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The main aim of this paper is to analysis of chemical performance of hydrogen peroxide based on numerical and parametric methods. The proper chemical function of the catalytic bed, as one of the components of monopropellant thruster, plays a significant role in achieving the two design main goals in (minimizing mass and maximizing the specific impulse). To this end, the effect of catalyst diameter (granules) on the bed chemical performance, optimal length and pressure drop, simulations for beds with different catalytic pellet diameters have been made to 0.4-0.9 cm diameters. Hydrogen peroxide with a concentration of 90% is defined as an inlet fluid at 0.014 m/s in simulations. The calculation of flow pressure drop across the catalyst bed is one of the activities undertaken in this study. The results of this study indicate that with increasing the pellet diameter, the reaction effective surface is reduced and the catalyst bed length is increased for complete decomposition of the propellant. In addition to the required length for complete decomposition of hydrogen peroxide, the pressure drop in various catalyst beds have also been calculated and evaluated. The results of the catalytic bed drop evaluation indicate that at a specific flow rate, a minimum pressure drop will be made in a specific diameter. The reason for this is the interaction of reaction surface and catalyst bed lengths on the pressure drop generated during the propellant decomposition process. Verification and validation of achieved results was conducted by comparing with experimental results.

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Due to increase in global demand of propylene, many extensive studies and research have been done to find alternative method for lower energy consumption and efficiency. In this research, gamma alumina is used for molybdenum catalyst base in oxidative dehydrogenation ‎process of propane, in order to produce the propylene. The catalysts are prepared based on wet impregnation method. The analysis of  FTIR, XRD, BET, SEM, and XRF are done to evaluate and determine the  characteristics of prepared catalysts. Central composition method is employed to study the influence of reaction temperature, molybdenum ‎loading percentage, oxygen to propane ratio, and the effect of interactions between them in propylene ‎production. The molybdenum in the range of 4-16%, propane to oxygen ratio in ‎the range of 1-3%, and temperature in the range of 380-540 ºC are the input parameters of ‎the central composite method. Finally, according to reactor test and analysis of the results of the Design expert software, it is shown that the predicted models for propane conversion, propylene selectivity, ‎and efficiency persentage of oxidation dehydrogenation are about 95%. Maximum of efficiency percentage with a value of 14.02% is obtained at 487 ºC, 11.22% molybdenum‎ percentage, and propane to oxygen ratio of 1.5, which in experimental results, achieving an accuracy of 94% is possible as compared to the optimal design of the test design model.

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Dehydrogenation of alkane to alkene is a key process in petrochemical industry. Propylene has intermediate role to production many industrial polymers. In this research applying oxidative dehydrogenation method for propylene production and CO2 used as oxidant. By use of XRD, Raman, TEM, BET and EDX techniques the results have been analyzed. In XRD and Raman tests anatase phase and Titania nanotubes have been distinguished. TEM confirmed TiNTs with pure structure. Vanadium catalyst with 5% of vanadia synthesized by impregnation method. Adding silicon in support increased thermal stability of catalyst. Raman and XRD method confirmed good distribution of active phase on supports. VSiTi catalyst have 28.31% conversion and 51% selectivity in 550 oC. Improvement in yield of propylene production would be in result of higher surface area and good distribution of vanadia over modified Titania nanotubes.  

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Abstract

Research Subject: Sulfide removal from sour water is essential, before reuse or release of sour water into the environment. Regarding the high costs of traditional methods, biological removal can be used as a reliable alternative.
Research Approach: Biological sulfide removal from sour water was investigated in a batch reactor using Thiobacillus sp. as a dominant species of a mixed culture. A conceptual model was developed to describe the process of H₂S removal from sour water in the batch reactor. The model considers H₂S and O₂ transfer between liquid and gas phases, biological oxidation of H₂S to sulfate and elemental sulfur, and chemical oxidation of H₂S to thiosulfate in the liquid phase. The governing equations were derived using the principles of mass conservation and biochemical reactions. Several batch runs were performed to obtain experimental data on the variation of sulfide, sulfate, thiosulfate, and oxygen concentrations in the system as a function of time, and an algorithm was devised to use the method of Particle Swarm Optimization together with the numerical solution of the model equations to estimate biokinetic parameters. Additional batch runs under different conditions were performed to verify the accuracy of the model. These results indicated reasonable accuracy of the model to predict the performance of a batch reactor for the removal of H₂S from sour water. The novelty of this model is considering different pathways for sulfide oxidation which includes product selectivity.
Main Results: The maxim specific oxygen uptake rate (SOUR=OUR/X) is one of the most important parameters in the evaluation of the biological activity of the microorganisms. The calculated value for this parameter was almost constant (16 mg DO g^-1 VSS min^-1) during all sulfide oxidation tests indicating that the maximum specific oxidation capacity of the biomass is independent of substrate and biomass concentration. Results exhibited bacteria prefer to partially oxidized sulfide to elemental sulfur, however this preference is a function of dissolved oxygen and substrate availability.

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Research subject: Mazut is widely used in petrochemical, power, and marine industries. The use of these fuels, in addition to causing widespread air and sea pollution in the country, has also led to severe international penalties, rising costs, and corrosion of equipment. Therefore, the use of mazut fuel with sulfur compounds of up to 0.5% in the world, as a refining mazut fuel at the origin (in refineries) and taking into account all aspects, is more important. There are limited industrial methods for the hydrotreating of mazut (Due to the heavy oil cut and the complexity of sulfur compounds in it), the most common of which is hydrogen desulfurization (HDS).
Research approach: The goal of this research, The simulation and economic evaluation of the hydrotreating plant from Mazut fuel with a capacity of 13.75 million barrels per year. The simulation of this process was performed in Aspen HYSYS petroleum refinery software. In this simulation, the effect of effective operating parameters such as pressure, hydrogen to mazut ratio, and finally catalyst consumption on the removal of sulfur compounds, production of by-products, net production costs, and total investment costs are investigated.
Main Results: The results showed that for the hydrotreating process of this mazut with sulfur compounds 3.5%, total capital investment is 308.9 million US$ and the net production cost of treated mazut fuel is estimated to be 114.5 million US$ per year. Also, economic sensitivity analysis showed that the operating parameter of the hydrogen to mazut ratio had the greatest effect on increasing the total capital investment and net production cost, which should be minimized as much as possible.

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Abstract
Research subject: Ethylene is a very important material in petrochemical industries, whose chief application is producing polymers such as polyethylene. The steam cracking of ethane or naphtha is commonly used to produce ethylene. A small amount of acetylene is produced in this process. The amount of acetylene in the product stream should not exceed 1 ppm, because it is harmful to polymerization catalysts in downstream units. The acetylene hydrogenation unit is designed for acetylene removal in industrial plants. In this unit, the removal of acetylene up to 1 ppm in the product stream and ethylene’s selectivity are of great importance.
Research approach: In this paper, the modeling and the dynamic simulation of acetylene hydrogenation reactors of Marun petrochemical complex with considering catalyst deactivation are presented. Then, here investigated is the effect of the operating conditions such as temperature, pressure and flow rate of the reactor feed on the amount of outlet acetylene as well as ethylene’s selectivity.
Main results: The simulation results show that in order to compensate for catalyst deactivation, it is necessary to gradually increase the reactor inlet temperature. With a linear increase in the inlet temperature of the reactors from 55 to 90 ˚C in a period of 720 operating days, the amount of outlet acetylene and ethylene’s selectivity are decreased. The reactions of acetylene to ethylene and ethylene to ethane are increased by increasing the inlet temperature of acetylene hydrogenation reactors. By increasing the feed flow rate from 50 to 100 kg/s, the amount of outlet acetylene and ethylene’s selectivity are increased. The residence time is decreased by increasing the feed flow rate and thus the conversion of acetylene to ethylene is decreased (increasing the outlet acetylene in the product). The amount of outlet acetylene and ethylene’s selectivity are decreased by decreasing the inlet pressure from 40 to 33 barg.

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Pea (Pisum sativum L.) is an important model plant for genetic as well as biochemical and physiological studies. A well-saturated map of pea consisting several morphological, biochemical and molecular markers has been constructed to date. Nevertheless, there are still several genes whose inheritance and map positions are not well understood. Orange cotyledon color in pea is an interesting characteristic whose precise nature of gene interactions is unknown. Genetic analysis using crosses between lines having orange cotyledon color and lines with yellow or green cotyledons revealed that the character is controlled by a single gene. It was also found that the gene i (producing green cotyledon color) shows an epistatic effect on the gene Orc (orange cotyledon color). Incomplete dominance and dominance were revealed in the loci Orc and I, respectively. Mapping analysis revealed that the gene Orc is located on linkage group 1 and 28.5 crossover units away from the gene Ans and 31.3 map units away from Idh. In addition, a significant linkage was detected between two genes Pur and Ans with an estimated distance of 9.9 map units. The distance between Orc and Pur was estimated as 38 map units.

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Research subject: Bio-hydrogen is a renewable energy source with many economic and environmental benefits as a fuel. Controlling the concentration of the substrate in the reactor has a significant effect on the amount of hydrogen production. However, bio-hydrogen production is a nonlinear process that requires the implementation of nonlinear control methods. In this paper, substrate concentration in an anaerobic bio-reactor is controlled using the feedback linearization method.
Research approach: The model employed for the simulation is a well-known model consisting of three state variables. The proposed controller is a globally linearized controller (GLC) designed based on the feedback linearization technique. In this method, the nonlinear system is precisely linearized by a transformation of the coordinate system. As a result, the linearized system can be controlled using a linear controller. In order to linearize the system, a nonlinear compensator is designed using the design model and applying the concepts of differential geometry. Proportional-integral (PI) controller is adopted as a linear controller. GLC controller performance has been compared with a nonlinear controller (NC) and a PI controller. The performance of these controllers has been studied by numerical simulation based on the integral of time-square error (ITSE).
Main results: The simulation results show that substrate concentration control can contribute to the hydrogen production. The control method applied has better set-point tracking than the other two control approaches. The ITSE performance index for the feedback linearization method is lower than the other two methods. The nonlinear feedback controller fails if the kinetic parameters are changed by 25%, but the PI method and the feedback linearization are robust against model uncertainty. An efficient controller guarantees stable bio-hydrogen production. Comparing open-loop and closed-loop simulation results shows that controlling the substrate concentration increases hydrogen production by 90%.

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Research subject: Because of the rising global demand for propylene, various extensive studies and research have been done in order to develop alternative ways that are both more energy-efficient and require less energy. In this research, CuBTC is used as a manganese catalyst base in the oxidative dehydrogenation of propane to produce propylene. The wet impregnation method is used to manufacture the catalysts.
Research approach: Wet impregnation is used to prepare the catalysts, which is a step in the manufacturing process. Analyses such as FTIR, XRD, BET, SEM, and EDX are used to examine and describe catalysts that have been created. On the basis of the central composition method, we have investigated the impacts of reaction temperature, manganese loading percentage, oxygen-to-propylene ratio, and their interactions on the synthesis of propylene in this study. The central composite method's input parameters include manganese concentrations ranging from 1 to 5 percent, a propane-to-oxygen ratio ranging from 1 to 3 percent, and a temperature ranging from 140 to 280 degrees Celsius.
Main results: After that, it is shown that the projected models for propane conversion, propylene selectivity, and oxidative dehydrogenation efficiency percentage are about 95 percent based on reactor testing and evaluation of the Design-Expert software results. It was possible to improve the efficiency of the oxidation dehydration process by 4.9 percent by using a conversion percentage of 28.38 percent, a selectivity of 18.14 percent at 278 degrees Celsius, a metal oxide loading of 3.74 percent, and propane to oxygen ratio of 1.5 percent. When laboratory data were compared to predicted data, the correlation coefficient was 93% in favor of the laboratory data.
 

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Research Subject: In recent years, industrial-scale production of propylene based on oxidative dehydrogenation of propane has been of particular importance due to the lack of thermodynamic limitations. In this regard, the use of natural zeolites with high abundance and low price has placed a special position. In this research, perlite natural zeolites were treated with ionic liquid solution and acid, then supported vanadium catalysis were synthesized. Performance of catalysis were investigated in oxidative dehydrogenation of propane to propylene process with a mixed feed of propane and air in a fixed bed quartz reactor under condition of atmospheric pressure and temperature of 500˚C with a flow rate of 40000 h^-1 (GHSV).
Research Approach: In this study, natural perlite support as a source of aluminum oxide (Al₂O₃) and silica (SiO₂) was ion exchanged by one molar solution of ammonium nitrate (NH₄NO₃ 1 M). Continuously, to investigate the effect of delamination, different acid molar concentrations of nitric acid (HNO₃) equal to 0.75, 1.5, and 2.25 were used and then compared with the just modified ion exchange sample without acid leaching (V/PERLIT-I). Dry vanadium impregnation, as an active metal, was carried out to synthesize 8% wt. catalysts. X-ray diffraction analyzes (XRD), scanning electron microscopy (FE-SEM), and ammonia Temperature-programmed desorption program (NH₃-TPD) were used to characterization and evaluate the properties of the catalyst.
Main Result: The results showed that the concentration of acid used affects the conversion and selectivity of the catalysis. In comparison, a significant difference was observed between the performance of V/PERLIT-I sample compared to V/PERLIT-IA samples. The maximum selectivity value for V/PERLIT-IA(2.25) was 74%. According to the results, the treated perlite support with suitable selectivity can be considered in the studies of use as an industrial support.

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Research Subject: The conversion of carbon dioxide into hydrocarbons is a potential process that can reduce and control greenhouse gases. According to the United Nations Development Program's sustainable development goals, liquefied gas is an environmentally friendly fuel. Hydrogenation of carbon dioxide over a suitable catalyst can be used directly to synthesize light hydrocarbons.
Research Approach: This study investigated the direct synthesis of liquefied petroleum gas from carbon dioxide hydrogenation using SBA-15 catalyst modified with copper and zinc nanoparticles. In this study, hydrogen and carbon dioxide were used as reactant gases, and the operation conditions such as reaction temperature and residence time were evaluated.
Main Results: The results showed that by modifying the catalyst with copper and zinc active sites, the active surface of the catalyst was reduced to 542 m².g^-1. Furthermore, SEM results revealed that the addition of metal oxides ZnO and CuO resulted in uniform distribution in the internal channels of the 1Cu1Zn/SBA-15 catalyst, with no aggregation. LPG production is optimal at a temperature of 360 ^oC and a residence time of 10 g.h.mol-1. These conditions yielded a CO₂ conversion rate of 24.6% and a LPG selectivity of 64.8%, respectively. The amount of LPG produced increases as the temperature rises, and after reaching the optimum temperature, there is no significant increase in the amount of LPG produced. The percentage of CO2 conversion does not change much when the residence time is increased after the optimum value, indicating that the reaction has reached its thermodynamic theoretical range. According to the catalytic lifetime test of 1Cu1Zn/SBA-15, CO₂ conversion percentage and LPG selectivity do not change after 85 hours. Based on the results of the experiments, the synthesized catalyst can hydrogenate CO₂ efficiently to LPG.
 

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Research subject: Propylene is one of the most prominent gases due to some valuable products and derivatives such as polymers, solvents, dyes, etc., which makes it one of the most important building blocks in the chemical industry. Due to the limitations of steam cracking and fluid catalytic cracking processes in terms of low selectivity, energy consumption, and significant CO₂ emission, these processes cannot fulfill the growing demand for propylene. In recent decades, the dehydrogenation of light alkanes to produce light olefins, especially propane dehydrogenation (PDH), has attracted much attention. Pt-Sn and CrOx catalysts, which are widely used in this process, have good dehydrogenation activity and selectivity; However, the limitations of price, deactivation, and environmental problems are serious and have led researchers to improve coking stability, sintering Pt catalysts, and find new and environmentally friendly catalysts.
Research approach: : One of the challenging issues in the PDH process is achieving
appropriate catalyst. Several solutions, including modification of the base and introduction of additives, have been proposed to enhance the catalytic performance overcome the problems, and increase the resistant stability of Pt, Cr catalysts. Understanding the structure-performance relationship of catalysts during the PDH reaction is essential to achieve innovation in new high-performance catalysts. This research aims to introduce the characteristics of the dehydrogenation reaction, the progress made in the development of the catalyst, and the existing challenges. This research provides a deep understanding of the reaction mechanism and its role in the development and future directions of the catalyst for practical and industrial development.
Main results: This study offers a detailed understanding of how the reaction mechanism works and its significance in the development and future directions of the catalyst for practical and industrial advancement.

 

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Ethanol as a renewable biofule is an appropriate and viable alternative to the challenging fossil fuels. Bacillus subtilis, a gram positive bacterium, seems to be a promising choice since it has many useful features. For example B.subtilis ferments broad range of sugars derived from lignocellulosic hydrolysis. Transformation of this cellulytic bacterium to an ethanologenic one was accomplished via metabolic engineering techniques and Ethanol production operon of Z.mobilis was introduced to the B.subtilis. SR1 and SR21 strains expressed plasmid-borne ethanologenic genes of Z.mobilis but the genes had been integrated into the SR22 genomic DNA. Also lactate dehydrogenase gene had been knocked-out in SR21 and SR22 strains. Defect of cell growth in SR21 and SR22, suggests that NAD+ oxidation by lactate dehydrogenase is important for anaerobic growth. Considering the impact of Fe2+ ion on alcohol dehydrogenase II activity, in further experiments Fe2+ was added to the culture media and improvement in growth rates was seen. Final yield of ethanol production of SR1, SR21, and SR22 strains were 53.8%, 86.7%, and 83.9% respectively.

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  The current approaches applied for removing nitrate from drinking water, commonly uses many chemical additives that may have undesirable or unknown effects on human health and in some cases caused many by products more dangerous than nitrate in drinking water. In present there are few appropriate and economic processes in this field and because of water scarcity in many areas development a suitable technology for treatment of nitrate contaminated drinking water for application in actual scales is crucial. In this research we develop an economic process with high selectivity for nitrate removal and minimum disturbance in other drinking water quality parameters that utilizes only hydrogen and carbon dioxide, produced in a methanol based electrochemical gas generator by applying a very low DC voltage (5-10 volt). We evaluate the ability of hydrogenotrophic denitrification for removal of nitrate in a bioreactor packed by light expanded clay aggregates known as LEACA. The results showed by proper coupling of electrochemical gas generator and denitrification bioreactor only by injection of tow clean and harmless gases, hydrogen and carbon dioxide and without any other chemicals addition for common concentrations of nitrate in natural waters by hydraulic retention time of 2-5 hr, removal efficiencies greater than 95% can be achieved. Also in comparison with other conventional methods such as ion exchange, reverse osmosis, electrodialysis, and biological heterotrophic with organic carbon source this system has several advantages such as: high selectivity for nitrate ion, low biomass yields, low electrical energy consumptions, without any problems resulted from organic carbon source addition (for example: taste and odor problem, carcinogenic THM production in disinfection process and rapid clogging of biofilter), easy operation, and compatibility with health issues in drinking water treatment.

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Research topic:
The disparity between supply and demand is one of the main obstacles in transitioning from fossil fuels to renewable energy. Underground hydrogen storage derived from renewable sources is a suitable method for storing energy from these sources. However, a portion of the stored gas remains in the reservoir as cushion gas, which can add to the operational costs. It is therefore recommended to replace this cushion gas with less expensive alternatives, such as CO₂ or sour gas, to reduce these costs. Nevertheless, this replacement can affect the purity and recovery factor of hydrogen, which can be controlled by specific operating parameters. This study will investigate how these parameters can be adjusted to maintain high purity and recovery factor for stored hydrogen.
Research Method:
In this section, a model of a partially depleted gas reservoir was initially constructed using the commercial simulator CMG. Following validation, this model was employed to evaluate the desired parameters. For this purpose, approximately 50% of the reservoir was depleted initially, followed by the injection of the cushion gas for one year. Subsequently, the hydrogen storage process was conducted over a period of 10 years. This research investigates the impact of various parameters, including the duration and rate of hydrogen injection and production, the soaking time and duration of cushion gas injection, the utilization of sour gas as the cushion gas, and the concentration of H₂S within it, on the purity and recovery factor of the produced hydrogen.
Main results:
The results showed that increasing the rate of hydrogen injection and production enhances its purity and recovery factor. Reducing the injection period while increasing the extraction period decreases purity but improves recovery, provided that the extraction period does not exceed the injection period. Extending the cushion gas injection time and the interval between injection and hydrogen storage supports the purity and recovery factor of hydrogen. Additionally, in the cushion gas composition, increasing the proportion of H₂S above 70% in the sour gas mixture reduces hydrogen purity and recovery by approximately 2% and 3%, respectively, confirming the potential of H₂S as a cushion gas.
 

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Patients afflicted by diabetes mellitus (DM) usually have more infections than those without DM. The course of the infections is also more complicated in this group of patients. One of the possible causes of increased infections prevalence is a deficiency in the immunity. Besides some decreased cellular responses in vitro, no disturbances in adaptive immunity in diabetic patients have been described. Different disturbances (low complement factor 4, decreased cytokine response after stimulation) in humoral innate immunity have been described in diabetic patients. In this research hydrogen peroxide (reactive oxygen mediator) and nitric oxide (reactive nitrogen mediator) in the neutrophil and macrophage culture of peritone in rats were evaluated against C.albicans. Via intravenous injection of streptozocin (65 mg/kg), a diabetic rat model was obtained. nitric oxide and hydrogen peroxide assay was performed by the Griess and Walter-Ruch methods respectively. C.albicans colony count on SCC medium was also done in two groups of healthy and diabetic mice. Macrophages of the healthy group reacted to C.albicans severly compared to the diabetic group which significantly produced more nitric oxide (P0.028). Neutrophils of the healthy group produced more No compared to the diabetic group against C.albicans (P0.165). No considerable difference was observed in production of hydrogen peroxide by macrophages in two groups of healthy and diabetic mice. Neutrophils of the diabetic group produced more hydrogen peroxide compared to the healthy group (P1). There was no significant difference in C.albicans colony count between the two healthy and diabetic groups (P0.058). Although nitrogen and oxygen related factors are changed or reduced after diabetic induction, changes in other immune system factors cannot be undermined.

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Antibiotics are widely used as bacteriostatic and bactericidal drugs in bacterial infections. Besides the respective interactions between the antibiotics and bacteria and between the immune system and bacteria, antibiotics also interact directly with the immune system and have various immunomodulatory effects on phagocytosis, chemotaxis, cytokine production and endotoxine release. In this study, monocytes and neutrophils were separated from the blood samples of the patients with urinary tract infection before and after the therapy with 500 mg ciprofloxacin tablets. The cells were cultured in the presence of ciprofloxacin and activators including IFN-γ and LPS or PMA (for in vitro evaluation) or only the activators (for ex vivo evaluation) . Supernatants of the cells were collected and then production of NO and H2O2 was measured. The effect of ciprofloxacin on NO and H2O2 production by monocytes and neutrophils in the patients with urinary tract infection caused by E. coli (E.Coli) was evaluated.The results were compared to an age- and sex-matched normal population. Peripheral blood samples from 45 patients with uUTI were collected at the time of diagnosis and at the end of the treatment. The results showed that NO and H2O2 production was significantly increased in the patients comparison to the control group, before and after the treatment (P<0.05). NO level in the post treatment group was also significantly raised compared to the pre-treatment group (P<0.05), but H2O2 level was not significantly altered (P>0.05). The results showed that the results of in vitro study was different from ex vivo. and alteration was not different between pre-treatment and post treatment groups significantly (P>0.05) .

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Introduction: Biofuel production from renewable resource has been extensively paid attention as a sustainable alternative for fossil fuel. As the feed of third-generation biofuels, microalgae can produce variety of lipids, proteins, and carbohydrates in large quantities and in a relatively short time. Regarding the compatibility of these microorganisms with culture diffrent conditions and independence from the seasons of the year, the rapid growth rate, absorbing carbon dioxide and improving air quality, renewablity, non-competing with food supplies, the existence of large quantities of lipid and carbohydrate inside their cells, and abillity of biofuels production, microalgae are known as one of the most suitable options for the biofuels production. Biofuel production from microalgae consists of several stages, including cultivation, harvesting, drying, cell disruption, extraction (lipids or carbohydrates), and the production of biofuels.
Conclusion: In the present study, by reviewing each stage of the biofuels production from microalgae, its importance and application for bioenergy production is discussed. Algal biofuel is not yet competitive with fossil fuels due to its high costs. Researchers are trying to produce economic algal biofuel by improving the growth of microalgae and enriching their reserves of oil and carbohydrates, creating genetic changes, improving the design of photobioreactros, developing harvesting and drying methods, improving methods of extracting lipid and carbohydrate, and producing valuable products.

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The response of five inbred sunflower seedling lines, including AC 4122, C, HA 89, HA 410, HA 411, to inoculation with Sclerotinia sclerotiorum culture filtrate containing endogenous oxalic acid was compared with the exogenous application of synthetic oxalic acid. The reaction of seedlings was evaluated in terms of dry and fresh plant weights and the total chlorophyll concentration relative to untreated controls. The expression of shikimate dehydrogenase in cotyledons was also assessed five days after treatment. The results indicated that exogenous oxalic acid inoculation caused more deleterious effects on stem rot, eliciting photosynthesis reduction and different isoenzyme patterns of shikimate dehydrogenase. A positive correlation was found between increased oxalic acid and shikimate dehydrogenase activity in both treatments. However, the excessive toxicity of the exogenously administrated acid suggests that Sclerotinia sclerotiorum infection triggers a more complex metabolic pathway involving oxalic acid secreted by the pathogen. These observations preclude the possibility of using the synthetic acid administration as a method of screening sunflower genotypes for resistance to Sclerotinia. In addition to these findings, the reactivation of shikimate dehydrogenase was observed in both treatments. In contrast to synthetic administration, expression during the first phase of growth may serve as a tool for rapid screening and selection of sunflower genotypes resistant to Sclerotinia sclerotiorum.