Methods for Extractive Distillation Flow Sheets Synthesis
Advancing the chemical engineering fundamentals
Distillation, Absorption & Extraction - II (T2-10b)
Keywords: extractive distillation, flow sheets synthesis
Distillation is one of the most widely used industrial methods in the separation of multicomponent mixtures. Those processes often accounts for up to 70% of the total productive power consumption in chemical industry. This raises the problem of flowsheet optimization with energy consumption decreasing. Thermodynamically reversible distillation is the least energy-consuming. However, it is unfeasible because of its specific features(1). One can only approach thermodynamic reversibility by optimizing flowsheet structure and/or using diabatic distillation. The well known examples are Petlyuk(1) and dividing-wall wall(2) columns. In this case only one feature of the reversible distillation was used – the components with the intermediate volatilities full distribution between the products. From this standpoint, of great interest are flow sheets with the partially thermally coupled columns which thermodynamic efficiency lies between the simple and Petlyuk columns. The investigation of partially thermally coupled complexes is difficult because of their quantity for the multicomponent mixtures distillation. More than that, the methods were proposed for their creation only some years ago(3). These ones are based on the initial graph transformation of the simple distillation columns sequences. Also one can use for this operation the residue curve graphs(4).
Now this graph-theoretical approach may be used not only for the zeotropic but also for the extractive distillation flow sheets construction. We propose to synthesize the extractive n-component distillation flow sheet manifold by transformation of graphs explicated the separation of (n+1)-component mixture in the sequence of simple columns. It is possible because in the extractive distillation there are no thermodynamical restrictions on the products composition (5). The initial flow sheet have to be presented as oriented graphs in which nodes characterize columns and fractions, and edges describe flow relationships between them.
This graph describes the separation sequence. The next step of the synthesis is to split the feed flow into two individual flows: the n-component mixture and the extractant. This structure already almost fully represents the extractive distillation flowsheet. It is only necessary to create a cycle for the extractant by merging two nodes which are explicated extractant flow. Thus we obtain the extractive distillation flow sheet graph. Note that, depending on the vapor–liquid equilibrium pattern, more than one edge may leave the “extractant”node (the extractant may also be necessary for separating the mixture in the second column of the system)
Since the extractive distillation system has a closed (without taking into account losses) cycle for the extractant, one of the conditions of operability of flow sheets synthesized using the proposed algorithm is the presence of a cycle in the orgraph. A cycle must include at least two nodes representing columns. A second condition of operability of the proposed flow sheets is the introduction of the extractant into the column in which an azeotropic pair of components is separated. Thus, we have developed a universal algorithm for synthesizing workable flowsheets for extractive distillation of multicomponent azeotropic mixtures. More than that now we can construct the extractive distillation flow sheets with partially coupled columns for separation of multicomponent mixtures using simultaneously two proposed methods of synthesis(3,5).
The part of this work was supported by the Russian Foundation for Basic Research.
References
(1) Petlyuk F.B., V.M. Platonov and Slavinskij D.M. Thermodynamically Optimal Method for Separating Multicomponent Mixtures, Int. Chem. Eng., 1965, 5(3), 555–561.
(2) Kaibel G., Schoenmarkers H. Process Synthesis and Design in Industrial Practice. Proc. ESCAPE-12 (Computer Aided Process Engineering,10), Eds. J. Grievink and J.V. Schijndel, Elsevier, Amsterdam (2002).
(3) Timoshenko, A.V. and Serafimov L.A., Flowsheet Synthesis Strategy for Irreversible Zeotropic Distillation, Theor. Found. Chem. Eng., 2001, 35(6), pp. 567–572
(4) Timoshenko A. V., Anokhina E. A. and Buev D. L. Application of Graphs of Distillation Trajectories to Synthesis of Separation Flowsheets, Theor. Found. Chem. Eng., 2004, 38(2), pp. 160–164
(5) Ivanova L.V., Timoshenko A.V., Timofeev V.S. Synthesis of Flowsheets for Extractive Distillation of Azeotropic Mixtures Theor. Found. Chem. Eng., 2005, 39(1), pp. 16–23.
Presented Monday 17, 16:20 to 16:40, in session Distillation, Absorption & Extraction - III (T2-10b).
