Cracking

About: Cracking is a(n) research topic. Over the lifetime, 15160 publication(s) have been published within this topic receiving 182860 citation(s).

A review of catalytic upgrading of bio-oil to engine fuels

Abstract: As the oil reserves are depleting the need of an alternative fuel source is becoming increasingly apparent. One prospective method for producing fuels in the future is conversion of biomass into bio-oil and then upgrading the bio-oil over a catalyst, this method is the focus of this review article. Bio-oil production can be facilitated through flash pyrolysis, which has been identified as one of the most feasible routes. The bio-oil has a high oxygen content and therefore low stability over time and a low heating value. Upgrading is desirable to remove the oxygen and in this way make it resemble crude oil. Two general routes for bio-oil upgrading have been considered: hydrodeoxygenation (HDO) and zeolite cracking. HDO is a high pressure operation where hydrogen is used to exclude oxygen from the bio-oil, giving a high grade oil product equivalent to crude oil. Catalysts for the reaction are traditional hydrodesulphurization (HDS) catalysts, such as Co–MoS2/Al2O3, or metal catalysts, as for example Pd/C. However, catalyst lifetimes of much more than 200 h have not been achieved with any current catalyst due to carbon deposition. Zeolite cracking is an alternative path, where zeolites, e.g. HZSM-5, are used as catalysts for the deoxygenation reaction. In these systems hydrogen is not a requirement, so operation is performed at atmospheric pressure. However, extensive carbon deposition results in very short catalyst lifetimes. Furthermore a general restriction in the hydrogen content of the bio-oil results in a low H/C ratio of the oil product as no additional hydrogen is supplied. Overall, oil from zeolite cracking is of a low grade, with heating values approximately 25% lower than that of crude oil. Of the two mentioned routes, HDO appears to have the best potential, as zeolite cracking cannot produce fuels of acceptable grade for the current infrastructure. HDO is evaluated as being a path to fuels in a grade and at a price equivalent to present fossil fuels, but several tasks still have to be addressed within this process. Catalyst development, understanding of the carbon forming mechanisms, understanding of the kinetics, elucidation of sulphur as a source of deactivation, evaluation of the requirement for high pressure, and sustainable sources for hydrogen are all areas which have to be elucidated before commercialisation of the process.

Catalytic pyrolysis of biomass for biofuels production

Abstract: Fast pyrolysis bio-oils currently produced in demonstration and semi-commercial plants have potential as a fuel for stationary power production using boilers or turbines but they require significant modification to become an acceptable transportation fuel. Catalytic upgrading of pyrolysis vapors using zeolites is a potentially promising method for removing oxygen from organic compounds and converting them to hydrocarbons. This work evaluated a set of commercial and laboratory-synthesized catalysts for their hydrocarbon production performance via the pyrolysis/catalytic cracking route. Three types of biomass feedstocks; cellulose, lignin, and wood were pyrolyzed (batch experiments) in quartz boats in physical contact with the catalysts at temperature ranging from 400 °C to 600 °C and catalyst-to-biomass ratios of 5–10 by weight. Molecular-beam mass spectrometry (MBMS) was used to analyze the product vapor and gas composition. The highest yield of hydrocarbons (approximately 16 wt.%, including 3.5 wt.% of toluene) was achieved using nickel, cobalt, iron, and gallium-substituted ZSM-5. Tests performed using a semi-continuous flow reactor allowed us to observe the change in the composition of the volatiles produced by the pyrolysis/catalytic vapor cracking reactions as a function of the catalyst time-on-stream. The deoxygenation activity decreased with time because of coke deposits formed on the catalyst.

Olefins from conventional and heavy feedstocks: Energy use in steam cracking and alternative processes

Abstract: Steam cracking for the production of light olefins, such as ethylene and propylene, is the single most energy-consuming process in the chemical industry. This paper reviews conventional steam cracking and innovative olefin technologies in terms of energy efficiency. It is found that the pyrolysis section of a naphtha steam cracker alone consumes approximately 65% of the total process energy and approximately 75% of the total exergy loss. A family portrait of olefin technologies by feedstocks is drawn to search for alternatives. An overview of state-of-the-art naphtha cracking technologies shows that approximately 20% savings on the current average process energy use are possible. Advanced naphtha cracking technologies in the pyrolysis section, such as advanced coil and furnace materials, could together lead to up to approximately 20% savings on the process energy use by state-of-the-art technologies. Improvements in the compression and separation sections could together lead to up to approximately 15% savings. Alternative processes, i.e. catalytic olefin technologies, can save up to approximately 20%.

Modeling the time-to-corrosion cracking in chloride contaminated reinforced concrete structures

Abstract: The time-to-cracking, from corrosion initiation to cracking of the reinforcing steel cover concrete, is one of the critical time periods for modeling the time to repair, rehabilitate, and replace reinforced concrete structures in corrosive environments. The time to corrosion cracking was experimentally determined from simulated bridge deck slabs. The experimental design considered corrosion rate, concrete cover depth, reinforcing steel bar spacing, and size. Three of the 14 slab series cracked during the 5-year study. Metal loss was determined for the cracked specimens that had cover depths of 1 in (25 mm), 2 in (51 mm), and 3 in (76 mm). A time to corrosion cracking model was developed that considered the critical amount of corrosion products that consists of the amount of corrosion products needed to fill the interconnected void space around the reinforcing bar plus the amount of corrosion products needed to generate sufficient tensile stresses to crack the cover concrete. The experimentally observed time to cracking is in good agreement with the model predicted time to cracking.

Cracking of thin bonded films in residual tension

Abstract: Solutions are obtained for two elastic plane strain problems relevant to the cracking of a thin film bonded to a dissimilar semi-infinite substrate material. The first problem is that of a crack in the film oriented perpendicular to the film/substrate interface with the crack tip touching the interface. The second problem is that of a crack of the same geometry, but with length less than the film thickness, so that the crack tip is within the film. These problems are used to model several modes of crack extension in thin films bonded to thick substrate materials. Complete results from the solution of each problem are given over the full range of practical elastic mismatches. Dimensionless quantities important in describing the cracking of thin films are introduced and accurate approximate formulas based on the solution results are given for them. Applications are discussed, including criteria for avoiding thin film crack extension and a formula for the curvature change induced by the cracking of a thin film bonded to a substrate of finite thickness. The solution results, approximate formulas and information on their application provide the details necessary for the analysis of practical thin film cracking problems.