(326k) Design Of Reactive Distillation Systems Using A Process Systems Engineering Approach

During the last years, process intensification received a great deal of attention because of the potential cost savings. By process intensification it is meant the significant improvement achieved by a process, at a lower total cost. For example, the entire plant design is upgraded by reducing the number of units involved in a conventional process to obtain the same products. Reactive distillation columns constitute a clear example, because it is possible to use a single piece of equipment in order to perform two tasks: reaction and separation of products, otherwise performed separately.

However, reactive distillation is not suitable for every process where reaction and separation steps occur. Operating conditions, such as pressure and temperature of the reactive and separation processes and perhaps other requirements, must overlap in order to assure the feasibility of the combined process. This limitation can be overcome by fixing adequate operating conditions in the cases where this is possible.

Reactive distillation has been presented as a multifunctional reactor, where the reactive and separation tasks are combined into a single unit, thus reducing investment costs. The advantages of using this configuration have already been reported (Stankiewicz, 2003). A thorough review on the design methods for reactive distillation can be found in Almeida-Rivera et al. (2004). However, a systematic analysis of the performance of reactive distillation columns changing structural variables has not been presented previously. The simplest case exhibits reactive and non-reactive sections with no interrelationship between them, except at the equipment level. Even when the interrelations between sections remain unchanged compared with traditional technology; significant savings (2.5 times as big as the equipment expenditure) can be obtained because of the smaller and cheaper plant needed to perform the same tasks. A clear example is the Urea2000plus? technology, by Stamicarbon B.V., where a pool reactor is used to combine the urea reactor, the carbamate condenser and the inerts scrubber. However, further improvements can be obtained by really integrating the tasks, on the following items:

? On the reaction: because there is an equilibrium displacement, since the products are being withdrawn

? On the separation: because of the changes in the driving force for mass transfer due to the reaction.

This case is formally presented as a reactive separation, also called a separative reaction. Both tasks occur at the same level. Several disadvantages can be overcome by using these combined schemes, because the selectivity and the reaction yield increase. This makes it possible to avoid thermodynamic constraints, such as azeotropes, thus allowing a considerable reduction not only in the investment costs but also in the operating costs (energy, water and solvents, otherwise required as entrainers). On the other hand, the mathematical model for this piece of equipment is more complex, and the requirements for simulating the combined process are higher. Non-linear coupling of reaction, phase equilibria, and mass and energy transport can give rise to many undesired effects, for example the appearance of reactive azeotropes (called arheotropes) and the multiplicity of steady states.

The performance of reaction with separation in one piece of equipment offers distinct advantages over the conventional, sequential approach (Podrebarac et al, 1998). As advantages of this integration, chemical equilibrium limitations can be overcome, higher selectivities can be achieved, the heat of reaction can be used for distillation, auxiliary solvents can be avoided, and azeotropic mixtures can be more easily separated than in conventionally distillation. This may lead to an enormous reduction of capital and investment costs and may be important for sustainable development due to a lower consumption of resources (Al-Arfaj and Luyben, 2000). Some industrial processes where reactive distillation is used are the sterification processes, such as the synthesis of methyl acetate (Agreda et al, 1990); and the preparation of ethers, like MTBE (Jacobs and Krishna, 1993), TAME and ETBE, used as fuel additives. An explanation for the occurrence of steady-state multiplicity was provided by Hauan et al (1995). A thorough review on the modeling aspects of reactive distillation can be found in Taylor and Krishna (2000), where special emphasis is put on the differences between equilibrium and non-equilibrium based models with their advantages and drawbacks. Doherty and Malone (2001) gave valuable commentaries on future trends and challenges, and a thorough review on the design methods for reactive distillation can be found in Almeida-Rivera et al. (2004).

We present the optimal design of a reactive distillation column, based on a model presented by Almeida-Rivera (2005) and further modified by Domancich et al (2006). The model comprises a combination of pure separation stages and a reactive distillation sector, where both reaction and separation take place at the same time.

For this study, we use a rigorous modeling of the column, with an equilibrium model for the reactive sector. The model of the column includes integer variables for defining the total number of stages, the number of reactive stages and feed locations. The economic optimization of the reactive distillation process is performed using a MINLP algorithm implemented in GAMS (Brooke et al, 2004) and the cost of the resulting scheme is compared to the cost of the conventional process considering that reaction and separation take place in different equipment, showing significant savings.

References

Agreda V.H., L.R. Partin and W.H. Heise (1990). Chemical Engineering Progress 40, 40-46.

Al-Arfaj M. and W.L. Luyben (2000). Industrial and Engineering Chemistry Research 39, 3298-3307.

Almeida-Rivera C.P., (2005) Designing reactive distillation processes with improved efficiency, PhD Thesis, Technical University of Delft.

Almeida-Rivera C.P., Swinkels P.L.J and J. Grievink, (2004), Designing reactive distillation processes: Present and future, Computers and chemical engineering, 28, (10), pp. 1997-2020.

Brooke A., D. Kendrick, A. Meeraus, R. Raman 2004, GAMS: A Userxs guide.

Doherty M. and M.F. Malone (2001). Conceptual design of distillation systems. McGraw-Hill, USA.

Domancich A.O., A.C. Olivera, N.B. Brignole, P.M. Hoch. 2006, ?Performance analysis of reactive distillation columns: adjustment of reactive and non-reactive stages?, Proceedings of the XXII Interamerican Congress of Chemical Engineering. ISSN 1850 3535.

Jacobs R. and R. Krishna (1993). Multiple solutions in reactive distillation for methyl tert-butyl-ether synthesis. Industrial and Engineering Chemistry Research 32 (8), 1706-1709.

Stankiewickz, A., (2003), Reactive separations for process intensification: An Industrial Perspective, Chemical Engineering and Processing, 42, pp. 137-144.

Taylor R. and Krishna R, (2000), Modelling reactive distillation, Chemical Engineering Science 55, pp. 5183, 5529.