What Is a Demister?

The demister / mist eliminator is mainly composed of fixed devices such as wave blades, plates, and clips. During wet desulfurization, the absorption tower is liable to generate a "fog" with a particle size of 10--60 microns during operation. "Mist" not only contains water, but it also dissolves sulfuric acid, sulfates, sulfur dioxide, etc. It also causes fouling and severe corrosion of fans, heat exchangers and flues. Therefore, the wet desulfurization process proposes to remove the absorption equipment Fog requirements, the purified gas must be defogged before leaving the absorption tower. The defogger nozzle is one of the main components of the defogging system. The defogger nozzle is mainly made of high-quality plastic, which has extremely high wear resistance and long service life.

The demister is an important part of the desulfurization tower. The function of the demister is to capture the mist particles and slurry droplets entrained in the smoke during the spray absorption process. The efficiency of the demister is not only related to the structure of the body, but also to the weight and particle size of the mist particles. The atomization particle diameter of the nozzle is related to the viscosity of the absorption liquid, the spray claw force and the nozzle structure. Only when the performance of the demister is matched with the diameter of the mist particles can a good defogging effect be achieved.

If the defogging is not good, the slurry entrained in the flue gas will be taken to downstream equipment, such as air heaters, induced draft fans (commonly used in industrial boiler desulfurization), and flue ducts, which will cause blockages, severe induced draft fan vibration, severely corroded flue ducts and scaling And so on, was forced to stop operations. ^[1]

(1) Defog efficiency. Refers to the ratio of the mass of droplets captured by the demister within the unit time to the mass of droplets entering the demister. The defogging efficiency is a key indicator for evaluating the performance of the defogger. There are many factors that affect the defogging efficiency, mainly including the flue gas flow rate, the uniformity of the airflow distribution through the section of the defogger, the blade structure, the distance between the blades, and the layout of the defogger. Form, etc.

The defogger layout usually includes: horizontal, herringbone, V-shaped, combined and other large-scale desulfurization absorption towers are mostly herringbone-shaped, V-shaped or combined (such as diamond, X-shaped). The horizontal section of the exit of the absorption tower is horizontal.

Baffle type demister

The baffle defogger uses the inertia of the moving particles in the moving airflow. By suddenly changing the flow direction of the mist-containing airflow, the fog particles deviate from the flow direction of the airflow under the action of inertia, and they are separated (removed) when they hit the baffle plate. . The mist-containing airflow changes the direction of flow under the action of the baffle. The foot uses the mist particles to inertially separate the mist particles, similar to an inertial dust collector.

If the turning angle is large, the air velocity is high, and the distance between the baffles is small, the defogging efficiency is high, but the resistance loss is large. In addition, too high speed will entrain the trapped mist particles into the air flow. These factors are matched to obtain a suitable and efficient baffle demister.

(1) Installation of baffle demister

1) Horizontal installation. The smoke flows vertically.

Advantages: easy installation and less space; up and down spray nozzles can be arranged for flushing, and the slurry and water flow into the circulation reaction tank together; small resistance loss, = 3.4m / s, p = 180Pa.

Disadvantages: Because the flue gas flows vertically upward, it is easy to entrain the flue gas twice, and the upward flow velocity is generally 3 ~ 5m / s.

2) Vertical installation. The flue gas flows horizontally.

Advantages: The flue gas flows in the horizontal direction, and the liquid for trapping mist particles flows vertically downward, which is not easy to cause secondary entrainment. The flue gas flow rate can reach 6 to 7m / s. The defogging element can be arranged in the horizontal flue to save space and height of the desulfurization tower.

Disadvantages: Vertical installation takes up more space.

(2) Flushing the deflector of the baffle

Because the desulfurization and absorption circulation liquid contains particles such as absorbent particles, soot, and desulfurization products, the baffles will stick to these particles and gradually scale and block when they collect mist particles.

In order to keep the defogging channel clean and unobstructed, without increasing resistance, without clogging, and capable of continuous operation, the defogger must be flushed regularly and effectively. Effective refers to the ability of the backwash water to wash down the deposits on the demister baffle. Timing means controlling the backwash rhythm. There is more sediment, which not only increases the resistance of the demister, but also increases the difficulty of flushing. This flushing cycle is related to various factors such as the structure of the mist eliminator, the viscosity of the mist particles, the pressure of the flushing water, and the performance of the nozzle. It is generally obtained through simulation tests and more running tests.

(3) Material of demister

The material selection of the demister should consider corrosion resistance, wear resistance, possible flue gas temperature fluctuations or short-term over-temperature. Generally use temperature-resistant glass fiber reinforced plastic, polypropylene, polysulfone and S316L, S317LM stainless steel. The saturated flue gas temperature is generally about 50 ° C. Considering various possible factors, generally, materials that can withstand 80 ° C and corrosive environment of demister (strong acidity, CI ^- ion concentration> 10g / L) should be used. ^[1]

(Defogger nozzle)

(1) Defogging efficiency: Under normal operating conditions, the droplet concentration in the flue gas at the outlet of the defogger is lower than 75mg / Nm ³ ;

(2) Pressure drop: Disregard the interference before and after the demister, and ensure that the pressure of the entire demister system is reduced to 120Pa under 100% smoke load.

(3) High temperature resistance: 80--95 ° C.

(4) Pressure resistance: ensure that the blade can work normally when the pressure of washing water is 0.3MPa.

(5) Flushing nozzle: Full-cone nozzle, flushing water spraying angle is 90-120 degrees, spraying solid cone, can ensure that the blade is completely covered. (The designed water consumption is at the maximum gas load. Taking into account the system water balance requirements, if the gas load is reduced, the water consumption can be reduced by half by increasing the flush interval).

The demister is used to separate the liquid droplets entrained by the gas in the tower to ensure mass transfer efficiency, reduce valuable material loss and improve the operation of the compressor behind the tower. Generally, a demister is installed at the top of the tower. It can effectively remove 3--5um mist droplets. If a mist eliminator is set between the trays, not only the tray's mass transfer efficiency can be guaranteed, but also the plate spacing can be reduced. Therefore, the demister is mainly used for gas-liquid separation. Air filters can also be used for gas separation. In addition, the wire mesh can also be used as a buffer for various instruments in the instrument industry to prevent electronic shields from radio wave interference.

Wet desulfurization, it also dissolves sulfuric acid, sulfate, sulfur dioxide and so on. If not properly resolved, any "fog" entering the chimney.

Actually, the sulfur dioxide is discharged into the atmosphere, and at the same time, the fan, heat exchanger and flue are polluted and severely corroded. Therefore, in the wet desulfurization process, requirements for defogging of the absorption equipment are required, and the purified gas must be defogged before leaving the absorption tower. The demister is a key device in the FGD system, and its performance directly affects the continuous and reliable operation of the wet FGD system. Failure of the demister will not only cause the shutdown of the desulfurization system, but may even lead to the entire unit (system shutdown).

The most common forms of defogger are flat-type and roof-type.