A Sequential and Hierarchical Approach for the Feasibility Analysis and the Preliminary Synthesis and Design of Reactive Distillation Processes

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1 A Sequential and Hierarchical Approach for the Feasibility Analysis and the Preliminary Synthesis and Design of Reactive Distillation Processes Raphaële Thery, Xuân-Mi Meyer 1, Xavier Joulia Laboratoire de Génie Chimique, UMR-CNRS 5503, INPT-ENSIACET 5 rue Paulin Talabot, Toulouse cedex 01, France Abstract A procedure that combines feasibility analysis, synthesis and design of reactive distillation columns is introduced. The main interest of this methodology lies on a progressive introduction of the process complexity. From minimal information concerning the physicochemical properties of the system, three steps lead to the design of the unit and the specification of its operating conditions. This methodology which provides a reliable initialization point for the optimization of the process has been applied with success to different synthesis. The production of Methyl-Tert-Butyl-Ether (MTBE) is presented here as an example Keywords : reactive distillation, process synthesis, feasibility analysis, process design 1. Introduction Despite the many advantages of Reactive Distillation (RD) processes (Stankiewicz, 2003) the industrial community still hesitates on firmly adopting that multifunctional process, mainly by lack of systematical and universal design tools. In that context, a state of the art on the thermodynamic fundamentals of reactive systems and methods and tools available for the analysis and the design of reactive distillation processes has been drawn up (Thery, 2002). This analysis shows that during the past decades, many studies have been published to provide systematic procedures for the feasibility analysis and the design of RD processes but only few of them propose a systematic procedure that combines feasibility analysis, synthesis and design. The methodology presented in this article contributes to fill this lack. Its application is currently limited to reactive systems where degree of freedom is less than 3. Most of the methodology exploits and enriches approaches found in the literature. Each step is described and our contribution is underlined. The procedure is then illustrated on the production of MTBE. 2. Detailed presentation of the methodology Prerequisite data of the methodology are: a thermodynamic model to describe the phase equilibria, the equilibrium constant of the chemical reaction and specifications concerning the purity of the products, the recovery rate or the yield of the reaction. Figure 1 presents a detailed flow chart of the approach adopted to carry out each step of the procedure and highlights their complementarities. 1 Author to whom correspondence should be addressed : XuanMi.Meyer@ensiacet.fr

2 OF REACTIVE COMPUTATION RESIDU AND CURVE E CURVE ANALYSIS MAPS MAPS OF REACTIVE RESIDU CURVE MAPS Existence of reactive azeotropes? Existence of of reactiveazeotropes? distillation boundaries? Existence of reactive distillation boundaries? Requirement of of a second a second feed feed plate plate?? Requirement of a of pure a stripping pure stripping or rectifying or rectifying section section?? YES NO Yield of the reaction Recovery rates Distillate and bottom molar flowrates Minimum numberofstage Favorable feed composition Bottom and distillate composition Location and size of the reactive zone MODIFIED MODIFIED Yield of the reaction Recoveryrates Distillateand Recovery rates bottom molarflowrates Distillate and bottom molar flowrates Favorable feed composition Bottom and distillate composition SYNTHESIS : BOUNDARY VALUE DESIGN METHOD Confirmation (or rejection) of the feasibility analysis Confirmation ( Minimum reflux Minimum ratio reflux ratio Couple Couple reflux reflux ratio ratio / Number / of theoretical stages stages Location Location of the feed plates Estimation of compositions and temperatures profiles Estimation of composition and temperature profiles DESIGN : SIMULATION WITH A M.E.S.H. MODEL operating parameters taking into account the energy balance Figure 1: Methodology for the design of Reactive Distillation processes 2.1 Feasibility Analysis 2.1.a Construction and analysis of reactive residue curve maps ( rrcm ) To initiate the feasibility analysis, reactive residue curve maps (Ung and Doherty, 1995a,b) are generated and analysed. A reactive residue curve is defined by the locus of the liquid compositions remaining from a simple reactive distillation process and the reactive residue curve map is obtained through the simulation of the reactive distillation process for various initial liquid compositions. A software was developed to generate complete reactive residue curves maps for ternary and quaternary mixtures involving one equilibrium reaction and to display the resulting distillation boundaries. Displaying the singular points of the system (pure component, reactive azeotropes, non reactive azeotropes ) and the distillation boundaries, the reactive residue curve map (rrcm) enables to define the most favourable feed composition and the column structure necessary to obtain the desired product (requirement of a pure separation section or of two feed plates) according to the rules enounced by Bessling et al. (1997) 2.1.b Static Analysis (SA) The second step provides a set of attainable compositions which satisfy the purity and conversion recovery ratio specifications. If non reactive sections are required, the rrcm must be completed by the static analysis (SA) proposed by Giessler et al. (2001). The assumption formulated in this step (infinite liquid and vapor flowrates, infinite number of trays) permits to consider the RD process as the combination of two successive operations. The first one involves only

3 the reaction, but thanks to the coupling between separation and reaction, the reaction yield may exceed the equilibrium. The second operation concerns the separation : because of the infinite flowrates, the composition profile in the column can be represented by the distillation lines. For various reactant ratios, the SA consists in identifying a feasible operation which leads to the best performances characterized by the yield and the recovery rate of the desired product. A separation is assumed to be feasible if : - regarding the separation, the global mass balance is satisfied on the column and if the distillate and the bottom compositions belong to the same distillation line - regarding the reaction, a part of the distillation line lies in the inside the forward reaction region (manifold of compositions favourable to the forward reaction) If pure separation sections are not required, a modified static analysis (msa) has been developed. It consists in the resolution of a set of mass balance and specification equations. Specifications concerning the yield of the reaction, the purity or the recovery ratio of products are inherited from the rrcm analysis. 2.2 Synthesis step: Boundary value design method (BVD) As it lies on many assumptions (total reflux ratio in the case of the analysis of the residue curve maps, infinite liquid and vapor flowrates in the case of the static analysis), the feasibility analysis must be completed by a more rigorous approach. The synthesis step is based upon the boundary value design method, introduced by Barbosa and Doherty (1987b) for entirely reactive columns and extended to hybrid processes by Espinosa et al. (1996). The interesting point of our methodology is that the specifications required for the synthesis (distillate, bottom and feed compositions, structure of the column) are inherited from the previous feasibility analysis. The synthesis provides more precise information concerning the process configuration: minimum reflux ratio, and, for a given reflux ratio, location of the reactive zone, number of theoretical stages, position of the feed plates. In the academic tool developped, a broad range of configurations can be considered : one or two pure separation sections, one or two feed plates, finite or infinite reflux ratio. The concept of minimum reflux defined by Barbosa and Doherty (1987a) and extended to hybrid columns by Thery (2002) has been applied. Within this framework, the Constant Molar Overflow (CMO) assumptions are formulated: all the thermal effect are neglected and composition profiles are deduced from the mass balance equations. The rectifying and stripping composition profiles are calculated considering a RD column, and calculating the mass balance between the top of the column and a rectifying stage and between the bottom of the column and a stripping stage respectively. The obtained profiles necessarily depend on the reflux ratio: the minimum reflux ratio is obtained as soon as we can observe a continuous path from the distillate to the bottom product. A feasible steady-state is found when the rectifying and stripping profiles intersect. As the synthesis step computes the profile stage by stage, the number of stages required in each section of the column can be estimated Design Step: simulation So far, the column configuration has been drawn up from the mass balances only. Thermal phenomena (heat of reaction, thermic loss, heat of mixture ) have been ignored. As their effect can not be excluded from the study, a third step is necessary.

4 The design step involves a simulation tool. Knowing the configuration of the required column which is not modified here, the design step consists in adjusting the operating parameters taking into account the overall complexity of the process. To perform this step, the PROSIM software commercialised by PROSIM S.A is exploited. Given the pressure and the column configuration, the degree of freedom of the MESH model is equal to 2: to saturate this degree of freedom, the purity and the partial flowrate of the desired component (at the top or at the bottom) are fixed. Then, the required reflux ratio and heat duties are deduced from the model resolution. To help the calculations, compositions and temperature profiles estimated during the synthesis step are used as initialization points. At the end of this step, a column structure and the associated operating parameters necessary to achieve the initial specifications are available. 3. Application The procedure is illustrated on the production of MTBE under 11 atm in the presence the (BU) as an inert compound: (MeOH) + (IB) MTBE 3.1. Feasibility analysis Figure 2 presents the rrcm of the MTBE system and the resulting distillation boundaries. This rrcm is represented using the reactive compositions (Ung and Doherty, 1995). Each point of the diagram characterizes an equilibrium mixture: for that reason, no vertex corresponds to pure MTBE because MTBE can not exist alone in an equilibrium mixture. Azeotrope methanol/ Azéotrop N-butane (84.5 C) region D1 N-butane (84.5 C) Region region F1 region F2 D2 (141 C) (75 C) (141 C) B1 B2 (75 C) Figure 2: rrcm and Distillation regions for the MTBE production system with n- butane as an inert Under 11 atm, the system presents three physical azeotropes but only the methanol/nbutane azeotrope, which involves two components which do not react, survives to the reaction. A quaternary reactive azeotrope appears next to the vertex. Another singular point can be noticed on the MeOH/IB edge. This point can not be considered as a reactive azeotrope because the residue curves do not stop on it. However, as the residue curves are heavily curved towards it, an infinite number of stages would be required to pass this point; from a practical point of view, it generates a distillation boundary which strongly influences the feasibility of the RD process. For that reason, it was called pseudo reactive azeotrope by Ung and Doherty (1995). Four distillation regions can be pointed out. If the feed has an excess of methanol (F 1 ), the lightest product, i.e. the azeotrope méthanol/ (D 1 ) can be recovered at the top and a

5 reactive mixture consisting of isobutene, methanol and MTBE at the bottom (B 1 ). If the feed has an excess of isobutene (F 2 ), the reactive pseudo-azeotrope can be recovered at the bottom (B 2 ) and a mixture made up of inert and isobutene at the distillate (D 2 ). In all cases, the recovering of pure MTBE in an entirely reactive process is impossible because it does not appear as a vertex of the reactive composition space. According to the Bessling et al., 1997, a pure separation section is then required to obtain pure MTBE. Consequently, according to the flow chart in figure 1, the Static Analysis is exploited. Table 1 presents the results obtained for the production of MTBE with a feed containing 20% of. Regarding these values, the equimolar feed appears to offer the most favourable conditions to obtain pure MTBE in reasonable quantities. Note that a too important excess of methanol would lead to the preferential recovery of pure methanol at the bottom because the azeotrope methanol/mtbe prevents MTBE to be recovered. Table 1: SA results (production of MTBE : 20% of nbu) Feed molar composition Distillate molar composition Bottom molar composition Reaction * Yield (%) Bottom Bottom MTBE MTBE Recovery ratio Recovery ratio (%) (%) IB MEOH IB MEOH MTBE nbu IB MEOH MTBE nbu 0,10 0,70 0,02 0,30 0,21 0,47 0,00 0,99 0,01 0,00 90,9 3,2 0,20 0,60 0,03 0,37 0,28 0,32 0,00 0,99 0,01 0,00 90,2 0,6 0,30 0,50 0,03 0,36 0,28 0,33 0,00 0,00 0,99 0,00 93,5 39,1 0,35 0,45 0,02 0,28 0,19 0,51 0,00 0,00 0,99 0,00 97,8 78,9 0,38 0,42 0,004 0,16 0,04 0,80 0,00 0,00 1,00 0,00 99,7 97,2 0,40 0,40 0,01 0,01 0,01 0,97 0,00 0,00 1,00 0,00 99,4 99,7 0,45 0,35 0,34 0,02 0,00 0,65 0,00 0,00 1,00 0,00 99,0 99,9 0,50 0,30 0,50 0,01 0,00 0,49 0,00 0,00 0,99 0,01 98,6 99,9 0,60 0,20 0,66 0,01 0,00 0,33 0,00 0,00 0,99 0,01 96,8 99,9 0,70 0,10 0,75 0,00 0,00 0,25 0,00 0,00 0,99 0,01 99,1 99,9 * The reaction yield is calculated according to the default reactant 3.2 Synthesis step Figure 3 present the influence of the reflux ratio on the liquid composition profiles. Here, again, these profiles are represented in the reactive composition space. r < r min r = 2 r > r min r = 3 Reactive Reactive rectifying rectifying profile profile Pure stripping profile Reactive stripping Reactive stripping stage Pure stripping profile (a) (b) r =r min r = 2,65 r = 2,65 zoom Reactive rectifying profile Reactive stripping Pure stripping profile (c) (d) Figure 3: Synthesis Step - influence of the reflux ratio on the composition profiles

6 In that case, the minimum reflux ratio is equal to 2,65. Then, for a reflux ratio equal to 3, the synthesis leads to the following configuration: 18 pure stripping stages, 8 reactive stripping stages and 17 reactive rectifying stages Design Step Applying these concepts to the MTBE production, we obtain a required reflux ratio equal to 7,5 (against 3 predicted by the synthesis step). As shown on figure 4a, the rise in the vapor flowrates on the reactive plates due to the heat of reaction compensates the positive effect of the reflux. Figure 4b shows that to obtain the same performances as those predicted during the synthesis step, it is so necessary to increase the reflux ratio. vapor flowrate(mol/s) 2 1,8 1,6 1,4 1,2 1 0,8 0,6 0,4 0, stage (from top to bottom) (a) Qr=0 Qr=-62,7 Kj/mol kj/mol Figure 4: Design step : (a) influence of the heat of reaction on the vapor flowrates (b) comparison of the profiles obtained through the design and the synthesis steps 4. Conclusion and outlooks A sequential methodology for the design of RD processes has been introduced: it has also been successfully applied to two feed plate column (Thery, 2002). This methodology leads to a reliable initialization point for the optimization of the operating conditions and the process design. Although it does not permit to design process where distillation boundaries can be crossed in a single column process, this methodology can be used to design a reactive column included in wider processes. For the moment, the procedure mainly relies on graphical analysis which restricts its application to a few number of systems: equilibrium systems containing no more than four components. To extend this procedure to multicomponent, multi reactions systems, a substitution of the graphical analysis by mathematic feasibility criteria is investigated. Then, concerning the description of chemical reactions, the extension of the procedure to kinetically controlled reactions is also planed. References Barbosa D, Doherty M.F, 1987a, Chem. Eng. Sci., Vol 43, No 7, pp Barbosa D, Doherty M.F.,1987b, Chem. Eng. Sci., Vol 43, No 9, pp Bessling B., Schembecker G., Simmrock K.H, 1997, Ind. Eng. Chem. Res., 36, Espinosa J., Aguirre P., Pérez G., 1996, Ind. Eng. Chem. Res, Vol.35, pp Giessler S., Danilov R.Y., Pisarenko R.Y., Serafimov L.A., Hasebe S., Hashimoto I., 2001, Comput. Chem. Eng., vol. 25, pp Stankiewicz A, 2003, Chem. Eng. Prog., vol. 42, pp Thery R., 2002, phd Thesis, INPT Toulouse Ung S., Doherty M.F, 1995, Ind. Eng. Chem. Res, 34, pp molar liquid fraction 1 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0, stage (from top to bottom) (b) isobutene methanol synthesis MTBE MTBE methanol isobutene design