Raw materials: yellow phosphorus, liquid chlorine and phosphorus trichloride are manufactured safely. After the yellow phosphorus is heated and melted, it is pumped into a reactor with a proper amount of phosphorus trichloride as mother liquid by a special reciprocating pump under the yellow phosphorus liquid, and chlorine gas is introduced for exothermic reaction. The generated phosphorus trichloride vapor enters the rectification tower for rectification, and phosphorus trichloride flows into the storage tank (see Figure 5-3).
equation
2P+3c L2→2pc L3+3 13.95 kj/mol
(yellow phosphorus) (chlorine) (phosphorus trichloride)
The process flow is shown in Figure 5-3.
Figure 5-3 Process Flow Diagram of Phosphorus Trichloride
(1) Melting phosphorus Put yellow phosphorus in a furnace and heat it to melt.
Yellow phosphorus is a spontaneous combustion product, which needs to be stored in water, and it is easy to spontaneously combust when it leaves the water surface. Therefore, the phosphorus melting tank must be filled with nitrogen or carbon dioxide before feeding to drive away the air. Feed quickly to prevent spontaneous combustion. The melting point of yellow phosphorus is very low, and the heating temperature need not be too high, so it can be heated with interlayer hot water.
(2) Yellow phosphorus reacts with chlorine gas in the reactor to generate phosphorus trichloride. Quick reaction, releasing a lot of heat, which is very dangerous.
(1) reaction, a proper amount of phosphorus trichloride must be added first, which is called "bottom phosphorus" in production. After yellow phosphorus and phosphorus trichloride are mixed, chlorine gas is introduced for reaction, which is safer. The content of "bottom phosphorus" must also be determined regularly in production to avoid being too small.
② The ratio of yellow phosphorus to chlorine must be appropriate. If the amount of yellow phosphorus is insufficient, chlorine will react with phosphorus trichloride to produce phosphorus pentachloride. The latter is a white solid, which is easy to sublimate and often blocks the pipeline, leading to accidents. If the amount of phosphorus pentachloride is already large, when yellow phosphorus is put in, it will react violently with yellow phosphorus immediately and be reduced to a large amount of phosphorus trichloride. The reaction is as follows:
3PCl5 +2p → 5PCl3
Phosphorus pentachloride (phosphorus trichloride)
In the reaction process, due to the high temperature, phosphorus trichloride is gasified in large quantities, which makes the pressure rise and the material easy to be washed. After washing, yellow phosphorus will be sprayed out together, and yellow phosphorus will spontaneously ignite when it meets air, which is easy to cause fire; Sometimes materials explode before they are washed, and the consequences are extremely serious. We must be fully alert to the lessons of production accidents. Therefore, a special immersion reciprocating pump should be used to transport molten yellow phosphorus. Using this pump to transport yellow phosphorus can not add too much phosphorus at one time, and can prevent explosion or material erosion caused by violent reaction with phosphorus pentachloride.
(3) The reaction generates a lot of heat and must be cooled in time. The cooling method is to spray cold water around the reactor wall, and it is not advisable to introduce cold water into the interlayer to prevent the reactor wall from leaking. The pressure of interlayer cooling water makes water enter the reactor and react violently with phosphorus trichloride, causing an explosion.
PCl3 + 3H2O → H3PO3+3HCl↑
(phosphorous acid) (hydrogen chloride)
Even if spraying method is adopted, the reactor should be overhauled frequently to prevent leakage. So is the condenser.
(4) The chlorine tube must be inserted into the bottom of the reaction solution. If the pipeline breaks, chlorine reacts with phosphorus trichloride on the liquid surface to generate phosphorus pentachloride:
PCl3+Cl2→PCl5
(phosphorus pentachloride)
Adding molten phosphorus is easy to explode or flush.
⑤ In order to control the "bottom phosphorus" level in the reactor, a bottom phosphorus buoy instrument or a "bottom phosphorus buoy" can be used. The "bottom phosphorus buoy" can generally be made according to the mixing ratio of yellow phosphorus and phosphorus trichloride at a certain temperature in the reaction tank. The upper end of the buoy is equipped with an iron bar and a solenoid transformer, which is then indicated by the instrument. According to the different heights of buoys, different currents are generated, and the height of "bottom phosphorus" liquid level can be judged.
(3) The hot gas phosphorus trichloride coming out of the reactor is rectified and enters a rectifying tower for rectification to obtain phosphorus trichloride condensate, which enters a storage tank.
If phosphorus trichloride contains high free phosphorus, it will burn and explode due to spontaneous combustion of yellow phosphorus in deacidification and subsequent chemical reaction, so the quality of phosphorus trichloride should be strictly controlled.