Species grouping in multicomponent adsorption calculations

The validity of the assumption of species grouping, which combines several adsorbable species into a fewer number of pseudo-species in adsorption calculations, was examined by comparing the equilibrium concentrations obtained from exact calculations with those obtained with the use of the assumption. It was found that if the adsorption affinities of the individual species exhibited by their single-species isotherms are of the same order of magnitude, the use of the species grouping assumption yields acceptable results. This procedure applies to three situations, including those in which (a) all the species are grouped into a single pseudo-species; (b) the various species are combined into several (but fewer) pseudo-species; and (c) only some of the species are grouped, while the others remain individual species.     

Einfluß der Produkteigenschaften auf die Trocknung

Influence of Product Properties on Drying. A number of items of information are required for selection and design of a drying process. Thermodynamic equilibrium as well as heat and mass transfer are the traditional prerequisites for describing the drying operation. In the development of a drying process further specific properties of the product must be known. These properties, such as chemical reactions or sticking properties, greatly reduce the number of possible drying processes. For this reason, the measurement of these properties plays an important role in the development of a drying process. Many of the bench scale tests in use do not have a physical basis, but have been developed empirically because the physical context is often unknown. Yet a number of physically well-defined methods are available for product characterization. In practical work a combination of empirical and scientifically defined methods is used.     

DISTILLATION COLUMN DESIGN BY METHOD OF McCABE AND THIELE: DISTMAN COMPUTER GRAPHICS PROGRAM

A computer graphics program developed for the IBM PC calculates equilibrium and operating lines and steps off the number of stages on screen using the McCabe-Thiele method. The results are obtained by graphics, not by numerical methods, so that the accuracy depends upon the screen resolution or pixel density. The equilibrium line is drawn from bubble-point temperature calculations using constants from the Wilson and Antoine equations. The operating lines and staircase are constructed by computer graphics according to the prescribed operating parameters. Design variables, which include the reflux ratio, quality of feed, Murphree efficiency, product purities and pressure, may be changed interactively.     

Modeling and simulation of an industrial slurry-phase catalytic olefin polymerization reactor series

In the present study, a comprehensive mathematical model is developed to simulate the dynamic behaviour of an industrial slurry-phase olefin catalytic polymerization loop reactor series. More specifically, the effects of various operating conditions on the dynamic reactor behaviour (i.e., reaction temperature and pressure, inflow rates of catalyst, monomers and diluent, etc.) as well as the on the molecular and rheological polyolefin properties (i.e., M_n, M_w, MWD, complex viscosity, etc.) are fully assessed. According to the proposed modeling approach, each loop reactor (i.e., consisting of the loop reactor and the settling legs) is modeled as an ideal CSTR in series with a semi-continuous product removal unit. Dynamic macroscopic mass species and energy balances are derived to calculate the dynamic evolution of the concentrations of the various molecular species as well as of temperature profiles and heat removal in the two loop reactors. The polymer molecular properties (i.e., number- and weight-average molecular weights and molecular weight distribution) are determined by employing a generalized multi-site, Ziegler–Natta (Z–N) kinetic scheme in conjunction with the well-known method of moments. All the thermodynamic calculations, regarding the equilibrium species concentration in the various phases (i.e., solids and liquid), are carried out using the Sanchez–Lacombe Equation of State (S–L EOS). It is shown that the proposed comprehensive model is capable of simulating the dynamic operation of an industrial slurry-phase cascade-loop reactor series under different plant operating policies (i.e., start-up, grade transition, etc.).     

Catalyst effectiveness in sulfur dioxide oxidation

The sensitivity of the relationship between catalyst effectiveness in SO₂ oxidation and the Thiele parameter to: (1) the kinetic expression with respect to the presence or absence of strong SO₃ inhibition, (2) the activation energy in the range of 16–32 kcal/g mol, (3) the conversion of SO₂ up to 90%, (4) the temperature at the exterior surface of the pellet in the range of 450–600°C, and (5) the feed composition ranging from 6–10% SO₂ in air, was investigated. A nonisothermal model was used and the calculations were carried close to equilibrium. Far from equilibrium, the relationship is essentially unaffected except by conversion at the pellet surface for the case of strong product inhibition. Close to equilibrium, the relationship is significantly influenced by conversion, temperature, and the inlet composition but the form of the kinetics is not important. The activation energy is not a significant variable. Catalyst size, surface area, and density were not specified while it was necessary to fix the ratios of the transport properties for the calculations. The properties of the American Cyanamid V₂O ₅ catalyst were used for this purpose.     

New approach for the prediction of azeotropy in binary systems

A new approach for the prediction of azeotrope formation between components in a mixture, that does not require vapor–liquid equilibrium calculations, is presented. The method employs neural networks to correlate azeotropic data for binary mixtures with a series of macroscopic and microscopic properties of the pure components, without explicit consideration of non-ideality of mixture. The model fails to make a clear prediction regarding azeotropy in only a relatively small number of situations in which structurally homologous molecules are known to exhibit quite distinct azeotropic behavior.     

HYDROGEN-BONDING COMPETITION IN ENTRAINER COSOLVENT MIXTURES

In chemical separation processes such as supercritical extraction, the use of an entrainer cosolvent can dramatically improve selectivity and yield. Ideally, an cntrainer cosolvent should solvate only the desired solute, pulling it from the feed. Prospective entrainers therefore are chosen for their hydrogen bonding tendency. But not all cosolvents are effective entrainers, and an entrainer that is effective for one application may not be effective for others. The effectiveness of a cosolvent as an entrainer can be limited by competition among various hydrogen bonding species in the mixture.

In this paper, experiment and theory are presented for hydrogen bonding in entrainer cosolvent mixtures. Concentrations of monomelic and hydrogen bonded species are determined using FTIR spectroscopy and these data are modeled using the Associated Perturbed Anisotropic Chain Theory (APACT). Liquid solvents with hydrogen bonding properties similar to those of supercritical fluids are used.

Using APACT, it is shown that the equilibrium constant, derived from activities, can be written as the product of a temperature dependent term and the ratio of concentrations: K = (RT)^v Π C^vi_i. This gives a statistical mechanical basis for the empirical observation that the equilibrium constant can be calculated as the ratio of concentrations of monomeric and hydrogen bonded species.     

Influence of combining rules on the cavity occupancy of clathrate hydrates using van der Waals–Platteeuw-theory-based modelling

In the current study, we report an extensive series of thermodynamic calculations using continuum-level models based on the van der Waals–Platteeuw theory. The calculations are performed along the three phase hydrate–liquid water–vapor (H–L_w–V) or hydrate–ice–vapor (H–I–V) equilibrium curve for a number of gases of industrial interest (e.g., methane, ethane, propane, nitrogen and carbon dioxide). We examine the effect of deviations from the classical Lorentz–Berthelot combining rules on the hydrate equilibrium conditions as well as the cavity occupancy of the hydrates and work towards quantifying it. Hydrate equilibrium predictions have a very strong sensitivity to deviations, while cavity occupancies exhibit a weaker sensitivity. Furthermore, the sensitivity is stronger on the small cavities compared to the case of the large cavities. Model calculations are compared against experimental data for selected systems with reasonable agreement.     

The effects of changes in problem parameters on chemical equilibrium calculations

A simple method is described for evaluating the first order changes in calculated chemical equilibrium compositions resulting from changes in the problem parameters of temperature, pressure, initial reactant compositions, and standard free energy data for the species. It may be used to calculate the first order derivatives of the equilibrium mole numbers with respect to the problem parameters, thus indicating the solution's sensitivity to small changes in these values. Using the temperature and pressure derivatives and appropriate data for the species, all first order thermodynamic derivatives of the equilibrium mixture properties may be calculated. Finally, surveys may be performed of the equilibrium system for ranges of the problem parameters. The technique involves the solution of a set of m + ? linear equations, where m is the number of elements and ? the number of phases.     

Acid-catalysed dehydration of alcohols in supercritical water

At pressures exceeding its critical pressure water retains its ionic properties to temperatures of 400 °C or more. In water under these conditions trace amounts of Arrhenius acids dissociate and selectively catalyse the dehydration of alcohols, diols, and polyols. High yields of the desired dehydration product (ethene from ethanol, propene from propanol, acetaldehyde from ethylene glycol, and acrolein from glycerol) can be obtained with a residence time of less than one minute. However, for ethanol the equilibrium conversion appears to be less than predicted by ideal solution thermochemical calculations. This may be due to catalyst deactivation, or it may be an effect of hydrogen bonding between the water and the reactant alcohol. The dehydration of n-propanol proceeds by a first order reversible reaction whose equilibrium is close to that predicted by thermodynamics. Because these dehydration reactions proceed rapidly with a high degree of specificity, they appear to be good candidates for industrial exploitation.     

复杂精馏塔的设计型算法

本文建立了用于复杂精馏塔(可有多股进料、汽、液相侧线出料和中间换热器的塔)的设计型算法.本算法可根据指定的独立变量值,通过优化计算确定能满足各项设计指标的一组优化设计参数值.本算法应用SHBWR(BWRS)及PR汽-液平衡和焓模型对金山乙烯装置中的典型塔等进行了考核,取得满意的结果.     

Special cases of determination of the minimum reflux ratio for distillation of binary mixtures

Specific features of the representation of liquid-vapor equilibrium data for binary mixtures and the realization of a pinch regime in minimum reflux calculations are considered. The pinch regime at minimum reflux is possible provided that the second derivative of the vapor composition with respect to the liquid composition is positive between the composition of the final product and the feed composition.     

Performance of fractionating reactors in the absence of rate limitations

A fractionating reactor for equilibrium‐limited reactions is studied theoretically. Reactant A is fed in the center of the countercurrent fractionating system. Product P is effectively transported with the auxiliary phase, while product Q is effectively transported with the main phase, in which the reaction takes place. Model calculations were based on partition and reaction equilibrium at all stages. These show that if the initial reactant concentration and the flow rates are properly selected, the extent of conversion will significantly exceed the corresponding batch conversion. To approach complete conversion in the fractionating reactor, and to recover both products in a pure form, net transport of reactant in either of the countercurrent directions should be prevented. However, irrespective of the number of equilibrium stages, this situation cannot be fully reached when the reactant feed stream is too large (compared with the main and auxiliary streams). Nonetheless, one of the two products may be recovered in a pure form even for such large feed streams. Copyright © 2004 Society of Chemical Industry     

White's method of thermodynamic calculations applied to ammoxidation of propylene

A method of steepest descent first applied by White et al. to the calculation of complex chemical equilibrium was used in this article to compute the equilibrium composition of a mixture resulting in the ammoxidation of propylene. As many as twenty species are likely to be present, some in insignificant quantities. A FORTRAN II Program was written for an IBM 1620 computer and calculations were made varying the feed composition, temperature and pressure. The results were discussed and qualitatively compared with those of the plant practice reported in the literature. The agreement is reasonably good when the oxygenated compounds are excluded from consideration.     

Nonlinear model based control of two-product reactive distillation column

Nonlinear feedback control scheme for reactive distillation column has been proposed. The proposed control scheme is derived in the framework of Nonlinear Internal Model Control. The product compositions and liquid and vapor flow rates in sections of the reactive distillation column are estimated from selected tray temperature measurements by an observer. The control scheme is applied to an example reactive distillation column in which two products are produced in a single column and the reversible reaction A+B=C+D occurs. The relative volatilities are favorable for reactive distillation so that the reactants are intermediate boilers between the light product C and the heavy product D. Ideal physical properties, kinetics, and vapor-liquid equilibrium are also assumed. It is shown that the proposed control scheme keeps tight product composition control.     

Solvation and chemical engineering thermodynamics

With the continued advances in computational chemistry, solvation calculation from first principle methods is becoming a promising route for phase equilibrium modeling. However, except in a few instances, such an approach has not been widely adopted, which may be a consequence of the abstractness of solvation itself and also of the lack of a simple bridge between solvation and other thermodynamic properties. Here, we establish and summarize the relationship between solvation and other properties frequently used in phase equilibrium modeling. An important quantity, called the total solvation free energy, is introduced so that one can easily derive engineering thermodynamic models (such as an equation of state) from solvation models, or vice versa. The equations presented here are of general validity and would be useful for obtaining existing model parameters from solvation calculations and for developing new models stemming from the ideas of molecular solvation.     

Influence of gas environment on reactions between sodium and silicon minerals during gasification of low-rank coal

Thermodynamic equilibrium calculations were performed to determine the possible compositions and conditions for formation of potential liquid phases responsible for fluidised bed agglomeration during gasification of a high-sulphur low-rank coal from South Australia. The coals from this region of Australia are typically characterised by containing high levels of sodium, silica and sulphur. The transformation behaviour of the form of sodium present in coal, as either a carboxylate forming part of the coal organic matter or as soluble salt (NaCl) and its reaction with silicon compounds (silica or kaolin) is presented. The influence of temperature and gas atmosphere on equilibrium composition was evaluated. Thermodynamic equilibrium calculations show that the distribution of sodium among the produced species will depend on the form of sodium in the coal, the gas atmosphere and the forms in which silicon is present in the coal. Steam was found to have the most significant effect causing a lowering of the sodium carbonate melting point temperature.