Thermoeconomic and Environmental Advantages of Ammonia Precooled Mixed Refrigerant in Liquefied Natural Gas Cycle

The Liquefied Natural Gas (LNG) processing is energy-intensive, such that any improvement of the process efficiency will result in reduction of energy consumption and greenhouse gas emissions. The specific energy consumption is calculated for an LNG plant with a natural gas mass flow rate of 100 kg/s (3.15 Million Tonnes Per Annum) utilizing the ammonia-mixed refrigerant liquefaction process (NH3MR). The proposed process is examined to determine optimal plant operations for different ambient temperatures. The results indicate that, the specific energy consumption of the NH3MR process is lower than the propane-mixed refrigerant (C3MR) process by 8.1% at nominal ambient temperature of 25°C, and on average by 7.8% at ten different ambient temperatures ranges from 0°C to 45°C. The lower SEC (specific energy consumption) value of NH3MR process has positive environmental impact by lowering the carbon emission on average by 7.8%. In summary, lower volumetric flow rate, lower specific energy consumption and subsequently lower carbon emission of NH3MR process makes ammonia a viable alternative to replace propane in LNG pre-cooling cycle. Flash intercooling (FIC) is evaluated in the precooling cycle to enhance the liquefaction process efficiency. Utilizing FIC reduces the precooling refrigerant temperature at the compressors inlet from a superheated to statured vapor state. Both propane and ammonia are examined as precooling refrigerants. The liquefaction process was modelled to minimize the SEC and determine optimal operating pressure for compressors. The results obtained using FIC show that at the nominal ambient temperature of 25°C, the SEC of C3MR and NH3MR is reduced by 12.6% and 4.0%, respectively. The findings from the exergy analysis reveal that compressors and heat exchangers are two major contributors to exergy loss which are responsible for 76% of the cycle exergy loss. Employing FIC shows that the largest impact in exergy loss saving occurs for the C3MR at 25°C by reducing the exergy loss by 23.5%, in which combination of compressors and heat exchangers exergy loss reduces by 121 kJ/kg.LNG.