1,720,966 research outputs found
Feedstock recycling di plastiche: recupero di monomero da polietilene e polistirene mediante trattamenti termici
Full text linkBy this work, we want to emphasize the importance of the chemical waste recycling, since it allows the reprocessing of the polymer waste into raw materials, which can be utilized again in the refineries or in the petrochemical industry: turning a polymer back into monomers, amenable to further polymerization, closes the life-cycle of the material.
Unfortunately, selective depolymerization is not easy to achieve for the largest class of polymers, i.e. those obtained by free-radical polymerization. Polyolefins belong to this category and xxPE and PP together are the largest production of synthetic polymer. Decomposition of polyolefins into olefins can be thermally achieved indeed, but the process is expected to operate through a random, rather uncontrollable, free-radical reaction mechanism.
Products distribution is dramatically affected by temperature and residence time.
A serious analysis of the state of the art in the waste recycling field and the reutilization of plastics in Europe, and particularly in Italy, shows that feedstock recycling is still little developed. Consequently recycling could be an interesting support in the traditional methodology of mechanical and energetic recycling.
Nowadays, the integrated waste management of plastics seems the best solution to safeguard the word environment.
With the target of demonstrating the chemical recycling effectiveness, the tests in laboratory have taken place starting from the depolymerization of plastic waste by heating in absence of oxygen: we gave great attention especially to the non-catalyzed pyrolysis of polyethylene, considering the great presence of this polymer in solid urban waste, due to the versatility and to the many uses allowed by this plastic.
Pyrolysis is a process of thermal decomposition in inert atmosphere (nitrogen, helium, argon) of a substrate, usually solid, and generally characterized by a fairly elevated molecular complexity.
This substrate, thanks to the heating action, produces a solid fraction (called char), a liquid fraction (formed by products called tars, which condense at room temperature) and gases.
During the experiment we tested different plant configurations in order to final out the best operating conditions to maximize the yield in the base monomer which constitutes the fed polymer.
Since we worked with polyethylene, it was obvious for us to try to maximize the productivity in ethylene. In this line of action, we also tried to look for optimal contact-time of the polymer in the reactor and optimal operative temperatures.
In fact, operating on some parameters, it is possible to improve the selectivity of some compound instead of others: this is the reason why we tried to identify the right level of heating, the suitable temperature of cracking and the appropriate contact-time in order to find out, more specifically, the thermal decomposition reaction towards the desired olefins .
Beside the study of the best plant processing, we also optimized the gas-chromatographic sampling technique, with the purpose to obtain a quality and quantity analysis of the gas mixture, produced by the pyrolysis reaction
Two-steps selective thermal depolymerization of polyethylene. 1: Feasibility and effect of devolatilization heating policy
No full textAiming at controlling the selective depolymerization via the thermal route, we investigated a two-stage process, where volatiles are produced in a low temperature (800 degrees C, residence time <1.3 s). Splitting the process allows to control the reaction times independently and circumvent limitations of a multiphase reacting system, where melting contrasts a rapid heating. The two-stage process with independent melting can dramatically improve the olefins yield, and particularly that of ethene. A proper heating policy in the 1st thermal decomposition affects the species distribution in the intermediate product (oils and waxes). The following gas phase, short contact time thermal cracking takes advantage of a 'tailored' feedstock, to drive the products distribution towards a larger amount of ethene. With the two-stages sequential process we achieved cumulated ethene yields from LLPDE in excess of 45 wt.% with respect to polymer fed. While gas yield approaches 85 wt.%, up to 75 wt.% of the gas can be ethene at suitable conditions. Indications to improve the interesting results are discussed. (C) 2010 Elsevier B.V. All rights reserved
On the understanding and control of the spontaneous heating of dried tannery wastewater sludge
No full textSelf-heating of dried industrial wastewater sludge: Lab-scale investigation of supporting conditions
No full textImproved storage and disposal of dried tannery sludges
No full textIn this work we quantitatively studied the opportunities to improve the compaction of dried wastewater treatment sludges from tanneries, possibly combining with pelletization. Measurements of bulk density have been carried out at the industrial and laboratory scale, using different packing procedures, amenable to industrial processes. Waste as powder, pellets and their mixtures have been considered. Mixtures of powder and pellets is the best packing policy. The best compaction results was achieved by controlled vibration of a 30/70%wt mixture of powders and pellets, leading to a final bulk density of 1 t/m3, i.e. an improvement of compaction by more than 54% with respect to poured powders, but also larger than 35% compared to poured pellets. That means increasing the mass storage capacity by a factor of 1.56
Improved compaction of dried tannery wastewater sludge
No full textWe quantitatively studied the advantages of improving the compaction of a powder waste by several techniques, including its pelletization. The goal is increasing the mass storage capacity in a given storage volume, and reducing the permeability of air and moisture, that may trigger exothermic spontaneous reactions in organic waste, particularly as powders. The study is based on dried sludges from a wastewater treatment, mainly from tanneries, but the indications are valid and useful for any waste in the form of powder, suitable to pelletization. Measurements of bulk density have been carried out at the industrial and laboratory scale, using different packing procedures, amenable to industrial processes. Waste as powder, pellets and their mixtures have been considered. The bulk density of waste as powder increases from 0.64 t/m3 (simply poured) to 0.74 t/m3 (tapped) and finally to 0.82 t/m3 by a suitable, yet simple, packing procedure that we called dispersion filling, with a net gain of 28% in the compaction by simply modifying the collection procedure. Pelletization increases compaction by definition, but the packing of pellets is relatively coarse. Some increase in bulk density of pellets can be achieved by tapping; vibration and dispersion filling are not efficient with pellets. Mixtures of powder and pellets is the optimal packing policy. The best compaction result was achieved by controlled vibration of a 30/70 wt% mixture of powders and pellets, leading to a final bulk density of 1 t/m3, i.e. an improvement of compaction by more than 54% with respect to simply poured powders, but also larger than 35% compared to just pellets. That means increasing the mass storage capacity by a factor of 1.56. Interestingly, vibration can be the most or the least effective procedure to improve compaction of mixtures, depending on characteristics of vibration. The optimal packing (30/70 wt% powders/pellets) proved to effectively mitigate the onset of smouldering, leading to self-heating, according to standard tests, whereas the pure pelletization totally removes the self-heating hazard
Self-heating of dried industrial tannery wastewater sludge induced by pyrophoric iron sulfides formation
No full textSimilarly to many powders of solids, dried sludge originated from tannery wastewater may result in a self-heating process, under given circumstances. In most cases, it causes a moderate heating (reaching 70-90°C), but larger, off-design residence times in the drier, in a suboxic atmosphere, extremely reactive solids can be produced. Tannery waste contains several chemicals that mostly end up in the wastewater treatment sludge. Unexpected and uncontrolled self heating could lead to a combustion and even to environmental problems. Elaborating on previous studies, with the addition of several analytical determinations, before and after the self-heating, we attempted to formulate a mechanism for the onset of heating. We demonstrated that the system Fe/S/O has been involved in the process. We proved that the formation of small quantities of pyrophoric iron sulfides is the key. They are converted to sulfated by reaction with water and oxygen with exothermic processes. The pyrite/pyrrhotite production depends on the sludge drying process. The oxidation of sulfides to oxides and sulfates through exothermic steps, reasonably catalyzed by metals in the sludge, occurs preferentially in a moist environment. The mechanism has been proved by reproducing in the laboratory prolonged heating under anoxic/suboxic atmosphere
Indagine sperimentale sulla decomposizione termica selettiva di polietilene e caffè
Full text linkIl lavoro sperimentale di tesi è stato sviluppato per comprendere il comportamento di polipropilene e caffè sottoposti a pirolisi. Si è indagato il comportamento nella produzione di gas, analizzando gli idrocarburi prodotti. Inoltre si è monitorata la distribuzione delle frazioni ottenute (residuo carbonioso, olii, cere e gas) per comprendere quali politiche di riscaldamento portano ad una maggiore produzione di oliiope
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