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Gas-to-liquids: unbridled stranded gas stands ready at the gate.

Technology advances within the energy industry have dramatically improved the discovery, recovery and refining of crude oil. However, despite these innovations, the petroleum industry continues to leave behind a vast quantity of unrecovered energy in the form of remote gas reserves. These 'stranded' reserves are unmarketable as the costs associated with production and movement to markets is prohibitive given the availability of lower cost alternatives. Nevertheless, stranded gas, including gas that is unnecessarily flared, is a significant energy resource that will be actively pursued over time. By some estimates, as much as 25 TCM of gas (i.e. 900 TCF) is beyond economic reach of commercial markets as gas or LNG. Some reserves also provide a trap for underlying oil.

Given the push towards energy self-sufficiency, many countries are now evaluating stranded gas. One of the technologies that has gained interest has been gas-to-liquids (GTL) as it offers an economical means to convert stranded gas into a series of super-clean fuels, chemicals, waxes and lubricants, each of which are ideally positioned against an industry where the drive towards higher performance and environmental safety is apparent.

In the 1920's, Franz Fischer and Hans Tropsch jointly developed a two-stage process to convert coal into liquid hydrocarbons. During World War II, Germany and other countries used this technology to produce transportation fuels, lubricants and other chemical products. After World War II, many countries studied Fischer-Tropsch as a process to address their own energy developments. Although many of these efforts were not commercially successful, their technical achievements laid the foundation for the current resurgence for this technology.

Gas-to-Liquids Processing

A GTL plant consists of three processes. The initial step is the production of hydrogen ([H.sub.2]) and carbon monoxide (CO), referred as synthesis gas or 'syngas' from a suitable feed. The conversion process involves the use of steam, oxygen or in selected cases air, each of which has its supporters and skeptics. Although there is interest in using stranded gas for syngas production, the original technology was developed using coal. Today, both conventional and exotic feeds are considered, including coal, gas, petroleum coke, biomass, wastes and refuses, visbreaker residue or deasphalting pitch.

The second step is the Fischer-Tropsch reaction, converting syngas into normal paraffins. Small quantities of olefins and alcohols are also produced that can be saturated or separated for chemical processing. The products from FT synthesis have a carbon range from methane to heavy paraffins, i.e. [C.sub.50] to [C.sub.200], The lighter products are reconverted to syngas while the heavier products ([C.sub.5.sup.+]) are isolated and upgraded to fuels, specialties and wax. The Anderson-Schulz-Flory equation defines the efficiency of the FT process, designated by an alpha value (a). As alpha increases, the distribution of products shifts towards higher carbon numbers. A comparison of predicted yields for different alpha values is given in Table 1.

The third step is to upgrade the liquid products and waxes into finished fuel products, such as diesel, jet and naphtha through hydroprocessing. For higher carbon numbers, the upgrading includes waxes, base stocks, solvents and white oils. Upgrading FT wax to base stocks is consistent with current process technologies as wax is a strategic feedstock to elevate Viscosity Index (VI) and other properties consistent with increasing performance demands.

A schematic for a GTL plant is provided in Figure 1.

[FIGURE 1 OMITTED]

GTL Commercial Processing

Three companies commercially use Fischer-Tropsch technology to produce 'synthetic' products. Schumann Sassol began in the 1950s when they developed a proprietary process to convert South African coal into diesel fuel. The process is still in production with a capacity of 175,f100 B/D producing diesel and jet fuels, solvents, waxes, and other hydrocarbons. A second South African company, PetroSA, produce 30,000 B/D of FT diesel from natural gas using Schumann Sassol process technology. Finally, Shell operates a 12,500 B/D Fischer-Tropsch plant in Bintulu, Malaysia with their Shell Middle Distillate Synthesis (SMDS) process to produce fuels, solvents, and waxes.

Recent GTL project announcements reflect the interest in this technology. In a recent Gas-to-Liquids News article, 40 global GTL projects were under construction, development or an initial discussion and feasibility study phase. Although many projects may never become commercial, it does reflect the economic viability and technical interest in FT technology. The combined production from all of these projects is ~ 1.6 million barrels a day of liquid products, equivalent to ~ 10 per cent of U.S. refining capacity based on 2002 statistics.

The most active region is the Middle East, particularly Qatar, with four projects equating to 0.5 MB/D of production. Australia, South America, Africa and Asia-Pacific also have developments as shown in Figure 2.

[FIGURE 2 OMITTED]

North America GTL interests are limited to the Alaskan North Slope. Nevertheless, there are several pilot plant demonstration units either wider construction or operation. Many majors are active in GTL research, including ExxonMobil, Shell, BP, ConocoPhillips, SasolChevron and Statoil. GTL technology companies include Rentech, Syntroleum, Synfuels International and Synergy Technologies Corporation, the latter with a small facility in Calgary, AB. Synergy Technologies Corporation recently filed for protection under Chapter 11 of the U.S. Bankruptcy Code, presenting a serious concern to 'technology based' GTL participants as the poor economy and political instability in many regions of the world has failed to provide investors for GTL mega projects.

GTL Products

GTL based fuels possess ideal characteristics for use in diesel engines. The 'sulfur-free' composition lends them as a blend component to meet ultra low sulfur diesel (ULSD) requirements in many world regions, including North America. Their fungibility with current fuels makes them compatible with fuelling systems and delivery infrastructures including tanks, pumps and fuelling stations.

Recent studies [3] have shown improvements in environmental pollutants with increasing diesel cetane index. This is important for GTL fuels whose cetane index of > 70 (NA fuels is 40-50) provides an opportunity for new engine designs where cleaner and higher performing fuels are required. The demonstrated performance of GTL based fuels in current and future model engines also provides an opportunity for backward compatibility, as the new fuels will not make older vehicles obsolete.

Fischer-Tropsch waxes are highly refined and conform to Federal Drug Authority (FDA) standards. Not surprisingly, base stocks produced from FT waxes demonstrate low toxicity and are consistent with White Oil qualities. GTL produced base stocks have excellent volatility and viscometric properties with VI > 140. By comparison, traditional solvent refined base stocks have a VI range of 90-95. When hydro-dewaxed, FT base stocks provide outstanding low temperature fluidity due to the absence of residual n-paraffin residual waxes. They are completely iso-paraffinic as measured by mass spectrometry and have demonstrated outstanding performance in bench engine oxidation, cleanliness and fuel economy testing, consistent with synthetic products.

The properties of base stocks produced from FT wax have been benchmarked against commercial API Group III base stocks with a VI > 120. Results show that GTL base stocks have the best combination of viscosity, volatility, low temperature and compositional features when compared with the best that industry can offer in each kinematic viscosity range.

When compared with polyalphaolefins (such as PAO, API Group IV), hydrodewaxed GTL base stocks match up extremely well when adjusted to the same kinematic viscosity. They have a slight VI, flash point and volatility advantage, while pour point is the only shortfall. This gap can be reduced through additive technology or more severe hydrodewaxing. Regardless of the approach that is used, GTL base stocks can provide features consistent with PAO.

Conclusions

Stranded gas, including gas that is flared, accounts for a significant energy resource that will be pursued over time, given the global push towards energy self-sufficiency. One technology that has gained interest has been gas-to-liquids (GTL) as it offers an economical means to convert stranded gas into super clean fuels, chemicals, waxes and lubricants, each of which are ideally positioned against an industry where there is a drive towards higher performance and environmental safety.

GTL fuels are sulfur free, low in emissions, and isoparaffinic in nature with a cetane index > 70. They are compatible with existing fuel distribution and delivery systems and provide an easy transition from current diesel fuels.

GTL represents the latest step in creating the ideal base stock and match performance with polyalphaolefins. Results suggest that hydrodewaxed GTL base stocks provide the best combination of viscosity, volatility, low temperature and compositional features compared to the best that industry can offer from crude based feedstock's and an excellent match to PAO.
Table 1: Product Distribution from Fischer-Tropsch reaction using
Anderson-Schulz-Flory predictions

Alpha Value 0.70 0.80 0.90 0.95
Predicted Yields, per cent

[C.sub.1]-[C.sub.2] 21.5 10.4 2.9 0.9
NGL ([C.sub.3]-[C.sub.4]) 25.5 15.9 5.4 1.9
Gasoline ([C.sub.5]-[C.sub.10]) 41.7 41.6 22.4 8.4
Diesel ([C.sub.11]-[C.sub.19]) 10.6 25.3 31.1 17.3
Lubes-Low Viscosity
 ([C.sub.20]-[C.sub.25]) 0.6 4.6 14.3 11.9
Lubes-Mild Viscosity
 ([C.sub.26]-[C.sub.35]) 0.1 1.9 14.1 18.9
Lubes-High Viscosity
 ([C.sub.36.sup.+]) 0.0 0.3 9.8 40.7


References

(1.) 'GTLN Project Listing as of January 27, 2003', Gas-to-Liquids News, Vol. 6, No. 2, p. 14, February 2003.

(2.) 'United States Refining Capacity' January 1. 2002, National Petrochemical & Refiners Association, Washington, DC, June 2002.

(3.) X. Montagne, 'GTL and their Application to Fuels', EUROFORUM France Gas-To-Liquids Conference, Paris, France, February 2003.

(4.) H.E. Henderson, 'Fischer Tropsch Gas to Liquids Base Stocks--Performance Beyond Current Synthetics', Hydrocarbon Engineering. August 2002.

(5.) American Petroleum Institute, Industry Services Department, 'Engine Oil and Licensing Certification System', API Publication 1509, 14th ed., New York, December 1998.

H. Ernest Henderson, FCIC, received his PhD in organometallic chemistry from the University of Windsor in 1977, after which he joined the research department of Imperial Oil Ltd. in Sarnia, ON as a research chemist. During his 19+ year association with Imperial Oil and Exxon, Henderson held several key technical and managerial positions in the lubricants business involving their research, marketing, logistics, and marketing technical services departments, and was associated with Exxon's North American and international lubricants business. In 1997, Henderson joined Petro-Canada where he was technically responsible for their API Group III Fluids, while maintaining a managerial role in the analytical and specialty fluids areas. In 2001, Henderson accepted a position with Syntroleum Corporation at their Tulsa, OK technology centre as director of refining technology and product development with their gas-to-liquids fuel and lubricant base stock upgrading programs. He has since left Syntroleum and is now providing consulting services in the areas of lubricants, base stocks, lubricant additives and petroleum waxes. He is also providing consulting assistance in the area of gas-to-liquids technology on a project-by-project basis.

Henderson is a Fellow of the CIC and the Society of Tribologist and Lubrication Engineers, a long-time member of the Society of Automotive Engineers, and an active participant in API and ASTM activities.
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Title Annotation:Articles
Author:Henderson, H. Ernest
Publication:Canadian Chemical News
Date:Sep 1, 2003
Words:1850
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