MULTI-STAGE SPRAY DRYING METHOD IN MANUFACTURING HEAVY DUTY DETERGENTS

Collision of droplets; counter-current spray dryer; drying rate; heat transfer; nonphosphated detergent; spray drying

The spray drying method of non-phosphated granular detergents is studied to decrease the amount of agglomerate particles. The formation of agglomerates is mainly influenced by the concentration of droplets in spray cloud and the water content of droplets at the time of collision. The overlaps of different spray clouds should be de- creased.

The drying rate near the nozzle zone is considerably faster than that calculated by Ranz-Marshal's equation. According to these phenomena, “Multi-stage spray drying” is developed, which is characterized by in stalling plural spraying stages in a spray dryer.

Consequently, non-phosphated detergents are manufactured with the same powder properties and productivity as phosphated detergents.     

RECIRCULATING REACTOR FOR THE STUDY OF MULTIPHASE KINETICS

A new laboratory reactor was set up to measure kinetic coefficients in a solid (catalyst)-liquid-gas reacting system.

The reactor consists of two parts: an absorber, where the liquid is partially saturated by the gas reactant and a reacting zone, where the liquid alone, containing the dissolved gas, flows through a fixed bed of catalyst.

The ricircle of the liquid in the absorber maintains a high concentration of the gas reactant in the liquid also in the zone of reaction, allowing the use of a high mass of catalyst (significative from a statistical point of view) and the achievement of sufficiently high conversion.

The tested reaction is the catalysed hydrogenation of ∝-metylstyrene: in order to consider a drastic situation and to verify the results with the literature data, the experimental conditions examined corresponded to very high chemical reaction rate (instantaneous reaction) at the surface of the pellets.

The tests were carried out with the reactor working both in batchwise and in continuous operative mode (steady state); the results show the reliability of the new reactor above all when the steady state operation is considered. For the use of the reactor in batchwise condition, the accumulation of the product inside the catalyst particles must be considered for an accurate measurement of the kinetic parameters,     

PREDICTION OF OIL SATURATION IN FIBROUS BED-COALESCER BY DYNAMIC PULSE TECHNIQUE

Average oil saturation in fibrous bed coalescer has been measured experimentally using pulse response technique. Comparable results were obtained with saturation values obtained under steady-state condition.

The experimental technique involved the injection of small amount of toluene-water dispersion in the upstream side of the coalescer and following the inlet and outlet concentration count for this dispersion. As a result of this work a coalescence rate constant, k, could be measured which gives an indication of the rate of coalescence in the bed.     

GAS MIXING IN SLUGGING AND TURBULENT FLUIDIZED BEDS

The gas phase mixing in a fluidized bed of glass beads (d_p = 0.362 mm) in the slugging and turbulent flow regimes has been studied in a 0.1 m-ID × 3.0 m high Plexiglas column.

The gas dispersion in the downstream of the bed has been described by a diffusion process with the axial and radial dispersion coefficients. The radial dispersion coefficient of the gas phase is nearly constant with the variation of gas velocity in the slugging flow regime, but it increases with an increase in gas velocity in the turbulent flow regime.

Appreciable backmixing of the gas phase is pronounced in the slugging flow regime whereas the lower gas backmixing is produced in the turbulent flow regime. The gas backmixing coefficient increases with an increase in gas velocity in the slugging flow regime, but it decreases slightly with an increase in gas velocity in the turbulent flow regime.

The radial mixing and backmixing coefficients of the gas in terms of Peclet numbers have been correlated with the relevant dimensionless parameters (U_g/U_mf, p_s/p_g, d_p/D_t).

The gas flow pattern in the bed has been well represented by a simplified model based on the two gas phases in the dilute and dense phases which are percolating through the bed in plug flow. The present model can predict the gas exchange coefficient between the phases, the fractions of the dilute phase, the interstitial gas in the dense phase, and the interstitial gas velocity in the bed.     

A KINETIC MODEL FOR THE PRODUCTION OF LIQUIDS FROM THE FLASH PYROLYSIS OF BIOMASS

A kinetically based prediction model for the production of organic liquids from the flash pyrolysis of biomass is proposed. Wood or other biomass is assumed to be decomposed according to two parallel reactions yielding liquid tar and ( gas + char) The tar is then assumed to further react by secondary homogeneous reactions to form mainly gas as a product

The model provides a very good agreement with the experimental results obtained using a pilot plant fluidized bed pyrolysis reactor

The proposed model is shown to be able to predict the organic liquid yield as a function of the operating parameters of the process, within the optimal conditions for maximizing the tar yields, and the reaction rate constants compare reasonably well with those reported in the literature     

PERFORMANCE OF A PRESSURIZED TWO-STAGE FLUIDIZED GASIFICATION PROCESS FOR PRODUCTION OF LOW-BTU GAS FROM COAL CHAR

A two-stage pressurized fluidized-bed gasification process has been developed to produce low-heating value gases from coal char. The reactor was 0.075 m id. and 1.4 m long, and gasification experiments were conducted under pressures up to 790 kPa and at temperatures up to 1323 K. A partition disc was used to divide the fluidized bed into two stages, using the first stage as a partial combuster and gasifier and the second stage as a gasifier. The disc was designed to control compositions of coal char particles in both stages so that the heat required for the endothermic gasification reaction in the second stage can be provided by the heat of combustion in the first

For conditions examined here, the disc with an opening ratio of 40° was found to give optimum distribution of the char particles in both stages without ash agglomeration. It was also shown that all oxygen gas was completely consumed within the first stage

The heating value of the product gas increased with the char feed rale. However, there may be an oplimum Teed ratio of char and sand-particles since the higher char feed rate causes more frequent ash agglomeration as well as less carbon conversion     

TWO STAGE DRYING OF SYNTHETIC DETERGENTS

A key ingredient in granular synthetic detergents is Sodium Tripolyphosphate (STP). The hexahydrate of STP is unstable during the spray drying process. The decomposition (reversion) is shown to be solely a function of particle temperature. The reversion products interact with calcium ions in the wash water and form insoluble species. This results in undesirable deposition onto the fabrics. The goal of this study is to develop a drying process with lower temperatures to reduce the decomposition. This, in turn, will improve product quality by reducing undesirable fabric deposits.

A two stage drying process is used to reduce the drying temperatures. Experimental evidence is presented to demonstrate the near-elimination of STP decomposition.

The design of a production scale fluid bed dryer uses a ‘normalization'procedure to calculate the rate constants at constant bed temperature from the data taken at constant inlet air temperature. The rate constants are corrleated to bed temperature using an Arrhenius-like equation.     

Numerical Spray Model of the Fluidized Bed Coating Process

In this study, a new model for the batch top-spray fluidized bed coating process is presented. The model is based on the one-dimensional (axial) discretization of the bed volume into different control volumes, in which the dynamic heat and mass balances for air, water vapor, droplets, core particles, and coating material were established. The coupling of the droplet phase's mass and heat transfer terms with the gas and solid phases was established by means of a droplet submodel in which droplet trajectories were individually simulated.

The model calculation method combines a Monte Carlo technique for the simulation of the particle exchange with the first-order Euler's method for solving the heat and mass balances, enabling the prediction of both the dynamic coating mass distribution and the one-dimensional (axial) thermodynamic behavior of the fluidized bed during batch operation. The simulation results were validated using experimental two-dimensional spatial air temperature and air humidity distributions, which were measured in a fluidized bed pilot reactor using a scanning probe.

Sensitivity analysis was carried out to study the effect of controllable process variables, such as fluidization air and atomization air properties, as well as the properties of the spraying liquid upon the simulated dynamic temperature and humidity distributions. Also, the effects of relevant process variables on growth rate uniformity and process yield were studied. Based on these sensitivity studies it was concluded that nozzle parameters, such as air pressure and positioning with respect to the bed, are as important as the fluidization air properties (humidity, temperature, and flow rate) for the coating growth rate uniformity and process yield.     

A MONTE-CARLO APPROACH TO THE INTERPRETATION OF BUBBLE PROBE SIGNALS IN FREELY BUBBLING FLUID BEDS†

The interpretation of the signals generated by a double probe may be done by the three characteristic times method:

t₁ the time duration of the pulses;

t₂ the time shift between the signals of the two probes;

t₃ the time interval between two pulses on one channel.

Each of these times is largely dispersed and the corresponding histograms may be constructed.

The present work is a trial to go over from the time histograms to physical properties of the bubbles combining a Monte-Carlo simulation and a flexible simplex optimisation procedure.

As a result, the percentage of oblique bubbles cutting just one level, the bubble size distribution, the average velocity-size relation, and the individual dispersion around it may be defined.

The procedure is finally applied to experimental results obtained with a light probe in a fluid bed of glass beads.     

RADIO FREQUENCY ENHANCED DRAUGHT TUBE SPOUTED BED DRYER

A study of the operation of a draught tube spouted bed in conjunction with dielectric heat addition was carried out. The addition of radio frequency allowed a greater amount of energy to be input into the particulate material in any given time, increasing the drying rate: however, the drying pattern was unaffected by the ratio of radio frequency to convective heat addition. The rate of moisture loss was proportional only to the total energy input. The synergy which often occurs with low levels of radio frequency addition does not occur with rf enhanced spouted beds.

The properties of spouted beds seem to be improved only for moisture contents at which packed bed drying can perform effectively.

Although the properties of spouted beds can be improved by radio frequency heat addition, their qualities although useful for specific operations, are unsuitable for more general use.     

ANALYSIS OF HEAT TRANSFER IN A NON-ISOTHERMAL SOLID-GAS REACTING MEDIUM

This article concerns the thermal phenomena occurring in a reacting solid-gas porous medium. The practical application envisaged is the use of these media in chemical heat pumps (CHPs).

The procedure chosen allowed us to almost totally separate the determination of the kinetic characteristics of the medium, presented in the previous article, from the study of the thermal phenomena presented here.

The thermal parameters were estimated or identified, depending on their influence, quite satisfactorily.

Moreover, we show that it is possible to exploit more limited experimental data, by modifying the procedure, to obtain a sufficiently good understanding of the reacting medium. This technique was applied in the study of the effect of mixing a conducting binder with the medium on its thermal parameters.

This model is also used to test configurations having different thicknesses of the reacting bed.     

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.     

EVALUATION OF CONDUCTIVE HEAT TRANSFER MECHANISMS BETWEEN AN IMMERSED SURFACE AND THE ADJACENT LAYER OF PARTICLES IN BUBBLING FLUIDIZED BEDS

The evaluation of the heat transfer coefficient h_wp between a heat exchanging surface immersed in a gas fluidized bed and the adjacent layer of dense phase particles is analyzed in this contribution. Gas convective and radiant effects are not included in the present analysis.

The inclusion of h_wp, or an equivalent formation, in mechanistic models describing heat transfer has been necessary because the sudden voidage variation close to the immersed wall restrains significantly the heat transfer rate. However, there is not at present a widely accepted expression to evaluate h_wp.

A precise formulation for h_wp accounting for transient conduction inside spherical particles, the Smoluchowski effect, the concentration of particles in the adjacent layer (N_p) and an effective separation gap (l₀) is developed here.

Although N_p can be estimated, in principle, from experimental evidence in packed beds, and it is reasonably expected that l₀ = 0, the analysis of experimental heat transfer rates in moving beds, packed beds, and bubbling fluidized beds indicate that values of h_wp are, in general, smaller than expected from these assumptions. Appropriate values of l₀ and N_p are then stimated by fitting the experimental data.

The probable effect of surface asperities is also discussed by analyzing a simplified geometrical model. It is concluded that the parameter l₀ can be also effective to account for particle roughness, independently of thermal properties.     

Drying Sawdust in a Pulsed Fluidized Bed

The objective of this work was the experimental and theoretical study of sawdust drying, in batch and continuous experiences, using a pulsed fluidized bed dryer.

In the batch experiences, a 2³ factorial design was used to determine the kinetics of drying, the critical moisture content, and the effective coefficients of both diffusivity and heat transfer, all of them as a function of the velocity and temperature of the air, the speed of turning of the slotted plate that generates the air pulses in the dryer, using sawdust with 65% moisture in each run.

In the continuous operation, a 2³ factorial design was used to study the effect of the solid flow and the velocity and temperature of the air on both the product moisture and the distribution of residence times. In order to determine these last ones, digital image processing was used, utilizing sawdust colored by a solution of methylene blue as tracer.

The statistically significant factors were the velocity and the temperature of the heating air, for both the continuous and batch operations. Although the speed of turn of the slotted plate was not significant, it was observed that the air pulses increased the movement of particles, facilitating its fluidization, especially at the beginning of drying.

The heat transfer coefficients were adjusted according to the equation Nu = 0.0014 Re_p^1.52, whose standard deviation of fit is 0.145.

The period of decreasing rate was adjusted to several diffusivity models, giving the best fit the simplified variable diffusivity model (SVDM). The curve of distribution of residence times was adjusted using the model of tanks in series, with values between 2.6 and 5 tanks.     

APPLICATION OF UF TECHNOLOGY TO LARGE-SCALE SYNTHESIS OF GT 267-004, A SEQUESTRANT FOR C. DIFFICILE TOXIN

GT 267-004 is a nonabsorbed, nonantibiotic, high molecular weight anionic polymer that is undergoing clinical evaluation as a Clostridium difficile toxin sequestrant. The API is a mixed salt form that consists of approximately 30 to 50% potassium and 70 to 50% sodium as the counterions on the polymer.

The initial polymerization process results in an aqueous polymer solution with the polymer in the 100% sodium form. It also contains some oligomeric impurities. UF technology was applied in a novel way to convert the single-salt polymer to the mixed salt form and to simultaneously remove the oligomers below the required specification limits in a single-unit operation.

Experiments with a UF lab unit validated the concept of simultaneously performing ion exchange and purification. An appropriate amount of potassium chloride was added to the polymer solution to carry out the ion exchange considering the selectivity of the polymer for the potassium ion over the sodium ion. The resulting mixed salts in solution were removed using ultrafiltration membranes. The process produced the API in excellent purity.

The lab data were used to scale up the process to produce several hundred kg of the API. The engineering analysis of the large-scale UF operation was carried out to run the UF process in the cyclic mode and in the diafiltration mode. The UF operation was optimized with respect to time, water usage, operability, and the concentration of product solution required for the subsequent processing.

The optimized UF process was found to be a very cost-effective and time-efficient route to produce the new API.     

MAGNETICALLY STABILIZED FLUIDIZATION, PART II: CONTINUOUS GAS FILTRATION

The magnetically stabilized Auidized bed can be used as a dust filter, as was suggested earlier by Rosensweig and coworkers (1983).

In this experimental study filtration tests were carried out under various conditions. It was shown experimentally that dust particles of about one micron can be filtered effectively in beds of magnetite particles of about 300 microns, with a bed height of 10 cm. The filtration efficiency was predicted reasonably well by existing theories.

By using a magnetic grid plate, proposed by Jaraiz and coworkers (1983), an even quasi continuous downflow of the solids can be realized. Since it was shown that the dust penetrates the bed for a few millimeters only, a very effective removal of the collected dust can be obtained. In this manner, the filter was used effectively in a continuous operation.

The results appear to be especially promising for dust filtration of gas flows under high pressure, and at elevated temperatures, limited only by the Curie point of the bed material.     

HEAT TRANSFER IN SPOUTED BEDS

Experiments were carried out in order to analyse the wall-to-bed and fluid-to-particle heat transfer coefficients in spouted Beds. wall-to-bed heat transfer coefficients were determined in cylindrical-conical and conical spouted beds for various gas flow rates, particle sizes and bed heights for spouted beds with and without draft tubes.

A new definition for wall-to-bed transfer coefficient was proposed baaed on experimental observations.

The heat tranefer area was also studied to ensure that a physically significant fluid-to-particle heat transfer coefficient was achieved.     

MAGNETICALLY STABILIZED FLUIDIZATION PART I GAS AND SOLIDS FLOW

A fluidized bed of magnetic particles, such as iron or magnetite, can be stabilized by applying an external magnetic field, as was shown earlier by Rosensweig and coworkers. The stabilization results in a suppression of bubble formation, little solids mixing and a much narrower residence time distribution of the gas flow; the gas flow rate in the dense phase is increased. In this experimental study the axial and radial mixing coefficients in the gas flow were determined as functions of several variables, such as gas flow rate and magnetic field strength. It appeared that the radial mixing coefficient is comparable to that in a fixed bed, and the axial mixing coefficient was greater than in a fixed bed but smaller than in a fluidized bed without stabilization. The axial mixing is the result of some channelling.

The mixing of the solids is very low, and if there is a continuous solids flow through the bed, deviations from plug flow can be reduced by increasing the magnetic field strength.

Apparently, the magnetically stabilized fluidized bed is well suitable for countercurrent gas-solid operations.     

CHARACTERIZATION OF FLUIDIZED BED REACTORS WITH GAS TRACER MEASUREMENTS

In a steady state bench scale fluidized bed the decomposition reaction of NaHCO₃ was carried out. The residence times distributions, DRT, of carbon dioxide (the gaseous product) and non adsorbing argon (the reference tracer) were mass spectroscopically measured as a function of the bed temperature. By means of single-, two- and three-phase dispersion models as well as by a cell model, the DRT's were evaluated on line by a computer.

The steady state transverse and longitudinal concentration profiles of these tracers upstream from the plane source were also measured and evaluated by a dispersion model as well as by a counter current back mixing model. Comparison of the steady state and nonsteady state dispersion coefficient measurements indicate that the longitudinal gas mixing is only partially due to backmixing. The experimentally determined wake fractions agree well with those published in the literature. Since the adsorption rate of CO₂ on the pore surface area of the particles in the dense phase is high no interphase transfer from the interstitial gas of the dense phase into the bubble phase takes place.

The desorption of CO₂ and its return into the interstitial gas and than into the gas phase occurs only slowly and with an initial time lag. The on-line DRT can be used as a diagnostical technique for investigation of the reactor during its operation, if operation disturbances or breakdowns occur.