Optimisation énergétique des procédés de séparation en raffinage et en traitement de gaz naturel Optimal Use of Energy in Separation Processes for Refining and Natural Gas Treatment

Abstract

Cet article présente une méthode d'optimisation des procédés de séparation basée sur une analyse thermodynamique. Cette analyse s'appuie sur un bilan exergétique qui est établi dans le cas général d'un système quelconque opérant en régime permanent. Les facteurs qui conditionnent le rendement exergétique d'un procédé de séparation sont ensuite examinés. Il en résulte une méthode d'optimisation basée sur une réduction des irréversibilités thermodynamiques. Des exemples concrets d'application en raffinage et en traitement de gaz naturel sont présentés, et on montre comment cette analyse peut déboucher sur la conception de procédés innovants. The optimization of separation units in refining and natural-gas processing must take into consideration new needs and constraints that may seem incompatible. New installations must be designed not only on the basis of energy optimization but also by seeking to minimize investments and to respect new rules concerning environmental protection. The optimization described in this article is based on a thermodynamic analysis of different material and energy exchanges, The energybalance provides a suitable basis for making this analysis. It leads to the defining of an exergy efficiency that is all the higher as the thermodynamic irreversibilities are reduced, and that tends toward one for an ideal reversible system. For a separation process, a separation exergyterm is defined that correspond to the minimum separation work . Distillation is the basic separation operation. The exergy efficiency of this operation is low, and we show that, even in a relatively favorable cas, it is no greater than a value of about 6%. For an atmospheric distillation operation of crude oil, the exergy efficiency is about 4%. This overall exergy efficiency is the product of an external exergy efficiency and an internal exergy efficiency. The external exergy efficiency can be improved by better thermal integration among the units or by using cogeneration for supplying heat to the reboiler. Another possibility, that is sometimes used for superfractionation, consists in providing the heat required by the reboiler by overhead steam condensation after recompression. The internal exergy efficiency depends on the number and arrangement of the plates as well as on the composition of the mixture. In particular, it is found that separation by distillation of a component present in low concentration is very inefficient. The use of a separation agent -solvent or adsorbent - serves to separate a constituent that cannot be separated directly by distillation under satisfactory conditions. The separation process is optimized in this case by applying a procedure similar to the one followed for distillation. The integration of several functions in the same piece of equipment or of several transformation steps in the same process often reduces both the energy expenditures and the investments required. Several examples are developed in the text - case of an exchanger-dephlegmator, reactive distillation and absorption, integrated process for treating natural gas. These examples show how a thermodynamic analysis aiming to reduce irreversibilities leads to the designing of innovating processes.