Liquefaction of coal by SRC-II process: Part IV: Steady-state thermal behavior of the reactor

An axial dispersion thermal model of an SRC-II reactor is developed. The model is validated by simulation of steady-state temperature profiles in the Fort Lewis (Tacoma, Washington) pilot-plant SRC-II reactor. The simulation shows that normal operations of the SRC-II reactors are thermally unstable. Changes in the process conditions can be utilized to achieve thermal stability for the reactor at normal SRC-II operating temperatures, i.e., reactor outlet temperature of less than 465°C and average reactor temperature of about 455°C. However, thermal stability of the reactor would always result in reduction in the rate of reaction. Regions of unstable steady-state reactor temperature over large ranges of ash concentrations in the reactor and reactor feed temperatures are also illustrated.     

Experimental study of the ignition behaviour of an adiabatic SRC-II coal liquefaction reactor

Experimental data are presented which describe the thermal behaviour of a bench-scale adiabatic coal liquefaction reactor operating in an open loop. The reactor employed external feedback control for maintaining adiabaticity. Conditions for both ignition and quench have been found for coal liquefaction at SRC-II operating conditions where repeated ignition/quench behaviour was demonstrated. No stable steady states were found between 450 °C and 475 °C for SRC-II operation. Ignition occurred at a feed temperature of ≈415 °C. The low steady states occurred at conditions of essentially no heat generation; at a very low extent of reaction. Some unexpected evidence of a preheater effect on reactor ignition was observed; however, the effect of preheater temperature profiles on reactor performance was not systematically studied.     

MASS TRANSFER EFFECTS IN LIQUEFACTION OF COAL BY SRC-II PROCESS PART II STUDY OF BUBBLE COLUMN REACTORS

The effects of hydrogen mass transfer resistance in large-scale SRC-II bubble column reactors (BCR), over large ranges of process variables, are studied. Due to the interactive effects of mass transfer resistance and gas hold up, the hydrogen consumption or liquid yield in a BCR has a maximum with respect to the specific mixing power. Under normal SRC-II process conditions a superficial gas velocity of about 0.01 m/s represents the optimum with respect to the hydrogen consumption or liquid yield. In general, the product quality requirement rather than the rate of hydrogen consumption determines the minimum specific mixing power requirement. Increase in hydrogen partial pressure can be used to reduce the level of mixing power required to maintain the desired product quality. Interrelations between mass transfer and gas hold effects and the variations in hydrogen concentration in slurry over large ranges of process conditions are also illustrated. This work provides some bases for the selection of reactor dimensions and process conditions for an SRC-II bubble column reactor (BCR).     

MASS TRANSFER EFFECTS IN LIQUEFACTION OF COAL BY SRC-II PROCESS PART II STUDY OF BUBBLE COLUMN REACTORS

The effects of hydrogen mass transfer resistance in large-scale SRC-II bubble column reactors (BCR), over large ranges of process variables, are studied. Due to the interactive effects of mass transfer resistance and gas hold up, the hydrogen consumption or liquid yield in a BCR has a maximum with respect to the specific mixing power. Under normal SRC-II process conditions a superficial gas velocity of about 0.01 m/s represents the optimum with respect to the hydrogen consumption or liquid yield. In general, the product quality requirement rather than the rate of hydrogen consumption determines the minimum specific mixing power requirement. Increase in hydrogen partial pressure can be used to reduce the level of mixing power required to maintain the desired product quality. Interrelations between mass transfer and gas hold effects and the variations in hydrogen concentration in slurry over large ranges of process conditions are also illustrated. This work provides some bases for the selection of reactor dimensions and process conditions for an SRC-II bubble column reactor (BCR).     

Free radicals in coal and coal conversions. 6. Effects of liquefaction process variables on the in-situ observation of free radicals

To determine the effects and relative importance of process variables in coal liquefaction, a uniquely designed and fabricated high-pressure/high-temperature electron spin resonance (e.s.r.) apparatus is used to monitor the in-situ formation and behaviour of free radicals, which are generally assumed to be the key factor. It is concluded that the temperature is the most significant single process variable that affects free radical formation; for Powhatan No. 5 coal there is a 9-fold increase in going from 400 to 460 °C. At 460 °C the other process variables tested can affect significantly the free radicals significantly, but at 400 °C these variables have essentially no effect on free radicals formation. The next most significant effect is due to the combination of solvent nature and residence time. Tetralin and the SRC-II heavy distillate quench the free radicals from Powhatan No. 5 to the same extent with one significant difference. In tetralin the maximum concentration is observed shortly after the slurry achieves its highest temperature, whereas in the SRC-II heavy distillate experiments the concentration is still increasing, at 460 °C, even after 1 h of reaction. The heating time, pressures and types of gas used affect the free radical concentration to a much smaller extent. The conversions obtained in the in-situ e.s.r. experiments using SRC-II heavy distillate as the solvent are somewhat lower than those obtained with tetralin as the solvent. The corresponding oil yields with tetralin are considerably higher than with SRC-II heavy distillate.     

气相法聚乙烯工艺冷凝态操作模式的稳定性和动态行为

气相法聚乙烯工艺冷凝态操作模式由于显著提高了循环气移热能力和反应器时空产率,已成为流化床乙烯聚合工艺的主流操作模式。建立了气相法聚乙烯工艺冷凝态操作模式的数学模型,包括流化床反应器模型,多级换热器模型和反应温度、压力以及循环气组成的控制模型。基于此,采用流程模拟方法,计算了系统在反应器温度采用闭环控制时的稳态解;根据系统对小扰动的动态响应特点,定性判断了反应器温度采用开环控制和闭环控制时聚合反应系统的稳定性;考察了系统对1-己烯分压和催化剂进料速率的阶跃响应特性。结果表明,反应器温度采用闭环控制时,聚合反应系统在所考察操作条件下均是稳定的,而采用开环控制时,解曲线被分叉点分割为稳定区域和不稳定区域。反应器温度对1-己烯分压阶跃变化的动态响应表明聚合反应系统存在长、短周期两类振荡,表明冷凝态操作模式下乙烯聚合反应过程是一个多控制回路耦合的复杂过程。     

Liquefaction of coal by SRC-II process: Part I: A new kinetic model

The kinetic experiments were carried out in a continuous stirred tank reactor. Practically important ranges of SRC-II reactor temperature (444–466°C), pressure (10.4–20.8 MPa), nominal slurry residence time (0.54–1.62 h), coal concentration in the feed slurry (25–35 wt%), and inorganic mineral matter concentration (4.75–13.43 wt%) were covered in a total of 43 experimental runs. In each of the experimental runs, the feed slurry was formulated by using vacuum tower bottoms from SRC-II pilot plants using the same feed coal (Powhatan No. 5), to obtain feed compositions similar to those obtained in SRC-II pilot-plant recycle operation. The kinetic model considers the overall conversion to be achieved in two stages. The first stage is the instantaneous dissolution of coal and in the rate controlled second stage all the reactive organic components in the liquid phase are initially assumed to react, each yielding components lighter than itself. The distribution of products in each reaction stage is considered to be independent of the operating conditions. The best rate controlled (second stage) reaction scheme and values of the unknown parameters are obtained by minimizing the overall difference (i.e. for all the components over all the runs) between the measured and model predicted mass fractions of the various components in the reactor. This analysis identifies the reaction of solvent refined coal (pyridine soluble organic matter boiling above 482°C) to be the only significant reaction in the second stage and its rate is determined to be ^-r_SRC = 1.567 × 10⁵ exp (-79.16/RT) · p^0.28 · X_ASH, kg/L h. Overall error in this analysis yielding the reaction scheme, r_SRC and values of product distribution coefficients for both the reaction stages is less than 8% absolute i.e. ±4%.     

SOME ASPECTS OF DESIGN OF COMMERCIAL REACTORS FOR DIRECT COAL LIQUEFACTION

In recent years, a number of direct coal liquefaction processes have been developed. All processes use a slurry type reactor. Although for lab-scale reactors of large length-to-diameter ratio the use of highly sophisticated slurry reactor model may be justified, simple considerations can meaningfully elucidate the behavior of industrial reactors. A simple analysis shows that the coal liquefaction is controlled by intrinsic kinetics. Both gas and slurry phases can be assumed to be completely backmixed in large diameter reactors. A simple analysis of the thermal behavior revealed multiplicity for a fairly wide range of operating conditions. In most cases, the intermediate unstable steady state is close to the temperature observed in adiabatic coal liquefaction reactors (with and without quench). Due to the unstable character of the operation, point pathological phenomena like runaway may be possible and a close feedback control of the commercial reactor may be required.     

Dynamic mathematical modeling of a novel dual-type industrial ethylene oxide (EO) reactor

In this paper, the dynamic behavior of a novel dual-type industrial ethylene oxide reactor has been proposed with taking catalyst deactivation into account. The configuration of two catalyst beds instead of one single catalyst bed is developed for conversion of ethylene to ethylene oxide. In the first reactor which is an industrial fixed-bed water-cooled reactor, the feed gas is partly converted to ethylene oxide. This reactor functions at very high yield and at a higher than normal operating temperature. In the second converter, the reaction heat is used to preheat the feed gas to the first reactor and a milder temperature profile is observed. The potential possibilities of a two-stage catalyst bed system are analyzed using a 1D heterogeneous dynamic model to obtain necessary comparative estimates. A differential evolution (DE) algorithm is applied as an effective and robust method to optimize the reactors length ratio. The results obtained from the simulation demonstrate that there is a desirable catalyst temperature profile along the dual-type reactor (DR) compared with the conventional single-type reactor (SR). In this way, the catalysts are exposed to less extreme temperatures and thus, diminishing the catalyst deactivation via sintering. Results from this study provided beneficial information about the effects of reactors configuration on catalyst lifetime and ethylene oxide production rate simultaneously.     

Dynamics of a continuous stirred tank reactor for styrene polymerization initiated by a binary initiator mixture. II: Effect of viscosity dependent heat transfer coefficient

The dynamic behavior of the solution polymerization of styrene in a continuous stirred tank reactor is analyzed with a mixture of tert-butyl perbenzoate and benzoyl peroxide as an initiator system. In the modeling of the reactor, a viscosity dependent reactor wall heat transfer coefficient is used to account for the changing heat transfer efficiency as monomer conversion and polymer molecular weight increase. The steady state and bifurcation behaviors have been investigated with the reactor residence time, initiator feed composition, initiator concentration, feed solvent volume fraction, and coolant temperature as bifurcation parameters. Unlike the reactors with constant heat transfer coefficient, the present system exhibits relatively simple steady state and dynamic bifurcation behaviors. Oscillatory behavior is observed only when the solvent volume fraction in the feed exceeds 0.2. The dynamic simulation of the reactor also indicates that a feedback temperature controller may fail to maintain the reactor temperature when the heat transfer coefficient changes as a result of process disturbances.     

Instationäre Prozeßführung kontinuierlicher Reaktoren

Non-steady-state operation of continuous reactors . The behaviour of continuous chemical reactors can be altered significantly by forced oscillation of the reactor parameters such as feed concentration of the reactants, flow rate, and temperature. In contrast to autonomous oscillations in unstable systems, the forced periodic disturbances are controlled and can be used as a further parameter for process optimization. The dynamic behaviour of individual steps of the overall reaction and of the reactor can be exploited to obtain performances and selectivities which cannot be accomplished in the traditional way on steady-state operation under comparable conditions. This is shown in discussing some simple kinetic schemes published in theoretical studies and comparing them with experimental results for homogeneous and heterogeneous reactions. The physical and chemical causes for the predicted and observed advantages of periodic operation are discussed.     

Dynamics and stability of ethylene polymerization in multizone circulating reactors

Multizone circulating bed reactors (MZCR) have the exclusive characteristics of producing polymers of different molecular weights in a single particle. Traditional fluidized bed reactors, on the other hand, can produce only one kind of molecular weight with relatively narrow distribution. A dynamic model for the MZCR is used to illustrate the basic dynamic behavior of the new reactor design used for polyethylene production. The model is used to study the copolymerization of ethylene with butene. Several parameter sensitivity analyses are performed to show the computer-simulated time responses for reactor temperature, number-average molecular weight, weight-average molecular weight, catalyst feed rate and the monomer/comonomer concentration along the reactor length. At certain operating conditions dynamic instability is observed and the results for the effect of cooling water temperature, catalyst feed rate, monomer and comonomer initial feed concentration on the reactor temperature and polymer molecular weight reveal that the system is very sensitive to disturbances in the heat exchanger coolant temperature. Also, at some operating conditions, the reactor temperature oscillates above the polymer melting temperature. Temperature runaway above polymer softening point is a serious problem which may cause polymer melting and hence reactor shutdown. The oscillatory behavior of the reactor temperature necessitates a suitable temperature control scheme to be installed.     

Dynamic simulation of a cascade fluidized-bed membrane reactor in the presence of long-term catalyst deactivation for methanol synthesis

In this work, a dynamic model for a cascade fluidized-bed hydrogen permselective membrane methanol reactor (CFBMMR) has been developed in the presence of long-term catalyst deactivation. In the first catalyst bed, the synthesis gas is partly converted to methanol in a water-cooled reactor, which is a fluidized-bed. In the second bed, which is a membrane assisted fluidized-bed reactor, the reaction heat is used to preheat the feed gas to the first bed. This reactor configuration solves some observed drawbacks of new conventional dual type methanol reactor (CDMR) and even fluidized-bed membrane dual type methanol reactor (FBMDMR) such as pressure drop, internal mass transfer limitations, radial gradient of concentration and temperature in both reactors. A dynamic two-phase theory in bubbling regime of fluidization is used to model and simulate the proposed reactor. The proposed model has been used to compare the performance of a cascade fluidized-bed membrane methanol reactor with fluidized-bed membrane dual-type methanol reactor and conventional dual-type methanol reactor. The simulation results show a considerable enhancement in the methanol production due to the favorable profile of temperature and activity along the CFBMMR relative to FBMDMR and CDMR systems.     

Kinetic rate model interpretation of the SRC-II liquefaction of an Ireland Mine coal

An experimental data base for characterizing the SRC-II liquefaction of an Ireland Mine coal is interpreted using an empirical rate model developed from SRC-II liquefaction data for a Powhatan No. 5 coal exclusively. The model predictions for hydrogen consumption and for the yields of the key components; C₁-C₄ gases, C₅-755 K liquids, and solvent-refined coal (SRC) are compared with the experimental values over a wide range of reactor operating conditions; temperature (703–743 K), pressure (10.2–20.4 MPa), and recycle ash (2.0–10.4 wt%). Model predictions are in good agreement with hydrogen consumption measurements at baseline conditions (728 K and 13.6 MPa) and at more severe conditions (743 K and 20.4 MPa) for a feed slurry containing 30 wt% coal and 10.4 wt% recycle ash (absolute error ± 6.4%). A similar result is obtained forthe C₁-C₄ gases and the C₅-755 K liquids at the baseline conditions; however, the error in the SRC yield prediction is higher (± 12.1%). The model predictions for the key components at the higher severity conditions deviate more than those for the baseline conditions.     

An improved weighted average reactor temperature estimation for simulation of adiabatic industrial hydrotreaters

A study on the improvement of the representative operating temperature from the temperature profile of an industrial adiabatic reactor is presented. This temperature is used to simulate the reactor performance by small scale laboratory isothermal reactors. An improved methodology for the estimation of a Weighted Average Bed Temperature (WABT) was elaborated to simulate an industrial multi-bed HDS reactor. The improved WABT, so called Weighted Average Reactor Temperature (WART), was compared with the most usually used WABT in a wide range of operational conditions as well as of kinetic parameters. In case of a multi-bed industrial hydrotreater, where quench zones are located between the beds and the H₂ flow rate, which enters each bed, is different, the optimal gas to oil ratio was estimated for the laboratory-scale reactor.     

High-temperature gasification of carbonaceous materials by flash pyrolysis: thermal aspects

Finely divided biomass was pyrolysed in heated tubé reactors at 700–1000 °C and 250–300 K s^{− 1} of the materials. It is concluded that the main thermal parameter is the reactor temperature at various heights. The range 800–900 °C mainly gives a fuel gas, while 900–1000 °C yields a synthesis gas. A secondary thermal parameter is the heating rate, which depends on the reactor temperature and the particle size. The results obtained are as good as at higher rates and can be compared with those under calculated equilibrium conditions. A third thermal parameter is the quenching effect on the entities immediately after formation. This effect depends on the vector gas flow and on the reactor geometry in the hot zone. The complex ‘chemical plasma’ generated by flash decomposition combines almost as fast as it is produced, under conditions which are very far from equilibrium. Generally, the shorter the residence time, the greater the difference from the equilibrium values. At high temperatures, short residence times of gases in the hot zone appear to increase the intensity of thermal shock to the particles, so some characteristics such as gasified carbon can exceed equilibrium values.     

A comparison of auto-thermal and conventional methanol synthesis reactor in the presence of catalyst deactivation

This study proposed a one-dimensional dynamic plug flow model to analyze and compare the performance of an auto-thermal and a conversional methanol synthesis reactor in the presence of catalyst deactivation. An auto-thermal two-stage industrial methanol reactor type is a system with two catalyst beds instead of one single catalyst bed. In the first catalyst bed, the synthesis gas is partly converted to methanol in a water-cooled reactor. In the second bed which is a gas-cooled reactor, the reaction heat is used to preheat the feed gas to the first bed. To analyze the effect of important control variables on the rector performance, steady state and dynamic simulations are utilized to investigate effect of operating parameters on the performance of reactors. The simulation results show that there is a favorable profile of temperature along the two-stage auto-thermal reactor type in comparison with conventional single stage reactor type. In this way the catalysts are exposed to less extreme temperatures and, catalyst deactivation via sintering is reduced. Overall, this study resulted in beneficial information about the performance of the reactor over catalyst life-time.     

Toward autothermal and hydrogen‐producing sorbent regeneration for calcium‐looping

This paper presents a simulation study on the kinetic performance of combined methane combustion, steam/dry reforming, and limestone calcination for autothermal, hydrogen‐producing, and rapid sorbent regeneration in turbulent fluidized bed reactors. The effects of key operational factors are investigated at reactor pressures of 1 bar to 5 bars, including reactor temperature, CaCO₃/total gas molar feed ratio, and sorbent residence time. The results are compared to those for conventional steam calciners, demonstrating the potential for superior performance of this novel sorbent regeneration technology under certain circumstances. A simple, but effective, design methodology is then suggested to determine the proper range of operating conditions and/or reactor dimensions for limestone calcination using this process.     

Improved Design of the Lurgi Reactor for Methanol Synthesis Industry

Current studies are devoted to promote the production yield of the methanol synthesis process for treating large feed capacities in Algerian methanol manufacture industry by designing new reactor technologies. In order to achieve a high yield of methanol, the performance of methanol synthesis is improved by substituting the quench reactor by a new Lurgi reactor. The design of operating parameters of the Lurgi reactor involves the effect of CO₂ injection on methanol production yield and the catalyst deactivation. The simulation results demonstrate that under the same industrial operating conditions the conversion rate of reactants increases from 23 % in the quench reactor to 37 % in the Lurgi reactor and the methanol yield can be increased by 33 % when substituting the quench reactor by the Lurgi reactor     

Anionic polymerization of styrene: The search for an industrial process

Anionic polymerizations were carried out in the laboratory using a CSTR reactor design and conditions typical of current commercial mass polystyrene plants. Polystyrene having excellent color and polydispersity was produced. Polymer quality, styrene conversion, and molecular weight control were all linked to use of polymerization feed of consistently high purity and polymerization in the 90–110°C temperature range. The results of this study clearly show that high quality polystyrene can be made utilizing anionic polymerization chemistry in existing well mixed mass polystyrene reactors of the CSTR design. The key to the successful practice of this technology is the ability to produce consistently high purity polymerization feed.