LOW TEMPERATURE OXIDATION OF LOW RANK COALS

The low temperature oxidation of a Montana subbituminous coal was investigated using round bottom 100 ml flasks in constant temperature baths. The experiments were carried out in normal and oxygen enriched air at 30°C, 45°C and 70°C with particle sizes ranging from 4 mesh to 100 mesh. Periodic analysis of gas samples from the flasks provided the rate data. The reactivity of the as received coal was compared with that of the same coal dried (i) in high pressure steam and (ii) in hot water

A rate equation has been proposed incorporating the effects of oxygen diffusion and surface reaction. For higher oxygen concentration and smaller particle sizes, the zero order surface reaction was found to be controlling. The temperature dependency of the reaction rate was found to be well represented by the Arrhenius equation. The activation energy varied slightly under various conditions between 15 to 20 kca)/g mole

The reactivity of the hot water dried coal was found to be similar to that of the as received coal. Steam dried coal however, was found to be much less reactive.     

RHEOLOGICAL BEHAVIOR OF LOW-RANK COAL-WATER SLURRIES

The rheological behavior of low-rank coal-water slurries was studied using a Brookfleld Rheolog viscometer. A Sarpy Creek subbituminous coal from Montana, with particle sizes ranging from 0.044 mm to 0.223 mm, was used. The solids concentration in the slurry varied from 15 to 65 weight % and the temperature was maintained constant at 82°F. The rheological behavior of the slurry made from as received coal was compared with that of the slurry made from hot water dried coal

Coal-water slurries, made from both as received and hot water dried coal exhibited pseudoplastic behavior. The shear stress was found to be related to the shear rate by the power law model. As expected, the apparent viscosity was found to increase sharply with solids loading. It was also observed that the apparent viscosities of the slurries made from hot water dried coal were lower than those made from as received coal.     

CARBON COMBUSTION RATES AND TEMPERATURES IN SHALLOW FLUIDIZED BEDS

The mechanism of combustion of carbon in shallow fluidized beds at temperatures 750-1000°C is studied by measuring burning rates and temperatures of spherical carbon particles ranging from 2 mm to 12 mm diameter directly in an experimental fluidized bed. Among variables investigated were inert particle size, superficial fluidizing velocity, temperature, the influence of neighbouring active particles and oxygen concentration in the fluidizing gas.

Under the experimental conditions explored, combustion was mainly kinetically controlled, so that with carbon particles larger than about 4 mm, burning rates are significantly higher than those predicted by combustion models which assume combustion to be controlled by the rate at which oxygen diffuses through a stagnant particulate phase surrounding the burning particle. The higher burning rate seems to arise because the greater mobility of particles in the bed causes the restriction to oxygen flow to the carbon surface offered by the particulate phase to be reduced and has important consequences for combustor design.

Measured carbon particle temperatures were influenced considerably by bed operating conditions ranging from 15 to 215°C higher than bed temperature.

Measured burning rates of carbon particles were found to be reduced significantly when other active particles were present in the bed. This sensitivity of burning rate to changes in active particle concentration in the bed was shown to be increasingly important once the concentration of carbon in the bed exceeded about 1%

Increasing the bed inert particle size, superficial fluidizing velocity, oxygen concentration in the fluidizing gas and bed temperature resulted in higher burning rates. The implication of these findings on combustor design are discussed.     

DEACTIVATION OF DRIED SUBBITUMINOUS COAL

Deep drying of inherent moisture from subbituminous coal produces a dried product which is friable and very reactive to oxygen in air. The major problems in handling dried coal, which are unique to deep drying of low-rank coal, are: (1) avoidance of spontaneous combustion, (2) prevention of moisture reabsorption, and (3) control of dust. Atlantic Richfield has developed, and successfully demonstrated, technologies which solve all three problems. However, this paper will be confined to the spontaneous combustion problem

Low-temperature 'oxidation may result in spontaneous combustion' of the coal during storage or transit. The increased reactivity to low-temperature oxidation of dried coal is due to two factors: (1) the reduction in moisture content, and (2) the increase in temperature. A laboratory technique was developed to measure the coal reactivity to low-temperature oxidation at temperatures near ambient. Laboratory data was then used to develop a pseudo-first order kinetics model for dried subbituminous coal as a function of temperature and moisture content. The influence of particle size was also determined

A one-dimensional spontaneous heating model was formulated to evaluate the effects of wind velocity, coal pile porosity, thermal conductivity of crushed coal, and geometry of coal piles on the dried coal's tendency to spontaneous combustion. This model was validated against two large-scale spontaneous heating tests. Each test consumed about 800 pounds of dried coal.

Based on a selected combination of three deactivation methods and predictions of the computer model, a special product treatment procedure was developed and successfully demonstrated in pilot plant tests in 1983. Dried coal stockpiles (about 100 tons per pile) were monitored for four months under severe weather conditions with no evidence of either moisture reabsorption or spontaneous combustion.     

SOLAR DRYING OF RAISING

The drying of Sultana seedless grapes was investigated under intermittent and continuous operating conditions in a laboratory solar installation involving a thermal storage bed and an auxiliary heater. The effect of pretreatment, and of the a i r velocity on the drying rate of the grapes at constant temperature was also studied in relation t o the quality of the dried product.

Solar drying of the grapes was accomplished in 30.5 to 60.5 h of intermittent operation, or 19 to 60 h of continuous drying, involving the thermal storage bed and the auxiliary heater. The shortest drying time (19 h ) and the highest quality dried product were obtained with grapes dipped in a hot (80 C ) solution of sodium hydroxide and ethyl oleate, which were dried continuously at 42°C and 2m/s, air temperature and velocity respectively.

The mean apparent diffusivity of moisture in raisins at 6o°C and air velocity 2m/s was estimated as 1.0.10-10 m2/s.     

DRYING RATE AND TEMPERATURE PROFILE FOR SUPERHEATED STEAM VACUUM DRYING AND MOIST AIR DRYING OF SOFTWOOD LUMBER

Two charges of green radiata pine sapwood lumber were dried, either using superheated steam under vacuum (90°C, 0.2 bar abs.) or conventionally using hot moist air (90/60°C). Due to low density of the drying medium under vacuum, the circulation velocity used was 10 m/s for superheated steam drying and 5.0 m/s for moist air drying, and in both cases, the flow was unidirectional. In drying, stack drying rate and wood temperatures were measured to examine the differences between the superheated steam drying and drying using hot moist air.

The experimental results have shown that the stack edge board in superheated steam drying dried faster than in the hot moist air drying. Once again due to the low density of the steam under vacuum, a prolonged maximum temperature drop across load (TDAL) was observed in the superheated steam drying, however, the whole stack dried slower and the final moisture content distribution was more variable than for conventional hot moist air drying. Wood temperatures in superheated steam drying were lower.     

SOME DESIGN ASPECTS OF STEAM-FLUIDIZED STEAM HEATED DRYERS

Results are given for a run in which approximately 10 tonnes of Loy Yang (Victoria, Australia) coal were dried in a continuous run. Interesting features are commented on.

Among matters Important for design Is the effect of the heat-transfer surface. The paper reports findings of studies In measuring local pressure-gradients and pressure fluctuations and comments on the significance for design.

Some calculations are also presented In regard to a multiple-effect dryer system with one dryer at elevated pressure, one at atmospheric pressure and one at atmospheric pressure but with the operating temperature at, say 76°C.     

ACETIC ACID AS AN OXYGEN CARRIER BETWEEN TWO PHASES FOR EPOXIDATION OF OLEIC ACID

Acetic acid was found to be an effective oxygen carrier for epoxidation of oleic acid. The reaction model of oleic acid epoxidation in the two-phase reaction system was systematically analyzed and the rate determining step was experimentally identified.

The results indicated that the rate of oxidation of the unsaturated acid was independent of the concentration of oleic acid and depended on the mixing rate and the rate of formation peracetic acid which in turn depended on the concentration of acetic acid, strength of acid catalyst and the oxygen source, hydrogen peroxide. In the region of reaction control, the rate equation of epoxidation was found to be

$

where k = 2.98 × 10^-2 M^-2 min^-1 at temperature of 35^°C.     

Drying Suspensions in a Pulsed Fluidized Bed of Inert Particles

The drying of carbohydrate suspensions on polypropylene particles in a pulsed fluidized bed was studied by means of a 2⁵ experimental design, to determine the effect of the air flow and temperature, suspension flow rate, and free section and rotating speed of the rotary plate on the Nusselt number, the moisture content of the product, and the percentage of solids retained inside the bed (which were minimized to 4.9 and 14.4%, respectively) with an air flow of 600 m³/h at 90°C and 720 mmHg, a suspension flow rate of 6 L/h, and a plate with 6% free section, rotating at 50 rpm.

Additionally, the effects of temperature, air flow, and suspension flow rate on the residence time distribution (RTD) were determined, using the stimulus-response methodology. The RTD was represented by 1.1 to 2 tanks in series, according to this model. The mean residence time of the dried carbohydrate particles was between 5.4 and 8.2 min.

Finally, an egg suspension could be dried at 4 L/h, with air at 90°C, with a mean residence time about 50% longer that that found for drying carbohydrate suspensions.     

EFFECT OF DRYING METHODS ON MOISTURE SORPTION ISOTHERMS OF FUFU AT THREE TEMPERATURES

Moisture sorption characteristics for dried fufu using three drying methods at 25°C, 32°C and 45°C were determined experimentally for water activity ranging from 0.11 to 0.96. At a given temperature and water activity, the rotary dried fufu had the highest equilibrium moisture content while the sun dried had the lowest.

Four linear and four nonlinear sorption models were used to analyze the data. The linear GAB model showed the best fit while the nonlinear Bradley model was the best over the whole range of water activity in their respective groups. Also estimates of the net isosteric heats of sorption and their dependence on moisture content were presented for each product.     

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).     

KINETICS OF THE HYDROLYSIS OF ACRYLONITRILE TO ACRYLAMIDE OVER RANEY COPPER

Measurements of the kinetics of the hydrolysis of acrylonitrileover Raney copper catalysts have been made in the temperature range 40 to 100°C in a tubular reactor operated differentially with and without recycle and over concentration ranges of 0-25 weight percent acrylonitrile, 0-35 weight percent acrylamide and 40-99 weight percent water.

For the concentration range 0 to 7 wt. % acrylonitrile and 0 to 7 wt. % acrylamide, data were fitted by an adsorption model of the type

A + K,CA + KCCC and at higher concentrations by the powers law expression

The activation energy for the reaction was found to be 49.2kJmol^-1. Both models showed that the product acrylamide strongly inhibits the reaction.     

DRYING KINETICS FOR SOME FRUITS: PREDICTING OF POROSITY AND COLOR DURING DEHYDRATION.

Drying kinetics of four fruits (prune, quince, fig and strawberry) were studied by using a simple mass transfer mathematical model involving a characteristic parameter (K) as a function of process variables. The model was tested with data produced in a laboratory air dryer, using non-linear regression analysis. The investigation involved three values of sample thickness (5, 10, 15mm) and three different air temperatures (50, 60, 70 °C).

The parameters of the model were found to be greatly affected by sample thickness and air temperature. The effect of moisture content on the porosity of three fruits (namely avocado, prune and strawberry) was also investigated. A simple mathematical model was used to correlate porosity with moisture content. It was found that porosity increased with decreasing moisture content.

Samples of three fruits (avocado, prune and strawberry) were investigated to estimate color changes during conventional drying at 70 °C. A first order kinetic model was fitted to experimental data with great success. It is found that the color of avocado and strawberry change while the color of prune remains the same.

Furthermore, the water sorption isotherms of three fruits (avocado, prune and strawberry) at 25° temperature were determined experimentally and the parameters at the GAB equation were evaluated by means of non-linear regression analysis. The use of the above equation produced a very good fit.     

TRANSPORT PROPERTIES OF CELLULAR FOOD MATERIALS UNDERGOING FREEZE-DRYING

The samples of sliced and mashed apples were freeze-dried by controlling their surface temperatures over the usual pressure range of commercial operations. The surface of sliced samples could not be maintained at above 10°C in order to prevent the frozen layer from melting, while that of mashed samples was allowed to heat up to 70°C.

Thermal conductivities and permeabilities were determined by applying the uniformly-retreating-ice front model to the dried layer of the samples undergoing freeze-drying. The values of permeability for the mashed samples were found to depend on the ice-crystallization time during freezing. The results indicated that the drying rate of sliced samples was limited by the transfer rate of water vapor flowing through the dried layer. A cellular structural model is proposed for predicting the permeability of the dried layer, based on the resistance of the cell membrane to molecular transfer of water vapor.     

CHANGE IN VIABILITY OF MAIZE DURING HIGH-TEMPERATURE DRYING

Germination and moisture content loss data were collected of maize with a moisture content ranging from 15 to 32% (w.b.), an air temperature from 40 to 75°C, and an exposure time from 0.5 to 180 minutes.

A germination-retention model was developed based on the normally distributed death-rate theory (NDD). The NDD model was combined with a concurrent-flow (CCF) dryer model, and tested against viability data of maize dried in a commercial two-stage CCF dryer. Acceptable agreement between the predicted and experimental viabilities was obtained.

The effect of the CCF dryer design, and of several operating parameters, on the loss of maize-seed viability was analyzed. Simulation with the NDD-CCF dryer model shows that high quality seed can be produced by drying at air temperatures well above 100°C.     

Room Temperature Curing Epoxy Adhesives for Elevated Temperature Service

Room Temperature curing compositions of epoxy resins with high temperature service capability (95-120°C) were formulated and evaluated. The compositions were based on selected high functionality atomatic epoxy polymers and multicomponent poly amine curing agent systems. Toughening was achieved by addition of a rubbery phase either by prereaction of the epoxy resin with carboxyl terminated (CTBN) or by amine terminated (ATBN) poly butadiene acrylonitrile. The latter elastomeric component served as a part of the poly amine curing agent.

Best results were achieved with an adhesive formulation comprising tetra glycidyl-4-4'-diaminodiphenylmethane (TGDDM) and triglycidyl ether of p-aminophenol with triethylenetetramine and addition of ATBN with a felt carrier.

Lap shear strengths of aluminum/aluminum specimens primed by silane coupling agent in the order of 22 MPa at 25°C and 11 MPa at 120°C with T-Peel strengths of 1.6N/mm at 25°C and 0.52 N/mm at 120°C, were obtained.

The thermal behaviour and transitions, the chemical and mechanical properties, the microstructure and morphology of the selected adhesive formulation were studied, using DSC, Gehman, FTIR, mechanical testing and SEM analysis, respectively.

Experimental results showed that the selected compositions could develop good high temperature (120°C) properties while cured at room temperature. Furthermore, their high temperature performance compares favorably or even exceeds that of commercially available room-temperature-curing adhesive compounds, and are competitive with elevated temperature cured film adhesives.     

SURFACE TENSION EFFECTS IN THE SEDIMENTATION OF COAL PARTICLES IN VARIOUS LIQUID MIXTURES

The sedimentation volume, V_sed, of coal particles in mixtures of pairs of non-polar and more polar organic liquid combinations as well as in aqueous solutions was determined at 20°C.

The liquid combinations were chosen such that the surface tension, γ_SV, of the coal particles fell between the surface tensions, γ_LV, of the two liquids. A constant mass of a given coal sample was suspended in constant volumes of liquid mixtures of different concentrations.

It was found that the sedimentation volume changed with varying composition of the liquid mixtures, as did the surface tension. A maximum or a minimum occurred in V_sed when the surface tension of the suspending liquid was equal to that of the coal particles, i.e. when γ_LV = γ_SV. Maxima occurred in more polar and minima in the non-polar or less polar liquid mixtures. The position of the extrema, and hence the surface tension, γ_sv, of the particles, was found to change with particle size, in agreement with findings from other independent techniques.

It was found that the surface tension of coal, γ_sv, depends on the surrounding liquid, i.e. coal is hydrophobic in organic and hydrophilic in aqueous media, again in agreement with the results from other techniques.     

KINETICS OF OXYDESULFURIZATION OF MINSHALL, INDIANA COAL

The kinetics of the oxidation of pyritic sulfur, organic sulfur and carbon for the Minshall, Indiana coal was investigated. Experiments were conducted in a semi-batch manner. The experimental data were taken in the temperature range of 19O-23O°C, oxygen partial pressure range of 0.86-3.44 MPa and for the batch times up to 3600 s.

The rate of organic sulfur removal was observed to be second order with respect to sulfur and first order with respect to the partial pressure of oxygen, and this reaction was found to have an activation energy of 75,000 kJ/kg-mole. The pyritic sulfur removal was observed to be much more rapid, and the experimental data fitted the continuous reaction model with second order dependence on pyritic sulfur. However, the coal combustion loss in the case of this coal was relatively high, probably because of the presence of larger amounts of oxygen containing compounds, and this coal did not meet the EPA standards for the heating value losses.     

Flexibilized Copolyimide Adhesives

Two copolyimides, LARC-STPI and STPI-LARC-2, with flexible backbones were prepared and characterized as adhesives. The processability and adhesive properties were compared to those of a commercially available form of LARC-TPI.

Lap shear specimens were fabricated using adhesive tape prepared from each of the three polymers. Lap shear tests were performed at room temperature, 177°C, and 204°C before and after exposure to water-boil and to thermal aging at 204°C for up to 1000 hours.

The three adhesive systems possess exceptional lap shear strengths at room temperature and elevated temperatures both before and after thermal exposure. LARC-STPI, because of its high glass transition temperature provided high lap shear strengths up to 260°C. After water-boil, LARC-TPI exhibited the highest lap shear strengths at room temperature and 177°C, whereas the LARC-STPI retained a higher percentage of its original strength when tested at 204°C [68% versus 50% (STPI-LARC-2) and 40% (LARC-TPI)].

These flexible thermoplastic copolyimides show considerable potential as adhesives based on this study and because of the ease of preparation with low cost, commercially available materials.