Safety analysis and risk identification for a tubular reactor using the HAZOP methodology

A model approach to Hazard and Operability (HAZOP) analysis is presented based on the mathematical modeling of a process unit where both the steady-state analysis, including the analysis of the steady states multiplicity and stability, and the dynamic simulation are used. Heterogeneous tubular reactor for the ethylene oxide production from ethylene and oxygen was chosen to identify potential hazards for real system. The computer code DYNHAZ was developed consisting of a process simulator and a generator of the HAZOP algorithm. Presented at the 33rd International Conference of the Slovak Society of Chemical Engineering, Tatranské Matliare, 22–26 May 2006.     

STEADY-STATE AND DYNAMIC BEHAVIOR OF FIXED-BED CATALYTIC REACTOR FOR FISCHER-TROPSCH SYNTHESIS II.STEADY-STATE AND DYNAMIC SIMULATION RESULTS

In this paper, the steady state and dynamic behavior of the F-T fixed-bed catalytic reactor were discussed. An investigation has been made on the impact of the operation conditions on the steady state behavior of the reactor. A study was made on the reactor dynamic behavior for step changes in the feed temperature, fluid rate and the wall temperature.     

固定床F-T合成反应器的稳态和动态行为特性 Ⅱ.稳态和动态模拟计算结果(英文)

讨论了固定床F－T合成反应器的稳态和动态行为。首先研究了操作条件对固定床F－T合成反应器稳态行为的影响,然后探讨了当反应器进口原料温度、原料流速和冷却介质温度突变后床层的动态响应行为。     

Investigation of coupling dehydrogenation and hydrogenation reactions in a fixed bed catalytic reactor with well-mixed catalyst pattern

The enhancement of ethylbenzene conversion by further displacement of the thermodynamic equilibrium via the influence of the dual-functionality of a well-mixed catalyst pattern has been investigated. A rigorous steady state mathematical model based on the dusty gas model is implemented for the simulation. The simulation results reveal that the introduction of the concept of the reaction coupling has significant effect on the displacement of the thermodynamic equilibrium and considerable enhancement of simultaneous production of styrene and cyclohexane. Almost 100% conversion of the ethylbenzene and benzene is achieved through the application of this approach. It is also found that considerable decrease in the reactor length is achieved by employing a reactor catalyst bed with different bed compositions. Effective operating regions with optimal conditions are observed. An effective reactor length criterion is used to evaluate the performance of the reactor under these optimal conditions. The effective reactor length is found to be sensitive and favored by high feed temperature and pressure. The sensitivity analysis shows that the key parameters of feed temperature, pressure, and the bed composition play an important role on the reactor performance. The results also show that almost 100% conversion of ethylbenzene and benzene at low temperature and shorter reactor length can be achieved by maintaining the reactor beds at different temperatures. This temperature switching policy may result in appreciable energy saving. Moreover, operating the reactor at low temperature protect the catalyst from the excessive temperatures which have destructive effects on the catalysts and the mechanical stability of the reactors. Also, the low temperature operation has significant contribution to the reduction of the operating cost.     

Steady state and dynamic simulation of a hybrid reactive separation process

Modelling and simulation of hybrid reactive separation system in steady state and in dynamic regime was carried out. The investigated hybrid process consisted of a reactive distillation column and a pervaporation membrane located in the distillate stream to remove water from the process. Heterogeneously catalysed esterification of propionic acid with propan-1-ol to propyl propionate and water was chosen as the model chemical reaction. Esterification reactions are a typical example of equilibrium-limited reactions producing water as a by-product. Using just a pervaporation membrane brings the biggest benefit in increasing the yield of one of the reactants due to the removal of water. To study reactive separation processes, a model of the hybrid system in steady state and in dynamic regime was developed. Steady state behaviour of the model was studied for different hybrid system configurations. The effect of catalyst amount doubling was also investigated. Dynamic behaviour of the system during the step changes of propionic acid feed flow rate and during the membrane module failure was investigated. For this reason, the conversion of propionic acid, purity of the product stream, mole fraction of water, and the temperature in three different parts of the reactive distillation column were monitored.     

Fault propagation behavior study of CSTR in HAZOP

This paper discusses the role of process modeling in safety analysis. Process modeling is applied in the fault propagation behavior study of CSTR chemical production. For that purpose, HAZOP methodology and continuation analysis were used. The proposed hazard identification methodology involves analysis of steady-state multiplicity and safe operating conditions as well as those which can shift process units from one steady state to another. All presented case studies are also supported by system dynamic simulations, essential to detect oscillatory thermal instability. In this paper, N-oxide alkylpyridines production process was chosen to identify potential hazard and operational problems. Presented dynamic simulations represent an analysis of the system response to step changes in the key operating parameters. The effect of deviations of three key parameters on the reactor safe operation was investigated. The proposed numerical algorithms represent a mathematical engine of the simulation module within an automated model-based HAZOP analysis tool.     

Theoretical study on transesterification in a combined process consisting of a reactive distillation column and a pervaporation unit

Steady state analysis of a combined hybrid process consisting of a reactive distillation column, pervaporation unit, and a distillation column is presented. This process configuration was first presented by Steinigeweg and Gmehling (2004) for the transesterification of methyl acetate and butanol to butyl acetate and methanol. This system is characteristic for its low reaction rate and complex phase equilibrium. Steinigeweg and Gmehling (2004) have shown that the combination of reactive distillation and pervaporation is favourable since conversions close to 100 % can be reached with a reasonable size of the reactive section in the reactive distillation column. The aim of this paper is to show that although high conversion can be achieved, very complicated steady state behaviour must be expected. The presented analysis is based on mathematical modelling of a process unit, where the steady-state analysis, including continuation and bifurcation analyses, was used. Multiple steady states were predicted for the studied system; three steady states with conversions higher than 98 %. However, not all predicted steady states met the maximal allowed temperature condition in the reactive section (catalyst maximal operation temperature of 393 K). The presence of multiple steady states reduces the operability and controllability of the reactive distillation column during its start-up and during the occurrence of any variation of operating parameters because the system can be shifted from one steady state to another one (concurrent exceeding the maximal allowed temperature) with unwanted consequences, e.g. production loss. Therefore, design and subsequent operation of such a complicated system is an ambitious task requiring knowledge of any possible system behaviour.     

Modeling, Simulation and Operation Performance of a Simulated Moving Bed for Enantioseparation of Fluoxetine on New β-Cyclodextrin Columns

A Simulated Moving Bed (SMB) setup, including 8 new -cyclodextrin columns, was used to achieve the enantioseparation of fluoxetine. The effects of feed concentration and feed flowrate on the operation performance parameters: purity, enrichment, recovery and productivity were studied. A simulation approach was applied to simulate the operation and performance of the simulated counter-current system. The model predicted the performance of the transient and cyclic steady state behaviour to a reasonably good extent. The adsorption isotherm and parameters determined by pulse experiments and moment analysis seem to be adequate in simulating both the transient and steady state behaviour. Generally, this model can provide guidance for designing operation condition of the SMB system.     

Simulation of free radical high-pressure copolymerization in a multi-zone autoclave reactor: compartment model investigation

A compartment model is used to describe the complex flow of a high-pressure ethylene copolymerization process in an industrial multi-feed multi-zone autoclave reactor at steady state operation conditions. To capture the imperfect mixing effects due to fresh initiator injection, each zone is considered as a set of three interconnected well mixed CSTRs with recycle streams. Volumes of the reactors and the recycle flow are adjusted to get the best fit with results of steady state well mixed analysis for each zone. Once the temperature and conversion as state variables in each reaction volume are known, the properties of polymer produced in each zone and those of final polymer can be determined. Using a realistic set of kinetic mechanisms, temperature, monomer conversion, molecular weights and short and long chain branching frequencies in each zone and at the exit point of the reactor are estimated. Some of the model results are compared with experimental data obtained for an industrial reactor.     

Using Unfold‐PCA for batch‐to‐batch start‐up process understanding and steady‐state identification in a sequencing batch reactor

In chemical and biochemical processes, steady‐state models are widely used for process assessment, control and optimisation. In these models, parameter adjustment requires data collected under nearly steady‐state conditions. Several approaches have been developed for steady‐state identification (SSID) in continuous processes, but no attempt has been made to adapt them to the singularities of batch processes. The main aim of this paper is to propose an automated method based on batch‐wise unfolding of the three‐way batch process data followed by a principal component analysis (Unfold‐PCA) in combination with the methodology of Brown and Rhinehart 2 for SSID. A second goal of this paper is to illustrate how by using Unfold‐PCA, process understanding can be gained from the batch‐to‐batch start‐ups and transitions data analysis. The potential of the proposed methodology is illustrated using historical data from a laboratory‐scale sequencing batch reactor (SBR) operated for enhanced biological phosphorus removal (EBPR). The results demonstrate that the proposed approach can be efficiently used to detect when the batches reach the steady‐state condition, to interpret the overall batch‐to‐batch process evolution and also to isolate the causes of changes between batches using contribution plots. Copyright © 2007 John Wiley & Sons, Ltd.     

Enhanced operation of a wastewater treatment process by periodic forcing input

This paper considers investigating the performance of a wastewater treatment plant consisting of a bioreactor with recycle followed by a clarifier under forced feed circulation. A previously developed dynamic model for the process is used to conduct the performance analysis. The static version of the model was utilized first to determine the optimal productivity conditions for the process. Numerical optimization is used to design the feed periodic function. The parameters of the feed cyclic functions are determined which resulted in improved productivity and substrate consumption. The improvement in productivity is marginal and is satisfactory for substrate conversion compared to that of the optimal condition and to the steady state condition, which corresponds to the average value of the periodic function.     

Dynamic simulation of pH in anaerobic processes

With the objective of contributing to the buildup of mathematical tools for anaerobic process simulation, an algorithm for the dynamic simulation of pH was developed. The dynamic simulation of the gaseous phase variables was also considered. The pH algorithm was validated for a watery system, obtaining good agreement between predicted and experimental data. The applied methodology provides a differential equation that allows the inclusion of pH as a state variable of the system that can be easily included in a general mathematical model of anaerobic digestion using matrix notation. This methodology also allows a noticeable decrease in computing time in simulations. A dynamic anaerobic digestion model of complex substrates taken from the literature was completed with the developed algorithms, and it was used to predict the response of an anaerobic reactor against overloading and against the presence of pH-dependent inhibitors with satisfactory results.     

Unseeded semibatch emulsion polymerization of butyl acrylate: Bimodal particle size distribution

Unseeded semibatch emulsion polymerization of butyl acrylate (BA) using sodium lauryl sulfate as emulsifier and potassium persulfate as initiator was carried out at the conditions where secondary nucleation was probable. This was achieved by using no emulsifier in the initial reactor charge. The effects of changes in monomer emulsion feed rate, initiator concentration and distribution, emulsifier concentration in the feed, and temperature on the evolution of particle size averages and distribution were investigated. Bimodal particle size distributions (PSD) were obtained for most of the latexes. Inhibition effects were found to be important in the development of PSD. Primary particle formation occurred through micellar nucleation, whereas secondary nucleation probably occurred through homogenous nucleation. The polydispersity index (PDI) of the latexes increased with the decreasing monomer emulsion feed rate. The application of a larger amount of initiator to the reactor charge or using a higher temperature, reduced the formation of secondary particles and resulted in a formation of an unimodal PSD. The overall steady‐state rate of polymerization was found to approach the rate of monomer addition (R_p ≈ R_a ), if the emulsifier concentration in the aqueous phase was appreciable. This is different from the correlation 1/R_p = 1/K + 1/R_a obtained for the BA semibatch process with neat monomer feed. This suggests that different rate expressions can be used for BA semibatch emulsion polymerization at different conditions. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 528–545, 2000     

从多重定态经复杂奇点向极限环型化学振荡的跃迁

在文Ⅰ~[1]的相平面分析的基础上, 利用线性稳定性分析和数值模拟的方法进一步分析了Schlögl模型在CSTR中的行为, 发现在适当条件下可以发生从定态向振荡态的不连续的突然跃迁现象。认为这种突然跃迁现象是通过鞍点-结点型复杂奇点的形成和消失而实现的。分析了这种鞍点-结点型跃迁与亚临界Hopf分支现象的差别, 并讨论了这种跃迁现象和实验上发现的某些突变型化学振荡现象之间的联系。     

Electron and nuclear spin dynamics in the thermal mixing model of dynamic nuclear polarization

A novel mathematical treatment is proposed for computing the time evolution of dynamic nuclear polarization processes in the low temperature thermal mixing regime. Without assuming any a priori analytical form for the electron polarization, our approach provides a quantitative picture of the steady state that agrees with the well known Borghini prediction based on thermodynamic arguments, as long as the electrons-nuclei transition rates are fast compared to the other relevant time scales. Substantially different final polarization levels are achieved instead when the latter assumption is relaxed in the presence of a nuclear leakage term, even though very weak, suggesting a possible explanation for the deviation between the measured steady state polarizations and the Borghini prediction. The proposed methodology also allows us to calculate nuclear polarization and relaxation times, once the electrons/nuclei concentration ratio and the typical rates of the microscopic processes involving the two spin species are specified. Numerical results are shown to account for the manifold dynamic behaviours of typical DNP samples.     

A study of nicotinic acid synthesis on a pilot installation and its simulation

Technological process parameters of the nicotinic acid synthesis by oxidation of β-picoline on a vanadium-titanium catalyst in a unit tube of a pilot installation were determined: conversion of β-picoline, yield and selectivity for products, and parametric sensitivity of the "hot point" temperature to variation of parameters at the reactor inlet. A mathematical simulation of the process was carried using the model of heat-and-mass transfer in a bed of a tubular reactor and the kinetic model of oxidation of β-picoline.     

Radiotracer investigation in a rotary fluidized bioreactor

A rotary fluidized bioreactor (RFBR) designed for treatment of wastewater was required to be investigated for its hydrodynamic behaviour and validation of design. A radiotracer investigation was carried out to measure residence time distribution (RTD) of wastewater in the RFBR using ⁸²Br as a radiotracer. The radiotracer was instantaneously injected into the inlet feed line and monitored at the inlet and outlet of the reactor using collimated scintillation detectors connected to a data acquisition system. The measured RTD data was treated and simulated using a tanks-in-series model and model parameters i.e. number of tanks describing the degree of mixing was obtained. The results of the investigation showed no flow abnormalities and the reactor behaved as an ideal continuously stirred-tank reactor at all the operating conditions. Based on the results, the design of the reactor was validated.     

Dynamical analysis of an elementary X + Y → P reaction in a continuously stirred tank reactor

This paper investigates the bifurcation behaviour of a model oxidation reaction in a continuously stirred tank reactor (CSTR). We assume that two gaseous chemical species are pumped separately into the CSTR, at constant total pressure, reacting to produce an inert product. The reaction is assumed to be a single step reaction that is described by Arrhenius kinetics. It is capable of producing oscillatory behaviour as well as steady state multiplicity in certain parameter regions. Bifurcation diagrams in various control parameter spaces are presented. We show that the system always possesses a globally attracting invariant region. The equivalence of a CSTR having n feed streams and the one pipe version, by appropriate rescaling, is also discussed.     

Evolution of products during the degradation of polyethylene in a batch reactor

The thermal degradation of two polyethylene samples (LDPE and HDPE) has been carried out in a batch reactor under dynamic conditions. The evolution of products generated after regular intervals of 5 min (temperature increments of approximately 25 °C) has been analyzed. The behaviour of LDPE and HDPE has been compared, and no differences in the quantity and weight fraction of the gaseous products obtained have been found. For both polymers, n-paraffins are the major products at the very beginning of the process, while as the decomposition proceeds 1-olefins are more abundant. The condensed fraction is much larger than the gaseous fraction and its analysis reveals some differences between the behaviour of LDPE and HDPE at the beginning of the degradation process. These differences disappear at higher temperatures where more similar trends are observed. 1-Olefins, n-paraffins, dienes and olefins with wide carbon number distributions are the most important condensed compounds obtained in the thermal degradation of both polyethylenes. The formation of 1-olefins and n-paraffins begins at slightly lower temperatures than for dienes and olefins. On the other hand, as the temperature increases, the amount of low and high molecular weight compounds increases at the expense of intermediate molecular weight products and the former become the most important by the end of the degradation process. This behaviour could be related to the thermal cracking of waxes through secondary reactions.