A Model-Based Methodology for Integrated Design and Operation of Reactive Distillation Processes

Process intensification is a new approach that has the potential to improve existing processes as well as new designs of processes to achieve more profitable and sustainable production. However, many issues with respect to their implementation and operation is not clear; for example, the question of operability and controllability.

Traditionally process design and process control are considered as independent problems and are solved sequentially. The process design problem is usually solved to achieve the design objectives,and then, the operability and process control issues are identified, analyzed and resolved. A new approach isto tackle process intensification and controllability issues in an integrated manner, in the early stages of process design.

This integrated and simultaneous synthesis approach provides optimal operation and moreefficient control of complex intensified systems that suffice the process design objectives. Furthermore, it may also suggest innovative process alternatives which are more economical and environmental sustainable.In this work, a systematic model-based methodology for integrated design and operation of reactivedistillation operations is presented.

Issues related to operation are addressed to ensure a stable and reliable process design at pre-defined operational conditions whereas controllability is considered to maintaindesired operating points of the process at any kind of imposed disturbance under normal operatingconditions. The methodology employs a decomposition-based method so that the complexity of the problemis reduced into a set of sub-problems that are solved sequentially.

The method consists of four hierarchicalstages: (1) pre-analysis, (2) steady state analysis, (3) dynamic analysis, and (4) evaluation stage. To illustratethe application of the proposed methodology, production of methyl-tert-butyl-ether (MTBE) using are active distillation column (RDC) is considered.

Simple graphical design methods that are similar inconcept to non-reactive distillation processes are used. The methods are based on the element concept, which is used to translate a ternary system of compounds (methanol, isobutene and MTBE) to a binary system ofelements (elements A and B). For a binary element system, a simple reactive McCabe-Thiele-type method (to determine the number of reactive stages) has been used.

The reactive equilibrium curve is constructed through sequential calculation of reactive bubble points. For an energy-efficient design, the driving-forc eapproach (to determine the optimal feed location) for a reactive system has been employed. For both thereactive McCabe-Thiele and driving force method, vapor-liquid equilibrium data are based on elements.

Thereactive bubble point algorithm is used to compute the reactive vapor-liquid equilibrium data set.The operation of the RDC at the highest driving force and other candidate points is compared through openloop and closed-loop analysis. By application of this methodology it is shown that designing the process atthe maximum driving force results in an energy efficient and operable design.

It is verified that the reactive distillation design option is less sensitive to the disturbances in the feed at the highest driving force and hasthe inherent ability to reject disturbances.