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The aim of this work was the experimental evaluation of different friction factor correlations for water flow through packed beds of spherical and non-spherical particles at ambient temperature. The experiments were performed by measuring the pressure drop across the bed. Packed beds made of monosized glass spherical particles of seven different diameters were used, as well as beds made of 16 fractions of quartz filtration sand obtained by sieving (polydisperse non-spherical particles). The range of bed voidages was 0.359-0.486, while the range of bed particle Reynolds numbers was from 0.3 to 286 for spherical particles and from 0.1 to 50 for non-spherical particles. The obtained results were compared using a number of available literature correlations. In order to improve the correlation results for spherical particles, a new simple equation was proposed in the form of Ergun's equation, with modified coefficients. The new correlation had a mean absolute deviation between experimental and calculated values of pressure drop of 9.04%. For non-spherical quartz filtration sand particles the best fit was obtained using Ergun's equation, with a mean absolute deviation of 10.36%. Surface-volume diameter (dsv) necessary for correlating the data for filtration sand particles was calculated based on correlations for d(v) = f(d(m)) and psi = f(d(m)).

In this paper, the minimum fluidization velocity and bed expansion of polydisperse sand mixtures fluidized with water were investigated. The fluidization experiments were carried out using 19 different fractions of quartz filtration sand with sieve diameters in the interval d(m) = 0.359 to 2.596 mm. The intercept fluid velocity U-E was determined from the plot of fluid superficial velocity vs. porosity. The results obtained indicated that U-E was about 11% higher than particle terminal velocity U-t, for all of the sand fractions investigated. Based on the experimental data, the new model for bed expansion of quartz filtration sand was proposed. Using the proposed model, the mean absolute deviation in predicting the bed porosity epsilon, for a given superficial velocity U, was 2.66%, while the mean absolute deviation in predicting U at a given epsilon was 8.75%. For the prediction of the minimum fluidization velocity, U-mf, several literature models were tested. The best results were obtained using the Ergun model, with mean absolute deviation in predicting U-mf of 12.42%. A new simple correlation for the prediction of the minimum fluidization velocity U-mf was proposed. The proposed correlation is the function only of the mean sieve diameter of the filtration sand fraction. The mean absolute deviation between the experimental and the predicted values of U-mf was 7.65%.

U ovoj disertaciji ispitivana je sinteza metil estara masnih kiselina (MEMK), odnosno biodizela u reakciji metanolize triglicerida (TG) u prisustvu čvrstog katalizatora. Sinteza katalizatora izvršena je mehanohemijskim putem polazeći od CaO i ZnO prahova uz dodatak vode, kako bi se dobio prekursor CaZn2(OH)6·2H2O, koji se ţarenjem na 700 °C prevodi u smešu oksida CaO∙ZnO. Nakon detaljne karakterizacije, CaO∙ZnO katalizator korišćen je u reakciji metanolize rafinisanog suncokretovog i korišćenog biljnog ulja (KBU) koja je izvedena u šarţnim uslovima. U cilju odreĎivanja parametara koji u potpunosti definišu brzinu metanolize i ukazuju na osnovne otpore od kojih ona zavisi, analiziran je uticaj temperature, molarnog odnosa metanol:ulje, mase katalizatora i intenziteta mešanja na brzinu procesa. UtvrĎeno je da brzina procesa zavisi od otpora prenosu mase TG do površine katalizatora i otpora hemijskoj reakciji na površini katalizatora, koji su definisani kao recipročna vrednost ukupnog zapreminskog koeficijenta prenosa mase, , i konstante brzine reakcije pseudo prvog reda, . Ovi kinetički parametri odreĎuju vrednost prividne konstante brzine koja se menja sa napredovanjem procesa metanolize, odnosno sa promenom stepena konverzije TG. Brzina procesa je na taj način opisana kinetičkim modelom nepovratne reakcije prvog reda sa promenljivom prividnom konstantom brzine koja uzima u obzir i prenos mase TG i brzinu hemijske reakcije izmeĎu TG i metanola. UtvrĎeno je da ukupan zapreminski koeficijent prenosa mase, , zavisi od specifične meĎufazne površine, kao i da se moţe predstaviti kao funkcija stepena konverzije TG. Ova pretpostavka se bazira na uočenom efektu da se dostizanjem odreĎene kritične konverzije TG nakon stvaranja prvih količina monoglicerida (MG), diglicerida (DG) i MEMK, brzina prenosa mase znatnije uvećava i postaje veća od brzine hemijske reakcije na površini katalizatora. Tada reakcija na površini katalizatora počinje da kontroliše brzinu ukupnog procesa..., The synthesis of fatty acid methyl esters (FAME), i.e. biodiesel, in the methanolysis of triglycerides (TG) in the presence of solid catalyst was studied. Heterogeneous catalyst was synthesized by mechanochemical treatment of CaO and ZnO powder mixture with the addition of water required for the formation of corresponding mixed hydroxides, CaZn2(OH)6·2H2O, followed by calcination at 700 °C in air to obtain CaO∙ZnO composite powder. After detailed characterization CaO∙ZnO catalyst was used in methanolysis of refined sunflower as well as used vegetable oil (UVO) in the batch conditions. Influence of temperature, agitation speed, catalyst amount and methanol to oil molar ratio on the rate of reaction was analyzed in order to determine parameters that completely define the rate of methanolysis and indicate the basic resistances on which it depends. It was revealed that the rate of the process depends on the two resistances – mass transfer of triglycerides to the catalyst surface and chemical reaction on the catalyst surface, which are defined as reciprocal value of , the overall triglyceride volumetric mass transfer coefficient and , the effective pseudo-first order reaction rate constant. These kinetic parameters actually determine the value of the apparent reaction rate constant, , which changes with the progression of the methanolysis process, i.e. with the change of triglyceride conversion. The rate of the process is thus described with the simple kinetic model of irreversible pseudo-first order reaction with the variable apparent rate constant, which takes into account both the triglycerides mass transfer and chemical reaction between TG and methanol. The overall volumetric mass transfer coefficient, , was found to depend on interfacial area between the oil and methanol, and can be presented as the function of TG conversion. This assumption is based on the observed effect that when critical conversion of TG is achieved, after formation of small amounts of MG, DG an

The synthesis of fatty acid methyl esters (FAME), i.e. biodiesel, in the methanolysis of triglycerides (TG) in the presence of solid catalyst was studied. Heterogeneous catalyst was synthesized by mechanochemical treatment of CaO and ZnO powder mixture with the addition of water required for the formation of corresponding mixed hydroxides, CaZn2(OH)6·2H2O, followed by calcination at 700 °C in air to obtain CaO∙ZnO composite powder. After detailed characterization CaO∙ZnO catalyst was used in methanolysis of refined sunflower as well as used vegetable oil (UVO) in the batch conditions. Influence of temperature, agitation speed, catalyst amount and methanol to oil molar ratio on the rate of reaction was analyzed in order to determine parameters that completely define the rate of methanolysis and indicate the basic resistances on which it depends. It was revealed that the rate of the process depends on the two resistances – mass transfer of triglycerides to the catalyst surface and chemical reaction on the catalyst surface, which are defined as reciprocal value of , the overall triglyceride volumetric mass transfer coefficient and , the effective pseudo-first order reaction rate constant. These kinetic parameters actually determine the value of the apparent reaction rate constant, , which changes with the progression of the methanolysis process, i.e. with the change of triglyceride conversion. The rate of the process is thus described with the simple kinetic model of irreversible pseudo-first order reaction with the variable apparent rate constant, which takes into account both the triglycerides mass transfer and chemical reaction between TG and methanol. The overall volumetric mass transfer coefficient, , was found to depend on interfacial area between the oil and methanol, and can be presented as the function of TG conversion. This assumption is based on the observed effect that when critical conversion of TG is achieved, after formation of small amounts of MG, DG an, U ovoj disertaciji ispitivana je sinteza metil estara masnih kiselina (MEMK), odnosno biodizela u reakciji metanolize triglicerida (TG) u prisustvu čvrstog katalizatora. Sinteza katalizatora izvršena je mehanohemijskim putem polazeći od CaO i ZnO prahova uz dodatak vode, kako bi se dobio prekursor CaZn2(OH)6·2H2O, koji se ţarenjem na 700 °C prevodi u smešu oksida CaO∙ZnO. Nakon detaljne karakterizacije, CaO∙ZnO katalizator korišćen je u reakciji metanolize rafinisanog suncokretovog i korišćenog biljnog ulja (KBU) koja je izvedena u šarţnim uslovima. U cilju odreĎivanja parametara koji u potpunosti definišu brzinu metanolize i ukazuju na osnovne otpore od kojih ona zavisi, analiziran je uticaj temperature, molarnog odnosa metanol:ulje, mase katalizatora i intenziteta mešanja na brzinu procesa. UtvrĎeno je da brzina procesa zavisi od otpora prenosu mase TG do površine katalizatora i otpora hemijskoj reakciji na površini katalizatora, koji su definisani kao recipročna vrednost ukupnog zapreminskog koeficijenta prenosa mase, , i konstante brzine reakcije pseudo prvog reda, . Ovi kinetički parametri odreĎuju vrednost prividne konstante brzine koja se menja sa napredovanjem procesa metanolize, odnosno sa promenom stepena konverzije TG. Brzina procesa je na taj način opisana kinetičkim modelom nepovratne reakcije prvog reda sa promenljivom prividnom konstantom brzine koja uzima u obzir i prenos mase TG i brzinu hemijske reakcije izmeĎu TG i metanola. UtvrĎeno je da ukupan zapreminski koeficijent prenosa mase, , zavisi od specifične meĎufazne površine, kao i da se moţe predstaviti kao funkcija stepena konverzije TG. Ova pretpostavka se bazira na uočenom efektu da se dostizanjem odreĎene kritične konverzije TG nakon stvaranja prvih količina monoglicerida (MG), diglicerida (DG) i MEMK, brzina prenosa mase znatnije uvećava i postaje veća od brzine hemijske reakcije na površini katalizatora. Tada reakcija na površini katalizatora počinje da kontroliše brzinu ukupnog procesa...

A three-phase (gas-liquid-solid) fluidized bed was used to study the heat transfer characteristics of a system consisting of low-density (290 kg/m(3)) spherical particles (2 cm diameter) in a 0.25 m cylindrical column with counter-current flow of water and air. The experimental investigation and mathematical modeling of heat transfer between the hot air and the cooling water was carried out. The experiments were conducted for a variety of different fluid flow rates and inlet air temperatures, while the air flow rate was kept constant. Based on the obtained experimental results, a new correlation for heat transfer in a three-phase fluidized system was proposed. The mean percentage error between the experimental and the correlated values of the j(Hp) obtained was 1.69%. The hydrodynamic parameters of the system were also calculated according to the available literature correlations.

The aim of this work was the experimental investigation of the particle friction factor for air flow through a packed bed of particles at ambient and elevated temperatures. The experiments were performed by measuring the pressure drop across the packed bed, heated to the desired temperature by hot air. Glass spherical particles of seven different diameters were used. The temperature range of the air flowing through the packed bed was from 20 to 350°C and the bed voidages were from 0.3574 to 0.4303. The obtained results were correlated using a number of available literature correlations. The overall best fit of all of the experimental data was obtained using the Ergun equation, with mean absolute deviation of 10.90%. Ergun's equation gave somewhat better results in correlating the data at ambient temperature with mean absolute deviation of 9.77%, while correlation of the data at elevated temperatures gave mean absolute deviation of 12.38%. The vast majority of the correlations used gave better results when applied to ambient temperature data than to the data at elevated temperatures. Based on the results obtained, the Ergun equation is proposed for friction factor calculation both at ambient and at elevated temperatures., Cilj ovog rada je bio eksperimentalno ispitivanje koeficijenta trenja fluid-čestice prilikom strujanja vazduha kroz pakovani sloj čestica, na sobnoj i povišenim temperaturama. Izvršeno je eksperimentalno merenje pada pritiska u pakovanim slojevima različitih temperatura zagrejanih korišćenjem vrelog vazduha. Kao materijal za pakovanje korišćene su sferične staklene kuglice 7 različitih prečnika. Temperaturni interval u kom su vršeni eksperimenti bio je od 20 do 350°C, dok su poroznosti sloja iznosile od 0,3574 do 0,4303. Dobijeni rezultati korelisani su korišćenjem većeg broja literaturnih korelacija. Najbolje slaganje sa eksperimentalnim podacima pokazala je Ergunova jednačina [1], sa srednjim procentnim odstupanjem od 10,90%. Ergunova jednačina je dala bolje rezultate prilikom korelisanja podataka na sobnoj temperaturi (srednja procentna greška 9,77%), dok je korelisanje podataka na povišenim temperaturama izvršeno sa greškom od 12,38%. Većina testiranih literaturnih korelacija je dala bolje rezultate pri korelisanju podataka dobijenih na sobnoj temperaturi u odnosu na podatke dobijene na povišenim temperaturama. Na osnovu dobijenih rezultata, predlaže se korišćenje Ergunove jednačine za izračunavanje koeficijenta trenja fluid-čestice kako na sobnoj, tako i na povišenim temperaturama.

Wall-to-bed heat transfer in particulate fluidized beds of spherical particles was studied. Experiments were performed using spherical glass particles of 0.80-2.98 mm in diameter with water in a 25.4 mm I.D. copper tube equipped with a steam jacket. Heat transfer data related to the fluid-particle interphase drag coefficient were obtained and compared with previous results for wall-to-bed mass transfer in fluidized beds [Bogkovit et aL, Powder Technol., 79 (1994) 217]. All the data for momentum, heat and mass transfer in particulate fluidized beds of spherical particles, showed the existence of an analogy among these three phenomena.

In this paper the process of the copper (II) sulphate monohydrate from copper (II) sulphate pentahydrate (also known as a Blue vitriol or Bluestone) production was analyzed. Copper (II) sulphate pentahydrate is one of the most important copper salts which has been known since the ancient Egyptians. In the nineteenth century its application as a fungicide was discovered which provoked wide industrial production. Molecule of the copper (II) sulphate pentahydrate is a crystalohydrate with five water molecules linked by chemical bonds to a molecule of the copper (II) sulphate. Copper (II) sulphate exists as a series of compounds that differ in their degree of hydratation. The anhydrous form is a pale green or gray-white powder, whereas the pentahydrate (CuSO4∙5H2O), the most commonly encountered salt, is bright blue. In order to obtain copper (II) sulphate monohydrate from copper (II) sulphate pentahydrate, it is necessary to remove four water molecules. To determine the optimum temperature and time required for the removal of four water molecules from a molecule of pentahydrate in this work thermogravimetric (TGA) analysis was performed. Thermogravimetric (TGA) analysis - dehydration of copper (II) sulphate pentahydrate is done using simultaneous TG-DSC thermal analyzer DTG-Q600 SDT from TA Instruments. Analysis was carried out for two types of samples, the samples containing particles of the average diameter equal to 0.17 mm and the particles of the average diameter 0.5 mm. In addition, fluidization and drying curve were determined using a semiindustrial fluidization column. On the top, the industrial fluidization column, aimed to produce 300 tonnes per month of copper (II) sulphate monohydrate, was designed. Material and energy calculations were performed using software packages Simprosys 3.0 and SuperPro Designer 5.1. Simprosys 3.0 is a software package designed for the modeling and simulation of a drying process, as well as for 20 different unit operations. SuperPr, U ovom radu analiziran je proces dobijanja bakar(II)-sulfat-monohidrata iz bakar(II)-sulfat-pentahidrata, poznatijeg kao 'plavi kamen', u sušnicama sa fluidizovanim slojem. Bakar( II)-sulfat-pentahidrat je kristalohidrat u čijoj su kristalnoj strukturi četiri molekula vode vezana za bakar(II)-jon dok je peti molekul vode slobodan, tj, vodoničnim vezama povezan za sulfat-anjon. Optimalna temperatura zagrevanja kao i dužina zagrevanja za uklanjanje četiri molekula vode iz molekula pentahidrata utvrđeni su na osnovu termogravimetrijske (TGA) analize. Za analizu su upotrebljene čestice različite granualcije: srednjeg prečnika 0,17 mm i 0,5 mm. U poluindustrijskom postrojenju određene su krive fluidizacije i krive sušenja analiziranih uzoraka. Nakon laboratorijskih i poluindustrijskih eksperimenata izvršeno je materijalno i energetsko bilansiranje realnog postrojenja sa kapacitetom proizvodnje od 300 t mesečno. Proračuni neophodne količine agensa za sušenje i dimenzija postrojenja izvršeni su korišćenjem programskih paketa SuperPro Designer 5.1 i Simprosys 3.0. Takođe, analizirana je i mogućnost recirkulacije agensa za sušenje.

Sistemi koji obuhvataju interakcije fluid-čestice nalaze široku primenu u različitim procesima. Jedan od najšire primenjenih sistema fluid-čestice u industriji je fluidizovani sloj čestica. Sistemi sa fluidizovanim slojem čestica primenjuju se u različitim granama industrije kao što su: separacija i klasifikacija čestica, adsorpcija, jonska izmena, katalitički kreking, operacijama u hidrometalurgiji, operacijama prerade otpadnih voda. Periodično ispiranje peščanih filtera je jedna od najšire primenjenih sistema fluidizovanog sloja čestica sa tečnošću. U okviru ovog rada primenjene su različite metode za karakterizaciju veličine i oblika nesferičnih polidisperznih smeša kvarcnog peska. Na osnovu dobijenih rezultata predložene su korelacione jednačine za određivanje ekvivalentnih prečnika čestica i faktora oblika. Za određivanje sferičnosti korišćenje su različite metode: metoda na osnovu eksperimentalnog određivanja pada pritiska u pakovanom sloju čestica i metoda bazirana na određivanju brzine taloženja nesferičnih čestica. Karakterizacija čestica peska razdvojenih po visini fluidizovanog sloja tokom fluidizacije polidisperzne smeše vršena je korišćenjem softverskog paketa SigmaScan (Image Analysis). U programu Sigma Scan obrađene su skenirane slike čestica upoređivanjem sa prethodno zadatim referentnim sistemom. Na osnovu obrade skenirane slike, kao izlazni podaci dobijaju se karakteristične veličine snimljenih čestica: veličina čestica (projektovani prečnik), faktor oblika čestica (odnosno dvodimenzione projekcije čestica), projektovana površina čestica, zapremina čestica, kao i broj snimljenih čestica. Na osnovu ovih podataka formirane su krive frakcione i kumulativne raspodele veličina čestica peska pre i tokom fluidizacije na osnovu čega se dobija jasna slika o segregacije čestica do koje dolazi tokom fluidizacije. Razdvajanje slojeva tokom fluidizacije izvršeno je hidrauličkom klasifikacijom materijala. Pokazano je da se raspodela veličine čestica frakcija pes, The systems involving fluid-particle interaction are widely used in different industrial processes. One of the most widespread fluid-particle systems is fluidized bed of particles. Fluidized beds are used in many applications, such as solids separation and classification, adsorption, ion exchange, catalytic cracking, hydrometallurgical operations, wastewater treatment etc. The periodic back-washing of down flow granular filters, especially sand filters is one of the widest uses of liquid fluidized beds . Different methods for particle characterization were applied to the polydisperse sets of particles of quartz filtration sand in order to derive the correlations between equivalent diameters, sphericity and 2D shape factor. For the determination of sphericity using different methods, the following characteristics of the particles were experimentally determined: pressure drop of the packed bed of particles, terminal velocity of the particles and porosity at minimal fluidization. The sphericities obtained were critically evaluated and the best method for particle characterization was proposed. Size distribution of sand particles for fractions obtained by sieving follows approximately modified normal (Gaussian) distribution with respect to the projected particle diameter, while the shape factor decreases linearly with the increase in the projected particle diameter. Expansion of sand particles during fluidization follows Richardson-Zaki relationship. Intercept velocity, i.e. extrapolated value of fluid superficial velocity at =1 on the plot log(U) vs. log(), agree quite well with the experientaly determined mean free settling velocity of the cloud of about 30 randomly selected particles. During fluidization the beds were hydraulicaly separated into 10 subfractions which were subsequently analyzed. The results indicate that the bed is partially segregated for the fraction dp=0.75-1.25 mm during the fluidization at overall bed expansion of 35% since the smallest particl

Wall-to-bed momentum, heat and mass transfer in the vertical liquid-solids flow, as well as in the single phase flow, were studied. The aim of this investigation was to establish the analogy among those phenomena. Also, effect of particle concentration on momentum, heat and mass transfer was studied. The experiments in hydraulic transport were performed in 25.4 mm I.D. cooper tube equipped with a steam jacket, using spherical glass particles of 1.94 mm in diameter and water as a transport fluid. The segment of the transport tube used for mass transfer measurements had internal coating made of benzoic acid. In the hydraulic transport two characteristic flow regimes were observed: turbulent and parallel particle flow regime. The transition between two characteristic regimes (Y* = 0), occurs at a critical voidage ε ≈ 0.85. The vertical two-phase flow was con­sidered as the pseudofluid, and modified mixture-wall friction coefficient (fw) and modified mixture Reynolds number (Rem) were introduced for system characterization. Experimental data show that the wall-to-bed momentum, heat and mass transfer coefficients, in vertical flow of pseudofluid, for the turbulent regime are significantly higher than in parallel regime. Wall-to-bed, mass and heat transfer coefficients in hydraulic transport of particles were much higher than in single-phase flow for lower Reynolds numbers (Re LT 15000), while for high Reynolds numbers (Re > 15000), there was not significant difference. The experimental data for wall-to-bed momentum, heat and mass transfer in vertical flow of pseudofluid in parallel particle flow regime, verify analogy among these three phenomena., U ovom radu prikazani su rezultati eksperimentalnih ispitivanja prenosa količine kretanja, toplote i mase zid-fluid u vertikalnom hidrauličkom transportu inertnih čestica i pri strujanju fluida u cevi istog prečnika. Eksperimentalna ispitivanja vršena su istovremenim merenjem potrebnih parametara za definisanje navedenih prenosa. Cilj ovih ispitivanja bio je određivanje koeficijenata prenosa pri vertikalnom hidrauličkom transportu inertnih čestica i uspostavljanje analogije tri prenosa. Eksperimentalni sistem predstavljala je vertikalna transportna cev prečnika 25,4 mm snabdevena omotačem za zagrevanje parom, kao i segmentom transportne cevi prepariranim rastopom benzoeve kiseline. Kao fluid korišćena je voda, a vršen je transport staklenih sfernih čestica prečnika 1,94 mm. U rezultatima su prikazani dobijeni koeficijenti trenja, prelaza toplote i prelaza mase. Korišćenjem koncepta pseudofluida, pokazano je postojanje analogije prenosa količine kretanja, toplote i mase u transportnoj cevi za paralelni režim strujanja.

Izučavanje procesa sušenja, zbog svoje kompleksnosti, i danas privlači pažnju istraživača širom sveta. Objašnjenje procesa sušenja svodi se na: uspostavljanje niza teorijskih i empirijskih modela koji pokazuju slaganja, u većem ili manjem stepenu sa eksperimentalnim podacima. Složeni procesi istovremenog prenosa mase i energije, koji su često nestacionarnarnog karaktera, kao i različita priroda i osobine materijala (higroskopnost, kapilarnost, distribucija i veličina pora, pojava skupljanja uz mogućnost nastanaka prslina prilikom sušenja oblikovanih keramičkih materijala...) još više komplikuju opisivanje procesa sušenja. Upravo to su razlozi zbog kojih jedinstveno teorijsko objašnjenje sušenja, koje bi univerzalno opisalo ovaj proces, za različite tipove glinenih materijala još uvek nije razvijeno. Modelovanje procesa konvektivnog sušenja sastoji se u rešavanju skupa parcijalnih diferencijalnih jednačina sastavljenih od jednačina kinetike procesa i jednačina bilansa mase i energije. S obzirom da se proces sušenja karakteriše nizom jednačina, koje izražavaju osnovne zakone fizike sušenja, a koje su izrazito ne linearne, uz napomenu da se vlažnost materijala menja na unapred nepoznat način (stohastički) i da ga karakteriše znatno vremensko kašnjenje, jasno se uočava da je izučavanje procesa sušenja, a pogotovu matematičko modelovanje kinetike sušenja, izuzetno složeno. Otuda postoji i potreba da se multidisciplinarnim pristupom i primenom savremenih postavki: fizike, hemije, nauke o materijalima, fenomena prenosa mase, teorije sušenja i računarskih tehnologija, prošire postojeća saznanja o procesu sušenja opekarskih proizvoda. Mehanizmi prenosa vlage za vreme sušenja u periodu konstantne, a naročito periodu opadajuće brzine sušenja, su vrlo kompleksni i do sada u literaturi nije postojalo opšte prihvaćeno objašnjenje koje bi moglo da identifikuje tačan prelaz i redosled delovanja svih potencijalno mogućih mehanizama sušenja. Predmet istraživanja ove doktorske diserta, The drying process is characterized by a series of equations, such as the drying process kinetic, mass and energy balance equations, which express the basic laws of drying physics. Additionally, it must be noted that during drying the moisture within the drying material is changing stochastically. Complex processes of simultaneous mass and energy transfer, which are often non-stationary and the distinct nature of the properties of the material (hygroscope, capillarity, pores size distribution, shrinkage effect, etc.) complicate even more the description of the drying process. Despite all these facts, which are indicating that the study of drying process and especially the mathematical modeling of drying kinetics is extremely complex, the drying process explanation still attracts the attention of the researchers around the world. That is a reason why only a multidisciplinary approach and the use of contemporary theoretical and practical knowledge from: physics, chemistry, materials science, mass transfer phenomenon, drying theory and computer technology, can lead to proper explanation of the drying process of porous materials. The mechanisms of moisture movement during drying in the constant and especially in the falling drying period are rather complex, and hitherto, there have been no generally accepted explanations that could identify the exact transition between possible drying mechanisms. The explanation of the drying process was reduced to the establishment of a series of theoretical and empirical drying models which show agreement, to a greater or lesser extent, with the experimental data. The goal of this thesis was to find a way to better understand the different drying mechanisms, to identify the exact transition between them and to estimate the time dependent effective diffusivity. In this work it is presented a literature review of available studies related to the determination of the effective diffusion coefficient, as well as a presentation of newly dev

In this paper, different methods for particle characterization were applied to 16 polydisperse fractions of quartz filtration sand obtained by sieving, with sieve diameters in the interval d(m) = 0359 to 2.415. For each fraction, volume diameter was measured and projected diameter and 2D shape factor were obtained using the scanned image of the projection of a large number of particles. The correlations between the volume diameter, projected diameter and 2D shape factor with sieve diameter were proposed. All of the correlations show linear dependence between the variables. The sphericity was determined for the fractions from packed-bed pressure drop measurements; from terminal velocity measurements using different correlations for the calculation of C-D, direct correlations for U-t and from direct empirical correlations. The sphericities obtained using different methods and correlations gave very different results. Both the sphericity and the 2D shape factor were shown to decrease with increase in particle sieve diameter.

The one-dimensional model of accelerating turbulent downward counter-current gas-solids flow of coarse particles was formulated and experimentally verified by measuring the pressure distribution along the transport tube. The continuity and momentum equations were used in the model formulation and variational model was used for the prediction of the fluid-particle interphase drag coefficient. Experiments were performed by transporting spherical glass particles 1.94 mm in diameter in a 16 mm i.d. acrylic tube at constant solids mass flux of 392.8 kg/m(2)s. Tube Reynolds number ranged from 170 to 5300 and the slip Reynolds number from 650 to 1060. Under these conditions loading ratio (G(p)/G(f)) varied between 66 and 2089. Visual observations show that particles flow downward in apparently homogenous dispersion. Experimental data for the static fluid pressure distribution along the transport tube agree quite well with the model predictions. The mean voidage and the particle velocity decrease, while the slip velocity increases with the increase in gas superficial velocity. The values of the pressure gradient, porosity, particle velocity and slip velocity along the tube were calculated according to the formulated model. In these calculations, particle-wall friction coefficient was determined indirectly by adjusting the f(p) value to agree with the experimental data. The effect of the value of fp on the model calculations was significant. Calculations show that the acceleration length for the same particles (1.94 mm) in downward counter-current gas-solids flow is about two times higher than the acceleration length in upward co-current gas-solids flow. In the system investigated, "choking" occurs at slip velocity which is about 73% of the single particle terminal velocity.

The equations for the overall particle circulation rate and the frequency of particle-particle collisions in the quasi two-dimenional water-fluidized bed were proposed. The equations were based on the experimental results obtained from the water fluidized beds of mono-sized spherical glass particles d(p) = 1.94, 2.98,4.00 and 6.00 mm in diameter and the correlation for the mean particle speed from our previous paper [14] The optimal porosity of the fluidized bed was defined as the porosity at which the overall circulation rate or the frequency of collisions shows their maximum. The optimal porosities were calcylated from the derivatives of the proposed correlations. The calculated optimal porosity was in the range 0.68-0.72 for the overall circulation rate, and 0.59-0.64 for the frequency of particle-particle collisions. The results obtained are in accordance with the experimental findings from the literature [2,3,23] which show that the maximum values of the heat and mass transport coefficients in the liquid fluidized beds are reached in the range of porosities between 0.6 and 0.8. By visual observations of the monolayer fluidized bed of spherical particles 10 mm in diameter we concluded that the collisions in the liquid fluidized beds in most cases are not pairwise, but that the particles which collide often move together for some time in agglomerates before separating. The majority of collisions in fluidized beds differ very much from the instantaneous collisions as regarded in the kinetic theory of gases. Therefore, the overall circulation rate is a better measure of the dynamics of the fluidized bed and the intensity of transport properties than the frequency of collisions.