Atık plastiklerin akışkan yatakta pirolizi

Abstract

Son yıllarda büyük boyutlara ulaşan ve günden güne artan çevre kirliliği yirmibirinci yüzyıla girmeye hazırlandığımız şu günlerde insanlığın en önemli problemlerin den biri haline gelmiştir. Özellikle plastik esaslı çöpler uzun süre bozunmadıkları için çevreye büyük zarar vermektedirler. Hem sağlıklı bir çevre oluşturarak ekolojik düzeni korumak, hemde katı artıkları, özellikle plastik atıkları değerlendirerek tekrar kullanıma sokmak büyük önem taşı maktadır. Bu amaçla bindokuzyüz yetmişli yılların başın dan itibaren çalışmalara başlanmış ve pek çok proses ortaya atılmıştır. Yapılan bu çalışmada plastik esaslı katı artıklar bir akışkan yatakta piroliz edilerek sisteme etkiyen parametreler saptanmış, bu parametrelerin ürün miktarına ve ürün dağılımına olan etkileri incelenmiştir.

Modern society has been trying to solve the solid waste problem which is caused by urbanization, increasing consumption and environmental pollution. Since the transportation of waste to long distance is not economical, several methods have been developed to dispose the solid waste. These methods include storage, hiogas and compost production, recovering of reco verable parts of waste, pyrolysis and incineration. All these methods, depending on the waste composition and local conditions, are being used in many countries. In countries where the topic is systematically approached, generally more than one methods are used for waste desposing. Incineration or burn out method has been used for centuries to burn the waste in the open areas. But in present times it is abondoned due to the environmental pollution considerations, I n order to burn the waste in closed cell or fixed bed, one-or multi-cell systems and tray furnaces are developed. In addition mowing beds, rotating furnaces, stoker and fluidised bed combustion are used. Since 1970's, there has been increasing studies in order to develop the new techniques for thermal treatment of wastes. The studies started much earlier in the USA, 3apon; England andDenmark. Consequently, numerous new waste processing system have been designed. Most of them are further developed, Incentivss behind the research which was sponsored by the public finds are; Vll a)- To develop new methods allowing the construction of very simple plants for the thermal treatment of run at a reseanable cost and would thus be suitable for waste disposal in small areas. 3)- New methods, apart from recovering energy, also enable to recovery of the valuable materials contained in the waste in different ways such as in the form of synthesis gas, oil coke or metals. c)- The methods sould be flexible to treate the wastes of varying compositions and quantity so that the wastes of varying properties can be disposed together. They also should reduce the size of the waste as much as possible. d)- In addition, the new methods should result in as little environmental pollution as possible Df these methods, incineration is well-known method, currently used. However, it has some weaknesses such as; the large quantity of produced gas to be cleaned low burning efficency, sensitivity of the grade type systems to the waste constituents such as glass and plastics, need for additional energy and lack of possibilities of storing energy, loss of metals separated from ashes. In the pyrolysis 5basically^ the large organic molecules are cracked and converted into small molecules while a small portion becomes coke. The obtained liquid and solid organic products are fractioned and used as a raw material in industry. The gaseous products are used as fuel. The coke residue (obtained during the pryrolysis) may be used as fuel either by direct combustion or by gasification. In this study, the pyrolysis of plastic materials which is one of the major constituent of wastes is studied Experiments were carried out ina batch fluidised bed in tie N " atmosphere. Parameters affecting product yield and composition and distribution of the products are found to be heating rate, fluidising gas velocity, the highest operation temperature and the type of vi 11 fluidising gas. The effect of heating rate on the yield and distri bution of products is investigated. Experiments ware carried out in a quartz fluidised Ted of 5cm. i.d. and 40 cm. long at atmospheric pressure with N2- The materials used in the experiments are polyethylene and polypropylene which are widely used in industry. The pyrolysis products from the bed are cooled down in order to sep arate liquid and gas fractions. The liquid fraction is collected in the traps, and gas products are collected in gas bags. The yield is calculated from the mass balance of the system. The qualitative and quantitative analysis of the liquid and gaseous products are done by a gas cromatograph (GC) Experiments were carried out for the plastics with 3,5,7,9, and 12 C/min heating rates and constant gas flow rate inorder to investigate the effect of heating rate on the yield and compostion of the products. Coke, liquid and gaseous products are obtained from the pyrolysis. The variation in liquid and gas products with heating rate is determined by mass balance. The structure of liquid product is investigated by several separation techniques and analysis. The first analysis is done by adsorption cromatography. Liquid sample is dissolved in a solvent consisted of n-hexane, toluene and methanol (3:2:1). A 50 cm long and 2 cm i.d. colum is filled up to 4/5 of height with silicagel (7Q7230 mesh) and is wetted with n-hexane. Dissolved samples is fed to the top Df the column and then, n_ -hexane (15D ml), tolune (IDG ml) and methanol (50 ml) are added to column in order to separete aliphatic, aromatic and polar fractions, respectively The amount of each fraction is determined after remov al of solvent by evaporation. The percentage of aliphatic, aromatic and polar fractions ara calculated accordingly. The aliphatic fractions separated by adsorption cromatography are injected into a nss cromatograph in order to determine the compounds ' contained and their percentage. IX The results show that the yield of liquid product exponentially increases with increasing heating rate for both plasti s. Yield increases rapidly at the beginning and slows after certain values and remains constant after wards. The maximum yield values obtained as a function of temperature are 90 % and 70 % for polyethylene and polypropylene respectively. The percentage of aliphatic, aromatic and polar groups increases with temparature^ reaches a maximum and then inclines af ter.uards. The maximum aliphatic percentage (BD %) is obtained with 5 C/min heating rate. The highest percentage values for aliphatics are obtained for 4-7 C/min heating rates. The percentage of aromatic decreases with temperature and reaches minimum value and increases afterwards. - The lowest aromatic percentage is abtained with 4-7 C/min heating rates. The constituents and thair percentage irr the aliphatic fractions are determined by gas cromatograph. The compounds are plotted as a function number and amount of carbon. It is seen that the liq uid product contains compound with carbon ranged from 9 to 35. But most of compounds consist of 15-20 carbon molecules. The compostion of the fraction does not change considerably with heating rate. The results show that the yield of liquid products not vary considerably with he 6 C/min. The percentage of the highest for this heating rate. does not vary considerably with heating rates higher than 6 C/min. The percentage of aliphatic fraction is These data suggest that a 6 C/min heating rate is the optimum value at which aliphatic groups and liquid products are obtained with a maximum percentage. In order to have a complete^, knowledge of operational conditions, the effet of gas flow rate also snoulri be investigated". On the other hand,pyrolysis can be carried out by uater-ujapor H", COr,... etc. and in the presence of a catalyst. Tnerefore the effect of these conditions on the distribution of the products can be understood.