Ice-melted water undergoes membrane pretreatment

When treating coking wastewater by cold method, the frozen ice is taken and centrifuged to separate the ice water. Since the ice is not eutectic with other substances, other substances in the water can be separated from the ice. After melting and membrane treatment, it can provide pre-treatment for membrane treatment and meet the conditions of membrane treatment. On the other hand, the concentration of water melted by ice has been reduced. After membrane treatment, the effluent concentration is even lower,

1.Process principle and flow analysis

（1）Freezing into ice and solute separation

Non eutectic characteristics: When water freezes at low temperatures, solutes (such as salts and organic compounds) rarely merge into the ice crystal lattice due to differences in intermolecular forces, but instead accumulate in the unfrozen mother liquor.

▶ Analogy: When seawater freezes, salt remains in the liquid water, and fresh water is obtained after the ice melts.

Freezing process:

Cool the wastewater to -10~-15 ℃ through a refrigeration unit, forming a mixed system of ice crystals and high concentration mother liquor.

Key equipment: scraper crystallizer, immersion freezer to ensure even growth of ice crystals.

（2）Centrifugal separation and ice crystal purification

Centrifugal effect: The ice mother liquor mixture is subjected to solid-liquid separation using a centrifuge (with a speed of 3000-5000rpm) to remove high concentration mother liquor carried on the surface of ice crystals, thereby increasing the purity of ice crystals to over 95%.

▶ Data: After centrifugation, the salt content of ice crystals can be reduced to below 500mg/L (the salt content of raw water is usually 10000-30000mg/L).

（3）Ice melting and membrane deep treatment

Melting water characteristics: The water quality after ice crystal melting has been preliminarily purified (COD reduced to below 500mg/L and salt content reduced to below 1000mg/L), meeting the requirements of membrane treatment for incoming water.

Membrane process selection:

Reverse osmosis (RO): removal rate>99%, effluent COD<50mg/L, conductivity<100 μ S/cm, can be directly reused.

Nanofiltration (NF): Pre treatment with salt separation, intercepting divalent salts (such as sodium sulfate) and passing monovalent salts (such as sodium chloride), creating conditions for subsequent salt separation crystallization.

2.Technical advantage: providing dual value of "pretreatment+concentration dilution" for membrane treatment

（1）Reduce membrane fouling and extend membrane lifespan

Freezing pretreatment removes over 80% of suspended solids, colloids, and large organic molecules, reducing membrane surface fouling load.

▶ Case: A coking plant adopts the "freezing+RO" process, which extends the RO membrane cleaning cycle from the traditional process of 15 days to 45 days, reducing membrane replacement costs by 60%.

（2）Reduce membrane treatment load and improve water production efficiency

After freezing, the concentration of wastewater decreases, and the osmotic pressure decreases during membrane treatment, resulting in a 30% to 50% increase in water production flux.

▶ Data: When the TDS of raw water is 20000mg/L, the water production rate of direct RO treatment is about 50%; After freezing pretreatment, the RO water production rate can be increased to over 80%.

（3）Quality based processing to reduce overall costs

The high concentration mother liquor (TDS>50000mg/L) separated by freezing can directly enter the evaporation crystallization system, reducing evaporation by 30%~50% and lowering energy consumption costs.

3.Key technical links and optimization directions

（1）Ice crystal growth control

Objective: To generate ice crystals with large particle size and regular shape, and improve centrifugal separation efficiency.

Optimization methods:

Control the degree of supercooling (1-2 ℃) to avoid the miniaturization of ice crystals;

Adopting a speed gradient control of the stirring blade (such as fast first and then slow) to promote ice crystal aggregation.

（2）Improvement of centrifugal separation efficiency

Technological innovation:

Using a disc centrifuge (separation factor>5000) instead of a traditional horizontal screw centrifuge, the separation accuracy is improved by 40%;

Add a small amount of flocculant (such as PAM) before centrifugation to improve the interfacial tension between ice and mother liquor and reduce carryover losses.

（3）Membrane process coupling

Recommended combination process:

Freezing+NF+RO: First, NF is used for salt separation, and then RO is used for deep desalination. The water reuse rate is>90%, which is suitable for high salt coking wastewater (TDS>10000mg/L);

Freezing+MBR+RO: For high organic wastewater (COD>1000mg/L), MBR is used to remove organic matter first, followed by freezing RO desalination.

4.Practical application cases and effects

（1）A coking plant in Tangshan (1000m ³/d)

Process route: Freeze crystallization (-12 ℃) → Disc centrifugation → Nanofiltration (NF90) → Reverse osmosis (SW30HR)

Processing effect:

Inlet water: COD=1500mg/L, TDS=18000mg/L;

Effluent: COD=32mg/L, conductivity=85 μ S/cm, meeting the reuse standard of "Emission Standards for Pollutants in Coking Chemical Industry" (GB16171-2012);

Cost advantage: The cost of treating one ton of water is 22 yuan, which is 18 yuan lower than the traditional "evaporation+RO" process, saving an annual cost of 6.5 million yuan.

（2）A coal chemical industrial park in Shanxi Province (5000m ³/d)

Innovation point: The refrigeration system is coupled with the waste heat of coke oven gas, utilizing the cooling waste heat of gas (50-60 ℃) to drive lithium bromide refrigeration units, reducing refrigeration energy consumption by 40%.

Economic data:

Freezing energy consumption: 6kWh/ton (traditional process 12kWh/ton);

Membrane method water production rate: 85%, annual recycled water consumption of 1.53 million tons, saving about 7.65 million yuan in water costs.

5. Process Challenges and Response Strategies

Challenge Point	Reason	Response strategy
Ice crystals carry high levels of mother liquor	Surface adsorption of ice crystals	Spray clean water before centrifugation to wash ice crystals and reduce salt content
Scaling in the refrigeration system	Ca ²+/Mg ²+low-temperature precipitation in wastewater	Pre treatment with scale inhibitors (such as polyphosphate) and regular acid washing of refrigeration equipment
Difficult to treat concentrated water by membrane method	Concentrate organic matter in the mother liquor after freezing	Mother liquor and raw water are mixed back in a ratio of 1:3 to reduce the concentration of organic matter before further treatment

6.Technology Outlook

（1）Low temperature membrane freezing coupling technology: Develop nanofiltration/reverse osmosis membranes that are resistant to low temperatures (-5~0 ℃), achieve the integration of "freezing membrane separation", eliminate the ice crystal melting process, and further reduce energy consumption by 15%~20%.

（2）Intelligent ice crystal monitoring: By monitoring the growth status of ice crystals online through infrared spectroscopy, dynamically adjusting freezing parameters, and improving separation efficiency to over 98%.

（3）Full chain resource utilization: After MVR evaporation of frozen mother liquor, industrial salt (purity>99%) is obtained, achieving a closed loop of "wastewater treatment resource recovery cost offset". For example, a project in Inner Mongolia achieved a profit of 15 yuan per ton of water through salt splitting sales, covering 80% of the treatment cost.

The collaborative process of freezing method and membrane method, with the advantages of "low energy consumption, high separation efficiency, and great potential for resource utilization", is becoming one of the core technologies for deep treatment and zero discharge of coking wastewater, especially suitable for areas with water scarcity or strict environmental protection requirements.

Costs and expenses

1.Core energy consumption process and data analysis

（1）Freeze crystallization stage (accounting for 60%~75% of total energy consumption)

Traditional mechanical refrigeration:

Adopting screw or centrifugal chillers to reduce wastewater temperature from 25 ℃ to -10~-15 ℃, with a power consumption of approximately 8-12 kWh per ton of water.

Case: A 300m ³/d coking wastewater project uses a screw unit with R22 refrigerant, with a refrigeration energy consumption of 11.5 kWh/ton and an annual electricity bill of approximately 1.2 million yuan.

Waste heat driven refrigeration:

By utilizing the waste heat from coke oven gas, steam, or industrial waste heat to drive lithium bromide absorption refrigeration units, the electricity consumption per ton of water can be reduced to 4-6 kWh (only the solution pump and fan need to be driven).

Case: In a coal chemical industrial park in Shanxi, coupled with coke oven gas waste heat (55 ℃), the lithium bromide unit consumes 5.8 kWh of electricity per ton of water, which is 50% more energy-efficient than mechanical refrigeration.

（2）Centrifugal separation stage (accounting for 10%~15% of total energy consumption)

Disc centrifuge: with a speed of 5000rpm, a processing capacity of 10m ³/h, a single device power of 15kW, and a power consumption of 1.5-2kWh per ton of water.

Horizontal spiral centrifuge: speed of 3000rpm, processing capacity of 20m ³/h, power of 30kW, power consumption of 1-1.5 kWh per ton of water (low efficiency but low investment cost).

（3）Membrane treatment stage (accounting for 15%~25% of total energy consumption)

Reverse osmosis (RO):

The inlet pressure is 1.5~2.0MPa, and the power consumption per ton of water is 2~3 kWh (when the water production rate is 80%).

Case: The RO system of a project in Tangshan consumes 2.8 kWh of electricity per ton of water, and after combined with refrigeration pretreatment, it saves 38% energy compared to direct RO (which consumes 4.5 kWh/ton of electricity).

Nanofiltration (NF):

The inlet pressure is 0.8~1.2MPa, and the power consumption per ton of water is 1~1.5 kWh. It is commonly used for salt pretreatment.

2. Comprehensive energy consumption of typical process combinations

Process route	Freezing energy consumption (kWh/ton)	Centrifugal energy consumption (kWh/ton)	Energy consumption of membrane method (kWh/ton)	Comprehensive energy consumption (kWh/ton)
Traditional freezing+RO (mechanical refrigeration)	10~12	1.5~2	2.5~3	14~17
Waste heat refrigeration+NF+RO (lithium bromide refrigeration)	4~6	1~1.5	3~3.5	8~11
Freezing+MBR+RO (low-temperature membrane coupling)	6~8	1~1.5	2~2.5	9~12.5

3.Energy consumption optimization technology and case studies

（1） Energy saving of refrigeration system

Multi stage freezing series: First, remove some moisture by pre freezing at -5 ℃, and then concentrate the mother liquor by deep freezing at -15 ℃, reducing total energy consumption by 15% to 20% compared to single-stage freezing.

▶ Data: A certain project adopts two-stage refrigeration, reducing the comprehensive energy consumption from 12kWh/ton to 9.8kWh/ton.

CO ₂ transcritical refrigeration: Using CO ₂ as a refrigerant, the refrigeration efficiency is 30% higher than traditional R22, and it is environmentally friendly without damaging the ozone layer. The power consumption per ton of water can be reduced to 7-8 kWh.

（2）Energy recovery and utilization

Ice crystal melting heat recovery: The heat generated by melting ice crystals (approximately 334kJ/kg) is used to preheat the wastewater to be treated through a plate heat exchanger, reducing the freezing load by 10% to 15%.

▶ Case: After recycling melting heat in a project in Shandong, the energy consumption for freezing decreased by 1.2 kWh/ton.

Membrane concentrated water waste heat utilization: RO concentrated water (temperature 25-30 ℃) is used to preheat the wastewater before freezing, reducing refrigeration consumption.

（3）Intelligent operation control

Through the PLC system, the load of the refrigeration unit is adjusted in real-time based on the temperature and concentration of the wastewater, avoiding the phenomenon of "big horses pulling small cars" and saving energy by 10% to 15%.

▶ Data: After the transformation of a certain automation system, the average load rate of the refrigeration unit increased from 70% to 85%, saving 150000 kWh of electricity annually.

4.Energy consumption comparison with other wastewater treatment technologies

Processing technology	Comprehensive energy consumption (kWh/ton)	Applicable scenarios
Freezing method (optimized)	8~12	High salt and high organic coking wastewater
Multi effect evaporation (MEE)	80~120	High concentration saline wastewater
Mechanical vapor recompression (MVR)	40~60	Medium concentration saline wastewater
Traditional "Biochemistry+RO"	15~20	Low salt (TDS<5000mg/L) wastewater

Conclusion: The energy consumption of freezing method is significantly lower than that of evaporation technology, comparable or slightly lower than the traditional "biochemical+RO" process, especially in the treatment of high salt wastewater (evaporation energy consumption is 5-10 times that of freezing method).

5.Key factors affecting energy consumption

（1）Initial temperature of wastewater:

For every 10 ℃ decrease in inlet water temperature, refrigeration energy consumption can be reduced by 8% to 10%. It is recommended to first pre cool the wastewater to below 25 ℃ through a cooling tower.

（2）Target desalination rate:

The desalination rate of the freezing method is usually 60%~80%. If the desalination rate is required to be greater than 90%, the freezing stage needs to be increased, and the energy consumption will correspondingly increase by 15%~20%.

（3）Regional climate conditions:

In winter, natural cold sources (outdoor temperature<-5 ℃) can be used to assist freezing in northern regions, reducing energy consumption by 20% to 30% compared to southern regions.

6.Future energy consumption optimization direction

（1）Adsorption freezing coupling technology: First, use activated carbon to adsorb and remove some organic matter, reduce ice crystal pollution during freezing, improve freezing efficiency, and estimate an additional 10% reduction in energy consumption.

（2）Magnetic freezing technology: Applying a magnetic field (0.5-1T) during the freezing process promotes the directional growth of ice crystals, reduces undercooling loss, and can reduce energy consumption by 5% to 8%.

（3）Photovoltaic refrigeration integration: By using photovoltaic power generation to drive refrigeration units, "zero external electricity purchase" treatment can be achieved in areas with sufficient sunlight. For example, a project in Xinjiang has piloted this mode, reducing the comprehensive energy consumption per ton of water to below 5kWh.

The energy consumption level of the freezing method has industrial application economy, especially after combining waste heat utilization and optimizing the process, the electricity consumption per ton of water treatment can be controlled within 10kWh, which has significant advantages compared to traditional high energy consumption technologies and is suitable for the low-carbon transformation needs of the coking industry.