Juan Gabriel Segovia-Hernández

A comparison of the feedback control behavior between thermally coupled and conventional distillation schemes

A comparative analysis of the feedback control responses to set point changes of three thermally coupled distillation schemes and two conventional distillation sequences for the separation of ternary mixtures is presented. Designs for the thermally coupled schemes were obtained and optimized for energy consumption to link their energy characteristics to their dynamic behavior. For the comparison of the dynamic behavior, responses to set point changes under closed loop operation with proportional-integral (PI) controllers were obtained. For each separation scheme, the parameters of the PI controllers of the three composition control loops were optimized using the integral of the absolute error criterion. The effects of feed composition and of the ease of separability index were considered. The dynamic tests showed that in many cases the thermally coupled distillation schemes outperformed the dynamic responses of the conventional distillation sequences. The results indicate that there exist cases in which the energy savings provided by the thermally coupled systems do not conflict with their dynamic properties.

Control Behaviour of Thermally Coupled Distillation Sequences

The controllability properties of thermally coupled distillation sequences for the separation of ternary mixtures are compared with those of the conventional direct and indirect sequences. Closed loop responses to set point changes were performed, and controllers were tuned to minimize their ISE values. The results indicate that the integrated systems exhibit better control properties than sequences based on conventional distillation columns. This result provides a further incentive for the use of those integrated systems.

Analysis of dynamic properties of alternative sequences to the Petlyuk column

The Petlyuk distillation configuration has received considerable attention because of its efficiency to reduce the energy required for the separation of ternary mixtures. The structure of the Petlyuk system offers some control challenges arising from the transfer of vapor streams back and forth between the columns. This means that neither of the columns can have a uniform lower nor higher pressure. Recently, some alternate Petlyuk-like schemes that might provide better operational properties than the Petlyuk column have been proposed. In this work, we analyze the control properties of six alternative thermally coupled distillation schemes to the Petlyuk system. The theoretical control properties are analyzed with the application of the singular value decomposition technique. Rigorous closed loop simulations are used to supplement the theoretical analysis. The results indicate that a reduction in the number of interconnections of the Petlyuk configuration does not necessarily provide an improvement of its controllability properties.

DESIGN STUDY OF THE CONTROL OF A REACTIVE THERMALLY COUPLED DISTILLATION SEQUENCE FOR THE ESTERIFICATION OF FATTY ORGANIC ACIDS

This article reports the esterification of lauric acid and methanol studied using a thermally coupled distillation sequence with a side rectifier and the Petlyuk distillation column. The product of the esterification can be used as biodiesel. It was found that the thermally coupled distillation sequence with a side rectifier can produce ester with a high purity (around 0.999) and also pure water, and the excess of methanol is recovered in the side rectifier. The results indicate that the energy requirement of the complex distillation sequence with a side rectifier can be reduced significantly by varying operational conditions. These reductions in energy requirements can be interpreted as reductions in carbon dioxide emissions. Moreover, dynamic tests for control of the composition of the ester and control of two temperatures for the thermally coupled distillation sequence with a side rectifier indicate that it is possible to eliminate disturbances in the feed composition, while the composition of the biodiesel remains at the desired value.

Simulation study on biodiesel production by reactive distillation with methanol at high pressure and temperature: Impact on costs and pollutant emissions

a b s t r a c t Recently, a two-step biodiesel production process which uses short-chain alcohols at supercritical con- ditions has been proposed. In addition, literature reports suggest that the COSMO-SAC thermodynamic model is a suitable alternative for the prediction of VLE for supercritical methanol/methyl esters mix- tures. Thus, in this work a simulation study of the two-step supercritical method for the production of biodiesel is performed by using the COSMO-SAC model. Further, alternative system configurations for biodiesel production based on reactive distillation are proposed and their total emissions are compared to those corresponding to the conventional catalytic method. The study demonstrates the benefits of using reactive distillation for the esterification step and discusses the environmental impact of the supercritical production process. It has been found that the intensified alternatives reduce the emissions considerably and, through the reuse of the excess methanol, the emissions level of the supercritical process can be compared to those of the catalytic method.

Dynamic analysis of thermally coupled distillation sequences with undirectional flows for the separation of ternary mixtures

The Petlyuk distillation system has been considered with special interest because of the high energy savings it can provide with respect to the operation of sequences based on conventional columns. The original design of the Petlyuk structure, however, shows two interconnections that seem to affect its operational and controllability properties. To overcome this problem, two alternate structures have been suggested that use unidirectional flows of the vapor or liquid interconnecting streams. In this work, a comparative analysis of the control properties of the Petlyuk column and the alternate arrangements with unidirectional interconnecting flows is presented. Through a singular value decomposition analysis, it is shown that the alternate schemes provide better theoretical controllability properties than the Petlyuk system. Closed loop tests using proportional-integral controllers were also carried out, and the results showed that, in most of the cases considered, the alternate arrangements improved the dynamic responses of the Petlyuk column. Such arrangements, therefore, show promising perspectives for its practical consideration.

The importance of the sequential synthesis methodology in the optimal distillation sequences design

Abstract The sequential design method is presented as a complementary tool of the systematical synthesis procedure that allows to define a clear connection among the different types of distillation column sequences. In particular, the connection with the simple column subspace is considered, since this subspace represents the comparison reference for all the alternatives considered. The sequential design procedure, based on the correspondence between the functionality of the columns section among the simple columns and the derived sequences, is compared with a mathematical based optimization algorithm. The separations of a four-component near ideal mixture and the azeotropic ethanol–water mixture are considered as case studies and the designs obtained applying both methods have been compared. The results confirmed that the sequential design method is a fast and reliable tool in the optimal design of the column sequence.

Synthesis and design of new hybrid configurations for biobutanol purification

Abstract The development of new technologies for biobutanol production by fermentation has resulted in higher butanol concentrations, less by-products and higher volumetric productivities during fermentation. These new technology developments have the potential to provide a production process that is economically viable in comparison to the petrochemical pathway for butanol production. New alternative hybrid configurations based on liquid–liquid extraction and distillation for the biobutanol purification were presented. The alternatives are designed and optimized minimizing two objective functions: the total annual cost (TAC) as an economical index and the eco-indicator 99 as an environmental function. All the new configurations presented reduced the TAC compared to the traditional hybrid configuration, in particular a thermally coupled alternative exhibited a 24.5% reduction of the TAC together with a 11.8% reduction of the environmental indicator. Also intensified sequences represented a promising option in the reduction of the TAC but with some penalty in the eco-indicator.

Simulation and optimization of a biojet fuel production process

Abstract Aviation sector contributes with 2% of the total CO 2 emissions due to human activities. Moreover, predictions estimate that air traffic will duplicate in the next 20 years, with the corresponding increasing in CO 2 emissions. The International Air Transport Association (IATA) has established four strategies to reduce CO 2 emissions; one strategy is the development of aviation fuel from renewable feedstocks, known as biojet fuel. In 2009 UOP Honeywell received a patent for its process to produce aviation fuel from renewable feedstocks. The process considers the transformation of vegetable oil through hydrogenating, deoxygenating, isomerizing and selective hydrocracking to generate propane and hydrocarbons fuels. The resulting aviation fuel is very similar to the fossil one, with the only difference that the first one does not contain aromatic compounds. Due to this, the ASTM standard established the use of biojet fuel in mixtures with fossil jet fuel with up to 50% of the bio-fuel. Also, it is important to remark that in this moment the process of UOP Honeywell is the only one certified for the production of aviation fuel from renewable feedstocks. In this work we propose a model for the production of biojet fuel, obtaining an estimation of the conversion of the reactions of the process of UOP Honeywell. Also, the optimization of the purification stage is performed using a multiobjective genetic algorithm with constraints, which is coupled to Aspen Plus process simulator, in order to generate results considering the complete models of the process. Results show a high conversion of the vegetable oil (castor oil) to biofuels (biojet fuel and green diesel); also, energy can be generated in the process as result of the conditioning of the stream that is fed to the distillation train.

Optimization of a reactive distillation process with intermediate condensers for silane production

Abstract This work presents a reactive distillation column for the catalytic disproportionation of trichlorosilane to silane which includes three consecutive reversible reactions. This reaction system is however characterized by a large distinction in the boiling points of the components, which make the reactive distillation extremely favored. Nevertheless, the normal reactive distillation column possesses the shortage of high refrigeration requirement. By removing heat at temperature higher than that at the condenser a superstructure representation, rigorous simulations, and optimization problems were combined to derive optimal reactive distillation columns which can realize heat integration between stages and utilities at several refrigeration conditions. An iterative simulation-optimization procedure was proposed to consider temperature changes in stages due to heat integration. The results showed that the installation of two inter-condensers results in the best option with economic savings up to 56%.