TRANSPORT OF PENETRANTS IN THE MACROMOLECULAR STRUCTURE OF COALS II. EFFECT OF POROUS STRUCTURE ON PYRIDINE TRANSPORT MECHANISMS

Pyridine transport in 600-850μm coal particles occurs by a non-Fickian mechanism which shows coupling of both diffusion and relaxation phenomena. In lignites (with carbon content less than 75%) the phenomenological pyridine transport mechanism is predominantly Fickian, whereas in bituminous coals (above 75% carbon) it approaches Case-II transport. However, when the porous structure of the coals is taken in consideration the true pyridine transport mechanism is Case-II transport.     

TRANSPORT OF PENETRANTS IN THE MACROMOLECULAR STRUCTURE OF COALS II. EFFECT OF POROUS STRUCTURE ON PYRIDINE TRANSPORT MECHANISMS

Pyridine transport in 600-850μm coal particles occurs by a non-Fickian mechanism which shows coupling of both diffusion and relaxation phenomena. In lignites (with carbon content less than 75%) the phenomenological pyridine transport mechanism is predominantly Fickian, whereas in bituminous coals (above 75% carbon) it approaches Case-II transport. However, when the porous structure of the coals is taken in consideration the true pyridine transport mechanism is Case-II transport.     

EFFECT OF SULPHIDING PARAMETERS ON THE INITIAL ACTIVITY BEHAVIOR OF A SUPPORTED NICKEL MOLYBDATE HYDROSULPHURIZATION CATALYST

The influence of some sulphiding parameters on the activity of a supported nickel-molybdate catalyst was studied for the gas phase hydrodesulphurization of thiophene carried out at atmospheric pressure. The optimum activation procedure involved prereduction with hydrogen at 300^°C followed by presutphidation with pure H₂S at 400^°C. The differences in activity observed were substantially maintained over the testing time when the reaction temperature was 300^°C. At 400^°C they were maintained only for limited periods. Catalyst deactivation was observed under either sulphiding or reducing atmospheres. The results are discussed in terms of possible structural changes involving molybdenum sulphide nucleation and crystal growth, which ted to the proposal of three stages for the initial catalyst transformation: oxide monolayer sulphidation, formation of small defected sulphide crystallites, and sulphide multilayer growth.     

DESORPTION ISOTHERMS OF APPLES AT SEVERAL TEMPERATURES

The desorption isotherms of apples of the Granny Smith variety, treated with 0.5% citric acid aqueous solution, were determined using the gravimetric static method. Experiments at three temperature levels: 20^°C, 40^°C and 60^°C and ten water activity levels from 0.112 to 0.856 were carried out. Along with five well known models - modified Chung-Pfost, modified Halsey, modified Oswin, modified Henderson and GAB (Guggenheim-Anderson-de Boer), a new fraction-linear model has been applied to fit the data and to calculate the isosteric heat of desorption determination.     

REDUCTION OF SULPHUR DIOXIDE BY METHANE OVER TRANSITION METAL OXIDE CATALYSTS

The reduction of sulfur dioxide by methane for the production of elemental sulfur using two hydrodesulfuriz-ation catalysts was studied. Oxides of cobalt-molybdenum and molybdenum alone supported on γ-alumina were employed. The reactions were carried out in a quartz reactor at 650^°C and 750^°C and at molar feed ratios (SO₂/CH₄ varying between 0.50 to 2.50. The physicochemical stale of the catalysts was examined by SEM, BET and X-ray diffraction. Molybdenum oxide was converted to sulphide after the chemical reaction. The cobalt molybdenum catalyst was found to be the more active of the two; however, the molybdenum catalyst exhibited higher elemental sulphur yield (99%). Side reactions resulted in the formation of H₂S, COS, CO, and elemental carbon. The production of these is minimized by operating at molar feed ratios greater than 1 and low temperature. The production of H2S is favoured over COS below 700^°C.     

REACTION KINETICS PARAMETERS DESCRIBING DISPLACEMENT BY ACIDS OF STRUCTURAL METALLIC IONS FROM CLAY MINERALS

Chemical kinetics parameters describing reactions of naturally-occurring clay minerals with acids were determined using batch reaction and analytical techniques. Clay minerals included sodium montmo-rillonite, kaolinite, and illite, reacting with hydrochloric acid, nitric acid, and sulfuric acid. Two acid concentrations were examined at 50 and 70^°C. Displacement by acids of aluminum and magnesium from clays was found to follow pseudo-first order kinetics. Data analysis verified the batch reaction technique for determining kinetics parameters of acid-clay reactions and demonstrated acid neutralization by naturally-occurring clays. For reactions of sodium montmorillonite with hydrochloric acid, nitric acid, and sulfuric acid, reaction rate coefficients for displacement of aluminum ranged from 1·73 × 10^-4 kg/mol-hr at 50^°C to 3·02 × 10^-3 kg/mol-hr at 70^°C. Displacement of magnesium from sodium montmorillonite by these acids occurred at rates ranging from 3·68 × 10^-4 kg/mol-hr at 50^°C to 1·12 × 10^-2 kg/mol-hr at 70^°C. Activation energies for displacement by acids of aluminum from sodium montmorillonite were calculated to range from 31,000J/mol to 112,000J/mol, while activalion energies for displacement of magnesium from sodium montmorillonite ranged from 36.000 J/mol to 103,000 J/mol. For reactions of illite with acids, reaction rate coefficients for displacement of aluminum were determined to be 1·45×10^-4Kg/mol-hr at 50^°C to 1·7×10^-3kg/mol-hr at 70^°C. Displacement of magneisum by acids from illite occurred at rates from 6·06 × 10^-4 kg/mol-hr at 50^°C to 5·97 × 10^-3 kg/mol-hr. Activation energies for reactions of acids with illite ranged from 33,000 J/mol to 62,000 J/mol for displacement of aluminum and from 9,400 J/mol to 29,000 J/mol for displacement of magnesium.     

A KINETIC STUDY OF THE SULPHURIZATION OF RESIDUAL ASPHALTS

The kinetics of the sulphurization reaction of residual asphalts were investigated at temperatures of 220^°-250^°C and reaction time of 1 hr. The apparent reaction order varied from 1.40-3.50 for the range of temperature studied. The maximum reaction order was obtained at 24O^°C which appeared to be the optimum reaction temperature. The apparent kinetic rate parameters estimated for the sulphurization reactions were activation energy of 99.5 kJ/mol and a frequency factor of 15.48 × 10⁸. The rate parameters show some good agreement with other work reported in the literature.     

The effect of penetrant activity and temperature on the anomalous diffusion of hydrocarbons and solvent crazing in polystyrene part I: Biaxially oriented polystyrene

The effects of temperature and penetrant activity on the sorption kinetics and equilibria of a series of alkanes in glassy, biaxially oriented polystyrene were studied. Normal isomers of pentane, hexane, and heptane cause crazing of polystyrene film samples at high penetrant activities (> 0.85). Crazing kinetics are identical to the kinetics of Case II transport. Transport of these normal hydrocarbons in glassy polystyrene in the temperature range 25 to 50°C is markedly non-Fickian; limiting Case II transport is observed at activities in exces of 0.6. Sorption appears to be controlled by highly activated relaxation processes including primary bond breakage at these high penetrant activities. Fickian diffusion behavior is approached, however, as penetrant activity is reduced. Sorption of the branched isomers of these compounds does not result in polymer microfailure. The sorption kinetics of the branched isomers, although time dependent, appear to be controlled primarily by thermally activated diffusion rather than large scale polymer relaxations which control Case II transport.     

PRODUCTION OF ACTIVATED CARBON FROM COAL CHARS USING MICROWAVE ENERGY

The microwave induced reaction of carbon with carbon dioxide has been investigated as a method of production of activated carbon using coal char. Factors which control the carbon-carbon dioxide reaction in a microwave environment were also studied.

Results indicate that the reaction rate is primarily controlled by the electric field strength. The reaction rate or conversion follows an Arrhenius type relationship with substitution of the electric field strength for temperature. In other instances, temperature is easily determined and therefore used in these calculations. In this case, the particle temperature is nearly impossible to measure. This means that we need an easily calculated variable to use in our calculations. The input microwave power will serve as this variable. It is easily determined, and is also useful in calculations because, as stated earlier, when input power (in the form of electric field strength) is substituted for temperature in equations of the Arrhenius type, the reaction rate is related in a similar manner, Because of difficulties associated with the direct measurement of particle temperature in the microwave energy field, only reactor skin temperatures were measured.

The char surface area, as calculated using the B.E.T. equation, was increased dramatically by the microwave process. The prod uct had a high degree of microporosity and was a good adsorbent of NO_x gas in air. The solids residence time for the coal char in direct microwave contact was on the order of minutes. Reactor skin temperature measurements at the reaction zone suggest that reaction occurred at approximately 600^°F(316^°C).     

Concentration and temperature dependence of diffusional deborah number during dodecane transport in crosslinked polystyrene

The diffusional Deborah number, De, defined as the ratio of characteristic relaxation time to characteristic diffusion time was determined as a function of concentration and temperature in the dodecane transport process in polystyrene. The characteristic relaxation time was obtained from the viscoelastic properties of dodecane/polystyrene systems measured by a dynamic mechanical analyzer. The characteristic diffusion time was obtained from the concentration and temperature dependence of the diffusion coefficient measured by NMR PGSE method. Above the room temperature the Deborah number changed significantly during isothermal transport process; however, the transport mechanism did not deviate from the Fickian transport as the order of De was still greater than 1. Around the room temperature the transport mechanisms represented by the resulting diffusional Deborah number changed significantly as the order of magnitude approached 1 with the temperature decreasing from 50°C to the room temperature. The transport mechanism predicted from the diffusional Deborah number was verified by the diffusional exponent, n, of an exponential time-dependence of the penetrant uptake.     

THERMODYNAMIC BEHAVIOR OF GREEN WHOLE AND DECAFFEINATED COFFEE BEANS DURING ADSORPTION

Differential and integral thermodynamic parameters were estimated for whole green coffee beans decaffeinated with either ethyl acetate (EADB) and methylene chloride (MCDB). Adsorption isotherms were prepared at 25, 35, and 45^°C. Changes in thermodynamic properties were estimated by the method of Othmer. Variations in the integral enthalpy with respect to moisture indicated that MCDB have a heterogeneous distribution of binding forces at the active sites. EADB showed smaller enthalpy values at lower moisture contents as compared to MCDB, and maintained a distribution of binding energy at the active sites similar to that of whole beans. We suggest storing MCDB at 25^°C with a moisture content of 0.9 g H₂O/100 g d.m., and of 2.3-4.0 g H₂O/100 g d.m. for EADB and whole beans, to obtain maximum stability.     

DETERMINATION OF MOISTURE DIFFUSIVITY AND BEHAVIOR OF TOMATO CONCENTRATE DROPLETS DURING DRYING IN AIR

The drying mechanism and diffusion coefficient of water in spherical droplets (1.73 - 2.08 mm diameter) of tomato concentrates were successfully interpreted and modelled by using Fick's law. Solids content of the initial concentrate (5-15% w/w), and drying temperature (60^° - 100^° C) were varied but the drying air was kept at constant velocity and humidity.

The effective moisture diffusivity was estimated from the drying rate curves and expressed by an Arrhenius relation. Further, it was observed that case hardening has a large effect on the diffusion process causing the effective diffusional distance and the rate of moisture accumulation in the hardened crust to vary with the moisture content, according to a sorption controlled mechanism.     

Time and temperature dependent sorption in poly-ether-ether-ketone (PEEK)

The possible multimodal sorption mechanisms in glassy amorphous poly-ether-ether-ketone (PEEK) are presented. By varying the penetrant-polymer affinity, experimental temperature, and external solvent activity, a broad range of sorption behaviors from ideal Fickian diffusion to limiting relaxation controlled kinetics is observed. In particular, water, methylene chloride, and n-heptane sorption kinetics are analyzed and interpreted on the basis of the multiple transport mechanisms. Low uptake liquid n-heptane sorption follows ordinary Fickian diffusion. Analogously, water vapor at low activity, is sorbed in small amounts in the same limiting mode while, at higher activities, the moderately higher penetrant uptakes induce slow relaxation coupled with ideal Fickian diffusion. The highly interacting methylene chloride leads to ideal Fickian diffusion only at very low activities, while anomalous non-ideal Fickian diffusion and limiting Case II and diffusion controlled swelling are observed at moderate and at high solvent activities, respectively. Limiting Case II sorption of methylene chloride in PEEK has been observed only at a very low temperature (?32°C). The optical microscopy observations of cryogenically fractured samples contacted with liquid methylene chloride at 5, 20 and 36°C revealed the presence of a sharp front moving linearly with the square root of time. Solvent induced crystallization in methylene chloride swollen samples was detected by means of differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). Finally, sorption from liquid methylene chloride/n-heptane solutions with varying compositions are presented. The progressive increase of the more high sorbing methylene chloride concentration in the solutions, leads to the same wide variety of sorption behavior observed in the methylene chloride vapor sorptions. The gas chromatographic (GC) analysis indicated that the presence of methylene chloride enhanced the n-heptane sorption in the polymer.     

Sorption and transport of linear and branched ketones in biaxially oriented polyethylene terephthalate

Equilibrium sorption and kinetics of acetone, methyl ethyl ketone (MEK), methyl n-propyl ketone (MnPK), and methyl i-propyl ketone (MiPK) uptake in uniform, biaxially oriented, semicrystalline polyethylene terephthalate films were determined at 35 °C and low penetrant activity. Sorption isotherms for all penetrants were well described by the dual-mode sorption model. Sorption and desorption kinetics were described either by Fickian diffusion or a two-stage model incorporating Fickian diffusion at short times and protracted polymer structural relaxation at long times. Diffusion coefficients and equilibrium solubility at fixed relative pressure decreased in the following order: acetone>MEK>MnPK>MiPK. Diffusion coefficients for each penetrant increased with increasing penetrant concentration.     

VAPOR-LIQUID EQUILIBRIA OF H2S AND C02 AND ETHYLENE GLYCOL AT ELEVATED PRESSURES

The solubility of H₂S and C0₂ in ethylene glycol has been measured at temperatures in the range 25^° to 125^°C at pressures up to 6.7 and 20 MPa respectively. The data have been correlated using the Peng-Robinson equation of state. Henry' coefficients have also been obtained from the experimental data. Values of the vapor phase composition predicted by the equation of state have been compared with some experimental data and with published data.     

Numerical simulation of anomalous penetrant diffusion in polymers

This work introduces a new numerical algorithm that can be used to analyze complex problems of penetrant transport. Penetrant transport in polymers often deviates from the predictions of Fick's law because of the coupling between penetrant diffusion and the polymer mechanical behavior. This phenomenon is particularly important in glassy polymers. This leads to a model consisting of two coupled differential equations for penetrant diffusion and polymer stress relaxation, respectively. If the polymer relaxation is the rate-limiting step, both the concentration and stress profiles are very steep. A new algorithm based on a finite difference method is proposed to solve the model equations. It features the development of a tridiagonal iterative method to solve the nonlinear finite difference equations obtained from the finite difference approximation of the differential equations. This method was found to be efficient and accurate. Numerical simulation of penetrant diffusion in glassy polymers was performed, showing that the integral sorption Deborah number is a major parameter affecting the transition from Fickian to anomalous diffusion behavior. © 1993 John Wiley & Sons, Inc.     

MATHEMATICAL SIMULATION OF STRESSES WITHIN A CORN KERNEL DURING DRYING

Thermal stress and hydro stress within a single com kernel during drying are analysed with the mechanics of elasticity and material. The mathematical model of the stress within a single com kernel during drying is given. A finite element method is developed in details to simulate the maximum principal stress and shear stress within a single com at 75^°C and 50^°C drying air temperatures. The locations of the predicted maximum stresses are described and stress fields at different drying time are obtained. The result shows that the predicted locations of the maximum stresses agree with published materials on stress-crack or burst of com kernels.     

Influence of copolymer composition on non-fickian water transport through glassy copolymers

The dynamic swelling behavior of poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) and poly(2-hydroxyethyl methacrylate-co-N-vinyl-2-pyrrolidone) in water was followed at 37°C. The results were analyzed in terms of a simple non-Fickian transport equation, which expresses the fractional penetrant uptake as an exponential function of the diffusion time. The exponent n, which indicates Fickian or non-Fickian transport mechanism, was correlated to the content of the more hydrophilic component of the copolymer. Photomicrographs obtained with polarized light offer new information about the position and movement of the penetrant front in glassy, hydrophilic polymers.     

THERMAL AND MASS DIFFUSIVITY FROM DYNAMIC SINGLE PELLET EXPERIMENTS

The single-pellet moment techniques previously developed for the evaluation of effective diffusion coefficient and effective thermal conductivity of a porous solid are combined and a procedure is proposed for the simultaneous evaluation of these constants from a single set of pulse response experiments. It is shown that both temperature and concentration response curves can be measured from a single pulse of oxygen injected into hydrogen carrier gas flowing past a catalyst pellet. It is shown that the zeroth moment of the concentration response peaks are sufficient for finding the effective diffusivity while both zeroth and first order moments are required for thermal conductivity. Experiments conducted at 110^°C with a boehmite pellet of porosity 0.57 gave a thermal conductivity value of 3.6 x 10^-4 cal/cm ^°K s and an effective diffusivity (for H₂O) of 0.054 cm2/s. The adsorption equilibrium constant of H₂O was determined as ρ_pK = 24.5 from the first moment data. If sorption resistance to diffusion through the pellet is included in the diffusion coefficient, an apparent value of 0.0011 cm2/s would be obtained for D_eff. This result indicates the importance of separation of diffusion and sorption parameters in the analysis.