Research on energy-saving and emission reduction application of steam compressor in lithium nitrate evaporation and concentration process
Aug,14,24
Abstract: In the process of preparing lithium iron phosphate by nitric acid method,
the lithium nitrate solution obtained by dissolving lithium hydroxide/lithium carbonate in nitric acid does
not meet the requirements for the mass fraction of lithium iron phosphate,
so an evaporation concentration process is required. This article tested the boiling point rise data of lithium nitrate solution.
Based on this boiling point rise data, a single-stage compressor pre concentration MVR evaporator and a dual compressor series
connected finished MVR evaporation concentration process were designed,
and a detailed operating cost analysis was conducted.
For high concentration and high boiling point lithium nitrate solutions, this article adopts a dual compressor series evaporation process,
which effectively replaces the traditional dual effect evaporation concentration process.
The series compression of dual compressors can increase the temperature of secondary steam by more than 34 ℃,
thereby overcoming the boiling point rise of lithium nitrate solution and achieving a power consumption of
approximately 110kWh for evaporating 1 ton of water without the need for steam replenishment.
Compared to the traditional MVR evaporation pre concentration+double effect evaporation concentration process,
the double effect evaporation concentration process requires 0.6 tons of steam to evaporate 1 ton of water.
By comparison, this process achieved a 22% reduction in production costs and achieved good economic results.
It has been confirmed that steam compressors have good energy-saving and emission reduction effects in high boiling point materials,
especially the innovative dual compressor series stable operation provides more choices for the industry.
With the rapid development of new energy vehicles in the past decade, China has achieved fruitful results in this field.
It is expected that the penetration rate of new energy vehicles will reach 50% by 2025.
The rapid development of new energy vehicles cannot be separated from the development of high-performance batteries,
especially lithium iron phosphate batteries with high safety and energy density [1,2,3,4].
The liquid-phase method for producing lithium iron phosphate products has excellent quality and is widely used.
According to anions, the main methods are sulfuric acid method and nitric acid method.
In the nitric acid method, nitric acid is mainly used to obtain the raw materials for the reaction,
such as high-quality fractions of lithium nitrate and iron nitrate solutions [5,6,7].
Lithium nitrate is mainly obtained by the reaction of nitric acid and lithium carbonate/lithium hydroxide,
resulting in a low mass fraction of lithium nitrate, only 15% to 25%.
However, the mass fraction of lithium nitrate in subsequent reactions needs to be increased to 60% to remove excess water.
Due to the high boiling point of lithium nitrate, the commonly used method is a multi effect evaporator with steam as the heating medium,
such as triple effect evaporation, double effect evaporation, or a combination of triple effect and double effect.
This method has high energy consumption and requires a large amount of steam.
In the context of carbon peaking and carbon neutrality, there is an urgent need for new energy-saving technologies.
Mechanical compression type (MVR) evaporator utilizes the characteristic of rising water vapor pressure and temperature to compress low-grade secondary steam,
increase the temperature and pressure of water vapor, and turn it into high-grade steam for reuse, which is an advanced evaporation technology [8,9,10].
At present, a single centrifugal mechanical compressor can increase the temperature of steam by up to 20 ℃.
For materials with a boiling point rise below 15 ℃, a single-stage steam compressor can achieve the reuse of secondary steam;
For high boiling point liquids, two-stage series connection is required.
This article conducts laboratory experiments to test the boiling point rise of lithium nitrate, and uses the obtained parameters for process design.
Finally, a single-stage vapor compression evaporator
and a two-stage series connected vapor compression evaporator are used to concentrate
the mass fraction of lithium nitrate solution from 15% to 25% to 60%, achieving low-cost production.
The boiling point rise test of lithium nitrate solution is carried out as follows:
take 1000mL of lithium nitrate solution with a mass fraction of 10% and a pH of 7.5, place it in a 2L beaker,
stir and heat it with a magnetic stirring heater to evaporate, control the heating power to make it in a slight boiling state,
place the thermometer at a position 1cm below the gas-liquid interface, wait for a certain amount of water to evaporate at regular intervals,
record the evaporation temperature of the material liquid, and calculate the corresponding concentration of the material liquid.
During the entire evaporation process, the liquid material changes from a colorless and transparent state to a slightly viscous liquid.
The boiling point rise of the feed solution is obtained by subtracting the boiling point of pure water from the boiling point of the feed solution.
The experimental results of the boiling point rise and mass fraction of lithium nitrate solution are shown in Figure 1.
From the experimental results, it can be seen that the boiling point rise of lithium nitrate solution follows
the numerical dependence of dilute solution and is proportional to the solute mass fraction when the mass fraction is low.
When the mass fraction exceeds 33%, the boiling point rise of lithium nitrate solution rapidly increases;
When the mass fraction of the liquid reaches 60%, its boiling point rises up to 30 ℃.
The boiling point rise of materials is the fundamental basis for designing MVR evaporators.
Its principle is to use a compressor (here a centrifugal compressor) to do work on the secondary steam in the separator,
which increases the temperature and pressure of the secondary steam, and then returns to the heater and material liquid for heat exchange, thus achieving the reuse of steam.
At present, single-stage centrifugal compressors can achieve a water vapor temperature rise of 16-20 ℃.
Even if the boiling point of the feed liquid reaches 15 ℃, the heat exchanger still has a heat exchange temperature difference of 5 ℃,
which can achieve heat exchange, such as a 45% mass fraction lithium nitrate solution.
If a two-stage series centrifugal compressor is used, a temperature rise of 34 ℃ can be achieved,
and a lithium nitrate liquid with a mass fraction of 60% can be obtained.
Based on an initial mass fraction of 20% for the feed, the overall process flow is designed as shown in Figure 2.
The material mass fraction ranges from 20% to 60%, requiring a large amount of water to be evaporated.
If you want to concentrate to the target mass fraction in one step, a three effect evaporator can be used, but this is a high energy consuming process.
In the case where the boiling point of the material does not rise high,
a single-stage steam compressor MVR can be used for pre concentration to evaporate a large amount of water,
and then a single effect evaporator, a two effect evaporator, or the two-stage series MVR evaporator with dual compressors mentioned in this article can be used.
Regardless of the type of MVR evaporator, it is recommended to design it as a two-stage evaporator when the evaporation capacity is large.
This means that the same evaporator contains two heaters and separators connected in series.
The material passes through the first stage heater&separator and the second stage heater&separator in sequence, and is evaporated twice;
The secondary steam from the separator enters the steam compressor in parallel, and after passing through the steam compressor,
it enters the first and second stage heaters in parallel.
This article focuses on the design of a lithium nitrate solution concentration process for a 50000 ton nitric acid method lithium iron phosphate.
In order to achieve this production capacity, 131.5 tons of lithium nitrate feed solution with a mass fraction of 60% is required per day.
According to the initial mass fraction of the feed material of 20%,
the total material relationship can be obtained based on this overall process, as shown in Table 1.
When designing, there is a certain margin for evaporation capacity, and the evaporation capacity is designed at 12t/h.
The feed liquid first enters the pre concentrated MVR evaporator, and its operating design parameters are shown in Table 2.
This evaporator uses a single-stage compressor MVR to concentrate the feed liquid to 40% to 45%, with a compressor temperature rise of 18 ℃.
The pre concentration MVR has an evaporation capacity of 10t/h and is divided into two stages.
The mass fraction of the first discharge is about 35%, at which point the boiling point of the liquid rises to 10 ℃, corresponding to an evaporation capacity of about 7.9t;
The second stage discharge is around 40% to 45%, corresponding to an evaporation rate of 2.1 tons. At this point, the boiling point of the feed liquid rises to 15 ℃.
The finished MVR evaporator adopts a two-stage steam compressor, with a compressor temperature rise of 17 ℃+17 ℃. Similarly, a two-stage MVR evaporator is used,
with the first stage discharging a mass fraction of about 55%, corresponding to an evaporation capacity of about 1.5t. At this time, the boiling point of the feed liquid rises to 25 ℃;
The mass fraction of the second stage discharge is about 60%, corresponding to an evaporation rate of 0.5t. At this point, the boiling point of the feed liquid rises to 30 ℃.
The project started operating at a company in Qujing, Yunnan in July 2022, and reached its design capacity during its first operation,
with stable operation and no compressor surge issues.
Due to a certain margin left in the calculation of heat transfer area, the actual evaporation rate is slightly higher than the design capacity.
As evaporation progresses, residual lithium carbonate will remain in lithium nitrate, and crystalline lithium carbonate will adhere to the wall of the heat exchanger tube.
Therefore, it is necessary to regularly use 1% to 2% dilute nitric acid to clean the equipment. After cleaning, the evaporation rate can be restored to the initial design value.
Under negative pressure conditions, the boiling point rise of lithium nitrate is lower than that tested under normal pressure,
so the actual compressor power during operation is lower than the design power.
After 6 months of stable operation, with an actual evaporation capacity of 12 tons, the power consumption of the pre concentrated MVR evaporator is about 570 kWh,
the steam consumption is about 0.5 t/h, and the power consumption of the finished MVR evaporator is about 200 kWh.
Compared with traditional three effect+two effect, MVR pre concentration+single effect evaporation concentration,
and MVR pre concentration+two effect evaporation concentration, this design greatly reduces the production cost of the enterprise.
The operating cost comparison is shown in Table 5.
From Table 5, it can be seen that the three effect+two effect process has the highest energy consumption during operation,
mainly because the three effect process evaporates one ton of water.
Although the electricity consumption is lower, it requires 0.4 tons of steam, which is a higher cost.
Nowadays, the vast majority of factories use natural gas boilers to generate steam.
A good natural gas boiler requires about 75m3 of natural gas to produce one ton of steam.
In areas with abundant natural gas, the price of natural gas is also around 3.5 yuan/m3, while in coastal areas it can be as high as 4-5 yuan/m3.
Therefore, in terms of operating costs, the operating cost of the three effect+two effect process is 2.2 times that of this design.
MVR pre concentration+two effect evaporation concentration, MVR pre concentration+single effect evaporation concentration,
and this design all contain MVR pre concentration,
so they are more energy-efficient than three effect+two effect.
This design uses two-stage MVR evaporation concentration instead of single effect evaporation or two effect evaporation,
greatly saving steam consumption and effectively reducing operating costs, achieving good economic benefits.
At the same time, from a management perspective, using less steam can eliminate or reduce boiler equipment, reduce management processes, and improve efficiency.
This design and process are conducive to achieving carbon peak carbon neutrality and reducing carbon dioxide emissions,
especially in areas with abundant hydropower resources such as Yunnan and Sichuan,
where its value will be greater.
By using two-stage compressors in series, high temperature rise can be achieved,
and this design can be used for concentration and evaporation crystallization of materials with high boiling points.
This article takes lithium nitrate as the design object and adopts a pre concentrated MVR evaporation+finished MVR evaporation mode.
The project has been running well in the enterprise, with production capacity and economic indicators meeting expectations,
achieving considerable economic benefits for the enterprise.
In this article, the MVR evaporation process of the finished product adopts a two-stage series steam compressor for the first time,
which achieves a temperature rise of 34 ℃ for the secondary steam, thus overcoming the boiling point rise of 30 ℃ for 60% lithium nitrate,
and utilizing the secondary steam to achieve the goal of energy conservation and emission reduction.
This article confirms that the series operation of high-speed two-stage compressors increases the temperature rise of secondary steam and has strong adaptability to materials.
In actual operation, there were no problems such as compressor surge, and the operation was stable.
This provides theoretical and engineering case support for the design of such devices in the future.
The evaporation capacity of the two-stage compressor connected in series with the MVR evaporator is not very high this time.
In the future, this design method can be introduced in larger devices to make corresponding contributions to energy conservation and emission reduction
