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In the group of multiphase contactors and reactors, an important place belongs to reciprocat-ing plate columns (RPCs), which consist of a set of perforated plates fixed on a carrier (the so-called reciprocating or vibrating agitator) moving periodically up and down through a column. This construction maximizes the positive effects of mechanical agitation and minimizes or eliminates the adverse effects characteristic of column-type contactors and reactors. In RPCs, the highest dispersed-phase holdup is achieved at a lower dispersed-phase velocity due to the influence of mechanical agitation on the bubble or drop comminution. Therefore, this device can be the most acceptable contactor or reactor for performing complex actions in multiphase systems. The paper reviews the fundamental research and application of RPCs in Serbia in the last fifty years, from 1970 to 2020. Hydrodynamic and mass-transfer characteristics are analyzed, such as the pressure variation at the column bottom, power consumption, dispersed-phase holdup, axial dispersion, liquid mass transfer coefficient, specific interfacial area, and volumetric mass transfer coefficient. The use of RPCs as reactors in bioprocesses and biodiesel production processes is also discussed.

Synthesis of biodiesel from various plant oils is realized by the transesterification of triglycerides with methanol or by a reaction usually defined as methanolysis. The usage of low-quality oils, such as waste cooking oil (WCO), is followed by undesirable side reactions as a result of the increased content of free fatty acids (FFA), and water. The presence of FFA in WCO usually requires a pretreatment stage before subjecting it to methanolysis. In the present work, heterogeneously catalyzed methanolysis of WCO with and without pretreatment was investigated. Removal of FFA from WCO was conducted by using only quicklime or with the addition of a small amount of methanol (FFA to methanol = 1:3 molar ratio). The obtained results showed that pretreatment of WCO with quicklime at 30 °C after 1 h reduces the FFA content by 72 %, while the adsorption capacity was determined to be 910 mg g-1. The adsorptive pretreatment, as a simple operation, using low-cost quicklime under mild conditions, had a positive effect on the transesterification rate with CaO∙ZnO as a catalyst, enabling the achievement of over 96 % of biodiesel yield in only 15 min, compared to 1 h without the pretreatment. Furthermore, pretreated WCO allows an increase in repeated catalyst use and overall savings in the necessary amount of catalyst. The present study showed that quicklime is an economic, environmental-friendly, and sustainable material for FFA removal from WCO.

The treatment of spent oil-in-water emulsion (SOWE) from a non-ferrous metal-processing plant by using aluminum sulfate and hydrated lime was studied to determine the purification efficiency, to optimize the operating conditions and to scale up the treatment process. The purification efficiency was estimated by comparing the compositions of the SOWE and the processed wastewater. The treatment efficiency does not depend on the type of mineral oil and filter aid. The optimum doses of aluminum sulfate and hydrated lime must be experimentally determined for each batch of SOWEs, but the results obtained at laboratory level are applicable at pilot level. The processed wastewater and the filter cake from the process can be safely disposed into public sewage systems and at municipal waste landfills, respectively. The purification efficiency was higher than 98% with respect to total suspended solids, chemical oxygen demand and oil and grease, and was comparable to the known treatment processes based on coagulation/flocculation followed by sedimentation.

Today, homogeneous base-catalyzed methanolysis is most frequently used method for industrial biodiesel production. High requirements for the quality of feedstocks and the problems related to a huge amount of wastewaters have led to the development of novel biodiesel production technologies. Among them, the most important is heterogeneous base-catalyzed methanolysis, which has been intensively investigated in the last decade in order to develop new catalytic systems, to optimize the reaction conditions and to recycle catalysts. These studies are a base for developing continuous biodiesel production on industrial scale in near future. The present work summarizes up-to-date studies on biodiesel production by heterogeneous base-catalyzed methanolysis. The main goals were to point out the application of different base compounds as catalysts, the methods of catalyst preparation, impregnation on carriers and recycling as well as the possibilities to improve existing base-catalyzed biodiesel production processes and to develop novel ones.

Continuous biodiesel production on laboratory and industrial scale was analyzed, with focus on their advantages and disadvantages. Attention was paid to specific characteristics of industrial processes in order to point out the advanced technologies. The well-known base-catalyzed continuous biodiesel production processes are related to problems caused by the immiscibility of the reactants (alcohol and oil), application of relatively high operating temperature (usually the boiling temperature of alcohol or one near it) and obtained yield of methyl ester yields lower than desired. One way to overcome these problems is to employ special reactor design favoring the emulsion process and increasing the overall rate of biodiesel production process, even at room temperature and atmospheric pressure. The second way is to apply heterogeneous catalysts in continuous processes, which will probably be the optimal approach to economically justified and environmentally friendly biodiesel production.

For distillation column design it is necessary to define all the variable parameters such as component concentrations in different streams temperatures, pressures, mass and energy flow, which are used to represent the separation process of some specific system. They are related to each other according to specific laws, and if the number of such parameters exceeds the number of their relationships, in order to solve a problem some of them must be specified in advance or some constraints assumed for the mass balance, the balance of energy, phase equilibria or chemical equilibria. Knowledge of specific elements which are the constituents of a distillation unit must be known to define the number of design parameters as well as some additional apparati also necessary to realize the distilation. Each separate apparatus might be designed and constructed only if all the necessary and variable parameters for such a unit are defined. This is the right route to solve a distilation unit in many different cases. The construction of some distillation unit requires very good knowledge of mass, heat and momentum transfer phenomena. Moreover, the designer needs to know which kind of apparatus will be used in the distillation unit to realize a specific production process. The most complicated apparatus in a rectification unit is the distillation column. Depending on the complexity of the separation process one, two or more columns are often used. Additional equipment are heat exchangers (reboilers, condensers, cooling systems, heaters), separators, tanks for reflux distribution, tanks and pumps for feed transportation, etc. Such equipment is connected by pipes and valves, and for the normal operation of a distillation unit other instruments for measuring the flow rate, temperature and pressure are also required. Problems which might arise during the determination and selection of such apparati and their number requires knowledge of the specific systems which must be separated. The experience of the designer and his creativity for proposing a solution for an industrial unit which enables optimal process operation accompanied with minimal energy consumption, as well as a small number of apparati, and minimal total length of pipes and number of instruments is always a valuable additional effect for minimal investment costs and an optimal production process.

St. John's Wort, the Hypericum perforatum L. is one of the most analyzed plant species today. Plant was characterized with a wide ecological spectrum and is a plant with beautiful yellow flowers. St. John's Wort was used and still is in used in traditional medicine all over the World. Many bioactive components from St. John's Wort like hypericine, hyperforine, qercetrine and essential oil, were isolated and have been used in medicine. The most popular use of Hipericum extract is as an antidepressant for the medicinal treatment of mild and high depression. The medical use of hyperforine in photodynamic therapy for cancer treatment has now been intensively analyzed. The extract of St. John's Wort showed high antimicrobial, even on pathogenic microorganisms as well as antiviral activity. The use of bioactive components from St. John's Wort depends on the possibility to isolate them in the pure state. It seems that supercritical extraction with carbon dioxide might to be the best solution for obtaining pure extract as well as some of the components present in the essential oil and extract of St. John's Wort. Such a conclusion is supported by the many results of recently performed and published in scientific journals.

The principles and kinetics of biodiesel production from vegetable oils using lipase-catalyzed transesterification are reviewed. The most important operating factors affecting the reaction and the yield of alkyl esters, such as: the type and form of lipase, the type of alcohol, the presence of organic solvents, the content of water in the oil, temperature and the presence of glycerol are discussed. In order to estimate the prospects of lipase-catalyzed transesterification for industrial application, the factors which influence the kinetics of chemically-catalysed transesterification are also considered. The advantages of lipase-catalyzed transesterification compared to the chemically-catalysed reaction, are pointed out. The cost of down-processing and ecological problems are significantly reduced by applying lipases. It was also emphasized that lipase-catalysed transesterification should be greatly improved in order to make it commercially applicable. The further optimization of lipase-catalyzed transesterification should include studies on the development of new reactor systems with immobilized biocatalysts and the addition of alcohol in several portions, and the use of extra cellular lipases tolerant to organic solvents, intracellular lipases (i.e. whole microbial cells) and genetically-modified microorganisms ("intelligent" yeasts).

The effects of operation conditions (the vibration intensity and gas flow rate) on the gas holdup in a 16.6 cm i.d. two-phase reciprocating plate column (RPC) were studied. Distilled water and aqueous solutions of carboxymethyl-cellulose of different concentration were used as the liquid phase and air as the gas phase in this investigation. The gas holdup was measured after the gas flow and the reciprocating action had been stopped. An empirical correlation which correlates the gas holdup with the specific power consumption and the superficial gas velocity showed that the aeration intensity had a greater influence on the gas holdup than the intensity of agitation. Because the gas holdup was approximately the same in RPC's of different diameters, an equation relating the gas holdup with the specific power consumption and the superficial gas velocity was derived. The correlation could be used in the scaling up of reciprocating plate columns.

One of the most interesting areas of supercritical fluids applications is the processing of novel materials. These new materials are designed to meet specific requirements and to make possible new applications in Pharmaceuticals design, heterogeneous catalysis, micro- and nano-particles with unique structures, special insulating materials, super capacitors and other special technical materials. Two distinct possibilities to apply supercritical fluids in processing of materials: synthesis of materials in supercritical fluid environment and/or further processing of already obtained materials with the help of supercritical fluids. By adjusting synthesis parameters the properties of supercritical fluids can be significantly altered which further results in the materials with different structures. Unique materials can be also obtained by conducting synthesis in quite specific environments like reversed micelles. This paper is mainly devoted to processing of previously synthesized materials which are further processed using supercritical fluids. Several new methods have been developed to produce micro- and nano-particles with the use of supercritical fluids. The following methods: rapid expansion of supercritical solutions (RESS) supercritical anti-solvent (SAS), materials synthesis under supercritical conditions and encapsulation and coating using supercritical fluids were recently developed.

Antifreeze based on ethylene glycol is a commonly used commercial product The classification of ethylene glycol as a toxic material increased the disposal costs for used antifreeze and life cycle assessment became a necessity. Life Cycle Assessment (LCA) considers the identification and quantification of raw materials and energy inputs and waste outputs during the whole life cycle of the analyzed product. The objectives of LCA are the evaluation of impacts on the environment and improvements of processes in order to reduce and/or eliminate waste. LCA is conducted through a mathematical model derived from mass and energy balances of all the processes included in the life cycle. In all energy processes the part of energy that can be transformed into some other kind of energy is called exergy. The concept of exergy considers the quality of different types of energy and the quality of different materials. It is also a connection between energy and mass transformations. The whole life cycle can be described by the value of the total loss of exergy. The physical meaning of this value is the loss of material and energy that can be used. The results of LCA are very useful for the analyzed products and processes and for the determined conditions under which the analysis was conducted. The results of this study indicate that recycling is the most satisfactory solution for the treatment of used antifreeze regarding material and energy consumption but the re-use of antifreeze should not be neglected as a solution.

Biodiesel is defined as a fuel which may be used as pure biofuel or at high concentration in mineral oil derivatives, in accordance with specific quality standards for transport applications. The main raw material used for biodiesel production is rapeseed, which contains mono-unsaturated acids (about 60%) and also poly-unsaturated fatty acids (C 18:1 and C 18:3) in a lower quantity, as well as some undesired saturated fatty acids (palmitic and stearic acids). Other raw materials have also been used in research and the industrial production of biodiesel (palm oil, sunflower oil, soybean oil, waste plant oil, animal fats, etc). The historical background of biodiesel production, installed industrial capacities, as well as the Directive of the European Parliament and of the Council (May 2003) regarding the promotion of the use of biofuels or other renewable fuels for transport are discussed in the first part of this article. The second part focuses on some new concepts for the future development of technology for biodiesel production, based on the application of non-catalytic transesterification under supercritical conditions or the use of lipases as an alternative catalyst for this reaction.

Garlic powder, essential oil and aqueous extracts from raw garlic and garlic powder prepared in accordance with the corresponding procedures were tested for antimicrobial activity on Staphylococcus aureus ATCC 6538, Escherichia coli ATCC 25922, Bacillus subtilis 6633, Sarcina lutea ATCC 4391, Pseudomonas aeruginosa ATCC 9027, Salmonella enteritidis, Klebsiel la pneumoniae, Proteus vulgaris, Candida albicans ATCC 10231 and Aspergillus niger ATCC 16404; and, three pathogen microorganisms isolated from human material were also used: Staphylococcus aureus, Escherichia coli and Candida albicans. The antimicrobial activities of the preparations were compared with that of commercial antibiotics and four commercial preparation based on garlic powder and essential oil. Aqueous extracts from raw garlic, from freshly prepared powder and powder stored for 18 months at +4°had the greatest antibacterial and antimycotic activities on all the microorganisms tested. The essential oil obtained by hydro distillation and commercial preparations based on garlic essential oil showed no antimicrobial activity on the microorganisms tested. Thermal oils showed a bacteriostatic or very weak bacteriostatic effect on all the microorganisms except Pseudomonas aeruginosa. The garlic powder and commercial preparations based on the powder have a strong antimicrobial effect on all the microbes except on Pseudomonas aeruginosa.

Biodiesel is defined as a fuel that might be used as a pure biofuel or at high concentration in mineral oil derivatives, in accordance with specific quality standards for transport applications. The main raw material used for biodiesel production is rapeseed, which contains mono-unsaturated (about 60%) and also, in a lower quantity, poly-unsaturated fatty acids (C 18:1 and C 18:3), as well as some amounts of undesired saturated fatty acids (palmitic and stearic acids). Other raw materials have also been used in the research and industrial production of biodiesel (palm-oil, sunflower-oil, soybean-oil, waste plant oil, animal fats, etc). The historical background of the biodiesel production, installed industrial capacities, as well as Directives of the European Parliament and of the Council (May 2003) regarding the promotion of the use of biofuels or other renewable fuels for transport are discussed in the first part of this article (Chem. Ind. 58 (2004)). The second part focused on some new concepts and the future development of technology for biodiesel production based on the use of non-catalytic transesterification under supercritical conditions. A literature review, as well as original results based on the transesterification of animal fats, plant oil and used plant oil were discussed. Obtained results were compared with the traditional concept of transesterification based on base or acid catalysis. Experimental investigations of transesterification with supercritical methanol were performed in a 2 dm3 autoclave at 140 bar pressure and at 300°C with molar ratio of methanol to triglycerides of about 41. The degree of esterification strongly depends on the density of supercritical methanol and on the possibility of reaction occurring in one phase.

An important step in waste water treatment is the removal of oil from an O/W emulsion and the separation of oil and water into two phases, a process usually called demulsification. Modern methods of oil removal from waste water were described in the present study. Typical demulsification techniques include thermal, electrical, chemical, acoustic, or mechanical methods of emulsion treatment. The freeze/thaw process was found to be very effective for demulsification. In the freeze/thaw process, water removal from the W/O emulsion depends on the initial oil content, freezing temperature, freezing time, thawing rate and temperature. The described method is a non-destructive and physical method of emulsion treatment and because there is no addition of chemical agents there is no problem of further waste water treatment, which usually exists when chemicals are applied for demulsification. Different methods for oil content determination in the O/W emulsion were also applied and compared in this study (gravimetric, volumetric, measurement of oxygen consumption and emulsion pH, IR analysis of the emulsion, electro analytical measurement).

The influence of different working parameters (vibration intensity superficial gas and liquid rate and content of the solid phase) on liquid flow in a multiphase (gas-liquid: RPC-II and gas-liquid-solid: RPC-III) reciprocating plate column was analyzed using step-response methods and sorbic acid as a tracer. The liquid flow was determined using a model of N-tanks in series followed by back mixing of the liquid phase between the tanks. The parameters of this model N, a and x were calculated by applying several methods: calculation of the moments of the residence time distribution function for a constant number of tanks in series (N=const) analysis of a set of linear equations for N *const and determination of the minimum of defined goal function using the optimization technique leastsq.m of MATLAB software.

The determination of specific characteristics of materials commonly used in the preparation of cloths for the protection of the human body, lungs and other human organs, based on the principles of the effective adsorption of highly toxic organic vapors initially requires the development of specific and reliable techniques for the preparation of a gas stream with the relevant highly toxic organic substances. The dynamic method, one of the techniques commonly used for material characterization, is based on the procedure of determining the concentration of different substances in the gas stream at the inlet and after passing through a specific thin layer of the adsorption material (outlet). Therefore, the generation of a gas phase containing highly toxic organic substances in a defined quantity during a specific time interval, is the requirement that must previously be fulfilled before using the dynamic method for testing specific materials for protection of the human body. Usually, a dynamic method of determining the quality of cloths prepared with specific adsorbents requires a relatively low volumetric flow rate of the gas phase containing the test substance which may be achieved by using a diffusion tube (DT) as the standard equipment for the generation of test substances. A very wide range of concentrations of the test substances in a specific gas phase (inert, air) can be obtained by varying the working conditions (temperature, pressure, gas flow rate) and the geometrical size of the diffusion tube, taking into account that the concentration of toxic substances, is determined as the ratio of their diffusion rate and from the DT and the carrier gas flow rate. The diffusion rate of the test substances from the DT is caused by the concentration gradient between the partial pressure of the saturated vapor at one end of the DT capillary and the partial pressure at the outlet of the diffusion tube according to Fick's law. The mass rate of the generated test substances from the DT is constant for constant conditions (temperature and pressure) in the reservoir of the DT On the basis of a detailed theoretical calculation such a rate (R) may be defined by the equation: R = 16.624 DM(P/T)(A/L)log [P/(P- pv)l, mg/min where: D - the diffusion coefficient, cm/s; M - the molar mass of the test substance, g/mol; P - the pressure in the DT kPa; pv - the vapor pressure of the test substance, kPa; A - the cross sectional area of the capillary of the DT cm2: L - the length of the capillary of the DT cm; T - the temperature, K. A procedure for the constant isothermal vapor generation of hydrogen cyanide (HCN) and 2-bromo-2-chloro-1,1,1-trifluoroethane (halotane) into air by a using diffusion tube is described. The DT was placed into a thermostat 0.5°C) and the diffusion rates of the substance were measured during some time interval and the above equation was used for the determination of the diffusion coefficient. The obtained data were correlated with the calculated ones on the basis of an empirical correlation from the literature. The relative differences in the experimentally determined and theoretically calculated values of the diffusion coefficient for hydrogen cyanide at different temperature (19-23.2°C) were 26-31 % and for halotane (26-45°C) 8-14%.

The power consumption in a gassed reciprocating plate column with Rashig rings placed in interplate spaces increases with both increasing vibration intensity and content of solid particles but decreases with increasing the superficial gas velocity, regardless of the rheological properties of the liquid phase. Under the same operating conditions, the power consumption is higher when the column is filled with a non-newtonian liquid than with distilled water.

The problem of sulfur compounds conversion, particularly heterocyclic compounds, from the middle fraction of crude oil (light gas oil) was analyzed and will be published in several articles in the Chemical Industry Journal. Published data in the literature, covering the conversion of tiophene, benzothiophene and dibenzothiophene are presented in the first paper of this series with the goal of comparing and critically analyzing their usage for reactor calculation.

Subcritical and supercritical extractions are novel, non destructive techniques which can be applied for the removal of hazardous compounds from contaminated soil without any changes of the soil composition and structure. The aim of the presented review paper is to give information on up-to day results of this method commonly applied by several institutions worldwide. Interest in the application of SC CO2 has been more expressed in the last two decades, which may be related to its favorable characteristics (non-toxic, non-flammable, increase diffusion into small pores, low viscosity under SC conditions, low price and others). However, interest in wet oxidation (WO) and especially in SCWO (the application of water under supercritical conditions with air) has also increased in the last few years. Interest in H2O as a SC fluid, as well as in extraction with water under subcritical conditions may also be related to specific characteristics and the enhanced rate of extraction. Moreover, the solubility of some specific compounds present in soil can be easily changed by adjusting the pressure and temperature of extraction. The high price of the units designed to operate safely at a pressure and temperature much higher than the a critical one of the applied fluids is the main reason why, at present, there is no more broader application of such techniques for the removal hazardous materials from contaminated soil. In the present paper, among many literature citations and their overall review, some specific details related to the development of specific analytical methods under SC conditions are also considered.

The pressure drop at the column bottom, filled with a Newtonian (distilled water) or non-Newtonian (aqueous solution of carboxymethylcellulose, sodium salt, CMC, 1 %) liquid, in the presence of Rashig rings, made of aluminum (0.8 cm in diameter), regularly distributed in the interplate spaces (volume content of the solid phase: up to 3.2 %) was studied. The average and total pressure drop at the column bottom increased with increasing vibration speed and content of the solid particles, but decreased with increasing superficial gas velocity, indepentently of the rheological properties of the liquid phase. The pressure drop at the column bottom filled with the non-Newtonian liquid was found to be higher than that in the case of distilled water.

Supercritical fluid extraction is an extraction process realized with supercritical fluids, which are at a temperature and pressure above their critical temperature and critical pressure. This process has shown to be very efficient one for the isolation of different substances of medium molecular weights and molecules of relatively low polarity. The solubility of more polar substances in supercritical fluids can be improved by the addition of small amounts of other polar solvents (cosolvent) to the supercritical fluids, which is the main solvent in extraction process. The advantage of supercritical extraction compared to other extraction procedures (the application of classical organic solvents hydrodistillation, distillation with steam) is that SFE is usually performed at moderate temperature (e.g. with SF CO2 at 40-70°C) so it can be applied for the separation of different substances which are thermally unstable and have a larger vapour pressure. All of these facts indicate that SFE is of special interest for the food and pharmaceutical industry.

The performed investigations were directed toward the analysis of the performance and activity of the fresh and regenerated Cyanamid HDS 20C catalyst in a laboratory batch reactor (2 dm3) for the HDS of the diesel fraction (light gas oil, LGO). Testing of the regenerated catalyst was performed with light gas oil (LGO) of different characteristics. The determined values of the reaction rate constant were compared to some published data in the literature for the HDS of specific sulfur compounds as well as the values of the activation energy. The rates of deactivation of the fresh and regenerated catalyst actually existed compared to some other results recently published in the literature. However, such an observed differences were not sufficient to derive a relation which could be used for the determination of the rate of catalyst deactivation.

Sol-gel synthesis, and the resulting materials (xerogels and aerogels) are finding increasing application in the synthesis of catalysts, due to their unique characteristics. The most important features of the sol-gel process are: the ability to achieve homogeneity at the molecular level, the introduction of several species in only one step and the ability to stabilize metastable phases. The supercritical drying process produces aerogels with structural features quite different to conventional materials. Some of these characteristics of aerogels can make them very effective catalysts.

The gas holdup was studied in non-newtonian liquids in a gas-liquid and gas-liquid-solid reciprocating plate bioreactor. Aqueous solutions of carboxy methyl cellulose (CMC; Lucel, Lučane, Yugoslavia) of different degrees of polymerization (PP 200 and PP 1000) and concentration (0,5 and 1%), polypropylene spheres (diameter 8.3 mm; fraction of spheres: 3.8 and 6.6% by volume) and air were used as the liquid, solid and gas phase. The gas holdup was found to be dependent on the vibration rate, the superficial gas velocity, volume fraction of solid particles and Theological properties of the liquid ohase. Both in the gas-liquid and gas-liquid-solid systems studied, the gas holdup increased with increasing vibration rate and gas flow rate. The gas holdup was higher in three-phase systems than in two-phase ones under otter operating conditions being the same. Generally the gas holdup increased with increasing the volume fraction of solid particles, due to the dispersion action of the solid particles, and decreased with increasing non-Newtonian behaviour (decreasing flow index) i.e. with increasing degree of polymerization and solution concentration of CMC applied, as a result of gas bubble coalescence.