- Email: [email protected]
GTL synthesis gas generation membrane for monetizing stranded gas
FEATURE
GTL synthesis gas generation membrane for monetizing stranded gas By Susanne Olsen and Dr Edward Gobina – Centre for Process Integration & Membrane Technology, Robert Gordon University, Aberdeen, Scotland This feature article provides details of the development of a high-performance membrane for use in gas-to-liquids (GTL) synthesis gas generation for monetizing stranded gas assets.
Without a viable market, the natural gas discovered by oil companies can become a costly nuisance (as ‘stranded gas’) which must be flared, re-injected or plugged. One attractive option for monetizing this gas is to use a gas-toliquids (GTL) process to produce a fuel that can be transported easily, which can can be used locally in existing fuel systems, and also is extremely clean.
Introduction In the first step of the gas-to-liquids process, syngas is produced from the natural gas. In the next step, Fischer-Tropsch chemistry is used to produce hydrocarbon from the syngas. Membrane technology can be applied to the syngas production reaction. An attractive application of membrane technology is that the conversion efficiency can be improved by using the membrane as the catalyst support, which then also increases the catalyst dispersion, resulting in an optimal catalyst load and complete consumption of the oxygen and methane. In this article, it is demonstrated that the oxygen is activated before contacting the methane inside the membrane. This often results in 100% oxygen conversion.
Liquefied natural gas In some cases, a conversion to liquefied natural gas (LNG) is carried out. Unfortunately, LNG has limited market opportunities, because its production and transportation require dedicated facilities and therefore a large capital investment. Without a viable market, the natural gas discovered by oil companies can become a costly nuisance which must be flared, re-injected or plugged.[1]
Attractive option One attractive option is to use Fischer-Tropsch gas-to-liquids conversion, which is where the development of improved syngas processes is so important. The fuel produced is easily transport-
ed, can be used locally, can be used in existing fuel systems and is extremely clean. An example of this process in shown in Figure 1. In the first step of the process, syngas is produced from the natural gas. In the next step of the process, Fischer-Tropsch chemistry is used to produce hydrocarbon from the syngas. In some cases a further hydro-cracking step is used to produce a liquid fuel. Approximately 60% of the costs for the overall processes are accounted in the first step, i.e. the production of syngas.[2] The development of an enhanced syngas technology is therefore absolutely critical for the commercialization of GTL processes.
Natural gas conversion to syngas Almost all options for the transformation of methane (the main component of natural gas) involve its initial conversion to syngas (CO + H2). Syngas can yield liquid hydrocarbons through the F–T reaction over Group VIII transition metal catalysts (see Reaction 1), or first to
Natural gas Water T
Air
Partial oxidation
Desulphurization
Waste heat system Water
Syngas FT-reactor 1
Steam FT-reactor 2
Stranded gas According to a recent estimate, almost 5000 significant reservoirs of natural gas have been found worldwide. Almost half of these are stranded because they are too remote and inaccessible to gas pipelines. Because of the high investment costs, the transportation of natural gas by pipeline is limited to those cases where reserves are located reasonably close to the customers.[1]
Membrane Technology June 2004
G
Off-gas
Water Diesel oil Wax
Figure 1. An example of the gas-to-liquids process using air.
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There are some unwanted reactions that occur simultaneously, namely carbon formation. Carbon deposited from the destabilization of CO into C and CO2, from the reduction of CO and from CH4 decomposition on catalytic surfaces, can be controlled by process conditions such as the steam concentration in the feed. Reactions 3 and 4 are involved in reforming processes, of which there are five: • • •
• •
Figure 2. The reaction rig system in use at the Centre for Process Integration & Membrane Technology, Robert Gordon University.
methanol over Cu/ZnO catalysts, and then to gasoline or petrol by the methanol-to-gasoline (MTG) process over ZSM-5 zeolites (see Reaction 2):
830°C. The reaction products H2, CO, CO2 and H2O are for all practical purposes stable under reaction conditions. There are two main reactions that are important in the conversion of natural gas to syngas: • •
Ammonia synthesis is still the largest single consumer of syngas, but the growing interest during the last decade in C1 chemistry and in large-scale conversion of natural gas into liquid fuels has created a need to explore the limits of the reforming technology.
Steam reforming. Partial oxidation.
The steam reforming is a highly endothermic reaction of methane and steam, whereas the partial oxidation is slightly exothermic:
Non-catalytic partial oxidation (NC-POM), in which only Reaction 4 is involved. Conventional steam reforming (SMR), where only Reaction 3 participates. Autothermal catalytic reforming (ATR), in which Reaction 4 and Reaction 3 take place simultaneously. Combined reforming (CMR), where Reaction 3 is followed by Reaction 4. Catalytic partial oxidation (POM), which mainly involves Reaction 4.
While most syngas is produced by steam reforming, some of the other routes may be more attractive, depending on factors such as the H 2/CO ratio; downstream use; product purity; the presence of CO2, N2, H2O and CH4; capacity; feedstock availability; purity and cost, including O2.[4]
Membrane technology in syngas production The primary advantage of using the enhanced membrane reactor configuration is that it provides a safer reaction environment, and this allows operations to be carried out under conditions that could be hazardous in a more conventional fixed-bed reactor. In the membrane reactor, it is possible to lower the methane/oxygen feed ratios without creating the potential for an explosion. It is also possible to operate at both high and low contact times.
Cost of generating syngas It has been estimated that in the above industrial applications, more than 60–70% of the cost of the overall process is associated with syngas production.[3] Therefore, reducing the syngas generation costs would have a large and direct influence on the overall economics of all these downstream industrial processes.
Stable Because methane (CH4) is a very stable molecule, it has to be processed under very severe conditions. Although its conversion to syngas can be conducted at temperatures even below about 430°C, high yields to syngas need substantially higher temperatures – typically about
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Figure 3. Cross-sectional view of the ceramic membrane — thickness.
Membrane Technology June 2004
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Figure 4. Cross-sectional view of the ceramic membrane — reaction.
Alternative strategy Preliminary cost estimates indicate that enhanced ceramic membrane reactors could reduce the capital cost for syngas by more than one-third. This reduction would have a significant impact on the costs of liquid transportation fuels derived from natural gas.[5] This factor definitely provides a substantial incentive for developing an alternative strategy for syngas production using membrane technology.
Ion-transport membrane One of the emerging membrane technologies is the ion transport membrane. For either partial oxidation or autothermal reforming, the cryogenic oxygen plant may be replaced by a high-temperature membrane that can separate oxygen from air and supply it directly into the reactor for converting CH4 to CO + H2. The extremely low partial pressure of oxygen in the reaction zone of an autothermal reformer (with mole fraction on the order of 10 –6 to 10 –18), creates a large driving force for oxygen separation in oxygenion conducting membranes. Using such membranes, only compression work, which needs to overcome the friction losses in the system, would be required.
Lower temperatures Attempts to further increase gas conversion using membranes in steam reformers have not proved to be economically viable. However, at lower temperatures an equilibrium shift towards the product gases could be obtained with H2selective membranes that remove the hydrogen formed in the reaction. Lower temperatures would allow the use of considerably cheaper tubing materials. Furthermore, the reformer size can be substan-
Membrane Technology June 2004
tially reduced because of the lower reformer duty and the lower reforming temperature, which would enable the use of more compact reformers of the heat-exchanger type. A decrease in reformer duty, which can be as high as 30–35%, can result in lower fuel gas consumption.[6]
Catalytic, dispersed porous membrane technology Another attractive application of membrane technology in GTL processes is using the Fischer-Tropsch reaction, which does not require air separation. The conversion efficiency can be improved by using the membrane as a catalyst support. This also increases the catalyst dispersion, resulting in optimal syngas ratios and the complete consumption of the oxygen and methane. This is because oxygen is activated prior to contacting the methane inside the membrane. It often results in 100% oxygen conversion.
Catalyst reactor Based on the knowledge obtained from previous work, a partial oxidation process, using a porous ceramic membrane catalyst reactor, was defined as the best outcome to be studied and on which experiments could be done. Conceptually, from an engineering viewpoint, a tubular configuration for the membrane reactor could give better performance than the flat format, because of the associated fluid-dynamic conditions.[7] Nickel-based and rhodium-based catalysts have presented the best results for partial oxidation of methane, indicating their suitability for use in a ceramic tubular membrane. The reactor section and heating system used in this study is shown in Figure 2.
Reactor design for the catalytic dispersed porous membrane technology A cross-sectional view of the ceramic membrane is shown in Figures 3 and 4. The pores are represented by cylindrical shapes, for simplified analysis. The thickness of the active porous layer and porous support layers are designated by t1, t2, t3, t4 and t5, respectively, while r1, r3, r4 and r5 are the pore radii of their respective layers. Active support layers are formed of rhodium-impregnated α-alumina and γ-alumina. The materials selected must have similar thermal coefficients of expansion as the adjacent layers.
Porous support layers If there is difference in thermal expansion coefficients of the active porous layers and porous support layers, there is an advantage to selecting materials for the intermediate porous support layers with expansion coefficients that gradually change from values near those for the active porous layer to values near those for the outer porous support layer. One way of achieving this is to prepare the intermediate layers from a mixture of the material used to form the active porous layer in decreasing amounts in successive porous support layers. For instance, the porous support layer could contain 75% by weight of the material used in forming the active porous layer.
Identical materials The above discussion does not exclude the use of identical materials in the active porous layer and porous support layer. Such a material selection will eliminate chemical compatibility
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and differential thermal expansion problems but typically entails sacrifices in strength and material cost. The number of porous support layers will depend on the porous radius of the adjacent active porous layer. They will vary from a single layer, for an active porous layer pore radius selected from the upper end of the specified range, to as many as four, for pore radii that are selected from the lower end of the specified range.
Syngas generation process design Figures 3 and 4 illustrate a composite membrane for syngas generation. An active porous layer is located on both sides facing the oxygen and methane-containing gas. Adjacent to this is a second active porous layer, which is supported by layers
with increasing pore radii in a similar way to that described above. Here, the active porous layer on the bore side enhances the reaction between permeated oxygen and fuel species. Adjacent to the final porous support layer is a layer containing the porous matrix of a reforming catalyst. A process gas stream, comprising a fuel such as hydrocarbons and carbon monoxide, steam and recycled gas (H 2, CO, and CO 2), flows next to or through the catalyst-impregnated layer. Because in this application fuel species have to diffuse to the bore side of the active layer and the adjacent porous layer, the gaseous environment at (and near) the bore of the active layer is less reducing than in the outer porous layers. As a result, a complete or partial oxidation reaction will take place, with some reforming occurring as the gas moves away from the active layers. It is advantageous to coat the pores of the final porous support layer with a reforming catalyst
such as rhodium in order to induce some endothermic reforming, as combustion products flow through the porous support layer. This will assist in removing the heat of the exothermic oxidation reaction from the surface of the active porous layer. The gradient in the oxygen activity in the porous layer will prevent damage to the active layers (from exposure to very low oxygen partial pressures), thus permitting a greater degree of freedom in the selection of materials for these layers.
Results and discussion Partial oxidation is an interesting and promising way of producing synthesis gas from natural gas and oxygen or air, to give an H2/CO ratio that is suitable for Fischer-Tropsch and methanol synthesis. The partial oxidation method is exothermic and therefore reduces energy consumption. It also has another advantage of fast start-up, compared with steam reforming, with a large endothermic reaction.
Safety The major problem of safety associated with partial oxidation arises because methane and air (or oxygen) should be fed into the reactor at the same time, and this means that there is the danger of an explosion. The membrane system being used in this study overcomes this problem since the oxygen and methane are fed separately, and their respective flow-rates can be adjusted independently of each other.
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This work 900.15 K membrane reactor Al2O3
30
SiO2
Benefits
TiO2 ZrO2
20
Y2O3
10
0
673.15
773.15
873.15
Temperature K
Figure 5. Effect of reaction temperature on the conversion of methane for the fixed-bed and membrane reactor.
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To illustrate the benefits of using the membrane reactor in syngas production, Figure 5 shows the effect of the reaction temperature on the conversion of methane over an iridiumloaded catalyst, on various supports. This was carried out with a fixed-bed flow-type quartz reactor (350–10 mm) at atmospheric pressure, using 60 mg of catalyst, 25 ml/min of O2, and a temperature range of 400–600°C. At 600°C the performances of TiO 2 and Al 2O 3 are roughly identical. [9] In the same Figure, experimental data are shown for our membrane system at about 630°C. The conversion values obtained using the fixedbed flow reactor are significantly lower than those obtained in the membrane reactor used in our study (which had a support made of Al2O3 with a TiO2 wash-coat) because of equilibrium limitation. This has been overcome in the membrane reactor system through controlled feeding of reactants and high dispersion of the catalytic species within the porous framework of the membrane. This achieves the expected 100% conversion of oxygen and a methane conversion of 41% as shown in Figure 5.
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It is well recognized that the partial oxidation of methane may occur via two distinct mechanisms – i.e. by direct partial oxidation or total oxidation, followed by reforming reactions.[10] In order to elucidate the mechanism governing the catalytic membrane reactor used in this study, the effect of the temperature on the methane conversion and product yields was studied. The results of the analysis are presented in Figures 6 and 7, respectively. Figure 6 shows that all the oxygen is consumed. This occurs before significant amounts of hydrogen and carbon monoxide are formed. Another important feature is that the conversion of methane, the yield of water and the yield of hydrogen all pass through a maximum at 750°C. This behavior suggests that below 750°C, water, carbon monoxide and hydrogen are primary products, while carbon dioxide is a parallel sidereaction as follows:
solutions to the various energy supply and utilization problems. However, the amount of hydrogen that can be produced by using renewable natural energy sources such as solar, wind and hydroelectric power is currently not sufficient to satisfy demand. In such a situation, the use of natural gas and/or the production of hydrogen from natural gas is seen to be a viable alternative and the most realistic solution, at least in the first half of this new century.[11–13]
natural gas involves the development of small cogeneration (combined heat and power, CHP) systems using micro gas turbines. In addition, fuel cells are expected to offer a highly efficient way of generating power. Fuel cells could be used in people’s homes, in addition to being installed in electric vehicles. A stationary fuel cell used in the home will be able to provide hot water and electricity simultaneously.
Abundant hydrogen Progress An example of progress in the widespread use of
To commercialize stationary fuel cells, it is vital to establish the technology needed to
1.2
1.2
1
1.0
y=1
Conversion
Kinetic modeling has shown that the overall reaction can be described well with the contribution of parallel oxidation and full oxidation, according this scheme. Above 750°C, the total oxidation reaction r2 is expected to dominate, with a significant increase in water and carbon dioxide. However, examination of Figure 3 reveals that above 750°C the carbon dioxide yield shows only a modest increase, while the yields for water and hydrogen fall above this temperature. This suggests that hydrogen, carbon dioxide and water are being consumed according to the following scheme:
Conversion CH4 Conversion O2 Yield H2O Yield CO Yield CO2 Yield H2 Expon. (conversion O2) Poly. (conversion CH4) Poly. (yield H2O) Expon. (yield CO2) Expon. (yield CO) Poly. (yield H2)
0.8
0.8
y = -2E-07x3 + 0.0005x2 - 0.3201x + 74.155 R2 = 0.9407 0.6
0.6
0.4
0.4
Yield
Reaction pathway
y = -2E-07x3 + 0.0004x2 - 0.2794x + 65.037 R2 = 0.9263 y = 0.0476e0.0016x 0.2 R2 = 0.916 y = 0.0238e0.00009x 3 + 6E-05x2 -0.0407x + 9.1173 y = -3E-08x R2 = 0.0616 R2 = 0.9571 0
0.2
0 600
650
700
750
800
850
Temperature ( C)
Figure 6. Effect of temperature on methane conversion – feed flow-rate ratio (CH4/O2) = 150/50.
(CH4/O2) feed = 150/15 2.5
Hydrogen/carbon monoxide ratio An important aspect in the conversion of synthesis gas to liquids is the hydrogen/carbon monoxide (H2/CO) ratio. A ratio of 2:1 is optimum for this conversion. Figure 6 shows a plot of H2/CO over the temperature range studied. The optimum for gas-to-liquids conversion is obtained at 750°C. Above this temperature, a ratio below 2.0 is attained, while below 750°C a value above 2.0 is obtained.
Hydrogen production from natural gas The hydrogen energy economy is expected to overcome global warming, as well as provide
Membrane Technology June 2004
2
Syngas ratio (H2/CO)
This scheme helps to explain the fall in the water and hydrogen yields, the modest CO2 yield increase, and the fall in methane conversion above 750°C.
1.5
1
0.5
0 600
650
700
750
800
850
Temperature ( C)
Figure 7. The effect of temperature on the syngas ratio.
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generate abundant supplies of pure hydrogen at low cost, in addition to the development of the fuel cells themselves. The catalytic dispersed ceramic membrane technology developed by the Centre for Process Integration & Membrane Technology at the Robert Gordon University can be reconfigured to generate this hydrogen by optimizing the feed flow-rate and temperature.
Recent developments in GTL technology On 8 December 2003, the US-based oil & gas company ConocoPhillips signed an agreement to build a $5 billion plant to convert gas to liquids in Qatar in the Persian Gulf, which is working to establish itself as the GTL capital of the world. This agreement, which initiates technical and commercial engineering and design studies for the plant, was the third GTL deal signed last year by state-owned Qatar Petroleum, following deals with Royal Dutch/Shell and Sasol of South Africa.
Gas-to-liquids projects Qatar has succeeded in becoming the target location for many gas-to-liquids projects. Such projects will also assist Qatar to monetize gas resources and become the GTL capital of the world. The plant, as described above, will initially include two trains producing around 33 000 barrels per day (bpd), comprising nearly 24 000 barrels of GTL diesel, 8000 barrels of GTL naphtha and 1000 barrels of LPG. The project is at its most advanced phase, with the engineering, procurement and construction contractor, and finance being secured. The project is seen as a strategic investment for Qatar because it will create many jobs – both skilled and non-skilled – for Qataris. It will also put Qatar in a unique position in the fast-developing GTL sector.
Carbon credits Other projects are being considered, which will enhance the production of environmentally friendly GTL fuel – mostly in preference to conventional diesel. During September 2003, Oklahoma-based Syntroleum Corporation reported that it had signed a memorandum of understanding to develop gas-to-liquids projects in Nigeria, using the country’s largely stranded natural gas reserves. In Nigeria an estimated volume of 2 billion ft3 (57 million m3) of gas is flared or vented every day. Syntroleum says that some 200 000 barrels per day of synthetic fuels could be produced from this gas. The project is seen as an excellent candidate to qualify for carbon credits that can be traded worldwide under the World Bank’s
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Prototype Carbon Fund. The design and construction of any mobile marine production facilities would fall under the planned joint venture between Syntroleum and Norwegian/ US-based Petroleum Geo-Services ASA.
Value-adding industry The gas-to-liquids projects are creating a new value-adding industry that requires specialist skills and expertise. It is predicted that there will be a significant demand for control room and process plant operators. Other occupational areas involved in the ongoing operations and maintenance of the new gas-to-liquids plants will include: •
• •
• •
Tradespersons and technicians, covering metals, electrical, electronic and instrumentation. Trades assistants and semi-skilled workers. Engineers (electrical and instrumentation, process, production, maintenance and mechanical). Management, warehouse, purchasing and administration personnel. Chemists and laboratory technicians.
Conclusions A catalytic membrane reactor has been developed and used to produce synthesis gas under various operating conditions with total consumption of oxygen. At lower feed ratios (CH4/O2), the syngas ratio is well above 2.0, while at a higher CH4/O2 ratio the syngas ratio is 2.0. This means that, depending on the application, the reactor is flexible to the extent that it could be applied in the Fischer-Tropsch process for converting natural gas to liquid hydrocarbons. For gas-to-liquids (GTL) conversion, an optimum temperature of 750°C has been established at which the hydrogen/carbon monoxide ratio is 2.0. In this project, we are focusing on the partial oxidation (POX) route. This technique follows an exothermic pathway, and is expected to reduce the energy consumed in the production of syngas. This POX method has other advantages such as fast startup (compared with steam reforming, which has a large endothermic reaction), enhanced flow rate of feedstock, and the possibility of using air rather than pure oxygen.[14] Most importantly, the process is able to overcome the problems of safety that are encountered in conventional reformers, where the air and methane are co-fed into the reformer. In the membrane process, the oxidant and methane are feed separately, thus overcoming the flammability constraint.
References 1. B. Macdonald: Syngas – not just a load of hot air, ECN Chemscope (1999) 24–25.
2. G. Parkinson: A new era for gas-to-liquids technology, Chemical Engineering (July 2002) 27–31. 3. J. Haggin: Chemical Engineering News 70 (1992) 33. 4. S. Michel: Hydrocarbon Processing 68(4) (1989) 37. 5. P.N. Dyer and C.M. Chen: Engineering development of ceramic membrane reactor systems for converting natural gas to hydrogen and synthesis gas for liquid transportation fuels, in: 2000 Hydrogen Program Review, Air Products & Chemicals Inc, Pennsylvania, USA (2000). 6. R. Bredesen: A technical and economic assessment of membrane reactors for hydrogen and syngas production, in: United Nations – Economic Commission for Europe Seminar, Cetraro, Calabria, Italy ( 1996). 7. A. Basile, L. Paturzo and A. Vazzana: Membrane reactor for the production of hydrogen and higher hydrocarbons from methane over Ru/Al2O3 catalyst, Chemical Engineering Journal, 4045 (2002) 1–9. 8. E. Gobina (Ed): Gas-to-liquids process for chemicals and energy production. Report E-101, BCC Inc, Norwalk, Connecticut, USA, October 2000. 9. K. Nakagawa et al.: Partial oxidation of methane to synthesis gas with iridium-loaded titania catalyst, Chemistry Letters 25(12) (1996) 1029–1030. 10. M. Prettre, C. Eichner and M. Perrin: Trans. Faraday Society 43 (1946) 335. 11. E. Gobina: The world natural gas business. Report E-104, BCC Inc, USA, September 2001. 12. E. Gobina: Merchant hydrogen utilization and on-site distributed generation. Report RC239, BCC Inc, USA, January 2004. 13. E. Gobina: Hydrogen as a chemical constituent and as an energy source. Report C-219R, BCC Inc, USA, February 2003. 14. S. Olsen and E. Gobina: An enhanced catalyst-dispersed ceramic membrane for lowcost synthesis gas production suitable for gas-to-liquids. 21st Annual Membrane/ Separations Technology Planning Conference, Boston, Massachusetts, USA, December 2003.
Contact: Dr Edward Gobina, Centre for Process Integration & Membrane Technology, School of Engineering, Robert Gordon University, Schoolhill, Aberdeen AB10 1FR, UK. Tel: +44 1224 262348, Fax: +44 1224 262444, Email: [email protected], Web: www.rgu.ac.uk/eng/cpi
The feature article is based on a paper entitled ‘Development of a high performance gas-to-liquids (GTL) synthesis gas generation membrane for monetizing stranded gas’, which was presented at the 21st Annual Membrane/Separations Technology Planning Conference in Boston, Massachusetts, USA in December 2003.
Membrane Technology June 2004
GTL synthesis gas generation membrane for monetizing stranded gas By Susanne Olsen and Dr Edward Gobina – Centre for Process Integration & Membrane Technology, Robert Gordon University, Aberdeen, Scotland This feature article provides details of the development of a high-performance membrane for use in gas-to-liquids (GTL) synthesis gas generation for monetizing stranded gas assets.
Without a viable market, the natural gas discovered by oil companies can become a costly nuisance (as ‘stranded gas’) which must be flared, re-injected or plugged. One attractive option for monetizing this gas is to use a gas-toliquids (GTL) process to produce a fuel that can be transported easily, which can can be used locally in existing fuel systems, and also is extremely clean.
Introduction In the first step of the gas-to-liquids process, syngas is produced from the natural gas. In the next step, Fischer-Tropsch chemistry is used to produce hydrocarbon from the syngas. Membrane technology can be applied to the syngas production reaction. An attractive application of membrane technology is that the conversion efficiency can be improved by using the membrane as the catalyst support, which then also increases the catalyst dispersion, resulting in an optimal catalyst load and complete consumption of the oxygen and methane. In this article, it is demonstrated that the oxygen is activated before contacting the methane inside the membrane. This often results in 100% oxygen conversion.
Liquefied natural gas In some cases, a conversion to liquefied natural gas (LNG) is carried out. Unfortunately, LNG has limited market opportunities, because its production and transportation require dedicated facilities and therefore a large capital investment. Without a viable market, the natural gas discovered by oil companies can become a costly nuisance which must be flared, re-injected or plugged.[1]
Attractive option One attractive option is to use Fischer-Tropsch gas-to-liquids conversion, which is where the development of improved syngas processes is so important. The fuel produced is easily transport-
ed, can be used locally, can be used in existing fuel systems and is extremely clean. An example of this process in shown in Figure 1. In the first step of the process, syngas is produced from the natural gas. In the next step of the process, Fischer-Tropsch chemistry is used to produce hydrocarbon from the syngas. In some cases a further hydro-cracking step is used to produce a liquid fuel. Approximately 60% of the costs for the overall processes are accounted in the first step, i.e. the production of syngas.[2] The development of an enhanced syngas technology is therefore absolutely critical for the commercialization of GTL processes.
Natural gas conversion to syngas Almost all options for the transformation of methane (the main component of natural gas) involve its initial conversion to syngas (CO + H2). Syngas can yield liquid hydrocarbons through the F–T reaction over Group VIII transition metal catalysts (see Reaction 1), or first to
Natural gas Water T
Air
Partial oxidation
Desulphurization
Waste heat system Water
Syngas FT-reactor 1
Steam FT-reactor 2
Stranded gas According to a recent estimate, almost 5000 significant reservoirs of natural gas have been found worldwide. Almost half of these are stranded because they are too remote and inaccessible to gas pipelines. Because of the high investment costs, the transportation of natural gas by pipeline is limited to those cases where reserves are located reasonably close to the customers.[1]
Membrane Technology June 2004
G
Off-gas
Water Diesel oil Wax
Figure 1. An example of the gas-to-liquids process using air.
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There are some unwanted reactions that occur simultaneously, namely carbon formation. Carbon deposited from the destabilization of CO into C and CO2, from the reduction of CO and from CH4 decomposition on catalytic surfaces, can be controlled by process conditions such as the steam concentration in the feed. Reactions 3 and 4 are involved in reforming processes, of which there are five: • • •
• •
Figure 2. The reaction rig system in use at the Centre for Process Integration & Membrane Technology, Robert Gordon University.
methanol over Cu/ZnO catalysts, and then to gasoline or petrol by the methanol-to-gasoline (MTG) process over ZSM-5 zeolites (see Reaction 2):
830°C. The reaction products H2, CO, CO2 and H2O are for all practical purposes stable under reaction conditions. There are two main reactions that are important in the conversion of natural gas to syngas: • •
Ammonia synthesis is still the largest single consumer of syngas, but the growing interest during the last decade in C1 chemistry and in large-scale conversion of natural gas into liquid fuels has created a need to explore the limits of the reforming technology.
Steam reforming. Partial oxidation.
The steam reforming is a highly endothermic reaction of methane and steam, whereas the partial oxidation is slightly exothermic:
Non-catalytic partial oxidation (NC-POM), in which only Reaction 4 is involved. Conventional steam reforming (SMR), where only Reaction 3 participates. Autothermal catalytic reforming (ATR), in which Reaction 4 and Reaction 3 take place simultaneously. Combined reforming (CMR), where Reaction 3 is followed by Reaction 4. Catalytic partial oxidation (POM), which mainly involves Reaction 4.
While most syngas is produced by steam reforming, some of the other routes may be more attractive, depending on factors such as the H 2/CO ratio; downstream use; product purity; the presence of CO2, N2, H2O and CH4; capacity; feedstock availability; purity and cost, including O2.[4]
Membrane technology in syngas production The primary advantage of using the enhanced membrane reactor configuration is that it provides a safer reaction environment, and this allows operations to be carried out under conditions that could be hazardous in a more conventional fixed-bed reactor. In the membrane reactor, it is possible to lower the methane/oxygen feed ratios without creating the potential for an explosion. It is also possible to operate at both high and low contact times.
Cost of generating syngas It has been estimated that in the above industrial applications, more than 60–70% of the cost of the overall process is associated with syngas production.[3] Therefore, reducing the syngas generation costs would have a large and direct influence on the overall economics of all these downstream industrial processes.
Stable Because methane (CH4) is a very stable molecule, it has to be processed under very severe conditions. Although its conversion to syngas can be conducted at temperatures even below about 430°C, high yields to syngas need substantially higher temperatures – typically about
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Figure 3. Cross-sectional view of the ceramic membrane — thickness.
Membrane Technology June 2004
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Figure 4. Cross-sectional view of the ceramic membrane — reaction.
Alternative strategy Preliminary cost estimates indicate that enhanced ceramic membrane reactors could reduce the capital cost for syngas by more than one-third. This reduction would have a significant impact on the costs of liquid transportation fuels derived from natural gas.[5] This factor definitely provides a substantial incentive for developing an alternative strategy for syngas production using membrane technology.
Ion-transport membrane One of the emerging membrane technologies is the ion transport membrane. For either partial oxidation or autothermal reforming, the cryogenic oxygen plant may be replaced by a high-temperature membrane that can separate oxygen from air and supply it directly into the reactor for converting CH4 to CO + H2. The extremely low partial pressure of oxygen in the reaction zone of an autothermal reformer (with mole fraction on the order of 10 –6 to 10 –18), creates a large driving force for oxygen separation in oxygenion conducting membranes. Using such membranes, only compression work, which needs to overcome the friction losses in the system, would be required.
Lower temperatures Attempts to further increase gas conversion using membranes in steam reformers have not proved to be economically viable. However, at lower temperatures an equilibrium shift towards the product gases could be obtained with H2selective membranes that remove the hydrogen formed in the reaction. Lower temperatures would allow the use of considerably cheaper tubing materials. Furthermore, the reformer size can be substan-
Membrane Technology June 2004
tially reduced because of the lower reformer duty and the lower reforming temperature, which would enable the use of more compact reformers of the heat-exchanger type. A decrease in reformer duty, which can be as high as 30–35%, can result in lower fuel gas consumption.[6]
Catalytic, dispersed porous membrane technology Another attractive application of membrane technology in GTL processes is using the Fischer-Tropsch reaction, which does not require air separation. The conversion efficiency can be improved by using the membrane as a catalyst support. This also increases the catalyst dispersion, resulting in optimal syngas ratios and the complete consumption of the oxygen and methane. This is because oxygen is activated prior to contacting the methane inside the membrane. It often results in 100% oxygen conversion.
Catalyst reactor Based on the knowledge obtained from previous work, a partial oxidation process, using a porous ceramic membrane catalyst reactor, was defined as the best outcome to be studied and on which experiments could be done. Conceptually, from an engineering viewpoint, a tubular configuration for the membrane reactor could give better performance than the flat format, because of the associated fluid-dynamic conditions.[7] Nickel-based and rhodium-based catalysts have presented the best results for partial oxidation of methane, indicating their suitability for use in a ceramic tubular membrane. The reactor section and heating system used in this study is shown in Figure 2.
Reactor design for the catalytic dispersed porous membrane technology A cross-sectional view of the ceramic membrane is shown in Figures 3 and 4. The pores are represented by cylindrical shapes, for simplified analysis. The thickness of the active porous layer and porous support layers are designated by t1, t2, t3, t4 and t5, respectively, while r1, r3, r4 and r5 are the pore radii of their respective layers. Active support layers are formed of rhodium-impregnated α-alumina and γ-alumina. The materials selected must have similar thermal coefficients of expansion as the adjacent layers.
Porous support layers If there is difference in thermal expansion coefficients of the active porous layers and porous support layers, there is an advantage to selecting materials for the intermediate porous support layers with expansion coefficients that gradually change from values near those for the active porous layer to values near those for the outer porous support layer. One way of achieving this is to prepare the intermediate layers from a mixture of the material used to form the active porous layer in decreasing amounts in successive porous support layers. For instance, the porous support layer could contain 75% by weight of the material used in forming the active porous layer.
Identical materials The above discussion does not exclude the use of identical materials in the active porous layer and porous support layer. Such a material selection will eliminate chemical compatibility
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and differential thermal expansion problems but typically entails sacrifices in strength and material cost. The number of porous support layers will depend on the porous radius of the adjacent active porous layer. They will vary from a single layer, for an active porous layer pore radius selected from the upper end of the specified range, to as many as four, for pore radii that are selected from the lower end of the specified range.
Syngas generation process design Figures 3 and 4 illustrate a composite membrane for syngas generation. An active porous layer is located on both sides facing the oxygen and methane-containing gas. Adjacent to this is a second active porous layer, which is supported by layers
with increasing pore radii in a similar way to that described above. Here, the active porous layer on the bore side enhances the reaction between permeated oxygen and fuel species. Adjacent to the final porous support layer is a layer containing the porous matrix of a reforming catalyst. A process gas stream, comprising a fuel such as hydrocarbons and carbon monoxide, steam and recycled gas (H 2, CO, and CO 2), flows next to or through the catalyst-impregnated layer. Because in this application fuel species have to diffuse to the bore side of the active layer and the adjacent porous layer, the gaseous environment at (and near) the bore of the active layer is less reducing than in the outer porous layers. As a result, a complete or partial oxidation reaction will take place, with some reforming occurring as the gas moves away from the active layers. It is advantageous to coat the pores of the final porous support layer with a reforming catalyst
such as rhodium in order to induce some endothermic reforming, as combustion products flow through the porous support layer. This will assist in removing the heat of the exothermic oxidation reaction from the surface of the active porous layer. The gradient in the oxygen activity in the porous layer will prevent damage to the active layers (from exposure to very low oxygen partial pressures), thus permitting a greater degree of freedom in the selection of materials for these layers.
Results and discussion Partial oxidation is an interesting and promising way of producing synthesis gas from natural gas and oxygen or air, to give an H2/CO ratio that is suitable for Fischer-Tropsch and methanol synthesis. The partial oxidation method is exothermic and therefore reduces energy consumption. It also has another advantage of fast start-up, compared with steam reforming, with a large endothermic reaction.
Safety The major problem of safety associated with partial oxidation arises because methane and air (or oxygen) should be fed into the reactor at the same time, and this means that there is the danger of an explosion. The membrane system being used in this study overcomes this problem since the oxygen and methane are fed separately, and their respective flow-rates can be adjusted independently of each other.
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This work 900.15 K membrane reactor Al2O3
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SiO2
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TiO2 ZrO2
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Y2O3
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0
673.15
773.15
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Figure 5. Effect of reaction temperature on the conversion of methane for the fixed-bed and membrane reactor.
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To illustrate the benefits of using the membrane reactor in syngas production, Figure 5 shows the effect of the reaction temperature on the conversion of methane over an iridiumloaded catalyst, on various supports. This was carried out with a fixed-bed flow-type quartz reactor (350–10 mm) at atmospheric pressure, using 60 mg of catalyst, 25 ml/min of O2, and a temperature range of 400–600°C. At 600°C the performances of TiO 2 and Al 2O 3 are roughly identical. [9] In the same Figure, experimental data are shown for our membrane system at about 630°C. The conversion values obtained using the fixedbed flow reactor are significantly lower than those obtained in the membrane reactor used in our study (which had a support made of Al2O3 with a TiO2 wash-coat) because of equilibrium limitation. This has been overcome in the membrane reactor system through controlled feeding of reactants and high dispersion of the catalytic species within the porous framework of the membrane. This achieves the expected 100% conversion of oxygen and a methane conversion of 41% as shown in Figure 5.
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It is well recognized that the partial oxidation of methane may occur via two distinct mechanisms – i.e. by direct partial oxidation or total oxidation, followed by reforming reactions.[10] In order to elucidate the mechanism governing the catalytic membrane reactor used in this study, the effect of the temperature on the methane conversion and product yields was studied. The results of the analysis are presented in Figures 6 and 7, respectively. Figure 6 shows that all the oxygen is consumed. This occurs before significant amounts of hydrogen and carbon monoxide are formed. Another important feature is that the conversion of methane, the yield of water and the yield of hydrogen all pass through a maximum at 750°C. This behavior suggests that below 750°C, water, carbon monoxide and hydrogen are primary products, while carbon dioxide is a parallel sidereaction as follows:
solutions to the various energy supply and utilization problems. However, the amount of hydrogen that can be produced by using renewable natural energy sources such as solar, wind and hydroelectric power is currently not sufficient to satisfy demand. In such a situation, the use of natural gas and/or the production of hydrogen from natural gas is seen to be a viable alternative and the most realistic solution, at least in the first half of this new century.[11–13]
natural gas involves the development of small cogeneration (combined heat and power, CHP) systems using micro gas turbines. In addition, fuel cells are expected to offer a highly efficient way of generating power. Fuel cells could be used in people’s homes, in addition to being installed in electric vehicles. A stationary fuel cell used in the home will be able to provide hot water and electricity simultaneously.
Abundant hydrogen Progress An example of progress in the widespread use of
To commercialize stationary fuel cells, it is vital to establish the technology needed to
1.2
1.2
1
1.0
y=1
Conversion
Kinetic modeling has shown that the overall reaction can be described well with the contribution of parallel oxidation and full oxidation, according this scheme. Above 750°C, the total oxidation reaction r2 is expected to dominate, with a significant increase in water and carbon dioxide. However, examination of Figure 3 reveals that above 750°C the carbon dioxide yield shows only a modest increase, while the yields for water and hydrogen fall above this temperature. This suggests that hydrogen, carbon dioxide and water are being consumed according to the following scheme:
Conversion CH4 Conversion O2 Yield H2O Yield CO Yield CO2 Yield H2 Expon. (conversion O2) Poly. (conversion CH4) Poly. (yield H2O) Expon. (yield CO2) Expon. (yield CO) Poly. (yield H2)
0.8
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y = -2E-07x3 + 0.0005x2 - 0.3201x + 74.155 R2 = 0.9407 0.6
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Reaction pathway
y = -2E-07x3 + 0.0004x2 - 0.2794x + 65.037 R2 = 0.9263 y = 0.0476e0.0016x 0.2 R2 = 0.916 y = 0.0238e0.00009x 3 + 6E-05x2 -0.0407x + 9.1173 y = -3E-08x R2 = 0.0616 R2 = 0.9571 0
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0 600
650
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Temperature ( C)
Figure 6. Effect of temperature on methane conversion – feed flow-rate ratio (CH4/O2) = 150/50.
(CH4/O2) feed = 150/15 2.5
Hydrogen/carbon monoxide ratio An important aspect in the conversion of synthesis gas to liquids is the hydrogen/carbon monoxide (H2/CO) ratio. A ratio of 2:1 is optimum for this conversion. Figure 6 shows a plot of H2/CO over the temperature range studied. The optimum for gas-to-liquids conversion is obtained at 750°C. Above this temperature, a ratio below 2.0 is attained, while below 750°C a value above 2.0 is obtained.
Hydrogen production from natural gas The hydrogen energy economy is expected to overcome global warming, as well as provide
Membrane Technology June 2004
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Syngas ratio (H2/CO)
This scheme helps to explain the fall in the water and hydrogen yields, the modest CO2 yield increase, and the fall in methane conversion above 750°C.
1.5
1
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0 600
650
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750
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Temperature ( C)
Figure 7. The effect of temperature on the syngas ratio.
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generate abundant supplies of pure hydrogen at low cost, in addition to the development of the fuel cells themselves. The catalytic dispersed ceramic membrane technology developed by the Centre for Process Integration & Membrane Technology at the Robert Gordon University can be reconfigured to generate this hydrogen by optimizing the feed flow-rate and temperature.
Recent developments in GTL technology On 8 December 2003, the US-based oil & gas company ConocoPhillips signed an agreement to build a $5 billion plant to convert gas to liquids in Qatar in the Persian Gulf, which is working to establish itself as the GTL capital of the world. This agreement, which initiates technical and commercial engineering and design studies for the plant, was the third GTL deal signed last year by state-owned Qatar Petroleum, following deals with Royal Dutch/Shell and Sasol of South Africa.
Gas-to-liquids projects Qatar has succeeded in becoming the target location for many gas-to-liquids projects. Such projects will also assist Qatar to monetize gas resources and become the GTL capital of the world. The plant, as described above, will initially include two trains producing around 33 000 barrels per day (bpd), comprising nearly 24 000 barrels of GTL diesel, 8000 barrels of GTL naphtha and 1000 barrels of LPG. The project is at its most advanced phase, with the engineering, procurement and construction contractor, and finance being secured. The project is seen as a strategic investment for Qatar because it will create many jobs – both skilled and non-skilled – for Qataris. It will also put Qatar in a unique position in the fast-developing GTL sector.
Carbon credits Other projects are being considered, which will enhance the production of environmentally friendly GTL fuel – mostly in preference to conventional diesel. During September 2003, Oklahoma-based Syntroleum Corporation reported that it had signed a memorandum of understanding to develop gas-to-liquids projects in Nigeria, using the country’s largely stranded natural gas reserves. In Nigeria an estimated volume of 2 billion ft3 (57 million m3) of gas is flared or vented every day. Syntroleum says that some 200 000 barrels per day of synthetic fuels could be produced from this gas. The project is seen as an excellent candidate to qualify for carbon credits that can be traded worldwide under the World Bank’s
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Prototype Carbon Fund. The design and construction of any mobile marine production facilities would fall under the planned joint venture between Syntroleum and Norwegian/ US-based Petroleum Geo-Services ASA.
Value-adding industry The gas-to-liquids projects are creating a new value-adding industry that requires specialist skills and expertise. It is predicted that there will be a significant demand for control room and process plant operators. Other occupational areas involved in the ongoing operations and maintenance of the new gas-to-liquids plants will include: •
• •
• •
Tradespersons and technicians, covering metals, electrical, electronic and instrumentation. Trades assistants and semi-skilled workers. Engineers (electrical and instrumentation, process, production, maintenance and mechanical). Management, warehouse, purchasing and administration personnel. Chemists and laboratory technicians.
Conclusions A catalytic membrane reactor has been developed and used to produce synthesis gas under various operating conditions with total consumption of oxygen. At lower feed ratios (CH4/O2), the syngas ratio is well above 2.0, while at a higher CH4/O2 ratio the syngas ratio is 2.0. This means that, depending on the application, the reactor is flexible to the extent that it could be applied in the Fischer-Tropsch process for converting natural gas to liquid hydrocarbons. For gas-to-liquids (GTL) conversion, an optimum temperature of 750°C has been established at which the hydrogen/carbon monoxide ratio is 2.0. In this project, we are focusing on the partial oxidation (POX) route. This technique follows an exothermic pathway, and is expected to reduce the energy consumed in the production of syngas. This POX method has other advantages such as fast startup (compared with steam reforming, which has a large endothermic reaction), enhanced flow rate of feedstock, and the possibility of using air rather than pure oxygen.[14] Most importantly, the process is able to overcome the problems of safety that are encountered in conventional reformers, where the air and methane are co-fed into the reformer. In the membrane process, the oxidant and methane are feed separately, thus overcoming the flammability constraint.
References 1. B. Macdonald: Syngas – not just a load of hot air, ECN Chemscope (1999) 24–25.
2. G. Parkinson: A new era for gas-to-liquids technology, Chemical Engineering (July 2002) 27–31. 3. J. Haggin: Chemical Engineering News 70 (1992) 33. 4. S. Michel: Hydrocarbon Processing 68(4) (1989) 37. 5. P.N. Dyer and C.M. Chen: Engineering development of ceramic membrane reactor systems for converting natural gas to hydrogen and synthesis gas for liquid transportation fuels, in: 2000 Hydrogen Program Review, Air Products & Chemicals Inc, Pennsylvania, USA (2000). 6. R. Bredesen: A technical and economic assessment of membrane reactors for hydrogen and syngas production, in: United Nations – Economic Commission for Europe Seminar, Cetraro, Calabria, Italy ( 1996). 7. A. Basile, L. Paturzo and A. Vazzana: Membrane reactor for the production of hydrogen and higher hydrocarbons from methane over Ru/Al2O3 catalyst, Chemical Engineering Journal, 4045 (2002) 1–9. 8. E. Gobina (Ed): Gas-to-liquids process for chemicals and energy production. Report E-101, BCC Inc, Norwalk, Connecticut, USA, October 2000. 9. K. Nakagawa et al.: Partial oxidation of methane to synthesis gas with iridium-loaded titania catalyst, Chemistry Letters 25(12) (1996) 1029–1030. 10. M. Prettre, C. Eichner and M. Perrin: Trans. Faraday Society 43 (1946) 335. 11. E. Gobina: The world natural gas business. Report E-104, BCC Inc, USA, September 2001. 12. E. Gobina: Merchant hydrogen utilization and on-site distributed generation. Report RC239, BCC Inc, USA, January 2004. 13. E. Gobina: Hydrogen as a chemical constituent and as an energy source. Report C-219R, BCC Inc, USA, February 2003. 14. S. Olsen and E. Gobina: An enhanced catalyst-dispersed ceramic membrane for lowcost synthesis gas production suitable for gas-to-liquids. 21st Annual Membrane/ Separations Technology Planning Conference, Boston, Massachusetts, USA, December 2003.
Contact: Dr Edward Gobina, Centre for Process Integration & Membrane Technology, School of Engineering, Robert Gordon University, Schoolhill, Aberdeen AB10 1FR, UK. Tel: +44 1224 262348, Fax: +44 1224 262444, Email: [email protected], Web: www.rgu.ac.uk/eng/cpi
The feature article is based on a paper entitled ‘Development of a high performance gas-to-liquids (GTL) synthesis gas generation membrane for monetizing stranded gas’, which was presented at the 21st Annual Membrane/Separations Technology Planning Conference in Boston, Massachusetts, USA in December 2003.
Membrane Technology June 2004