Conceptual design of an autonomous once-through gas-to-liquid process — Comparison between fixed bed and microchannel reactors Mohammad Ostadi, Kristin Dalane, Erling Rytter, Magne Hillestad ⁎ Department of Chemical Engineering, Norwegian University of Science and Technology (NTNU), Sem Sælandsvei 4, N-7491 Trondheim, Norway abstract article info Article history: Received 8 June 2015 Received in revised form 2 July 2015 Accepted 17 July 2015 Available online 13 August 2015 Keywords: Gas-to-liquid Fischer–Tropsch Autothermal reformer Heat exchange reformer Fixed bed reactor Microchannel reactor Distributed hydrogen feed Autonomous FPSO Remote gas A novel process concept is proposed for converting natural gas to liquid Fischer–Tropsch products. An autothermal reformer with enriched air as oxidant is applied for synthesis gas (syngas) production, and because of the inert nitrogen a once-through Fischer–Tropsch synthesis is the preferred option. In order to maximize the syngas conversion and the production of heavy hydrocarbons, a staged reactor path with distributed hydrogen feed and product withdraw is proposed. The hydrogen is produced by steam methane reforming in a heat exchange reformer (gas heated reformer), heat integrated with the hot effluent stream from the autothermal reformer. Tail gas from the last Fischer–Tropsch stage is sent to a gas turbine for power production. The hot exhaust gas from the gas turbine is used for natural gas preheating. The process is autonomous in the sense that it is self sufficient with power and water, and therefore well suited for production in remote locations such as a floating production unit. The process concept is simple and inexpensive since cryogenic air separation and fired heaters are not required. For the Fisher–Tropsch synthesis, both the conventional shell and tube fixed bed reactors and microchannel reactors are considered and compared. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Due to the depletion of easily accessible oil, and steadily increasing energy consumption worldwide, focus is turned on untapped resources that are unused for technical or economic reasons, such as associated and stranded gas reserves. One of the biggest challenges in exploiting remote gas reserves is transportation of the gas. Converting natural gas to liquid fuels, gas-to-liquids, is one possibility to bring remote natural gas reserves to the market. If a floating production vessel is to be used for gas-to-liquid process- ing, there are several requirements that are not necessarily equally restrictive for an onshore plant. There are restriction with respect to space and the total weight of equipment. The floating production vessel needs to be autonomous in the sense that all production utilities, such as water and power, need to be available onboard the unit. Due to safety issues a cryogenic air separation unit may be problematic onboard a floating production vessel because of the possibility of the presence of pure oxygen in the vicinity of hydrocarbons. Also high columns with liquid inventory on board a rolling vessel may create problems. There have been some investigations looking at the feasibility of installing a gas-to-liquid (GTL) process on a floating production storage and offloading (FPSO) vessel. Daewoo Shipbuilding & Marine Engineering together with RES Group Incorporated, has completed conceptual design package of GTL process for FPSO application producing 20,000 bbl/day of a Fischer–Tropsch liquid syncrude product. They considered steam-CO 2 combined reforming for syngas production and slurry bubble column as Fischer–Tropsch (FT) synthesis [14]. Velocys, which is one of the pioneers of commercializing microchannel technology, proposes the use of microchannel technology on FPSO [15,31]. Velocys together with Toyo Engineering and Mitsui Ocean Development & Engineering Co is working on commercializing Micro-GTL technology which is applicable for small scale gas reserves. CompactGTL is another leading company in modular small scale GTL. Together with Petrobras, they built a fully integrated small scale GTL facility using associated gas. SBM Offshore together with CompactGTL is cooperating on offshore projects to increase productivity and to reduce flaring. The concept utilizes CompactGTL technology for conversion of associated gas into syncrude. Loenhout et al. [16] proposed to use air instead of pure oxygen in the reforming step. Three-phase slurry bubble column reactors were used for the two stages of the FT reaction. Use of air in the reformer resulted in very large equipment downstream the reformer. Masanobu et al. [20] proposed to use oxygen blown autothermal reformer (ATR), which requires an air separation unit onboard the ship. Syntroleum Corporation has developed an offshore gas-to-liquid conversion process that uses air in a reforming process step to produce syngas [11]. The feasibility assessment of utilizing associated gas and converting it into Fischer–Tropsch liquids on the Fuel Processing Technology 139 (2015) 186–195 ⁎ Corresponding author. E-mail address: magne.hillestad@chemeng.ntnu.no (M. Hillestad). http://dx.doi.org/10.1016/j.fuproc.2015.07.022 0378-3820/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc