Oct-2013

StarLNG standard small-scale 
LNG plants

Driven by growing domestic natural gas reserves, favourable gas prices and stricter emission regulations in North America, LNG is beginning to replace traditional oil-based fuels in marine or heavy vehicle engines, power generation and process industries. This emerging “Merchant LNG” market calls for decentralised small-to-mid scale LNG plants and is now growing beyond a niche market.

Linde Engineering

Article Summary

Linde’s Air Separation Units (ASUs) have been in use for more than a century, and over time, have been optimised and standardised. To take advantage of LNG’s current position as a clean fuel on the verge of a breakthrough in the North American market, Linde has applied this standardisation idea to Small-Scale LNG:
StarLNGTM provides a small-to-mid scale LNG plant suitable for a wide set of capacity and process variations. A generic LNG plant design for a 200 tpd (net liquefaction capacity) base case is surrounded by many alternatives, with pre-engineered documents including a 3D CAD-model for a fully modularised plant. A “Process Toolbox” approach was designed to cover about 90% of real-life boundary conditions, with the following major benefits:
• Safety levels matching World Scale LNG plants
• Fast-track EPC time schedule
• Reduction of Capex
• Highly efficient processes that are easy to operate
• Modularised units for pre-treatment, processing and main pipe racks
• Configuration flexibility with many options

The StarLNG standardised plant concept covers a capacity range from 0.03 to 0.5 MTPA and uses a robust and easy to operate single mixed refrigeration process, based on proven technology.

Industrial standardisation is a necessary response to clients requiring shorter delivery times for a previously custom-designed product. Unlike ASU’s, there is considerable variation in the feed compositions and conditions for an LNG plant. To cover the anticipated range while still avoiding overdesign (and high cost) based on a combination of worst-case conditions, the process was considered as a series of modules – a ‘Toolbox’ approach.

• With an ‘ideal’ feed composition (a close match to the LNG product), a straight-through liquefaction can be applied as the only processing step. This is the simplest possibility and so was chosen as base case (Liquefaction Unit).

• If a feed gas contains heavier hydrocarbons (HHCs) like Hexane or heavier, freezing will occur during liquefaction if the concentration has not been lowered sufficiently. This is handled by adding a knock-out drum to the base case, operated at a controlled temperature below the dew point of HHCs (HHC Separator).

• Some of the less heavy hydrocarbons like ethane or propane may be undesirable to have accumulating at higher concentrations in the LNG product e.g. if it is to be used as vehicle fuel. Those components can be removed via the addition of a Stripping Column to the base case liquefaction process.

• High nitrogen concentration in the feed gas may require removal to avoid increased process energy consumption, reduced LNG product heating value or to address storage safety concerns (stratification and roll-over in the tank). A nitrogen rejection column can then be added to the base case liquefaction process (N2-removal Unit).

Combining these modules as needed in the liquefaction cold box makes it possible to design a StarLNG plant for most pipeline gas compositions worldwide. The plant configuration comprises all systems typically needed for a small-scale LNG business including LNG storage and off-loading, utilities and infrastructure.

The core of an LNG plant is its refrigeration system. LIMUM is Linde’s high efficiency single mixed refrigerant process. This closed cycle refrigeration system provides cryogenic temperatures via two staged compression followed by Joule-Thomson expansion and liquid evaporation of the mixed refrigerant. Energy efficiency is similar to competing two-staged MR-processes, about 5 to 10% higher than single-staged processes (such as PRICO) and 20 to 30% higher than nitrogen expander plants. Linde also builds nitrogen expander plants but for the given capacity range a mixed refrigerant compressor is simpler and more robust than large compander machines or split compressors and expanders respectively. The LIMUM process also has a lower equipment count than any competing process of the same.

A road transportable, generic module concept was developed for StarLNG to serve markets with high on-site construction costs. The StarLNG 3D-CAD model targets minimum hook-up work and moderate crane lifting capacities on site while facilitating road transport, including escorts or special permits if necessary. Process modules are typically manufactured in-house, and so can be split into smaller units for transportation is needed.

In view of LNG being a flammable hydrocarbon with a very high energy density, plant safety is the number one priority under all circumstances. Significant effort has thus been put into safety of design and safety reviews of the process and the plant layout, all of which is reflected in standard documents such as:
• Detailed HAZOP Report
• Detailed HAZAN Report
• Hazardous Area Plan
• Fire Fighting Plot Plan
• Fire & Gas Detection Plot Plan
• Quantitative Risk Assessment (QRA) Report

Understanding all safety requirements and constraints on this basis at the outset of a new LNG project helps to satisfy these by attentive plant design rather than by adding expensive protection systems during later project stages. During execution of a previous LNG plant it was found how modifications of the plant layout would have resulted in savings for passive fire protection – unfortunately, the differences that could have been cost-neutral were only considered at an execution stage where such layout modifications were no longer feasible.

Risks from new LNG plants to surrounding populations are a concern faced in any LNG project development. This is generally a more serious issue for small-scale LNG plants, as they are typically located close to their LNG end users, in more populated areas than world-scale LNG plants or terminals. Just following the design guidelines provided by LNG standards such as. 49 CFR 193 or NFPA 59A does not tackle such concerns, as they do only define technical minimum requirements but do not provide quantification of the risks to surrounding population. Quantitative risk assessment (QRA) for StarLNG plants has proven a valuable basis for activities such as comparing the risks of different LNG plant design alternatives, assessing the adequacy of separation distances, benchmarking the risks of an LNG facility against the risk profiles accepted in other industries, etc.