From emission reduction to emission limitation to zero emission, the standard of wastewater discharge is gradually rising. To achieve "zero discharge", the focus is on the full recovery of highly saline wastewater, which essentially means separating water and salts from the wastewater.
Currently, concentration and crystallisation technologies, as well as coupled and synergised technologies, are more often used to achieve zero discharge of highly saline wastewater. Of course, sometimes, depending on the actual situation of the high-salt wastewater, it is necessary to add pre-treatment technology before the technology in order to provide better treatment conditions for the subsequent process.
Concentration, the core process for the resource treatment of high-salt wastewater, is divided into thermal concentration and membrane concentration depending on the target and the scope of application.
Among them, thermal concentration technology is suitable for treating wastewater with high TDS and COD of up to several hundred grams per litre, by heating to concentrate the ions in high-salt wastewater at a high rate, mainly including multi-stage flash evaporation MSF, multi-effect evaporation MED and mechanical vapour recompression evaporation MVR.
Multi-stage flash evaporation (MSF)
Multi-stage flash technology began in the 1950s as a process whereby highly saline wastewater heated to a certain temperature is sequentially flash vaporised in a series of vessels at progressively lower pressures, before the vapour is condensed to give fresh water.
As the first distillation technology to be applied, multi-stage flash MSF is a mature process, reliable in operation and suitable for large-scale, but with relatively low thermodynamic efficiency, high energy consumption and the presence of equipment fouling and corrosion limiting the MSF first effect steam temperature and affecting operating costs.
Multi-effect evaporation (MED)
Basic principles: The principle of multi-effect evaporation (hereinafter referred to as MED) is to connect multiple evaporators in series, the secondary steam of the previous evaporator as the heating steam of the next evaporator, and the heating chamber of the next evaporator is the
Condenser of the previous evaporator.
In the multi-effect
Evaporation System, only fresh steam needs to be added at the first effect, and the secondary steam produced at the top of the evaporation tower in front of the first effect afterwards is directly used as the heating medium of the subsequent reboiler of the first effect evaporation tower, and there is no need to introduce fresh steam into the evaporation tower after the first effect, and the steam at the top of the last effect tower can be used as a low pressure grade heat source.
Therefore, its biggest advantage is the multiple use of secondary steam vaporisation and condensation, which can significantly reduce the consumption of fresh steam.
MED evaporator type MED evaporator types are many, according to the evaporation pressure, evaporator type, evaporation effect number and material flow direction classification, a total of four categories of fifteen kinds:
According to the steam pressure is divided into: normal pressure evaporation, pressurized evaporation and reduced pressure evaporation;
According to the evaporator type is divided into: tube type evaporation, plate type evaporation and tube plate combined evaporation;
According to the number of effect is divided into: two effect, three effect, four effect, five effect and six effect evaporation;
According to the material flow is divided into: parallel flow, countercurrent, mixed flow and flat flow.
So, MED evaporator in the end how to compare the selection?3 principles:
1, countercurrent and mixed flow effect are better than parallel flow system. The energy consumption of counter-current multi-effect evaporation is the smallest, and the energy consumption of parallel-flow multi-effect evaporation is the largest; the characteristics of mixed-flow multi-effect
Evaporation system are better than parallel-flow multi-effect evaporation system.
2, evaporation effect number is not more the better. When the number of effects increases, the efficiency of heat utilization also decreases, considering the increase in the number of effects, the investment of equipment increases, so the actual number of effects should have an optimal point. For example, for some high boiling point material system, can only use two or three effect evaporator.
3, consider material characteristics, heat balance and non-condensing gas retention degree and other factors to choose evaporation pressure. Some research shows that the pressure of each effect is related to the material and heat balance of the evaporator, but also related to the characteristics of the material and the size of the non-condensable gas throttling degree above and below each effect.
the advantages and disadvantages of MED 1.The advantages of MED are mainly reflected in the following 5 aspects:
Pre-treatment is simple, less chemical consumption, add scale inhibitor can be.
Short heating time, mostly using the double lateral variable heat transfer method of condensing in the tube and boiling outside the tube, small heat transfer area and high heat transfer coefficient.
The system can provide 40% to 110% of the design value of the product water, while neither multi-stage flash nor reverse osmosis has such a large operational flexibility.
Good treatment effect, the salt is thoroughly analysed out during the treatment process and the coolant is removed by more than 90% after cooling, making it difficult for microorganisms to be inhibited by salt.
Operation reliability is high, the whole process using full automation operation, and in the operation process the pressure inside the tube is greater than the pressure outside the tube, even if the phenomenon of corrosion of the heat exchanger tube, the cooling water will not pollute the product water.
2. The disadvantages of MED are mainly reflected in the following 3 aspects:
Easy scaling inside the tube, about 10d to clean up once, need to be timely descaling treatment.
The number of effects increases, the steam utilization rate is low. When the number of effects increases, each effect of the heat transfer temperature loss will increase, such as each evaporation of 1t of water consumed by the ratio of steam for an effect of 1.1, two effects of 0.57, three effects of 0.4, four effects of 0.3, five effects of 0.27, equipment production capacity decline.
multi-effect evaporation MED common three technical problems and countermeasures Generally speaking, multi-effect evaporation MED often appear in the device blistering, evaporator scaling, salt ion end-effect steam corrosion equipment and other three major problems.
1, for the device in the blistering problem, the solution -
Physical defoaming mainly has high temperature and low temperature defoaming method, acoustic defoaming method, liquid spray dispersion defoaming method and mechanical vibration method. Although physical defoaming is effective in the case of particularly large treatment volume, its device and its operating costs are high;
Chemical defoaming method mainly refers to the use of defoamer, but the use of expensive defoamer, high production costs and complex production processes;
Mechanical defoaming method mainly uses rotation to change the pressure and shear force acting at the bubble to achieve the removal of foam, because of its low cost, good defoaming effect, more popular at present.
2, for the evaporator fouling problem, the solution -
Some researchers have the evaporator outer wall scale sample (sodium sulfate and calcium carbonate) for acid cleaning plus neutral cleaning, the end-effect heat exchanger inner wall scale sample (calcium carbonate) acid cleaning, hanging piece of analysis found that the average corrosion rate of each effect hanging piece are less than 1g/m2-h, the total corrosion amount are less than 10g/m2.
It is worth mentioning that the method is better than the "industrial equipment chemical cleaning quality standards" (HG/T2387-2007) and "corrosion sample preparation, cleaning and assessment standards".
3, for the problem of salt ions containing end-effect steam corrosion equipment, the solution -
Can use low-chloride ion content condensate for low-temperature, regular, quantitative replacement and replenishment, and the addition of high-efficiency corrosion inhibitors in the circulating water.
Mechanical Vapour Recompression Evaporation (MVR)
Basic principles Mechanical steam recompression technology (hereinafter referred to as MVR) is to use the secondary steam generated by the evaporation system itself and its energy to upgrade the low-grade steam to a high-grade steam heat source through the mechanical work of the compressor. This cycle provides heat energy to the evaporation system, thus reducing the need for external energy as an energy-saving technology.
MVR process flow
In this system, the heat source for the pre-heating phase is provided by the steam generator until the material starts to evaporate to produce steam.
The material is heated to produce secondary steam, which is compressed by the compressor to become high temperature and high pressure steam. The high temperature and high pressure steam produced here is used as the heat source for heating, and the material in the evaporation chamber is continuously evaporated by heating, while the high temperature and high pressure steam passing through the compressor is cooled to condensate, i.e. treated water, through continuous heat exchange.
Compressor as the heat source of the whole system, to achieve the conversion of electrical energy to thermal energy, to avoid the whole system on the outside raw steam dependence and ingestion.
MVR system equipment composition From the MVR evaporation process, it is easy to see that the MVR evaporation system is composed of various equipment connected together in series, and the equipment should be skillfully matched with each other in terms of thermodynamics and heat transfer to make the whole system achieve the best effect. There are four main pieces of equipment in the system as follows:
1. compressors. the MVR compressors are selected from two main types: roots compressors and centrifugal compressors.
Roots blower is often used to compress small flows of steam, belonging to is a volumetric compressor, which provides a small air volume and a large temperature rise, suitable for evaporation of small amounts of material with a large boiling point rise.
The centrifugal compressor is a differential pressure fan, which provides a small pressure difference, a large flow rate, a small temperature rise, a uniform exhaust, and a non-pulsating airflow, suitable for materials with a large evaporation volume and a small boiling point rise.
Comprehensive view, the stability of centrifugal compressor is better than roots compressor, but centrifugal compressor will sometimes occur wheezing phenomenon, will lead to unstable compressor.
2, evaporator. The type of evaporation processing device is generally divided into ascending film evaporation and descending film evaporation.
It is mainly based on the characteristics of the treatment material, energy consumption to choose. At present, the main domestic use of falling film evaporation method.
3.Heat exchanger. In the MVR heat pump evaporation process, the heat exchanger used is mostly inter-wall heat exchanger.
In this kind of heat exchanger, hot and cold fluids do not directly contact, but through the wall between the heat exchange. Commonly used in the production of wall heat exchanger types are: tube heat exchanger, corrugated heat exchanger and
Spiral Heat Exchanger.
4, gas-liquid separator. Gas-liquid separator is to provide materials and secondary steam separation site.
Its role is mainly to gather the solution in the mist froth into droplets and separate the droplets from the secondary steam. It is worth mentioning that the design of the separator should fully consider the evaporation volume, evaporation temperature, material viscosity, separator level and other factors.
the technical advantages of MVR 1, compared with the traditional evaporation system, MVR system only needs to start, pass into the raw steam as a heat source, and when the secondary steam generated, the system stable operation, will not need external heat source, the energy consumption of the system on the compressor and all kinds of pump energy consumption, so the energy-saving effect is quite significant.
2.The energy consumption of MVR evaporator system is mainly the electricity consumption of the compressor, the operation cost drops significantly, the operation and maintenance cost is low, because the system does not need industrial steam, its safety aspect is low hidden danger, the operation is simple.
3, in the same evaporation processing volume, MVR evaporator required area is far less than the traditional multi-effect evaporation equipment.
MVR treatment of high salt wastewater in common technical problems Although MVR technology plays a good role in the treatment of high-salt wastewater, there are still some technical problems in operation that have an impact on the operation effect.
1. System fouling problem
Scaling on the wall of the heat exchanger is one of the main reasons why the evaporation efficiency of the system is reduced, mainly because the heating heat source is the use of secondary steam, scaling and coking will make the heat transfer effect drop, the evaporation volume per unit of time is reduced, which makes the amount of compressed secondary steam available reduced, the impact on production capacity will be more obvious.
Due to the special nature of the MVR evaporator, can not clean the equipment on time is more common, which is one of the reasons for the unstable production capacity.
2.Temperature rise problem problem
The temperature rise problem in the MVR system is an important factor affecting its application in the treatment of saline wastewater.
When MVR technology is used to treat high concentration saline wastewater, due to its high concentration and large boiling point rise, the corresponding steam compressor needs to be raised to a higher temperature to overcome the impact of the boiling point rise, putting forward higher requirements for the compressor and significantly increasing the energy consumption of the system.
Studies have shown that using MVR evaporation technology, a reasonable temperature rise range is 8°C to 20°C. If the boiling point rises above 18℃, MVR technology will lose its advantages.
3.Material physical properties of the MVR selection matching problem
Due to the different sources of industrial wastewater, MVR needs to be selected according to the physical properties of different materials.
Material characteristics analysis mainly includes: the composition of the material; whether the material is accompanied by crystallization precipitation in the evaporation process; the viscosity, specific heat, density and boiling point of the material.
A single material can be obtained by consulting the relevant tables, but industrial high salt wastewater is mostly a mixed type of material and liquid, its relevant data can only be estimated through simulation, therefore, accurate analysis and calculation of material properties is a key factor to ensure the normal operation of the MVR plant.
(a) For materials with a large boiling point temperature rise, MVR single effect evaporation is generally used;
High concentration materials require the use of forced circulation to prevent coking if the material flow rate is too slow;
Heat-sensitive materials require to stay in the evaporator for as short a time as possible, etc.
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