Optimization of the polycondensation step of polyethylene terephthalate formation with continuous removal of condensation products

The polycondensation step of poly(ethylene terephthalate) (PET) formation is assumed to include various side reactions in addition to the usual polymerization reaction. Polymerization in the batch reactor at constant pressure has been modeled to flash volatile components at the end of small discrete time intervals with polymerization occurring during the interval. The Hamiltonian has been written for the time interval and optimum temperature history computed for batch reactors for various reactor pressures. For low pressures, computed optimum temperature history is found to be of similar nature as that used in industry.     

Optimization of the polycondensation step of polyethylene terephthalate formation in semibatch reactors

The polycondensation stage of polyethylene terephthalate (PET) formation is assumed to include side reactions leading to the formation of diethylene glycol, vinyl end groups, and acid end groups, in addition to the usual polymerization of bis (2-hydroxyethyl) terephthalate (BHET) in semi-batch reactors. A, flexible objective function has been proposed with temperature and pressure as control variables. Computations from the first variation technique show that the pressure should be reduced to the lowest limit under all possible conditions. Consequently, optimal temperature profiles in batch reactors are obtained for various lower limits of reactor pressures using the combination of first and second variation techniques. For the first variation technique, the vector iteration method of computation was used, and the near optimal profile so obtained was used as the initial guess for the second variation technique. The result of optimization shows that the lower limit of pressure and weighting parameters appearing in the objective function have profound effects on the optimum profiles. For higher pressures, it is shown that a high temperature must be used initially; but must be lowered later to minimize the formation of side products. However, for lower pressures, the temperature must be increased from a low value initially; but for large polymerization times, the temperature must be further reduced to minimize the formation of side products. It is thus seen that the optimum temperature profile for low pressures exhibits a broad maximum.     

Steady-state and transient behavior of a continuous polyethylene terephthalate condensation polymerization process. I. Melt transesterification stage

The steady-state and transient behavior of a continuous stirred-tank reactor for melt transesterification of dimethyl terephthalate with ethylene glycol in the presence of metal acetate catalyst is presented. The kinetic model includes the main transesterification reactions and side reactions leading to diethylene glycol and carboxylic acid end groups. The effect of various reactro operating parameters such as [EG]/[DMT] mole ratio and feed catalyst concentration on the product distribution under steady-state reactor operating conditions is analyzed. The dynamic process model has also been solved and the reactor transients to step changes in various reactor parameters are reported.     

Adsorptive reactors for enhancing equilibrium gas-phase reactions—two case studies

The conversion enhancement potential of fixed-bed adsorptive reactors has been evaluated for two heterogeneously catalysed gas-phase equilibrium reactions: the Claus reaction used in sulphur recovery and the direct synthesis of hydrogen cyanide from carbon monoxide and ammonia. It was found that kinetics of both reaction and adsorption as well as adsorbent capacity have to be very compatible to achieve high conversions. The specific parameters of the reaction/catalyst system, such as the deposition of solids (e.g. sulphur, coke) or the formation of undesirable by-products have to be taken into account for the successful application of adsorptive reactor concepts. A crucial point is the selection of the reaction conditions (i.e. temperature), since the occurrence of side reactions may be enhanced in adsorptive reactor operation due to the inherently distorted concentration profiles.     

A modeling of semibatch reactors for melt transesterification of dimethyl terephthalate with ethylene glycol

A mathematical model of a semi batch reactor was developed to investigate the oligomerization reactions in the melt transesterification of dimethyl terephthalate with ethylene glycol catalyzed by metal acetate catalyst. The detailed kinetic scheme based on the molecular species model is used to estimate the conversion of methyl ester groups and the concentrations of various oligomeric species. The numerical simulation of the model shows that the oligomerization reactions lower the overall conversion of methyl ester end groups. Effects of ethylene glycol/dimethyl terephthalate mole ratios, reaction temperature and, catalyst concentration on the conversion, oligomer concentration, oligomer molecular weight, and molecular weight distribution were also analyzed.     

Solution of final stages of polyethylene terephthalate reactors using orthogonal collocation technique

The formation of polyethylene terephthalate (PET) has been modeled to have reactions with monofunctional compounds, redistribution, and cyclization reactions in addition to the usual polycondensation step. In the final stages, the overall polymerization is mass-transfer controlled and solution of the reactor performance equations have been determined through the orthogonal collocation technique. This technique is found to be considerably more efficient for PET reactors compared to the finite difference method; the use of ten collocation points gives results which are close to the exact solution.     

A novel reverse flow reactor coupling endothermic and exothermic reactions: Part II: Sequential reactor configuration for reversible endothermic reactions

The new reactor concept for highly endothermic reactions at elevated temperatures with possible rapid catalyst deactivation based on the indirect coupling of endothermic and exothermic reactions in reverse flow, developed for irreversible reactions in Part I, has been extended to reversible endothermic reactions for the sequential reactor configuration. In the sequential reactor configuration, the endothermic and exothermic reactants are fed discontinuously and sequentially to the same catalyst bed, which acts as an energy repository delivering energy during the endothermic reaction phase and storing energy during the consecutive exothermic reaction phase. The periodic flow reversals to incorporate recuperative heat exchange result in low temperatures at both reactor ends, while high temperatures prevail in the centre of the reactor. For reversible endothermic reactions, these low exit temperatures can shift the equilibrium back towards the reactants side, causing ‘back-conversion’ at the reactor outlet.The extent of back-conversion is investigated for the propane dehydrogenation/methane combustion reaction system, considering a worst case scenario for the kinetics by assuming that the propylene hydrogenation reaction rate at low temperatures is only limited by mass transfer. It is shown for this reaction system that full equilibrium conversion of the endothermic reactants cannot be combined with recuperative heat exchange, if the reactor is filled entirely with active catalyst. Inactive sections installed at the reactor ends can reduce this back-conversion, but cannot completely prevent it. Furthermore, undesired high temperature peaks can be formed at the transition point between the inactive and active sections, exceeding the maximum allowable temperature (at least for the relatively fast combustion reactions).A new solution is introduced to achieve both full equilibrium conversion and recuperative heat exchange while simultaneously avoiding too high temperatures, even for the worst case scenario of very fast propylene hydrogenation and fuel combustion reaction rates. The proposed solution utilises the movement of the temperature fronts in the sequential reactor configuration and employs less active sections installed at either end of the active catalyst bed and completely inactive sections at the reactor ends, whereas propane combustion is used for energy supply. Finally, it is shown that the plateau temperature can be effectively controlled by simultaneous combustion of propane and methane during the exothermic reaction phase.     

CO氧化偶联反应器模拟与优化

CO与亚硝酸甲酯(MN)氧化偶联制草酸二甲酯(DMO)是合成气制乙二醇过程的关键步骤，现有工业装置存在效率低的问题。采用包括副反应(生成碳酸二甲酯和甲酸甲酯)动力学的动力学模型和二维两相反应器模型，对CO氧化偶联的移热式固定床反应器进行建模，研究了换热方式及操作条件对反应器性能和安全性的影响。结果表明，以温度的二阶导数作为飞温的判据是灵敏和可靠的。与常规的逆流和恒温移热方式相比，并流移热使反应器形成更为均匀的温度分布，有利于提高反应器产能。增加入口MN含量会升高反应器MN转化率和热点温度；但是，由于CO、NO和反应物MN之间存在竞争吸附，增加入口CO和NO含量会导致MN转化率和热点温度降低，所以增加入口压力导致MN转化率降低。且热点温度对MN和NO的含量更为敏感，应严格控制入口MN和NO的含量。采用遗传算法进行反应器工况寻优，确定了最优的反应条件，可提高单台反应器对应的乙二醇(EG)年产能至12万吨。     

用于甲烷偶联的SrCe0.95Yb0.05O3-α中空纤维膜反应器建模

Proton-hole mixed conductor, SrCe0.95Yb0.05O3-α(SCYb), has the potential to be used as a membrane for dehydrogenation reactions such as methane coupling due to its high C2-selectivity and its simplicity for fabricating reactor systems. In addition, the mixed conducting membrane in the hollow fibre geometry is capable of providing high surface area per unit volume. In this study, mechanism of methane coupling reaction on the SCYb membrane was proposed and the kinetic parameters were obtained by regression of experimental data. A mathematical model describing the methane coupling in the SCYb hollow fibre membrane reactor was also developed.With this mathematical model, various operating conditions such as the operation mode, operation pressure and feed concentrations affecting performance of the reactor were investigated. The simulation results show that the cocurrent flow in the reactor exhibits higher conversion of methane and higher yield of ethylene compared to the countercurrent flow. In order to achieve the highest C2 yield, especially of ethylene, pure methane should be used as feed and the operating pressure be 300 kPa. Air can be used as the source of oxygen for the reaction and it soptimum feed velocity is twice of the methane feed velocity. The air pressure in the lumen side should be kept the same as or slightly lower than the vressure of shell side.     

The kinetics of formation of diethylene glycol in preparation of polyethylene terephthalate and its control in reactor design and operation

The kinetics of formation of diethylene glycol (DEG) (ether bond) which is an important side reaction in preparing polyethylene terephthalate was investigated. The following empirical rate expression is proposed for convenience of engineering applications It has been found by experiment that n = 0.9; apparent activation energy E = 8.2 kcal/mol. The equation can be used to predict the quantity of DEG formed in the reaction. The influence of the process conditions and reactor types on DEG content in polymer are also discussed.     

Diffusion of butanediol in poly(butylene terephthalate) (PBT) melt and analysis of pbt polymerization reactor with surface renewal

A reactor with surface renewal, originally designed for poly(ethylene terephthalate) (PET) polymerization, was applied for poly(butylene terephthalate) (PBT) polymerization. A comprehensive model including side reactions was developed and compared with the experimental results. The diffusivity of butanediol (BD) in PBT melt was measured separately by desorption experiments (D_b ? 1.08 × 10⁶ exp(?32600 / RT) (m²/min)). Optimum operating temperature for PBT polymerization was found to be around 250°C in order to avoid degradation.     

五釜工艺聚酯工业装置的稳态模拟

针对大型连续PTA直接酯化法PET工艺过程装置,以Aspen Plus和Polymers Plus为模型开发工具,建立了以反应和传质过程机理为基础的稳态模型。结果表明:该模型中包括了酯化反应、缩聚反应、二甘醇生成反应、链降解反应和乙醛生成等主副反应,且考虑了端羧基对酯化反应的自催化效应;更重要的是模型考虑了酯化阶段PTA在酯化反应器中的溶解过程和终缩聚阶段小分子的脱挥,并建立了小分子脱挥的传质系数与缩聚反应器内聚合度、黏度、温度和搅拌器转速等的关联;在此模型基础上模拟研究了第一酯化反应操作温度对各反应器出口指标的影响,指出酯化段的酯化率有一个适宜的控制范围。     

Multiobjective optimization of an industrial wiped film poly(ethylene terephthalate) reactor: some further insights

Multiobjective optimization of an industrial third-stage, wiped-film poly(ethylene terephthalate) reactor is carried out, using a pre-validated model. The two objective functions minimized are the acid and vinyl end group concentrations in the product. These are two of the undesirable side products produced in the reactor. The optimization problem incorporates an end-point constraint to produce polymer having a desired value of the degree of polymerization (DP). In addition, the concentration of the di-ethylene glycol end group in the product is constrained to lie within a certain range of values. The possible decision variables for the problem are the reactor pressure, temperature, catalyst concentration, residence time of the reaction mass in the reactor and the speed of rotation of the agitator. The nondominated sorting genetic algorithm (NSGA) is used to solve this multiobjective optimization problem. It is found that this algorithm is unable to converge to the correct solution(s) when two or more decision variables are used, and we need to run the code several times over (with different values of the computational variable, S_r, the seed for generating the random numbers) to obtain the solutions. In fact, this is an excellent test problem for future multiobjective optimization algorithms. It is found that when temperature is kept constant, Pareto optimal solutions are obtained, while, when the temperature is included as a decision variable, a global unique optimal point is obtained.     

Formation of poly(butylene terephthalate): Secondary reactions studied by model molecules

The secondary reactions occurring in the formation poly(butylene terephthalate), leading to thermal degradation (in the temperature range 210°–280°C) or giving rise to tetrahydrofuran (in the temperature range 160°–190°C) have been kinetically studied with the aid of model molecules: 1,4-butylene dibenzoate and 4-hydroxybutyl benzoate. Some kinetic parameters have been determined; the effect of temperature and of catalysts and stabilizers has been considered and a mechanism is proposed for the formation of tetrahydrofuran from hydroxyl end groups.     

Transesterification of dmt with ethylene glycol catalyzed by amberlyst 15

The catalytic effects of the macroreticular cation exchange resin Amberlyst 15 (H+) on the transesterification of dimethyl terepbthalate with ethylene glycol have been investigated in a batch reactor and at the temperature of 146°C with the aid of extensive chemical analysis. The resin exhibited significant intraparticle diffusion effects for the disappearance of dimethyl terephthalate and promoted the dehydration and subsequent etherification and polymerization reactions of ethylene glycol to a very significant extent. The side reactions of ethylene glycol rendered the transesterification process in the presence of Amberlyst 15 (H+) inefficient.     

Kinetics studies of ketazine formation: Effect of temperature and catalyst concentration

Formation of methyl ethyl ketazine is a distinct case of homogeneous catalyzed gas–liquid–liquid reactions. Kinetics studies of methyl ethyl ketazine formation has been carried out in a semi‐batch reactor. The effects of temperature and catalyst concentration on the percentage yield of ketazine have been studied extensively. The yield of ketazine is found to increase with increase in temperature and then levels off. Increase in catalyst concentration favours the formation of ketazine. The conversion of peroxide is found to increase with increase in temperature thus indicating that chemical reaction is rate‐limiting step in the system. The desired temperature for carrying out the reaction is found to be 60°C and the required catalyst to peroxide ratio is 2.5. The activation energy for the reaction is 24.5 kJ/mol.     

The effect of liquid flow distribution on catalytic hydrogenation of cyclohexene in an adiabatic trickle-bed reactor

The earlier formulated mathematical model of the distributed-flow adiabatic trickle-bed reactor has been used to predict the course of liquid-phase hydrogenation of cyclohexene. Computed solutions for independently obtained parameters and reaction kinetics have been compared with experiments on a laboratory trickle-bed reactor 3 cm in diameter packed with 3% Pd on activated carbon pellets. The model has been able to predict the experimentally found tailings on the cup-mixing mean outlet concentration versus bed depth curves. The existence of these tailings at the high conversion end cannot be accounted for by other mechanisms than the flow distribution effects. The model has been found useful for predicting the required depth of the catalyst under the real flow maldistribution conditions.     

A comparison of co-current and counter-current modes of operation for a novel hydrogen-permselective membrane dual-type FTS reactor in GTL technology

In this work, a comparison of co-current and counter-current modes of operation for a novel hydrogen-permselective membrane reactor for Fischer-Tropsch Synthesis (FTS) has been carried out. In both modes of operations, a system with two-catalyst bed instead of one single catalyst bed is developed for FTS reactions. In the first catalytic reactor, the synthesis gas is partly converted to products in a conventional water-cooled fixed-bed reactor, while in the second reactor which is a membrane fixed-bed reactor, the FTS reactions are completed and heat of reaction is used to preheat the feed synthesis gas to the first reactor. In the co-current mode, feed gas is entered into the tubes of the second reactor in the same direction with the reacting gas stream in shell side while in the counter-current mode the gas streams are in the opposite direction. Simulation results for both co-current and counter-current modes have been compared in terms of temperature, gasoline and CO₂ yields, H₂ and CO conversion, selectivity of components as well as permeation rate of hydrogen through the membrane. The results showed that the reactor in the co-current configuration operates with lower conversion and lower permeation rate of hydrogen, but it has more favorable profile of temperature. The counter-current mode of operation decreases undesired products such as CO₂ and CH₄ and also produces more gasoline.     

The synthesis of poly(butylene terephthalate) from terephthalic acid,part I: The influence of terephthalic acid on the tetrahydrofuran formation

An in depth study is performed on the origin of and influences on the formation of tetrahydrofuran (THF) during the first stage of the terephthalic acid (TPA) based synthesis of poly(butylene terephthalate) (PBT). Although many improvements on the synthesis process of PBT have been reported in literature to suppress this undesired side reaction, only few studies reported on the actual mechanism of the THF formation, which is not completely understood. Low molecular weight compounds have been used to model the side reactions occurring during the polymerization reaction. It could be concluded that, in contrast to previous reportage, only the THF formation from the monomer, 1,4‐butanediol, is directly influenced by the use of TPA as a starting material for the production of PBT. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

The study of oxazolidone formation from 9,10-epoxyoctadecane and phenylisocyanate

The formation of oxazolidone from 9,10-epoxyoctadecane and phenylisocyanate was studied. One branch of epoxidized vegetable oil with one epoxy group per chain corresponds to 9,10-epoxyoctadecane. This model could explain the probability of oxazolidone formation from natural oil-derived epoxides. Epoxidized natural oils are TG consisting of glycerin and three FA with or without one to three epoxy groups in the middle of the chain. To study oxazolidone formation from an internal epoxy group without possible interference from the side reactions on the ester group, 9,10-epoxyoctadecane was selected as the most appropriate model compound. Epoxy groups in the middle of allong aliphatic chain are of low reactivity toward isocyanates, and preparation of oxazolidones requires fairly harsh conditions such as high temperatures and catalysts, which also promote side reactions. The dominant side reaction is rearrangement of the epoxy groups. We found that the direction and magnitude of the rearrangement and the yield of any particular product depended on the catalyst used. Lithium chloride, aluminum trichloride, and zinc iodide catalyzed oxazolidone formation, along with the catalysis of side reactions such as ketone and carbonate formation. Aluminum trichloride showed the highest conversion of 9,10-epoxyoctadecane to oxazolidone. Aluminum triisopropoxide, triphenylantimony iodide, and imidazole did not catalyze the formation of oxazolidone. They were effective as catalysts of epoxy group rearrangement and promoted the formation of hydroxyl, ketone, and carbonate compounds. Hydroxyl groups reacted with isocyanate to produce urethane.