The application of plate heat exchangers in sugar mills

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Plate heat exchangers are widely used in many industries. Many sugar mills in Europe, America, South Africa, and India also use it for heating clear syrups, heating thin syrups produced by the extraction method, heating syrup molasses before bottling, and cooling the final molasses (before it is placed in the molasses storage tanks). It yields good results, and some sugar mills have also tried using it for heating mixed syrups. Below are some examples of good applications. 1. For heating the syrup, the Dalton sugar factory in South Africa uses plate heat exchangers manufactured in Sweden, of the AM20 model; these exchangers consist of 44 plates, each with a heating area of 0.79 m2. The flow rate of the clear juice is 130 t/h; it is heated from 92°C to 104°C. The saturated temperature of the waste steam used is 111°C, and the heat transfer coefficient is 3400 W/m2·K. The pressure drop of the clear juice as it passes through the heat exchanger is 76 kPa ； When in use, the steam valve is not fully opened; thus the actual temperature difference is small, and the heat transfer coefficient is even higher. After running continuously for 5 weeks, the clear juice heater was opened; there was some sediment deposited inside, and a black film covered the surface, which could be easily washed away with water. At the sugar factory in southern Germany, the beet juice flow rate is 600 t/h. Before entering the evaporation tanks, the juice is heated in 4 stages using 4 A30HBM plate heat exchangers; the steam from the 1st to 4th evaporation tanks is used as the heat source for each heating stage. The temperature reached after each stage of heating is only 3°C lower than the temperature of the steam, with the final temperature reaching 127°C. Guangzhou Overseas Chinese Sugar Factory (carbonation process) employed a Swedish-made plate heat exchanger in the 1980s for heating the molasses; the model was AM-20FM, with a plate area of 0.79 m2, 50 plates in total, and an effective heating area of 37.9 m2. The first-effect juice steam is used; the clear juice flow rate is 117 t/h, the inlet juice temperature is 68.7°C, the outlet juice temperature is 100.7°C, resulting in a temperature rise of 32°C. The inlet steam temperature is 114.2°C, and the discharge water temperature is 108.4°C. The heat transfer coefficient K is 4010 W/m2·K, and the pressure drop across the heat exchanger for the clear juice is 36 kPa. Its discharge temperature is about 6°C lower than the inlet steam temperature, and there is a slight amount of water accumulation inside the device. If the drainage is smooth, the K value will be higher as well. After being used continuously for over ten days and then taken apart for inspection, the surface of the vast majority of the plates remained as smooth as new. There is very little dirt, which can be wiped away with a soft cloth. The Guangdong Shunde Sugar Factory (sulfite process) used a plate heat exchanger manufactured in Lanzhou, with a plate area of 1.07 m2, for the third-stage heating of the molasses. The average sugarcane juice flow rate is 254 t/h; the temperature of the waste steam used is 129–130°C. The juice temperature before heating is 106–108°C, and it rises to 120–126°C after heating. The heat transfer coefficient ranges from 2400 to 3900 W/m2·K. The Guangdong Zhujiang Ganhua Plant uses a domestic plate heat exchanger for the primary heating of the juice, with a heat transfer area of 35 m2. After heating, the juice temperature is only 2–4°C lower than the steam temperature; after 15 days of use, this difference increases to 4–6°C. The heating temperature is higher than that of the 60 m2 tubular heater previously used. Its heat transfer coefficient is 2700–4000 W/m2.K. Another sulfite sugar factory uses a domestically produced plate heat exchanger with a plate area of 0.36 m2; it has 90 plates, giving an effective heat transfer area of 31.68 m2. This heat exchanger is used to heat the clear juice using steam from the first stage of heating. Based on seven days of actual measurements, the average values are as follows: clear juice flow rate of 37.2 t/h, inlet juice temperature of 76°C, outlet juice temperature of 107°C, resulting in a temperature rise of 31°C; inlet steam temperature of 113°C, and steam condensate temperature of 112.5°C. The heat transfer coefficient is 2300 W/m2·K. After using it for over ten days, I opened it up to check, and there was very little sediment on the surface of the plate, which could be removed with water and a soft brush. The heat transfer performance of using a plate heat exchanger to heat the clear juice is more than twice that of a tubular heat exchanger; therefore, the heat transfer area required can be reduced by half or more. It is better to use the vapor from the first effect here, as this offers certain energy-saving benefits (compared to using waste vapor for heating). When the heat exchanger performs well and is located close to the evaporation tank, heating with juice vapor can bring the temperature of the clear juice close to that of the juice vapor, with a difference of only 3–5°C, which is sufficient to meet the requirements of evaporation. Heating with syrup vapor can also prevent the coking of the syrup caused by excessively high steam temperatures. Some sugar factories in China use waste vapor; in one factory, improper operation led to severe coking, and the resulting coked substances were very difficult to remove. 2. For heating the mixed juice: The plate heat exchangers used at the Dalton sugar factory in South Africa to heat the mixed juice are of the AM20 model manufactured by the Swedish company Alfa-Laval, with 50 plates. After two months of use, it is cleaned once a week. The sugar juice flow rate is 103–130 t/h; the temperature is raised from 72–76°C to 94–97°C. The saturated temperature of the waste steam used is 109–111°C, the heat transfer coefficient is 2900–3380 W/m2·K, and the pressure drop across the sugar juice is 0.05–0.1 MPa. When the unit is shut down and opened, sediment made up of silt and bagasse accumulates inside, and a thin layer of scale forms on the surface; however, this is less severe than in tubular heaters, and it can be cleaned by rinsing with water and brushing. A \"backwash\" is performed once per working shift, which can wash away some of the sediment. But it is best to first remove the fibrous impurities from the sugarcane juice using a sieve. Cuba tested plate heat exchangers for heating mixed juices, with the model AM-20SFM being used. It is equipped with 34 plates, offering a heat exchange area of 26 m2. The sugarcane juice flows in four directions inside (two upward and two downward), which increases its flow velocity between the plates to reduce deposition. Every 4 hours of operation, the sugarcane juice is flowed in the reverse direction to flush out the deposits inside. The sugarcane juice flow rate is about 55 t/h. The steam used comes from a third-effect evaporator; its temperature ranges from 101 to 108°C. The incoming sugarcane juice has a temperature of 60–68°C, and it can be heated to 96°C at first, after which the temperature gradually decreases, dropping to around 80°C by the fourth day. The heat transfer coefficient decreases from 3200 at the beginning to 1500 by the fourth day, but it remains much higher than that of ordinary heaters (which need to be stopped for cleaning after just one day of use). The pressure drop of the sugarcane juice through the heat exchanger was 0.15 MPa on the first day, and gradually increased to 0.2 MPa thereafter. Although deposits gradually form inside the heat exchanger, it can still be used. For materials such as mixed juices that contain a high amount of solid suspended particles, plate heat exchangers with wide gaps perform better. In a sugar factory abroad that uses the sulfite process, this type of heat exchanger, together with steam from the third-effect evaporation used to heat the mixed juice and the sulfurized juice, allows for continuous operation for 30 days. The pressure of the steam used is very low, at only 40 kPa absolute pressure, with a steam temperature of 75°C. The heat transfer coefficient ranges from 2090 to 2440 W/m2.K, and its heating area is more than half smaller than that of a tubular type. Two beet sugar factories in Poland (with a daily processing capacity of over 2,000 tons of beets) adopted wide-gap plate heat exchangers in the mid-1990s. They used the steam from the sugar boiling tanks (at a temperature of around 60°C) to heat the extract, raising its temperature from about 30°C to around 50°C; afterwards, ash was added, and the extract was further heated to around 88°C using steam condensate and steam from the third effect. It performs well and can be used continuously for one pressing season; after shutting down, spraying it with high-pressure water can remove all deposits from the surface of the plates. It makes use of a large number of low-temperature heat sources, significantly reducing the fuel consumption of sugar mills. Plate heat exchangers with wide gaps can use vacuum-based low-temperature steam as a heat source to heat low-temperature sugarcane juice, offering significant energy-saving benefits. 3. Used for cooling molasses – it cools the molasses before it enters the storage tank, and this is a key measure to prevent spoilage during storage. However, molasses has high viscosity, making heat transfer difficult, and ordinary heat exchangers are not very effective in this case. Plate heat exchangers tested in South Africa and Australia have shown good performance in cooling molasses. Both the Gladhow Sugar Mill and the Sezela Sugar Mill in South Africa use plate heat exchangers manufactured by the British company APV to cool molasses. The molasses flows in a single pass through the vessel, while the cooling water passes through in five passes. The plate sizes used are 1556×416 mm, with 96 and 160 plates respectively; the heat transfer areas are 50 and 83 m2 respectively. The amount of molasses processed is 11.5 and 12.6 t/h respectively, and the consistency of the molasses is around 81. The temperature of the molasses decreases from 59.8°C to 42.8°C in the former case, and from 62.1°C to 36.7°C in the latter case. The inlet and outlet temperatures of the cooling water are 30.2 and 36.3°C respectively for the former case, and 23.3 and 46°C respectively for the latter case. The consumption of cooling water is 13.8 and 6.1 t/h respectively ； The heat transfer coefficients are 96.3 and 112 W/m2.K, respectively. The heat exchanger needs to be opened and cleaned with water once a month. In 1990, the Bingera sugar factory in Australia used plate heat exchangers manufactured by the German company GEA to cool molasses; the model was B12, and it consisted of 149 plates with V-shaped corrugations measuring 1400×400 mm each. The heat exchange area was 67.6 m2, with both molasses and water flowing in a single pass. The actual amount of molasses processed is 10–15 t/h; the temperature of the molasses drops from 57–62°C to around 37°C. The temperature of the cold water used is 32–35°C, and the amount of water is adjusted automatically based on temperature, being 1.5–3 times the amount of molasses. The heat transfer coefficient is 200–550 W/m2·K. The pressure drop across the molasses is 50–150 kPa. The equipment operated normally throughout the entire harvesting season (it was disassembled for inspection and cleaning at intervals), and the stainless steel plates (model 316SS) showed no visible signs of corrosion. The cold water used must be clean; lower-temperature river water was tried, but it caused soft deposits to form between the plates, hindering heat transfer. 4. Heat transfer coefficients for different materials: The plate heat exchangers manufactured by the German company GEA are used to determine the average heat transfer coefficient when heating various materials in sugar mills (an average value over a single crushing season, in W/m2.K). The values can be obtained from the table below: For ordinary plate heat exchangers – for sugar syrup without solids: 1280–1690; for concentrated sugar syrup: 870–1450; for molasses: 460–1400; for syrups: 870–1450; for dilute sugar syrup: 3840–4180. For plate heat exchangers with wider gaps – for circulating syrup: 1680–2030; for sugar syrup containing solids: 1220–1450; for syrup with ash added: 1980–2440; for diluted syrup after pressing: 2560–3020; for crude sugar syrup: 2210–2670