Exergetic Analysis of a Cryogenic Air Separation Unit

Abstract

This case study analyzes a cryogenic air separation unit (ASU) with a production of V˙O2=58300 [m3Nh] of gaseous oxygen with a concentration greater than 98.5%, operating in Romania on a steel plant platform. The goal of the paper is to provide an extensive model of exergetic analysis that could be used in an optimization procedure when decisional parameters are changed or structural design modifications are implemented. For each key part of the Air Separation Unit, an exergetic product and fuel were defined and, based on their definition, the coefficient of performance of each functional zone was calculated. The information about the magnitude of the exergetic losses offers solutions for their future recovery. The analysis of the exergy destructions suggests when it is worth making a larger investment. The exergetic analysis of the compression area of the ASU points out an exergy destruction and loss of 37% from the total plant’s electrical energy input. The exergy loss with the heat transferred to the cooling system of compressors can be recovered; for the exergy destruction portion, the challenge between investment and operating costs should be considered. The exergy destruction of the air separation columns found the High Pressure Column (HPC) to be more destructive than the Low Pressure Column. The share of the exergy destruction in the total plant’s electrical energy input is 8.3% for the HPC. The local COP of the HPC, calculated depending on the total exergy of the local product and fuel, is 62.66%. The calculus of the air separation column is performed with the ChemSep simulator.

The Use of Organic Rankine Cycles for Recovering the Heat Lost in the Compression Area of a Cryogenic Air Separation Unit

“…An analysis of the evacuated heat recovery potential was performed for the cryogenic air separation unit (ASU) ( Figure 1 ) operating on the platform of the Galati Iron and Steel Works, Romania [ 26 ].…”

“…Highlighting as final products of the installation the currents of oxygen gas, nitrogen gas and liquid argon ( Figure 1 and Table 1 ), and as resource (fuel) the electricity consumed to drive the compressors ( Figure 1 and Table 2 ), the exergetic analysis of the installation showed a global exergetic performance coefficient of 14% (Equation (1)) [ 26 ].…”

“…Table 3 shows the exergy destructions due to the irreversibility of the compression processes in the compression stages, and Table 4 shows the exergy losses with the heat transferred in the intermediate coolers and the final of the compression stages [ 26 ].…”

“…It was observed that 37% of the electricity consumption of the global installation was dissipated in the irreversible processes of the compression zone (Equations (2) and (3)) [ 26 ].…”

“…Of this exergy consumption, more than half (21% of global consumption) represented the exergy loss with the heat transferred to the environment (Equation (4)) [ 26 ]. …”