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 the process equipment used to extract refrigerant components from the feed gas as required for mixed and multi-refrigerant cycles. 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 warmer circuit indicated in red and a low temperature circuit shown in blue. Chilled gases from expanders EX1 and EX2 are routed to the cold box for cooling duty and then returned to the expanders by the recycle compressor CP1. Flash gas is recaptured to the system by a small compressor CP2 and routed through the cold box to the suction of the recycle compressor for return to the expanders. The expanders are configured as companders and operate in series with the recycle gas compressor (Fig 2), producing 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 EX2 – 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. As an option, a liquid turbine may also be installed in the LNG run down line further to improve efficiency.
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; the highest methane cycle efficiency commercially available, equivalent or better than single mixed refrigerant processes, and 25-30% lower than dual nitrogen expander processes. ZR-LNG achieves this whilst 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 (IPL)
All liquefaction technologies consume more power at lower feed gas pressures. ZR-LNG can boost low pressure feed gases by routing feed gas to a suction point in the recycle gas compression train. IPL thus provides a higher liquefaction pressure decoupled from the feed gas pressure, which enhances liquefaction efficiency and LNG production without need for a separate feed gas compression plant (Fig 3).
Integrated Heavies Removal (IHR)
LNG liquefaction systems require removal of C5+ to <0.1 mol% and benzene to <1 mol ppm to avoid freezing and plugging of the main cryogenic exchanger and ancilliary equipment. This is typically carried out in a scrub column upstream of liquefaction, operating at liquefaction pressure and heat integrated with the system. For feed-gases near their critical pressure, achieving effective vapour/liquid separation and satisfactory removal of C5+ and aromatics in a scrub column can be problematic and may require operation of the scrub column at a reduced pressure which reduces liquefaction efficiency. Leaner feed-gases with reduced levels of C2 and C3+ can also create instability due to lack of liquid reflux. Where there are concerns regarding scrub column performance a typical solution is to install a separate upstream NGL removal unit, in which the feed-gas is expanded to a sub-critical pressure, the liquids condensed, and the depleted gas re-compressed to recover liquefaction efficiency. This adds cost and complexity. Many applications of the ZR-LNG process allow the heavy components to be removed by passing the feed-gas plus recycle gas through the warmer gas expander EX1 and separating the condensed liquids from the expander outlet at sub-critical pressure. This IHR 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. Specifically, the expander and re-compression facilities (CP1) already exist in the basic ZR-LNG configuration. In addition to cost, the weight and footprint reduction is particularly relevant to FLNG applications.
Comparative Features and Advantages of ZR-LNG
Elimination of the entire refrigerant infrastructure reduces the equipment count and complexity of ZR-LNG schemes but such are the merits of its configuration that the system achieves a high energy efficiency.
An indication of the relative simplicity and efficiency of ZR-LNG compared to mixed and multi refrigerant cycles is provided in the table below.
|Process||Total Equipment Count Single train IBL + OBL||Power Demand kWh/tonne LNG|
|C3MR/DMR – Cascade||64-68||280-310|
In terms of power demand ZR-LNG represents a reduction of 15-25% relative to the triple and dual nitrogen expander processes.
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