Gas to liquids (GTL)

Converting gas to liquids (GTL) through the Fischer-Tropsch (FT) route to monetize stranded gas has received increasing attention over the past few years. FT technology is a process that rearranges carbon and hydrogen molecules in a manner that produces a liquid, heavier hydrocarbon molecule.

In general, GTL through the FT route refers to technology for the conversion of natural gas to liquid; however, GTL is a generic term applicable to any hydrocarbon feedstock. This page focuses on GTL processes based on natural gas feedstock. The FT GTL process produces petroleum products such as:

• Naphtha
• Kerosene
• Diesel
• Lubricants
• Solvents
• Waxes
• Other specialty products

History

FT chemistry originated during the early 1920s from the pioneering work of Franz Fischer and Hans Tropsch at the Kaiser Wilhelm Inst. for Kohlenfirschung in Germany. They used a precipitated-cobalt catalyst at normal pressure. It was further developed by various German companies with sintered and fused iron catalyst, resulting in the manufacturing in Germany during World War II of 600,000 tonnes per annum of FT products, mainly motor fuels. Further development in the FT GTL process took place in Brownsville, Texas, producing 365,000 tonnes per annum from a fluidized-bed process during 1948–1953. Subsequently, Sasol in South Africa developed various FT plants with fixed-bed; circulating fluidized-bed; and recently, slurry-type reactor with iron, as well as cobalt, catalysts. The Sasol GTL process for the production of middle distillates is known as the slurry phase distillate process. Later, between 1973 and 1990, Shell developed a cobalt-based process in their Amsterdam research facility. Shell’s GTL technology is based on the Shell middle distillate synthesis (SMDS) process. ExxonMobil’s research, which ultimately led to today’s AGC-21™ process, started in the early 1980s. Besides Sasol, Shell, and ExxonMobil, several major oil companies, as well as smaller companies, are developing their own GTL technology.

Gas-to-liquid process

Fig. 1 shows the three major steps in a GTL process. These steps are described here.

• 

  Fig. 1—Simplified block-flow diagram for FT gas-to-liquid process.

Syngas generation

The first step in a GTL process is to convert the natural gas feed into synthesis gas or syngas. Before being fed to the syngas generation unit, the natural gas is typically processed to remove impurities such as:

• Sulfides
• Mercaptans
• Mercury
• Any impurities that will poison the various catalysts that are used in the GTL conversion steps

The cleaned feed gas is then fed to a syngas generation unit. In this step, the bond between the carbon and hydrogen is broken, and two separate molecules (CO and H₂) are formed. The ratio of H₂ to CO in the syngas is a critical factor in the FT process.

There are several ways to produce synthesis gas from natural gas and air or oxygen. These include steam reforming of feedstock in the presence of a catalyst,

and the partial oxidation process in which air or oxygen is burned together with natural gas at high temperatures and pressure. No catalyst is used.

For GTL plants that require large quantities of oxygen, a cryogenic air separation plant is currently the most economical option. Natural gas and oxygen are preheated and compressed (if necessary) to required conditions before being sent to the synthesis gas reactor.

Another method is autothermal reforming, which involves partial oxidation, coupled with steam reforming.

The syngas fed to the downstream FT synthesis unit must have a ratio of H₂ to CO of approximately 2. This ratio has favored the development of partial oxidation and autothermal reformer (ATR) processes (by themselves or in combination with other processes) over the steam-reforming process because the latter requires additional processing to achieve the desired H₂:CO product ratio. Even though the technology for syngas generation is considered proven, its application in GTL plants is complex and costly. Significant research is ongoing in this area to reduce cost.

Fischer-Tropsch synthesis

The FT synthesis section involves the conversion of synthesis gas to long-chain, heavy paraffinic liquid. Paraffin is a mixture of high-molecular-weight alkanes (i.e., saturated hydrocarbons with the general formula C_nH_2n+2, where n is an integer). Large quantities of water are produced as a byproduct, which is required to be treated before disposal or reuse. Small quantities of CO₂, olefins, oxygenates, and alcohols are also produced as byproducts. The reaction is highly exothermic, with heat of reaction of approximately −39.4 kcal/gmol of CO. Large quantities of heat are generated in the process that must be removed. This energy is partially recovered by the production of steam.

The product slate from a FT reactor is dependent on the type of catalyst and the operating conditions of the reactor. Generally, an iron-based or cobalt-based catalyst is used for FT synthesis. The choice of the catalyst is to some extent related to the type of feed to the GTL plant. For natural gas feed, a cobalt-based catalyst is more likely to be used.

There are several different reactor types to produce FT products:

• Fixed-bed
• Fluidized-bed
• Slurry-phase reactors

Several publications discuss the pros and cons of the various reactor designs. The operating conditions of the FT reactors typically range from 220 to 250°C and pressure of 20 to 60 bar. The operating conditions vary depending on:

• Desired product mix
• Type of catalyst
• Reactor type

The FT product is totally free of the sulfur, nitrogen, metals, asphaltenes, and aromatics that are normally found in the petroleum products produced from crude oil. Table 1 compares the quality of the products from the FT process with that of conventional refinery-based products.

• 

  Table 1—Quality of FT GTL products

Product upgrading

The hydrocarbon products produced in the FT reactor consist of a mix of:

• Light hydrocarbons
• Olefins
• Liquid hydrocarbons
• Waxy, long-chain paraffinic molecules that cannot be sold directly as products

These products are processed further in the product-upgrading unit to primarily produce naphtha, kerosene, and diesel. There is a variety of specialty products that can be produced from FT products such as:

• Solvents
• Wax
• Lube oils

The market for these products is limited. The product-upgrading step involves processes very similar to processes used in a crude-oil refinery.

Besides the three process steps detailed in this section, the GTL facility includes a large utility plant, offsites, and infrastructure. GTL production can be described as utility intensive; it is both a large producer and consumer of energy. The magnitude of the utilities for a GTL plant is evident from the large amount of power required to operate these plants. A 74,000 bbl per stream day SMDS-based GTL plant requires approximately 360 MW of power.

The GTL plant is not based on just one technology but brings together several technologies on a large scale. These technologies include:

• Gas processing
• Industrial gas production
• Syngas generation
• Catalytic reactors
• Refining
• Power generation
• Effluent treatment

Screening criteria

The size of GTL plants can vary from small (5 to 15,000 B/D) to large (> 50,000 B/D). GTL plants produce petroleum products, which are sold in a commodity market. The size of the market is large, on the order of 1,240 million tonnes per annum. A world-scale GTL plant with a capacity of approximately 50,000 B/D (1.95 million tonnes per annum) contributes a very small fraction of the total market. GTL technologies available from different licensors differ in process configuration, thermal efficiencies, and capital cost; hence, the amount of gas required to produce a specific amount of liquid varies. The gas consumption for a GTL plant ranges from 8,500 to 12,000 scf/bbl. The range of 8,500 to 10,000 scf/bbl is typical of oxygen-based GTL processes.

Key considerations

Economics

The key parameters that determine the economic viability of a GTL plant are:

• Gas price
• Capital cost
• Operating cost

Other parameters that play a key role in the economics of a GTL plant are:

• Product premiums
• Tax incentives
• Shipping cost
• Crude prices
• Environmental aspects

Gas price

With only two commercial GTL plants built in the past 10 years, there is little information available on the capital cost of these facilities. However, it is widely believed that technology developments in syngas generation, FT reactor, and catalyst technology have resulted in significant reduction in capital costs of GTL plants in recent years.

Capital costs

Capital costs are also dependent on factors such as location of the plant, infrastructure requirements, plant capacity, technology selected, quality of gas, and site development. GTL plants benefit significantly from economies of scale, which is driving most technology suppliers toward building larger plants. All major technology suppliers have announced GTL plants in the capacity range from 50,000 to 100,000 B/D. The capital cost of the GTL plants quoted by various technology suppliers for a fuels-based plant range from U.S. $20,000 to $35,000 per bbl per stream day (US Gulf Coast location), depending on the plant capacity and technology. The production of specialty products in a GTL plant, while improving revenue, will increase the capital cost of the plant.

Operating costs

Operating costs vary depending on several factors such as location, technology (catalyst), and product slate. Typically, the operating cost of a GTL plant ranges from U.S. $3 to $5/bbl excluding the cost of feed gas.

Commercialization of gas-to-liquid technology

Although the FT process was developed in the 1920s, the commercialization of this technology is still evolving, with only three companies currently operating commercial plants. There is currently a great deal of interest in GTL, and a number of companies believe that this is a technology whose time has come.

Proprietary nature of technology and licensing

Technology providers consider GTL technology highly proprietary. There are significant barriers to new entrants developing GTL processes because of the high cost of technology development and the extensive patent protection of existing processes. Currently, the technology for GTL is not widely licensed. Most technology suppliers leverage their technology to gain access to gas assets.

Crude oil pricing

The products from a GTL facility are in direct competition with products produced by crude-oil refining; therefore, the growth of GTL is dependent on the price of crude oil. One way to review GTL products is to compare the cost of producing these GTL products with the cost of products from a crude-oil refinery. Hence, a minimum crude-oil price level will be required to support future GTL projects. GTL technology providers claim that a crude-oil price of U.S. $15 to $20/bbl results in a profitable GTL project. The impact of crude-oil price on GTL product prices is one of the major obstacles to widespread commercialization of GTL.

GTL is an emerging technology. Although there are few plants in construction phase, there is considerable activity around the world by major oil companies. Reduction in capital costs and reasonable projections of the crude-oil price will be instrumental in the success of GTL as a gas monetization option.

Noteworthy papers in OnePetro

Use this section to list papers in OnePetro that a reader who wants to learn more should definitely read

External links

Use this section to provide links to relevant material on websites other than PetroWiki and OnePetro

See also

Gas utilization options

Gas to methanol

Gas to power

Gas pipelines

Gas as fertilizer feedstock

Monetizing stranded gas

Stranded gas

Transporting stranded gas as hydrates

PEH:Monetizing_Stranded_Gas