IMPROVED OUTGASSING FOR HYDROPROCESSING VESSELS

The increasing use of high-temperature, high-pressure hydroprocessing has stimulated the need to minimize process unit turn-around time. Both the restreaming of active catalyst at turn-around and the discharging of spent catalyst must be preceded by a cooling of the vessel wall to ambient temperature. Consequently, temperature-time and pressure-time profiles must be selected which transform the unit from operating temperature and hydrogen pressure to ambient temperature and zero hydrogen pressure without: (1) generation of excessive thermal stress in vessel walls, (2) coke formation on catalyst, (3) overlay disbonding or (4) crack initiation and growth due to base-metal hydrogenembrittlement. This paper describes a method for selecting the temperature-time and pressure-time profiles which produce outgassing in minimum time consistent with these constraints. The outgassing time computed by this method for a particular unit is a theoretical minimum in the sense that a shorter time is not possible without violating one of the constraints; this procedure can therefore be used as a standard of reference against which other procedures can be compared. Conversely, the procedure is also practical and can be implemented on actual process units. Improved outgassing methods can result in significant cost savings by reducing equipment downtime. The U.S. Patent and Trademark Office has advised the authors that this outgassing procedure is patentable and that a patent will be issued in due course.     

MODELLING OF AXIALLY DISPERSED NON-ISOTHERMAL TUBULAR REACTORS FOR HYDROPROCESSING OF SOLID-LIQUID MIXTURES

A mathematical model for hydroprocessing of coal-oil slurries in non-isothermal, axially dispersed tubular reactors is developed and numerically solved, considering a five component-six reaction model representing the chemistry of the process, and, dispersive, corrective and interfacial mass and energy transfers. Several simulation results are presented.     

SHALE OIL HYDROPROCESSING TO PRODUCE SYNTHETIC JET FUELS

The crude shale oil fractions below 343°C available in the United States, (a) Geokinetics, (b) Occidental, (c) Paraho, and (d) Tosco II, were catalytically hydroprocessed at low, medium and high severities. The hydroprocessed oils were fractionated to the jet fuel cuts range of 121-300°C. Shale-oil jet fuels were characterized and compared with petroleum jet fuels to evaluate their suitability as future jet fuel oil substitutes. Nitrogen content in shale-oil jet cuts was in the range of 0.03 to 1.15 wt%. The lowest nitrogen content, 0.03 wt% in high severity Occidental jet fuel, was considerably higher than that of the petroleum jet fuel cuts (1-5 ppm). The sulfur content and mercaptan sulfur content in shale-oil jet fuel was significantly lower than in petroleum jet fuel (total sulfur = 0.3 wt% maximum, mercaptan sulfur = 0.003 wt% maximum), the hydrogen content (13-14wt%) in the shale-oil jet fuel cut was lower than that of petroleum jet fuel (15-16 wt%). The jet fuel distillates, volume percent of the shale-oil and petroleum, that were operated at the same temperatures were comparable; with the exception of the freezing points of the shale-oil fuel cut which were much higher than those of the petroleum jet fuel.     

MODELING AND SIMULATION OF BINARY APPROACH BY ENERGY CONSERVATION ANALYSIS

A new model for approach processes of two unequal or equal sized drops or bubbles is proposed based on a parallel film concept and energy conservation analysis. This model can estimate the interaction time and the maximum film area for an approach and the effects of Weber number, size ratio and curvature of film. It also gives a good explanation why the effective virtual mass coefficienl in the model of Chesters and Hofman (1982) changes with the size ratio of drops or bubbles.     

ISOTHERMAL AXIAL DISPERSION REACTORS IN COAL LIQUEFACTION HYDROPROCESSING

The purpose of this study is to provide a better understanding of reactors for hydroprocessing of liquid-solid mixtures, and to assist in the design and scale-up of large scale reactors from the information obtained from small scale experimental units. The paper describes a linear, isothermal, cocurrent upflow tubular reactor model with axial dispersion. The model is applied to coal liquefaction, assuming that the complex liquefaction kinetics can be represented by five lumped components-six first order reactions, and that the three phase (coal-solvent-gas) flow can be approximated by a two phase (slurry-gas)flow.     

ISOTHERMAL AXIAL DISPERSION REACTORS IN COAL LIQUEFACTION HYDROPROCESSING

The purpose of this study is to provide a better understanding of reactors for hydroprocessing of liquid-solid mixtures, and to assist in the design and scale-up of large scale reactors from the information obtained from small scale experimental units. The paper describes a linear, isothermal, cocurrent upflow tubular reactor model with axial dispersion. The model is applied to coal liquefaction, assuming that the complex liquefaction kinetics can be represented by five lumped components-six first order reactions, and that the three phase (coal-solvent-gas) flow can be approximated by a two phase (slurry-gas)flow.     

THE CHARACTERIZATION OF COMPLEX SYSTEMS OF CHEMICAL REACTIONS

A method is presented for the enumeration of possible overall chemical reactions and mechanisms, based on an initial choice of elementary reactions connecting a set of chemical species. Such a procedure furnishes an important tool for the study of complex reaction systems. The method is presented in such a way as to be readily implemented by computer solution, which is indispensable for many systems.     

MODELING OF EVAPORATION FROM V-SHAPED MICROGROOVES

A one-dimensional model of evaporation from V-grooved microchannels is presented herein. The integrated model incorporates the evaporation from the capillary as well as the transition region of the extended meniscus. The amount of evaporation from these two regions are calculated as function of the input heat flux, groove geometry and the liquid-solid combination. The evaporation from the capillary meniscus region is found to be substantially higher than that from the extended thin film region. The effects of the groove geometrical parameters on the performance (e.g., critical heat fluxrpar; of the microchannels have been studied. It has been found that including the specific heat requirement in the model equations has negligible effect on the evaluated parameters.     

MATHEMATICAL MODELING OF DIFFUSION-CONTROLLED PNEUMATIC-CONVEYING DRYERS

A mathematical model for a continuous pneumatic-conveying dryer has been developed for removing internally bound moisture from solid particulates. The dryer relies on a recirculating carrier gas stream for entrainment. Drying is carried out by injecting into the gas loop a fresh stream of conditioned drying gas while an equal amount of wet gas is vented out. Because pneumatic-conveying dryers usually employ huge gas velocities, the particulates are well dispersed in the gas. Therefore, for solids absent of surface moisture, the drying kinetics is controlled by intraparticle diffusion. The mode! developed based on the diffusion mechanism relates the moisture reduction in the solids to various process parameters (diffusiv-ity, partition coefficient, particle size, residence time, solids loading, drying gas usage, and carrier gas recirculating rate), and is fully predictive. Therefore, it can be used to study the effects of these variables. The model was compared with the plant data and found to match the data within ± 15%.     

MODELING A CASCADE OF CONTINUOUS MSMPR CRYSTALLIZERS WITH AGGLOMERATION

Cascade models have been used to account for real flow patterns in crystallizers that are not well-mixed and to better control the characteristics of the crystal size distribution (CSD). Such models have been solved under two different sets of boundary conditions describing nucleation in each stage. In this article, we have solved (numerically and analytically) population balance equations that include the mechanism of agglomeration in each stage of the cascade without considering classification or recycle, and have derived expressions for the moments of the CSD in closed forms. We then obtained analytic solutions to our model equations for two stages in the cascade under two different sets of boundary conditions. When there were more than two stages in the cascade, we found out that it was necessary and also efficient (computing timewise) to use numerical schemes to solve and analyze our model equations. Our results show that the agglomeration kernel has a significant influence on the coefficient of variation of the CSD, and hence it should be considered in applying the cascade models to describe the mixing and performance of crystallizers with significant particle agglomeration.     

MATHEMATICAL MODELING OF FLUIDIZED BED HEAT REGENERATORS

Based on the two-phase theory of fluidization, a fairly rigorous model is developed to describe the dynamic behavior of fluidized-bed heat regenerators. This model takes into account the major hydrodynamic aspects of bubbling fluid beds. Based on the assumption that the gas leaves the bed in thermal equilibrium with the solids, a rather simplified model is presented. Exact analytical solutions are obtained for both models. The predictions of the more rigorous model are found to be in good agreement with many experimental observations. On the other hand, the simplified model gives satisfactory results only when the value of the parameter m₁is relatively large. Using the rigorous model, it has been shown that the bed thermal efficiency can be improved by reducing the time of operation, increasing the mass of solids or decreasing the bed aspect ratio. Also it has been found that when the gas flow-rate is increased, the amount of heat transferred to the solids displays a non-monotonic behavior and passes through a maximum.     

MODELING MASS TRANSFER OF FITC-LABELED DEXTRAN FROM POLYELECTROLYTE MICROCAPSULES

Layer-by-layer adsorption of oppositely charged polyelectrolytes has been used for the manufacturing of hollow microspheres or capsules for controlled release studies. Using this self-assembly technique, microsphere core material was encapsulated with poly styrenesulfonate (PSS) and poly allylamine (PAH) multilayers or PAH and polyvinyl sulfate (PVS) multilayers and the core dissolved to produce hollow microcapsules. The microspheres were loaded with fluorescein-isothiocyanate (FITC)-labeled dextran of different molecular weights for different lengths of time to quantify the release properties. After loading, the capsules were immersed in water and the FITC-dextran was allowed to diffuse into the mother liquor. The FITC-dextran concentration of the mother liquor was measured over a period of hours and days. In this study, one- and two-compartment models were developed, based on a species mass balance, to predict the concentration of dextran release from the microcapsules over time. The two-compartment model was found to be superior to the one-compartment model in its fit to the observed data. The model was applied to experimental data in order to characterize the release properties of microcapsules with different numbers of layers and constituent architectures.     

牙舟古陶的造型及其审美特征

牙舟陶在漫长的历史发展过程中在逐步地、默默地变化和改进着,目前已渐渐形成了具有自己的独特风格、格调、造型特点及审美特色。本文主要从艺术设计中美的形式规律和审美要求对牙舟陶的造型及其审美特点做深入地研究分析,得出以生活器具为主的古牙舟陶造型简洁、质朴;千姿百态的牙舟古陶品类繁多,极具艺术性和观赏性,有着自然古朴的韵味与较好地秩序感,有着典雅和谐的审美情感特征。对牙舟陶造型及其审美特征的研究,使作为非物质文化遗产的牙舟陶得以更好的保护和在应用中传承,从而为社会经济等全面发展发挥出更大的作用。     

MODELING, SIMULATION AND CONTROL OF CRUDE TOWERS

Crude towers are one of the most energy consuming, complex distillation columns in a refinery. A mathematical model for the rigorous dynamic simulation of a crude tower is presented. Because of the large dimension, large stiffness and non-linearity of the model, dynamic simulation of a crude tower has proven to be very difficult. In this paper, the Sum of Rates algorithm is shown to be suitable for obtaining the steady states of a crude tower. Once a steady state is approached, the dynamic responses of a crude tower can be simulated by the Bubble Point algorithm. The dynamic simulation predicts strong, one-way interaction among the product streams. Through dynamic simulation, it is shown that the interaction can be eliminated by a decoupler.     

MATHEMATICAL MODELING OF MULTISTAGE DIFFUSION-CONTROLLED PNEUMATIC-CONVEYING DRYER TRAIN

Based on the single stage dryer model (Qi, 1996), a model for a multi-stage diffusion-controlled pneumatic-conveying dryer train was developed. The model consisted of a set of algebraic equations describing the relationship between the moisture level in solids and major process parameters (i.e., diffusivity, partition coefficient, particle size, number of stages, solids loading, residence time, etc.rpar;. The effects of these parameters on drying were studied using the model. Equilibrium and other special or asymptotical conditions (e.g., infinitesimal injection of drying gas, uniform partition coefficient and Fourier numbers, etc.rpar; were analyzed. Model calculations have been shown to compare very well with data from an actual plant HOPE drying process.     

ROBUSTNESS ANALYSIS OF A FIXED-BED TUBULAR REACTOR: IMPACT OF MODELING DECISIONS

A robustness analysis technique is applied to the control of a tubular fixed-bed reactor. Critical modeling decisions such as lumping and order reduction are evaluated in terms on their impact on the model/plant mismatch and its consequences in closed-loop stability and performance. It is shown that a previously proposed control scheme lacks robustness and thus is very difficult to control in the presence of modeling errors.     

MATHEMATICAL MODELING OF FLUID SPRAY CURTAINS FOR MITIGATION OF ACCIDENTAL RELEASES

In this article experimental and theoretical investigations on liquid spray curtains are presented, in the context of absorbing and dispersing accidental releases of chlorine in air. A mathematical model of a two-phase jet is developed to evaluate the entrained air rate in connection with the liquid flow rate. The model was successfully compared by means of replicated wind tunnel experimental runs adopting spray nozzles suitable to create a two-blade barrier. The experimental work demonstrated that it is possible to distinguish two regions in the barrier. One region is dominated by the liquid inertia and is comparable to an airplane jet. The other region is dominated by gravitational effects, and its dimensions do not vary appreciably. An analytical solution to the problem is obtained, taking into account the instantaneous and nonreversible chemical reactions, i.e., chlorine absorption in alkaline solutions, in the case of still air. The developed methodology could be applied to more complex situations, allowing the attainment of a more generalized approach for the design of a curtain given the release parameters, the site layout, and vulnerable target specifications.     

MATHEMATICAL MODELING OF FLUID SPRAY CURTAINS FOR MITIGATION OF ACCIDENTAL RELEASES

In this article experimental and theoretical investigations on liquid spray curtains are presented, in the context of absorbing and dispersing accidental releases of chlorine in air. A mathematical model of a two-phase jet is developed to evaluate the entrained air rate in connection with the liquid flow rate. The model was successfully compared by means of replicated wind tunnel experimental runs adopting spray nozzles suitable to create a two-blade barrier. The experimental work demonstrated that it is possible to distinguish two regions in the barrier. One region is dominated by the liquid inertia and is comparable to an airplane jet. The other region is dominated by gravitational effects, and its dimensions do not vary appreciably. An analytical solution to the problem is obtained, taking into account the instantaneous and nonreversible chemical reactions, i.e., chlorine absorption in alkaline solutions, in the case of still air. The developed methodology could be applied to more complex situations, allowing the attainment of a more generalized approach for the design of a curtain given the release parameters, the site layout, and vulnerable target specifications.