Gas-To-Liquids Update
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 93580, "GTL Update," by G. Apanel, SRI Consulting, prepared for the 2005 14th SPE Middle East Oil & Gas Show and Conference, Bahrain, 12–15 March.
The world's proved natural-gas reserves, currently exceeding 5,000 Tcf, have grown at a faster rate than proved oil reserves. Approximately 3,000 Tcf of the gas reserves is considered stranded (i.e., accessible by drilling but too far from potential markets for economical transportation to those markets). The chemical conversion of methane to liquid fuels and other higher-value products or derivatives, which may be more cost-effective to transport long distances from remote gas sources, has attracted renewed interest. Even though Fischer-Tropsch (FT) synthesis (FTS) is a technically proven gas-to-liquids (GTL) technology, the conversion of natural gas to liquid fuels such as diesel and gasoline has only recently been perceived as a potentially viable commercial proposition.

Introduction
Generally, GTL technology entails chemical conversion of natural gas into readily transportable liquids, such as methanol or conventional petroleum-refinery-type distillate fuels. Recently, the term GTL has been loosely applied to physical-conversion processes, such as that for liquefied natural gas, and to chemical-conversion processes yielding products that may not be in a liquid state under ambient conditions. This paper focuses on GTL technologies that produce diesel fuel by use of FTS.
FTS generally involves the synthesis of hydrocarbons and oxygenates from synthesis gas (syngas) consisting primarily of a mixture of CO and H2. The types of hydrocarbons produced can include olefins, such as ethylene and propylene, and an extremely wide range of saturated hydrocarbons ranging from methane and ethane to long "straight-chain" paraffinic waxes. The oxygenated materials primarily are alcohols, such as methanol, and ketones, such as acetone. When configured to maximize the production of paraffinic hydrocarbons, the resulting intermediate-product mix is often described as "synthetic crude oil" (syncrude). Such syncrudes can be refined by conventional petroleum-refining techniques into desirable distillate-fuel fractions, such as kerosene, naphtha, and heating oil. The kerosene can be further refined or blended into high-quality diesel or jet-fuel products, while the naphtha can be further refined into gasoline or used as a thermal-cracking feedstock for olefins production.
Worldwide demand for diesel fuel is projected to grow more rapidly than that for gasoline, while maximum allow-able sulfur levels and other specifications for diesel are becoming more stringent. FTS-derived diesel can exceed such specifications. These factors appear to be stimulating more interest in FTS-based GTL-diesel production than in other FTS-based products.

GTL-Diesel Production
Fig. 1 shows a general GTL-processing scheme. Four key process steps are required for FTS-based GTL-diesel pro-duction.Syngas generation with a minimum H2/CO ratio just greater than 2:1.FTS of paraffin-rich hydrocarbon liquids.Mild hydrocracking of the waxy paraffins to a diesel-rich middle-distillate fraction.Fractionation of the distillate into the predominantly diesel product, along with some byproduct kerosene, naphtha, heating oil, and liquified-petroleum-gas (LPG) -type fuels.