Torben Ravn Andersen

Principles of control thermodynamics

The article presents a partial synthesis of progress in control thermodynamics by laying out the main results as a sequence of principles. We state and discuss nine general principles (0–8) for finding bounds on the effectiveness of energy conversion in finite-time.

Positioning heat exchangers in binary tray distillation using isoforce operation

The best way of adding two interstage heat exchangers to a binary distillation column is studied using irreversible thermodynamics. A distillation column is simulated with a computer program using the tray-to-tray calculation method. The purpose of the analysis is to find the locations of the two interstage heat exchangers which give the minimum entropy production rate in the column. According to the isoforce principle, minimum entropy production rate is obtained in distillation when the driving forces are uniformly distributed over the trays. This implies that the entropy production rate, in the optimum case, varies according to the value of the phenomenological coefficient. Therefore, locations with the largest deviations from this behavior are good locations for additional heat exchangers. A column separating n-pentane and n-heptane is used to demonstrate how the optimum variation in the coefficient may be used in practice.

Energy efficient distillation by optimal distribution of heating and cooling requirements

Abstract In this paper investigations regarding optimal placement and duty distribution of internal heat exchangers in distillation columns have been carried out based on simulations. In particular the effect of adding 2, 4 and 19 internal heat exchangers to a 19 tray distillation column separating 2-propanol and methanol was studied. It is concluded that the minimum entropy production placement of the heat exchangers depends strongly on the degree of separation. The optimum is however rather flat, which indicates that optimal placement for a nominal operating point may remain nearly optimal in terms of entropy production even for significant changes in feed and product compositions. When compared to a conventional column run under the same operating conditions, entropy production for an optimally operating column with four thermally active stages (4TA) is reduced by 25% as compared to 37% for a column with six thermally active stages (6TA) column and 49% for a column in which all the trays are equipped with heat exchangers.

Distillation control using passivity

Publisher Summary This chapter presents the design of an asymptotically stable control system for homogeneous multicomponent distillation. The control system is designed using passivity and thermodynamic based analysis. One design solution is presented to illustrate the use of passivity and thermodynamics to better understand the distillation control problem. The configuration presented is based on the fundamentals of thermodynamics where pressures are used for mass balance control, temperatures for energy balance control, and chemical potential are related to composition control. The chemical potential and its relation to composition are explored to show complications that can occur in azeotropic distillation. The temperature and temperature difference can be updated in the controllers from either concentration measurements, where possible, or from a model of vapor–liquid equilibrium (VLE) data.

Static Multiplicities in Heterogeneous Azeotropic Distillation Sequences

Abstract In this paper the results of a bifurcation analysis on heterogeneous azeotropic distillation sequences are given. Two sequences suitable for ethanol dehydration are compared: The ‘direct’ and the ‘indirect’ sequence. It is shown, that the two sequences, despite their similarities, exhibit very different static behavior. The method of Petlyuk and Avet’yan (1971), Bekiaris et al . (1993), which assumes infinite reflux and infinite number of stages, is extended to and applied on heterogeneous azeotropic distillation sequences. The predictions are substantiated through simulations. The static sequence performances are compared for minimal impurities in both products, where the direct sequence exhibits output multiplicity, while the indirect sequence exhibits state multiplicity. The latter multiplicity may be avoided by accepting a slightly increased impurity in the ethanol product.