Technical Program Paper Detail

[23a] - Integrated Column Designs for Minimum Energy and Entropy Requirements in Multicomponent Distillation

Presented at: [23] - Distillation Modeling and Processes # 2

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Author Information:

Ivar J Halvorsen NTNU Chemical Engineering Trondheim, N7491 Norway Phone: +47 7359 4154 Fax: Email:		Sigurd Skogestad (speaker) Norwegian University of Science and Technology (NTNU) Chemical Engineering Trondheim, B7491 Norway Phone: +47 7359 4154 Fax: +47 7359 4080 Email: skoge@chembio.ntnu.no

Abstract:

Integrated column designs for minimum energy and entropy requirements in multicomponent distillation

Ivar J. Halvorsen and Sigurd Skogestad
Department of Chemical Engineering
Norwegian University of Science and Technology (NTNU)
N7491 Trondheim
Norway

What is the minimum energy requirement for separating a given multicomponent mixture by distillation? This is a fundamental question of significant practical importance, yet it remains unsolved even for the case of ideal mixtures, at least when we consider the practical case with adiabatic column sections. We conjecture that, with constant pressure and without internal heat integration, the generalized Petlyuk arrangement requires less energy than any other adiabatic arrangement. The energy requirement is then easily computed as the being equal to the most difficult binary split(!). The minimum energy requirement when we allow for internal heat exchange remains unknown.

However, minimum energy (1st law) is by itself not sufficient as a measure, because also the quality (temperature) of the energy matters. Specifically, we prefer a process where the energy may be supplied at a low temperature and cooling may be supplied at a high temperature. To take this into account, we also consider the entropy production or lost work (2nd law). This leads us to the reversible Petlyuk arrangement. The total required heat supply is higher than for the adiabatic Petlyuk arrangements, but the reversible arrangement has a potential for further energy reduction by use of internal heat integration. This principle can also be applied to general arrangements (not only reversible).

We here consider the separation of ideal mixtures for which we may assume constant relative volatility and constant molar flows. We assume infinite number of stages because resulting minimum vapor rate Vmin provides a lower bound on the energy requirement for ideal mixtures. In practice, with a finite number of stages, the actual energy requirement may be about 10% higher.

To illustrate the results we consider the energy requirement (given in terms of the vapor rate Vmin/F) and entropy production (given in terms of the relative entropy production Sr= Stotal/S where S is the entropy of mixing) for a specific case, namely the separation of an equimolar ternary mixture (components A, B and C) with relative volatilities 4 and 2 into its three pure products.

Let us first consider the conventional "direct split" arrangement with two columns. Here we in the first column take A as the top product and B/C as the bottom product. The second column separates B and C, and we get B in the top and C in the bottom. The combined energy requirements in the two column reboilers is Vmin/F=2.072 and the entropy production is Sr=0.59.

Some energy reduction may be achieved in this case with the "indirect split" where we in the first column take C as the bottom product and A/B as a vapor top product. The second column separates A and B. Here Vmin/F=2.032, but the entropy production is higher, Sr=1.21, because the temperature difference between heat supply and cooling is larger.

The adiabatic directly coupled Petlyuk column with a single reboiler achieves more than 30% energy reduction, Vmin/F=1.366. This is the best adiabatic arrangement in terms of energy. However, the entropy production remains high, Sr=0.72, because all the heat is supplied at the highest temperature (boiling point of component C) and all the cooling is at the lowest temperature (boiling point of component A). To further reduce the energy requirements we must allow for internal heat exchange. For example, for this specific mixture we may in the Petlyuk column preheat the feed with the sidestream product and achieve Vmin/F=1.181 and Sr=0.49, but it is not known whether further reductions are possible.

To significantly reduce the energy consumption further we must allow for non-adiabatic sections with continuous heat exchange. For example, for this specific mixture, if we allow for heat exchange between the middle four sections in the Petlyuk column, then we may achieve Vmin/F=1.000 and Sr=0.26.

To achieve further reductions let us consider reversible distillation where the entropy production is zero, Sr=0. There are many possible reversible configurations, all of which require nonadiabatic column sections. One is the reversible Petlyuk column where Vmin/F=1.566, so its energy requirement is higher than for the adiabatic Petlyuk column (Vmin/F=1.366), which is no big surprise since we no longer supply all the heat at the highest temperature. To reduce the energy requirements we may introduce internal heat pumps so that again all heat supply is at the highest temperature. In this way we may reduce the energy requirement to Vmin/F=0.793, which is the theoretical minimum for the separation of this mixture by distillation at constant pressure.

In the paper we derive general expressions for computing the energy and entropy, and also discuss practical issues related to operation.

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