In this PhD thesis, seawater desalination by Membrane Distillation (MD) has been explored from the perspective of process and membrane. Regarding the process, an innovative, energy efficient, and environmentally friendly Zero Thermal Input Membrane Distillation (ZTIMD) process was proposed. ZTIMD uses thermal energy stored in seawater, which makes the process sustainable by being independent of the external sources of thermal energy, which is one of the major contributors to the cost and energy consumption of conventional MD desalination processes. Economic feasibility study was carried out for the ZTIMD process, and it was demonstrated that drinking water could be produced with a cost of $0.28/m3, which is approximately half of the cost of conventional desalination processes. Regarding the membrane, novel MD membranes were developed through incorporation of nanomaterials in polyvinylidene fluoride (PVDF). Different nanomaterials including superhydrophobic SiO2, amine modified hydrophilic SiO2, CuO, and CaCO3 were used for this purpose. It was shown that membrane structure and consequently its performance could be affected by the nanoparticle properties, concentration, presence of backing material, PVDF blend ratio, and penetration time. In a best membrane developed in this work, almost 2500% increase was observed in the Vacuum Membrane Distillation (VMD) flux over that of the neat PVDF membrane at a feed temperature of 27.5 °C and vacuum pressure of 1.2 kPa, when 7.0 wt.% hydrophilic SiO2 nanoparticles were added into a PVDF membrane supported with Non-Woven Fabric (NWF) polyester. The membrane possessed near perfect selectivity.

Heat-integrated crude oil distillation systems – the atmospheric and vacuum distillation towers and associated heat recovery system – are energy and capital intensive. The structures of the distillation columns are very complex and the distillation columns interact strongly with the preheat train. There are many degrees of freedom in this system, most of which are interlinked with each other and cannot be considered separately. A systematic design approach is necessary to exploit these design issues for increasing the efficiency with which energy and capital are employed in the overall system. This thesis develops an optimisation-based methodology for the simultaneous design of crude oil distillation systems. Both new design and retrofit scenarios are considered. This design approach considers some significant design issues and generates design solutions that are realisable and industrially practicable. Robust and more accurate models have been developed to represent the distillation columns and heat exchanger networks (HENs) within an optimisation framework, compared with previous work. Facilitated by the decomposition approach (Liebmann, 1996), simplified models (Suphanit, 1999; Gadalla et al., 2003a; Rastogi, 2006), based on the Fenske-Underwood-Gilliland method, were developed previously to model the atmospheric distillation unit and the vacuum unit. This work extends and modifies these simplified models to account more accurately for the effect of pump-around location on the separation performance in atmospheric units. Moreover, the simplified model has been extended to consider an atmospheric distillation column with a pump-around located above the top side-stripper. This work also proposes a new methodology to incorporate product specifications following refining conventional in the simplified models. The proposed approach enables systematic identification of key components and associated recoveries to match specified boiling temperature profiles, as these are normally used as indicators of separation performance in the refining industry. The new simplified models are validated by comparison with rigorous simulation results of atmospheric distillation columns. Multi-segmented stream data are implemented in the design and analysis of heat exchanger networks, in which the thermal properties of streams are temperature dependent and cannot be assumed constant. Two existing promising HEN design approaches, the simulated annealing optimisation-based approach (Rodriguez, 2005)and the network pinch approach (Asante and Zhu, 1996), are modified and extended to apply to the HEN design with multi-segmented stream data. In the modified network pinch approach, the bottleneck of an existing HEN configuration is better overcome by varying stream split fractions and heat exchanger loads at the same time, rather than simply redistributing heat loads. The modified network pinch approach also combines structural modifications and cost optimisation in a single step to avoid missing cost-effective design solutions. An optimisation framework, applying a stochastic optimisation method – multiple simulated annealing runs – is developed to generate grassroots and retrofit designs of the heat-integrated crude oil distillation systems. The heat integration of the system is accounted for more accurately than previously by using multi-segmented stream data. Operating conditions and pump-around locations of distillation columns are optimised, together with structural options and continuous variables of heat exchanger networks as appropriate, in a single optimisation framework. The new degrees of freedom considered in this work include key components and associated recoveries (used in simplified models of distillation columns to express the separation of products) and operating pressures of distillation columns. The optimisation of key components and recoveries allow the systematic exploitation of product distributions and product slate in order to maximise net profit. Including operating pressures in the optimisation facilitates creation of heat recovery opportunities in configuration studies. Product specification constraints are imposed in the optimisation so that product quality is not compromised during design. A novel distillation configuration, with a liquid side-draw prefractionator column upstream of an atmospheric distillation column, is proposed in this work. The case study shows a very promising performance with respect to energy efficiency. Case studies illustrate the beneficial application of the proposed approach in both grassroots and retrofit design of crude oil distillation systems, with respect to energy demand and net profit improvement. Comparisons are made between different configurations, and results are given as proof of principle.

This work addresses the challenges in design of heat integrated low-temperature separation processes. A novel, systematic and robust methodology is developed, which contributes to the design practice of heat-integrated separation sequence and the refrigeration system in the context of low-temperature separation processes. Moreover, the methodology exploits the interactions between the separation and refrigeration systems systematically in an integrated design context. The synthesis and optimisation of heat-integrated separation processes is complex due to the large number of design options. In this thesis, task representation is applied to the separation system to accommodate both simple and complex distillation columns. The stream conditioning processes are simulated and their associated costs are included in the overall cost of the process. Important design variables in separation systems, such as the separation sequence, type and operating conditions of the separation units (e.g. the operating pressure, feed quality and condenser type) are optimised. Various refrigeration provision strategies, such as expansion of a process stream, pure and mixed multistage refrigeration systems and cascades of multistage refrigeration cycles, are considered in the present work. A novel approach based on refrigeration system database is proposed, which overcomes the complexities and challenges of synthesis and optimisation of refrigeration systems in the context of low-temperature separation processes. The methodology optimises the key design variables in the refrigeration system, including the refrigerant composition, the number of compression stages, the refrigeration and rejection temperature levels, cascading strategy and the partition temperature in multistage cascaded refrigeration systems. The present approach has selected a matrix based approach for assessing the heat integration potentials of separation and refrigeration systems in the screening procedure. Non-isothermal streams are not considered isothermal and stream splitting and heat exchangers in series are taken into account. Moreover, heat integration of reboiler and condenser of a distillation column through an open loop heat pump system can be considered in this work. This work combines an enhanced simulated annealing algorithm with MILP optimisation method and develops a framework for simultaneously optimising different degrees of freedom in the heat integrated separation and refrigeration processes. Case studies extend the approach to the design of heat integrated separation sequences in above ambient temperature processes. The robustness of the developed framework is further demonstrated when it is utilised to design the LNG and ethylene plant fractionation trains.

A large number of distillation sequences can be generated to separate a multicomponent azeotropic mixture. However, there is no systematic and efficient method for synthesising promising sequences, which also consider recycle connections and flowrates. In this work, a systematic procedure is developed for synthesising economically promising distillation sequences separating multicomponent homogeneous azeotropic mixtures. The procedure uses spherically approximated distillation boundaries, a shortcut column design method, and allows recycle and sequence alternatives to be screened. Both feasibility and design are addressed. Approximation of a distillation boundary as a spherical surface is a simple nonlinear, yet more accurate representation of the actual boundary than a linear approximation. For shortcut column design, azeotropes are treated as pseudocomponents and the relative volatilities of all singular points of the system are characterised, based on the transformation of vapour-liquid equilibrium behaviour in terms of pure components into that in terms of singular points. Once the relative volatilities of singular points are obtained, the classical Fenske-Underwood-Gilliland method can be used to design columns separating azeotropic mixtures. This method is extremely computationally efficient and can be applied to homogeneous azeotropic mixtures with any number of components; the results are useful for initialising rigorous simulations using commercial software and for assessing feasibility of proposed splits. Together with the spherical approximation of distillation boundaries, this shortcut method provides a basis for evaluating distillation sequences with recycles. Analysis of feasibility requirements of splits, component recovery requirements and the effects of recycles on the performance of proposed splits allows rules and procedures for selecting recycles to be proposed. Recycles with compositions of either singular points or mixtures of singular points are identified that are beneficial to the feasibility of sequences and the recovery of components. The principles are applicable to azeotropic mixtures with any number of components; using these procedures, recycle structures can be generated and are much simpler than the superstructures of recycle alternatives. The sequence synthesis procedure of Thong and Jobson (2001c) allows all potentially feasible sequences to be generated. To screen among these sequences, a split feasibility test and a two-step screening procedure are proposed. In the first step, feasibility of splits is tested efficiently and sequences containing either infeasible or sloppy splits are eliminated. In the second step, sequences containing sloppy splits are generated, based on the evaluation of sequences containing only feasible sharp splits. Using this procedure, the number of distillation sequences identified using the procedure of Thong and Jobson (2001c) can be significantly reduced. A systematic methodology is proposed for the synthesis and evaluation of multicomponent homogeneous azeotropic distillation sequences. The methodology is computationally efficient. It is demonstrated through a case study, the synthesis of distillation sequences separating a five-component mixture, in which two homogeneous azeotropes are formed, and for which over 5000 sequences producing pure component products can be generated. Using this methodology, only ten sequences are evaluated to identify three promising sequences. The evaluation of each sequence using the shortcut column design method is extremely efficient compared with that using the boundary value method.

To extend the knowledge of complex distillation arrangements for the separation of ternary mixtures and make them closer to implementation in industry is the motivation of this thesis work. Some design, operation, and control aspects are analysed, having the conventional distillation arrangements as comparison basis. The DWC is a complex distillation arrangement very attractive in terms of energy and cost savings. However, the DWC require large columns in order to be economically advantageous. Therefore, this arrangement is more attractive when the energy cost is high compared to the trays cost. From rigorous simulations of different separation examples, some synthesis rules have been obtained.Designs that minimise the distillation cost have been given special importance. DWC design procedures reported in the literature were based on only two decision variables. A new procedure that uses three decision variables for design optimisation has been proposed. It has been seen that the use of three decision variables is important to avoid excluding possible optimum designs.The control of the DWC is more complex than the control of the other distillation arrangements because the DWC has more operation degrees of freedom. The approach in this work has been to solve the DWC control by levels: stabilisation, composition control, and optimising control. When the manipulated variables for DWC stabilisation control are the distillate and the bottoms flowrate, which is the most common stabilisation control structure in the literature, the DWC presents high directionality and interaction. Besides, this stabilisation control structure is not appropriate to the DWC because the DWC has typically high reflux ratios. When the manipulated variables for stabilisation control are the reflux flowrate and the boilup, the DWC has better controllability and larger stability margins. Linear analysis tools are useful to select the set of manipulated variables for the DWC composition control. The preferred set of manipulated variables and the controllability of the corresponding control structure depend on the nominal operation. The DWC has two extra operation degrees of freedom that permit an operation optimisation. This optimisation is used to minimise the boilup. At a non-optimal operation, the controllability is improved. Therefore, a trade-off appears between operation optimisation and controllability. Changes in the design of the DWC can be used to improve its controllability. High directionality is a problem associated to DWC that may be improved using a large number of trays. On the other hand, DWC optimal designs have an equilibrated distribution of distillation effort between sections which, if broken, may also present improve the controllability. As found for operation, also for design, leaving optimal conditions permits to improve the controllability. Alternatively, for the composition control of the DWC, Dynamic Matrix Control is analysed. It is found that DMC performs worse than the diagonal feedback control strategy. The DWC extra operation degrees of freedom can be used dynamically for optimising control. Due to a marked directionality found in the response surface, DWC operation may be kept close to optimal operation using only one of the operation degrees of freedom. Optimising control of the DWC through the feedback control of a variable that characterises the optimal operation is possible. Some measurable variables are able to maintain optimal conditions with certain accuracy.The controllability of the DWC is compared to the controllability of other distillation arrangements. In general, the complexity of a distillation arrangement makes its energy efficiency better and its controllability worse. However, important exceptions have been found: - The DWC may give important energy savings as well as the best controllability if it is operated at non-optimal conditions. - When the manipulated variables for stabilisation are internal flows (reflux rates and boilups), the controllability of the DWC is better than the controllability of the other distillation arrangements. The DWC controllability is found to be more interesting for long columns. Therefore, the DWC needs long columns to be really attractive in terms of energy as well as in terms of controllability. In the literature, DWC controllability was misevaluate. In this thesis work, it is seen that, thanks to the arrangement complexity, the DWC may give at the same time energy savings and a good controllability, what makes it very attractive. To know the influence of the non-modelled detail and to obtain experimental results for the DWC is the most important step in the way to DWC implementation in industry.

No description available.

Reproduction de : Thèse de doctorat : Génie des procédés et de l'environnement : Toulouse, INPT : 2006. / Titre provenant de l'écran-titre. Bibliogr. 127 réf.

Reproduction de : Thèse de doctorat : Genie des procédés et de l'environnement : Toulouse, INPT : 2006. Reproduction de : Thèse de doctorat : Genie des procédés et de l'environnement : Budapest, Université des Sciences techniques et économiques : 2006. / Thèse soutenue en co-tutelle. Titre provenant de l'écran-titre. Bibliogr. 110 réf.

No description available.

In this thesis, an alternative control method based on Fuzzy Inference System (FIS) is proposed to keep the product composition of a distillation column constant. This study compares a proposed FIS to traditional PID (proportional, integral, derivative) control technique and analyzes the results. The FIS is applied to the control of the tray temperature of the distillation column by using indirect feed split control structure to modulate the steam flow with management of the tray temperature. In turn, this modulation maintains the composition of product at specified levels. Rockwell fuzzy designer is used to develop the fuzzy logic controller. Both a fuzzy logic controller and a PID controller are downloaded in the Rockwell ControlLogix L62 process controller. Chemstations Chemcad simulation software is used to run the distillation column simulations. Simulation results are programmed into L62 process controller to behave as a dynamic distillation column. Results show that the proposed fuzzy logic controller is more tolerant to disturbances in the feed flow and feed composition of the distillation column than the PID controller.