Process Description of ZR-LNG
The patented GASCONSULT ZR-LNG (Zero Refrigerant LNG) process is highly differentiated; unlike competing processes it uses no external refrigerants, using the natural gas feed as the refrigerant medium in an optimised system of expanders. This eliminates refrigerant storage and transfer systems and for mixed refrigerant cycles, the process equipment used to extract refrigerant components from the feed gas. This reduces equipment count, capital cost and footprint. Make-up refrigerant is low cost natural gas as opposed to nitrogen or a mixture of liquid hydrocarbons; reducing operating cost. The absence of liquid hydrocarbon refrigerant also makes for a safer operating environment. A simplified schematic of the process is provided in Fig 1.
Refrigeration is effected in two expander circuits, a high temperature circuit indicated in red and a low temperature circuit shown in blue. Chilled gases from expanders CX1 and CX2 are routed to the cold box for cooling duty and then returned to the expanders by the recycle compressor CP1. Flash gas is also routed through the cold box and recaptured to the system by a small compressor CP2 which feeds the suction of the recycle compressor. The expanders are configured as companders and operate in series with the recycle gas compressor (Fig 2), recovering approximately 35% of the power required to run the system.
ZR-LNG is similar in concept to nitrogen schemes. However it enjoys a fundamental advantage as methane has a higher specific heat than nitrogen. This significantly reduces circulating flows which in turn reduces power consumption and pipe sizes.
A patented feature is that a partial liquefaction takes place in the low temperature expander CX2 – this very efficiently converts latent heat directly into mechanical work and also permits a reduction in heat transfer area and cost of the main heat exchanger HX1. An optional liquid turbine TU1 in the LNG run down line also improves efficiency by providing a significant chilling effect.
These features, together with the optimised distribution of flows, temperatures and pressures in the expander circuits makes for a highly efficient system, around 300kWh/tonne in temperate climates; equivalent or better than single mixed refrigerant processes, and 25-30% lower than dual nitrogen expander processes. ZR-LNG achieves this without feed gas pre-cooling, providing a very simple low equipment count facility. The low power demand also reduces CO2 emissions.
Because of its open methane cycle configuration ZR-LNG also lends itself to three very useful alternate process configurations.
Integrated Pressure Liquefaction for Low Pressure Feed Gases
All liquefaction technologies consume more power at lower feed gas pressures. ZR-LNG can boost low pressure feed gases by routing feed gas after liquids separation in SP1 back to an inter-stage suction point of the recycle gas compressor, instead of to Zones 2 & 3 of the liquefaction section of the cold box (Fig 3). This provides a higher liquefaction pressure decoupled from the feed gas pressure, enhancing liquefaction efficiency without need for a separate feed gas compression plant.
Integrated Heavies Removal
To achieve satisfactory C5+ and aromatics removal may require expansion of the feed gas to a lower pressure, condensing the heavy material and recompressing the depleted gas for liquefaction. This pressure reduction can be required to effect satisfactory vapour/liquid separation with feed gases close to their critical pressure. With the ZR-LNG process heavy components are removed by passing the feed gas plus recycle gas through the high temperature gas expander CX1 (Fig 4) and separating the condensed heavy material from the expander outlet at around 10-15 bar. This solution de-couples the vapour/liquid separation and feed gas pressures and saves a large part of the equipment and cost of a separate expander based NGL removal unit. It also reduces footprint which is particularly relevant to FLNG applications.
Integrated CO2 Removal
Integrated CO2 Removal (ICR) eliminates the need for a CO2 removal unit as part of the feed gas pre-treatment train. LNG liquefaction schemes typically incorporate an amine scrubbing system to remove CO2 to below 50 mol ppm and prevent it solidifying in the main cryogenic heat exchanger. The size, weight and cost of the amine unit are all very significant in the context of the overall liquefaction plant. The feasibility of ICR arises from the ZR-LNG process configuration and is facilitated by the particular temperature and CO2 concentration profiles within the ZR-LNG cryogenic exchanger. This permits CO2 removal within the basic ZR-LNG configuration and elimination of the amine unit. The process is applicable to feed gases containing up to at least 5% CO2. To accommodate higher CO2 feeds the new technology can be combined with a relatively coarse CO2 pre-removal process such as a membrane system.
Comparative Features and Advantages of ZR-LNG
Elimination of an external refrigerant infrastructure reduces the equipment count and complexity of ZR-LNG schemes but such are the merits of its configuration that this is achieved with a quite limited sacrifice of energy efficiency compared to base load plants. In terms of power demand ZR-LNG is equivalent or better than SMR processes and represents a step change improvement over the dual nitrogen expander processes (see chart).
In addition to its low power demand and reduced equipment count a further set of advantages stems from the Zero Refrigerant concept:
- There are no refrigerant logistics issues in remote or offshore locations. Shipments of light and heavy hydrocarbons; and segregated storage to facilitate blending a mixed refrigerant are not required; and absolute security of refrigerant supply is also assured
- There are no propane or other liquid hydrocarbon refrigerants – a major safety plus relative to mixed refrigerant schemes, particularly for FLNG where personnel exit options are limited
- Single phase refrigerant makes the system motion tolerant and well suited to FLNG
- Reduced footprint from the absence of refrigerant infrastructure and simpler C5+ removal makes the system particularly suited to FLNG
- Several operational benefits relative to mixed refrigerant schemes; no refrigerant make-up cost or composition adjustments, shorter start-up time, reduced flaring