Bir işletmede enerji ve kullanılabilir enerji çözümlemesi: Elazığ Şeker Fabrikası örneği

Bu çalışmanın amacı pancar işleyen bir şeker fabrikasında üretim süreçlerini belirlemek ve bu süreçlere termodinamiğin birinci ve ikinci kanunlarını uygulamak ol muştur. Çalışmada önce termodinamiğin birinci kanunu, termodinamiğin ikinci kanunu ve kullanılabilir enerji denklemleri açıklanmıştır. Pancardan şeker üretimi süreçleri tek tek ele alınarak, Elazığ Şeker Fabrikası verileriyle enerji ve kullanılabilir enerji çözümlemesi yapılmıştır. Pancardan şeker üreten fabrikaların genel akım şemalarına göre. üretim süreçleri: Şerbet üretimi, şerbetin arıtılması, şerbetin koyulaştırılması, şekerin kristalleştirilmesi ve eldesi, küspe kurutma ve buhar üretimi olarak alınmıştır. Bu çalışmada geliştirilen bilgisayar programında süreçlerin birinci ve ikinci kanun verimleri hesaplanarak karşılaştırılmıştır. Süreçlerin enerji ve kullanılabilir enerji çözümlemeleri modüler biçimde yapıldığından, istenildiğinde bası kontrol hacimleri hesap dışı bırakılabilir. Bundan başka daha dar kapsamlı kontrol hacimleri seçilerek kurulu fabrikalarda yenileştirmeler, kurulacak fabrikalarda projelendirme yönlendirilebilir. Yapılan enerji ve kullanılabilir enerji çözümlemeleri sonucunda şerbet üretimi ve şerbet arıtma süreçlerindeki bası ısıtıcılarda daha. düşük sıcaklıkta ısıtıcı buhar kullanımının mümkün olabileceği saptanmıştır. Şerbetin koyulaştırılması sürecinde ise tüketiciler için çekilen buhar miktarlarının azaltılması ile koyulaştırma için gereken çürük buhar miktarı düşecektir. Küspe kurutma sürecinde tersinmezlikler yanma odasının modernize edilme si, karışım odasının buhar üretici olarak kullanılması ile azaltılabilir. Buhar üretimindeki kullanılabilir enerji kayıpları kazan veriminin düşüklüğünden, yanmanın tam olmamasından kaynaklanmaktadır. Şeker üretiminde nominal çalışma noktasından sapmalarda ortaya çıkan buhar tüketimi fakları veya buhar üreticisinde ısıtıcı yüzeyler üzerinde biriken katmanlar nedeniyle yüksek sıcaklıkta a- tık gazların oluşması, kayıplara yol açar. Bu atık gazların küspe kurutmada kullanılması kayıpları, dolayısıyla da yakıt tüketimini azaltır. Yapılan enerji ve kullanılabilir enerji çözümlemeleri sonucunda enerji çözümlemelerinin, tersinmezlikleri gözönüne almaması nedeniyle, kullanılabilir enerji çözümlemeleriyle tamamlanmadığı zaman yetersiz kaldığı ve hatta yanıltıcı sonuçlar verdiği saptanmıştır. The object of this study was to identify the pro cesses in the production of beet sugar and apply to these processes the first and second laws of therm.0dynam.ic3 to determine the losses and irreversibilities. The sugar industry is dependent on agriculture and for this reason is a preferred industrial sector in deve loping countries because both agricutural and industrial development can be achieved. Furthermore the by-products of sugar factories can be utilised in other agricultural area.3 In parallel with increasing population and national income, it has been necessary to modernise the existing stigar factories and to boild new factories in light of the experience gained. Decrease in the supply of fossil fuels ami environmental considerations have forced industry to take measures for energy recovery and to utilise energy sources at lower temperatures. For this reason/ the mass and energy balances of sugar production processes based on practical estimations of dry substances, sacca rose ratio and purity is inadequate. In the first section of this thesis the first and second laws of thermodynamics and the availability equation are stated and their application to closed and open systems are shown by examples. Information on the properties of the sugar beet and processes related to extraction of sugar from beets is given next. Later the processes are considered separately and the energy and available energy analysis are made for each process. The thermodynamic properties of the substances taking place in this processes are expressed in equation form by using the tables ami graphs available in the literature, BALOH [1], IRVINE and LILEY [2]. The development of sugar industry in Turkey and its present state are also accounted. The capacities and production rates of the sugar factories are given m a table. In the third section of the thesis the operating parameters of the Elazığ Sugar Factory are used as input., to do a first and second law analysis of the production processes in a sugar factory. The processes of sugar production from beets is shown in the diagram below. vi ıWater Beets J L_ JUICE PRODUCTION Water. C02 - Lime - Pressed Molasses.JUICE PURIFICATION Fuel-*- DRYING OF Air - MOLASSES Dried `*` Molasses Sludge.JUICE CONCENTRATION Make-up water Fuel Air _1 L REFINERY Waste water ELECTRICITY AND STEAM GENERATION i r Electrical Stack Energy Gases Molasses Sugar Block Diagram of Processes in a Sugar Factory Processing Beets In the Elasig Sugar Factory 48.79 k¥h (175643 kJ) heat energy and 3.02 kWh (10872 kJ) electrical energy was used to produce 14.83 kg sugar from 100 kg of beets during the 1990 campaign. 100 kg of beets has been used as a reference quantity and referred to as unit beet (BP) in this study. The daily sugar production capacity of the Elasig Sugar Factory is 1650 tons of beets. During the 1990 campaigne the daily production rate of the factory was 165 tons of crystal sugar and 26 tons of cube sugar. The daily average thermal energy used for this production was 628415 k¥h (2262.3 GJ) and the daily average electrical energy requirement was 38898 klh (140 GJ). A steam turbine cogeneration plant was used to supply the heat and electricity requirements. 18 m3/h of steam at 2.5 MPa pressure and 380°C temperature i3 produced in three steam boilers. Electricity is produced by three 1.2 HW five stage turbines. The back pressure in the turbines is between 0.8 and 2.2 bars. The average power output of the turbines during the 1990 campaign was 2. 3 MW. VI 11For comparison., a sugar factory in German uses 11.811 k¥h (42520 kJ) of thermal energy and 6.668 kWh (24005 kJ) of electrical energy to produce 14.46 kg sugar per 100 Kg beets, AUERSWALD [3J. The capacity of this factory is 4440 tons beet per day and the daily thermal and electrical energies used in' this factory are 524000 kWh (1886.4 GJ) and 296000 kWh (1066 GJ) respectively. The reason for the high electric energy usage in this factory compared to Elazığ Sugar Factory is that heat pumps are used to regain heat at low temperatures. The main reason for the low thermal energy usage in this factory is that electiricity used is largely bought from the national grid and therefore, the amount of steam required to generate electricity for the turbines is lowered. In the Elazığ Sugar Factory a total of 51.81 kWh (186516 kJ) of thermal and electrical energy was used to process 100 kg of beets. The reasons for high energy usage are the low capacity of the factory, the old technology U3ed and design faults such as the unnecessarily long steam supply lines. With today' s technology it is estimated that the total energy requirement to process 100 kg of beets can be lowered` to 40.56 k¥h (146016 kJ), BÂLOH [4]. This represents a energy saving of 21.7 % over the present ene rgy r e qui r emen t s. With the computer program developed in this study it is possible to perform energy and exergy (available energy) analysis for sugar factories. The processes (open systems) and the inlet and outlet streams associated with these processes are identified first and then the entalpies and available energies of these streams are calculated. Application of the first law of thermodynamics and the availability equation to these processes yield the heat transfer, irreversibilities., first and second law efficiencies of these processes. In the analysis of the Elazığ Sugar Factory, the general flow diagram for factories producing sugar from beets was taken as basis.. Ö2KAN [5]. This flow diagram given in section 3.2.2 show very little variations in different factories producing sugar from beets. The computer program that has prepared in this thesis is modular and therefore these variations can easily be incorporated into it. It is also possible, to chance the control volumes so that any portion of a process can be analysed in more detail. By using this computer program it is expected that a first and second law analysis can be made In the planning stage so that the construction and layout of te factory can be optimised. The first law efficiency of the processes concerned has been defined as the ratio of the total enthalpy of the masses leaving the control volume to the sum of the total enthalpy of the masses entering the control volume and the electric energy input to the control volume. This is a measure, of the heat transfer to the IXenvironment. If heat transfer to the environment can be lowered., the first law efficiency can be improved. Equation 2.7 expresses the first law efficiency. The second law efficiency of the processes concerned has been defined as the ratio of the availability of the masses leaving the control volume less the availability of heat transfer to the environment to the availability of the masses entering the control volume plus the electrical energy input. The second law efficiency is a measure of the irreversibilities in the process. Second law efficiency increase as the heat transfer to the environment and the internal irreversibilities in the process are lowered. Equation 2.27 expresses the second law efficiency. Results of the first and second law analysis of the processes in the Elazığ Sugar Factory are summarised below. In the juice, production process the first law efficiency is 95 % and second law efficiency 55 %. In this process third stage steam at 103°C is used as the heating medium. Since the temperature of the water or juice being heated is at least 21°C lower than this temperature, irreversibilities due to heat transfer at a finite temperature difference increase. The use of fourth stage steam at 90°C, by increasing the heating surfaces will lower the irreversibilities. In the jxiice purification process the first law efficiency was calculated as 90 % and the second law efficiency was calculated a3 70 %. In the limed raw juice heating section of this process fourth stage steam at 90°C, instead of third stage steam at 103°C should be used to reduce irreversibilities. For the same reason the third stage steam at 103°C can be used instead of the second stage steam at 115°C in the carbonation heating section. In the juice concentration stage, the first law efficiency was calculated as 96 % and the second law efficiency was calculated as 91 %. The reason for higher efficiencies is that concentration is done in stages and that the vapour seperated from juice in one stage is used to heat juice in the next stage. The frequent cleaning of the heaters to prevent the formation of deposits will decrease the quantity of stream required in the heaters. This will help increase the first and second law efficiencies. In the refining process., the first law efficiency was calculated as 78 % and the second law efficiency was cal culated as 67 %. The reason for higher irreversibilities in the crystalliser is that the vapour separated from the juice i 3 condenced at a high temperature difference inthe condensers and that the low temperature condensates cannot be used for further heating. Furthermore high temperature dif frence s are needed in the vacuum pans. In the drying of molasses, the first law efficiency was calculated as 71 % and the second law efficiency as 20 %. The fuel expenditure for drying molasses in sugar factories that process beets is quite high. In the E la sığ Sugar Factory 3.84 küflı (13424 kJ) of thermal e- nergy is used to obtain 1 kg of dry molasses and 3.29 k¥h (11844 kJ) of thermal energy is used to obtain 1 kg of sugar. The energy balance in the process of drying molasses show that a larger portion of the fuel energy is passed to the evaporating water. The second law analysis show that the ava i lability of the resulting vapours is low due to low temperatures. As a result the loss of available energy in drying molasses is high. The irreversibilities originate from incomplete combustion., mixing of cool air with the drying air and the high temperature difference between the molasses and the drying air stream. The irreversibilities in the mixing chamber can be prevented by using the combustion chamber as a boiler for steam generation. Thus the combustion gases need not be mixed with cool air to attain the proper temperature for drying but are colled to the required tempetarure during the production of steam. The production rate of dry molasses of the factory is 1700 tons per year. The rest of the molasses is given to the livestock growers to be used as animal feed. In the steam generation process the first law efficiency was calculated as 79 H and the second law efficiency was calculated as 35 %. In the process of steam generation 4. 37 kg of fuel oil with a lower heating value `of 40193 kJ/kg` per 100 kg of beets. The irreversibilities in steam generation result from the combustion process itself. The deposition of layers on steam generating surfaces and the high temperature stack gase3 also in drying molasses decrease the losses and hence the fuel consumption. The results of the first and second law analysis made showed that the second law efficiencies could be much lower than the first law efficiencies and that conclusions based on first law analysis alone could be misleading. 150