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5Membrane Technology June 2004

Without a viable market, the natural gas dis-covered by oil companies can become a costlynuisance (as ‘stranded gas’) which must beflared, re-injected or plugged. One attractiveoption for monetizing this gas is to use a gas-to-liquids (GTL) process to produce a fuel thatcan be transported easily, which can can beused locally in existing fuel systems, and also isextremely clean.

IntroductionIn the first step of the gas-to-liquids process, syn-gas is produced from the natural gas. In the nextstep, Fischer-Tropsch chemistry is used to pro-duce hydrocarbon from the syngas. Membranetechnology can be applied to the syngas produc-tion reaction.

An attractive application of membrane tech-nology is that the conversion efficiency can beimproved by using the membrane as the cata-lyst support, which then also increases the cata-lyst dispersion, resulting in an optimal catalystload and complete consumption of the oxygenand methane.

In this article, it is demonstrated that the oxy-gen is activated before contacting the methaneinside the membrane. This often results in 100%oxygen conversion.

Stranded gasAccording to a recent estimate, almost 5000 sig-nificant reservoirs of natural gas have been foundworldwide. Almost half of these are strandedbecause they are too remote and inaccessible togas pipelines.

Because of the high investment costs, thetransportation of natural gas by pipeline is limit-ed to those cases where reserves are located rea-sonably close to the customers.[1]

Liquefied natural gasIn some cases, a conversion to liquefied naturalgas (LNG) is carried out. Unfortunately, LNGhas limited market opportunities, because its production and transportation require dedi-cated facilities and therefore a large capitalinvestment. Without a viable market, the natur-al gas discovered by oil companies can become acostly nuisance which must be flared, re-injectedor plugged.[1]

Attractive optionOne attractive option is to use Fischer-Tropschgas-to-liquids conversion, which is where thedevelopment of improved syngas processes is soimportant. The fuel produced is easily transport-

ed, can be used locally, can be used in existingfuel systems and is extremely clean. An exampleof this process in shown in Figure 1.

In the first step of the process, syngas is pro-duced from the natural gas. In the next step ofthe process, Fischer-Tropsch chemistry is used toproduce hydrocarbon from the syngas. In somecases a further hydro-cracking step is used toproduce a liquid fuel.

Approximately 60% of the costs for the over-all processes are accounted in the first step, i.e.the production of syngas.[2] The developmentof an enhanced syngas technology is thereforeabsolutely critical for the commercialization ofGTL processes.

Natural gas conversion to syngasAlmost all options for the transformation ofmethane (the main component of natural gas)involve its initial conversion to syngas (CO + H2).

Syngas can yield liquid hydrocarbons throughthe F–T reaction over Group VIII transitionmetal catalysts (see Reaction 1), or first to

GTL synthesis gas generation membrane formonetizing stranded gasBy 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-performancemembrane for use in gas-to-liquids (GTL) synthesis gas generation for monetiz-ing stranded gas assets.

Natural gas

Air

Partialoxidation

Desulphurization

Water

T G

Water

Steam

Waste heat system

Syngas

FT-reactor 2FT-reactor 1

Off-gas

Water

Diesel oil

Wax

Figure 1. An example of the gas-to-liquids process using air.

methanol over Cu/ZnO catalysts, and then togasoline or petrol by the methanol-to-gasoline(MTG) process over ZSM-5 zeolites (seeReaction 2):

Ammonia synthesis is still the largest single con-sumer of syngas, but the growing interest duringthe last decade in C1 chemistry and in large-scaleconversion of natural gas into liquid fuels hascreated a need to explore the limits of thereforming technology.

Cost of generating syngas It has been estimated that in the above indus-trial applications, more than 60–70% of thecost of the overall process is associated withsyngas production.[3] Therefore, reducing thesyngas generation costs would have a large anddirect influence on the overall economics of allthese downstream industrial processes.

StableBecause methane (CH4) is a very stable mole-cule, it has to be processed under very severeconditions. Although its conversion to syngascan be conducted at temperatures even belowabout 430°C, high yields to syngas need substan-tially higher temperatures – typically about

830°C. The reaction products H2, CO, CO2and H2O are for all practical purposes stableunder reaction conditions.

There are two main reactions that are impor-tant in the conversion of natural gas to syngas:

• Steam reforming.• Partial oxidation.

The steam reforming is a highly endothermicreaction of methane and steam, whereas the partial oxidation is slightly exothermic:

There are some unwanted reactions that occursimultaneously, namely carbon formation.Carbon deposited from the destabilization ofCO into C and CO2, from the reduction ofCO and from CH4 decomposition on catalyt-ic surfaces, can be controlled by process conditions such as the steam concentration in the feed.

Reactions 3 and 4 are involved in reformingprocesses, of which there are five:

• 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), inwhich Reaction 4 and Reaction 3 take placesimultaneously.

• Combined reforming (CMR), whereReaction 3 is followed by Reaction 4.

• Catalytic partial oxidation (POM), whichmainly involves Reaction 4.

While most syngas is produced by steamreforming, some of the other routes may bemore attractive, depending on factors such asthe H2/CO ratio; downstream use; productpurity; the presence of CO2, N2, H2O andCH4; capacity; feedstock availability; purityand cost, including O2.

[4]

Membrane technology insyngas production The primary advantage of using the enhancedmembrane reactor configuration is that it provides a safer reaction environment, andthis allows operations to be carried out underconditions that could be hazardous in a more conventional fixed-bed reactor. In themembrane reactor, it is possible to lower themethane/oxygen feed ratios without creatingthe potential for an explosion. It is also possible to operate at both high and low con-tact times.

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6Membrane Technology June 2004

Figure 2. The reaction rig system in use at the Centre for Process Integration & Membrane Technology,Robert Gordon University.

Figure 3. Cross-sectional view of the ceramic membrane — thickness.

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7Membrane Technology June 2004

Alternative strategyPreliminary cost estimates indicate thatenhanced ceramic membrane reactors couldreduce the capital cost for syngas by more thanone-third. This reduction would have a signifi-cant impact on the costs of liquid transportationfuels derived from natural gas.[5]

This factor definitely provides a substantialincentive for developing an alternative strategyfor syngas production using membrane tech-nology.

Ion-transport membraneOne of the emerging membrane technologies isthe ion transport membrane.

For either partial oxidation or autothermalreforming, the cryogenic oxygen plant may bereplaced by a high-temperature membranethat can separate oxygen from air and supply it directly into the reactor for converting CH4to CO + H2. The extremely low partial pres-sure of oxygen in the reaction zone of anautothermal reformer (with mole fraction onthe order of 10–6 to 10–18), creates a large driving force for oxygen separation in oxygen-ion conducting membranes. Using such mem-branes, only compression work, which needsto overcome the friction losses in the system,would be required.

Lower temperaturesAttempts to further increase gas conversionusing membranes in steam reformers have notproved to be economically viable. However, atlower temperatures an equilibrium shift towardsthe product gases could be obtained with H2-selective membranes that remove the hydrogenformed in the reaction.

Lower temperatures would allow the use of considerably cheaper tubing materials.Furthermore, the reformer size can be substan-

tially reduced because of the lower reformerduty and the lower reforming temperature,which would enable the use of more compactreformers of the heat-exchanger type. Adecrease in reformer duty, which can be as high as 30–35%, can result in lower fuel gasconsumption.[6]

Catalytic, dispersed porousmembrane technologyAnother attractive application of membranetechnology in GTL processes is using theFischer-Tropsch reaction, which does not requireair separation.

The conversion efficiency can be improvedby using the membrane as a catalyst support.This also increases the catalyst dispersion,resulting in optimal syngas ratios and thecomplete consumption of the oxygen andmethane. This is because oxygen is activatedprior to contacting the methane inside the membrane. It often results in 100% oxy-gen conversion.

Catalyst reactorBased on the knowledge obtained from previouswork, a partial oxidation process, using a porousceramic membrane catalyst reactor, was definedas the best outcome to be studied and on whichexperiments could be done.

Conceptually, from an engineering viewpoint,a tubular configuration for the membrane reac-tor could give better performance than the flatformat, because of the associated fluid-dynamicconditions.[7] Nickel-based and rhodium-basedcatalysts have presented the best results for par-tial oxidation of methane, indicating their suit-ability for use in a ceramic tubular membrane.The reactor section and heating system used inthis study is shown in Figure 2.

Reactor design for thecatalytic dispersed porousmembrane technologyA cross-sectional view of the ceramic membraneis shown in Figures 3 and 4.

The pores are represented by cylindricalshapes, for simplified analysis. The thickness ofthe active porous layer and porous support layersare designated by t1, t2, t3, t4 and t5, respectively,while r1, r3, r4 and r5 are the pore radii of theirrespective layers.

Active support layers are formed of rhodi-um-impregnated α-alumina and γ-alumina.The materials selected must have similar thermal coefficients of expansion as the adja-cent layers.

Porous support layersIf there is difference in thermal expansion coeffi-cients of the active porous layers and porous sup-port layers, there is an advantage to selectingmaterials for the intermediate porous supportlayers with expansion coefficients that graduallychange from values near those for the activeporous layer to values near those for the outerporous support layer.

One way of achieving this is to prepare theintermediate layers from a mixture of thematerial used to form the active porous layerin decreasing amounts in successive poroussupport layers. For instance, the porous sup-port layer could contain 75% by weight of the material used in forming the active porouslayer.

Identical materialsThe above discussion does not exclude the useof identical materials in the active porous layerand porous support layer. Such a materialselection will eliminate chemical compatibility

Figure 4. Cross-sectional view of the ceramic membrane — reaction.

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8Membrane Technology June 2004

and differential thermal expansion problemsbut typically entails sacrifices in strength andmaterial cost.

The number of porous support layers willdepend on the porous radius of the adjacentactive porous layer. They will vary from a single layer, for an active porous layer poreradius selected from the upper end of the specified range, to as many as four, for poreradii that are selected from the lower end ofthe specified range.

Syngas generation process designFigures 3 and 4 illustrate a composite membranefor syngas generation.

An active porous layer is located on bothsides facing the oxygen and methane-contain-ing gas. Adjacent to this is a second activeporous 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 sideenhances the reaction between permeated oxy-gen and fuel species. Adjacent to the final poroussupport layer is a layer containing the porousmatrix of a reforming catalyst.

A process gas stream, comprising a fuel suchas hydrocarbons and carbon monoxide, steamand recycled gas (H2, CO, and CO2), flowsnext to or through the catalyst-impregnatedlayer. Because in this application fuel specieshave to diffuse to the bore side of the activelayer and the adjacent porous layer, the gaseousenvironment at (and near) the bore of theactive layer is less reducing than in the outerporous layers. As a result, a complete or partialoxidation reaction will take place, with somereforming occurring as the gas moves awayfrom the active layers.

It is advantageous to coat the pores of the finalporous support layer with a reforming catalyst

such as rhodium in order to induce someendothermic reforming, as combustion productsflow through the porous support layer. This willassist in removing the heat of the exothermicoxidation reaction from the surface of the activeporous layer.

The gradient in the oxygen activity in the porous layer will prevent damage to theactive layers (from exposure to very low oxygenpartial pressures), thus permitting a greaterdegree of freedom in the selection of materialsfor these layers.

Results and discussionPartial oxidation is an interesting and promis-ing way of producing synthesis gas from natural gas and oxygen or air, to give an H2/COratio that is suitable for Fischer-Tropsch andmethanol synthesis.

The partial oxidation method is exothermicand therefore reduces energy consumption. Italso has another advantage of fast start-up, com-pared with steam reforming, with a largeendothermic reaction.

SafetyThe major problem of safety associated with par-tial oxidation arises because methane and air (oroxygen) should be fed into the reactor at thesame time, and this means that there is the dan-ger of an explosion.

The membrane system being used in thisstudy overcomes this problem since the oxygenand methane are fed separately, and their respec-tive flow-rates can be adjusted independently ofeach other.

BenefitsTo illustrate the benefits of using the mem-brane reactor in syngas production, Figure 5shows the effect of the reaction temperature onthe conversion of methane over an iridium-loaded catalyst, on various supports. This wascarried out with a fixed-bed flow-type quartzreactor (350–10 mm) at atmospheric pressure,using 60 mg of catalyst, 25 ml/min of O2, anda temperature range of 400–600°C.

At 600°C the performances of TiO2 andAl2O3 are roughly identical.[9] In the sameFigure, experimental data are shown for ourmembrane system at about 630°C. The conversion values obtained using the fixed-bed flow reactor are significantly lower thanthose obtained in the membrane reactor used in our study (which had a support madeof Al2O3 with a TiO2 wash-coat) because ofequilibrium limitation.

This has been overcome in the membranereactor system through controlled feeding ofreactants and high dispersion of the catalyticspecies within the porous framework of themembrane. This achieves the expected 100%conversion of oxygen and a methane conversionof 41% as shown in Figure 5.

0 673.15 773.15 873.15

10

20

30

40

Temperature K

This work 900.15 Kmembrane reactor

Al2O3

SiO2

TiO2

ZrO2

Y2O3

Figure 5. Effect of reaction temperature on the conversion of methane for the fixed-bed and membrane reactor.

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9Membrane Technology June 2004

Reaction pathwayIt is well recognized that the partial oxidation ofmethane may occur via two distinct mecha-nisms – i.e. by direct partial oxidation or totaloxidation, followed by reforming reactions.[10]

In order to elucidate the mechanism govern-ing the catalytic membrane reactor used in thisstudy, the effect of the temperature on themethane conversion and product yields wasstudied. The results of the analysis are presentedin Figures 6 and 7, respectively.

Figure 6 shows that all the oxygen is con-sumed. This occurs before significant amountsof hydrogen and carbon monoxide are formed.Another important feature is that the conversionof methane, the yield of water and the yield ofhydrogen all pass through a maximum at 750°C.This behavior suggests that below 750°C, water,carbon monoxide and hydrogen are primaryproducts, while carbon dioxide is a parallel side-reaction as follows:

Kinetic modeling has shown that the overallreaction can be described well with the contribu-tion of parallel oxidation and full oxidation,according this scheme.

Above 750°C, the total oxidation reaction r2 isexpected to dominate, with a significant increasein water and carbon dioxide. However, examina-tion of Figure 3 reveals that above 750°C the car-bon dioxide yield shows only a modest increase,while the yields for water and hydrogen fallabove this temperature. This suggests thathydrogen, carbon dioxide and water are beingconsumed according to the following scheme:

This scheme helps to explain the fall in the waterand hydrogen yields, the modest CO2 yieldincrease, and the fall in methane conversionabove 750°C.

Hydrogen/carbon monoxide ratioAn important aspect in the conversion of syn-thesis gas to liquids is the hydrogen/carbonmonoxide (H2/CO) ratio. A ratio of 2:1 is opti-mum for this conversion. Figure 6 shows a plotof H2/CO over the temperature range studied.The optimum for gas-to-liquids conversion isobtained at 750°C. Above this temperature, aratio below 2.0 is attained, while below 750°Ca value above 2.0 is obtained.

Hydrogen production from natural gasThe hydrogen energy economy is expected to overcome global warming, as well as provide

solutions to the various energy supply and utilization problems.

However, the amount of hydrogen that canbe produced by using renewable natural energysources such as solar, wind and hydroelectricpower is currently not sufficient to satisfydemand. In such a situation, the use of naturalgas and/or the production of hydrogen fromnatural gas is seen to be a viable alternative andthe most realistic solution, at least in the firsthalf of this new century.[11–13]

ProgressAn example of progress in the widespread use of

natural gas involves the development of smallcogeneration (combined heat and power, CHP)systems using micro gas turbines. In addition,fuel cells are expected to offer a highly efficientway of generating power.

Fuel cells could be used in people’s homes, in addition to being installed in electric vehi-cles. A stationary fuel cell used in the homewill be able to provide hot water and electrici-ty simultaneously.

Abundant hydrogenTo commercialize stationary fuel cells, it isvital to establish the technology needed to

2.5

2

1.5

0.5

0600 650

1

700 750 800 850

Temperature ( C)

(CH4/O2) feed = 150/15

Syn

gas

ratio

(H

2/C

O)

Figure 7. The effect of temperature on the syngas ratio.

0.8

1.0

1.2

0.6

0.4

0.2

0650 700 750 800 850600

Temperature ( C)

y = 1

1.2

1

0.8

0.6

0.4

0.2

0

y = -2E-07x3 + 0.0005x2 - 0.3201x + 74.155R2 = 0.9407

y = -2E-07x3 + 0.0004x2 - 0.2794x + 65.037R2 = 0.9263

y = 0.0238e0.00009x R2 = 0.0616

y = 0.0476e0.0016x R2 = 0.916

y = -3E-08x3 + 6E-05x2 -0.0407x + 9.1173 R2 = 0.9571

Con

vers

ion

Conversion CH4Conversion O2Yield H2OYield COYield CO2Yield H2Expon. (conversion O2)Poly. (conversion CH4)

Poly. (yield H2O)Expon. (yield CO2)Expon. (yield CO)Poly. (yield H2)

Yie

ld

Figure 6. Effect of temperature on methane conversion – feed flow-rate ratio (CH4/O2) = 150/50.

generate abundant supplies of pure hydrogenat low cost, in addition to the development of the fuel cells themselves. The catalytic dispersed ceramic membrane technology devel-oped by the Centre for Process Integration &Membrane Technology at the Robert GordonUniversity can be reconfigured to generate thishydrogen by optimizing the feed flow-rate andtemperature.

Recent developments inGTL technologyOn 8 December 2003, the US-based oil & gascompany ConocoPhillips signed an agreementto build a $5 billion plant to convert gas to liq-uids in Qatar in the Persian Gulf, which isworking to establish itself as the GTL capital ofthe world.

This agreement, which initiates technical andcommercial engineering and design studies forthe plant, was the third GTL deal signed last yearby state-owned Qatar Petroleum, following dealswith Royal Dutch/Shell and Sasol of SouthAfrica.

Gas-to-liquids projectsQatar has succeeded in becoming the target loca-tion for many gas-to-liquids projects. Such pro-jects will also assist Qatar to monetize gasresources and become the GTL capital of theworld.

The plant, as described above, will initiallyinclude two trains producing around 33 000barrels per day (bpd), comprising nearly 24 000barrels of GTL diesel, 8000 barrels of GTLnaphtha and 1000 barrels of LPG.

The project is at its most advanced phase,with the engineering, procurement and construction contractor, and finance beingsecured. The project is seen as a strategicinvestment for Qatar because it will createmany jobs – both skilled and non-skilled – forQataris. It will also put Qatar in a unique posi-tion in the fast-developing GTL sector.

Carbon creditsOther projects are being considered, which willenhance the production of environmentallyfriendly GTL fuel – mostly in preference to con-ventional diesel.

During September 2003, Oklahoma-basedSyntroleum Corporation reported that it hadsigned a memorandum of understanding todevelop gas-to-liquids projects in Nigeria, usingthe country’s largely stranded natural gasreserves. In Nigeria an estimated volume of 2 billion ft3 (57 million m3) of gas is flared orvented every day.

Syntroleum says that some 200 000 barrels perday of synthetic fuels could be produced fromthis gas. The project is seen as an excellent candi-date to qualify for carbon credits that can betraded worldwide under the World Bank’s

Prototype Carbon Fund. The design and con-struction 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 industryThe gas-to-liquids projects are creating a newvalue-adding industry that requires specialistskills and expertise. It is predicted that there willbe a significant demand for control room andprocess plant operators. Other occupationalareas involved in the ongoing operations andmaintenance of the new gas-to-liquids plants willinclude:

• Tradespersons and technicians, coveringmetals, electrical, electronic and instrumen-tation.

• Trades assistants and semi-skilled workers.• Engineers (electrical and instrumentation,

process, production, maintenance andmechanical).

• Management, warehouse, purchasing andadministration personnel.

• Chemists and laboratory technicians.

ConclusionsA catalytic membrane reactor has been devel-oped and used to produce synthesis gas undervarious operating conditions with total con-sumption of oxygen.

At lower feed ratios (CH4/O2), the syngasratio is well above 2.0, while at a higher CH4/O2ratio the syngas ratio is 2.0. This means that,depending on the application, the reactor is flex-ible to the extent that it could be applied in theFischer-Tropsch process for converting naturalgas to liquid hydrocarbons. For gas-to-liquids(GTL) conversion, an optimum temperature of750°C has been established at which the hydro-gen/carbon monoxide ratio is 2.0.

In this project, we are focusing on the partialoxidation (POX) route. This technique followsan exothermic pathway, and is expected toreduce the energy consumed in the productionof syngas.

This POX method has other advantages suchas fast startup (compared with steam reforming,which has a large endothermic reaction),enhanced flow rate of feedstock, and the possi-bility of using air rather than pure oxygen.[14]

Most importantly, the process is able to overcome the problems of safety that areencountered in conventional reformers, wherethe air and methane are co-fed into thereformer. In the membrane process, the oxidantand methane are feed separately, thus overcom-ing the flammability constraint.

References1. B. Macdonald: Syngas – not just a load of hotair, ECN Chemscope (1999) 24–25.

2. G. Parkinson: A new era for gas-to-liquidstechnology, 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: Engineeringdevelopment of ceramic membrane reactorsystems for converting natural gas to hydrogenand synthesis gas for liquid transportationfuels, in: 2000 Hydrogen Program Review, Air Products & Chemicals Inc, Pennsylvania,USA (2000).6. R. Bredesen: A technical and economic assess-ment of membrane reactors for hydrogen andsyngas 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 hydro-gen and higher hydrocarbons from methane overRu/Al2O3 catalyst, Chemical Engineering Journal,4045 (2002) 1–9.8. E. Gobina (Ed): Gas-to-liquids process forchemicals and energy production. Report E-101,BCC Inc, Norwalk, Connecticut, USA, October2000.9. K. Nakagawa et al.: Partial oxidation ofmethane to synthesis gas with iridium-loadedtitania 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 utilizationand on-site distributed generation. Report RC-239, BCC Inc, USA, January 2004.13. E. Gobina: Hydrogen as a chemical con-stituent and as an energy source. Report C-219R, BCC Inc, USA, February 2003.14. S. Olsen and E. Gobina: An enhanced cat-alyst-dispersed ceramic membrane for low-cost 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, RobertGordon University, Schoolhill, Aberdeen AB10 1FR, UK.Tel: +44 1224 262348, Fax: +44 1224 262444, Email:e.gobina@rgu.ac.uk, 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 mon-etizing stranded gas’, which was presented at the21st Annual Membrane/Separations TechnologyPlanning Conference in Boston, Massachusetts,USA in December 2003.

FEATURE

10Membrane Technology June 2004