The three main families of FT technologies. Published by Axens at PTQ Q2 2008.
GTL technology involves conversion of methane to high molecular weight hydrocarbons from LPG to waxy paraffins. The process itself consists of mainly 3 steps, syngas production through steam reforming or partial oxidation, liquid production through Fischer-Tropsch (F-T) reactions with cobalt or iron-based catalyst followed by product updgrading through mild hydrocracking to convert high molecular weight waxes to LPG, naphta and diesel. The processes call for important utilities like steam and oxygen. Large air separation plant is needed in order to satisfy the oxygen need to convert methane to syngas.
What kind of reactors?
There are three types of commercial FT reactors in the market: a fixed bed reactor, where the catalyst is located inside the tubes, a fluidized bed reactor where the catalyst is maintained in suspension by the syngas and a slurry bubble column of three-phase reactor with synthesis gas, waxes, liquid products and solid catalyst. The operating temperatures range from high-temperature (around 350°C and above) to low-temperature (220–240°C). The type of catalyst and reactor temperature define the final products obtained (after FT upgrading) to middle distillates (diesel, paraffinic naphtha, waxes) and gasoline (olefins, chemicals).
Challenges facing valves in GTL
In Fischer-Tropsch synthesis process valves encounter many demands, such as high temperatures (around 350°C, depending on reactor type), high pressures (ANSI CL 900/1500), high pressure differentials, noise and cavitation. Combining these requirements to slurry and/or waxy services require a lot from valve designs, which should not be prone to clogging to keep the required tightness and control performance over long periods. This calls for careful selection of valve type, seat and materials. There needs to be reasonable amount of valve types to meet the variety of all requirements, which again sets challenges to find manufacturers that can provide products with long lasting performance and take single valve manufacturer responsibility of these ciritical valves.
Metal seated rotary valves have been chosen in recent GTL projects by Qatar Petroleum and Shell in Qatar, Shell Bintulu in Malaysia and Sasol Oryx GTL in Qatar. Featuring metal-seated rotary technology, provide long-lasting performance for the demanding GTL industry. They are less vulnerable to impurities and produce low VOC emissions through the gland packing. Simple construction and small body size give benefits to piping designs. Trim designs, such as Neles Q-trim provide the means to reach high capacity requirements, eliminate cavitation and reduce noise to levels fullfill noise level requirements. Dedication to technology development at ever demanding process requirements will benefit further the GTL industry in to meet the toughest noise level requirements and ensure safe plant operations.