Abstract
Air compression, ammonia oxidation, heat recovery, nitrogen-oxide absorption and product-acid bleaching are strongly coupled in medium-pressure nitric acid plants. This study formulates the redistribution of process air in an AK-72M unit as a multicriteria retrofit-selection problem. Five alternatives were compared against seven criteria covering production potential, energy performance, capital demand, implementability, process safety, environmental performance and controllability. A normalized weighted-sum model identified partial replacement of the T-5–K-47 bleaching-air stream by filtered ambient air, with the released M-10A compressor air redirected to the R-12/1,2 contact section, as the leading concept (score 7.49/9). Control-only optimization ranked second (7.18/9). Robustness was evaluated by 200,000 Monte Carlo samples in which all criterion weights were perturbed independently by ±20% and renormalized. The preferred concept ranked first in 99.9895% of samples; its 95% score interval was 7.444–7.535 and the 95% score gap over the second-ranked alternative was 0.102–0.515. The result is robust to the stated weight uncertainty but remains conditional on the expert ratings and on hard technical constraints. A front-end engineering and validation framework is therefore proposed instead of claiming an implemented industrial improvement.