Award-winning, patented zero-refrigerant LNG production process
“Many countries in Africa have undeveloped gas resources and could use ZR-LNG to their benefit to monetise stranded gas reserves that would not be economically viable using conventional LNG production processes”.
Process Description of ZR-LNG
This content is summarised from Gasconsult. For more detail please see http://www.gasconsult.com/
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 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.