partial oxidation of methane to syngas (POM)
· Partial oxidation of heptanes to hydrogen (POH)
· Selective oxidation of ethane to ethylene (SOE) and selective oxidation of propane to propylene (SOP)
The aim of this work is to compare the oxygen release and uptake and by inference oxygen permeation obtained using both an La0.6Sr0.4Co0.2Fe0.8O3-δ hollow fibre membrane (referred to hereafter as LSCF6428-HFM) under an oxygen chemical potential difference (obtained by using air and inert feeds) at 850°C and 900°C. Besides, study the impact of surface modification by catalyst on the shell side membrane surface. Furthermore a kinetic model will be used to estimate rate constants for oxygen transfer in both unmodified and modified LSCF6428 membranes.
Follow-up work also includes methane oxidation experiments obtained with LSCF6428-HFM and a 5%Ni/LSCF6428-HFM at 850°C. Pre- and post-operation HF membranes samples are characterised by techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS). These techniques provide data on micro structural changes of the membrane and catalysts employed and show possible phase change development of LSCF6428 following operating conditions.
1.3 Organization of the Dissertation
This thesis is divided into following eight chapters.
Background of the Research and Problem Statement,
Organization of the Dissertation
Reviews the mixed ionic- electronic conducting (MIEC) ceramic membranes for oxygen separation and chemical reactions such as partial oxidation of methane
Theory of the oxygen permeation through MIEC ceramic membrane
Principal fundaments of various techniques of the characterization used
Describes the methodology use in this thesis such as the different catalyst preparation methods, module assembly, catalytic surface modification, catalytic reaction conditions and characterization techniques employed in this work
In this chapter is discussed the oxygen release and oxygen uptake behaviour from unmodified LSCF6428 hollow fibre membrane using two different modes of operation (co-current and counter- current.) at different temperatures. It is also calculated the overall reaction rate constant for both mode of operation.
Oxygen release in different zone to large of the unmodified LSCF6428 hollow fibre membrane at 850°C and 900°C is also determined.
A study of the oxygen release, oxygen uptake and oxygen permeation through LSCF 6428 hollow fibre membrane-modified with cobalt and nickel catalyst at different temperature are described in this chapter and also the determination of the overall rate constant for both cases.
A comparison of the oxygen release and uptake measurements between unmodified and catalytically modified membranes is discussed.
Results for partial oxidation of methane from 5%Ni/LSCF6428 hollow fibre membrane are also analyzed. A comparison the overall rate constant between oxygen flux and methane oxidation
Here is given conclusions and recommendations for future works