How to improve the capacity of centrifugal drying

Saving Energy, Reducing Consumption and Improving Efficiency through New Filtration Technologies – Zhou Rong (Hunan Yueyang Changling Petrochemical Engineering Design Co., Ltd.) By replacing valveless filters with efficient fiber filters that use fiber materials as filter elements, the problems associated with raw water treatment systems such as limited treatment capacity, high turbidity of the treated water, high water consumption for backwashing filtration equipment, and blockages in heat exchangers were addressed. This ensured the proper operation of the raw water treatment system and subsequent water treatment equipment, resulting in significant savings in production costs and energy consumption. Raw water, valveless filtration, fiber filters, high efficiency and energy savings. 1. Overview: The raw water system is an important part of chemical water treatment in power plants. The quality of the raw water directly affects the performance of the subsequent water treatment equipment as well as the quality of the deionized water, which in turn has an impact on the safe operation of thermal equipment in power plants. The design capacity of our plant’s raw water treatment system is 480 t/h. Due to the expansion and renovation efforts carried out over the past five years, the increased volume of raw water along with higher quality requirements mean that the two existing gravity valveless filters, each with a processing capacity of 240 t/h, can no longer meet the production needs. Moreover, there are issues with the current system, so renovation has become an urgent priority ； After research and thorough evaluation, it was decided to use a new type of bundled soft filler—fiber—as the filter element in a pressure-type high-efficiency fiber filter. Current status and issues of raw water treatment systems: The valveless filter used for raw water filtration is a type of rapid filter that operates on the principle of equal flow velocity; it uses granular quartz sand as the filtering medium, with a flow velocity of 6–10 m/h ； The process flow of its raw water treatment system is as follows: the raw water flows from the system to the heat exchanger in the low-temperature power plant, where it exchanges heat with low-temperature hot water and its temperature rises to around 40°C; thereafter, it passes through a valveless filter before reaching the clean water tank, from where it is pumped by a clean water pump to subsequent equipment for further water treatment ； This system has the following three problems: (1) Valveless filters occupy a large area; the external dimensions of a valveless filter with a processing capacity of 240 t/h are 8×4 m², while those of one with a processing capacity of 400 t/h are 10×5 m². Due to limitations in space and production conditions, it is not possible to build another valveless filter ； Furthermore, when granular filter media is used for filtration, its filtering precision is limited by the particle size of the media; as a result, the turbidity of the filtered water can reach up to 5 mg/L, and the cleaning process for such media is quite labor-intensive ； Additionally, valveless filters have a large surface area, and granular quartz sand can easily cause uneven layering and distribution; in severe cases, this leads to flow deviation and affects the quality of the effluent water. (2) Valveless filter backwashing consumes a large amount of water, and it also carries away a significant amount of heat, resulting in substantial waste of heat. (3) Due to the poor quality of the raw water supplied to the system, both heat exchangers are severely clogged and covered with sediment; it is often necessary to shut down one of them for maintenance. Moreover, the low capacity of each individual heat exchanger results in the raw water temperature not meeting the required levels, thereby increasing the costs of water production. 3. Feasibility Analysis 3.1 Limitations of valveless filters using quartz sand as filter media The entire operation process of the filtration unit consists of two steps: the first step is the adsorption and removal of suspended particles from the water by the filter media, and the second step is the cleaning of the filter media. The filtering efficiency of granular filter media such as quartz sand can be expressed by the following formula: C = Co·e^-3/2·(1-ε) ·η·ηc·L/dc. Where Co and C represent the concentrations of suspended particles before and after filtration, respectively ； 　　Thickness of L-filter media layer ； 　　ε – Porosity of the filter media layer ； 　　dc – Particle size of filter media ； 　　η – the ratio of effective collision counts to total collision counts, and is related to the coagulation efficiency of water ； 　　ηc – the ratio of the rate at which suspended particles in water collide with a certain filter element to the rate at which they move toward that filter element ； As can be seen from the formula, for filters, the main way to improve filtration efficiency is to reduce the particle size of the filter media; however, due to constraints such as operating resistance and backwashing procedures, it is difficult to decrease the particle size of granular filter media. Additionally, when granular filter media is cleaned, the hydraulic screening effect of the backwash water results in a distribution within the filter media layer in which the particle sizes increase from small to large along the direction of the filtration flow. This leads to an unreasonable decline in filtration efficiency. The fine filter media located at the upper part of the layer has a high filtration efficiency, and its small pores are easily filled by impurity particles, thereby increasing operational resistance and causing the impurities in the water to move downward. On the other hand, the coarser filter media situated in the middle and lower parts have a lower filtration efficiency and are unable to remove particles effectively, leading to the failure of the filter. As a result, it becomes difficult to further improve indicators such as the filtration rate, efficiency, and dirt-trapping capacity of the filter. 3.2 Characteristics of fiber filters: A high-efficiency fiber filter is a filtering device that uses bundles of expanded fiber filaments (with diameters of 20–50 um), attached with weights at their lower ends, and suspended on a porous plate located at the upper part of the filter; these filaments serve as the filtering medium. Several capsules are placed within the fiber bundle clusters; during filtration, water is first pumped into these capsules to create pressure that forces the surrounding fiber bundles together, compacting them. Then, water flows from bottom to top through the pores in the fibers, enabling upward flow filtration. During cleaning, first drain the water from the sac, remove the pressure applied to the fiber bundle, allowing the fibers to return to their fluffy state under the combined effect of gravity and the elasticity of the expanded fibers. Then, clean it thoroughly by using a combination of air and water for shaking and scrubbing, so as to restore its original properties. Since the fibers used as filter elements are a flexible and curved material, the diameter of these filter media can be as small as several dozen micrometers. There are numerous void spaces within the filter media layer, and by controlling the compression conditions of the fiber bundles during filtration, different fiber porosities can be achieved. This allows the efficiency and resistance of the filter to be kept within specified ranges, addressing the issue inherent in traditional filtering devices such as valveless filters, siphon filters, and mechanical filters, which all use granular filter media like quartz sand; the filtering precision of these devices is limited by the relatively large size of the filter media particles ； The small diameter of the filter media greatly increases its specific surface area and surface free energy, thereby increasing the chances of contact between impurity particles in water and the filter media as well as the adsorption capacity of the latter, and thus improving the filtration efficiency and dirt retention capacity. The main performance comparisons of the two types of filters are as follows: Valveless filter bed vs. High-efficiency fiber filter. Filter media name: Quartz sand particles, Fiber. Filter media diameter (mm): 0.5–1.0, 0.02–0.05. Flow rate (m3/h): 5–10, 30. Effluent turbidity (mg/L): ≤5, ≤2. Pollution retention capacity (kg/m3): 1–1.2, 5–10. As can be seen from the table, the raw water quality tolerance and pollution retention capacity of the fiber filter are much higher than those of the valveless filter bed ； Furthermore, allowing raw water to pass through a fiber filter before entering the heat exchanger can address the shortcomings present in the original process flow. Based on research and analysis, and adhering to the principles of advanced process technology and high levels of automation, we utilized PLCs (Programmable Logic Controllers) to modify the process flow of the raw water system. In this new setup, the raw water first passes through 4 sets of 3000-high-efficiency fiber filters, after which it is sent to the low-temperature power plant where it exchanges heat with low-temperature hot water before entering the clean water tank. A single 3000 high-efficiency fiber filter is designed with an output of 210 t/h. 4. Economic and social benefits: Over the past year since the renovation and operation of this raw water treatment system, the problems existing in the original process have been resolved, the quality and quantity of the treated water have improved, and production costs have been reduced. 4.1 The water quality of the effluent from the high-efficiency fiber filter has improved, resulting in a stable turbidity of the effluent at levels below 2 NTU. This reduces the load on the cation exchange resins, allows for an increase in the amount of water that can be processed by the ion exchangers per cycle, reduces the consumption of acids and bases, and minimizes pollution of the resins. 4.2 Due to the adoption of the new process, the backwash water used for the filtration equipment is not heated, thereby reducing heat loss. Annual backwash water volume for valveless filters = Annual water production volume × (1 – water production rate) = 2,500,000 × (1 – 0.974) = 425,000 tons. Heat loss due to backwash water = Temperature rise of raw water × Backwash water volume = 15 × 425,000 × 4.18 = 2.66×10^6 (MJ). Equivalent amount of standard oil = 2.66×10^6 / 41.87 = 63.53 tons. Here, 41.87 represents the heat value of 1 kg of standard oil ； 4.3 High-efficiency fiber filters produce a large volume of water per cycle, with low backwashing water consumption, thereby reducing water usage ； Reduced waste discharge. Periodic water production volume (t): Backwashing water volume (t): Water production efficiency. High-efficiency fiber filter: 10,000; 170; 0.983. Valveless filter: 3,500; 92; 0.974. Water saved by the filters throughout the year = (Water production efficiency of filters – Water production efficiency of filter beds) × Total annual water production volume = (0.983 – 0.974) × 2,500,000 = 22,241.603 tons. Savings in discharge fees for the whole year = 22,241.603 × 4.5 = 100,000 yuan. 4.4 This solution addresses the problem of blockages in heat exchangers, improves their performance, and reduces maintenance costs. Before the renovation, the two heat exchangers often became clogged due to the poor quality of the raw water; each one required maintenance 2 to 3 times per year. The raw water flowed directly into the valveless filter through a bypass around the heat exchangers, and the low-temperature hot water needed to be cooled by circulating water, which increased the electrical load on the circulation water system. Additionally, because of the low temperature of the raw water, the volume of water that could be processed by the subsequent ion exchange units decreased, leading to an increase in the consumption of acids and bases. Since its operation half a year after the renovation, no blockages have occurred, which has saved on maintenance costs and reduced the workload for workers. 5. Conclusion: Compared with valveless filters using quartz sand as the filter media, high-efficiency fiber filters using fibers as the filter media feature high filtration efficiency, fast filtration speed, large dirt holding capacity, high water production per cycle, low self-consumption of water, and small floor area. They also address the problems existing in the original process flow, offering high economic and social benefits ； It is gradually being adopted in the raw water treatment processes of power plants. If the cleaning technology for fiber filters can be improved and the internal structure of the filtration equipment simplified to enable it to handle water with high turbidity directly, along with the development of larger-scale fiber filter systems, it can be widely applied in wastewater treatment.