IMPROVEMENT OF ENERGY EFFICIENCY IN SUGAR PROCESSING LANKA SUGAR INDUSTRIES LIMITED, PELWATTE, SRI LANKA

Transcription

1 IMPROVEMENT OF ENERGY EFFICIENCY IN SUGAR PROCESSING AT LANKA SUGAR INDUSTRIES LIMITED, PELWATTE, SRI LANKA By N.M.Lasantha KTH Supervisor Professor Andrew Martin Local Supervisor Mr.Ruchira Abeyweera 2018 Master of Science in Engineering Thesis Department of Energy Technology Royal Institute of Technology, KTH Stockholm, Sweden 0

2 IMPROVEMENT OF ENERGY EFFICIENCY IN SUGAR PROCESSING AT LANKA SUGAR INDUSTRIES LIMITED, PELWATTE, SRI LANKA A dissertation submitted to the Department of Energy Technology, Royal Institute of Technology, Sweden for the partial fulfillment of the requirement for the Degree of Master of Science in Engineering By N.M.Lasantha KTH Registration No: A518 KTH Supervisor Professor Andrew Martin Local Supervisor Mr.Ruchira Abeyweera 1

3 Declaration The work submitted in this thesis is the result of my own investigation, except where otherwise stated. It has not been accepted for any other degree and not being concurrently submitted for any other degree. N.M.Lasantha Date: 2018 We/I endorse declaration by the candidate. Professor Andrew Martin (KTH) Mr.Ruchira Abeyweera 2

4 Master of Science Thesis EGI 2009: IMPROVEMENT OF ENERGY EFFICIENCY IN SUGAR PROCESSING AT LANKA SUGAR INDUSTRIES LIMITED, PELWATTE, SRI LANKA N.M Lasantha Approved Examiner Professor Andrew Martin Commissioner Supervisor Mr. Ruchira Abeyweera Contact person 3

5 Abstract One of the drawbacks of the process house in a sugar factory is the potential for exhaust steam pressure drop during operation, leading to temporary loss of production and concomitant energy waste. Identified exhaust steam demand for existing exhaust steam distribution system is larger than exhaust steam generation, therefore there is potential to minimize or avoid such issues. This investigation examines process house steam systems via in-field measurements at a sugar factory located in Sri Lanka. Five options for modifying the exhaust steam distribution network were considered, with the most favorable one implemented in year Considering operational data this year as compared with the previous year, measures to improve steam utilization yielded a 3.5% increase in crushing rate and nearly eliminated mill stoppage due to juice accumulation all while maintaining steam parameters at nominal levels. FÖRBÄTTRING AV ENERGIANVÄNDING INOM SOCKERPRODUKTIONEN VID LANKA SUGAR INDUSTRIES LIMITED, PELWATTE, SRI LANKA Sammanfattning En begränsning hos processerna inom en sockerfabrik gäller tryckförluster i utsläppsångan, som kan leda till tillfälliga produktionsförluster och energislöseri. Den identifierade efterfrågan av utsläppsångan gällande det befintliga ångdistributionssystemet är högre än den tillförd ångan, därför finns det potential för att minimera eller undvika sådana situationer. Denna undersökning analyserar processångan genom mätningar hos en fabrik i Sri Lanka. Fem alternativ för modifiering av ångsystemet undersöktes och den lämpligast genomfördes under Jämfört med presentanda från året innan ledde dessa åtgärder till en ökning av 3,5% i produktivitet med bibehållen ångdata. 4

6 ACKNOWLEDGMENTS I would like to express my thanks to Professor Torsten H. Fransson, KTH Department of Energy Technology, for expanding the Distance Sustainable Energy Engineering Program to Sri Lanka and making it functional to the world of sustainable energy engineering. I would like to extend my thank to professor N. R. Athayake, previous dean of the Open University of Sri Lanka for taking opportunity to give the facility to conduct the Distance Sustainable Energy Engineering Program in Sri Lanka. I would like to thank to coordinator of Sustainable Energy Engineering Program at OUSL, Mr. Ruchira Abeyweera for giving more help us to continue this program, and supervision and the guidelines given in completing the project as my project supervisor at OUSL. I would like thank for all staff members who is given support for us to continuous this program. I would like to thank for Dr. N S Senanayake for him sense understanding, supervision and the guidelines given in completing the project. I would like to thank for Mr Eng. WR. Demel for giving support to completing the project. I would like to thank for my project supervisor at KTH, Dr. Sad Jarall for him sense understanding, supervision and guidelines given in completing the project. I would like to thank Professor Andrew Martin for kindly accepted to grade my thesis and give instruction to complete thesis. I have to mention about my wife, elder daughter, younger daughter, mother and father for given support to continuous this program. I would like to thank to previous chief operation engineer, present factory manager Mr. Eng. E. P. Suranga, who granted the opportunity to take leaves for examination and presentation for this program. I would like to thank to Former factory manager Palitha Prematunga and present engineering manager Mr. Eng. Sisiwath also. I should mention about my working place Lanka Sugar Company Limited, Pelwatte. I got more experience related to energy engineering field as well as Mechanical engineering. I will never forget this great opportunity in my life time. I should thank to my friend who is Mr. Eng. Ghanapriya Mahendra for giving massage about Sustainable energy engineering program. Otherwise I could not continuous this program. I should thank for my friend Mr. Eng. Hemakumara and Mr. Eng. Wasantha for cover up my duties during examination period and presentation period. Finally, I would like to thank many individual friends and colleagues who have not been mentioned here personally in making this education success. I could not have to complete successfully without their support. 5

7 NOMENCLATURE List of Abbreviations A B J B S B V B XI C C P D P E E T J KTH L M P 1 P 2 P 3 P 4 P 5 Q Q P R S S P S W T Heating surface area Brix of juice Brix of syrup Average Brix of juice leaving the vessel Outlet juice brix of each body Specific evaporation coefficient for evaporator real temperature drop Specific evaporation coefficient True temperature difference Weight of juice to be evaporated per cent cane Total quantity of water evaporated per metric ton of Cane quintuple effect Weight of juice obtain per cent cane Kungliga Teknika Hogskoolan (Royal Institute of Technology) Latent heat of steam Quantity of juice to be heated Vapour bleeding from 1 st vessel Vapour bleeding from 2 nd vessel Vapour bleeding from 3 rd vessel Vapour bleeding from 4 th vessel Vapour bleeding from 5 th vessel The quantity of direct or enhance steam consumed by the evaporator Quantity of vapour generation The quantity of direct or enhance steam usage for pans and juice heaters Specific heat of juice Heating surface area Weight of thick syrup per cent cane Log means temperature difference 6

8 T S U V X Saturation temperature of heating stream Heat transfer coefficient Velocity of juice in tube The quantity of vapour passing from the last effect to the condenser 7

9 Units 0 C Degree Celsius Ton K s H Metric tons Kelvin Meters Second Hours % Percentage kj/kg Kilojoules per kilogram kj/kgk Kilojoules per kilogram Kelvin Ton/h Tones per hour Kg/m 2 h Kilogram per square meter hour 8

10 TABLE OF CONTENTS 1 INTRODUCTION OBJECTIVES METHODOLOGY LITERATURE REVIEW Sugar production and cogeneration in Southeast Asia Steam economy and cogeneration in cane sugar factories Conventional raw sugar factory base on the Monymusk factory in Jamica... (case 1) Conventional sugar factory with condensate heat recovery for juice heating (case 2) Quadruple effect falling film evaporator with condensate heat recovery for juice heating (case 3) Quadruple effect falling film evaporator with condensate juice heating and continuous vacuum pans (case 4) Improving energy efficiency in sugar mills Making use of lower grade vapours for heating purposes Improving Boiler Efficiency Replacement of steam driven mill drives with electric DC motor SUGAR MANUFACTURING PROCESS IN LANKA SUGAR INDUSRTIES LIMITED, PELWATTE Description of sugar factory under study Cane Preparation Extraction Clarification of juice Evaporation Crystallization-Pan Boiling Centrifuging Sugar drying and storage ENERGY CONSERVATION AND UTILIZATION Estimation of total exhaust steam generation Estimation of pressure drop in evaporators and exhaust steam requirement for evaporators and heater Estimation of quantity of vapor or exhaust steam requirement for juice heating Estimation of individual evaporation in each vessel

11 5.5 Estimation of juice brix from each vessel Estimate of the specific evaporation rate according to Dessin formula in each vessel Estimate of heating surface area of individual evaporator vessel Estimate of Total exhaust steam requirement Results from EES Option No 01 of the exhaust steam distribution network Option no 02 of the exhaust steam distribution network Option No 03 of the exhaust steam distribution Option No 03 of the exhaust steam distribution Option No 05 of the exhaust steam distribution Summary of all the option of exhaust steam distribution network Summary of exhaust steam requirement and generation Calculation of exhaust steam reduction SUMMARY OF OPERATIONAL DATA Summary of operational data on year Summary of operational data on year Analyzing of operational data Average value of operational data in year 2012 and year DISCUSSION AND CONCLUSION REFERENCES APPENDIX EES Calculation

12 LIST OF FIGURES Figure 3-1 Sugar manufacturing process with co-generation plant [12] Figure 3-2 Conventional factory based on the Monymusk factory in Jamica [5] Figure 3-3 Conventional Factory with condensate heat recovery [5] Figure 3-4 Quadruple effect falling film evaporators with condensate heat recovery for juice heating [5] Figure 3-5 Quadruple effect falling film evaporators with condensate juice heating and continuous pans [5] Figure 3-6 Evaporators [4] Figure 4-1 Sugar manufacturing process flow chart Figure 4-2 Evaporators Figure 4-3 Crystallization-Pan Boiling Figure 4-4 Boiling Scheme Figure 5-1 Process house steam balance in sugar factory Figure 8-1 Average crushing rate in year 2012 and Figure 8-2 Average steam flow rate in year 2012 and Figure 8-3 Average boiler pressure in year 2012 and Figure 8-4 Average exhaust steam pressure in year 2012 and Figure 8-5 Average mill stoppage due to juice accumulation in year 2012 and

13 LIST OF TABLES Table Present steam distribution in Lanka Sugar Company (pvt) Ltd Table Distribution of pressure drop in each vessel as follows Table Specific heat capacity of juice Table Estimation of total exhaust steam generation Table Option no Table Option no Table Option no Table Option no Table Option no Table Summary of all the option Table Summary of the exhaust steam requirement and generation Table Summary of Operational data on 10 days July Table Summary of Operational data on 10 days July Table Average value of operation data

14 1 INTRODUCTION Sugar cane is a type of grass with large stems. It is similar to bamboo cane. It is harvested yearly when it has matured with height around 3m.It has a 70-75% of moisture content,15.4% of total sugar content and 12.2 % of fiber content [21]. Around 70% of the global sugar supply is derived from sugar cane. The remaining 30% is derived from sugar beet. Sugar cane is grown in more than 100 countries worldwide. Considering sugar producers in 2014, Brazil is the world s leading sugar producer with 25% of global sugar production and 50% of world sugar exports. India is the second leading sugar producer and leading sugar consumer. The countries among EU-28 are the third leading sugar producer and second leading consumer. Thailand is third largest sugar producer. China is fourth largest sugar producer and third largest sugar consumer [21]. The sugar requirement of Sri Lanka is estimated around Ton per annum and 94% of the sugar requirement is imported. Around Rs.55 million in foreign exchange is expended for imported sugar. The sugar requirement is expected to increase over the next 10 to 15 years with increase in population and income. The demand may be approached 1 million Ton per annum of sugar by year 2020 [22]. There are three sugar factories available in Sri Lankan. Lanka Sugar Company Ltd, Pelwatte; Lanka Sugar Company Ltd Sevanagala are controlled by The Sri Lanka Government. Gal-Oya Plantations (Pvt) Limited has 51% of share for Sri Lankan Government and 49% of share for Brown and Company PLC and Lanka ORIX Leasing Company PLC [23]. Lanka Sugar Company Ltd, Pelwatte is located in Buttala in Sri Lanka. The main product of the company is commercial sugar, and alcohol is produced as a by-product. Daily sugar cane consumption is around 3600 metric tons.during the financial year 2012 the company crushed metric tons of sugar cane. The total sugar production is around metric tons per year. Approximately 8.5 tons of sugar was produced from 100tons of sugar cane and this value is called rendement in the sugar industry. Sugar plant energy demand for electrical, mechanical and process heat is met by a steam power plant. This plant has two boilers, two power turbines and five turbines solely used to supply mechanical power for running the machinery. Boiler pressure is 28bar and exhaust steam coming out of all the turbines is used to meet the process heat demand. It has been observed that exhaust steam pressure often drops from the normal operating pressure of 1.5bar due to high demand of process heat during production process. In this situation extra heat demand is supplied by direct feeding of super-heated steam to the process house. This in turn unfavorably affects the turbines leading to the stoppages of plant. In order to ensure continuous operation of the plant it is necessary either to prevent direct supply of super-heated steam extracted from the boiler or maintain steady process heat demand. In view of maintaining process heat demand at a low value, rearranging the steam distribution network within the process house is supposed to be a more practicable solution. Therefore, the main focus of this study was to analyze the steam distribution network of the process house, and to design and implement an alternative method of steam distribution network so that heat supplied to the process house is optimally utilized. 13

15 2 OBJECTIVES The main objectives of the study were as given below. 1. Analyze the process house steam balance in the sugar factory 2. Identify the causes that impact on the energy efficiency in the process house in sugar processing and establish corrective measures especially in the steam distribution network. 3. Implement corrective measures and evaluate the performance 14

16 2 METHODOLOGY In this study, the main consideration was to analyze the energy utilization in the process house in the sugar factory and to redesign the process heat supply chain for optimal use of available process heat. The following methodology was used to achieve the intended objectives of the study. Investigation of relevant literature related to the energy balancing of sugar factories together with related theoretical aspects in energy usage in sugar processing. Analysis of different arrangements of major processing steps where steam is used as process heat. Selection of the layout of steam flow that allows maximum usage of process heat available in the process steam supplied to the process house. Implement the most suitable option. Collection and analysis of operational data with the implementation of the proposed system. Based on data, estimation of exhaust steam savings. Evaluation of the feasibility of the new system for long term operation of the plant. 15

17 3 LITERATURE REVIEW 3.1 Sugar production and cogeneration in Southeast Asia In early days, most of the sugar factories were designed purposely with lower efficiency cogeneration systems in order to eliminate all the bagasse produced in the plant, as there was no possibility to sell excess electricity to the grid or sell the bagasse for other manufacturing process like paper, plywood etc. In the last 15 years, due to technological developments it has been possible to sell excess electricity to national grid [12]. The sugar mills use very low pressure cycle for cogeneration plant in Southeast Asia and they are inefficient. When considering some of the sugar mills every tone of sugar milling up to 40kWh electricity and up to 600kg of steam are consumed and 30kWh of electricity is produced. With cogeneration technologies up to kWh of electricity could be produced from one tone of sugar cane [12]. Figure 3-1 Sugar manufacturing process with co-generation plant [12] One of the sugar milling groups in Thailand started to employ the first modern sugar cogeneration plant with 67bar steam cycle in There are two 41MW identical Cogeneration Plants in the sugar mill. Now, high pressure steam power plant of up to 110bar has come into the industry [12]. Multi fuel boilers are used in new cogeneration plants due to unavailability of bagasse during off season in the sugar industries. Wood waste, rice husk, sugar cane trash, cassava wastes etc. are widely used along with the bagasse as fuels for boilers. The most common technologies such as traveling gate system, vibrating gate systems are also used [12]. 16

18 Table 3-1 shows the variation of steam production, power production and bagasse requirement with steam pressure and steam temperature in each bagasse fired steam power plant. Power generation per Ton of bagasse was increasing with increase working pressure and temperature in each steam cycle. Bagasse requirement per one megawatt hour power production was decreased with increasing working pressure and temperature of steam cycle [12]. Table 3.1 Power generation, steam production and bagasse requirement with boiler pressure and variation Power generation and steam production from bagasse Steam Sugar Mill Pressure (bar) Temperature ( C) Steam production(ton) for Ton of bagasse No Power generation (KW) for Ton of bagasse Bagasse required (Ton/MWh) 17

19 3.2 Steam economy and cogeneration in cane sugar factories According to Ogden et al. [5] most cane sugar factories had been designed with the following conditions in early days: Sugar as the primary product Energy self sufficient Medium pressure (15 bar 20 bar) Small steam turbines Generated all the electricity and steam requirement for the factory Boiler has been designed to somewhat inefficient. Because prevent the excess bagasse accumulate problem and disposal problem. When growing number of sugar factories are manufacturing one or more by products from sugar cane like molasses and electricity, electricity is supplied to main grid from excess power from sugar factory. Alcohol is generated by using molasses [5]. A certain amount of energy was required to manufacture by product of sugar cane. Therefore energy efficiency of sugar factory can become much more important. Some implication of energy efficiency improvements were discussed for raw sugar factory with cogeneration system. Two type of bagasse fired cogeneration system were discussed [5]. High pressure condensing extraction steam turbine system Steam injected gas turbine run on gasified bagasse. The following sections provide an overview of four major sugar mill configurations found worldwide, presented in order of increasing heat recovery complexity. 18

20 3.2.1 CONVENTIONAL RAW SUGAR FACTORY BASE ON THE MONYMUSK FACTORY IN JAMICA (CASE 1) Figure 3-2 shows the conventional raw sugar factory base on the Monymusk factory in Jamica. It is produced 157 Ton of mixed juiced per hour in the sugar mill and mixed juice temperature is 34 o C. It is produced 13.7 bar pressure and 250 o C temperature superheated steam in the boiler section. Turbo alternator is consumed 34.3 Ton/h and mill turbine is consumed 36.5 Ton/h of superheated steam. There is 4.2 Ton/h of steam loss from steam turbines and 1.9 Ton/h of steam loss from lines [5]. In this factory, exhaust steam is consumed 0.7 Ton/h for deaerator, 56.4 Ton/h for evaporator no 01 and 17 Ton/h for vacuum pan. Evaporator no 1 is generated 32.5 Ton/h of first vapour and it is consumed 27.2Ton/h for evaporator no 02, 5.3 Ton/h for heater no 03, 17 Ton/h for vacuum pan and 4 Ton/h for clear juice heater. Evaporator no 02 is generated 28.6Ton/h second vapour and it is consumed 6 Ton/h for juice heater no 02 and 22.6 Ton/h for evaporator no 03. Evaporator no 03 is generated 24.1 Ton/h of third vapour and it is consumed 8.8 Ton/h for heater no1 and 15.3 Ton/h for evaporator no 04.Finaly forth vapour is condensed by condenser and make syrup at brix 62 [5]. Figure 3-2 Conventional factory based on the Monymusk factory in Jamica [5] 19

21 3.2.2 CONVENTIONAL SUGAR FACTORY WITH CONDENSATE HEAT RECOVERY FOR JUICE HEATING (CASE 2) Process steam requirement is calculated by using heat recovery from the condensate. Reduction of the overall heat demand is 405 kg/tc to 350 kg/tc. The heating surface area requirement is 979m 2 for plate and gasket heat exchanger. Entrap electricity pretention is 600kw due to condensate heat recovery. The low pressure steam demand can be reduced by 4 to 12% [5]. Figure 3-3 shows that conventional sugar factory with condensate heat recovery for juice heating. In the sugar mill, 157 Ton/h of mixed juiced is produced with 34 o C temperature. In the boiler, 62.8 Ton/h of super-heated steam is produced with 13.7 bar pressure and 250 o C temperature. Turbo alternator, 34.3 Ton/h is consumed and mill turbine 36.5 Ton/h is consumed superheated steam. There is 4.2 Ton/h of steam loss from steam turbines and 1.9 Ton/h of steam loss from pipe lines [5]. In this factory, exhaust steam is used 0.7 Ton/h for deaerator, 56.4 Ton/h for evaporator no 01 and 17 Ton/h for vacuum pan. First vapour 45.9Ton/h is generated in evaporator no 1 and it is used 25Ton/h for evaporator no 02, 17 Ton/h for vacuum pan and 4 Ton/h for clear juice heater. Second vapour 24.3 Ton/h is is generated by evaporator no 02 and it is used for evaporator no 03. Third vapour 24.1 Ton/h is generated by evaporator no 03 and it is used for evaporator no 04. Fourth vapour is generated by evaporator no 04.Finaly forth vapour is condensed by condenser and deliver syrup at brix 62 from evaporator no 04. There is a possibility to heat mixed juice by using first vapour. Mixed juice temperature is increased 34 o C to 94 o C by absorbing heat from total condensate of evaporators and vacuum pan and condensate temperature is decreased 100 o C to 40 o C [5]. Figure 3-3 Conventional Factory with condensate heat recovery [5] 20

22 3.2.3 QUADRUPLE EFFECT FALLING FILM EVAPORATOR WITH CONDENSATE HEAT RECOVERY FOR JUICE HEATING (CASE 3) Considering this case, Quadrupled effect falling film evaporator is installed. The juice heating is done by condensate. Reduction of medium pressure steam requirement is 313kg/Ton of sugar cane, 23% of steam can be saved by this arrangement. Saving amount of steam is large due to under pressure of the 1 st two effects. Vapor is bled from 2 nd effect rather than 1 st effect. Vapor blending is not required for juice heaters. The area of evaporator and juice heater are 4800m 2 and 970 m 2. In this arrangement pay bare time is 7.2 years [5]. Figure 3-4 shows that quadruple effect falling film evaporator with condensate heat recovery for juice heating. In the sugar mill, 157 Ton/h of mixed juiced is produced with 34 o C temperature. In the boiler, 54.8 Ton/h of super-heated steam is produced with 13.7 bar pressure and 250 o C temperature. Usage of superheated steam for turbo alternator is 14.7 Ton/h and mill turbine is 40.7 Ton/h. There is 3.3 Ton/h of steam loss from steam turbines and 1.6 Ton/h of steam loss from pipe lines [5]. In this factory, exhaust steam is used 0.7 Ton/h for deaerator, 50.7 Ton/h for evaporator no 01.First vapour 40.1Ton/h is generated in evaporator no 1 and it is used for evaporator no 02. Second vapour 41.3 Ton/h is generated by evaporator no 02 and 15.4Ton/h is used for evaporator no 03 and 24 Ton/h is used for vacuum pan and 4 Ton/h is used for clear juice heater.third vapour 17.0 Ton/h is generated by evaporator no 03 and it is used for evaporator no 04. Fourth vapour 20.4Ton/h is generated by evaporator no 04.Finaly forth vapour is condensed by condenser and deliver syrup from evaporator no 04. Mixed juice temperature is increased 34 o C to 102 o C by absorbing heat from total condensate of evaporators and vacuum pan and condensate temperature is decreased 116 o C to 40 o C [5]. 21

23 Figure 3-4 Quadruple effect falling film evaporators with condensate heat recovery for juice heating [5] QUADRUPLE EFFECT FALLING FILM EVAPORATOR WITH CONDENSATE JUICE HEATING AND CONTINUOUS VACUUM PANS (CASE 4) In this arrangement quadruple effect falling film evaporator, condensate juice heater and continuous vacuum pan could be installed. The total area is 4800m 2 in the evaporator and total area of primary mixed juice condensate heater is 378m2. Total reduction of steam requirement is 258kg/Ton of sugar cane, and 36% saving. The export electricity is increased 12% as compared with case 01. Payback time of this design is mention as around 5.5 years [5]. Figure 3-5 shows that quadruple effect falling film evaporator with condensate juice heating and continuous pans. In the sugar mill, 157 Ton/h of mixed juiced is produced with 34 o C temperature. In the boiler, 45.1 Ton/h of super-heated steam is produced with 13.7 bar pressure and 250 o C temperature. Usage of superheated steam for turbo alternator is 5.1 Ton/h and mill turbine is 40.7 Ton/h. There is 2.6 Ton/h of steam loss from steam turbines and 1.2 Ton/h of steam loss from pipe lines [5]. 22

24 In this factory, exhaust steam is used 0.7 Ton/h for deaerator, 40.5 Ton/h for evaporator no 01.First vapour 33.1Ton/h is generated in evaporator no 1 and it is used for evaporator no 02. Second vapour 34.3 Ton/h is generated by evaporator no 02 and it is used for evaporator no 03.Therd vapour 36.2 Ton/h is generated by evaporator no 03 and 17 Ton/h is used for vacuum pan, 4 Ton/h is used for clear juice heater, 3.7 Ton/h is used for mixed juice heaters and 10.7 Ton/h is used for evaporator no 4. Fourth vapour 10.3 Ton/h is generated by evaporator no 04 and it is used for evaporator no5.finally fifth vapour is condensed by evaporator no 5 and it is condensed in condenser and deliver syrup from evaporator no 05. Mixed juice temperature is increased 34 o C to 90 o C by absorbing heat from total condensate of evaporators and vacuum pan and condensate temperature is decreased 117 o C to 40 o C [5]. Figure 3-5 Quadruple effect falling film evaporators with condensate juice heating and continuous pans [5] 23

25 3.3 Improving energy efficiency in sugar mills The traditional factory is designed to balance fuel, minimize the supplementary coal and avoid generating excess bagasse. Active and efficient in-house energy management is a good way of improving energy efficiency in very low cost. Following are the some of the practices help to achieve target. Establishment of in-house energy management committees or group Data collection for processes Improved maintain ace practices Review of operational efficiency The following are some of process component improve energy efficiency [4] MAKING USE OF LOWER GRADE VAPOURS FOR HEATING PURPOSES Steam consumption within the mill processes can be reduced by using low grade vapour for heating purposes. The number of effect can be increased by increasing number of evaporator effect. The greater will be the steam economy. Quintuple effect evaporator is used in the present day mills [4]. Juice heaters and vacuum pans are heated by vapour bleeding from evaporators. Efforts have been taken to reduce steam consumption in the modem sugar mills. Steam or vapour demand is reduced by increasing the energy efficiency of the process. The efficiency can be improved by doing optimal design of the evaporator configuration. Maximum efficiency can be achieved zero vapour in the condenser and this achievement can be done by over concentrating the juice by heating to low grade vapour [4]. Figure 3-6 Evaporators [4] 24

26 The temperature and pressure should be increased to improve the steam condition. The more energy can be supplied to grid by improving the efficiency of the system. The tons of steam per tons of bagasse can be increased by increasing steam temperature more power can be produced for export [4]. Steam condition is needed to increased temperature C and pressure 4500kPa which are still maximum design condition in South Africa sugar factory. Mauritius and other countries are already in used high pressure and temperature in sugar plant. The high boiler pressure and temperature increased the efficiency of steam cycle. The different operating condition indicating that as the amount of steam produced per ton of bagasse increases and the amount of electrical power generated increases per ton of bagasse [4]. The crystallization pan is one of the most important areas to consider steam consumption in the sugar mills. More electricity can be produced to export by using low grad steam to operate crystallizer pans. Two processes of milling and diffusion can be used to juice extraction. These processes will be defending on various consideration of the particular factory. But experience shows that diffusion is significantly better. High pressure steam is required for mill extraction to full fill the power requirement for mill drivers. Advantages of diffusion extraction are process steam efficient; low maintain ace costs and mechanical reliability. The energy efficiency is improved by mud recycle, because evaporation load is reduced and heat losses are eliminated from mud filter [4]. After introduce mud recycle and diffuser make additional steam available and less exhaust steam required for sugar process.continuous pans are characterized by constant boiling point and constant heating surface to volume ratio. But in the batch pan boiling temperature increases with rise in material level in the pan. As well as ratio of the heating surface and volume reduces the material level rises in the batch pans. The continuous fan is more economical because pan heated by low grad vapours.automation is easy in continuous fan. And boiling tine can be reduced by automation [4] IMPROVING BOILER EFFICIENCY The boiler efficiency can be increased by efficient heat recovery and excess air control in the boiler. Boiler efficiency can be increased by adding heat recovery equipment like economizer and more surface area in the air heater. Every 10 0 C of final gas temperature is reduced 1% of steam per tons of bagasse in the boiler. Due to modern improvement of the boiler achieved complete combustion with 27% excess air,4% oxygen level maintain in the flue gas.85% of efficiency in low calorific value on bagasse of 50% moisture. The net calorific value of bagasse depends on the moisture content of bagasse. The net calorific value of bagasse is 7670 kj/kg at around 48% moisture [4]. 25

27 3.3.3 REPLACEMENT OF STEAM DRIVEN MILL DRIVES WITH ELECTRIC DC MOTOR Conventional sugar factory use single stage impulse type steam turbines to drive mills. These steam turbines efficiency around 25-30%.It is very low compare with modern multistage power turbine. These turbines are replaced by high efficient electrical DC motor. Power turbines are replaced by 65-70% efficient multistage steam turbine to generate electrical power. Additional electricity is supplied to main grid and earns extra revenue for sugar plant [4]. 26

28 4 SUGAR MANUFACTURING PROCESS IN LANKA SUGAR INDUSRTIES LIMITED, PELWATTE 4.1 Description of sugar factory under study Lanka Sugar Company (pvt) Ltd is located in Buttala in Monaragala District in the Uva province of Sri Lanka, about 225km from Colombo. It was incorporated on 1981 as Pelwatte Sugar Company Ltd. Raw material of sugar factory is sugar cane and the main product of the company is commercial sugar, and alcohol is produced as a by-product. Daily sugar cane consumption is around 3600 metric tons.during the financial year the company crushed metric tons of sugar cane. The total sugar production is around metric tons per year. Approximately 8.5 tons of sugar was produced from 100tons of sugar cane and this value is called rendement in the sugar industry In the sugar factory, two numbers of supper heated water tube boilers are available with 45Ton/h of design capacity per one boiler, 28bar of working pressure and 3800 C of working temperatures. Shredder is use to fiberized the sugar cane and it is run by a steam turbine and its mechanical output is 430kW. Sugar factory is self-sufficient with electricity; it is generated around 3.2MW electricity power by 1.6MW capacity two numbers of steam turbine. Sugar Mills are powered by 4 numbers of steam turbines. Four numbers of mills tandem use to extract juice from sugar cane. Capacity of each steam turbine is 430kW.Feed water pump is also powered by steam turbine during operation of the boilers. Exhaust steam is generated with 1.5bar pressure from the mill turbines, the shredder turbine, the feed water turbine and power turbine. This is not enough to full fill the heating requirement in the process house. Therefore supplementary low pressure steam required to generate from the superheated steam directly obtained from the boilers and mixed with hot water. Therefore, the heating system should be optimized to improve the sugar making process. Table Present steam distribution in Lanka Sugar Company (pvt) Ltd Description Distillery 8 Power Turbine 45 Mill Turbine 18 Shredder Turbine 4.5 Steam Losses(3% of total) 2.3 Total Steam generation from Boiler 77.8 Total exhaust steam generation 65.5 Steam amount(ton/h) Therefore, this project focused on the analysis of energy utilization of sugar plant and the performance of the co-generation system, and thereby improving the overall plant efficiency. 27

29 Flow diagram and outline of manufacturing process of the plant studied are given in Figure 4-1. The present system of sugar manufacture can be divided into following stages. 1. Cane preparation 2. Extraction 3. Clarification of juice 4. Evaporation 5. Crystallization-Pan Boiling 6. Centrifuge 7. Storage 28

30 Figure 4-1 Sugar manufacturing process flow chart 29

31 4.1.1 CANE PREPARATION There are two feeder tables and two unsolders available in the sugar feeding system in Lanka Sugar Industries Limited, Pelwtta, Sri Lanka. The sugar cane is loaded into both feeder tables by both unsolder and a ramp. Sugar cane in the cane carrier is very tangled. It should be cut, volume reduced and open up the sugar rich juice cells. This process is called preparation of the cane and is done by two sets of rotating knives and one set of rotating hammers. Preparation of sugar cane is critical for successful sugar extraction during the sugar milling EXTRACTION Sugar cane received at the factory is weighed and fed into auxiliary cane carrier by two unloading machines and two feeder tables. The cane is prepared for crushing during its travel to mill. The preparation device consists of a kicker in each feeder table, two cutters and a shredder. First cutter consists of 32 numbers of knives and second cutter consists of 64 numbers of knives. The both cutter knives rotate at 750rpm for cutting the crane traveling on the carrier. The cut cane is delivered to the shredder for final preparation before the mill. The quality of cane prepared has a very significant effect to the mill capacity and maximum extraction. The mill plant consists of three rollers which are hydraulically loaded and each of one has a guide roller. The prepared cane passes through these mills with repeated application of high pressure. The processed sugar cane is discharged from the last mill and sent to the boiler to be used as a fuel. Mixed juice is collected in sand extractor to separate sand and juice. This sand free juice is then pumped to the process house for sugar making process. Hot water is used in the mill to increase the extraction of sugar. Hot water is spread over the blanket of cane before the last mill. The diluted juice from this mill is spread on the blanket to the just out of 2 nd mill. Juice from 3 rd mill is spread on the blanket to the just out of 1 st mill. Hence, the hot water system gives a counter current process of milling to improve the extraction of sugar. Average amount of extraction is around 92%. The bagasse from last mill contains on average 34% fiber, 2.5% sugar and 50% moisture. The bagasse is discharged to boiler furnace for burning. Heat is utilized to generate superheated steam at 28 bar pressure and C temperatures. The super-heated steam is used for generation of electric power, drive mill turbine and shredder turbine. The exhaust steam from turbines is utilized for process heating and evaporation. 30

32 4.1.3 CLARIFICATION OF JUICE Juice extracted from 1 st mill is called 1 st expressed juice. And juice extracted from 2 nd mill is called 2 nd expressed juice. Both of juices are combined and go through the rotary screen filter to separate bagasse particles. This clear juice is collected in the sand extractor to separate sand. This clear juice is called mixed juice. The mixed juice is a sugar solution; it consists of 85 percent water, 12 percent sucrose and 3 percent non-sugar. Its ph range is from 4.7 to 5.7. In this ph, the juice is prone to microbial and chemical inversion. Therefore, the process should be done as soon as possible to prevent the loss of crystal liable sugar. Large amount of sucrose lost in final molasses due to low purity of juice. The impurities can be removed by clarification treatment. Lime is applied to neutralize the juice. Flocculants is added before the clarifier to settle down mud in the clarifier. Off flow juice from clarifier is called clear juice or clarifier juice. Mud is extracted from bottom of the trace in clarifier. Mud mixed with bagacillo and sends to vacuum filters to separate mud and juice. This separated juice send to mixed juice tank and filter mud is send to sugar cane field use as fertilizer EVAPORATION Figure 4-2 [9] is shown that evaporators system. The clear juice or clarifier juice contains water and sucrose together. The clear juice is about 100 percent on cane with 12 percent brix. The clear juice is pumped to the first evaporator through pre-heater. The function of the pre-heater is to raise the temperature of clear juice to boiling point temperature in first vessel. Figure 4-2 Evaporators 31

33 The evaporation carried out in quintuple effect evaporators with single vapor cells. The all sells are vertical tube evaporator based on Rillieux s principles. The exhaust steam is admitted in the calandria of the first vessel. Its latent heat is absorbed to the juice inside the vertical tube and gets condensed. The condensates are continuously removed and fresh exhaust steam intake continuously. The juice intake tube is already at boiling point, absorbs the latent heat from exhaust steam and evaporates water in the juice. The vapor thus liberated by the boiling juice passes through pipe into calandria of next vessel and juice is evaporated to the second time. The process is repeated in subsequent vessel generally. The vapor space of quintuple effect is connected to the multi jet condenser. The vapor are condensed mix with cold water. The non-condensable gasses are removed under vacuum the calandria to save condensers. The condensate of the first calandria is free from the sugar traces and has higher temperature. It is sent to the boiler feed water tank through the pump. And the condensate of the second calandria is also low in sugar traces. Therefore it is also send to the boiler feed water tank. But regularly check for sugar traces of the condensate by laboratory. The condensate 2 nd, 3rd, 4th and 5 th calandria are removed by pump through equalizing system. These calandria works under vacuum. The 3 rd, 4th 5 th condensates are used for mills and sugar making process. The high density syrup is being accumulated in the last vessel. It is evacuated under vacuum by a pump. And it is sent to the storage tank. 32

34 4.1.5 CRYSTALLIZATION-PAN BOILING Figure 4-3 [9] is shown that crystallization-pan boiling system. The syrup is subjected to boiling in the vacuum pans. Its concentration leads to the super saturation and formation of sugar crystals. A three stage boiling scheme is available in pan station and it is fully automated. Massecuite A, B, C are after boiling in the pans treated in the crystallizers. Figure 4-3 Crystallization-Pan Boiling CENTRIFUGING The curing of A, B, C Massecuite is done in centrifugal machine to separate molasses from sugar crystal. The curing of A Massecuite is done in vertical batch type machines with automated. The sugar in the basket is washed with hot water and steam. During this continuous operation, A Massecuite is separated into A sugar and A molasses. A sugar is called the commercial sugar. 33

35 Figure 4-4 Boiling Scheme 34

36 B Massecuite is cured in vertical continuous low grad centrifugal machine. B Massecuite is separated into B sugar and B molasses. C Massecuite is also cured in continuous centrifugal machines. C Massecuite is separated in to C sugar and C molasses. C molasses is called final molasses. C sugar is melt and used for A and B boiling. B sugar is melt and used for A boiling SUGAR DRYING AND STORAGE Sugar drying is done by continuous type horizontal drier. Its air is heated by exhaust steam. The sugar packed in 50kg a twill gunny bags. The sugar packing is done semi-automated machine. 35

37 5 ENERGY CONSERVATION AND UTILIZATION Lanka sugar company (pvt) ltd, Pelwatte represents one of the most important economic sectors in Sri Lanka and ethanol is produced for local markets. Thermal and electrical energy are produced by using sugar cane bagasse as a fuel in cogeneration plant for the own factory consumption. This sugar factory is self-sufficient in thermal, mechanical and electrical energy during the crushing season. The steam power plant is operating at 28bar pressure, C temperatures, 90Ton per hour with super-heated steam. In this section to be estimated exhaust steam generation in the sugar factory and exhaust steam requirement for each options. 36

38 Figure 5-1 Process house steam balance in sugar factory 37

39 5.1 Estimation of total exhaust steam generation ANALYSIS OF STEAM DISTRIBUTION IN EVAPORATORS AND HEATERS Firstly amount of water to be evaporated from the juice in evaporators are estimated as follows. E MJ B B 5 M 150,B 12 %,B 65 %,J 95 % E Ton h 5.2 Estimation of pressure drop in evaporators and exhaust steam requirement for evaporators and heater Exhaust pressure 2.5, Last body pressure 0.23 Therefore Total pressure drop bar Therefore pressure drop in each vessel bar bar Table Distribution of pressure drop in each vessel as follows Exhaust Vapor Description Steam 1 st 2 nd 3 rd 4 th 5 th Pressure Temperature Boiling point elevation Boiling Point Temperature in juice

40 The 1 st vapor is used for pan boiling and steam requirement for pan floor is say approximately 23 per cent cane. Therefore 1 st vapor requirement for pan, P P Tn Tn 34.5 h h 5.3 Estimation of quantity of vapor or exhaust steam requirement for juice heating Table Specific heat capacity of juice Temperature Brix % Purity % Specific Heat Capacity(kJ/kgK) Considering option no 04 3 rd body vapors are used for primary heating from 30C to 50C Q L 5.19 tonse/h 1 st body vapors are used for secondary heating from 50C to 75C Q st body vapors are used for final heating from 75C to 105C Q tonse/h 39

41 5.4 Estimation of individual evaporation in each vessel E Tn h,e Tn h,e Tn h,e Tn h,e Tn/h 5.5 Estimation of juice brix from each vessel Inlet juice Brix to 1 st body is B Outlet juice Brix of each body is BX BX 22.59, BX 28.20, BX 37.54, BX 47.59,BX Estimate of the specific evaporation rate according to Dessin formula in each vessel C kg m h,c kg m h,c kg m h,c kg m h,c 1.08 kg/m h 5.7 Estimate of heating surface area of individual evaporator vessel S 1328 m,s m,s m,s m,s m 5.8 Estimate of Total exhaust steam requirement Total exhaust steam requirement E Tn/h Total exhaust steam generation was equal to steam consumption of turbines and it can be calculated as follows Table Estimation of total exhaust steam generation Turbine Exhaust generation one(ton/h) steam per Number of turbine Power Turbine Mill Turbine Shredder Turbine Total exhaust steam generation Total Steam Generation per each type of turbine(ton)

42 Assume 3% loss of the exhaust steam from turbine station to process from drain traps and use for exhaust steam for sugar driers, centrifugal machines and heat massecuite lines. Total exhaust steam generation Ton/h Results from EES In following tables are shown that total exhaust steam requirement for each option and way of exhaust steam and vapour usage OPTION NO 01 OF THE EXHAUST STEAM DISTRIBUTION NETWORK In the option No 01, exhaust steam was used for the first evaporator calandria and final heater, and first vapor was used for primary and secondary heaters. Total exhaust steam requirement for option no 01 was 68.60Ton/h Table Option no 01 Vapour (Ton) Exhaust team Primary Heater Vapour from 1 st evaporator 2 nd vapour 3 rd vapour Secondary Heater Final Heater Pans 34.5 Evaporator Total

43 5.9.2 OPTION NO 02 OF THE EXHAUST STEAM DISTRIBUTION NETWORK In the option no 02, exhaust steam was used for the first evaporator calandria and final juice heater, first vapour used for pan boilers, secondary juice heaters, and primary juice heater. Total exhaust steam requirement for option no 02 was 67.50Ton/h Table Option no 02 1 st Exhaust steam(ton) 2 nd vapour(ton) 3 rd vapour (Ton) vapour(ton) Primary Heater Secondary Heater Final Heater Pans 34.5 Evaporator Total OPTION NO 03 OF THE EXHAUST STEAM DISTRIBUTION In the option no 03, exhaust steam used only for first evaporator calandria. First vapor used for secondary juice heater and final juice heater. Second vapor used for primary juice heater. Total exhaust steam requirement for this option was 65.8Ton/h. Table Option no 03 Exhaust steam(ton) 1st vapour(ton) 2nd vapour(ton) Primary Heater Secondary Heater Final Heater 8.07 Pans 34.5 Evaporator 65.8 Total rd (Ton) vapour 42

44 5.9.4 OPTION NO 03 OF THE EXHAUST STEAM DISTRIBUTION In the option no 03 exhaust steam used for evaporator calandria. First vapor used for pan boilers, final juice heater and secondary juice heater. Third vapor is used for primary juice heater. Total exhaust steam requirement for this option was 64.72Ton/h. Table Option no 04 Exhaust steam(ton) 1st 2nd vapour(ton) 3rd vapour vapour(ton) (Ton) Primary Heater 5.19 Secondary Heater Final Heater 8.07 Pans 34.5 Evaporator Total OPTION NO 05 OF THE EXHAUST STEAM DISTRIBUTION In the option no o5 exhaust steam used for first evaporator calandria. First vapor use for the pan boilers and final juice heater. Second vapor was used for secondary juice heater. Third vapor used for primary juice heater. Total exhaust steam consumption was 63,34Ton/h. Table Option no 05 Exhaust steam(ton) 1st vapour(ton) 2nd vapour(ton) 3rd vapour (Ton) Primary Heater 5.1 Secondary Heater Final Heater 8.07 Pans 34.5 Evaporator Total

45 5.9.6 SUMMARY OF ALL THE OPTION OF EXHAUST STEAM DISTRIBUTION NETWORK Following table included the summary of the exhaust steam requirement for each option of exhaust steam distribution network. Table Summary of all the option Option No 01 Exhaust Steam Secondary Heater Final Heater 02 Final Heater 03 Final Heater 04-1 st Vapor 2 nd vapor 3 rd vapor Primary Heater Primary Heater Secondary Heater Secondary Heater Secondary Heater Final Heater 05 - Final Heater Primary Heater Primary Heater Secondary Heater Primary Heater Exhaust steam Requirement(Ton/h) SUMMARY OF EXHAUST STEAM REQUIREMENT AND GENERATION In this table included required exhaust steam amount of each option of the exhaust steam distribution network and amount of exhaust steam generation. Table Summary of the exhaust steam requirement and generation Option Number Total Exhaust steam Required(Ton/h) Total Exhaust steam generate(ton/h)