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.     

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

Recently, a two-step biodiesel production process which uses short-chain alcohols at supercritical conditions 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 mixtures. 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.     

液化天然气分离的节能蒸馏工艺

采用严格模拟计算的方法,对液化天然气分离过程的现有常规蒸馏工艺与热耦蒸馏工艺、侧线提馏工艺、双效蒸馏工艺和热集成工艺进行了模拟,并比较了其能量消耗。模拟计算结果显示,复杂蒸馏工艺都比常规蒸馏工艺减少了能耗和操作成本。其中,热耦蒸馏工艺可比常规蒸馏工艺节能21.4%,侧线提馏工艺节约13.3%,双效蒸馏工艺可节约34.7%,热集成工艺节约则达37.6%左右,表明新工艺在此分离过程中都有较好的应用前景,尤以多效蒸馏和热耦蒸馏最有优势。     

Revalorization of glycerol: Comestible oil from biodiesel synthesis

High dependence on fossil fuel has caused increase of carbon dioxide concentration in the atmosphere. The actual political trends are towards an increased use of renewable fuels from agricultural origin. One of the main products of the European biorefineries is biodiesel. The main reaction involved in biodiesel synthesis produces a large amount of glycerol as by-product. Two aspects are arising in this respect: the glycerol obtained as residue and the food conversion to fuel. This paper deals with the revalorization of the residual glycerol stream to obtain triacetin (glyceryl triacetate), the lightest comestible oil. The application of glycerol as raw material to produce triacetin is not new. The goal of this paper is to check the feasibility of this transformation in an efficient integrated continuous process which is suitable for processing high quantities of glycerol. A kinetic model was determined experimentally for the production of triacetin from glycerol and acetic acid in the absence of catalyst. The results showed that by process integration of the reaction and distillation in the same unit (reactive distillation), a more sustainable process can be developed. The proposed configuration output is checked by rigorous simulation.     

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.     

Simulation and optimization of the thermally coupled reactive distillation column for producing toluene diisocynate

Toluene diisocyanate is an important chemical intermediate prepared by phosgenation reaction with two steps named cold and thermal phosgenation, respectively. The thermal phosgenation reaction is a complex reactive distillation process with high energy consumption and multiple targets of reaction and separation, so it is of great significance to optimize the step. The reactive distillation model of the thermal phosgenation was established for an industrial installation by aspen plus, and the effects of the different parameters of temperatures, residence times, etc., were investigated firstly; then a thermally coupled reactive distillation column was presented to perform the thermal phosgenation process, and the optimal operation and configuration parameters were obtained by simulation. The results showed that the proposed process can save the heat and cold energy with 7.29% and 32.78%, respectively, and reduce the total annual cost by about 17.11%.     

A review of biodiesel production by integrated reactive separation technologies

Biodiesel is a biodegradable and renewable fuel, emerging as a viable alternative to petroleum diesel. Conventional biodiesel processes still suffer from problems associated with the use of homogeneous catalysts and the limitations imposed by the chemical reaction equilibrium, thus leading to severe economic and environmental penalties. This work provides a detailed review—illustrated with relevant examples—of novel reactive separation technologies used in biodiesel production: reactive distillation/absorption/extraction, and membrane reactors. Reactive separation offers new and exciting opportunities for manufacturing the fatty acid alkyl esters involved in the industrial production of biodiesel and specialty chemicals. The integration of reaction and separation into one operating unit overcomes equilibrium limitations and provides major benefits such as low capital investment and operating costs. These reactive separation processes can be further enhanced by heat‐integration and powered by heterogeneous catalysts, to eliminate all conventional catalyst related operations, using efficiently the raw materials and the reaction volume, while offering higher conversion and selectivity, as well as significant energy savings compared with conventional biodiesel processes. Remarkable, in spite of the high degree of integration, such integrated reactive‐separation processes are still very well controllable as illustrated by the included examples. Copyright © 2012 Society of Chemical Industry     

Conversion of waste cooking oil to biodiesel using ferric sulfate and supercritical methanol processes

In this comparative study, conversion of waste cooking oil to methyl esters was carried out using the ferric sulfate and the supercritical methanol processes. A two-step transesterification process was used to remove the high free fatty acid contents in the waste cooking oil (WCO). This process resulted in a feedstock to biodiesel conversion yield of about 85-96% using a ferric sulfate catalyst. In the supercritical methanol transesterification method, the yield of biodiesel was about 50-65% in only 15 min of reaction time. The test results revealed that supercritical process method is probably a promising alternative method to the traditional two-step transesterification process using a ferric sulfate catalyst for waste cooking oil conversion. The important variables affecting the methyl ester yield during the transesterification reaction are the molar ratio of alcohol to oil, the catalyst amount and the reaction temperature. The analysis of oil properties, fuel properties and process parameter optimization for the waste cooking oil conversion are also presented.     

Heat-integrated reactive distillation process for synthesis of fatty esters

Catalytic reactive distillation (RD) offers novel opportunities for manufacturing fatty acid alkyl esters involved in specialty chemicals and at a larger scale in biodiesel. The integration of reaction and separation into one RD unit, corroborated with the use of a heterogeneous catalyst, provides major benefits such as low capital investment and operating costs. This work presents a novel heat-integrated process based on reactive distillation that aims to reduce furthermore the energy requirements for biodiesel production, leading to competitive operating costs. Despite the high degree of integration, the process is well controllable using an efficient control structure proposed in this work. Rigorous simulations embedding experimental results were performed using computer aided process engineering tools, such as AspenTech Aspen Plus and Aspen Dynamics. The RD column was simulated using the rigorous RADFRAC unit with RateSep (rate-based) model, and explicitly considering three phase balances. Steady-state and dynamic simulation results are given for a plant producing 10 ktpy fatty acid methyl esters (FAME) from methanol and waste vegetable oil with high free fatty acids (FFA) content, using sulfated zirconia as green catalyst. The heat-integrated RD process eliminates all conventional catalyst related operations, efficiently uses the raw materials and the reactor volume offering complete conversion of the fatty acids and allowing significant energy savings. Remarkably, compared to previously reported RD processes, the energy requirements of this process are about 45% lower - only 108.8 kW h/ton biodiesel - while the capital investment cost remains the same as no additional equipment is required.     

Production of biodiesel with glycerol carbonate by non‐catalytic supercritical dimethyl carbonate

New biodiesel production processes comprising one‐step and two‐step supercritical dimethyl carbonate methods have been pioneered. The use of dimethyl carbonate allows the reaction conditions to be mild and thus avoid unwanted deterioration of substrates during reaction. In this process, without any catalyst applied, supercritical dimethyl carbonate converts triglycerides (rapeseed oil) into fatty acid methyl esters (FAME) along with glycerol carbonate as a value‐added by‐product, instead of glycerol. Free fatty acids could be also converted into FAME so that the total yield of biodiesel for both methods resulted in over 96 wt%. In addition, the produced FAME satisfy the fuel requirements for the international standards of biodiesel specification.     

C3选择性加氢能量耦合催化精馏结构与分析

提出了一种C₃选择性加氢能量耦合催化精馏新工艺,首先将催化精馏构件放置在丙烯精馏塔的提馏段,再将丙烯塔与脱乙烷塔通过气液流股连接成热耦合结构。与传统加氢工艺相比,能量耦合催化精馏工艺通过分离和加氢反应的结合使丙炔、丙二烯加氢过程的选择性得到较大幅度的提高,并通过热量耦合消除丙烯在脱乙烷塔内的返混,从而降低分离能耗。采用Aspen Plus化工流程模拟软件对该流程进行模拟。模拟结果表明,能量耦合催化精馏工艺可以使丙烯收率提高0.74%～2.19%,年度冷剂费用降低2.44%～3.61%。同时,热量耦合催化精馏工艺对于重质裂解原料油具有更好的适用性。     

The heterogeneous advantage: biodiesel by catalytic reactive distillation

The incentives for using biodiesel as renewable fuel and the difficulties associated with its production are outlined. The pros and cons of manufacturing biodiesel via fatty acid esterification using solid acid catalysts are investigated. Finding a suitable catalyst that is active, selective, and stable under the process conditions is the major challenge. The most promising candidates were found to be the sulfated metal oxides that can be used to develop a sustainable esterification process based on catalytic reactive distillation.     

Controllability analysis of alternate schemes to complex column arrangements with thermal coupling for the separation of ternary mixtures

Thermally coupled distillation systems (TCDS) have been proposed to perform distillation separation tasks with the incentive of achieving lower energy consumption levels with respect to conventional distillation sequences. In particular, the presence of recycle streams for TCDS schemes has influenced the notion that control problems might be expected during the operation of those systems with respect to the rather well-known behavior of conventional distillation sequences. That has been one of the main reasons for the lack of industrial implementation of thermally coupled distillation schemes. Recently, some alternatives to thermally coupled distillation arrangements that might provide better operational properties than the complex columns have been proposed. In this work, we analyze the control properties of two alternatives to the coupled systems. The results indicate that a reduction in the number of interconnections in alternate configurations does not necessarily provide an improvement of controllability properties.     

差压热耦合反应精馏塔的模拟研究

为了降低反应精馏塔能耗,将差压热耦合技术与反应精馏技术结合,提出了一种新型的差压热耦合反应精馏的流程,并将其应用于乙酸正丁酯的合成中。应用Aspen Plus模拟软件对新工艺流程以及常规反应精馏流程进行了模拟,通过考察压缩比、进料位置、进料醇酸摩尔比等因素对差压热耦合反应精馏合成乙酸正丁酯工艺的影响,得到最优条件。同时,将该工艺与常规反应精馏工艺进行比较,结果显示,新工艺能够大幅度降低能耗,与常规反应精馏装置相比可节能40%左右。     

A Transesterification Double Step Process — TDSP for biodiesel preparation from fatty acids triglycerides

This study introduces a two consecutive steps basic–acid transesterification process, (denominated Transesterification Double Step Process — TDSP) for biodiesel production from vegetable oils. The process involves homogeneous consecutive basic–acid catalysis steps and is characterized by formation of well-defined phases, easy separation procedures, high reaction velocity and high conversion efficiency. The proposed TDSP is different in relation to other traditional two-step procedures which normally include acid esterification followed by basic transesterification, or enzymatic or even supercritical transesterification conditions. The biodiesel (fatty acid methyl esters) was analyzed by standard biodiesel techniques in addition to ¹H-NMR, indicating high quality and purity biodiesel products. The transesterification of sunflower and linseed oils resulted in oil conversions higher than 97% corresponding to yields of 85%. A probable reaction mechanism responsible for the process is presented.     

Analysis of control properties of intensified distillation sequences: Reactive and extractive cases

In this work, we obtain and compare the control properties of thermally coupled reactive distillation sequences and thermally coupled extractive distillation sequences with those of conventional reactive and extractive distillation configurations. All sequences have been designed using a multiobjective genetic algorithm with restrictions. The theoretical control properties of those schemes were obtained using the singular value decomposition technique in all frequency domain. In order to complete the control study, the distillation options were subjected to closed-loop dynamic simulations. The effects of total stages, reactive stages, and extractant/feed ratio on the energy consumption and control properties are obtained for the intensified distillation options. The results show that there are cases in which integrated reactive and extractive sequences do not only provide significant energy savings with respect to the conventional reactive and extractive arrangements, but also may offer dynamic advantages in high energy consumption conditions.     

多效反应精馏过程生产氯化苄的能量集成

以甲苯氯化生产氯化苄为研究对象,对带侧反应器的反应精馏与精制塔串联工艺(CSRRT)进行研究及能量分析,建立了分段反应精馏与精制塔串联生产氯化苄的新工艺。利用精制塔塔顶蒸汽潜热加热第一段反应精馏塔的塔釜,建立了多效反应精馏(MERD);进一步利用侧反应器的甲苯氯化反应热加热第一段反应精馏塔的塔板物料,建立了多效透热反应精馏(MEDRD)。在相同生产要求下,对3种工艺的能耗进行比较。结果表明,MERD和MEDRD过程实现了能量的优化利用,与CSRRT过程相比,塔釜总再沸器热负荷分别降低16.8%和33.7%。     

Reactive distillation for biodiesel production from soybean oil

Biodiesel, which is regarded as a promising alternative to a conventional petroleum-based diesel fuel, can be produced from transesterification of vegetable oils and alcohol in conventional batch and continuous reactors. Since the transesterification is an equilibrium-limited reaction, a large excess of reactants is usually used to increase the production of biodiesel, thereby requiring more expensive separation of unreacted raw materials. This study proposed the use of a reactive distillation for transesterification of soybean oil and methanol catalyzed by sodium hydroxide to produce biodiesel. The simulation results showed that a suitable configuration of the reactive distillation column consists of three reactive stages. The optimal conditions for the reactive distillation operation are at the molar feed ratio of methanol and oil at 4.5: 1, reflux ratio of 3, and reboiler duty of 1.6×10⁷ kJ h⁻¹. Methanol and soybean oil should be fed into the column at the first stage. The effect of important operating and design parameters on the performance of reactive distillation was also presented.     

Preparation of biodiesel from soybean oil using supercritical methanol and co-solvent

Transesterification of soybean oil in supercritical methanol has been carried out in the absence of catalyst. A co-solvent was added to the reaction mixture in order to decrease the operating temperature, pressure and molar ratio of alcohol to vegetable oil. With propane as co-solvent in the reaction system, there was a significant decrease in the severity of the conditions required for supercritical reaction, which makes the production of biodiesel using supercritical methanol viable as an industrial process. A high yield of methyl esters (biodiesel) was observed and the production process is environmentally friendly. Furthermore the co-solvent can be reused after suitable pretreatment.     

Optimal integration for biodiesel production using bioethanol

The production of biodiesel from algae is optimized using bioethanol following four different transesterification paths: alkali, enzymatic, and heterogeneous catalysts and supercritical conditions. The reactors are modeled using response surface methodology based on experimental results from the literature. These reactor models are implemented together with short‐cut methods for the other equipment (distillation columns, gravity separators, etc.) in order to recover the ethanol, separate the polar and nonpolar phases, and purify the glycerol and biodiesel produced to formulate the problem as a superstructure of alternatives. The aim is to simultaneously optimize and heat integrate the production of biodiesel using ethanol in terms of the reaction technology and the operating conditions. The optimal conditions in the reactors differ from the ones traditionally used because these results take the separation stages into account. In terms of the optimal process, the alkali catalyzed process is the most profitable, while the enzymatic one is also promising due to the lower consumption of energy and water, although it requires significant enzyme cost. © 2012 American Institute of Chemical Engineers AIChE J, 59: 834–844, 2013