Preface
Pages 4-6
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New approach to modelling and simulation of chemical and mass transfer processes in column apparatuses
Original Research Article
Pages 7 – 23
Chr. Boyadjiev, B. Boyadjiev
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Abstract
The classical mass transfer theory is not applicable for the modeling the mass transfer of chemical, absorption, adsorption and catalytic processes in column apparatuses, where the velocity distributions and interphase boundaries are unknown. The modeling of these processes is related with the creation of new type of convection-diffusion models (for qualitative analysis) and average-concentration models (for quantitative analysis), where the surface reactions are replaced by equivalent volume reaction, while the velocity and concentration distributions are replaced by average velocity and concentrations. The effect of the radial non-uniformity of the velocity in the average-concentration models is introduced by model parameters, which must be obtained experimentally. The new convection-diffusion and averageconcentration models are obtained in the cases of different processes in column apparatuses: simple and complicated chemical reactions, physical and chemical absorption, physical and chemical adsorption, heterogeneous catalytic processes (physical and chemical adsorption mechanism). These models are presented in the monograph Chr. Boyadjiev, M. Doichinova, B. Boyadjiev, P. Popova-Krumova, “Modelling of Column Apparatus Processes” (Second edition), Springer-Verlag, Berlin Heidelberg, 2018. Two hydrodynamic situations are considered, when the radial velocity component is equal to zero, in the cases of an axial modification of the radial non-uniformity of the axial velocity component and when the radial velocity component is not equal to zero. The use of experimental data, for the average concentrations at the column end, for a concrete process and column, permits to be obtained the model parameters, related with the radial non-uniformity of the velocity. These parameter values permit to be used the average-concentration models for modeling of different processes.

Multi-period deterministic model of sustainable integrated of hybrid first and second generation bioethanol supply chains for synthesis and renovation
Original Research Article
Pages 24 – 35
B.Ivanov
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Abstract
This paper focuses on designing mathematical model ofan integrated bioethanol supply chain (IBSC) that will account for economic and environmental aspects of sustainability. A mixed integer linear programming model is proposed to design an optimal IBSC. Bioethanol production from renewable biomass has experienced increased interest in order to reduce Bulgarian dependence on imported oil and reduce carbon emissions. Concerns regarding cost efficiency and environmental problems result in significant challenges that hinder the increased bioethanol production from renewable biomass. The model considers key supply chain activities including biomass harvesting/processing and transportation. The model uses the delivered feedstock cost, energy consumption, and GHG emissions as system performance criteria. The utility of the supply chain simulation model is demonstrated by considering a biomass supply chain for a biofuel facility in Bulgarian scale. The results show that the model is a useful tool for supply chain management, including selection of the optimal bioethanol facility location, logistics design, inventory management, and information exchange.

Characteristics of boiling heat transfer on hydrophilic surface with SiO2 coating
Original Research Article
Pages 36 – 44
A. S. Surtaev, V. S. Serdyukov, A. N. Pavlenko, D. V. Kozlov, D. S. Selishchev
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Abstract
The paper presents the results of an experimental study of the effect of wetting properties on the local and integral characteristics of heat transfer at water boiling on the saturation line under atmospheric pressure. To control the wetting characteristics, the nanocoatings of SiO2 were synthesized on the sapphire substrate surface using various chemical methods including dip coating and spin coating. New experimental data on dynamics of vapor bubble growth and detachment, evolution of microlayer and dry spot regions, nucleation site density and bubble emission frequency, heat transfer, etc., were obtained using the high-speed imaging techniques, including infrared thermography and video recording from the bottom side of transparent heater. The analysis of experimental data on the local and integral characteristics of the boiling process made it possible to determine the mechanisms of the influence of deposited hydrophilic coatings on the heat transfer intensity.

Scaling of the mechano-chemical process of production of silicon chelates from plant raw materials
Original Research Article
Pages 45 – 48
E. G. Trofimova, E. M. Podgorbunskikh, T. S. Skripkina, A. L. Bychkov, O. I. Lomovsky
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Abstract
In order to scale up the mechanochemical process of production of biologically active silicon chelates from plant raw materials the transfer from laboratory mechanochemical reactors to industrial equipment with a productivity of 15-70 kg / h was carried out. The efficiency of raw material grinding (by size characteristics of the product) and activation effects (by solubility of silicon dioxide) were studied depending on the technological treatment regimes (processing intensity and feed rate). The characteristics of the product obtained at laboratory scale (equilibrium concentration of soluble silicon chelates of 28 mg / l) were improved with a rotor speed of 1500 rpm and a productivity of 15-20 kg / h. Taking into account the energy costs, the rotation speed of 1200 rpm and feed rate of 43 kg / h can be considered as the optimal mode, which provides the obtaining of product with an equilibrium concentration of soluble chelates of 24 mg / l.

Hydrodynamics and heat transfer in a centrifugal film evaporator
Original Research Article
Pages 49 – 57
V. G. Rifert, P. A. Barabash, A. S. Solomakha, V. Usenko, V.V. Sereda, V.G. Petrenko
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Abstract
Evaporators with a rotating surface (a disk or a cone) are used for the concentration of liquids in the food, pharmaceutical industries and bioindustry. They are also relevant for water recovering from liquid waste in life support systems for spacecraft and space stations. The paper reviews the works on the study of characteristics of a liquid film (thickness, wave parameters) flowing under the action of a centrifugal force and heat transfer during film condensation and film evaporation. In most theoretical and experimental studies, the flow of a film on a rotating surface was investigated when R/Ri (the ratio of the radius of the entire surface to the radius of the jet irrigation) is less than 5, which is typical for installations with a small radius of the rotating surface. The authors of the paper give new data on the film characteristics at R/Ri > 5, which is relevant for the food and pharmaceutical industries.

Restoration of correctness and improvement of a model for film condensation inside tubes
Original Research Article
Pages 58 – 69
V. G. Rifert, V. V. Sereda, V. V. Gorin, P. A. Barabash, A. S. Solomakha
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Abstract
This work is devoted to the experimental studies aimed at increasing the efficiency of horizontal tube condensers by strictly accurate evaluation of heat transfer and regime parameters in various condensing refrigerants in the horizontal tubes of such devices. The unique measurements of heat fluxes and heat transfer coefficients local by circumference were carried out during condensation of Freon R-22 and steam which varies over a wide range of the main regime parameters (G, х, q, Rel). The improved model of film condensation inside the horizontal tubes for prediction of heat transfer with application of the results of numerical solutions of Bae et al. is suggested. In this model more precise definition of the interphase friction coefficient as the main parameter crucial for condensation is given. This more precise definition contains experimental substantiation of βq – prediction for calculation of pressure losses by friction and correction βq that takes into account surface suction at the interphase. Heat exchange predicted by the improved method was compared with the experimental data of various authors for 13 fluids (steam, R-22, R-123, R-134a, R-245fa, carbon dioxide, propylene, propane, ether, isobutene, refrigerants FC-72, Novec®649, HFE-7000) in annular and intermediate modes. Good agreement of the experiments with calculations (divergence within 25%) proves the correctness of the proposed method for both laminar flow of condensate film and turbulent flow.

Mathematical modelling of heat and mass transfer processes in wastewater biological treatment systems
Original Research Article
Pages 70 – 75
A. P. Safonyk, O.O. Hrytsyna, V. A. Voloshchuk, V. V. Sereda
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Abstract
In this paper, a mathematical model is presented for wastewater biological treatment of multicomponent pollutants. The proposed model includes a simulation of the wastewater clarification in the clarifier; simulation of the wastewater aerobic treatment in the porous medium; numerical-asymptotic approximation for solutions of spatial model problems of the wastewater aerobic treatment; simulation of singularly perturbed processes of convective diffusion, taking into account mass transfer and temperature regime; computer modeling of the wastewater biological treatment process in the aerotank regenerator. The idea of perturbation theory is used for constructing a mathematical model of an appropriate nonlinear problem (this theory is based on the consideration of new factors and effects, by perturbing the output, well-known backgrounds, but not by solving new appropriate intricate modeling problems). The relevant processes, such as “filtration-convection-diffusion-heat and mass transfer”, are considered in the media (one connected areas, bounded by equipotential lines and flow lines), which can be deformed depending on the certain process characteristics. This, by turn, predetermine the process nature, i.e. there is a mutual influence of the medium characteristics and the process. Filtration flow is regarded as a certain background for the convective transport of soluble substances (pollutants), taking into account small diffusion effects. Based on the above, a mathematical model of the wastewater aerobic treatment, taking into account the interaction of bacteria, organic and biologically non-oxidizing substances in conditions of diffuse and mass-transfer disturbances and the influence of temperature regimes, has been constructed.

Mathematical model of a plate heat exchanger for condensation of steam in the presence of non-condensing gas
Original Research Article
Pages 76 – 82
L.L. Tovazhnyanskyy, P. O. Kapustenko, O. A. Vasilenko, S. K. Kusakov, O. P. Arsenyeva, P. Y. Arsenyev
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Abstract
The process of vapour condensation from its mixture with noncondensing gas is analysed and mathematical model for condensation in PHE channels is proposed. The model is developed with accounting for the variation of local parameters of heat and mass transfer processes along condensation surface and features of these processes intensification in PHEs channels. The model is accounting for the effects of plate corrugations geometry on process intensity. The system of ordinary differential equations with considerably nonlinear right parts is solved by the numerical method of finite differences. The solution is implemented with the software developed for personal computer. The model validation is performed by comparison with experimental data for condensation of steam from its mixture with air in a sample of PHE channel.

Recovery of antioxidant phenolic compounds from avocado peels by solvent extraction
Original Research Article
Pages 83 – 89
S. Boyadzhieva, S. Georgieva, G. Angelov
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Abstract
The extraction of a vegetal waste (avocado peels) is studied aimed at quantification of polyphenolic content and antioxidant activity of the extracts. Ethanol-water mixtures are used as solvents for the reason that ethanol is the most common “green” solvent having high solubility power, and being completely biodegradable. The main task is to determine the optimal operational conditions, at which the extraction of antioxidant substances from avocado peels is maximized and correspondingly, extracts with higher antioxidant capacity are obtained. A simplified four-step experimental procedure is applied for optimization of the main process parameters, at which the yield of the target bioactive components (polyphenols) is maximized. The process kinetics is studied with the use of Peleg’s equation, and model equilibrium concentration is calculated. The experimental concentration at equilibrium state fits well the model results (2.5 % difference).

ANN modeling of a two-stage industrial ATAD system for the needs of energy integration
Original Research Article
Pages 90 – 99
E. G. Kirilova, N. Gr. Vaklieva-Bancheva
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Abstract
This study proposes an approach for Artificial Neural Network (ANN) modeling of a two-stage industrial Autothermal Thermophilic Aerobic Digestion (ATAD) system for wastewater treatment which can be incorporated in an energy-saving framework operating under uncertainties. For this purpose, ANN models with different architectures are developed for bioreactors from the first and second stages of the ATAD system. Then, they are connected in a common model of the two-stage ATAD system. Models are trained and validated using one-year records of data for a real ATAD system. Three different measures are used to assess their efficiency. The best models are used to create the model of the two-stage ATAD system which is validated with selected weekly data. The designed ATAD system model is able to capture the fluctuations in the parameters of the fresh sludge incoming into the system and to predict the expected target temperatures of the treated sludge at the end of each batch. The latter makes it suitable for use in energy-saving framework under uncertainties.

An approach for reduction of computational complexity of a two-stage stochastic optimization problem for capturing parameters uncertainty in an ATAD system
Original Research Article
Pages 100 – 105
R.K. Vladova, E.G. Kirilova, N.Gr. Vaklieva-Bancheva
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Abstract
Аn approach for reduction of the computational complexity of a two-stage stochastic optimization problem for capturing parameters uncertainty in a conventional ATAD system is proposed in this study. The main aim is to find the boundary values of the variables of the first stage of the approach which will result in solutions into the boundaries of the stochastic space. The boundaries of variation of the first stage variables determine the variation of the parameters of the main equipment (heat exchangers surfaces and operating volumes of heat storage tank) which are affected by the change in stochastic parameters. The computational complexity is reduced as in any scenario vertex in the stochastic space a deterministic optimization problem is formulated and solved. As an optimization criterion, the minimum capital costs for purchase of heat exchangers and heat storage tank are used. For the purpose of the study, data from measurements in a real ATAD system were used. As a result of the deterministic optimization problems solution, the values of the parameters of main equipment corresponding to the minimum capital costs are determined. Based on these values the lower and the upper boundaries of the variables of the first stage of the approach are determined.

Adopting environmental transportation practice in biodise lproduction as key factor for sustainable development: A Bulgarian case study
Original Research Article
Pages 106 – 111
E. I. Ganev, B. B. Ivanov, D. G. Dobrudzhaliev, Y. R. Dzhelil, D. Nikolova
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Abstract
During the last decades the Bulgarian transport policy is directed mainly towards the construction of modern fast and safe road infrastructure. It corresponds to the geopolitical location of the state in Southeast Europe. Thus, reliable transport corridors will be provided to neighbour states and EU member states. The construction of transport corridors is directly related to the usage of internal combustion engine fuels, used for the vehicles in Bulgaria. The vehicles are based mainly on petrol and diesel engines as the entry of other engines goes at a slow pace. Diesel has become a fuel for trucks and agricultural machinery due to the low fuel consumption and the high efficiency of the diesel engines. On the other hand, the diesel exploitation inevitably reflects on the environment and its pollution because of the volume and the specificity of the waste gases emitted during combustion. Directive 2020 / EU provides the permanent increase of biofuels use. The developed toolbox consists of mathematical model and its optimization for production and use of biofuels, as follows: production and technological criteria, territorial distribution of logistic and production units and environmental pollution with waste gases, consisting mainly CO2.The results of the toolbox implementation may be used for future effective transport policy for permanent and balanced development of Bulgaria in the next years.

Reaction kinetics in chemical engineering
Original Research Article
Pages 112 – 119
Christo Boyadjiev
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Abstract
In the paper is presented a theoretical analysis of the role of the reaction kinetics in chemical engineering for the solution of the main problems in the chemical industry (biotechnology, heat energy), i.e. the optimal design of new devices and the optimal control of active processes. The thermodynamic and hydrodynamic approximations for the modeling of the industrial process rates are presented and analyzed. The relation between the Onsager’s linearity coefficient and mass transfer coefficient is presented.