Energy efficiency in sugar manufacturing process

Contents

1. Introduction
1.1 Historical development of cane sugar
1.2 Process description of sugar and ethanol production
1.3 Objective of the project
1.4 Methodology
1.5 Steam in sugar factories

2. Metahara sugar factory
2.1 History & location
2.2 Agricultural operations
2.3 Factory operations
2.4 marketing
2.5 Human resources
2.6 Sugar Manufacturing Processes in MSF
2.7 Main causes for the high amount of steam consumption and loses in the factory
2.7.1 Evaporator
2.7.2 Vacuum pans (Batch pans)
2.7.3 High moisture content of bagasse
2.7.4 Steam turbines
2.7.5 Problem of excess air in the boiler
2.7.6 Molasses

3. Possible solutions of the stated problems
3.1 Increase the number evaporator effects
3.1.1 Quintuple effect evaporator design
3.2 Adding first effect falling film evaporator
3.2.1 Design
3.3 Installation of Fully Automated Continuous
3.3.1 Design of the pan
3.4 Bagasse dryer
3.4.1 Advantages of bagasse drying
3.4.2 Bagasse Drying Methods
3.4.3 Preliminary design
3.5 Replacement of Steam Driven Mill Drives with Electric DC
3.6 Flue Gases Analysis

Lists of figures

Figure2.1: Metahara sugar factory partial view

Figure 2.2: Sugar cane farm

Fig 2.3 Series of mills

Figure 2.4: Schematic diagram of sugar manufacturing process

Figure 3.1: Falling Film Evaporator

Figure 3.2: Bagasse drier

Lists of tables

Table 3.1: choice of temperature for standard quintuple effect

Table 3.2: properties of vapor (steam)

Table 3.3: Interpolated value of properties of vapor (steam)

Table 3.4: Temperature drop

Table 3.1: Required parameters for the calculation of heat transfer & design of the dryer

Table3.3: Heating value of bagasse at different moisture content obtained from experimental data

Table 3.4: Calorific values of bagasse substitute’s resources

1. Introduction

1.1 Historical development of cane sugar

People were arguing that whether sugarcane is native to India or New Guinea. They do agree that ancient people liked it and carried with them in their migration and spread throughout south pacific area. Although sugar cane was possibly known in the holy land in biblical time only syrups could be obtained from it. In the 7th- 10th centuries AD, the Arabs spread sugarcane throughout their region of influence in the Mediterranean and eastwards. By the 12th century sugarcane reached Europe and Marco polo reported advanced sugar refining in china toward the end of 13th century. The ancient process for obtaining sugar consisted of boiling the juice until solids formed as the syrup cooled. Egyptians were using lime as purifying agent and carrying out recrystallization, which is still the main step in refining. The development of the sugar industry from the 16th century onward is closely associated with slavery, which supplied the largest amount of labor used at the time. The law cost of labor and price for sugar made many fortunes. The abolition of slavery introduces steam power as a replacement for the animal or human power that drove the cane mills. The use of steam in steady of direct firing was soon applied for evaporating the cane and following this vacuum pans and centrifuge were applied. The manufacturing of sugar is an energy intensive process which was the cause for deforestation, and then later replaced by bagasse burning and using energy efficiently by designing a multiple effect evaporators.

The sugar industry represents one of the most important economic activities that consume higher quantity of energy. The production of sugar from sugar cane involves several steps. The present system of sugar manufacturing process is essentially a combination juice extraction from sugarcane by milling or diffusion or a combination of both, clarification of juice, concentration of juice by evaporation of syrup, crystallization of sucrose by vacuum pans boiling, centrifugal separation of sugar & molasses from massecuite, drying & cooling of sugar & sugar grading & packing.

All the above process needs relatively large amount of energy in different forms such as power for prime movers. This power is mainly in the form of electricity for the electric motors and medium to high pressure steams for the steam engines & steam turbines. The processing of raw cane also requires a large amount of heat provided by low pressure steam for the processing of juice in to commercial sugar. The distillation system of ethanol process also consumes a large amount of process steam for its heating requirement. Unless the sugar industry uses its’ by products to produce value added products and minimize production energy consumption to use energy efficiently, it will face economical crises due to high production cost. This problem can be simplified by using cogeneration activities and process steam demand reduction. Our project gives much emphasis to the process steam reduction. Benefits of process steam demand reduction includes, increases the crushing capacity of the mill, and increases the capacity of the evaporator; reduced sugar losses etc.Ethiopia produces sugar since 1945. The factories production capacity nowadays of 280 000 tons a year is much below the local demand of 460 000 tones per annum.Local sugar production comes from Wenji-Shoa Sugar Factory (80,000 tons per annum) and two state-owned factories, Metahara and Fincha, which produce the remaining 200,000 tones. The gap between demand and local supply is covered with imported sugar. The government already has ambitious plans and expects a lot from the sugar industry. The development of the sugar industry includes the expansion of three of the existing sugar factories - Fincha, Wonji and Metahara sugar factories - and the development of a new sugar factory in Tendaho, which will have an annual production capacity of six million quintals per annum. The production capacity of the new factory will make it one of the biggest sugar factories around the globe. Aside boosting the production capacity of the sector, the project will also play an important role in creating employment opportunities for more than 80,000 skilled, semi-skilled and unskilled laborers, generating about 100 MW electric power, and substituting up to 20 percent of gasoline import with fuel ethanol. Upon completion the ongoing project, the sugar factories will be able to produce up to 243 million liters of ethanol, an alcohol which can be blended with benzene with a five percent proportion. (A car engine running on benzene can do the same with five percent ethanol blended benzene.) Efficient utilization of steam has always been a topic in the cane sugar industry. With the advent of refineries it became necessary to employ additional fuels (generally fuel oil or coal) in order to satisfy the additional process steam demand. In the face of rising fuel costs however, factories the world over have increasingly striven to reduce process energy consumption, i.e. use steam (and electrical power) more efficiently. The reduction of process steam requirements and the use of more efficient cogeneration systems are alternative to increase the surplus electricity generation. So that the Ethiopia sugar factories expected to make analysis on energy saving by process integration and produce extra products like electricity and alcohol. Since cogeneration has been proven to be a viable alternative efforts have been made for reducing energy demand in all areas of the Ethiopian sugar factory in order to maximize the export of electricity. In our country where cogeneration is still not economically viable (or there are no alternative uses for excess bagasse) raw sugar typically has process steam demands in excess of 50% on cane. By employing available technologies it is possible to reduce process steam demand to values at or even below 30 % on cane. Some advantageous technologies in this respect would be: Environmental implications, including air pollution, CO2 emission and the protection of the forests are good reasons why bagasse should be used as raw material for electrical cogeneration, for paper, cellulose board, charcoal, furfural or cattle feed. Therefore, it is reasonable to reduce its combustion in the boiler as much as possible.

1.2 Process description of sugar and ethanol production

Cane sugar is the name given to sucrose, a disaccharide produced from the sugar cane plant. In the production scheme for cane sugar, the cane cannot be stored for more than a few hours after it is cut because microbiological action immediately begins to degrade the sucrose. This means that the sugar mills must be located in the cane fields. The raw sugar produced in the mills is the item of international commerce. Able to be stored for years, it is handled as a raw material shipped at the lowest rates directly in the holds of ships or in dump trucks or rail road cars and pushed around by bulldozers. There are another category of cane sugar called direct – consumption (white sugar) and non – centrifugal sugar (whole sugar). The principal by product of cane sugar production is molasses. Final molasses is about 35 – 40% sucrose and slightly more than 50% total sugars. It is used to produce alcohol.

Sugarcane preparation and juice extraction: washing system is used to remove excessive amounts of soil, rocks and trash delivered with the sugarcane before entering the extraction system. After washed, sugarcane is prepared using rotating knives and shredders that reduce the cane fed to the mill into small pieces suitable for the subsequent extraction process. Juice extraction system separates the bagasse and the juice by compression of the sugarcane. The bagasse is used as fuel at the cogeneration system and the raw juice produced is delivered to the treatment system.

Juice treatment: some non-sugar impurities are separated by the addition of some chemical reactants as sulphur, lime, among others, being juice heating necessary for the purification reactions. After that, the juice passes through a flash tank, before entering the clarifier. The precipitate formed into the clarifier is separated from the clarified juice and directed to filters. After filtration, part of the juice returns to the process ahead the clarifier, and filter cake is rejected. The clarified juice can be then directed to the evaporation system. Treatment of juice for ethanol and sugar production can be very similar deferring on the sulphur addition step, used exclusively for sugar production.

Juice evaporation: juice for sugar production is concentrated in a multiple-effect evaporator. Exhausted steam from the cogeneration system is used as thermal energy source in the first evaporation effect, separating part of the water presented in the juice that is used as heating source for the next evaporation effect. The system works with decreasing pressure due to a vacuum imposed in the last effect, producing the necessary difference of temperature between each effect. Vapor bleed can be used to attend heat requirements of other parts of the process, as the juice treatment heaters and sugar boiling system. Part of the juice for ethanol production is concentrated in five effects evaporation systems to reach the concentration necessary for the fermentation process. The other part of the juice for ethanol is by-passed to the fermentation process, to be mixed with concentrated juice and molasses for the mash preparation.

Sugar boiling, crystallization, centrifugal separation and sugar drying: syrup is boiled in vacuum pans for crystal formation and then directed to crystallizers to complete crystal enlargement. After that, sugar crystals are separated from molasses using centrifugals. Sugar dryer consumes exhaust steam to reduce the moisture content of the sugar.

Fermentation: integrated sugar and ethanol plants use a mixture of molasses and juice for mash preparation. Part of juice is concentrated to reach optimum solid content level necessary for the fermentation process. Good quality water is also needed during the mash preparation and for CO2 scrubber. Fermented liquor produced has around 8% of ethanol concentration in mass basis and is directed to the distillation system to be separated from the water.

Distillation: ethanol produced at the fermentation process is recovered by distillation. Before entering the first distillation column, fermented liquor is heated to reach the adequate temperature for the distillation process. Hydrous ethanol is obtained by stripping and rectification stages. In order to remove the remaining water and obtain the anhydrous ethanol, dehydration process is required. A large amount of stillage is produced and must be handled as an effluent with high biochemical and chemical oxygen demand.v Condensates tank and water cooling system: the condensate tank receives all the condensates generated in the process excluding the exhaust steam condensate which returns to the cogeneration system. Separate tanks are used for hot condensates storage, like those originated with in the condensation of vapors from 1st, 2nd, 3rd and 4th effects of evaporation, being used as imbibitions water in the juice extraction system and washing water in sugar and molasses centrifugal separation and in the juice treatment filter. The water cooling system is composed by spray ponds that reduce condensate water temperature to be re-circulated in the process as cooling water for fermentation, distillation, sugarcane washing and vacuums systems.

1.3 Objective of the project

The general objective of this project is to identify the problem associated with the energy generation and utilization of the sugar factories and to recommend the possible solutions. Consequently the factories and in general the country could be benefited by:

- Reduce energy generation cost
- Minimizing resource wastage
- Exporting the surplus electricity to the grid
- Exploit other energy resources
- Getting additional profit

And the specific objectives are:

1. To survey the condition of the selected industry
2. To assess the way that the factories energy generation and utilization
3. To identify ways of process steam demand reduction
4. To compute investment required for implementing process steam demand reduction
5. To compute the saving that the factory could get due to the reduction
6. To show the general benefits to other existing as well as new sugar factories.

1.4 Methodology

The methods employed to achieve the objectives of this project are:

1. Literature Survey: - A review of literature is conducted on the area of industrial energy use and efficiency in related to sugar factory. Available books, journals, case studies, previous research works, policies & guidelines are surveyed in order to have a clear understanding of the subject matter
2. Data collection

The necessary data for the project are collected from different sources. The necessary data are:

- Amount and composition of bagasse generated
- Energy generation capacity
- Energy consumption
- Energy equipments
- Factory energy efficiency
- Problems concerning energy efficiency
- Amount and capacity of miscellaneous energy sources and
- Amount of molasses produced in the factory

These data can be collected by

1. Conducting interviews with the respective personal of the factory
2. By observation in the factory
3. Data from previous papers and research documents
3. Analysis and Evaluation. The collected data is analyzed quantitatively and qualitatively
4. Conclusion & Recommendation

After detail analysis of the collected data conclusion and recommendation is forwarded.

1.5 Steam in sugar factories

Nowadays Steam is used in nearly every industry, and it is well known that steam generators and heat recovery boilers are vital to power and process plants. It is no wonder that with rising fuel and energy costs engineers in these fields are working on innovative methods to generate electricity, improve energy utilization in these plants, recover energy efficiently from various waste gas sources, by products, and simultaneously minimize the impact these processes have on environmental pollution and the emission of harmful gases to the atmosphere.

Heat sources in industrial processes can be at very high temperatures, 1000–2500 0F, or very low, on the order of 250–500 0F, and applications have been developed to recover as much energy from these effluents as possible in order to improve the overall energy utilization. Heat recovery steam generators form an important part of these systems. (Note: The terms waste heat boiler, heat recovery boiler, and heat recovery steam generator are used synonymously). Waste gas a stream sometimes heat industrial heat transfer fluids, but in nearly 90% of the applications steam is generated, that is used for either process or power generation via steam turbines.

For steam production bagasse being the only fuel, it is very important to ensure that uniform feed of the fuel of not exceeding 50% moisture content is assured always to the boilers. Whatever may be the operating parameters of the boilers, the need for maintaining the boilers in excellent health to work at the rated efficiencies does not require over emphasis. Typically in a well maintained boiler there is a variation of 0.8% in the efficiency of boiler for every percentage of moisture variation in bagasse. A moisture level of 50% in bagasse is generally taken as the benchmark as all the sugar mills as well as boiler manufacturers adopt this for their designs. Though bagasse drying can improve boiler efficiency further, it has to be carefully planned in the heat balance of the plant, while optimizing the power cycle.

It is important to take note of the following thumb rules for quantifying the energy conservation in boiler operations. Every 20°C reduction in back end temperature: 1.0% increase in boiler efficiency. 10% reduction in excess air: 0.4% increase in boiler efficiency. 1% reduction in bagasse moisture: 0.8% increase in boiler efficiency.

2. Metahara sugar factory

2.1 History & location

Metahara Sugar Factory is located on the eastern part of Addis Ababa, about 200 km away. The state is located at 940 meter above sea level. The average rainfall is about 600mm, so the area to be irrigated is 100% conforte for crops to grow. It is located in the Oromyia region. The establishment history tells us that, it was established in 1965 on the agreement between the Dutch company called Hangler Vondr Amsterdam (HVA), and the Ethiopian government on share basis. In 1969, the factory started its first operations, which is crashing canes. At that time, the crashing capacity of the factory was 17 000 quintal per day, or 1700 tons per day. Gradually the factory took its expansion. In 1973, the first expansion program has been implemented, enabling the factory to crash 24 000 quintal of cane a day, or 2 400 tones a day, and producing 2 400 quintal of white sugar plantation. In 1975, the company was nationalized by the military government. At that time, it was purely Ethiopian sugar factory and not a share company anymore. It was administered by Ethiopian Sugar Corporation. In 1976, the second phase of the factory took place. In this level, the factory was to crash 30 000 quintal or 3 000 tons of cane a day, and produced 3 000 quintal of sugar a day. In 1980, the last expansion took place, this time the factory was able to crash 50 000 quintal or 5 000 tones a day, and producing 5 000 quintal of sugar a day. This is the capacity the factory owns at this time. This is one of largest sugar factory even in Africa. Simultaneously, along with the expansion of the factory, the expansion of the cane area was also conducted. During the beginning of the company's crashing, the cane area was 2,700 hectares. This time through many expansions, it has reached 10000 hectares of cane area. They produce plantation white sugar, and cane molasses as by produced. After the company has been nationalized, all the employees were Ethiopians, and through all its historical progress, the factory has shown ups and downs in the level of production, due to many reasons, especially during the past government. This happened, due to acute shortage of spare parts, impossibility to produce up to the capacity level. There was also lack of important chemicals for cane, like agro-chemicals, their timely application, and sometimes their access. Finally, after the change of the government and the dissolution of the cooperation took place, the company got its autonomy. It can buy, and sell its spare parts, and its products. Secondly, the board of management was established and the management was to discuss with the board and decide shortly. The chain of command is short; the problems are easily understood and solved. The factory has planned and decided the rehabilitation program, seven years ago.

illustration not visible in this excerpt

Figure2.1: Metahara sugar factory partial view

The mission of the organization is to Produce sugar of standard quality at a least possible cost and satisfy its customers; and utilizes all resources at disposal and to provide best service to the society at large and remain competitive & profitable, to be environmentally friendly in its process as well as to provide affordable living standard to its employees.

And the main Purposes of the Enterprise are to:

- Grow sugar cane and other sugar yielding plants
- Process and produce sugar & sugar by products
- Distribute and sell sugar and sugar related products within the country and abroad
- Study, plan and implement various sugar development programs
- Carry on scientific, industrial and agricultural research and surveys to enhance its programs
- Possess and develop agricultural lands in the country whenever it is deemed appropriate to fulfill its purposes
- Engage in other activities that enables the attainment of its purpose

2.2 Agricultural operations

Agricultural operations comprises of various activities like topographic survey, land preparation, sugar cane cultivation and harvesting which are essential to the sustainable supply of sugar cane required by the factory. Currently eleven commercial and semi commercial cane varieties have been used by the Enterprise. These varieties have been selected on the basis of compatibility to the soil characteristics of the area and their ability to resist prevalent diseases. Most of the plains in Metahara are gentle and suitable for gravity irrigation where 81.7% irrigated by gravity. There are 1200km irrigation canals on the cane fields. Water drawn from Awash River is stored in 23 reservoirs whose water holding capacity ranges from 6500m to 93,000 m. The average land productivity is about 165 tons of cane per hectare, which makes the Enterprise one of the highest cane producing farm in the world. About 1,091,100 tons of cane is supplied to the factory annually Along with the cane plantation, the Enterprise owns 140 hectares of land covered with various types of fruits such as orange, Mango, Lemon, Grape-fruit, etc… About 3000 tons of fruits are produced annually (Factory documents).

2.3 Factory operations

The factory operates for about 8 and 1/2 months annually and produces 120,000 tons of sugar and other useful by products. By-products that have economic value are molasses, bagasse and filter cake. The average annually production of molasses is about 35,580 tons and it is a basic material from which ethanol, baker’s yeast, fodder yeast, organic fertilizer, lysine, ethylene, etc… can be produced. Apart from these molasses can be mixed with materials like wheat barn, wheat meddling and oil cake to make a concentrated cattle feed. Bagasse is a fibrous residue that remains after cane is crushed and the juice is extracted. It is used as a boiler fuel where steam needed to run the factory is produced. Part of the steam so produced is used to run steam turbines that generate about 5.2 MW of electricity. On the average, 312,115 tons of bagasse is extracted as by-products of the sugar production process.

When cane juice is treated chemically for clarification, most of the solid impurities that settle down are removed in the form of filter cake. Filter cake is rich in organic materials and in its dried form is used for soil amendment in cane fields. On average 35,590 tons of filter cake is removed as by-products of the sugar production process (Factory production manual).

2.4 marketing

Marketable products of the Enterprise are sugar, molasses and various types of fruits.

1. Sugar

Sugar is supplied to domestic and foreign markets. The annual sales volume stands at

120,000 tons and about 65% of the annual sales volume goes to domestic market while the remaining is exported

2 Molasses

The major part of molasses produced (20,000 tons) is exported and small part of the annual production goes to domestic cattle fatteners and distilleries

3 Fruit

Fruit such as orange, lemon, mango and mandarin are subsidiary products of the Enterprise. About 3,000 tons of those fruits are marketed annually.

2.5 Human resources

The workforce includes professionals, semi professionals, clericals and manual laborers.

At peak time the workforce reaches 11,000 where 3700 are permanent employees and the remaining are seasonal. The population consists of workers, their families and their dependents. The total number is estimated at 35,000(Factory documents).

2.6 Sugar Manufacturing Processes in MSF

The sugar manufacturing from sugar cane is started from harvesting sugar cane plant by the sugar factory itself. The bulk of sugar cane is cut by hand (manually) with a cane knife but sometimes mechanical cutting is also being practiced in some other countries. The cut sugar cane is then loaded to vehicles and transported to the mill. At the first end of the factories the cane is usually weighted, washed and chopped in to smaller pieces before the cane is feed to mills (Tandems) for juice extraction.

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Figure 2.2: Sugar cane farm

Juice extraction is mostly done by passing the chopped cane through a series of three roller horizontal mills. The rollers are laid in triangles which are supported by a mill housing made of cast iron or cast steel, and they revolve in water cooled bearings made up of brass or bronze.

The prime objective in sugar cane milling is to extract the greatest possible amount of sucrose from sugar cane, and to make the final bagasse as dry as possible so that it will burn readily in the boilers.

The tandems are a train of six mills preceded by various combinations of cane preparation devices. The power required by the mills is obtained from steam turbines followed by gear boxes for speed reduction.

During the last few years diffusers have been installed in various sugar factories instead of mills. This new method of extracting sugar from sugar cane has proved advantages from the technical point of view. Diffusion has been accepted as an efficient way of achieving high extraction. The capital investment and maintenance costs of diffusers are lower than those of mills but the moisture of the bagasse obtained here is high hence reducing boiler efficiency. Metahara sugar factory has one diffuser in tandem B between the mills.

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Fig 2.3 Series of mills

Next to this important process, juice extraction, the raw cane juice is weighted and carried to liming process. The fibrous part called bagasse is transported to furnaces for burning.

The liming station of the cane juice is one of the most important stations in a raw-cane sugar factory. Raw sugar cane juice is composed of a great number of organic and inorganic compounds, acids, salts etc in varying amounts. When it comes from the mill tandem, the juice is an opaque liquid varying in color from greenish-gray to dark green, and it carries suspended matter such as fine bagasse (bagacillo), gums, albumin, wax, coloring matter, particles of soil sand clay and muck. The normal cane juice has PH 5.2 – 5.4.

The gums, wax and albumin make the raw sugar juice rather viscous and it cannot, therefore, be readily filtered when cold. Liming and heating causes many impurities in the juice to become coagulated and precipitated out. At the same time the acids are neutralized and any phosphates present are flocculated, adsorbing a large amount of coloring matter, solids and other impurities. Usually the lime is added to the raw sugar cane juice in the form of milk of lime, for better dispersion and quicker reaction.

The next process after liming of sugar cane juice is clarification. Without good clarification of sugar cane juice, the production of good quality raw sugar is impossible.

The purpose of clarification is the precipitation and removal of all possible non sugars, (organic & inorganic) and the preservation of the maximum sucrose and reducing sugars possible in the clarified juice.

The greatest part of sugar cane consists of soluble inorganic compounds or ashes. A certain amount of fiber, mainly cellulose, also remains in sugar cane juice after crushing, which passes through the cush-cush screen in the form of bagacillo. The raw cane juice is generally limed to PH 8 in order to obtain clarified juice of about PH 6.8-7.2 clarified juice is concentrated to a syrupy consistency before it is sent to the vacuum pans to be crystallized in to raw sugar. The concentrate is made in several evaporators connected in series called a multiple effect. The juice travels from one vessel to another because of the gradual increase of vacuum. The vapors obtained in each body of the multiple effects serve to heat the calanderia tubes and to evaporate additional water in the following vessel.

And after being evaporated in a multiple effect evaporator to be a syrupy consistency, clarified juice must be evaporated further for the sugar to crystallize. This is accomplished in a vacuum to form a heavy mixture of crystals and mother liquor, called massecuite.

The raw sugar massecuite is then crystallized by cooling. On this process residual syrup incapable of crystallizing called black strap molasses is separated.

And finally Batch & continuous centrifugals are used to separate the liquid and hard phases of raw sugar.

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