REV . CHIM. (Bucureº ti) ♦ 59 ♦ Nr. 7 ♦ 2008 812 Energy Reduction in a Divided Wall Distillation Column RALUCA ISOPESCU*, ALEXANDRU WOINAROSCHY, LOREDANA DRÃGHICIU University „Politehnica” of Bucharest, Department of Chemical Engineering, 1-7 Gh. Polizu Str., 011061 Bucharest, Romania The paper presents the analysis of the thermal efficiency of a dividing wall distillation column and the influence of the feed composition on the reduction of energy consumption compared to a classical scheme of multicomponent mixture separation. The study relays on rigorous simulations in HYSYS TM using thermodynamically equivalent schemes. A case study is presented for the separation of a hydrocarbon mixture: benzene, toluene, ethylbenzene, o-xylene separated in three products in a dividing wall column. The dividing wall column solution led to about 40 % energy saving. The efficiency of a dividing wall column increases when the middle component is in large amount in the feed. Keywords: divided wall distillation column, energy savings, thermal coupling * email: r_isopescu@chim.upb.ro, Distillation is the most commonly used method for multicomponent mixture separation in chemical and petrochemical industries. The main disadvantage of this operation is its large energy requirements; distillation processes represent more than 40 % of the total energy consumption in these industries. Multiple mixtures separation sequences are generally designed considering some empirical rules that can make the operation more efficient from economical point of view: (a) the light component is separated first, (b) the component in the greatest amount is separated first, (c) the difficult separations are left at the end of the sequence, etc. Considering for instance a mixture of three components, A, B and C, where A is the lightest and C is the heaviest, the separation into pure products is commonly performed using simple column sequencing. For some mixtures, and especially when B is the major component, these solutions are not thermally efficient. The remixing effect that occurs in a conventional distillation sequences is associated with higher energy consumption [1]. In the conventional direct separation sequence the concentration of component B in the first column reaches a high value below the feed stage and then decreases towards the bottom of the column as the concentration of the heavy component increases. In the second column, a supplementary amount of energy is required to concentrate component B as a top product. In a thermally coupled distillation scheme this effect is diminished and a higher efficiency in energy use can be realised. Typical thermally coupled solutions are the side striper, side rectifiers, already in large use in oil refineries, and the Petlyuk column, which is a total thermal coupled solution (fig. 1a). The Petlyuk column is a well- known concept [2] but the practical solution that includes the prefractionater and the main column in a single shell is a new and challenging design option for distillation systems. This solution (figure 1b) represents the dividing wall distillation column (DWC) and it saves both energy and capital costs. A DWC column looks like a common distillation column with a side-draw. In reality it is a column that has a vertical separating wall for a given part of its length, which defines the “prefractionating” region and the “main column” region. The reflux that comes from the condenser will split on both sides of the separating wall and will make the reflux in the two parts of the distillation column. The vapour from the reboiler will also split, in the lower part of the column, in accordance with the liquid split and hydrodynamic conditions on both sides of the dividing wall. The main advantage of a DWC is that the middle component B splits in the prefractionator section, so that a high composition of B can be reached in the middle product that is drawn out from a given tray on the right hand side of the dividing wall. Assuming that the heat transfer across the dividing wall is negligible, DWC is thermodynamically equivalent to the Petlyuk column. When compared to conventional two-column system a capital cost saving of up to 40% can be expected [3]. To evaluate if DWC is a good solution, the thermodynamic properties of the components and the composition of the feed and products must be considered. Some guidelines are proposed concerning product purity requirements and feed composition. -Product purity: the purity of the middle product is higer then can be achieved in a simple side draw column. The DWC is a good solution when high purity of the middle product is required; Fig. 1. Fully thermally compled sequence a) Petlyuk Column b) Dividing Wall Column