Influence of electrical potential on the crystallization and adhesion of potassium hydrogen tartrate crystals

Interfacial interactions between a hydroalcoholic solution of potassium hydrogen tartrate (KHT) and a stainless steel surface are studied, when an electrical potential is applied to the metal substrate. The capacitive domain of the metal–solution interface is determined by cyclic voltammetry. In order to study the influence of the potential on KHT nucleation and crystal adhesion, the solid–liquid interfacial energy is assessed from contact angle and capillary rise measurements. Experimentally, the contact angle between a NaF solution and a stainless steel vs. the potential has a parabolic behaviour. The metal topography has no apparent influence on physicochemical properties of the interface when ethanol is present in a KHT solution. The metal substrate promotes the formation of KHT crystals, which is improved by the application of an anodic potential. The adhesion of crystals becomes more effective when wires of 25 μm diameter are used in comparison with those of 250 μm

Dynamic Behaviour of a Continuous Heat Exchanger/Reactor after Flow Failure

The intensified technologies offer new prospects for the development of hazardous chemical syntheses in safer conditions: the idea is to reduce the reaction volume by increasing the thermal performances and preferring the continuous mode to the batch one. In particular, the Open Plate Reactor (OPR) type “reactor/ exchanger” also including a modular block structure, matches these characteristics perfectly. The aim of this paper is to study the OPR behaviour during a normal operation, that is to say, after a stoppage of the circulation of the cooling fluid. So, an experiment was carried out, taking the oxidation of sodium thiosulfate with hydrogen peroxide as an example. The results obtained, in particular with regard to the evolution of the temperature profiles of the reaction medium as a function of time along the apparatus, are compared with those predicted by a dynamic simulator of the OPR. So, the average heat transfer coefficient regarding the “utility” fluid is evaluated in conductive and natural convection modes, and then integrated in the simulator. The conclusion of this study is that, during a cooling failure, a heat transfer by natural convection would be added to the conduction, which contributes to the intrinsically safer character of the apparatus

Evaluation of an intensified continuous heat-exchanger reactor for inherently safer characteristics

The present paper deals with the establishment of a new methodology in order to evaluate the inherently safer characteristics of a continuous intensified reactor in the case of an exothermic reaction. The transposition of the propionic anhydride esterification by 2-butanol into a new prototype of ‘‘heatexchanger/ reactor’’, called open plate reactor (OPR), designed by Alfa Laval Vicarb has been chosen as a case study. Previous studies have shown that this exothermic reaction is relatively simple to carry out in a homogeneous liquid phase, and a kinetic model is available. A dedicated software model is then used not only to assess the feasibility of the reaction in the ‘‘heat-exchanger/reactor’’ but also to estimate the temperature and concentration profiles during synthesis and to determine optimal operating conditions for safe control. Afterwards the reaction was performed in the reactor. Good agreement between experimental results and the simulation validates the model to describe the behavior of the process during standard runs. A hazard and operability study (HAZOP) was then applied to the intensified process in order to identify the potential hazards and to provide a number of runaway scenarios. Three of them are highlighted as the most dangerous: no utility flow, no reactant flows, both stop at the same time. The behavior of the process is simulated following the stoppage of both the process and utility fluid. The consequence on the evolution of temperature profiles is then estimated for a different hypothesis taking into account the thermal inertia of the OPR. This approach reveals an intrinsically safer behavior of the OPR

Toxic release dispersion modelling with PHAST : parametric sensitivity analysis

Recent changes to French legislation, concerning the prevention of technological and natural risk, require industrial sites to calculate the safety perimeters for different accident scenarios, based on a detailed probabilistic risk assessment. It is important that the safety perimeters resulting from risk assessment studies are based on the best scientific knowledge available, and that the level of uncertainty is minimised. A significant contribution to the calculation of the safety perimeters comes from the modelling of atmospheric dispersion, particularly of the accidental release of toxic products. One of the most widely used tools for dispersion modelling in several European countries is PHASTTM[1]. This software application is quite flexible, allowing the user to alter values for a wide range of model parameters. Users of the software have found that simulation results may depend quite strongly on the values chosen for some of these parameters. While this flexibility is useful, it can lead different users to calculate effect distances that vary considerably even when studying the same scenario. In order better to understand the influence of these input parameters, we have carried out a parametric sensitivity study of the PHAST dispersion models. This allows us to obtain global sensitivity indices for the input parameters, which quantify the level of influence of each parameter on the output of the model and the interactions. The FAST (Fourier Amplitude Sensitivity Test) sensitivity analysis method that we have applied (using the SimLab software tool [2]) provides both first order indices (that characterize the parameter’s influence on the model output when it varies in isolation) and total indices (that characterize one parameter’s influence including its joint interaction with other input parameters). We shall present results of this analysis on a number of toxic gas dispersion scenarios. The analysis has considered parameters related to the physical release scenario (release rate, release height, etc.), to weather conditions (wind speed, stability class, atmospheric temperature, etc.) and to the numerical resolution (step size, etc.). We compare the results of several sensitivity analysis methods, both local one-at-a-time methods and global methods. We discuss the importance of selecting an appropriate model output value when studying the model sensitivity (output measures considered include the concentration of the released gas at a long distance, at a short distance, and the maximal distance at which a specified concentration is attained). Our experimental results assume that input parameters to the dispersion model are independent. However, correlations exist between several input parameters that we have analysed, such as wind speed and atmospheric stability class. We discuss various approaches to calculate sensitivity indices that take this correlation into account. [1] DNV Software, London, UK [2] Saltelli, A., Chan, K., Scott, E. M. Sensitivity Analysis, 2004, John Wiley & Sons Publishers

Study of wine tartaric salt stabilization by addition of carboxymethylcellulose (CMC). Comparison with the « protective colloïds » effect

Aims : Inhibition of potassium hydrogen tartrate (KHT) crystallization by carboxymethylcellulose (CMC) is tested in a model solution and in wines. Tartaric acid salt crystallization risk is assessed by computing the supersaturation, saturation temperature and excess KHT with respect to the saturation equilibrium using MEXTAR® (Mesure de l’EXces de TARtre) software. Materials and results : Firstly, the time for crystals to appear was recorded by monitoring the conductivity in a model solution and in a wine, and the inhibition ratio was computed. At 11,5 °C, 0,5 mg.L-1 CMC inhibited KHT crystallization. The inhibitory effect increased exponentially with increasing CMC concentration and was several times greater than that of polysaccharides and polyphenols, the protective colloids in wine (Gerbaud et al., 1997). At 2 °C, 30 mg.L-1 CMC had the same inhibitory effect than 10 mg.L-1 at 11.5°C.Secondly, 20 red and white wines were refrigerated for 3 weeks at -4 °C with CMC or metatartaric acid. Results show that the addition of 20 mg.L-1 CMC has an inhibitory effect at least equivalent to 100 mg.L-1 metatartaric acid. Furthermore, for 10 wines preheated for 8 days at 30 °C and then refrigerated for 2 months at 0 °C, 5 and 20 mg. L-1 CMC maintains its inhibitory efficiency, unlike metatartaric acid which is hydrolysed Significance and impact of the study : The OIV-OENO 366-2009 and OIV-OENO 02/2008 resolutions recently authorized the use of CMC to prevent tartaric acid salt precipitation. With no impact on health, and stable under heating and in acid solution, CMC is an efficient candidate for tartaric stabilization. The optimal concentration of 20 mg.L-1 (2 g.hL-1) should however be adapted to local wine storage conditions and KHT crystallization risk

Application d’un champ électrique de faible amplitude pour l’amélioration de la cristallisation et de la filtration sur supports métalliques en oenologie.

En œnologie, on assiste à l’apparition de matériaux métalliques, utilisés comme support de cristallisation ou média filtrant, qui ont permis la mise en œuvre de méthodes électrochimiques. Des dispositifs expérimentaux ont permis à la fois de moduler le potentiel électrique relatif métal-solution et d’atteindre diverses grandeurs étroitement liées à la physico-chimie de l’interface. Le premier montage permet la mesure de l’angle de contact entre une goutte de solution électrolytique et une plaque métallique polie ou rugueuse. Le deuxième dispositif permet de déduire l’ascension capillaire de la mesure de la masse de solution incorporée dans un matériau métallique poreux. Le dernier est une cellule, où le média filtrant est enchâssé entre deux électrodes métalliques, dans laquelle le débit de filtration peut être aisément mesuré. Les résultats concernent la variation de l’énergie interfaciale avec le potentiel électrique appliqué d’où l’on déduit le point de charge nulle et la capacité différentielle de la double couche électochimique. Pour la cristallisation, on a constaté une dépendance entre la masse de cristaux de bitartrate de potassium déposée sur le support métallique et le potentiel électrique. Pour la filtration, le champ électrique a fait apparaître simultanément une série de phénomènes qui augmente le flux de filtration des vins

Using dynamic simulation for risk assessment: application to an exothermic reaction

In this paper, a solution strategy based on an optimization formulation is proposed for the design of Water Allocation and Heat Exchange Networks (WAHEN) in the process industries. Such typical large problems involve many processes, regeneration units and multi-contaminants. For this purpose, a two-stage methodology is proposed. The first step is the Water Allocation Network (WAN) design by multi-objective optimization, based on the minimization of the number of network connections and of the global equivalent cost (which includes three criteria, i.e., freshwater, regenerated water and wastewater). The ɛ-constraint method is used to deal with the multi-criteria problem. In a second step, the Heat Exchange Network (HEN) is solved by two approaches, Pinch analysis and mathematical programming (MP). In both cases the HEN structure is found when the minimal energy requirement and the total annual cost are minimized for Pinch and MP, respectively. These results are compared and the best HEN network is then coupled to the WAN to verify the feasibility of the network. A case study including a change of phase among the streams is solved. The results show that this two-step methodology can be useful for the treatment of large problems

Safety enhancement by transposition of the nitration of toluene from semi-batch reactor to continuous intensified heat exchanger reactor

The behaviour of a continuous intensified heat exchanger (HEX) reactor in case of process failure is analysed and compared to the behaviour of a semi-continuous reactor. The nitration of toluene is considered as test reaction to identify the main failure scenarios that can lead to thermal runaway in both processes using the HAZOP method.No flow rate of process fluid and utility fluid in the continuous process. No stirring during feeding of the reactor followed by normal stirring for the semi-continuous reactor. These scenarios are simulated for both processes and the temperature profiles are observed. This study shows that the temperature is better controlled in the continuous process because of the intrinsic characteristics of the HEX reactor. In fact, this device has a low reactive volume relative to the mass of the reactor, allowing a good dissipation of the heat produced by the reaction, even in case of failure. This characteristic of the intensified reactor is confirmed by an experimental work

Towards the modelling of a heat-exchanger reactor by a dynamic approach

The aim of this paper is to present the development of a simulation tool in order to assess the inherently safe characteristics of a heat‐exchanger reactor(HEX) operating reaction systems. The modelling of steady and transient states of a HEX reactor is performed following a hybrid dynamic approach. The global dynamic behaviour of this reactor can be represented by several continuous models, which are bounded by state or time events. Each continuous model is defined as a system of partial differential‐algebraic equations. The numerical scheme is based on the method of lines. Special attention is paid to the model initialization and a simulation strategy of the start‐up phase is presented. The validation of the model is made by numerous examples, such as the simulation of an exothermic reaction

Deviation propagation analysis along a cumene process by using dynamic simulations

The dynamic response of benzene alkylation process to a set of deviations is analyzed with Aspen Plus Dynamics. A quantitative risk assessment is developed through simulations of deviation scenarios. The process comprises a reactor and three distillation columns with a recycle stream. The simulation scenarios are determined according to lessons learnt from accidents. This study underlines the conditions that induce an overpressure or a flooding in a distillation column. Three scenarios are proposed: feed flowrate variations, coolant flowrate reduction and cooling of the reboiler steam. Thereafter, the results allow calculating a set of risk indexes related to flooding and overpressure phenomena. This study underlines the deviation propagation effects that can be expected in all the process equipment. Moreover, it represents a significant contribution to the definition of the process control strategy and the necessary safety barriers