Summary of 7 major problems in evaporative crystallization

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1. How do wastewater evaporators prevent tube blockage and scaling? Answer: There are generally two reasons for tube blockage: one is the deposition of crystalline salts, and the other is scaling caused by ions such as calcium and magnesium. Therefore, preventing tube blockage requires addressing these two aspects. First, choosing the right type of evaporation process is the first step in avoiding blockages; film evaporators such as falling film evaporators are not suitable for environments where crystals form, as the use of such evaporators in such situations inevitably leads to tube blockage. In situations where crystals tend to form, a forced-circulation evaporator or a scraped-surface evaporator should be used; among these, the forced-circulation evaporator is preferred due to its large evaporation surface area and ease of operation. Secondly, the design of the evaporation pipeline is also a key factor in reducing salt accumulation. The pipeline is designed without any dead corners, in line with the principles of salt flow and deposition, ensuring that all crystallized salt is collected in the salt collector and finally discharged from the salt outlet. If scaling is caused by ions such as calcium and magnesium, the measures that can be taken are: 1. Soften the wastewater to a certain extent in order to reduce the concentration of calcium and magnesium ions ； 2. Use a forced-circulation evaporator ； 3. Use the gypsum seeding method to prevent scaling, or add a small amount of scale inhibitor ； 4. Perform routine cleaning of the evaporation equipment. 2. Evaporator operating costs and how to reduce them? Answer: Method 1: Use a multi-effect evaporation process. For each additional effect, the operating costs decrease, but the initial investment increases. For the evaporation of salts from wastewater, three effects are usually sufficient, with four effects being the maximum. Method 2: The thermal compression type evaporator TVR can reduce the energy consumption of the first stage. Method 3: Mechanical vapor compression evaporators, which significantly reduce operating costs. Method 4: Apply external insulation to the evaporation equipment to minimize heat loss. Method 5: Utilization of the sensible and latent heat of condensate. Utilize the preheater to recover the sensible heat from the condenser, thereby increasing the feed temperature of the evaporating solution ； A flash system is used to recover the latent heat of the condensate. 3. What is the situation regarding salt evaporation? Answer: There are generally two ways to evaporate salt: one is salt discharge via a centrifuge; the moisture content of salt discharged from a centrifuge is low ； Another method involves using a salt output device together with a crystal collection tank to produce salt; the salt is precipitated in crystalline form, with less moisture present. 4. Evaporator material selection? Answer: The choice of generator material is closely related to the composition of the wastewater. Salts can be divided into chloride salts (such as sodium chloride and ammonium chloride) and non-chloride salts (sodium sulfate, sulfuric acid, sodium carbonate, nitrates, etc.). For chloride resistance, the preferred materials in order are titanium, duplex stainless steel, carbon steel, and ordinary stainless steel. In terms of cost-effectiveness, the preferred materials are carbon steel, titanium, and duplex stainless steel; ordinary stainless steel is the least suitable option. For non-heat exchange equipment, carbon steel coated with enamel or PTFE can be used, while polypropylene and fiberglass-reinforced plastic can be employed in low-temperature applications. For corrosion resistance in the presence of non-chloride ions, the preferred order is stainless steel 316L, followed by stainless steel 304, and then carbon steel. In terms of cost-effectiveness for non-chloride environments, stainless steel 304 or stainless steel 316L are the preferred choices, with carbon steel coming next. At low temperatures, materials such as UPVC, PE polypropylene, and fiberglass can be used. 5. How to choose the right evaporator? Answer: For salt evaporation, a forced-circulation evaporator is the preferred choice; if the salt concentration is low, a pre-film evaporator combined with a forced-circulation evaporator can also be used. For the evaporation of other non-salt substances, falling film evaporators are preferred. 6. The issue of COD removal rate during the evaporation process? Answer: The COD in wastewater is primarily determined by the amount of organic matter present in it; the organic matter in wastewater can be either high-boiling or low-boiling point. If high-boiling-point organic compounds enter the solid waste and wastewater systems along with salts during evaporation, the COD of the condensed water decreases ； If the wastewater contains low-boiling-point organic compounds, they enter the condensate system along with the condensate during the evaporation process, and as a result the COD of the condensate does not decrease. Therefore, the COD removal rate during the evaporation process is related to the specific composition of organic substances in the wastewater. If the client can provide information regarding the specific components and concentrations of these organic substances, it is possible to make a preliminary estimate of the COD removal rate during evaporation. However, the most scientific approach is to conduct small-scale evaporation tests in order to determine the actual COD removal rate. 7. What is the difference between MVR and multi-effect evaporation in terms of steam reuse? Answer: Both MVR and multi-effect evaporation are aimed at reducing the operating costs of wastewater evaporation. The basic principle of MVR is to re-compress the secondary steam, thereby increasing its pressure and temperature; this secondary steam is then reused to heat the heat exchanger, ultimately achieving energy savings. The basic principle of multi-effect evaporation is that the secondary steam enters the next effect evaporator to provide heating, thereby enabling the reuse of steam. Due to various factors, the number of effects in multi-effect evaporation is often limited. MVR saves steam consumption compared to multi-effect evaporation.