Conceptual Design of Reactive Distillation Columns with Non-Reactive Sections

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1 Conceptual esign of Reactive istillation Columns with Non-Reactive Sections R. M. ragomir, M. Jobson epartment of Process Integration, UMIST, PO ox 88, M60 Q, Manchester, UK Abstract Reactive distillation has received intense attention due to the well known benefits of integrating distillation with reaction in a single unit. While there are procedures available for the synthesis of non-reactive columns and of reaction-separation systems, the design of reactive distillation columns is still a challenge. This work presents a new synthesis and design methodology for reactive distillation columns featuring both reactive and non-reactive sections. Two-feed column configurations are also addressed. The design method allows rapid and relatively simple screening of different reactive distillation column configurations. The results are also suitable for initializing more rigorous calculations. The methodology is illustrated for MTE and ethyl formate production. Keywords: process synthesis, stage composition lines, hybrid columns. Introduction As reactive distillation becomes an increasingly promising alternative to conventional processes, there has been increased interest in conceptual design of such processes, providing accessible tools for design engineers. Many methods for optimisation (e.g. Ciric and Gu, 995; Jackson and Grossman, 200) and design of reactive distillation columns (arbosa and oherty, 988; Espinosa et al., 995; Lee, 2000) have been developed, but there is still a lack of systematic conceptual design methods for complex column configurations, especially for double-feed columns. Investigating different column configurations for a given separation task is a very important part of reactive distillation column design. A hybrid configuration (featuring both reactive and nonreactive sections) might be much more economic than a fully reactive column for a given chemical system. Furthermore, some systems must use hybrid columns to obtain the desired products (Espinosa et al., 995). This work develops a design methodology, based on the oundary Value Method (VM) of Levy et al. (985). The design procedure allows rapid screening for the best configuration and operating parameters for a reactive distillation column with a specified performance (e.g. yield and product purities). Hybrid configurations top or bottom section reactive and columns with a reactive core are considered, for both single- and double-feed columns. From a synthesis point of view, the methodology developed allows the generation and evaluation of various column configurations and selection of appropriate operating conditions.

2 2. Synthesis and esign Methodology for Single-Feed Reactive istillation Columns 2.. Intersections of SCLs with the reactive surface The VM is based on the premise that a continuous composition profile between the top and bottom product compositions must be possible for a column to be feasible (Levy et al., 985). Stage composition lines (SCLs) are a reformulation of the information contained in composition profiles and can be applied for column design (Castillo et al., 998). Groemping (2002) developed a systematic methodology for conceptual design of fully reactive columns, using SCLs represented in transformed variables, applicable to reactive systems with two degrees of freedom. However, non-reactive composition space has a higher dimension than reactive equilibrium space. Therefore the methodology of Groemping (2002) cannot be directly applied for hybrid columns. Methods for design of fully reactive columns are extended in this work. Chemical equilibrium is assumed on all stages. The approach exploits the fact that in a hybrid reactive distillation column the composition of the liquid leaving the interface stage (between reactive and non-reactive sections) is on both a non-reactive SCL and a reactive SCL, and hence on the reactive surface. A necessary condition for a continuous profile to exist, for the specified product compositions, is that at least one non-reactive SCL intersects the reactive surface. The point of intersection is characterised by a specific reflux or reboil ratio, as shown in Figure. An intersection is thus associated with design parameters (number of non-reactive stages and reflux or reboil ratios) for a feasible hybrid column containing one reactive and one non-reactive section. Columns with a reactive core can be addressed by extending these ideas. In this case, the intersection with the reaction surface will be calculated for both rectifying and stripping non-reactive SCLs. Reactive surface X Intersections of nonreactive SCLs with the reactive surface Nonreactive SCLs s reboil ratio m number of stripping stages m = 5 2 s = m = 6 s = A x 6 C X A (a) (b) Figure. Non-reactive SCLs and their intersection with the reactive surface for an ideal system A + C (inert ). (a) Real mole fraction space; (b) Transformed mole fraction space.

3 2.2. esign methodology for single-feed reactive distillation columns Figure 2 shows the configurations of reactive distillation columns that are addressed in this work. Although hybrid columns usually have the feed situated at the interface between the reactive and the non-reactive sections (as for configurations Types I and III in Figure 2), feeds located within the reactive section can also be accommodated. (T) () (T) () (T) () Type I I Type IV Figure 2. Hybrid configurations for single-feed reactive distillation columns (T) Top section reactive; () ottom section reactive Continuous composition profiles for the above configurations can be achieved if suitable intersections exist between reactive SCLs or middle section profiles (if the configuration has a middle reactive section) and points of intersection of non-reactive SCLs with the reactive surface. All but Type I configurations have middle reactive sections. Table summarises the types of intersection and associated feasibility criteria leading to column designs for single feed hybrid reactive distillation columns. Fully reactive columns, designed using the methodology of Groemping (2002), can supplement these design options. The intersections provide sufficient design data to allow operating and capital costs to be estimated for each alternative. The alternatives may then be ranked. Table. Intersection types and feasibility criteria used for the different types of hybrid single-feed reactive distillation column shown in Figure 2 Hybrid Feasibility Intersection Type Configuration Criterion Type I (T) Rectifying SCLs + Stripping SCL-RS points () X n = X intersect () Stripping SCLs + Rectifying SCL-RS points () X m = Xintersect (T) Rectifying SCLs + Middle stripping CPs (2) X m = X n+ () Stripping SCLs + Middle rectifying CPs (2) X m = X n+ I (T) Middle rectifying CPs (2) + Stripping SCL-RS points () X n = X intersect () Middle stripping CPs (2) + Rectifying SCL-RS points () X m = X intersect Type IV - Middle rectifying CPs (2) + Middle stripping CPs (2) X m = X n+ () Intersection points of non-reactive SCLs with the reactive surface (2) Composition profiles calculated from intersection points n No. of reactive rectifying stages; m No. of reactive stripping stages

4 The design procedure assumes a constant pressure in the column. Constant molar overflow (in transformed variables for reactive sections) is a good assumption for most of the reactive systems. However, in this work, the assumption is first verified for arbitrary values of reflux and reboil ratios, for both reactive and non-reactive sections. If the test shows that the assumption does not hold, heat balances are included in the calculation of composition profiles. The approach is illustrated by application to MTE production. A hybrid configuration is needed, as MTE is not located on the reactive surface (Espinosa et al., 995). Figure 3 shows the rectifying SCLs, the intersections of stripping non-reactive SCLs with the reactive surface and the middle section profiles calculated bottom-up from the intersections. For the specified product compositions, two types of configurations are possible: Type I (T) and (T). The results obtained from the design procedure offer very good initialisation for rigorous simulation (ragomir, 2004). n-c MeOH 0. X Region of feasible designs Reactive rectifying SCLs Middle composition profiles r reflux ratio s reboil ratio q F feed condition Intersections of nonreactive SCLs with the reactive surface i-c 4 F q F = 3 r =.3 r =.4 F 8 q F = s=3.2 7 s=2.8 Type I (T) (T) Figure 3 MTE system: (a) SCLs and middle section composition profiles in transformed variables; (b) Single-feed hybrid configurations. etails of the best design for each configuration are shown, with respect to total annualised cost. 3. Synthesis and esign Methodology for ouble-feed Reactive istillation Columns ouble-feed columns are widely used for reactive distillation. A double-feed configuration can improve contact between the reactants, leading to a more uniformly distributed reaction in the column. The configurations addressed in this work are illustrated in Figure 4. One feed stage is assumed to be located at the interface between the reactive and the non-reactive sections; the other may be within the reactive section. The concepts developed for single-feed columns are extended to accommodate doublefeed configurations. Stage composition lines can be calculated from specified product compositions, for both rectifying and stripping sections. For the middle section, SCLs are calculated from a specified feed stage for fully reactive columns or, for hybrid columns, middle section composition profiles are calculated from intersections of nonreactive SCLs with the reactive surface. Table 2 summarises the types of intersections leading to feasible designs and the associated feasibility criteria.

5 F U F U F U F L F L F L F L FL F L F U F U F U (T) () Fully reactive Type I F U Upper feed; F L Lower feed (T) () I Figure 4. Configurations for fully reactive and hybrid double-feed reactive columns. (T) Top section reactive; () ottom section reactive. Table 2. Intersection types and feasibility criteria for the different types of double-feed reactive distillation columns shown in Figure 4 Hybrid Intersection Type Feasibility Configuration Criterion FR - Rectifying SCLs + Middle SCLs X m = X n+ Type I (T) Rectifying SCLs + Middle stripping CPs () X m = X n+ () Stripping SCLs + Middle rectifying CPs () X m = X n+ (T) Middle rectifying CPs (2) + Middle stripping CPs () X m = X n+ () Middle rectifying CPs () + Middle stripping CPs (2) X m = X n+ I - Middle rectifying CPs (2) + Stripping SCL-RS points (3) X n = X intersect () Composition profiles from intersection points calculated from mass and energy balance with the feed (2) Composition profiles from intersection points calculated from mass and energy balance without the feed (3) Intersection points of non-reactive SCLs with the reactive surface n No. of reactive rectifying stages; m No. of reactive stripping stages X E-Formate Middle section composition 0. 8 profiles from intersection points (calculated upwards) r =.3 r =.6 Stripping SCLs Water Intersections of nonreactive rectifying SCLs with the reactive surface - Type I () Fully reactive Formic Acid Ethanol (a) (b) Figure 5. Ethyl formate system: (a) SCLs and middle composition profiles in transformed variables; (b) double-feed configurations. etails of the best design, with respect to total annualised cost, are shown for each configuration. FAc EtOH s=.6 FAc 8 EtOH 3 6 s=.8

6 The approach is illustrated for the example of ethyl formate production from formic acid and ethanol (system described in Rhim et al., 985). oth fully reactive and hybrid double-feed configurations can be used. Figure 5(a) shows the stripping SCLs calculated from the bottom product composition and the middle section profiles calculated from intersections of non-reactive rectifying SCLs with the reactive surface. Intersections between the profiles result in the Type I () configuration as shown in Figure 5(b). Fully reactive columns can also be used for this system (for clarity, the rectifying SCLs were not represented). However, a hybrid configuration will always be preferred, due to the lower capital cost. The results obtained from the conceptual design method were used to initialise a rigorous simulation. Very good agreement was obtained, showing that the methodology can provide fast and reliable results, even for highly non-ideal systems and complex column designs (ragomir, 2004). 4. Conclusions A new design methodology is presented that is able to assess feasibility and obtain feasible design alternatives for hybrid reactive distillation columns, including both single-feed and double-feed configurations. The method, using simple graphicallybased concepts, is systematic and provides fast and reliable results, offering very good initialisation for rigorous simulation. Also, the method provides a basis for extending the analysis to more complex configurations, such as side-draw columns, sidestrippers/rectifiers or dividing wall reactive columns. References arbosa,. and M.F. oherty, 988a. esign and minimum reflux calculations for single-feed multicomponent reactive distillation, Chem. Eng. Sci., 43, arbosa,. and M.F. oherty, 988b. esign and minimum reflux calculations for double-feed multicomponent reactive distillation, Chem. Eng. Sci., 43, Castillo, F.J.L.,.Y-C. Thong and G.P. Towler, 998, Homogeneous azeotropic distillation.. esign procedure for single-feed columns at nontotal reflux. Ind. Eng. Chem. Res., 37(3), Ciric, A. R. and. Gu, 994, Synthesis of non-equilibrium reactive distillation by MINLP optimisation. AIChE J., 40, oherty, M. F. and G. uzad, 992, Reactive distillation by design, Trans. IChemE., Part A, 70, ragomir, R.M., 2004, Synthesis and design of reactive distillation columns, Ph Thesis, UMIST, UK. Espinosa, J., P. Aguirre, and G. Perez, 995, Some aspects in the design of multicomponent reactive distillation columns including non-reactive species, Chem. Eng. Sci., 50, Groemping, M., 2002, Synthesis and design of reactive distillation processes, Ph Thesis, UMIST, UK. Jackson, J.R. and I.E. Grossman, 200, A disjunctive programming approach for the optimal design of reactive distillation columns, Comp. Chem. Eng., 25, Lee, J.W., 2000, Graphical synthesis methods for reactive distillation systems, Ph Thesis, Carnegie Mellon University, USA. Rhim, J.N., H.T. Lee and S.Y. ae, 985, Isothermal VLE accompanied by esterification. Ethanol-formic acid system, Int. Chem. Eng., 25, Taylor, R. and R. Krishna, 2000, Modelling reactive distillation, Chem. Eng. Sci., 55,