Development of a distillation column for a dual mode LOX-LIN plant at BARC, Mumbai

A dual mode air separation plant producing either liquid oxygen (LOX) or liquid nitrogen (LIN) using a common single distillation column for both the LOX and LIN modes, is conceptualized by BARC in order to address the lack of indigenous technology in this particular domain. The dual mode air separation plant is based on a cryogenic process cycle involving a high speed turbo expander and a pair of plate-fin heat exchangers. The product output is expected to be about 50 l/hr in the LOX mode and 75 l/hr in the LIN mode. The purity of LIN and LOX would be in excess of 99% and 96% (by volume) respectively. The present work describes process calculations for sizing & optimization of a packed bed distillation column for LOX & LIN modes of operation. The basic calculation of the number of theoretical stages is a precursor to a more detailed analysis of a packed bed column using the HETP & HTU-NTU methods employing correlations for mass transfer coefficients. Along with the column, the thermal designs of the reboiler & condenser have also been worked out. The results from the cryogenic process calculations act as inputs for the design of the column. A single, optimized column catering to all possible modes of operation provides flexibility and compactness, increases plant availability and at the same time reduces capital cost. (author)