In large-scale industrial production, a significant amount of water often needs to be evaporated, which requires a substantial energy input to heat the water into steam. To reduce the consumption of heating steam, multi-effect evaporation can be employed. However, in practice, both the operation and design of multi-effect evaporation can encounter various real-world challenges. Today, Xiaoqi has compiled several classic multi-effect evaporation issues, hoping to resolve similar problems encountered by our friends!
About Material:
The multi-effect evaporator is used to treat wastewater containing 12% sodium chloride with a pH of about 1. What material is preferable? If adjusting the pH, what level is most suitable, and what materials are needed? If employing multi-effect evaporation, what is the typical feed liquid temperature control for the most economic operation?
Sodium chloride tends to crystallize during the concentration process, thus a forced circulation external circulation evaporator is chosen. Due to the high chloride ion content and acidity in the solution treated by evaporation and concentration, a heater made of dual-phase stainless steel is required to meet production requirements. However, to reduce the cost of the complete set of equipment, equipment made with part graphite and 316L materials can be selected.
In the multi-effect evaporator system, a preheater is installed to utilize the heat from the first or third effect evaporation to preheat the feedstock, which can be done without limitation.
The specific equipment selection and materials are as follows:
The first-effect heater is selected with a graphite heater due to its high evaporation temperature. The second and third-effect heaters use shell-and-tube heaters, with the tube side and tube plate materials being made of selected duplex stainless steel. The shell side material is 304/8mm stainless steel.
(2) Evaporator: The evaporator is made of 316L stainless steel material. It is equipped with manholes, sight glasses, thermometers, vacuum gauges, and other devices.
(3) Preheater: The preheater features a tube-type heating design, with both the tube and tube plate made of duplex stainless steel material. Shell material: 304/6mm stainless steel material.
(4) Feed Pump: The feed pump is made of fluoro plastic material.
(5) Recirculation Pumps, Recirculation Discharge Pumps:
Recirculation pumps, recirculation discharge pumps, require excellent sealing, temperature resistance, and ensure continuous discharge of high-concentration materials or crystalline materials under negative pressure conditions. Materials are made of fluoroplastic.
(6) Condenser: Made of 321 stainless steel material.
(7) Liquid Seal Groove: Made of carbon steel material.
(8) Vacuum Unit: Water-jet vacuum unit with variable frequency control.
Cooling Crystallizer: Reduces discharge temperature while achieving more crystallization; material choice is fluoro-plastic.
(10) Process Components: The process piping is made of 316L/fluoroplastic stainless material.
Regarding Materials:
How to control the feed when using a multi-effect evaporator to concentrate materials? How should the flow rate be adjusted when the first-effect material enters the second-effect evaporator? What issues should be considered when designing a multi-effect evaporator for material concentration?
Regardless of the efficiency of the evaporator, the fundamental purpose is to concentrate the material. Typically, there is a concentration requirement for the material after evaporation. A densimeter is installed at the tail end to control the material concentration. The densimeter is interconnected with the direct current (DC) and reflux valves. When the material reaches the set requirements, the discharge pump operates in DC mode; otherwise, it returns.
Adjust the feed amount based on the level of the sight glass; this can be used in situations with lower automation. The evaporation system is a closed system, as long as a level gauge and a regulating valve on the discharge pipe are linked to the tail effect, there is no need for such complexity; no flow control is required from stage one to stage two.
The key to multi-effect evaporation lies in the balance of heat between each stage. For instance, the secondary steam generated by one stage should be used to heat the next, with the steam being perfectly utilized by the next stage. If it's not, the pressure of the first stage must increase; otherwise, the evaporation intensity of the second stage won't meet the requirements.
I'm not familiar with the level of automation on your equipment, but I'd like to share some of my own experience in hopes it's helpful.
Our facility utilizes vacuum multi-effect evaporation, operated manually; please note the following points:
Steam pressure is stable; monitor regularly.
Vacuum and temperature meet the requirements, especially under the negative pressure operation, fluctuations were detected; an immediate cause was sought.
Adjust the discharge valve to ensure the discharge density meets specifications. Measure at regular intervals; increase measurements when the equipment is unstable. Under normal circumstances, there should be minimal variation.
Adjust the feed valve to stabilize the liquid level at the center of the sight glass.
Vacuum pumps, circulating pumps, and condensate pumps are operating normally.
If temperatures keep rising, first check if the vacuum has tripped, then see if condensate hasn't been drained. If all is well, inspect if steam pressure has suddenly increased; if not, it may be leaking. Stop and hold pressure, and confirm the issue.
Upon detecting a rapid increase in temperature, immediately shut off the steam valve, check if the vacuum pump trips. If it does, close the vacuum pump intake valve. If not, inspect all states, and then restart once the temperature drops.
Pressure Design
The multi-effect evaporation system includes common configurations such as two-effect and three-effect evaporators. Each effect evaporator experiences a certain pressure drop. During design calculations, the pressure of the heating steam and the pressure in the final effect evaporator are known. It is then assumed that the pressure drops in each effect are equal, allowing for the calculation of the pressure conditions within each evaporator. However, in actual production, the pressures within the evaporators are not equal to the design pressures. What are your experiences with the pressure distribution in evaporation systems?
Certainly, here is the translation: "In reality, there is indeed a certain degree of discrepancy between design pressure and actual pressure. Generally, considering the increased boiling point and various losses, once the area is determined, the ultimate vacuum, and the heating steam are set, the actual pressure should be the standard. Essentially, the evaporation volume is roughly the same, with the design taking into account a certain margin for safety."
They calculate using the general steam pressure when heating, and the vacuum at the end is usually 0.02. Then, it's just allocated. Generally, the pressure is used to calculate the area after distribution. Once your area is determined, and the end vacuum is set, the pressure is only related to the initial steam pressure. Once the initial steam pressure is fixed, all pressures are determined. There is no fixed pressure for each stage. Hope this is clear. What's the use of asking this?
Based on the temperature corresponding to the saturated vapor pressure of the medium at a certain concentration.
Based on the varying feedstock quantities, first calculate the material balance for each stage of the evaporator, and then derive the heat balance from the material balance.
After theoretical calculations, a phenomenon of boiling point elevation was identified. Subsequently, the pressure difference was redistributed for correction, thereby aligning the basic theory with practicality.
About Flow Rate:
Seeking expertise: What is a reasonable flow rate for the secondary steam in a separator during the design of a multi-effect evaporator?
Please refer to the relevant design specifications. The secondary steam velocity in the chamber is generally 1~7 m/s, with the first and second stages typically ranging from 1~2 m, and the final stage from 5~7 m. The values of 20~35 m refer to the flow velocity in the secondary steam vapor pipeline. Your opinions are welcome for review.
Dual-effect Evaporation Process: Feed rate of 2100 KG/hour, solid content of 100g/L, 30℃. Output solid content of 370g/L, heat source: 0.6Mpa saturated steam, heat transfer coefficient K=500W/m².℃. Final-effect vacuum evaporation, vacuum level -680mmHg. Calculate the area of each effect and draw the process flow diagram.
Material input: 2100, solid content in input: 10%, solid content in output: 37%. The calculated evaporation volume = material input x (1 - input concentration/output concentration). The evaporation volume is calculated as kilograms per 1532.432 hours.
Calculate the concentrations of various effects.
Due to its downstream dual-effect, the equipment does not have a heat pump system, nor does it extract additional steam. The calculated hypothetical evaporation ratio is W1:W2 = 1:1.1.
W1=729.7297 W2=1532.432-729.7297=802.7027
So the concentrations are as follows: 15.32544% for the first effect, and 37% for the second effect.
Determine the Boiling Point of the Solution
The heating steam is 0.6, and the vacuum is -90.4. I just explained the methods.
Establish total pressure by equating the two effective pressure drops.
Next, calculate the pressures of each efficiency, then consult the enthalpy damage table, the physical properties of the second gas, the latent heat of vaporization, and the temperature will be determined. Calculate the total temperature difference loss.
The content provided does not include any Chinese text; therefore, it is returned as is: Split into three sections: 1. Increase in boiling point, Duhring's rule, either consult the property manual or conduct experiments in engineering, as this is very important.
2. The boiling point of the still night is rising; there's a formula, figure it out yourself.
3. Resistance and pipeline losses, an estimated few degrees.
Third: Steam Consumption for Heating
No fluff, straight to the formulas. Calculate the heat enthalpy, list three formulas, and complete the calculations for the two-effect evaporation volume and steam consumption. Compare the results with the set evaporation volume.
Fourth: Evaporation Area
Heat transfer coefficients provided, no need to say more; divide the heat by yourself.
When calculating for two areas with significantly different sizes, we redistribute the temperature differences and recalculate until the areas are roughly consistent. This has led to severe heating chamber-like evaporation, which can be troublesome during the debugging process.
