Vladan Mićić

Modeling the Extraction Process of Sage ( Salvia Officinalis L.) with Supercritical CO 2 at Different Temperatures

In this paper an investigation was done on the extraction of sage (Salvia officinalis L.) using supercritical carbon dioxide at different extraction temperatures (313, 323 and 333 K). For the qualitative and quantitative analysis of the obtained extracts methods GC-FID and GC-MS were applied. Qualitative and quantitative composition of the obtained CO2 extracts and essential oils isolated from CO2 extracts at different temperatures of extraction was also determined. The extraction system was modeled by using the modified equation Reverchon - Sesti Osseo, whereby the effect of temperature on the yield of the extract was studied. Based on the results of modeling the overall yield of the extract depending on the temperature, it is concluded that a given model approximates the experimental results very well in all cases.

The influence of the vanadium content on the toughness and hardness of high-alloyed Cr-Mo steel

Characteristics of air hardening steel are high hardness and low impact toughness. In order to increase the impact toughness while retaining replace with the high hardness value numerous research added. One of the ways to achieve this is the alloying of specified steel with vanadium and application of appropriate heat treatment. Vanadium affects the solidification process of these alloys by narrowing of the temperature interval of crystallization. In addition, vanadium formed V6C5 carbides that block the growth of austenitic dendrites and structure makes fine-grained. Vanadium which forms V6C5 carbides is partly distributed between present phases in the steel, carbide (CrFe)7C3 and austenite. Also, the presence of vanadium can enable the formation of (CrFe)23C6 carbide and its precipitation in austenite during the cooling process. In local areas around fine carbide particles, austenite is transformed into martensite. In other words vanadium reduces the amount of remained austenite and so improves hardenbility of the steel. In this way, better technical characteristics of these steels are obtained.

On some other preferred method for optimizing the welded joint

The paper shows an example of performed optimization of sizes in terms of welding costs in a characteristic loaded welded joint. Hence, in the first stage, the variables and constant parameters are defined, and mathematical shape of the optimization function is determined. The following stage of the procedure defines and places the most important constraint functions that limit the design of structures, that the technologist and the designer should take into account. Subsequently, a mathematical optimization model of the problem is derived, that is efficiently solved by a proposed method of geometric programming. Further, a mathematically based thorough optimization algorithm is developed of the proposed method, with a main set of equations defining the problem that are valid under certain conditions. Thus, the primary task of optimization is reduced to the dual task through a corresponding function, which is easier to solve than the primary task of the optimized objective function. The main reason for this is a derived set of linear equations. Apparently, a correlation is used between the optimal primary vector that minimizes the objective function and the dual vector that maximizes the dual function. The method is illustrated on a computational practical example with a different number of constraint functions. It is shown that for the case of a lower level of complexity, a solution is reached through an appropriate maximization of the dual function by mathematical analysis and differential calculus.

Catalytic activity of sulfuric acid on the morphology of electrochemically obtained copper powder

The subject of the research is influence of sulfuric acid on morphology of electrochemically obtained copper powder during galvanostatic electrolysis mode. In this paper electrodeposition of copper powder from sulphate electrolyte was carried out. Electrolytes with three different concentrations of Cu2+ (7 g/dm3, 18 g/dm3 and 33 g/dm3) were used, each in combination with three different concentrations of H2SO4 (100 g/dm3, 150 g/dm3 and 200 g/dm3), so that copper powder was deposited from nine electrolytes. Copper wire with surface area of 1cm2 was used as cathode, and copper anode in the form of metal sheet was pressed against the wall of electrochemical cell that had volume of 500 cm³. Experiments were carried out at room temperature, and electrolyte stirring was performed using a magnetic stirrer. Copper powder was deposited galvanostatically at current densities of j = 1500 A/m2, j=2000 A/m2 and j=2500 A/m2. Produced copper powder was removed from cathode every three minutes. All the characteristics of powder depend on the size and shape (morphology) of particles and they are mutually correlated. Research of morphology and size of particle was carried out using scanning electron microscopy (SEM). Increase of H2SO4 concentration causes increase in branching of copper powder. It was concluded that the size of copper powder grains reduces with the increase in concentration of H2SO4.

Investigation of corrosion effect on the dynamic strength of some structural steels

By corrosion effect, the surface of the metal becomes so rough that there are reduction in metal strength as a consequence of nicking. It should be added that the continuing effects of corrosion produces deep and sharp notches in the metal. It causes tiny cracks that allow further penetration of corrosion. As results of this activities, the dynamic hardness by simultaneously highly corrosion effect substantially decreases so that it may lead to fatigue failure, as shown in this paper. It is also not possible to establish any qualitative or quantitative legality between dynamic strength and corrosion effects. This hold for the effect of corrosion on unloaded metal element. This must be tested in all chance.

Possibility of Using Fourier’s Differential Equation in Cooling Process of Meat Steaks

Summary: In this paper, based on derived differential equation for the heat transfer and a suitable boundary condition (equation of heat exchange), the appropriate model of cooling of meat stakes in the form of a flat plate is set up. By using theory of similarity, Fourier and Biot criterion is defined, which allowed setting criterial equation of temperature field which included an unnamed temperature and geometric criterion. For solving the obtained criterial equation, existing diagrams of temperature functions were utilized, which were specified by analytical method for the core, the surface and the interior of the observed model. The proposed model is used for analytical solving of a number of practical problems in the cooling process of meat steaks in a gaseous environment. Particular focus has been on the temperature of the surface and core of steak as well as ambient temperature and cooling time. Also, it was shown that proposed model can be used to define temperatures or temperature field along the thickness of steak, depending on the distance from the central plane. Special possibility of applying the model is for the case of preventing freezing steaks, when their temperature is maintained within specified limits.

O JEDNOJ ANALOGIJI IZMEĐU SLOŽENIH HEMIJSKIH REAKCIJA I ELEKTRIČNIH KOLA

U ovom radu, kao za karakteristican primer iz hemijske kinetike, za reakciju prvog reda, sistem složenih hemijskih jednacina matematickim transformacijama sveden je na sistem homogenih i nehomogenih diferencijalnih jednacina prvog i drugog reda, koje su pogodne za predstavljanje preko elektricne analogije. Nakon toga, posmatrana su elektricna kola sa rednom i paralelnom vezom u kojima su prisutne sve tri karakteristicne otpornosti i to sa i bez strujnog izvora. Primenom Kirhofovih zakona, izvedene su pogodne diferencijalne jednacine u zavisnosti od naelektrisanja, napona i jacine struje, a koje predstavljaju opsti model za uspostavljanje analogije sa ranije izvedenim slicnim hemijskim jednacinama za određivanje koncentracije jedinjenja, u kojima su ukljucene i konstante brzine reakcije. Na kraju, na primeru jedne od karakteristicnih izvedenih elektricnih relacija u diferencijalnom obliku, dat je tabelarni prikaz analogije između elektricnih i velicina koje figurisu u hemijskim jednacinama, kao i neke mogucnosti primene prikazanih analogija.

REŠAVANJE JEDNOG PROBLEMA U ORGANSKOJ HEMIJI METODOM EKVIVALENTNIH JEDNAČINA

U teorijskoj organskoj hemiji, zakoni molekulsko – orbitalne teorije omogucuju da se odrede energetska stanja π – elektrona u konjugovanim (nezasicenim) jedinjenjima. Pri ovome energetski nivoi elektrona, dobijaju se razvijanjem odgovarajuce determinante koja se formira koristeci tri karakteristicne matrice po definiciji. Svakom jedinjenju, pri ovome pridružuje se odgovarajuci polinom, dok ce energetski nivoi zavisiti od njegovih nula. Kod velike vecine jedinjenja koja se susrecu u teoriji i praksi, dobijeni polinom uvek ima bar jednu racionalnu nulu pa se problem resava jednostavno metodom faktorizacije. Problem nastaje kada karakteristicni polinom nema ni jednu racionalnu nulu. U ovom slucaju moguca je primena gotovih matematickih formula koje su međutim nepodesne za prakticnu upotrebu. Isto tako moguca je i primena numerickih metoda, koje daju samo približno realno resenje koje nije pogodno za primenu s obzirom da se gotovo uvek traži tacno realno resenje. Uzimajuci u obzir predhodno, u radu bice dat predlog za resavanje gore navedenog problema na bazi formiranja ekvivalentnih jednacina preko odgovarajucih matematickih transformacija, koje omogucuju da se na relativno jednostavan nacin dobiju tacno sve realne nule polinoma. U prikazanom radu, resavanje matematickog problema bice ilustrovano na nekoliko karakteristicnih primera. Tacnost dobijenih rezultata prema predloženoj metodi bice izvedena poređenjem sa približnim numerickim resenjem koristeci odgovarajuci racunarski program, odnosno numericke metode.

JEDAN PREDLOG ZA EFIKASNIJI PRORAČUN PARAMETARA KOD SLOŽENIH HEMIJSKIH REAKCIJA

U prvom delu rada, za karakteristican primer složene hemijske reakcije, proanalizirano je nekoliko matematickih metoda za resavanje sistema diferencijalnih jednacina kojima se opisuje proces, a koje se najcesce koriste u inženjerskoj teoriji i praksi. Analiza je izvrsena sa aspekta neophodnog matematickog znanja, potrebnog vremena za resavanje problema, mogucnosti greske kao i kontrole dobijenih rezultata. U drugom delu rada, dat je predlog za efikasnije resavanje posmatranog problema pri cemu je primenjen princip redukcije sistema na jednu diferencijalnu jednacinu, koju je najpogodnije posmatrati kao nehomogenu jednacinu I reda sa konstantnim koeficijentima, zavisnu od vremena. Pokazano je da je predložena metoda pogodnija u odnosu na postojece, s obzirom da ima određene prednosti i može se koristiti kao alternativa istim.