Cryogenic air separation is a process that can be integrated into other processes. Yet, many processes that include a cryogenic air separation unit (ASU), such as gasification, still utilize the standard design typically used to produce relatively pure nitrogen and oxygen products for retail purposes. The energy efficiency of processes with a cryogenic ASU could be improved by using a non-standard design, where nitrogen and oxygen products of lower purity are produced and used in downstream process units. However, it is complex to simultaneously optimize an ASU with other processes. Therefore, a simple model of an ASU that can be integrated with other process units is developed in this work. The most complex units in a cryogenic ASU are the distillation units and the multistream heat e...[ Read more ]

Cryogenic air separation is a process that can be integrated into other processes. Yet, many processes that include a cryogenic air separation unit (ASU), such as gasification, still utilize the standard design typically used to produce relatively pure nitrogen and oxygen products for retail purposes. The energy efficiency of processes with a cryogenic ASU could be improved by using a non-standard design, where nitrogen and oxygen products of lower purity are produced and used in downstream process units. However, it is complex to simultaneously optimize an ASU with other processes. Therefore, a simple model of an ASU that can be integrated with other process units is developed in this work. The most complex units in a cryogenic ASU are the distillation units and the multistream heat exchanger.

Distillation models were reviewed followed by the development of a new model. A novel, non-rigorous distillation model is developed without using constant molal overflow assumptions. The novel model was developed with insight from the models reviewed, as well as material and energy balances and could predict the change in liquid and vapour flow throughout a distillation column without rigorous stage-by-stage modelling. It was tested against other models and also compared to a rigorous model obtained from an ASPEN simulation. Results showed that the novel model was accurate and easy to solve.

Following that, equation-oriented heat integration models were studied. A comparison between three heat integration models was carried out and the Multi-M model proved to be the best in terms of solution quality, speed and accuracy. Although the model was traditionally applied to heat exchanger networks, a slight modification by setting the hot and cold utilities to zero enabled the use of the model to represent a multistream heat exchanger.

After suitable models for distillation and heat integration were identified, case studies were carried out on the optimization of a cryogenic ASU. The results of the case studies were encouraging as comparison with ASPEN simulations consistently showed close agreement. Furthermore, the solutions obtained were in close agreement with literature data.