1,720,971 research outputs found
Safety Challenges on Runaway Reactions: The Esterification of Acetic Anhydride with Methanol
No full textEsterification reactions are commonly used in industrial practice. These reactions are fast and moderately exothermic thus they are prone to exhibit a runaway behavior, that is a thermal loss of control of the synthesis reactor. This scenario may lead to either quality issues, such as formation of undesired side products, or safety concerns, as pressurization and rupture of the reactor itself.
To mitigate the risk, proper recipes should be designed and optimized for a safe conduction of the reaction at the full-plant scale. This can be done by knowing the kinetics of the involved reactions, both desired and undesired.
The aim of this work is to determine the kinetic parameters of the esterification of acetic anhydride with methanol (forming methyl acetate) in the presence of sulfuric acid as a catalyst to be used in a cost-effective safe optimization protocol. This reaction has been analyzed in the past because of its exothermicity, but without fully describing the involved reactions (both desired and side).
In this work, calorimetric measurements are used to observe both the thermic and quality characteristics of the overall synthesis run in a semi-batch, lab-scale reactor. Both a detailed kinetic scheme and the associated parameters are determined to provide safe and productive operating conditions for the process, properly considering all the side reactions than could emerge during the synthesis. The study also required the implementation of a dedicated mathematical model for the simulation of the lab-scale semi-continuous reactor
Dispersion in coiled tubular reactors: A CFD and experimental analysis on the effect of pitch
No full textThis work investigates axial dispersion in coiled tubes when the coil pitch is non-negligible compared to the other geometrical parameters. The axial dispersion model (ADM) is used for the description of residence time distributions (RTDs) and an analytical expression for the closed-closed (CC) ADM is presented and used for the analysis of coiled reactors. Correlations, based on validated computational fluid dynamics (CFD) results, are developed to take the coil pitch effect into account. The dispersion coefficient is evaluated numerically and experimentally for different coils, with a good agreement between predicted and observed values. Coiling not only reduces the dispersion significantly with respect to straight tubes, but also the ADM can be applied in a wider region of the common conditions used in practice. The results are summarized in a nomogram that displays applicability regions of the ADM and enables estimation of the dispersion coefficient with a simple graphical procedure
Kinetics-free process intensification: From semi-batch to series of continuous chemical reactors
Full text linkA kinetics-free procedure is developed to transform a reaction recipe carried out in an isothermal discontinuous semi-batch reactor into one based on a series of continuous tubular or tank reactors. Using a suitable number of reactors and a correct feeding policy, it is possible to reproduce any discontinuous recipe in flow reactors without knowing the kinetics of the system. The developed procedure allows to determine the number of reactors in series (either tubular or stirred tank) able to reproduce the performance of a semi-batch recipe in terms of selectivity, keeping the same productivity of the original semi-batch process. It was found that using 15 reactors in series allows to keep unchanged the performance of a large number of semi-batch processes once a correct policy of intermediate reactant feeding is implemented. This large number of reactors in series with intermediate feeding can be easily implemented using tubular reactors instead of stirred tank ones. Finally, the synthesis of an amine/epoxy resin was investigated as a case study, fully supporting the reliability of the proposed procedure
Increasing Safety by Shifting Semi-Batch Polymerizations into Semi-Continuous Production
No full textIntensification of Processes for the Production of Active Pharmaceutical Ingredients: Increase in Safety and Sustainability
Full text linkFine chemical compounds and so-called "specialties" are generally synthesized through batch or semi-continuous processes. This is largely because such syntheses often involve complex and highly exothermic reactions, to be performed in semi-batch reactors for safety and/or selectivity reasons. An effective way to reduce costs and improve the reproducibility of such batch processes is to transform them into their continuous counterparts to reduce volumes and investment costs, while increasing the inherent safety of the process thanks to fewer hold-ups. The “shift to continuous” allows to reduce both the overall process times, with a general decrease in operating costs, and the content of solvents used as thermal flywheels, thanks to the greater efficiency of the heat exchange systems. All these aspects are defined as process intensification. In this work, the intensification of the production process of N-(4-nitro, 2-phenoxyphenyl) methanesulfonamide (NIM) by nitration in glacial acetic acid of N-(2-phenoxyphenyl) methanesulfonamide (FAM) will be proposed. Starting from the original semi-batch recipe two different continuous configurations will be proposed: a series of tubular reactors and a series of continuous reactors with complete mixing, in both cases with intermediate injections. The solvent content (glacial acetic acid) has been drastically reduced (from 82.5% to 50% by weight) to increase the levels of environmental sustainability of the synthesis. The high exothermicity of the process and the extremely rapid reaction kinetics were two fundamental aspects which had to be considered in the transition to the continuous process of the new formulation with reduced solvent content. For this reason, an ad hoc procedure was developed which allows the semi-batch recipe to be transformed into a corresponding one conducted in a tubular reactor with continuous lateral injections; this reactor was then discretized in the two reactor configurations mentioned above. The results obtained have shown how it is possible to obtain the desired product with practically unitary conversions using: a) a series of 4 isoperibolic tubular reactors, each with 4 discrete lateral feeds; b) a series of 5 mixed reactors with discrete side feeds. In both cases, the correct distribution of both the flow rate fed between the reactors in series and the temperatures of the cooling fluid (defined on the basis of the procedure developed for the passage of the process from discontinuous to continuous) was decisive for obtaining the desired performance. The series of tubular reactors was found to be optimal from the point of view of thermal control of the process, confirming that a series of tubular reactors is to be preferred in terms of safety compared to its counterpart with mixed reactors
A novel Static Mixer for photochemical reactions
No full textPhotochemical reactions have become a widely studied topic in the past decade, as they are employed in a wide range of processes and are recognized to be a fundamental tool in the field of sustainable chemistry. Moreover, in recent years there is an increasing interest in turning discontinuous photochemical processes into continuous ones, since continuous processes can ensure better results in terms of productivity and selectivity. In this work, a novel static mixer useful to reach a plug-flow behaviour inside a square-based reactor, while not obstructing the photon flux coming from the light sources, was designed. Through RANS based CFD simulations, the residence time distribution of molecules inside the reactor was evaluated. Moreover, the effect of both absorption and scattering phenomena, which play a major role in photochemical reactions, was assessed. Finally, a case study was considered and the photochemical degradation of 2-hydroxybenzoic acid (2-HBA) was simulated. The conversion obtained in the reactor equipped with SMs was compared to an empty reactor, finding that the degradation efficiency is doubled
Jet fires and reaction runaway interaction: A multiscale approach
Full text linkIn industrial safety, large engulfing jet fires can cause damages and collapse of process vessels with catastrophic consequences; however, even smaller jet fires can be extremely dangerous due to their ability to induce hazardous conditions when chemical reacting systems are involved. This work investigates the ethoxylation of 1-dodecanol as a case-study to investigate the potentialities of a multiscale approach able to analyse scenarios of accidental impingement on a chemical reactor involving a jet fire. In particular, the investigated system was a Venturi tower reactor, where the reacting fluid is pumped, cooled via an external heat exchanger and sprayed from the top of the reactor together with ethylene oxide. The aim is to understand if and when the cooling power of the external heat exchanger can prevent dangerous temperature build-up in the chemical reactor. The multiscale approach used in this work involves a series of mathematical simulations aimed at evaluating the effects of different jet fires in terms of heat flux entering the Venturi reactor impinged by a jet fire. These simulations were carried out using a mathematical model developed in the framework of computational fluid dynamics (CFD). The results of such simulations were used to carry out a parametric study with a 0D model able to foresee the dynamic behaviour of the reactor impinged by the jet fire. Preliminary results confirmed the possibility of linking jet fires characteristics with the time available before dangerous conditions in the Venturi reactor occur, therefore allowing for making an estimation of the time available for activating proper mitigation measures
Influence of Buoyancy Effects on the Mixing Process and RTD in a Side-Injection Reactor Equipped with Static Mixers
Full text linkThe mixing process and the residence time distribution (RTD) of molecules inside reactors are well-known topics in chemical engineering; good radial mixing and poor axial mixing of chemical species are the essential conditions to achieve a plug flow behavior in a tubular reactor, which is often highly desirable. While the influence of viscosity and spatial velocities on mixing and RTD has been investigated in the literature, the influence of density differences between the streams to be mixed has been much less investigated, especially considering laminar regimes. Thus, the mixing and RTD of two miscible liquids with different densities and viscosities in a side-injection tubular reactor equipped with Sulzer Static Mixers were studied by RANS-based computational fluid dynamics (CFD) simulations. The results obtained show that if adequate configurations are used, it is possible to well-approximate radial mixing and a plug flow behavior, even when large differences in densities are involved. Moreover, graphics for a fast estimation of the maximum mixing length involved as well as for the corresponding Pe-1 value were obtained as a function of the Re number
A CFD hybrid approach to simulate liquid-phase chemical reactors
Full text linkDesign and safety assessments of chemical reactors can be done using Reynolds Averaged Navier-Stokes (RANS) type equations. The averaging procedure of transport equations gives rise to unclosed terms, which must be properly modeled independently on the computational cell size. In particular, presence of chemical reactions leads to an additional source term in species equation. The averaged value of this term involves effects of both chemical kinetics and turbulence. Turbulence-kinetics interaction (TKI) models must be then developed in order to close species transport equations, so that Computational Fluid Dynamics (CFD) can be used to reduce the number of experiments required to design a chemical reactor.Many TKI models have been developed in the past, mainly for gaseous systems, while liquid-phase models have been less investigated because of demanding theoretical challenges. Therefore, the purpose of this work is the development of a new TKI model for liquid phase reactions, which combines the Laminar Rate model (for kinetic controlled systems) with the Multiple Time Scales model (for turbulence controlled systems) allowing its use also when kinetic and turbulent mixing characteristic times are comparable. An analysis of the influence of the turbulence model coupled with the proposed model was carried out to identify the most suitable turbulence model, and two different case studies were investigated to show the potentialities of the proposed approach
- …
