The basis of microwave chemical wastewater treatment technology is the "polar molecule theory". In addition, the microwave field can cause these polar molecules to oscillate due to dipolar forces and consume energy to produce heat. Whether a substance is absorbed in a microwave field, and the strength of the absorption, is related to the electrical properties of the substance.
Experimental results have shown that the microwave power Pa absorbed by the substance per unit volume is directly proportional to the electric field strength E, tangent of the loss angle tgδ of the substance, and frequency f. All the microwave energy absorbed by the substance in the microwave field can be converted into heat energy, so Pa is the energy generated per unit volume of substance in a unit time. This quantity is related to the substance's dielectric constant, dielectric loss, and other factors.
The microwaves in a microwave digestion instrument, a type of lab automation equipment, are evenly distributed inside the cavity, but microwaves have a certain penetration thickness. If there are two digestion vessels surrounding the main vessel, there are six reflection surfaces in the cavity. Two of the reflected microwaves are absorbed by the two digestion vessels, reducing the power absorbed by the main vessel by one-third.
Each acid and solvent has a different absorption efficiency for microwaves. Under the same conditions, some solvents have a high absorption efficiency, causing inconsistency in temperature between the digestion vessels and the main vessel in a microwave digestion system. If the temperature is higher than that of the main vessel in the automated microwave digestion system, pressure may build up. For example, if nitric acid is used in the main vessel and distilled water is in the blank, the efficiency of distilled water absorbing microwaves is high. Under the same conditions, the temperature of the distilled water may exceed 260℃ when the temperature of the main vessel is only 170℃.
The temperature can be monitored in real-time up to 300℃ with an accuracy of ±0.5℃. The microwave digestion system use a direct-contact pressure measurement to reflect the pressure in real time accurately, which can reach up to 900psi with a control accuracy of 0.01MPa.
The microwave digestion system can handle 10 samples simultaneously in one batch. The safety film protects the pressure up to 200psi for medium pressure and 400psi for high pressure. The microwave digestion system adjusts the output power by changing the microwave frequency through PID control, improving the efficiency of the power supply and reducing energy consumption during standby by more than 20% compared to traditional microwave ovens. In addition, the introduction of frequency conversion technology has also reduced the operating noise of the instrument.