Optimisation of complex distillation colomn systems using rigorous models

Since distillation is still the most widely used separation technique used in the petrochemical industry, optimisation of these unit operations are important to minimise costs and maximise production. This thesis focuses on the development of a tool using rigorous non-equilibrium distillation models to optimise complex columns. Non-equilibrium distillation models are usually avoided in optimisation studies due to the time required to solve them, but this has been overcome by using a technique called orthogonal collocation in which the profiles in the columns are represented by polynomials of a lower order than would be required normally. This significantly reduces the process times and makes the use of non-equilibrium models a possibility in optimisation studies. The orthogonal collocation technique was applied to a packed distillation column model and shown to be effective in modelling the system. A system consisting of a distillation column with integrated external side reactors was chosen as a case study to investigate the use of the methods. These systems have been shown to be effective in certain circumstances in literature, when comparing them to other forms of process intensification, such as reactive distillation. The toluene disproportionation reaction was considered as a potential use for the technology and the optimisation tool was used to find optimum system configurations for achieving maximum toluene conversions and minimum costs. Nonlinear programming techniques were used initially to optimise these systems, but due to the discontinuities associated with multiple side streams, they were replaced by a genetic algorithm. Various system configurations were identified as achieving maximum conversions and minimum costs. These results were used in a comparison with results obtained from a literature study and the results showed significant promise. Unfortunately, the two studies did not have enough in common to truly produce a comprehensive result. This iv lead to further comparisons with another system using the same information. The results obtained in the toluene disproportionation case study showed that there was some possible benefits for using the side reactor systems, but the conventional system was still 30 and 60% cheaper in terms of capital and utility costs respectively. Another case study was investigated that looked at the synthesis of methyl acetate from acetic acid and methanol. The packed collocation model was used as a comparison with another investigation performed in literature (using equilibrium distillation models). Both showed comparable results, but still had significant differences. Costs were also compared between the side reactor system and a more conventional system for methyl acetate synthesis. The side reactor systems were found to be more cost effective than the conventional system. Additionally, an increase in the number of external reactors resulted in lower utility costs (mainly as a result of lower flow rates in the side streams). Overall, the reaction and process conditions are important considerations when deciding whether or not to use a side reactor system. For the gas phase toluene disproportionation reaction, the side reactor systems were not cost effective, when compared to the conventional system. However, the liquid phase methyl acetate reaction proved to be more conducive to side reactor systems in terms of cost. This thesis has shown the applicability of using rigorous disequilibrium distillation models in optimisation studies. The side reactor systems have been found to be complex systems that require a holistic approach to find optimum configurations instead of optimising individual process units

Reduced order modeling of distillation systems

The concept of distillation separation feasibility is investigated using reduced-order models. Three different models of nonequilibrium rate-based packed distillation columns are developed, each with progressive levels of complexity. The final model is the most complex, and is based on the Maxwell-Stefan theory of mass transfer. The first and second models are used as building blocks in the approach to the final model, as various simplifying assumptions are systematically relaxed. The models are all developed using orthogonal collocation. The order reduction properties of collocation are well documented. A low order model is desirable as the subsequent generation of data required for assessing the separation feasibility is fast. The first model is the simplest as constant molar overflow is assumed. This assumption is relaxed in the subsequent models. The second and third models differ in their respective mass and energy transfer. The second model uses a constant bulk phase approximation for an overall gas phase transfer coefficient. The third model uses rigorous Maxwell-Stefan mass transfer coefficients, which vary throughout the column. In all models, the bootstrap equation for the energy balance across the two-phase film is used after the appropriate modifications are made based on the system assumptions. Starting point solutions and minimum height and flows analysis are presented for all models. The first model is used to develop an azeotropic methodology for identifying and characterizing pinches. Different numerical techniques are also compared, and the accuracy of orthogonal collocation is verified. Ternary and pseudo McCabe-Thiele diagrams are used to represent the result$ for the multicomponent models 2 and 3. The results for models 2 and 3 are similar. This is expected as they differ only in the mass and heat transfer definitions. An argument is made for a specific definition of an objective function for models 2 and 3, which is subsequently used to generate separation surfaces. This function is defined such that there will always be a solution and for this reason is deemed superior to any alternatives. Feasible regions are identified using a grid projection of the relevant sections of the separation surfaces. The data set contained within the feasible region will be used in an optimizer in future work. In general, this work involves an understanding and application of the collocation mathematics to distillation systems. A further understanding of distillation systems, the associated mathematics and degrees of freedom is essential. A large section of this work is devoted to explaining and manipulating the available degrees of freedom, such that the desired end result of a feasible region for a specific separation can be obtained. Other complicating factors include the use of the collocation boundary conditions, and the relationship between these and the overall degrees of freedom for the system. In the literature, collocation is largely applied to staged columns. The resulting feed stage discontinuities are smoothed out using various interpolation routines. Both of these approaches are incorrect. It is shown that the use of collocation in staged columns is fundamentally flawed due to the underlying theory of staged distillation and the implications of collocation assumptions. Further, the feed discontinuities present in all the results are intrinsic features of the system and should be preserved. It is further concluded that Models 2 and 3 were correct in comparison with each other. Finally it was shown that the separation feasibility was successfully determined using the optimal objective function. This success was based on the accuracy and order reduction achieved through the use of collocation. Further work will involve optimizing the data found in the feasible region using Non-Linear Programming

Methanol synthesis from CO2 and H2 by reactive distillation

The widely use of fossil fuel generates large amount of CO2, which causes greenhouse effect. Since fossil fuel is nonrenewable, a potential energy crisis is no doubt coming. Thus, the reuse of CO2 has generated a lot of attention. Among them, CO2 hydrogenation is one of the most attractive ways to transform CO2 into hydrocarbons, which can be used as an alternative for conventional fossil fuel. Methanol is one of promising candidate, which has various applications in transport, chemical industry, pharmaceutical industry, and other fields. Recent years has seen a rapid growth in methanol production. The conventional methanol process uses fossil fuel, such as coal and natural gas, as the raw material. Due to the similarity to conventional methanol process, there are lot of researches in CO2 hydrogenation to methanol. There is already one industrial CO2 to methanol plant operating in Iceland. Most CO2 to methanol studies consider gas–phase reaction, which has however relatively low conversion and severe conditions. In contrast, liquid phase CO2 hydrogenation to methanol has in principle milder conditions and higher conversion and requires less equipment when implemented in pilot or industrial scale. Moreover, reactive distillation process, which combines liquid phase CO2 hydrogenation and separation of products, can significantly improve conversion and overcome thermodynamic limitations. In the thesis, reactive distillation technology is used to enhance the CO2 hydrogenation. A simulation model is built to study the process using Aspen Plus. The process includes the main reactive distillation column and a downstream unit to purify methanol product and recover excess raw materials. Besides, a detailed solution is given to separate the azeotropic water–butanol solution, which is formed during the process. Moreover, the whole process is optimized to obtain the best operating parameters and suitable equipment variables. In conclusion, the one–way methanol conversion is much higher and required conditions are much lower than in conventional gas–phase methanol process. The whole process does not contain any complex circulating lines and most unreacted raw materials can be recycled

Optimal design and operation of reactive distillation systems based on a superstructure methodology

A novel methodology for the simultaneous optimisation of design and operation of a complex reactive distillation process, considering a number of process alternatives (e.g. pre-/side-reactor, side-stripper, additional columns etc.), is presented. The methodology is based on a superstructure approach, and a detailed cost-based objective function, solved by MINLP optimisation. The methodology is illustrated using different case studies of industrial interest with varying separation and reaction characteristics. For easy separations, in terms of relative volatilities and boiling points order, a single reactive distillation column is found to be optimal for both fast and slower kinetics. However, when the separation is more challenging (i.e. product is a middle-boiler), the design is more complex, even for fast kinetics, and additional processing units, such as a pre-reactor and/or additional distillation columns, are required to meet the product quality specifications. It is found that the design, i.e. the capital cost, mainly depends on the relative boiling point rankings. For operation, chemical reaction equilibrium is the dominant factor. It is demonstrated, however, that the combined effects of separation and reaction must be considered carefully when designing a reactive distillation process. The liquid holdup has an impact on the reaction performance, and proper choice of holdup can lead to a more flexible design, able to mitigate production failure issues even for slower reactions

Diseño y optimización de procesos de purificación de propileno mediante membranas de alto rendimiento PVDF-HFP/BMImBF4/AgBF4

RESUMEN: Resumen El propileno es el segundo producto químico con mayor volumen de producción a nivel mundial, principalmente debido a la producción de resinas de polipropileno. El impacto de este producto químico básico en la economía mundial ha estado creciendo en las últimas décadas y la notable importancia de su larga lista de productos derivados garantiza una continuación de esta tendencia. Sin embargo, la separación de mezclas gaseosas propano/propileno conlleva grandes desafíos desde el punto de vista de la sostenibilidad económica y medioambiental del proceso de producción de propileno y sus productos derivados. Estos retos surgen de los elevados requerimientos de energía y capital de los procesos de separación actuales, basados principalmente en destilación criogénica o a alta presión, y que están causados por la similitud entre las propiedades fisicoquímicas de ambas sustancias. Esta tesis tiene como objetivo la síntesis y el desarrollo de materiales de membrana innovadores y el análisis de su capacidad de separación cuando se implementan en procesos de membranas alternativos.ABSTRACT: Abstract Propylene is the second-largest-volume chemical produced globally, mainly driven by the production of polypropylene resins. The impact of this commodity chemical in the world’s economy has been growing in the last decades and the remarkable importance of its wide list of derivative products guarantees that this trend will continue. However, the separation of propane/propylene gaseous mixtures entails great challenges from the point of view of the economic and environmental sustainability of the manufacture processes of propylene and its derivative products. These challenges arise from the large energy and capital intensity of the current separation processes, mainly based on cryogenic and high pressure distillations, and caused by the similar physico-chemical properties of both substances. This thesis aims at the synthesis and development of innovative membrane materials and the assessment of their separation performance when implemented in alternative membrane-based processes.This research has been financially supported by the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund through the projects: CTQ2012-31639 (MINECOFEDER, UE), “New separation processes with kinetic control based on the use of functionalized materials”; CTQ2015-66078-R (MINECO-FEDER, UE), “Advanced separation applications. Modeling and experimental validation”; CTQ2016-75158-R (MINECO-FEDER, UE), “Composite selective membranes and their implementation in microfluidic devices”

Methodologies for the optimisation, control and consideration of uncertainty of reactive distillation

The work presented in this thesis is motivated by the current obstacles hindering the implementation of reactive distillation in industry, mainly related to the complexities of its design and control, as well as the impact of uncertainties thereupon. This work presents a rigorous methodology for the optimal design and control under uncertainty of reactive distillation. The methodology can also be used to identify and investigate mitigation strategies for process failures arising due to design and/or operation deficiencies under changed processing conditions, based on the evaluation of different design and/or control alternatives. The first step of the methodology is the simultaneous (MINLP) optimisation of the design and operation of a reactive distillation process superstructure, used to explore the possible steady-state design alternatives available, including ancillary equipment such as pre- and side-reactors, side-strippers and additional distillation columns, based on product-related constraints and a detailed objective cost function. The next step is the investigation of the dynamic control performance of this optimal system, where conventional and advanced process control strategies are considered in order to investigate how robust the system is towards operational disturbances, or whether revising the optimal steady-state design is required. As the optimisation depends heavily on accurate data for reaction kinetics and separation performance, the final step of the methodology is the evaluation of the impact of parameter uncertainty on the performance of the optimal controlled system, including redesigning the controlled system if required. The methodology is demonstrated using a number of industrially relevant case studies with different reaction and separation characteristics in order to investigate how these determine the design and control of an economically attractive and rigorous reactive distillation process. It is demonstrated that the process characteristics have a significant impact on the design of the system, and that auxiliary equipment may be required to meet production specifications and/or to ensure robust controlled behaviour. It is also shown that, under parameter uncertainty, an optimal controlled system may nevertheless face performance issues, and revising the design and/or operation of the process may be required in order to mitigate such situations