Optimum Expanded Fraction for an Industrial, Collins-Based Nitrogen Liquefaction Cycle

Abstract

Industrial nitrogen liquefaction cycles are based on the Collins topology but integrate variations. Several pressure levels with liquefaction to medium pressure and compressor–expander sets are common. The cycle must be designed aiming to minimise specific power consumption rather than to maximise liquid yield. For these reasons, conclusions of general studies cannot be extrapolated directly. This article calculates the optimal share of total compressed flow to be expanded in an industrial Collins-based cycle for nitrogen liquefaction. Simulations in Unisim Design R451 using Peng Robinson EOS for nitrogen resulted in 88% expanded flow, which is greater than the 75–80% for conventional Collins cycles with helium or other substances. Optimum specific compression work resulted 430.7 kWh/ton of liquid nitrogen. For some operating conditions, the relation between liquid yield and specific power consumption was counterintuitive: larger yield entailed larger consumption. Exergy analysis showed 40.3% exergy efficiency of the optimised process. The exergy destruction distribution and exergy flow across the cycle is provided. Approximately 40% of the 59.7% exergy destruction takes place in the cooling after compression. This exergy could be used for secondary applications such as industrial heating, energy storage or for lower temperature applications as heat conditioning.

Dynamics of Hydrogen Storage through Adsorption: Process Simulation and Energy Analysis

“…The mechanical energy consumed to store hydrogen by compression in a system operating at elevated pressures of 300 and 700 bar was derived using Equation (48). The energy consumption for liquid nitrogen production is based on an industrial Collins-based process, 0.474 KWh•kg −1 [59], which is more than twice the ideal consumption, 0.21 KWh•kg −1 [60]. When comparing the energy consumption with another compound of similar boiling temperature (77 K for N 2 ), such as methane, whose boiling temperature is 111 K, the energy consumption is lower: 0.29 KWh•kg −1 [61]; thus, the data agree with thermodynamic laws.…”