(1) The basic principle of physical absorption
Gas absorption can be divided into physical absorption and chemical absorption. If there is no significant chemical reaction in the absorption process, it is simply the process of dissolving the absorbed gas in the liquid, which is called physical absorption, such as absorbing SO2 with water. The characteristic of physical absorption is that as the temperature increases, the amount of absorbed gas decreases.
The degree of physical absorption depends on the gas-liquid equilibrium. As long as the partial pressure of the absorbed gas in the gas phase is greater than the partial pressure of the gas when the liquid phase is in equilibrium, the absorption process will proceed. Because the driving force of the physical absorption process is very small and the absorption rate is low, the gas-phase partial pressure of the purified gas is required to be greater than the gas-liquid equilibrium partial pressure in engineering design. The rate of physical absorption is low, and physical absorption alone is rarely used in modern flue gases.
(2) The basic principle of chemical absorption method
If the absorbed gas components react chemically with the components of the absorbing liquid, it is called chemical absorption, for example, using lye to absorb SO2. The process of using solid absorbent to chemically react with the absorbed components and separating them from the flue gas is also chemical absorption. For example, the flue gas desulfurization by spraying calcium (CaO) in the furnace is also chemical absorption. During the chemical absorption process, the absorbed gas reacts with the liquid phase components, which effectively reduces the partial pressure of the absorbed gas on the surface of the solution. The driving force of the absorption process is increased, that is, the absorption efficiency is improved and the gas phase partial pressure of the absorbed gas is reduced. Therefore, the chemical absorption rate is much larger than the physical absorption rate.
Both physical absorption and chemical absorption are affected by the gas phase diffusion velocity (or gas film resistance) and the liquid phase diffusion velocity (or liquid film resistance). . In flue gas desulfurization, it is necessary to continuously purify a large amount of flue gas containing low concentration SO2 in an instant. If physical absorption is used alone, it is difficult to meet the SO2 emission standard due to its low purification efficiency. Therefore, chemical absorption method is widely used in flue gas desulfurization technology. Flue gas desulfurization by chemical absorption method is relatively mature in technology, rich in operating experience and strong in practicability, and has become the most widely used and common flue gas desulfurization technology.
In the desulfurization tower and desulfurization dust collector, the flue gas containing SO2 is washed with lye, and the SO2 in the flue gas is chemically absorbed. In order to strengthen the absorption process, improve the desulfurization efficiency, and reduce equipment investment and operating costs, the desulfurization tower and desulfurization dust collector should meet the following basic requirements:
(1) There is a large contact area and a certain contact time between the gas and liquid;
(2) The gas-liquid disturbance is strong, the absorption resistance is small, and the absorption efficiency of SO2 is high;
(3) The operation is stable, and there must be suitable operation flexibility;
(4) The pressure drop when the airflow passes through should be small;
(5) Simple structure, convenient manufacture and maintenance, low cost and long service life;
(6) No scaling, no clogging, wear resistance and corrosion resistance;
(7) Low energy consumption and no secondary pollution.