Process analytics in gas-to-liquid (GTL) plants – Case Study

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NG, FT and GTL

Natural Gas (NG) is a vital component of the world's supply of energy. It is one of the cleanest, safest, and most useful of all energy sources. NG is colorless, shapeless, and odorless in its pure form. It is combustible, and when burned it emits lower levels of potentially harmful byproducts into the air than other fuels. NG is a mixture of hydrocarbon gases. While it is formed primarily of methane, it can also include ethane, propane, butane, pentane and certain impurities. NG has been widely used to make commodity products such as methanol or ammonia. But in light of environmental and economic climate today it´s conversion to synthetic liquid hydrocarbons has become a most important objective worldwide. It was already in 1923, when Fischer and Tropsch (FT) developed the process of converting coal into „syngas“ and from there into gasoline. But it took many decades from this FT-process origin before the first commercial FT-based plant, using NG instead of coal, was put into operation. Meanwhile, new technologies have been and are being developed to convert NG to liquids in Gas-to-Liquid (GTL) processes. Commercial interest in using these new technologies arise from e.g. increasing consumer demand for cleaner burning fuels or from the opportunity to develop gas reserves remote from existing markets. Consequently, many GTL plants exist today, are under development or in design phase using different FT-process technologies (Sasol, Shell, Exxon, e.a.). Whatever technology is applied, the process steps require always to be monitored and controlled continuously. Process analyzers play an important role for that. Hundreds of process analyzers, most of them process gas chromatographs, are in use in a typical GTL plant.

As premium-grade hydrocarbon feedstocks prices rise, synfuels and novel petrochemical processes are becoming increasingly valuable. Natural gas represents an abundant alternative hydrocarbon source to crude oil. It is distributed throughout the world and represents a cleaner fuel compared to crude oil.

Therefore, the method of converting natural gas to marketable liquid hydrocarbons (GTL) gets increasing interest worldwide. Large plants have been erected and are in the design phase with a tremendous need in process instrumentation including process analyzer systems.

Siemens, a leader in process analytical instrumentation, has proven over decades its capability to plan, engineer, manufacture, implement and service such analyzer systems.

This Case Study provides details about the GTL process and related analyzer tasks.

Process chain

The GTL process chain consists of a number of fundamental processing steps each of which is important to achieve the final goal of producing high quality synthetic liquid hydrocarbons. The Fischer-Tropsch (FT) reaction is considered the heart of the processchain. Its overall efficiency depends strongly on the type of reactor technology used as well as on the catalyst material applied.

The three main process steps and associated utilities include (Fig. 1):

Syngas generation

Syngas (Synthesis gas) is composed of hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2), whereas the ration H2/CO is important in view of the process efficiency using a certain catalyst material. Syngas is produced from natural gas (NG) through a reforming process. Various technologies are used for that, with or without air or oxygen, such as Steam Methane Reforming (SMR), Partial Oxidation (POX), Autothermal Reforming (ATR), e.a.

Syngas conversion

The conversion of syngas to hydrocarbons comprises the process of H2 and CO molecules to form -CH2- alkyl radicals and water in an exothermic reaction. The -CH2- radicals then immediately combine in a preferably iron or cobalt based catalytic reaction to make synthetic olefin and/or paraffin hydrocarbons of various chain-lengths (high boiling point wax and olefinic naphta). This process is called the Fischer- Tropsch (FT) synthesis process.

Synthesis product upgrading

The longer straight-chain paraffins are pure solid waxes at room temperature with a limited market only. Therefore, to obtain a better usable scope of hydrocarbons, the waxy paraffins need to be upgraded to recieve products with shorter chain length and lower boiling points. This is realized through e.g. catalytic hydrocracking of the wax streams and hydrotreating of the naphta.

Utilities and by-product treatment

Utilities are pieces of equipment to provide services such as heat or electricity necessary to fulfill the plants main goal. By-products of the FT-process are the FT-tail gas and the FT water each of which is treated in order to improve plant efficiency.

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