全混流反应器热稳定性的参数敏感性分析

有许多伴随着大量的热量释放的化学反应过程,是在多相反应器(非均相催化反应器、气液反应器)中完成的。从安全的角度来看,它们代表了化学工业中最危险的操作单元。对这类全混流特质反应器,操作参数对对其热稳定性的影响程度可以用参数热敏感性加以表述。本文推到了操作过程中变化的参数V_0、C_(A0)、T_i、T_c、U、(-H_r)、x_A对热稳定性的影响。     

Parametric sensitivity in fixed-bed reactors

Parametric sensitivity in fixed-bed reactors has been analysed by previous workers using the quasihomogeneous model [i.e. for the kinetic regime only]. In the present paper a general treatment is proposed in which transport limitations are also considered. It is shown that the critical inlet partial pressure is strongly dependent on the transport processes. Based on the analysis presented, four regimes of fixed-bed reactor operation are postulated and conditions are stated to identify the regime in which a given system would operate. These conditions are applied to some industrially important reactions to show that the regimes represent realistic systems. Procedures are developed to obtain the critical inlet partial pressure for systems lying in the various postulated regimes. Illustrative examples are then solved to demonstrate the procedures developed in the text. The correctness of the procedures is established by comparing the values of the critical inlet partial pressure obtained from the present analysis with those obtained from numerical solution.     

Parametric sensitivity in fixed-bed catalytic reactors

This paper introduces a criterion for runaway in fixed-bed catalytic reactors based on the behaviour of sensitivity coefficients along the reactor. The criterion can be used when the temperature variation in a co-current cooling medium is taken into account; and also, it is not restricted to simple kinetics. In addition, multiple reactions may be analysed. Some examples of industrial significance are studied and the results are compared with those obtained by other authors.     

Parametric sensitivity in tubular polymerization reactors

A recent technique for studying the parametric sensitivity of chemical reactors is applied to tubular chain homopolymerization reactors. The sensitivities of the temperature maxima with respect to various parameters of the model are computed. Using conditions typically encountered for high-pressure polyethylene systems, it is found that the temperature sensitivities with respect to all the nine parameters have their maxima at approximately the same value of the feed initiator concentration, thus leading to a generalized sensitivity-based constraint for design. It is also found that, under usual conditions of operation, no significant design constraints on the feed temperature are indicated. Detailed sensitivity plots are presented, which could be used to obtain “safe” operating conditions. The effects of changing the most important parameters, the dimensionless heat of reaction and the dimensionless activation energy (ϵ), on the sensitivity envelope are also investigated. Our studies reveal that better estimates of ϵ than are available presently are required. Sensitivities of the number-average chain length maxima with respect to the same nine parameters are also computed. Under conditions where the steady-state hypothesis applies, estimates of these sensitivities can be obtained analytically. However, for the usual values of the parameters, and close to “sensitive” values of the feed initiator concentration, this hypothesis does not apply, and the chain length sensitivities need to be obtained numerically. In the absence of the gel effect, chain length sensitivities do not usually provide design constraints because of the very low monomer conversions encountered.     

Parametric sensitivity in co-currently cooled tubular reactors

An intrinsic criterion was derived for parametric sensitivity and runaway in co-currently cooled tubular reactors. It is essentially based on the approach of Van Welsenaere and Froment (1970) developed for the constant wall temperature case and which makes use of fundamental properties of the trajectory in the temperature vs. partial pressure plane. The criterion limits the maximum allowable temperature on an objective basis and permits the calculation of the corresponding critical inlet values for the operating variables, like partial pressure of the reactant, temperature of the reactor fluid, temperature of the cooling medium, coolant flow rate, reactor throughput ... . In addition to the rigorous treatment, a simple extrapolation procedure is presented, which proves to be very useful and accurate, provided the conditions are not too severe.     

Refolding of proteins in a CSTR

A model to predict refolding of proteins in a continuous stirred tank reactor (CSTR) was developed and compared to a batch refolding process with simple dilution of the protein in a stirred tank reactor. For experimental verification of the model a continuous refolding of a model protein (α-lactalbumin) was performed in a CSTR. The refolding process of denatured and fully reduced α-lactalbumin could be accurately predicted by a set of differential equations assuming a first order reaction rate for folding and a second order reaction rate for aggregation. The system composed of a CSTR with an additional diafiltration circuit for removal of denaturing agents from the feed solution and to maintain constant refolding conditions. Based on the folding kinetic the dynamic behavior of such a continuous refolding reactor was simulated under different operating conditions. It was shown that the refolding efficiency was higher compared to batch dilution under certain conditions, namely high residence times. The yield of refolded protein could further increased by recycling the outlet stream containing unfolded protein to the reactor entrance.     

乙烯环氧化反应器操作参数敏感性分析

采用乙烯环氧化制取环氧乙烷单管一维数学模型计算和实验,研究乙烯环氧化反应操作参数如原料入口温度、冷却介质温度、总传热系数、空速以及入口乙烯、氧和二氧化碳体积分数对列管式反应器轴向热点温度分布和转化率、选择性的影响.结果表明:冷却介质温度、空速以及总传热系数对反应器热点温度有较大影响.在原料入口温度120-210℃、乙烯...     

Regeneration of polymeric adsorbents in a CSTR

The regeneration step is crucial in the design of cyclic fixed bed adsorbers. In order to understand the regeneration process of a polymeric adsorbent (Duolite ES861) saturated with phenol by using sodium hydroxide simple dynamic experiments in a CSTR were done. Results were explained roughly by a simple equilibrium model and more accurately by a reaction front model which will be later used in a package for the design of cyclic fixed bed adsorption processes.     

Double-command fuzzy control of a nonlinear CSTR

In this research, double-command control of a nonlinear chemical system is addressed. The system is a stirred tank reactor; two flows of liquid with different concentrations enter the system through two valves and another flow exits the tank with a concentration between the two input concentrations. Fuzzy logic was employed to design a model-free double-command controller for this system in the simulation environment. In order to avoid output chattering and frequent change of control command (leading to frequent closing-opening of control valves, in practice) a damper rule is added to the fuzzy control system. A feedforward (steady state) control law is also derived from the nonlinear mathematical model of the system to be added to feedback (fuzzy) controller generating transient control command. The hybrid control system leads to a very smooth change of control input, which suits real applications. The proposed control system offers much lower error integral, control command change and processing time in comparison with neuro-predictive controllers.     

Parametric sensitivity of a fixed bed catalytic reactor: Cooling fluid flow influence

Temperature variation in a cocurrent cooling medium and its influence on the operation of a fixed bed catalytic reactor of the type used for orthoxylen oxydation is examined. Two characteristic regimes are found: a first regime where temperature shows a maximum at a finite axial reactor position (MFARP and a second regime where temperature is always increasing with the axial reactor coordinate: pseudoakiabatic operation (PO). The relative importance o ach operating mode may be observed by modifying the cooling flow. Two equations are derived from these findings. One of them estimates the limiting condition between MFARP temperature curves and PO curves. The other equation establishes an a prioi run away criterion for hot spot operation, being an extension of a previous formula derived by Van Welsenaere and Froment.     

Parametric sensitivity and runaway in fixed bed catalytic reactors

The paper introduces two intrinsic criteria for runaway in fixed bed tubular reactors based on the occurrence of characteristic points in the temperature profile along the reactor. Their transposition in the partial pressure-temperature phase plane allows the derivation of some very simple formulae for the prediction of the critical values for the operating variables. The results are compared with those obtained from Barkelew's empirical criterion. The paper also treats the related problem of a hot spot ehich has to be limited for reasons after the runaway. The simple formulae mentioned above can also be used in this case to determine the operating conditions.     

Intricate CSTR dynamics

Various dynamical characters of continuous stirred tank reaclors (CSTR) are introduced with respect to the effects of reaction types, extra thermal capacitance, periodic forcing, and coupling of CSTRs. The subject includes the classical dynamics of two-dimensional model and the variety of complex dynamics in three or higher dimensional systems such as periodic bifurcations to torus or chaos, aperiodic oscillations on invariant torus, and universal dynamics of alternating periodic-chaotic sequences with k.2^v-cycles for every natural number k. Particularly this review intends to bring about the problems that the engineers must be prepared to encounter in solving various physical systems including chemically reacting systems.     

Parametric sensitivity of chain polymerization reactors exhibiting the trommsdorff effect

The recently developed mathematical technique for studying the parametric sensitivity of reactors is extended to include chain polymerization systems exhibiting the gel and glass effects, as well as physical property variations. The sensitivities of the two temperature maxima with respect to various parameters are computed. It is found that, for a sample system, poly(methyl methacrylate), all the sensitivities of the gel effect induced temperature peak attain their maxima at the same conditions—this leads to a generalized temperature sensitivity constraint applicable to reactor design or operation. This sensitivity boundary is associated with high conversions and high molecular weights. The analysis shows that the dimensionless propagation activation energy, ?_p, and the dimensionless initiation activation energy, ?_d, are the two most important parameters governing the system performance. Sensitivities of the gel effect-induced number average chain length peak with respect to various parameters are also obtained. Again, all of these chain length sensitivities show maxima at the same condition, leading to the concept of a generalized chain length sensitivity criterion of constraint. Most importantly, the temperature and chain length sensitivity boundaries are virtually identical.     

Micromixing effects on complex reactions in a CSTR

Several errors and mistakes are pointed out in a paper published by Rao and Rao under the above title, concerning the influence of micromixing on conversion and yield for consecutive-competing second order reactions. The influence of the stoichiometric ratio and the existence of limiting conversion and yield for instantaneous reactions with unmixed feed are especially overlooked in their simulations. Several of their plots are erroneous. A correct treatment of these different points is given, using in particular a deterministic micromixing model (IEM) which is found to show definite advantages over random coalescence simulation models.     

Parametric sensitivity analysis of complex process flowsheets using sequential modular simulation

An algorithm for parametric sensitivity analysis of complex process flowsheets has been developed for use with sequential modular simulators. The method reduces the calculational effort when several sensitivities are calculated from the same flowsheet. A study of the efficiency of these methods is presented in the context of two example problems. Computational savings of order 100 are reported relative to sensitivity analysis by the continuation (restart) method. The method presented allows direct analysis of dependent variable variations which can be used to examine the effects of model prediction error. Furthermore, the algorithm lends itself to a structural analysis to quickly identify which variables in a flowsheet yield zero sensitivities and may be eliminated a priori from the sensitivity calculation.     

Micromixing effects on complex reactions in a CSTR

Micromixing effects on consecutive-competing second order reactions in an ideally macromixed CSTR are investigated by the use of the Two Environment Model of Ng and Rippin for mixed feed, and the model of Spielman and Levenspiel for unmixed feed.General plots are obtained for various values of reaction and mixing parameters. Yield of the desired product is found to show a maximum at intermediate states of micromixing for unmixed feed conditions. For mixed feed condition, micromixing effects are found to be very small.     

Experimental evaluation of a model-based degree of segregation in a CSTR

Conversions measured at the outlet of a CSTR, in which sodium bromoacetate reacted isothermally with sodium thiosulfate, were found to lie between the macro and microfluid conversion limits.On the basis of a reactor model, these data led to evaluations of a degree of segregation within the reacting mixture. This degree of segregation decreases linearly with increasing propeller speed. This rate of decrease is greater the larger the propeller diameter and the higher the propeller height above the reactor bottom. Feed jet velocity had no significant effect on this rate.This degree of segregation is inversely proportional to Reynolds' number to the $\text{1}\text{6}$th power.     

Micromixing and multiple steady state transitions in a CSTR

Using the micromixing concept introduced by Zwietering we describe the effect of incomplete stirring on consecutive reactions exhibiting bistability transitions in a CSTR. In particular we explain the difference in the behaviour of the system when reactant feed streams are premixed or non-premixed. A perturbation expansion around the fast stirring limit provides analytical results.     

On-line monitoring of a CSTR polymerization reactor using multivariable filters

Three multivariable filters are evaluated for on-line monitoring of a CSTR polymerization reactor. The first filtering algorithm is the Kaiman filter. This linear filter is simple to implementation, but cannot exactly estimate the dynamic behavior of the polymerization reactor. To compensate the state model inadequacies, nonlinear models can be considered in the filtering algorithm. The precise state estimation can be guaranteed by the extended Kaiman filter (EKF). Finally, the auto-regressive exogenous inputs model based filter (ARXF) is developed to reduce the modeling cost. These different filters are applied to the continuous solution polymerization of a MMA-AIBN-EA system as a case study. The ARXF is easy to implement and shows satisfactory results.