Eduardo S. Pérez-Cisneros

A robust PI control configuration for a high-purity ethylene glycol reactive distillation column

The control problem of an ethylene glycol reactive distillation column is studied in this work. The control objective is to regulate the ethylene glycol composition in the product by manipulating the reboiler boil-up ratio. A preliminary input/output bifurcation analysis reveals the existence of an unique equilibrium point at low reboiler heat inputs and three equilibrium points (two stable and one unstable) at high reboiler heat inputs. Moreover, the existence of input multiplicities at moderate and high product purity is also revealed. A first-order output-feedback compensator is designed which is shown to be equivalent to a PI controller. Tuning issues in terms of two-time constants are given, and the ability of the controller to regulate the product composition in the face of sampled/delayed measurements is shown via numerical simulations on a full dynamical model.

Aspects of simulation, design and analysis of reactive distillation operations

Various aspects related to simulation, design and operation of reactive distillation processes are presented through a systematic analysis of the model parameters and the important design/operational variables. The need for reliable, versatile and consistent simulation tools that allow the analysis of design/operation of reactive distillation processes are highlighted through two well known examples (the MTBE process (the production of Methyl Ter-Butyl Ether from the reaction of isobutene and methanol with the presence of one inert component) and an esterification process (the production of ethyl acetate and water from ethanol and acetic acid)). The model parameters for the estimation of the properties related to physical and chemical equilibrium have been found to be highly sensitive to the design and operation of the two systems studied. The reactive phase diagrams and the existence of reactive azeotropes have been found to influence the choice of the condition of operation and the existence of multiple solutions.

A reactive distillation process for deep hydrodesulfurization of diesel: Multiplicity and operation aspects

A systematic study of the operating conditions and parameter sensibility under which multiple steady states may occur in a reactive distillation column (RDC) for diesel deep hydrodesulfurization is presented. The multiplicity analysis is performed through bifurcation diagrams for two feed case scenarios. The main variables that affect the steady state behavior are the reflux ratio, the hydrocarbon feed flowrate, the reboiler heat duty, and the catalyst load (liquid holdup). For the first case only a single steady state was found, while in the second case input and output multiplicities were determined. It is shown that the introduction in the feed of the recalcitrant 4,6-DMDBT, plays a crucial role in the RDC design and operation. A set of values of the bifurcation parameters for an appropriate operation of the RDC are recommended to avoid the multiplicity region and to reach the required output targets.

A generalized reactive separation unit model. Modelling and simulation aspects

The modelling and simulation of a generalized reactive separation unit has been developed. The generalized reactive separation unit is able to consider multiple configurations where the reaction and separation may occur simultaneously or where only a single separation process (multiphase) or single reaction process (multireaction) may take place. Therefore, through the generalized reactive separation unit model, it is possible to represent systems with or without chemical reaction, systems where homogeneous or heterogeneous chemical reactions occur (kinetically and equilibrium controlled) and also multiphase (LLV) and multireaction systems. The generalized reactive separation unit has been constructed such that it can also be employed in the modelling and simulation of reactive distillation columns, reactive extraction columns and sequences of reactor-separator flowsheets.

An Integrated Reactive Distillation Process for Biodiesel Production

Abstract An integrated reactive distillation process for biodiesel production is proposed. The reactive separation process consists of two coupled reactive distillation columns (RDCs) considering the kinetically controlled reactions of esterification of the fatty acids (FFA) and the transesterification of glycerides with methanol, respectively. The conceptual design of the reactive distillation columns was performed through the construction of reactive residue curve maps in terms of elements. The design of the esterification reactive distillation column consisted of one reactive zone loaded with Amberlyst 15 catalyst and for the transesterification reactive column two reactive zones loaded with MgO were used. Intensive simulation of the integrated reactive process considering the complex kinetic expressions and the PC-SAFT EOS was performed using the computational environment of Aspen Plus. The final integrated RD process was able to handle more than 1% wt of fatty acid contents in the vegetable oil. However, results showed that the amount of fatty acids in the vegetable oil feed plays a key role on the performance (energy cost, catalyst load, methanol flow rate) of the integrated esterification–transesterification reactive distillation process.

Nonlinear PI control of an ethylene glycol reactive distillation column

Abstract The control problem of an ethylene glycol reactive distillation column is studied in this work. The control objective is to regulate the ethylene glycol composition in the product by manipulating the reboiler boil-up ratio. By using an estimator of the input-output modeling error, a first-order output-feedback compensator is designed which is shown to be equivalent to a PI controller with antireset windup structure. The ability of the controller to regulate the product composition in the face of unknown nonlinearities and sampled/delayed measurements is shown via a numerical simulation on a rigorous dynamical model.

Static/dynamic analysis and controllability issues in reactive distillation columns

Abstract The design, operation and control of reactive distillation columns are more complex than those involved for either reactors or conventional distillation columns. Therefore, a good un derstanding of the process helps to avoid unsafe operation conditions and further facilitates various, subsequent studies such as control, monitoring, data reconciliation, parameter estimation and optimization of existing plants. In this work the study of the steady state and dynamic behavior as well as the analysis of some controllability issues for reactive distillation columns, based on the applicability of the (equilibrium) element-based approach and thermodynamic issues, are presented. The MTBE process is selected as case study, so that previous reported, results can be compared and verified. Results are obtained using ICAS and ASPEN simulators.

A reactive distillation process for co-hydrotreating of non-edible vegetable oils and petro-diesel blends to produce green diesel fuel

Abstract A reactive distillation (RD) process for the hydrotreating (HDT) of vegetable oils and sulphured petro-diesel to produce green diesel is developed. Process intensification (PI) of a reactive separation unit that combines both, the hydrodeoxigenation (HDO) of triglycerides and free fatty acids and the hydrodesulfurization (HDS) of petroleum-diesel reactions, is carried out. PI considers the thermodynamic analysis of model mixtures of vegetable oils with hexadecane and hydrogen to determine the appropriate operating conditions (temperature, pressure and composition blends) of the RD process. Two different hydrotreating RD column configurations are proposed. The simulation of the hydrotreating RD processes is performed with the Aspen Plus environment using the PC-SAFT option to modelling the phase equilibria. Simulation results show that the performance of the hydrotreating RD process is more energy efficient and higher yields are attained when blends of vegetable oils with high free fatty acids content and petro-diesel are premixed and further hydrotreated.

Design of a Reactive Distillation Process for Ultra-Low Sulfur Diesel Production

Abstract The analysis of the applicability of the reactive separation concepts to the deephydrodesulfurization process has been performed. Through the computation of the reactive residue curve maps a basic conceptual design of a reactive distillation column was obtained. This preliminary design considers two reactive zones, each one packed with a different catalyst, a Ni−Mo based catalyst for the bottom reactive section and a Co−Mo for the top reactive section. Six different column configurations were validated through numerical simulations. The results show that the complete elimination of the organo-sulfur compounds from a HC mixture is possible with appropriate design specifications for the reactive distillation column, i.e., reflux ratio, feed location and condenser duty.

Exploring the interaction between flows and composition in reactive distillation

In this paper a new equilibrium approach is used to simulate the closed loop behaviour of the MTBE production process to study the interactions between flows and composition. This will facilitate the application of the existing methods for analysis of distillation systems. Results show that the operating pressure is an important factor in improving conversion and that ignoring the presence of inerts in the mixture introduces major errors.