Yeraltı sularında bulunan uçucu organiklerin dolgulu kulelerde giderilmesi üzerine bir araştırma
A Study about removal of volatile organic compounds from ground water in packed towers
- Tez No: 21909
- Danışmanlar: YRD. DOÇ. DR. İSMAİL TORÖZ
- Tez Türü: Yüksek Lisans
- Konular: Çevre Mühendisliği, Environmental Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 109
Özet
ÖZET Sularda bulunan uçucu organik maddelerin giderilmesi oldukça önemli bir konudur. Zira, bu maddelerin insan sağ lığı üzerine olan olumsuz etkileri yapılan bilimsel çalış malarla gözlenmiştir. Ülkemizde, genel olarak içme suyu ih tiyacı yeraltı ve yüzeysel sulardan karşılanmaktadır. Öy le ise, insan sağlığını olumsuz yönde etkileyen bu maddele rin sulardan uzaklaştırılması gerekmektedir. Bu çalışmada, sularda bulunan ve üzerinde en fazla araştırma yapılan 7 uçucu organik madde üzerinde durulmuş, bunların bazılarının insan sağlığı üzerindeki olumsuz etki lerinden bahsedilmiş ve bu maddelerin sulardan uzaklaştırıl ması için yöntemler araştırılmıştır. Sonuç olarak bu yön temlerin en uygunu olan hava ile sıyırma metodunun üzerinde çalışmalar yapılmış ve bu metodun teorisinden ayrıntılı ola rak bahsedilmiştir. Daha sonra çalışma prensibi hava ile sıyırma olan dolgulu kuleler gözönüne alınarak farklı gider me verimi, farklı sıcaklık farklı sıyırma faktörü ve farklı basınç düşmeleri gibi değişen koşullarda bilgisayar destek li hesap ve tasarımlar yapılmıştır. Bu çalışmanın birinci bölümünde, çalışmanın amaç ve kapsamı açıklanmıştır. İkinci bölümde, uçucu organik mad delerin meydana geliş şekilleri ve insan sağlığı üzerinde ki olumsuz etkilerinden bahsedilmiştir. Üçüncü bölümde uçucu organik maddelerin sulardan giderilmesinde kullanı lan hava ile sıyırma yöntemi tüm ayrıntıları ile incelen miş ve dolgulu kuleler üzerinde durulmuştur. Dördüncü bö lümde hava ile sıyırma prensibi ile çalışan dolgulu kule lerde tasarım esasları incelenmiştir. Beşinci bölümde he sap sonuçları tablolar ve grafikler halinde verilmiştir. Altıncı bölümde maliyet hesapları üzerinde durulmuştur. x
Özet (Çeviri)
SUMMARY A STUDY ABOUT REMOVAL OF VOLATILE ORGANIC COMPOUNDS FROM GROUND WATER IN PACKED TOWERS. As it known, water is the most important factor for the living organism. In recent years, because of the ' mdustrilazition and increasing the population, the neces sity of the water has been gradually increased. As a result of this, has been faced water pollution problems. However using a clean water is an natural rights for the people. It is the fact that, T ground. water has to be con sidered as an drinking water source which other water source are gradually decreasing in our world. Consequently, ground waters has begun to considered as an important natu ral water source in recent years. In USA, 125.000-300.000 m3 drinking water had been obtained from -ground between 1950 and 1990. Today in USA, 1/4 of the fresh water are being obtained from ground and water need of the %50 people are obtained from underground. In Turkey, according to the DSİ datums, annual drawable water potential is 11.6 billion m3. But allocated of these water is only 6.6 billion m3. Lost of the.ground waters are clean and suitable for use. But in recent years, the pollution of these water has increased and faced regional and local pollution problems. It has been deter mined more than 200 pollutant m some water resources and the contamination of drinking water by low molecular weight, volatile organic chemicals has been reported in many areas of the world. It is known that, contaminants which are found in ' ground and surface waters are volatile and harmful for the human life. Concern over this type of drinking water contamination led the U.S. Environmental protection Agency (EPA) to propose maximum contaminant levels for eight volatile organic compounds and recommended maximim contaminant levels in drinking waters. As a result of scientific studies, because of having a volatile and harmful properties of these compounds have XIto be removed from the waters. In this study, it has been investigated methods of removal volatile organic compounds from the water and made a design in Packed Tower which working principle is based on air stripping, that is the best method for removing volatile organic compounds from the water. Design is based on computer programme. In the first chapter, it is given introduction of the study, aim and the contents. In the second chapter, effect of the volatile organic compounds on human life and treatment alternatives has been investigated. Some of the volatile organic compounds in formed by reacting with chlorine of some organic in a form of humic and fulvic acid. At the same time it is known that Trihalomethanes (THMs) is formed in a disinfection process. Other sources of volatile organic compounds which contaminates the water are different production process and distribution, urbanization and agriculture activities and the last, litter leakage water. Removal of volatile organic compounds from water is an important subject. Because existence of these organics in water which is vital for people form a great danger for human life. These organic' s negative effects on human life has been proved by the scientific studies. It has seen that some of these organics effects on a nerveous system, stomoch and liver. It also has seen that some of volatile organic compound cause cancer and effect on human body. In a raw or finished water, some volatile compounds can be found. Main of volatile organics and often seen in waters as follows. - Trichlorocthylene - Tetrachloroethylene - 1,2-Dichloroethane - 1,1,1- Trichloroethane - Carbon tetrachloride - Vinyl chloride - Chloroform Some countries has certain standarts for these com pounds and have an acceptable maximum volatile organic compounds concentration. For example for. Trichloroethy- lene, acceptable maximum concentration is 0,005 mg/L in USA XIIand 0,03 mg/L for the world health organisation. In Tur key, there is no standards for any volatile organic com pounds. Other acceptable concentrations chatiging with countries can be seen table 2.1. There are different methods in order to remove volatile organics which are found in waters. These are can be exp lained as follows, - Air stripping - Reverse osmosis - Ozonitation - Oxidation with hydrogen peroxide - Ultraviolet radiation - Land Treatment Between these methods, air-stripping is the best and found place itself in most applications. It has been observed that, by using air-stripping method can be reached high removal efficiencies. In the third chapter, air, stripping is explained in details. Working principle of the air-stripping process is to provide water to get in touch with air in a close position. Then it can be seen that volatile organic com pounds will pass to gas phase from the liquid phase. The kinetic theory of gases stares that molecules of dissolved gases can readily move between the gas and liquid phases. Consequently, if water contains a volatile conta minant in excess of its equilibrium level, the contaminant will move from the liquid phase to the gas phase until equilibrium is reached. Leaving from the equilibrium level cause mass transfer. Equilibrium concentration of the substance depends on Henry constant. Henry's law gives idea about action tendency between gas and liquid phases, so Henry constant can be defined as an action coefficient. Henry constant is quite important while designing the packed tower so it is recommended to determine its value from experimental studies. Air-strigping can bedene different ways. These, are, - Packed Tower Aeration - Diffused Aeration - Tray Aeration - Spray Aeration xmIn this study, Packed Towers been considered because of the best way and high removal efficiency. The design of an air stripping column can also be described mathema tically, the equaitons have been well-developed in the literature. The equaitons are derived by setting up a mgss balance in the air stripper. Four basic assumptions are incorporoted in these equa tions. 1, The influent air is free of volatile organic com pound. (VOCs) 2- Plug flow conditions hold for the air water flow, the use of an inlet water distribution system helps to preserve this condition. 3- The changes that occur in the liquid and air volumes during mass transfer are negligible. 4- Henry's law holds true for these conditions. To obtain the best stripping, it must be provided a good mixture and high superficial contact area between the phases. Packed Towers are widely used to obtain high mavss transfer between the phases. Mass transfer rate from the liguid phase to the gas phase can be explained with an equation as follows, JA" KLa The overall liquid mass transfer coefficient K, demon strates, the system in which rate acts to the equilibrium. This is called mass transfer rate. Kl determinations is based on a two-layer theory. Knowledge about two-layer theory can be seen in 3.2.2.1. in details. Kl is the func tion of the system's geometric and physical characters, temperature, flow rate and stripped compounds. Mass transfer coefficient is quite important in desig ning packed towers, so it is recommended to calculate by pilot studies. In the lack of datums, ' tnere are. met hods in order to determine mass transfer rate constants. These are, Sherwood-Holloway Model, Shulman Model, Onda Model xivResults from the onda model and pilot studies confirm each other so Onda equations has been considered in designing packed towers in this study. As it seen the equations, there are terms as diffusion coefficient in liguid and gas phase (Dl,Dg). These terms can be determined different methods. Wilke-Chang method had been used in calculations of the D, and Hirscfelder-Bird-Spots method used in calcu lations of the Dç, These methods are recommended xn lite rature xn calculations of Dl and Dq. Packed towers, used for continous contact of liguid and gas xn both countercurrent and cocurrent flow, are vertical columns which have been filled with packing or devices of large surface as in Fig. 3.1. The liquid is distributed over, and trickles down through, the packed bed, exposing a large surface to contact the gas. Meanwhile gas is pum ped at the bottom of the coloumn. This pumprd air go for ward through coloumn providing uniform air loading. When Henry Law is considered, while water is going forward through packing material and contacting with air, volatile organic concentration which is found in water will decrease, nevertheless the influent air which is free of volatile organic compounds, will have a concentration of organic compounds. In order to remove contaminants in gases, active carbon adsorption is used. In designing packed towers, there are some factors have to be considered carefully to have economic and high efficiency. These factors are packing material, pressure drop, stripping factor and temperature. Packing material effects efficiency, economy and life of the tower. Packings which have a great surface, area is prefered in designing packed towers. Because it can be achieved high mass transfer rate in great surface area. The two broad categories of packing are randomly dumped packing and stacked packing. Dumped packing uses small, randomly placed plastic, metal or ceramic packings to provide a high surface area and a high void volume. Dumped packing has been much more common, but stacked pac kings may offer some advantages. According to manufac turers, stacked packings are less susceptible to biological and mineral fouling. This is due to the higher void space and the fact that stacked packings do not have horizontal surfaces as do dumped packings. In general Rasching rings are widely used in air strippingg process. Packing materials and properties can be seen in Tablo. 3.1 xvPressure drop is a function of the gas and liguid flow rates and the size and the type of the packing. A stripper operating at a high pressure drop will require a smaller volume than a similiar stripper at a lower pressure drop. This reduces capital cost for the tower, but increases the blower cost. Generally pressure drop are kept between 50-400 N/m2/m in packed towers. Otherwise can be faced some problems as flooding and loading. Pressure drop curves is shown in Fig. 3. 4. The stripping factor R, is a ratio of the actual opero- ting air-water ratio to the theoretical minimum ratio. It is quite important for the economy and efficiency of the tower. Generally it is kept between 2-5 in water systems. In the fourth chapter it is mentioned design princip les in packed tower. In this part it had been designed packed towers for removal 7 compounds which are widely found in surface and undeground waters. During the design, required physical and chemical values of compounds, has obtained from the literature. Design had been done in different temperature as 10,20,30°C different pressure drop and different flow rate. Other factors changes during the design are, stripping factor and removal efficiency. Meanwile packing material is the only factor which is constant. Ceramic Rasching rings with a size of 13 mm had been used in design because of having high surface area and suitable for Onda equations. It is known that Onda equations are valid with a size of 10-50 mm for packing materials. The procedure in designing in packed towers can be lined up as follows. - Select compound to be stripped, - Select packing material, - Determine minimum air-water ratio, - Select stripping factor, (R) - Determine operating air-water ratio, - Select design pressure drop, - Determine air and water loading on tower - Determine tower diameter, (D), - Determine mass transfer coefficient (kTa) - Determine height of transfer unit, (HTU) - Determine number of transfer units, (NTU) - Determine packing height (Z) - Go to step 4. Iterate stripping xactor and/or gas pressure drops to determine design parameters that optimize cost effectiveness. XVIFormulas of the values used in designing packed towers can be seen in details in chapter four. Design results and comments according to results has given in chapter five. As said before, packed tower design had been done by computer programme. Computer programme is given appendix 1. About 30.000-35.000 values are determined from the programme exits, so it was impossible to consider all the values so the first 150 step had been considered which obtained from the programme exits. According to the programme exits, achiewed results can be summarized as follows, - When the removal efficiency increases, air-water ratio vaues increases. - While removal efficiency is increasing and stripping factor is constant, it was observed that packing height increases. - Tower diameter increases with flow rate. - Conditions which removal efficiency constant and stripping factor increases it was observed that packing height decreases. - When the stripping factor increases air-water ratio values increase. - While air-water ratio is increasing, miss transfer rate decreases. - When the temperature increase, liquid and gas diffu- sivity increase. - Organic compands which have low Henry constant a has a low volatile properties so air-water ratio values are higher to remove these compounds from the water. In this condition it must be given much air to the system. Bromoform is a good example for these organics. - Compounds like a carbon tetrachloride, tetrachloro- ethylene needs lower air-water ratio values because of having high Henry constants. As can be seen, compounds which have a high Henry constants value has a high volatile properties. - When the removal efficiency constant and air-water ratio increase, it was obs-erved packing height decrease. In the sixth chapter, cost of air-stripping is presen ted. One of the main benefits of air-stripping as a xvi itreatment technology for contaminoted groundwater is its cost-effectiveness compared to other clean up methods, such as activated carbon. However, the cost of air-stripping is dependent on many factors and is therefore highly site specific. Cost can vary widely, but air stripping is very often the least costly clean up method for a particular site. The total cost of any treatment method is a combina tion of of the initial capital costs and operating costs. Capital costs are all costs associated with the startup of the air-stripping facility. Included costs are, - The process equipment, such as the tower and packing material, air blowers, pumps, piping, valves and electrical equipment. - A clearwell and holding tank. - Ony site-related costs, such as land purchase, bull dozing, and access. - Vapor-phase control (if required) - Moterials and construction cost for housing ( If required) - Miscellaneous cost such as painting, plumbing and cleanup. Operating cost are costs are basically comprised of power for the pumps and blowers and maintenance costs (labor and materials) As a result of this study, xt has been seen that air- stripping is the best way removing volatile organic com pounds from water and packed towers are suitable for this process. XVlll
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