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03
2020
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03
Reverse osmosis (SWRO) seawater desalination high pressure pump selection method
The 21st century will be the century of water. At the beginning of the 20th century, there was a saying in the world that “the battle for coal in the 19th century, the battle for oil in the 20th century, and the battle for water in the 21st century”. The 47th United Nations General Assembly designated March 22 every year as “World Water Day”, calling for Countries around the world have aroused a high degree of alertness to the global shortage of fresh water resources. From a global perspective,
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Reverse osmosis (SWRO) seawater desalination high pressure pump selection method
The 21st century will be the century of water. At the beginning of the 20th century, there was a saying in the world that “the battle for coal in the 19th century, the battle for oil in the 20th century, and the battle for water in the 21st century”. The 47th United Nations General Assembly designated March 22 every year as “World Water Day”, calling for Countries around the world have aroused a high degree of alertness to the global shortage of fresh water resources. From a global perspective, according to United Nations statistics, global freshwater consumption has increased by about 6-7 times since the beginning of the 20th century, which is twice the rate of population growth. Currently, 1.4 billion people in the world lack safe and clean drinking water, that is, every One out of five people lacks water.
Asking for fresh water from the ocean has become the consensus of coastal countries in the world. In the past 30 years, seawater desalination technology has provided humans with a large amount of freshwater resources. According to the International Desalination Association
According to the latest data provided by (InternationalDesalinationAssociation), since 1997, the compound annual growth rate of seawater desalination capacity was 16.8%. In 2007, the growth rate of desalination capacity was 24.5%, a total increase of 6.7 million cubic meters per day compared to 2006. (1770MGD). There are 13,869 desalination plants in operation in the world. Their total production capacity is 63.6 million cubic meters per day (16800 MGD) of potable water, of which 53 million cubic meters per day are already in operation-only 0.5% of the global Water consumption. About half of desalination plants are in the Middle East, about 20% are in the United States, 13% are in Europe, and 12% are in Asia.
China is recognized by the United Nations as one of the 13 most water-poor countries in the world. The total amount of fresh water resources in my country ranks sixth in the world, but the per capita share is only 1/4 of the world average, ranking 109th in the world, and the distribution of water resources in time and region is very uneven. There are 10 provinces, The water resources of cities and autonomous regions are already below the minimum survival level, and the per capita water resources there are less than 500 cubic meters.
At present, there are 300 cities in our country who are short of water, 110 of which are severely short of water. They are mainly distributed in North China, Northeast, Northwest and coastal areas. Water has become a bottleneck for economic development in these areas. After 2010, my country will enter a period of severe water shortage. Some experts estimate that China’s water shortage will reach 60 billion cubic meters by 2030. Therefore, in order to ensure the sustainable development of our country's economy, the solution to the problem of freshwater resources is urgent.
The analysis of relevant experts pointed out that desalination of seawater is one of the effective ways to solve the serious shortage of freshwater in coastal areas of our country. Seawater desalination has become a rising industry in my country.
Today, when seawater desalination technology is mature, economy is an important factor that determines its wide application. At present, among more than 20 kinds of desalination technologies that have been developed, reverse osmosis (SWRO), multi-stage flash evaporation (MSF) and low-temperature multi-effect (MED-TVC) have reached industrial-scale production applications. Among them, reverse osmosis technology is a highly efficient and easy-to-use technology. Its biggest advantage is energy saving. As shown in Table 1, the energy consumption of reverse osmosis is the lowest when producing the same amount of fresh water.
In the application of reverse osmosis seawater desalination technology, high-pressure pumps are one of the key equipment. When the reverse osmosis membrane is selected, the energy consumption index of the reverse osmosis desalination system mainly depends on the energy consumption index of the high pressure pump, booster pump and energy recovery device. In the reverse osmosis seawater desalination plant, electricity costs account for 1/2 to 2/3 of water production costs, and equipment investment accounts for about 1/4 of water production costs. High-pressure pumps are the main energy-consuming equipment, and their electricity consumption accounts for about system operating costs. 35% of it is one of the main factors affecting the cost of product water.
Combined with the reverse osmosis seawater desalination project, for the selection and analysis of the high-pressure pump used in the project, selecting the appropriate pump type is of great significance for reducing the operating cost of the system.
1. Technical parameters of the pump
1. Reverse osmosis desalination process
Figure 1 is a schematic diagram of the reverse osmosis (SWRO) seawater desalination process. In the reverse osmosis desalination process, the raw seawater to be treated is pressurized by a high-pressure pump and enters the reverse osmosis membrane module: the water passing through the reverse osmosis membrane is what is needed Fresh water is produced water; the remaining part of the water that has not passed through the membrane is seawater with a higher concentration, that is, concentrated seawater. This part of the concentrated seawater with high pressure energy is directly boosted by the PX energy recovery device to part of the raw seawater to be treated, and then the booster pump is used to compensate the pressure lost through the membrane stack and the pipeline. This part of the boosted raw seawater and The raw seawater after being boosted by the high pressure pump is mixed and sent to the reverse osmosis membrane group.
2. Technical parameters
The flow rate of the high-pressure pump used in the reverse osmosis desalination system of different scales is determined by its daily processing capacity and a small part of the margin. The pressure varies according to the selected membrane model and flux, operating conditions, raw water quality and water temperature, etc. The operating pressure range of reverse osmosis is usually 5.0-7.2MPa. Table 2 lists the parameters of high-pressure pumps used in different scale seawater desalination systems.
Let's take a 50,000 tons/day seawater desalination system as an example. The system recovery rate is 42%. It is divided into 5 rows of reverse osmosis units, each with a capacity of 10,000 tons/day. Determine the technical parameters and reasonable selection of the high-pressure pump.
Assuming that seawater is standard seawater, the water temperature is 20℃.
The water production index of the desalination unit is close to the flow rate of the high-pressure pump, that is, the flow rate of the high-pressure pump is Q=425m3/h.
The required head of the high-pressure pump varies according to the selected membrane model and flux, operating conditions, raw water quality and water temperature, etc. The operating pressure range of reverse osmosis is usually 5.0-7.2MPa. The higher the operating pressure of seawater reverse osmosis, the higher the operating cost and the higher the equipment investment. The system requires a high-pressure pump head of 67.2bar, that is, the high-pressure pump flow Q=425m3/h; the head H=685m, and the efficiency index is not less than 80%.
Two, water pump selection
1. Choose the type of pump
At present, there are two main types of high-pressure pumps used in reverse osmosis seawater desalination systems: plunger pumps and multistage centrifugal high-pressure pumps. These products are relatively mature in foreign technology, and the products have been serialized.
We select the segmented multi-stage centrifugal pump type in the multi-stage centrifugal high-pressure pump according to the high-pressure pump parameter requirements of each column of the 50,000-ton desalination system (Q=425m3/h, H=685m).
2. Choose water pump series
In the segmental multi-stage centrifugal pump, mainly out of the requirements for efficiency, we chose the PWTD (N) series, which adopts an efficient hydraulic model, energy saving and environmental protection; modular design, all adopt diaphragm type extension The shaft device is easy to maintain; the structure is reasonable and reliable, and the life is long. It is a product with excellent performance and reliable structure provided by Pentair Group for the vast Chinese market.
Flow rate: ~950m3/h
Head: ~1400m
Medium temperature: -80～180℃
Maximum working pressure: ~150bar
3. Determine the pump model
According to the determined pump parameters and types, we choose the pump model: 6PWTD(I)*6
The parameters of the pump can be obtained from Figure 2 as follows:
Flow rate: 425m3/h
Head: 69.2-2=67.2bar, converted to meter water column is 67.2*10.2=685m
Efficiency: 80%
Required cavitation allowance: 10m
Energy consumption per unit of water produced by the high-pressure pump:
Where:
W- energy consumption per unit of water production, kWh?m-3
p-pressure difference of high pressure pump Mpa
η-High pressure pump efficiency%
It can be calculated that the energy consumption per unit of water produced by the high-pressure pump is 2.33kWh?m-3.
4. Determine the technical parameters of the motor
According to the performance Q and H of the selected pump, the shaft power required by the pump can be calculated according to the following formula:
Q-Flow (m3/h) 425m3/h
H-head (m) 685m
ρ-density (kg/m3); standard sea water density is calculated as 1.03
g-local acceleration of gravity, calculated according to 9.8
η-Efficiency at rated operating point, 80%
It can be calculated that the shaft power of the water pump at this operating point is 1020KW.
When selecting the motor power, the safety margin should be added according to ISO5199. When selecting the motor size, select P2 which should be greater than the pump shaft power and similar to it. So we choose the motor power to be 1120KW.
5. Optional high-efficiency COMPACT high-voltage motor.
The voltage, frequency, Hz, etc. of the high-voltage motor are determined by the power supply on site. We choose 6KV, 3-phase, 50HZ according to the domestic common power supply. In order to reduce the energy consumption per unit of water production, we use high-efficiency motors as much as possible. Therefore, the internationally leading compact COMPACT high-efficiency motors are selected, and their efficiency is as high as 96%, while the power of the commonly used YKK series in China is 93%. In addition, the COMPACT series has a protection level of IP55, small size, light weight, easy maintenance for users, and simple installation. In this system, the purchase price difference of using high-efficiency motors compared with ordinary motors is about 50,000 yuan. If the price of 1 kWh of industrial electricity is 0.5 yuan, the increased cost of high-efficiency motors can be recovered in 4 months.
3. Energy consumption analysis of the entire system
In this system, the booster pump is also part of the energy consumption. In Figure 4, through the energy consumption analysis of the high-pressure pump and booster pump, it can be calculated that the unit water consumption of the entire system is 2.68kWh/m3
It can be seen from Table 6 that through the reasonable selection of the high-pressure pump of the reverse osmosis desalination system and the selection of high-efficiency pumps and motor series, the energy consumption of the reverse osmosis desalination device with a daily production capacity of 50,000 tons can be reduced by 0.5kWh /m3 or more, greatly reducing the energy cost of reverse osmosis desalination pumps. The daily electricity bill can reduce 25,000 kilowatt-hours of electricity, saving a very considerable cost.
4. Conclusion
Today, when seawater desalination technology is mature, economy is an important factor that determines its wide application. In view of the operating conditions of reverse osmosis desalination systems, especially for desalination systems with a daily output of more than 10,000 tons, the reasonable selection of high-pressure pumps plays a key role in the energy consumption of the entire system. The selection of high-efficiency pumps and motors will increase A small part of the initial investment, but from the perspective of long-term operating costs, this part of the increased initial investment can be recovered in a short period of time. Therefore, whether it is from environmental protection or operating cost savings, the choice of efficient equipment will bring the highest return on investment to the owner.
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