Modular design of smaller-scale GTL plants
Modularisation is opening up opportunities for GTL plants to monetise small-scale natural gas deposits both on- and offshore
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Modularisation is a construction method that involves designing and building refinery and gas processing facilities as series of unitised (or modularised) process and utility systems. The units are designed and fabricated in a controlled shop environment, then shipped to the project site, where they are installed on foundations and connected together. Compared to traditional field construction methods, where all materials are delivered to the project site and then built on location, modularisation is proving to be an effective solution for achieving fast-track project schedules, and for building refining and gas processing facilities in remote â€¨locations. For example, Ventech Engineers International LLC (Ventech), based in Pasadena, Texas, is an engineering, procurement and construction company that specialises in the design and fabrication of modularised facilities.
A project in Siberia illustrates â€¨the capabilities of modularisation â€¨when dealing with remote and challenging environments. Ventech designed, fabricated and shipped a 2000 b/d crude processing plant to the remote Siberian town of Urai in just six months after contract signing. Urai’s temperatures drop as low as -45°C (-50°F), so the modules were designed and fabricated to accommodate these conditions. In particular, the crude process module was completely enclosed and heated. Ventech personnel, assisted by local labour, installed the modules at the project site and started refinery operations four weeks after the module’s delivery.
Ventech has applied modular design and construction most recently in the Kurdistan region of northern Iraq. A large-scale expansion to a refinery in Kurdistan started during 2010, with Ventech providing a 20 000 b/d modularised crude processing plant that was added to the existing 20 000 â€¨b/d refinery. Made up of â€¨26 modules, the new crude unit was designed and fabricated at Ventech’s Pasadena fabrication site. The modules were shipped to the port of Mersin, Turkey, then transported by truck to the project site in Kurdistan, where the refinery was erected in 16 weeks.
The refinery has continued to expand in a modular fashion; once completed in 2013, total capacity will be over 185 000 b/d and it â€¨will remain the sole producer of unleaded gasoline in Iraq. In all, Ventech has provided modularised crude distillation units, naphtha hydrotreaters, catalytic reformers, isomerisation units, demercaptanisation systems, gas plants and supporting utilities to this three-phase project. Modularisation has made the difference in this geographically and politically challenging location, enabling expansion work to be accomplished quickly.
Design concepts for modularisation
In modular plant design, the different process units are contained in separate modules. For ease of shipment to any location, Ventech fabricates its modules to a standard size of 13.5ft wide x 12ft high x 40ft long. Each of the modules and â€¨its associated equipment has a reference code, which helps communicate where the module will be installed on the plot plan. Connections between the modules are designed to be similar in configuration so that construction is relatively straightforward. Ventech estimates that, with modularisation, approximately 70% of a project is already complete even before the modules are shipped from their facility. This greatly decreases field construction time to deliver an operational facility (see Figure 1).
These methods also facilitate easy disassembly and relocation, if necessary, at some point in the future. For example, a remotely located gas processing facility could be easily taken apart and moved to a new natural gas source if an existing supply was depleted in its current location.
Applying modularisation to refinery construction has advantages with regard to productivity, product quality and ensuring the safety of construction personnel. Since the modules are built in a well-lit, climate-controlled environment, work can continue around the clock regardless of weather conditions, for greater productivity and easier quality control. Since module height is restricted, safety is enhanced, as workers build at limited heights within the fabrication facility.
The same advantages of modular construction of refineries are being applied to the construction of distributed GTL plants. The GTL process involves two operations: the conversion of natural gas to a mixture of carbon monoxide (CO) and hydrogen (H2), known as syngas, followed by a Fischer-Tropsch (FT) process to convert the syngas into paraffinic hydrocarbons that can be further refined to produce a wide range of â€¨hydrocarbon-based products, including clean-burning, sulphur-free diesel and jet fuel. Speciality products including food-grade waxes, solvents and lubricants can also be produced from the paraffinic hydrocarbons.
Large, commercial-scale GTL plants, including the Sasol Oryx and the Shell Pearl plants (both located in Qatar), have been built at enormous capital cost. The Oryx plant, designed for production levels of â€¨34 000 b/d, cost around $1.5 billion to build. The Shell Pearl plant, with an ultimate design capacity of 140 000 b/d of GTL products and 120 b/d of natural gas liquids, cost around $18-19 billion. Conventional GTL plant designs rely on economies of scale to drive positive financial returns and are viable only where there are large supplies of low-priced natural gas.
However, another option being developed — smaller-sized and distributed GTL plants — shows promise for deriving value from smaller accumulations of unconventional gas that would otherwise be left underground, such as shale gas, tight gas, coal bed methane and stranded gas (gas fields located too far from existing pipeline infrastructure). A small, modularised GTL plant has the flexibility to be installed close to the trapped resource and then used to process that resource locally. Associated gas (gas produced along with oil) is another area of opportunity for modularised GTL plants. This gas is typically disposed of either by re-injection, at considerable expense, back into the reservoir or by the wasteful and environmentally damaging practice of flaring, which is subject to increasing regulation. Modularised GTL plants enable this otherwise wasted gas to be converted into additional revenue.
In the larger economic picture, a modular GTL capability can be the key factor that enables the construction of upstream projects that would otherwise be cancelled because of poor results derived from economic models. For example, some shale gas discoveries are being hampered by high development costs, which result in marginal economics due to gas prices that are often low. These projects can be enhanced by converting the gas to higher-value clean fuels produced in the GTL process.
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