Engineering gas-to-liquids projects – Part 1
An insight into the R&D, optimisation and engineering activities needed to take Fischer-Tropsch pilot plant data through all development stages
Simon C Clarke, Foster Wheeler Energy
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Gas-to-liquids (GTL) projects are unique in their development and execution. The projects that ultimately arise from R&D initiatives are frequently of the billion-dollar variety (if not greater) and are often part of large integrated projects including upstream development and facilities. Other projects are targeted at remote areas, which will be totally greenfield, quite often in inhospitable conditions (either environmentally or politically).
The other unique aspect of GTL technology is the seemingly relentless push by oil majors, governments and technology companies to develop their own technologies from scratch. A recent survey easily found over 25 references to technology development programmes of this type, and this excludes general research being carried out at academic institutions. And yet, many of these technologies have yet to reach even the pilot plant phase at sufficient scale to generate sufficient data to allow for “reasonable” engineering to fully define the scope and extent of the entire GTL facility, sufficient for cost estimating and award of a FEED contract.
To date, only one project has successfully reached the EPC phase, with congruence of both technical definition for award of the EPC contract, and closure of financing to enable the enterprise to proceed.
GTL pilot plants often fall into the three distinct categories of bench scale, work scale and semi-commercial scale (capacities are approximate and are for order of magnitude comparisons).
The bench scale (or slightly larger) often include larger scale laboratory type equipment, capable of continuous production but mainly used for catalyst performance testing. These can be considered as the workhorse of most catalyst development initiatives and are used to produce preliminary perform-ance data for early facility scope definition. The work scale (larger size pilot plant facilities) include specially designed equipment capable of continuous production of up to around 100bpd. These are mainly used to test reactor design parameters, produce sufficient liquid products for further testing or upgrading, and for catalyst life-cycle testing. Semi-commercial scale pilot plants are large stand-alone plants of reduced commercial size capacity, designed for extended larger scale capa-city of around 500 to 2000bpd. They are mainly used for larger scale prove-out of technology and catalyst performance, and to check integration of key technologies.
These plants are sufficiently large to produce economic quantities of product and therefore are often integrated into existing plant operations, such as refineries. If constructed, they allow for reduced risk in scale-up and improves bankability. The downside of semi-commercial scale pilot plants is cost, with such facilities costing up to $100m.
The level of data that can be derived for suitable engineering definition will obviously be of different quality, depending on what type of plant is being used. It is also worth noting that the pilot plant may or may not include the key components of the GTL facility, including: Syngas generation – if a newer technology is being considered this may be part of the overall pilot plant, or if the technology is developed by another party this might be done in isolation. If the process is excluded from the GTL pilot plant, there is additional effort required to match the syngas compositions and impurities (especially trace components) between the facilities.
Fischer-Tropsch – the core component of the pilot plant. However, is the technology the same as being considered for the eventual project? Is a fixed-bed reactor being used to prove-out catalyst performance or is a three-phase liquid slurry reactor being used to prove reactor scale-up issues?
Product finishing and upgrading – if a newer technology is being considered, this may be part of the overall pilot plant, or if the technology is being developed by another party, this might be done in isolation. If the process is excluded from the GTL pilot plant, there is additional effort required to match the interfaces (compositions and impurities, especially trace components) between the facilities.
Byproduct disposal – this is an area often overlooked in pilot plant facilities in general. The focus will be on the main process technologies and major products. With GTL facilities, the produced water from the reaction should not be overlooked, and if the pilot plant does not include facilities specifically designed with the eventual complex in mind, adequate measures are needed to ensure that water characteristics can be analysed. Utilities and offsites – almost without exception, these are specified within pilot plant facilities as sufficient to ensure continued stable operation of the pilot plant facility and bear no resemblance to the configuration being considered for the eventual plant. If the pilot plant is small enough, then existing systems will simply be tapped for requirements.
The main purpose of the pilot plant is to generate data, which is either used to confirm existing predictions of catalyst or reactor performance, expand the known operating range of a particular process, or from the standpoint of development of the eventual project, used for additional engineering design definition (what data is available and how is it used to define the plant from then on).
In some respects, the other aspects of the pilot plant performance are irrelevant to this definition phase of the project, as they effectively represent go/no-go decisions as to the commercial attractiveness of the venture. The pilot plant facility acts as a proving ground for the technology and only once satisfactory, repeatable and predictable performance is achieved will the project move forward. However, data produced by the pilot plant facility must be analysed once this milestone has been achieved and more importantly, how it affects the engineering definition of the proposed eventual project at this stage.
Looking at the GTL facility from an engineering standpoint, the pilot plant really needs to provide the data shown in the panel below as a minimum for the synthesis gas, Fischer-Tropsch and hydrocarbon upgrading sections.
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