Novel process for biodiesel by reactive absorption

Process integration and intensification is increasingly applied also in the biodiesel production, as shown by recent research papers. This study takes previous work on reactive separation processes for biodiesel production to a new level by proposing an innovative technology based on reactive absorption using solid acid catalysts. This is a major step forward since reactive absorption offers significant benefits over reactive distillation, such as: reduced capital investment and operating costs due to the absence of the reboiler and condenser, higher conversion and selectivity as no products are recycled in the form of reflux or boil-up vapors, as well as no occurrence of thermal degradation of the products due to a lower temperature profile in the column. Rigorous simulations embedding experimental results were performed in AspenTech AspenONE engineering suite to design this novel reactive absorption process and evaluate the technical and economical feasibility. The main results are given for a plant producing 10 ktpy biodiesel from waste vegetable oil with high free fatty acids content, using solid acids as green catalysts. This innovative process eliminates all conventional catalyst-related operations, and efficiently uses the raw materials and the reactor volume in an integrated setup that allows significant savings in CapEx and OpEx of the plant.     

Feasibility study of a thermally coupled reactive distillation process for biodiesel production

Biodiesel fuel represents an interesting alternative as a clean and renewable substitute of fossil fuels. A typical biodiesel production process involves the use of a catalyst, which implies high energy consumptions for the separation of the catalyst and the by-products of the reaction, including those of undesirable side reactions (such as the saponification reaction). A recently proposed process involves the use of short-chain alcohols at supercritical conditions, avoiding the use of a catalyst and the occurrence of the saponification reaction. This process requires fewer pieces of equipment than the conventional one, but its high energy requirements and the need of special materials that support the reaction conditions makes the main product, biodiesel fuel, more expensive than petroleum diesel. In this work, a modification of the supercritical process for the production of biodiesel fuel is proposed. Two alternatives are proposed. The process involves the use of either reactive distillation or thermally coupled reactive distillation. Simulations have been carried out by using the Aspen One™ process simulator to demonstrate the feasibility of such alternatives to produce biodiesel with methanol at high pressure conditions. A design method for the thermally coupled system is also proposed. Both systems have been tested and the results indicate favorable energy performance when compared to the original scheme. Furthermore, the thermally coupled system shows lower energy consumptions than the reactive distillation column.     

An integrated reactive distillation process for biodiesel production

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.     

Esterification of fatty acids in a thermally coupled reactive distillation column by the two-step supercritical methanol method

Biodiesel fuel has been shown as a clean energy alternative to petroleum diesel. Conventional biodiesel production involves the use of catalyst, which implies high energy consumptions for the separation of both the catalyst and the by-products of the reaction, including those of the undesirable reaction of saponification. Recently, a process involving the use of short-chain alcohols at supercritical conditions has been proposed (Saka-Dadan process); one of the main advantages of that process is that it avoids the need for a catalyst as well as the occurrence of the saponification reaction. However, although the process requires less pieces of equipment than the conventional one, its energy requirements are still high, making biodiesel fuel more expensive than petroleum diesel. This work proposes the use of reactive distillation and thermally coupled reactive distillation configurations to produce biodiesel fuel by the supercritical methanol method. First-order kinetics is used to represent the esterification reaction, obtaining high conversions in a single shell. Both of the configurations proposed reduce energy requirements when compared to the conventional (Saka-Dadan) process. Calculations were also performed to estimate CO₂ emissions, thermodynamic efficiency and cost. The thermally coupled reactive distillation configuration shows to be the best alternative in terms of energy consumption, CO₂ emissions and thermodynamic efficiency. Further, cost estimations also show that the use of a thermally coupled scheme considerably reduces both utilities and capital costs.     

Dynamics and control of a biodiesel process by reactive absorption

Integrated biodiesel processes based on reactive separations powered by solid acid/base catalysts are available nowadays, offering significant advantages such as minimal capital investment and operating costs, as well as no catalyst-related waste streams and no soap formation. However, the controllability of the process is just as important as the capital and operating savings. In such processes the small number of degrees of freedom is a drawback which makes it difficult to correctly set the ratio of reactant feeds and consequently to avoid impurities in the products. This work considers the process control of biodiesel production by reactive absorption, the main result being an efficient control structure that ensures the excess of methanol that is necessary for the total conversion of the fatty acids and for prevention of the difficult separations, while maintaining high purity of the water by-product. Rigorous simulations were performed - using Aspen Plus and Aspen Plus Dynamics as efficient computer-aided process engineering tools - for a plant producing 10 ktpy biodiesel from waste vegetable oil with high free fatty acids content, using solid acids as green catalysts. This reactive absorption process eliminates all conventional catalyst-related operations, and efficiently uses the raw materials and the reactor volume in an integrated setup that is well controllable in spite of the reduced number of degrees of freedom.     

A generalized method for the synthesis and design of reactive distillation columns

Owing to the combination between the reaction operation and the separation operation involved, it is extremely difficult to determine in advance the optimum configuration of a reactive distillation column and this makes process synthesis and design a great challenging task. Currently, no easy-to-use and yet effective methods are available to guide process synthesis and design, restricting considerably the applications and therefore the impacts of reactive distillation columns to the chemical process industry. In this paper, a generalized method is proposed for the synthesis and design of reactive distillation columns in terms of the insights from process intensification. The method is initiated from a simple process design with all feeds of reactants at the middle of the process and all stages as reactive ones. In terms of an economical objective function, it can be evolved into the optimum process design via sequential structure adjustments, including reactive section arrangement, feed stage relocation, feed splitting, and catalyst redistribution. The generalized method proposed is characterized by great simplicity in principle, the capability to tap the full potentials of process intensification, and the high robustness to the initial guess of process configuration as well as the thermodynamic properties of the reacting mixtures separated. Four example systems are employed to evaluate the generalized method proposed and the obtained outcomes demonstrate its effectiveness and applicability to the synthesis and design of various reactive distillation columns.     

Transesterification of palm oil with methanol in a reactive distillation column

The higher feedstock and processing costs for biodiesel production can be reduced by applying reactive distillation (RD) in transesterification process. The effects of reboiler temperature, amount of KOH catalyst, methanol to oil molar ratio and residence time on the methyl ester purity were determined by using a simple laboratory-scale RD packed column. The results indicated that from the empty column, the system reached the steady state in 8 h. Too high reboiler temperature and the amount of catalyst introduce more soap from saponification in the process. The optimal operating condition is at a reboiler temperature 90 °C, a methanol to oil molar ratio of 4.5:1.0, KOH of 1 wt.% respect to oil and 5 min of residence time in the column. This condition requires the fresh feed methanol 25% lower than in the conventional process and produces 92.27% methyl ester purity. Therefore this RD column can be applied in small or medium biodiesel enterprise.     

Optimization and control of a reactive distillation process for the synthesis of dimethyl carbonate

Dimethyl carbonate is an environmentally benign and biodegradable chemical. Based on integration of reactive distillation and pressure-swing distillation technologies, a novel process for synthesis of dimethyl carbonate through transesterification with propylene carbonate and methanol has been developed by Huang et al. In this work, the optimization of this process was performed by minimizing the total TAC. The results show that the op-timal design flowsheet can save energy consumption by 18.6％with the propylene carbonate conversion of 99.9％. Then, an effective plant-wide control structure for the process was developed. Dynamic simulation results dem-onstrate that the temperature/flow rate cascade control plus with simple temperature control can keep not only product purity but also the conversion of the reactant at their desired values in the face of the disturbance in re-actant feed flow rate and feed composition.     

Comparison between a conventional process and reactive distillation for naphtha hydrodesulfurization

Reactive distillation is a useful operation in which reactions and separation take place in the same unit. In this paper, the feasibility of using reactive distillation for hydrodesulfurization of naphtha is explored by simulation and experimentation. It was found that the process can be attractive since initial and operation costs are reduced substantially while the specification of the products can be met.     

Design of entrainer-enhanced reactive distillation for the synthesis of butyl cellosolve acetate

An innovative entrainer-enhanced reactive distillation (RD) process is presented, which aims to the production of high-purity butyl cellosolve acetate from butyl cellosolve and acetic acid via an esterification reaction. This entrainer-enhanced RD process can procure technical advantages from both heterogeneous azeotropic distillation and RD. Solvents such as cyclohexane, ethylene dichloride, toluene, and octane are considered as candidates in this esterification RD process. The function of entrainers is to simplify the separation between water and acetic acid. For this purpose, the proper entrainer to use is thus evaluated based on its mutual solubility with water in two liquid phases. Simulation results reveal that total annual cost can be substantially reduced when cyclohexane, toluene, and octane are used as entrainers in the RD column. The octane-enhanced RD provides the most economical design in this studied case.     

A reactive distillation process with a sidedraw stream to enhance the production of isopropyl acetate

This paper designs an entrainer combined with a sidedraw to enhance the reactive distillation (RD) process of isopropyl acetate (IPAc). Acetic acid (HAc) reacts with isopropanol (IPOH) to generate IPAc and water (H₂O). The ratio of IPAc to H₂O in the products of esterification is smaller than that in the minimum boiling IPAc–IPOH–H₂O azeotrope, resulting in a mass of organic phase reflux to remove the surplus H₂O from the top of the RD column. This process consumes a high amount of energy. For better energy efficiency, a feasible design flowsheet includes an RD column, a stripper, a top decanter, a middle decanter, and a sidedraw stream to intensify the azeotropic separation where an entrainer is introduced to carry out the surplus water from the middle of the RD column in the form of a liquid phase. The key design variables in the proposed flowsheet are determined to obtain a minimal total annual cost (TAC). As a result, an optimal process design is drawn out while satisfying the stringent specifications for product purity. These results show that the energy requirements of the IPAc system can be decreased by 27.55%.     

A fundamental principle and systematic procedures for process intensification in reactive distillation columns

A fundamental principle is developed for process intensification through internal mass and energy integration in reactive distillation columns and three systematic procedures are devised for process synthesis and design. For reactive distillation columns involving reactions with highly thermal effect, process intensification can be achieved with an exclusive consideration of internal energy integration between the reaction operation and separation operation involved. However, in the case of a highly endothermic reaction with an extremely low reaction rate and/or small chemical equilibrium constant, internal mass integration has also to be considered between the reactive section and stripping section. For reactive distillation columns involving reactions with negligibly or no thermal effect, process intensification can be performed with an exclusive consideration of internal mass integration. For reactive distillation columns involving reactions with moderately thermal effect, process intensification must be conducted with a careful trade-off between internal mass and energy integration. Five hypothetical and two real reactive distillation systems are employed to evaluate the principle and procedures proposed. It is demonstrated that intensifying internal mass and energy integration is really effective for process intensification. Not only can the thermodynamic efficiency be improved substantially, but also the capital investment can be further reduced.     

Determination of catalytic packing characteristics for reactive distillation

Catalytic distillation still expands its field of applications. New structured catalytic column internals have been developed in recent years and new studies have been reported. A modern structured catalytic packing MULTIPAK^® is a subject of the investigations presented in this paper.

Important parameters of MULTIPAK^® have been examined experimentally in a 250 mm ID laboratory column: pressure drop for dry, prewetted and irrigated packings, flooding line and mass transfer coefficients for the gas and liquid phases.

The correlations obtained have been incorporated into a software tool for the simulation of catalytic distillation processes and simulations have been performed assuming methyl acetate synthesis as a model process. The calculations have been verified using the experiments performed for the same synthesis in a 50 mm ID catalytic distillation column operating continuously and thus reflecting industrial applications. It is concluded that the model represents the real process with satisfactory accuracy, although some deviations can be observed, especially within the reactive zone.     

Optimization and control of a reactive and extractive distillation process for the synthesis of isopropyl acetate

A conventional reactive distillation column will not be able to produce high purity isopropyl acetate (IPAc) due to the existence of a minimum boiling azeotrope in the system. In this work, a novel reactive and extractive distillation (RED) process was proposed and used for the synthesis of IPAc. Results showed that the purity of IPAc reached 99.5%. Then, the RED flowsheet was optimized with minimum total annual cost (TAC), and a number of key variables were determined with the assistance of program written in Visual Basic 6.0 (VB). After that, two control structures of the RED process were developed: a basic control structure with temperature/proportional cascade control and an improved control structure with composition/temperature cascade control. The integral of squared error (ISE) was introduced to evaluate the performance of control systems, it revealed that the improved control structure had better controllability.     

反应精馏技术的研究进展

针对国内外反应精馏技术及其应用的最新研究情况,较全面地归纳和分析了反应精馏工艺的研究进展,并介绍了SYNTHESISER、ASPEN PLUS等计算机模拟软件在反应精馏工艺的应用情况。     

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

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%.     

脂肪酸乙酯合成工艺研究

研究了大豆油和乙醇进行酯交换反应合成脂肪酸乙酯的工艺。考察了催化剂类型、催化剂用量、n(醇)∶n(油)、反应温度以及反应时间对酯交换率的影响,结果表明最佳合成工艺条件为反应温度60℃,n(醇)∶n(油)=5∶1,w(NaOMe)=1.0%,反应时间4h,在此条件下,酯交换率可达到96.7%。     

一种通用的反应蒸馏塔综合与设计方法

反应精馏是反应过程和分离过程耦合为一体的单元操作,已成为当今研究的重要领域。然而,到目前为止并没有一套通用简便的方法去指导反应精馏过程的综合与设计,严重限制了它的广泛应用。本文在过程强化原理的基础上提出了一种反应蒸馏塔通用的综合与设计方法,并利用2种反应蒸馏系统来评价所提出的设计策略。结果表明,该综合设计方法可以简便高效地搜索出反应蒸馏塔的最优结构,适用于不同类型的反应蒸馏塔的综合与设计。     

A reactive distillation column with double reactive sections for the separations of two-stage consecutive reversible reactions

In this paper, a novel reactive distillation column with double reactive sections (RDC-DRS) is proposed for the separations of two-stage consecutive reversible reactions. The arrangement of two reactive sections not only allows the careful coordination of the two reaction operations involved, but also provides additional degrees of freedom for the reinforcement of internal mass integration and/or internal energy integration between the reaction operations and the separation operation involved, which could facilitate the RDC-DRS to be more advantageous than the conventional reactive distillation column with a single reactive section (RDC-SRS) in operating cost and capital investment. A representative hypothetical and real two-stage consecutive reversible reactions are chosen to evaluate the steady-state performance of the RDC-DRS. With the constraints of the same total number of stages and the same total number of reactive stages, the RDC-DRS is demonstrated to require less utility consumption than the RDC-SRS and this outcome indicates that the former could be a competitive alternative to the latter in the separations of the two-stage consecutive reversible reactions. The number of reactive sections should therefore be viewed as an important decision variable for the synthesis and design of reactive distillation columns especially in the separations of complicated reacting mixtures involving multiple reversible reactions.     

Balancing design and control of an olefin metathesis reactive distillation column through reactive section distribution

In this contribution, balance of design and control is investigated through reactive section distribution in an olefin metathesis reactive distillation column. Four designs with different strategies for reactive section distribution are studied and these include one with all stages as reactive ones (design-I), one with the distribution of reactive section according to the principle of internal mass integration (design-II), and ones with the extension of the reactive section of the design-II by five stages to either the stripping section (design-III) or the rectifying and stripping sections simultaneously (design-IV). The design-II appears to be more thermodynamically efficient than the design-I but with considerable degradation in process controllability. The design-III and design-IV retain most of the economical advantages of the design-II with only slight deterioration in controllability as compared with the design-I. This outcome demonstrates the fact that the strategy for reactive section distribution could be an effective decision variable for the balance of design and control in the development of reactive distillation columns.