How to prevent pipe blockage and scaling in wastewater evaporators?
Answer: There are generally two reasons for pipe blockage: one is the deposition of crystal salts;
Another type is the scaling formed by calcium, magnesium ions, etc., so preventing pipe blockage should also focus on these two aspects:
Firstly, choosing the right evaporation method is the first step in preventing tube blockage. Film evaporators such as falling film evaporators are not suitable for situations where crystals are produced. If falling film evaporators are used in these situations, tube blockage is inevitable. In situations where crystals are prone to occur, forced circulation evaporators or scraper evaporators should be used. Among them, forced circulation evaporators are highly recommended due to their large evaporation area and easy operation. The flow rate inside the heat exchange tube should be maintained at 2m/s. Although this project claims to be a forced circulation type, the flow rate of the pump used is far from sufficient, resulting in salt deposition when the flow rate is not specified.
Secondly, the setting of the evaporation pipeline is also an important factor in reducing salt. The pipeline we designed has no dead corners and conforms to the principles of salt flow and deposition, ensuring that all crystalline salts are collected in the salt collector and finally discharged from the salt outlet.
If scaling is caused by calcium, magnesium ions, etc., the measures that can be taken are:
Soften the wastewater to reduce the concentration of calcium and magnesium ions;
Adopting a forced circulation evaporator;
Using gypsum crystal seed method for scale prevention or adding a little scale inhibitor;
Perform routine cleaning on the evaporation equipment. Generally, calcium and magnesium deposits can be cleaned with dilute acid.
02
Operating costs of evaporators and how to reduce them?
Answer: Method 1: Adopting a multi effect evaporation process, with each additional effect, the operating cost decreases, but the initial investment increases. For wastewater salt evaporation, it generally does not exceed three effects, with a maximum of four effects.
Method 2: Thermal compression evaporator TVR can reduce the energy consumption of the first effect. The system operation will become more complex.
Method 3: Mechanical steam compression type evaporator can reduce operating costs, but due to the immaturity of domestic steam compressors, equipment costs are high and maintenance difficulties become greater. In addition, mechanical vapor compression evaporators are not suitable for evaporating salts and bases with high boiling points. The use of mechanical vapor compression evaporators for the evaporation of saline wastewater should be carefully selected.
Method 4: Ensure proper external insulation of the evaporation equipment to reduce heat loss.
Method 5: sensible heat and latent heat of condensate and their utilization. Utilize a preheater to recover the sensible heat from the condenser and increase the feed temperature of the evaporated liquid; Adopting a flash evaporation system to recover the latent heat of condensate.
What is the evaporation of salt in 03?
Answer: There are generally two ways to evaporate salt: using a centrifuge to remove salt, which has a low moisture content but increases initial investment and operating costs. For those with high quality requirements for evaporated waste salt, this method can be used;
Another method is to use a salt extractor with a crystal collection tank to extract salt, which precipitates in the form of crystals with less external moisture. The equipment is simple and easy to operate. Trace amounts of water can be returned to the original liquid tank through the filter and evaporated again.
What is the material selection for evaporator 04?
Answer: The selection of evaporator material is closely related to the composition of wastewater. For salts, they can be divided into chloride ion salts (such as sodium chloride and ammonium chloride) and non chloride ion salts (sodium sulfate, ammonium sulfate, sodium carbonate, nitrate, etc.).
Chloride ions are preferred in the order of corrosion resistance: titanium, duplex stainless steel, carbon steel, and ordinary stainless steel.
Chloride ions are preferred in order of cost-effectiveness: carbon steel, titanium, duplex stainless steel, followed by ordinary stainless steel.
For non heat exchange equipment, carbon steel enamel and PTFE can also be used, while polypropylene and fiberglass can be used for low temperatures.
The preferred order for corrosion resistance of non chloride ion salts is stainless steel 316L, stainless steel 304, and carbon steel.
Non chloride ions are preferred in order of cost-effectiveness: stainless steel 304 or stainless steel 316L, followed by carbon steel. Low temperature options include UPVC, PE polypropylene, fiberglass, etc.
How to choose a suitable evaporator?
Answer: According to our company's situation, for salt evaporation, the forced circulation evaporator is preferred. If the salt concentration is low, a pre falling film evaporator+forced circulation evaporator can also be used to reduce operation and initial investment. For other non salt evaporation, a falling film evaporator is preferred.
What is the service life of the 06 evaporator made of carbon steel material?
Answer: From the perspective of reducing the initial investment of evaporation equipment for users and improving the cost-effectiveness of the equipment, and based on corrosion data and production experience, some of the evaporators produced by our company for certain wastewater will be partially or completely made of carbon steel material.
Explanation as follows: The use of carbon steel material does not mean that it has the strongest corrosion resistance among all materials, but rather that our company recognizes it as a cost-effective material; The corrosion data table suggests that the corrosion rate of sodium chloride salt water on carbon steel is 0.1-0.5mm/year. According to this data, the service life of carbon steel in the sodium chloride wastewater evaporator is 5 years without any problem. Due to the complex composition of wastewater, the corrosion of materials by wastewater is more complicated. Scientific analysis and previous engineering experience cannot fully prove the accuracy of the corrosion rate;
Actual situation: Most of our company's carbon steel sodium chloride wastewater evaporators have been in use for more than two to three years, with some exceeding five years, and they have been operating normally ever since. But the operation of other companies cannot represent the actual situation of your company; Our company's warranty period for electromechanical products is one year.
Is the 07 evaporator a pressure vessel? Does your company manufacture it according to pressure vessels?
Answer: Our company can process the equipment according to the requirements of pressure vessels, but evaporators are not typical pressure vessels. Except for the first effect heat exchange chamber, which may have a certain pressure, the rest of the parts are atmospheric or negative pressure devices and do not need to be executed according to the requirements of pressure vessels.
If the customer wants to follow the pressure vessel procedures, future annual inspections, security checks, etc. will cause a lot of trouble for the customer, and it is usually the customer who voluntarily requests not to go through the pressure vessel procedures. We have many sets of equipment that are processed according to pressure vessels and have relevant inspection documents, but the end customers do not go through the pressure vessel inspection and approval procedures.
In addition, although the pressure vessel procedures have not been completed, the production is still strictly required and has nothing to do with whether the pressure vessel procedures can be completed or not.
What is the issue of COD removal rate during the evaporation process?
Answer: The COD in wastewater is mainly determined by the content of organic matter in the wastewater, which may have high or low boiling points. If high boiling point organic matter enters the solid and waste liquid system with salt during evaporation, the COD of condensed water will decrease; If there are low boiling point organic compounds in the wastewater, they will enter the condensate system with the condensate during the evaporation process, and the COD of the condensate will not decrease.
Therefore, the removal rate of COD during the evaporation process is related to the specific composition of organic matter in the wastewater. If the construction party can provide the specific composition and content of organic matter in the wastewater, it can make a basic judgment on the COD removal rate during the evaporation process. The most scientific way is to conduct an evaporation test to determine the COD removal rate during the evaporation process.
Is the issue of using MVR and multi effect evaporation for 09 wastewater evaporation?
Answer: MVR and multi effect evaporation are both aimed at reducing the operating costs of wastewater evaporation. The basic principle of MVR is to re compress the secondary steam, increase the pressure and temperature of the secondary steam, reuse the secondary steam to heat the heat exchanger, and ultimately achieve energy saving.
The basic principle of multi effect evaporation is that the secondary steam enters the next effect evaporator for heating, achieving the reuse of steam. Multi effect evaporation often has limited effectiveness due to various factors. From the perspective of steam conservation alone, MVR definitely saves steam consumption compared to multi effect evaporation.
Initial cost: Compared to multi effect evaporation, MVR requires an increase in steam compressor and single effect heat exchange area, reducing the related equipment and condensing equipment for multi effect. Due to the high cost of steam compressors and the increased heat exchange area, the total cost of treating MVR evaporators of the same scale is often three to four times that of multi effect evaporation.
Operating cost: MVR is suitable for the evaporation of liquids with little increase in boiling point with concentration. In the food industry such as juice and the evaporation of low concentration liquids (including salt solutions), MVR has an advantage in operating cost. If the boiling point of wastewater increases rapidly with concentration, due to the increase in temperature required for compression and the high power of the compressor, the economy will significantly decrease.
10. Why is a combination of falling film evaporator and forced circulation evaporator sometimes used for salt water evaporation?
Answer: The main starting point is to reduce operating costs and initial costs. The forced circulation pump has a large flow rate, complex structure, and high cost. The initial cost of a falling film evaporator without a forced circulation pump can be appropriately reduced. The electric power of the forced circulation pump is relatively high, and after removing the forced circulation pump, the power consumption of the evaporation system is reduced, resulting in a decrease in operating costs.
The falling film evaporator is only suitable for the concentration of wastewater and is not suitable for situations with salt crystallization. Therefore, using a falling film+forced circulation combination evaporator actually sacrifices the reliability of salt water evaporation. Choosing a falling film+forced circulation evaporator to treat salt containing wastewater should be more cautious. In general, we only use a combination of falling film and forced circulation evaporation for wastewater with a salt concentration below 10%.
11. Should three effect or four effect wastewater evaporation equipment be used, and their respective advantages and disadvantages?
Answer: Overall, the initial cost of the four effects will be higher, and the operating cost of the four effects will be relatively lower. Specifically, in the evaporation process of saline wastewater, due to the problem of increasing boiling point with increasing concentration, excessive efficiency is not suitable. The energy savings caused by excessive efficiency are becoming increasingly limited, and the required steam pressure is increasing. At the same time, the initial cost increases significantly. Taking into account the initial and operational costs, evaporators with an evaporation capacity exceeding 100T should adopt three effect or four effect design, while small evaporators (less than 20T/d) can adopt two effect design, which should be determined based on the actual situation of the manufacturer.
