Zeolit-su ikilisini kullanan adsorpsiyon ısı pompalarında ısıve kütle iletimini geliştirmek için yeni bir yaklaşım
A new approach to enhance heat and mass transfer in adsorption heat pumps
Yazar: MELKON TATLIER
Danışman: PROF.DR. AYŞE ERDEM ŞENATALAR
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Kimya Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 1996
Konu: Kimya Mühendisliği = Chemical Engineering
Dizin: Isı pompası = Heat pump ; Su = Water ; Zeolitler = Zeolites

Özet:
ÖZET Günümüzde yaygın olarak kullanılmaya devam edilen fosil yakıtların tükenme tehlikesinin mevcut olması ve çevre kirliliği gibi sorunların varlığı, alternatif enerji kaynağı araştırmalarını hızlandırmıştır. Bu amaçla geliştirilen ısı pompaları birçok sorunu halledebilecek özelliklere sahiptir. Buhar-kompresörlü ve absorpsiyon ısı pompalarından daha sonra geliştirilmiş olmalanna rağmen adsorpsiyon ısı pompalarının avantajları oldukça fazladır. Dışarıdan hiçbir müdahale olmadan uzun süre çalışabilecek özelliğe sahip bu cihazlar için en uygun adsorban-adsorbat seçeneklerinden biri hiç kuşkusuz zeolit-su ikilisidir. Adsorpsiyon ısı pompaları enerji kaynağı olarak herhangi bir atık ısı kaynağından yararlanabildikleri gibi güneş enerjisi ile de çalıştırılabilirler. Hem güneş enerjisi, hem de diğer enerji kaynakları ile çalıştırılan adsorpsiyon ısı pompalarında ısı ve kütle iletimi ile ilgili sorunlar mevcuttur. Adsorpsiyon ısı pompalarında verimliliği kısıtlayan ısı ve kütle iletimi sorunlarını giderebilmek için çeşitli düzenlemeler yapılabilir. Bu çalışmada güneş enerjisi ve diğer enerji kaynakları ile çalışan ısı pompaları için iki ayrı sistem önerilmektedir. Güneş enerjisi ile çalışan adsorpsiyon ısı pompalarında, adsorban olarak kullanılan zeolit metal elekler üzerinde ince bir tabaka halinda sentezlen- mekte ve böylece homojen bir ısı dağılımı elde edilmektedir. Bunun sonucunda, ulaşılabilen maksimum sıcaklık ve dolayısıyla da elde edilen enerji miktarı artmaktadır. Enerjinin ısı değiştiriciler aracılığı ile iletildiği sistemlerde ise zeolit ince ısı değiştirici borular üzerinde sentezlenmekte ve homojen bir sıcaklık dağılımının yanısıra ısıl iletkenliğin yüksek olması nedeniyle zeolit tabakasının çok çabuk ısınması da sağlanmaktadır. Ayrıca bu durumda kütle iletimi de sorun olmaktan çıkmaktadır. Bu şekilde, çok kısa sürede tamamlanabilecek olan çevrimler sayesinde belli bir sürede elde edilebilen enerji miktarı önemli oranda artacaktır. Her iki sistemde de zeolit ile birlikte önemli ölçüde metal kullanımı gerekmektedir. Ancak bunun gerek ağırlık gerekse de yer kaplama açısından önemli bir sorun yaratmayacağı ve enerji kazanımının her iki durumda da artış göstereceği anlaşılmıştır.

Abstract:
A NOVEL APPROACH TO ENHANCE HEAT AND MASS TRANSFER IN ADSORPTION HEAT PUMPS SUMMARY Although the demand for enegy has shown a sharp increase in recent years, the fact that conventional energy sources are going to be used up completely in the near future has caused a common inquietude. This fact has been an impetus for the researchers to direct their interest into this field. Undoubtedly the renewable energy sources will play a major role in supplying the world's energy needs in the near future. Solar energy, which is one of the major renewable energy sources, has always drawn great attention. However it's only recently that solar energy has been begun to be captured and used efficiently. In addition to efficient use of the energy sources, environmental aspects should also be taken into account. The fossil fuels and the conventional refrigerators contribute a great deal to the environmental pollution by means of the emission of CO2 and chlorofluorocarbons. Zeolites are hydrated crystalline aluminosilicates. They are finding widespread application in a diversity of areas. The largest use of zeolites is as detergent builders. They are also useful as catalysts and catalyst supports. Moreover, bulk separation and purification operations can be achieved with zeolites. Zeolites are useful as adsorbents because they are molecular sieves, contain large void fractions and are hydrophilic. Adsorption is brought about by the forces acting between the atoms and ions composing the solid and the molecules of the gas. These forces are physical and chemical and give rise to physical adsorption and chemisorption respectively. The discovery and commercialization of synthetic zeolites as adsorbents provided the major stimulus to the development of adsorption as a separation process. The primary requirement for development of an adsorption separation process is an adsorbent that has sufficient selectivity, capacity and stability to achieve the required separation economically over a prolonged period of time. Heat pumps have been developed to eliminate the undesired aspects of the fossil fuels and to provide a primary energy diversification. There are mainly three types of heat pumps, namely compression, absorption and adsorption heat pumps. These different types of heat pumps all operate on the same principles, but make use of different concepts. XIA heat pump constitutes of an adsorber, evaporator and a condenser. The energy is supplied to the adsorber and the circulating fluid is reversibly vaporized in the evaporator and condensed in the condenser. During these phase changes, the fluid gives off heat in the condenser and draws heat from the environment in the evaporator. In order to obtain the cooling and heating effects, some arrangements should be made so that the heat can be captured effectively. Several advantages and disadvantages of the different types of heat pumps exist. The compression heat pump generally uses electricity as an energy source and has problems such as emitting chlorofluorocarbons, being noisy, having a relatively short expected life-time. Absorption heat pumps bring solution to some of the problems like noise, vibration and strong dependence on electricity, but in turn have other disadvantages such as crystallization, need of high pressures and reduced reliability. On the other hand, adsorption heat pumps eliminate most of these disadvantages and seem most proper for the environment. Once installed, they operate continuously without any need of interference for a longtime, following the reversible cycles of adsorption and desorption respectively. One of the most important aspects of adsorption heat pumps is choosing the proper adsorbent-adsorbate couple. Because of their unique properties, zeolites are very suitable to be used in adsorption heat pumps. Since water has a high latent heat of vaporization and is readily available, it's the most common adsorbate used. Therefore, zeolite-water pair is one of the most preferred adsorbent-adsorbate couple. Unlike other adsorbents, zeolites have almost nonlinear pressure dependance which means that the amount of water adsorbed will be independent of pressure after saturation. So when the zeolite is at ambient temperatures it can adsorb large quantities of water vapor even at low partial pressures. On the other hand, when the zeolite is heated it desorbs most of the water vapor even at high partial pressures. Therefore, the difference in adsorbed gas between the high and low temperature states is high and depends only slightly on condenser pressure. The adsorbent is placed in the adsorber and the whole system is evacuated before operation begins. The system is heated until the pressure in the adsorber reaches the pressure determined by the temperature of the condenser. Then the valve between the adsorber and the condenser is opened to allow water vapor to be introduced into the condenser where it is liquefied. After this step, liquid water is transferred into the evaporator and when the pressure in the adsorber reaches the value determined by the temperature of the evaporator, the valve between the adsorber and the evaporator is opened to allow vaporization of water. Water vapor is again adsorbed on the zeolite and the latent heat of water is given off in the condenser while heat is drawn from the environment during evaporation. The first step of the process can be described as isosteric heating while the second step is isobaric desorption. The third and fourth steps are, isosteric cooling and isobaric adsorption, respectively. The operating conditions of the system can be determined by analyzing the isotherms and isosters of the adsorbent-adsorbate couple used. Temperature of the evaporator and the condenser, ambient temperature and maximum XIItemperature to be attained have to be determined before operation. Otherwise, the efficiency of the system may even go to zero without our knowledge. The heat of adsorption can be determined from the slopes of the isosters and the amount of adsorbate in circulation may also be calculated using these isosters. Although solar radiation has been used extensively ever since mankind moved from the hunter-gatherer mode to one of settlement agriculture it is only in recent decades that major scientific and technical effort has been dedicated to harnessing the power of the sun. Once solar radiation has been absorbed by a suitable surface and converted to heat it is possible to use this for a wide range of heating purposes at varying temperature levels. Solar radiation can also be used directly via the photovoltaic effect to produce electricity or indirectly through the solar thermal electric conversion process. Although solar radiation is often spoken of as a free resource this is evidently only true when considering the free availability of solar radiation as opposed to the prices of competing fossil fuels. In reality it is far better to talk of solar technologies as being inherently capital-intensive but of low running cost. There are three radiation components at the earth's surface. Direct radiation comes directly from the sun, whereas diffuse radiation comes from all angles within the celestial hemisphere. Reflected radiation can only be captured after being reflected from earthly objects such as trees, mirrors or snow. Active solar collectors can be classified into those that are mounted so that they track the sun as it appears to move through the sky, or those that are fixed at optimum angles of tilt so that they collect as much energy as possible throughout the day. Tracking collectors can further be divided into those that focus the sun's rays and those that do not. Flat plate collectors are the most commonly used type of solar collectors which have the advantage of being able to operate in both bright sunshine and cloudy conditions. On the other hand, the focusing collectors can reach much higher temperatures. Solar energy is very suitable to be used as an energy source in the adsorption heat pumps. Although solar energy is a free source, some arrangements should still be made to increase the efficiency of the system. Whatever energy source they use, adsorption heat pumps have some problems to be solved. Heat and mass transfer are the major causes of the insufficient level of efficiency in this system. For this reason a homogeneous temperature gradient should be obtained in the adsorbent layer. In order to achieve such a result, several attempts have been made, none of them being completely successful. Since adsorption is limited by diffusion and desorption is controlled by heat flow, a good compromise should be achieved between the high density required for good thermal conductivity and high porosity necessary for fast vapor diffusion. There are mainly two types of resistances to heat transfer. One of them occurs at the metal-adsorbent interface and depends on the physical contact between the materials. The other resistance is associated with heat transfer inside the solid adsorbent bed and is inversely proportional to the thermal conductivity of the bed. In order to improve the heat transfer inside the adsorber, it is necessary to increase the thermal conductivity of the system and to provide a means for a better contact between the metal and the adsorbent. XIIIOn the other hand, there are also a few difficulties associated with the thermodynamics of the adsorption cycle. These are the intermittent nature of the energy supplied to the user and the variability of the temperature of heat during the adsorption phase. Many efforts have been made to obtain a cycle in which the output energy is both released cotinuously and is within a small range of temperature variation. As a result regenerative cycles have been developed, in which at least two reactors operate out of phase with internal heat recovery. Using the regenerative cycle, an improvement of COP, which is a measure of the efficiency of the system, may also be obtained. In this study, Zeolite 4A-water is used as the adsorbent-adsorbate couple. Zeolite 4A is strongly hydrophilic and has a quite high adsorption capacity for water. Although Zeolite 13X is the most commonly used adsorbent because of its very high adsorption capacity, 4A is also appropriate and it is readily available. Prior to theoretical studies performed, some experiments were carried out in order to obtain the isotherms and isosters of the zeolite 4A-water couple. For this aim an apparatus designed particularly for adsorption experiments was used. The aim of this study is to design new arrangements for adsorption heat pumps, so that heat and mass transfer problems may be solved completely. For this reason, two separate designs have been proposed concerning the arrangement inside the adsorber or collector, whatever it's called. One of them involves an ordinary heat pump while the other is designed for adsorption heat pumps using solar energy. The zeolite is synthesized on metal supports using sodium aluminusilicate mixtures of desired composition by way of hydrothermal crystallization. This act is performed at temperatures between 50-1 00°C and for a period ranging from 30 minutes to 24 hours. In an adsorption heat pump, the zeolite is synthesized on heat exchangers which have very thin walls. The thickness of the zeolite layer will be approximately 5 microns which means that an important amount of metal should be used. This arrangement ensures that the two prominent resistances present in the adsorber will lose their influence. Since the zeolite is synthesized on the metal surface, there will be no contact resistance. Moreover, the composite thermal conductivity of the system will increase by a considerable amount. Additionally there will be no mass transfer problems in the system due to the very thin zeolite layer used. The model proposed is solved by using the finite difference method. The negligible terms in the differential equations and the boundary conditions are determined by the scaling method. The results are obtained with the help of a computer program. As a result the amount of time necessary for a single cycle is calculated using a model designed for adsorption heat pumps. The amount of time necessary to complete one cycle is reduced drastically using the new arrangement. In this way, in a specific period of time more cycles are available so that the energy obtained enhances by a considerable amount. Moreover, the xivenergy supplied to the user will be no more of intermittent nature. An alternate arrangement for adsorption heat pumps using solar energy has also been designed. The zeolite is again synthesized on metal supports. In this case metal supports are wire gauzes with very high mesh numbers. With this arrangement the thermal conductivity of the system will improve and a homogeneous temperature distribution will be obtained. As a result the maximum temperature attained will increase causing more adsorbate to circulate in the system. Although the mass of the inert parts of the collector increases by such an arrangement, the energy obtained enhances free of this fact. COPcyc|e is reduced by a small amount whereas COPso|ar increases as a result of this arrangement. Although more inert material should be used in such a system the dimensions of the collector stay almost the same and the increase in the amount of mass is tolerable. Also there is no additional cost necessary for this arrangement. It is possible to enhance heat and mass transfer in adsorption heat pumps by using alternate arrangements. This improvement may even be extended if a metal with higher thermal conductivity is used. In this way, a more efficient use of the energy received will be provided. xv