Dividing wall column application for platformate splitter A case study

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1 20 th European Symposium on Computer Aided Process Engineering ESCAPE20 S. Pierucci and G. Buzzi Ferraris (Editors) 2010 Elsevier B.V. All rights reserved. Dividing wall column application for platformate splitter A case study Igor Dejanović a, Ljubica Matijašević a, Žarko Olujić b a Faculty of Chemical Engineering and Technology, University of Zagreb, Savska cesta 16, HR Zagreb, Croatia, ideja@fkit.hr b Process and Energy Laboratory, Delft University of Technology, Leegwaterstraat 44, 2628 CA Delft, the Netherlands, Z.Olujic@tudelft.nl Abstract This paper presents results of a simulation study indicating that a DWC would be a beneficial alternative to conventional two columns in series configuration for a platformate splitter application in a petroleum refinery. The DWC was designed using commercial software, complemented by a fundamentally sound shortcut method proposed by Halvorsen et al. [1-3] which allows generation of reliable initial values for rigorous simulations. Compared to actual, two-columns-in-series configuration, a DWC requires approximately 33.6 % less energy to deliver three fractions at required product specifications. Keywords: distillation, thermally coupled columns, dividing wall columns 1. Introduction Energy used for continuous distillation processes comprises approximately 40% of total energy use in chemical process industry [4]. This leads us to conclude following: distillation is still the most common method for separation of liquid mixtures and it is very energy intensive. Together with a fact that many successful separation systems were invented by experience alone [5], this makes distillation, which is considered to be the most mature among separations technologies, still an amenable subject for process intensification. Most impressive development in this respect is implementation of so called dividing wall column (DWC), which not only leads to energy saving but also to capital saving. Upon establishing itself as packed column in various chemicals separations performed mainly under vacuum, DWC is now making an inroad into refinery world dominated by large tray columns [6], where in many situations multicomponent feeds are separated into three products. In case of sharp product specifications, this often means use of two columns connected in series. Figure 1 shows schematically a typical (indirect sequence) two-columns-in-series arrangement for obtaining three pure products from a three component feed. Where appropriate, this sequence can be realized as a so called Petlyuk column, which, as shown in Fig.2, consists of a prefractionator column fully thermally coupled to the main column. This configuration implies that only one condenser and one reboiler are sufficient, which means a reduction in the number of main equipment and accompanying investment costs. A DWC, shown schematically in Fig.3, is a single shell thermodynamic equivalent of a fully thermally coupled column, which allows separation of three or more components into high purity products within one shell. This is achieved by using a vertical partition wall that divides mainly central part of the column into prefractionator and main column sections. In each section, two components with greatest difference in volatility are separated, while others are allowed to distribute.

2 I. Dejanovic et al. A, B A A, B, C B B, C Figure 1. Conventional three component separation arrangement Figure 2. Petlyuk arrangement C Figure 3. DWC arrangement The overall thermodynamic efficiency of Petlyuk column and its thermodynamic equivalent DWC is significantly larger than that of conventional configuration. This is mainly because of its configuration, which allows avoidance of the entropy of mixing formation caused by composition differences between feed stream and feed stage, as well as minimization of re-mixing effect of middle boiling components in separation of mixtures with more than 2 components [7], which is effectively a direct loss of separation work. Although the idea of thermal (heat) coupling is older, F. Petlyuk was instrumental in its implementation and therefore, this and more complex fully thermally coupled columns are known as Petlyuk columns. Most of the work, Petlyuk and coworkers have devoted over the years to distillation theory and its application to arrive at optimal design of columns and column configurations or sequences, which was not readily available because it was described in publications written mainly in Russian, can be found completely and consistently presented in English in a recently published book by Petlyuk [8]. Recent advances in design of DWCs including non-welded partition walls that can be placed in off-centre positions to allow optimal design for unsymmetrical (with respect to composition), and partly vaporized feeds, are described elsewhere [9]. The objective of this paper is to demonstrate the usability of commercial simulation software for performance evaluation and conceptual design of a DWC column for a refinery application, concerning separation of a multicomponent aromatics rich mixture into three fractions, according to given specifications. 2. Problem statement Platforming process product (platformate) comprises approximately 40 components, which however have been reduced for the purposes of this simulation study into a 15 component feed mixture with composition shown in Table 1. This table contains also the compositions of three product streams, i.e. fractions: 1. C5-C6 gasoline containing no more than 1.5 mass % of benzene; 2. Benzene rich cut (BRC) containing 68 mass % benzene; 3. Heavy reformate containing no more than 0.5 mass % of benzene. These represent specifications as encountered in actual refinery case employing a conventional configuration shown in Fig.1.

3 Dividing wall column application for platformate splitter A case study 3. Base case To have a reference point for comparison of non-conventional distillation configurations, base case model of the process was developed in Chemcad. Compositions of feed stream and resulting product streams are shown in Table 1. Results are in good agreement with real plant data. Total required reboiler duty is 5548 kw, and condenser duty kw. Table 1. Base case feed and product compositions Stream Name Platformate C5-C6 BRC Heavy Temperature [ C] Pressure [bar] Total flow [kg h -1 ] Component mass frac. N-Butane [-] Isopentane [-] N-Pentane [-] Methylpentane [-] N-Hexane [-] Benzene [-] Methylhexane [-] Toluene [-] Ethylbenzene [-] P-Xylene [-] M-Xylene [-] O-Xylene [-] M-Ethyltoluene [-] Trimethylbenzene [-] Diethylbenzene [-] DWC design Regarding the simulation approach, as indicated in Fig.4, a DWC has significantly more degrees of freedom than a conventional distillation column. That is why a good initialization method (shortcut model) is essential to give rough design estimates, which can be used to set up rigorous simulation. According to own evaluations, this appeared to be a rather simple but in this respect very effective method introduced recently by Halvorsen and Skogestad [1-3] Halvorsen s V min diagram method Halvorsen and Skogestad [2] proposed a design method based on Underwood s equations to determine minimum vapour and liquid flows needed for binary separation occurring in particular column sections, with infinite number of stages. Their basic postulate is that minimum vapour flow required for separation of n components feed into n pure products, corresponds with that required for the most difficult binary split. Halvorsen s method is available as a commercial software package, which can be used to gain insight into requirements to separate n-component feed mixture into n-products. However, V min diagram can also be developed using rigorous simulation [1].

4 I. Dejanovic et al. I R A N I LII:LIII A, B, C II IV III V S B N II N III N IV VIV:VV N V VI N VI Q R C Figure 4. DWC design parameters Figure 5. 4 Colum DWC model For the purpose of 3-product DWC design, 15 components from the feed were grouped in three representative key components: n-pentane, benzene and toluene. Then, V min diagram was constructed by rigorous simulation of possible binary splits, with stage number being at least 4N min, and key component s mass recoveries set to From that diagram, initial design parameters for rigorous simulation with infinite number of stages can easily be obtained. According to Halvorsen s method, energy saving estimate for this three component - three pure products case compared to conventional arrangement were around 14%, and by rigorous simulation around 18%. Present actual case with fractions as products, specified to maximize recovery of main components, allows more in this respect. 5. Rigorous performance simulation Commercial process simulators such as Chemcad, Aspen or HYSYS allow a DWC to be modelled as a sequence of simple columns. Several approaches can be used, ranging from one to four column models, each exhibiting some good and bad sides. As a general rule, one-column models with pump-arounds have better convergence properties than models with more columns. Four-column model is more difficult to initialize and exhibits a slower convergence, but it offers most flexibility. In this study, four-column model, i.e. the configuration shown in Fig 5 was used in conjunction with Chemcad. In simulating DWC according to initial design parameters from V min diagram, first step was to restore feed composition to full 15 components. Benzene mass fractions in distillate and side draw liquid flow rate were set to be the same as in base-case simulation, while initial value for reboiler duty was set to give required minimal vapour flow. Liquid and vapour split ratios were adjusted, until minimal possible mass fraction of benzene in bottoms was achieved, which was approximately the same as in base case. Next step was to determine actual number of stages in each section. This was done by reducing number of stages in sections, using composition profiles as guidelines, keeping mass fractions of benzene in distillate and bottoms, as well as liquid side draw flow rate constant.

5 Dividing wall column application for platformate splitter A case study For every converged case, an optimization was performed using optimization tool built in Chemcad, objective function being Min Qr, and independent variables liquid and vapour split ratios. To determine the optimal ratio between energy cost and number of stages, empirical objective function, Min N(R+1) was used, which approximates minimum of total annualized cost of a conventional distillation column. 6. Results Dependence of proposed objective function used to approximate minimum of total annualized cost of DWC on total number of stages is shown in Fig. 6. It can be seen that minimal value is obtained with total of 64 equilibrium stages. This is well bellow 78, i.e. the total number of equilibrium stages contained in two actual sieve tray columns connected in series. Comparison of simulation results of base-case and DWC design is shown in Table 2, indicating that a DWC would require approximately 33.6 % less energy, based on reboiler duties. N(R+1) N Figure 6. Dependence of objective function on total number of stages Table 2. Base case and DWC simulation results Base case DWC Column 1 Column 2 Product name C5-C6 mas % BRC mas % Heavy reformate mas % Reflux ratio Reboiler duty kw Total kw Condenser duty kw Total kw Number of stages This clearly indicates that an increase in thermodynamic efficiency of distillation leads not only to a significant saving in energy consumption but also in significant reduction

6 I. Dejanovic et al. of overall column dimensions. The financial aspects of designing a DWC for present case will be elaborated in detail in forthcoming full scale paper. One interesting option in this particular industrial case is to consider retrofit of first, larger platformate column, with a tangent to tangent height of 40 m into a DWC, by employing high performance structured packings. This could even enable a considerable capacity increase with minimum investment. Namely, originally both columns have been designed to operate at two operating pressures (approximately 1 bar difference) to be able to accommodate different product purity requirements. This means that the diameter of first column is much larger than required for present separation task, and thus may be larger than needed to accommodate internal vapour flows as associated with DWC designed for this purpose. If this proves to be feasible, the second column would be available for reassignment. This would lead to a maximized gain compared to a new design situation. For that purpose, however a detailed sizing of the proposed configuration needs to be performed, which is not in the scope of present work. 7. Conclusions In this paper we have demonstrated that a commercial simulator can be used in conjunction with initial guesses for governing variables obtained from a simple but theoretically founded short-cut method, to generate without computation difficulties an optimized internal configuration of a DWC. Compared to conventional two-column-in series configuration for obtaining benzene and toluene rich fractions from a 15- component feed, a DWC requires 33.6 % less energy to get the same product specifications. This as well as the fact that less capital and space will be needed to install new DWC makes this option highly interesting for implementation. References [1] I.J. Halvorsen and S. Skogestad, Ind. Eng. Chem. Res. 42 (2003) 596. [2] I.J. Halvorsen and S. Skogestad, Ind. Eng. Chem. Res. 42 (2003) 605. [3] I.J. Halvorsen and S. Skogestad, Ind. Eng. Chem. Res. 42 (2003) 616. [4] J.L. Humphrey and G.E.Keller II, Separation Process Technology, McGraw-Hill, New York, [5] M.F. Doherty and M.F. Malone, Conceptual Design of Distillation Systems 1st ed., McGraw-Hill, New York, 2001, pp [6] Ž. Olujić, M. Jödecke, A. Shilkin, G. Schuch, B. Kaibel, Chem. Eng. Proces. 48 (2009) [7] C. Triantafyllou and R. Smith, Trans IChemE -Chem. Eng. Res. Des. 70 (1992) 118. [8] F.B. Petlyuk, Distillation Theory and Its Application to Optimal Design of Separation Units, Cambridge Press syndicate of the University of Cambridge, [9] B. Kaibel, Dividing Wall Columns. Encyclopedia of Separation Science, I.D.W. Colin and F. Poole (eds.), Elsevier Science Ltd., Oxford, 2007.